Geology of the Mexican Republic Moran

THE GEOLOGY OF THEMEXICAN REPUBLIC By Dante Morán-Zenteno Universidad Nacional Autonoma de México (U.N.A.M.) Collaborators: From U.N.A.M.: J. Urrutia Fucugauchi E. Cabral Cano G. Silva Romo S. Alarcoa Parra C. Caballero Miranda G. Mora Alvarez S. Campos From Instituto Nacíonal de Estadistica, Geografía e Informatíca (INEGI): J. Alvaro lruretagoyena E. Campos Madrigal J. Luís Moreno J. Uríbe Luna J. Olivera Translated and with additional annotated bibliography by James Lee Wilson and Luis Sanchez-Barreda AAPG Studies in Geology #39 Published by The American Association of Petroleum Geologists 1994 Printed in the USA. Translation of: Geología de la República Mexicana First Spanish edition, 1984, copublished by INEGI & UNAM Second Spanish edition, 1985 First Spanish reprinting © 1990 First English edition © 1994, by the AAPG AH Rights Reserved ISBN: 0-89181-047-1 AAPG grants permission for a single photocopy of an item hom this publication for personal use. Authorization for additional copies of items from this publication for personal or internaI use is granted by AAPG provided that the base fee of $3.00 per copy is paid directly to the Copyright Clearance Center, 222 Rose\"-'ood Orive, Danvers, Massachusetts 01923. Fees are subject to change. Any form of electronic or digital scanning or other digital trans­ formation of portions of this publicabon into computer-readable and/ or transmittable form for personal or corpo­ rate use requires special permission from, and is subject to fee charges by, the AAPC. Association Editor: Kevin T. Biddle Science Director: Richard Steinmetz Publications Manager: Cathleen P. Williams Special Projects Editor: Arme H. Thomas Production: Custom Editorial Productions, lnc., Cincinnati, Ohio On the cover: Canyon of Sumidero, incised in the Lower and middle Cretaceous Sierra Ma.dre Limestone near Iuxtla Gutiérrez, State of Chiapas, southern Mexico. Photo by R. K. Goldhammer, Exxon E&P Research. Inset photo shows the Middle Cupido Formation in Cortinas Canyon, Sierra Madre Oriental. Ihis and other AAPG publications are élvailable fram: The AAPG Bookstore P.O. Box 979 Iulsa, OK 74101-0979 Telephone: (918) 584-2555; (800) 364-AAPG (USA-book orders only) FAX: (918) 584-0469; (800) 898-2274 (USA-book orders oniy) About the Author • Dante J. Morán-Zenteno has been a research scientist at the Geophysics Inshtute of the National University of Mexico (UNAM) since 1992. He spent eight years working in the geoIogical mapping program of the NationaI Institute of Statistics, Geography and Information Technology (INEGI). During this time he prepélred most of the present book. Since 1984 he has participated in research projects of the Geophysics lnstitute dealing with the tectonic structure and paJeogeographic evolution of southern Mexico. As part of these projects, he has applied paleomag­ netic and isotope geochemical analyses to the study of the tectonic evolution of the southwestern continental margin of Mexico. Since 1983 he has been lecturing on geology at the Engineering Félculty of UNAM. From 1988 to 1990 he was at the University of Munich, in Germany, carrying out isotope analyses of Mexican rocks In 1992 he received his Ph.D. About the Translators • James Lee WiIson was born in Waxahatchie, Texas, é1nd raised in Houston. He attended Rice University and the University of Texas at Aushn, where he received his B.A. and M.A. degrees. He received his Ph.D. fram Yale University in 1949. Jim Wilson was a field geologist in the Rocky Mountains, Associate Professor at the University of Texas at Austin, and from 1953 to 1966 worked as a research geol­ ogist for Shell Development Company in Houston. During this period he spent three years in the Netherlands working on Mesozoic geology of the Middle East. In 1966 Jim returned to academia as Professor of Geology at Rice University; he joined the University of Michigan in 1979. In 1975 he completed él book, Carbonate Facies in Geologic History (Springer- VerJagl. Jim was President of sEPM in 1975-1976, became an Honorary member in 1980, élnd was elected an Honorary member of AAPG in 1987. In 1990 he received the Twenhofel Medal from SEPM. He has participated in carbonate field élnd lecture courses 'with the Laboratory of Comparative Sedimentation of Miami Unjversity, Florida; with ERICO of London; the University of Houston; AAPG; and MASERA Corp. of Tulsa, Oklahoma. His field experience ineludes work in Mexico, New Mexico, North Africa, the I ~ o c k y Mountains, the Austroalpine area, and the Middle East. Jim is now Professor Emeritus at the University of Michigan and adjunct Professor at Rice University in Houston. He resides in New Braunfels, Texas. As a consultant, he is working on the geology of Mexico and is involved in a world­ wide study of carbonate platforms. Luis A. Sanchez-Barreda is currently senior consultélnt for Barreda and Associates, Navasota, Texas. He received his B.5. degree in Oceanography in 1972 from the University of Baja California and in 1976 an M.A. degree in geology from Rice University. He began his career as él field geologist in Libya and Spain. After receiving his Ph.D. from the University of Texas at Austin in 1981, he worked as an exploféltionist for Pecten lnternational (Shell Oil Company). In 1987 he left Pecten to fome his own consulting company. Luis has more than 20 years of geologic experience working in Mexico, and presently specializes in frontier exploration throughout Mexico, the Caribbean, and Central and South America. His main areas of interest focus on seismic/structural interpretation of sub­ Andean, forearc, and passive margin basins of Latin America. iii Table of Contents • Preface vi Section 1 The Geology of the Mexican Republic Introdudion 3 1. GeoJogy of the Northwest Region of Mexico 5 2. Geology oi the Northern and Northeastern Regíons of Mexico 35 3. Geology oi the Central Regíon oi Mexíco 55 4. Geology of the Southeastern Regíon oi Mexico .75 Section II The Bibliography of Mexicéln Geology, 1983-1993 Introductíon to the Bibliography oi Mexican Geology, 1983-1993 87 1. Stratígraphic and PaleontoJogic Studíes, Structural Controls of Sedímentation 89 2. Plate Tectonícs, Paleomagnetic Studies, and Regional Structural Terrane Studies .. .. 117 3. Mineral Resources, Studies oi Igneous Metamorphic Rocks, Neovolcanic lnvestígatíons, Basement Studíes, and Age Dating 135 4. Petroleum and Gas, Structure and Mapping, Engineering and Environmental Geology, Hydrology, and Remote Sensing 147 v Preface • PRE5ENTATION The N<ltionallnstitute of Statistics, Geography, and Technical Inforrnation Technology ClNEGI) and the Faculty of Engineering of the National Autonomolls University of Mexico offer this work as él joint effort to contribute to knowledge of the geology of Mexico, employing the new concepts relélted to the dynamics of the earth and as a step toward the teachmg and fur­ ther development of professionals in Eélrth Science. This volume collects and interprets a large part of the information gathered during more than 15 years of geological mapping by the General Directory of Geogrélphy and forms él compendium of scienhfic contributions related to the Geology of MexiCo, many of which result from research investigations within the National Autonomous University of Mexico. TRAN5LATOR5' NOTE Even though ten yeélrs old, Dante Morán-Zenteno's summary of the Geology of the Mexican Republic remains the most complete report of this very lé1fge, structurally complex, and economicéllly important area thélt forms the southwestern margin of the North American craton. Because Morán-Zenteno's work was in Spanish only, it has not become wen known north of the border. Zoltan De Cserna's excellent 32-page Outline of the Geology of Mexico (1989, DNAG Volume A) was until this point the only other good general descri ption of Mexican geology in English. The translators have attempted to render an accurate and readable English text while retaining some of the use of passive voice and indirect style of the elegant Spanish languélge. The translators and tILe American Association of Petroleum Geologists are grateful to Dr. Dante Morán-Zenteno for his work and for rus review of the translation. We are also grateful to the Instituto Nacional de Estadística, Geografía, e Informática (INEGI) for permission to publish this version, which is complete except for outcrop photos whose originals were lost after a move of lNEGI heildquarters from Mexico City to Aguascalientes. The geological map at the scale of 1/1,000,000 (eio-ht sheets) that accompanies the original text s h ~ w s the general features of the geologic struch.ue of the country, treating not only the different types of rocks that outcrop at the surface, but also the geologic times in which they were formed, i.e. their relative positions within the stratigraphic column. However, the geological map a t 1/1,000,000 scale described aboye has been recently superceded by a map of 1/2,000,000 scale that includes important changes, particularly in southern Mex1co. Th1S map lS accompanied by text describing the vanous forma­ tions in the néltion. For this reason the appendix of the original work, "Methodology of Formulation of the Geologic Mélp at a Scale of 1/1,000,000," has not been inc1uded in the English translation. A fairly complete and briefly annotated bibliogra­ phy of Mexican geology from 1983 to 1993 has been added to update the original text references and com­ prises Section 11 of this publica tion. DI. James Lee Wilson Professor Emeritus, University of Michigan Ann Arbor, Michigan, USA and Adjunct Professor, Rice University, Houston, Texas, USA Dr. Luis A. Sanchez-Barreda Barreda and Associates Nélvasota, Texas, USA vii • Section 1 The Geology of the Mexican Republic • • Introduction • Map of the Republic of Mexico showing the regions described in each chapter. Available knowledge concerning the origin and geologic stmcture of Mexico is still incomplete. Each day scientific discoveries, advances jn mapping, and new techniques of explorabon offer more information toward the development of our understanding. Nevertheless, it is still difficult to achíeve a complete description of the geologic character of the territory of the nabon, as well as to work out functional models to explain the origin of its geologic structure. Along with the development of geologic studies that science has made in Mexico, there have been a few attempts to fonnulate general works covering the many aspects of geology that the country presents. Nevertheless, one must recognize that the lack of informabon about cer­ tain periods in the geological history of the national territory, and the numerous unexplored aTeas, have constituted sorne princi pal obstacles toward achiev­ ing a finíshed work of this type. In reviewing past information, ít is worth indicat­ ing that in 1896 a somewhat unsettled state of knowl­ edge resulted in a work in Spanish entítled A Sketch of the Geology af Mexica formulated by Jose Guadalupe Aguilera and Ezequiel Ordonez of the Geologic Instítute of Mexico, a descripbve work that constitut­ ed a11. important complement to and summary for the Geologic Map of Mexico, which had been published ear­ lier. Nevertheless, it was not until1949 that V. Garfias and T.e. Chapin published the work entitled Ce%gy 3 4 Sectíon 1 The Geology of lhe Mexican Republic of Mexico, in which reconstructions of the events that occurred d uring the geological history oE the Republic are included. A more recent work is The Geolog}-/ ofMexico, whose author was lng. Manuel Alvarez, Jr., and which the Faculty oE Engineering of UNAM printed as notes of the subject matter of the Geologt) ol Mexico, presented by the same author. Fina1ly, in 1979, lng Ernesto López-Rarnos published his work, The Geology ol Mexico, in two volumes. That publication constitutes at present the most widely known text because it con­ tains detailed descriptions of lithostratígraphic units and references to numerous unpublished works, prin­ cipally from Petróleos Mexicanos. The present book has the double objective of offer­ ing a geologícal synthesis oi Mexico as a general refer­ ence work for a11 readers, and a presentation of themes in organized and dídactic form so that it can be utilized in upper level courses related to the geolo­ gy of Mexico. The first edition of this work was the responsibility of the National Institute of Statistics, Ceography, and TechnicaJ Information (TNECl) as a complement to the geological maps that the Ceneral Department of Ceography had prepared. The preparation oE the text was the responsibility of lng. Dante J. Morán-Zenteno, then chief of Petrography and PaleontoJogy of the same department. This second edítion is the result oE combined forces of INECl, an administrative unít decentralized between the Secretaríat of Planning and Budget, and the Faculty oE Engíneering of UNAM. 1ng. Dante J. Morán-Zenteno gives courses on the geology oE Mexico and physical geology at UNAM and in addi­ líon is a researcher in the Institu te oE Ceophysics at the same university. To deveJop the present work it was necessary to divide the Republic into difierent regions, defined by natural limits, which are descríbed in each of the chapters that Eorm the work. This division does not correspond to that of the original eight maps of the Republic at the 1:1,000,000 scale, which were designed at a scale to show topography, culture, and use oE the substrate. In this work, the geologic maps that pertain to each chapter are mentioned. The information in this second edition can be used as a point oE departure Eor regional projects oE investigation and guidance. It offers, together with the geologic maps at 1:1,000,000 scale, a general key for localizing areas and objectives of economic or particular scientiEic interest. It should make c1ear, furthermore, key characteristics that are pertinent for geological interpretation oE certain regions, and it comments on the most recent models concerning tectonic evolution and the geologic orígin of signifi­ cant economíc deposits. The bíbliography that accompanies each chapter makes it possible to orga­ nize a wide varíety of consulting work aimed at studying in depth certain aspects or certain particu­ lar areas. • 1. Geology of the Northwest Region of Mexico • .....; ".:: " .. . DURANGO • • .... ,::: : ~ . • ;-'., ,,' NORTHI¡VESTERN RE'GION GENERAL CüNSIDERATIüNS For the description of northwestern Mexico the fol­ lowing natural limits have been selected in this work: to the east, the volcanic seq uence of the Western Sierra Madre; to the west, the Pacific coasts of Baja Céllifornia and Sinaloa; and to the south, the northern edge of the Neovolcanic axis. In accordance with the physiographic division of the General Department of Geography (see Figure 1.1), the provinces of Baja California, the Sonoran Oesert, Sierra Madre OccidentaL élnd the Pacific Coastal Plain me included within this region. The cli­ mate varies in general fram dry in Baja California, Sonora, and northern 5inaloa, to subhumid in the higher parts of the Sierra Madre Occidental and south of Mazatlán, In almost all the region rainfall comes in GUADALAJARA the summer, except in the north of Bajél California where the rain is in winter. PENINSULA The peninsula of Baja CélLifornia as shown on the geologic map at 1:],000,000 scale (General Department of Geography, DGG) offers él high struc­ tural complexity and rocky outcrops, which mélke it difficult to reconstruct a stratigraphic column for this region and to ascertain events that have occurred, Nevertheless a subdivision has been mélde, élS rational as possible, that permits explélnation, \vith a certélin clarity, of the geoLogic concepts of this province and that coincides in lmge part with the physiographic divisions of the OGG and with the di vision into the geologic provinces of López-Ramos (1979). 5 6 Section 1 The Geology of lhe Mexican Republic PHYSIOGRAPHICAL PROVINCES I BAJA CALIFORNIA PENINSULA 11 SONORAN DESERT 111 SIERRA MADRE OCCIDENTAL IV SIERRAS AND PLAINS OF THE NORTH V SIERRA MADRE ORIENTAL VI GREAT PLAIN OF NORTH AMERICA VII PACIFIC COASTAL PLAIN VIII COAS AL PLA'N OF ''''11I::. ..) IX CENTRAL MESA X NEOVOLCANIC AXIS XI YUCATAN PENINSULA XII SIERRA MADRE DEL SUR VIII XIII SOUTHERN GULF COASTAL PLAIN XIV CHIAPAS MOUNTAINS XV CENTRAL AMERICAN CORDILLERA XI XIII XII V XIV Figure 1.1. Physiographic framework of the Republic of Mexico. A Portion of Northem Baj a California In this zone exposures of a stratigraphic sequence whose geochronologic range varies from Paleozoic to Recent ¿¡re encountered. The configuration of the dif­ ferent units forms three pre-Tertiary belts (Figure 1.2) that run the length of this part of the peninsula and that present dearly differentiated petrographic, struc­ tmal, and stratigraphic characteristics. These belts are covered indiscriminately by volcanic bodies and Ter­ tiary and Quaternary sedimentary deposits. Ihe hrst belt, Jocated in the extreme western penin­ sula, is composed of a sequence of marine and conti­ nental Upper Cretaceous sediments that are poorly consolidated and laek appreeiable tectonic deforma­ hon. Ihis band of outcrops is of maximum width at the lahtude of Punta San Antonio, a httle less than laL 30 0 N (Figure 1.3). The sequence was designated by Beal (1948) as the Rosario Formabon and consists of subhorizontal strata oi silty, shaly, and eonglomeratic sandstone that contélin both marine iossils and sauri­ an bones. Al! this attests to the development of envi­ ronments that vary hom continental out to the platform and slope with fluctuating coastlines orient­ ed more or less paral!el to the line that divides this belt from the terranes located to the east. The latter constitute the source of supply for the sediments that comprise this sequence since in this time emergence occurred and formed mountainous masses exposed to erosiono Gastil et al. (1975) defined the outer limit of the cited belt as the "Santillan-Barrera Line" (Figure 1.2) and considered this feature to have controlled the depositional history of Baja California for long time periods. Ihese authors cite numerous paleontolagicaJ determinations that stratigraphieally position the Rosario Formaban in the Campanian and Maastrich­ tian Stages. Mina (1957) correlated this formation with clastic sediments that erop out on the western border of the State of Southern Baja California and called it the Valle Formation. Ihe sequence that comprises this western portion of Baja California covers, in angular discordance, older volcanic and sedimentary rocks, as well as intrusives; it underlies Quaternary volcanics and con­ tinental and marine sediments of the Tertiary and Quaternary. The next belt is located to the east of that described above and is made of sequences oi volcanic rocks, volcanidastics, and sedimentary rocks whose age is principally Lower Cretaceous (see Figure 1.4). The upper and more extensive part of the sequence was 7 1. Geology of the Northwest Region of Mexico 117' 114' I 30'­ 28'- -1-­ - - j j - - - ~ - - - , Figure 1.2. Pre-Tertiary terranes oí Northem Baja California. 117' I MeXlcal1 114' Ensenada 30' .­ I ¡ I I I I ¡j 28' - -t - - - - - - - - ..Al..Ji----- J I o Rosario Fomlatlcn 01 Ihe Upper Crelaceous Figure 1.3. Distribution of outcrops oí marine sedi­ mentary rocks oí the Upper Cretaceous. 114' Ensenada ~ Sed,memary rocks 01 Aptian-Albien age Figure 1.4. Distribution oí volcanic, volcanoclastics, and sedimentary rocks oí the Lower Cretaceous. originally named by Santillán and Barrera (1930) the Allsitos Formation from exposures on the Rancho Alisitos located to the south of Ensenada It is consti­ tuted chiefly of pyroclastic rocks and l¿lVas of dacite­ andesite composition, by bodies of reefaJ limestone with Aptian and Albian fossils, as well as clasbc rocks derived from volcanics. This forma han covers discor­ dantly in some localities vo1canic and sedimentary rocks of Triassic and Jurassic age. It is deformed and partly metamorphosed, and it is affected by numer­ ous faults and by emplacement of bodies of intrusive granite of Cretaceous age. It underlies discordantly the Rosario Formaban and extends persistently along al! northern parts of the Baja California peninsula. Numerous outcrops of this type of sequence exist, correlative with the Alisitos Formation, mainly along the western border of Mexico. Rangin (1978) has interpreted this seguence as one of the volcano-sedimentary belts that were devel­ oped in northwest Mexico during the Mesozoic, that formed in a similar manner to vu1canism in Sonora, and that evolved on continental crust. These belts have been related to subduction and partial fusion associated with one or more con vergent borders (see Figure 1.5) developed in northwest Mexico. The con­ vergent borders seem to be tectonic features com­ mon to al] of western Mexico since there exist numerous volcano-sedimentary outcrops along this side of tI1e country. 8 Seetion 1 The Geology of the Mexiean Republie .. ­..-----. .:-.­.... : - ~ : .... : . ~ - Figure 1.5. ldealized block diagram that shows the tectonic situation of northwest Mexico for the Late ]urassic. Based on ideas of Gastil et al. (1980), Márquez-Casteñeda (1984), and R. Garza (in Márquez-Castañeda, 1984). This process developed during the opening of the Atlantic Ocean and the movement oE North America toward the northwest. The sediments that form the Alisitos Formation were subjected to a period of com­ pression at the beginning of the Upper Cretaceous. They were folded and partially metamorphosed. The terranes that form this second belt emerged during this period; and to the west of them sediments formed that were to become the Rosario Formation. The third belt, located on the eastern border of the nortbern part oE the peninsula of Baja California, is eomposed of complex outcraps of intrusive raeks and metamorphies derjved principally fram the regional metélmorphism of sedimentary rocks. To tbis beIt belong the Mesozoic béltholiths (Figure 1.6) of the northern part oE Baja California and the pre­ batholithic metamorphic rocks formed before the Alisitos Formation. Their élge has still not been well defined. The plutonic racks that eomprise the batholiths vmy in mineralogical composition from tona lites to granodiorites and granites. In contrast, in some localíties sma1l plutons of diorite and gélbbro are mapped. Some authors (Gastil and Krummenacher, 1978; Sjlver and Anderson, 1978), citing radiometric studies, have postuJélted that in northwestern Mexico there occurred a migration in time and space of this type of plutonic emplacement from the Cretaceous in Baja California to the Cenozoic in Chihuahua. The major part of this b,Hholithic emplélcement occurred during and aher the sedi­ mentation and magmatic extrusions that originated the Alisitos Formation. Pre-batholithic metamorphic sequences associated witb this third belt present various metamorphic facies, but their ages have not been determined. However, McEldo\vney (1970) reported the presence of Pélleozoic crinoids, corals, and bivalves in sedimen­ tary rocks that crop out southeast of Ensenada. There also exist on the eastern edge of the peninsulél some -'­ Ensenada _1 I 1 1 I 1 1 1 I 1 30'- ¡-------. 1 1 1 í 1 1 1 I 28.1- ~ --Q""+--1 I C] Mesozolc granlllc racks Figure 1.6. Distribution of Mesozoie plutonic out­ erops in northem Baja California. outcrops oE metamorphic ealcareous racks that proba­ bly are related to the Paleozoic limestones which crap ou t in the State of Sonora. The Cenozoic history of the northern part of Bajél California is characterized by the accumulation of great thicknesses of continental sediments that crap out in numerous localities; by the development of marine deposits, particularly on the western edge of 9 1. Geology of lhe Northwest Regíon of Mexíco the peninsula; and by importémt volcaníc activity that partly covers the Mesozoic belt described aboye. During the Paleocene and Eocene, sediments accu­ mulated in nearshore and deltaic environments (Gastil et a!., 1975) on the western border of the north­ ern portion of the peninsula, following a coastline that is located slightly to the east of the present shore. These sediments came from emergent areas to the east where time-equivalent continental sediments are encountered. Santillán and Barrera (1930) termed the marine Paleocene sediments that were encountered between Punta San Isidro and Mesa de San Carlos the Sepultura Formation. This formabon can be correlat­ ed with the Santo Domingo, Tepetate, and Malarrimo formations described by Mina (1956) in the southern half of Baja California. The Pliocene and Miocene sed­ iments correspond, it seems, to greélt thicknesses of fluviatile and eolian strata that crap out at laL 31° in the area of San Augustín and some localities located about élt the latitude of the bays of Las Animas and San Rafael. These sediments are found generally capped by lélvél. extrusions of Miocene and Pliocene age. The Miocene contains outcrops of marine sedi­ ments that are the oldest Cenozoic strata to appear in the northeastern part of the Peninsula and that rnarked the earliest advances of the sea over the area that would become the Gulf of California. According to Gastil et a!. (1975), in the Eocene the Mesozoic mountains were completely denuded and formed only small isolated hills. These areas were drained by stream courses that flowed toward the Pacific and fed their sediments into the marine deposits on the western edge. Later in the Eocene, the eastern part of the region experienced some subsj­ dence where the Gutf of California later developed. Some of the interior fluviatile stream courses were directed toward this area. Coastal deposits that formed in the littoral of the Pacific during the Pleistocene élre found aboye a series of terrélces developed in that epoch. Some of these reach up to 500 m in altitude. These terraces have been related to glacial changes nf sea level (Gastil et él!., 1975) that were superimposed on a tectonic setting of a series of upi i fts and downwarps in the coastal zone of the peninsula during the Pleistocene (Ortlieb, 1978). In contrast, in the interior of the peninsula dur­ ing this time éllluviaL, colian, and lacustrine deposits accumulated. Manv of these sediments continue to develop today. J The Cenozoic vulcanism of the northern part of the peninsula can be referred principally to four zones where wide exposure of the volcanic rocks thélt origi­ nated in this area are encountered and that mark the Miocene as the méljor epoch of volcanic activity (see Figure 1.7). The first zone, located in the southern part of Sierra de ]uárez between latitude parallels 31°and 32°, contains an important sequence of silíceous pyro­ c1astic rocks of diverse types that are found capped in some localities by basal tic flows of Pliocene and Quaternary age (Figure 1.7). The second zone, located on the coast of the Gulf of California él.t the latitude of MeXlcal1 • .. ''\1 Tlluana I c> I I I I I 28' i - - - - - -~ ­- - Ensenada J I I I I I I I I I 30-r---­ i --ó.<+-----:;;;"';";..;....IIL-l I r-n Slllceous and Jnlermedlale grade rocks of Mlocene age ~ ~ Basallle raeks 01 Pho·Plel$!ocene age Figure 1.7. Distribution of outcrops of Cenozoic vol­ canics of northern Baja California. 30°, is represented by siliceous pyrocJastic sequences that are seen to cover andesite flows in some localities and are capped in other places by basaltic flows of Pliocene-Quaternary age. The third zone corresponds to extensive flows of éllkéllíne basalts of the upper Tertiary located in the central part of the peninsula at the léltitude of Canoas Point. These flows are similar to those that are localízed with minor distribution around the Mesa de San Carlos and San Quintín. The last zone is composed of outcrops of basaltic and pyrocJélStiC rocks, principally rhyolites, tha t occur on the island Angel de la GUélTdél élnd to the south of the 29° parallel. These cover great thicknesses of conti­ nental and mixed sedirnentary rocks. This zone can be considered as a northward extension of the Miocene sequence that constitutes the Sierra de la Giganta in southern Baja California. Sierra de la Giganta The Sierra de la Giganta, located in South Baja California, is composed of éln impressive sequence of pyroclastic rocks, lava f1ows, and continental sand­ stones that together reach 1200 m in thickness. Outcrops of this seCjuence are persistent in most of the eastern half of the southern peninsula. Originally, Heim (1922) termed these deposits the Comondú Fonnation 10 Section 1 The Geology of the Mexican Republic (see Figure 1.8) and assigned their stratigraphic position geochronologic ranges vary from Triassic to Recent. to upper Miocene. Later, Escandón (1977) indicated that Structurally, these areas constitute two large synclinal the upper member of this formation belongs to the depressions with general northwest-southeast orien­ lower Pliocene. This sequence presents strong lateral tation. They are composed of Cretaceous and Ceno­ variation and is composed principally of volcanic zoic rocks (see Figure 1.9). Lozano (1976) has agglomerate, pumice-tuffs, ignimbrites, basalts, interpreted the existence, at depth, of an uplifted litharenite sandstones, and conglomerates. On the other block of ophiolitic rocks that separates these two hand, the strata cover discordantly the main sedimenta­ structural depressions, based on geophysical data and ry sequences of the Tertiary that crop out more widely wells drilled by Petróleos Mexicanos. This block in the basins of Purísima-Iray and Vizcaíno, and the might have an orientation perpendicular to the gener­ plutoníc rocks that are a southern continuation of the al structural tendency of the península and would be batholiths of northern Baja California. Mina (1956) con­ located between parallels 27° and 28°. Above this sidered that the source of supply of this great quantity structural high, the Cretaceous sequences wedge out, of volcanic sediments should have been located in a but these strata reach great thicknesses in the center of volcanic belt to the east of the present coast of the Gulf both depressions. The southwestern flank of these of California. major structures is represented by outcrops of older The sequence that makes up the Comondú rocks that form ophiolitic complexes and partly meta­ Formation does not show strong tectonic deformation. morphosed Triassic and Jurassic seguences (see However, it reveals accentuated epeirogenic uplift and Figure 1.10). In the axial portion of the structures, out­ an inclinabon of its strata gently toward the west. crops of the younger Cenozoic formations occur, while on the northeastern flank some bodies of the The Basins of Vizcaíno and Ballenas-hay-Magdalena batbolithic complex of Baja California are found, although generally these are covered by the Miocene The basins of Vizcaíno and Ballenas-Iray­ and Plíocene sequence of the Comondú Formation. Magdalena, which take in the western haH of the larg­ The oldest seguence of this region is composed of er part of the State oE South Baja California, are partly metamorphosed volcanic and sed imentary represented by ZOnes of low and smooth topography rocks that crop out in Punta Prieta, Punta San in which exposed sequences are encountered whose Hipólito, and Cedros Island (Figure 1.10). Originally 111· -n::--i----r-----r- - - - - - +­ I I I I I I I Volcaníc and sedimentary rocks 01 lhe Comondu Formalion Sedlmenlary manne rocks 01 Tertlary age In Punsima-Iray and Magdalena baslns Cabo San Lucas Figure 1.8. Distribution of principal outcrops of Tertiary rocks of southern Baja California. 11 1. Geology of the Northwest Region of Mexico VIZCAINO BASIN, BC SECTION 1-1' NE SW SISMOLOGIA I GEOLOGIA SUPERFICIAL GRAVIMETRIA ~ GEOL. SUPo SIERRA GN-I SAN ANDRES OPHIOLlTE COMPLEX Ca BASEMENT COMPLEX Cb UPPER JURASSIC DEPOSITS -- Js LOWER CRETACEOUS DEPOSITS - KI MIDDLE CRETACEOUS Km UPPER CRETACEOUS Ks 1t-.l.--'--'----'I o S 10 1S 20 Km PLlO-PLEISTOCENE Plp TERTIARY (PALEOCENE-EOCENEj- Tpe IRAY-MAGDALENE BASIN, B.C. SECTION 11-11' E W ISLA MAGDALENA 1-4 M-2 M-3 x x x TERTIARY OPHIOLlTE COMPLEX LC ~ ~ . I (PALEOCENE-EOCENE) IT¡>;I BASEMENT COMPLEX laS,bo 1 TERTIARY/MIOCENE MARINE DEPOSITS 8 UPPER JURASSIC DEPOSITS -I'js: 1 TERTIARY/MIOCENE ITmv·1 VOLCANICS , , ~ LOWER CRETACEOUS I'K.t,' 1 I - P I P ~ I PLlO-PLEISTOCENE UPPER CRETACEOUS IGNEOUS INTRUSIVES ~ 00 Figure 1.9, Vizcaíno Basin, Re. sect. 1-1' and Iray-Magdalena Basin, B.e. sed. 11-11' taken fmm Petroleum Evaluation of the Peninsula of Baja California, by F. Lozano (1976). ------------ 12 Section 1 The Geology of the Mexican Republic 114' 111' = - + ~ ~ - . . . . , - - - - - -1­ 1 Sta Rosalla I I I I I I 1 I ~ 26'- -1- - - - -­ I I 1 1 1 1 I 1 ~ Ophlollle complex rn Metamorphosed Jurasslc rocks Cabo San Lucas rn Lower Cretaceous sedlmentary rocks ~ Granlllc rocks Figure 1.10. Distribution of the principal outcrops of Mesozoic rocks of southern Baja California. Mina (1956) designated these rocks the San Hipólito There also exist other outcrops of sequences with Formation. They crop out in the type locality and cor­ oceanic affinity that resuIted in ophiolites and relate lithologically with the Franciscan Formation of melanges; these also have been attributed to the California. For this reason he tentatively designated Jurassic because of their radiolarian content (Rangin, these rocks as Jurassic. Later, Lozano (1976) reported 1978). These units crap out both on Cedros lsland and a fauna of the Late Triassic at the top of the sequence on the Vizcaíno Peninsula. On the islands of Santa ami for this reason considered the rocks to be of this Margarita and Magdalenil, partially serpentinized age. ultramafic rocks crop out and are a pparently a frac­ In Cedros lsland, the Vizcaíno Peninsula, and the tion of an ophiolitic complex related to those of area of Magd¡:¡lena Bay, rocks exist that petrographi­ Vizcaíno and Cedros. There have been recognized in cally resemble those of the Mesozoic and that form an this regíon combined volcanics, volcaniclastics, and intricate mosaic of terranes of both oceanic and vol­ sedimentary rocks of Late Jurassic and Early Cre­ canic arc affinity. Although Mina originally had pos­ taceous age with an ophiolitic basement-forming tulated correlation with the Upper Jurassic Franciscan sequence originally termed the Eugenia Formation by Formation, Finch ami Abbott (1977) later placed these Mina (1956). beds in the Upper Triassic, because of their content of The Upper Cretaceous is represented in the region macrofossils and radiolarians. The association of of southern Baja California by a detrital sequence of chert, litharenitic volcanics, and the inclusion of reefal Cenomanían to Maastrichtian age that overlies earlier limestone blocks forming a sequence underlain by pil­ sequences with apparent angular discordance. This low basalts, as well as the apparent absence of detri­ unit was designated the Valle Formation by Mina tus derived from the craton, indicate that this unit (1956) and inc1udes turbiditic toe-of-slope fan deposi­ was deposited in an oceanic basin associated with a tion (Patterson, 1979). lt has been recognized in out­ volcanic island arc within a convergent tectonic crops of the Vizcaíno Peninsula and in the subsurface framework (Finch et aL, 1979; Castil et al./ 1981). of the two Cenozoic basins of this region. 14 SecHon 1 The Geology of the Mexican Republic 1. Dominantly 01 oceanic affinity 2. Dominantly volcanics, volcanoclastics and sedimentary rocks of Jurasslc-Cretaceous age 3. Dominantly metasedimentary rocks 01 Paleozoic?-Triassic age Figure 1.11. Tectonic beIts of Baja California (l) dominantly of oeeanie affinity; (2) dominantly vol­ eanie, volcanoclastie, and sedimentary roeks of the ]urassic and Cretaceous; (3) dominantly metasedi­ mentary roeks of Paleozoie?-Triassie age. combinations in California that include the Franciscan Formation. 2. To the east of the aboye combinations of sediments, a volcanic-plutonic fringe of Jurassic to Early Cretaceous age is developed, at least partly, on oceanic crust and constitutes an ancient calcalka­ line volcanic are like those that typieaJly evolve as fringes parallel to convergent borders. This vol­ eanic-plu tonie belt occurs in the western half of nortnern Baja California, is extended beneath tne volcanic cover of the Sierra de la Giganta, and stretches probably to the El Cabo region. 3. To the east of the aboye domain, a belt of metasedi­ mentary continental border clastic sequences appears (Gastil et aL, 1981), overlapped partially by the combined volcanie-plutonic rocks. This belt is probably of Triassic age and forms the eastern half of northern Baja California. In the extreme east, sorne isolated outcrops of calcareous and very badly deformed detrit,11 rocks occur that have been a ttributed to the PaleozoiCo The tectonic evolution of Baja California during the Paleozoie seems to have been related to the Cordilleran continental margin of the western border of North America, but nevertheless offers sorne distinctive details in its own evolution. There are sequences exposed in the east of northern Baja California, in addi­ tion to the ealcareous and detrital sequences of Sonora, whieh reveal the presence during this era of a passive margin domain for northwest Mexieo. This type of tec­ tonic situation has been interpreted for a large part of the North American Cordillera. Two episodes of oro­ genic deformation have been identified for this region. The first of them oecurred in the Devonian­ Carboniferous (Antler Orogeny) and the second in the Permian-Triassic (Sonoma Orogeny). Both events have been interpreted reeently as marking pathways of eolli­ sions of intraoeeanic arcs against the passive margin of North America that induced the emplacement of allochthons of the Roberts and Galconda Mountains over the miogeoclinal sequenees of the Cordillera (Diekinson, 1979). In contrast, passive margin conditions are recog­ nizable between these two events. In northern Baja California, neither episodes of collision nor allochtho­ nous arc sequences have been identified. However, Gastil and his eo-workers (1981) nave suggested the possible existence of a trench or basin marginal to the cratonic edge. In nortnern Baja California, passive margin condi­ tions persisted during the Triassic, evidenced by an apparent tectonie stability in Sonora and by the absence of volcanic arc components in the metasedi­ mentary sequence of the Peninsula. Only the Upper Triassic San Hipólito Formation in the Vizcaíno region reveals a probable boundary of convergence toward the ocean interior, later accreted landward (Gastil et al., 1981). In the Jurassic, the development of an islélnd arc dominion was initiated to the west of the eratonie bor­ der and its Triassic sedimentary wedge. This are apparently evolved eontemporélneously with the one reported aboye the continental crust in Sonora (Rangin, 1978). Gastil et al. (1981) consider that these two arcs were associated with different zones of sub­ duchon that evolved in parallel, one of them related to an intraoceanic treneh and the other to a treneh bordering the craton (see Figure 1.5). The collision of the intraoeeanic arc relélted to the Alisitos Formation against the eratonic margin appar­ ently occurred in distinet episodes owing to the pres­ ence of transform faults between trenches that displaced distinct segments of the are. The principal episodes of collision seem to have oceurred in Bajél California during the Cenomanian. This phenomenon generated a primary phase of deformation that fold­ ed, metamorphosed, and elevated the volcélnie, vol­ eanoclastic, and eélrlier sedimentélry seguences at the same time of the principal emplaeement of batholiths (Gastil et aL, 1981). 13 Thc Cenozoic sedimentélrv formations form the larger piHt of the bélsin-fill of Vizcaíno and PurísimCl­ lray-Magdalenél downwarps élnd are principally char­ acterized by little consolidation, subhorizontaJ position of the strata, and a marine dastic lithology. Outcrops of Paleocene sediments are rare, notwith­ standing that a.thickness of more than 2000 m can be recognized in the su bsu rface, thanks to the welJs drilled by Petróleos Mexicanos (Lozano, 1976). These wells encountered diverse lithologies with predomi­ nance of slope shale facies. To this epoch belong the Santo Domingo and Malarrimo formations (Minél, 1956), the latter of which rests in discordm1Ce on the Cretaceous forméltions. The outcrops of the Eocene are represented principally by sandy and shaly sequences that have been designated as the Bateque Formation in the area of Vizcaíno and as the Tepetate Formation in the La Purísima Mea where the 100ver part of the sequence belongs to the Paleocene. Sedi­ ments corresponding to this epoch have been recog­ nized in the Pemex wells (Lozé\no, 1976), principally in the La Purísima area where they reach a thickness of IIp to 500 m. In this pMt of Baja California, Oligo­ cene sedimentary rocks do not crop out, attesting to a period of emergence for that time. The Miocene is íOllnd Clmply exposed in the regions of VizcClíno and PurísimCl and consists of sedi­ mentMY and volcanic rocks. The lower Miocene is represented in the area of Vizcaíno by agglomerates, sands, and shales of the Zacarías, Santa Clara, La Zorra, and San Joaquín formations (Mina, 1956). In the area of Purísima it is composed of lutites with diatomite intercalations of the Monterrey Formation (Oarton, 1921) and white sandstones of the San Gregorio Formation (Heim, 1922). The middle Miocene is formed by diverse sequences that show much lélteral variéltion élnd are composed of tuffél­ ceous sélndstones, bentonitic shaJes, and sandstones of the lsidrio (Beal, 1948), San Ignacio, Tortugas, and San Raymundo form<ltions (Mina, 1956) that represent the constal and lagoonal platform environments. The élbove formations underlie discordantly the continentéll sedimentary and volcélnic deposits of the Comondú Formation that reach maximum develop­ ment in the Sierra de la Giganta, located to the east of this described region. Ouring the Pliocene, sediments of COélStéll environ­ ments were deposited in the Vizcaíno and Purísima basins, with discordance over the Miocene fonnations. These are represented by the Almejas Formation in the area of Vizcaíno (Mina, 1956) and the SaIadél Formation in the Purísima areél (Heim, 1922). Region of the Cape (El Cabo) The extreme south of the Béljél Céllifornia peninsula breaks abruptly across the general geologic aspect of the basins described aboye; the area is forroed bv a mas­ si ve batholith that is expressed in the form of ~ mOlU'\­ tainous complex élnd thélt is interrupted in the central part by a depression known as the Santiago Valley and in the northern part by the La Ventana Valley. 1. Geology of the Northwest Region of Mexico The batholith thélt composes this mountainolls zone has characteristics similar to those masses that crop out in the north of Baja California and is made up of granodioritc and granite. The rectilinear borders of these mountains suggest faults of great displace­ ment that jllxtapose and eIevate the batholithic region aboye the leve] of the areas of Cenozoic outcrops. In the northern and western portions of the Sierra de In LagunCl, one can recognize the existence of a pre­ batholithic metamorphic complex formcd principally by metasedimentary rocks derived from lutites, sand­ stones, and limes tones with some apparently metavo!canic bodies bearing epidote and amphibole. In the metasedimentary sequence, Ortega-Gutiérrez (982) has identified (l dosely juxtaposed succession of isograds of biotite, andaIucite, sillimanite, and cordierite. There exist also catac1élstic belts and diorit­ ic and gabbroic intrusions that form north-south-ori­ ented lineations. In the neighborhood oí the main batholithic body, inside the metamorphic complex, zones of migmatites and numerouS intrusions of fel­ sitic character Me present élpparently associated \vith the batholith. The Santiago Valley is structuréllly él tectonic graben in which the principal sedimentary sequences of the region are developed. The bélse of these sequences is formed by conglomeratic deposits that are correlative with the Comondú Formation and that rest above the crystalline basement represented by intrusive Cretaceous rocks. The outcrops of these deposits are located chiefly in the ex treme north of Sélntiago Valley. Over the élbove sequence, the sedi­ ments of the Pliocene Trinidad Formabon rest with angular discordance (Pantoja-Alor and Carrillo­ Bravo, 1966). These sediments form a sequence of sandy clays and silts with some diéltornite horizons, all of which attest to a marine depositiona] environ­ mento Above this unit rests in concordance a sequence of marine sandstones that represents the Sajada Formation (Heim, 1922), which has isolated outcrops along the Sélntiago Valley. The Cenozoic sedimentélry sequence that fills this tectonic graben is covered dis­ cordantly by a series of sandy-conglomeratic deposits of the Pleistocene that are seen in the form of ancient piedmont fans and belts. Tectonic Summary The principal tectonic elemcnts of the Bélja California Peninsula céln be summed up as follows (see Figure 1.11): 1. It is possible to recognize on the western border at Cedros lsland, the Vizcaíno Peninsula, and the islands in Magdalenél Bay the presence of com­ bined tectonical1y controlled rock types oE oceanic affinity that include portions of ophiolite complcx­ es élnd typicnl melange sequences that range from Triassic to Late Jurassjc. Tbese combinations of sediments have been interpreted to result from the structural evolution of a paleo-oceélnic crust and to mark an ancient line of convergence. They have been related, furtherroore, to similéH Iithologic 15 In the Late Cretaceous ilnd part of the Cenozoic¡ only a subduction zone persisted¡ located on the west­ ern margin of Baja California and marking the con­ vergent boundary between the Farallon and North American plates Generally the uplifts of Baja Ca1lfornia and northwest Mexico during the Late Cretaceous made for an important contribution of detritus directed toward the east within a general framework of eastward marine regression. The vul­ canism associated with the subduction on the western margin of Baja California during the Late Cretaceous and Paleogene has been recognized¡ chiefly in the continental part of Mexico¡ and as late as Miocene time it is expressed in the Peninsula by pyroclastic sequences in the Sierra de la Giganta and other erup­ tive centers in northern Baja California. In üligocene time the collision of the Pacific ridge with the North American plate was initiated. This ridge divided the Farallon plate¡ now extinct¡ from the Pacific plate and is apparently formed of segments dis- A FARALLON PLATE l. Geology of the Northwest Region of Mexico placed by numerous transform faults. According to a model of McKenzie and Margan (1969) and Atwater (1970)¡ the collision of the first segment of the ridge against the North American plate was initiated approx­ imately 30 million years ago¡ at a point located in pre­ sent-day Baja California. Starting from the first contact of the Pacific and North American plates, there began a right lateral movement along the growing border of both plates with a velocity of 6 cm per year (see Figure 1.12). This lateral movement could have occurred in its initial stages along the continental border of North America and later couId have occupied the present belt of the San Andreas System and the Gulf of California (Atwater, 1970). The opening of the Gulf of California and the development of its ridge system was initiated about 4 million years ago. This system is the manifesta­ tion of the relative movement between the North American and Pacific plates and is the southern pro­ longation of the San Andreas System. The movement of Baja California toward the northwest is possibly B NüRTH AMERfCAN PLATE Figure 1.12. Tectonic evolution of northwest Mexico in the Tertiary. Oifferent stages in the co11ision of the eastern Pacific oceanic crest, and the development of right lateral movement between the North American plate and the Pacific plateo S = Seattle, SF = San Francisco, LA =Los Angeles, GS =Guaymas¡ MZ =Mazatlán (after Atwater, 1970). (A) 10 million years before the present; (B) 20 million years before the present; (O 30 million years before the present; (O) 40 million years before the presento 16 Section 1 The Geology of the Mexican Republic o NO' TH AMERICAN PLATE related to the tectonic lineaments that cut the Peninsub diagonally and to the alkaline basaltic extru­ sions of the Pliocene-Pleistocene that are encountered in numerous localities. Economic Deposits According to Gastil et al. (975), the northern por­ tion of the Baja California Peninsula can be divided into five mineral provinces (Figure 1.13). The most western of these comprises deposits of mesothermal iron and copper sulfides as well as oxides of iron. These deposits are found emplaced in the partially metamorphosed volcanic sequence of Mesozoic a.ge and have been attributed to hydrothermal origin related to the Cretaceous granitic intrusions. The prin­ cipal known localities thElt manifest this type are: El Sueño mine (loe 1), San Antonio (loc. 4), Mision SEln Vicente (loe 11), San Isidro Point (loe lO), R"ncho Rosario (loc. 12), zones to the east of El Rosario (locs. 17, 18), Elnd to the southeast of San Fernando (locs. 19-21). To this province also belong the deposits of the El Arco mine (loe 29), which accounts for one of the rnost important copper reserves of the nation. The second province eomprises veins of gold con­ tElined in metasedimentary rocks that are distributed along the axis of the peninsula. Their occurrence, restricted to metasedimentary rocks, leElds to the pos­ sibility that they have been reworked from ancient placers before their metélmorphism. The principal localities known for this type of deposits are: Las Cruces (loe. 7), El Alamo (loe 9), Socorro (10e. 13), Arroyo Caléll"najú (loe. 23), Cerro San Luis (loc. 24), Desengaño (loc. 25), León Grande (loe. 26), and Coltunbia Mine (loc. 27). The third province comprises tungsten deposits rela ted to contact metamorphism of pre-batholithic calcareous rocks where precious stones may be found. The intrusives that affect the calcareous sequences were emplélced chiefly in the Cretaceous. The locali­ ties known are: La Olivia (loe. 3), Los Gavilanes and El Fenomeno (loe. 6), as well as in the Sierra de los Cucapá, Sierra Mayor, and Sierrél San Pedro Mártir. The fourth province includes superficial deposits of travertine with sulfides of manganese, copper, silver, and lead as well as deposits of wulfenite, stibnite, and other minerals. These deposits have the peculiarity of having formed in the Cenozoic near the edge of the Gulf of Californiél. The distribution of these deposits is very complex Elnd the localities are numerous. The final province includes deposits of placer gold developed in the Cenozoico The principal localities 17 1. Geology of the Northwest Region of Mexico .2 • 3 4 ' • "&; 5 6 ~ _ . 7 eL\. - ir­ , • O 114· ----./ ,.,9 I I I I I I I I I I I I 30·_ --...¡. 1­ ~ Copper-Iron • Gold e Tungslen • Gold-sllver O Placer gold L\. Hot springs Figure 1.13. Principal mineral deposits known in Baja California: copper­ iron, gold, tungsten¡ gold-silver¡ placer gold¡ hot springs. are: Campo Juárez (loe. 2), Los Pinos and Campo because Mesozoic rocks are less exposed (Figure 1.14). Nacional (loe. 8), Socorro (loe. 12), Valledores (loe. 36), On the coast of the Gulf of California sOrne deposits of Los Enjambres (loe. 40), Real del Castillo (loe. 14) and manganese exist, but these are of little importance. Pozo Alemán (loe. 28). They are in the form of oxides occurring as hydrother­ In the southern pan of the Peninsula of Baja mal veins. The principallocalities are: Lucifer (loe. 1), California, occurrences of mineral deposits are rarer Mulegé (loe. 21), and Misión San Juan (loe. 3) 18 Seclion 1 The Geology oí the Mexican Republic 111' 1 o ,&---- - - - - - - - ­ I I o Gold O Areas 01 eopper I O Gypsum I '1'/ Mn oxide D Tale I • Areasol Mg I • Phosphonte I .... Gold-sllver ' ~ - , I ,~ Figure 1.14. Principal mineral deposits known in soulhern Baja California (taken from the metaloge­ netic map of the Mexican Republic, 1ng. Guillermo P. Salas, 1975). Gold, copper deposits, gypsum, man­ ganese oxides, talc, magnesite deposits, phosphorite, gold-silver. The most important copper deposits are in the form of the sulfides of El Boléo (loe. 4) developed in Mesozoic voicanic rocks of the Santa Rosalía area. Their metallic deposits are represented by hydrother­ mal deposits of gold and silver in the Cabo region (loes. 5-7), as well as in the Vizcaíno area. Various occurrences of nonmetallic deposits exisl including the magnesite deposits found in Magdalena Bay (loes. 8, 9) and Eugenia roint (loes. lO, 11), the ta1c deposits of Comondú (loe. 12), and the phospho­ rite deposits of San Hilarla area. These last constítute the major reserves of phosphorite in Mexico. The most important petroleum occurrences are localized in Paleocene sediments in the Purísima basin, as seen in exploratory wells drilled by Petróleos Mexicanos (Lozano, 1976) and in some oil seeps of this same region. SONORA AND SINALOA In the Sta tes of Sonora and Sinaloa, one observes as in Baja California a greal complexity of rocky out­ crops owing to similarly intricate slructure and to the great lithologic heterogeneity of the various uníts; particularly those of the pre-Tertiary form highly variable stra tigra phic columns in this region. This geologic terrane contrasts markedly with that observed to the east of the Sierra Madre Occidental where the structures are more regular and the stratig­ raphy more homogeneous. To describe effectively the geologic characleristics of this region, lhe physiographic division of provinces defined by the General Department of Geography have been followed because they present suitable nat­ urallimits for better descri ption. Sonoran Desert This zone is characterized by the presence of com­ plex mountains, separated by alluvial valleys that become wider toward the northwest portion of the state, where they contain important eolian deposits. The complex mountains are found to conform with pTe-Tertiary terranes that are covered toward the east by the piles of Cenozoic volcanics forming the Sierra Madre Occidental. Here they turn up in the form of isolated outcrops, under the ignimbrite cover. In the State of Sonora, units of rock are exposed that ha ve d geochronologic range varying hom Precambrian to Recent. The Precambrian is represented by two well­ defined groups of rocks (see Figure 115): one older group composed of metamorphic rocks derived from igneous and sedimentary rocks, and a younger group composed of sedimentary sequences of quartzite and dolomite tha t overlie díscordantIy the earlier strilta. The metamorphic Precambrian occurs in northwest Mexico as an extension of the Precambrian shield that crops out widely in the Unlted States and Canada. This Precambrian basement in North America shmvs a series of provinces that are older toward the nucleus of the Cl'aton, suggesting accretionary development of the continental crust of this region. In northern Sonora, two Precambrian metamorphic terranes exist that aTe of different ages and are structurclily juxta­ posed along one major strike-slip zone that originated in the Jurassic. It crosses the northern part of Sonora diagonally with a northwest-southeast orientation. This strike-slip zone has been proposed by Silver and Anderson (1974) as the "Mojave-Sonora Megashear," with a left-lateral movement that is extended toward the states of Arizona and California (see Figure 1.16). The Precambrian block located to the southwest of the "megashear" is represented by outcrops of meta­ morphics in the area of Caborca, where the oldest known rocks of Mexico are located. This block has rectilínear limits, both to the south and to the .vest, marked by the sudden disappearance of Precambrian outcrops. As far away as Sinaloa, rocks this oJd ap­ pear again as outcrops, represented by the Sonobari Complex (Rodríguez and Córdoba, 1978); however, their age has not yet been confirmed. The Precambrian metamorphic outcrops of the Caborca area are represented by ígneous rocks and by metasedimentary rocks of greenschist and amphibo­ lite facies (Anderson et aL, 1978) formed during a period of from 1700 to 1800 milJion years (Silver and Anderson, 1979). These metamorphic units llave been designated by Longoria et al. (1978) as the Bamori Complex. They have suggested that the existence of massive anorthositic rocks could correspond to the unificabon of two Precambrian continental areas. In opposition to the aboye ideas of unification, there exist to the northeast of the zone of "mega­ shear" Precamb(Ían metamorphic rocks, such as those 19 1. Geology of the Northwest Region of Mexico J I I j I I ¡ I I I I I I I I I I I 28, __ 1 1 --­ --­ r ~ Precambrian melamorphic rocks D Precambrian sedlmentary rocks lJ ·· . . . :; --­ 111' I --1---­ J I --­ Q Cd. Obr;egon Figure 1.15. Distribution of outcrops of Precambrian rocks in Sonora. cropping out in the Sierra de los Ajos, whose ages range between 1600 ,md 1700 mil1ion years and that have been correlated with the Pinal del Sur schists of southern Arizona. A sedimentary group of late Precambrian age crops out in the Caborca area and covers, with tectonic dis­ cordance, the metamorphic Precambrian (Longoria et al., 1978). Originally this sequence was named by Keller and Wellings (922) as the Gamuza beds, and later Stoyanow (1942), on the basis of Collenia algal reefs, placed it in the late Precambrian. The seguence ineludes the Pitiqui to and Gamuza formations (Longoria and Pérez, 1978) and is composed princi­ pally of dolomite with stromatolites (lnd with quartz sandstone and shale. The upper contact of the Gamuza Forrnntion is discordant with the overlying Paleozoic seqLlence. The Paleozoic sequence crops out in numcrous localities in the State of Sonora and is eomposed prin­ cipally of limes tones and sandstones tha t were • --- • 20 Section I The Geology of the Mexican Republic 114' 109' I I O I , .' º I :-1 ' ." :..-=.':'- • " - r .'" . ' ... ". -, .. " .• - t· ." • iI t t" 4 "'&1 ""1 • ". " j "",,. I ,,". '" j: • , , • '... ':.,." l 2 I • • + " ' • 1" J , , ' •• ' " . r -:.'-.;.....:.L:...\-, I " . .: n.¿' '., " - - -J- r I I " , . .. ... ." I I . .. " ,,; ., ! I • + J I " "; :" .. t I I " :. I I I I I I I I I I I I Hermosillo ' , I I I • I r , ... I ' I f I I I <t: I i ::> SONORA J I --- .1 _ J Guaymas Citles 6. Locatlon 01 ,ndiVidual samples o Terrane underlain by rocks between 1600 and 1700 m y. old O Terrane underlaln by rocks belween 1700 and 1800 m.y old SINALOA Figure 1.16. Localities of Precambrian crystalline rocks (ages established by means of isotopic studies of Anderson and Silver, 1979). Cities, localities of individual samples, lerranes underlain by rocks of ages between 1600 and 1700 million years, terranes underlain by rocks with ages of 1700-1800 million years. !J NavoJoa • deposited in él plalform environment (see Figure 1.17). Thís ancient continental platfonn would be a south­ ward continuabon of the miogeosynclinal Cordilieran belt. Fries ("1962) proposed lhe n<lme Sonoran Trough for this southern extension of lhe Cordilleran Geosyncline and indicated that during the whole :r <t: ::> :r r ü 24 Section 1 The Geology of the Mexican Republic • Magmalle are al end 01 Jurass/e (San Andres-Cedros Formation) Dominantly paleo-oeeanle Magmatie are of Early Cretaeeous (AlIsitos Formatlon) U.SA Magmatle are 01 early Middle Jurasslc (Sonora) Cretaeeous (Aptian-Albian) 01 Chihuahua basin Coaslal basin-Paleozolc and Preeambnan Texas platform Figure 1.19. Schematic paleogeography of a portion of northwest Mexico during the Mesozoic. After C. Rangin (1978). whose composition varies from rhyolite to andesite. According to Rangin (1978), a t the beginning of the The appearances of vulcanism in this Epoch seem to Tertiary an important assemblage of plutonic-volcanic extend toward the base of the volcanic sequence of rocks that is responsible for the mineralization of dis­ the Sierra Madre Occidental, where the presence of seminated copper developed in northeastern Sonora. rocksl00 million years old has been reported The volcanic rocks are generally related to intrusive (McDowell «nd Clabaugh, 1979). bodies tll«t affect and mineralize them (Si llitoe, 1973). ---- 21 1. Geology of lhe Northwest Region of Mexico ¡ Hermosillo b I I IJ Caborca - - - __ I ! i I I I I I I I I 28' ! I D Paleozolc sedímenlary outcrops -.J 111' 109' ¡--------L I I ! No'gales ! I I I I í I I I I I I I I I I I i f ------1-- Figure 1.17. Distribution of outcrops of Paleozoic rocks in Sonora. Pélleozoic this arca underwent a slow élnd uninter­ rupted subsidence According to F. Rangin (1978), at the time of the Cambrian and Ordovician periods there existed él grada tion from platform facies in the north of Sonora to a more iuternal facies toward the south of the state where the Paleozoic facies have a tectonic style of much llLOre intense deformabon. The calcareous facies nf the Carboniferous and Permian constitute a more homogeneous facies over al! the state. The two above major intervélls are separated by a phase of major deformation that occurred in the Devonian. The Pa leozoic of the Caborca area is represented in <lscending stratigraphic order by the following forma­ tions: Puerto Blanco, Provedora Quartzite, Buelnil, Cerro Prieto¡ Arroyos, and Tren. AH of these belong to the Cambrian (Cooper et al., 1952) and consist of sequences principally of calcareous-detritaJ lithology. Small isolated outcrops of calcareous strata thélt repre­ sent parts of the Ordovician, Silurian, Devonian, éllld Míssissippían Systems (Cooper and Arellano, 1946) exist in the BiSélni area. In the vicinity of Antimonio, a Permian sequence crops out that consísts of beds of shale and sandstone with limestone lenses and that was termed the Monos Formatíon by Cooper and Arellano (1946). In the mineral district of Cananea there exists él Cambrian sequence of quartzite and limestone comprising the Capote Quartzite and Esperanza Limestone (MuJchay and Velasco, 1954; Valentine, 1936) as well as límestones of Devonian, Mississippiéln, Pennsylvanian, and Permian age. In northwest Sonora there exist calcareous outcrops of Paleozoic strata in Cabullona (Taliefferro, 1933), Sierrél del Tigre, N acozélfi, a nd the Sierra de Moctezuma (lmlay, 1939). In the Hermosillo area and in the region loca ted more to the sou th, isolated ou t­ crops of Ordovician and Permiéln exist (King, 1939). The first deposits after the Paleozoic in the state of Sonora are continental sediments of the Upper Triassic and Lower Jurassic belonging to the Barranca Formabon that crops out in the centréll and southern portions of the sta te. There are in addition marine deposits of sandstone, Ijmestone, and shale in the areas of Antimonio and Santa Rosa in northwest Sonora. The deposits of the Antimonio area form a marine sequence sorne 300-400 m thick, They contain amrnonites, belemnites, í'\nd bivalves whose age vélfies !rom Late Triassic to Early Jurassic, are exposed príncipaJ1y in the Sierra de El Alamo, and have been informally designated as the Antimonio Formation by González (1979). This sequence is cor­ rdative with the lower part of the sedimentary and 22 Section 1 The Geology of the Mexican Republic volcanoclastíc sequence of the Rajon Group (Longoria and Pérez, 1978) that crops out in the hi11 of the same name located to the southeast of Caborca. According to Alencaster (1961), the region that ineludes these localities constituted a former bav in which 'Nere deposited sediments coming from the northern and northeastern parts of the sta te. In the area of San Marcial, to the southeast of Hermosillo¡ a sequence crops out that consists of fine-grained clastics with coal beds and with limestone intercalations; this was deposited in a swampy basin contemporaneously with the marine deposits of the Sierra de El Alamo. This sequence was named by King (1939) the Barranca Formation, and Alencaster (1961) later ele­ vated its deposits to the rank of Group. The absence of Lower Triassic deposits and the disconformable relationship observed within the Upper Triassic see¡uence above Permian rocks in the Sierra de El Alamo reveal important tectonic movements in the region during the close of the Paleozoic and the initia­ tion of the Mesozoic. The Jurassic in the State of Sonora is characterized by the development of an important volcanic-plutonic arc with a general northwest-southeast direction, evi­ denced by numerous outcrops of andesitic volcanic and vo\canoclastic rocks (see Figure 1.18). The devel­ opment of this arc has been related to episodes of sub­ duction occurring on the Pacific margin of Mexico where an oceanic plate was being subducted under the continental crust of Mexico. In the area of Cucurpe, to the southeast of Santa Ana, Rangin (1977a) reported a sedimentary see¡uence with volcanic intercalations that contain Jurassic ammonites. In the Sierras de la Gloria (Corona¡ 1979), El Alamo (González, 1979)¡ and in various localities in northwest Sonora, Mesozoic voicanic and voicanoclas­ tic rocks of prob(lble Jurassic age have been reported but unconfirmed. In sorne localities these rocks are partly affected by dynamic metamorphism and are genercdly of andesitic composition. Anderson and Silver (1978) have r port U-Pb ,\ es in various locali­ uf lL<-llllL clIlu ulC:dnoL1a'l e I'vck" Ve rylni;; between 180 and 150 million years. According to these authors the voicanic-plutonic acti vity in the Jurassic originated owing to the presence of a zone of plate con­ vergence to the west and was interrupted by the initia­ tion of lateral displacement along the so-called Mojave-Sonora Megashear. The prevalence of the mag­ matism tha trema ined in the convergent zone is also evidenced by vO\c(lnic and volcanoclastic Cretaceous rocks that a1so crop out in various localities in Sonora. In the Cretaceous, two realms in Sonora that have clearly distinctive characteristics can be defined (Rangin, 1978) (Figure 1.19) The first of these is found in the central and western belts in the state and evolved over a permanently emergent band of Jurassic volcanic and volcanoclastic rocks. In this belt were developed lava extrusions, principally of andesite, which in the central and southern portions of the state contaín intercalations of Lower Cretaceous marine sedimentary rocks (King, 1939; Roldán and Solano, 1978). The second real m, located in the east­ ern belt of the state¡ is composed of Lower Cretaceous marine sediments tha t afford evidence of a marine transgression coming from the Chihuélhua basin dur­ ing the Aptian-Albian (King¡ 1939; Rangin, 1978). These strata partly cover the volcanic and volcan­ oclastic Jurassic terranes. In the Upper Cretaceous, both realms are affected by compressioné11 deformation ilnd by granitic pluton­ ism accompanied by andesitic lava extrusions that become more intense toward the western pélrt of the region of the Sierra Madre Occidental. Evidence exists, in various outcrops, of volcanic activity that occurred in Sonora during the Eilrly Cretaceous. In the Cerro Lista Blanca, south of Hermosillo, a unit of andesitic volcanic rocks of prob­ ilble Early Cretaceous age crops out. Earlier, Dumble (1900) designated these beds the Lista Blanca Division and assigned an Upper Triassíc posítion to them Later King (1939) assigned to them the name Lista Blanca Formation and positioned them stratigraphi­ cally in the Lower Cretaceous. This same author noted numerous outcrops in the central and southern zones of the state where volcanic rocks of the Lü\ver Cretaceous appear intercalated in marine sedimentary sequences also indicating that volcanic rocks of this epoch incre(lse proportionally toward the west and southwest. In the northwest coasts of the state, Silver and Anderson (1978) recognízed some volcanic rocks dated by radiometric methods as belonging to the Upper Jurassic and Lower Cretaceous. Marine sedimentary sequences are seen to crop out in diverse localities and contain generally fossil faunas of the Aptian and Albi<1l1 Stages. In the northwest of the state and southwest of Arizona, a sequence crops out that constitutes the Bisbee Group, formed in élscending stratigraphic arder by the following: Glance Conglom­ erate; sandy shales and quartzose and feldspathic sand­ stones of the Morita Formation; límestones of the Mural Formation that vary from forereef to backreef; sandy shales and redbed sandstones of the Cintura FOIT(l(ltion 1904; Rangin and Córdoba, 1976; Gamper dll(l Lunguria, 1 In the ilhudripa area, a sequence of more than 3000 m exists¡ composed of conglomerate, shale¡ sandstones, and limes tone that represent the Palmar Formation in its lower part élnd the Potrero Formation in the upper part (King¡ 1939). Other marine sequences occur in the areas of Cucurpe, Sta. Ana, and Sierra Azul. These are formed chieny of ca1careous and sandy sediments of the Lower Cretaceous. During the Late Cretaceous the territory of Sonora State underwent an uplift and general emersion resulting from a phase of compressional deformation that was active over a large part of western Mexico at the beginning of this epoch. The principal igneous activity consisted of granitic emplacements that migrated in time toward the east and the extrusion of lavas that varied from andesite to rhyolite, chiefly along the eastern belt of the state and tmvard the base of the Sierra Madre Occidental. The exposures of Cretaceous batholithic bodies in Sonora constitute one of the outstanding characteris­ tics of the region. These granite-granodiorite bodies --- 23 1. Geology of the Norlhwest Region of Mexico D Paleozoic outcrops, some melamorphosed I 1 ! i I I I f I I I r J I I I I I 28'--1 _ I I [§] Jurassic andesltic volcanic rocks 111' I I f i - --. --­ -­ 1--------__ r I Figure 1.18. Dislribution of outcrops of Mesozoic igneous rocks of Sonora, have obscured in large part the deformational phe­ nomena that occurred before their emplacement. There is exposed in the Agua Prieta area a conti­ nental sedimentary sequence of Upper Cretaceous Ihal covers with angular discordance the deformed units of the Bisbee Group. This sequence was desig­ 109' ¡ I I I J I I I J I I I I I I I I f I I I J I I ¡ I ---1­ I nated by Taliefferro (1933) as the Cabullona Group and is made up of continental clastic sediments with intercalations of volcanic rocks, and contains dinosaur bones and an Upper Cretaceous flora (Rangin, 1978). Over al! the northeastern part of Sonora, numerous ou tcrops exist of U pper Cretaceous volcanic rocks 25 1. Geology of the Northwest Region oi Mexico 114" 111" I 109" ! I ---r-------- I I I i)'i'.o . I O I I <::1 t'!. J Caborca'C \Ji r Figure 1.20. Cenozoic voIcanic rocks in Sonora. . t>. (J O' ~ I I<11 Olígocene-Mlocene vulcanlsm D Basic vulcanlsm In the Plío-Qualernary --­ I I I r i ! ! I I I -J­ I I I I I I J I I 28'- L __ r These volcanic rocks vary in composition from in only slightly cansolidated strata (Dumble, 1900; andesítes and trachytes to dacites and rhyoJites. In the King, 1939). central and southern part af the sta te, volcanic rocks At the end of the Tertiary and beginning of the of the lower Tertiary crap out \vhose composition is Quaternary, an impartant episode of aJkaline mainly intermedia te. They caver with angular discor­ bClsaltic vulcanism took place, together with tectonic dance the deformed Mesowic sequences. distension of the normal faults, concurrent with The principal volcanic eyent of the TertiélfY of episodes of opening of the Culf of California (Clark Sonora consists predorninantly of ignimbrite extru­ et aL, 1980; Rangin, 1978). Ihis vulcanism has its sions of Oligocene-Miocene age that become a \vest­ clearest example in the Serrania del Pinacate located ward C'xtension of the yolcanic episodes responsible in the Altar Desert. for the Sierra Madre Occidental (see Figure 1.20). The exposures of this c1ass of volcanic units gener­ Pacific CoastaJ Plain ally form dissected tablelands that coyer in large part older terranes and geological structures. This regian is characterized by the development of During the late Tertiary the whole regio n of a plain constructed by the evolution of a system of Sonora was subjected to a series of normal faults that deltas that have advanced gradually toward the \vest. cut complctdy independently acrass élll e,nlier struc­ These deltas were formed at the mouths of the rivers tural units. This phenomenon resulted in a system of Mayo, Fuerte, Sinaloa, Culiacán, San Lorenz.o, é1nd northwest-southeast faults and consequent forma­ Mocorito and have surrounded rocky prarninences han of depressions that were fiHed by continental that formed old islands. detrital sediments of the Baucarit Formation. Ihis This zone is bordered on the west by a shoreline unit crops out in various localities of the state and is that developed sandy sediments, the sand being a made up generaHy of lithic fragments of diverse product af the action of littoral currents, tides, and composition that vary from subangular to rounded \vaves that have reworked the deltaic sediments and 26 Section 1 The Geology of the Mexican Republíc caused formation of bars, tombolos, and hooks. The eastern border of this zone is made up by the foothills of the Sierra Madre Occidental; here appears a moun­ taín chain formed from rock units whose geochruno­ logic range varíes from Precambrian to lower Tertiary. These units are partially covered by the vol­ canic sequence of the Sierra Madre Occidental, a sequence that becomes dominant eastward. The history of thc pre-Tertiary terranes tha tare exposed on the eilstern edge of Sinaloa share many affinities wíth the tectonic and paleogeographical styles that prevailed in Sonora and B<1ja California, which were united befare the Pliocene. It appeéus that the most ancient rocks that crup out in the State of Sinaloa are the metamorphics in the Sierra de San Francisco to the north of Los Mochis. A Precambrian élge has been assigned to these roeks by previous authors (Rodríguez and Córdoba, ]978). De Cserna and Kent (1961) termed these rocks the Sonobari Complexo The unit consists of intercalations of muscovi!e and biotite gnciss with amphibolites. In addition, intrusions by bodies of gabbro and granodi­ orite are present and development of pegmatites and migmiltites is observed. According to Rodríguez and Córdoba (1978), the gneisses were derived from sandy and argillilceous sedimentary rocks with possi­ ble intercalations of basie lava that have undergone at least two metilmorphic events. Along the eastern border of the Pacifíe Coastal Plain there exists a series of isolated OlltCropS of mod­ erilte extension that are of marine Paleozoic roeks. These sequences are eomposed principal1y of sand­ stones, shales, silts, and limestones; in sorne localities they have been affected by various grades of meta­ morphism. The stratigraphic relationship between this sequence and the Sonobari Metamorphic Complex hilS not been observed, and it is apparent that the contact with Mesozoic rocks is generally tee­ tonie. Rodríguez and Córdoba (1978) report the dis­ covery of the fuslllinid Millerella sp., which indica tes that the lower part of the sequence is probably of Late Mississippian to Early Pennsylvanian age. These authors indicate that the Paleozoie sequenees of Sinaloa were deposited in shallow water platform cond itions. In general, one can consider tha t these sequences were deposited in a miogeosynclinal belt that could be a southward continuabon of the Paleozoie Cordilleran geosyncline developed in the western United States. The Mesozoie in Sinaloa exists as a very heteroge­ neous series of rack types tha! consists seemingly of a volcano-sedimentary assemblage which becomes a southeilstward contin uation of the volcano-volcan­ oclastic and sed imentary arc rocks of the Alisitos Formiltion of Baja California (Rangin, 1978). Along the eastern border of the coastal plain, extensive out­ crops of volcanic rocks are observed. There are both lavas and pyroclastics whose composition varies from acidic to bilSic and thilt show the effects of both regional and contact metamorphism (Figure 1.21). The Mesozoic sedimentary rocks are represented by sequences of limes tones that in some localities are observed to be partialJy metamorphosed. Outcrops of these rocks are isotated; they are present aboye the intrusives in the form of roof pendants or of windovvs under the Tertiary eover. In sorne loealities they are apparently intercalated within the Mesozoic metavol­ eanie sequenee, but the contaets are not clearly observed. The major part of the ealcareous rocks that crap out in Sinaloa are seemingly of Cretaceous age, but Rodríguez and Córdoba (978) consider that some of these rocks could be Jurassie, or perhaps older. AH the Mesozoie volcanic and sedimentarv assem­ blages are affected by the emplacement of Mesozoie and Tertiary plutons. These intrusive bodies are the units that erop out most extensively in the State of Sinaloél. Their petrographic classification varies fram granite to monzonite, with biotite and homblend as principal mafie minerals. The extensive outcrops of this unit disappear under the volcanic eover of the Sierra Madre Occidental. The periods of emplacement of the intrusive bodies appear to be similar to those oecurring in Sonora. These emplaeements migra te in age from Cretaceous in Baja California to early Tertiary at the border of Sinaloa and Chihuahua (Sil ver and Anderson, 1978). During the Tertiary, important volcanic episodes oceurred in the State of Sinaloa; chief among them are those that oeeurred in the middle Tertiary and that gave rise to the ignimbrite cover of the Sierra Madre Occidental. This ignimbrite sequence covers () lc1fge part of the Mesozoie rocks of the eastern border of the Pacific coastal plain and the intennediate and basic volcanics of the lower Tertiary. Tectonic Surnmary The outcrops of Preeambrian metamorphic rocks in northern Sonora eonstitute one of the most character­ istic fea tu res of this region. Aceording to Anderson and Silver (979), these metamorphie rocks form two magmatic and orogenic belts with a northeast-south­ west orientation, truncated and juxtaposed by move­ ment a10ng a 112ft lateral zone of slippage thilt was active d1..lring the Jurassic in a northwest-southeast direchon. These two orogenic belts form part of the Precambrian terranes with orientation similar to that eneountered in the southwest portian of the North American craton. According to rad iometric da tes obtained by Anderson and Silver (1978), these sequences were d eformed and me tamorphosed 1650-1660 million years ago. However, periods of igneous intrusion between 1410 and 1440 mili ion years ago have also been recognized, as well as one that is about 1100 mil­ !ion years old. This last episode of intrusion consti­ tutes the first report of Grenvillian rocks in this region of the North American craton (Anderson et al., 1978). These metamorphic tenanes make up the base­ ment, aboye which episodes of marine platform sedi­ mentation oecurred at the end of the Precambrian and d uring the Paleozoie. Accord ing to Fries (962) this platform eonstituted a southern extension of the mio­ 27 1. Geology oE the Northwest Region of Mexico 26'- ­ Ic r ~ 1 Cretaceous plutons Figure 1,21. Outcrops of igneous rocks in Sinaloa. 24' - D Volcanics-volcanoclastics of Mesozoic age geosynclíne of the Cordilleran Geosyncline. He desig­ nated this area the Sonoran Trough. This trough underwent a slow subsidence during the whole Paleozolc, with some interruptions marked by hiatus in the sequences that crop out in Sonora and Sinaloa. Fries (1962) considers that at the end of the Permian, a period of gentle folding occurred, resulting in uplift and block-faulting that destroyed the earlier geosyn­ cJinal pa ttern. In the Late Triassic-Early Jurassic, two paleogeo­ graphic elements provided the setting for processes of sedimentation in Sonora. On the one hand, the ancient bay of Antimonio existed in the present north­ western part of the state, in which a thick marine sedi­ mentary sequence accumulated to the east, derived from positive areas. In contrast, tbere existed the swampy basin of San Marcia!, located southeast of Hennosillo, in which beds of coal, gypsiferous lime­ stones, sandstones, and shales accumulated (Alencaster, 1961). Overlying the sedimentary seguence of Antimonio there is él package of volcanic and volcanoclastic rocks. These together with the Lower Jurassic volccmic intercalations crap out in tbe Cerro Rajón southeast of Caborca and indicate the initiation of volcanic activity in the Mesozoic. This volcanic activity has been attrib­ uted by many authors to the presence of a zone of convergence located to the west. The subsidence of a paleopacific plate beneath the continental crust of Mexico, cmd the partial fusion of a plate at the leve! of the asthenosphere, originated the construction of a magmatic arc that was active during the Mesozoic. The magmatic activity related to this arc is interrupt­ ed only by the development of a left-lateral strike-slip zone, the Mojave-Sonora Megashear of Silver and Anderson (1974). The setting of convergence of the Paleopacific and North American plates is developed in two principal phases of deformation whose relationships can be clecHly observed in the Cabullona area to the south of Agua Prieta and Naco. The first of these occurred at the beginning of the Late Cretaceous and is indicated by angular discordance between the sandy calcareous Lower Cretaceous sediments and the detrital conti­ nental sediments of the Upper Cretaceous that crop out in the Cabullona basin. The second phase corre­ 28 Seetion 1 The Geology oi the Mexiean Republie sponds to the compressive deformabon from the end of the Cretaceous to tbe beginning of the TertiMY. This deforméltion originélted foJds with a northwest­ sOlltheast axial direction; they can be observed on the western flank of the Sierra Madre Occidental as well as in the overthrusting of the Lower Cretaceous and Paleozoic sequences aboye the Upper Creti1ceous Cabullona Group of tbe Naco ilnd Agua Prieta region (Rangin, 1977b). According to Rangin (1978) there seems to hélve developed between the Late Jurassic and the Early Cretaceous a phase of deformation, still not well known in Sonora, that correlates with the NevadclJ1 orogeny developed in North America. At the beginning of the Li1te Cretaceolls, the conti­ nental history of Sonora and Sinaloa comrnenced. ln this epoch the most important plutonic emplacements of the region occurred. These become youngcr east­ wc1rd. Also in this epoch the first volcanic episodes occurred that form the base of the Sierra Madre Occidental, whose principal period of construction W(1S ignimbrite activity occurring in the late Oligo­ cene (McDowell and Clabaugh¡ 1979). In the Miocene, activity of the convergence zonc to the west seems to have ceased and the development of the Culf of Ccdifornia was initiated. This is aCC0111­ panied in the adjacent regions of Sonora and Sinilloa by d istensive tectonics consisting of horsts and grabens that "vere active into the Quaternary and are responsible for the present-day distribution of the chicf orographic elements of the Sonoran deserto The setting of this type of tectonism gives rise to the deposition of great thicknesses of conglomeratic continental sediments, the Bilucarit Forrnation. Economic Resources In the State of Sonora (Figure 1.22), the most important resoUfces Me the deposits of copper and molvbdenum that are chieflv located in an eastern belt 'of the state. The origin o{ most of these resources has been attributed to the emplacernent of granite and granodiorite porphyries that occurred at the end of the Late Cretaceous and the beginning of the Tertiary. The principill rocks encompassed by this minerillizil­ tion me Cenozoic volcélnic rocks of intermediate com­ position, intrllsives of the same type, and in sorne cases sedimentary rocks of marine origino The chid reserves of copper porphyry in Sonora are encOllntered in the areas of Cananeél and Nacozari Oocalities 1, 2, élnd 3); other more minor deposits are locélted filrther south and west of these localities (loe. 4). The origin of this order of resources has been attribllted by Sillitoe (1975) to the partial fusion of the oceilnic crust under the continent and the li1ter ascen­ sion of magmatic material with solutions rich in cop­ per and molybdenum that formed "stockwork" deposits in roof pendants of large plutons and breccia pipe deposits. The deposits of lead and zinc in the State of Sonora are present in zones of rnetasornatic replacement and in hydrothermal veins. A major parl oi the first are of Laramide age while the second are generally associat­ ed witb middle Cenozoic volcanic rocks (Echavarri et a1., 1977). The principal localities with these types of deposits are: C<1nanea, San Felipe, El Tecolote, Sierrél de Cabullona, Lampazos, and San Javier. The gold élnd sil ver deposits are located at shallow levels in hvdrothermal veins Ihat contain the <1bove-rnentioned of lead and zine. Principal locéllities with these types of deposits are El Tigre, Las Chispas, Lampazos, and San Javier. Tungsten is an element with significant occurrencE' in the zones of contact metamorphism in the State of Sonora. Genera]]v it occurs in the form of the mineral scheelite and on' occasions is found associated with metasomatic deposits of copper, zinc, and collapse breccias ilssociilted with deposits of copper-bearing porphyry (Echavarri et al., 1977). The most important deposits of tungsten are found near Baviacora. The 1110St irnportant nonmetallic deposits me fluo­ rite élnd gr<1phite. The first mineral is of hydrothennal origin and consists of veins that are exploited princi­ pally in Esqueda and Santa Rosa; the second is found élssociated with coal and is present élS intercalations in the paludal sequence of the Barranca Group of the Upper Triassie. In the State of Sinaloél (Figure 122), deposits of copper and molybdenum form resources of the cop­ per-bearing porphyry type such as occur in Santo Tomás-Cuchicari and Tameapa. Deposits associated with stocks or veins of quartz with the presence of wolfr<1mite and tungsten are found in the mines of El Magistral, La Guadalupana, Siln José del Desierto, élnd El GuaYélbo. Deposits of molybdenum in stock­ work form exist in the mines of Los Chicharrones and Las Higueras and breccia and hydrothermal vein deposits occur in El Magistral (loe. 5), Bahuitil, Las Pastillas, La India, as well as the regions of Sinaloa de Leyva, Culiacán, San Ignacio, and Plomoso. There are important amounts of lead, zinc, and silver in deposits found in hydrothennal veins. These !atter veins are part of a belt that runs along the eilstern half of the state and that includes, further­ more, deposits of Ihe western borders of Chihuahua and Durango. Epithermal veins predominate in this belt. They contain gold, silver, lead, and zinc, which are most important in the State of S.inilloa. The enclos­ ing rocks of tbis class of deposits are generally andesites at the base of the vo!canic sequence of the Sien<1 Madre Occidental, and in some plutonic rocks. The zones of GU<1dalupe élnd Céllvo, Rosariltilo, Guadalupe de los Reyes, Pánuco, and Tayoltita con­ tain these types of deposits. The latter site contains the richest gold mineral district in the nation. SIERRA MADRE OCCIDENTAL The Sierra Mild re Occidental is formed from an extensive vo!canic meseta affected by normal faults and grabens that detract from its homogeneous and pseudohorizontal appearance, especially along its flanks <Figure 1.23). The eastern border of the Sierfé.'l grades into the Basin and Range Province of 29 1. Geology of the Northwest Region of Mexico 114 3Z' .- 111 --J- 108' -L I I NOGA S I O eCADORCAj O : Cananea O tJ 2 N_coz", I ,O I . O:> , I , O O ,O La Caridad 30' ­ - - - - - ~ . I . ~ - "- ­ L .1. I />.., O I • .1. ,l:,. • I • I [j /:..e HERMOSILLO • I So, Jose ¡ • D.. ,je Moradlna\; • • • I 28' - GUA.YMAS_D.._ ---0­ o Cu-Mo I • CIJ, Cu·Fe Cu-Ag J La Re10rma Au-Ag, Ag O O D.. W,W-Mo 0D.. A. Pb-Zn-Ag • I __ ~ d > __ O Au • • Graphlle .! '" O Sb I • f:¡J Cu-Au 4- Fe Note Slze of svmbol denotes ItS Impor1ance . Figure 1.22. Known mineral deposits in the States of Sonora and Sinaloa (taken from the metalogenic map of the Republic of Mexico, G,P, Salas, 1975). Chihuahua, while the western border consists of an abrupt termination with normal faults of major dis­ placement and zones of deep borrancas (steep-walled canyons). Accord ing to McDO\vell and Clabaugh (1979), the Sierra Madre Occidental is composed of two impor­ tant igneous sequences, whose contad marks an inter­ med ia te period of volcanic calmo The older sequence is formed main),\, fmm voIcanic rocks of intermediate composition, a ~ d igneous bodies whose ages vary between 100 and 45 million years. The more recent is Cümposed of rhyolitic élnd rhyodacitic ignimbrites in generally horizontclI position or slightly inclined and whose ages vary between 34 and 27 million ye<lrs. The lowE'r volcanic complex is dominantly in the form of lava flows and pyroclastic units and also con­ tains intercalations of siliceous ignimbrites. This lower complex contrasts in large me<lsure with the upper one, because of its slightly deformed character and intense faul ting élnd dist\,Hbed aspecto The sequences that comprise it ore, in general, the host rocks for the principal mineralization of él large port of this region of Mexico. The outcrops of this lower complex are, as expected, more restricted than those of the upper but have been recognized over aH the slope toward the Pacific in Sonora and Sinaloa. The upper contact displays <ln irregular surface \vith strong relief and shOl'vs marked contrast in the degree of alteration of the sequences. The upper complex constitutes the most continu­ OLlS dnd extensive ignimbrite cover in the world and is observed to form an area elongated northwest­ SOll theast abOLl t 250 km brocld and more than 1200 km long. Toward the north, this blanket has its lasl out­ crops about <lt the border with lhe United States and to the SOLllh it disappears beneath rocks of basic élnd intermediate composition that make L1p the Neovolcanic axis. According to Demant <lnd Robin (975), lhe thick­ ness of these ignim.brites in some localities approaches 30 Sedion 1 The Geology of the Mexican Republic Figure 1.23. Volcanic cover of the Sierra Madre Occidental. more than 1000 m. McDowell and Clabaugh consider The lower volcanic complex constitutes a typical that the number of calderas originated during the calcalkaline magmatic arc related to a convergent con­ emission of this great volume of rack ought to have tinental margin where the Farallon plate is buried been between 200 and 400. Many of these have a under the continental crust of Mexico. This phenome­ diameter greater than 40 km, although the semicircu­ non of convergence lasted until 29 miJlion years ago, lar configuration today is obscured by the presence of when the system of the expanding eastern Pacific normal fauIts and recent alluvial deposits. impinged against the western margin of Mexico 31 (Atwater, 1970). Nevertheless, the jnterruption of magmatism in the interval of 45 to 34 million years demonstrates a break in the continuity of these processes. McDowell and Clabaugh (1979) consider that this period of calm has two possible causes: one of these is a lowered amount of convergence or a change of indinaban of the plate being subducted; the other is the subduction of an active oceanic ridge. These same authors do not set forth a satisfactory tec­ tonic explanabon for the sudden voJcanic activity indicated by the upper complex and the bimodal character of this volcanic secluence when compared with a silica and anorthosite standard. Demant and Robin (1979) explain the origin of the ignimbrite blanket as the typical vulcanism of a rift zone behind an andesitic arc caused bv the reaction of the crust to the subduction m o v e m e ~ t s and indicate the coexistence of compressive and distensive vuJcan­ lsm. The principal mineralization within the Sierra Madre Occidental was partly discussed in the sections aboye, but the discussion is cornplemented by consid­ eration of the whole Chihuahua mea. It is thus conve­ nient to indicate some generalities related to this subject. The larger part of the mineral masses that are locat­ ed in the Sierra Madre Occidental are strictly related to the lower volcanic complexo The copper-bearing porphyries of Cananea and Nacozari are related to the episodes of intrusive emplacement at the Cretaceous-Tertiary boundary and the hydrothermal deposits belong to a period that fluctuates between 49 and 28 miUion yea rs (Clark et aL, 1980) With respect to the hydrothermal veins, belts are encountered situ­ ated on both flanks of the Sierra Madre Occidental. The first of them, located to the west, comprises the deposits of gold and silver of Sinaloa and Sonora, such as those of Tayoltita and San José de Grácia. The second belt, on the east side of the Sierra, includes areas with lead, zinc, and silver, such as Santa Barbara and San Francisco del Oro. BIBLIOGRAPHY AND REFERENCES Abbreviation UNAM is Universidad Nacional Autónoma de México Alencaster, Gloria, 1961, Estratigrafía del Triásico Superior de la parte central del Estado de Sonora. Paleontología Mexicana 11, parte 1, Instituto de Geología, UNAM, 18 P Anderson, D.L., 1971, La FaIJa de San Andrés, in Tuzo Wilson, 1974, Deriva Continental y Tectónica de Placas. Selecciones de Scientific American, W.H. Freeman and Company, San Francisco y Londres, p.163-179. Anderson, T.H., and L.T. Silver, 1978, junlssic mag­ matism in Sonora, Mexico: Geological Society of America Abstract with Programs, v. 10, p. 359. Anderson, TH., and L.T Silver, 1979, The role of the Mojave Sonora Megashear in the tectonic evolution 1. Geology of the Northwest Regíon of Mexico of northern Sonora, Guidebook Field Trip no. 27, Geology (lf Northern Sonora: GeologicaJ Society of America. Annual Meeting in San Diego, p. 59-66. Anderson, T.H., J.B. Eels, L.T. Silver, 1978, Rocas Precárnbricas y Paleozóicas de la región de Caborca, Sonora, México. Libreto Guia del Primer Simposio sobre la Geología y Potencial Minero del Estado de Sonora. Hermosillo, Sonora, Instituto de Geología, UNAM, p. 5-34. Atwater, T, 1970, Implications of plate tectonics for the Cenozoic tectonic evolution of western North Arnerica: Geological Society of America Bulletin V. 81, p. 3513-3536. BeaJ, CH., 1948, Reconnaissance of the geology ,lI1d oil possibilities of Bélja California: Geological Society of America Memoir 31, 138 p. (Original not consulted, cited in R.G. Gastil et aL, 1975, Recon­ naissance geology of the State of Béljél Californiél, Geological Society of America Memoir 140, 170 p.) Clark, K.F., PE. Damon, S.R. Schutte, and M. Shaffiquillah, 1980, Magmatismo en el norte de México en relación con los yacimientos metalíferos. Revista, Geomimet, no. 106, p. 49-71. Cooper, G.A., and AR. Arellano, 1946, Stratigl'aphy near Caborca, northwest Sonora, Mexico. AAPG Bulletin, v. 30, p. 606-611. Cooper C.A, et al., 1952, Célmbrian stratigraphy and paleontology near Caborca, northwest Sonora, Mexico: Smithsoniéln Miscellaneous Collections, V. 119, 184 p. Corona, F., 1979, Preliminal)l reconnaissance geology of Sierra La Gloria and Cerro Basura, northwestern Sonora, Mexico. Guidebook Field Trip 27, Geology of Northern Sonora: Ceological Society of America Annual Meeting in San Diego, p. 41-58. Darton, N.H., 1921, Geologic reconnaissance in Baja California: Journal of Geology, V. 29, p. 720-748. (Originéll not consulted, cited in F. Lozano, 1976, Evaluación petrolifera de la península de Bajél California, Mexico. Boletin Asociación Mexicana Geólogos Petroleros, v. 27, no. 4-6, p. 106-303.) De Cserna, Z., and B.H. Kent, 1961, Mapa geólogico de reconocimiento y secciones estructurélles de la región de San Bias y el Fuerte, Estados de Sinaloa y Sonora. Instituto de Geologia, UNAM, cartas geo­ logicas y minerales, no. 4. Demant, A., and C Robin, 1975, Las fases del v01­ canismo en México; una síntesis en relación con la evolución geodinámica desde el Cretácico: Revista Instituto de Geologia, UNAM, V. 75, p. 70-83. Dickinson, W.R., 1979, Plate tectonics and the conti­ nental margin of California, il1 W.G. Ernest, ed., The Geotectonic Development of California: Rubey volume 1, Prentice Hall, p. 1-28. Dumble, E.T., 1900, Notes on the geology of Sonora: Geological Society of America Bulletin, V. 11, p. 122-152. Echavarri, A., AO. Saitz, and C.A. Salas, 1977, Mapa Metalogenético de Estado de Sonora: Revista Geomimet 2a Epocha July-August, no. 88, Consejo de Recursos Minerales. 32 Section 1 The Geology ai the Mexican Republic FlIlCh, J.W., and P.L. Abbolt, 1977, Petrology of a Triassic marine scction, Vizcaíno Peninsula, B<lj<l California Sur, México: Sedimentary Geology, v. 19, p. 253-273. Finch, J.W., E.A. Pessagno, and P.L. Abbott, 1979, San Hipolito Forrnation: Triassic marine rocks of the Vizcaíno Peninsula. Field Guides and Papers of B<lja California, Geological Society of America Annual Meeting in San Diego, p. 117-120. Fries, e., 1962, Reseña geológica del Estado de Sonora, con énfasis en el Paleozóica: Boletín Asociación Mexicana Geólogos Petroleros, v. 14, p. 257-273. Gamper, M., and F.J. Longoria, 1980, Bioestratigrafía y f<lcies sedimentárias del Cretácico Inferior de Sonor<l: Resúmenes de la V Convención Geológica Nacional, México, D.F, p. 14-15. Gastil, R.G., and D. Krummenacher, 1978, The migra­ tíon of the axis of Pacific margin magmatism across Baja California, Sonora and Chihuahua. Resúmenes del Primer Simposio sobrc la Geología y Potencial Minero del Estado de Sonora, Hermosillo, Sonora: Instituto de Geologia, UNAM, p. 63-64. Gastil, R.G, RP PhiUips, and E.e. Allison, 1975, ReconnaisSclnce geology of the State of Baja California: Geological Society of America Memoir 14, 170 p. Gastíl, R, C. Morg<ln, and D. Krummenacher, 1981, The tectonic history of peninsular California, in W.G. Ernest, ed., The Geotectonic Development of Californi<l: Rubey volume 1, Prentice Hall, p. 285-305. Gonzé1lez, C, 1979, Geology of the Sierra del Alamo. Guidebook Field Trip no. 27, Geology of Northern Sonor<l: Geological Society of America Annual Meeting in San Diego, p. 23-31. Heim, A., 1922, Notes on the TertiiHV of southern Lower California: Geological M a g a ~ i n e . (Original not consulted, cited in F. Mina, 1956, Bosquejo geológico de la parte sur de la peninsula de Baja California, Excursion A-7 of the XX Congreso Geológico Internacional, México, p. 11-42.) Tmlay, R.W., ]939, Paleogeographic studies in north­ eastern Sonora: Geological Society of America BuUetin, v. 50, p 1723-1744. Keller, W.T., and FE. Wellings, 1922, Sonora: Cía Petrolera el Aguila, Gcological Report no. 180,38 p. (unpublished). (Original not consulted, cited in T.H. Anderson, J. H. Eells, and L.T. Silver, 1978, Rocas Precámbricas y Paleo;¡;óicas de la región de Caborca, Sonor<l, México. Guia del Primer Simposio sobre la Geología y Potencial Minero del Estado de Sonor<l, Hermosillo, Instituto de Geología, UNAM, p 5-34.) King, R.E., 1939, Geological reconnaissance in north­ ern Sierra Madre Occidental of Mexico. Geologicéll Society of America Bulletin, v. 50, p 1625-1722. Longoria, F.J., Clnd V.A. Peréz, 1978, Bosquejo geológi­ co de los cerros Chino y Rajón, cuadrángulo Pitiquito-Lel Primavera (NO de Sonora): Bulletin Department of Geology, UNI-SON, v. 1, no 2, p. 119-144. Longoria, F.J., M.A. Conzále¿, J.]. Mendoz<I, and V.A. Pércz, 1978, Consideraciones estructurales en el cUéldrángulo Pitiquito-La Prim<lvera, (Noroeste dc Sonora): Bulletin Department of Geology, UNT­ SON, v. 1, no. 1, p 61-67 López-Ramos, E., 1979, Geologí<l de México, 2a edi­ ción, México D.F., Edición escolar, 3 volúmenes. Lozano, F., 1976, Evaluación petrolífera de la peninsu­ la de Baja California, México Boletín Asociación Mexicana Geólogos Petroleros, v. 27, n. 4-6, p. ]06-303. Márquez-Castañeda, 13, 1984 Estudio geológico del áreél de Santa Barbara, Chihuélhuél. Unpublished Report of the Faculty of Engineering, UNAM. McDowell, FW., and S. Clabaugh, 1979, Tgnimbrites of the Sierra Madre Occidental and their relation to the tectonic history of western Mexico, in e.E. Chapin and W.E.Elston, eds., Ashflow Tuffs: Geological Society of America Special Pelper 180. McEldowney, RC, 1970, An occurrence of Paleozoic fossils in Baja California, Mexico: Geological Society of America Abstracts with Programs, v. 2, p. 117. (Original not consulted, cited in Re. GastiJ et al., 1975, Reconnaissance geoIogy of the State of Baja California: Geological Society of America Memoir 140, 170 p.) McKenzie, DP, and W.]. Morgan, 1969, Evolution of triple junctions: Nature, v. 224, p 125-133. Mina, F., 1956, Bosquejo geológico de la parte sur de la penínsulél de Béljél California: Excursion A-1 of the XX Congreso Geológico Internacional, México. p 11-42. Mina, F, 1957, Bosquejo geológico de la parte sur de la península de Baja California, Boletín Asociación Mexicana Geólogos Petroleros, v. 9, p 139-269. Mulchay, R.B, and J.R Velasco, 1954, Sedimentary rocks at Canélnea, Sonora, Mexico with the sections at Bisbe and Swisshelm Mountains, Arizona: AIME Transactions, v. 199, p. 628-632 (Original not con­ sulted, cited in Cía Minera Cananea, SA, 1978, Geología del Distrito Minero de Cananea, Sonora, Libreto Guia del Primer Simposio sobre la Geología y Potencial Minero del Estado de Sonora, Instituto de Geología, UNAM, p. 57-70.) Ortega-Gutiérrez, F., 1982, Evolución magmática y metamórfica del complejo cristalino de La Paz, B.CS.: Resúmenes de la VI Convención Geológica Nacional de la Sociedad Geológica Mexicana, p. 90. Ortlieb, L., 1978, Reconocimiento de las terrazas mari­ nas Cuaternárias de la parte central de B<ljél Cali­ fornia: Revista, Instituto de Geología, UNAM, v. 2, no. 2, p. 200-211. Pantoja-Alor, J., and J. Célrrillo-Bnwo, 1966, Bosquejo geológico de la región de Santiago, San Jose del Cabo, Baja California: Boletin AsociaCIón Mexical1é1 Geólogos Petroleros, v. 17, nos. 1-2, p. 1-11. Pa tterson, D. L., 1979, The Valle Formation: physical stratigraphy and depositional modeL southern Vizcaíno Peninsula, Baja California Sur: Field Guides and Papers of Baja California: Geological Society of America Annual Meeting in San Diego, p.73-76. 33 Rangin, C, 1977a, Sobre la presencia del Jurásico Su perior con amon ita:; en Sonora :;epten trional, México: Revista, Instituto de Geología, UNAM, v. 1, p. 1-4. Rangin, C, 1977b, Tectónicas sobrepuestas en Sonora septentrional: Revista, Instituto de Geología, UNAM, v. 1, p. 44-47. Rangjn, C, 1978, Consideraciones sobre la evolu tión geológicas de la parte septentrional del Estado de Sonora. Libreto Guia del Primer Simposio sobre la Ceología y Potential Minero del Estado de Sonora, Hermosi1Jo, Sonora, Instituto de Geología, UNA M, p.35-56. Rangin, C, 1979, Evidence for superimposed subduc­ tion and colljsíon processes during Jurassic­ Cretaceou:; time along Baja C¡:¡lifornia continental borderland. Field Guides and Papers of Baja California: Geological Society of America Annual Meeting in San Diego, p. 37-52 Rangin, F., 1978, Consideraciones sobre el Paleozóico sonorense: Resúmenes del Primer Simposio sobre de Geología y Potential Minero del Estado de Sonora, HermosilJo, Sonora: 1nstituto de Geología, UNAM, p. 35-56. Rangin, F., and D.A Córdoba, 1976, Extensión de la Cuenca Cretácico Chihuahuense en Sonora septentrional y sus deformaciones: Memória del Tercer Congreso Latinoamericano de Geología, México, 14 p. Ransomc, F.L., 1904, Description of the Bisbee quad­ rangle, Arizona: US Geological Survey, v. 112, 17 p. Rodríguez, R., and D.A. Córdoba, 1978, eds., Atlas geológico y evaluación geológico-minero del Estado de Sinaloa: Instituto de Geología, UNAM y Secretaría del Desarrollo Económico del Estado de Sinaloa, 702 p. 1. Geology of ¡he Northwest Region of Mexico Roldán, J., and Solano B., 1978, Contribución a la estratigrafía de las rocas volcánicas del Estado de Sonora: Bu1Jetin of thc Dcpartment of Ceology, UNI-SON, v. 1, p. 19-26. Salas, G.P., 1980, Carta y provincias metaJogenéticas de la Republica Mexicana, Consejo de Recursos Minerales 2nd Edición, 199 p. (Mapa con texto.) Santillán, M., and T. Barrera, 1930, Las posibilidades petrolíferas en lo costa occidental de la Baja California, entre los paralelos 30 y 32 de la ti tud norte: Anales de Instituto de Geología, v. 5, p. 1-37. Sillitoe, R.H., 1973, The tops and bottoms of porphyry copper deposits: Economic Geology, v. 68, p. 709-715. Sillitoe, R.H., 1975, A reconnaissance of the Mexican porphyry copper belt. Silver, L.T., and TH Anderson, 1974, Possible left-lat­ eral early to rniddle Mesozoic disruption of the soutl1western North A01erican craton margin: Geologícal Society of America Abstracts with Programs, v. 6, p. 955. Silver, LT., and T.H. Anderson, 1978, Mesozoic mag­ matism and tectonísm in northern Sonora and their implications for mineral resources, Resúmenes del Primer Simposio sobre la Geología y Potencial Minero del Estodo de Sonora, Hermosillo, Sonora: Instituto de Geología, UN AM, p. 117-118. Stoyanow, A., 1942, Paleozoic paleogeography of Arizona: GeoJogical Society of America Bulletin, v. 53, p. 1255-1282. Taliefferro, N., 1933, An occurrence of Upper Cretaceous sediments in northern Sonora, Mexico: Journal Geology, v. 41, p. 12-37. Valentine, W.C., 1936, Geology of the Cananea Mountoins, Sonoro, Mexico: Geological Society of America Bulletin, v. 47, p. 53-86. • 2. Geology of the Northern and Northeastern Regions of Mexico • / ( /'" TORREaN <... , •• ./.. NORTHERN AN'O • MATAMOROS 'REG!QN . ".,--,... ,/ ' ....... ", /. SAN LUI:,>·.. \ ,.... <., POTOSI MONTERREY ) . .,>.. \.,) .... . ./ GENERAL CONSIDERATIONS For description of the northern and northeastern regions of Mexico, one can use the following natural limits: to the west, the Sierra Madre Occidental; to the east, the Gulf of Mexico coast; and to the south, the northern edge of the Neovolcanic axis. The region comprises, according to the physio­ graphic division of the DGG (see Figure 1.1), the provinces of Sierras and Plains of the North (Basin and Range), the Sierra Madre Oriental, the Great Plain of North America, the Central Mesa, and the Coastal Plain of the northern Gulf of Mexico. Nevertheless, the division that is used is based fundamentally on the paleogeographical elements of the Mesozoic in this part of Mexico. In vorious forms, these elements have a general correspondence with the physiographic provinces mentioned aboye, chíefly in the orígín of orographic forms thot are particular expressíons of the types of geologieal phenomena that generate them. The climate of aH the region varies in general from hot to temperate, and regular summer rains oceur. "'.:" Additíonal1y, precipitation records reveal that the clj­ mates vary from dry to semi-arid in the west of this zone and from humid to subhumid in the Sierra Madre Orientol and the Coastal Plaín of the northern Gulf of Mexico. CHIHUAHUA AREA General Geology The area of the State of Chihuahua is eharacterized, particularly in its eastern part, by the presenee of fold­ ed mountains formed from marine Mesozoic strata. These mountains make up prominent topographic peaks that oecur separated by great plains that were elevated as thev were filled. These are tectoníc troughs filled \vÚh continental sediments and sorne lava debris blocks. They originated as local filled basins termed bolsones. The folded sedimentary sequenees gradually disappear toward the western margin of the state, to an edge under the ignimbrite cover of the Sierra Madre Occidental. 35 --------- 36 Seclion 1 The Geology oi lhe Mexican Republic The folded sediment<uy rocks, which crop out in the majc)f pclft of the arca, developed aboye a Precambrian and l'aleozOlc basement that is exposed in some localities and that also héls been reported in wells drílled by Pemex. In the area of the Sierra del Cuervo, Mauger a.nd co-workers (1983) obtained a K­ Ar c1ge corresponding to the Grenville from ct meta­ morphic block included in a Permictn sequence. Quintero and Guerrero (1985), in addition, rcported the exposure of a sírnilar metcllnorphic unit to the south of Mina Plomosas that could be an outcrop of the Chihuahua Precambrian basement. The Paleozoic rocks that crop out widcly in sorne areilS of Texas have in northeastern Mexico very restricted exposures wüh the result that it is difficult to reconstruct the paleogeographic elements of thot c1H'a (Figu re 2.1). Gonzáles (1976) considers tha t the outcrops of limes tones and dolornites of the lower Paleozoic of Chihuahua reflect a platform environ­ 108' I ment similar to the focies deveJoped across the North American craton and considers it logical that this ete­ ment continucs toward Mexico. In contrast, he indi­ cates tha! in the Pennsylvaniiln-Permian interval, the sedimentological pattern prescnts more irregular con­ ditions caused by the action of block faulting that devcloped intracraJonic platforms and ba.sins on \·vhich were deposited respectively carbonates and terrigenous sediments. The Diablo Platfonn dates from this time; its southwest border forms a rnarked lineament thélt coincides approximately ""ith the course of the Rio Grélnde along a belt from Ciudad Juárez to üjinilga. This tectonic feature has main­ tained its influence on sedimentological events and deformation during the Mesozoic and even the Cenozoic. DeFord (] 969) makes note of the suelden disappearance, at the Mexican irontier, of the üUélchita belt composed of deformed Paleozoic ter­ rigenous sediments. Ihis leads onc to believe that this 105' CO. JUAREZ 1 1 1 I 1 I I I I I I I 1 I I CHIHUAHUA • 1 I ·-1-·------­ I I I I 1 I I ----t I - - - - '30' bJINAGA· j 1 I • PARRAL] I I I I I I ¡ I - - - - L ~ - - - - - í - - - ­ 26' I D Sedlmenlary rocks Figure 2.1. Paleozoic sedimentary rocks of Chihuahua. 37 2. Geology of the Northern and Northeastem Regions of Mexico bdt continues under the Mcso7.oic sequencc ol ChihuClhud to the east of the calcélfeous outcrops oí the Paleozoje at Plilccr de Guadi\lupe-Minél Plomosas. Nevertheless, one Cdnnot rule out the pns<;ibílity that thi::; bdt pélsses to the west of the élbove-cited lOCéllity, siJlce in the area of Aldélmél the presence of a consid­ erable seqw'nce oE délfk grélY siltstones similar to those üutcropping on the northern flank of the OUélchitél belt in -, exas ha:, been reported. The dilliculty in defíning the Paleozoic tectonic e1e­ ments in Chihuahua is Glused by the very scarce out­ crnps ano the fact thélt this region encompasses the confluence of the North Ameríc,m ([aton, the OUélch..ita belt, élnd ["he Cordilleran miogeosyncline, al! uf whose ínterrelahonships are still somewhcl.t confusing. Thl-' pflncipéil Pólleozoic outcrops of Chihuahllél inelude the seqlll:'nces cxposed in Ihe areél around Minél Plomoséls northeélst of Chihllélhué1 (mostly Iime­ stones); Aldélma sillsl mes, \vhích are present to the north of Chihuélhuil Citv; élnd variollS minor loca!íties that occur in the corner of the state where ]jmcslone and dolomite plélltorm félcies appeélr. In the oil wells Moyotes no. 1 élnd Chinos no. 1, I'élleozoic c;"quencl:S \Vere drilled. In the latter well Ordoviciéln, Cambriéln, and metélmorphic Precélmbrian rocks \Vere ene luntered (N,warro élnd Tovar, 1973). Thc urugenic dcformation of the Marathon­ Ouachila olC'osyncJine during the Pennsylvélnian ¡nterval élnd the normal faulting of toe soutbern portion of the North American créllon "vere followed by <l long periocl of eme(sion (PermiiHI to lVIiddle Jurassid during wbich redbeds \Vere deposit­ ed within él tectonic setting of intense normal faulting. This episodc of continental deposition is \'1 1 1dely known ayer the neighboring regions of Torreon clnd northern Zélcalecas, At lhe bq;innm¡?; of Kim meridgi 11, lhe eastern pl1rtion of Chihuahua IOrmed él marim' basin as a con­ sequence of a transgression initiilted during this time (DeFord, 1969). This basin limited to the norlheélst bv the Diélblo l'Jatform, to the south"vest by the J.\Jdama Peninsulé1, élnd to the eélst by the Coa'huila P 'ni nsula or nd (see hgure 2.4). éonzcíJes (976) clles cm unpublished \·vork by R. Carza of Pemex in which it is sughcsted thélt the Aldélmél Peninsula élmi the COélhuila lslélnd might h,we constituted él single positive element be:,ide which the Chihué1hua basin might have had communication élcross t11(' Gulf uf S,lbinas In _oahuila. The [irst sLlgl'" (lf th pp '1" lTldflne tr,ll1sgress'ion coveríng ti e Lll11u.'ilm, basin g,we rise to deposition of an sequence thcll is nolV mélnifested by diélpiric struetures of salt dnd gypsum Jocélted lo ('he south uf Ojinag<l and Ciudéld JuéÍrez, élS well as encountered in oi! wells clrilled by Pemcx in the Sierra de Cuchillo Pélféldo (DeFord, 1969). The chicf outcrops of the Upper Jurassic contain argiJlélceolls élnd eéllcélfeoLls sequences and are locélted principéllly between Ciudad Juárez élnd Chihuahua, mainly to the llorth of Sierra de Si1Ll1alaYllca, in lhe Sierra de Jél Alcc1pélrrél, in the Sierra El Kilo, élnd in the Sierrél La Mojína. At the beginning uf the Cret<1Ceous, dllring the f\ t'uconlian, mélfine sed il11l'nla tion con ti nueo in Ihe Chihuahua basin, chiefJy with deposlts of lil11eslone and gypSlll11 of the Alcaparra hHmation as w(,11 as with the clays élnd sdndstones of the Lél c' V'né1S Formabon. During this epoch the Coahuilél ¡si wd remélined still emcrgent and the Aldélm,' Pcnin:::lll,1 was covered by a l11élrine trélns -1,,'. sion. At the end oi the Neocomjéln élnd beginning uf the Aptian, the seas begélll él very impo l<lnl transgression oV't'r 11. COélhLlilél IsJélnd élnd the inlernéll terrélnes of Sunor, and SinaJoa (Rangin élnd Córdoba, 1976) In thr Chihuahuél basin él very thiek, fllndamentillJy . kMl' ous sequence then developed that inch¡des ¡he Cuchillo Forméltion and the Chihuahuél Group (Benigno and Lágrima forméltions, the Fin <l)' Llmestone, and Benavides Forméltion) (Córdoba, 1Y70). This transgression élcross the positivc c1emenb reaches its maximum d '\'eloprnent in lhL' Albian-Cenomélniéln intervéll, during which impor­ tant red facies were developed over the AldélJn<I Platform (Frélnco, 1978). During the _, le Crcl<l'l'DUS, lerrigenolls sedimentation in the Chihudhua r , ion marks the uplift and volcélnic élctivily of the vVl'stern part of Mexico. These tcrrigenous deposits the Ojinagél Group, rewgnized in the ,He,l oi the it". of this name; they rcfJect él deltélic environment th< t in the Célmpanian marks the ,1dvélnee of lhe COlO:; line lo lhe Célst (Gonólcz, 1976). The absenct. (lf L'pper retélceOllS sedlments in the éHeél of Ihe Aldélm,1 Peninsulél sLIggests th,11 lhis region remélincd ('ni 'r­ gent duríng most of Lélte Cret¡ll'cous lime. The end of the ErJ is markecl by folding of the Meso:¿oic cover (Jigure 22) ZlS él result 01 a "decollement" or sliding élt the leV'L'! of thL' eVilp­ orite seqllence. On the other hand, the strucluraJ axes present gencréllly a northwest-southeélsl orientatioll. The recumbence of the inh'rse faulls, which hél\'(;' vergence on bolh sides of the ba.sin, hélS been interpreted lo be dlle to the up-éItehing of el central beJt élt lhe level of the basement, which originah-'d the slippélge toward the Diablo ,1nd Aldama respectively (Gries and ':i 711,. In lhe Cenozoic Eri\ Ihe Chihuahua region evoJved as an emergent 20ne that was panly covcrcd (almo:',[ complete]y so in the western portion) by uf i::.,nimbrites of Olígocene-Míocene age. McDowell and CJaballgh (1979) indicnte that the volcanic rocks to the casI side of Chihuélhu, {Figure 2.12) hélve ,1 ditfcrent chemicéll composition from lhe majnr VOlc.llÜC .1rC<1S iínl1 L'O :,titu! ,In inll'r¡,nCl11éll" prov I1lt· bd 'l't' t\ lb'.' calcéllké1line series of the Sierra Madre OLcidcnl,l! (west of Chihuahuél) élnd the éllkélline series of Pecos Texas. The distention;,l teelLlnics of the late TertiéJry caused the formiltion of grélbens in which considerable thicknesses of continental sediments \Vere deposited. Economic Resources Aecording to the mi)p of Meléllk)bl'níc Provinces of the Mexícan Republie (Sélléls, 197<;), the Stilte of Chihuahua beJongs to the Metélllosenie Province of 38 Section 1 The Geology of the Mexican Republic 108' I TTTTT Normal fault Y'TT'T' ¡nverse lauH + Sync1lne +Antlcline IG Tertlary volcanlc rocks Figure 2.2. Structures in Mesozoic rocks of Chihuahua. the Sierra Madre Oriental. The princi pal deposits of this region ore the hydrothermal deposits of silver, lead, zinc, and gold that are localized principally in the central belt of the state and that follow in large part the axis of the Mexican Republic (see Figure 2,3). The mineral districts of Santa Eulalia, Naica, Hidalgo del ParraL Santa Barbara, and San Francisco del Oro belong to this central belt of Chihuahua sta te, According to Clark et o!. (1980), during the end of the Mesozoic and a large part of the Cenozoic, there occurred an eastward migration in time and space and a la ter return of the magmotic arc related to the convergent morgin that was developing along the western border of the country. The episodes of hydrothermal mineralization mentioned aboye occurred 28-40 million years ago (Clark et al., 1980). These episodes were related to the magmatic activity that is a product of the partíal fusion induced by the Farallon plate under the continental crust of Mexico during which time a return to westward migration of the magma tic arc was occurring. The hydrothermal deposits of manganese, con­ tained generally in ignimbrites, also form a belt of deposits within which the localities of Talamantes, Tenantes, and Casas Grandes occur. The iron deposits of eastern Chihuahua belong to a belt associated with the return to the west of magma­ tism related to arc migration (Clark et a!., 1980) The chief deposits are encountered in the mines oE La Perla and Hercules in the vicinity of Coahuila. The uranium-bearing volcanogcnic deposits of Chihuahua are linked to the migration of magmatisrn toward the east and are related to rhyolitic and tra­ chytíc lavas about 40 million years in age. The princi­ pal locabon of this type of deposit is the Sierra de Peña Blanca, located in the north of Chihuahua. With respect to petroleum reservoirs, tile calcare­ ous Paleozoic rocks that possess platform facies, chief1y those that crop out in the northwest part of the state, occur at depth with petroleum possibilities. They show considerable porosity and resemble pro­ ductive rocks of western and central Texas (González, 1976). Information from petroleum wells has revealed possibilitíes in the Jurassic of Chihuahua since they have penetrated calcareous-argillaceous sequences with high organic content, which could serve as source rocks as well as some porous sequences of platform facies that could be reservoirs (González, 1976). COAHUILA AND NUEVO LEÓN General Geology This region is characterized by the predominant presence of folded Mesozoic sedimentary rocks that rest upon a Paleozoic and Precambrian basement. The most significant physiographic feature is the flexure that the Sierra Madre Oriental undergoes at the lati­ tude of Monterrey; fram here the fold belt acquires a general east-west orientation. To the north of this flex­ ure the orographic elements become more widely sep­ arated and the geologic structures less narrow. Furthermore, the topographic relief diminishes grad­ ually to the east grading into the coastal plclin of the Gulf of Mexico. The Paleozoic basement, above which the Mesozoic seguence of this region developed, has been interpret­ ed as a continuation of the Ouachito belt of the south­ eastern United States. Denison et al (1970) have indicated that the Granjeno schists of Peregrina Canyon rnaintain a great similarity to the eastern inter­ nal zones of the Ouachita belt. This affirmation seems lo be corroborated by the metamorphic basement reported in the petroleum wells of the States of Nuevo León and Tamaulipas. In contrast, the detrital Pennian sediments reported fram the Delicias-Acatita area are similar to those of the frontal belt east of the Ouachita geosyncline. Periods of emersion and normal faulting occurring during the Triassic and part of the Jurassic controlJed the paleogeography of the upper part of the Mesozoic and gave way to continental redbed deposition that has been reported chiefl y within and to the sou th of the Monterrey-Torreon transverse sector. 39 2. Geology of lhe Northem and Norlheaslem Regions of Mexico 108' I 105' CO.JUAAEZ I ,'" I I , I o Au-Ag .... • Mn A Fe L:::,.W I 1- - - - 26' I - - - - ~ - - - - A Sta. Eulalia I . A . I I , I S!m frBn¡¡í5Cil d.QI Úru • I I I - --1- I I • I I ~ A L. I I I I I I I I I , I I - - ~ - - - - I I A Iorrn I Pb-Zn-Ag • Hg O Pb-Cu Figure 2.3. Distribution of principal rninerallocalities known in the State of Chihuahua (taken froro the MetaUogenic Map of the Republic of Mexico by G.P. Salas, 1975). In the Late furassic, a transgression occurred over northeastern Mexico; it concurred with the formation of the Sabinas Gulf, the Coahuila lsland, and the Peninsula and Archipelago of Tamaulipas (see Figure 2.4). This phenomenon has been related by various authors to the opening of the western extremity of the Tethyan Sea during the initial disintegration of the supercontinent Pangea. During this process the Paleogulf of Sabinas was defined in the Oxfordian. 11 possesses the characteristics of an intracratonic basin developed in the more tectonically stable southern North American cralon. Terrigenous and calcareous evaporite deposits were developed in the first stages of transgression in the Sabinas Gulf. They formed under conditions of strong evaporation (González, 1976), mainly in the Oxfordian (see Figure 2.5). The Minas Viejas, Novillo, Olvido, Zuloaga, and La Gloria formations belong to this epoch. The last two forma­ tions represent respectively the extra-littoral and near-coastal facies of the upper Oxfordian (Rogers et al., 1961). The advance of the marine transgression during the I<immeridgian and Tithonian created deposits of the open sea, the La Caja and Pimienta formations, which are composed of calcareous-argilla­ ceous sequences with carbonaceous horizons, just as in the La Casita Group (see Figure 2.6). During the beginning of the Early Cretaceous, lhe marine transgression of the Late Jurassic continued 40 Section 1 The Geology of the Mexican Republic i I so 160 240 km Figure 2.4. Paleogeography of Upper Jurassic, after Raúl González-Garcia, 1976. and gave way thcoughout ClI1 the Neocomian to depo­ sitian of a heterogeneous sequence camposed of vari­ ous formations. The S,\l1 Mc1fCOS llrkose constitutes a littoréll and continental facies encompassing Cl major part of the f\Teocomian deposited simultaneously with other diverse forméltions. The configuration of the San Marcos arkose permits the observation of intercala­ tions of the unit between formations deposited con­ temporaneollsly in marine platform environments. This is indicélted by various lentils formed off the coastline of the COilhuila Islélnd in different strati­ graphic levels. The MenchClca Formation is formed by a seguence of limestones and some intercalations of mar! and shale. This formation constitutes the base of the plalform sequence of the Neocomian that is repre­ sented higher up by the shales and sandstones of the Barril Viejo Formation; the limestones and shales of the Padilla Forméltion; the calcareous-argillaceous sequence of the La Mula; and the limestones, dolomites, and evaporites of the LCl Virgen Fonnation. In the southeast sector of the Sabinas Gulf, the argilla­ ceous limestones of the Télfaises Formation ,vere deposited during the Berriasian-Hauterivjan interval. From the H,lUterivian to the Aptian, in al! of north­ east Mexico, caJcareous deposits fonned; these occur in various facies. In a large part of the Sabinas Gulf, limestones of the Cupido Formation were deposited in a platform environment. Furthermore, there devel­ oped a reeiéll lineament that trends from Lélfedo to Monterrey and from there to the west toward Torreon. This is considered an integral part of the Cupido. Finally, outside of the reefal margin that bounds this formation, facies of the open sea devel­ oped, constituting the upper Tamaulipas Formation (see Figure 2.7). At the Aptian-Albian boundary, there suddenly appeared a general influx of fine terrigenous clastics into the Gulf of Sabinas; this forms the La Peí\a Formation. This infiux could be a response to epeirogenic uplifts of the surround ing positive ele­ rnen ts or to a eustatic féll1 in sea level (Smith, 1970; Chélfleston(1973). During the Albian-Cenomanian intervaL an impor­ tant marine transgression, cornpletely covering ele­ ments that until then had remained positive, began the development of thick sequences of carbonates 41 2. Geology of the Northem and Northeastem Regions of Mexico • CO VICTORIA 1"AMAULlPAS NINSULA ZACATECAS o 50 100 km ~ Plalform carbonates-evapontes 01 Olvido-Novillo Formallon ~ ~ Shelf margln oollte banks 01 ZLJloaga GroLJp (?) ~ ~ Plafform clastlcs 01 La Glolla Formatlon ~ ArglllaceoLJs open manne Ilmeslone of unknown lormatlon Figure 2.5. Paleogeographical configuration of northeast Mexico during the Oxfordian, after Alfonso­ Zwanziger,1978. 42 Seetion 1 The Geology of the Mexiean Republie .00 VICTO /A l.' .1, °:1 Terngenous La Casita Group o so 100km 1 ÍI R3 Argillaceous sandy carbonate 01 Pimienta Formalíon t:s:::::d Argíllaceous carbonates 01 La Caja Formallon ~ Figure 2.6. Paleogeographical configuration of northeast Mexico during the Kirnrneridgian and Tithonian, after Alfonso-Zwanziger, 1978. 43 2. Geology of the Northern and Northeastern Regions of Mexico Reef 100 200 km \ _J 1 i ­ .... -1 l ..... ..... .... __... -, o / / ... J ( ( I ,/ I . r ... _. . "/ J .... • .0:--......./ ... [!9 . ..... \ \ \ \ ) \ J \ \ '). ... / ­ I \ I '­.... \ I ,..... ­ . .... , t \ \ \ \ I I \J I I I ( \ Dolomlte Evaporiles Temgenous deposlts Platform Ilmestone Baslnalllmes\one Conllnenlal depos,ts H RMOSllLO • Figure 2.7. Paleogeography of Neocomian-lower Aptian for northern and northeast Mexico, after Raúl González-Garcia,1976. over al! of northeast Mexico. Over the Burro platform tow8rd the east, successive cOélstlines and deltas (Tamaulipas Peninsulél) and the COélhuila Island, developed with consequent detrital clélstic deposi­ sequences of shallow marine and evaporite félcies tion (see Figure 2.9). In the Lél Popa élnd Parréls were deposited owing to the presence of reefs that basins slow subsidence induced the accumulation of bordered the tectonic elements. The Aurora, Acatita, greélt thicknesses of shales and sandstones. The for­ and upper Tamaulipas formations belong to this mations of Del Rio, Suda, Indidura, Eagle Ford, intervéll (see Figure 2.8). Caracol, Austin, Parras, Upson, San Miguel, Olmos, During the Late Cretaceous, over aH the region in Escondido, and Difunta are units belonging to the general, terrigenolls sediments coming from western Upper Cretélceous. Sediments that constitute the last Mexico were deposited. These underwent orogenic formation have been considered bv Tardv et al. deformation at the beginning of this epoch and later (1974) as flysch deposits that the él general uplift With the gradual retreat of th€ seas deformations. 44 Seclion 1 The Geology of lhe Mexican Republic r_t-.....l.-_,--........ ....... 2 v v Platforrn Anhydnle I Dolomlte HERMOSILLO • Reef Basln o 100 200 km Figure 2.8. Paleogeography of the Albian-Cenomanian for northern and northeastern Mexico, after Raúl González-G arcia,1976. Deformations of the Laramide Orogeny occllrred mainly in the early Cenozoic. The anticJinal and syn­ clinal structures so characteristic of the Coahuila. land­ scape belong to these epochs. The style of deformabon of the Sélbinas Culf Mea is less intense than thélt observed in the front of the Parras basin where recuUlbent folds and overthrusts have signifi­ cant development. The folds are somewha.t narrow and onlv are recumbent and overthrust toward the positive' elements at the edges of the Sabinas Culf. Over the old posjtive elernents the structures are even more gentle and can be observed to have the form of large periclines. From the time of these orogenic deform<l.tions the continental cvolution of the region begéll1 with importélnt continental dcposits índuced by Late Cretaceolls normal faulting. During the Cenozoic, isolated pulses of igneous acbvíty occurred in this part of the country, chiefly in the Oligocene, when intrusions of nepheJinc syen­ ite were emplaced (Bloomfield and Cepeda, 1973). Clarl< and co-workers 09RO) consider these igneolls bodíes élS part of an all<alic igneous belt that is extended from New Mexico into Mexico. These authors consider that this alkaline magmabsrn was caused by the subduction that was occllrring in western Mexico and that constitutes the most distant manifestabon of the ancient ocean trench tbat was forming more Ihan 1000 km away. There <1lso exist Oligocene volcanic occurrences similar lo the siliceous rocks east of Chihuahua, in addition to small basaltic emissions in Ihe Pliocene and Quaternary. 45 -... -, \_---'­ \ ,....) . \ (J CD. VICTORIA l_, ..., ¡ ¡.... / I MONCLOVA • 1 ..­ / / /' ( l \ \ I I \ \ ,.J • HERMOSILLO 1 I I ./ \ \ , \ '>" / '­ / \ / f ...... " \ 2. Geology of the Northern and Northea.stern Regions of Mexico PIEDRAS NEGRAS '\ I .... "1 "1..._\ .MONTE,RREY l MATAMORAS ) -" ,,-/ ) (-' \ EARLY LATE LATE CAMPANIAN CAMPANIAN MAASTRICHTIAN o 200 400 Km Figure 2.9. Paleogeography of the Upper Cretaceous for northern and northeastern Mexico, after Raul González, 1976. Economic Resources Thc Meil of thc S,lbinas palcogulf and thc platforms of Coahuila and Tamaulipas have been the object of very petroleum exploratíon because they contain favorable characteristics for the dcvelopmcnt uf this resource, imd, furthermore, abundant commer­ cial production has been obtained in adjacent regions. The stratigraphic levels with major possibiJities are in the Upper jurilssic and Lower Cretaceous, since in thesc rocks both SOllrcc beds and strata with reservoir characteristics appear. Petróleos Mexicanos has drilled numerous exploriltion wells <1nd hilS encoun­ tered important shows of hydrocarbons in the Sabinils paleogulf Mea. The coal-bearing zone of Sabinas is formed by sedi­ mentary deposits located within the deltélic sequence of the Upper Cretilceous. Specificillly, these bcds belong to the Olmos Formabon of Maastrichtian age thilt was deposited in a dominantIy swampy environ­ mento This zone is the principal producer of coal in the nation and its major reserve (see Figure 2.'13). There also exist numerous deposits of t1uorite and barite distributed in belts generally oriented north­ 46 Section 1 The Geology of the Mexican Republic west-southeast. These belts c1early parallel mineral­ ization belts developed in northern Mexico as a conse­ quence of the magmatism associated with subduction in the west. These fissures are hydrothermal yeins that occur general1y within the host rock of Lower Cretaceous limes tone. This area is the principal pro­ ducer of fluorite in Mexico. The State of Coahuila also contains resources of phosphorite of sedimentar)' ori­ gin th(lt forms horizons in strata of the La Caja Formation of the Upper Jurassic; this constitutes one of the most important sources of this material in the nation. SIERRA MADRE ORIENTAL, GULF COASTAL PLAIN, AND MESA CENTRAL General Geo1ogy The Eastern Sierra Madre and the adjacent areas are composed chiefly of Mesozoic sedimentar)' rocks that were deposited and developed over a Paleozoic and Precambrian basement. The Sierra consists of an orogenic mountain belt that follows, in its southern segment, a general northwest-southeast trajectory, and at the latitude of Monterrey, turns to follow an east-west trajectory toward Torreon. The Sierra Madre is composed oE narrow foJds ""ith an orienta­ tion that folJows tbe general strike of the mountain belt. In the direction of the Mesa Central the valleys are wider, the Sierra anticlines less narrow, and toward the west they become gradually covered by the volcanic rocks of the Sierra Madre Occidental. The Precambrian and Paleozoic basement can be observed in isolated outcrops that occur in erosional windows in the folded Mesozoic sequence (Table 2.1). In the area of Ciudad Victoria, numerous authors have described an important sequen ce of upper Paleozoic rocks that rests on metamorphic rocks of early Paleozoic and Precambrian age (Carrillo-Bravo, 1961; Fries and Rincón, 1965). The lower metamorphic sequence is composed of the Novillo gneiss oE Precambrian age, La Pres¿¡ quartzite of the Cambrian, and by the Granjeno schist, Iater than both of the aboye units. According to radiometric dating of Denison et al. (971), the Granjeno schist originated by metamorphic processes that occurred in the Pennsylyanian-Permian interval. These authors sug­ gest thM the schist was positioned tectonically in jux­ taposition with the sedimentary sequen ce of the upper Paleozoic that is contemporaneous with it. De Cserna et al. (1977) consider this unit to be allochtho­ nous, positioned in tectonic contact on the Novillo gneiss in an episode later than the formation of the La Presa quartzite and before the deposition of the upper Paleozoic sequence. Furthermore, these authors have dated the metamorphism of the Granjeno schist and have assigned it to the Ordovician (446 million years). Ramírez-Ramírez (978) has suggested that the tec­ tonic emplacement uf the Granjeno schist occurred in the late Paleozoic, at the time of intense folding uf the upper Paleozoic sedimentary sequence. According to the model of this author, the Granjeno schist comes from an eastern belt belonging to an internal zone of the Ouachita trend that was metamorphosed during the Carboniferous. The sedimentary sequence deposited in the Silurian-Permian interval has been considered by most authors as a tectonic autochthon developed aboye continental basement represented by the Novlllo Gneiss and belonging to the Ouachita belt of southern North America. Other Paleozoic outcrops of the Sierra Madre Oriental are located in the area of Huayacocotla at the latitude of the 21st parallel At this l o ~ a l i t y Paleozoic rocks are seen exposed in the nucleus of a major anti­ clinorium whose flanks are composed of a thick sequence of Mesozoic sediments. Here the Paleozoic constitutes él metamorphic sequence of gneiss, schist, and metaconglomerate probably belonging to the early part of the era and a flysch sequence more than 2000 111 thick that is Permian in agc. In addition, Mississippian shales, sandstones, and conglomerates havc been reported in the area of Calnali, Hidalgo (Carrillo-Bravo, 1965). The Triassic is represented in the Sierra Madre Oriental and in neighboring areas by redbeds belong­ ing to the Huizachal Formation. These continental sed­ iments attest to a long period of emersion in this part of the country. This originated later than the orogenic deformation at the end of the Paleozoic (Table 2.1). The Mesa Central contains numerous ou tcrops of metamorphie sequences that could belong to the Triassic or the end of the Paleozoic. In the area of Zacatecas City, above a metamorphic sequence, there rest marine partially metamorphosed sedimentary rocks that contain fossils of Carnian (Late Triassic) age (Burckhardt, 1930). These, together with those at Peñon Blanco and Charcas, S.L.P., constitute the only known outcrops of marine Triassic in this part of Mcxico. Other outcrops of schistose rocks of probable late Paleozoic or Early Triassic age are located in the areas of Caopas, Zacatecas, and Guanajuato. During the beginning of the Jurassic, continental deposition continued in this part of Mexico with redbed sedimentaban, except in the region of the Huayacocotla anticlinorium, where an advance of the seas that induced marine sedimentation of an argilla­ ceous and sandy sequence is recorded. Carrillo-Bravo (1971) termed this area the "Liassic Basin of Huayacocotla." These sedimentary strata, named by the smne author the Huayacocotla Formation, were deformed at the end of the Early Jurassic leading to the predominance oE continental deposition in the whole region during the Middle Jurassic. In the Lilte Jurassic, there is recorded generally in all of nortbern and northeastern Mexico a marine transgression that Tardy (1980) related to the western opening of the Tethyan Sea during the disintegration oE the supercontinent Pangea (Table 2.1). Pilger (978) indicated that the opening oE tbe Gulf of Mexico was earlier than the opening of the Atlantic, from whicb it might be supposed that the marine transgression of the first half oE the Mesozoic ought to have come from the Pacific rather than from the east. The affinity of the faunas of eastern Mexico with those of the Pacific STRATIGRAPHIC CORRELATIONS FOR NORTHEASTERN MEXICO CHIHUAHUA BASIN Complled by Ennque Cabral. 1984 PARRAS SECTOR GULF OF SABINAS 'M , " DIFUNTA GP FULlJ'J. fM ANMi u " l PARRAS FM SN FELIPE FM EAGLE FORD OJINAGAGP WA$HITAGP FM CUESTA OE:l CURA FM LOMA PLATA FM KIAMICHI FM BENAVlDES FM TAMAU· FM C AURORA L1PAS FM LAGn¡MII. FM BENIGNO FM LA PENA fM LA PENA FM CUCHILLO e CUPIDO C cuelDO FM LAS HA LA MULA • PAOIlI FM VIGAS FM BARRIL VIEJO e MENCHACA TARAI5ES FM ALCAPARRA LA CASITA GP LACAJA FM ALEJA (LA CASITA) GNEISS VALLES - SAN LUIS P. PLATFORM HUAYACOCOTLA AREA FM NAZAS N Pomon NO DEPOSITION. ANDIOR EROSION central and E Por\lon • C. MEXICO MESOZOIC BASIN FM lA· FM i i ICATECAS I FM MENDEl FM MENDEl FM CARACOL I FM CARACOL FM SAN FELIPE FM SAN FELIPE fM SOYJo.TA,L I FM INDIDURA I fM INDlmJRA. FM A(>UI\ NUEVA fM AGuA NUL:VA A I I FM CUESTA I FM CUESTA I FM CUESTA I FM EL ABRA FM EL ABRA (REEFAL DEL DEL DEL (POST­ AND POST· CURA CURA CURA REEFAL) REEFAL) ? ? PM fM TA""AtJlIPAS reu Tol,lIfl.lAif>AS U. PENA. Sl.Wt=:hK;H FM LA PENA " ? FM orATES FJ.I O1,l¡,U;S FM FM lAf'.IAUUPAS FII, fl\MAUUPAS GUAXCAMA ? INFE>lIOR JNFERIOF! FM TAMAULI­ FM TAMAULI­ --? PAS INFERIOR 14. I FM TARAISES fM lAA.AISES PAS INFERIOR FM PIMIENTA FM PIMIENTA (?l FM TAMAN FM TEPEXIC FM TAMAN (MIXTO) FM CAHUASAS 208 245 PlIOCENE MIOCENE OUGOCENE EOCENE NOOUTCROP PN...ECCENE d: >-, RECEN! I a PlElSTOC'"E () o N o Z w () MAASTRICHTlI>N a: CAMPANIAN w SANTONtMI o­ CQNIAC1AN "'­ :::> TtJRONlAN (f) I CENOMANIAN ::J o w I I ALBIAN Ü 1­ w a: a: w Ü () o APTlAN o --' - N NEOCO- o MIAN (f) W ffi 1<2 TITHONIAN n. "'< Kll'&1EAlOOlAN CI. «w Ü :::> "'z CALLOVIAN iñ BATONIAN (f) MIDDLE « BIoJDC..... a: TOAFtClo\i'..I ::J -, LOWER pue,.lSM0:J4M SINEMUAIAN HEnArK1JA.\j I UPPER º (f) (f) I MIDDLE « oc PEA· MIAN vllNII>.N () IJIIlSI o sg¡:P!J<.N N o DEVO­ UJ NIAN ....J « SILU­ Q. RIAN -­ üRDO VICIAN eAM PRECAM­ BRIAN ERAISYS-I SERIES IEUROPEAN TEM STAGE Im.a. t'"' CJ ro O O­ O ..... ;c. ro Z O ro .., ::J ª" III ::J O­ Z O >-i ;c. ro III 'J; ., ro ::: :;::1 ro ()Q o' ::J 'J; O ...... a:: ro ;:;. () O >Po Table 2.1. Stratigraphíc correlations for northeast Mexico. -..¡ 48 Section 1 The Geology of lhe Mexican Republic (LonSllJlrl, pcrsonéll eommunication) is a félet that sup­ j'orts Ihis -;uppasition. In a time eClrlier than the [1 Trido>sic continental d"'posilion, a part of whélt is now lVIexico l,,-'Io to the \'\ 'stern sector of the above-mtc'n­ lioned cuntinl'nl ílngea. With the of the Lite Jurélssic seélS over 1.1\0<; uf northern and northeac,lern Mexico, the major pclleogeographic e!elllents begrlll to be defined-ele­ ments .lchve during the whole Mesozoic and that cal1­ lmlkd :, dimentation at this time and later tectoníc d<c'tonll<'ltion. Among the principal elements that were ,1,live during the lVIesozoic in the pr '5l'nt area of the ,'ierra M,ldre Orientéll and éldjacent regions are the \1psozoic basin of Mexico ar Mexican geosyncline, the Luis Potosí platform, the Peninsula ar IsJílnd of'oilhuilél, the Peninsula or Archipclago of Tcl'llziUliFil::', <1nd tlll' anci'nt Guli of Mexica. The Mesozoic basin, deveJoped in the zone of the Ml::'-;a -':'I1tra! and the Sierra Madre Oriental, hélS been by nUJl)erous cluthors as il geosyncline, in st'nst: of el linear belt of subsidence where a con­ siderable lhickncss nf sediment élccumulated élnd thélt laler WdS d "slro:,ed by orogeny. Burkbard t (L 930) consid"r d tbilt tor the Lélte Jurassic in this res ion él entrance of the seil that bordered the positive le nds took place, except in the southcélst toward the Slók' of Verélcrllz. rmlay (] 938) mentions the ex istence in this region of a M"xiGln separdted from thilt devel­ oped in w 'stern Mexico, that WélS termed the Pacific geosyncli[1(:. This iluthor indiciltes, furthermore, thélt in the Jurassic and Cretaceous periods communicél­ tion developed with portals bctwcen both geosyn­ clines; lhe . clre evidenced by the migriltÍon of cl1arélcl('ristic Mediterralleéln biotil tOvv<lrd the Pacific provine' ot th(! north and vice vcrSél. More recently, Télrdy (l ':180) cOllsidered that tbe eJst-central portion of Mexico evolved élS él geosyn­ cline in whidl there were individuéllized t\Vo bilsins (Ancestrill Culf of Mexico élnd the Mcsozoic bilsin of Mexico), Thesp possessed él NNVV-SSE orientéltion, "'vH' si 's 0.- Gdc, rl::'OUS pelé1gic sedimentéltioll, illld were S 'pélfated by il cresta I Mea (the Véll\es-Séln T llis over which neritic sedimentation dcvelop-'d. Schmidt-Effing (1980) belie\'cd in tbe presencc of ,1n auJilcogell in the area of Hu,lyacocotlil dllri ,!.., the )-o.Mly Jurélssic; lhe possibility was suggest­ ed tb<lt this piHt of the country might hélve evolved as an (luJ?lcogen syslcm dllring the first half of the Ml'sozoic, i.e., élS él series nf marine-lIlvaded tectonic derressions ,1$ -ociilt ,d with the initiéll expélnsion of tl1l::' Atlclntic. '1Ill? constant élctivity of these troughs cilLlsed draslic changes in the bathymetry that pro­ vok -d, in cert,llfl ,1rlCC\S, dl::'position of pelagÍl': stmtél cstabIJshed 011 continenLll crust without re<lching éln ultil1late proc 's., of oceanizatiol1. in a large part of the Mes%le bilsin of Mexico (f\,[ '. el Cl'ntréll élnd the wcstern belt of the Sierra Madre hiental), the Oxlordiéln marine trilnsgression is m, rked bv an ini­ kll deposit of the Minas Viejas, indicating a shélllow water basin under climatic conditions of strong evaporation. These gypsum deposits playa very important role in the orogenic deformation ¿¡t the end of the Mesozoic, according to the modeJs ot \'ari­ ous authors, In the Ancestral Culf of Mexico, Hw Oxfordian transgression illso is initiatel by evaporil . deposits. OnJy at tbis tirne did shallow w<Jter ckposib on'ur in both bélsins, since during lIw resl of the Mesozoic sedimentary conditions were in the Culf bélsin in contrast to the inIand bas.in where nerit­ ic deposits den'loped 011 llll' Valll's-San Lui. Télmaulipas, élnc! COilhuilil platforms. The {Trper Jurélssic deposits in the Mesozoic bélsin of consist in stratigrélphic order of the Mineb Viejas Gypsum, tbe Zuloagél Lirnestol1cs, and lhe silt­ stones, limestones, élnd shales 01 the L'I Caja Formation, lhe clastic strata equivalent lu ne,1f cOilstal facies nf tbe last two formations are respec­ tively th,: La Cloriél ilnd [ a Cilsita forllliltions. Thc marine transgression initialcd in tbe Oxfordian die! not completely cover the Véltles-San Luis Potosí plilt­ forlll, ancl some ,1r(',lS rell1.lined emcrgent during all of the Lille Jura 'sic (Carrillo-Brélvo, 1971). In élddition, the platform or archipelago of T<llllaulip<1s WilS pélrtly emcrgent and the CO.lhuilél pliltform totillly so. During the first pi1rt of the Early Cret;lceous (Nl'ocomiéln--Aptiiln), open sea deposition occurred in tht' Mesozoic bilsin nf Mexico (Taraises anJ lower Télmaulipas fornléltions) while over the Vall?S-Siln Luis POlllSÍ platforlll ,1 sequence that was chiefly evaporitic (Cnuxcamá Formation) occurred. With the ,.... Ibi.ln there came a general marine transgression that covered the last positive elements and that cxp,lnded over the western portions of Mexico (Rangin and Córdoba, 1976). On the perimeter of the Vall's-San Lui Potosi pliltform there developed at this tinll' a 1b 'I:t flélnked by bilck-reef ilnd forereef deposlts (5ee Figure 2.10). This whole facies assemblage was terllled the El Abra Formél tion (Carrillo-Bravo, 1971). Similélr re (,'1.1 developments have been interpreted to occur around the Coahuila and Burro ptllforms contemporilneous with the Aurora élnd Cuesta del Cura formiltions, and éllso in the arCél of Tuxpan where él reef in the form (lf an atoll forllls the reservoir for hydrocélfbons in the so­ céllled ColeJen Lane (Figure 2.1 n At the beginning (Jf the Late Cretilceous, the re 'i­ men of scdimentation in eilstern Mexico changed drélstically with él influx of detritcll sedimcnts cominh from the west \\fhere an uplift il sociélted with vol­ cilni, and plutonic élctivity was lodking place. During this epoch the seas retreélted gradually toward the east, with associated prograding deltas. [n the areélS of the MesoLoic basin, d posits of the Turonian lndidurél Formation (!irncstoncs and shaJes) were laid down, followed bv the Carilcol Fürmiltion in the ('h.lIes and sé'lIldstones), lhe Pilrras ShaJe from the Santonian, and the Dilunta in the CampClnian to Mélastrichtié111 (shales anu nd­ stones). Over the Va Iles-Sa n lui Potosí pla tform during most of the Late Cretaceous, Cl céllcareous corn­ plex of pliltform type developed, This is composed of pure limes tones and argilldcéou'i límestones of the TéllllélSOpél Formation belon['.ing to the intervéll from 49 2. Geology of the Northem and Northeastem Regions of Mexico Arrec,le La Asurlclon • Cd Vlctona Ansia· MESOZOIC 8ASIN OF CENTRAL MEXICO San LUIS Polosl Approxlmale Ilmls ol plalform dunrlg the ('\ Arrecife Early Crelaceous wlth \J Tollman reel growth on Ihe margins Platform Ilmll dunng Ihe mlddle Crelaceous w'lh reel development Arreclle on lhe marglns El Doctor Approxlmate platlorm Ilmlls dunng the Lale Crelaceous wlth developmenl o( reefs dunng Turorllan and Senonlan • Cd Mante ANCESTRAL GULF OF MEXICO 0====50 km Figure 2.10. Limits of the Valles-San Luis Potosí platform during the Cretaceous. upper Turonian to upper Senonian. This underlies the shales, sandstones, and limestones of the Cárdenas Formation of Campanian-Maastrichtian age. These formations are the platform equivalents of the Agua Nueva, San Felipe, and Mendez formations of the Ancestral Gulf of Mexico. The first manifestations of the orogenic deforma­ tion at the beginning of the Cenozoic are flysch deposits associated with prograding deltas of the Late Cretaceous and with the foredeeps formed in the areas of Parras (Campanian-Maastrichtian) and the Chicontepec (Paleocene) where thick terrigenous setluences were deposited in deep water. These defor­ mahons began the construction of the Sierra Madre Oriental and initiated the continental history of a large part of this sector of the country. In the period of maximum orogenic deformation in the Mesa Central area, deposits of molasse-type con­ glomerates belonging to the Ahuichila Formation and the Red Conglomera tes of Guanajuato beg<:1l1 to formo Generally these are polymictic conglomera tes derived fram erosion of the folded Mesozoic fO,rmations. Great thicknesses of continental alluvium accumulat­ ed in the synclinal depressions and tectonic troughs that imprinted a characteristic geomorphology on the landscape of the Mesa Central. The extreme western part of this zone of Mesozoic folds appears covered by mesetas of ignimbrites dissected and split into trib­ utaries from the Sierra Madre Occidental. These sedi­ ments originated chief1y in the Oligocene. In the Ancestral Gulf of Mexico, two principal Tertiary sedimentary basins are separated from each otber by the Laramide folds of the Sierras de Tamaulipas and San Carlos. The Burgos basin, Iocated to the north, contains marine sequences that are chieflv detrital and more than 1500 m thick, ,vith the of numerous growth faults contempora­ neous with sedimentation, recognizable in the wells 50 Seclion 1 The Gealagy ai the Mexican Republic REEFS U.SA. I I I i o 100 200 km GULF OF MEXICO CARIBBEAN SEA 2: <{ llJ Ü O () ~ z :5 ~ Figure 2.11. Distribution of reefs of Lower and middle Cretaceous around the margins oi the Gulf oi Mexico, after Carrillo-Bravo, 1971. drilled by Petróleos Mexicanos (López-Ramos, 1979) In the Tampico-Misantla basin, thick marine sandy argillaceous sediments developed. This basin is seen to be limited to the following geographic borders, principally by orogenic structures from the early Cenozoic: to the north by the Sierra Tamualipéls, to the east by the Sierra Madre Oriental and the Chicontepec foredeep, and to the south by the Tezuitlan massif. In the two basins, the Tertiary deposits occur within the setting of general eastward regression that left successive belts of outcrops poréll­ lel to the present coastline. The pJutonic and volcanic activity in the Sierra Madre Oriental and Gulf of Mexico coastal plain was very minor during the Cenozoic and is represented only by isolated plutons emplaced in the Mesozoic strata, and SOme flows in the areas of the Sierra Méldre Oriental and the Neovolcanic axis. These are mineralogically similar to the alkaline province of eastern Mexico. The most important plutonic emplacements Me located in the Sierra de San Carlos, in Tamaulipas, where nepheline syenites, gabbros, and monzonites disposed in laccoliths, dikes, and mantos are found (see Figure 2.12). These rocks con­ stitute a southward continuation of the alkaline province that begins toward the north in tlle Big Bend orea in Texas (Clmk et éll., 1980). Radiometric studies of intrusive rocks in the Tamaulipas area, published by Bloomfield and Cepeda (973), reveal dates that vary between 28 and 30 million years. The flows of alkaline basalt located to the north of Tampico repre­ sent a later event attributélble to él distension (Cantagrel and Robin, 1979). Tectonic Summary The characteristics of the Precambrian and Paleozoic basement, aboye which the great Mesozoic sequence of eastern Mexico evolved, are not dear because in general there are very scarce outcrops. The belts that form this basement should ha ve been strongly dislocated by lateral and vertical movements during the first half of the Mesozoic. These tectonic movements prepared the paleogeographic distribu­ tion of basins and platforms that would control the sedimentation and the Laramide deform,ltions during the end of the Mesozoic. In whatever structural form and orientation they might have, the Paleozoic out­ crops of the Sierra Madre Oriental have been consid­ ered to be a prolongation of the Ouachita belt of the southeastern United States, since many writers have pointed out similarities with the rocks of this belt (De Cserna, 1956; Flawn, 1961; Denison et aL, 1971; Ramírez- Ramírez, 1978). This belt was formed as a consequence of the closing of the proto-A tlantic Ocean. During the Triassic this part of the country evolved in continentéll aspect with the development of disten­ sive tectonics that caused the formatlon of troughs filled with thick continental sediments. In the Jurassic, 51 2. Geology of lhe Northern and Northeastern Regions of Mexico 1 Ollgocene Ignimbntes 2 Lower Cenozoic granitic intrusives 3 Alkaline Intruslve rocks associated with lower Cenozoic granileS 4 8asaltic-alkallne rocks 01 the upper Tertiary and Quaternary Figure 2.12. Distribution oi outcropping igneous rocks in northeastem Mexico. two important domains were estab1ished in Mexico as a result of the opening of the Atlantic and the Gulf of Mexico, as weIJ as by the northwestward movement of North America. The first of these, located in west­ em Mexico, was represented by a convergent margin and a zone of magmatic arc of Andean type, resulting from the subsidence of the Farallon plate under the North American continent. The second domain of geosynclinal type or of an aulacogenic system origi­ nated during the marine transgression over the east­ ern part of tbe country at the time of the opening of the Gulf of Mexico. This transgression caused a great thickness of calcareous deposits within a setting of intermittent subsidence and the presence of emergent cratonic elements and marine "highs." At the beginning of the Late Cretaceous, a marked change occurred in the sedimentary regimen in this region, as a consequence of the uplift and deformation of the western area where there was continuouslv active subduction of the Farallon plate under the c o ~ ­ tinental portion of Mexico. The detrital sediments that began to cover the G¡]careous sequence of the east were distributed widely and came to reélch greélt thick­ nesses in the foredeeps of Parras (Upper Cretaceous) and Chicontepec (Pélleocene). Their formation presages the orogenic activity thélt would affect aH the region. In this way the western and eastern domains of Mexico, which had acted in a relatively indepen­ dent mélnner, eélch wilh its own characteristics, are seen to be closelv interrelated with the final Mesozoic deformations. ' According to a model proposed by Coney (1983), the orogenic deformations élt the end of the Cretaceous and beginning of the Tertiélry coincide with a change in movement of the tectonic plates, since the North American and Farallon plates that converged obliquely in western Mexico began to face each other frontéllly and to move with greater veloci­ ty. De Cserna (1956) considers that the folds of the Mesozoic sequence increased in intensity from the Mesa Centréll toward the Sierra Madre Orien tal because of the presence, in the time of deformabon, of the massive cratons of the Coahuila platform and the Tamaulipas Peninsula. The forces coming from the southwest provoked the deformation of the sequence from the base of the Oxfordian evaporites on up. The 52 SectioIl T The Geology oi the Mexican Republic eVClporiles served as a surrace of sliding or decolle­ ment in the style of the Jura Mountains of Europe (De Cserna, 1979). Tardy el al. (1975) supposed the exis­ tence of a nappe with NNE direction, i.e., Cln over­ thrust of hundreds of kilometers that relocated the pelagic sequence of the internal bélsin (Mesél Centréll Clnd High Chain of the Sierra Madre Oriental) over the Coahuila and Valles-San Luis Potosí platforms with reef(ll and subrecfal sequcnccs that form similar pale­ ogeographical uplifts. The model of this author pre­ sumes the unbuckling of the internal basinaJ sequence at the level of the Oxfordian gypsum and establishes the possibility that the basement might have taken part in this tectonic phenomenon. According to the rnodel of Padilla y Sánchez (1982), the distribution of the tolds and ovcrthrusts of northeast Mcxico can be explained by movement of North America toward the northwest with respect to Mexico, more than by the action of compressive forces coaxial with a southwest­ northeast orientaban. Economic Resources The Gulf Coastal Plain and adjacent areas consti­ tute a very important region for petroleum produc­ tion, which has been obtained from both Mesozoic and Tertiary sequences. The Faja de Oro (Golden Lane) has been tradition­ ally a productive zone that years ago constituted the principal source of hydrocarbons in the nation. The productive unit is a reef that developed in the Early Cretaceous and that extends in a semicircular form out to the continental sheif at the latitude of Tuxpan. Tbe Tam<1br<1 belt in the Poza Rica Mea that consists of iln ilncient forereef zone has also been éln importélnt source of hydrocarbons. fn the Burgos basin zone to the north of Tamaulipas and east of Nuevo Leon, <1n import<1nt petroleum-producing TertÍilrY sequence is encountered. Furthennore, the Paleocene sequence of the Chicontepec area consists in actuality of an assem­ bl<1ge of very important reserves. Other ZOnes with petroleum potentials, principally in Mesozoic rocks, are the Valles-San Luis Potosí platform, where impor­ tant reefal development is present in the Lower Cretaceous and the Mesa CentraJ has <1 significant mMine sedimentary sequence of ]urassic élnd Cretaceous age. In sllmmary, the oil and gas productive districts of the region north of the GulfCO<1stal Plélin are Pánuco­ Ebano, Faja de Oro, Poza Rica, and Veracruz (Dfaz, 1980). Mineral deposits include notable hydrothcrmaJ developments in the Tertiary of the Mesa CentraJ élrea and on the \-vestern flank of the Sierra Madre Oriental. The most important recognized resources of leéld, sil­ ver, and zinc cHe localized in the eneas of Fresnillo, Zacatecas, Sierra de Catorce, Charcas, and Zimapán, in addition to the mine,al district of GUilnajU<1to, where the principal association is silver and gold. Likewíse, of outstanding importance are the hydrothermal deposits of fluorite of the area of Las Cuevas and Río Verde. These constitllte a sOllthern continuation of the northwest-southeast-oriented belt that developed in Coahuíla during the Tertiary, the time in which the m<1gmatic arc reached its most east­ ern position. Finally, it is appropri<1te to indic<1te that the barite deposits that were developed on the eastern slopes of the Sierra Madre Oriental are also substan­ tial and bear reJationshi p to this episode of vO!c<1nic and subvo!canic activity (see Figure 2.13). BIBLIOGRAPHY AND REFERENCES Abbreviation UNAM is Universidad Nacional Autónoma de México Alfonso-Zwanziger, ].A., 1978, Geología regional del sistema sedimentario Cupido: Boletín Asociación Mexic<lna Geólogos Petroleros, v. 30, n. 1 and 2. Bloomfield, K., and D. L. Cepeda, 1973, Oligocene alkaline igneous activity in N.E. Mexico: GeoJogical Magazine, v. 110, p. 561-559 Burckh<lfdt, C, 1930, Etude Synthétique sur de Mesozoique Mexicain: Memoire Société PaJeontologique Suisse, v. 49-50,280 p. Cantélgrel, ].M., and C. Robin, 1979, K-Ar dating on eastern Mexican volcanic rocks-relations between the andesitic and the alkaline provinccs: Journal of Vulcanology GeothermResearch., v. 5, p. 99-114. Cilrrillo-Bra VOl L 1961, Geología del Anticlinorio Huizachal-Peregrina al noroeste de Ciudad Victoria, Tamaulipas. Boletín Asociación Mexicana Geólogos PetroJeros, v. 13, p. 1-98. Carrillo-Bravo, L 1965, Estudio de una parte del Anticlinorio de Huayacocotla: Boletín Asociación Mexicana Geólogos PetroJeros, v. 17, p. 73-96. C<1rrillo-BraVO, L 1971, L<I plat<lforma Valles-San Luis Potosí: BoJetin Asociilción MexiCélnél Geólogos Petroleros, V. 23, nos. 1-6, p. 1-112. Charleston, S., 1973, Stratigraphy, tectonics, and hydro­ carbon potential of the Lower Cretélceous, Coahuila Series, Coahuila, Mexico. Doctoral Dissertation, University of Michígan, Ann Arbor, 268 p. Cl,1rk, K, P. Délmon, S Shutter, élnd M. Sh<1fiquíllah, 1980, Magmatismo en el norte de México en relación con los yacimientos metalíferos: Revista, Geomimet, no. 106, p. 49-71­ Conev, P., 1983, Plélte tectonics <1nd the Laramide O r ~ g e n y : New Mexico Geological Society, Special Publica tion no. 6, p. 5-10. Córdobél, D., 1970, Mesozoic stratigraphy oí nortb­ eastern Chihuahua, Mexico: The Geologic Framework of the Chihuahua Tectonic Belt. Symposium West Texas Geological Society, in honor of Prof RK DeFord, p. 91-96. De Cserna, Z" 1956, Tectónica de la Sierra Méldre Oriental de México, entre Torreón y Monterrey, XX Congreso lnternacionat 87 p. De Cserna, Z" 1979, Cuadro tectónico de la sedi­ mentación y magmatismo en algunas regiones de México durante el Mesozoico: Programas y resúmenes del V Simposio sobre la Evolución Tectónica de México: Instituto de Geología, UNAM, p.11-14. 53 2. Geo(ogy of the Northern and Northeastern Regions of Mexico PIEDRAS NEGRAS e IHUAHU' • Sla i=ulaJ' \ \ \ \ CO VICTOR1A '-­ ""---, , I _" ,..- t \ J Fe Pb. Cu D ~ 8a .. Pnnclpally Pb-Zn-Ag @ F ª O Sall Au-Ag O Na. Mg ~ p Carbon • I§ O Hg (ID Pnnclpally Mn Figure 2.13. Distribution of the principal known mineral resources found in northeastem Mexico (taken from the Metallogenic Map of the Republic of Mexico, G.P. Salas, 1975). De Cserna, Z., TL. Graf, and F. Ortcga-Gutiérrez, northern Chihuahua, New Mexico Geological 1977, Alóctono dcl Pa leozóÍCo Inferior en la región Society Guidebook, 20th Field Trip, p. 61-65 de Ciudad Victoria, Estado de Tarnaulipas: Revista DenisOl1, R.E., et aL, 1971, Basement rock framework del [nstituto de Geología, UNAM, v. 1, p. 33-43. of parts of Texas, southern New Mexico, and north­ DeFord, R.K, 1969, Sorne keys to the geology of ern Mexico: GeoIogic Framework of the Chihuahua 54 Section 1 The Geology of the Mexican Republic Tectonic Belt, Midland: West Texas Geological Society, p. 3-14. Diaz, J., 1980, ¿En qué consiste una reserva petrolera? El Petróleo en México y en el Mundo, CONACYT, 2nd edition, p. 221-223. Flawn, P.T., 1961, Rocas metamórficas en el armazón tectónico de la parte septentrional de México. Boletín Asociación Mexicana Geólogos Petroleros, v. 13, p. 105-116. Franco, M., 1978, Estra tigra fía del Albiano­ Cenomaniano en la región de Naica, Chihuahua: Revista, Instituto de Geología, UNAM, v. 2, no. 2, p.132-149. Fries, c., and O.c. Rincón, 1965, Nuevas aportaciones geocronológicas y tectónicas empleadas en el Laboratorio de Geocronometria: Boletín Instituto de Geología, UNAM, no. 73, p. 57-133. González-Garcia, R., 1976, Bosquejo geologico de la Zona Noreste: Boletín Asociación Mexicana Geólogos Petroleros, v. 28, no. 1 and 2, p. 2-49. Gries, c.J., and W.T. Haenggi, 1970, Structural evolu­ tion of the eastern Chihuahua tectonic belt: Geologic Framework of the Chihuahua Tectonic Belt: Symposium of West Texas Geological Society, in honor of R.K. DeFord, p. 119-137 Imlay, R.W., 1938, Studies of the Mexican Geosyn­ cJine: Geological Society of America BulLetin, v. 50, p. '1-77 López-Ramos, E., 1979, Geología de México, 2nd edi­ tion, México, D.F.: Scholastic edition, 3 volumes. Mauger, R L., F. McDowell, and J.c. Blount, 1983, Grenville Precambrian rocks of the Los Filtros near Aldama, Chihuahua, Mexico. Geology and Mineral Resources of north central Chihuahua: El Paso Geological Society, p. 165-168. McDowell, F.W., and S.E. Clabélugh, 1979, 19nimbrites of the Sierra Madre Occidental and their relation to the tectonic history of western Mexico, in C.E. Chapin and WE. Elston, eds., Ash Flow Tuffs: Geological Society of America Speciéll Pé\per 180. Navarro, A., and R. J. Tové1T, 1975, Stratigraphy and tectonics of the State of Chihuahua: Exploréltion from tlle Mountains to the Basin: El Paso GeoJog­ iCéll Society, p. 23-27. Padilla y Sánchez, R., 1982, Geologic evolution of the Sierra Madre Oriental between Linares, Concepción del Oro, Saltillo, and Monterrey, Mexico: Ph.D. Dissertation, University of Texas at Austin, 217 p. Pilger, RH., Jr., 1978, A dosed Gulf of Mexico, pre­ Atlélntic Ocean plate reconstruction élnd the early fift history of the Gulf and the north Atlantic: Transactions Gulf Coast Association Geological Societies, v. 8, p. 385-593. Quintero, O., élnd J. Guerrero, 1985, Una localidad de bélsamento Precámbrico de Chihuahua, en el área de Carrizalillo: Revis ta, Insti tu to de Geología, UNAM., v. 6, p. 98-99 Ramírez, J.c., and F.O. Acevedo, 1957, Notas sobre la geología de Chihuahua: Boletín Asociación Mexicana Geólogos Petroleros, v. 9, p. 583-766. Ramírez-Ramírez, c., 1978, Reinterpretación tectónica de] Esquisto Granjeno de Ciudad Victoria, Tamaulipéls: Revista, Instituto de Geología, UNAM, v. 2, p. 31-36. Rangin, c., and D.A. Córdoba, 1976, Extensión de la cuenca Cretácica Chihuahuense en Sonora septen­ trional y sus deformaciones: Memória del Tercer Congreso Latinoamericana de Geología, México, 14 p. Rogers, L.c., et aL 1961, Reconocimiento geológico y depósitos de fosfatos del norte de Zacatecas y áreas adyacentes en Coahuila, Nuevo León, y San Luis Potosí: Boletín de Consejo de Recursos Naturales no Renovables, no. 56. Salas, G.P., 1975, Mapél Metalogenético de la Repúblicél Mexicana: Consejo de Recursos Minerales. Schmidt-Effing, R., 1980, The Huayacocotla aulocogen in Mexico (Lower Jurélssic) and the origin of the Gulf of Mexico, in RH. Pilger, ed., Proceedings of a Symposium, The Origin of the Gulf of Mexico in the Eélrly Opening of the centré\] North Atlantic Ocean. Louisiana State University, Baton Rouge, Louisiana, p. 79-86. Smith, c.I., 1970, Lower Cretaceous stratigraphy, northern COélhuila, Mexico: Bureau of Economic Geology, University of Texas, Report of Investigation no. 65. 1al p. Tardy, M, 1980. La transversal de Guatemala y la Sierra Madre de México: J. Aubouin, Tratado de Geologia, v. III Tectónica y Tectonofísica, y Morfología-David Serret, translator, Barcelona, Españu Editorial Omega, p. 117-182. Té1Tdy, M., J. Siga!, élnd G Gacon, 1974, Bosquejo sobre la estratigrafía y pélleogeograffa de los fiysch Cretácicos del sector tranversal de Pc1rras, Sierra Madre Oriental, México: Instituto de Geología, UNAM, Serie Divul, no. 2. Tardy, M., et él1., 1975, Observaciones generales sobre la estructuras de la Sierra Madre Oriental. La alóc­ tonia del conjunto Cadena Alta-Altiplano Centréll, entre Torreon, Coahuila y San Luis Potosí, S.L.P., México: Revista, Instituto de Geologia, UNAM, v. 75, no. 1, p. 1-11. • 3. Geology of the Central Region of Mexico • GUADALAJARA ..:.:: ../ ,- .- (.,.,,? <•••• ­ ( ./ .. I<> > VERACRUZ ,.--. ....,..... ...._.,/··-<,.../....!PUEBLA\.,...,.,',. .-' CENTRAL \., :' ,.,> " REGlaN . ,.... v GENERAL CONSIDERATIONS In describing the geology of the central mea of Mexico, the foUowing limits have been used: to the north, the northern edge of the Neovo1canic axis; to the west and south, the coastlines of the Pacific; and to the east, the shore of the Gulf of Mexico and the Isthmus of TehUimtepec. The physiographic provinces of the Neovo1canic axis, the Sierra Madre del Sur, and the northern part of the Southern Gulf Coastal Plain are included in this region (see Figure 11). In terms of the division of geo­ logical provinces used by López-Ramos (1979), the provinces of the Veracruz basin (with the subprovince of Sierra de Juárez), the province of San Andres Tuxtla, the Tlaxjaco basin, the Sierra Madre del Sur, the Altiplano of Oaxaca, the Guerrera-Morelos basin, and the Neovolcanic axis are induded. The climate of the region is highly variable owing to the complex physiography On the slopes of the Gulf of Mexico the dimate changes fram humid tem­ perate in high parts of the Sierra Madre Oriental, to semi-hot and humid in the lowlands. On the Pacific slopes the climates vary fram hot and subhumid on the southeast flank of the Sierra Madre del Sur and the banks of the Río Balsas to semi-arid, hot, and very hot in the Valley of Oaxaca and the major part of the Balsas bélsin. In the regions of basins within the Neovolcanic axis, the climate is in general subhumid and varies from temperate to semi-frigid and cold. In the central Mexico region, sequences outcrop that élttest to diverse domains of various stratigraphic leveis, which in some regions are observed to be superimposed, This makes general descriptions rather fruítless. For this reason, this chapter treats eélch of the six domains of this region separately. This format facilita tes description and synthesis, since within each one of the domains stratigraphic and tectonic condi­ tions are more or less homogeneous wíth well­ defined limits. These domains coincide in large part with the geological provinces proposed by López­ Ramos (1979) for this region. NEOVOLCANIC AXIS The Transmexican Neovolcanic axis is composed of an upper Cenozoic belt that transversely crosses the Republic of Mexico at the 20th paraUel (see Figure 3.1). It is formed by a large variety of volcanic rocks that were emitted along a significant number of vol­ canos, some of which constitute the highest peaks of the country. The volcanic actívíty in this belt has given rise to a large number of internal basins, with the consequent occurrence of lakes thi1t give the geo­ morphic landscape a very characteristic appearance. The principal volcanos located in this province are stratovo1canos of híghly variable dimensíons, such as El Pico de Orizaba, El Popocatépetl, Ellztaccíhuatl, El Nevado de Toluca, and El Nevado de Colima (see Figure 3.2). AlI of them were built by alternating pyroclastic emissions and lava flows. In addition, there exist vents of cinder cone type that are generally small, such as Paricutín, and rhyolitic volcanos such as are encountered southwest of Guadaléljara. In addi­ tíon to these types of centralized emissions, there is evidence of numerous fissure emissions and adventi­ tious developments on the sides of the great stratovol­ canos. There are also some calderas caused by both collapse and explosion; examples of the largest are La Primélvera in the State of Jéllisco and Los Húmeros in the State of Puebla. According to Mooser (1972), the Neovolcanic axis has a zigzag pattern caused by the presence of a fun­ 55 56 Section 1 The Geology of the Mexican Republic Figure 3.1. Distribution of rocks forming the Transmexican Neovolcanic axis. datnentaJ system of orthogonal fragmentation with a northwest and northeast direction of the fractures. The latter orientabon seems to be related to transcur­ rent movements principally in the eastern and central portions of the belt. This impiHts the zigzag aspect to the axis. The great stratovolcanos¡ as at Tancítaro¡ Nevado de Toluca, Popocatépetl, and Nevado de Colimél, would be situated on the southern apices of this system, while the great mining centers of the region, such as Guanajuato and Pachuca, would remain situated on the northern apices. Oemant (1978) considers that the Neovolcanic axis, more than just fonning il continuous belt of volcanic rocks, constitutes a group of five principal focal points of activity with distil}ct orientabon and charac­ teristics. Within these five principill centers it is possi­ ble to recognize two types of volcanic structures: (l) those represented by great stratovolcanos in north­ south alignment, and (2) those represented by numer­ ous small volcanos aligned in a northeast-southwest trend, developed over tensional fractures. The first volcanic manifestations in the area of the Valley of Mexico¡ in the upper Oligocene, are observed to be principally associated with fractures of west-northwest and east-southeast orientation ilnd with influence of frélctures with northeast-southwest orientéltion. In contrast, the last volcanic episodes of the Pleistocene and Quaternary in this portion of the axis seem to be related to a system of fractures with east-west orientabon as in the Siern1 de Chicl1inautzin (Mooser et al., 1974). In the central portion of the axis, seven phases of vulcanism have been recognized (Table 3]) that have occu rred since the Oligocene. The most important of these is the fifth, which occurred at the end of the Miocene and that gave rise to the mountain ranges of Las Cruces, Río Frío, and Nevada. During the sixth phase the cones and domes of Iztaccihuatl and the active cone of Popocatépetl were developed. The last phase, egual to the former¡ was developed in the Quaternary and is responsible for the volcanic acti vity tha t cut off t he drainage of the basin of Mexico toward the basin of the Río Balsas and caused the enclosed interior drainage of the for­ mer feature (Mooser et al., 1974). In its western part, the Neovolcanic axis is bor­ dered by the tectonic troughs of Tepic-Chélpala and Colima. The first has a northwest-southeast orienta­ tion and is associated with the volcanos of San JUéln, Sanganguey¡ Ceboruco, and Tequila. The second pos­ sesses a north-south orientation and is associated with 57 3. Geology oí the Central Region of Mexico Figure 3.2. Distribution oí the principal vents in the Mexican Neovolcanic axis. Table 3.1. Sequences oí volcanic groups and tectonic events oí the I 11 I 3<S{f)4 i I I 20' -.¡--­ I . I 18-, ­ lOS' 1 SAN JUAN 2 SANGANGUEY 3 TEPETILTIC HILL 4 CE80RUCO 5 TEOUILA 6 PRIMAVERA CALDERA 7 NEVADO DE COLIMA 8 COLIMA I I I TECTüNISM AGES MODERN FILL <{ o <{ > VULCANITES L.U CHICHINAUTZIN OF NORTHERN Z GROUP <{ PART OF a: 8ASIN a: L.U üi SIERRAS MAYORES GROUP SIERRAS MENORES GROUP MIDDLE TERTIARY GROUP w Z o f­ <:1 Z ir :J f­ (.) <t: a: l.L I I I o f- o zw ¡¡oz t;w <{ ([ LL >­ ([ « z ([ w f­ « o -­ - >­ ([([ w« f­ ---­ wi';: ..J« 0­ of­ -([ :>ow f­ -­ - I I I >­ ([([ W<{ BALSAS GROUP basin of Mexico. the Nevado de Colima and the Volcán de Fuego (Colima Volcano). The tatter vent constitutes, in the judgment of Demant (1978), the IDost dangerous vol· cano of the Neovolcanic axis, since it is a vent of the ¡ 17 i l.J 19 OC)16 10 .. 15 '\720 $,32 1 f€l12 1 q24 q11 ¡B' G) @13 I 014 \3)23 .l3? 27 . • 25 1 • - 30 I l 9 APAXTEPEC 10 PALAMBAN HILL 11 PARICUTIN 12 TANCITARO HILL 13 BUENA VISTA HILL 14 JORULLO 15 CAPAXTIRO 16 GRANDE HILL I I I I .991 O 50 km '----' 17 SEAS OF SANTIAGO VALLEY 18 CULlACAN HILL 19 GAVIA HILL 20 LOS AZUFRES· SIERRA DE SAN ADRES 21 AMEALCO CALDERA 22 HUICHAPAN CALDERA 23 NEVADO DE TOLUCA 24 XITLE Mt. Pelee type with a plug of dacite lava that might cause the development oE Nues Ardentes. Toward the east the axis is bordered by the vol­ canic rocks of the San Andrés-Tuxtla region, although I I 0 22 21 ¡ -----,------­ I 25 SIERRA CHICHINAUTZIN 26 27 POPOCATEP8l... 28 LA MALlNCHE 29 DERRUMBADAS HILL 30 PICO DE ORIZABA 31 COFRE DE PEROTE 32 TEZI ,lAN (LOS HUMEROS) CALDERA 58 Section 1 The Geology oi the Mexican Republic Demant and H.obin (1975) consider the rocks of this region to belong to the Eastern Alkaline Province since they fix the eastern limit of the province at the latitude of Pico de Orizaba and at Cofre de Perote. The petrographic composition of the rocks forming the Transmexican Neovolcanic axis is highly variable. Flows and pyroclastic products of andesitic composi­ tion are abundant, although numerous dacite and rhyolite units also existo Some units traditionally rec­ ognized as basalts, such as the Chichinautzin Group, have been considered recently to be andesitic in view of chemical analyses of rock samples (Mooser et aL, 1974). In addition, local isolated occurrences of recent rhyolitic volcanics exist, such as those localized in the dómes of the Primavera Caldera in Jalisco, in the area of Azufres in Michoacán, as well as in Los Húmeros and Puebla (Demant, 1978). From a chemical point of view, the Transmexican Neovolcanic axis is consid­ ered by numerous authors as a calcalkaline province characterized by its abundance of andesites and dacites and by the ratio maintained by content of sili­ ca and sodium and potassium oxides. Most authors agree that the activity of the Neovol­ canic axis began in the Oligocene and that it has con­ tinued up to the Recent (Mooser et a!., 1974; Negendank, 1972; Bloomfield, 1975). Within this activity two principal cycles have been recognized: (1) Oligocene-Miocene and (2) Pliocene-Quaternary. Demant (1978) considers that the vulcanism of the axis is solely Pliocene-Quaternary, since the lower cycle of the Oligocene-Miocene constitutes the south­ ern prolongation of the volcanic system of the Sierra Madre Occidental. This author indica tes that the andesites of the Oligocene are folded, as in the Sierra de Mil Cumbres in the Lake Chapala region and in the Tzitzio-Huetamo anticlinorium. In contrast, he notes that in the eastern segment of the axis, outcrops of these andesites are very rareo Ihis author does not clearly establish the relationship of these intermediate rocks with the Oligocene ignimbrites of the Sierra Madre Occidental, where the real andesitic activity had ceased by the end of the Eocene, about 40 million years ago (McDowell and Clabaugh, 1979). The origin of the Neovolcanic axis has been reIated chiefly to the subduction of the Cocos plate beneath the continental crust of Mexico, which at the level of the asthenosphere induced partial fusion and origi­ nated the magmas of the axis (Mooser, 1975; Urrutia­ Fucugauchi and Del C<1stillo, 1977; Demant, 1978) (Figure 3.3). The calcalkaline character of this province supports the aboye hypothesis, although the obligue position of the axis with respect to the Acapulco trench does not result in a feature typical of this type of phenome­ non. Urrutia-Fucugauchi and Del Castillo (1977) explain this lack of parallehsm by means of a model which demonstrates that the direction of movement of the Cocos and American plates is not perpendicu­ lar to the Acapulco trench and that the northwest and southeast extremes of the Cocos plate become more dense, less warm, and older as well as of greater thickness and rigidity. Al! this is responsible for a gradual decrease in the angle of subduction toward the southeast end of the trench and causes a horizon­ tal angle of 20° between the Acapulco trench and the Neovolcanic axis. According to Demant (1978), the subduction of the Cocos plate along the Acapulco trench commenced to develop progressively in the Oligocene, in the for111 of <1 zone of left-Iateral dis­ placement between the American plate and the Caribbean plate, which still is active along the system of the Polichic-Montagua-Cayman trench. The lateral movement within this system reflects the rotation of North Americc1 t0',vard the west with respect to the Caribbean plate, which includes the continental por­ tion of Central America. Negendank (1972) supposes, based on the chemical characteristics of the rocks of the Neovolcanic axis, that this calcalkaline province originated as c1 result of the partial fusion of materials from the lower crust, more than from partial fusion of the Cocos plate at the level of the asthenosphere. Some c1uthors have indicated that the Neovolcanic axis coincides with a zone of lateral slippage that was active during the past. According to a model of Gastil and Jensky (1973), importan t right-Ia teral d isplace­ ments occurred in the axis in the Late Cretaceous and in the early Tertiary, in concordance with the move­ ments observed in the \"lestern United Sta tes. Nevertheless, Urrutia-Fucugauchi (personal communi­ cation) considers that the movement has been left-Iater­ al, and he calls attention to the available paleomagnetic data. This author believes that the zone of lateral dis­ placement indicated aboye couId have operated as a structural control for the exit of the magmas produced by the subduction of the Cocos plate under the American plateo Mooser (1975) considers that the Neovolcanic axis could have coincided with the scar (geosuture) that marks the union between two ancient cratonal masses and whose zigzag arrangement would reflect the fact thc1t the Cocos plate, after foundering in the Acapulco trench, would have been divided into slightly overlapping zigzagging fragments. THE MORELOS-GUERRERO PLATFORM The area of the Morelos-Guerrero platform, in which important marine Mesozoic deposits are devel­ oped, is located for the most part within the State of Marelos and in small portions of the northeastern State of Guerrero and southeastern State of Mexico. Ihe marine sedimentary seguence exposed in this region covers a chronostratigraphic range from the Upper Jurassic to the Upper Cretaceous. This sequence rests on Precambrian metamorphic base­ ment represented apparently by the Iaxco Schist (Fries, 1960; De Cserna et al., 1975), which in a similar area underlies a lightly metamorphosed andesite unit that Fries (1960) termed the Old Taxco Greenstone Campa (978) indica tes much similarity between the Taxco Schist rocks described by Fries and the voIcano­ sedimentary rocks of the Lower Cretaceous that crop out to the west of Teloloapan. This would indicate that the age of the Iaxco Schist is not Precambrian, 59 3. Geology oi the Central Region of Mexico ... PACIFIC PLATE NAZCA PLATE / AMERICAN PLATE \ \ SOUTH AMERICAN PLATE ~ A San Andres-Gulf of California Fracture System B Rivera Fracture C Rivera Triple Junctlon D Clarion Fracture E Orozco Fracture F Siqueiros Frac1ure G Clipperton Fracture H Galapagos High I Panama Fracture J Tehuantepee High Figure 3.3. Tectonics oí the Caribbean and central Pacifico and in that case this unit would not form part of the of the metamorphic basement aboye which evolved the Mesozoic sedirnentary sequence oE the Morelos­ Guerrero platform. Toward the borders of Guerrero and Oaxaca, the sedimentary marine sequence oE the MoreJos-Guerrero platform rests over a metamorphic basement oE Paleozoic strata represented by the Acatlán Complexo The marine sedimentary units of this region are covered discordantly by Cenozoic continental K Cocos High L Carnegle HIgh M Nazca High N Meso-American Trench O Peru-Chrle Trench P Polochle-Motagua Fault O Cayman o Bartlett Fault R Pequenas Antillas Subductlon Zone S Puerto Rico Treneh T Oea - El Pilar Faull deposits and voIcanic rocks oE the Neovolcanic axis, as well as by sorne Oligocene remnants of rhyolitic volcanism. The base of the Mesozoic marine package is repre­ sented by the Acahuitzotla Formabon of Late Jurassic age (Fries, 1956), which js formed by calcareous and argillaceoils sedjrnents that crop out in isolated locali· tieso This formabon underlies with erosjonaJ discor­ dance calcareous shales of the Neocomian Acuitlapan Formation. 80th formations show the effect of weak 60 Section 1 The Geology of the Mexican Republic dynarnic metamorphism. The Xochicalco Formation of Aptian élge, éllso in rare outcrops, is formed by a sequence of thin limes tone beds that rest upon the Acuitlapan Forméltion. After deposition of the Xochicalco Formation an uplift occurred in the region that gave rise to the paleopeninsul,) of Taxco (Fries, 1956) and a period of erosion markecl by tbe presence of éln unconformity that places the Xochicalco Forméltion in contact wilh véHious p<lIts of the Morelos Forrnation. ThlS latter formation cont>ists of a calcareolls unit that accounts for the most extensive outcrops of the region. lts name hélS been applied to scquences of limestone that exlend toward Michoacán, Jalisco, and Colima, aJthough its characteristics are not always the Sélme. H is formed of thick beds of limes tone and dolomite théll in one sequence reach up to 900 m thick élnd thélt have in the base an anhvdrite member some rneters tbick. The litboJogic cb;racteristics clnd lhe fauna reve,)1 that this unit formed from shallow water marine deposition during the Albiéln-Cenomanian intervalo At the end of the Cenomanian an emersion occurred in the area with the emplacernenl of various lncks )1 gr<lnik and with differential erosion of tbe top 01 the Morelos Formation (Fries, 1956). During the Turon ia n a n i nvasion of tbe sea was repeélted ancl calcareous sedimentation was reestab­ hshed with the development of a calcareous bank toward the west of a line trending from Cuernavaca to Huit7uco. The end. of the Turoniélll marked a drastic change in the sedimentiltion of lhe Morelos-Guerrero pJat­ form rcsulting from uplift of a majar pOIt of the "01­ cano-sedimentélry areas located in the western regíon of this pilrt of MeAico. The deposits of shélle, siltstone, "and stone, a nd conglomerate Cilme to form a sequence more th,m 1200 m thick developed in the Tu roniéln-Campilnian interval. At the end of the Cretc'lceous í.md beginning of the Tertic1fY, compres­ sionill deformiltion occurred tha t resulted in the for­ mation uf élnticlinal élnd synclinal folds. In tllt' Olig0cene-Eocene interval, intense norrnal félultmg occui-red, accompéllücd by continental c1astic sdimentation over the low parts of the newly created topography. This clastic continental sedimentabon was initiated in the rniddle of the Cretilceous in the ilreas located west of this region. Deposition of conglomeréltjc materials \Nas contemporélneous with some basaltic 1,wél flows that gave rise to the lithostratigraphic ilssem­ blage termed the Balsas Group (Fries, 1960). These deposits ''''ere followcd by importanL siliceous voJcanic emissions that formed the ignimbrite cover of the Taxco area termed the Tilzapotla Rhyolite and by volcanics and voleilnoclastic deposits of the Tepoztl,ln Formation. According to Campa (1978), this region suffered consid­ erable warping during the Miocene thilt is evidenced by the dipping beds of the Balsas Group and by the ilbnor­ mal elevél tion of the Oligocene ignirnbrites. The upper Tertiary and Quaternary are character­ Ized in this region bv the influence of volcanic activitv of the Neovolcanic J axis and by the development tectonic trenches thélt caused the deposition of conti­ nental c1astic sediments of the Cuernavaca Formation. METAMORPHIC REGION Of ACATLÁN The region that includes the higher part of the Béllsas basin, drained by the Mixtcco and Acateco rivers, is characterized by extensive ou tcrops of meta­ morpbic rocks of various types that form a complcx of early Paleozoic ilge (Ortega-Gu tiérrez, 1978) (see Figure 3.4). This metamorphic unit was termed origincllly the Acatlán Schist by Sélléls (1949). Lélter, Fries and Rincón (965) defined ii as the Acatlán Formation. Recentlv, Ortega-Gutiérrez (1978) elevated this unit to the ra¡{k of a complex, pointing to its varied lithology and structure. This author divided the Acatlán Complex into two subgroups termed Petlancingo ancl Acateco. In the lithostratigraphic division that Ortega­ Gutiérrez introduced at tbe formé1tional level, he employed some names that had already been utilized by Rodríguez (1970) in an informal sllbdivision that included in the ACClteco Group the forrné1tions Esperélnza, Acatlán, Salado, ilnd Tecomilte. The formation that constitutes the structurillh lower part of the Acatlán Complex is the Migmatite, a classic migmatite derived from sedimen­ tary rocks. The Chazumba Formation is fonned prin­ cipally by biotite schists with intervals of quartzite, differentiated metélgilbbro, and pelitic schist. Thl' Cosoltepec Formation, which togcther with tbe two aboye units makes up the structurally lower Pellancingo Subgroup, is composed of pSilmmitic and pelitic schists with the presence of greenstones, tale schists, ca1careous schists, metilmorphosed chert, ilnd manganiferous rocks (Ortega-Gutiérrez, 1978). The ACé1teco Subgroup is composed of the Xayacatlan, Tecomate, and Esperanza grMtitoid forma­ tions as wcll as the Totoltepec stock and the San Miguel dikes. The first formation is composed of greenschists, amphibollte, metagabbro, eclogite, ser­ pentinite, mylonite, pelitic schists, élnd qUélrtzite in an élsscmblage that, according to Ortegél-Gutiérrcz (1978), possibly makes up an ophiolite complex and is of great importance, since this is the first time in Mexico where tbe presence of eclogite rocks has been report­ ed. The Tecomilte Formation is composed of metaren­ ite, pelites, and semipelites partially of tuffaceous origin, as weH as metamorphosed Iirnestone and meta­ conglomerate. The Esperé1nZél grélnitoids ilre formed by granitic, aplitic, and pegmatitic rocks that are cata­ c1élStiC and metamorphosed and in certain élreélS have been considered by Rodríguez (1970) as part of the Oaxacan Complex. The Totoltepec stock is éln intrusive of trondhjemitic composition with slight foliabon élnd could have resulted from the differentiation of a tholeiitic gabbro <Ortega-Gutiérrez, 1978). Fries el al. (1970) ind icated éln age of 440 ± 50 Ma for this intru­ sive, which would be in the Ordovician. The name "Siln Miguel dikes" has been applied to a series of tab­ ular intrusive bodies of grélnitic and tonalitic composi­ tion thélt affect some units of the Acatlrín Complexo 61 3. Geology of the Central Region of Mexico 99' 2 0 ' - - - ' ~ - - - ---- - --1- ­ I I I I I I - - ~ - - _ . j 1 16°- - r- - - -- o_-=-.;:;; I Mesozoic Teloloapan-Ixtapan complex Paleozoic-Mesozolc Mazaleco complex Paleozoic-Mesozolc Xolapa complex lIIIIll Paleozoic Acallan complex ~ Precambnan Oaxaqueno complex Figure 3.4. Metamorphic complexes in central Mexico. Thc assernblage of the Acatlán Complex is found covered discordantly by numerous igncous and sedi­ mentary units that inc1ude an age range which varies from the late PaJeozoic to Quaternary and constitutes the basement of an cxtcnsive region that includes parts of the sta tes of Puebla, Oaxaca, Guerrero, and Morelos. Ihis group correlates with the Chacús Group of Guatemala and with the metamorphic rocks of the Sierra de Omoa in Honduras. Tt seems to have no similar relationship with the Xolapa Complex of the Sierra Mad re del Sur, nor with the Oaxacan Complex (see Figure 3.4). According to Ortega-Gutiérrez (1978), the character of the Acatlán Complex leads to the supposition t!tat it is an ancient marine eugeosynclinal deposit with a style of tectonie deformation nnd metamorphism resembling that of the internéll 01' deep zoncs of an alpine orogenic belt. SIERRA MADRE DEL SUR AND ADJACENT AREAS Ihe Sierra Méldre del Sur, from Colimn to Onxacél élnd contiguous areas of northwestern Guerrero, Michoacán, and the State of Mexico, makes up a region of high strueturnl eomplexity that contains var­ ious juxtaposed teetonic domains (Figure 3.5). Ihe most northern segment of the Sierra Madre del Sur ís formed by outerops of Mesozoic seguences, both platform sediments and vo1caníc rack sediments of ísland are type. Areas found in northwestern Guerrero, west of the State of Mexico and sonth of Michoacán, form a region vvith partially meta mor­ phosed volcnno-sedimentary rocks of ]urassic élnd Cretélceous élge. Ihese are covered by Cenozoic conti­ nental volcanic and sedimentary rocks. This region borders on the east the area of the Cretaceous Marelos-Guerrero plntform at the latitude of the linea­ ment of Ixtapan de la Sal-Taxco-lgunJa. The southern segment of the Sierra Madre del Sur is formed by extensive Olltcrops of metamorphie rocks that have n geochronologic range vnrying from Pélleozoie to Mesozoic and that nre seen to be affeeted by batholith­ ie emplncements of late Mesozoic and eve'n Cenozoic age. Tlle Pacific area of the Sierra Méldre del Sur includes the States of Colima, Miehoncán, and north­ em Guerrero and contains outerops of andesitic vol­ eanic rocks interstratified with 5iH)' redbeds, vo1canic 62 Sectíon 1 The Geology oí the Mexican Republic HONDURAS ( ) TZI- TZIO TELOLOAPAN GUERRERO - MORELOS Sierra Madre del Sur HUETAMO IXTAPAN DE LA SAL PLATFORM Continental Crust Figure 3.5. Schematic tectonic model oí the Sierra Madre del Sur. conglomerate, and subreef (slope) limestone beds that contain an Albian fauna. These outcrops form part of what Vidal et al. (1980) have called the Petrotectonic Assemblage of Zihuatanejo¡ Guerrero¡ Coa1comán¡ Michoacán. There exist furthennore, in this northern portian of the Sierra¡ extensive outcrops of sedimenta­ 0' sequences of platform limestones with Albian fauna and rhythmjc sequences of terrigenous sandy muds. In areéls situated in the neighborhood of Colima City, the platform limestones contain great thicknesses of intercalated evaporites. These underlíe¡ in apparently transitíonaJ contact¡ continental terrigenous Upper Cretaceous sediments. In a large part of the Sierra Madre del Sur¡ from the northern tributarjes to the area near Zihuatanejo¡ Campa and Ramírez (1979) have reported the existence of numerous mountains formed by andesitic materíals interstratified with some beds of limestone and terrigenous clastics dis­ semínated in small areas in the Sierra. This Mesozoic vulcanism continues toward the north bordering the Pacific coast until it becomes blurred with similar areas of the North American Pacific Cordillera (Campa and Ramírez¡ 1979). Ferrusquía and co-workers (1978) have reported the presence¡ in the area of Playa Azul, Michoacán¡ of él transitional volcanic-sedimentary sequence with dinosaur footprints that indjcate perhaps a Middle Jurassíc to Early Cretaceous age. In addition they indjcate that this is the first record of dinosaur tracks in Mexico and constitutes the southernmost trace of dinosallrs in North America. Most authors have reported the volcanic-sedimen­ tary sequences of this Pacific region of Mexico as being of Mesozoic age. Nevertheless, De Cserna at al. (1978a) obtained a Rb-Sr radiometric age of 311 ± 30 mi Ilion years for intrusive rocks strictly related to vol­ canjc rocks belonging to the metavolcanic complex of Zapotillo, enst of Zihuatanejo. Campa and Ramírez (1979) as weH as Vidal and co­ workers (1980) consider that the Mesozoic vo1cano­ sedimentary sequences of a majar part of the Sierra Madre del Sur nre the result of mélgméltic activity from convergent edges of pla tes developed in this part of Mexjco during the Early Cretaceous. The southern haH of the Sierra Madre del Sur is forrned from metamorphic rocks that constitute the Xolapél Complex (De Cserna, 1965). This is found to be intruded by batholiths of granite (see Figure 3.4). De Csernél (1965) reported the Xolélpél Complex on the highway from Chilpancingo to Acapulco as an assem­ blage of metasedimentary rocks formed of bjotite schists and gneiss with sorne quartzite and cipolin marble horizons¡ and including the presence of peg­ matites. Nevertheless, Guerrero and co-workers (1978) consider that in the majar part of this region the cornplex is forrned trom quartz-feldspa thjc orthogneiss of granodiorite composition. In the most southern section of the Sierra Madre del Sur¡ corre­ sponding to southern Guerrero and western Oaxaca, the Xolapa Complex bas an ampholite facies derived from sedimentary rocks and orthogneiss \·vith abun­ dant migmatites. De Cserna considers this metamorphic cornplex to be of Paleozoic age¡ given that it underlies the volcano­ sedimentélry sequence of the Chapolapa Formation¡ which is probably of Triassic age. As weH, the complex is never seen in a locality where it underljes Paleozoic sedimentary rocks. Nonetheless, the stratigraphic range of tbis complex has not been precisely deter­ mined because the geochronologic studies have given very disparate radiometric ages indicatíng that thermal events occurred in the Paleozoic (Halpern et a1.¡ 1974)¡ in the Mesozoic (Guerrero et al., '1978), and in the Tertiary (De Cserna, 1965). Guerrero et al (1978) rely on the existence of a thermal event in the Tertiary (about 32 miHion years ago) in the area of the highway of Chilpancingo¡ and in their radiometric determina­ tions¡ which failed to indicate Paleozoic or Precam­ brian ages as suggested by other authors. These authors recognized the oldest thermal event élS Jurassic by means of uranium-lead (165 ± 3 million years) and rubidium-strontium (180 ± 84 million years), 63 In the Tierra Caliente region and adjacent areas of the western part of the State of Mexico and southeast Michoacán, extensive outcrops of partly meta mor­ phosed volcano-sedimentary sequences exist that are juxtaposed against other extensive outcrops of marine Cretaceous platform sequences from the areas of MoreJos ami Huetamo-Coyuca, along the borders of Guerrero and Michoacán. In the Teloloapan-Arcelia sector¡ a sequence of élndesitic volcanic rocks as weU as calcareous-argilla­ ceous foliated sedimentary rocks and graywackes constitute deposits of an isJand volcanic arc and mar­ ginal seas developed in the La te Jurassic and Early Cretaceous (Campa and Ramírez, 1979) (Figure 3.5). These volca no-sedimen tary sequences crop ou t in continuous pattern toward the north, up to the area of Tejupilco. From here the olltcrops become isolated and less extensive. They may also be observed in the areas oE Ixtapan de la Sal, Zitácuaro, and Tlalpujahua. In the Huetamo-Covuca section, a Jurassic­ Cretaceous sequence is exposed thaJ gradllaUy becomes more sedimentary toward its topo The base contains detrital sedimentary rocks interstratified with lavas and andesitic tuffs of the Jurassic that constitute the Angao Formation (Pantoja, 1959). Above this formation rest interbedded shales and sandstones with some tuffaceous horizons and with siltstones and reefal limestones deposited in the Lower Cretaceous (Neocomian-Aptian-Iower Albiéln). These deposits make up the San Lucas Formation (Pantoja, 1959). FinalJy, the top of the sequence is formed by beds of argillaceolls limestone attributed to the Morelos Formabon of Albian age (Pantoja, 1959). The Huetamo-CoyuCél sector forms a transitional zone between the external Mesozoic domain repre­ sented by the Guerrero-Morelos platform and the Mesozoic island arc represented by the volcano-sedi­ mentary outcrops of the Sierra Madre del Sur. The volcano-sedimentélrY seqllences of Teloloapan and lxtapan, situated to the east of Huetamo, would then be considered as tectonic allochthons transportcd ovcr the platform of the external domain (Campa and Rélmírez, 1979) and to ha ve come from the western island arc domain De Cserna (l978b) believes that the absence of platform limestone in the Morelos Formation to the west of Teloloapan is due to a facies change into a basin in this area during the Albian and Cenomanian. This author considers that the volcanic rocks of the Teloloapan-ArccIia area, which form the volcano-sedimentary seqllence of the marginal sea and island arc proposed by Campa and Ramírez, belong to a stage of Cenomanian-Turonian vulcanism (Xochipala Formabon) or could weU be a basement of ancient vo1canic rocks, aU this in a model withollt major tectonic complications. OAXACA AND AD]ACENT ZONES In the central region of Oaxaca and adjacent areas of southern Puebla and eastern Guerrero, ém impor­ tant sedimentary Mesozoic sequence crops out that 3. Geology oE the Central Region oE Mexico attests to the development of a basin beginning in the Early Jurassic (Figures 3.6-3.8). This region of Mesozoic outcrops is limited by vari­ ous metamorphic complexes that are exposed in this part of the counrry. To the northeast, metamorphics of the Acatlán Complex are present, belonging to the lower Paleozoic and resulting from marine eugeosyncli­ nal deposi tion (Ortega-Gu tiérrez, 1978). Above this metamorphic complex rest sedimentary rocks of the Jurassic ,md Cretaceous émd some unmetélmorphosed units of the Paleozoic. To the west and south, the nonsedimentary Mesozoic exposures me bordered by the XoJapa CompJex, composed of gneiss, migmabte, élnd biotite schists with amphibolite metamorphic facies (Ortega-Gutiérrez, 1976). The age of this complex is apparently Mesozoic, but thermal events of Paleozoic, Jurassic, and Tertiary have been reported (Héllpern et al., 1974; Guerrero et al., 1978; De Cserna et al., 1962). To the southeast, the Oaxacan Complex forms the limit of the basin. lt is formed by banded gneiss meta­ morphosed from granulite facies to transitionéll gmn­ ulite-amphibolite, including charnockite, anorthosite, and pegmatite. Fries and co-workers (1962) carried out radiometric studies of the Oaxacan Complex that resulted in age dates of 1100 ± 125,920 ± 3D, and 940 million years (Precambrian). Additionally, these authors indicate thélt the pegmatites and the Ié1st stage of metamorphism that affected the host rocks are equivalent to the Grenville metamorphic province of the eastern United States and Canada. The outcrops of this complex [orm a considerable part of the rooun­ tainous zone that is located to the west of the dty of Oaxaca. Finally, to the northeast, the basin is found to be bordered by the metamorphic outcrops of the western flank of the Sierra de Juárez with a markedly rectilinear contact thélt forms the Oaxacan Ravine; this probably is a regional tectonic feature. These meta­ morphic rocks have been traditionally assigned to Precambrian (orthogneiss) and Paleozoic (phyl1ites and incipient meta-arkose) (López-Ramos, 1979). However, CharIeston (1980) reported the existence of an ample metamorphic complex derived from eugeo­ synclinal deposition of sandstone, clays, and volcélnic flows of Cretaceous age. Radiometric studies of these rocks gave ages [or the metamorphism corresponding to Upper Cretaceous and lower Tertiary (Charleston, 1980). According to this author, this complex is formed by aUochthonous blocks whose provenance is to the west and that have been thrust over miogeo­ synclinal sediments of Jurassic and Cretaceous during the Laramide Orogeny. The Puebla, Oaxaca, Guerrero, and Morelos regions, underlain by the Acatlán and Oaxacan com­ plexes, contain extensive outcrops of Mesozoic sedi­ mentary units élrranged in north-northeasterly foJds. Under this Mesozoic sequence there have been report­ ed, in isolated exposures, sorne Paleozoic sedimentary units resting discordantly aboye the metamorphic basement. Above the Acatlán Complex, Corona (1981) and Flores élnd Buitrón (1982) discovered in the Olinalá area a sequence of detrital and calcareous rocks with Pennsylvanian and Permian fossils. AIso 64 Seehon 1 The Geology of the Mexiean Republie 99" 96' I I I I 20' '----1------­ I GULF I I CUERNAVACA Vt o¿)lj'O H,-- CLI _ II!: . p I ;;Ir::, CH'!f'ANCINGO\ \1. • OAXACA : I i -----,­ I Figure 3.6. Sedimentary rocks oí the Upper Cretaceous oí the Guerrero-Morelos platíorm¡ Tlaxiaco basin¡ and southem section oí the Sierra Madre Oriental. I 20--- 1 ---- GULF OF MEXICO I PUEBLA ·1 • {7 (] l> \? P,LJC/ F /co Ct,LJfv 16'- - - L - -­ I 99 Figure 3.7. Sedirnentary rocks oí the Lower Cretaceous of the Guerrero-Morelos platíorm¡ 96' I _.J_ 20' ---------­ I GULF OF I MEXICO PUi=BLA .UERN lACA • VERACRUZ. I • I I I I I • I _._----+­ 18 o I .. I OAXACA • I (J . " I P", C/Jo¡e OCl("'N I 16' - - +- - - - - - - - ... - - Figure 3.8. Sedimentary rocks of the Jurassic oí the Guerrero-Morelos platíorm¡ Tlaxiaco basin¡ and southern sector of the Sierra Madre Oriental. there have been reported aboye this complex discov­ eries oí upper Paleozoic sedimentary rocks in Mixtepec, Oaxaca (Flores and Buitron, 1984), and in Tuxtepeque, Puebla (De la Vega, 1983). The Matzitzi Formation¡ with Pennsylvanian plant fossils (De Cserna, 1970), is found covering apparently both the Acatlán and Oaxacan complexes; its main outcrops oecur to the southwest of TehuaeéÍn. In the Noehixtlán region aboye the Oaxacan Complex, Pantoja-Alor and Robison reportcd in 1967 the discovery oí a marine sequenee with Cambri<m­ Ordovieian trilobites that was tenned the Tiñú Fonnation. Above this unit a sequenee made up of the Santiago, lxtaltepec, and Yododeñe formations rests diseordantly. These units íorm more than 1000 m of clastics belonging to the Mississippian, Pennsylvanian, and Permian systems (Pantoja-Alor, 1970). The base of the Mesozoie sequence that is exposed in the Tlaxiaeo basin is represented by detrital sediments of the lower part oí the Rosario Formation, whieh is of continental origin and con­ tains coal horizons (Erben, 1956). Aecording to this author¡ the sediments oí the Rosario Formation were deposited in i1 coal basin that developed dur­ ing the Early Jurassie in northwestern Oaxaea, northeastern Guerrero, and southwestern Puebla. On the western and eastern borders of the basin, the lower strata of the formatíon were not deposited. Above the Rosario Forma tion rests the Cualac Conglomera te which, together with the middle and upper strata of the former formation, belongs to the Tlaxiaco basin¡ and southem sector of the Sierra Middle Jurassic. Both formations constitute the Madre Oriental. Consuelo Group, which underlies the Tecocoyunca Group whose formations erop out in varioLls loeali­ 65 bes 11l the Tlélxiacn basin and also beJong to the Middlc Jurassic. This group is composed of bolh derrita] alld car­ bonate both continentéll and marine, and contains plant fossils and ammonites thé'lt indicélte several marine invasions and regressions. Dllring tbe Late Jllrassic, clec1rly mmine sediments werc deposit­ cd in 50111(' ilre<lS of the basin, such as the Cidilris Limestone i11 the Mixtepec-Tia xiaco area (Erben, 1956) and the Chimeco ami Mapache del Sur de Puebla, for­ mation::; formed of Iimestone, clrgillaceous limestone, alld shales (Pérez et Cl1., 1965). The Teposcululé'l f.imestone, considered originally }urClssic by SalCls (949) é'lnd later by Erben (956), h<ls been recently <l'-signed to the Albian-Cenomé'lnian ([errusquía, 1970) on account of its faunal contento On the other hand, the Cidaris Limestone has been con­ firmed as belonging to the Lé'lte Jurassic beGluse of its echinoid fcluna of Oxfordian, C<lJlovian, and KimmeridgiCln (Buitron, 1970) lt should be noted that these Juras ie units that outcrop in the region of the ACCltlán Complex are not reported to be represent­ ed by similélf slTClta above the Oaxacan Complexo Tbe Upper Cretaceous also is represented by marine sediments; in some localities the Neocomié'ln and Aptüm i1re prescnt. In the Tehuacán area i1 ciastic­ calcareous sequence with beds of limestone crops out and constitlltes the Zapotitlán Formatiol1. Above this unit lie BOO m of both fine and coarse clastic-cé'llcare­ ous beds of the Aptian San Juan Raya Formation. The ocomié'ln é'lnd Aptian formations of the central area of Oaxaca and central élnd south of Puebla have been included within the group termed Puebla. However, in various 10caJities this group is absent and Albian limestones re<;t discordantly above the Jllrassic sec¡uence. (1979) mentions that the wells Yacudá no. 1 and Teposcolula no. 1 cut él sequence oí more than 2500 m of Upper Jurassic and Lnwer Cn' dceous evaporites. During the Aibian-Cenomanian interval é'I seqllence of thick-bedded limestones developed in él transgressive se<l. These hé'lve received dif­ L'r 'nt names in different Me<lS. C<llderón (1956) desig­ né'lted i1 widespread sequence of massive micritic and biomicritic limestone with chert nodules that crops out in the ehuacán region aS the Cipiapa Formation. Ferrusquí<l (197(}) designated a mé'lssive biomicrite as th' Teposcolulil Limestone. This crops out in a similar area and Wé'lS cOl1sidered Jurassic by Salas (1949). Finéllly, Pérez é'lnd co-workers (965) applied the nélme Morclos Formation to these limestones in the region of Acatl<l and reJated them to the .'-\Ibi<ln-Ccnomanian str<lta that crop out on the Cuerrero-Morelos platform. Above the Albian-Cenomé'lni¿lt1 limestones líes <l sequence of marly limestone designated by ¡- .rrllsquÍé'I (197ñ) as the YucUnal11él Formation. lt con­ tains fossils of the Coniacié'ln-Mé'lastrichtian stages ilnd crops out northwest of Nochixtlán. It can be cor­ related with the Tilantongo Marls (Salas, 1949) that are expoO'cd southeast oí Nochixtlán and with the Mexcala Formation of the Guerrero-Morelos platform. 3. Geology oí the Central Region oE Mexico The tolded Mesozoic seqllencc of the.: Tlaxié'lco basin is covered with angular djscord,Il1Ci.:' by exten­ sive OlltCropS of continental deposils. ThesC' re sand­ stone-conglomerate ami argillé'lceous sandy be'!s of the Tertiary élnd include siliceolls, intennediale, ilnd mafic volcanic rocks. The Tertiary continental deposits hé'lve been assigned to the Yanhllitlán and forma­ tions (Salas, 1949) tha according to Erben 11956) élre distinct facies of the same unit. The first is formed by c!ays with S0111e intercalations of sc1ndstone and vol­ canic ash, <lfgillaceous sandstones, élnd beds of con­ glorneré'lte and breccia. Ferrusqllía (197ñ) mt:ntions <l radiometric age of ±49.0 million years for a tuff inter­ stratified within the Yanhuitl¿in Forl11,1tion of Sayultepec, dating tbis formation as late Paleocene-middle Eocene. This author indicates that tlw formabon has a stratigraphic position simi¡;u to that of the Tehuacán Formation (C<llderón, 1956) <lnd the Eal as Croup (Fries, 1960) The Oligocene, in various localities in the state of Oaxaci1, was deveJoped in a period of é'lctive vulcan­ ism thé'lt originé'lted initially with the emission of siJiceous and intermedia te tuffs and l<lter and ';,itic lava flows. The volcanic é'lctivity culmin<lted with some bé'lsaltic flows in the Neogene THE SECTOR SOUTH OF THE SIERRA MADRE ORIENTAL ANO' THE COASTAL PLAIN OF THE SOUTHERN GULF On the eastern flank of the sector south of the Sierra Madre Oriental (Sierra de Ju<Írel) ,1 thick sequence of Mesozoic sedimentary rocks is exposed that rests cm a metamorpbic basement composed of schists, gn and phyllites. These have been derived prjncipally froro sedimenté'lry rocks <lnd have been tr¡'lditionally attributed to the Paleozoic and Precambrian. However, in a section locé'lted é'lt the 18th parallel, Charl (1980) recognized a thick sequence of schists and metavolcanic rocks that he attributed to the Lower Cretélceous. The sedimentary sequence of the €c1stern flank of the sector, which forms folds asymmetric toward the east, has, in the ZongoJica-Tehuacán sector, a bas<ll unit of dark-colored slates with some interc,llatj()J1s of fine-grained sand ilnd calcareous shales. This is widc­ ly exposed and has been tentatively attributed to the Middle Jurassic (López-Ramos, 1971,1). In the sectür located soutb of the 18th parallel and down to the region of the Isthmus of Tehuantepec, the base 01 the Mesozoic is formed by the Todos Santos Formabon, which is a sequence of continental redbeds with cross­ stratified sandstones, conglomerate, ilnd Shc1k. Thi_ forma tion h<ls, furthermore, been recognized in Chiapas and northern Central AmCflCél where its lower part is consjdered Lower and Middle Jura. sic (MulJeried, 1957), However, López-Ramos believ s that ir could extend to the Triassic. The Upper Jurassic is exposed in the Zongolica area (Vinicgra, 1965) in the fmm of marine sequences 66 Section 1 The Geology of the Mexican Republic of bituminous limestones with intercalations of sandy-argillaceous limestones and with ammonites. However, in the southern sector of the eastern flank of the Sierra de Juárez, outcrops of this age have not been reported. The marine Cretaceous sequence, which crops out in the northern portian of the Sierra de Juárez, is com­ posed principally of calcareous rocks that have been recognized by Petróleos Mexicanos in both surface and subsurface studies. These rocks include the fol­ lowing formations: Tuxpanguillo (Neocomian), Capolucan (Aptian), Orizaba Limestone (Albian­ Cenomanian), Maltrata Limestone (Turonian­ Coniacian), Guzmantln Unit (Turonian-Senonian) as well as the Necoxtla and Atoyac formations of Senonian-Campanian and Campanian-Maastrichtian age (Viniegra, 1965) Additional1y, the marine Cretnceous is represented in the area of the lsthmus of Tehuantepec by neritic fossiLiferous limes tones that López-Ramos (1979) inc1uded within the series of middle Cretaceous limes tones of Nizanda-Lagunas. In the portion of the Gulf Coastal Plain that borders the Sierra de JuéÍ.rez, Petróleos Mexicanos has driHed exploratory wells that have afforded recognition of Mesozoic units in the subsurface. From these it has been possible to reconstruct a paleoplatform termed the Córdoba Platform, which formed the marine sea floor during the second haH of the Mesozoic (Figures 39-3.10). The western half of the platform is exposed in the Sierra Madre Oriental and the eastern half is buried under the Coastal Plain of the Gulf. In addi­ bon, it is limited on the west by the Zongolica pale­ obasin and on the east by the Veracruz paleobasin (González-Alvarado, J976). More than 5000 m of sed­ iments accumulated in this latter basin. Petroleum production has been obtained from these strata, chiefly in fields located in its eastern portion (González-Alvarado, 1976). During the Tertiary, in a setting of eastward marine regression, terrigenous sediments were deposited in the Gulf Coastal Plain. These are: Chicontepee- Velasco (Paleoeene); Aragon¡ Guayabal, and Chapopote (Eocene); Horcones and La Laja (Oligocene); Depósito, Encanto, Concepción, FilisoLa, and Paraje (Miocene). These deposits began to fonn at the inception of the orogenic deformation of the Sierra Madre Oriental during the beginning of the Cenozoico The igneous activity of the southern sector of the Sierra Madre Oriental, which is manifested in the form of granitic intrusions at the end of the Mesozoic and beginning of the Cenozoic, is restrieted to alkaline basaltie emissions in the area of Tuxtlas in the upper Tertiary and Quaternary. Demant (1978) related this volcanie zone with the alkaline province of the Gulf of Mexico, more than wi th the eas tern extreme of the Neovolcanic axis as some other authors have indicated. TECTONIC SUMMARY The eomplicated structural and stratigraphie set­ ting of the central-southern portion of Mexico makes difficult a paleogeograpbic and tectonic reconstruc­ tion that permits a clear explanation of the origin of fea tures in this part oí the nation. Recently the structure of the region has been inter­ preted in terms of a mosaie of tectonostratigraphic terranes (see Figure 3.11) that were acereted in differ­ ent episodes during the tectonic evolution of this part of Mexico (Campa et al., 1981; Campa and Coney, 1983). Each terrane contains a distinctly different basement and their limits have been generally inter­ preted as tectonic boundaries. GULF OF MEXICO Figure 3.9. Situation of the Córdoba Platform. 67 3. Geology of the Central Regíon of Mexico PLATFORM DEPOSITS (Step 1) ZONGOLlCA Ks BASIN-? Km K. 'IR 2 / . , , i . , / \ ... ... ..... TILTING (Slep 11) FOLOING ANO EROSION (Steplll) NR , \ Figure 3.10. Tectonic evolution of the Córdoba Platform. 1. Guerrero Terrane 2. Mixteco Terrane 3. Oaxaca Terrane 4. Juarez Terrane 5. Maya-Yucatan Terrane 6. Complex 01 minar terranes including Xolapa Terrane Figure 3.11. Tectonostratigraphic terranes from southern Mexico accord­ ing to the divisions of Campa and Coney (1983). The terrane with the otdest basement is the evolution of a rift with sedimentation on ancient con­ Oaxacan, which occupies part of the state of that tinental crust and later metamorphism to the gran­ name and contains unmetamorphosed sequences of ulite facies. This resu1ted from an ensialic evolution or Cambrian-Ordovician and Mississippian-Pennsyl­ from continental collision (Ortega-Gutiérrez, 1981). vanian. The metamorphic basement, formed in the This complex is considered to be a southern continua­ Oaxacan Complex of Precambrian age (900-1100 mit­ tion of the Grenvillian belt (Fries et al., 1962). !ion years), has been interpreted as the result of the However, the trilobite fauna of the Cambrian­ 68 Section r The Geology of the Mexican Republic Ordovician caver shaw more affinity with the fauna al' Europe and South America théln vvith the félunél uf urth AmeriGl (Whittington and Hughes, 1974). Bazán (1984) does not discélrd the idea of the existence of arc-type rocks in this complex, based on the in ter­ pretéllion of greenstone belts in Precambriéln shield arcas. To the west of the OaXaCc'll1 Terrane lies the Mixtecan Terrc""lI1e, \vhich has the lower Paleozoic Acatlán Complex as basement (Campél and Coney, 1983) and which, in contrast to contemporaneous rocks in the Oélxacan Terrane, contains diverse gréldes of metamorphic rocks. The boundary between these two terranes hc'ls been interprcted to be tectonic (Ortega-Cutiérrez, 1981). The time of its accretion has not yet been confirmed, but it has been suggested to be Devonian (Ortega-Gutiérrez, 1981). As well, the time of accretion has been placed in the Late Jurassic-Earl y Cretaceous in terVul (Ramírez, 1984). The first paleomagnetic dates for Permian units in both terranes indicc'lte similar directions of primary magnetismo This does not discount totally a later accretion (of the blocks) along the same magnetic paleolatitude (Urrutia-Fucugauchi and Morán­ Zenteno, 1984). The AcatlAn CompIex has been interpreted as an élggregation of petrotectonic assemblages resulting from the opening and closure of an ocean basin (Ortega-Cutiérrez, 1981). The PetJancingo Subgroup would constitlJte a seguence of an élutochthonous pas­ sive nlc1rgin and the Acateco Subgroup would form the allochthonous assemblage, including the Xayacatlán Forméltion élS the vestige of an élncient oceanic lithosphere consumed in the subduction process. To the soutlnvest, the Mixteco and Oaxacan ter­ ranes are bordcred in tectonic contact by the Xolapa Complex, whose élge ami time of accretion to the tec­ tonic mosélic of southern Mexico are not well known, but whose characteristics identify thcm as roots of a mountélin rélnge from éln ancient magmatic arc (Halpern et c'll., 1974). In the extreme ec""lst of the cen tra l-southern portion of Mexico, deformed Mesozoic marine sequcnccs arc recognized that revei'll a paleogeogrélphie setting of interspersed deep Elnd shallow marine SLlbstrates developed over the Paleozoic bélsement, which has traditionally been considercd relatcd to Appalachian dcformabon. These assemblages form pélrt of the Moya Terrane, which extends into south and south­ east Mexico (Campa and Coney, 1983). Separating the Maya and Oi1xi'lcan terranes, él belt of apparently Mcsozoic stra.t o has been recognized. These are mé1.rine beds that inc1ude cél,lcareous, detritéll, and voI­ canie rocks. They are highly deformed and have a general ei1stwi1rd vergence. The western boundi1rY of this belt fonns a mylonitic band that separates it from the Oélxacan Terrane. In the central-southern portion of Mexieo, two principal íVIesozoic domains with clearly distinct characteristics are recognized. In the west an andesitic island arc Wc""lS developed, associated with the subduc­ tíon of ocea nic lithosphere (Campél élnd Ramírez, 1979). This is a feature that is common to a m<1jor pélrt of western North America and thélt originélted during the initial breakup of Pangea. Additionally, in the east an external zone with marine sedimentation evolvcd over the Guerrero-Morelos platform, the Tlaxiaco basin, and the area of the cast flank of the Sierra de Juárez, the coastal plélin and the platform of the Gulf of Mexico-all developed on continental crust. Mélrine sedimentabon of this externéll zone was initi­ c'lted with the opening of the Culf of Mexico and the marine transgression over this part of Mexico. The partly metamorphoscd volcanic and sedimentélry assemblages of the Sierra de Juárez alter the homo­ geneity of this domain, and their presence is not clear­ Iy understood. Carfantan (983) has suggested that this petrotectonic assemblage is the resuIt of the open­ ing ami closing of an ocean bélsin occu rring between Portlandian and Turonian and causcd by deveIop­ ment of a rift that "vas connected to a triple junetion over él ridge located between Yucc'ltiÍn and South Amcrica. Two alternative models have been postlllated in order to explain the development uf c'I volcanic island arc in the western domain of the central-sollthern por­ tion of Mexieo. One of them proposes the accrction by obduction of an island arc system developed in the Pc""lcific élnd displaced in the direction of its col1jsion with the Mexican continental crust (Urrutiél­ Fucugéluchi, 1980; Coney, 1983). In the other mode!, the development of an arc domain in the vicinity of the continental crust of Mexico is proposed, limited to the southwest by éln eastward subduction (Campa and Ramírez, 1979). Preliminary paleomagnetic délta of the volcano-sedimentary sequence of Ixtapan­ TeloJapan (Urrutia-Fucugauchi and Vc""llencio, 1986) seem to point to the first hypothesis, although no report exists of assemblages of oceanic affinity thilt would indicate a suture. According to Campa ami Ramírez (1979), in the northwest region of Guerrero ami adjoining regions of other states, five phases of deformatían can be rec­ ognized that were c""lctive in Mesozoic and Cenozoic time. Thc first of thcsc occurred at the end of the Jurassie, élffected the Jurassie volC<1no-sedimentary deposits, and manifests itseU by the presence of folds refolded in two generéltions with a relatively increas­ ing metamorphism in some zones. Thc second phase OCCLlrred in the Cenomanian and is manifested in the Teloapéln-Ixtapéln élrea by metamorphism that folded and foliated the volcano-sedimentary sequence. This phase, in the Sierra Madre del Sur, caused the cmcr­ gence of the island arc terranes and ma.rginal seas. During this time marine sedimentatjon continued in the Guerrero-Morelos platform, and to its east, con­ temporaneously with a méljor introducbon of terrige­ nous sediment comíng from the western emergcnt region. The next phase occurred in the Paleocene and deformed the ,'\'hole Mesozoie blanket of the two domains and is responsible fol' the folds in the exter­ néll zone as well as the overthrusting of the internal domain over the externa] zone. 69 Campa (1978) has proposed two alternative models to explain the presence of the volcano-sedimentary assemblage of Ixtapan- Teloloa pan between the Guerrero-Morelos and Huetamo platforms. In one of these ít is suggested that the Ixtapan-Teloloapan assemblage is the result of the evoh-üion of an arc between the two platforms¡ but this does not explain the metamorphism of this é1ssemblage between the unmetamorphosed seqllences of the two platforms and the absence of facies changes from these plat­ forms to the volcanic éHC. In the otber model, the author sllggests that the Guerrero-Morelos and Huetélmo sequences, belonging to the Albian­ Cenomanian, could be part of a single platform and that the lxtapan-Teloloapan assemblage would be a tectonic allochthon of the compressional phase of Paleocene age. At the end of the Miocene there occurred a phase of deformation that rcsulted in a warping that is observed in the Arcelia-Altamirano region and evi­ denced by the abnormaUy elevated metamorphic sequences and the pre-Miocene lithostratigraphic units. The origin of the great strLlctllral élnticline of Tzitzio-Tiqllicheo of southeastern Michoacán is attrib­ uted to th.is phase because uf the consideratioll that the continental sequence on the flanks of the structure is correlative with the 1m,ver Tertiary Balsas Group, Campos (1984) has a ttributed the folding to the Paleocene compressional phase since he cónsiders that the continental sequence on the f1anks of the struch.lfe belongs to the Upper Cretaceous and not to the Tertiarv. Campa 'and co-workers (1980) believe that in the weStern part of the central-southern portion of Mexico on e can recognize tectonostra tigraph ic ter­ ranes that are characterized by homogeneity and continuity of interna] stratigraphy, but that have (In obscure and poorly understood relationship o Au-Ag • Hg A Fe OAu ... Pb Zn-Ag ~ S • Mn 3. Geology of the Central Regíon of Mexíco between themselves. The borders of each terrane separate sequences that have djfferent physical and temporal characteristics. The discontinllities of thcse borders can not be explained clea rl y by conven tiOlléll facies changes or unconformities. These authors have recognized in this region the following funda­ mentaL terranes: the Assemblélge ot Guerrero­ Morelos platform, Assemblage of Teloloapan, Assemblage of Huetamo-Cutzamala, Assemblage of Zihuatenjo, and Assemblage of Taxco and Taxco Viejo, aH of them integrated into the Guerrero com­ posite lerrane. In the Pliocene-Quaternary intervaJ, the central­ southern region of Mexico has been affected by nor­ mal falllting and lateral displacement within a setting of general uplift and very great geodynamic ilctivity. ECONOMIC RESOURCES The principal minerill resources known in the central-southern region of Mexjco are the sulfides of lead, zinc, él nd sil ver in a cen tra I belt, ilS well as iron oxides localized chiefly in the Sierra Madre del Sur (Figure 3.12). To the first category belong the miner­ al deposits of the Pachuca D1ining district, which is located at the northern edge of the Neovolcanic axis and has been one of tile principal silver producers in the world. To the south of the Neovolcanjc axis, mineral districts of hyd rothermal sulfides appear along a belt with north-northwest and south-south­ east orientation in the sta tes of México, Cuerrero, and MichoacéÍn. The band includes the field areas of Taxco, Xitingil, Zacualpan, Temascilltepec, Angangueo, élnd Tlapujahua. Within this belt, the mercury deposits of Huitzuco and Huahuaxtla also are developed. These hydrothermal deposits are attributed by Campa and Ramírez (1979) to the end of the Miocene period, contemporaneous with the Figure 3.12. Dístribution of the principal mineral deposits known in the central part of Mexico. 70 Section 1 The Geology of the Mexican Republic warping that affected pre-Miocene strata. Another group of sulfide mineral deposits exists in this region. Their origin has been attributed to vol­ canogenic processes that do not have a preferred orientation but are encountered associated with vol­ cano-sedimentary Upper Jurassic and Lower Cretaceous rocks. To this group belong the fie!ds of Pinzán Morado, T!apehuilla-Las Fraguas, Campo Morado-La Suriana, Rey de la Plata, Teloloapan, and Cuetzalán del Progreso, as weU as the volcano field in the north of Michoacán. The deposits men­ tioned are considered to be contemporaneous with the volcanic activity that occurred in the island arc zone formed during the Mesozoic in this part of Mexico (Gaytán et aL, 1979; Campa and Ramírez, 1979). In a belt situated along the Sierra Madre del Sur, numeroliS deposits of iron are located; these make up the major reserves of the cOllntry. The origin of these deposits is attribllted to processes of contact metaso­ matism unleashed by the effect of silicic and interme­ diate intrllsions of the lower Cenozoic on the Cretaceous limestones (Gómez, 1961; Mapes, 1959; Pineda et aL, 1969; Zamora et al., 1975). Among the ) \ ,/ , \ \ • 1 , " , \ ,1 I I I ? , 1 ? AGlJi\SCJl.I \ , I NA'Y'ARIT I .­ I • I 1 ? " 1 I I -" I I ,. I ~ 1 ,. / / / , " ....... :y / , , / ¡ I , , \ I ~ I ; :J Ó 1 1_, Q y ~ - , Ú O , 5 more important iron deposits that are known in this region are those of Peña Colorada, in Colima; Pihuamo, in Jalisco; Las Truchas in Michoacán; and El Violín and Tiber in Guerrero. Also, deposits of copper such as those oi Inguarán and La Verde in Michoacán exist in this belt Fllrthermore, the zone of major petrolellm interest is in the coasta! plain of the Gulf where petrolellm has been extracted in fields !ocated along the eastern edge of the Córdoba Platform in sedimentary rocks of the Cretaceous and where there exist good pros­ pects in sediments deeper than the Upper Jllrassic (González, 1976). In considering possibilities of obtaining geotherma! energy, the Mexican Neovolcanic axis constitutes the geologic province with the major manifestations and potentials in the country, owing to its contemporane­ ous igneous activity. The principal thermal manifesta­ tions are related to acid igneous activity. Sorne of these are located in the areas of La Primavera, Jalisco; Ixtlán de los Hervores; Negritos, and Lago de Cuitzeo, Michoacán; Los Húmeros, Puebla; and San Bártolo de los Baños, Queretaro (see Figure 3.13 and Table 3.2). \ I SAN IUIS POTOSI I I -, .. / E '1 $'-­ ~ - - b \ I '" .. , I I 1 I .. J J , , I I \ , I l' / , , O J .. I " JALISCO , r \ ,--­ , ESTA.DO DE MEXICO GUANAJUArO , 1 , ,. , , I , I '1 , ¡ 01'1 , 1, , MIGHOAGAN I , I • I \ , ~ ~ [) F I , , ,-..... ' I \ . , , /" I I 1 1. CERRO PRIETO, S.C.N. 2. DESIERTO DE ALTAR. SON. 3. HERVORES DE LA VEGA, JAL 4 LA PRIMAVERA, JAL 5 SAN MARCOS. JAL. 6 LA SOLEDAD, JAL 7. LOS NEGRITOS, MICH 8. IXTLAN DE LOS HERVORES, MICH. 9. LAGO DE CUITZEO y ARARO, MICH. 10. LOS AZUFRES. MICH. , 1. EL GOGORRON, S.l.P 12. SAN BARTOLO, ORO. 13. PATHE, HGO. 14. EL CHICHONAL, CHIS. 15. TOLlMAN, CHIS. 16. LOS HUMEROS. PUE. Figure 3.13. Location of the most important geothennal fields in the Republic of Mexico. STRATIGRAPHIC CORRELATIONS FOR SOUTHERN MEXICO DIVISION XOLAPA M I X T E e o MAYA ERA GUERRERO OAXACA JUAREZ Guerrero-Morelos Mexc.aJa-Ollnala Cordoba Platlorrn Central Yucalan ;o- RECENT I 1/ Andeslles <3 PLEISTOCENE m Cuernavaca Fm Chllapa ü A San Marcos Fm Sosola - PLlOCENE A Zampoala Fm Oapan A Yucadoac O r Fm AIQUllranlFm Papagayo Fm Sur; Frn Yanhulllan Frt... So.:¡·CorooePOC>f' MIOCENE Fm ef(O ;:'(19 _ Er><:ao1l0 La Lala N a: Chaet,arando Fm. Tepoxtlan A Buenavlsta Fm lIar.a,-de Lobo!'; I I I I O « Frn Agua de Obispo Fm Yannulllan - OLlGOCENE Fm Tilzapotla Fm Tilzapolla Fm H,Hll:'lNl:> Z f- Balsas Gp F". Tam:nu1()Pal,1 Fm Fm. Tecomallan Fm Ch.• W a: fm Fm P,sle EOCENE Balsas Gp. Gpo Balsas Fm Aragon Fm Icaclle Ü W f- PALEOCENE Balsas Gp Fm Velasco Fm Chlcnen liza Fm Telelel(' o Frn Telelcinqo Fm Aloyac r.J) MAASTRICHTIAN Flyseh Sequence Llmestone Frn. Mexcala Fm Mexcala Fm Mexcala Marga Marga Sedlmenlary SENONIAN Fm Mol Yucunama Tllantongo I I Fm Guzmaolla Soquence O .. W TURONIAN Fm. Cuautla(?) Frn Cuautla - I I CENOMANIAN Fm. MOlelosI Fm Teposcolula !fm CIPI<l.OO Fm Orizaba Fm Yucalan ü Fin. Morelos Fm Morelos Fm, Morelos Fm. Teposcolula _ f- ALBIAN Fm So Locas t Tierra OW Fm Xochlcalco Frn HUlizuOO I ,cm ZlCapa Fm Sn Juan Raya Puebla Gp Fm Xonamanca a: APIlAN Fm AcahurzoUa Caliente Frn ACUltlapan Fn'I lapollllan N ü NEOCOMIAN I Complex Fm AcahUlzotla ._ Arco and Cuenca Fm San Pedro O Ir I Fm Mapache Seql,ence (f)<¿ UPPER Fm Angao? Fm C.Cidaris (/) I Tecocoyunca Gp fm po lOCCQ",.lC,J Fm Etlaltongo W(/) MIDDLE « :2g¡ I C&(;.ua1&s 11 0::: -;¡ulllG Ophlollles LOWER ? R V Taxco VieJo fm. Todos Sanlos Fm Todos Santos -, ü UPPER Fm Chapolapa I (/) -- ? U I (/) MIDDLE « :: LOWER Turnblscallo Sequenc:e Undlfferenllaled Cm C_'=""" Cm Frn Yododene PERMIAN Melarnorph.e ü PENNSYLVANIAN Xolapa ? Frn Ixlaltepec Com pie. Complex O MISSISSIPIAN Fm Santiago N O DEVONIAN SJLURIAN -< ORDOVICIAN a... Frn Tlnu CAMBRIAN PRECAMBRIAN Oaxaqueno Complex Recopdado pOr S Alarco y G Mora (1984) Table 3.2. Stratigraphic correlations for southern Mexico. w CJ '" 2.­ O O ...... g. lb n lb .., e. ro 03. O O ..... ro "O j 72 Section 1 The Geology of Ihe Mexican Republic BIBLIOGRAPHY AND REFERENCES Abbreviation UNAM is Universidad Nacional Autónoma de México Bazán, 8.S., 1981, Distribución y metalogénesis de la provincia uranífcra del Mesozóico de México: Geomimet 3a. epoca Julio/ Agosto, n. 112, p. 65-96. Bazá n¡ B.s., 1984, Litoes tra tigra fía y rasgos estruc­ turales del Complejo Oaxaqueño¡ Mixteca Alta, Oaxaca: Geomimet¡ no. 129, p. 35-63. Bloomfield, K., 1975¡ A late QUélternary monogenetic field in Centréll Mexico: Geologische Rundschau, v. 64,n.2, p476-497 Buitrón, BS., 1970, Equinóides del Jurásico Superior y del Cretácico Inferior de Tlaxiélco, Oaxélcél: Libro Guia de la Excursión México-Oélxaca de la Sociedad Geológica Mexicana, p. 154-163. Bullard, E¡ 1969, El origen de los océélnos in J.T Wilson, 1976, Deriva Contincntéll y Tectóniél de Placas. Selections of Scientific American, e. Martin­ E. ond A. González-U., translators, 2nd edition¡ Madrid: H. Blume editions, p. 98-109. Calderón, A., 1956, Bosquejo geológico de la región de Son Juan Raya, Puebla: XX Congreso Geológico Internacionélt México. Libro Guia de la Excursion A-11¡ p. 9-33. Campél, M.F., 1978, La evolución tectónica de Tierra Caliente. Bulletin Geological Society of Mexico, v. 39, n. 2. Campa, M.F., and PJ. Caney, 1983, Tectono-strélti­ graphic terranes and mineral resource distributions in Mexico: Canéldian Journal of Earth Science, v. 20, p.l 040-1 051. Campa, M.F., and J. Rélmírez, 1979, La evolución geológicél y la mctalogénesis del norocciden te de Guerrero. Série técnico-científica de la Universidéld Autónoma de Guerrero, n. 1, 102 p. Campal M.F, et al., 1977¡ La evolución tectóniC<l y la mineralización en la región de Valle de Bravo, México, e Iguala¡ Gro: Asoc. Ing. Min. Met.Geol. Méx. Memoria de la XII Convención Nacionéll, p. 143-170. Campa, M.F., J. Ramírez, R. Flores, and P. Coney¡ 1980, Conjuntos estratotectónicos del occidente de Guerrero y oriente de Michoélcán: Resúmenes de la V Convención Geológica Nacionat México¡ O.F.¡ p.106-107. Campa, MF, et al., 1981, Terrenos tectono-estratigráfi­ cos de la Sierra Madre del Sur, región comprendida entre los estados de Guerrero, Michoacán, México, y. Morelos: Série técnico-científica de la Universidad Autónoma de Guerrero, n. 10,28 p. Campos, E., 1984, Estudio geológico regional del área de Valle de Bravo-Tzitzio, estados de México y Michoacán. Tesis Profesional Facultad de Ingeniería¡ UNAM. CMfantan¡ J.e., 1983, Les ensembles géologiques du Mexique meridiona.l. Evolution géodynamique durante le Mesozóique et le Cénozique: Geofísica Internacional v. 22, n. 1, p. 39-56. Charleston¡ S., 1980, Stratigraphy and tectonics of the Rio Sonto Domingo areél, State of Oaxaca, Mexico: 26th Congrés Géologique International (ParisL Abstracts, v. 1¡ sections 1-5,324 p. Caney, P., 1983, Un modelo tectónico de México y sus relaciones con América del Norte, América del Sur y el Caribe: Revista, Instituto Mexicano del Petróleo, v. 15, n. 1, p. 6-15. Corona, J.R., 1981 (delayed untiI1983), Estratigrafía de la región OlinaJá-Tecocoyunca, noreste del Estéldo de Guerrero. Revista del 1nstituto de Geología, UNAM, v. 5, n.1, p. 17-24 De Cserna, Z., 1965, Reconocimiento geológico en la Sierra Mildre del Sur de México, entre Chilpancingo y Acapu!co¡ Estado de Guerrero: Boletín del Instituto de Geología, UNAM, n. 62, 77 p. De Cserna, Z., 1970, Reflexiones sobre algunas proble­ mélS de la geologia de la pé'\fte centréll-meridional de México. Libro GUiél de la excllfsión México-Oaxaca de la Sociedad Geología Mexicana, p. 37-50. De Cserna, Z, 1979, Cuadro tectónico de la sedi­ mentación y milgmatismo en algunils regiones de México durante el Mesozóico: Programas y Resúmenes del V Simposio sobre lo Evolución Tectónica de México: Instituito de Geología, UNAM, p. 11-14. De Cserna, Z., et al., 1962¡ Edades isotópicas de rocas metamórficas del centro y sur de Guerrero y de unél manzanita cUé'\fcífera del norte de Sinaloa: Boletín del Instituto de Geología, UNAM, n. 64, p. 71--84. De Cserna, Z., et al, 1975, Edad Precámbrica Tardia del Esquisto Taxco, Estado de Guerrero: Boletin Asociación Mexicana de Geólogos Petroleros, v. 26, p.183-193. De Csernil, Z., et a!., 1978a, Rocas metavo!cánicas e intrusivos relacionados Paleozóicos de la región PetatlAn, Estado de Guerrero: Revista, Insti tuto de Geología, UNAM, v. 2, n. 1, p. 1-7. De Cserna, Z., et al., 1978b. Relaciones de facies de las roCélS Cretácicas en el noroeste de Guerrero y en las áreas colindantes de México y Michoacán: Revista, Instituto de Geología, UNAM, v. 2, n. 1, p. 8-18. De lél Vega, E., 1983, Una nueva localidad Pérmicél en México fechadél con fusilinidos, porción meridional del Estado de Puebla: Resúmenes de la VI L Convención Geológica Nacional Sociedad Geológica Mexicana, p. 51. Demant, A., 1978. Características del Eje Neovo!cánico Transmexicano y sus probleméls de interpretación.. Revista, Instituto de Geología, UNAM, v. 2, n. 2, p. 172-187. Demant¡ A., and e. Robin, 1975, Las félses del vo1con­ ismo en México; una síntesis en relación con La. evolución geodinámica desde el Cretácico: Revista, Instituto de Geología, UNAM, v. 75, n. 1, p. 70-83. Oietz, R.s., and Je. Holden, 1970, La disgregación de la Pangea, in JT. Wilson, 1976, Deriva Continental y Tectonica de Placas. Selecciones de Scientific American, C. Martin-E. and A. Gonzalez-U., translators, 2nd editíon, Madrid, H. Blume Ediciones, p. 154-167. Erben, H.K., 1956, El Jurásico Medjo y el CCll10viana de México: XX Congreso Geológico Internacional, México: Contribution to the Congress by the Instituto de Geología de UNAM¡ 140 p. 73 Ferrusquía, J., 1970, Geología del área Tamazulapan­ Teposcolula-Yanhuitlán. Mixteca AHél, Estado de Oaxaca: Libro Guia de la excursión México-Oaxaca de la Sociedad Geológica Mexicana, p, 97-119. Ferrusquía, J., 1976, Estudios geológico-paleontológi­ cos en la Región Mixteca, Part 1: Geología del área Tarnazula pa n- Teposcolula-Ya nhuitlán, Mixteca Alta, Estado de Oélxaca, Mexico: Boletín del Instituto de Geología, UNAM, n. 97, 106 p. Ferrusquíél, J., S.P Appelgate, and L. Espinosa, 1978, Rocas volcélnosedirnentarias Mesozóicas y huellas de dinosaurios en la región suroccidentéll Pacífica de México: Revista, Instituto de Geología, UNAM, v,2,n. 2,p. 150-162. Flores, L.A" and B.E, Bui trón, 1982, Revisión y aportes de la estratigrafia de la montaña de Guerrero: Serie técnico-científica de la Universidad Autónoma de Guerrero, n. 12,28 p. Flores, L.A" and UE. Buitrón, 1984, Una nueva locali­ dad del Paleozoico Superior de 1i1 región de la Mixteca Oaxaqueña: I,esúmenes de la VIl Convención Geológica Nacional Sociedad Geológica Mexicana, p 207. Fries, C, 1956, Bosquejo geológico de la región entre Mexico D,F. y Acapulco, Gro: Excursions A-9 and C-12, XX Congreso CeoJógico Internacional México, p, 7-53. Fries, C, 1960, Ceología del Estado de Morelos y de partes adyacentes de México y Guerrero, región central meridional de México: Boletín del Instituto de Geologia, UNAM, n. 60,236 p, Fries, C, 1966, Hoja Cuernavaca 14 Q-h (8), Estado de Morelos, Carta Ceológica de México: Instituto de Geología, UNAM, Serie 1/100,000 mapa con texto. Fries, C, and O.C Rincón, 1965, Nuevas aportaciones geocronológicas y técnicas empleadas en el Li1boratorio de Gecronometria: Boletín del Instituto de Geología, UNAM, n. 73, p 57-133. Fries, c., et al., 1962, Rocas Precámbricas de ed<td Grenvi\liana, de la parte central de Oaxaca en el sur de México. Boletín del Instituto de Geología, UNAM, n. 64, part 3, p. 45-53. Fries, c., et al. 1970, Una edud rad iométrica Ordovícica de Totoltepec, Estado de Puebla: Libro Guia de la excursión Méxjco-Oaxaca de la Sociedad Ceológica Mexicana, p. 164-166, Gastil, G.R, and W. )ensky, 1973, Evidence for strikeslip displacement beneath the Trans-Mexicéln vo1canic belt: Stanford Universitv, Publications in Geologic Science, v, 13, p. 171-1SÓ. Gaytán, ],E., E. Garza-V de lél Arévalo, and A. Roséls, 1979, Descubrimiento, geología, y génesis del Yacimiento Vulcano, Michoacán: Memória de la xllr Convención Nacional de la Asociélción de Ingeníeros de Minas, Metalogénesis, y Geología de México, Acapulco, Gro., p. 58-113 Gómez, D., 1961, Inventario de los yacimientos fer­ ríferos de México, Consejo de Recursos Natur<lles no Renovables, Publication 3E. González-Alvarado, J., 1976, Resultados obtenidos en la exploración de la Plat¿¡forma de Córdobél y prin­ cipales campos productores: Boletín, Sociedad Geológica Mexicana. v. 37, n. 2, p. 53-60. 3, GeoJogy of lhe Central Region of Mexico Guerrero, J.C, L.T. Silver, and T.H. Anderson, 1978, Estudios geocronológicos en el Complejo Xolapa: Boletín, Sociedad de Gcología Mexicana, Resúmenes de la Convención Geológica Nacional, v. 39, p. 22-23. Halpern, M" J.e Guerrero, and M. Ruiz-Castellanos, 1974, Rb-Sr dates of igneous élnd metamorphic rocks from southeastern and central Mexico, a progress report: Unión Geofísica Mexicana. l,eunión Anual, Resúmenes, p, 30-31, Lopez-Ramos, E" 1979, Geología de México, 2nd edi­ tion, Mexico D.F., Scholastic Edition, 3 volumes. Mapes, E., 1959, Los Yélcimientos ferríferos de Las Truchas, Michoacán: Consejo de Recursos no Renovables. Boletín 46. Mauvois, 1977, Cabalgamiento Miocénico (1) en la parte centro-meridional de México. Instituto de Geología, UNAM, v, 1, n. 1, p. 48-63. McDowell, F.W., cll1d S,E. Clabc)Ugh, 1979, 19nimbrites of the Sierra Madre Occidental and their relation to the tectonic history of western Mexico, il1 CE, Chapin and W.E. Elston, eds" Ash-Flow Tuffs, Geological Society of America Special Pélper 180. Mooser, F., 1972, El Eje Neovolcánico Mexicano, debilidad cortical pre-Pa.1eozóica reactivada en el Terciario: Memária de la II Convención Nacional de la Sociedéld Geológica Mexicana, Mazatlán, Sinaloil, p. 186-187, Mooser, E, 1975, Historia geológica de la Cuenca de México: Memoria de las Obras del Sistema de Drenaje Profundo del Distrito Federéll, v. 1, DDF, p. 7-38, Mooser, F., A,E. Nairn, and J.F. Negendélnk, 1974, Paleomagnetic investig<ttions of the Tertiary and Quaternary igneous rocks, VII, él paleomagnetic and petroJogic study of vo1canics of the Valley of Mexico: Geologische v. 63, n. 2, p, 452-483. Mulleried, F,K,G., 1957, La Geología de Chiapas: Publicación del gobierno del Estado de Chiapas, Negendank, ].EW., 1972, Vo1canics of the ValJey of Mexico: Neues Jahrbuch Mineréllogische Abhandlungen, v. 116, p. 308-320 Ortega-Gutiérrez, E, 1974, Notél preliminar sobre las eclogitas de Acatlán, Puebla: Boletín, Sociedad Gcológica Mexicana, v. 35, p, 1-6, Ortega-Gutiérrez, F., 1976, Los cornplejos metamórfi­ cos del sur de México y su significado tectónico: Resúmenes del In Congreso Latinoamericano de Geología, México, Ortega-Gutiérrez, F, 1978, Estratigrafia del Complejo Acatlán en la Mixteca Baja, estados de Puebla y Oaxaca: Revista, Instituto de Ceología, UNAM v. 2, n, 2, p, 112-131, Ortega-Gutiérrez, F., 1979, La evolución tectónica pre­ Misisípica del sur de México: V Simposio Evolución Tectónica de México, Programas y Resúmenes, p. 27-29. Ortegél-Gutiérrez, F., 1981, Metamorphic belts of southern Mexico and their tectonic significance: Geofísica lnternélcional, v. 20, n. 3, p. 177-202. Pan toja-Alor, L 1959, Estudio geológico de reconocimiento de la región de Huetamo, Estado 74 Section T The Geology of the Mexican Republic de Michoacán: Consejo Recursos Naturales no Renovables, Boletín 50, 36 p. Pantoja-Alor, L 1970, Rocas sedimentárias Paleozóicas de la región centro-septentrional de Oaxaca, México. Libro Guia de la Excursión México-Oaxaca de la Sociedad Geológica Mexicana, p. 67-84. Pantoja-Alor, J., and KA. Robison, 1967, Paleozoic sedimentary rocks in Oaxaca, Mexico: Science, v. 157, p. 1033-1035 Pérez, J.M., A. Hokuto, and Z. de Cserna, 1965, Reconocimiento geológico del área de Petlalcingo­ Santa Cruz, municipio de Acatlán, Estado de Puebla: Instituto de Geología, Paleontología Mexicana, UNAM, n 21, part 1, 22 p. Pineda, A., H. López, and A. Peña, 1969, Estudio geológico-magnetométrico de los yacimientos fer­ ríferos de Peña Colorada, municipio de Minatitlán, Colima: Consejo de Recursos Naturales no Renovables, Boletín 77. Ramírez, L 1984, La acreción de los terrenos Mixteca y Oaxaca durante el Cretácico Inferior, Sierra Madre del Sur: Resúmenes de la VII Convención Geológica Nacional Sociedad Geológica Mexicana, p. 59. Rodríguez, K, 1970, Geología metamórfica del áreas de Acatlán, Estado de Puebla: Libro Guia de la Excursión México-Oaxaca de la Sociedad Geológica Mexicana, p. 51-66. Salas, G.P., 1949, Bosquejo geológico de la cuenca sed­ imentaria de Oaxaca: Boletín Asociación Mexicanos Geólogos Petroleros, v. 1, p. 79-156. Urrutia-Fucugauchi, J., 1980, Paleomagnetic studies of Mexican rocks: Ph. O. Oisserta hon, University Newcastle upon Tyne, England, 689 p. Urrutia-Fucugauchi, J., ed., 1981, Paleomagnetism and tectonics of Middle America and adjacent regions, Part 1: Geofísica Internacional, v. 20, n. 3, p.139-270. Urrutia-Fucugauchi, J., ed., 198321, Paleomagnetism and tectonics oE Middle America and adjacent regions, Part 2: Geofísica Intemacional, v. 22, p. 87-110. Urrutia-Fucugauchi, L 1983b, On the tectonic evolu­ tion of Mexico: Paleomagnetic constraints: American Geophysical Union, Geodynamics Series, v. 12, p. 29-47. Urrutia-Fucugauchi, L and L. del Castillo, 1977, Un modelo del Eje Volcánico Mexicano: Boletín de la Sociedad Geológica Mexicana, v. 38, p. 18-28. Urrutia-Fucugauchi, J., and O. J. Morán-Zenteno, 1984, Resultados preliminares paleomagnéticos para el sur de México y sus implicaciones tectóni­ cas: Resúmenes de la VII Convención Geológica Nacional de Sociedad Geológica Mexicana, p. 5. Urrutia-Fucugauchi, L and O.A. Valencio, 1986, Paleomagnetic study of Mesozoic rocks from Ixtapán de la Sal, México: Geofísica Internacional, v. 25, p. 485-502. Vidal, K, et al., 1980, El Conjunto petrotectónico de Zi hua tanejo, Guerrero-Coa1comá n, Michoacá n. Sociedad Geológica Mexicana: Resúmenes de la V Convención Geológica Nacional, p. 111-112. Viniegra-Osorio, F., 1965, Geología del Macizo de Teziutlán y la Cuenca Cenozoica de Veracruz: Boletin Asociación Mexicana de Geólogos Petroleros, v. 17, p. 103-163. Whittington, H.B., and c.P. Hughes, 1974, Geography and faunal provinces in the Tremadoc Epoch, in C.A. Ross, ed., Paleogeographic Provinces and Provincia lit y: SEPM Special Publication 21, p. 203-218 Zamora, S., et al., 1975, Los yacimientos en el Cerro del Violín, municipio de Mochitlán, Gro, Geomimet n. 78. • 4. Geology of the Southeastern Region of Mexico • \ CHETUfV1AL. VILLAHERMOSA ',,/\ ' ............ .- --! ./ GENERAL CONSIDERATIONS For the description of the region of southeastern Mexico, the following limits have been selected: on the west, the Isthmus of Tehuantepec; to the north, the shores oí the Gulf of Mexico; and to the south, the Pacific coast. The region ineludes the physiographic provinces of the Chiapas Mountains, the Central American Cordillera, the Yucatán Peninsula and the eastern extreme of the coastal pLain of the southern Gulf (see Figure 1.1). The climate of this region varíes fram temperate and semi-arid in the high parts of the Sierra de Soconusco and Sierra de Chiapas to hot in the coastal plain of the Gulf and Pacific as we1l as in the central depression of Chiapas. In this last area the clima.tes are subhumid and different fram the coastal plams where they are generally humid. In the Yucatán Peninsula the elimate 1S typically hot and subhumid. In a11 places in southeastern Mexico the rainy season is ín the summer except in sorne areas of the Gulf Coastal Plain where raíns occur a1l year. CHIAPAS AND TABASCO In the regíon that ineludes the sta tes of Chiapas and Tabasco, a great sequence of Mesozoic and Cenozoic rocks crops out. lt consists principalIy of marine sedimentary rocks that are folded and faulted. This sequence rests discordantly on a crystalline base­ ment of Precambrian and Paleozoic rocks that crops out to the southwest of the regíon, where the crys­ talline rocks of these Eras form an outlying metamor­ phic and plutonic complex constitutíng the nueleus of the Sierra de Soconusco. Mulleried (1957) considers that a large part of the Sierra de Soconusco is formed by Precambrian igneous and metamorphic rocks. However, the major­ ity of radiometríc ages obtained from intrusive samples reveal Paleozoic dates for the pnnClpal events of igneous intrusíon. Castro and co-workers (1975) report an age of 242 ± 9 mi Ilion years for a dio­ rite (analysis of biotite by K/ Ar method) that forms part of the batholithic complex of the Sierra de Soconusco and that was discovered in the base of a section located at the borders of the states of Oaxaca and Chiapas. Damon and co-workers (1981) report dates from 17 samples from eight areas of the batholithic complex that were studied by the K/ Ar and Rb/Sr methods. After analyzing ten samples of the complex, these authors recognized an isochron oí apparent age at 256 ± 10 million years, which indicates that these intrusions originated from the same Permían magma that was isotopically homogeneous and perhaps derived from the mantle. These authors mention unpublished dates for the eastern part of the Sierra Mad re del Sur in Chiapas that indicate Carboniferous plutonic activíty 75 76 Section 1 The Geology of the Mexican Republic In this élreél. Furthermore, they consider thélt the bathnlithic emplacernent in Chiapa" would ha ve been associated with the c10sing of the Proto-Atlantic ocean at the end of the Paleozoic, at the tirne of the Appéllélchian Orogeny. Carfantan (1977) believes that the empLacement of the batholith should have occurred in the Appalachian pbase of deformation and considers that the metamorphic rocks affected by this intrllsion ought to have originated in the GrenvilJe phase of the Prccambrian. This is in Jccord with radiometric déltes from gneiss samples in ChIapas as well élS from vari­ ous samples from the base of the complex in Oaxaca th(lt have been correlélted with these rocks (De Cserna, 1967, 1971). In the extreme southeast of the Sierra de Soconusco, a sedimentary sequence of lélte Paleozoic age crops out that has been recognized in the areél of Chicomuselo (Hernández-García, 1973) and that extends toward Guatem<1la. Strata in the base of this sequence com­ prise the Santa Rosa Formation élnd form a lower member consisting of a sequence of slates with some metaquartzite intercalations. The llpper member is formed of slates, sandstone, and some beds of fossilif­ erous li¡nestone. The fOH1.1a.ti0I1 appears to be partially metamorphosed and has been assigned an age of Mississippian-Pennsylvanian based on the reported fossils (Hernándcz-García, 1973). A sequence of shales and Jimestones of the Grupera Formation that contains Lower Permiéln fusulinids rests unconformably on the Santa Rosa Formation (Gutiérrez, 1956). The Vainilla Limestone overlies this forma tion. 1t con tains crinoids, bra­ chiopods, and various fllsulinid species and is cov­ ered discordantly by the Paso Hondo Formation. This liltter unit is compost'd of massive limestones with fusulinids of the Middle Permian and basal Upper Permian (Glltiérrez, 1956). In a large péHt of the north­ eastern edge of the Sierra de Soconusco, an important continental sequence is exposed that consists of red conglomercltes, sandstones, silts, and clay. These out­ craps reach to the Isthmus of Tehuélntepec and even to the eastern edge of the southern sector of the Sierra MAdre Oriental. This sequence has been named the Todos Santos Formation and constitutes the base of the Mesozoic, which crops out chief]y in Chiapas. Most authors have assigned this formiltion to él strati­ graphic interval that varies from Triassic to Jurassic (Mulleried, '1957; Gutiérrez, 1956; Castro et aL, 1975; López-Ramos, 1979). In central Chiapas, above the Todos Santos Formc1tion, an Upper Jurassic sedimentary marine sequence occurs. This is formed by limestones of shal­ low water félcies with some intercalated continental beds. The Tithonian sediments indicate an open plat­ form environment with a pelc1gic fauna over the whole area where the states of Chiapas, Oaxacél, and Veracrlt¿ converge, but to the southeast of Chiapas the facies become more sandy (Castro et ill., 1975). Viniegra-Osorio (1981) has interpreted the existence of a saline Oxfordian basin that occupied a major part of the present Sierril de Chiapas, the coastal plain of the southern Gulf, ilnd the continental platform of Tabasco (see Figure 4.1). These saline dcposits played a very important role in the deformation of the la ter Mesozoic sequence and in the development of petrole­ um traps. At present these bodies of séllt form two great uplifts, which Viniegra (1981) termed the Campeche dome and the Jalpa dome (see Figure 41). In the petroleum areas of Tabasco and Campeche, Pemex has drilled Upper Jurassic sequences, principal­ Iy of platfonn facies, é\l1d has obtained petrolellm pro­ duction from them (Figure 4.2) Above the Upper Jurassic sediments there rests a Neocomian sequence that gives evidence of the exis­ tence of marginal marine ilnd continental deposits in northwestern Chiapas and eastern Veracrúz. In the Yucatán PeninSllla and a large part of the State of Chiapas, a major ca1careous bank was created by the marine transgression initiated i.n the Cretaceous. This resuIted in the deposition of carbon­ ate and anhydrite in these regions as well as sedimen­ tation of slope deposits in a belt that bordered the Great Calcareous Bank (Viniegra-Osorio, 1981). This bel t is loca ted in the subsurface of the eastern hAlf of the State of Tabasco and in parts of northeilstern Chiapas and the marine platform of Campeche where these sed iment types are important producers of hyd rocarbons (scc Figure 4.3). In the area of Cintalapa, the Neocomian sequence has been termed the San Ricardo Formation (Richard, 1963) and is composed of sandy shaJes, red sand­ stones, intercalations of Iimestone and dolomite, and sorne horizons of gypsum. The Bclrrernian-Aptian interval seems to be absent in the immediate vicinity of Sierra de Soconusco because rocks of this age have not been identified; tbere is a resulting discordance between the lower Neocomian units and the Albian-Cenomé1l1ian sequence. According to Castro et al. (1975), this discordance is élccentuated toward the west ,vith the disappearance of units corresponding to the Lower Cretaceous and Upper Jurassic, owing to probable erosion occurring at the end of the Aptian. Limestones crap out extensivcly in il central belt in the State of Chiapas and reveal shallow ' ....'ater bank environments belonging to the Albiiln élnd Cenomanian Stages (Figure 4.4). During this time, seas transgressed numerous areas that had been erod­ ed during the Barremian-Aptian. They extended to cover up the crystalline rocks of the Sierra de Soconusco. These Lm,ver Cretaceous sequences that crap out acrass Chiapas disélppeélr under the Tertiary deposits in the areaS of Tabasco and Campeche but have been recognized in petrolellm-praducing wells. In the subsurface of this last-mentioned region, sequences reported by Petróleos Mexicano show slope facies that follow a persistent sedimentary pat­ tem during Neocomian and Aptian and the continued existence of the Great CaJcareous Bank of Yucatán during Albian and Cenomanian time. In central Chiapas abundant cakareolls sediments of the Upper Cretaceous are exposed. These show bank facies with peri-reefal lirnestones and rudist fragments (Castro et al., 1975) (Figure 45). In the 77 4. Geology of Ihe Soulheastern Region oE Mexico GULF OF MEXICO :,\0 1 Santa Ana Massll PALEOCENE í'é- G O 200 km , , 2 T u"lla Masslf 3 Chiapas Mas,,1 A Slgsbee "Knolis' [:51 Terreslnal areas 8 Campeche Oome e Jalpa Dome Metamorptlosed PaJeozolc rocks D Nancrlllal Dome E Chiapas Sal,ne 8asln Unmeramorphosed Paleozolc rocks Llmlt of Isopach conlours or sall Edgellne of sa!1 Redbeds oITodos Sanlos Formatlon Figure 4.1. The large Saline Basin of Campeche during the Ca llovian-Oxfordian. Reforma ()rea the edges of the Creat Calc()reous Bank were exposed and eroded during the Upper Cretilceous because sorne Pemex wells in this areil enCOllnter Paleocene overlying Albian-Cenomaniél11 sediments (Viniegr(l-Osorio, 1981). In offshore wells, recognition of the Upper Cret(lceous has not been possible owing to dolomitiz(ltion that has affected the Mesozoic scqllencc in lhis portion of the marine plilt­ form (Viniegra-Osorio, 1981). During the Tertiary, in most of Chiapas and Iab(lsco, marine terrigenous sedimentation was initj­ ated (Figure 4.6). These dastics are products of lIplift of western Mexico and the folding of the Sierra Madre Oriental. At this same time, deposition of c(lrbon(ltes was continuing in the Yucatán Peninsula with the graduétl emersion of its central parto Tv>'O basins of Iertiary age were developed in the Gulf of Mexico coastal plain (Comacalco and Macuspana). These are separated by a high formed by the "Villahermosa Horst," a result of normal faLllting at the fiose of the Chiapas anticlinoriunl. This antic1inoriurn is divided in sections by normal faults at the foot of the Sierra. Ihe faulting has induced its subsidence into the Culf Coastal Plain. YUCATÁN PENINSULA A Iertiary calcareous sequence crops out in a large part of Yucatán. The strata have no significant defor­ mation and are horizontal. 130th the Cretaceous sequence recognized in the subsurface a nd the Cenozoic sequence shO\·v no major structllral pertur­ bation and overlie a crysta lline m(lSS tha t has remained stable from the Paleozoic on. Ihe CretaceOllS recognized in the Pemex wells is composed principally of anhydrites, limestones, dolomites, and intercalations of bentonites and sorne pyroclastic materials. Especially toward the base, the section consists of the YlIcatán Evaporites (López­ Ramos, 1979). AH the CretaceoLlS sediments that have been encountered in the Pemex wells belong to the middle and upper parts of this system. During the second half of the Cretaceous and a large part of the Cenozoic, the Yucatán Peninsula and its marine platform formed a calcareOllS bank. Ihis was a marine high that extended to Chiapas cJl1d to the south of Veracrúz. A shelf margin developed that has beeo the principal petroleum objective in Tabasco and on the Campeche marine platform. 78 Seclion 1 The Geology of lhe Mexican Republic ~ Redbeds o M31nly marme IZI Edgehne 01 sal! I'ZI Emergent area o 200 km '-'-----', Figure 4.2. Map showing facies distribution in southeastem Mexico of Upper Jurassic facies. Under this Cretaceous sequence, the wells Yucatán No. 1 and 4 cu t through silts tones and sandstones with sorne intercalations of quartzose sand and gravel as well as green bentonite and dolomitic limes tone. López- Ramos (1979) original! y considered these as belonging to the Jurassic-Cretaceous interval. These redbeds rest aboye a crvstalline basement that was reached by the well Yucatán No. 1, at 3200 m depth (López-Ramos, 1979). From a sample of rhyo­ lite porphyry from this welt a Rb/Sr date of 410 mil­ !ion years (Silurian) was obtained. This porphyry seems to have intruded a quartz and chlorite schist (López-Ramos, 1979). The Yucatán No. 4 well cut 8 m of slightly metamorphosed quartzite that underlies the Triassic-Jurassic redbeds (López-Ramos, 1979). The Cenozoic deposits of the Yucatán Peninsula are represented principally by cakareous and dolomitic sequences with evaporite intercalations. Butterlin and Bonet (1963) formulated a column that extends from Paleocene to Quaternary. This column includes in ascending order: the Chichén Itzá and lcaiche formations (Paleocene-Eocene); the Bacalar, Estera, Franco, and CarjlJo Puerto formations (upper Miocene and Pliocene); and molluscan limestones of Pleistocene-Holocene. The Oligocene has not been recognized on the surface but was cut in the explo­ ration wells of Chicxulub No. 1 and Cacapuc No. 1 (Butterlin and Bonet, 1963). The surface distribution of the Cenozoic units clearly shows a gradual retreat of the seas toward the present coast line, and onl1' in the Eocene did the seas transgress and cover almost completely the Yucatán Peninsula (Butterlin and Bonet, 1963). TECTüNIC SUMMARY The metamorphic rocks that crop out in the Sierra de Soconusco have been related to a metamorphic event contemporaneous with the Grenvillian defor­ mation, which is well known in the eastern United States (Carfantan, 1977), and they have also been cor­ related with the metamorphic events that formed the Oaxaca Complex (Fries et al., 1962) An important phase of defonnation occurred at the end of the Paleozoico This affected the Mississippian and Pennsylvanian sedimentary sequences of south­ east Chiapas, and the chief plutonic él.ctivity began in the present-day Sierra de Soconusco. This phenome­ non was followed by él prolonged interval of cont.i­ nental environments during which the lower beds of the Todos Santos Formation were deposíted. Damon and co-workers (1981) relate the emplacement of the Sierra de Soconusco ba tholith to the closing of the Proto-Atlantic ocean and the unification of South 79 4. Geology of the Southeastern Region of Mexico ~ Deep water sed,ments g Carbonales-evaporiles o Edge of Ihe platform or bank ~ External edge 01 zone of dolomitlzed slope sedlmenls B Nearshore line ~ Nearshore volcanlcs '1Z] Emergenl areas HONDURAS o 200 km 'L- -.J' Figure 4.3. Map showing facies distribution of Neocomian-Aptian facies in southeastem Mexico. America and Africa with Narth America, an actíon that culminated in the Appalachian Orogeny at the end of the PaleozoiCo During the Late Jurassic a trans­ gression occurred that gave rise to marine sedimenta­ tion, especiaUy in the localities near the Culf Coa.st in Tabasco and Veracrúz_ In the Sierra Madre Oriental and other regions in the east of Mexico, a Jurassic transgression has been related to the opening of the western extreme of the Tethys (Tardy et aL, 1975; Tardy, 1980; Campa and Ramírez, 1979) during the disintegration of Pangea. In the Cretaceous there was general marine sedi­ mentation that, in a majar part of the state of Chiapas, is represented by the platform Sierra Madre Limestone. The area of the Yucatitn Peninsula remained stable but submerged and had shallow water deposition, forming the Creat Calcareous Bank that extended toward Chiapas and south of Veracruz. Viniegr a-Osorio (1981) believes tha t the Crea t Calcareous Bank of Yucatán tilted southwestward during its evolutlon. This interpretation is sustained by the fact that in the Pemex wells the basement was encountered at increasing depths from east to west across the marine platform of Campeche and finally reaches depths greater than 6500 m, with stj]] greater thickness of the whole Mesozoic and Tertiary sequences. Dengo (1968) recognizes a partial deforma­ tion of the Mesozoic sequence at the end of the Albian 80 Section 1 The Geology of the Mexican Republie Deep ,¡a1lf' Sedl ".en., LZl Carbol12,le·evaparlles Q Edgeline of rul!fs 01 banks Probable Iomll 01 bank claslrcs g EO gaaf IHlp wai r E:J earshoi <ItIlcanlGs [Z], i::mergel'il a eas G kten Lal1 carbonale bank [TI] YLJcatan carbonate bank [TI Figure 4.4. Map showing facies dístribution of Albian-Cenomanian in southeastern Mexico. that was accompi1J1ied by granitic intrusions that <o'xtend lo the Sierra Madrc del Sur in Chiapas and central GU<ltemala. _arfa ntan (1977) men tioned a phé1se of Cel1oD1"nian deformation that placed él volcé1nic-plu­ tonic complex of probable Mesozoic age in allochthonolls positi n over the L'rodcd Chiapas rlatform. This located. in the arca of Motozint!<.'l. his complex corresponds to a volcal1ic arc similar to tho . rt'<': >gnized in the northwest and western pélrts of Mexico. After this deformation, and dllring the Late Cretoceolls, Paleocene, and Eocene, the Mesoznic st'quence was aHected by Laramide orogenic defor­ mation. At this time an elongat",d marine basin WélS developed as a Foredeep with flysch deposition of the Ocozocuautla Forméltion (Dengo, 1968). Seemingly, the sélUne sediments ot the base of the Mesozoic played a very important role in these defor­ mations, sincc they ser ved as plastic material during the developml.:nt of the deCüllement in which time thE' Mesozoic and Cenozoic sequences were fold 'el (Viniegra-Osorio, 1981). In the Reforma-Campc-he beH, the origin of the domnl dnd pillow-like stnlctural system is related to verticéll movement impelied by the subjacent salto During the Cenozoic, the Chiopas regio n was opparently caught in tectonism involving nOTméll and strike-slip félulting, w"hich complica tes lhe structural rclations of the Mesozoic <lnd Cenozoic sequences. 81 4. Geology of the Southeastern Region of Mexico GULF OF MEXICO Carbonates wllh rudlsts and evaponles Globigerl.nld deep water facies O Melange In deep water Nearshore Nearshore volcanlcs o f:6I D 200 km IIIIII E3 Volcanlc Intruslves 1 ~ : 1 ¡;z¡ Emergenl areas Figure 4.5. Map showing distribution of Upper Cretaceous facies in southeastern Mexico. GUlF I Carbonate evaporl1e Open sea ciay, sands, rnarls ~ I Open sea rnarls g Bank edge clasllcs o 200 km [ZJ Emergent areas ~ Flysch facies Figure 4.6. Map showing distribution of Paleocene facies in southeastern Mexico. 82 Section 1 The Geology of the Mexican Republic , I , , , I GULF OF I I I MEXICO , I , 1 , 1 I I I , I ' .... .,I'-J I ARENQUE I / / I , I / /' / 1 / /' , 1 \ , , \ , \ \ '- FAJA DE ORO \ \ \ (D Sierra Madre Oriental o Sierra Madre del Sur @ Sierra de Chiapas Veracruz basln ~ Productlve areas 01 Ihe Upper Crelaceous -. Productlve areas 01 (he Upper Jurass,c and Upper Crelaceous ¿fj7 Areas 01 polentlal accumulallon , \ \ , \ \ , / \ , /_.-' / s­ ~ ~ "V; O LI 200 km ~ l ..::>"" o Figure 4.7. Map showing producing areas and oi! wells in southeastern Mexico. The directions of the faults of this period seem to be associated \vith the northwestern movement of North Americé'l in respect to the Caribbean plate along the Polochic-Monté'lgua fault system of Guatemala and the southern border. ECONOMIC RESOURCES The principal petroleum reserves of the nation are located in the subsurface of the Reforma areé'l at the Chiapas and Tabé'lsco border, as well as on the marine Campeche platform (Figure 4.7). Most of the produc­ tion comes from rocks of Late Jurassic and Cretaceous age as well as from Ihe base of the Paleocene. In these areas the sequences have slope facies composed of fractured and dolomitized detrital material. The belt containing the slope where these sequences were deposited developed during the Cretaceous along the edge of the Great Calcareous Yucatán bank, which extends to Chiapas and VeracrÚz. It has long been considered that the source rocks of these hydrocarbons are ]urassic and that the reservoirs were developed in many varied traps resulting from a complex stratigraphic and structural evolution. In the southeast region of Mexico, there exist some mineral deposits of known hydrothermal origin that show the association of silver-Iead-zinc-gold-copper. In gp.neral they are small and localized generally in the south of Chiapas, in the localities of Pijjjapan, Nueva Morelia, Lajeria, Payacal, and Almagres. In additio11., metasomatíc deposits of iron exist in the localities of Ventosa, Niltepec, and FololapilJa. Iron and copper are found in Aniaga and copper, lead, and zinc in Ixtapa. 83 BIBLIOGRAPHY AND REFERENCES Abbrevlation UNAM is Universidad Nacional Autónoma de México Butterlin, J., and F. Bonet, 1963, Mapas geológicos de la Peninsula de Yucatán: Ingeniería Hidráulica en México. Campa, M.F., and J. Ramírez, 1979, La evolución geológica y la metalogénesis del noroccidente de Guerrero: Serie técnico-científica de la Universidad Autónoma de Guerrero, n. 1, 102 p. Carfantan, eL 1977, La cobijadura de Motozintla­ un pa leoarco volcánico en Chiapas: Revista, Instituto de Geología, UNAM, v. 1, n. 1, p. 133-137. Castro, J., ej. Schlaepfer, and E. Martínez, 1975, Estratigrafía y microfacies del Mesozoico de la Sierra Madre del Sur, Chiapas: Boletín Asociación Mexicana de Geólogos Petroleros, v. 27, n. 1-3, p. 1-103. Damon, P.R., M. Shafiquillah, and K. F. Clark, 1981, Age trends of igneous activity in relation to metal­ logenesis in southern Cordillera, in W.R Dickinson and D. Payne, eds.: Arizona Geological Society Digest, v. 14, p. 137-154. De Cserna, Z., 1967 (1969), Tectonic framework of southern Mexico and its bearing on the problem of continental drift Boletín de la Sociedad Geológica Mexicana, v. 30, p. 159-168. De Cserna, Z., 1971, Precambrian sedimenta ton, tec­ tonics and magmatism in Mexico: Geologische Rundschau, v. 60, p. 1488-1513. Dengo, G., 1968, Estructura geológica, história tectóni­ ca y morfología de América Central: Guatemala Instituto Centroamericano de Investigación y 4. Geology of the Southeastem Region of Mexico Tecnología Industrial: Centro Regional dE' Ayuda Técnica, Agencia para el Desarrollo Internacional, 45 p. Fries, e, et al., 1962, Rocas Precámbricas de edad Grenvilliana de la parte central de Oaxaca en el sur de México: Boletín, Instituto de Geología, UNAM, n. 64, parte 3, p. 45-53 Gutiérrez, R, 1956, Bosquejo geológico del Estado de Chiapas: XX Congreso Geológico Internacional, México: Excursion C-15, Geologia del Mesozóico y Estratgrafía Pérmica del Estado de Chiapas. Hernández-Garcia, R., 1973, Paleogeografía del Paleozóico de Chiapas: Boletin, Asociación Mexicana Geólogos Petroleros, v. 25, p. 79-113. López-Ramos, E., 1979, Geologia de México, 2nd edi­ tion: Scholastic Edition, v. IIl, 446 p. Mulleried, F.K.G., 1957, La Geología de Chiapas: Publicación del gobierno del Estado de Chiapas. Richard, H.G., 1963, Stratígraphy of Early Mesozoic sediments in southwest Mexico and western Guatemala: AAPG Bulletin, v. 47, p. 1861-1970. Tardy, M., 1980, La transversal de Guatemala y las Sierra Madre de México, in J. Auboin, R. Brousse, J.P. Lehman, Tratado de Geologia, v. Ill, Tectónica, Tectonofísica, y Morfolgía. D. Serrat Translatíon, Barcelona, España: Editorial Omega, p. 117-182. Tardy, M., et al., 1975, Observaciones generales sobre la estructura de la Sierra Madre Orientéll. La alocto­ nía del conjunto cadena alta-alta plano central, entre Torreon, Coah. y San Luis Potosi, S.L.P., Mexico Revista del Instituto de Geología, UNAM, v. 75, p. 1-11. Viniegra-Osorio, F., 1981, El gran banco calcáreo yucateco: Revista Ingenieria n. 1, p. 20-44. THE GEOLOGY OF THE MEXICAN REPUBLIC By Dante Morán-Zenteno Universidad Nacional Autonoma de México (U.N.A.M.) Collaborators: From U.N.A.M.: E. Cabral Cano S. Alarcoa Parra G. Mora Alvarez S. Campos J. Urrutia Fucugauchi G. Silva Romo C. Caballero Miranda From Instituto Nacíonal de Estadistica, Geografía e Informatíca (INEGI): J. Alvaro lruretagoyena E. Campos Madrigal J. Luís Moreno J. Uríbe Luna J. Olivera Translated and with additional annotated bibliography by James Lee Wilson and Luis Sanchez-Barreda AAPG Studies in Geology #39 Published by The American Association of Petroleum Geologists 1994 Printed in the USA. Translation of: Geología de la República Mexicana First Spanish edition, 1984, copublished by INEGI & UNAM Second Spanish edition, 1985 First Spanish reprinting © 1990 First English edition © 1994, by the AAPG AH Rights Reserved ISBN: 0-89181-047-1 AAPG grants permission for a single photocopy of an item hom this publication for personal use. Authorization for additional copies of items from this publication for personal or internaI use is granted by AAPG provided that the base fee of $3.00 per copy is paid directly to the Copyright Clearance Center, 222 Rose\"-'ood Orive, Danvers, Massachusetts 01923. Fees are subject to change. Any form of electronic or digital scanning or other digital trans ­ formation of portions of this publicabon into computer-readable and/ or transmittable form for personal or corpo­ rate use requires special permission from, and is subject to fee charges by, the AAPC. Association Editor: Kevin T. Biddle Science Director: Richard Steinmetz Publications Manager: Cathleen P. Williams Special Projects Editor: Arme H. Thomas Production: Custom Editorial Productions, lnc., Cincinnati, Ohio On the cover: Canyon of Sumidero, incised in the Lower and middle Cretaceous Sierra Ma.dre Limestone near Iuxtla Gutiérrez, State of Chiapas, southern Mexico. Photo by R. K. Goldhammer, Exxon E&P Research. Inset photo shows the Middle Cupido Formation in Cortinas Canyon, Sierra Madre Oriental. Ihis and other AAPG publications are élvailable fram: The AAPG Bookstore P.O. Box 979 Iulsa, OK 74101-0979 Telephone: (918) 584-2555; (800) 364-AAPG (USA-book orders only) FAX: (918) 584-0469; (800) 898-2274 (USA-book orders oniy) About the Author • Dante J. Morán-Zenteno has been a research scientist at the Geophysics Inshtute of the National University of Mexico (UNAM) since 1992. He spent eight years working in the geoIogical mapping program of the NationaI Institute of Statistics, Geography and Information Technology (INEGI). During this time he prepélred most of the present book. Since 1984 he has participated in research projects of the Geophysics lnstitute dealing with the tectonic structure and paJeogeographic evolution of southern Mexico. As part of these projects, he has applied paleomag­ netic and isotope geochemical analyses to the study of the tectonic evolution of the southwestern continental margin of Mexico. Since 1983 he has been lecturing on geology at the Engineering Félculty of UNAM. From 1988 to 1990 he was at the University of Munich, in Germany, carrying out isotope analyses of Mexican rocks In 1992 he received his Ph.D. About the Translators • James Lee WiIson was born in Waxahatchie, Texas, é1nd raised in Houston. He attended Rice University and the University of Texas at Aushn, where he received his B.A. and M.A. degrees. He received his Ph.D. fram Yale University in 1949. Jim Wilson was a field geologist in the Rocky Mountains, Associate Professor at the University of Texas at Austin, and from 1953 to 1966 worked as a research geol­ ogist for Shell Development Company in Houston. During this period he spent three years in the Netherlands working on Mesozoic geology of the Middle East. In 1966 Jim returned to academia as Professor of Geology at Rice University; he joined the University of Michigan in 1979. In 1975 he completed él book, Carbonate Facies in Geologic History (Springer- VerJagl. Jim was President of sEPM in 1975-1976, became an Honorary member in 1980, élnd was elected an Honorary member of AAPG in 1987. In 1990 he received the Twenhofel Medal from SEPM. He has participated in carbonate field élnd lecture courses 'with the Laboratory of Comparative Sedimentation of Miami Unjversity, Florida; with ERICO of London; the University of Houston; AAPG; and MASERA Corp. of Tulsa, Oklahoma. His field experience ineludes work in Mexico, New Mexico, North Africa, the I~ocky Mountains, the Austroalpine area, and the Middle East. Jim is now Professor Emeritus at the University of Michigan and adjunct Professor at Rice University in Houston. He resides in New Braunfels, Texas. As a consultant, he is working on the geology of Mexico and is involved in a world­ wide study of carbonate platforms. Luis A. Sanchez-Barreda is currently senior consultélnt for Barreda and Associates, Navasota, Texas. He received his B.5. degree in Oceanography in 1972 from the University of Baja California and in 1976 an M.A. degree in geology from Rice University. He began his career as él field geologist in Libya and Spain. After receiving his Ph.D. from the University of Texas at Austin in 1981, he worked as an exploféltionist for Pecten lnternational (Shell Oil Company). In 1987 he left Pecten to fome his own consulting company. Luis has more than 20 years of geologic experience working in Mexico, and presently specializes in frontier exploration throughout Mexico, the Caribbean, and Central and South America. His main areas of interest focus on seismic/structural interpretation of sub­ Andean, forearc, and passive margin basins of Latin America. iii Petroleum and Gas. Structure and Mapping.. Geology of the Southeastern Regíon oi Mexico Section II The Bibliography of Mexicéln Geology. GeoJogy of the Northwest Region of Mexico vi 3 5 35 55 . Geology oi the Northern and Northeastern Regíons of Mexico 3. Basement Studíes. Mineral Resources. and Regional Structural Terrane Studies 3. 87 89 . Structural Controls of Sedímentation 2. Geology oi the Central Regíon oi Mexíco 4. and Remote Sensing 135 147 v . .75 2. 117 Plate Tectonícs. 1983-1993 Introductíon to the Bibliography oi Mexican Geology. Engineering and Environmental Geology. Stratígraphic and PaleontoJogic Studíes. 1983-1993 1. Hydrology.Table of Contents • Preface Section 1 The Geology of the Mexican Republic Introdudion 1. and Age Dating 4.. Studies oi Igneous Metamorphic Rocks. Neovolcanic lnvestígatíons. Paleomagnetic Studies. the geological map a t 1/1. Geografía. The geological map at the scale of 1/1. Houston.000." has not been inc1uded in the English translation. Texas. Sanchez-Barreda Barreda and Associates Nélvasota. DNAG Volume A) was until this point the only other good general descri ption of Mexican geology in English. Michigan. Because Morán-Zenteno's work was in Spanish only.e. We are also grateful to the Instituto Nacional de Estadística. USA TRAN5LATOR5' NOTE Even though ten yeélrs old. many of which result from research investigations within the National Autonomous University of Mexico. and economicéllly important area thélt forms the southwestern margin of the North American craton. Rice University.000.000. DI. translation. Texas. James Lee Wilson Professor Emeritus. and Technical Inforrnation Technology ClNEGI) and the Faculty of Engineering of the National Autonomolls University of Mexico offer this work as él joint effort to contribute to knowledge of the geology of Mexico. Zoltan De Cserna's excellent 32-page Outline of the Geology of Mexico (1989. which is complete except for outcrop photos whose originals were lost after a move of lNEGI heildquarters from Mexico City to Aguascalientes.Preface • PRE5ENTATION The N<ltionallnstitute of Statistics. Geography.000 scale described aboye has been recently superceded by a map of 1/2. their relative positions within the stratigraphic column. structurally complex.000 (eio-ht sheets) that accompanies the original text sh~ws the general features of the geologic struch. However. This volume collects and interprets a large part of the information gathered during more than 15 years of geological mapping by the General Directory of Geogrélphy and forms él compendium of scienhfic contributions related to the Geology of MexiCo. employing the new concepts relélted to the dynamics of the earth and as a step toward the teachmg and fur­ ther development of professionals in Eélrth Science. i. The translators and tILe American Association of Petroleum Geologists are grateful to Dr. A fairly complete and briefly annotated bibliogra­ phy of Mexican geology from 1983 to 1993 has been added to update the original text references and com­ prises Section 11 of this publica tion. treating not only the different types of rocks that outcrop at the surface. particularly in southern Mex1co. e Informática (INEGI) for permission to publish this version. For this reason the appendix of the original work.000. USA vii . Luis A. "Methodology of Formulation of the Geologic Mélp at a Scale of 1/1. Th1S map lS accompanied by text describing the vanous forma­ tions in the néltion. it has not become wen known north of the border.000.ue of the country. University of Michigan Ann Arbor. USA and Adjunct Professor. The translators have attempted to render an accurate and readable English text while retaining some of the use of passive voice and indirect style of the elegant Spanish languélge.000 scale that includes important changes. Dante Morán-Zenteno for his work and for rus review of the Dr. Dante Morán-Zenteno's summary of the Geology of the Mexican Republic remains the most complete report of this very lé1fge. but also the geologic times in which they were formed. • Section 1 The Geology of the Mexican Republic • . Garfias and T. have constituted sorne princi pal obstacles toward achiev­ ing a finíshed work of this type. Nevertheless. and new techniques of explorabon offer more information toward the development of our understanding. one must recognize that the lack of informabon about cer­ 3 tain periods in the geological history of the national territory. Along with the development of geologic studies that science has made in Mexico. ít is worth indicat­ ing that in 1896 a somewhat unsettled state of knowl­ edge resulted in a work in Spanish entítled A Sketch of the Geology af Mexica formulated by Jose Guadalupe Aguilera and Ezequiel Ordonez of the Geologic Instítute of Mexico. Each day scientific discoveries. In reviewing past information. Available knowledge concerning the origin and geologic stmcture of Mexico is still incomplete. and the numerous unexplored aTeas. Nevertheless. it was not until1949 that V. Chapin published the work entitled Ce%gy . advances jn mapping. a descripbve work that constitut­ ed a11. which had been published ear­ lier.e. as well as to work out functional models to explain the origin of its geologic structure. important complement to and summary for the Geologic Map of Mexico. there have been a few attempts to fonnulate general works covering the many aspects of geology that the country presents.• Introduction • Map of the Republic of Mexico showing the regions described in each chapter. it is still difficult to achíeve a complete description of the geologic character of the territory of the nabon. Nevertheless. whose author was lng. The first edition of this work was the responsibility of the National Institute of Statistics. Fina1ly. In this work. Dante J. which are descríbed in each of the chapters that Eorm the work. in two volumes.000 scale. furthermore. . prin­ cipally from Petróleos Mexicanos. The preparation oE the text was the responsibility of lng. an administrative unít decentralized between the Secretaríat of Planning and Budget.4 Sectíon 1 The Geology of lhe Mexican Republic of Mexico. a general key for localizing areas and objectives of economic or particular scientiEic interest. Dante J. the geologic maps that pertain to each chapter are mentioned.000 scale. in 1979. and which the Faculty oE Engineering of UNAM printed as notes of the subject matter of the Geologt) ol Mexico. Morán-Zenteno. The present book has the double objective of offer­ ing a geologícal synthesis oi Mexico as a general refer­ ence work for a11 readers. lng Ernesto López-Rarnos published his work. and use oE the substrate. in which reconstructions of the events that occurred d uring the geological history oE the Republic are included. 1ng. and it comments on the most recent models concerning tectonic evolution and the geologic orígin of signifi­ cant economíc deposits. culture.. The Geology ol Mexico. Ceography. The information in this second edition can be used as a point oE departure Eor regional projects oE investigation and guidance. To deveJop the present work it was necessary to divide the Republic into difierent regions.000. and a presentation of themes in organized and dídactic form so that it can be utilized in upper level courses related to the geolo­ gy of Mexico. Manuel Alvarez. A more recent work is The Geolog}-/ ofMexico. which were designed at a scale to show topography. This division does not correspond to that of the original eight maps of the Republic at the 1:1. and TechnicaJ Information (TNECl) as a complement to the geological maps that the Ceneral Department of Ceography had prepared. Morán-Zenteno gives courses on the geology oE Mexico and physical geology at UNAM and in addi­ líon is a researcher in the Institu te oE Ceophysics at the same university. key characteristics that are pertinent for geological interpretation oE certain regions. defined by natural limits. presented by the same author. then chief of Petrography and PaleontoJogy of the same department. together with the geologic maps at 1:1. It offers. That publication constitutes at present the most widely known text beca use it con­ tains detailed descriptions of lithostratígraphic units and references to numerous unpublished works. Jr. and the Faculty oE Engíneering of UNAM. This second edítion is the result oE combined forces of INECl. The bíbliography that accompanies each chapter makes it possible to orga­ nize a wide varíety of consulting work aimed at studying in depth certain aspects or certain particu­ lar areas.000. It should make c1ear. the provinces of Baja California. . The cli­ mate varies in general fram dry in Baja California..::: :~.. .. the Pacific coasts of Baja Céllifornia and Sinaloa. Geology of the Northwest Region of Mexico • NORTHI¡VESTERN RE'GION . except in the north of Bajél California where the rain is in winter... and to the south. to the west. ". • . which mélke it difficult to reconstruct a stratigraphic column for this region and to ascertain events that have occurred.. that permits explélnation.. PENINSULA The peninsula of Baja CélLifornia as shown on the geologic map at 1:]. In almost all the region rainfall comes in 5 the summer. DGG) offers él high struc­ tural complexity and rocky outcrops. the northern edge of the Neovolcanic axis. Nevertheless a subdivision has been mélde. Sonora. Sierra Madre OccidentaL élnd the Pacific Coastal Plain me included within this region. the volcanic seq uence of the Western Sierra Madre. to subhumid in the higher parts of the Sierra Madre Occidental and south of Mazatlán.-'. .000. \vith a certélin clarity. In accordance with the physiographic division of the General Department of Geography (see Figure 1. of the geoLogic concepts of this province and that coincides in lmge part with the physiographic divisions of the OGG and with the division into the geologic provinces of López-Ramos (1979).000 scale (General Department of Geography. • GUADALAJARA GENERAL CüNSIDERATIüNS For the description of northwestern Mexico the fol­ lowing natural limits have been selected in this work: to the east..' .• 1.. DURANGO • . the Sonoran Oesert. and northern 5inaloa.:: " ..1). élS rational as possible.. ) IX CENTRAL MESA X NEOVOLCANIC AXIS XI YUCATAN PENINSULA XII SIERRA MADRE DEL SUR XIII SOUTHERN GULF COASTAL PLAIN XIV CHIAPAS MOUNTAINS XV CENTRAL AMERICAN CORDILLERA VIII XI XIII XII V XIV Figure 1.ULt--~'" . struc­ tmal.6 Section 1 The Geology of lhe Mexican Republic PHYSIOGRAPHICAL PROVINCES I 11 111 IV V VI VII VIII BAJA CALIFORNIA PENINSULA SONORAN DESERT SIERRA MADRE OCCIDENTAL SIERRAS AND PLAINS OF THE NORTH SIERRA MADRE ORIENTAL GREAT PLAIN OF NORTH AMERICA PACIFIC COASTAL PLAIN COAS AL PLA'N OF ''''11I::. The next belt is located to the east of that described abo ve and is made of sequences oi volcanic rocks. A Portion of N orthem Baj a California In this zone exposures of a stratigraphic sequence whose geochronologic range varies from Paleozoic to Recent ¿¡re encountered. and stratigraphic characteristics. Al! this attests to the development of envi­ ronments that vary hom continental out to the platform and slope with fluctuating coastlines orient­ ed more or less paral!el to the line that divides this belt from the terranes located to the east.. The sequence was designated by Beal (1948) as the Rosario Formabon and consists of sub horizontal strata oi silty. a httle less than laL 30 N (Figure 1. Jocated in the extreme western penin­ sula.2) and considered this feature to have controlled the depositional history of Baja California for long time periods.. The latter 0 constitute the source of supply for the sediments that comprise this sequence since in this time emergence occurred and formed mountainous masses exposed to erosiono Gastil et al.1. and sedimentary rocks whose age is principally Lower Cretaceous (see Figure 1.. as well as intrusives.2) that run the length of this part of the peninsula and that present dearly differentiated petrographic. it underlies Quaternary volcanics and con­ tinental and marine sediments of the Tertiary and Qua ternary . older volcanic and sedimentary rocks.4). Physiographic framework of the Republic of Mexico.~. and eonglomeratic sandstone that contélin both marine iossils and sauri­ an bones.1. Ihe hrst belt. is composed of a sequence of marine and conti­ nental Upper Cretaceous sediments that are poorly consolidated and laek appreeiable tectonic deforma­ hon. in angular discordance.. shaly. The upper and more extensive part of the sequence was . Ihis band of outcrops is of maximum width at the lahtude of Punta San Antonio. The configuration of the dif­ ferent units forms three pre-Tertiary belts (Figure 1. . Ihe sequence that comprises this western portion of Baja California covers.. These belts are covered indiscriminately by volcanic bodies and Ter­ tiary and Quaternary sedimentary deposits. volcanidastics. Mina (1957) correlated this formation with clastic sediments that erop out on the western border of the State of Southern Baja California and called it the Valle Formation. (1975) defined the outer limit of the cited belt as the "San tillan-Barrera Line" (Figure 1. Ihese authors cite numerous paleontolagicaJ determinations that stratigraphieally position the Rosario Formaban in the Campanian and Maastrich­ tian Stages.3).. by bodies of reefaJ limestone with Aptian and Albian fossils. as well as clasbc rocks derived from volcanics... ~ Sed.Al.Ji-----J o Rosario Fomlatlcn 01 Ihe Upper Crelaceous Figure 1. and that evolved on continental crust. Distribution oí volcanic.1.. . Rangin (1978) has interpreted this seguence as one of the volcano-sedimentary belts that were devel­ oped in northwest Mexico during the Mesozoic. and it is affected by numer­ ous faults and by emplacement of bodies of intrusive granite of Cretaceous age. Pre-Tertiary terranes oí Northem Baja California.5) developed in northwest Mexico. ­ ¡ I I I I I 28' - -t . Ensenada 30' . Distribution of outcrops oí marine sedi­ mentary rocks oí the Upper Cretaceous.4.. These belts have been related to subduction and partial fusion associated with one or more con vergent borders (see Figure 1.. mainly along the western border of Mexico. It underlies discordantly the Rosario Formaban and extends persistently along al! northern parts of the Baja California peninsula. Geology of the Northwest Region of Mexico 117' 114' 7 I 114' Ensenada 30'­ 28'- -1-­ --jj---~---.2. that formed in a similar manner to vu1canism in Sonora. and sedimentary rocks oí the Lower Cretaceous. 117' I MeXlcal1 114' Figure 1.I ¡j . This forma han covers discor­ dantly in some localities vo1canic and sedimentary rocks of Triassic and Jurassic age.memary rocks 01 Aptian-Albien age Figure 1. Numerous outcrops of this type of sequence exist.3.. volcanoclastics.. It is deformed and partly metamorphosed. originally named by Santillán and Barrera (1930) the Allsitos Formation from exposures on the Rancho Alisitos located to the south of Ensenada It is consti­ tuted chiefly of pyroclastic rocks and l¿lVas of dacite­ andesite composition.. correlative with the Alisitos Formation. The con­ vergent borders seem to be tectonic features com­ mon to al] of western Mexico since there exist numerous volcano-sedimentary outcrops along this side of tI1e country. The major part of this b. 1 1 1 1 í 1 1 28. Their élge has still not been well defined. is eomposed of complex outcraps of intrusive raeks and metamorphies derjved principally fram the regional metélmorphism of sedimentary rocks. in some localíties sma1l plutons of diorite and gélbbro are mapped. but their ages have not been determined. Based on ideas of Gastil et al. by the development of marine deposits. Sjlver and Anderson. However. 1978. The terranes that form this second belt emerged during this period. located on the eastern border of the nortbern part oE the peninsula of Baja California.6) of the northern part oE Baja California and the pre­ batholithic metamorphic rocks formed before the Alisitos Formation... :-~: . have postuJélted that in northwestern Mexico there occurred a migration in time and space of this type of plutonic emplacement from the Cretaceous in Baja California to the Cenozoic in Chihuahua.6. They were folded and partially metamorphosed. citing radiometric studies. particularly on the western edge of . The Cenozoic history of the northern part of Bajél California is characterized by the accumulation of great thicknesses of continental sediments that crap out in numerous localities. (1980).. ldealized block diagram that shows the tectonic situation of northwest Mexico for the Late ]urassic. Distribution of Mesozoie plutonic out­ erops in northem Baja California. The sediments that form the Alisitos Formation were subjected to a period of com­ pression at the beginning of the Upper Cretaceous. In contrast. Pre-batholithic metamorphic sequences associated witb this third belt present various metamorphic facies.5.. This process developed during the opening of the Atlantic Ocean and the movement oE North America toward the northwest...8 Seetion 1 The Geology of the Mexiean Republie . ­. : . The third belt. ~ - Figure 1. and to the west of them sediments formed that were to become the Rosario Formation.­. To tbis beIt belong the Mesozoic béltholiths (Figure 1. 1984). Garza (in Márquez-Castañeda. McEldo\vney (1970) reported the presence of Pélleozoic crinoids. and R. The plutonic racks that eomprise the batholiths vmy in mineralogical composition from tona lites to granodiorites and granites.1I 1 I ~ Mesozolc granlllc racks --Q"" +--1 -'­ C] Figure 1.:-. 1978). Márquez-Casteñeda (1984).Hholithic emplélcement occurred during and aher the sedi­ mentation and magmatic extrusions that originated the Alisitos Formation. Some authors (Gastil and Krummenacher. and bivalves in sedimen­ tary rocks that crop out southeast of Ensenada. .-----.. outcrops oE metamorphic ealcareous racks that proba­ bly are related to the Paleozoic limestones which crap ou t in the State of Sonora. There also exist on the eastern edge of the peninsulél some Ensenada _1 I 1 1 I 1 1 1 I 30'- ¡-------. corals. in the Eocene the Mesozoic mountains were completely denuded and formed only small isolated hills.7).. is composed of éln impressive sequence of pyroclastic rocks. The Cenozoic vulcanism of the northern part of the peninsula can be referred principally to four zones where wide exposure of the volcanic rocks thélt origi­ nated in this area are encountered and that mark the Miocene as the méljor epoch of volcanic activity (see Figure 1. Originally. Coastal deposits that formed in the littoral of the Pacific during the Pleistocene élre found aboye a series of terrélces developed in that epoch.. Geology of lhe Northwest Regíon of Mexíco 9 the peninsula.<+-----:. Outcrops of this seCjuence are persistent in most of the eastern half of the southern peninsula.I --ó. The second zone. following a coastline that is located slightly to the east of the present shore... lava f1ows.. Distribution of outcrops of Cenozoic vol­ canics of northern Baja California. principally rhyolites. in the interior of the peninsula dur­ ing this time éllluviaL. ' \1 • Ensenada J I I I I I I I I I 30-r---­ i I I c> I I I I 28' i .7). These sediments came from emergent areas to the east where time-equivalent continental sediments are encountered. the eastern part of the region experienced some subsj­ dence where the Gutf of California later developed. According to Gastil et a!. extrusions of Miocene and Pliocene age. and by importémt volcaníc activity that partly covers the Mesozoic belt described aboye. sediments accu­ mulated in nearshore and deltaic environments (Gastil et a!.... Heim (1922) termed these deposits the Comondú Fonnation ..7. These cover great thicknesses of conti­ nental and mixed sedirnentary rocks. 1975) that were superimposed on a tectonic setting of a series of u pi i fts and down warps in the coastal zone of the peninsula during the Pleistocene (Ortlieb. 1978)."'.. (1975). In contrast. The Miocene contains outcrops of marine sedi­ ments that are the oldest Cenozoic strata to appear in the northeastern part of the Peninsula and that rnarked the earliest advances of the sea over the area that would become the Gulf of California.. Later in the Eocene. located in South Baja California. located in the southern part of Sierra de ]uárez between latitude parallels 31 ° and 32°.. tha t occur on the island Angel de la GUélTdél élnd to the south of the 29° parallel.. These sediments are found generally capped by lélvél. Sierra de la Giganta The Sierra de la Giganta.. This formabon can be correlat­ ed with the Santo Domingo. contains an important sequence of silíceous pyro­ c1astic rocks of diverse types that are found capped in some localities by basal tic flows of Pliocene and Quaternary age (Figure 1. colian.1. Tepetate. Some of these reach up to 500 m in altitude. These terraces have been related to glacial changes nf sea level (Gastil et él!. The last zone is composed of outcrops of basaltic and pyrocJélStiC rocks...IIL-l r-n ~ ~ Slllceous and Jnlermedlale grade rocks of Mlocene age Basallle raeks 01 Pho·Plel$!ocene age Figure 1.. and continental sand­ stones that together reach 1200 m in thickness. This zone can be considered as a northward extension of the Miocene sequence that constitutes the Sierra de la Giganta in southern Baja California. is represented by siliceous pyrocJastic sequences that are seen to cover andesite flows in some localities and are capped in other places by basaltic flows of Pliocene-Quaternary age. Some of the interior fluviatile stream courses were directed toward this area. located on the coast of the Gulf of California él.". 1975) on the western border of the north­ ern portio n of the peninsula. to greélt thicknesses of fluviatile and eolian strata that crap out at laL 31° in the area of San Augustín and some localities located about élt the latitude of the bays of Las Animas and San Rafael. The third zone corresponds to extensive flows of éllkéllíne basalts of the upper Tertiary located in the central part of the peninsula at the léltitude of Canoas Point. Manv of these sediments continue to develop today. These flows are similar to those that are localízed with minor distribution around the Mesa de San Carlos and San Quintín. 30°.~ ­. and lacustrine deposits accumulated. These areas were drained by stream courses that flowed toward the Pacific and fed their sediments into the marine deposits on the western edge.t the latitude of J Tlluana MeXlcal1 . The first zone. and Malarrimo formations described by Mina (1956) in the southern half of Baja California. it seems. Santillán and Barrera (1930) termed the marine Paleocene sediments that were encountered between Punta San Isidro and Mesa de San Carlos the Sepultura Formation. During the Paleocene and Eocene. The Pliocene and Miocene sed­ iments correspond. The sequence that makes up the Comondú Formation does not show strong tectonic deformation. although generally these are covered by the Miocene and Plíocene sequence of the Comondú Formation. They are composed of Cretaceous and Ceno­ zoic rocks (see Figure 1.10). The oldest seguence of this region is composed of partly metamorphosed volcanic and sed imentary rocks that crop out in Punta Prieta. the Cretaceous sequences wedge out.- - - - - +­ I I I I I I I Volcaníc and sedimentary rocks 01 lhe Comondu Formalion Sedlmenlary manne rocks 01 Tertlary age In Punsima-Iray and Magdalena baslns Cabo San Lucas Figure 1. pumice-tuffs. Originally 111· -n::--i----r-----r. The Basins of Vizcaíno and Ballenas-hay-Magdalena The basins of Vizcaíno and Ballenas-Iray­ Magdalena. Later. In the axial portion of the structures. The southwestern flank of these major structures is represented by outcrops of older rocks that form ophiolitic complexes and partly meta­ morphosed Triassic and Jurassic seguences (see Figure 1. these areas constitute two large synclinal depressions with general northwest-southeast orien­ tation. litharenite sandstones. at d epth. and conglomera tes.9).8.10). However. Above this structural high. which take in the western haH of the larg­ er part of the State oE South Baja California. the strata cover discordantly the main sedimenta­ ry sequences of the Tertiary that crop out more widely in the basins of Purísima-Iray and Vizcaíno. it reveals accentuated epeirogenic uplift and an inclinabon of its strata gently toward the west. are represented by ZOnes of low and smooth topography in which exposed sequences are encountered whose geochronologic ranges vary from Triassic to Recent. based on geophysical data and wells drilled by Petróleos Mexicanos. Punta San Hipólito. while on the northeastern flank some bodies of the batbolithic complex of Baja California are found. and Cedros Island (Figure 1. Lozano (1976) has interpreted the existence. ignimbrites. Escandón (1977) indicated that the upper member of this formation belongs to the lower Pliocene.10 Section 1 The Geology of the Mexican Republic (see Figure 1. out­ crops of the younger Cenozoic formations occur. and the plutoníc rocks that are a southern continuation of the batholiths of northern Baja California. basalts. but these strata reach great thicknesses in the center of both depressions. Mina (1956) con­ sidered that the source of supply of this great quantity of volcanic sediments should have been located in a volcanic belt to the east of the present coast of the Gulf of California. This block might have an orientation perpendicular to the gener­ al structural tendency of the península and would be located between parallels 27° and 28°. of an uplifted block of ophiolitic rocks that separa tes these two structural depressions. This sequence presents strong lateral variation and is composed principally of volcanic agglomerate. Distribution of principal outcrops of Tertiary rocks of southern Baja California. Structurally. . On the other hand.8) and assigned their stratigraphic position to upper Miocene. B. . Lozano (1976). sect. Geology of the Northwest Region of Mexico VIZCAINO BASIN. sed.' 1 ~ VOLCANICS PLlO-PLEISTOCENE IGNEOUS INTRUSIVES I-PIP~I 00 Figure 1..Js TERTIARY (PALEOCENE-EOCENEj. SUPo OPHIOLlTE COMPLEX BASEMENT COMPLEX LOWER CRETACEOUS DEPOSITS MIDDLE CRETACEOUS UPPER CRETACEOUS PLlO-PLEISTOCENE Ca Cb KI Km Ks Plp UPPER JURASSIC DEPOSITS . 11-11' taken fmm Petroleum Evaluation of the Peninsula of Baja California.e..1.--' o S 10 1S 20 Km IRAY-MAGDALENE BASIN. SECTION 11-11' W ISLA MAGDALENA M-2 E 1-4 M-3 x x x OPHIOLlTE COMPLEX .9. 1-1' and Iray-Magdalena Basin.I TERTIARY (PALEOCENE-EOCENE) IT¡>.l.~ L C~~. Vizcaíno Basin.bo 1 TERTIARY/MIOCENE MARINE DEPOSITS UPPER JURASSIC DEPOSITS -I'js: 1 TERTIARY/MIOCENE BASEMENT COMPLEX LOWER CRETACEOUS UPPER CRETACEOUS 8 ITmv·1 I'K.Tpe 1 --'----'I t-.t. by F. BC SECTION 1-1' SW GEOLOGIA SUPERFICIAL GRAVIMETRIA SISMOLOGIA NE 11 I SIERRA SAN ANDRES GN-I ~ GEOL. B.C. Re.I laS. Lozano (1976) reported a fauna of the Late Triassic at the top of the sequence ami for this reason considered the rocks to be of this age.. Distribution of the principal outcrops of Mesozoic rocks of southern Baja California. and the inclusion of reefal limestone blocks forming a sequence underlain by pil­ low basalts.12 Section 1 The Geology of the Mexican Republic 114' 111' =-+~~-. the Vizcaíno Peninsu la.. 1978). volcaniclastics.­ I I 1 1 I I I I I I I 1 ~ 1 1 -----------1 I rn ~ Ophlollle complex rn ~ Metamorphosed Jurasslc rocks Cabo San Lucas Lower Cretaceous sedlmentary rocks Granlllc rocks Figure 1. There also exist other outcrops of sequences with oceanic affinity that resuIted in ophiolites and melanges./ 1981). Later.10. The Upper Cretaceous is represented in the regio n of southern Baja California by a detrital sequence of Ceno manían to Maastrichtian age that overlies earlier sequences with apparent angular discordance.. lt has been recognized in out­ crops of the Vizcaíno Peninsula and in the subsurface of the two Cenozoic basins of this region. There have been recognized in this regíon combined volcanics.. Although Mina originally had pos­ tulated correlation with the Upper Jurassic Franciscan Formation.. Finch ami Abbott (1977) la ter placed these beds in the Upper Triassic. On the islands of Santa Margarita and Magdalenil.Sta Rosalla -1­ 1 26'- -1. 1979). and sedimentary rocks of Late Jurassic and Early Cre­ taceous age with an ophiolitic basement-forming sequence originally termed the Eugenia Formation by Mina (1956)... 1979. The association of chert.- . For this reason he tentatively designated these rocks as Jurassic.. litharenitic volcanics. They crop out in the type locality and cor­ relate lithologically with the Franciscan Formation of California.. indicate that this unit was deposited in an oceanic basin associated with a volcanic island arc within a convergent tectonic framework (Finch et aL. This unit was designated the Valle Formation by Mina (1956) and inc1udes turbiditic toe-of-slope fan deposi­ tion (Patterson. rocks exist that petrographi­ cally resemble those of the Mesozoic and that form an intricate mosaic of terranes of both oceanic and vol­ canic arc affinity. Mina (1956) designated these rocks the San Hipólito Formation. In Cedros lsland. .. and the area of Magd¡:¡lena Bay. these also have been attributed to the Jurassic beca use of their radiolarian content (Rangin. Castil et al. partially serpentinized ultramafic rocks crop out and are a pparently a frac­ tion of an ophiolitic complex related to those of Vizcaíno and Cedros. because of their content of macrofossils and radiolarians. These units crap out both on Cedros lsland and on the Vizcaíno Peninsula. as well as the apparent absence of detri­ tus derived from the craton. . This belt is probably of Triassic age and forms the eastern half of northern Baja California. combinations in California that include the Franciscan Formation. but nevertheless offers sorne distinctive details in its own evolution. 3. is extended beneath tne volcanic cover of the Sierra de la Giganta. To the east of the aboye domain. The principal episodes of collision seem to have oceurred in Bajél California during the Cenomanian. overlapped partially by the combined volcanie-plutonic rocks. In northern Baja California. 1981). passive margin condi ­ tions persisted during the Triassic. metamorphosed. and eélrlier sedimentélry seguences at the same time of the principal emplaeement of batholiths (Gastil et aL. The first of them oecurred in the Devonian ­ Carboniferous (Antler Orogeny) and the second in the Permian-Triassic (Sonoma Orogeny). (1981) consider that these two arcs were associated with different zones of sub­ duchon that evolved in parallel. at least partly. Dominantly metasedimentary rocks 01 Paleozoic?-Triassic age Figure 1.14 SecHon 1 The Geology of the Mexican Republic sorne isolated outcrops of calcareous and very badly deformed detrit. (3) dominantly metasedi ­ mentary roeks of Paleozoie?-Triassie age. To the east of the aboye combinations of sediments. 1978). Dominantly 01 oceanic affinity 2. In contrast. evidenced by an apparent tectonie stability in Sonora and by the absence of volcanic arc components in the metasedi­ mentary sequen ce of the Peninsula. Dominantly volcanics.11.. 1979). In the extreme east. the development of an islélnd arc dominion was initiated to the west of the eratonie bor ­ der and its Triassic sedimentary wedge. This are apparently evolved eontemporélneously with the one reported aboye the continental crust in Sonora (Rangin. 1981). later accreted landward (Gastil et al. (2) dominantly vol­ eanie. The collision of the intraoeeanic arc relélted to the Alisitos Formation against the eratonic margin appar ­ ently occurred in distinet episodes owing to the pres ­ ence of transform faults between trenches that displaced distinct segments of the are. one of them related to an intraoceanic treneh and the other to a treneh bordering the craton (see Figure 1. In nortnern Baja California. Two episodes of oro­ genic deformation have been identified for this region. 1981). Gastil and his eo-workers (1981) nave suggested the possible existence of a trench or basin marginal to the cratonic edge. in addi­ tion to the ealcareous and detrital sequences of Sonora. Both events have been interpreted reeently as marking pathways of eolli­ sions of intraoeeanic arcs against the passive margin of North America that induced the emplacement of allochthons of the Roberts and Galconda Mountains over the miogeoclinal sequenees of the Cordillera (Diekinson. volcanoclastie. a volcanic-plutonic fringe of Jurassic to Early Cretaceous age is developed. and elevated the volcélnie. and stretches probably to the El Cabo region. This type of tec­ tonic situation has been interpreted for a large part of the North American Cordillera. whieh reveal the presence during this era of a passive margin domain for northwest Mexieo. In the Jurassic. passive margin conditions are recog­ nizable between these two events. on oceanic crust and constitutes an ancient calcalka ­ line volcanic are like those that typieaJly evolve as fringes parallel to convergent borders. Gastil et al. There are sequences exposed in the east of northern Baja California. vol­ eanoclastic. 1. 2. However. neither episodes of collision nor allochtho­ nous arc sequences have been identified.11 rocks occur that have been a ttribu ted to the PaleozoiCo The tectonic evolution of Baja California during the Paleozoie seems to have been related to the Cordilleran continental margin of the western border of North America. This phenomenon generated a primary phase of deformation that fold­ ed. volcanoclastics and sedimentary rocks of Jurasslc-Cretaceous age 3. a belt of metasedi­ mentary continental border clastic sequences appears (Gastil et aL. Only the Upper Triassic San Hipólito Formation in the Vizcaíno region reveals a probable boundary of convergence toward the ocean interior.5). This vol­ eanic-plu tonie belt occurs in the western half of nortnern Baja California. Tectonic beIts of Baja California (l) dominantly of oeeanie affinity. and sedimentary roeks of the ]urassic and Cretaceous. These sediments form a sequence of sandy clays and silts with some diéltornite horizons. Oligo­ cene sedimentary rocks do not crop out. The middle Miocene is formed by diverse sequences that show much lélteral variéltion élnd are composed of tuffél­ ceous sélndstones. sillimanite. all of which attest to a marine depositiona] environ­ mento Above this unit rests in concordance a sequence of marine sandstones that represents the Sajada Formation (Heim. The élbove formations underlie discordantly the continentéll sedimentary and volcélnic deposits of the Comondú Formation that reach maximum develop­ ment in the Sierra de la Giganta. and limes tones with some apparently metavo!canic bodies bearing epidote and amphibole. furtherroore. with discordance over the Miocene fonnations. thanks to the welJs drilled by Petróleos Mexicanos (Lozano. 1956) and the SaIadél Formation in the Purísima areél (Heim. 1976).thickness of more than 2000 m can be recognized in the su bsu rface. Sedi­ ments corresponding to this epoch have been recog­ nized in the Pemex wells (Lozé\no. and sandstones of the lsidrio (Beal. The Miocene is íOllnd Clmply exposed in the regions of VizcClíno and PurísimCl and consists of sedi­ mentMY and volcanic rocks. There exist also catac1élstic belts and diorit­ ic and gabbroic intrusions that form north-south-ori­ ented lineations. the area is forroed bv a mas­ sive batholith that is expressed in the form of ~ mOlU'\­ tainous complex élnd thélt is interrupted in the central part by a depression known as the Santiago Valley and in the northern part by the La Ventana Valley. These are represented by the Almejas Formation in the area of Vizcaíno (Mina. one can recognize the existence of a pre­ batholithic metamorphic complex formcd principally by metasedimentary rocks derived from lutites. sediments of COélStéll environ­ ments were deposited in the Vizcaíno and Purísima basins. 1966). In the northern and western portions of the Sierra de In LagunCl. Santa Clara. which has isolated outcrops along the Sélntiago Valley. the latter of which rests in discordm1Ce on the Cretaceous forméltions. To this epoch belong the Santo Domingo and Malarrimo formations (Minél. subhorizontaJ position of the strata. Tortugas. In the neighborhood oí the main batholithic body. 1956). In the area of Purísima it is composed of lutites with diatomite intercalations of the Monterrey Formation (Oarton. attesting to a period of emergence for that time. 1948). located to the east of this described region. It is possible to recognize on the western border at Cedros lsland. The outcrops of these deposits are located chiefly in the ex treme north of Sélntiago Valley. The outcrops of the Eocene are represented principally by sandy and shaly sequences that have been designated as the Bateque Formation in the area of Vizcaíno and as the Tepetate Formation in the La Purísima Mea where the 100ver part of the sequence belongs to the Paleocene. They have been related. zones of migmatites and numerouS intrusions of fel­ sitic character Me present élpparently associated \vith the batholith. The bélse of these sequences is formed by conglomeratic deposits that are correlative with the Comondú Formation and that rest above the crystalline basement represented by intrusive Cretaceous rocks. Ouring the Pliocene. The Cenozoic sedimentélry sequence that fills this tectonic graben is covered dis­ cordantly by a series of sandy-conglomeratic deposits of the Pleistocene that are seen in the form of ancient piedmont fans and belts. Region of the Cape (El Cabo) The extreme south of the Béljél Céllifornia peninsula breaks abruptly across the general geologic aspect of the basins described aboye. The rectilinear borders of these mountains suggest faults of great displace­ ment that jllxtapose and eIevate the batholithic region aboye the leve] of the areas of Cenozoic outcrops. La Zorra. 1922). and a marine dastic lithology. In this pMt of Baja California. the sedi­ ments of the Pliocene Trinidad Formabon rest with angular discordance (Pantoja-Alor and Carrillo­ Bravo. the Vizcaíno Peninsula. The batholith thélt composes this mountainolls zone has characteristics similar to those masses that crop out in the north of Baja California and is made up of granodioritc and granite. These wells encountered diverse lithologies with predomi­ nance of slope shale facies. principally in the La Purísima area where they reach a thickness of IIp to 500 m. 1922). and the islands in Magdalenél Bay the presence of com­ bined tectonical1y controlled rock types oE oceanic affinity that include portions of ophiolite complcx­ es élnd typicnl melange sequences that range from Triassic to Late Jurassjc.11): 1. The lower Miocene is represented in the area of Vizcaíno by agglomerates. and San Raymundo form<ltions (Mina. 1976). 1922). and shales of the Zacarías. 1921) and white sandstones of the San Gregorio Formation (Heim. Ortega-Gutiérrez (982) has identified (l dosely juxtaposed succession of isograds of biotite. to similéH Iithologic . Over the élbove sequence. The Santiago Valley is structuréllly él tectonic graben in which the principal sedimentary sequences of the region are developed. and cordierite. inside the metamorphic complex. notwith­ standing that a. and San Joaquín formations (Mina. Tbese combinations of sediments have been interpreted to result from the structural evolution of a paleo-oceélnic crust and to mark an ancient line of convergence. bentonitic shaJes. andaIucite. Geology of the Northwest Region of Mexico 13 Thc Cenozoic sedimentélrv formations form the larger piHt of the bélsin-fill of Vizcaíno and PurísimCl­ lray-Magdalenél downwarps élnd are principally char­ acterized by little consolidation. 1956). 1956) that represent the constal and lagoonal platform environments. sands. Tectonic Summary The principal tectonic elemcnts of the Bélja California Peninsula céln be summed up as follows (see Figure 1. In the metasedimentary sequence.1. Outcrops of Paleocene sediments are rare. sand­ stones. San Ignacio. 1970). Starting from the first contact of the Pacific and North American plates. According to a model of McKenzie and Margan (1969) and Atwater (1970)¡ the collision of the first segment of the ridge against the North American plate was initiated approx­ imately 30 million years ago¡ at a point located in pre­ sent-day Baja California. This lateral movement could have occurred in its initial stages along the continental border of North America and later couId have occupied the present belt of the San Andreas System and the Gulf of California (Atwater. (B) 20 million years before the present. and the development of right lateral movement between the North American plate and the Pacific plateo S = Seattle. Tectonic evolution of northwest Mexico in the Tertiary. The movement of Baja California toward the northwest is possibly A FARALLON PLATE NüRTH AMERfCAN PLATE B Figure 1. (O 30 million years before the present. The vul­ canism associated with the subduction on the western margin of Baja California during the Late Cretaceous and Paleogene has been recognized¡ chiefly in the continental part of Mexico¡ and as late as Miocene time it is expressed in the Peninsula by pyroclastic sequences in the Sierra de la Giganta and other erup­ tive centers in northern Baja California. (A) 10 million years before the present. Geology of the Northwest Region of Mexico 15 In the Late Cretaceous ilnd part of the Cenozoic¡ only a subduction zone persisted¡ located on the west­ ern margin of Baja California and marking the con­ vergent boundary between the Farallon and North American plates Generally the uplifts of Baja Ca1lfornia and northwest Mexico during the Late Cretaceous made for an important contribution of detritus directed toward the east within a general framework of eastward marine regression. LA = Los Angeles.12). This system is the manifesta­ tion of the relative movement between the North American and Pacific plates and is the southern pro­ longation of the San Andreas System.12.l. 1970). there began a right lateral movement along the growing border of both plates with a velocity of 6 cm per year (see Figure 1. This ridge divided the Farallon plate¡ now extinct¡ from the Pacific plate and is apparently formed of segments dis- placed by numerous transform faults. The opening of the Gulf of California and the development of its ridge system was initiated about 4 million years ago. GS =Guaymas¡ MZ =Mazatlán (after Atwater. Oifferent stages in the co11ision of the eastern Pacific oceanic crest. (O) 40 million years before the presento . SF = San Francisco. In üligocene time the collision of the Pacific ridge with the North American plate was initiated. 25). 7).16 Section 1 The Geology of the Mexican Republic NO' TH AMERICAN PLATE o related to the tectonic lineaments that cut the Peninsub diagonally and to the alkaline basaltic extru­ sions of the Pliocene-Pleistocene that are encountered in numerous localities. copper. Elnd to the southeast of San Fernando (locs. El Alamo (loe 9). San Isidro Point (loe lO). León Grande (loe. These deposits are found emplaced in the partially metamorphosed volcanic sequence of Mesozoic a. and Coltunbia Mine (loc. and other minerals. 4). The locali­ ties known are: La Olivia (loe. which accounts for one of the rnost important copper reserves of the nation. The intrusives that affect the calcareous sequences were emplélced chiefly in the Cretaceous. the northern por­ tion of the Baja California Peninsula can be divided into five mineral provinces (Figure 1. Mision SEln Vicente (loe 11). Their occurrence. and Sierrél San Pedro Mártir. Socorro (10e. Economic Deposits According to Gastil et al. 24). zones to the east of El Rosario (locs. San Antonio (loc. 13). as well as in the Sierra de los Cucapá. restricted to metasedimentary rocks.ge and have been attributed to hydrothermal origin related to the Cretaceous granitic intrusions. silver. Los Gavilanes and El Fenomeno (loe.13). 3). The principal localities known for this type of deposits are: Las Cruces (loe. Arroyo Caléll"najú (loe. 26). R"ncho Rosario (loc. 18). Sierra Mayor. leElds to the pos­ sibility that they have been reworked from ancient placers before their metélmorphism. The fourth province includes superficial deposits of travertine with sulfides of manganese. Desengaño (loc. The prin­ cipal known localities thElt manifest this type are: El Sueño mine (loe 1). 6). The final province includes deposits of placer gold developed in the Cenozoico The principal localities . 23). To this province also belong the deposits of the El Arco mine (loe 29). Cerro San Luis (loc. 12). 19-21). These deposits have the peculiarity of having formed in the Cenozoic near the edge of the Gulf of Californiél. The most western of these comprises deposits of mesothermal iron and copper sulfides as well as oxides of iron. 17. The second province eomprises veins of gold con­ tElined in metasedimentary rocks that are distributed along the axis of the peninsula. The distribution of these deposits is very complex Elnd the localities are numerous. (975). The third province comprises tungsten deposits rela ted to contact metamorphism of pre-batholithic calcareous rocks where precious stones may be found. stibnite. and lead as well as deposits of wulfenite. 27). • 3 ir ­ 114· 7 • eL\. tungsten¡ gold-silver¡ placer gold¡ hot springs. 2). 8). 1). - O . Figure 1. are: Campo Juárez (loe. The principallocalities are: Lucifer (loe.. Principal mineral deposits known in Baja California: copper­ iron. In the southern pan of the Peninsula of Baja California. 3) . 14) and Pozo Alemán (loe.. Mulegé (loe.1. 6 . but these are of little importance.. They are in the form of oxides occurring as hydrother ­ mal veins.9 30·_ ----. 28). Socorro (loe. 12)./ I I I I I I I I I I I I --. occurrences of mineral deposits are rarer beca use Mesozoic rocks are less exposed (Figure 1. Valledores (loe. 40). and Misión San Juan (loe. Geology of the Northwest Region of Mexico 17 .. 21).2 4 • "&. On the coast of the Gulf of California sOrne deposits of manganese exist. Los Enjambres (loe.13.¡. gold.' 5 ~_. Los Pinos and Campo Nacional (loe.14). 1­ ~ Copper-Iron Gold Tungslen Gold-sllver Placer gold Hot springs • e • O L\. 36). Real del Castillo (loe. as seen in exploratory wells drilled by Petróleos Mexicanos (Lozano. where they contain important eolian deposits. In opposition to the aboye ideas of unification. I ~ . suggesting accretionary development of the continental crust of this region. 12). man­ ganese oxides. This strike-slip zone has been proposed by Silver and Anderson (1974) as the "Mojave-Sonora Megashear. phosphorite. 4) developed in Mesozoic voicanic rocks of the Santa Rosalía area. there exist to the northeast of the zone of " mega ­ shear" Precamb(Ían metamorphic rocks. This Precambrian basement in North America shmvs a series of provinces that are older toward the nucleus of the Cl'aton. These metamorphic units llave been designated by Longoria et al. separated by alluvial valleys that beco me wider toward the northwest portion of the state.&---. The most important copper deposits are in the form of the sulfides of El Boléo (loe.16). 1979). under the ignimbrite cover. copper deposits. magnesite deposits. 1975). To describe effectively the geologic characleristics of this region. The Precambrian block located to the southwest of the "megashear" is represented by outcrops of meta­ morphics in the area of Caborca. lO. It crosses the northern part of Sonora diagonally with a northwest-southeast orientation. The Precambrian is represented by two well­ defined groups of rocks (see Figure 115): one older group composed of metamorphic rocks derived from igneous and sedimentary rocks. 11). 1ng. (1978) as the Bamori Complex.. These last constítute the major reserves of phosphorite in Mexico. Guillermo P. 5-7). Salas. two Precambrian metamorphic terranes exist that aTe of different ages and are structurclily juxta­ posed along one major strike-slip zone that originated in the Jurassic. I I I I I I I - ­ Gold Areas 01 eopper O Gypsum '1'/ Mn oxide D Tale • Areasol Mg o O defined by the General Department of Geography have been followed because they present suitable nat­ urallimits for better descri ption. In northern Sonora. 9) and Eugenia roint (loes. lhe physiographic division of provinces .vest. however. In the State of Sonora.18 Seclion 1 The Geology oí the Mexican Republic 111' o . The complex mountains are found to conform with pTe-Tertiary terranes that are covered toward the east by the piles of Cenozoic volcanics forming the Sierra Madre Occidental. 1978). This block has rectilínear limits.14. talc. 8. As far away as Sinaloa. gold-silver. and the phospho­ rite deposits of San Hilarla area. The most important petroleum occurrences are localized in Paleocene sediments in the Purísima basin. one observes as in Baja California a greal complexity of rocky out­ crops owing to similarly intricate slructure and to the great lithologic heterogeneity of the various uníts. their age has not yet been confirmed.. The metamorphic Precambrian occurs in northwest Mexico as an extension of the Precambrian shield that crops out widely in the Unlted States and Canada. Here they turn up in the form of isolated outcrops. Gold-sllver Figure 1. The Precambrian metamorphic outcrops of the Caborca area are represented by ígneous rocks and by metasedimentary rocks of greenschist and amphibo­ lite facies (Anderson et aL. represented by the Sonobari Complex (Rodríguez and Córdoba." with a left-lateral movement that is extended toward the sta tes of Arizona and California (see Figure 1. This geologic terrane contrasts markedly with that observed to the east of the Sierra Madre Occidental where the structures are more regular and the stratig­ raphy more homogeneous.- - - - - - 1 '~. rocks this oJd ap­ pear again as outcrops. Various occurrences of nonmetallic deposits exisl including the magnesite deposits found in Magdalena Bay (loes. Their metallic deposits are represented by hydrother­ mal deposits of gold and silver in the Cabo region (loes. SONORA AND SINALOA In the Sta tes of Sonora and Sinaloa. They have suggested that the existence of massive anorthositic rocks could correspond to the unificabon of two Precambrian continental areas. gypsum. 1976) and in some oil seeps of this same region. such as those • Phosphonte . particularly those of the pre-Tertiary form highly variable stra tigra phic columns in this region. both to the south and to the . Gold. marked by the sudden disappearance of Precambrian outcrops. the ta1c deposits of Comondú (loe. units of rock are exposed that ha ve d geochronologic range varying hom Precambrian to Recent. Principal mineral deposits known in soulhern Baja California (taken from the metaloge­ netic map of the Mexican Republic.. as well as in the Vizcaíno area. where the oldest known rocks of Mexico are located. and a younger group composed of sedimentary sequences of quartzite and dolomite tha t overlie díscordantIy the earlier strilta. 1978) formed during a period of from 1700 to 1800 milJion years (Sil ver and Anderson. Sonoran Desert This zone is characterized by the presence of com­ plex mountains. Distribution of outcrops of Precambrian rocks in Sonora. on the basis of Collenia algal reefs. . Originally this sequen ce was named by Keller and Wellings (922) as the Gamuza beds. with tectonic dis­ cordance. --­ --­ --­ Cd. _ _ 1 --­ 1 r · lJ . placed it in the late Precambrian.md 1700 mil1ion years and that have been correlated with the Pinal del Sur schists of southern Arizona. Geology of the Northwest Region of Mexico 19 111' I --1---­ J J I I I j I I ¡ I I I I I I I I I I I 28. The upper contact of the Gamuza Forrnntion is discordant with the overlying Paleozoic seqLlence. cropping out in the Sierra de los Ajos. A sedimentary group of late Precambrian age crops out in the Caborca area and covers. :. 1978) and is composed princi­ pally of dolomite with stromatolites (lnd with quartz sandstone and shale. 1978). whose ages range between 1600 . Obr. the metamorphic Precambrian (Longoria et al. The seguence ineludes the Pitiqui to and Gamuza formations (Longoria and Pérez. and later Stoyanow (1942). The Paleozoic sequence crops out in numcrous localities in the State of Sonora and is eomposed prin­ cipally of limes tones and sandstones tha t were .1..egon Q ~ Precambrian melamorphic rocks D Precambrian sedlmentary rocks Figure 1.15. . Thís ancient continental platfonn would be a south­ ward continuabon of the miogeosynclinal Cordilieran belt... -. lerranes underlain by rocks of ages between 1600 and 1700 million years. .. ... " ." :.-~ ·~I I .: n. t :.... ~'.. .• • iI '" j: J • • ~" :-1 ' .::'-:. " UJ$-~I j ':. deposited in él plalform environment (see Figure 1.L:. Localities of Precambrian crystalline rocks (ages established by means of isotopic studies of Anderson and Silver. " - .. . • I I I f I ' SONORA I J ---~-- --- ..:.. ". 'DPIJÓI~'J . • NavoJoa O Terrane underlaln by rocks belween 1700 and 1800 m.'-. ...ndiVidual samples Terrane underlain by rocks between 1600 and 1700 m y.. I I I I I I I I I I r " ". terranes underlain by rocks with ages of 1700-1800 million years. " " .'" . ' •• ' " ~" "'&1 ~ ~ ""1 • ".¿' '." ' • " r I I "TI~~~:':"':"-'-:. Fries ("1962) proposed lhe n<lme Sonora n Trough for this southern extension of lhe Cordilleran Geosyncline and indicated that during the whole ... • '. old o 6.-J- I I .. . Cities.y old • SINALOA Figure 1..1 _ ü r :r <t: ::> :r <t: ::> --Guaymas • Citles Locatlon 01 .. l 1" I r .':'. " .....-=.. • ." t· ' .'-~" .. ..\-.' . :" ~ _. "". 1979)....17). localities of individual samples... .16. " • + ! J . I I I I I I I I I I i J Hermosillo ..20 Section I The Geology of the Mexican Republic 114' 109' " . . .".'~.... I I I 28'~ ~I~ ~ ' !J ... ~ º ~ 2 O r I I I I t t" 4 + • ~ . "~" ." . According to Rangin (1978). Rangin (1978).24 Section 1 The Geology of the Mexican Republic U. The appearances of vulcanism in this Epoch seem to extend toward the base of the volcanic sequen ce of the Sierra Madre Occidental. 1973). .19. a t the beginning of the Tertiary an important assemblage of plutonic-volcanic rocks that is responsible for the mineralization of dis­ seminated copper developed in northeastern Sonora. where the presence of rocksl00 million years old has been reported (McDowell «nd Clabaugh. After C. 1979). whose composition varies from rhyolite to andesite. The volcanic rocks are generally related to intrusive bodies tll«t affect and mineralize them (Si llitoe.SA • Dominantly paleo-oeeanle Magmatie are of Early Cretaeeous (AlIsitos Formatlon) Magmalle are al end 01 Jurass/e (San Andres-Cedros Formation) Magmatle are 01 early Middle Jurasslc (Sonora) Cretaeeous (Aptian-Albian) 01 Chihuahua basin Coaslal basin-Paleozolc and Preeambnan Texas platform Figure 1. Schematic paleogeography of a portion of northwest Mexico during the Mesozoic. N acozélfi. Distribution of outcrops of Paleozoic rocks in Sonora.17. éllld Míssissippían Systems (Cooper and Arellano. The deposits of the Antimonio area form a marine sequence sorne 300-400 m thick. and shale in the areas of Antimonio and Santa Rosa in northwest Sonora. Devonian. and Tren. í'\nd bivalves whose age vélfies !rom Late Triassic to Early Jurassic. and Permian age.I I I í I ¡ Hermosillo I b I f ------1-I I i I ! ! I I I I I I I I I I D Paleozolc sedímenlary outcrops Figure 1. In northwest Sonora there exist calcareous outcrops of Paleozoic strata in Cabullona (Taliefferro.__ -. AH of these belong to the Cambrian (Cooper et al. 1952) and consist of sequences principally of calcareous-detritaJ lithology. Buelnil. In the vicinity of Antimonio. In the mineral district of Cananea there exists él Cambrian sequen ce of quartzite and limestone comprising the Capote Quartzite and Esperanza Limestone (MuJchay and Velasco. a nd the Sierra de Moctezuma (lmlay. a Permian sequence crops out that consísts of beds of shale and sandstone with limestone lenses and that was termed the Monos Formatíon by Cooper and Arellano (1946). Provedora Quartzite. Valentine. This sequence is cor­ rdative with the lower part of the sedimentary and . Geology of lhe Northwest Region of Mexico 21 .1. The first deposits after the Paleozoic in the state of Sonora are continental sediments of the Upper Triassic and Lower Jurassic belonging to the Barranca Formabon that crops out in the centréll and southern portions of the sta te. 1939). The two abo ve major intervélls are separated by a phase of major deformation that occurred in the Devonian.J I ! i IJ 111' 109' ¡--------L No'gales I Caborca I I I I I I I I 28' -!. Silurian. 1939).. at the time of the Cambrian and Ordovician periods there existed él grada tion from platform facies in the north of Sonora to a more iuternal facies toward the south of the state where the Paleozoic facies have a tectonic style of much llLOre intense deformabon. Small isolated outcrops of calcareous strata thélt repre­ sent parts of the Ordovician. isolated ou t­ crops of Ordovician and Permiéln exist (King.. The Pa leozoic of the Caborca area is represented in <lscending stratigraphic order by the following forma­ tions: Puerto Blanco. In the Hermosillo area and in the regio n loca ted more to the sou th. Cerro Prieto¡ Arroyos. Pennsylvanian. 1933). They contain amrnonites. and have been informally designated as the Antimonio Formation by González (1979). Sierrél del Tigre. belemnites. The calcareous facies nf the Carboniferous and Permian constitute a more homogeneous facies over al! the state. 1946) exist in the BiSélni area. Pélleozoic this arca underwent a slow élnd uninter­ rupted subsidence According to F. Ijmestone.. Mississippiéln. are exposed príncipaJ1y in the Sierra de El Alamo. 1936) as well as límestones of Devonian. There are in addition marine deposits of sandstone.. Rangin (1978). 1954. principally of andesite. In the area of Cucurpe. sandy shales and quartzose and feldspathic sand­ stones of the Morita Formation.\ es in various locali­ ll~~ uf lL<-llllL clIlu ulC:dnoL1a'l e I'vck" Ve rylni. 1978) that crops out in the hi11 of the same name located to the southeast of Caborca. In the Cretaceous. Ana. this was deposited in a swampy basin contemporaneously with the marine deposits of the Sierra de El Alamo. In the Sierras de la Gloria (Corona¡ 1979). límestones of the Mural Formation that vary from forereef to backreef. During the Late Cretaceous the territory of Sonora State underwent an uplift and general emersion resulting from a phase of compressional deformation that was active over a large part of western Mexico at the beginning of this epoch. Marine sedimentary sequences are seen to crop out in diverse localities and contain generally fossil faunas of the Aptian and Albi<1l1 Stages.18). two realms in Sonora that have clearly distinctive characteristics can be defined (Rangin. Silver and Anderson (1978) recognízed so me volcanic rocks dated by radiometric methods as belonging to the Upper Jurassic and Lower Cretaceous. The second real m. Rangin (1977a) reported a sedimentary see¡uence with volcanic intercalations that contain Jurassic ammonites. The exposures of Cretaceous batholithic bodies in Sonora constitute one of the outstanding characteris­ tics of the region. sandy shales and redbed sandstones of the Cintura FOIT(l(ltion (R<~nsome. of volcanic activity that occurred in Sonora during the Eilrly Cretaceous. Dumble (1900) designated these beds the Lista Blanca Division and assigned an Upper Triassíc posítion to them Later King (1939) assigned to them the name Lista Blanca Formation and positioned them stratigraphi­ cally in the Lower Cretaceous. These strata partly cover the volcanic and volcan­ oclastic Jurassic terranes. and Sierra Azul. The prevalence of the mag­ matism tha trema ined in the convergent zone is also evidenced by vO\c(lnic and volcanoclastic Cretaceous rocks that a1so crop out in various localities in Sonora. 1978). to the southeast of Santa Ana. Rangin and Córdoba. Mesozoic voicanic and voicanoclas­ tic rocks of prob(lble Jurassic age have been reported but unconfirmed. located in the east­ ern belt of the state¡ is composed of Lower Cretaceous marine sediments tha t afford evidence of a marine transgression coming from the Chihuélhua basin dur­ ing the Aptian-Albian (King¡ 1939. between 180 and 150 million years. 1978). and Alencaster (1961) later ele­ vated its deposits to the rank of Group.. Other marine sequences occur in the areas of Cucurpe. The absence of Lower Triassic deposits and the disconformable relationship observed within the Upper Triassic see¡uence abo ve Permian rocks in the Sierra de El Alamo reveal important tectonic movements in the region during the close of the Paleozoic and the initia­ tion of the Mesozoic. which in the central and southern portions of the state contaín intercalations of Lower Cretaceous marine sedimentary rocks (King.22 Section 1 The Geology of the Mexican Republic volcanoclastíc sequence of the Rajon Group (Longoria and Pérez. In the Cerro Lista Blanca. shale¡ sandstones. both realms are affected by compressioné11 deformation ilnd by granitic pluton­ ism accompanied by andesitic lava extrusions that become more intense toward the western pélrt of the region of the Sierra Madre Occidental. The devel­ opment of this arc has been related to episodes of sub­ duction occurring on the Pacific margin of Mexico where an oceanic plate was being subducted under the continental crust of Mexico. Gamper dll(l Lunguria. According to these authors the voicanic-plutonic activity in the Jurassic originated owing to the presence of a zone of plate con­ vergence to the west and was interrupted by the initia­ tion of lateral displacement along the so-called Mojave-Sonora Megashear. evi­ denced by numerous outcrops of andesitic volcanic and vo\canoclastic rocks (see Figure 1. Evidence exists. to the southeast of Hermosillo¡ a sequence crops out that consists of fine-grained clastics with coal beds and with limestone intercalations. 1976. a sequence crops out that constitutes the Bisbee Group. and limes tone that represent the Palmar Formation in its lower part élnd the Potrero Formation in the upper part (King¡ 1939). In the northwest of the state and southwest of Arizona. In the Upper Cretaceous. south of Hermosillo. chiefly along the eastern belt of the state and tmvard the base of the Sierra Madre Occidental. in various outcrops. a unit of andesitic volcanic rocks of prob­ ilble Early Cretaceous age crops out. Rangin. El Alamo (González. In the northwest coasts of the state. The Jurassic in the State of Sonora is characterized by the development of an important volcanic-plutonic arc with a general northwest-southeast direction. The principal igneous activity consisted of granitic emplacements that migrated in time toward the east and the extrusion of lavas that varied from andesite to rhyolite. 1939. 1979)¡ and in various localities in northwest Sonora. In this belt were developed lava extrusions. a sequence of more than 3000 m exists¡ composed of conglomerate. the region that ineludes these localities constituted a former bav in which 'Nere deposited sediments coming from the northern and northeastern parts of the sta te. Sta. This same author noted numerous outcrops in the central and southern zones of the state where volcanic rocks of the Lü\ver Cretaceous appear intercalated in marine sedimentary sequences also indicating that volcanic rocks of this epoch incre(lse proportionally toward the west and southwest. These are formed chieny of ca1careous and sandy sediments of the Lower Cretaceous. According to Alencaster (1961). In the area of San Marcial. Earlier. In sorne localities these rocks are partly affected by dynamic metamorphism and are genercdly of andesitic composition. In the ilhudripa area.19) The first of these is found in the central and western belts in the state and evolved over a permanently emergent band of Jurassic volcanic and volcanoclastic rocks. Roldán and Solano. 1978) (Figure 1. 1904. This sequence was named by King (1939) the Barranca Formation. Anderson and Silver (1978) have r port U-Pb . formed in élscending stratigraphic arder by the following: Glance Conglom­ erate. These granite-granodiorite bodies . 1~O). have obscured in large part the deformational phe­ nomena that occurred before their emplacement.18.1.r i ¡ I I I J I I I J I 1 I ! i I I I f I I I r J I I I I I I I I f I I I I I 28'--1 _ I I I J I I I I ¡ I ---1­ I D [§] Paleozoic outcrops. 1978). numerous ou tcrops exist of U pper Cretaceous volcanic rocks . some melamorphosed Jurassic andesltic volcanic rocks Figure 1. This sequence was desig­ nated by Taliefferro (1933) as the Cabullona Group and is made up of continental clastic sediments with intercalations of volcanic rocks. Geology of the Norlhwest Region of Mexico 23 111' 109' I I f - --.. There is exposed in the Agua Prieta area a conti­ nental sedimentary sequence of Upper Cretaceous Ihal covers with angular discordance the deformed units of the Bisbee Group. and contains dinosaur bones and an Upper Cretaceous flora (Rangin. Dislribution of outcrops of Mesozoic igneous rocks of Sonora. --­ -­ 1--------__ . Over al! the northeastern part of Sonora. The exposures of this c1ass of volcanic units gener­ ally form dissected tablelands that coyer in large part older terranes and geological structures. volcanic rocks of the lower Tertiary crap out \vhose composition is mainly intermedia te. the sand being a product af the action of littoral currents.o O' I . The principal volcanic eyent of the TertiélfY of Sonora consists predorninantly of ignimbrite extru­ sions of Oligocene-Miocene age that become a \vest­ ward C'xtension of the yolcanic episodes responsible for the Sierra Madre Occidental (see Figure 1. Cenozoic voIcanic rocks in Sonora. During the late Tertiary the whole regio n of Sonora was subjected to a series of normal faults that cut complctdy independently acrass élll e. At the end of the Tertiary and beginning of the Quaternary. 1900. \Ji I I I I I J I I I r i 28'- I I L __ r --­ ! ! I I I -J­ I D I<11 Basic vulcanlsm In the Plío-Qualernary Olígocene-Mlocene vulcanlsm Figure 1.o. 1939).1. é1nd Mocorito and have surrounded rocky prarninences that formed old islands. Fuerte. Geology of the Northwest Region oi Mexico 114" 111" 25 ---r-------I I r I ! 109" . Sinaloa. O I I I I I J Caborca'C <::1 t'!. t>. Culiacán. 1978). Ihis vulcanism has its clearest example in the Serrania del Pinacate located in the Altar Desert. 1980. In the central and southern part af the sta te. King. together with tectonic distension of the normal faults.nlier struc­ tural units. Pacific CoastaJ Plain This regia n is characterized by the development of a plain constructed by the evolution of a system of deltas that have advanced gradually toward the \vest. (J ~ i)'i'. This zone is bordered on the west by a shoreline that developed sandy sediments. tides. Rangin.20. and \vaves that have reworked the deltaic sediments and . These volcanic rocks vary in composition from andesítes and trachytes to dacites and rhyoJites. This phenomenon resulted in a system of northwest-southeast faults and consequent forma­ han of depressions that were fiHed by continental detrital sediments of the Baucarit Formation. concurrent with episodes of opening of the Culf of California (Clark et aL. These deltas were formed at the mouths of the rivers Mayo.20). They caver with angular discor­ dance the deformed Mesowic sequences. an impartant episode of aJkaline bClsaltic vulcanism took place. Ihis unit crops out in various localities of the state and is made up generaHy of lithic fragments of diverse composition that vary from subangular to rounded in only slightly cansolidated strata (Dumble. San Lorenz. silts. tombolos. The periods of emplacement of the intrusive bodies appear to be similar to those oecurring in Sonora. Along the eastern border of the coastal plain. Aceording to Anderson and Silver (979). This last episode of intrusion consti­ tutes the first report of Grenvillian rocks in this region of the North American craton (Anderson et al. shales. a sequence that becomes dominant eastward. The extensive outcrops of this unit disappear under the volcanic eover of the Sierra Madre Occidental. A Precambrian élge has been assigned to these roeks by previous authors (Rodríguez and Córdoba. Accord ing to Fries (962) this platform eonstituted a southern extension of the mio­ . These intrusive bodies are the units that erop out most extensively in the State of Sinaloél. extensive out­ crops of volcanic rocks are observed. The Mesozoie in Sinaloa exists as a very heteroge­ neous series of rack types tha! consists seemingly of a volcano-sedimentary assemblage which becomes a southeilstward contin uation of the volcano-volcan­ oclastic and sed imentary arc rocks of the Alisitos Formiltion of Baja California (Rangin. as well as one that is about 1100 mil­ !ion years old. In general. Their petrographic classification varies fram granite to monzonite. here appears a moun­ taín chain formed from rock units whose geochruno­ logic range varíes from Precambrian to lower Tertiary. and limes tones.lring the Jurassic in a northwest-southeast direchon. one can consider tha t these sequences were deposited in a miogeosynclinal belt that could be a southward continuabon of the Paleozoie Cordilleran geosyncline developed in the western United States. aboye which episodes of marine platform sedi­ mentation oecurred at the end of the Precambrian and d uring the Paleozoie.. Tectonic Surnmary The outcrops of Preeambrian metamorphic rocks in northern Sonora eonstitute one of the most character­ istic fea tu res of this region. In addition. in sorne localities they have been affected by various grades of meta­ morphism.21). periods of igneous intrusion between 1410 and 1440 mili ion years ago have also been recognized. AH the Mesozoie volcanic and sedimentarv assem­ blages are affected by the emplacement of Mesozoie and Tertiary plutons. The stratigraphic relationship between this sequence and the Sonobari Metamorphic Complex hilS not been observed. According to Rodríguez and Córdoba (1978). These metamorphic tenanes make up the base­ ment. they are present aboye the intrusives in the form of roof pendants or of windovvs under the Tertiary eover. During the Tertiary. There are both lavas and pyroclastics whose composition varies from acidic to bilSic and thilt show the effects of both regional and contact metamorphism (Figure 1. It appeéus that the most ancient rocks that crup out in the State of Sinaloa are the metamorphics in the Sierra de San Francisco to the north of Los Mochis. chief among them are those that oeeurred in the middle Tertiary and that gave rise to the ignimbrite cover of the Sierra Madre Occidental. The major part of the ealcareous rocks that crap out in Sinaloa are seemingly of Cretaceous age. Outcrops of these rocks are isotated. which were united befare the Pliocene. these sequences were d eformed and me tamorphosed 1650-1660 million years ago.. and hooks. 1978). The Mesozoic sedimentary rocks are represented by sequences of limes tones that in some localities are observed to be partialJy metamorphosed. Rodríguez and Córdoba (1978) report the dis­ covery of the fuslllinid Millerella sp. which indica tes that the lower part of the sequence is probably of Late Mississippian to Early Pennsylvanian age. these metamorphie rocks form two magmatic and orogenic belts with a northeast-south­ west orientation. According to rad iometric da tes obtained by Anderson and Silver (1978). This ignimbrite sequence covers () lc1fge part of the Mesozoie rocks of the eastern border of the Pacific coastal plain and the intennediate and basic volcanics of the lower Tertiary. These units are partially covered by the vol­ canic sequence of the Sierra Madre Occidental. These sequences are eomposed principal1y of sand­ stones. the gneisses were derived from sandy and argillilceous sedimentary rocks with possi­ ble intercalations of basie lava that have undergone at least two metilmorphic events. 1978). These emplaeements migra te in age from Cretaceous in Baja California to early Tertiary at the border of Sinaloa and Chihuahua (Sil ver and Anderson. Along the eastern border of the Pacifíe Coastal Plain there exists a series of isolated OlltCropS of mod­ erilte extension that are of marine Paleozoic roeks. These two orogenic belts form part of the Precambrian terranes with orientation similar to that eneountered in the sou th west portian of the N orth American craton. 1978). These authors indicate that the Paleozoie sequenees of Sinaloa were deposited in shallow water platform cond itions. ]978). with biotite and homblend as principal mafie minerals. However.. and it is apparent that the contact with Mesozoic rocks is generally tee­ tonie.26 Section 1 The Geology of the Mexican Republíc caused formation of bars. important volcanic episodes oceurred in the State of Sinaloa. The history of thc pre-Tertiary terranes tha tare exposed on the eilstern edge of Sinaloa share many affinities wíth the tectonic and paleogeographical styles that prevailed in Sonora and B<1ja California. but Rodríguez and Córdoba (978) consider that some of these rocks could be Jurassie. De Cserna and Kent (1961) termed these rocks the Sonobari Complexo The unit consists of intercalations of muscovi!e and biotite gnciss with amphibolites. but the contaets are not clearly observed. intrusions by bodies of gabbro and granodi­ orite are present and development of pegmatites and migmiltites is observed. or perhaps older. In sorne loealities they are apparently intercalated within the Mesozoic metavol­ eanie sequenee. The eastern border of this zone is made up by the foothills of the Sierra Madre Occidental. truncated and juxtaposed by move­ ment a10ng a 112ft lateral zone of slippage thilt was active d1. Geology oE the Northwest Region of Mexico 27 26'.1. He desig­ nated this area the Sonoran Trough. with some interruptions marked by hiatus in the sequences that crop out in Sonora and Sinaloa. In the Late Triassic-Early Jurassic. Outcrops of igneous rocks in Sinaloa. cmd the partial fusion of a plate at the leve! of the asthenosphere. a period of gentle folding occurred. and shales accumulated (Alencaster. the Mojave-Sonora Megashear of Silver and Anderson (1974). On the one hand. sandstones. Fries (1962) considers that at the end of the Permian. In contrast. 1961). This trough underwent a slow subsidence during the whole Paleozolc. geosynclíne of the Cordillera n Geosyncline. the ancient bay of Antimonio existed in the present north­ western part of the state. The second phase corre­ . The magmatic activity related to this arc is interrupt­ ed only by the development of a left-lateral strike-slip zone. two paleogeo­ graphic elements provided the setting for processes of sedimentation in Sonora. located southeast of Hennosillo. derived from positive areas. tbere existed the swampy basin of San Marcia!. originated the construction of a magmatic arc that was active during the Mesozoic. gypsiferous lime­ stones. resulting in uplift and block-faulting that destroyed the earlier geosyn­ cJinal pa ttern. Overlying the sedimentary seguence of Antimonio there is él package of volcanic and volcanoclastic rocks. The first of these occurred at the beginning of the Late Cretaceous and is indicated by angular discordance between the sandy calcareous Lower Cretaceous sediments and the detrital conti­ nental sediments of the Upper Cretaceous that crop out in the Cabullona basin.­ 24' - D Icr~1 Volcanics-volcanoclastics of Mesozoic age Cretaceous plutons Figure 1. The setting of convergence of the Paleopacific and North American plates is developed in two principal phases of deformation whose relationships can be clecHly observed in the Cabullona area to the south of Agua Prieta and Naco. This volcanic activity has been attrib­ uted by many authors to the presence of a zone of convergence located to the west. in which a thick marine sedi­ mentary sequence accumulated to the east. The subsidence of a paleopacific plate beneath the continental crust of Mexico. These together with the Lower Jurassic volccmic intercalations crap out in tbe Cerro Rajón southeast of Caborca and indicate the initiation of volcanic activity in the Mesozoic.21. in which beds of coal. Culiacán. and in place~ some plutonic rocks. whose principal period of construction W(1S ignimbrite activity occurring in the late Oligo­ cene (McDowell and Clabaugh¡ 1979). deposits of copper and molybdenum form resources of the cop­ per-bearing porphyry type such as occur in Santo Tomás-Cuchicari and Tameapa. élnd El GuaYélbo. The principal localities with these types of deposits are: C<1nanea. San Ignacio. They contain gold. The zones of GU<1dalupe élnd Céllvo. Principal locéllities with these types of deposits are El Tigre. Lampazos. Tungsten is an element with significant occurrencE' in the zones of contact metamorphism in the State of Sonora. In the State of Sinaloél (Figure 122). Pánuco. the second is found élssociated with coal and is present élS intercalations in the paludal sequence of the Barranca Group of the Upper Triassie. ln this epoch the most important plutonic emplacements of the region occurred. Genera]]v it occurs in the form of the mineral scheelite and on' occasions is found associated with metasomatic deposits of copper. intrllsives of the same type.inilloa. activity of the convergence zonc to the west seems to have ceased and the development of the Culf of Ccdifornia was initiated. the conti­ nental history of Sonora and Sinaloa comrnenced. The origin of this order of resources has been attribllted by Sillitoe (1975) to the partial fusion of the oceilnic crust under the continent and the li1ter ascen­ sion of magma tic material with solutions rich in cop­ per and molybdenum that formed "stockwork" deposits in roof pendants of large plutons and breccia pipe deposits. and in sorne cases sedimentary rocks of marine origino The chid reserves of copper porphyry in Sonora are encOllntered in the areas of Cananeél and Nacozari Oocalities 1. The enclos­ ing rocks of tbis class of deposits are generally andesites at the base of the vo!canic sequence of the Sien<1 Madre Occidental. and Plomoso. Also in this epoch the first volcanic episodes occurred that form the base of the Sierra Madre Occidental. According to Rangin (1978) there seems to hélve developed between the Late Jurassic and the Early Cretaceous a phase of deformation. The eastern border of the Sierfé. At the beginning of the Li1te Cretaceolls. other more minor deposits are locélted filrther south and west of these localities (loe. 5). In the Miocene. Epithermal veins predominate in this belt. and collapse breccias ilssociilted with deposits of copper-bearing porphyry (Echavarri et al. and zinc.. The latter site contains the richest gold mineral district in the nation. San Felipe. These become youngcr east­ wc1rd. This is aCC0111­ panied in the adjacent regions of Sonora and Sinilloa by d istensive tectonics consisting of horsts and grabens that "vere active into the Quaternary and are responsible for the present-day distribution of the chicf orographic elements of the Sonoran deserto The setting of this type of tectonism gives rise to the deposition of great thicknesses of conglomera tic continental sediments. especially along its flanks <Figure 1.28 Seetion 1 The Geology oi the Mexiean Republie sponds to the compressive deformabon from the end of the Cretaceous to tbe beginning of the TertiMY. élnd 3). which are most important in the State of S. silver. La India. Las Chispas. as well as the regions of Sinaloa de Leyva. La Guadalupana. 2. Bahuitil. These !atter veins are part of a belt that runs along the eilstern half of the state and that includes. they can be observed on the western flank of the Sierra Madre Occidental as well as in the overthrusting of the Lower Cretaceous and Paleozoic sequences aboye the Upper Creti1ceous Cabullona Group of tbe Naco ilnd Agua Prieta region (Rangin. Economic Resources In the State of Sonora (Figure 1. The principill rocks encompassed by this minerillizil­ tion me Cenozoic volcélnic rocks of intermedia te com­ position. El Tecolote. deposits of Ihe western borders of Chihuahua and Durango. There are important amounts of lead. The first mineral is of hydrothennal origin and consists of veins that are exploited princi­ pally in Esqueda and Santa Rosa. Deposits associated with stocks or veins of quartz with the presence of wolfr<1mite and tungsten are found in the mines of El Magistral. the Bilucarit Forrnation. still not well known in Sonora. 1977b). Deposits of molybdenum in stock­ work form exist in the mines of Los Chicharrones and Las Higueras and breccia and hydrothermal vein deposits occur in El Magistral (loe. zinc. Sierrél de Cabullona. Lampazos. The gold élnd sil ver deposits are located at shallow levels in hvdrothermal veins Ihat contain the <1bove-rnentioned r~serves of lead and zine. and San Javier. lead. further­ more. 4). 1977). The deposits of lead and zinc in the State of Sonora are present in zones of rnetasornatic replacement and in hydrothermal veins. and silver in deposits found in hydrothennal veins. A major parl oi the first are of Laramide age while the second are generally associat­ ed witb middle Cenozoic volcanic rocks (Echavarri et a1. Guadalupe de los Reyes. SIERRA MADRE OCCIDENTAL The Sierra Mild re Occidental is formed from an extensive vo!canic meseta affected by normal faults and grabens that detract from its homogeneous and pseudohorizontal appearance.. and Tayoltita con­ tain these types of deposits. The origin o{ most of these resources has been attributed to the emplacernent of granite and granodiorite porphyries that occurred at the end of the Late Cretaceous and the beginning of the Tertiary. The 1110St irnportant nonmetallic deposits me fluo­ rite élnd gr<1phite. Siln José del Desierto.22). This deforméltion originélted foJds with a northwest­ sOlltheast axial direction. The most important deposits of tungsten are found near Baviacora. the most important resoUfces Me the deposits of copper and molvbdenum that are chieflv located in an eastern belt 'of the state.23). Las Pastillas. 1977). and San Javier. Rosariltilo. that correlates with the NevadclJ1 orogeny developed in North America. zinc.'l grades into the Basin and Range Province of . Toward the north. The lowE'r volcanic complex is dominantly in the form of lava flows and pyroclastic units and also con ­ tains intercalations of siliceous ignimbrites.brites in some localities approaches . ". The more recent is Cümposed of rhyolitic élnd rhyodacitic ignimbrites in generally horizontclI position or slightly inclined and whose ages vary between 34 and 27 million ye<lrs.O La Caridad O:> . because of its slightly deformed character and intense faul ting élnd dist\. • • GUA. 1975). W.O -~.1. I I . Chihuahua. The outcrops of this lower complex are. whose contad marks an inter­ med ia te period of volcanic calmo The older sequence is formed main)..1.O . G. Cu·Fe Cu-Ag • I I ---0­ I J La Re10rma Au-Ag._ o O Cu-Mo CIJ.22. This lower complex contrasts in large me<lsure with the upper one. the host rocks for the principal mineralization of él large port of this region of Mexico. while the western border consists of an abrupt termination with normal faults of major dis­ placement and zones of deep borrancas (steep-walled canyons).I. Salas.. • • • I I So. this blanket has its lasl out­ crops about <lt the border with lhe United States and to the SOLllh it disappears beneath rocks of basic élnd intermediate composition that make L1p the Neovolcanic axis. The upper contact displays <ln irregular surface \vith strong relief and shOl'vs marked contrast in the degree of alteration of the sequences. in general. Ag O D.Hbed aspecto The sequences that comprise it ore....O - . lhe thick­ ness of these ignim. as expected.l:. I />. Geology of the Northwest Region of Mexico 114 3Z' . more restricted than those of the upper but have been recognized over aH the slope toward the Pacific in Sonora and Sinaloa.! '" I • 4- Note Slze of svmbol denotes ItS Impor1ance . Accord ing to McDO\vell and Clabaugh (1979). HERMOSILLO D.W-Mo • O f:¡J • A.1. .~ I ..\.e I .P.- 29 111 --JI NOGA S 108' -L I I O eCADORCAj O 30' ­ O : Cananea . O Sb Cu-Au Fe .je Moradlna\. Jose ¡ • . Known mineral deposits in the States of Sonora and Sinaloa (taken from the metalogenic map of the Republic of Mexico. O L [j 28' - • • /:. Pb-Zn-Ag Au Graphlle • __ I ~d> __ 0D.YMAS_D.. fmm voIcanic rocks of intermediate composition.­ tJ 2 N_coz". According to Demant <lnd Robin (975). the Sierra Madre Occidental is composed of two impor ­ tant igneous sequences.. a~d igneous bodies whose ages vary between 100 and 45 million years. Figure 1. The upper complex constitutes the most continu­ OLlS dnd extensive ignimbrite cover in the world and is observed to form an area elongated northwest ­ SOll theast abOLl t 250 km brocld and more than 1200 km long. when the system of the expanding eastern Pacific impinged against the western margin of Mexico . The lower volcanic complex constitutes a typical calcalkaline magma tic arc related to a convergent con­ tinental margin where the Farallon plate is buried under the continental crust of Mexico. more than 1000 m.23. McDowell and Clabaugh consider that the number of calderas originated during the emission of this great volume of rack ought to have been between 200 and 400.30 Sedion 1 The Geology of the Mexican Republic Figure 1. although the semicircu­ lar configuration today is obscured by the presence of normal fauIts and recent alluvial deposits. This phenome­ non of convergence lasted until 29 miJlion years ago. Many of these have a diameter greater than 40 km. Volcanic cover of the Sierra Madre Occidental. H. Selecciones de Scientific American.. A. 27. 1979. 1921. Mexico.. but the discussion is cornplemented by consid­ eration of the whole Chihuahua mea. p. Anderson. Estados de Sinaloa y Sonora. 29. Darton. N. Geological Society of America Memoir 140. 30. no. PE.T. V. and C Robin. Corona. 88. 606-611. 1946. V. (Original not consulted. 1970. Implications of plate tectonics for the Cenozoic tectonic evolution of western North Arnerica: Geological Society of America Bulletin V. Demant and Robin (1979) explain the origin of the ignimbrite blanket as the typical vulcanism of a rift zone behind an andesitic arc caused bv the reaction of the crust to the subduction moveme~ts and indicate the coexistence of compressive and distensive vuJcan­ lsm. and AR. 70-83. San Francisco y Londres.. Geomimet. Sonora. Geologic reconnaissance in Baja California: Journal of Geology.G.. Nevertheless. p. 1970).. 1-28. 1980) With respect to the hydrothermal veins. 1975. 1975.) De Cserna. p. T. v.. northwestern Sonora. E. 138 p. includes areas with lead. v. T. Mapa Metalogenético de Estado de Sonora: Revista Geomimet 2a Epocha July-August. Arellano. Mexico. The role of the Mojave Sonora Megashear in the tectonic evolution of northern Sonora. Kent. and L. northwest Sonora. La FaIJa de San Andrés. T. V. . p. 27. The larger part of the mineral masses that are locat­ ed in the Sierra Madre Occidental are strictly related to the lower volcanic complexo The copper-bearing porphyries of Cananea and Nacozari are related to the episodes of intrusive emplacement at the Cretaceous-Tertiary boundary and the hydrothermal deposits belong to a period that fluctuates between 49 and 28 miUion yea rs (Clark et aL. AO. BeaJ.. Guidebook Field Trip 27.R.A. L.. Sonora. Mexico: Geological Society of America Abstract with Programs. UNAM.H. 81. Geology of Northern Sonora: Ceological Society of America Annual Meeting in San Diego. Demant. AAPG Bulletin.H. Dickinson. Anderson. 720-748. Geology of the Northwest Regíon of Mexico 31 (Atwater. Silver. Silver. CH. and silver. no. il1 W. p. Mapa geólogico de reconocimiento y secciones estructurélles de la región de San Bias y el Fuerte. p. junlssic mag­ matism in Sonora. Saitz. V. 4-6. 59-66.T. cartas geo­ logicas y minerales. cited in F. Guidebook Field Trip no. 3513-3536.A. TH.. 359. and M. UNAM. Cooper C. una síntesis en relación con la evolución geodinámica desde el Cretácico: Revista Instituto de Geologia.T Silver. Gastil et aL. Lozano. Boletin Asociación Mexicana Geólogos Petroleros. Echavarri. p. Cooper. 170 p. F. Preliminal)l reconnaissance geology of Sierra La Gloria and Cerro Basura. Damon. and B. no. 1980. Mexico: Smithsoniéln Miscellaneous Collections. Las fases del v01­ canismo en México. Dumble. and L.. 1977. Paleontología Mexicana 11. Geology (lf Northern Sonora: GeologicaJ Society of America. p. Gloria. K. 41-58. in Tuzo Wilson. northwest Sonora. Evaluación petrolifera de la península de Bajél California.. G. zinc.1. 18 P Anderson. 49-71. UNAM. 184 p. parte 1. p. BIBLIOGRAPHY AND REFERENCES Abbreviation UNAM is Universidad Nacional Autónoma de México Alencaster. Instituto de Geología.L. 1976. Schutte.R. on the east side of the Sierra. v. the jnterruption of magmatism in the interval of 45 to 34 million years demonstrates a break in the continuity of these processes. Célmbrian stratigraphy and paleontology near Caborca. Mexico. The first of them. such as Santa Barbara and San Francisco del Oro. et al. Magmatismo en el norte de México en relación con los yacimientos metalíferos.H. 1948. Hermosillo. Deriva Continental y Tectónica de Placas. UNAM. Freeman and Company. 4.A. 1978. 1961. Consejo de Recursos Minerales. p. Instituto de Geologia. 119. 1900. such as those of Tayoltita and San José de Grácia. Stratigl'aphy near Caborca. 5-34. 1979.) Clark. Salas. 10.G.. Reconnaissance of the geology . Eels. no. 1952. Annual Meeting in San Diego. located to the west. Instituto de Geología. W. 106. The second belt. comprises the deposits of gold and silver of Sinaloa and Sonora. W. 106-303. Notes on the geology of Sonora: Geological Society of America Bulletin.. 1971. Atwater. McDowell and Clabaugh (1979) consider that this period of calm has two possible causes: one of these is a lowered amount of convergence or a change of indinaban of the plate being subducted. Shaffiquillah. Anderson.B.F. J. D. 11.. (Originéll not consulted. 1961. Estratigrafía del Triásico Superior de la parte central del Estado de Sonora.. It is thus conve­ nient to indicate some generalities related to this subject. Rocas Precárnbricas y Paleozóicas de la región de Caborca. These same authors do not set forth a satisfactory tec­ tonic explanabon for the sudden voJcanic activity indicated by the upper complex and the bimodal character of this volcanic secluence when compared with a silica and anorthosite standard.lI1d oil possibilities of Bélja California: Geological Society of America Memoir 31. p. Plate tectonics and the conti­ nental margin of California. A. Libreto Guia del Primer Simposio sobre la Geología y Potencial Minero del Estado de Sonora.T. The principal mineralization within the Sierra Madre Occidental was partly discussed in the sections aboye. Prentice Hall. México. ed. 1978. S. The Geotectonic Development of California: Rubey volume 1. 1974.H. cited in R. belts are encountered situ­ ated on both flanks of the Sierra Madre Occidental. p. p. 122-152. 75.. and C.163-179. the other is the subduction of an active oceanic ridge. Ernest. Recon­ naissance geology of the State of Béljél Californiél. Revista. 1979. Z. J.) McKenzie. and L. 1956. Geología del Distrito Minero de Cananea. con énfasis en el Paleozóica: Boletín Asociación Mexicana Geólogos Petroleros. p. 119-144. 1956. Excursion A-7 of the XX Congreso Geológico Internacional.Elston. cited in Re. p. Geologicéll Society of America Bulletin. F. GastiJ et al.e. UNAM. and P. 17. México: Sedimentary Geology. Sonora. Sonora and Chihuahua.T. Geology of the Sierra del Alamo. The Geotectonic Development of Californi<l: Rubey volume 1. J..J. Bosquejo geológico de la región de Santiago. ]06-303. Morg<ln. cited in F. Krummenacher... R. and S. Wellings. R. Lozano. Gastil. Sonora: Cía Petrolera el Aguila. (Noroeste dc Sonora): Bulletin Department of Geology.A. Gamper. M. 1978. SA. Anderson. F. 1982. México. J. p. p 11-42.CS. The tectonic history of peninsular California. nos. Geological reconnaissance in north­ ern Sierra Madre Occidental of Mexico.A. 117. 50. Guidebook Field Trip no. Hermosillo. Instituto de Geología.F. 1979. 1978. Gcological Report no. 1. Mendoz<I. McDowell. p 61-67 López-Ramos. R. 1979. and J. Notes on the TertiiHV of southern Lower California: Geological Maga~ine. H. .. M. 19. v. 1980. Prentice Hall. México.) Ortega-Gutiérrez. v. and P.A. Clabaugh. p. Bosquejo geológico de la parte sur de la península de Baja California. Mina. FW. Pa tterson. E. p. 117-120. ed. Arizona: AIME Transactions. 14-15. Instituto de Geología. J. 2.. in e. 1977. 1. Pantoja-Alor. Clnd V. v. 224. Geological Society of America Annual Meeting in San Diego. 1954. 1978.W. UNT­ SON. 57-70.T. Mulchay. p 1723-1744.. Evaluación petrolífera de la peninsu ­ la de Baja California. Reconocimiento de las terrazas mari­ nas Cuaternárias de la parte central de B<ljél Cali­ fornia: Revista. The migra­ tíon of the axis of Pacific margin magmatism across Baja California. Baja California: Boletin AsociaCIón Mexical1é1 Geólogos Petroleros. p.38 p. Silver. México Boletín Asociación Mexicana Geólogos Petroleros. 1978. eds. Ortlieb. v. Célrrillo-Bnwo. ]939. Abbott. (Original not consulted. Sedimentary rocks at Canélnea. Ashflow Tuffs: Geological Society of America Special Pelper 180. no. and V. Geologí<l de México. 2a edi ­ ción. Evolución magmática y metamórfica del complejo cristalino de La Paz. and W.J.. Keller. p.G. Chapin and W . p. Bosquejo geológi­ co de los cerros Chino y Rajón. 1975. R. 63-64. DP. v. v. Krummenacher. and J.W. p 139-269. 13. Geology of Northern Sonor<l: Geological Society of America Annual Meeting in San Diego. 1-11. Ernest. 257-273. J. Hermosillo. 1.].. 9. Bioestratigrafía y f<lcies sedimentárias del Cretácico Inferior de Sonor<l: Resúmenes de la V Convención Geológica Nacional. Mina. F. 1976. The Valle Formation: physical stratigraphy and depositional modeL southern Vizcaíno Peninsula. México D. 1979.. Gastil. (Original not consulted. (unpublished). J.E. in W. Mina. Unpublished Report of the Faculty of Engineering. Tgnimbrites of the Sierra Madre Occidental and their relation to the tectonic history of western Mexico. Consideraciones estructurales en el cUéldrángulo Pitiquito-La Prim<lvera. Morgan. Allison. Baja California Sur: Field Guides and Papers of Baja California: Geological Society of America Annual Meeting in San Diego. McEldowney. Gonzé1lez. v. cited in T.. E. San Hipolito Forrnation: Triassic marine rocks of the Vizcaíno Peninsula.. B. v. p.. Mexico: Geological Society of America Abstracts with Programs. 2. A. 1962. n. p. 27. Gastíl. e. 1922. México. Finch.. 170 p. 1957.óicas de la región de Caborca. Paleogeographic studies in north ­ eastern Sonora: Geological Society of America BuUetin. Chihuélhuél. Vizcaíno Peninsula. D. RC. and D.R Velasco. Reseña geológica del Estado de Sonora. Conzále¿. and FE. p 5-34. 1978. p 1625-1722. 285-305. ReconnaisSclnce geology of the State of Baja California: Geological Society of America Memoir 14. D.L. p.E. p. p. Petrology of a Triassic marine scction. San Jose del Cabo. v.: Resúmenes de la VI Convención Geológica Nacional de la Sociedad Geológica Mexicana. Peréz. 1978. 27. UNAM.. 50. p 125-133. 90. F.W. 200-211.32 Section 1 The Geology ai the Mexican Republic FlIlCh.. Libreto Guia del Primer Simposio sobre la Geología y Potencial Minero del Estado de Sonora.L.E. 14. Instituto de Geología. UNAM. p. C. Bosquejo geológico de la parte sur de la peninsula de Baja California.. Mexico with the sections at Bisbe and Swisshelm Mountains.) Tmlay. F. Longoria. cited in Cía Minera Cananea.. Pércz. México.. 1922. v. B<lj<l California Sur.]. UNAM. and D. 628-632 (Original not con­ sulted. Rocas Precámbricas y Paleo.73-76.. 1979.F. Bosquejo geológico de la parte sur de la penínsulél de Béljél California: Excursion A-1 of the XX Congreso Geológico Internacional.B. Sonora: Instituto de Geologia.¡. Boletín Asociación Mexicana Geólogos Petroleros. 1975. Evolution of triple junctions: Nature. C. no.H. no 2. UNI-SON. p. and F. W. L.G. UNAM. 3 volúmenes. 180. Longoria. p. v. 1984 Estudio geológico del áreél de Santa Barbara.. 2. 11-42. 1-2. Sonora. An occurrence of Paleozoic fossils in Baja California. L.. R. Fries. Longoria. Pessagno.G. F.A.) King. R. Sonor<l. p. 1939. Heim. 23-31. 1969. 1979. Reconnaissance geoIogy of the State of Baja California: Geological Society of America Memoir 140. 170 p. and E. 1966. Guia del Primer Simposio sobre la Geología y Potencial Minero del Estado de Sonor<l.. 199. Field Guides and Papers of B<lja California. v. cuadrángulo Pitiquito-Lel Primavera (NO de Sonora): Bulletin Department of Geology. 1981. Edición escolar. Resúmenes del Primer Simposio sobrc la Geología y Potencial Minero del Estado de Sonora. Abbolt. 253-273. 1970.. (Original not consulted. Eells. Márquez-Castañeda. RP PhiUips. v. 4-6. 5. Rodríguez. p. C. Instituto de Geología. C. v. v. 1980. 117-118. 44-47. and D. Instituto de Geología. UNAM. México.. 19-26. 1978.) Santillán.. 17 p. W.. 53-86. Geology of ¡he Northwest Region of Mexico 33 Rangin. Resúmenes del Primer Simposio sobre la Geología y Potencial Minero del Estodo de Sonora.. 1979. Mexico: Geological Society of America Bulletin. Ransomc. 1976. Consideraciones sobre el Paleozóico sonorense: Resúmenes del Primer Simposio sobre de Geología y Potential Minero del Estado de Sonora. . Contribución a la estratigrafía de las rocas volcánicas del Estado de Sonora: Bu1Jetin of thc Dcpartment of Ceology. L. México: Revista. The tops and bottoms of porphyry copper deposits: Economic Geology. 1-37. 112. Salas. Consideraciones sobre la evolu tión geológicas de la parte septentrional del Estado de Sonora.H. Rangjn. Anderson. UNAM. Barrera. C. UNI-SON. Sonora: 1nstituto de Geología. (Mapa con texto. R. F. v. eds. Stoyanow.. F.A Córdoba. An occurrence of Upper Cretaceous sediments in northern Sonora. LT. Rangin. Carta y provincias metaJogenéticas de la Republica Mexicana.. p. v. 1978. 1.H.C. and TH Anderson.. Sobre la presencia del Jurásico Su perior con amon ita:. 41. G. R. A reconnaissance of the Mexican porphyry copper belt. 1936. v. Las posibilidades petrolíferas en lo costa occidental de la Baja California. p. 1978. N. p. entre los paralelos 30 y 32 de la ti tud norte: Anales de Instituto de Geología.. p. 199 p. Sillitoe. 35-56. 68. Mesozoic mag­ matism and tectonísm in northern Sonora and their implications for mineral resources. 1974. Sonoro. Geology of the Cananea Mountoins. Silver. 1978. Evidence for superimposed subduc­ tion and colljsíon processes during Jurassic­ Cretaceou:. 6. and Solano B. Possible left-lat­ eral early to rniddle Mesozoic disruption of the soutl1western North A01erican craton margin: Geologícal Society of America Abstracts with Programs. Sillitoe. UNA M. UN AM. Libreto Guia del Primer Simposio sobre la Ceología y Potential Minero del Estado de Sonora. 53. Valentine. v. Hermosillo.T. Arizona: US Geological Survey. Atlas geológico y evaluación geológico-minero del Estado de Sinaloa: Instituto de Geología. F. UNAM y Secretaría del Desarrollo Económico del Estado de Sinaloa. and T. Consejo de Recursos Minerales 2nd Edición.. and D. 1. Paleozoic paleogeography of Arizona: GeoJogical Society of America Bulletin. Description of the Bisbee quad­ rangle. 47. UNAM.P. 1933. 1977a. v. 702 p. Mexico: Journal Geology.H.. A. 1978. 955. time along Baja C¡:¡lifornia continental borderland. p.L. en Sonora :.A. Field Guides and Papers of Baja California: Geological Society of America Annual Meeting in San Diego. p.. p. 1.. 1942. 1973. M. J. Sonora. Roldán.35-56. HermosilJo. Hermosi1Jo. Córdoba. 709-715. 1-4. p. and T. v. v.. Rangin. 1904.. 1930. R. p. 1977b. p. Instituto de Geología.1.. p. Extensión de la Cuenca Cretácico Chihuahuense en Sonora septentrional y sus deformaciones: Memória del Tercer Congreso Latinoamericano de Geología. p. Tectónicas sobrepuestas en Sonora septentrional: Revista. Rangin. 12-37.epten trional. Taliefferro. 1255-1282. 37-52 Rangin. 14 p. 1975. v. Sonora: Instituto de Geología. C.. Silver. to the east.. and to the south.. /.. In vorious forms. 35 Additíonal1y... the provinces of Sierras and Plains of the North (Basin and Range). the Great Plain of North America... They originated as local filled basins termed bolsones. according to the physio ­ graphic division of the DGG (see Figure 1.. precipitation records reveal that the clj­ mates vary from dry to semi-arid in the west of this zone and from humid to subhumid in the Sierra Madre Orientol and the Coastal Plaín of the northern Gulf of Mexico./ GENERAL CONSIDERATIONS For description of the northern and northeastern regions of Mexico. . ~.) . (~ . . the Sierra Madre Occidental. Nevertheless. .. the Sierra Madre Oriental.. These mountains make up prominent topographic peaks that oecur separated by great plains that were elevated as thev were filled. The region comprises. •• /'" . and regular summer rains oceur.- .• 2. .>·. ~.. the Central Mesa. The folded sedimentary sequenees gradually disappear toward the western margin of the sta te. Geology of the N orthern and Northeastern Regions of Mexico • ( / TORREaN <. CHIHUAHUA AREA General Geology The area of the State of Chihuahua is eharacterized. the division that is used is based fundamentally on the paleogeographical elements of the Mesozoic in this part of Mexico. the Gulf of Mexico coast. . and the Coastal Plain of the northern Gulf of Mexico.. to an edge under the ignimbrite cover of the Sierra Madre Occidental. MONTERREY "..\ . ".. / . The clima te of aH the region varies in general from hot to temperate.1). ~t NORTHERN AN'O NO~THEASTERN • 'REG!QN .--. particularly in its eastern part.. \./ ) ' . by the presenee of fold­ ed mountains formed from marine Mesozoic strata... These are tectoníc troughs filled \vÚh continental sediments and sorne lava debris blocks.:" ~'\. POTOSI .>. the northern edge of the Neovolcanic axis. one can use the following natural limits: to the west. <.. these elements have a general correspondence with the physiographic provinces mentioned aboye.... "'.. MATAMOROS . SAN LUI:. chíefly in the orígín of orographic forms thot are particular expressíons of the types of geologieal phenomena that generate them. sins on \·vhich were deposited respectively carbonates and terrigenous sediments. Gonzáles (1976) considers tha t the outcrops of limes tones and dolornites of the lower Paleozoic of Chihuahua reflect a platform environ­ ment similar to the focies deveJoped across the North American craton and considers it logical that this ete­ ment continucs toward Mexico. in addition. which crop out in the majc)f pclft of the arca. the sedimentological pattern prescnts more irregular con­ ditions caused by the action of block faulting that devcloped intracraJonic platforms and ba. its southwest border forms a rnarked lineament thélt coincides approximately ""ith the course of the Rio Grélnde along a belt from Ciudad Juárez to üjinilga.1).­ 26' D I Sedlmenlary rocks Figure 2. at the Mexican irontier. of the üUélchita belt composed of deformed Paleozoic ter­ rigenous sediments.nd co-workers (1983) obtained a K­ Ar c1ge corresponding to the Grenville from ct meta­ morphic block included in a Permictn sequence. DeFord (] 969) makes note of the suelden disappearance.. This tectonic feature has main­ tained its influence on sedimentological events and deformation during the Mesozoic and even the Cenozoic. Paleozoic sedimentary rocks of Chihuahua. The Paleozoic rocks that crop out widcly in sorne areilS of Texas have in northeastern Mexico very restricted exposures wüh the result that it is difficult to reconstruct the paleogeographic elements of thot c1H'a (Figu re 2. rcported the exposure of a sírnilar metcllnorphic unit to the south of Mina Plomosas that could be an outcrop of the Chihuahua Precambrian basement. Quintero and Guerrero (1985)..1. developed aboye a Precambrian and l'aleozOlc basement that is exposed in some localities and that also héls been reported in wells drílled by Pemex. ..36 Seclion 1 The Geology oi lhe Mexican Republic The folded sediment<uy rocks. In the area of the Sierra del Cuervo..í . Ihis leads onc to believe that this 108' I 105' 1 CO. Mauger a.. he indi­ cates tha! in the Pennsylvaniiln-Permian interval. In contrast.. The Diablo Platfonn dates from this time. JUAREZ 1 I 1 1 I 1 I I I --------- I I I I 1 ----t I j 1 I I - - - - '30' bJINAGA· I I 1 CHIHUAHUA • I I I I I I ·-1-·------­ I I I • ¡ PARRAL] I I ----L~ I . Nevertheless. Economic Resources Aecording to the mi)p of Meléllk)bl'níc Provinces of the Mexícan Republie (Sélléls. respectively (Gries and Hélcn~gi. In the latter well Ordoviciéln.11 lhis region remélincd ('ni 'r­ gent duríng most of Lélte Cret¡ll'cous lime. The dilliculty in defíning the Paleozoic tectonic e1e­ ments in Chihuahua is Glused by the very scarce out­ crnps ano the fact thélt this regio n encompasses the confluence of the North Ameríc. mainly to the llorth of Sierra de Si1Ll1alaYllca.lIls uf i::.1 was covered by a l11élrine trélns -1.1 ditfcrent chemicéll composition from lhe majnr VOlc.1 Peninsulél sLIggests th. the seas begélll él very impo l<lnl transgression oV't'r 11.llÜC . Carza of Pemex in which it is sughcsted thélt the Aldélmél Peninsula élmi the COélhuila lslélnd might h. Geology of the Northern and Northeastem Regions of Mexico 37 bdt continues under the Mcso7.titu! .ita belt. During the _. The absenct.He. {Figure 2.ide which the Chihué1hua basin might have had communication élcross t11(' Gulf uf S.we constituted él single positive element be:. which hél\'(.4). élnd to the eélst by the Coa'huila P 'ni nsula or J~la nd (see hgure 2. 1976) In thr Chihuahuél basin él very thiek. On the other hand.. COélhLlilél IsJélnd élnd the inlernéll terrélnes of Sunor.. of ~11e Kim meridgi 11.m ([aton.1rC<1S iínl1 L'O :.ibílity that thi::.' calcéllké1line series of the Sierra Madre OLcidcnl.In inll'r¡.12) hélve . Aldélma sillsl mes. 1978).lbinas In _oahuila."quencl:S \Vere drilled. élnd ["he Cordillera n miogeosyncline. line lo lhe Célst (Gonólcz.warro élnd Tovar.l teelLlnics of the late TertiéJry caused the formiltion of grélbens in which considerable thicknesses of continental sediments \Vere deposited. 1976). dllring the f\ t'uconlian. of this name. 1.l! (west of Chihuahuél) élnd the éllkélline series of Tran~­ Pecos Texas. exas ha:.'. chiefJy with deposlts of lil11eslone and gypSlll11 of the Alcaparra hHmation as w(. élS well as encountered in oi! wells clrilled by Pemcx in the Sierra de Cuchillo Pélféldo (DeFord. one Cdnnot rule out the pns<. and Benavides Forméltion) (Córdoba. McDowell and CJaballgh (1979) indicnte that the volcanic rocks to the casI side of Chihuélhu. lhe eastern pl1rtion of Chihuahua IOrmed él marim' basin as a con­ sequence of a transgression initiilted during this time (DeFord. 1973). Cambriéln. they rcfJect él deltélic environment th< t in the Célmpanian marks the . in lhe Sierra de Jél Alcc1pélrrél. The [irst sLlgl'" (lf th pp '1" rllr¡l~sic lTldflne tr.oic sequencc ol ChihuClhud to the east of the calcélfeous outcrops oí the Paleozoje at Plilccr de Guadi\lupe-Minél Plomosas. 1 élnd Chinos no. basin g. Thl-' pflncipéil Pólleozoic outcrops of Chihuahllél inelude the seqlll:'nces cxposed in Ihe areél around Minél Plomoséls northeélst of Chihllélhué1 (mostly Iime­ stones)..). Thc urugenic dcformation of the Marathon­ Ouachila olC'osyncJine during the jVlí~i:. (lf L'pper retélceOllS sedlments in the éHeél of Ihe Aldélm.'ilm.. siJlce in the area of Aldélmél the presence of a consid­ erable seqw'nce oE délfk grélY siltstones similar to those üutcropping on the northern flank of the OUélchitél belt in -. A t lhe bq. The distention. al! uf whose ínterrelahonships are still somewhcl. ion marks the uplift and volcélnic élctivily of the vVl'stern part of Mexico. 197<.11 as with the clays élnd sdndstones of the Lél c V'né1S ' Formabon..' oppo~itl' vergence on bolh sides of the ba. and metélmorphic Precélmbrian rocks \Vere ene luntered (N. I'élleozoic c. ':i 711. During this epoch the Coahuilél ¡si wd remélined still emcrgent and the Aldélm. the Fin <l)' Llmestone.1dvélnee of lhe COlO:. bdt pélsses to the west of the élbove-cited lOCéllity.sippian­ Pennsylvélnian ¡nterval élnd the normal faulting of toe soutbern portion of the North American créllon "vere followed by <l long periocl of eme(sion (PermiiHI to lVIiddle Jurassid during wbich redbeds \Vere deposit­ ed within él tectonic setting of intense normal faulting. At the beginning uf the Cret<1Ceous. mélfine sed il11l'nla tion con ti nueo in Ihe Chihuahua basin. fllndamentillJy .2. élnd in the Sierrél La Mojína.i1! eVilp­ orite seqllence. At the end oi the Neocomjéln élnd beginning uf the Aptian.' Pcnin:::lll.nimbrites of Olígocene-Míocene age.on~litut the Ojinagél Group. in the Sierra El Kilo.nCl11éll" prov I1lt· bd 'l't' t\ lb'. The recumbence of the inh'rse faulls.we rise to deposition of an ev~porite sequence thcll is nolV mélnifested by diélpiric struetures of salt dnd gypsum Jocélted lo ('he south uf Ojinag<l and Ciudéld JuéÍrez. to the south"vest by the J. le Crcl<l'l'DUS.l oi the it". In lhe Cenozoic Eri\ Ihe Chihuahua region evoJved as an emergent 20ne that was panly covcrcd (almo:'. sion. kMl' ous sequence then developed that inch¡des ¡he Cuchillo Forméltion and the Chihuahuél Group (Benigno and Lágrima forméltions. élnd variollS minor loca!íties that occur in the north~vest corner of the state where ]jmcslone and dolomite plélltorm félcies appeélr. This episodc of continental deposition is \'11 1dely known ayer the neighboring regions of Torreon clnd northern Zélcalecas. 1969). the Stilte of Chihuahua beJongs to the Metélllosenie Province of . during which impor­ tant red facies were developed over the AldélJn<I Platform (Frélnco. 1969). The end of the Me~ozoic ErJ is markecl by folding of the Meso:¿oic cover (Jigure 22) ZlS él result 01 a "decollement" or sliding élt the leV'L'! of thL' bd~. lerrigenolls sedimentation in the Chihudhua r . The chicf outcrops of the Upper Jurassic contain argiJlélceolls élnd eéllcélfeoLls sequences and are locélted principéllly between Ciudad Juárez élnd Chihuahua. the OUélch. hélS been interpreted lo be dlle to the up-éItehing of el central beJt élt lhe level of the basement. and SinaJoa (Rangin élnd Córdoba. These tcrrigenous deposits . This transgression élcross the positivc c1emenb reaches its maximum d '\'eloprnent in lhL' Albian-Cenomélniéln intervéll. 1Y70).ll1sgress'ion coveríng ti e Lll11u. the strucluraJ axes present gencréllly a northwest-southeélsl orientatioll.inn m¡?. éonzcíJes (976) clles cm unpublished \·vork by R. been reported. This basin i~ limited to the norlheélst bv the Diélblo l'Jatform. \vhích are present to the north of Chihuélhuil Citv.1nd Aldama pléltform~.t confusing. which originah-'d the slippélge toward the Diablo .sin.[ complete]y so in the western portion) by emis~i.\Jdama Peninsulé1. rewgnized in the . In the oil wells Moyotes no. Information from petroleum wells has revealed possibilitíes in the Jurassic of Chihuahua since they have penetrated calcareous-argillaceous sequences with high organic content. above which the Mesozoic seguence of this region developed. the detrital Pennian sediments reported fram the Delicias-Acatita area are similar to those of the frontal belt east of the Ouachita geosyncline. 1976). The iron deposits of eastern Chihuahua belong to a belt associated with the return to the west of magma­ tism related to arc migration (Clark et a!. (1980). With respect to petroleum reservoirs. Denison et al (1970) have indicated that the Granjeno schists of Peregrina Canyon rnaintain a great similarity to the eastern inter­ nal zones of the Ouachita belt. the topographic relief diminishes grad­ ually to the east grading into the coastal plclin of the Gulf of Mexico. The most significant physiographic feature is the flexure that the Sierra Madre Oriental undergoes at the lati­ tude of Monterrey.2. The episodes of hydrothermal mineralization mentioned aboye occurred 28-40 million years ago (Clark et al. during the end of the Mesozoic and a large part of the Cenozoic. To the north of this flex­ ure the orographic elements become more widely sep­ arated and the geologic structures less narrow.38 Section 1 The Geology of the Mexican Republic 108' I deposits within which the localities of Talamantes. 1980) The chief deposits are encountered in the mines oE La Perla and Hercules in the vicinity of Coahuila. The hydrothermal deposits of manganese. zinc. 1980). there occurred an eastward migration in time and space and a la ter return of the magmotic arc related to the convergent morgin that was developing along the western border of the country.. which could serve as source rocks as well as some porous sequences of platform facies that could be reservoirs (González. fram here the fold belt acquires a general east-west orientation. con­ tained generally in ignimbrites.. occur at depth with petroleum possibilities. chief1y those that crop out in the northwest part of the state. has been interpret­ ed as a continuation of the Ouachito belt of the south­ eastern United States. In contrast. Hidalgo del ParraL Santa Barbara. The princi pal deposits of this region ore the hydrothermal deposits of sil ver. and San Francisco del Oro belong to this central belt of Chihuahua sta te. Tenantes. TTTTT Y'TT'T' Normal fault ¡nverse lauH COAHUILA AND NUEVO LEÓN General Geology This region is characterized by the predominant presence of folded Mesozoic sedimentary rocks that rest upon a Paleozoic and Precambrian basement. Periods of emersion and normal faulting occurring during the Triassic and part of the Jurassic controlJed the paleogeography of the upper part of the Mesozoic and gave way to continental redbed deposition that has been reported chiefl y within and to the sou th of the Monterrey-Torreon transverse sector. located in the north of Chihuahua. Structures in Mesozoic rocks of Chihuahua. They show considerable porosity and resemble pro­ ductive rocks of western and central Texas (González. According to Clark et o!. These episodes were related to the magmatic activity that is a product of the partíal fusion induced by the Farallon plate under the continental crust of Mexico during which time a return to westward migration of the magma tic arc was occurring. the Sierra Madre Oriental. Naica. and Casas Grandes occur. The princi­ pal locabon of this type of deposit is the Sierra de Peña Blanca. The uranium-bearing volcanogcnic deposits of Chihuahua are linked to the migration of magmatisrn toward the east and are related to rhyolitic and tra­ chytíc lavas about 40 million years in age. also form a belt of . + IG + Sync1lne Antlcline Tertlary volcanlc rocks Figure 2. The mineral districts of Santa Eulalia. Furthermore. The Paleozoic basement.3). This affirmation seems lo be corroborated by the metamorphic basement reported in the petroleum wells of the States of Nuevo León and Tamaulipas. lead. tile calcare­ ous Paleozoic rocks that possess platform facies. 1976). and gold that are localized principally in the central belt of the state and that follow in large part the axis of the Mexican Republic (see Figure 2. 1961). During this process the Paleogulf of Sabinas was defined in the Oxfordian..3. the La Caja and Pimienta formations..P. They formed under conditions of strong evaporation (González. Olvido. The advance of the marine transgression during the I<immeridgian and Tithonian created deposits of the open sea. I A Iorrn I I S!m frBn¡¡í5Cil d. Eulalia I I .. .26' o • Au-Ag I . 11 possesses the characteristics of an intracratonic basin developed in the more tectonically stable southern North American cralon.W Hg Pb-Cu O Figure 2. During the beginning of the Early Cretaceous. and the Peninsula and Archipelago of Tamaulipas (see Figure 2. ... a transgression occurred over northeastern Mexico. In the Late furassic. and La Gloria formations belong to this epoch. I I I I I I I --~---I ----~---- A A Sta.JUAAEZ I . which are composed of calcareous-argilla­ ceous sequences with carbonaceous horizons. Terrigenous and calcareous evaporite deposits were developed in the first stages of transgression in the Sabinas Gulf. 1976).'" I I I I • I I ~ I A L.5)... Zuloaga. lhe marine transgression of the Late Jurassic continued . This phenomenon has been related by various authors to the opening of the western extremity of the Tethyan Sea during the initial disintegration of the supercontinent Pangea. mainly in the Oxfordian (see Figure 2. Novillo.2. The last two forma­ tions represent respectively the extra-littoral and near-coastal facies of the upper Oxfordian (Rogers et al. just as in the La Casita Group (see Figure 2. . Pb-Zn-Ag Mn Fe A • L:::. the Coahuila lsland. The Minas Viejas. Distribution of principal rninerallocalities known in the State of Chihuahua (taken froro the MetaUogenic Map of the Republic of Mexico by G.--1- . 1975).QI Úru I I I • I I 1. Geology of lhe Northem and Norlheaslem Regions of Mexico 108' 39 I 105' CO. Salas.4). I I I I .6). it concurred with the formation of the Sabinas Gulf. dolomites. The S. outside of the reefal margin that bounds this formation. Furthermore. these occur in various facies. this forms the La Peí\a Formation. The MenchClca Formation is formed by a seguence of limestones and some intercalations of mar! and shale. Paleogeography of Upper Jurassic. This is considered an integral part of the Cupido.7). limes tones of the Cupido Formation were deposited in a platform environment. Chélfleston(1973). 1970. In a large part of the Sabinas Gulf. and evaporites of the LCl Virgen Fonnation. During the Albian-Cenomanian intervaL an impor­ tant marine transgression. constituting the upper Tamaulipas Formation (see Figure 2.40 Section 1 The Geology of the Mexican Republic so i I 160 240 km Figure 2. At the Aptian-Albian boundary. after Raúl González-Garcia. From the H. there suddenly appeared a general influx of fine terrigenous clastics into the Gulf of Sabinas. the limestones and shales of the Padilla Forméltion. 1976. This is indicélted by various lentils formed off the coastline of the COilhuila Islélnd in different strati­ graphic levels.4. caJcareous deposits fonned. This infiux could be a response to epeirogenic uplifts of the surround ing positive ele­ rnen ts or to a eustatic féll1 in sea level (Smith. and gave way thcoughout ClI1 the Neocomian to depo­ sitian of a heterogeneous sequence camposed of vari­ ous formations. This formation constitutes the base of the plalform sequence of the Neocomian that is repre­ sented higher up by the shales and sandstones of the Barril Viejo Formation. cornpletely covering ele­ ments that until then had remained positive. the argilla­ ceous limestones of the Télfaises Formation .vere deposited during the Berriasian-Hauterivjan interval. The configuration of the San Marcos arkose permits the observation of intercala­ tions of the unit between formations deposited con­ temporaneollsly in marine platform environments. and the limestones.\l1 Mc1fCOS llrkose constitutes a littoréll and continental facies encompassing Cl major part of the f\Teocomian deposited simultaneously with other diverse forméltions. there devel­ oped a reeiéll lineament that trends from Lélfedo to Monterrey and from there to the west toward Torreon. in al! of north­ east Mexico.lUterivian to the Aptian. Finally. facies of the open sea devel­ oped. the calcareous-argillaceous sequence of the La Mula. In the southeast sector of the Sabinas Gulf. began the development of thick sequences of carbonates . 5. .1978. after Alfonso­ Zwanziger. Geology of the Northem and Northeastem Regions of Mexico 41 1"AMAULlPAS NINSULA o 50 100 km ZACATECAS • CO VICTORIA ~ ~ Plalform carbonates-evapontes 01 Olvido-Novillo Formallon ~ ~ ~ Shelf margln oollte banks 01 ZLJloaga GroLJp (?) Plafform clastlcs 01 La Glolla Formatlon ~ ArglllaceoLJs open manne Ilmeslone of unknown lormatlon Figure 2.2. Paleogeographical configuration of northeast Mexico during the Oxfordian. .6. 1978.42 Seetion 1 The Geology of the Mexiean Republie o so 100km 1 ÍI .1. after Alfonso-Zwanziger. Paleogeographical configuration of northeast Mexico during the Kirnrneridgian and Tithonian.' . °:1 Terngenous La Casita Group R3 t:s:::::d ~ Argillaceous sandy carbonate 01 Pimienta Formalíon Argíllaceous carbonates 01 La Caja Formallon Figure 2.00 VICTO /A l. . r . / ­ \ I . __ ...... / / \ _J i ­ . Geology of the Northern and Northeastern Regions of Mexico 43 H RMOSllLO • \ \ \ \ I I \J I I I ( -~ \ \ \ I '). and upper Tamaulipas formations belong to this intervéll (see Figure 2.. Sediments that constitute the last formation have been considered bv Tardv et al. .... Indidura. . OO~o\CAN ..... San Miguel.... Caracol. sequences of shallow marine and evaporite félcies were deposited owing to the presence of reefs that bordered the tectonic elements. '­ .. In the Lél Popa élnd Parréls basins slow subsidence induced the accumulation of greélt thicknesses of shales and sandstones. .. Over the Burro platform (Tamaulipas Peninsulél) and the COélhuila Island. During the Late Cretaceous. (1974) as flysch deposits that preced~ the o~ogenic deformations. _. after Raúl González-Garcia.. Suda. successive cOélstlines and deltas developed with consequent detrital clélstic deposi­ tion (see Figure 2. \ \ ( \ \ ) I . and Difunta are units belonging to the Upper Cretélceous. terrigenolls sediments coming from western Mexico were deposited... Baslnalllmes\one Evaporiles o Conllnenlal depos.9). [!9 .ts 100 200 km Figure 2. Eagle Ford.. Parras. ... -.... • "/ . t .. -1 l 1 ~ \ J . . over aH the region in general.7. . Upson. over al! of northeast Mexico.0:--. The for­ mations of Del Rio./ I ( J . Acatita. .. .1976.8). Austin. Olmos. / .... These underwent orogenic deformation at the beginning of this epoch and later él general uplift With the gradual retreat of th€ seas tow8rd the east. Temgenous deposlts Platform Ilmestone Dolomlte Reef \.. . The Aurora.. Escondido. J .... Paleogeography of Neocomian-lower Aptian for northern and northeast Mexico.... ­ I ..2. ..~. There <1lso exist Oligocene volcanic occurrences similar lo the siliceous rocks east of Chihuahua.... The style of deformabon of the Sélbinas Culf Mea is less intense than thélt observed in the front of the Parras basin where recuUlbent folds and overthrusts have signifi­ cant development..-_. after Raúl González-G arcia. Deformations of the Laramide Orogeny occllrred mainly in the early Cenozoic..L_...--. These authors consider that this alkaline magmabsrn was caused by the subduction that was occllrring in western Mexico and that constitutes the most distant manifestabon of the ancient ocean trench tbat was forming more Ihan 1000 km away. Paleogeography of the Albian-Cenomanian for northern and northeastern Mexico....------''-v~ HERMOSILLO 2 v v • Platforrn Anhydnle I Dolomlte ~ Reef Basln o 100 200 km Figure 2...l.8. The folds are somewha. From the time of these orogenic deform<l. in addition to small basaltic emissions in Ihe Pliocene and Quaternary. when intrusions of nepheJinc syen­ ite were emplaced (Bloomfield and Cepeda. land­ scape belong to these epochs..JL-.. During the Cenozoic..1976.tions the continental cvolution of the region begéll1 with importélnt continental dcposits índuced by Late Cretaceolls normal faulting. . Over the old posjtive elernents the structures are even more gentle and can be observed to have the form of large periclines..--""'_rl-""'_J6--r--.. chiefly in the Oligocene.. The anticJinal and syn­ clinal structures so characteristic of the Coahuila... 1973). Clarl< and co-workers 09RO) consider these igneolls bodíes élS part of an all<alic igneous belt that is extended from New Mexico into Mexico.44 Seclion 1 The Geology of lhe Mexican Republic r_t-. isolated pulses of igneous acbvíty occurred in this part of the country..t narrow and onlv are recumbent and overthrust toward the positive' elements at the edges of the Sabinas Culf. ./ I I PIEDRAS NEGRAS \ \ .MONTE.... since in thesc rocks both SOllrcc beds and strata with reservoir characteristics appear.-/ ) (-' \ \ /' / ( \ (J l_.) CD. .stern Regions of Mexico 45 \ \ I I \ \ .. The stratigraphic levels with major possibiJities are in the Upper jurilssic and Lower Cretaceous. after Raul González.. There also exist numerous deposits of t1uorite and barite distributed in belts generally oriented north ­ ....­ ...2. imd. VICTORIA .. .J • HERMOSILLO 1 ~-.. \ / '>" ' ­ ¡.'13). abundant commer ­ cial production has been obtained in adjacent regions..... l -. -.lbinas palcogulf and thc platforms of Coahuila and Tamaulipas have been the object of very extel~sive petroleum exploratíon because they contain favorable characteristics for the dcvelopmcnt uf this resource. Geology of the Northern and Northea. ¡ . • "\ / . "1 "1.. Specificillly.. Petróleos Mexicanos has drilled numerous exploriltion wells <1nd hilS encoun ­ tered important shows of hydrocarbons in the Sabinils paleogulf Mea. I / 1 '\ MONCLOVA I / / \ f .. The coal-bearing zone of Sabinas is formed by sedi­ mentary deposits located within the deltélic sequence of the Upper Cretilceous. these bcds belong to the Olmos Formabon of Maastrichtian age thilt was deposited in a dominantIy swampy environ­ mento This zone is the principal producer of coal in the nation and its major reserve (see Figure 2.9. Economic Resources Thc Meil of thc S. furthermore.. Paleogeography of the Upper Cretaceous for northern and northeastern Mexico. \_---'­ EARLY CAMPANIAN LATE CAMPANIAN LATE MAASTRICHTIAN o 200 400 Km Figure 2...RREY l ) -" MATAMORAS .. .._\ . 1976. Hidalgo (Carrillo-Bravo. from whicb it might be supposed that the marine transgression of the first half oE the Mesozoic ought to have come from the Pacific rather than from the east. Carrillo-Bravo (1971) termed this area the "Liassic Basin of Huayacocotla. Zacatecas. 1961. the Granjeno schist originated by metamorphic processes that occurred in the Pennsylyanian-Permian interval. These fissures are hydrothermal yeins that occur general1y within the host rock of Lower Cretaceous limes tone. The Mesa Central contains numerous ou tcrops of metamorphie sequences that could belong to the Triassic or the end of the Paleozoic. 1965). During the beginning of the Jurassic. this constitutes one of the most important sources of this material in the nation. in its southern segment. The Sierra Madre is composed oE narrow foJds ""ith an orienta­ tion that folJows tbe general strike of the mountain belt. Pilger (978) indicated that the opening oE tbe Gulf of Mexico was earlier than the opening of the Atlantic. Furthermore. (971).L. This originated later than the orogenic deformation at the end of the Paleozoic (Table 2. The sedimentary sequence deposited in the Silurian-Permian interval has been considered by most authors as a tectonic autochthon developed aboye continental basement represented by the Novlllo Gneiss and belonging to the Ouachita belt of southern North America. Here the Paleozoic constitutes él metamorphic sequence of gneiss. numerous authors have described an important sequen ce of upper Paleozoic rocks that rests on metamorphic rocks of early Paleozoic and Precambrian age (Carrillo-Bravo. SIERRA MADRE ORIENTAL. In the direction of the Mesa Central the valleys are wider. above a metamorphic sequence.46 Section 1 The Geology of the Mexican Republic west-southeast. Mississippian shales. (1977) consider this unit to be allochtho­ nous. the Sierra anticlines less narrow. these authors have dated the metamorphism of the Granjeno schist and have assigned it to the Ordovician (446 million years). The Precambrian and Paleozoic basement can be observed in isolated outcrops that occur in erosional windows in the folded Mesozoic sequence (Table 2. According to radiometric dating of Denison et al. were deformed at the end of the Early Jurassic leading to the predominance oE continental deposition in the whole region during the Middle Jurassic. These authors sug­ gest thM the schist was positioned tectonically in jux­ taposition with the sedimentary sequen ce of the upper Paleozoic that is contemporaneous with it. In the area of Zacatecas City. sandstones. In addition.. The State of Coahuila also contains resources of phosphorite of sedimentar)' ori­ gin th(lt forms horizons in strata of the La Caja Formation of the Upper Jurassic. AND MESA CENTRAL General Geo1ogy The Eastern Sierra Madre and the adjacent areas are composed chiefly of Mesozoic sedimentar)' rocks that were deposited and developed over a Paleozoic and Precambrian basement. In the Lilte Jurassic. These belts c1early parallel mineral­ ization belts developed in northern Mexico as a conse­ quence of the magmatism associated with subduction in the west. De Cserna et al. there is recorded generally in all of nortbern and northeastern Mexico a marine transgression that Tardy (1980) related to the western opening of the Tethyan Sea during the disintegration oE the supercontinent Pangea (Table 2. a general northwest-southeast trajectory. Other outcrops of schistose rocks of probable late Paleozoic or Early Triassic age are located in the areas of Caopas. where an advance of the seas that induced marine sedimentation of an argilla­ ceous and sandy sequence is recorded. named by the smne author the Huayacocotla Formation. Ramírez-Ramírez (978) has suggested that the tec­ tonic emplacement uf the Granjeno schist occurred in the late Paleozoic." These sedimentary strata. turns to follow an east-west trajectory toward Torreon. schist.P. The lower metamorphic sequence is composed of the Novillo gneiss oE Precambrian age. and conglomerates havc been reported in the area of Calnali. constitute the only known outcrops of marine Triassic in this part of Mcxico. and at the latitude of Monterrey. continental deposition continued in this part of Mexico with redbed sedimentaban. and by the Granjeno schist. The affinity of the faunas of eastern Mexico with those of the Pacific . GULF COASTAL PLAIN. positioned in tectonic contact on the N ovillo gneiss in an episode later than the formation of the La Presa quartzite and before the deposition of the upper Paleozoic sequence.1). except in the region of the Huayacocotla anticlinorium. there rest marine partially metamorphosed sedimentary rocks that contain fossils of Carnian (Late Triassic) age (Burckhardt. This area is the principal pro­ ducer of fluorite in Mexico. These continental sed­ iments attest to a long period of emersion in this part of the country. at the time of in tense folding uf the upper Paleozoic sedimentary sequence. Iater than both of the aboye units. and toward the west they become gradually covered by the volcanic rocks of the Sierra Madre Occidental. S. La Pres¿¡ quartzite of the Cambrian. 1965). together with those at Peñon Blanco and Charcas. In the area of Ciudad Victoria. The Sierra consists of an orogenic mountain belt that follows.1). Other Paleozoic outcrops of the Sierra Madre Oriental are located in the area of Huayacocotla at the latitude of the 21st parallel At this lo~ality Paleozoic rocks are seen exposed in the nucleus of a major anti­ clinorium whose flanks are composed of a thick sequence of Mesozoic sediments. and metaconglomerate probably belonging to the early part of the era and a flysch sequence more than 2000 111 thick that is Permian in agc. the Granjeno schist comes from an eastern belt belonging to an internal zone of the Ouachita trend that was metamorphosed during the Carboniferous. According to the model of this author. and Guanajuato. The Triassic is represented in the Sierra Madre Oriental and in neighboring areas by redbeds belong­ ing to the Huizachal Formation. These. 1930).1). Fries and Rincón. .. PLATFORM GULF SABINAS .\j 208 FM º « (f) (f) UPPER lA~Ai~Ei~~~ i i I FM lA· O Z CATECAS I I MIDDLE ro .H FM orATES FM LA PENA FII.I O1.. «w 1<2 "'z I TITHONIAN Kll'&1EAlOOlAN OX~ORDIAN LA CASITA GP LACAJA (LA CASITA) FM ALEJA CJ (f) iñ CALLOVIAN BATONIAN ::J « a: MIDDLE BIoJDC. FM MENDEl FM CARACOL fM SOYJo. 1984 >Po -.. ro O ~N a:: PRECAM­ BRIAN ~ NOOUTCROP Table 2..1.L a: CAMPANIAN FM MENDEl FM SAN FELIPE fM AGuA NUL:VA FULlJ'J... .a.LLU'{~'Jf'" OF PARRAS SECTOR CHIHUAHUA BASIN PlElSTOC'"E PlIOCENE MIOCENE w OUGOCENE EOCENE PN. DEVO­ NIAN :.lAif>AS I FM EL ABRA (POST­ REEFAL) ? FM EL ABRA (REEFAL AND POST· REEFAL) ? FM KIAMICHI C AURORA FM LA PENA FM TAMAU· L1PAS fM LA PENA FM ~INLA't' FM LAGn¡MII. o' O .c.TA. u TtJRONlAN CENOMANIAN FM SAN FELIPE FM A(>UI\ NUEVA CRETAA~TIN " l PARRAS I I FM INDIDURA A II FM SN FELIPE OJINAGAGP FM CUESTA OE:l CURA FM LOMA PLATA FM BENAVlDES FM EAGLE FORD WA$HITAGP ::J o w ~ Ü I a: w ~ I ALBIAN 1­ FM CUESTA DEL CURA PM U.. FM BENIGNO FM CUCHILLO o N o (f) W ~ Ü CI. Stratigraphíc correlations for northeast Mexico.SAN LUIS P.IAUUPAS INFE>lIOR 14. MEXICO MESOZOIC BASIN central and E Por\lon o N o Z () a~ >-. ::J III ~ PEA· MIAN f>l:NNS~L oc FM NAZAS 245 ª" Z ::J O­ . fl\MAUUPAS JNFERIOF! FJ. o --' - APTlAN NEOCOMIAN fM TA""AtJlIPAS PENA..N () o N o UJ « Q..J IJIIlSI sg¡:P!J<.. () NO DEPOSITION.. • GNEISS .STRATIGRAPHIC CORRELATIONS FOR NORTHEASTERN MEXICO ERAISYS-I SERIES EUROPEAN TEM STAGE Im.I pue.Wt=:hK. ~ d: RECEN! I N Pomon HUAYACOCOTLA AREA VALLES ..N ro .:. LOWER O ...c.. ANDIOR EROSION Complled by Ennque Cabral.¡ . .. III >-i O ro vllNII>...ECCENE () MAASTRICHTlI>N 'M .l¡. I C.. ::: ()Q 'J.. " DIFUNTA GP "'­ :::> (f) o­ w SANTONtMI CQNIAC1AN I I fM ANMi FM CARACOL fM INDlmJRA. TOAFtClo\i'.S " FM lAf'. I --? FM TARAISES fM lAA.AISES FM TAMAULI­ PAS INFERIOR FM PIMIENTA FM TAMAN FM TEPEXIC FM CAHUASAS FM GUAXCAMA ? ? FM TAMAULI­ PAS INFERIOR FM PIMIENTA (?l FM TAMAN (MIXTO) e e CUPIDO • PAOIlI HA LA MULA FM BARRIL VIEJO C cuelDO FM TARAI5ES ~M FM LAS VIGAS FM ALCAPARRA MENCHACA t'"' ro O :::> ffi "'< n... a: () Ü w I FM CUESTA DEL CURA I FM CUESTA DEL CURA reu Tol.U. Sl..::1 ro SILU­ RIAN -­ üRDO VICIAN eAM ::J 'J.. SINEMUAIAN HEn ArK1JA.lIfl.lSM0:J4M O­ ~ ro -.b. the )-o. lhe possibility was suggest­ ed tb<lt this piHt of the country might hélve evolved as an (luJ?lcogen syslcm dllring the first half of the Ml'sozoic. lhe . and lhe silt­ stones. More recently. followed bv the Carilcol Fürmiltion in the C()niacian-M~élstrichtian('h.: La Cloriél ilnd [ a Cilsita forllliltions.1r('. Ibi.ln~~r('~"ion. uf northern and northeac. "'vH' si 's 0. During the first pi1rt of the Early Cret. d posits of the Turonian lndidurél Formation (!irncstoncs and shaJes) were laid down.ing to the intervéll from . the Oxlordiéln marine trilnsgression is m.lceous (Nl'ocomiéln--Aptiiln). During this epoch the seas retreélted gradually toward the east. rm lay (] 938) mentions the ex istence in this region of a M"xiGln geo~yncline. ancl some . nd­ stones).1f cOilstal facies nf tbe last two formations are respec­ tively th.ldre Orientéll and éldjacent regions are the \1psozoic basin of Mexico ar Mexican geosyncline. clre evidenced by the migriltÍon of cl1arélcl('ristic Mediterralleéln biotil tOvv<lrd the Pacific provine' ot th(! north and vice vcrSél.e.. On the perimeter of the Vall's-San Lui Potosi pliltform there developed at this tinll' a n~(:!f 1 b 'I:t flélnked by bilck-reef ilnd forereef deposlts (5ee Figure 2.live during the lVIesozoic in the pr '5l'nt area of the . and éllso in the arCél of Tuxpan where él reef in the form (lf an atoll forllls the reservoir for hydrocélfbons in the so­ céllled ColeJen Lane (Figure 2. dl::'position of pelagÍl': stmtél cstabIJshed 011 continenLll crust without re<lching éln ultil1late proc 's. deveJoped in the zone of the Ml::'-. that WélS termed the Pacific geosyncli[1(:. In a time eClrlier than the [1 '?~~ic !T.. the re 'i­ men of scdimentation in eilstern Mexico changed drélstically with él influx of detritcll sedimcnts cominh from the west \\fhere an uplift il sociélted with vol­ cilni. of oceanizatiol1.a -':'I1tra! and the Sierra Madre Oriental. in a large part of the Mes%le bilsin of Mexico (f\. With the invC\~ion of the Lite Jurélssic seélS over 1.1 sequence that was chiefly evaporitic (Cnuxcamá Formation) occurred. In the Ancestral Culf of Mexico. lhe clastic strata equivalent lu ne. except in the southcélst toward the Slók' of Verélcrllz.1rlCC\S. '1Ill? constant élctivity of these troughs cilLlsed draslic changes in the bathymetry that pro­ vok -d.. Télrdy (l ':180) cOllsidered that tbe eJst-central portion of Mexico evolved élS él geosyn­ cline in whidl there were individuéllized t\Vo bilsins (Ancestrill Culf of Mexico élnd the Mcsozoic bilsin of Mexico). tbe Zuloagél Lirnestol1cs.Mly Jurélssic. 1971). and the Dilunta in the CampClnian to Mélastrichtié111 (shales anu ~. separdted from thilt devel­ oped in w 'stern Mexico. indicating a shélllow water basin under climatic conditions of strong evaporation.'1. deposits. Similélr re (.[ '. pcrsonéll eommunication) is a félet that sup­ j'orts Ihis -.lyacocotlil dllri . Télmaulipas.lhuilél pliltform totillly so. Among the principal elements that were .uppasition. the platform or archipelago of T<llllaulip<1s WilS pélrtly emcrgent and the CO. i. The Mesozoic basin.lined emcrgent during all of the Lille Jura 'sic (Carrillo-Brélvo. the Peninsula or Archipclago of Tcl'llziUliFil::'.-'Io ~"l to the \'\ 'stern sector of the above-mtc'n­ lioned cuntinl'nl ílngea.1\0<.in where nerit­ ic deposits den'loped 011 llll' Valll's-San Lui.lll'~-San Luis Potosí platform. with associated prograding deltas. élnc! COilhuilil platforms.1 developments have been interpreted to occur around the Coahuila and Burro ptllforms contemporilneous with the Aurora élnd Cuesta del Cura formiltions.48 Section 1 The Geology of lhe Mexican Republic (LonSllJlrl. 1976).ed by orogeny. rl::'OUS pelé1gic sedimentéltioll. In élddition..1$ -ociilt . el Cl'ntréll élnd the wcstern belt of the Sierra Madre hiental).Gdc.. . a lélf~e part of whélt is now lVIexico l. This whole facies assemblage was terllled the El Abra Formél tion (Carrillo-Bravo. limestones. With the .1 n At the beginning (Jf the Late Cretilceous. duriJlc~ Trido>sic continental d"'posilion. thélt in the Jurassic and Cretaceous periods communicél­ tion developed with portals bctwcen both geosyn­ clines.ln there came a general marine transgression that covered the last positive elements and that cxp. The {Trper Jurélssic deposits in the Mesozoic bélsin of Mexi~o consist in stratigrélphic order of the Mineb Viejas Gypsum.. Cl céllcareous corn­ plex of pliltform type developed. élS él series nf marine-lIlvaded tectonic derressions .lern Mexico. Over the Va Iles-Sa n lu i Potosí pla tform during most of the Late Cretaceous. the Peninsula ar IsJílnd of'oilhuilél. [n the areélS of the MesoLoic basin.llfl .1. according to the modeJs ot \'ari­ ous authors. the major pclleogeographic e!elllents begrlll to be defined-ele­ ments . This iluthor indiciltes. in thl~ st'nst: of el linear belt of subsidence where a con­ siderable lhickncss nf sediment élccumulated élnd thélt laler WdS d "slro:. lhe Pilrras ShaJe from the Santonian.lS rell1. furthermore. dimentation at this time and later tectoníc d<c'tonll<'ltion. hélS been considl'r~d by nUJl)erous cluthors as il geosyncline.'ierra M.lchve during the whole Mesozoic and that cal1­ lmlkd :. <1nd tlll' anci'nt Guli of Mexica. the V. and plutonic élctivity was lodking place.10). rked bv an ini­ kll deposit of the gyps~m Minas Viejas.lIes and sé'lIldstones). Thesp possessed él NNVV-SSE orientéltion. Pot()~i. OnJy at tbis tirne did shallow w<Jter ckposib on'ur in both bélsins. élnd shales 01 the L'I Caja Formation. 1971).1n auJilcogell in the area of Hu. This is composed of pure limes tones and argilldcéou'i límestones of the TéllllélSOpél Formation belon['. Schmidt-Effing (1980) belie\'cd in tbe presencc of .. Burkbard t (L 930) consid"r d tbilt tor the Lélte Jurassic in this res ion él m~jor entrance of the seil that bordered the positive le nds took place. Hw Oxfordian transgression illso is initiatel by evaporil . open sea deposition occurred in tht' Mesozoic bilsin nf Mexico (Taraises anJ lower Télmaulipas fornléltions) while over the Vall?S-Siln Luis POlllSÍ platforlll . illld were S 'pélfated by il cresta I Mea (the Véll\es-Séln T llis Poto~ll'liltform) over which neritic sedimentation dcvelop-'d. in cert.d with the initiéll expélnsion of tl1l::' Atlclntic. These gypsum deposits playa very important role in the orogenic deformation ¿¡t the end of the Mesozoic.!. Thc marine transgression initialcd in tbe Oxfordian die! not completely cover the Véltles-San Luis Potosí plilt­ forlll.. since during lIw resl of the Mesozoic sedimentary conditions were peiél~ic in the Culf bélsin in contrast to the inIand bas.lnded over the western portions of Mexico (Rangin and Córdoba. contains marine sequences that are chieflv detrital and more than 1500 m thick. The first manifestations of the orogenic deforma­ tion at the beginning of the Cenozoic are flysch deposits associated with prograding deltas of the Late Cretaceous and with the foredeeps formed in the areas of Parras (Campanian-Maastrichtian) and the Chicontepec (Paleocene) where thick terrigenous setluences were deposited in deep water. In the Ancestral Gulf of Mexico.2. Limits of the Valles-San Luis Potosí platform during the Cretaceous. . sandstones.vith the devel~pment of numerous growth faults contempora­ neous with sedimentation.10. recognizable in the wells . and limestones of the Cárdenas Formation of Campanian-Maastrichtian age. Great thicknesses of continental alluvium accumulat­ ed in the synclinal depressions and tectonic troughs that imprinted a characteristic geomorphology on the landscape of the Mesa Central. deposits of molasse-type con­ glomerates belonging to the Ahuichila Formation and the Red Conglomera tes of Guanajuato beg<:1l1 to formo Generally these are polymictic conglomera tes derived fram erosion of the folded Mesozoic fO. These sedi­ ments originated chief1y in the Oligocene. The Burgos basin. This underlies the shales. These defor­ mahons began the construction of the Sierra Madre Oriental and initiated the continental history of a large part of this sector of the country. upper Turonian to upper Senonian. Iocated to the north. The extreme western part of this zone of Mesozoic folds appears covered by mesetas of ignimbrites dissected and split into trib­ utaries from the Sierra Madre Occidental.le La Asurlclon • Cd Vlctona 49 • Cd Mante Ansia· MESOZOIC 8ASIN OF CENTRAL MEXICO ANCESTRAL GULF OF MEXICO San LUIS Polosl Approxlmale Ilmls ol plalform dunrlg the Early Crelaceous wlth reel growth on Ihe margins ( ' \ Arrecife Tollman \J 0====50 km Platform Ilmll dunng Ihe mlddle Crelaceous w'lh reel development on lhe marglns Approxlmate platlorm Ilmlls dunng the Lale Crelaceous wlth developmenl o( reefs dunng Turorllan and Senonlan Arreclle El Doctor Figure 2. In the period of maximum orogenic deformation in the Mesa Central area.rmations. Geology of the Northem and Northeastem Regions of Mexico Arrec. two principal Tertiary sedimentary basins are separated from each otber by the Laramide folds of the Sierras de Tamaulipas and San Carlos. These formations are the platform equivalents of the Agua Nueva. San Felipe. and Mendez formations of the Ancestral Gulf of Mexico. This basin is seen to be limited to the following geographic borders. after Carrillo-Bravo. Ramírez. . since many writers have pointed out similarities with the rocks of this belt (De Cserna. and SOme flows in the areas of the Sierra Méldre Oriental and the Neovolcanic axis. Tectonic Summary The characteristics of the Precambrian and Paleozoic basement. Flawn. This belt was formed as a consequence of the closing of the proto-A tlantic Ocean. published by Bloomfield and Cepeda (973).50 Seclion 1 The Gealagy ai the Mexican Republic U. The most important plutonic emplacements Me located in the Sierra de San Carlos. 1956. 1971. The belts that form this basement should ha ve been strongly dislocated by lateral and vertical movements during the first half of the Mesozoic. 1961. <{ 2: llJ Ü O () ~ ~ :5 z GULF OF MEXICO CARIBBEAN SEA REEFS I I I i o 100 200 km Figure 2. where nepheline syenites. These tectonic movements prepared the paleogeographic distribu­ tion of basins and platforms that would control the sedimentation and the Laramide deform. and to the south by the Tezuitlan massif. 1979) In the Tampico-Misantla basin. The flows of alkaline basalt located to the north of Tampico repre­ sent a later event attributélble to él distension (Cantagrel and Robin. are not dear because in general there are very scarce outcrops. the Paleozoic out­ crops of the Sierra Madre Oriental have been consid­ ered to be a prolongation of the Ouachita belt of the southeastern United States. thick marine sandy argillaceous sediments developed. During the Triassic this part of the country evolved in continentéll aspect with the development of disten­ sive tectonics that caused the formatlon of troughs filled with thick continental sediments. dikes. the Tertiary deposits occur within the setting of general eastward regression that left successive belts of outcrops poréll­ lel to the present coastline.12).SA. 1978).11.Ramírez. In whatever structural form and orientation they might have. to the east by the Sierra Madre Oriental and the Chicontepec foredeep. Radiometric studies of intrusive rocks in the Tamaulipas area. The pJutonic and volcanic activity in the Sierra Madre Oriental and Gulf of Mexico coastal plain was very minor during the Cenozoic and is represented only by isolated plutons emplaced in the Mesozoic strata. These are mineralogically similar to the alkaline province of eastern Mexico. In the two basins. These rocks con­ stitute a southward continuation of the alkaline province that begins toward the north in tlle Big Bend orea in Texas (Clmk et éll. gabbros. principally by orogenic structures from the early Cenozoic: to the north by the Sierra Tamualipéls. Distribution of reefs of Lower and middle Cretaceous around the margins oi the Gulf oi Mexico. reveal dates that vary between 28 and 30 million years. 1971. Denison et aL. 1980). drilled by Petróleos Mexicanos (López-Ramos. and monzonites disposed in laccoliths. aboye which the great Mesozoic sequence of eastern Mexico evolved. and mantos are found (see Figure 2. in Tamaulipas.ltions during the end of the Mesozoic. 1979). In the Jurassic.. Geology of lhe Northern and Northeastern Regions of Mexico 51 1 Ollgocene Ignimbntes 2 Lower Cenozoic granitic intrusives 3 Alkaline Intruslve rocks associated with lower Cenozoic granileS 4 8asaltic-alkallne rocks 01 the upper Tertiary and Quaternary Figure 2. as weIJ as by the northwestward movement of North America. The second domain of geosynclinal type or of an aulacogenic system origi­ nated during the marine transgression over the east­ ern part of tbe country at the time of the opening of the Gulf of Mexico. as a consequence of the uplift and deformation of the western area where there was continuouslv active subduction of the Farallon plate under the co~­ tinental portion of Mexico. since the North American and Farallon plates that converged obliquely in western Mexico began to face each other frontéllly and to move with greater veloci­ ty. in the time of deformabon. two important domains were estab1ished in Mexico as a result of the opening of the Atlantic and the Gulf of Mexico. The first of these. The forces coming from the southwest provoked the deformation of the sequence from the base of the Oxfordian evaporites on up. Their formation presages the orogenic activity thélt would affect aH the region. are seen to be closelv interrelated with the final Mesozoic deformations. ' According to a model proposed by Coney (1983). eélch wilh its own characteristics.2. De Cserna (1956) considers that the folds of the Mesozoic sequence increased in intensity from the Mesa Centréll toward the Sierra Madre Orien tal beca use of the presence. a marked change occurred in the sedimentary regimen in this region. was represented by a convergent margin and a zone of magmatic arc of Andean type. The . The detrital sediments that began to cover the G¡]careous sequence of the east were distributed widely and came to reélch greélt thick­ nesses in the foredeeps of Parras (Upper Cretaceous) and Chicontepec (Pélleocene). In this way the western and eastern domains of Mexico. of the massive cratons of the Coahuila platform and the Tamaulipas Peninsula. resulting from the subsidence of the Farallon plate under the North American continent. This transgression caused a great thickness of calcareous deposits within a setting of intermittent subsidence and the presence of emergent cratonic elements and marine "highs. the orogenic deformations élt the end of the Cretaceous and beginning of the Tertiélry coincide with a change in movement of the tectonic plates. Distribution oi outcropping igneous rocks in northeastem Mexico. which had acted in a relatively indepen­ dent mélnner." At the beginning of the Late Cretaceous.12. located in west­ em Mexico. . 1979). fn the Burgos basin zone to the north of Tamaulipas and east of Nuevo Leon. 1979. Z" 1956. Faja de Oro. p. Mexico: The Geologic Framework of the Chihuahua Tectonic Belt. the oil and gas productive districts of the region north of the GulfCO<1stal Plélin are Pánuco­ Ebano. 5. P. 1930. of outstanding importance are the hydrothermal deposits of fluorite of the area of Las Cuevas and Río Verde. the Paleocene sequence of the Chicontepec area consists in actuality of an assem­ bl<1ge of very important reserves. Délmon. The productive unit is a reef that developed in the Early Cretaceous and that extends in a semicircular form out to the continental sheif at the latitude of Tuxpan..280 p. In sllmmary.A. 13. Mexico. principally in Mesozoic rocks. are the Valles-San Luis Potosí platform. 30. p.. K. 268 p. According to the rnodel of Padilla y Sánchez (1982). L. 1970. p. nos. 49-71­ Conev. Córdobél. 73-96. . p. the distribution of the tolds and ovcrthrusts of northeast Mcxico can be explained by movement of North America toward the northwest with respect to Mexico. 91-96. C. These constitllte a sOllthern continuation of the northwest-southeast-oriented belt that developed in Coahuíla during the Tertiary. Tectónica de la Sierra Méldre Oriental de México.. v. p. Economic Resources The Gulf Coastal Plain and adjacent areas consti­ tute a very important region for petroleum produc­ tion.13). Poza Rica. 99-114. 1983. University of Michígan. D. Cilrrillo-Bra VOl L 1961. Symposium West Texas Geological Society.1rk. Coahuila Series. v. i. Cln over­ thrust of hundreds of kilometers that relocated the pelagic sequence of the internal bélsin (Mesél Centréll Clnd High Chain of the Sierra Madre Oriental) over the Coahuila and Valles-San Luis Potosí platforms with reef(ll and subrecfal sequcnccs that form similar pale­ ogeographical uplifts. Charcas. and C. sil­ ver. L 1965. The Faja de Oro (Golden Lane) has been tradition­ ally a productive zone that years ago constituted the principal source of hydrocarbons in the nation. De Cserna. Robin. Mexico: GeoJogical Magazine. The model of this author pre­ sumes the unbuckling of the internal basinaJ sequence at the level of the Oxfordian gypsum and establishes the possibility that the basement might have taken part in this tectonic phenomenon.al district of GUilnajU<1to. Cuadro tectónico de la sedi­ mentación y magmatismo en algunas regiones de México durante el Mesozoico: Programas y resúmenes del V Simposio sobre la Evolución Tectónica de México: Instituto de Geología. in honor of Prof RK DeFord. 1980. UNAM. Mesozoic stratigraphy oí nortb­ eastern Chihuahua. Boletín Asociación Mexicana Geólogos PetroJeros. the time in which the m<1gmatic arc reached its most east­ ern position.e. BIBLIOGRAPHY AND REFERENCES Abbreviation UNAM is Universidad Nacional Autónoma de México Alfonso-Zwanziger. p. 6.11-14. 561-559 Burckh<lfdt. v.M. p. Special Publica tion no. L<I plat<lforma Valles-San Luis Potosí: BoJetin Asociilción MexiCélnél Geólogos Petroleros. Tardy el al. Sh<1fiquíllah. S. 17. v. 1-112.. ]. Etude Synthétique sur de Mesozoique Mexicain: Memoire Société PaJeontologique Suisse. it is appropri<1te to indic<1te that the barite deposits that were developed on the eastern slopes of the Sierra Madre Oriental are also substan­ tial and bear reJationshi p to this episode of vO!c<1nic and subvo!canic activity (see Figure 2. <1n import<1nt petroleum-producing TertÍilrY sequence is encountered. (1975) supposed the exis­ tence of a nappe with NNE direction. where impor­ tant reefal development is present in the Lower Cretaceous and the Mesa CentraJ has <1 significant mMine sedimentary sequence of ]urassic élnd Cretaceous age. n. 1978. where the principal association is silver and gold. Mineral deposits include notable hydrothcrmaJ developments in the Tertiary of the Mesa CentraJ élrea and on the \-vestern flan k of the Sierra Madre Oriental. P. Geomimet. Oligocene alkaline igneous activity in N. 1-98. 23. Geología del Anticlinorio Huizachal-Peregrina al noroeste de Ciudad Victoria. 110. ]. Zacatecas. 5-10. Cantélgrel. Bloomfield. Tbe Tam<1br<1 belt in the Poza Rica Mea that consists of iln ilncient forereef zone has also been éln importélnt source of hydrocarbons. and Vera cruz (Dfaz. and Zimapán. Estudio de una parte del Anticlinorio de Huayacocotla: Boletín Asociación Mexicana Geólogos Pe troJeros..52 SectioIl T The G eology oi the Mexican Republic eVClporiles served as a surrace of sliding or decolle­ ment in the style of the Jura Mountains of Europe (De Cserna. entre Torreón y Monterrey. Cepeda. Tamaulipas. p. more than by the action of compressive forces coaxial with a southwest­ northeast orientaban.. Charleston. Geología regional del sistema sedimentario Cupido: Boletín Asociación Mexic<lna Geólogos Petroleros. and hydro­ carbon potential of the Lower Cretélceous. 1980). Z" 1979. K. Likewíse. no. Stratigraphy.E. v. De Cserna. K-Ar dating on eastern Mexican volcanic rocks-relations between the andesitic and the alkaline provinccs: Journal of Vulcanology GeothermResearch. S Shutter. XX Congreso lnternacionat 87 p. v. 1973. 1-6. V. and D.. Doctoral Dissertation. élnd M. 106. L 1971. Finally. 49-50. Carrillo-Bravo. Plélte tectonics <1nd the Laramide Or~geny: New Mexico Geological Society. Cl. Furthennore. Other ZOnes with petroleum potentials. in addition to the mine. Ann Arbor. Sierra de Catorce. The most important recognized resources of leéld. tectonics. which has been obtained from both Mesozoic and Tertiary sequences. 1 and 2. and zinc cHe localized in the eneas of Fresnillo. Magmatismo en el norte de México en relación con los yacimientos metalíferos: Revista. Coahuila. C<1rrillo-Bra VO. p. 1973. v. and north­ ern Mexico: GeoIogic Framework of the Chihuahua . Geo(ogy of the Northern and Northeastern Regions of Mexico 53 e IHUAHU' • PIEDRAS NEGRAS Sla i=ulaJ' \ \ \ \ CO VICTOR1A '-­ ""---. southern New Mexico. p. Alóctono dcl Pa leozóÍCo Inferior en la región de Ciudad Victoria.. O O t I . 1971. De Cserna. ~ . New Mexico Geological Society Guidebook. 61-65 DenisOl1.E.13. 20th Field Trip.2. 1969. 33-43.. p. G. 1975). Salas. TL.P.. Z. Graf. UNAM._" \ . R. Sorne keys to the geology of • (ID Carbon northern Chihuahua. Estado de Tarnaulipas: Revista del [nstituto de Geología. and F. 1. Distribution of the principal known mineral resources found in northeastem Mexico (taken from the Metallogenic Map of the Republic of Mexico.. Basement rock framework of parts of Texas. Ortcga-Gutiérrez. J Fe D @ Pb.K. DeFord.. Cu Pnnclpally Pb-Zn-Ag Au-Ag F Na. et aL. Mg p Hg Pnnclpally Mn O I§ ~ ª 8a Sall Figure 2. R. 1977. K.O. R. 1-11.. Mexico: Ph. 6. p. c. UNAM. v.. McDowell.. in RH. 28. v.132-149.. v. Tardy. and O. Tardy.W. M..P. no. y Morfología-David Serret. p. Rangin. p. Proceedings of a Symposium.. Chapin and WE. 1.E. Pilger. v. 119-137 Imlay.c. and W. 165-168. p. v.W. in honor of R. Nuevas aportaciones geocronológicas y tectónicas empleadas en el Laboratorio de Geocronometria: Boletín Instituto de Geología. III Tectónica y Tectonofísica. Louisiana State University. Structural evolu ­ tion of the eastern Chihuahua tectonic belt: Geologic Framework of the Chihuahua Tectonic Belt: Symposium of West Texas Geological Society. Ash Flow Tuffs: Geological Society of America Speciéll Pé\per 180.. F. F. no. Geología de México. México. p.J. élnd G Gacon. Geologic evolution of the Sierra Madre Oriental between Linares.. p. Concepción del Oro. Barcelona. Sierra Madre Oriental. v. 1978.A. Report of Investigation no.P. eds. 2. p. no. 98-99 Ramírez. Bosquejo geologico de la Zona Noreste: Boletín Asociación Mexicana Geólogos Petroleros. J. 2nd edi ­ tion. Smith.. Elston. and J. 9. Tamaulipéls: Revista. Mapél Metalogenético de la Repúblicél Mexicana: Consejo de Recursos Minerales... O. 57-133. p.. y San Luis Potosí: Boletín de Consejo de Recursos Naturales no Renovables. Chihuahua: Revista. c.. 2-49.. no. c.E.F. UNAM. Clabélugh. ed. p. Nuevo León. 1 and 2. 31-36. Córdoba. 1978. 1978. and S. Stratigraphy and tectonics of the State of Chihuahua: Exploréltion from tlle Mountains to the Basin: El Paso GeoJog­ iCéll Society. Boletín Asociación Mexicana Geólogos Petroleros. G. Schmidt-Effing. Ramírez-Ramírez. p. McDowell.. Quintero. 217 p. Una localidad de bélsamento Precámbrico de Chihuahua. Padilla y Sánchez. northern COélhuila. 1976. p. p.. 14 p. Tratado de Geologia. 79-86. 583-766. Estra tigra fía del Albiano ­ Cenomaniano en la región de Naica. Navarro. 1980. 50. 19nimbrites of the Sierra Madre Occidental and their relation to the tectonic history of western Mexico. p. 1970. Instituto de Geología.T. Instituto de Geologia. Jr. 221-223. 1975. 1965. Siga!. élnd J. 1957. and Monterrey. A dosed Gulf of Mexico. 3 volumes. 1976. Fries.. México. p. Midland: West Texas Geological Society. A. RH. Serie Divul. Salas. Mexico. Acevedo. Dissertation.. Reinterpretación tectónica de] Esquisto Granjeno de Ciudad Victoria. 1985. Rocas metamórficas en el armazón tectónico de la parte septentrional de México. 1975. Notas sobre la geología de Chihuahua: Boletín Asociación Mexicana Geólogos Petroleros. Louisiana.L.: Scholastic edition. University of Texas. c.D. 1982. J. v. Españu Editorial Omega. M.. J. Coahuila y San Luis Potosí. M. R. 23-27. Lower Cretaceous stratigraphy. Mauger. Studies of the Mexican Geosyn­ cJine: Geological Society of America BulLetin. 13. 75.54 Section 1 The Geology of the Mexican Republic Tectonic Belt. 117-182. DeFord.. The Origin of the Gulf of Mexico in the Eélrly Opening of the centré\] North Atlantic Ocean. México: Instituto de Geología. Tové1T. México: Revista. 1979.c. '1-77 López-Ramos. . Franco. 1980. et él1... The Huayacocotla aulocogen in Mexico (Lower Jurélssic) and the origin of the Gulf of Mexico. J. translator. v. Grenville Precambrian rocks of the Los Filtros near Aldama.. 1980. 385-593. P.. p. 1983. UNAM. entre Torreon. 73. 56. Rogers.c..I. 1al p. en el área de Carrizalillo: Revis ta. c. M. Rincón. CONACYT. et aL 1961. D.. 2nd edition.T.c. Reconocimiento geológico y depósitos de fosfatos del norte de Zacatecas y áreas adyacentes en Coahuila. 1975. R. 3-14. Haenggi. Observaciones generales sobre la estructuras de la Sierra Madre Oriental. 2. 1938. no.. E. 1961. 8. 65. Té1Tdy. University of Texas at Austin. L. and F.. Blount. in C. Geology and Mineral Resources of north central Chihuahua: El Paso Geological Society. and D. Aubouin. Bosquejo sobre la estratigrafía y pélleogeograffa de los fiysch Cretácicos del sector tranversal de Pc1rras. González-Garcia. Diaz. 2. Pilger. Saltillo. Gries. UNAM. Flawn. UNAM. v. Baton Rouge. 1970. p. 105-116. 1974. Extensión de la cuenca Cretácica Chihuahuense en Sonora septen ­ trional y sus deformaciones: Memória del Tercer Congreso Latinoamericana de Geología. La alóc­ tonia del conjunto Cadena Alta-Altiplano Centréll. S.. Guerrero. 2. p. pre­ Atlélntic Ocean plate reconstruction élnd the early fift history of the Gulf and the north Atlantic: Transactions Gulf Coast Association Geological Societies. Chihuahua. R L. v. ¿En qué consiste una reserva petrolera? El Petróleo en México y en el Mundo. Insti tu to de Geología. 1979. UNAM. and R. La transversal de Guatemala y la Sierra Madre de México: J. Mexico: Bureau of Economic Geology. no. Instituto de Geología. R. to semi-hot and humid in the lowlands. examples of the largest are La Primélvera in the State of Jéllisco and Los Húmeros in the Sta te of Puebla. such as Paricutín..{~~. and rhyolitic volcanos such as are encountered southwest of Guadaléljara. the coastlines of the Pacific. and the northern part of the Southern Gulf Coastal Plain are included in this regio n (see Figure 11).... the Sierra Madre del Sur..~~~:t (. REGlaN . _. since within each 55 one of the domains stratigraphic and tectonic condi­ tions are more or less homogeneous wíth well­ defined limits. the climate is in general subhumid and varies from temperate to semi-frigid and cold. The physiographic provinces of the Neovo1canic axis. It is formed by a large variety of volcanic rocks that were emitted along a significant number of vol­ canos. The volcanic actívíty in this belt has given rise to a large number of internal basins. ..\. the Tlaxjaco basin.9LIMA ( ". NEOVOLCANIC AXIS The Transmexican Neovolcanic axis is composed of an upper Cenozoic belt that transversely crosses the Republic of Mexico at the 20th paraUel (see Figure 3.~. the Guerrera-Morelos basin. the Sierra Madre del Sur. In the regions of basins within the Neovolcanic axis.:: .<~. Geology of the Central Region of Mexico • GUADALAJARA .. hot.. this chapter treats eélch of the six domains of this region separately. For this reason.. According to Mooser (1972)... :' v I<> > VERACRUZ . the Altiplano of Oaxaca.~. On the Pacific slopes the climates vary fram hot and subhumid on the southeast flank of the Sierra Madre del Sur and the banks of the Río Balsas to semi-arid.2). These domains coincide in large part with the geological provinces proposed by López­ Ramos (1979) for this region... ~ . and very hot in the Valley of Oaxaca and the major part of the Balsas bélsin.• 3.... the shore of the Gulf of Mexico and the Isthmus of TehUimtepec. some of which constitute the highest peaks of the country..../ <•••• ­ .> " GENERAL CONSIDERATIONS In describing the geology of the central mea of Mexico.. In the central Mexico region. there exist vents of cinder cone type that are generally small. the provinces of the Vera cruz basin (with the subprovince of Sierra de Juárez). the northern edge of the Neovo1canic axis./. the foUowing limits have been used: to the north. El Popocatépetl.. the Neovolcanic axis has a zigzag pattern caused by the presence of a fun­ . El Nevado de Toluca.../ .:..- .. !PUEBLA\. such as El Pico de Orizaba. In addi­ tíon to these types of centralized emissions. and the Neovolcanic axis are induded.. This makes general descriptions rather fruítless. the province of San Andres Tuxtla... There are also some calderas caused by both collapse and explosion.-.. which in some regions are observed to be superimposed..? MEXIC?~z..1). Ellztaccíhuatl. . and El Nevado de Colima (see Figure 3. AlI of them were built by alternating pyroclastic emissions and lava flows... and to the east..'. sequences outcrop that élttest to diverse domains of various stratigraphic leveis. with the consequent occurrence of lakes thi1t give the geo­ morphic landscape a very characteristic appearance.-' CENTRAL \.. In addition. to the west and south.. The principal volcanos located in this province are stratovo1canos of híghly variable dimensíons. In terms of the division of geo­ logical provinces used by López-Ramos (1979).. This format facilita tes description and synthesis.j./··-<. The clima te of the region is highly variable owing to the complex physiography On the slopes of the Gulf of Mexico the dimate changes fram humid tem­ perate in high parts of the Sierra Madre Oriental..--.. ... there is evidence of numerous fissure emissions and adventi­ tious developments on the sides of the great stratovol­ canos.. The first has a northwest-southeast orienta­ tion and is associated with the volcanos of San JUéln. and Tequila. such as Guanajuato and Pachuca. seven phases of vulcanism have been recognized (Table 3]) that ha ve occu rred since the Oligocene. The last phase. The second pos­ sesses a north-south orientation and is associated with . constitutes a group of five principal focal points of activity with distil}ct orientabon and charac­ teristics. the Neovolcanic axis is bor­ dered by the tectonic troughs of Tepic-Chélpala and Colima. The most important of these is the fifth. while the great mining centers of the region. Sanganguey¡ Ceboruco. more than just fonning il continuous belt of volcanic rocks. and Nevada. The first volcanic manifestations in the area of the Valley of Mexico¡ in the upper Oligocene.. the last volcanic episodes of the Pleistocene and Quaternary in this portion of the axis seem to be related to a system of fractures with east-west orientabon as in the Siern1 de Chicl1inautzin (Mooser et al. Within these five principill centers it is possi­ ble to recognize two types of volcanic structures: (l) those represented by great stratovolcanos in north­ south alignment. Distribution of rocks forming the Transmexican Neovolcanic axis. and (2) those represented by numer­ ous small volcanos aligned in a northeast-southwest trend. would remain situated on the northern apices. This impiHts the zigzag aspect to the axis. Oemant (1978) considers that the Neovolcanic axis. are observed to be principally associated with fractures of west-northwest and east-southeast orientation ilnd with influence of frélctures with northeast-southwest orientéltion. developed over tensional fractures. and Nevado de Colimél. egual to the former¡ was developed in the Quaternary and is responsible for the volcanic acti vity tha t cut off the drainage of the basin of Mexico toward the basin of the Río Balsas and caused the enclosed interior drainage of the for­ mer feature (Mooser et al. The great stratovolcanos¡ as at Tancítaro¡ Nevado de Toluca. 1974). During the sixth phase the cones and domes of Iztaccihuatl and the active con e of Popocatépetl were developed. Río Frío.1. In its western part. In contrast. In the central portion of the axis. The latter orientabon seems to be related to transcur­ rent movements principally in the eastern and central portions of the belt.56 Section 1 The Geology of the Mexican Republic Figure 3.. datnentaJ system of orthogonal fragmentation with a northwest and northeast direction of the fractures. which occurred at the end of the Miocene and that gave rise to the mountain ranges of Las Cruces. 1974). Popocatépetl. would be situated on the southern apices of this system. J« 0­ :>ow f­ -­ I I ([([ W<{ >­ I BALSAS GROUP ~~ -'~ Table 3. 15 . 29~~31 • - $..U üi SIERRAS MAYORES GROUP SIERRAS MENORES GROUP MIDDLE TERTIARY GROUP <{ CHICHINAUTZIN GROUP VULCANITES OF NORTHERN PART OF 8ASIN I I I ¡¡oz ~"" o zw <{ « f­ ([ z w t.2.w ([ w -­ o « ~ - LL o­ f <:1 Z :J <t: l.. Pelee type with a plug of dacite lava that might cause the development oE Nues Ardentes.991 '----' 50 km 1 2 3 4 5 6 7 8 SAN JUAN SANGANGUEY TEPETILTIC HILL CE80RUCO TEOUILA PRIMAVERA CALDERA NEVADO DE COLIMA COLIMA 9 10 11 12 13 14 15 16 APAXTEPEC PALAMBAN HILL PARICUTIN TANCITARO HILL BUENA VISTA HILL JORULLO CAPAXTIRO GRANDE HILL 17 SEAS OF SANTIAGO VALLEY 18 CULlACAN HILL 19 GAVIA HILL 20 LOS AZUFRES· SIERRA DE SAN ADRES 21 AMEALCO CALDERA 22 HUICHAPAN CALDERA 23 NEVADO DE TOLUCA 24 XITLE 25 26 27 28 29 30 31 32 SIERRA CHICHINAUTZIN IZTACCIHUA~l POPOCATEP8l. The tatter vent constitutes.~ 20' -. ­ lOS' I I 17 l. Toward the east the axis is bordered by the vol­ canic rocks of the San Andrés-Tuxtla region.L Z ([([ >­ w« ~~ ~w f­ ir f­ (. Sequences oí volcanic groups and tectonic events oí the basin of Mexico. I 18-.l3? ¡ B' 27 • I I I O 25 1 G) ~8 . TECTüNISM AGES MODERN FILL > Z o <{ L.lAN (LOS HUMEROS) CALDERA Figure 3.) ---­ of­ -([ a: wi'. the Nevado de Colima and the Volcán de Fuego (Colima Volcano).U <{ o f- >­ ([ a: a: L.3.. although .. in the judgment of Demant (1978). LA MALlNCHE DERRUMBADAS HILL PICO DE ORIZABA COFRE DE PEROTE TEZI .1.­ I I I I . Geology oí the Central Region of Mexico 57 I 11 I~ O~ I 3<S{f)4 i ¡ i OC)16 10 . the IDost dangerous vol· cano of the Neovolcanic axis. Distribution oí the principal vents in the Mexican Neovolcanic axis.: .32 30 l ~ I I . since it is a vent of the Mt.-----.¡-.­ 21 0 22 I I I 1 q11 f€l12 @13 1 '\720 I 014 \3)23 q24 .J 19 18~V ¡ -----. Negendank (1972) supposes. local isolated occurrences of recent rhyolitic volcanics exist. According to a model of Gastil and Jensky (1973).obin (1975) consider the rocks of this regio n to belong to the Eastern Alkaline Province since they fix the eastern limit of the province at the latitude of Pico de Orizaba and at Cofre de Perote. This would indicate that the age of the Iaxco Schist is not Precambrian. in the for111 of <1 zone of left-Iateral dis­ placement between the American plate and the Caribbean plate. 1974. Urrutia­ Fucugauchi and Del C<1stillo. This author believes that the zone of lateral dis­ placement indicated aboye couId have operated as a structural control for the exit of the magmas produced by the subduction of the Cocos plate under the American plateo Mooser (1975) considers that the Neovolcanic axis could have coincided with the scar (geosuture) that marks the union between two ancient cratonal masses and whose zigzag arrangement would reflect the fact thc1t the Cocos plate. in the area of Azufres in Michoacán. Al! this is responsible for a gradual decrease in the angle of subduction toward the southeast end of the trench and causes a horizon­ tal angle of 20° between the Acapulco trench and the Neovolcanic axis. is located for the most part within the State of Marelos and in small portions of the northeastern State of Guerrero and southeastern State of Mexico. The origin of the Neovolcanic axis has been reIated chiefly to the subduction of the Cocos plate beneath the continental crust of Mexico. The petrographic composition of the rocks forming the Transmexican Neovolcanic axis is highly variable. Demant (1978) considers that the vulcanism of the axis is solely Pliocene-Quaternary. 1978). about 40 million years ago (McDowell and Clabaugh. 1960. The lateral movement within this system reflects the rotation of North Americc1 t0'. more than from partial fusion of the Cocos plate at the level of the asthenosphere. Demant. This author indica tes that the andesites of the Oligocene are folded.vard the west with respect to the Caribbean plate. such as those localized in the dómes of the Primavera Caldera in Jalisco. In addition. would ha ve been divided into slightly overlapping zigzagging fragments. In contrast. 1974). have been considered recently to be andesitic in view of chemical analyses of rock samples (Mooser et aL.. 1972. De Cserna et al. THE MORELOS-GUERRERO PLATFORM The area of the Morelos-Guerrero platform.. after foundering in the Acapulco trench. 1978) (Figure 3. Nevertheless. From a chemical point of view. he notes that in the eastern segment of the axis. im portan t right-Ia teral d isplace­ ments occurred in the axis in the Late Cretaceous and in the early Tertiary. in which important marine Mesozoic deposits are devel­ oped. 1977. based on the chemical characteristics of the rocks of the Neovolcanic axis. 1975). Urrutia-Fucugauchi (personal communi­ cation) considers that the movement has been left-Iater­ al. . since the lower cycle of the Oligocene-Miocene constitutes the south­ ern prolongation of the volcanic system of the Sierra Madre Occidental. Within this activity two principal cycles have been recognized: (1) Oligocene-Miocene and (2) Pliocene-Quaternary. less warm. Urrutia-Fucugauchi and Del Castillo (1977) explain this lack of parallehsm by means of a model which demonstrates that the direction of movement of the Cocos and American plates is not perpendicu­ lar to the Acapulco trench and that the northwest and southeast extremes of the Cocos plate become more dense. which includes the continental por­ tion of Central America.58 Section 1 The Geology oi the Mexican Republic Demant and H. that this calcalkaline province originated as c1 result of the partial fusion of materials from the lower crust. and he calls attention to the available paleomagnetic data. in concordance with the move­ ments observed in the \"lestern United Sta tes. Most authors agree that the activity of the Neovol­ canic axis began in the Oligocene and that it has con­ tinued up to the Recent (Mooser et a!. although the obligue position of the axis with respect to the Acapulco trench does not result in a feature typical of this type of phenome­ non. outcrops of these andesites are very rareo Ihis author does not clearly establish the relationship of these intermediate rocks with the Oligocene ignimbrites of the Sierra Madre Occidental. although numerous dacite and rhyolite units also existo Some units traditionally rec­ ognized as basalts. which in a similar area underlies a lightly metamorphosed andesite unit that Fries (1960) termed the Old Taxco Greenstone Campa (978) indica tes much similarity between the Taxco Schist rocks described by Fries and the voIcano­ sedimentary rocks of the Lower Cretaceous that crop out to the west of Teloloapan. as well as in Los Húmeros and Puebla (Demant. the subduction of the Cocos plate along the Acapulco trench commenced to develop progressively in the Oligocene. According to Demant (1978). Some c1uthors have indicated that the Neovolcanic axis coincides with a zone of lateral slippage that was active during the past. 1975. 1975). The calcalkaline character of this province supports the aboye hypothesis. Ihe marine sedimentary seguence exposed in this region covers a chronostratigraphic range from the Upper Jurassic to the Upper Cretaceous. Flows and pyroclastic products of andesitic composi­ tion are abundant. as in the Sierra de Mil Cumbres in the Lake Chapala region and in the Tzitzio-Huetamo anticlinorium. Negendank. This sequence rests on Precambrian metamorphic base­ ment represented apparently by the Iaxco Schist (Fries.3). 1979). the Transmexican Neovolcanic axis is consid­ ered by numerous authors as a calcalkaline province characterized by its abundance of andesites and dacites and by the ratio maintained by content of sili­ ca and sodium and potassium oxides. such as the Chichinautzin Group. Bloomfield. which at the level of the asthenosphere induced partial fusion and origi­ nated the magmas of the axis (Mooser. which still is active along the system of the Polichic-Montagua-Cayman trench. and older as well as of greater thickness and rigidity. where the real andesitic activity had ceased by the end of the Eocene. The base of the Mesozoic marine package is repre­ sented by the Acahuitzotla Formabon of Late Jurassic age (Fries. 1956).3. the sedimentary marine sequence oE the MoreJos-Guerrero platform rests over a metamorphic basement oE Paleozoic strata represented by the Acatlán Complexo The marine sedimentary units of this region are covered discordantly by Cenozoic continental deposits and voIcanic rocks oE the Neovolcanic axis.. Tectonics oí the Caribbean and central Pacifico and in that case this unit would not form part of the of the metamorphic basement aboye which evolved the Mesozoic sedirnentary sequence oE the Morelos­ Guerrero platform. 80th formations show the effect of weak .3. \ \ NAZCA PLATE SOUTH AMERICAN PLATE ~ A B C D E F G H I J San Andres-Gulf of California Fracture System Rivera Fracture Rivera Triple Junctlon Clarion Fracture Orozco Fracture Siqueiros Frac1ure Clipperton Fracture Galapagos High Panama Fracture Tehuantepee High K L M N O P O R S T Cocos High Carnegle HIgh Nazca High Meso-American Trench Peru-Chrle Trench Polochle-Motagua Fault Cayman o Bartlett Fault Pequenas Antillas Subductlon Zone Puerto Rico Treneh Oea .. which js formed by calcareous and argillaceoils sedjrnents that crop out in isolated locali· tieso This formabon underlies with erosjonaJ discor­ dance calcareous shales of the Neocomian Acuitlapan Formation. as well as by sorne Oligocene remnants of rhyolitic volcanism. Geology oi the Central Region of Mexico 59 AMERICAN PLATE / PACIFIC PLATE . Toward the borders of Guerrero and Oaxaca.El Pilar Faull Figure 3. 1978). Thl' Cosoltepec Formation. The litboJogic cb. Lélter.racteristics clnd lhe fauna reve. The end. ca 1careous schists. Salado. Jalisco. aJthough its characteristics are not always the Sélme. 1956) and a period of erosion markecl by tbe presence of éln unconformity that places the Xochicalco Forméltion in contact wilh véHious p<lIts of the Morelos Forrnation. tale schists.) of Taxco (Fries. mylonite. The Tecomilte Formation is composed of metaren­ ite. eclogite. 1978) (see Figure 3. since this is the first time in Mexico where tbe presence of eclogite rocks has been report­ ed. After deposition of the Xochicalco Formation an uplift occurred in the region that gave rise to the paleopeninsul. Tecomate. The Chazumba Formation is fonned prin­ cipally by biotite schists with intervals of quartzite. Fries and Rincón (965) defined ii as the Acatlán Formation. is formed by a sequence of thin limes tone beds that rest upon the Acuitlapan Forméltion. élnd qUélrtzite in an élsscmblage that. In tllt' Olig0cene-Eocene interval. ser­ pentinite.60 Section 1 The Geology of the Mexican Republic dynarnic metamorphism. Fries el al. H is formed of thick beds of limes tone and dolomite théll in one sequence reach up to 900 m thick élnd thélt have in the base an anhvdrite member some rneters tbick. The upper Tertiary and Quaternary are character­ Ized in this region bv the influence of volcanic activitv of the Neovolcanic axis and by the development ~f J tectonic trenches thélt caused the deposition of conti­ nental c1astic sediments of the Cuernavaca Formation. he employed some names that had already been utilized by Rodríguez (1970) in an informal sllbdivision that included in the ACClteco Group the forrné1tions Esperélnza. a classic migmatite derived from sedimen­ tary rocks.md beginning of the Tertic1fY. this region suffered consid­ erable warping during the Miocene thilt is evidenced by the dipping beds of the Balsas Group and by the ilbnor­ mal elevél tion of the Oligocene ignirnbrites. which togcther with tbe two aboye units makes up the structurally lower Pellancingo Subgroup. The Xochicalco Formation of Aptian élge. drained by the Mixtcco and Acateco rivers. and Esperanza grMtitoid forma­ tions as wcll as the Totoltepec stock and the San Miguel dikes. This metamorphic unit was termed origincllly the Acatlán Schist by Sélléls (1949). The formation that constitutes the structurillh lower part of the Acatlán Complex is the Magdalen~ Migmatite. is composed of pSilmmitic and pelitic schists with the presence of greenstones. and semipelites partially of tuffaceous origin. accompéllücd by continental c1astic sdimentation over the low parts of the newly created topography. At the end of the Cretc'lceous í. Recentlv. These deposits ''''ere followcd by importanL siliceous voJcanic emissions that formed the ignimbrite cover of the Taxco area termed the Tilzapotla Rhyolite and by volcanics and voleilnoclastic deposits of the Tepoztl. "and stone. metilmorphosed chert. This author divided the Acatlán Complex into two subgroups termed Petlancingo ancl Acateco. The first formation is composed of greenschists. The ACé1teco Subgroup is composed of the Xayacatlan. and pegmatitic rocks that are cata­ c1élStiC and metamorphosed and in certain élreélS have been considered by Rodríguez (1970) as part of the Oaxacan Complex. differentiated metélgilbbro. a nd conglomerate Cilme to form a sequen ce more th. compres­ sionill deformiltion occurred tha t resulted in the for­ mation uf élnticlinal élnd synclinal folds. (1970) ind icated éln age of 440 ± 50 Ma for this intru­ sive. metagabbro. in tense norrnal félultmg occui-red. 1960). According to Campa (1978). and pelitic schist. siltstone.ln Formation. ThlS latter formation cont>ists of a calcareolls unit that accounts for the most extensive outcrops of the region. possibly makes up an ophiolite complex and is of great importance. In the lithostratigraphic division that Ortega­ Gutiérrez introduced at tbe formé1tional level. 1978). amphibollte. of the Turoniélll marked a drastic change in the sedimentiltion of lhe Morelos-Guerrero pJat­ form rcsulting from uplift of a majar pOIt of the "01­ cano-sedimentélry areas located in the western regíon of this pilrt of MeAico. éllso in rare outcrops. During the Turon ia n a n i nvasion of tbe sea was repeélted ancl calcareous sedimentation was reestab­ hshed with the development of a calcareous bank toward the west of a line trending from Cuernavaca to Huit7uco. This clastic continental sedimentabon was initiated in the rniddle of the Cretilceous in the ilreas located west of this region. aplitic. Deposition of conglomeréltjc materials \Nas contemporélneous with some basaltic 1. ilnd manganiferous rocks (Ortega-Gutiérrez. and Colima. as weH as metamorphosed Iirnestone and meta­ conglomerate. pelites. pointing to its varied lithology and structure. The Esperé1nZél grélnitoids ilre formed by granitic. Ortega-Gutiérrez (1978) elevated this unit to the ra¡{k of a complex. according to Ortegél-Gutiérrcz (1978). lts name hélS been applied to scquences of limestone that exlend toward Michoacán.wél flows that gave rise to the lithostratigraphic ilssem­ blage termed the Balsas Group (Fries. which would be in the Ordovician. is characterized by extensive ou tcrops of meta­ morpbic rocks of various types that form a complcx of early Paleozoic ilge (Ortega-Gu tiérrez.m 1200 m thick developed in the Tu roniéln-Campilnian interval.)1 that this unit formed from shallow water marine deposition during the Albiéln-Cenomanian intervalo At the end of the Cenomanian an emersion occurred in the area with the emplacernenl of various lncks )1 gr<lnik and with differential erosion of tbe top 01 the Morelos Formation (Fries.4). ilnd Tecomilte. The Totoltepec stock is éln intrusive of trondhjemitic composition with slight foliabon élnd could have resulted from the differentiation of a tholeiitic gabbro <Ortega-Gutiérrez. The name "Siln Miguel dikes" has been applied to a series of tab­ ular intrusive bodies of grélnitic and tonalitic composi­ tion thélt affect some units of the Acatlrín Complexo . The deposits of shélle. 1956). METAMORPHIC REGION Of ACATLÁN The regio n that includes the higher part of the Béllsas basin. pelitic schists. Acatlán. vo1canic SIERRA MADRE DEL SUR AND ADJACENT AREAS Ihe Sierra Méldre del Sur. from Colimn to Onxacél élnd contiguous areas of northwestern Guerrero. and Morelos.:. and north­ em Guerrero and contains outerops of andesitic vol­ eanic rocks interstratified with 5iH)' redbeds.. west of the State of Mexico and sonth of Michoacán.4). Michoacán. Guerrero. Ihis group correlates with the Chacús Group of Guatemala and with the metamorphic rocks of the Sierra de Omoa in Honduras. Ihe most northern segment of the Sierra Madre del Sur ís formed by outerops of Mesozoic seguences. and the State of Mexico. Geology of the Central Region of Mexico 61 99' 20'---'~. both platform sediments and vo1caníc rack sediments of ísland are type. . According to Ortega-Gutiérrez (1978).. j 1 16°. Ihese are covered by Cenozoic conti­ nental volcanic and sedimentary rocks.-- o_-=-.3.. Metamorphic complexes in central Mexico. Oaxaca. This region borders on the east the area of the Cretaceous Marelos-Guerrero plntform at the latitude of the linea­ ment of Ixtapan de la Sal-Taxco-lgunJa. Areas found in northwestern Guerrero. The southern segment of the Sierra Madre del Sur is formed by extensive Olltcrops of metamorphie rocks that have n geochronologic range vnrying from Pélleozoie to Mesozoic and that nre seen to be affeeted by batholith­ ie emplncements of late Mesozoic and eve'n Cenozoic age. Miehoncán.4. nor with the Oaxacan Complex (see Figure 3. form a region vvith partially meta mor­ phosed volcnno-sedimentary rocks of ]urassic élnd Cretélceous élge. Tlle Pacific area of the Sierra Méldre del Sur includes the States of Colima.5). I Mesozoic Teloloapan-Ixtapan complex Paleozoic-Mesozolc Mazaleco complex Paleozoic-Mesozolc Xolapa complex lIIIIll Paleozoic Acallan complex ~ Precambnan Oaxaqueno complex Figure 3.--1. the character of the Acatlán Complex leads to the supposition t!tat it is an ancient marine eugeosynclinal deposit with a style of tectonie deformation nnd metamorphism resembling that of the internéll 01' deep zoncs of an alpine orogenic belt. Thc assernblage of the Acatlán Complex is found covered discordantly by numerous igncous and sedi­ mentary units that inc1ude an age range which varies from the late PaJeozoic to Quaternary and constitutes the basement of an cxtcnsive region that includes parts of the sta tes of Puebla.- - ---- . Tt seems to have no similar relationship with the Xolapa Complex of the Sierra Mad re del Sur.­ I I I I I I --~--_. makes up a region of high strueturnl eomplexity that contains var­ ious juxtaposed teetonic domains (Figure 3.- r.. In a large part of the Sierra Madre del Sur¡ from the northern tributarjes to the area near Zihuatanejo¡ Campa and Ramírez (1979) have reported the existence of numerous mountains formed by andesitic materíals interstratified with some beds of limestone and terrigenous clastics dis­ semínated in small areas in the Sierra. These underlíe¡ in apparently transitíonaJ contact¡ continental terrigenous Upper Cretaceous sediments. the platform limestones contain great thicknesses of intercalated evaporites. . De Csernél (1965) reported the Xolélpél Complex on the highway from Chilpancingo to Acapulco as an assem­ blage of metasedimentary rocks formed of bjotite schists and gneiss with sorne quartzite and cipolin marble horizons¡ and including the presence of peg­ matites. the stratigraphic range of tbis complex has not been precisely deter­ mined because the geochronologic studies have given very disparate radiometric ages indicatíng that thermal events occurred in the Paleozoic (Halpern et a1. and in the Tertiary (De Cserna. In addition they indjcate that this is the first record of dinosaur tracks in Mexico and constitutes the southernmost trace of dinosallrs in North America. The southern haH of the Sierra Madre del Sur is forrned from metamorphic rocks that constitute the Xolapél Complex (De Cserna. Guerrero et al (1978) rely on the existence of a thermal event in the Tertiary (about 32 miHion years ago) in the area of the highway of Chilpancingo¡ and in their radiometric determina­ tions¡ which failed to indicate Paleozoic or Precam­ brian ages as suggested by other authors. De Cserna at al. Most authors ha ve reported the volcanic-sedimen­ tary sequences of this Pacific region of Mexico as being of Mesozoic age. enst of Zihuatanejo. 1965). In areéls situated in the neighborhood of Colima City. These outcrops form part of what Vidal et al. Guerrero and co-workers (1978) consider that in the majar part of this regio n the cornplex is forrned trom quartz-feldspa thjc orthogneiss of granodiorite composition.5. This is found to be intruded by batholiths of granite (see Figure 3. This Mesozoic vulcanism continues toward the north bordering the Pacific coast until it beco mes blurred with similar areas of the North American Pacific Cordillera (Campa and Ramírez¡ 1979). the complex is never seen in a locality where it underljes Paleozoic sedimentary rocks. De Cserna considers this metamorphic cornplex to be of Paleozoic age¡ given that it underlies the volcano­ sedimentélry sequence of the Chapolapa Formation¡ which is probably of Triassic age. Ferrusquía and co-workers (1978) have reported the presence¡ in the area of Playa Azul.¡ 1974)¡ in the Mesozoic (Guerrero et al.62 Sectíon 1 The Geology oí the Mexican Republic HONDURAS ( ) Sierra Madre del Sur TZI. Michoacán¡ of él transitional volcanic-sedimentary sequence with dinosaur footprints that indjcate perhaps a Middle Jurassíc to Early Cretaceous age. Schematic tectonic model oí the Sierra Madre del Sur.4). Campa and Ramírez (1979) as weH as Vidal and co­ workers (1980) consider that the Mesozoic vo1cano­ sedimentary sequences of a majar part of the Sierra Madre del Sur nre the result of mélgméltic activity from convergent edges of pla tes developed in this part of Mexjco during the Early Cretaceous. In the most southern section of the Sierra Madre del Sur¡ corre­ sponding to southern Guerrero and western Oaxaca. Nevertheless. (1978a) obtained a Rb-Sr radiometric age of 311 ± 30 miIlion years for intrusive rocks strictly related to vol­ canjc rocks belonging to the metavolcanic complex of Zapotillo. 1965). and subreef (slope) limestone beds that contain an Albian fauna. the Xolapa Complex bas an ampholite facies derived from sedimentary rocks and orthogneiss \·vith abun­ dant migmatites. conglomera te. Nevertheless.TZIO HUETAMO TELOLOAPAN IXT APAN DE LA SAL GUERRERO . There exist furthennore. Nonetheless.MORELOS PLATFORM Continental Crust Figure 3. These authors recognized the oldest thermal event élS Jurassic by means of uranium-lead (165 ± 3 million years) and rubidium-strontium (180 ± 84 million years). As weH. in this northern portian of the Sierra¡ extensive outcrops of sedimenta­ 0' sequences of platform limestones with Albian fauna and rhythmjc sequences of terrigenous sandy muds. (1980) have called the Petrotectonic Assemblage of Zihuatanejo¡ Guerrero¡ Coa1comán¡ Michoacán.. '1978). De Cserna et al. 1979) (Figure 3. and Tertiary have been reported (Héllpern et al. in isolated exposures. metamorphics of the Acatlán Complex are present. These meta­ morphic rocks have been traditionally assigned to Precambrian (orthogneiss) and Paleozoic (phyl1ites and incipient meta-arkose) (López-Ramos. However. a Jurassic­ Cretaceous volcano-sedj¡~cntarysequence is exposed thaJ gradllaUy becomes more sedimentary toward its topo The base contains detrital sedimentary rocks interstratified with lavas and andesitic tuffs of the Jurassic that constitute the Angao Formation (Pantoja. anorthosite. 1978. Above this metamorphic complex rest sedimentary rocks of the Jurassic . ém impor­ tant sedimentary Mesozoic sequence crops out that attests to the development of a basin beginning in the Early Jurassic (Figures 3. this probably is a regional tectonic feature. To the west and south. lt is formed by banded gneiss meta­ morphosed from granulite facies to transitionéll gmn­ ulite-amphibolite. 1979) and to ha ve come from the western island arc domain De Cserna (l978b) believes that the absence of platform limestone in the Morelos Formation to the west of Teloloapan is due to a facies change into a basin in this area during the Albian and Cenomanian. 1962). situated to the east of Huetamo. including charnockite. Oaxaca. Guerrero et al. the Oaxacan Complex forms the limit of the basin. In the Teloloapan-Arcelia sector¡ a sequence of élndesitic volcanic rocks as weU as calcareous-argilla­ ceous foliated sedimentary rocks and graywackes constitute deposits of an isJand volcanic arc and mar­ ginal seas developed in the La te Jurassic and Early Cretaceous (Campa and Ramírez. sorne Paleozoic sedimentary units resting discordantly aboye the metamorphic basement. migmabte. Under this Mesozoic sequence there have been report­ ed. belonging to the lower Paleozoic and resulting from marine eugeosyncli­ nal deposi tion (Ortega-Gu tiérrez. and 940 million years (Precambrian). The Huetamo-CoyuCél sector forms a transitional zone between the external Mesozoic domain repre­ sented by the Guerrero-Morelos platform and the Mesozoic island arc represented by the volcano-sedi­ mentary outcrops of the Sierra Madre del Sur. Above this formation rest interbedded shales and sandstones with some tuffaceous horizons and with siltstones and reefal limes tones deposited in the Lower Cretaceous (Neocomian-Aptian-Iower Albiéln). To the northeast. 1974. would then be considered as tectonic allochthons transportcd ovcr the platform of the external domain (Campa and Rélmírez. Fries and co-workers (1962) carried out radiometric studies of the Oaxacan Complex that resulted in age dates of 1100 ± 125. This region of Mesozoic outcrops is limited by vari­ ous metamorphic complexes that are exposed in this part of the counrry. this complex is formed by aUochthonous blocks whose provenance is to the west and that have been thrust over miogeo­ synclinal sediments of Jurassic and Cretaceous during the Laramide Orogeny. 1959).. 1959). up to the area of Tejupilco. to the northeast. these authors indica te thélt the pegmatites and the Ié1st stage of metamorphism that affected the host rocks are equivalent to the Grenville metamorphic province of the eastern United States and Canada. Guerrero. The outcrops of this complex [orm a considerable part of the rooun­ tainous zone that is located to the west of the dty of Oaxaca. From here the olltcrops beco me isolated and less extensive. Geology oE the Central Region oE Mexico 63 In the Tierra Caliente region and adjacent areas of the western part of the State of Mexico and southeast Michoacán. Finally. To the southeast.920 ± 3D. along the borders of Guerrero and Michoacán. the top of the sequence is formed by beds of argillaceolls limes tone attributed to the Morelos Formabon of Albian age (Pantoja.8). and volcélnic flows of Cretaceous age. These deposits make up the San Lucas Formation (Pantoja. composed of gneiss. They may also be observed in the areas oE Ixtapan de la Sal.5). 1979). but thermal events of Paleozoic. Additionally. According to this author.6-3. This author considers that the volcanic rocks of the Teloloapan-ArccIia area. AIso . the basin is found to be bordered by the metamorphic outcrops of the western flank of the Sierra de Juárez with a markedly rectilinear contact thélt forms the Oaxacan Ravine. belong to a stage of Cenomanian-Turonian vulcanism (Xochipala Formabon) or could weU be a basement of ancient vo1canic rocks. Zitácuaro. 1978). aU this in a model withollt major tectonic complications. 1980). CharIeston (1980) reported the existence of an ample metamorphic complex derived from eugeo­ synclinal deposition of sandstone. In the Huetamo-Covuca section. extensive outcrops of partly meta mor­ phosed volcano-sedimentary sequences exist that are juxtaposed against other extensive outcrops of marine Cretaceous platform sequences from the areas of MoreJos ami Huetamo-Coyuca. and Tlalpujahua.md Cretaceous émd some unmetélmorphosed units of the Paleozoic. underlain by the Acatlán and Oaxacan com­ plexes. Corona (1981) and Flores élnd Buitrón (1982) discovered in the Olinalá area a seq uence of detrital and calcareous rocks with Pennsylvanian and Permian fossils. which form the volcano-sedimentary seqllence of the marginal sea and island arc proposed by Campa and Ramírez. The volcano-sedimentélrY seqllences of Teloloapan and lxtapan. Above the Acatlán Complex.. These volca no-sedimen tary seq uences crop ou t in continuous pattern toward the north. 1959).3. clays.. Jurassic. FinalJy. and Morelos regions. élnd biotite schists with amphibolite metamorphic facies (Ortega-Gutiérrez. contain extensive outcrops of Mesozoic sedi­ mentary units élrranged in north-northeasterly foJds. OAXACA AND AD]ACENT ZONES In the central region of Oaxaca and adjacent areas of southern Puebla and eastern Guerrero. The Puebla. Radiometric studies of these rocks gave ages [or the metamorphism corresponding to Upper Cretaceous and lower Tertiary (Charleston. the nonsedimentary Mesozoic exposures me bordered by the XoJapa CompJex. The age of this complex is apparently Mesozoic. 1976). and pegmatite. :!~7t­ (] l> \? P.. lxtaltepec. 1984).UERN •I lACA PUi=BLA • VERACRUZ. Pantoja-Alor and Robison reportcd in 1967 the discovery oí a marine sequenee with Cambri<m­ Ordovieian trilobites that was tenned the Tiñú Fonnation. The base of the Mesozoie sequence that is exposed in the Tlaxiaeo basin is represented by detrital sediments of the lower part oí the Rosario Formation. _. ._----+ ­ . the lower strata of the formatíon were not deposited. ~ I +. . 20--- 1 I ---I GULF OF MEXICO CUERNA~ACA PUEBLA ·1 • I~ ~ {7 \+ljh~. and southwestern Puebla.. • C/Jo¡e OAXACA : OCl("'N 16' - -----. These units íorm more than 1000 m of clastics belonging to the Mississippian.J_ I I GULF OF MEXICO I 20' I '----1------­ CUERNAVACA H.p ~ I I I I GULF I ME~~CO I 18 . together with the middle and upper strata of the former formation. northeastern Guerrero.. and Yododeñe formations rests diseordantly. I I I I I _ P". 1956).. Aecording to this author¡ the sediments oí the Rosario Formation were deposited in i1 coal basin that developed dur­ ing the Early Jurassie in northwestern Oaxaea.64 Seehon 1 The Geology of the Mexiean Republie I 99" 96' 20' ---------­ _. Sedirnentary rocks oí the Lower Cretaceous of the Guerrero-Morelos platíorm¡ Tlaxiaco basin¡ and southem sector of the Sierra Madre Oriental. • I I CLI o¿)lj'O . Puebla (De la Vega. and Permian systems (Pantoja-Alor.-­ I 99 96' Figure 3. belongs to the Middle Jurassic. Both formations constitute the Consuelo Group.. Oaxaca (Flores and Buitron. is found covering apparently both the Acatlán and Oaxacan complexes..-". its main outcrops oecur to the southwest of TehuaeéÍn.7. Sedimentary rocks oí the Upper Cretaceous oí the Guerrero-Morelos platíorm¡ Tlaxiaco basin¡ and southem section oí the Sierra Madre Oriental. \1. and in Tuxtepeque.-- II!: Vt . Pennsylvanian.. In the Noehixtlán region aboye the Oaxacan Complex.Ir::. Sedimentary rocks of the Jurassic oí the Guerrero-Morelos platíorm¡ Tlaxiaco basin¡ and southern sector of the Sierra Madre Oriental. Above the Rosario Forma tion rests the Cualac Conglomera te which.. Above this unit a sequenee made up of the Santiago. 1983).LJfv - 16'.LJC/ F /co Ct. o OAXACA I I I CH'!f'ANCINGO\ • " (J . On the western and eastern borders of the basin. 1970)..~.- L .. The Matzitzi Formation¡ with Pennsylvanian plant fossils (De Cserna. i Figure 3.­ I Figure 3. there have been reported aboye this complex discov ­ eries oí upper Paleozoic sedimentary rocks in Mixtepec.. which underlies the Tecocoyunca Group whose formations erop out in varioLls loeali­ . whieh is of continental origin and con­ tains coal horizons (Erben.- I -~-.6. 1970).8. was deveJoped in a period of é'lctive vulcan­ ism thé'lt originé'lted initially with the emission of siJiceous and intermedia te tuffs and l<lter and '. in a section locé'lted é'lt the 18th parallel. lt con­ tains fossils of the Coniacié'ln-Mé'lastrichtian stages ilnd crops out northwest of Nochixtlán. Pérez é'lnd co-workers (965) applied the nélme Morclos Formation to these limestones in the region of Acatl<l and reJated them to the . However. 1956) and the Chimeco ami Mapache del Sur de Puebla. C<lJlovian. intennediale. During the Aibian-Cenomanian interval é'I seqllence of thick-bedded limestones developed in él transgressive se<l. Ferrusquí<l (197(}) designated a mé'lssive biomicrite as th' Teposcolulil Limestone. This is widc­ ly exposed and has been tentatively attributed to the Middle Jurassic (López-Ramos. Dllring tbe Late Jllrassic.rrllsqu Íé'I (197ñ) as the YucUnal11él Formation.1tion of Sayultepec. 1960) The Oligocene. the Cidaris Limestone has been con­ firmed as belonging to the Lé'lte Jurassic beGluse of its echinoid fcluna of Oxfordian. Ferrusqllía (197ñ) mt:ntions <l radiometric age of ±49.'-\Ibi<ln-Ccnomanian str<lta that crop out on the Cuerrero-Morelos platform.imestone. In the sectür located soutb of the 18th parallel and down to the region of the Isthmus of Tehuantepec. Finéllly. 1970) lt should be noted that these Juras ie units that outcrop in the region of the ACCltlán Complex are not reported to be represent­ ed by similélf slTClta above the Oaxacan Complexo Tbe Upper Cretaceous also is represented by marine sediments. furthermore. However.1). such as the Cidilris Limestone i 11 the Mixtepec-Tia xiaco area (Erben. The Teposcululé'l f. in the ZongoJica-Tehuacán sector. This group is composed of bolh derrita] alld car­ bonate s~diments. in various 10caJities this group is absent and Albian limestones re<. The volcanic é'lctivity culmin<lted with some bé'lsaltic flows in the Neogene THE SECTOR SOUTH OF THE SIERRA MADRE ORIENTAL ANO' THE COASTAL PLAIN OF THE SOUTHERN GULF On the eastern flank of the sector south of the Sierra Madre Oriental (Sierra de Ju<Írel) . conglomerate. 1949) tha according to Erben 11956) élre distinct facies of the same unit. It can be cor­ related with the Tilantongo Marls (Salas. Above this unit lie BOO m of both fine and coarse clastic-cé'llcare­ ous beds of the Aptian San Juan Raya Formation.t discordantly above the Jllrassic sec¡uence. considered originally }urClssic by SalCls (949) é'lnd later by Erben (956).llatj()J1s of fine-grained sand ilnd calcareous shales.:' by exten­ sive OlltCropS of continental deposils.. These lime~tones hé'lve received dif­ L'r 'nt names in different Me<lS.. This crops out in a similar area and Wé'lS cOl1sidered Jurassic by Salas (1949). dating tbis formation as late Paleocene-middle Eocene. the base 01 the Mesozoic is formed by the Todos Santos Formabon. clec1rly mmine sediments werc deposit­ cd in 50111(' ilre<lS of the basin.: Tlaxié'lco basin is covered with angular djscord. ilnd mafic volcanic rocks. élnd beds of con­ glorneré'lte and breccia. 1 and Teposcolula no. Thi_ forma tion h<ls. The first is formed by c!ays with S0111e intercalations of sc1ndstone and vol­ canic ash. The Upper Jurassic is exposed in the Zongolica area (Vinicgra.u to that of the Tehuacán Formation (C<llderón. 1971.itic lava flows. both continentéll and marine. alld c. clrgillaceous limestone. and KimmeridgiCln ~lge (Buitron. 1949) that are expoO'cd southeast oí Nochixtlán and with the Mexcala Formation of the Guerrero-Morelos platform. Above the Albian-Cenomé'lni¿lt1 limestones líes <l sequence of marly limestone designated by ¡. This author indicates that tlw formabon has a stratigraphic position simi¡. However. a bas<ll unit of dark-colored slates with some interc. 1965) in the fmm of marine sequences . The Tertiary continental deposits hé'lve been assigned to the Yanhllitlán and HUili~hlFiH1 forma­ tions (Salas. gn ~isses.0 million years for a tuff inter­ stratified within the Yanhuitl¿in Forl11. The tolded Mesozoic seqllencc of the. has.1 thick sequence of Mesozoic sedimentary rocks is exposed that rests cm a metamorpbic basement composed of schists. In the Tehuacán area i1 ciastic­ calcareous sequence with beds of limestone crops out and constitlltes the Zapotitlán Formatiol1. in various localities in the state of Oaxaci1.Il1Ci. 1 cut él sequence oí more than 2500 m of Upper Jurassic and Lnwer Cn' dceous evaporites. and contains plant fossils and ammonites thé'lt indicélte several marine invasions and regressions. The sedimentary sequence of the €c1stern flank of the sector. 1970) on account of its faunal contento On the other hand. 1956) <lnd the Eal as Croup (Fries. Charl ~ton (1980) recognized a thick sequence of schists and metavolcanic rocks that he attributed to the Lower Cretélceous. 1965). ThesC' re sand­ stone-conglomerate ami argillé'lceous sandy be'!s of the Tertiary élnd include siliceolls. and phyllites.~lcareous shales (Pérez et Cl1.. in some localities the Neocomié'ln and Aptüm i1re prescnt. <lfgillaceous sandstones. formed of Iimestone. ilnd Shc1k. which is a sequence of continental redbeds with cross­ stratified sandstones. These have been derived prjncipally froro sedimenté'lry rocks <lnd have been tr¡'lditionally attributed to the Paleozoic and Precambrian. The ocomié'ln é'lnd Aptian formations of the central area of Oaxaca and central élnd south of Puebla have been included within the group termed Puebla. López-Ramos (l~79) believ s that ir could extend to the Triassic. sic (MulJeried. C<llderón (1956) desig­ né'lted i1 widespread sequence of massive micritic and biomicritic limestone with chert nodules that crops out in the ehuacán region aS the Cipiapa Formation.3. which forms folds asymmetric toward the east. for­ mation::. h<ls been recently <l'-signed to the Albian-Cenomé'lnian ([errusquía. 1957). Geology oí the Central Region oE Mexico 65 bes 11l the Tlélxiacn basin and also beJong to the Middlc Jurassic. Lóp~z-Ramos (1979) mentions that the wells Yacudá no. been recognized in Chiapas and northern Central AmCflCél where its lower part is consjdered Lower and Middle Jura. Each terrane contains a distinctly different basement and their limits have been generally inter­ preted as tectonic boundaries. 1983). Depósito. These are: Chicontepee. These rocks include the fol­ lowing formations: Tuxpanguillo (Neocomian). In the portion of the Gulf Coastal Plain that borders the Sierra de JuéÍ.9. Campa and Coney. Recently the structure of the region has been inter­ preted in terms of a mosaie of tectonostratigraphic terranes (see Figure 3.10). . chiefly in fields located in its eastern portion (González-Alvarado. Petroleum production has been obtained from these strata. 1965) Additional1y. FilisoLa. it is limited on the west by the Zongolica pale­ obasin and on the east by the Veracruz paleobasin (González-Alvarado. which is manifested in the form of granitic intrusions at the end of the Mesozoic and beginning of the Cenozoic.Velasco (Paleoeene). 1976). more than wi th the eas tern extreme of the Neovolcanic axis as some other authors have indicated.rez. Maltrata Limestone (Turonian­ Coniacian). 1981. Orizaba Limestone (Albian­ Cenomanian). Horcones and La Laja (Oligocene). During the Tertiary. These deposits began to fonn at the inception of the orogenic deformation of the Sierra Madre Oriental during the beginning of the Cenozoico The igneous activity of the southern sector of the Sierra Madre Oriental. Encanto.11) that were acereted in differ­ ent episodes during the tectonic evolution of this part of Mexico (Campa et al. Concepción. in the southern sector of the eastern flank of the Sierra de Juárez. and Chapopote (Eocene). Capolucan (Aptian). In addi­ bon. is com­ posed principally of calcareous rocks that have been recognized by Petróleos Mexicanos in both surface and subsurface studies. terrigenous sediments were deposited in the Gulf Coastal Plain. Guzmantln Unit (Turonian-Senonian) as well as the Necoxtla and Atoyac formations of Senonian-Campanian and Campanian-Maastrichtian age (Viniegra.. is restrieted to alkaline basaltie emissions in the area of Tuxtlas in the upper Tertiary and Quaternary.66 Section 1 The Geology of the Mexican Republic of bituminous limestones with intercalations of sandy-argillaceous limestones and with ammonites. which crops out in the northern portian of the Sierra de Juárez. TECTONIC SUMMARY The eomplicated structural and stratigraphie set­ ting of the central-southern portion of Mexico makes difficult a paleogeograpbic and tectonic reconstruc­ tion that permits a clear explanation of the origin of fea tures in this part oí the nation. Petróleos Mexicanos has driHed exploratory wells that have afforded recognition of Mesozoic units in the subsurface. outcrops of this age have not been reported. From these it has been possible to reconstruct a paleoplatform termed the Córdoba Platform. Aragon¡ Guayabal. which formed the marine sea floor during the second haH of the Mesozoic (Figures 39-3. The western half of the platform is exposed in the Sierra Madre Oriental and the eastern half is buried under the Coastal Plain of the Gulf. The marine Cretaceous sequence. the marine Cretnceous is represented in the area of the lsthmus of Tehuantepec by neritic fossiLiferous limes tones that López-Ramos (1979) inc1uded within the series of middle Cretaceous limes tones of Nizanda-Lagunas. in a setting of eastward marine regression. Situation of the Córdoba Platform. However. More than 5000 m of sed­ iments accumulated in this latter basin. and Paraje (Miocene). Demant (1978) related this volcanie zone with the alkaline province of the Gulf of Mexico. J976). GULF OF MEXICO Figure 3. . The terrane with the otdest basement is the Oaxacan. which occupies part of the state of that name and contains unmetamorphosed sequences of Cambrian-Ordovician and Mississippian-Pennsyl­ vanian... . formed in the Oaxacan Complex of Precambrian age (900-1100 mit­ !ion years). . Guerrero Terrane Mixteco Terrane Oaxaca Terrane Juarez Terrane Maya-Yucatan Terrane Complex 01 minar terranes including Xolapa Terrane Figure 3.. Geology of the Central Regíon of Mexico PLATFORM DEPOSITS (Step 1) 67 ZONGOLlCA BASIN-? Ks Km K.10. However. 2 .3.11. The metamorphic basement. . \ Figure 3. . 3. 6.i . 4. / 'IR .. \ / .. Tectonostratigraphic terranes from southern Mexico accord­ ing to the divisions of Campa and Coney (1983).. Tectonic evolution of the Córdoba Platform. the trilobite fauna of the Cambrian­ ... TILTING (Slep 11) FOLOING ANO EROSION (Steplll) NR . has been interpreted as the result of the evolution of a rift with sedimentation on ancient con­ tinental crust and later metamorphism to the gran­ ulite facies. 5. 2. This complex is considered to be a southern continua­ tion of the Grenvillian belt (Fries et al. 1. This resu1ted from an ensialic evolution or from continental collision (Ortega-Gutiérrez. 1962). 1981). Thc first of thcsc occurred at the end of the Jurassie. Carfantan (983) has suggested that this petrotectonic assemblage is the resuIt of the open­ ing ami closing of a n ocean bélsin occu rring between Portlandian and Turonian and causcd by deveIop­ ment of a rift that "vas connected to a triple junetion over él ridge located between Yucc'ltiÍn and South Amcrica.t o has been recognized. 1974). which extends into south and south­ east Mexico (Campa and Coney. the development of an arc domain in the vicinity of the continental crust of Mexico is proposed. The AcatlAn CompIex has been interpreted as an élggregation of petrotectonic assemblages resulting from the opening and closure of an ocean basin (Ortega-Cutiérrez. Additionally. detritéll. in the Sierra Madre del Sur. In the extreme ec""lst of the cen tra l-southern portion of Mexico. the coastal plélin and the platform of the Gulf of Mexico-all developed on continental crust. and voI­ canie rocks. The western boundi1rY of this belt fonns a mylonitic band that separa tes it from the Oélxacan Terrane. To the soutlnvest. Thc second phase OCCLlrred in the Cenomanian and is manifested in the Teloapéln-Ixtapéln élrea by metamorphism that folded and foliated the volcano-sedimentary sequence. 1974). associated with the subduc­ tíon of ocea nic lithosphere (Campél élnd Ramírez. élffected the Jurassie volC<1no-sedimentary deposits. This phase.'\'hole Mesozoie blanket of the two domains and is responsible fol' the folds in the exter­ néll zone as well as the overthrusting of the internal domain over the externa] zone. in the east an external zone with marine sedimentation evolvcd over the Guerrero-Morelos platform. These assemblages form pélrt of the Moya Terrane. 1983). The time of its accretion has not yet been confirmed. and the area of the cast flank of the Sierra de Juárez. which has traditionally been considercd relatcd to Appalachian dcformabon. 1984). including the Xayacatlán Forméltion élS the vestige of an élncient oceanic lithosphere consumed in the subduction process. although no report exists of assemblages of oceanic affinity thilt would indicate a suture. deformed Mesozoic marine sequcnccs arc recognized that revei'll a paleogeogrélphie setting of interspersed deep Elnd shallow marine SLlbstrates developed over the Paleozoic bélsement. two principal íVIesozoic domains with clearly distinct characteristics are recognized. Preliminary paleomagnetic délta of the volcano-sedimentary sequence of Ixtapan­ TeloJapan (Urrutia-Fucugauchi and Vc""llencio. Two alternative models have been postlllated in order to explain the development uf c'I volcanic island arc in the western domain of the central-sollthern por­ tion of Mexieo. The next phase occurred in the Paleocene and deformed the . con­ temporaneously with a méljor introducbon of terrige­ nous sediment comíng from the western emergcnt region. Bazán (1984) does not discélrd the idea of the existence of arc-type rocks in this complex. During this time marine sedimentatjon continued in the Guerrero-Morelos platform. The PetJancingo Subgroup would constitlJte a seguence of an élutochthonous pas­ sive nlc1rgin and the Acateco Subgroup would form the allochthonous assemblage. 1981).rginal seas. The partly metamorphoscd volcanic and sedimentélry assemblages of the Sierra de Juárez alter the homo­ geneity of this doma in. Separating the Maya and Oi1xi'lcan terranes. whose élge ami time of accretion to the tec­ tonic mosélic of southern Mexico are not well known. 1986) seem to point to the first hypothesis. In the central-southern portion of Mexieo. They are highly deformed and have a general ei1stwi1rd vergence. 1984).rine beds that inc1ude cél. This is a feature that is common to a m<1jor pélrt of western North America and thélt originélted during the initial breakup of Pangea.lcareous. and their presence is not clear­ Iy understood. . The boundary between these two terranes hc'ls be en interprcted to be tectonic (Ortega-Cutiérrez. 1980.68 Section r The Geology of the Mexican Republic Ordovician caver shaw more affinity with the fauna al' Europe and South America théln vvith the félunél uf urth AmeriGl (Whittington and Hughes. the Tlaxiaco basin. 1981). The first paleomagnetic dates for Permian units in both terranes indicc'lte similar directions of primary magnetismo This does not discount totally a later accretion (of the blocks) along the same magnetic paleolatitude (Urrutia-Fucugauchi and Morán­ Zenteno. caused the cmcr­ gence of the island arc terranes and ma. Coney. the time of accretion has been placed in the Late Jurassic-Earl y Cretaceous in terVul (Ramírez. él belt of apparently Mcsozoic stra. Mélrine sedimentabon of this externéll zone was initi­ c'lted with the opening of the Culf of Mexico and the marine transgression over this part of Mexico. In the west an andesitic island arc Wc""lS developed. and manifests itseU by the presence of folds refolded in two generéltions with a relatively increas­ ing metamorphism in some zones. According to Campa ami Ramírez (1979). based on the in ter­ pretéllion of greenstone belts in Precambriéln shield arcas. One of them proposes the accrction by obduction of an island arc system developed in the Pc""lcific élnd displaced in the direction of its col1jsion with the Mexican continental crust (Urrutiél­ Fucugéluchi. limited to the southwest by éln eastward subduction (Campa and Ramírez. 1983). 1979). contains diverse gréldes of metamorphic rocks. five phases of deformatían can be rec­ ognized that were c""lctive in Mesozoic and Cenozoic time. the Mixteco and Oaxacan ter­ ranes are bordcred in tectonic contact by the Xolapa Complex. but whose characteristics identify thcm as roots of a mountélin rélnge from éln ancient magmatic arc (Halpern et c'll. in contrast to contemporaneous rocks in the Oélxacan Terrane. As well. These are mé1. To the west of the OaXaCc'll1 Terrane lies the Mixtecan Terrc""lI1e. \vhich has the lower Paleozoic Acatlán Complex as basement (Campél and Coney.. 1981). and to its east. In the other mode!. 1979). in the northwest region of Guerrero ami adjoining regions of other states. but it has been suggested to be Devonian (Ortega-Gutiérrez. 1983) and which. Xitingil. mineral districts of hyd rothermal sulfides appear along a belt with north-northwest and south-south­ east orientation in the sta tes of México. the central­ southern region of Mexico has been affected by nor­ mal falllting and lateral displacement within a setting of general uplift and very great geodynamic ilctivity. and MichoacéÍn. and Assemblage of Taxco and Taxco Viejo. Zacualpan.ver Tertiary Balsas Group. but that have (In obscure and poorly understood relationship 69 between themselves.12).. These hydrothermal deposits are attributed by Campa and Ramírez (1979) to the end of the Miocene period. The discontinllities of thcse borders can not be explained clea rl y by con ven tiOlléll facies changes or unconformities. Assemblage of Zihuatenjo. To the south of the Neovolcanjc axis. could be part of a single platform and that the lxtapan-Teloloapan assemblage would be a tectonic allochthon of the compressional phase of Paleocene age. . zinc..3.12. aH of them integrated into the Guerrero com­ posite lerrane. the mercury deposits of Huitzuco and Huahuaxtla also are developed. Geology of the Central Regíon of Mexíco Campa (1978) has proposed two alternative models to explain the presence of the volcano-sedimentary assemblage of Ixtapan. él nd sil ver in a cen tra I belt. the author sllggests that the Guerrero-Morelos and Huetélmo sequences.Teloloa pan between the Guerrero-Morelos and Huetamo platforms. ilS well as iron oxides localized chiefly in the Sierra Madre del Sur (Figure 3. To the first category belong the miner­ al deposits of the Pachuca D1ining district. Angangueo. Dístribution of the principal mineral deposits known in the central part of Mexico. The borders of each terrane separate sequences that have djfferent physical and temporal characteristics. These authors have recognized in this region the following funda­ mentaL terranes: the Assemblélge ot Guerrero­ Morelos platform. Assemblage of Teloloapan. Temascilltepec. Assemblage of Huetamo-Cutzamala.is phase because uf the consideratioll that the continental sequence on the flanks of the structure is correlative with the 1m.lfe belongs to the Upper Cretaceous and not to the Tertiarv. Campos (1984) has a ttributed the folding to the Paleocene compressional phase since he cónsiders that the continental sequence on the f1anks of the struch. Campa 'and co-workers (1980) believe that in the weStern part of the central-southern portion of Mexico on e can recognize tectonostra tigraph ic ter­ ranes that are characterized by homogeneity and continuity of interna] stratigraphy. In the Pliocene-Quaternary intervaJ. Pb Zn-Ag • Mn Figure 3. The origin of the great strLlctllral élnticline of Tzitzio-Tiqllicheo of southeastern Michoacán is attrib­ uted to th. which is located at the northern edge of the Neovolcanic axis and has been one of tile principal silver producers in the world. élnd Tlapujahua. contemporaneous with the o A Au-Ag Fe • Hg OAu ~S . In one of these ít is suggested that the Ixtapan-Teloloapan assemblage is the result of the evoh-üion of an arc between the two platforms¡ but this does not explain the metamorphism of this é1ssemblage between the unmetamorphosed seqllences of the two platforms and the absence of facies changes from these plat­ forms to the volcanic éHC. At the end of the Miocene there occurred a phase of deformation that rcsulted in a warping that is observed in the Arcelia-Altamirano region and evi­ denced by the abnormaUy elevated metamorphic sequences and the pre-Miocene lithostratigraphic units. The band includes the field areas of Taxco. belonging to the Albian­ Cenomanian. Cuerrero. In the otber model. Within this belt. ECONOMIC RESOURCES The principal minerill resources known in the central-southern region of Mexjco are the sulfides of lead. :y I :J O Q 5 . 1969. the zone of major petrolellm interest is in the coasta! plain of the Gulf where petrolellm has been extracted in fields !ocated along the eastern edge of the Córdoba Platform in sedimentary rocks of the Cretaceous and where there exist good pros­ pects in sediments deeper than the Upper Jllrassic (González.1 I I I \ \ " .. I I . Mapes. . I . Los Húmeros. /" 9. Ixtlán de los Hervores. 15. ORO. 1.. . 14. .70 Section 1 The Geology of the Mexican Republic warping that affected pre-Miocene strata. 2. 1 I ~ " . The deposits men­ tioned are considered to be contemporaneous with the volcanic activity that occurred in the island arc zone formed during the Mesozoic in this part of Mexico (Gaytán et aL. ' 1 I ~\ [) F . / / / / ¡ / 1_. S. owing to its contemporane­ ous igneous activity. MICH. I ~ • I ¡ I I . Among the more important iron deposits that are known in this region are those of Peña Colorada. numero liS deposits of iron are located..2). 1 . ESTA. EL GOGORRON. and El Violín and Tiber in Guerrero. and Cuetzalán del Progreso. Pineda et aL.13 and Table 3. JAL LOS NEGRITOS. I \ ~ . In a belt situated along the Sierra Madre del Sur. Another group of sulfide mineral deposits exists in this region. and San Bártolo de los Baños. CHIS..­ I " 1 ? $'-­ \ .1 ? I I SAN IUIS POTOSI '1 AGlJi\SCJl. LOS AZUFRES. Negritos. I . I . PUE. I l' O J / I .C. .. 3. Zamora et al. . \ ~ Ú - y .DO DE MEXICO 01'1 . I CERRO PRIETO. HERVORES DE LA VEGA. T!apehuilla-Las Fraguas..b -" I I I '" . . r . JAL LA PRIMAVERA. Rey de la Plata.. . 1961. S... LOS HUMEROS. I . JAL SAN MARCOS. the Mexican Neovolcanic axis constitutes the geologic province with the major manifestations and potentials in the country. The origin of these deposits is attribllted to processes of contact metaso­ matism unleashed by the effect of silicic and interme­ diate intrllsions of the lower Cenozoic on the Cretaceous limestones (Gómez. 1979). \ •1 . HGO. Teloloapan. EL CHICHONAL.J GUANAJUArO I \ Ó . CHIS. MICH IXTLAN DE LOS HERVORES.l. The principal thermal manifesta­ tions are related to acid igneous activity. 1976). as weU as the volcano field in the north of Michoacán. 1959. TOLlMAN. ) \ . deposits of copper such as those oi Inguarán and La Verde in Michoacán exist in this belt Fllrthermore. 4 5 6 7. LAGO DE CUITZEO y ARARO./ . . MICH. Their origin has been attributed to vol­ canogenic processes that do not have a preferred orientation but are encountered associated with vol­ cano-sedimentary Upper Jurassic and Lower Cretaceous rocks. Pihuamo.. .. Figure 3.. 1975). Also. 13. and Lago de Cuitzeo.. 12.... In considering possibilities of obtaining geotherma! energy.N..I E ? -. To this group belong the fie!ds of Pinzán Morado. .--­ . . in Jalisco.. 1979. 8. .P SAN BARTOLO. DESIERTO DE ALTAR. Sorne of these are located in the areas of La Primavera.13. 16. 10. \ JALISCO . Las Truchas in Michoacán.. these make up the major reserves of the cOllntry. Michoacán.. I / I I 1 I ~-.. MICH. Jalisco. Queretaro (see Figure 3. in Colima. Campa and Ramírez. LA SOLEDAD. Campo Morado-La Suriana.. . 1... I I 1 NA'Y'ARIT • . 1.. I . J . . SON. \ . PATHE. Location of the most important geothennal fields in the Republic of Mexico. Puebla. \ '1 MIGHOAGAN . JAL.1 I " . .lC.arando Fm. Fm Mexcala Fm.:¡·CorooePOC>f' Er><:ao1l0 La Lala Frn Agua de Obispo Balsas Gp Fm Yannulllan F".. Morelos Frn HUlizuOO Fm Teposcolula !fm CIPI<l. ~ O ro ~ .ua1&s fm T~'t...1 Fm 1"~I.OO Fm Sn Juan Raya Caliente Fm Xochlcalco Frn ACUltlapan Fm AcahUlzotla .• ~IEill(l fm (llJay!l~l W Balsas Gp.:. ro ~ " O ~... ~l"... MIOCENE OLlGOCENE EOCENE PALEOCENE MAASTRICHTIAN Fm ef(O ..1t('.aJa-Ollnala OAXACA Andeslles Fm Sosola Frn Yanhulllan JUAREZ 1/ MAYA Cordoba Platlorrn Central Yucalan ü <3 ~ RECENT PLEISTOCENE PLlOCENE m Cuernavaca A Zampoala Fm AIQUllranlFm Papagayo I a: f- O r N Fm Oapan Fm Chllapa A San Marcos A Yucadoac Fm Sur...1 Xolapa Complex Comple x Fm Angao? I ? I R V Taxco VieJo C. j . O 03.'-':.cm ZlCapa Fn'I lapollllan Fm Mapache Fm r:""1.­ O . MOlelos Fm So Locas I Tierra II Fm Morelos Fm Orizaba Fm Xonamanca Fm San Pedro Fm Yucalan ALBIAN APIlAN NEOCOMIAN OW ü Fm AcahurzoUa t I I Fm. Stratigraphic correlations for southern Mexico. CJ ? Frn Ixlaltepec O N O ~? g.:'(19 _ O « Z Ü W a: fr.a.. ~ Fm Santiago ~ a.o- XOLAPA GUERRERO M I X Guerrero-Morelos T E e o Zapotitlan-T'ax~aco Mexc.) O (f)<¿ UPPER MIDDLE LOWER UPPER MIDDLE LOWER PERMIAN Fm Chapolapa (/) W(/) :2g¡ -. Tam:nu1()Pal.sle Fm Icaclle Fm Chlcnen liza ~ SENONIAN TURONIAN CENOMANIAN Frn.. Teposcolula Puebla Gp Arco and Cuenca Seql.... Cuautla(?) Fm Mexcala Marga Marga Yucunama Tllantongo I Fm..Cidaris Tecocoyunca Gp C&(. e.e Com pie.zJchll po lOCCQ".ence I Fm Guzmaolla Llmestone Sedlmenlary Soquence Frn Cuautla Fm. Table 3.h. Mexcala O Fm Mol ~ W f- ü _ N ~ a: Fin.. P~"~!. Todos San los Fm Todos Santos I -? Turnblscallo Sequenc:e w (/) :: « U I '~:~ Cm C_'=""" Cm ~"~'C:":"'" Frn Yododene ü PENNSYLVANIAN MISSISSIPIAN DEVONIAN SJLURIAN ORDOVICIAN CAMBRIAN ~ Undlfferenllaled Melarnorph. ~ O '" 2.-de Lobo!'.. So.J Fm Etlaltongo Ophlollles 11 Gp!~ 0::: -. ü (/) « Ir Fmlxc\J""'l'J~ar. Tepoxtlan Fm Tilzapotla A Buenavlsta Fm Tilzapolla Fm lIar._ I . Balsas Gp Fm Telelel(' o Gpo Balsas Frn Telelcinqo Flyseh Sequence Fm Aragon Fm Velasco Fm Aloyac Fm P.2.:J1 I Fm.1~'0.. -< Frn Tlnu PRECAMBRIAN Oaxaqueno Complex Recopdado pOr S Alarco y G Mora (1984) ~~ ~ n lb ~ lb ..Hll:'lNl:> Fm Ch. Tecomallan I I I Fm H.STRATIGRAPHIC CORRELATIONS FOR SOUTHERN MEXICO ERA DIVISION .¡ulllG fm.) Frt. Morelos .ni.J) Chaet.. Demant¡ A. v. región comprendida entre los estados de Guerrero. 1. Blume editions. v. p.. Erben. Estratigrafía de la región OlinaJá-Tecocoyunca.. Blume Ediciones. 154-163... Campél. v. una síntesis en relación con La. v. p476-497 Buitrón.324 p. R. H. Puebla: XX Congreso Geológico Internacionélt México. Bosquejo geológico de la región de Son Juan Raya.2. 1981. in JT. UNAM.. M. 1978. p. 1984. 51. Selecciones de Scientific American. C. U na nueva localidad Pérmicél en México fechadél con fusilinidos. Las félses del vo1con­ ismo en México. p. Instituto de Geología. and PJ. n. BS. Rocas metavo!cánicas e intrusivos relacionados Paleozóicos de la región PetatlAn. Flores. México. A. 1956. 8. 2.. p. Z. González-U. 129. UNAM. M. Z. Z. Oaxaca: Geomimet¡ no. UNAM. 1975¡ A late QUélternary monogenetic field in Centréll Mexico: Geologische Rundschau. Rélmírez. De Csernil. v. Insti tuto de Geología. et al. et a!.. and P. Campa.F. 1981. 6-15.. 102 p. Estado de Guerrero: Boletin Asociación Mexicana de Geólogos Petroleros. México: Contribution to the Congress by the Instituto de Geología de UNAM¡ 140 p. y. Oaxélcél: Libro Guia de la Excursión México-Oélxaca de la Sociedad Geológica Mexicana. n. 39-56. 1979. El Jurásico Medjo y el CCll10viana de México: XX Congreso Geológico Internacional. A. 8-18.l 040-1 051.. Características del Eje Neovo!cánico Transmexicano y sus probleméls de interpretación. n. La disgregación de la Pangea. Caney. 112. 9-33. Campal M. 154-167. v.. n.. Bazá n¡ B. e Iguala¡ Gro: Asoc. Tesis Profesional Facultad de Ingeniería¡ UNAM. estados de México y Michoacán. et al.. 77 p. J. p. Demant.. Libro GUiél de la excllfsión México-Oaxaca de la Sociedad Geología Mexicana. Z. De lél Vega. Coney¡ 1980. UNAM. Caney. E. 1977¡ La evolución tectóniC<l y la mineralización en la región de Valle de Bravo. Campa. State of Oaxaca. p..¡ p. 64. Evolution géodynamique durante le Mesozóique et le Cénozique: Geofísica Internacional v. translators.72 Section 1 The Geology of Ihe Mexican Republic BIBLIOGRAPHY AND REFERENCES Abbreviation UNAM is Universidad Nacional Autónoma de México Bazán. Madrid. 35-63. p. Litoes tra tigra fía y rasgos estruc ­ turales del Complejo Oaxaqueño¡ Mixteca Alta. Tectono-strélti­ graphic terranes and mineral resource distributions in Mexico: Canéldian Journal of Earth Science.. p. Stratigraphy and tectonics of the Rio Sonto Domingo areél. CMfantan¡ J. et al.. Terrenos tectono-estratigráfi­ cos de la Sierra Madre del Sur. Revista del 1nstituto de Geología. 10. 1. 2. 2. 1970. J. n. n.K. UNAM. Michoacán. 1978b. Memoria de la XII Convención Nacionéll. Deriva Contincntéll y Tectóniél de Placas.F. Campa. v. 2. porción meridional del Estado de Puebla: Resúmenes de la VI L Convención Geológica Nacional Sociedad Geológica Mexicana. 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Ferrusquía. 1. Fries. Estudio geológico de reconocimiento de la región de Huetamo. Acapulco.3. l.. Gastil. and O. Mooser. González-Alvarado. Halpern. UNAM. n. p. il1 CE. Estado . Buitrón. 37. 13. v. La Geología de Chiapas: Publicación del gobierno del Estado de Chiapas. Flores. l~evista. Nuevas aportaciones geocronológicas y técnicas empleadas en el Li1boratorio de Gecronometria: Boletín del Instituto de Geología. M" J. Michoacán: Memória de la xllr Convención Nacional de la Asociélción de Ingeníeros de Minas. Los cornplejos metamórfi­ cos del sur de México y su significado tectónico: Resúmenes del In Congreso Latinoamericano de Geología. 1976. Flores. v. 1966. p 207. and M. Metamorphic belts of southern Mexico and their tectonic significance: Geofísica lnternélcional. 1957. v. E. DDF. 39. Fries. Resúmenes. eds" Ash-Flow Tuffs. G.G. Una edud rad iométrica Ordovícica de Totoltepec. n. Ortega-Gutiérrez. Elston. 112-131. 20. n. . Urrutia-Fucugauchi. Pantoja-Alor. municipio de Minatitlán. J. v. 103-163. López. p. and c.. La acreción de los terrenos Mixteca y Oaxaca durante el Cretácico Inferior. Paleozoic sedimentary rocks in Oaxaca. v. Un modelo del Eje Volcánico Mexicano: Boletín de la Sociedad Geológica Mexicana. G. p. 36 p. Guerrero-Coa1comá n. México. H. J. L 1983b. v.P. 1980.. Paleontología Mexicana. p. Urrutia-Fucugauchi.. 20. part 1. 1. Morán-Zenteno. Geodynamics Series. p. Michoacá n. Reconocimiento geológico del área de Petlalcingo­ Santa Cruz. 1977.. Urrutia-Fucugauchi.. Estado de Puebla: Libro Guia de la Excursión México-Oaxaca de la Sociedad Geológica Mexicana. p. v. p. K. Bosquejo geológico de la cuenca sed­ imentaria de Oaxaca: Boletín Asociación Mexicanos Geólogos Petroleros. 1981. et al. Gro.A. Pantoja-Alor. Hughes. L and L. K. 1986. v. O.139-270.74 Section T The Geology of the Mexican Republic de Michoacán: Consejo Recursos Naturales no Renovables. Sierra Madre del Sur: Resúmenes de la VII Convención Geológica Nacional Sociedad Geológica Mexicana. et al. 157. 5. University Newcastle upon Tyne. J. 17. 22 p. v. municipio de Acatlán.. Vidal. S. and Z. 67-84. Geography and faunal provinces in the Tremadoc Epoch.P. Paleomagnetism and tectonics oE Middle America and adjacent regions. 38. p. 485-502. 1033-1035 Pérez. Resultados preliminares paleomagnéticos para el sur de México y sus implicaciones tectóni­ cas: Resúmenes de la VII Convención Geológica Nacional de Sociedad Geológica Mexicana. 1984. Estudio geológico-magnetométrico de los yacimientos fer­ ríferos de Peña Colorada. p. On the tectonic evolu­ tion of Mexico: Paleomagnetic constraints: American Geophysical Union.. Boletín 50. Oisserta hon. v. Viniegra-Osorio. and O. Part 2: Geofísica Intemacional. Peña. Paleomagnetic study of Mesozoic rocks from Ixtapán de la Sal. ed. 18-28. Rodríguez. 51-66. 1974. Paleomagnetic studies of Mexican rocks: Ph. n. 1975. 689 p.. Whittington. Ross.M. 111-112. Los yacimientos en el Cerro del Violín. p. 1967. 1949.. p. Pineda. Geomimet n. 22. Estado de Puebla: Instituto de Geología. J. Sociedad Geológica Mexicana: Resúmenes de la V Convención Geológica Nacional. in C. 3. 78. Rocas sedimentárias Paleozóicas de la región centro-septentrional de Oaxaca. 1965. p..B. L and O. J. 198321. El Conjunto petrotectónico de Zi h ua tanejo. n 21.. 1980. del Castillo. L 1970. 87-110. and KA. ed. 12. 59. Paleogeographic Provinces and Provincia lit y: SEPM Special Publication 21. Geología metamórfica del áreas de Acatlán. 203-218 Zamora. p. J. Part 1: Geofísica Internacional. Salas. Urrutia-Fucugauchi. v. 79-156. J. 1970. England. Urrutia-Fucugauchi. Urrutia-Fucugauchi. A.. Valencio. 25. Paleomagnetism and tectonics of Middle America and adjacent regions. Geología del Macizo de Teziutlán y la Cuenca Cenozoica de Veracruz: Boletin Asociación Mexicana de Geólogos Petroleros. municipio de Mochitlán. and A.. Boletín 77.. p. L 1984. Libro Guia de la Excursión México-Oaxaca de la Sociedad Geológica Mexicana.. p. H. A. ed. UNAM.A. de Cserna. 1965. Mexico: Science. Ramírez. Hokuto.. México: Geofísica Internacional. Urrutia-Fucugauchi. 1969. F. 29-47. Colima: Consejo de Recursos Naturales no Renovables. Robison. Geology of the Southeastern Region of Mexico • \ CHETUfV1AL. CHIAPAS AND TABASCO In the regíon that ineludes the sta tes of Chiapas and Tabasco.--! . the Central American Cordillera./\ ' "-yJ(. The clima te of this region varíes fram tempera te and semi-arid in the high parts of the Sierra de Soconusco and Sierra de Chiapas to hot in the coastal plain of the Gulf and Pacific as we1l as in the central depression of Chiapas. . the Yucatán Peninsula and the eastern extreme of the coastal pLain of the southern Gulf (see Figure 1.... These authors mention unpublished dates for the eastern part of the Sierra Mad re del Sur in Chiapas that indicate Carboniferous plutonic activíty . Mulleried (1957) considers that a large part of the Sierra de Soconusco is formed by Precambrian igneous and metamorphic rocks. the Pacific coast. the shores oí the Gulf of Mexico.• 4. these authors recognized an isochron oí apparent age at 256 ± 10 million years. the Isthmus of Tehuantepec...tes are subhumid and different fram the coastal plams where they are generally humid. the following limits have been selected: on the west. lt consists principalIy of marine sedimentary rocks that are folded and faulted. Castro and co-workers (1975) report an age of 242 ± 9 miIlion years for a dio­ rite (analysis of biotite by K/ Ar method) that forms part of the batholithic complex of the Sierra de Soconusco and that was discovered in the base of a section located at the borders of the sta tes of Oaxaca and Chiapas.. 75 This sequence rests discordantly on a crystalline base­ ment of Precambrian and Paleozoic rocks that crops out to the southwest of the regíon./ GENERAL CONSIDERATIONS For the description of the region of southeastern Mexico. and to the south...::~""-"""--""'-:.. which indicates that these intrusions originated from the same Permían magma that was isotopically homogeneous and perhaps derived from the mantle. where the crys­ talline rocks of these Eras form an outlying meta mor­ phic and plutonic complex constitutíng the nueleus of the Sierra de Soconusco..1). In the Yucatán Peninsula the elimate 1S typically hot and subhumid. Damon and co-workers (1981) report dates from 17 samples from eight areas of the batholithic complex that were studied by the K/ Ar and Rb/Sr methods..r . In this last area the clima. The region ineludes the physiographic provinces of the Chiapas Mountains. In a11 places in southeastern Mexico the rainy season is ín the summer except in sorne areas of the Gulf Coastal Plain where raíns occur a1l year.. VILLAHERMOSA '. However. the major­ ity of radiometríc ages obtained from intrusive ~ock samples reveal Paleozoic dates for the pnnClpal events of igneous intrusíon. to the north. After analyzing ten samples of the complex..~//::::. a great sequence of Mesozoic and Cenozoic rocks crops out. López-Ramos. bra­ chiopods. this discordance is élccentuated toward the west . In the area of Cintalapa. sandstone. . These show bank facies with peri-reefal lirnestones and rudist fragments (Castro et al. 1967. These saline dcposits played a very important role in the deformation of the la ter Mesozoic sequence and in the development of petrole­ um traps. Carfantan (1977) believes that the empLacement of the batholith should have occurred in the Appalachian pbase of deformation and considers that the metamorphic rocks affected by this intrllsion ought to have originated in the GrenvilJe phase of the Prccambrian. In central Chiapas. 1963) and is composed of sandy shaJes. seas transgressed numerous areas that had been erod­ ed during the Barremian-Aptian. 1975). red sand­ stones. 1973). sandstones. In the extreme southeast of the Sierra de Soconusco. which Viniegra (1981) termed the Campeche dome and the Jalpa dome (see Figure 41). 1956). In central Chiapas abundant cakareolls sediments of the Upper Cretaceous are exposed. Pemex has drilled Upper Jurassic sequences.ver Cretaceous sequences that crap out acrass Chiapas disélppeélr under the Tertiary deposits in the areaS of Tabasco and Campeche but have been recognized in petrolellm-praducing wells. Strata in the base of this sequen ce com­ prise the Santa Rosa Formation élnd form a lower member consisting of a sequence of slates with some metaquartzite intercalations.76 Section 1 The Geology of the Mexican Republic In this élreél. 1975. These out­ craps reach to the Isthmus of Tehuélntepec and even to the eastern edge of the southern sector of the Sierra MAdre Oriental. sequences reported by Petróleos Mexicano show slope facies that follow a persistent sedimentary pat­ tem during Neocomian and Aptian and the continued existence of the Great CaJcareous Bank of Yucatán during Albian and Cenomanian time. and sorne horizons of gypsum. which crops out chief]y in Chiapas. silts. tbere is a resulting discordance between the lower Neocomian units and the Albian-Cenomé1l1ian sequence. 1981). This liltter unit is compost'd of massive limestones with fusulinids of the Middle Permian and basal Upper Permian (Glltiérrez. the coastal plain of the southern Gulf. ilnd the continental platform of Tabasco (see Figure 4. Viniegra-Osorio (1981) has interpreted the existence of a saline Oxfordian basin that occupied a major part of the present Sierril de Chiapas.vith the disappearance of units corresponding to the Lower Cretaceous and Upper Jurassic. but to the southeast of Chiapas the facies become more sandy (Castro et ill.ti0I1 appears to be partially metamorphosed and has been assigned an age of Mississippian-Pennsylvanian based on the reported fossils (Hernándcz-García. above the Todos Santos Formc1tion. This sequence has been named the Todos Santos Formation and constitutes the base of the Mesozoic.1). '1957. an important continental sequence is exposed that consists of red conglomercltes. (1975). 1973) and that extends toward Guatem<1la. and clay. Limestones crap out extensivcly in il central belt in the State of Chiapas and reveal shallow ' . A sequence of shales and Jimestones of the Grupera Formation that contains Lower Permiéln fusulinids rests unconformably on the Santa Rosa Formation (Gutiérrez. According to Castro et al.. and some beds of fossilif­ erous li¡nestone. This resuIted in the deposition of carbon­ ate and anhydrite in these regions as well as sedimen­ tation of slope deposits in a belt that bordered the Great Calcareous Bank (Viniegra-Osorio. owing to probable erosion occurring at the end of the Aptian. intercalations of Iimestone and dolomite.n the Cretaceous. the Neocomian sequence has been termed the San Ricardo Formation (Richard. principal­ Iy of platfonn facies. and various fllsulinid species and is cov­ ered discordantly by the Paso Hondo Formation.. 1975) (Figure 45). Furthermore. In a large péHt of the north­ eastern edge of the Sierra de Soconusco. These Lm. The Tithonian sediments indicate an open plat­ form environment with a pelc1gic fauna over the whole area where the states of Chiapas. The llpper member is formed of slates. The fOH1. This is formed by limestones of shal­ low water félcies with some intercalated continental beds. 1971). 1t con tains crinoids. a sedimentary sequence of lélte Paleozoic age crops out that has been recognized in the areél of Chicomuselo (Hernández-García.4). a major ca1careous bank was created by the marine transgression initiated i. In the Yucatán PeninSllla and a large part of the State of Chiapas. The Vainilla Limestone overlies this forma tion. 1956. they consider thélt the bathnlithic emplacernent in Chiapa" would ha ve been associated with the c10sing of the Proto-Atlantic ocean at the end of the Paleozoic.3). at the tirne of the Appéllélchian Orogeny. é\l1d has obtained petrolellm pro­ duction from them (Figure 4. Oaxacél. This bel t is loca ted in the subsurface of the eastern hAlf of the State of Tabasco and in parts of northeilstern Chiapas and the marine platform of Campeche where these sed iment types are important producers of hyd rocarbons (scc Figure 4. They extended to cover up the crystalline rocks of the Sierra de Soconusco. 1979). In the petroleum areas of Tabasco and Campeche. During this time. In the subsurface of this last-mentioned region. Castro et aL. and Veracrlt¿ converge. This is in Jccord with radiometric déltes from gneiss samples in ChIapas as well élS from vari­ ous samples from the base of the complex in Oaxaca th(lt have been correlélted with these rocks (De Cserna. Gutiérrez. ' ater bank environments belonging to the Albiiln élnd Cenomanian Stages (Figure 4. Most authors have assigned this formiltion to él strati­ graphic interval that varies from Triassic to Jurassic (Mulleried. The Bclrrernian-A ptian interval seems to be absent in the immediate vicinity of Sierra de Soconusco because rocks of this age have not been identified. At present these bodies of séllt form two great uplifts.2) Above the Upper Jurassic sediments there rests a Neocomian sequence that gives evidence of the exis­ tence of marginal marine ilnd continental deposits in northwestern Chiapas and eastern Veracrúz. 1956). In the .1a. an Upper Jurassic sedimentary marine sequence occurs. " a result of normal faLllting at the fiose of the Chiapas anticlinoriunl. The large Saline Basin of Campeche during the Ca llovian-Oxfordian. The strata have no significant defor­ mation and are horizontal.\0 ~G O . These dastics are products of lIplift of western Mexico and the folding of the Sierra Madre Oriental. 200 km . Ihe CretaceOllS recognized in the Pemex wells is composed principally of anhydrites. In offshore wells.«"\)~ G PALEOCENE í'é- :. During the Tertiary.1. [:51 Terreslnal areas Metamorptlosed PaJeozolc rocks Unmeramorphosed Paleozolc rocks Llmlt of Isopach conlours e D Nancrlllal Dome E Chiapas Sal. AH the CretaceoLlS sediments that have been encountered in the Pemex wells belong to the middle and upper parts of this system. . Ihis was a marine high that extended to Chiapas cJl1d to the south of Veracrúz.4. These are separated by a high formed by the "Villahermosa Horst. Especially toward the base. This antic1inoriurn is divided in sections by normal faults at the foot of the Sierra. 1981). in most of Chiapas and Iab(lsco. YUCATÁN PENINSULA A Iertiary calcareous sequence crops out in a large part of Yucatán. 1979). Reforma ()rea the edges of the Creat Calc()reous Bank were exposed and eroded during the Upper Cretilceous beca use sorne Pemex wells in this areil enCOllnter Paleocene overlying Albian-Cenomaniél11 sediments (Viniegr(l-Osorio.ne 8asln or sall ~ Edgellne of sa!1 Redbeds oITodos Sanlos Formatlon Figure 4. the Yucatán Peninsula and its marine platform formed a calcareOllS bank. A shelf margin developed that has beeo the principal petroleum objective in Tabasco and on the Campeche marine platform.6). marine terrigenous sedimentation was initj­ ated (Figure 4. recognition of the Upper Cret(lceous has not been possible owing to dolomitiz(ltion that has affected the Mesozoic scqllencc in lhis portion of the marine plilt­ form (Viniegra-Osorio. the section consists of the YlIcatán Evaporites (López­ Ramos. Ihe faulting has induced its subsidence into the Culf Coastal Plain. deposition of c(lrbon(ltes was continuing in the Yucatán Peninsula with the graduétl emersion of its central parto Tv>'O basins of Iertiary age were developed in the Gulf of Mexico coastal plain (Comacalco and Macuspana). Geology of Ihe Soulheastern Region oE Mexico 77 GULF OF MEXICO 1 Santa Ana Massll 2 T u"lla Masslf 3 Chiapas Mas.. At this same time.1 A Slgsbee "Knolis' 8 Campeche Oome Jalpa Dome ~~~'e. During the second half of the Cretaceous and a large part of the Cenozoic. and intercalations of bentonites and sorne pyroclastic materials. 130th the Cretaceous sequence recognized in the subsurface a nd the Cenozoic sequence shO\·v no major structllral pertur­ bation and overlie a crysta lline m(lSS tha t has remained stable from the Paleozoic on. limestones. dolomites. 1981). 78 Seclion 1 The Geology of lhe Mexican Republic o ~ Redbeds o - - - - - '200 km , '-' M31nly marme IZI I'ZI Edgehne 01 sal! Emergent area Figure 4.2. Map showing facies distribution in southeastem Mexico of Upper Jurassic facies. Under this Cretaceous sequence, the wells Yucatán No. 1 and 4 cu t through silts tones and sandstones with sorne intercalations of quartzose sand and gravel as well as green bentonite and dolomitic limes tone. López- Ramos (1979) original! y considered these as belonging to the Jurassic-Cretaceous interval. These redbeds rest aboye a crvstalline basement that was reached by the well Yucatán No. 1, at 3200 m depth (López-Ramos, 1979). From a sample of rhyo­ lite porphyry from this welt a Rb/Sr date of 410 mil­ !ion years (Silurian) was obtained. This porphyry seems to have intruded a quartz and chlorite schist (López-Ramos, 1979). The Yucatán No. 4 well cut 8 m of slightly metamorphosed quartzite that underlies the Triassic-Jurassic redbeds (López-Ramos, 1979). The Cenozoic deposits of the Yucatán Peninsula are represented principally by cakareous and dolomitic sequences with evaporite intercalations. Butterlin and Bonet (1963) formulated a column that extends from Paleocene to Quaternary. This column includes in ascending order: the Chichén Itzá and lcaiche formations (Paleocene-Eocene); the Bacalar, Estera, Franco, and CarjlJo Puerto formations (upper Miocene and Pliocene); and molluscan limes tones of Pleistocene-Holocene. The Oligocene has not been recognized on the surface but was cut in the explo­ ration wells of Chicxulub No. 1 and Cacapuc No. 1 (Butterlin and Bonet, 1963). The surface distribution of the Cenozoic units clearly shows a gradual retreat of the seas toward the present coast line, and onl1' in the Eocene did the seas transgress and cover almost completely the Yucatán Peninsula (Butterlin and Bonet, 1963). TECTüNIC SUMMARY The metamorphic rocks that crop out in the Sierra de Soconusco have been related to a metamorphic event contemporaneous with the Grenvillian defor­ mation, which is well known in the eastern United States (Carfantan, 1977), and they have also been cor­ related with the metamorphic events that formed the Oaxaca Complex (Fries et al., 1962) An important phase of defonnation occurred at the end of the Paleozoico This affected the Mississippian and Pennsylvanian sedimentary sequences of south­ east Chiapas, and the chief plutonic él.ctivity began in the present-day Sierra de Soconusco. This phenome­ non was followed by él prolonged interval of cont.i­ nental environments during which the lower beds of the Todos Santos Formation were deposíted. Damon and co-workers (1981) relate the emplacement of the Sierra de Soconusco ba tholith to the closing of the Proto-Atlantic ocean and the unification of South 4. Geology of the Southeastern Region of Mexico 79 ~ HONDURAS Deep water sed,ments Carbonales-evaporiles Edge of Ihe platform or bank o B '1Z] g o 'L- 200 -.J' km ~ External edge 01 zone of dolomitlzed slope sedlmenls Nearshore line ~ Nearshore volcanlcs Emergenl areas Figure 4.3. Map showing facies distribution of Neocomian-Aptian facies in southeastem Mexico. America and Africa with Narth America, an actíon that culminated in the Appalachian Orogeny at the end of the PaleozoiCo During the Late Jurassic a trans­ gression occurred that gave rise to marine sedimenta­ tion, especiaUy in the localities near the Culf Coa.st in Tabasco and Veracrúz_ In the Sierra Madre Oriental and other regions in the east of Mexico, a Jurassic transgression has been related to the opening of the western extreme of the Tethys (Tardy et aL, 1975; Tardy, 1980; Campa and Ramírez, 1979) during the disintegration of Pangea. In the Cretaceous there was general marine sedi­ mentation that, in a majar part of the state of Chiapas, is represented by the platform Sierra Madre Limestone. The area of the Yucatitn Peninsula remained stable but submerged and had shallow water deposition, forming the Creat Calcareous Bank that extended toward Chiapas and south of Vera cruz. Viniegr a-Osorio (1981) believes tha t the Crea t Calcareous Bank of Yucatán tilted southwestward during its evolutlon. This interpretation is sustained by the fact that in the Pemex wells the basement was encountered at increasing depths from east to west across the marine platform of Campeche and finally reaches depths greater than 6500 m, with stj]] greater thickness of the whole Mesozoic and Tertiary sequences. Dengo (1968) recognizes a partial deforma­ tion of the Mesozoic sequence at the end of the Albian 80 Section 1 The Geology of the Mexican Republie LZl I~I Deep ,¡a1lf' Sedl ".en., Carbol12,le·evaparlles Edgeline of rul!fs 01 banks Probable Iomll 01 bank claslrcs EO gaaf Q g IHlp wai r E:J ~ ~,,8CiI"sho.rB earshoi <ItIlcanlGs [Z], [TI [TI] i::mergel'il a eas G kten Lal1 carbonale bank YLJcatan carbonate bank Figure 4.4. Map showing facies dístribution of Albian-Cenomanian in southeastern Mexico. that was accompi1J1ied by granitic intrusions that <o'xtend lo the Sierra Madrc del Sur in Chiapas and central GU<ltemala. _arfa ntan (1977) men tioned a phé1se of Cel1oD1"nian deformation that placed él volcé1nic-plu­ tonic complex of probable Mesozoic age in allochthonolls positi n over the L'rodcd Chiapas rlatform. This i~ located. in the arca of Motozint!<.'l. his complex corresponds to a volcal1ic arc similar to tho . rt'<': >gnized in the northwest and western pélrts of Mexico. After this deformation, and dllring the Late Cretoceolls, Paleocene, and Eocene, the Mesoznic st'quence was aHected by Laramide orogenic defor­ mation. At this time an elongat",d marine basin WélS developed as a Foredeep with flysch deposition of the Ocozocuautla Forméltion (Dengo, 1968). Seemingly, the sélUne sediments ot the base of the Mesozoic played a very important role in these defor­ mations, sincc they ser ved as plastic material during the developml.:nt of the deCüllement in which time thE' Mesozoic and Cenozoic sequences were fold 'el (Viniegra-Osorio, 1981). In the Reforma-Campc-he beH, the origin of the domnl dnd pillow-like stnlctural system is related to verticéll movement impelied by the subjacent salto During the Cenozoic, the Chiopas regio n was opparently caught in tectonism involving nOTméll and strike-slip félulting, w"hich complica tes lhe structural rclations of the Mesozoic <lnd Cenozoic sequences. 4. Geology of the Southeastern Region of Mexico 81 GULF OF MEXICO o f:6I Carbonates wllh rudlsts and evaponles Globigerl.nld deep water facies Melange In deep water Nearshore Nearshore volcanlcs D IIIIII O 200 km E3 1~:1 Volcanlc Intruslves Emergenl areas ¡;z¡ Figure 4.5. Map showing distribution of Upper Cretaceous facies in southeastern Mexico. GUlF I Carbonate evaporl1e Open sea ciay, sands, rnarls ~I Open sea rnarls g [ZJ Bank edge clasllcs Emergent areas o 200 km ~ Flysch facies Figure 4.6. Map showing distribution of Paleocene facies in southeastern Mexico. . \ \ .::>"" o Productlve areas 01 Ihe Upper Crelaceous . \ '.. . lead. Iron and copper are found in Aniaga and copper. \ \ \ 1 . \ \ . ~~ s­ O LI ~l 200 km (D Sierra Madre Oriental Sierra Madre del Sur Sierra de Chiapas Veracruz basln o @ ~ . there exist some mineral deposits of known hydrothermal origin that show the association of silver-Iead-zinc-gold-copper. Productlve areas 01 (he Upper Jurass. ECONOMIC RESOURCES The principal petroleum reserves of the nation are located in the subsurface of the Reforma areé'l at the Chiapas and Tabé'lsco border. Niltepec. .. I I . I 1 / / I / / / / /' . \ I I .. In the southeast region of Mexico. . .. The directions of the faults of this period seem to be associated \vith the northwestern movement of North Americé'l in respect to the Caribbean plate along the Polochic-Monté'lgua fault system of Guatemala and the southern border. It has long been considered that the so urce rocks of these hydrocarbons are ]urassic and that the reservoirs were developed in many varied traps resulting from a complex stratigraphic and structural evolution.neral they are small and localized generally in the south of Chiapas. I I I I 1 1 I GULF OF MEXICO / /_. In these areas the sequences have slope facies composed of fractured and dolomitized detrital material. which extends to Chiapas and VeracrÚz. Map showing producing areas and oi! wells in southeastern Mexico.-' .FAJA DE ORO \ \ \ "V.I'-J .. metasomatíc deposits of iron exist in the localities of Ventosa. Lajeria. Most of the produc ­ tion comes from rocks of Late Jurassic and Cretaceous age as well as from Ihe base of the Paleocene. The belt containing the slope where these sequences were deposited developed during the Cretaceous along the edge of the Great Calcareous Yucatán bank. and Almagres.7. and zinc in Ixtapa. in the localities of Pijjjapan.7). Nueva Morelia.82 Section 1 The Geology of the Mexican Republic .. and FololapilJa. . I I I ARENQUE . In gp. ' . .. In additio11. /' . Payacal.c and Upper Crelaceous ¿fj7 Areas 01 polentlal accumulallon Figure 4. as well as on the marine Campeche platform (Figure 4.. 1979.. v. J. Bonet. La Geología de Chiapas: Publicación del gobierno del Estado de Chiapas. J. Tectonic framework of southern Mexico and its bearing on the problem of continental drift Boletín de la Sociedad Geológica Mexicana. p. 1956. Brousse. Schlaepfer. Stratígraphy of Early Mesozoic sediments in southwest Mexico and western Guatemala: AAPG Bulletin.. 1861-1970. Clark.K. 1979. Geology of the Southeastem Region of Mexico 83 BIBLIOGRAPHY AND REFERENCES Abbrevlation UNAM is Universidad Nacional Autónoma de México Butterlin. Instituto de Geología. Tardy. México: Excursion C-15. et al. La alocto­ nía del conjunto cadena alta-alta plano central. 2nd edi­ tion: Scholastic Edition. v. Tectonofísica. S. Carfantan. 1973.. El gran banco calcáreo yucateco: Revista Ingenieria n.R Dickinson and D. 117-182. Geologia del Mesozóico y Estratgrafía Pérmica del Estado de Chiapas.4. Campa. 1967 (1969).G.. and K. Agencia para el Desarrollo Internacional. 1980. Precambrian sedimenta ton. Instituto de Geología. 1957. Z.. Ill. Dengo. M. n. 45-53 Gutiérrez. España: Editorial Omega. v. Mapas geológicos de la Peninsula de Yucatán: Ingeniería Hidráulica en México. R.. Bosquejo geológico del Estado de Chiapas: XX Congreso Geológico Internacional. 133-137. p. e. eL 1977.. in W. E. 1968. F. 1488-1513. y Morfolgía. 1975. Lehman. 75. p. parte 3. 79-113. 1981. v. 1963. 1963. p. 1. 64. 1-11. Ramírez.. Age trends of igneous activity in relation to metal­ logenesis in southern Cordillera. 1. Coah. F.P. v. v. Asociación Mexicana Geólogos Petroleros. and J. Castro. 1. 446 p. Mexico Revista del Instituto de Geología. Estratigrafía y microfacies del Mesozoico de la Sierra Madre del Sur. UNAM.. et al.. and E. eds.. F. 137-154..L. Fries. M. v. Payne. La cobijadura de Motozintla ­ un pa leoarco volcánico en Chiapas: Revista. 159-168. n. Geologia de México. IIl. UNAM. 60. Viniegra-Osorio. entre Torreon. G.F. 45 p. 27. Tectónica. n. R. p. ej. 1. p. De Cserna. Richard. y San Luis Potosi. p. v. Chiapas: Boletín Asociación Mexicana de Geólogos Petroleros.P. López-Ramos. 1971. M. 47.. tec­ tonics and magmatism in Mexico: Geologische Rundschau. 20-44. .: Arizona Geological Society Digest. Z. Estructura geológica. 102 p. p. v.. p. p. Tratado de Geologia. p. UNAM. Damon.. and F. n.. R. Hernández-Garcia. Tardy. D. M. 1962. Paleogeografía del Paleozóico de Chiapas: Boletin. 1975. Serrat Translatíon. La evolución geológica y la metalogénesis del noroccidente de Guerrero: Serie técnico-científica de la Universidad Autónoma de Guerrero. 30. v. La transversal de Guatemala y las Sierra Madre de México. Barcelona. P.R. história tectóni ­ ca y morfología de América Central: Guatemala Instituto Centroamericano de Investigación y Tecnología Industrial: Centro Regional dE' Ayuda Técnica. De Cserna. in J. 1981. Shafiquillah. Auboin. J. Rocas Precámbricas de edad Grenvilliana de la parte central de Oaxaca en el sur de México: Boletín. Mulleried.G.. 1-103. 14. 1-3. 25. Martínez. Observaciones generales sobre la estructura de la Sierra Madre Orientéll. H.