Carbonate Ramp Depositional Systems

Sedimentary Geology, 79 (1992) 3—57 3Elsevier Science Publishers B.V., Amsterdam Carbonate ramp depositional systems T.P. Burchette a and V.P. Wright b a BP Exploration, 4/5 Long Walk, Stockley Park, Uxbridge, Middlesex UB1I 1BP, UK b Postgraduate Research Institute for Sedimentology, The University, P.O. Box 227, Whiteknights, Reading RG6 2AB, UK (Received January23, 1992; revised version accepted April 7, 1992) ABSTRACT Burchette, T,P. and Wright, V.P., 1992. Carbonate ramp depositional systems. In: B.W. Seliwood (Editor), Ramps and Reefs. Sediment. Geol., 79: 3—57. The classification, tectonic settings, stratigraphy and early diagenesis of carbonate ramp systems are reviewed. Carbonate ramps are common in all geological periods, but were dominant at times when reef-constructing organisms were absent or inhibited. Ramps can be subdivided into inner-, mid-, and outer-ramp environments. The mid-ramp zone extends from fair-weather wave base to normal storm wave base, although the water depths which these boundaries represent vary. An additional outer-slope environment occurs on distally steepened ramps. As with siliciclastic shelves, a range of wave-, storm-, and tide-dominated ramps can be recognised and this forms the most convenient basis for ramp classification. The carbonate productivity profile of ramps differs from that of rimmed shelves, with the inner ramp showing lower production rates than comparable shallow-water facies on rimmed shelves. The zone of greatest organic carbonate sediment production appears to have shifted from the mid-ramp to the inner ramp since the Late Jurassic. Carbonate ramps occur in most types of sedimentary basin but are best developed where subsidence is flexural and gradients are slight over large areas, as in foreland and cratonic-interior basins and along passive margins. The featureless depositional profiles of many ramps means that sequence geometries are best observed on regional seismic lines. Some show low-angle sigmoidal or shingled clinoforms, suggesting that ramps may seldom be “homoclinal”, but possess subtle slope geometries which reflect depositional environments. Because of their low-angle slopes, ramps respond differently to rimmed shelves during relative sea-level changes although results seem to be strongly dependent on the rate of relative sea-level change. During a minor fall, shallow-ramp facies belts will simply shift basinwards in a “forced regression”. In contrast, the whole surface of a steep-sloped, flat-topped rimmed shelf may be exposed so that sediment production ceases or is drastically reduced. During a major fall, shallow ramp-bounded basins may empty completely. Conversely, ramps also flood gradually, whereas rimmed shelves do so more rapidly. Homoclinal ramps develop no resedimented lowstand deposits; rimmed shelves and distally steepened ramps, in contrast, may develop lowstand talus or turbiditic wedges. Distally steepened ramps may behave more like homoclinal ramps during minor base-level falls and like rimmed shelves during major base-level falls. Many ramps consist of layered successions of several ramp sequences stacked one upon the other. Ramp “stacks” of this sort may show gross vertical accretion, but individual ramp sequences seldom appear to develop in a “keep-up” style, apart from minor organic buildups, as with many rimmed shelves. Steepening of the outer-ramp margin due to tectonism, slope inheritance, or differential sedimentation may promote the development of a distally steepened ramp or rimmed shelf. A wide variety of organisms have constructed buildups in mid- and outer-ramp environments. Isolated buildups may seed early in ramp development, accrete to wave base or sea level, and continue growth by stacking through successive ramp sequences so that depositional and diagenetic features within them are in concert with those of the shallow ramp. The location of isolated buildups on ramps is governed by tectonism, halokinesis, antecedent topography, or by the subtle slope geometry of the previous ramp sequence. Diagenesis on ramps shows some major variations compared with diagenesis on steep-sloped, flat-topped carbonate platforms. Correspondence to: T.P. Burchette, BP Research (140/105), Chertsey Road, Sunbury on Thames, Middlesex TW16 7LN, UK. 4 T.P. BURCHETFE AND V.P. WRIGHT Ramp-bounded basins may form prolific petroleum sourcing and reservoiring systems and offer a range of subtle stratigraphic play types and lateral facies variations which determine reservoir quality and distribution. Isolated buildups in the mid- and outer-ramp environments represent one of the commonest petroleum reservoirs in ramp systems and tend to have their foundations in transgressive systems tracts. Grainstone and packstone reservoirs are widespread and range from shoreline carbonate sandbodies to major detached shoal complexes or shoals over offshore highs. Grainstone sandbodies occur in both highstand systems tracts and in prograding lowstand wedges. Introduction nized (Read, 1982a, 1985). Ramp deposits form the foundation phases for many large-scale car- The original concept of the carbonate ramp bonate platforms and in some settings occur as (Ahr, 1973), as an alternative to the steep-sloped, major basin fills. They also host significant reef-rimmed shelf (Table 1), was of a simple petroleum and mineral deposits. Despite their carbonate depositional system with a low-gradi- abundance and undoubted economic importance, ent slope (<1°) from shoreline to basin. Many however, carbonate ramps have been the subject ramp-like carbonate platform have since been of surprisingly little focussed study and are still identified in all parts of the geological record and poorly understood. the continuity in morphology between ramps and In this paper we review the major characteris- other carbonate platform types has been recog- tics of carbonate ramp depositional systems and, TABLE 1 Glossary of main terms as used in this paper a Term Definition Carbonate platform An informal term used for all major shallow-water carbonate successions, including ramps, rimmed shelves, and isolated buildups, particularly where these cannot immediately be assigned to one, or a single one, of these categories. Carbonate shelf A carbonate depositional system which develops perceptible constructional relief above the sea floor (tens to thousands of metres). The transition from shallow water to basin is marked by a distinct break in slope and occurs over a relatively short distance (metres to a few kilometres). Slope angles vary from a degree or so to almost vertical. Carbonate ramp A gentle slope in a carbonate depositional system which extends from the shoreline, or a platform surface, to the adjacent basin. The angle of slope is commonly much less than 1°(although there may be steeper dips locally) and may be inherited or constructed. Buildup A constructional mound consisting of organic skeletal framework and/or bound sediment, commonly further indurated by inorganically precipitated cement. May develop in a wide range of sizes from metres to kilometres across, and metres to hundreds of metres high. Ramp stack Informal term assigned to a vertically layered carbonate succession consisting of stacked ramp sequences. Ramp slope crest Subtle slope change in a carbonate ramp profile recognizable on some regional seismic lines, and exceptional outcrops. May coincide in some ramps with fair-weather wave base. Inner ramp Zone of ramp deposition between upper shoreface (beach or lagoonal shoreline) and fair-weather wave base. The sea floor in this zone experiences almost constant wave agitation. Mid-ramp Zone of ramp deposition between fair-weather wave base and storm-wave base, in which bottom sediment is frequently reworked by storm waves and swells. Outer ramp Zone of ramp deposition below normal storm-wave base, characterized by mudstone deposition and few storm beds. a For more detailed definitions see Ahr (1973) and Read (1980). CARBONATE RAMP DEPOSITIONAL SYSTEMS 5 using ancient and Holocene examples, discuss terized by its very gentle slope of 0.1°”(Bates and likely controls on their development, their tee- Jackson, 1987). The importance of such ramp tonic settings, sequence stratigraphy, and hydro- profiles in siliciclastic regimes has recently been carbon significance. We also suggest a classifica- appreciated (Van Wagoner et al., 1990). tion for these enigmatic carbonate systems and Read (1982a, 1985) divided carbonate ramps address in particular the problem of why ramps into two groups, homoclinal (Greek: “same appear to be more abundant in some parts of the slope”, i.e. with the same gradient from shoreline geological record than in others. to deeper water) and distally steepened, with an offshore slope break between the shallow ramp and the basin. A distally steepened ramp, there- Ramps versus shelves: a semantic problem fore, has a similar configuration to many silici- elastic shelves and, in hindsight, a more logical In the 1960s and early 1970s a range of car- distinction might have been between flat-topped bonate depositional models were developed based (aggradational) carbonate shelves, “sloping” car- largely on modern analogues from the Bahamas, bonate shelves (both categories with a slope- the Florida Shelf, and Yucatan (see Bathurst, break), and ramps (i.e. homoclinal variety). This 1975; Wilson, 1975). These focussed on “barrier” would entail two practical problems, however. reef or shoal-rimmed carbonate systems, the inte- Firstly, ramps and distally steepened ramps have riors of which consisted of shallow-water, low-en- more in common with each other sedimentologi- ergy “lagoonal” and peritidal deposits formed on cally than with flat-topped shelves. Secondly, the extensive aggrading, flat platform tops. In the two are difficult to distinguish in the rock record simplest models the rim bordered a steep slope unless the presence or absence of slope or slope which dropped away to a deep basin into which apron deposits can be demonstrated. This distinc- shallow-water carbonates were resedimented. The tion would be additionally hindered where the geological record contains many successions de- shelf-break zone, commonly a tectonic feature, posited in analogous settings (see e.g. Wilson, had been deformed by later tectonism. 1975). Ahr (1973) noted that the “rimmed shelf” Ramp classification models bore little relevance to the interpretation of many carbonate successions and that a differ- ent model was necessary, in particular for many Environmental subdivisions ancient “epeiric” carbonate depositional environ- ments (Shaw, 1964; Irwin, 1965; Laporte, 1969). Several schemes have been offered for the The term ramp was adopted to describe a gently subdivision of carbonate ramp profiles, using var- sloping depositional surface (generally < 1°) ious water-depth criteria. Markello and Read which passes gradually offshore, with no slope (1981), for example, subdivided a Cambrian ramp break, from a shallow, wave-agitated setting into in Virginia into three zones: peritidal platform, a deeper-water, lower-energy environment. Shoal surrounding the basin; shallow ramp, above fair- deposits in this model occur close to the shoreline weather wave base (FFWB); and deep ramp, be- and not at some potentially considerable distance low FWWB, which passed into a “shale” basin. from the shoreline as in many rimmed shelves; In the Triassic Upper Muschelkalk of Germany, the Arabian Gulf (Purser, 1973) appeared to rep- Aigner (1984) recognised a deep ramp environ- resent a modern environment which conformed melt with storm deposits, a wave-influenced shal- to the ramp ideal, In some respects this ramp low ramp, and a back-bank lagoonal-peritidal model is also analogous to the familiar continen- zone. For lower-energy Upper Muschelkalk ramps tal “shelf” which is defined as “that part of the of the Catalan Basin, Spain, Calvet and Tucker continental margin that is between the shoreline (1988) defined a shallow ramp zone above and the continental slope [or 200 ml... charac- FWWB, and a deep ramp below, within which 6 T.P. BURCHETFE AND V.P. WRIGHT they recognized three further subdivisions: proxi- criteria for the classification of siliciclastic mal deep ramp mainly above storm wave base “shelves”, and because ramps are morphologi- (SWB), intermediate deep ramp between SWB cally and hydrodynamically similar to siliciclastic and a poorly oxygenated zone, and distal deep shelves, it seems appropriate to subdivide them in ramp below the suboxic zone. Buxton and Pedley a similar way. On this basis, and following previ- (1989), on Tertiary ramps in the eastern Mediter- ous use (Wright, 1986; Burchette et al., 1990), we ranean, recognized inner- and outer-ramp zones, suggest four subdivisions which should be appli- the former within the photic zone and above cable to most ramp successions (Fig. 1 and Table SWB, and the latter below this level (see also 1): Somerville and Strogen, 1992). Inner ramp. This is the zone above FWWB Most of these classifications recognize two crit- dominated by sand shoals or organic barriers and ical interfaces: fair weather wave base and storm shoreface deposits, and back-barrier peritidal ar- wave base. The actual depth of water in which eas. these boundaries occur in marine environments Mid-ramp. This is the zone between FWWB varies in relation to local hydrodynamic/climatic and SWB where the sea floor is affected by storm conditions, and with time, but the process do- waves but not by fair-weather waves. Sediments mains that they define leave readily identifiable show evidence of frequent storm reworking. A sedimentary features in the rock record. In Se- variety of storm-related features typically occur, quences deposited in protected settings (e.g. in- including graded beds and hummocky cross- trashelf basins) or tidally dominated settings, or stratification. Proximal—distal trends can corn- where bioturbation is pervasive (e.g. Pedley, monly be recognized in ancient mid-ramp de- 1992), location of these depth/energy-related posits (Aigner, 1984; Burchette, 1987; Faulkner, lithofacies may be more problematic although 1988). they generally are identifiable. Because many an- Outer ramp. This zone extends from the cient ramps appear to have been storm- depth-limit to which most storms influence the dominated, these interfaces represent perhaps the sea floor down to the basin plain. Sediments most widely recognisable “yardsticks” in ramp show little evidence for direct storm reworking successions. Since they also represent the main but a variety of storm-related deposits, such as Fig. 1. The main environmental subdivisions of a “homoclinal” carbonate ramp. MSL = mean sea level; FWWB = fair-weather wave base; SWB = storm wave base; PC = pycnocline (not always identifiable in the rock record). Water depths corresponding to these boundaries are variable. CARBONATE RAMP DEPOSITIONAL SYSTEMS 7 sparse, graded, distal tempestites, may occur in water ramp environments. In distally steepened the upper part (e.g. Aigner, 1984; Calvet and ramps, the slope break is commonly located in a Tucker, 1988). In deeper zones restricted bottom position around the mid- or outer ramp and so conditions may develop, perhaps in association these depositional zones should also be present. with suboxic basinal waters due to density stratifi- The slope-break in this case should be recogniz- cation in the basin, able in the rock record by evidence for slumping Basin. The identification of truly “basinal” de- and slope apron construction; the outer-slope en- posits is a persistent problem. The character of vironment would clearly form an appropriate ad- the deposits will, of course, depend on the nature ditional subdivision. and depth of the basin itself, but generally they lack coarse “tempestites”. Turbidites are mostly Classification absent in basins adjacent to ramps. In deep, rapidly subsiding basins, sediments may be Read (1985) recognized six major ramp types siliceous, while in shallow basins they may consist based on the character of the highest-energy fa- of bioturbated lime mudstones. In restricted cies: these were ramps with fringing skeletal basins, the outer-ramp and basin-centre deposits “banks”, barrier—bank complexes, shallow and may consist of cyclic organic rich facies (Droste, deeper water buildups, fringing ooid shoals, 1990) or may be pervasively bioturbated and mis- ooid—peloid shoal barriers, and coastal takable for lagoonal facies. beach/dune complexes. This classification ad- These simple facies subdivisions may be com- dressed at least one critical factor which has the plicated by biological accumulations (e.g. mud- potential to influence the relatively simple hy- and reef-mounds) in the deeper- or shallower- draulic controls on carbonate ramp environments Fig. 2. Ternary diagram showing suggested classification for carbonate ramps based on the degree of storm, wave, or tidal influence which they exhibit in the mid- and inner-ramp zones. An additional axis accommodates the various lithologies which dominate ramp sediments and seem to reflect the level of environmental energy (see arrow). Several representative ramps have been entered. See text for source references on the characteristics of individual ramps. the Trucial Coast of the Arabian wind and wave energy. How. al. form energy barn. or tide-dominated or mixed systems of the sort outlined by Read (1982a).P. fore. he shown to have changed during the period of 2). Organisms Britain. with the addition of a third dimen- several ramp sequences or parasequences and sion to account for the varied sediment types hydraulic and biological conditions can commonly which may be dominant in carbonate ramps (Fig. All these boundaries are gradational. 1990). (C) shoreface or shoal cross-laminated oolitic or bioclastic grainstones and packstones. 3. . McRory and additional lateral transitions from one to the other Walker. (B) bioturbated and variably bedded lagoonal lime mudstone. then. and have changed significantly through time. there- redistribution as sandbodies. storm. Mid-ramp: (D) amalgamated coarse.. Mississippian inner-ramp deposits in southwest parts: organic sediment production. is a modern this classification for carbonate inner-ramp set- example which shows this lateral facies variability tings has recently been discussed (Burchette et (Purser and Evans. In another example. 1986). storm-. Modern siliciclastic shelves are classi- Burchette et al. graded tempestites interbedded with bioturbated or laminated lime or terrigenous mudstone. with its varied shoreline styles. on the classification already established for silici- ally represent “stacks” (or “sets”) consisting of elastic shelves. ramp development. be applicable to a local shoaling succession. 1979. Outer ramp: (E) fine-grained.WRIGHT when compared with their siliciclastic counter. 1973). and wackestone. commonly with hummocky cross-stratification. or ocean-current domi- wave and storm influences on a series of Early nated (Johnson and Baldwin. (1990) documented fluctuating fied as wave. graded tempestites. again to favour a process-related approach based Many thick carbonate ramp successions actu. should ramps in successions the location and character of the buildups which of this sort be classified? Our studies lead us they create. packstone. Most are Fig. the differences being controlled along its length and at different times within the largely by tidal range and its interaction with ramp stack. ers. 1986). but may not define the ramp as a depositional both in their role as sediment producers and in system. 1985) may. “Homoclinal” carbonate ramp showing main sedimentary facies. tide. (F) laminated or sparsely rippled silt-grade carbonate sediment or quartz silt in a predominantly terrigenous mudstone succession. with wave—tide regimes (Hayes.8 T. Inner ramp: (A) peritidal and sabkha facies with stromatolitic algae and evaporites. One major ramp succession Modern siliciclastic shorelines can be classified may therefore contain a number of depositional as wave-. A simple term such as “barrier bank” or create high rates of local sediment supply for “fringing ooid shoal” (Read. BURCHETFE AND V. The appropriateness of Gulf.P. and in the inner-ramp linear sand ridges sandbodies. typical of early Carboniferous and Jurassic. hummocky the Paris Basin (Purser. characteristic of those in the Paleogene and Early Neogene. Faulkner.CARBONATE RAMP DEPOSITIONAL SYSTEMS 9 storm-dominated and have relatively low tidal storm reworking in the mid-ramp environment influence. In the mid-ramp. 1988. mid-. Such systems record (e. The Middle forms accumulate.g. plex overall hydraulic regime. tenized by decametre-scale tabular. may in part reflect the relatively high frequency In tidal regimes. and the rate of sediment supply. land (Sellwood et al. 1986) and lated deposits such as tempestites. 1979. 1988. A modern example commonly show little evidence for significant is the unnimmed northwestern margin of Aus- Fig. and outer-ramp depositional environments and how these are related to fair-weather wave base (FWWB) and storm wave base (SWB). (C) Grainstone-dominated ramp. Based on Aigner (1983). 1986. typical of early Palaeozoic and later Mesozoic successions. 1975. 1985. Johnson and Baldwin. suggesting a corn- the latter type occur. and swaley cross-stratification probably represent such systems. (B) Skeletal boundstone-dominated ramp. Calvet are best regarded as protected ramps. Sellwood. (1982).g. representing shallow tidal sand waves (e. Profiles are valid for several scales of sequence. 1989) cross-stratification. ribbons and sheets of storm events in the tropical and subtropical may develop.. 1978). 1979) represents a ramp deposits do not appear to be stronglys tidal modern example where carbonate sandbodies of (Palmer. Aigner. contemporaneous inner- Shelf (Ward and Brady. Laville et al. The Yucatan and deltas. Faulkner. Handford. 2) and Records of tidally dominated ramps are rare. However. 4. Highly schematic vertical sections through several end-member ramp depositional Systems. and Burchette et. and some modern ramps. (A) Proterozoic stromatolite-dominated ramp.g. . This also appears to have been true of since limits on wave fetch or height have pre- many carbonate ramp sequences in the geological vented significant storm reworking. showing variation of facies within inner-. 1984. Baria et a!. Based on Ahr (1973). (D) Large-foraminiferan shoal-dominated ramp. cross-bedded 1988). Another possible logues of mid-ramp storm-generated sand ridges example of a tidally dominated inner ramp is the are rare or have possibly been misinterpreted. a!. Based on Burchette (1981) and Burchette and Britton (1985). (1989).. Aigner. Monteagle Limestone in the Black Warrior Basin Ramp sequences associated with intrashelf basins of Alabama (Handford. but ancient carbonate ana. 3 and 4) exhibit varied storm-re. and Tucker. and are charac- (e. successions Jurassic oolite-dominated ramps of southern Eng- of this sort (Figs. 1973. 1975). sand waves. Purser. 1986. showing variations in stromatolite morphology with depth. Based on Grotzinger (1989). 1986). Fig. depending on tidal current strengths latitudes in which most major carbonate plat. Handford. 1985. wacke-. or packstone lithologies. grade. reflecting the tendency of high. mi. Inner-ramp tively larger volumes of muddy carbonate sedi- deposits typically consist of oolitic or bioclastic ment in the offshore environment of ramps com- shoal. successions of this type are either uncom. monly bioturbated. from one area to another in the offshore zones of stand ramp shoreface sediments to migrate or carbonate ramps.P. WRIGHT tralia (Dix. storm influence depending on the water depth Some ramp successions do not easily fit into and the depth of wave base. suspension dominates (Fig. Organic buildups in inner-ramp severe storms affect the sea floor and so evidence environments tend to be biostromal. consisting cause of unusual hydraulic conditions or possibly largely of lime or terrigenous mud. mid-ramp zone of many ancient ramp successions exceeds that in the inner ramp. 3D) remains enigmatic and stromatolitic and evaporitic lagoons and playas has been little studied. This may be largely a question of be compared with studies of comparable modern scale and timing. In. cyclic deposi.or outer-ramp. Although largely of mud- via a zone of storm reworked sediments corn. are commonly with a restricted biota. able carbonate sediments deposited below fair- mon in the geological record or have been consis. weather wave base and reflect varying degrees of tently misinterpreted. many of which seem to argillaceous carbonate and terrigenous mud from have developed into flat-topped. Fairweather phases the above attempts at classification. prominent peritidal intervals with tidal channels. storm-influenced mid. Detailed studies of sediment and in arid climates may become evaporitic. The origin of carbonate muds in mid-ramp ramp deposits passed landward into inner-ramp environments (Fig. pestite “couplets”. as on siliciclastic shelves.10 T. and are com- because of intrinsic differences between siliciclas. for wave reworking is sparse. 3). However. Such successions have been dian of Greenland and Scotland. either be. Only the most tional systems. composition in the mid. BURCHETFE AND V. Shoal deposits commonly form sheet-like shore currents in transporting material laterally grainstone units. The outer ramp is a zone where deposition of long-lived ramp systems. while peritidal also largely unevaluated. thin lami- .and outer-zones of an- ner-ramp deposits of modern ramp settings differ cient carbonate ramps are clearly required in from those found in modern flat-topped “keep. 1989). Mid-ramp deposits (Fig. 3E). pared with the inshore zone. This may reflect the generation of rela- gradual water-depth changes (Fig. or of storm-generated currents prograde rapidly. and back-barrier sediments (Figs. Such work could narrow lagoons. for example. again reflect. but are potentially of sediments are commonly microbially laminated major importance. mocky cross-stratification or form graded tern- crotidal. Fairchild (1989) and packstone sediments consist largely of au- Fairchild and Hetherington (1989). however. This regime described in detail by Aigner (1984) and Faulkner generated a succession lacking near-shore shoal (1988). mid. barrier. tochthonous bioclasts and typically show hum- have described an unusual low-wave energy. 3D) consist of van- Again. Associated grainstone or tic and carbonate systems. where higher rates of car- bonate production in shallow water ensure that Ramp facies reflect the protracted offshore these environments have the greatest accretion energy gradients which are a consequence of potential. This situation is the reverse of that seen in most other carbonate Ramp facies depositional systems. are dominated by suspension fall-out. the volume of carbonate sediment in the posed of intraclast-rich dolomites. order to determine the significance of these dif- up” shelves in that they are typically restricted to ferent sediment transport paths. The roles of long- 3A—3C). are actually poor analogues for many ancient. so that low-energy. The marginal successions of ing the shallow water depths and large substrate tidally dominated inner ramps should display areas. belts. storm-dominated ramp from the Ven.P. since modern ramps carbonate and siliciclastic settings. Lagoonal sediments comprise a in transporting material from the shoreline to the range of mud-. or climatic factors may have been form style to rimmed shelf morphologies (Ahr. In may have favoured the massive oolite production modern environments a similar effect is achieved which characterise the ramp-dominated time in. for example. Pedley. is ally absent or scarce. eu. oceanographic (e. 5). The resulting decrease in biological car. 1992). The appearance in the later poorly understood.or wackestone-textured offshore sediments.. means that in distally steepened ramps. 1984) and other rimmed shelves. would have (temporarily) eliminated the 1987. They differ from slope aprons or base-of-slope aprons adjacent to shallow-water platform mar- gins which are composed largely of shallow-water materials (cf. Shinn et al. One possibility is that major Mississippian. there have been times during rates of biological carbonate production in shal- the Phanerozoic when they constituted the most lower-water environments at these times (Wright prominent carbonate platform type (Fig. particularly during rela- tive sea-level highstands. and the exists between ooid generation and biological change in production profile from shoreline to productivity in Holocene settings (Lkyd et al. The Trucial Coast of the ability of carbonate platforms to develop shelf Arabian Gulf. upwelling. The mostly hemipelagic sediment composition in outer-ramp settings. lacking a compared with the time-distribution of reefs (after Heckel. slope deposits consist largely of mud. Bar chart showing the major ramp-dominated periods agenetic potential” of such deposits. Fig. The same phenomenon also ap- organisms from shallow water environments and pears to have occurred again in the Jurassic. be the basis for the antithetic relationship which bonate productivity in such settings. Extinctions were clearly a factor in determining the relative importance of ramp depositional systems. of more effective reef extinctions. because of The widespread oolite-dominated ramps of the high ambient salinity.g. in areas of environmental restriction and this may tervals. although other possible causes (see text) should not Organic influence on ramps be overlooked. 5. 1990). and Shark Bay in Western Aus- morphologies and favoured ramp development. This and Faulkner. Both correspond to periods temporal alternation between ramps and other when shallow-water framework reefs were glob- platform types. Impact offramebuilders on ramps Although carbonate ramps are common in all Mississippian and Jurassic probably reflect low geological periods. Note that subjected to meteoric water. significant aragonitic component and less readily 1974. unless significant quantities of shallow- water material are generated at the slope break during relative sea-level lowstands. is also lower than in ramps are the dominant platform types following major or- shelf aprons ganic extinctions (crosses). Aprons at the slope toes of distally steepened ramps thus probably represent less attractive play types for petroleum exploration than the slope aprons of many rimmed shelves. constructors saw a widespread transition in plat- trophication). James..CARBONATE RAMP DEPOSITIONAL SYSTEMS nated and rippled beds of carbonate or siliciclas- tic silt or very fine-grained sand in offshore ramp sediments may be the expression of storm re- working (see e.g. The “di. tralia.. particularly framework reefs. Calvet et al. 1990). constitute case-examples where. corals and other typical . 5). responsible for the exclusion of frame-building 1989) (Fig. 1990). basin. Plot of predominant organism distribution versus time showing how the role and location of organic sediment producers on ramps has changed. Bar widths provide a qualitative guide to the relative sedimentary importance of each group of organisms. 6. Note how the main sediment producers appear to have migrated from mid.to inner-ramp niches since the Late Jurassic. . Data derived from numerous sources. Bottom inset shows ramp subenvironments defined in this paper. This coincides with the rise of the calcareous pelagic foraminifera. although it is unclear whether a causal relationship can have existed.Fig. Wright et al.and late Palaeozoic. poroids. 1982b). If shallow-water aching implications for the development of ramps ramps had similar productivity profiles to other facing oceanic basins in that sedimentation rates platform types. Waulsortian facies. 1981.g. distally steepened ramp parts on ancient ramps. outer-ramp important features and sources of sediment in buildups were dominated by stromatoporoids and inner-ramp environments only since the late in some areas are up to several kilometres across Palaeogene and Neogene and must have had a (e. features in both shallow and deeper ramp settings tion in the inner ramp produce steeper ramp (Read. Stromatolites were im- framebuilding organisms produce most of the portant mound builders in inner-. were mate. they would presumably also show in post-Cretaceous outer-ramp environments may marked potential to aggrade rapidly into shelves have been significantly elevated in comparison even in the absence of a reefal rim. Read. In both. mid-. They do not appear to have had counter- Australia is a high-energy.. while Ordovician. On the other hand. crinoids. Heckel. 6). 1991. 6). and sediment. 1990). outer-ramp settings of ramps in the Precambrian ity of the mid-ramp setting would also favour and early Palaeozoic (Cecile and Campbell. Sea-grass banks. Grotzinger ability of the system to accrete vertically would be (1989) has reviewed mound occurrence in the reduced. many persisted as ramps may in some cases re- flect overriding tectonic or eustatic controls (see Organisms contributing to buildups on ramps later section). (Fig. The above hypothesis is without doubt too The evolution of calcareous pelagic foraminifera simplistic. have been Wilson. but would mostly seem to support the view that the rate of carbonate sediment A wide range of organisms have constructed production on many ramps is lower and more biological buildups in ramp settings throughout evenly distributed than in systems in which the Phanerozoic (Fig. The proportionately higher productiv.. 1991). Crinoids. allowing the maintenance of ramp reefs (Davies. 1975) and in the Devonian. (Elrick and Read. tive sediment producers in these zones (Fig. tant. Ramp mounds in the . major sediment producers in mid-ramp environ- ate sediment producers although sea-grass banks ments during the mid. These and as isolated buildups in mid-ramp locations or show that models in which sediment production even as conical pinnacle reefs of 50 m relief in (or accumulation) is uniform across the ramp outer-ramp environments. while Markello and Read. 1974. the whole ramp system progrades.. 1983. 1974. also lacks the key prolific inshore carbon. 1986. In the Cambrian and produce more “homoclinal” geometries. The latter point has been demonstrated Precambrian when stromatolites developed both using two-dimensional computer simulations in inner-ramp settings. The micrite-dominated core facies of these From the Late Jurassic onwards. as fringing biostromes. 1970. profiles. maintenance of the shallow ramp profile. Belperio et al. 1979). there has buildups resemble those of Mississippian been an interesting shift in the locations pre. bryozoa and slopes or rimmed shelves and a flat-topped plat. and bryozoans. Ricketts. The fact that with their pre-Cretaceous counterparts. possibly leading to local rates of carbon- marine platforms such as the Bahamas or the ate production as high as those of coral—algal Florida Shelf. Read et al. stromato- which.CARBONATE RAMP DEPOSITIONAL SYSTEMS 13 Late Cenozoic carbonate producers are excluded dramatic effect on ramp sediment production or are less prolific than at the margins of open profiles. 6).to more inner-ramp settings corals and stromatoporoids (e. 1978. ferred by bioclastic carbonate sediment producers Silurian ramps contain reef mounds formed by on ramps from mid. for example. biological buildups were prominent those which use higher rates of sediment produc. on the other hand.g. but are important locally in inshore areas (James and have no modern counterparts which are as effec- Bone. 1991). unidentified lime mud generators were impor- form morphology. Sears and Lucia. because of an essentially temperate cli. The answer may lie in the productivity in the Cretaceous may also have had wide-re- of ramp systems as a whole. the southern coast of 1988). although the Moshier. deeper-water 1985). 1985. Framework reefs are virtually unknown Waulsortian mudmounds (Lees and Miller. and wave-formed crinoid banks were prominent oped in a range of water depths (Lees and Miller. 1986.Mississippian are mostly distinctive. 1990) which devel. in mid. from this time (James. King.to outer-ramp settings (Wright and . 1984). although current George and Ahr. 3 = basinal deposits commonly bioturbated. Edwards. while River Formations) at the margins of intrashelf later Permian mounds were constructed largely basins. ied as the settings in which they occur. more widespread in cratonic-intenior basins than matolite-sponge and coral buildups occur widely in other settings and are commonly developped in the Maim of southern Germany (Barthel. Heckel. inward of inner-ramp oolite shoals. 6 = ramp inherits distally steepened margin along the shelf slope break of underlying carbonate shelf. settings the communities comprised capninids and Triassic ramps were marked by the occurrence of radiolitids. Aigner. late Cretaceous and early Tertiary.. Harris and Crevello.g. wave-swept inner-ramp by stromatolites (e. corals. 1990). and Devils algae and Problematica such as Tubiphytes. 1969. and algae in occur on low-energy ramps of the eastern North the Tertiary of the Tethyan and Mediterranean Atlantic margins and developed in water depths realms. banks (e. noting displacement down-ramp of some of possibly several hundred metres (Jansa et al.and mid-ramp buildups appear to be southern England (Sun and Wright. 1990). 1984. especially in Deeper-water buildups were widespread on inner. 1990). They also occur in intrashelf basins and in some over ramp of the US Gulf Coast (Baria et al. Calvet et al. orbitolinids. Upper Jurassic ramps in several areas.g. a!- 1982. 2 = swarms of isolated buildups (“pinnacle reefs”) form in mid. Aigner. passive margin and foreland basin settings. though in these latter locations they commonly veloped on basement or salt-controlled highs bas. In the Gulf Coast mid-Creta. 6). 3 = ramp in younger post-rift sequences. 1983).. Outer ramp and basin may show episodes of sediment starvation coincident with major subsidence phases. as swarms of hundreds of small mounds each a Meyer and Schmidt-Kaler. (A) Extensional basin: I = ramps developed on the shallow dip slopes (“rollovers”) of fault block. Wilson. 1983) and have also few hundred metres to several kiiometres across. on small and larger benthic foraminifera were important ramps.and outer-ramp environments.and mid-ramp settings (e. generally with low rates of subsidence promote extensive ramp development. 1. were Reid (1987) has described microbial-coral mounds important contributors to buildups throughout the from the Triassic of Canada. 1990). 2 = small rimmed shelf developed along steep footwall escarpment of fault block.. Pennsylvanian and Early Per. Ellis et al. forms during the Tertiary by new genera in shal- 1989. mounds occur on an Upper Jurassic ramp in Outer..and outer ramp. 4. The components in mid. . but it Fig. The latter de. 1975). 3. and sediment production on.g. Low-gradient margin. while requienids were dominant in outer-ramp mudmounds and mid-ramp cninoid lagoonal biostromes. Sunniland. Larger benthic foraminifera. Multiple unconformities develop in the mid. been described from the Upper Jurassic Smack. Fig. 1983). 2 = ramps developed as prograding wedges in older post-rift sequences.g. alveolinids and nummulites. Intracratonic basin. 5 = buried sediment fans generated through erosion of. 1974. The inner ramp only is shaded in each example.and inner-ramp buildups reasons for this distribution are probably as var- (Scott. ceous a variety of rudist buildups are associated mian ramp mounds were dominated by phylloid with ramps (e. Siliciclastic wedge generated through erosion of thrust stack. Stro. In higher-energy. 3 = ramp progradation limited by synsedimentary antithetic fault in dip slope. D.CARBONATE RAMP DEPOSITIONAL SYSTEMS 15 Faulkner. (B) Passive margin: 1. the footwall escarpment. Schematic cross-sections through various styles of sedimentary basin showing the locations and character of associated carbonate ramps. Similar coral-microbial lower-water niches. 1989). 4 = remnant rift topography and halokinesis localise development of isolated shoals/reefs in the mid. 2. 7. have a tectonic or halokinetic foundation.and inner-ramp areas over the peripheral bulge. commonly along clinoform edges. (C) Foreland and compressional back-arc basins: I = stacked ramps seed on gentle slope at margin of depressed foreland and prograde into the basin. 4. 1990.g. 7 = base of slope apron developed at the base of escarpment. Minor Buxton and Pedley (1989) have discussed the coral-stromatoporoid or thrombolitic bioherms distribution of foraminifera. Gerard and Buhrig. or in hypersaline intrashelf basins. Buildups are rarer on ramps in extensional set- During the Cretaceous both rudist bivalves tings (e. Ramps therefore occur consistently in isolated buildups are to develop and survive. (A) Example of a ramp in an extensional basin which has prograded over infilled half-graben topography. gentle cratonic downwarps. . Footwall uplift locally exposes the ramp carbonates to karstification. 8. WRIGHT would seem that the lower the energy and the slight. 16 T. optimal situations for are known from all of the above settings and ramp development are those in which subsidence models are outlined below (Fig. Disruption in this example was accompanied by renewed influx of siliciclastic sediment which contributed to the extinction of the platform. gradients are opment in these settings is reinforced where un- Fig. Based on Mississippian examples in Ebdon et aL (1990). and basinal water depths are relatively gentler the depositional slope. Ramps regardless of other factors. the mar- Tectonic setting gins of shallow intrasheif/intracratonic basins. post-rift phases of The low carbonate-sediment productivity of passive margins. and the dip slopes (hanging-wall most ramps (Elrick and Read. 1991) means that. tectonic regimes charactenised by gentle flexural subsidence. the more likely shallow. BURCHEI-rE AND V. Ramp devel- is continuously or episodically slow.P. This is only possible where siliciclastic or carbonate basinal sediments reduce the amount of accommodation space available in the basin and generate a suitably low-gradient substrate for carbonate ramp growth. (B) Subsequent reactivation of faults disrupts the ramp. “rollovers”) of extensional fault blocks. creating local steep scarps from which material is reworked into debris and turbiditic deposits. such as orogenic forelands. 7).P. or a cool basins within dip slopes. northern Norway bonates may be intercalated with shallow-marine (Gerard and Buhnig. typically develop in transfer or more than 50 km across and have deep. the Palaeocene of the Sirte Basin. 1988). 1991). Burchette. 1988). 1983. motes the growth of rimmed shelves and isolated Examples of ramps in extensional (Table 2) buildups. Early Miocene of the Gulf of Suez (Burchette. 1990). 1988. the Silurian of North humid climates. elastic alluvial or submarine fans. Large ramps on passive. margins or in foreland or cratonic interior basins At this stage ramps may prograde from the rift commonly form linear sediment prisms trending margin across buried extensional faults which. high half-grabens are occupied by small rimmed rates of elastic input. urian of North Greenland (Hurst and Surlyk.. hypersalinity. which inhibits ramp development.g. Gawthorpe. but pro. or on the dip slopes of They are characterized by high rates of subsi- tilted fault blocks (Fig. the mid. settings are known from: the Cambrian of Sicily elastic sediment input may be high. subsidence. 1984). 1990) (Fig. or evaponites (Reading. and relief is reduced or infilled. 1988) or distally steepened make these regimes unfavourable for widespread ramps (e. characteristics of the settings discussed above.. 7A). The high subsidence rates.. Permian of the Finnmark Shelf. high Antithetic faulting. the basin centre. Jacquin et al. lacustrine sedi- 1986. Simo. the Early Tniassic fan-delta sediments as in the Miocene of the Gulf (Anisian) of the Dolomites. or a layered tings develop during quiescent phases or in the character. Where fault trends remain active. narrow fault tip-zones. the are thus restricted to drowned rifts with low Mississippian of the Bowland Basin (Gawthorpe. 1989). or to intrabasinal highs isolated 1990) in northern England. 1990). ramp development. Ebdon et al.g. as in the Sil- are high. reactivated. Locally car. 1975).g. 1986. A possible ancient example . The availability of shallow-dipping substrate is 1988). raphy. Carbonate platforms of all types Greenland (Hurst and Surlyk. rather Subsidence rates in active extensional basins than rimmed shelf development. while ele.. flexural. minimal (Hurst. and small substrate areas shelves (Burchette. may be period. if for hundreds of kilometres along the basin mar. 1991). that ramps in such settings may be dominated by Libya (Bebout and Pendexter. Small ramps. the Early Cretaceous of the Ver- in arid climates also commonly become restricted cours. In ramps can prograde or may contribute to distal some of these regimes. where throws and gradients are geometries and normal fault-bounded margins. esis (e. French Alps (Jacquin et a!. arid settings where elastic input is 1986) and the Widmerpool Gulf (Ebdon et al. Allen and vated footwalls facing the deepest portions of Allen. Due to footwall uplift the rate of silici. 7A). the (cf. with the development of marked topog. Marine rift basins Hsü. Chatellier..g.to Late from marginal elastic sediment input.g. 1980. may control the distance climate) inhibit rapid carbonate production. Burchette. ments.g.CARBONATE RAMP DEPOSITIONAL SYSTEMS 17 favourable environmental conditions (e. 8). areas of small or flexural Extensional basins displacement may be the sltes of ramp. so (Simo. comprising alternating prograding post-rift stage when subsidence becomes largely progradational and drowning events (see e. 1986). ically high and this may be reflected in the short Most ramps associated with extensional set- duration and small size of ramps. and the hypersaline facies. limited in extensional basins and this contributes Major strike-slip zones possess many of the to the small sizes of ramps in such settings (Fig. a characteristic Late Cretaceous of the south-central Pyrenees accentuated by their compartmentalization. Tertiary of the Red Sea). may influence deposition and diagen- gin. 1973). high subsidence rates are restricted to Ebdon et al. a few kilometres or tens of Smaller strike-slip extensional basins are rarely kilometres across. steepening of the shallow slopes (Fig. 7~t). 1984). and prograde into dence and are typically infilled by coarse silici- the adjacent half-grabens (see e. 1990). 1987). particularly in (Bechstädt and Boni. Italy (Bosellini and of Suez (e. or the development of sub- rates of elastic input. only periodic. elastic supply. Australia Huqf Group E. Cambrian 10 (est.P. WRIGHT TABLE 2 A compendium of published information on selected carbonate ramp systems. 700 Formation Geosyncline. Anadarko Late Silurian — ?Post-rift (remnant 40—50 ?200 50 Formation Basin. Central Oman Infracambrian — Intrashelf basin — — 1500 due to extension along passive margin CAMBRIAN: Shady Dolomite Virginia. 140 Group chians. and setting Stratigraphic Location Age Duration Tectonic setting Width Length Thick- name (m. U.W.A.) 100 400 Formation ORDOVICIAN: Whiterock W. Early Cambrian 15 Post-rift 800 1600 + Nebida S. development.) ?Passive margin 40 (est.E. Utah E.18 T. of Oklahoma Oklahoma aulacogen) Heldergerg Central Appala. Sardinia L. Virginia— Silurian—Early foreland basin. Late 8—10 Late-stage 100—300 300 + Max.y.) (km) (km) ness (m) PRECAMBRIAN: Wonoka Adelaide Late Proterozoic — Early post-rift — 100 H. including data on their geometry.) Passive margin 150—200 200 + Several Land 100 Group Henryhouse N.S. S. Greenland Silurian 20 (est. BURCHETrTE AND V. — Passive margin 600 1000 60 Series Ordovician modified by fault reactivation SILURIAN: Washington N. New York Devonian ramps on both cratonic and orogenic margins .P.—M. Lateral transition Vertical — Hurst and oolitic grainstones. peritidal lime gradation 35—50 m/1000 yr. crinoid-bryozoan Dolomitized beneath uncon. dominated stromatoporoid mounds. Wave.CARBONATE RAMP DEPOSITIONAL SYSTEMS 19 Character Facies types Distinctive features Dominant Climate/ Source accretion paleolatitude reference style Storm-dominated Lime silts. 1986 mudstones. Foundation for Calathic algal build. calcareous transgressive/regressive sandstones and shale. influenced dolomitized grainstones. Pervasive lime siltstones. of foreland basin. siliceous mudstones . Intraclast hummocky cross-lamination. ?Wave and tidally Laminated dolomite. coral. Location 1984 floatstones. Mixed siliciclastic/carbonate. 1989 beds proximally. Local development of 1990 stromatolites. 1969. formity. 1989 oolites mudstones. siltstones. wacke-stones. Ramps developed on both Lateral and ?Sub-humid Laporte. 1980. locally isolated in terrigenous Boni. 1990. Skeletal grainstones. Archaeocyathid mounds Vertical ?Humid Bechstädt and archaeocyathid mounds. mudstones. sequences. Highly tectonized Terrigenous mudstone.and ?tide. stylonodular clayey ?Milankovitch cyclicity.(stacked ramp) Dorobek and pack-stones. Mud mounds in few-km Vertical Sub-arid Barnaby and stones. Three Read. intraclast escarpment shelf.. Massive siliciclastic ?Vertical Humid and Ross et al. — Arid Wright et al. Possible tidal channels calcareous mudstones mappable in oolite shoals from subsurface thickness data. 1983. storm 1989 buildup mud mounds. dominated — Stromatoporoid biostromes. Rates of pro. calc. Vertical ?Sub-humid Morgan. calcareous sand. cyclic red and outer ramp. tempestites — Nodular lime wacke-. co-sedimentation. 1985 pack-. at each shoaling cycle base. conglomerates. bioclastic-peloidal wacke-. Stromatolites. lime into rimmed and Surlyk. terrigenous controlled by unfaulted flexure. major carbonate ups. peritidal Evolves from ramp to laminites rimmed shelf. evaporites.. intraclast breccias. and biostromes. cyathid mounds. wide zone between inner (ramp stack) Read. Adjacent to major salt basin. mudstones ?Tidal Oolite. Read. stacked archaeo. stromatolite bioherms bituminous dolomites. Ramp shows cyclic shoaling. glauconitic Condensed glauconitic deposits limestones. eastern and western margins vertical Read. ?Vertical Arid Haines. pack. 1988 stones. ) 700 Quarry Mississippian basin (basin fill) Beds (Brigantian) ea. Late 2—3 (est.K.) Limestone Holkerian) basin Lodgepole Williston Basin. Canada Basin.) Extensional. Mississippian — Cratonic-interior 300 1000 + 500 Group western USA/ basin southern Canada Pitkin Limestone Ozark Mountains. Mississippian 23 Back-are basin— 50 150 1000 Carboniferous (Tournaisian— foreland (max.) (km) (km) ness (m) DEVONIAN: Nisku West Canada Basin.20 T.P. Frasnian — Cratonie-interior 150 200 50—100 Formation Alberta basin basin Grosmont W. Wales E. with 100 + 40 + 30 Formation New Mexico ramps on dip slopes of fault blocks Station Derbyshire. Late Mississippian 8 (est. Late <10 Small intrashelf 10 20 (est.) Cratonie-interior 200 + 800 . Mississippian 10 (est. WRIGHT TABLE 2 (continued) Stratigraphie Location Age Duration Tectonic setting Width Length Thick- name (m. 15 km wide Lisburne Arctic Alaska Late 20 (est..) Foreland basin 80—150 300 + 50 + and northern Arkansas Fayetteville Shale Morgan Northern Utah and Mid-Pennsylvanian 8—15 Cratonie-interior 200 Formation Colorado basin . 170 Formation Alberta Devonian (late basin at Frasnian) passive margin CARBONIFEROUS: Caballero Sacramento Mnts. 150 1100 + 4—500 Group Mississippian (Meremecian and Chesterian Stages) Exshaw/Banff West Canada Basin. Mid-Mississippian — Extensional 230 + 900 + 150—800 Formation Alberta (Tournasian) (?intracratonie) Lower SW.y. U. E.) ?Passive margin. BURCHETrE AND V.P. . 1982. steepened (fault control).t.CARBONATE RAMP DEPOSITIONAL SYSTEMS 21 Character Facies types Distinctive features Dominant Climate/ Source accretion paleolatitude reference style Muddy bound. angle 1986 0. 1990 ?Leeward. lagoonal (ramp stack) arid/humid Burchette lime mudstones. 1974. Lepain fasciculate corals. buildups ?Leeward Calcareous mudstones. Most sub-humid seismic.140. Aeolian No carbonate aeolianites. 1990 stone ramp. bioclastic wacke-. pack-. Basinal nodular lime mudstones facies suboxic and cyclic. . 5—15°N. quartz sandstones. Pronounced shoaling cyclicity on Lateral and Arid. grading and bioturbation sequence (progradational). 20°N. Wave base data. Progrades over in small intrashelf basin) correlative Ireton shale basin fill. and mud-stones Adjacent to shale basin. southern margin distally dolomitized laminates. and grain. — Argillaceous crinoid. bioclastic Adjacent to deep flysch basin. evaporites homoclinal to distally steepened (Hondo Formation possibly ramp morphology. — — — Smith. Ahr. Northern Vertical ?Sub-humid Gutteridge. bryozoan calcarenites. 1986.r.. margin homoclinal ramp. pack-. effects pervasive. Windward Oolites. pack-. 1975. shale. oolites between cycles. Stacked ramps with buildups Vertical ?Sub-arid/ Watts. crinoid-bryozoan shoreline progradation. — Spicule-pellet lime mud. Amphipora/peloidal longer term cycles. laminated Shoaling beach depositional Vertical and Arid Wilson. wacke-. laminated shows internal steepening from ramp stack) mudstones. grainstones. 1989 crinoidal grainstones. 1983 coral-stromatoporoid float. Lateral ?Sub-humid. Bioclastic and oolitic Distally steepened in west Lateral Sub-arid/ Chatellier. ?Lee. Oolitic grain. Leeward w. wacke-stones. dominated peloidal mustone. Arid Driese and trade winds? wacke-. Cutler. oolitic and sequences in gross transgressive lateral Lindsayand bioclastic pack-. grainstones regressive cycles terminating in (ramp stack) Kendall. Progrades downslope coral/strom. Basin fault controlled. Forms coarsening-upwards (ramp sheet). Each cycle gradational packstones. wave-dominated. 5—7°N Dott. 1990 offshore shales ?Wave Silty mudstone. lithofacies argillaceous. Storm Overall slope Handford. local part argillaceous. Shoaling cycles representing Vertical Arid. grainstones with HCS. 10—25 m scale superimposed on 5 vertical (pro- stones. Stromatoporoid pack-.or diurnal stone.and crinoidal Cyclic alternation of aeolian Vertical. packstones. et al. 1976. events on seismic. (ramp stack) 1989 resedimented carbonates. et al. Exposure 1977. Waulsortian mounds — Bioclastic wacke-. ?30°N. 1985 basin-filling evaporites Storm. 1988 Progrades on grainstones. Evaporites. pelloidal and (fault controlled).08_0. Lower (ramp stack) Bird and Jordan. bioclastic Barrier islands and shoals Vertical Alternating Wright. Jehn and dominated pack-. stones. Armstrong. sandstones and ramp carbonates. Oolitic grainstones. and grain-stones. Young. 1984 Storm. developed during drowning (ramp stack) arid ward. P. Permian 16. Midland L. Kimmeridgian on passive margin Smackover U.) Extensional 150 450 + 30 + Musehelkalk (Anisian) (cratonic interior) JURASSIC: Mem Martins Lusitania Basin. German Basin Mid-Triassic 1—2 (est. WRIGHT TABLE 2 (continued) Stratigraphic Location Age Duration Tectonic setting Width Length Thick- name (my. Muschelkalk 2—3 each Extensional 10 + 1000 120 Capafona Spain (Anisian/ sequence Units Ladinian) Upper S. Finnmark.) Oxfordian CRETACEOUS: Mishrif Southern Arabian Gulf Cenomanian— 7 Cratonic-interior 100 400 + 200 Formation earliest basin at (max.) (km) (km) ness (m) PERMIAN: Un-named N. 12 Extensional 200 140 + 220 Callovian— (max. Platform. Spain M. BURCHETIE AND V. Norway TRIASSIC: El Brull— Catalan Basin. Jurassic.) Turonian passive margin .) Early post-rift 20 + 25 + 100 + Formation Portugal Berriasian Hanifa Southern Arabian L. modified by New Mexico campian) reactivation of basement features Un-named E.S.P. (early Wolf. Kimmeridgian/ 3—4 (est.—U. E. Gulf Coast U.) Intrashelf basin 100 — 30—40 Formation Gulf E. Early to — Late synrift/ 100 300 + up to Barents late Permian early post-rift 200 Shelf. — Early passive — — 300 Formation Oxfordian margin JURASSIC: Un-named Iberian Chain. SE.22 T.5 Foreland basin 50 50 750 Basin. Jurassic. Oxfordian/ 2 (est. evaporites movement. Redbeds. Local rudist Source facies developed seismic. 1984 packst. halokinesis and fault 1982 seismic. bioclastic Uplifted and karsted due to Lateral Humid Burchette and facies hydrocar. Bind. hurri. foram. marly 1.. bioclastic pack-. marl Vertical Arid. and peripheral bulge growth. Development influenced by Lateral Arid Baria et at. 1990 wacke-. — Stromatolitic and hydrozoan Shallow-water carbonates back. stones. Numerous buildups up to 2 km stones. intercalations. L. pack-. Calvet and with cyclic facies grainstones. across. wacke-stones. arrangement mudstones. 1988. (ramp stack) Buhrig. grain-. pack. Smackover is source facies. Thinly (ramp stack) Reid. buildups 1990 Storm-dominated Oolitic grainst. (echinoderms. bioclastic/fusulinid step between E.. Source facies developed Vertical Arid Moshrif. evaporites in L. 1985 bon source rock. Overlies shale. depos.. Syn-sedimentary salt doming. and L. karst. Arid/semi-arid.. basin fill paekstones and portion suggests steepening humid 1990 wackestones Basinal facies Bioturbated lime mud..Vertical Arid Gerard and mounds. downslope activity Calvet et at. Bedded bioclastic Resedimented breccias in Lateral ?Semi-arid/ Ellis et at. hydrocarbon stones. and grain. oncolite Graded grainstone sheets. location controlled by Permian faults. Aigner.CARBONATE RAMP DEPOSITIONAL SYSTEMS 23 Character Facies types Distinctive features Dominant Climate/ Source accretion paleolatitude reference style Foundation for Dolomitized lime muds Stacked and distally Vertical Humid Mazzulo and a major carbonate and packstones. Permian. 1991 ahermatypic corals). bioclastic grsts. Progrades on stones. (ech. basin and siliciclastic hinterland. Black steepened ramps. biostromes. Adjacent to shale 35—43°N. molluscs). lime mud-. marlstones sequences. lime Riba reefs) seeded in TST some lateral low storm Tucker. Karsted. shoaling (ramp stack) caine belt.. 1989 buildup shales. Two sequences Bioclastic and oolitic Organic/cement buildups (La Vertical. 1990 source section ?Windward Oolitic/peloidal grain. wackestone. lime mud. interbedded over shale basin fill.. Welt-developed lowstand ?Lateral ?Semi-arid— Aurell. ?Leeward./glauconitic condensed humid 2. Oolitie grst. pack-. cyclically in transgressive phases . ?Leeward. coralgal sections reefs.. subaqueous cyclically in transgressive Droste. 1984. vertical Progrades on stones. and Britton. Basinal Pelagic basin. Downslope buildups. sponge. 200 600 + <2 Peninsula Mexico present occurs in the Cretaceous of the Pyrenees crease exponentially as the passive margin ma- (Puigdefàbregas and Souquet.) Syn-rift 10 15 + Max. WRIGHT TABLE 2 (continued) Stratigraphic Location Age Duration Tectonic setting Width Length Thick- name (my. Ingersoll. 1986). 60 with Pleisto- cene.008 Foreland basin 200 400 + Few Gulf cm—few metres. Gulf of Tertiary— 25 Passive margin ca.P. However.24 T. Phases of siliciclastic sediment-starvation during The rift to drift transition is characterized by early passive margin development also favour relatively uniform subsidence rates which de. (Fig.) (km) (km) ness (m) TERTIARY Darai Limestone SW. this . short-lived ramps might conceivably form at a 1988).008 Passive margin 100 200 <3 Yucatan Mexico.P. late stage in such basins. Once syn-rift relief has been eliminated. Papua Late Oligocene— 15 (est.) wide Rudeis Formation Gulf of Suez Burdigalian/ 3 (est. 7B) which favour the development of car- nant. Harding. bonate ramps or low-gradient rimmed shelves hundreds of kilometres across and trending po- Passive continental margins tentially thousands of kilometres along strike. BURCHETFE AND V. 1978. 1983. widespread carbonate deposition. 80 Langhian Un-named West Florida Tertiary— 25 Passive margin 200 800 + up to Slope present adjacent to 1000 enclosed oceanic basin (Gulf of Mexico) QUATERNARY: Trucial Coast Southern Arabian Recent 0. (max. Shark Bay Western Australia Recent 0. 700 kin. Small tures (Pitman. when much of the initial this regime is characterized by large areas over topography has been eliminated or in areas where the old rift shoulders with very gradual slopes flexural subsidence rather than faulting is domi.) Foreland basin 500 1000 + 2000 New Guinea mid-Miocene ca. Restricted basin. central Texas (Ahr. and tide-dominated. elastic basinal facies. Max. sandwaves. Vertical Humid. grainsts... Seasonal oblique lithoclast sand. mid. 1974. 1975 water depth ca. 1975. (Ross Proterozoic Wonoka Formation of South Aus. Mid- development of rimmed shelves (Fig. ooze. leeward Coralline algal—large foram Drowned due to rapid sub. Multins et al. Transition to basin stage. muddy generated by salt diapirs. Rem. Texas (Loucks and Anderson. covered by elastics which 1989 overflowed foredeep. grainstones. 1987. Progrades sand. loci of isolated buildup or shoal growth in mid. 1989) and the mid-western U. coral/ behind reefat rimmed margin arid 1989 algal biostromes on footwall escarpment Leeward with winter Winnowed planktic foram Intraformational slumps. bioherms. Hypersatine basin Lateral Semi-arid/ Logan. 100 m. Barnaby and Read. pellet arid. karst during 20—28°N. 1990). coastal marine currents. dle to Upper Cambrian algal carbonates of the nant syn-rift topographic highs may become the Moore Hollow Group.or the Early Ordovician Ellenburger Formation of outer-ramp situations. Subsequently Pigram et at.CARBONATE RAMP DEPOSITIONAL SYSTEMS 25 Character Facies types Distinctive features Dominant Climate/ Source accretion paleolatitude reference style Monsoonal. stromatotites. bioct. sands. strong skel. Topography inherited from ? Sub-humid Logan et at. Galloway et Examples of ramps developed in this tectonic a!. as with the Oxfor- . or oblique to winter pack-. Ramps also developed exten- The latter includes: the Early Cambrian Shady sively during the Late Jurassic. Thin veneer of ooid—pel. is likely to promote the and Read.. Bird Palaeozoic around the North American craton. nodular grounds. Storm-dominated. storms. facies distribution over these. Thick over foreland flexure. may reduce basinal sedimentation which.. Davies et at. 1988) and much of the early Pennsylvanian of Alaska (Armstrong. Outer ramp topography Lateral Arid. 1977).S. Evans. 1988 on shallow seismic. 1989). and Jordan. with Dolomite of the Virginian Appalachians (Bova continued subsidence. grainstones. 1980. trade winds Local mounds. Grades to Tertiary platform 1969 windward. Low tidal pelagic ooze at margin. 1989. lagoon..A. 25°N. et al. 14—19°S. hard.- energy. Distally steepened over inherited margin Windward (winds Oolitic barriers and beaches. and the Late Mississippian and tralia (Haines.. Oct. Mixed carbonate Burrowed sandy foram Covers fault-block dip slope Vertical Semi-arid/ Burchette. foram ooze. 7B). 1971). sabkha.. rare turbidites lowstands. 1974 Seagrass banks.. Windward—leeward Purser. barriers. Purser and seasonal). siliciclastic pack-. Vertical Humid.and Outer ramp. wacke-. 1983). Wave. the Ordovician of the Appalachians setting are numerous (Table 2) and include the (Read. 25°S. Leeward Oolitic barriers and shoals. sidence during late starved. This setting. Gulf Coast ent and subsidence rates are lowest. ramp depositional systems which may prograde Handford. (Budd and Loucks. Fig. carbonate ramps settings may be up to many tens of kilometres commonly form as linear belts seeded along the across and several hundreds of kilometres along peripheral “bulge” (Fig. the Arabian Gulf in front of the Zagros Moun- margin ramp of this character occurs on the Yu. 1982) and subsequent stages of ment features in the foreland may be reactivated the basin fill may be affected by salt pillowing. 1985) which of the Gulf of Papua (Pigram et al. Meendsen et al.. as in the case of the Tertiary—Holocene campian) of the North Platform of the Midland ramp of the West Florida Shelf (Mullins et a!. tario (Brookfie!d and Brett.. ify ramp growth. line of the Arabian Gulf. Wolf- locally.. (Table 2). a modern example is the southern coast- and Blome. 1988). Basin. or 1986.g. the later Tertiary of southeastern Sicily (Pedley. and associated growth faulting (Fig. Gutschick and Sandberg. also experiences relatively low rates of shelf of North America (Jansa.. ner-ramp areas (see e. Gamboa et al. the process generating basement. 1969. and the Middle East (e. 7C) where slope gradi. Where drowned. Examples of ramps in such settings are: 1982. 1989). 1989).S. 1979. 1989). 1981. of uplift and subsidence in response to thrust grade (Humphris. Another modern passive. Ramps in these In marine foreland basins. and displays both high-energy windward and low-en. land basins may be a hundred kilometres or more North Africa. in the is one seen repeatedly in the geological record Smackover Formation (Ahr. to the previous platform margin to form a the Marathon foldbe!t (Ross. 7C). as subtle extensional topography which can mod- diapirism. Read.. Jackson and Talbot. such flat-topped deposited on the foreland to the Ouachita Moun- platforms can be succeeded by equally large-scale tains (Glick. the Lower Permian (L. 1987) and in the Permian the mid-Ordovician Trenton Limestone of On- of the southern Barents Shelf (Gerard and Buhrig. 1973. deposited on the foreland to 1988). dovician/Early Silurian of the Appalachians Rimmed shelves which develop during the later (Read. 1981). The foreland basin setting for carbonate ramps 7B). Ellis et al. Examples of such systems occur in: the . 1986). Ramps within fore- 1985. strike.g. compactional. In some situations.P. 1990). Mussman and Read. Distal portions of ramps developed over oping bulge may be subject to repeated episodes buried rift basins may therefore be affected by of uplift and drowning. ergy leeward shoreline aspects. 7B). the Late Or- 1990). 1988). Baria et a!. Read.. Such phenomena are commonly accentuated The peripheral bulge may experience episodes by sediment loading as depositional systems pro.. modify the seismic expression of the ramp (Fig. 1986). emplacement. the Oligocene Asmari Formation of slope apron (Fig. terrigenous sediment supply. Ramps associated with the devel- 1986). 1989. Watts across. lated organic buildups or grainstone shoals. This has occurred. 1981). for example.P. and the Late Jurassic isolated by the foredeep from the main siliciclas- and Early Cretaceous of Tethyan and Atlantic tic source area in the thrust zone (cf. Rift basins are commonly the sites of halite pre-existing passive margin or even older base- deposition (Rona. 1983. Mazzullo and distally steepened ramp with a distinct base of Reid. Ramp systems also develop in compressional Compressional basins back-arc basins where depositional gradients on the cratonward side are gentle. the Oligocene catan Peninsula (Logan. New Mexico. 1986. 1981). 1990. 1989). 1985. the Silurian to Carboniferous of the stages of passive margins may be hundreds of Timan-Pechora Basin in the foreland to the Urals kilometres across and have significant relief above (Ulmishek. Brett et a!.. WRIGHT dian Smackover Formation of the U. 1978). or halokinetic highs complex karsted unconformities in mid. Pigram et passive margins along the eastern continental a!. BURCHETTE AND V..and in- which promote the development of scattered iso. 7B). Hubbard et a!. tains (Szabo and Kheradpir. the Late Mississippian Pitkin the surrounding basin floor along the slope break Limestone and Batesvi!le Formation of Arkansas (Jansa.26 T. 1979. seems likely for the Ordovician and Silurian of ceous of the Neuquén Basin of western Ar. Although cratonic-interior basins up to a few hundred kilometres across which may be over 1000 km across. 1981) and a similar origin 1990). and in the Late Jurassic and Early Creta.A.. 1990) so that depositional gradients are shallow platform top. the Missis. 1975). 1983). important on wide plat- Siberia. and are characterized Intrashelf basins are shallow (mostly <200 m by slow overall rates of flexural subsidence (Klein deep). 1983. This is perhaps one of proportion of the basin fill are the Palaeozoic the few situations where true ramps are likely to Michigan. Riding. or between basement !inea- low-gradient rimmed shelves which are commonly ments). 1981). Lindsay and sippian (Tournaisian) of the western Rocky Kendall. the mid. The their origin appears to be largely tectonic (e. 1987). 1979. few contain more develop within major carbonate platforms and than several kilometres of sediment (Allen and are commonly only indirectly connected via the Allen. 1975. Cambrian of Oman (Wright et al. relatively short-lived sedimentary basins and Hsui. 1987). or are lo.and outer-ramp environments (Fig. Orogen (cf. the Shoal belt deposits in intrashelf basins may re- Proterozoic. sides of the basins (Scott.g. Burchette et a!. the ple. Illinois and Williston Basins of the be backed by broad shelf-lagoons (see Markello central U. such monest along passive continental margins and basins also commonly become evaporitic. but intrashelf Examples of cratonic-interior basins in which basins within such systems are commonly bounded carbonate ramps of several ages form a major by ramp margins (Fig. 9). 7D. of the Michigan Basin (Whitaker. The gentina (Mitchum and Uliana. 7D). Early Cretaceous Habshan Formation (Connally 1991). The basin fills of . 1977. Larger intrasheif basins cated adjacent to orogens.. 1985) and the mid-Cretaceous Mishrif Formation (Burchette and Britton. the Mississippian of southwest cratonic-interior basin adjacent to the Caledonian Britain (see e. 1990). developed preferentially on the windwards Duperow Formation of the Williston Basin (Wil. Basin (Wilson. 1985) of the Cra tonic-interior basins southern Arabian Gulf are additional examples. Leeder. ramp-like depositional slopes. Because of their relative isolation.. the Frasnian Grosmont the windward margins. Flat-topped carbonate shelves seldom posses data in Table 2). although their development may be modi- characterized by numerous isolated buildups in fied by subsequent carbonate platform growth. with the gentle and basina! water depths mostly shallow open ocean environment. for exam- of Alberta (Cutler. The Visby Silurian ramp carbon- Mountains (Gutschick et a!. 1985). geometries of carbonate depositiona! systems late movement on extensional faults. Cratonic-interior basins are broad. the Silurian of the Michigan Basin (Sears forms (Pratt and James. Gutschick and ates of Scandinavia may have accumulated in a Sandberg. Watts.CARBONATE RAMP DEPOSITIONAL SYSTEMS 27 Early Silurian of the eastern Arctic Islands and son. 1979).g. 1988).. Documented examples of ramps such settings might conceivably show much higher in such settings include: the Precambrian— rates of progradation than windward margins. over within such basins are biased towards ramps or marginal basins. 1990). the Devonian/Mississippian West and Read. Cambrian and Ordovician of eastern flect either tidal activity. 1988) and the 1984).. 1980. and the Rub al Khali the platform top. and Vest. Legarreta. or a re-entrant. the Mississippian Strawn Formation western North Greenland (Hurst and Surlyk. persistent Intrasheif basins depressions which may overlie older rifts or sags between upwarps or basement blocks. the mid-Silurian of the Welsh Borderlands Lodgepole and Madison Groups of the Wi!!iston and Gotland (Anderton et a!. or wave activity on and Lucia. Smith. highstand leeward margins in Basin of Arabia. Middle Albian rudist and Nisku Formations of the Winterburn Basin banks in the East Texas Embayment. 1985. 1980). Such basins are com- (Fig. Due to sediment supply from Canada Basin of Alberta. the Illinois basin (Klein and Hsui. 1988). 1987.S. 1986. ists in a Late Mississippian carbonate platform in ing platform may be stimulated to vertical growth. With source complexes developed at passive continental mar- and inner-ramp reservoir in juxtaposition. Burchell.P. em Europe and the Middle East (M.P. BURCHETFE AND V. McKnight. 1969. Droste. 1976.000 km2 Markello and sea-level !owstands and the sites of organic-rich Read. 1989). with a act to reinforce the expression of the basin. Mitchell-Tapping. Ramps prograde centripetalty from the basin margins during highstand.28 T. Basin may become density-stratified with the development of cyclic suboxic or anoxic basin centre sediments. Tyler and Er- caused by very shallow wave bases and/or density win. pers. 1990. 9.. 1990). Fig. Where basin development is associated with teresting example of a small intrashe!f ramp ex- high rates of relative sea-level rise. 9).g. The surrounding platform may be exposed and karsted. suboxic centre. 1991). and several basins (e. commun. 1989). the sedimentation during transgressions or relative Tethyan Triassic (Dolomia Principale) of south- sea-level highstands (e. (Table 2) with well-developed ramp-like margins Because of their isolated nature. stratification caused by dense brines discharged All of these occur within major rimmed-shelf from the surrounding platform top. The with the development of isolated “pinnacle reefs” platform formed over a series of extensional fault within the basin and margins bounded by rimmed blocks which were reactivated in the early Brigan- shelves rather than ramps. ronment and surrounded by evaporitic platform East Texas. On the margins small ramps de- Fig. SL1 and SL2: relative sea-level stands. the surround. due to either the lack of mixing margin (Fisher and Rodda. intrashelf occur in: Cambrian and Ordovician platforms of basins may become evaporitic during relative the Appalachians (150. intrasheif basins may become hypersaline at behind the mid-Cretaceous Stuart City platform other times too. Ramps in intrashelf basins. 1981. England (Gutteridge. Mazzullo and Read. WRIGHT shallow intrashelf basins may be mistaken for the Documented examples of intrashelf basins deposits of lagoons. (B) During lowstand. forms with ramp-like depositional slopes.g. Scott. such gins and were infilled by smaller carbonate plat- settings are commonly prolific hydrocarbon sys. Derbyshire. . These two processes tian to form a small basin 5—10 km wide. Sea-level draw-down may lead to isolation of the basin and towstand deposition of subaqueous evaporite sediments. Where poorly connected to the open marine envi. These commonly correspond with parasequence-scale cycles in the shallow ramp. and South Florida basins) tops. (A) During flooding and highstand. An in- tems. Field outcrops are rarely large enough (e. Chevron Standard. tres across flanked by time-equivalent diapiric illustrations in Hurst and Surlyk. 1984) to view uplifts. 10. 1968).S. control the development of carbonate peripheral sinks to individual salt diapirs or as ramp sequences are listed in Table 3. Scale sections through the Frasnian Grosmont Formation. broader depressions up to several tens of kilome. a carbonate ramp in the West Canada Basin. Carbonate Salt-withdrawal basins ramps represent just one end-member in this Salt-withdrawal basins are seldom more than a continuous spectrum. It is the Cretaceous Mishrif Formation of the south. and show correspond- steepened profile (Gutteridge. therefore commonly necessary to determine ramp em Arabian Gulf (Burchette and Britton. 1979). sequence stratigraphies and geometries from Other likely examples exist in the Upper Creta. based on log correlations. Small ramps phies to be determined even from seismic data developed around the margins of the Sir Abu without the use of special processing or display Nuair salt withdrawal basin in the upper part of techniques (cf.g. 1989). widely spaced vertical sections using regional sed- ceous Austin Chalk around the Hainesville Dome imento!ogical. 200 m. thin (Table 2). Note that the section with only small vertical exaggeration (top) shows insignificant geometry. or that gross ramp geometries are seldom observ- in cratonic-interior or foreland basins. where the able other than on regional seismic lines. ranging from near vertical successfully mapped using careful wireline log. tureless for sequence geometries and stratigra- velop at the margins of such basins. Note too how slope angle and slope height show cumulative increase upwards through the sequence. Many overlie significant halite deposits. 1989). 1979) or the Late to support interpretations. In some cases clinoforms (not necessar- Carbonate depositional systems exhibit widely ily seismic) and sequence geometries have been variable slope shapes. so major carbonate platforms at passive margins. Only small (< ramps are too extensive. After Cutler (1983). several kilometres high to epeiric were later tectonically modified to a distally platforms with little relief. Salt withdrawal basins may occur within major portions of ramp depositiona! systems. Gulf of facies successions and the unequivocal identifi- Coast (Hughes. low-energy ramp systems may de. Vertical thickness of the Grosmont Formation c. . This involves analysis Jurassic Smackover Formation of the U..CARBONATE RAMP DEPOSITIONAL SYSTEMS 29 veloped which were originally homoclinal. The major factors which. in few tens of kilometres across and may develop as our view. and tea- 20 km across). ingly variable responses to relative sea-level changes (Kenter and Schiager. 1985). but escarpments. biostratigraphic and seismic data in south Texas (Halbouty et a!. cation of sequence boundaries on all scales in core or field sections from which a model for the Ramp geometry and seismic character larger ramp sequence architecture can be con- structed. but that with large exaggeration (bottom) the ramp exhibits subtle sigmoidal “clinoforms”. Fig. tonic or depositionat slopes and relatively stow subsidence rates. etc. (B) Banff Formation. between arrows) overlain by low-gradient rimmed shelf sequences (2—5). Jurassic. 1990). exclusively those which generate tow gradient . lands and passive margins. (A) Nisku Formation. such as orogenic fore- regime. Thickness c. 1985).S. Many ramps are thin and “stacked”. during rapid base-level rises. Ramps have been common in alt periods but dominant when framebuilders mental trends which deter. Sediment produc . Thickness c. Evidenced by the tectonic locations in ited from preceding ted. between arrows) shows a wedge-like geometry with parallel-continuous internal character which thins towards the basin (to left). Several small reflector-free areas (R) represent outer-ramp reefat buildups. storms. 1988). Oolite grainstone ramp. U. in. Grainstone ramp. Norway (Gerard and Buhrig. which pass laterally into continuous and discontinuous parallel facies of the outer ramp. This line shows well developed sigmoidal clinoforms indicating strong progradation with minimal aggradation. slopes and moderate to high basinal sedimentation rates (sediment production can comfortably fill all available accommodation space). Mississippian. Sigmoidal clinoforms indicate a progradational ramp with some aggradation (TR top ramp). Biogenic vs. ment is maintained at the shoreline.and storm-dominated and grainy sedi- regime tides. Tracings of portions of regional seismic lines showing the geometries of various carbonate ramps. BURCHETFE AND V. representing algal-dominated isolated buildups up to 1—2 km across. F) Portions of two lines from an Upper Jurassic/Lower Cretaceous grainstone carbonate platform. 200 m. WRIGHT TABLE 3 Major controls on the development of carbonate ramp systems Control Expression Significance Antecedent slope Low-gradient slope inher. Thickness of units 1—2 c. Ramp sediment production rates show lower shore to organic sediment. Thickness of ramp unit c. . 150 m. High rates of sediment mine presence/absence of production associated with reefs enable “keep-up” -sedimentation major shallow-water car. Base-level changes Rates of relative sea-level Ramps drown readily even without ecological “inhibition”. Rapid rise/tall due to subsidence base-level rises cause incipient drowning whereas high-productivity or eustacy.30 T. Tidal ramps have not been widely documented. drown readily. form aeolian dunes. (G. Frasnian. Important in alt settings.P. which ramps occur. (C) Smackover Formation. This section contains several Nisku depositional sequences. Sediment production and redistribution on ramps is still poorly understood. basin differential than in rimmed shelves. Argentina (Mitchum and Utiana. (E) Initial ramp (1. FR = top ramp. reef-constructors were rare or inhibited. Rate of in situ sediment Ramps are low productivity carbonate systems and may therefore tion generation. 11. isms. Shallow basin margins promote ramp development by limiting accommodation space. Thickness c. Gulf Coast (unpublished). H) Portions of two lines from the mid. West Canada Basin (Chevron Standard. 200 m. These show areas of mounding. This squeezed line shows gently sigmoidal ctinoforms which represent largely outer-ramp deposits (OR) with inner-ramp (IR) more proximally. Oceanographic Windward/teewardness. 200 m. reflecting such incipient drowing events. U. continental shelf. 100—200 m.S.. Wind-entrained sediment may oceanic currents. This shows welt-developed clinoforms and a strongly progradational character with minor aggradation.and Late Permian of the Finnmark area. wave energy. 300 m. (D) Upper Jurassic. a low-energy boundstone-dominated muddy ramp. Framework construction promotes the bonate producing organ.A (Gamboa et at. (E. development of rimmed shelves. The Jurassic ramp may become distally steepened and is succeeded by a reefal sequence (R).P. Windward ramps are wave. West Canada Basin (Chatellier. rimmed shelves can continue to accrete vertically or may evolve from ramp. eastern U. Neuquen Basin. 1979). Leeward ramps seem to be low energy and muddy with tow grainstone content. Fig. Thickness c. Barents Shelf. (F) More distal portion of ramp (1. Water depth Basinal water depth or Shallow basins are commonly characterized by gentle depositionat basinal sedimentation rate. 1985). Presence/absence Evolutionary or environ. 10). The thin depositiona! profiles of carbonate tions. 1985. is clearly only viable where an adequate database tions (e. and hardgrounds in distal loca. 1983. Cutler. 1988. ramps provide little opportunity for the develop- tions together with biostratigraphic correlation. Dix. can be used to enhance such interpreta. Chate!lier. such as condensed Ramp geometries sections and subaerial exposure surfaces in proxi- mal situations.CARBONATE RAMP DEPOSITIONAL SYSTEMS 31 biostratigraphic. ment of pronounced sequence geometries. Gross This technique for addressing ramp stratigraphy ramp geometries determined from seismic data .g. Stoakes. 1990) - (Fig. Burchette exists. and Britton. 1980. Distinctive features. and sedimentological correla. and the Mississippian Limerick ramp in may be in tens of m water). outer ramp (i. 150 km. or can even be recognised as a definite facies Carbonate ramps in such settings may form the change in the field or core. Chevron Standard. Burchette. except in determining platform geometry. In contrast to many rimmed shelves. which 1992). Length of section c. possibly corresponding to able accommodation space and so allowed shal- the inner-. convincingly where they amalgamate as a trend to form a shelf demonstrated for isolated buildups and rimmed Fig. BURCHETFE AND V. Grosmont. 1983). 1990. After Stoakes (1980). “slope”. and deep mor. Jurassic rimmed shelves. as on most rimmed 1cr. Such classification. showing how accommodation space created during the early Frasnian (Cooking Lake. ramp sedimentation and 11C). although the term ramp for very low. On this basis. the Mississippian ramp stack in south- shelves. many carbonate platforms classified as progradationa!. 1992). Whether Calmar Formations in the Winterburn Basin (see the seismic ramp “slope crest” corresponds to the e. Schematic section through the Frasnian succession in the West Canada Basin. Examples are the Nisku. 12.g. and Camrose Member) could prograde into the basin. 1979. as seems more likely.e. or represents the transition from inner to west Britain (Burchette et al. 1975). paradoxically. Stoakes. resemble “flattened” or tabular over large distances (e. Infitling of accommodation space in this fashion is the only way in which late Frasnian age shallow-water ramps (Grosmont. up to several hundred metres thick and tens drowning of the ramp as.or lens-like (Fig. ally has little effect on the gross geometry of The role of windwardness versus !eewardness ramp depositional systems (Fig. thick be mappable. Mitchum and Uliana. Leduc. thinning gently Devonian of the West Canada Basin (ci. Many ramps are thus divisible even on progradation appears to have been facilitated by seismic lines into shallow.g. and outer-ramp zones of our low-water ramps to prograde (Figs. fair-weather wave base. 11A). On laterally compressed displays of regional seis. towards both the basin centre and basin margin. with subtle Smackover Formation) and the thickest part of slope “crests” (or “offlap breaks”) and gently one depositional sequence lies basinward with sigmoidal clinoforms (Figs. been introduced to the system by lateral trans- ally exist. Where each ramp highstand system is strongly mic lines. Duvernay Formations) transgression was infilled by a progradationat sequence set. each sequence of which consists of distal terrigencus mudstones (stippled) and proximal carbonates (bricks). ening siliciclastic or mixed carbonate—siliciclastic tion. 12).g. Internally. 1966. WRIGHT and geological studies are sheet. Cut- shal!owest-water sediments. McGillavray and Mountjoy. mid-. terrigenous muds which infilled most of the avail- phological segments. . seem to be major marginal portions of large-scale basinwards-thick- untested aspects of carbonate ramp sedimenta. 1980. margin or evolve to larger structures following 11). the geometry may seem sheet-like ramps.P. western Ireland (Somerville and Strogen. ramp successions commonly comprise a number topset reflectors are commonly insignificant or of discrete ramp sequences with similar geome- absent or seismically unresolvable (Figs..P. The presence of downslope buildups gener. In some basins. Murray. it seems doubtful terrigenous sediment commonly seems to have that homoclinal ramps in the strictest sense actu. Nisku Formations. 1985).32 T. and have plat tops. 8. hA— tries. for example. and gradient slopes remains perfectly valid. wedges. in the to hundreds of kilometres across. port. 1 1A— 1 1D) which are respect to that in the preceding sequence (see visible in both time and depth displays and may e. To illustrate this point. high-amplitude continu- abundant grainstone than those in windward set. or in evaporite-filled basins. subparalle!. 1 iF). low-angle.or outer-ramp mudstones. or associated with base- consequent seismic-energy loss in the overlying ment or salt highs. 1 hA. reflection characteristics.g. tions where ramp sequences have stacked to build dation or even lower slope angles. Individual prograding rated in the onshore dune belt of the Rub al inner-ramp units will be mostly below vertical Khali. h hA. isolated buildups . or tuning effects might provide longshore drift or wind deflation of beaches or clues to the existence of inner-ramp units interca- shoals. and oth- subdued morphologies (see Table 2). outer-ramp zones may be distinguishable in situa- ably. Purser. reflectors are mostly continuous and regular. 1989. In thicker. deeper water sediment drifts of such mate. Pilskaln et al. hA). 1988). ous events (with parallel.or ate platforms. interpretations can clearly be made with greater tion from beaches and shoals along the windward confidence if facies transitions are suggested by Trucial Coast of the Arabian Gulf. For this erwise may be interpreted as parallel or subpara!- reason. indicating low rates of vertical Seismic stratigraphic and seismic facies inter. Presum. has yet to be properly investigated between predominantly inner. portions of a!. and outer-ramp locations.. a substantial basin-margin prism of distinctive dency towards more tabular geometries. lel (locally basinwards-divergent). we estimate that lated with mid. 1990). their effective analysis. Seismic examples section. 1 1B). Moreover. These may be con. ramps occur Areas of reflection-free. hiD). Reflector amplitudes may decrease ba- of such features from the margin of the Great . 11). fairly continu- Wilbur et a!. Figs. aggradation. low-gradient sediment bodies with a (Figs. regional geological data. lateral ampli- can be removed from the coastal environment by tude changes. gins. in windward locations. (Burchette et a!. (1980). or basin- tings. 1973. may point to discrete organic successions means that ramps in such situations buildups or grainstone shoals. 1981). and less lithology. lens-like. ally has parallel. For example. sinwards if lithologies become shalier (see e. ihE.g. Bahama Bank have been published by Mu!!ins et Sheridan. hiB.CARBONATE RAMP DEPOSITIONAL SYSTEMS 33 shelves (e. In the outer ramp. the ramp. consist of a seismically unresolvable condensed cies information is unavailable. with a ten. The basina! ramp section gener- wedging. Such the volume of oolitic sediment removed by defla. leeward ramps might show stronger progra. but rial formed by ocean currents may generate may diverge slightly towards the basin margin large-scale. seismic resolution but their occurrence can be tional progradation of the shoreline by up to 20 predicted in a limited fashion if it is known where km. they might be located in sedimentary sequences A large proportion of hemipelagic mud and as these relate to possible seismic geometries silt adjacent to carbonate platforms is redis. and incorpo. indicating stacking of study. hA. good-quality seismic data are essential for le! continuous reflectors (e. 1990). As in other settings. small thicknesses of ramp sequences and their which accentuate the subtle geometries. 1975. Eberli and Ginsburg. Seismic facies variations 1987. particularly in mid. Such zones may are commonly inadequately imaged for detailed occur one above the other.g. ramp-like geometry. bioherms influenced by compaction. 1977) Seismic character of carbonate ramp successions which show distinct toplap and little resolvable topset (Fig. On leeward platform mar. mounded. or Carbonate ramps possess no unique seismic halokinesis. 1 iF). tributed from their flat tops by storm action Inner-ramp seismic reflectors tend to be paral- (Neumann and Land. would have been sufficient to allow addi. character (Figs. or chaotic widely as the foundation phases to major carbon. These features are clearest on pretations of many ramps are hindered by the “squashed” regional lines (Figs. tectonism. In such cases.. ous and regular (Fig. sediment wards-divergent seismic reflectors)...and predominantly with respect to carbonate ramp systems. continuous character or may fused with shallow-water carbonate ramps if fa. gently sigmoidal or shingled clinoforms may be present (Mitchum et al. on the high-order sequence at parasequence scale. phosphates. and comprise siliciclastic mudstones. On distally steepened ramps such sions with such facies contrasts can be less prob- zones may signify resedimented slope deposits lematic. 1977). the internal ma! areas.WRIGHT are identifiable by a variety of direct and indirect In many ramps. surface. rests directly upon another. Drowning at sequence missed completely.P. as zones with this response are clearly important. 1 iF). and the Late Jurassic and Early on single lines in a regional survey. however. southern France size and shape determination problematic. 1988). and occurred via a migratingbrackish interval frequency and scale of base-level excursions and which is characterized by early dolomitization. unpublished (Loutit et a!. Early Mississippian ramps. Identification of flooding surfaces in succes- be created. the response of the depositional system to changes . The pres- data). such zones should form isolated shifts from inner-ramp shoreline carbonates to lenses rather than regional trends. on!ap/down!ap. as a in areas of wide seismic grid spacing.. velopment for the simple reasons that shallow-water facies Sedimentary expression of parasequence and Se. The flat tops of rimmed tinctive lithologies (soils. BURCHETIE AND V.P. the Early the encasing facies (Bubb and Hat!e!id. Prominent cx- various velocity effects resulting in acoustic amples include the Frasnian Nisku Formation of impedance contrasts between the buildups and the West Canada Basin (Watts. to the next inner-ramp impression of a continuous platform margin may unit. by abrupt upward facies When mapped. h992).. Mississippian ramp of southwest Britain Their small size means that they may appear only (Burchette. drape. or be (Jaquin et al. show marked variations in the degree of pale. bonate-dominated while outer-ramp sediments reflective outlines. in siliciclastic systems. due to the ramp geome. calcretes.P. events are expressed uniformly over the platform possibly with concentrated biotic assemblages). Drowning in the latter cases was sequence architecture and sequence stacking pat- demonstrably protracted. at least Sequence stratigraphy and controls on ramp de. inner-ramp deposits are car- criteria (Fig. flood or become exposed ily identifiable condensed sections (sedimentary rapidly during relative sea-level changes so that ironstones. As in other depositional systems. similar lithology. the false result of progradation. in contrast. than in and will generally occur near the base of the clay-free successions where one carbonate unit sequence or within the slope. terns of carbonate ramps (Fig. but they do not always hold sequence exposure and flooding surfaces on ramps are gen. particularly on wireline logs. g!auconites. The context and geometry of boundaries in such cases is commonly marked. The presence of dis- erally diachronous. 1991). Wright et al. black shales. quent rises. have shown that paleokarsts ence of clearly defined condensed sections can be and paleosols developed uniformly at sequence particularly useful where nearshore sediments and boundaries during relative sea-level !owstands and the overlying transgressive offshore facies are of that the buildups drowned rapidly during subse. Burchette. for example. in these settings drown less readily or are signifi- quence boundaries cantly elevated above the basin floor where most fine-grained siliciclastic sediment accumulates. with the most mature in proxi. Rimmed shelf sys- tems show this relationship more rarely. Due to the presence of a gentle depositiona! Such transitions abound in ramp shoreline sue- slope and the absence of a slope break. 13) reflect the try.. but where outer-ramp terrigenous mudstones or argilla- “swarms” of closely spaced mounds are present ceous limestones which then shoal upwards. making initial Cretaceous of the Vercours. stratigraphic significance. Studies of Late Mississippian flat-topped may allow the further recognition of sequence shelves and isolated platforms in southwest and boundaries and flooding surfaces. karst) or read- shelves. including interval thickening. h987). respectively central Britain (V. 1988. Sequence and parasequence organization okarst and paleoso! development in transects up the regional dip. major cessions.34 T. Likely sequence stratigraphic components of a “homoclinat” carbonate ramp using the example of a grainstone-donlinated system with outer-ramp buildups and a well-developed lowstand systems tract. The locations of outer.Fig. Note that lowstand inner-ramp grainstones rest abruptly upon outer-ramp mudstones (although erosion may be slight). These may comprise barrier or beach grainstones on a high-energy ramp or be packstone-dominated on a tow-energy ramp. In the mid-ramp zone. the vertical succession generated by ramp progradation’coarsens up from outer-ramp mudstones to inner-ramp grainstones overtain by thin tagoonal sediments. . 13. The offshore equivatent of the transgressive systems tract may be a phosphatic or glauconitic condensed section. and well-developed. backstepping shorelines in the transgressive systems tract.and mid-ramp buildups may be influenced by the “slope crest” morphology of the underlying sequence or by extrinsic factors such as hatokinesis or tectonism. flooding events are less clearly been documented in oo!itic grainstones (Van expressed) (Fig. 199h). isolated cult to distinguish between shoreface or shoal buildups (e.. The fall (4/5th order). Read. It may thus be diffi- flooding events. 1990). strongly progradationa! may be amalgamated with the distal portion of ramp sequences separated by incipient drowning the previous highstand inner ramp (Fig.e. In response to a small relative sea-level distally steepened ramps (Jacquin et a!.g. and the depth of the basin. Whole ramp “stacks”. with a range of subtle geometries proximal part of the lowstand inner-ramp tract comprising superposed. Because of the able in the adjacent basins during their develop. A larger relative sea-level fall (3rd order). h990) and outer-ramp Waulsortian exhibit transgressive and highstand systems tracts.. sea-level fall is rapid. reflects outer ramp to form the shallow-water portion of the generally limited accommodation space avail. In . Probable examples of this have clearly defined (i.P. 1991). Individual ramp sequences also show little character of ramp sediments between highstand potential for vertical (-keep-up) growth during and !owstand systems tracts. sediments of the highstand systems tract and those how~ver. The whole of the inner ramp becomes exposed and karstified during a 3rd-order lowstand. ceeding the depth of fair-weather wave base. rarely more than 200 m. although these may be a feature of some ments. the duration of the low.P. 13). 10. in the late transgressive and highstand systems indicating the development of a !owstand pro- tracts in which high-order sequences are less grading wedge.g. Because slope angles on homo- magnitude of the fall. 13). hhB). there may be abrupt “out- stepping (retrogradational) fourth. and the character of inner-ramp sedi. or zones of meteoric diagenesis. Burchette et al.. 13). of-sequence” shallowing of the depositional envi- order sequences.and fifth. exposed (Figs. 1989. low slope angle.. in the Mississippian of Europe and might be ed fourth. dent on the rate of the fall. the gradient of the ergy” ramps show clear transgressive systems slope. and fluvia! Characteristics of lowstand systems tracts si!iciclastic sediments may also overlie or incise Carbonate ramps show varied responses to rel. and the successions. but the expression of dent on the environmental energy of the basinal such a “forced regression” will be strongly depen- setting in which the they develop. 1985). 1986). is In common with siliciclastic systems. cours. the sedi. except locally as small.and outer ramp (Fig. A tract of the inner ramp will become (cf.and ramp sequences. Some “high-en. but a broad substrate will The relatively small thickness of “third-order” remain over the sites of the previous mid. be completely divorced from those of the previ- ture. An example following paragraphs represent summaries of the from a distally steepened ramp has been docu- main characteristics of ramp depositional systems mented from the Early Cretaceous of the Ver- tracts in an “idealized” ramp sequence.. 1991). the accommodation space remaining within “aprons” of resedimented material do not de- the basin. buildups (Faulkner. Where relative tracts characterized by discrete stacked or back. velop. lowstand “slope fans” or stand. southern France (Jacquin et al. clinal ramps are so low. The sought in ramp sequences elsewhere. Read et a!. into the previous highstand inner-ramp sediments ative sea-level fall. depending on the rate and (cf. there may be little change in the ment. cx- water facies. ca!cretiza- (e. and to a degree the architec. followed by rapid progradation ronment over the mid. generating layered tion.may show cumulative vertical accretion of any subsequent !owstand succession unless this in response to long-term relative sea-level rises is revealed by the distribution of karst.and fifth-order sequences in both. facies belts on a homoclina! basic building blocks of ramp sequences similarly ramp are likely to shift basinwards in an offlap consist of small-scale shoaling depositional cycles fashion. Burchette et al.36 T. 1990) although with poorly defined packstone-dominat. WRIGHT in accommodation space on several scales. necessary for lowstand inner-ramp sediments to mentary character. 1991. Elrick and Read. “Low-energy” ramps also Steenwinkel. BURCHETFE AND V. or backstepping events characterized by deeper. Wright. ous highstand systems tract. a strongly progradationa! system. of carbonate ramp systems tracts are depen. g. the quences culminating in shoreface grainstones or “bank” foundation stages for many Devonian iso. ing. 13) capped by lated buildups in the West Canada Basin (e. highstand systems tracts.. Burchette et al. 1981). 13) (e. models those in the transgressive systems tracts (see e.CARBONATE RAMP DEPOSITIONAL SYSTEMS 37 restricted basins. through the highstand succession may show melt production in shoreline facies belts. The potential for slope sist mostly of packstone and wackestone sedi. ment production must infi!! accommodation space mated by offshore bioclastic sediment production created during the transgressive and early high- (echinoderm—bryozoan. 1990). relative sea-level lowstands may Cooking Lake and Swan Hills platforms) and the lead to the deposition of evaporites. or glauconitic or chamositic fluctuations within a longer-term relative sea-level ironstones (Burchette.or fifth-order se- mian San Andres Formation of New Mexico. A vertical section packstones). barrier-is. 1991). In these situations. isolated continuity of backstepping depends on the rate buildups may continue to develop during several any relative sea-level rise and sediment rework.g. Swift. perhaps reflecting slow rates of sedi. p. and situations occur in which shorelines are laterally widely spaced in dip section. coarsening and ples of this depositiona! style occur in the Per. One example Devonian “Schwe!m” facies of Europe occurs in the Jurassic Hanifa Formation of the (Burchette. or dissolution. phos- On high-energy ramps. palaeosols. Clearly. developed for siliciclastic sedimentation during Aigner. since sedi- as local shoals. the style and highstand systems tract. they may . ramp depositional sequences.g. 1991). cumulative small-scale phatic mudstones. Such sequences are similarly dom. tres to tens of metres in thickness. commonly Isolated buildups in ramp depositional systems preserved intact by “in-place drowning”. than at other times. thin lagoonal sediments (Figs. 1990. of barrier islands or in some inner ramps.or fifth. h984. 150). Elrick and faces are mostly readily identifiable. They may several kilometres may occur even within ramp become swamped by progradation during the later transgressive systems tracts. seaward (Fig.S. Nevertheless. the karstic surfaces which become more pronounced . 1990) and seem to the Holocene transgression on the eastern shelf be more commonly oo!itic. is rarely evident except in the subaerial portions order sequences consisting of beach. 1984. tional systems and restricted or lagoona! facies On low-energy ramps. 13). steepening and clinoforming is greater at this ments and contain high-energy grainstones only stage in ramp sequence development. In transgressive settings where drowning of aqueous evaporite complexes may develop. h98h). terized by pa!aeokarst. or barrier-shoal carbonate grainstones and ramps show little potential for continuous vertical associated shoreface and transitional sediments accretion during transgressions. stacked. charac- rise may generate a set of stacked or backstep. upward-shal!owing. when sediment input to Aigner. and the Triassic (Muschelka!k) Arabian Gulf (Wright et a!. the flooding sur- (Fig... 13).A. 1975. Heward. 1992). Burchette et a!. fourth or fifth-order make up a larger proportion of the inner ramp sequences in the transgressive systems tract con. ping and onlapping (retrograde) fourth. 1975.g. and develop mostly during the transgressive and early dominated by bioc!astic sediment production (e. inner-ramp setting. Exam. Droste. closely Characteristics of highstand systems tracts spaced. even in the Read. Exposure. are useful for comparison (Sanders dominate over beach or barrier-island deposi- and Kumar. prograding sabkha and sub. be capped by thin condensed sections consisting Characteristics of transgressive systems tracts of distinctive facies such as black shales. 13). of Spain (Calvet et a!. high-order ramp sequences entails strong land- ward shifts of the shorelines (Fig. or vertically stacked. 4. or even replaced by Highstand ramp facies belts prograde strongly simple ravinements overlain by offshore carbon. Since land. Shoal deposits tend to of the U. Shoreline progradation for the offshore environment is reduced. For this style of deposition. 1990.. Such units are generally a few me. are commonly grainier than ate sands. thickening-upwards fourth. or large foraminiferan stand systems tracts (Fig. BURCHETFE AND V. their histories have been largely lowstands (Haak and Schiager. and it is not sensible to use modern 1991).g.P.. Note how even a small relative sea-level fall may expose the whole surface of a flat-topped rimmed shelf and restrict lowstand carbonate sedimentation to a narrow rim along the old platform slope unless the tower slope and basin floor enter the photic zone. but at sequences? a reduced rate and is restricted to the old plat- form slope. Carbonate stand and transgressive systems tracts of ramp production during lowstands may continue. the scale of many 4th. The absence of she!f-lagoonal systems responses in sediment production for different with which to “boost” highstand sediment pro- sea-level stands on carbonate rimmed shelves and duction in most ramp systems suggests that this ramps are therefore markedly different. and the magnitude of (e. On a ramp. Stippled areas represent notional lowstand deposits. WRIGHT upwards within the stack as the upper sequence Sediment production on most carbonate boundary is approached. also Table 3). facies belts will shift basinward over the ramp surface without significant interruption producing a “towstand prograding wedge”. 1991. 1985. rimmed shelves (Fig.38 T. Boardman et the fall (see Sch!ager. dreds of kilometres across (Fig. Schlager. unless lower slope and basin floor Responses to relative sea-level changes: ramps vs. transgressive.P. 1989.and !owstands for rimmed shelves and carbonate system to a relative sea-level change is commonly expressed as an alternation in the are the steepness of its basinward slope. and (B) a ramp showing the effects of minor changes in relative sea level. environments are forced into the photic zone. the composition of sediments in the adjacent basin depth of the adjacent basin. 14A) is greater during rela- Because Holocene carbonate platforms have tive sea-level highstands (when the whole plat- barely reached equilibrium with the post-glacial form surface is flooded) than during intervening sea-level rise. highstand vs lowstand contrast in periplatform A Accretionary rimmed shelf Fig. A base-level fall of just a few metres. 1986). on ramps as exact analogues for the highstand de. . Is sea-level variations. Schematic profiles of (A) a rimmed shelf. may expose the whole inte- the ramp model of a depositiona! system in which nor of a flat-topped carbonate platform (inc!ud- the high-energy zone is displaced towards the ing those with ramp margins).and 5th-order relative posits of ancient ramp depositional systems. Droxler and Schlager. 14. potentially hun- shoreline therefore really only typical of the low. rimmed shelves This causes marked asymmetry in the rate of sediment production with respect to relative sea- Factors which determine the response of a level high. The al. h4A). 1991) as a “shelf-margin - fall in relative sea-level would expose a significant wedge” (Figs. . Great Bahama Bank). unstable strips along the old platform glacial sea-level stand (Schlager. and 200 m (blank) based on sub-sea contours derived from nautical charts. (E) West Florida Shelf. Arrows represent the dominant wind directions. rimmed shelf. between the different platform types can be mod. narrow. 1 1A. proportion of most flat-topped platforms (e. homoclinat ramp. a!. Australia. 1981). basins with homoclinat ramps would be drained. (A) Arabian Gulf. (D) Yucatan Peninsula. the depth of the basin.CARBONATE RAMP DEPOSITIONAL SYSTEMS 39 sediments is likely to be less pronounced. and the steepness of the original depositional slope. though the topic is largely uninvestigated. white the margins of rimmed shelves and distally steepened ramps adjacent to deep basins would retain some substrate for towstand sediment production. only the marginal areas of all ramps will be exposed. showing the proportion of the platform surface which woutd be exposed for relative sea-level falls of 10 m (black areas). Note that for a 10 m fall. 15C). in relative sea level (the scale of many fourth- although it is worth re-emphasising in this context order cycles). 15.g.g. A 1—2 m margin (Cook and Taylor. Mexico. distally steepened ramp. Sketch maps of modern carbonate platforms drawn at the same scale. Facies belts simply shift seawards. southern Arabian Gulf or Shark Bay). while the whole of the flat rimmed shelf platform top (C) would be exposed. but would have little effect The contrast in response to base level changes on the character of a “homoclinal” ramp (e. (C) Great Bahama Bank. For a 100 m fall. 100 m (stippled areas). The expression of these effects would clearly depend on the rate and scale of the base level fall. 15A—h5C). relegate shallow water carbonate production to since they have barely re-equilibrated with post. homoctinal ramp. The same relative sea- Fig. distally steepened ramp. A 10 m fall elled using modern examples (Figs. would expose the whole of the that modern carbonate platforms offer no good platform interior of a major rimmed shelf and analogues for highstand sedimentation on ramps. (B) Shark Bay. forming a progradational and aggrada- 11B. slope-apron to inner ramp are rare in such sys- h5D. or due to tectonic mode as narrow shelves rimming the foot of the movement. the system will tend to herited (e.40 T.. Different responses might be ramp would remain in or enter a favourable envi. or it may occur commodation space in the basin increases. water depth. 1985) and along the northwestern margin therefore. would behave as pography in order to prograde over the -break in ramps during minor base-level falls. extensional faulting). must intrinsically. 1988). Modern examples of distally steepened deeper -basins may become. rates of sediment accumulation in A common phenomenon in the geological the inner and mid-ramp may still be significantly record is the evolution of a “homochinal” ramp higher than in the adjacent basin.P.P. 1982a. In all cases. the next. Major rimmed shelves adjacent to Late Cretaceous and Tertiary of the Gulf of deep basins would continue growth in !owstand Mexico (Mullins et al. (Gawthorpe. in. Kenter. 1SB). Basin (Chatellier. as in the Evans. occur in the Mississippian of northern England plications for sedimentation patterns and sedi. Ebdon et a!. into a distally steepened ramp or rimmed shelf where unable to prograde beyond the critical (Read. ramps which have inherited a pre-existing slope the sites of shallow-water carbonate production occur on the Yucatan and Florida peninsulas during relative sea-level lowstands. 1990). as ac- or carbonate platform slope). (Read..and outer tiona! sequence set. 1986. since mud will build much h5A. expected between windward and leeward mar- ronment for lowstand shallow-water carbonate gins. Mullins et Major relative sea-level falls of the order of a!.. so that full vertical sections from base-of- become rimmed shelves during larger falls (Figs.g. BURCHE~FEAND V. 1988). and the Early Creta- ceous of the Vercours. although there have been downward of Australia (Dix. 1988). The cumulative effect. Presumably. 1979) position on the ramp in which the slope break and the Caribbean (Eberli and Ginsburg. Basins such as the Arabian Gulf and lower-gradient slopes than grainy material Shark Bay would empty completely to become (Kenter and Schlager. or be reactivated as. or where the outer ramp has a higher sedi- production. and the documented in Belize (James and Ginsburg.. Drowned highs in the outer ramp in tems. The nature of the sediments generated 100 m. ramp facies to step out. there are marked im. siliciclastic shelf. h5B). an antecedent delta. as in Late well increase. the final geometric effect on the ramp old platform slope or escarpment (Fig. 1991). typical of the most recent glaciation. although basinal restriction might mentation rate due to pelagic input. 1988). The latter process seems to be cumulative ramp distally or promoting its further develop- . i5E). through basinal topography sufficiently to allow inner- differential subsidence. 1985). as a result of differential sedimenta. Ancient examples of shifts in coastal onlap. Ramps rarely seem capable of generating Modern distally steepened ramps. 1990). Jurassic and Cenozoic examples (cf. but much of the mid. “stick and stack”.g. Purser and Where the slope break is inherited. WRIGHT level fall on a carbonate ramp would expose a and becomes accentuated from one sequence to 20—50 km tract of the inner and mid-ramp (Figs. 1989).g. steepening the centre. h5A. but might slope. 1989. would probably also influences the ramp-to-rimmed entirely expose modern carbonate ramps (Figs. shelves Although overall carbonate productivity on ramps is low. occurs. with shal!owing of offshore tectonically controlled distally steepened ramps depositional environments. Steepening of the basinward point at which sediment production fails to fill slope may be tectonica!!y driven (e. the Mis- ment volumes produced for different scales of sissippian Banff Formation of the West Canada relative sea-level fall. h5C). because they adequate volumes of sediment to infihl such to- lie adjacent to deep basins. shelf transition. as will depend on slope height. southern France (Jacquin Evolution to distally steepened ramps and rimmed et a!. be to enhance elevation between the carbonate- tion between the basin margin and the basin dominated margin and the basin. lacustrine or fluvial systems (e. 1973). 1980.. 1974). Faulkner.. Chevron Standard. 1988). relative sea-level rise decreases. difficult to generalise factors which might initiate mont Formation of the Alberta Basin (Cutler. Watts. by deeper-water biotas (e. 1981. but they do com- ment production rate of the buildup. appear to seaboard (Gamboa et a!. 1983). may have controlled their loca- topographic relief. Early diagenesis water btotas (pioneer stage) and muddy facies at the base with an upwards transition to progres. before maximum rates of relative sea- velopment of isolated buildups on ramps are level rise are achieved. (e. in Argentina (Mitchum and Uliana. Given this diversity. and maintain mound growth within ramp settings. Fig.g. or compaction low-water sediments (cf. Buildups which grow through of this phenomenon occur in the Silurian of the several ramp sequences also reflect relative sea- Michigan basin (Mesolella et a!. Toomey. tending to expand during highstands.A. Read. the U. 1981) (Fig. partially drown during transgressive events (Sears Some buildups are constructed predominantly and Lucia. and the Late have accreted predominantly vertically and have Jurassic/Early Cretaceous of the Neuquén Basin thus maintained pace with relative sea-level rises. structures are diverse and presumably may seed Few isolated buildups seem to grow directly from at any stage within a ramp sequence as long as surviving portions of the preceding shallow-water suitable substrates are available and the rate of ramps as widely assumed (cf. as the rate of ramps may stack one upon the other (cf. and Canada. such as Si!.. Sears and those for rimmed shelves and isolated buildups.g. This suggests that where there are no over-riding tectonic controls (extensional fault/salt move- Isolated buildups ment) many buildups seed at the start of a trans- gression (or conceivably during the preceding The factors which control the growth and de. the Early Jurassic of the US eastern Many outer-ramp buildups.. Isolated buildups on During late highstand phases. and other mudmounds). Presumably. Such rigenous pollution in the outer ramp is minimal. 1992).S. show deep. it is might be inferred are the Late Devonian Gros. Burchette and Britton. some Waulsortian Watts et a!. while others are decid. possibly a previous ramp slope “crest” buildups development will be those of elevated or lowstand shoal. In reality mon!y seed around the margins of drowned ramps they are commonest during the transgressive sys. over a previous buildup. James. .. many Creta. and develop during the poorly known and little has been published on transgressive systems tracts of depositional se- the locations of such buildups in a sequence quences when the shallow ramp is backstepping stratigraphic context. Upper Devonian of the West Canada Basin). tion (cf. Kendall and relative sea-level rise does not exceed the sedi. 1990. and the West Canada Basin (Chevron Standard. 13). a remarkable fact in view of or drowning and fine-grained carbonate or ter- their economic importance in many areas. 1981. and higher nu. but not necessarily exposure.CARBONATE RAMP DEPOSITIONAL SYSTEMS 41 ment as a rimmed shelf. Documented examples 1985. Published examples from Lucia. . 1981. trient or -light availability. different tectonic settings where this process Dixon and Graf. of Europe. This commonly reflects either a persistent may expand or be engulfed by prograding shal- tectonic or halokinetic control. 1980. but drown or 1979). !owstand). tems tracts of ramp sequences. isolated mounds 1984). 1992). related to ceous and Tertiary buildups). Burchette. Buildups in deeper-water. Sch!ager. Some. and in level fluctuations in concert with the shallow ramp the Upper Devonian of Europe (Burchette. Burchette et urian and Late Devonian stromatoporoid buildups a!. 13). as in suggesting in many cases that subtle slope mor- other settings situations most favourable for phology. relative sea-level lowstands (see e. 1979. 1989). 1979. lowing.g. h982a). 1985). commonly with late-stage exposure. however. 1985).g.g. Conceptual models for early diagenesis in ramp sively shallower-water biotas (domination stage) depositional systems are significantly different to and grainy or boundstone facies (e. outer-ramp settings may show evidence for shal- edly shallow water structures (e. low turbidity. BURCHETITE AND V. early marine -cementation and sea-water dotomitiza- tion are likely to be less prominent. mechanisms also vary in importance depending level changes (Figs. h6A. Highstand porewater move- Major rimmed and isolated platforms exhibit a ment within the Great Bahama Bank is driven by range of porewater circulation mechanisms due several mechanisms: Kohout convection. but because they are mostly attached to land masses they may be subject to more meteoric influence. 16B. and ramps during a relative sea-level highstand. deep sea-water column. These diagrams illustrate the simpler hydrology expected in ramps compared with steeper-margined rimmed platforms and isolated buildups. h7B). Ramps are probably less influenced by marine porewater circulation than other platforms.P.P. These morphologies and the influence of relative sea. even during relative sea-level highstands. 14). HIGHSTAND Fig. reflux. Likely porewater circulation systems for isolated buildups.42 T. . 16. on sea-level stand. 13. exposure to a within the end-member platform types are di. 17A.WRIGHT This is because porewater circulation patterns to their steep-sided morphology. and variable geothermal recthy related to the markedly different platform heat flow (-Figs. rimmed shelves. The isolated platform model is based on the example of the Great Bahama Bank. Conversely. Note the distribution of meteoric tenses in the different platform types and the development of buoyant circulation systems which promote seawater mixing. Fluid circulation in many ramps compared with broad. . 1990). flat-topped ramps will depend much more on wave and tidal rimmed shelves will also result in reduced oppor- pumping. h6C. and ramps during a relative sea-level towstand. Likely porewater circulation systems for isolated buildups. The lack largely by tidal. Note that meteoric water. shelves.CARBONATE RAMP DEPOSITIONAL SYSTEMS 43 and head differences on several scales driven circulatory mechanism in ramp settings. 17. By virtue of their shallow depositional LOWSTAN D Fig. rimmed shelves. Kohout convection is not a significant h7C). which radically affects the distribution tunity for brine generation. oceanic. driven by hydrostatic head. so that large-scale of diagenetic phases such as early cementation reflux is only likely to occur where the ramp is and dolomitization compared with rimmed backed by a broad hypersahine lagoon (Figs. and storm currents of extensive hypersaline platform interiors in (Whitaker and Smart. can extend downdip beyond the shoreline and discharge at the sea floor. or patch reefs isolated within sealing facies of inner or mid ramp (e.A. (G) Carbonate grainstone bodies.g. southern Arabian Gulf).). fault block. may form Fig. Fig. (C) Pinchout of inner-ramp carbonate sands into tight offshore facies where regional dip has been reversed (e. 1988). U.)..g. Ordovician.A. except black: tight mid. Potential locations of stratigraphic traps within sequence tracts of a carbonate ramp with a well-developed lowstand prograding system (cf. and small. Upper Devonian. (D) Karstification of mid.S. below sea-level.44 T. Key as in Fig. 16C. Upper Jurassic. etc. (B) Isolated buildup or shoal developed over high (salt diapir. 1989) scale tidal pumping. typically attached to a shoreline. 13. Gulf Coast. Ellenburger Formation. Upper Jurassic. U. . Mississippian. with their corn- if the hinterland is sufficiently elevated to gener. England (Sun.P. 1990). Oklahoma.A.). ramps may meteoric recharge of porewaters is a significant be preferred sites for mixing-zone diagenesis or factor in all stages of sea-level stand. Delaware Basin. 17C) and. particularly leaching (Figs.S. 16C. mottled: organic buildup. (A) Isolated buildups developed during drowning of parasequence or sequence and encased in onlapping or downlapping highstand mudstones (e.g.A. The extensive systems will be a major mechanism creating a early marine cementation associated with reef porewater flux capable of forcing diagenetic fronts in rimmed shelves and large isolated change. 13). Pembina trend. monly sheet-like sequence geometries. Formation of the US Gulf Coast (Moore. For this reason. (Chafetz et al. Paris Basin.g. shoals.) in offshore location and covered by mudstones (e. channels. 18. west Texas).g. tidal bars. (B) Truncation of dipping inner.g. France). and as has been shown for and minor buildups where porewater circulation several ancient systems including the Smackover is largely due to wave-. Silurian. 17C). southwestern U. Buoyant marine porewater circulatory systems capable of circulation between the meteoric and seawater causing seawater dolomitization. Smackover Formation. Mishrif Formation. barrier islands.. The meteoric phreatic zone may extend buildups will be absent from ramp systems and offshore.and mid-ramp carbonates at a regional unconformity (e. e.and inner-ramp sediments at major sequence boundary (e.g. horizontal lines: terrigenous mudstones (seal).and outer-ramp facies (notional seal). Upper Devonian West Canada Basin). Grosmont Formation. and the Corallian (Upper Jurassic) of southern In ramps. San Andres Formation. ramps will also be less likely to develop ate hydraulic head (Figs. (I) Updip pinchout of inner-ramp shoreline grainstone into tight inner-ramp facies (e. West Canada Basin). mid-Cretaceous. WRIGHT slopes. (H) Individual carbonate grainstone shoals. U. BURCHETFE AND V.S.S. storm-surge.P.g.g. (F) Lowstand deposits isolated from previous highstand inner ramp and sealed by onlapping or downlapping outer-ramp or basinal muds (conceptual—examples unknown to the authors). isolated by shoreline facies disposition (e. as in the Arabian Gulf restricted to the shoreface (beach rock). Paradox and Williston Basins. most prominent of which were the phylloid product of upwarping of Mesozoic or Tertiary and related algae. reservoirs in outer-ramp particularly favourable. Williston (Wilson. High-energy ramps in buildups are rare and most occur in inner-ramp contrast commonly possess a wide range of reefoid settings. Many giant and Early Permian. In the Late Carboniferous sites of structural or halokinetic traps.or outer-ramp locations com- monly form ideal stratigraphic traps sealed by Reservoirs in organic buildups onlapping basinal facies or by downlapping. because they have such mud-textured and seem to form relatively few wide areal distribution. Basin. dolomitization. matoporoids and corals. Well-developed outer- ties for stratigraphic and structural trapping and ramp buildup reservoirs occur in the Silurian and lateral variations in reservoir quality (Fig. 1985). and the models applied mostly and Michigan Basins (Sears and Lucia. A common location for petroleum reservoirs in 10. fracturing) of the US Gulf Coast (Baria et a!. West Texas or over diapirs rooted in the deeply buried Pro. and the Timan terozoic Hormuz salt. Kazakhstan) form prolific petroleum reservoirs in tia! reservoir facies unless downslope buildups or mid-ramp locations. karst. 1988). and Canada.S. formed a larger propor- bonate rimmed shelves or in large isolated tion of such buildups in both shallow and buildups than in ramp systems. Late Permian. additional or alter- ramp systems is in organic buildups (Fig. native seal may be provided by subsequent low- These features have exhibited a wide range of stand deposits where these comprise tight periti- . ments. Silurian Although the controls on source rock and reser.CARBONATE RAMP DEPOSITIONAL SYSTEMS 45 ready conduits for basin-derived fluids expressed organic and sedimentary facies through time (Fig. 1975) p!etely understood. 6) and this diversity is reflected in variations in their reservoir potential. mud-textured distal highstand sediments (Figs. created texturally varied carbonate ramp sediments over basement highs buildups which in some areas (e. are the isms. 1979).g. however. Com. 1985). examples occur in the intracratonic settings of the voir distribution in such settings are still incom. 18). ramps deeper-water settings and generated textural di- too form major reservoir zones in some of the versity which improved their primary reservoir world’s most prolific petroleum provinces and potential and susceptibility to secondary diage- offer different. in the West Canada Basin and the Wi!liston ments will be restricted to situations where origi. 17). Pechora and North Caspian Basins in Russia and Low-energy ramps are mostly sparse in poten. In the later Ordovician. some generalities can be made Devonian ramp-buildup reservoirs occur widely and are discussed in the following sections.. Florida (Halley. mud-textured buildups were dominant and restricted to outer-ramp environ- More emphasis is traditionally placed by cx.S. commonly more subtle play types netic alteration. and the Triassic. Illinois (Whitaker. Silurian and Early plorationists on depositiona! models for Devonian. However. In the Cambrian and Economic aspects Early Ordovician. Petroleum reservoirs also oc- incipient organic rims are developed (Read. cur in outer-ramp coral—algal buildups in some or they lie in situations where the timing of Jurassic ramps. such as the Smackover Formation diagenesis (e. these ages are common. However. framebuilding organisms. Petroleum-bearing buildups of than many rimmed shelves. In the in relation to petroleum migration have been Cretaceous and Tertiary.g. a variety of organ- fields in the Arabian Gulf. 1988). for example.A. in the U. poorly predictive. such as stro- petroleum exploration and development in car. 1982). - Handford. by compaction and thermal maturation. In the Late Devonian and Early Carbonif- na! ramp depositional environments have been erous. with wide opportuni.g.A. Buildups in mid.and largely responsible for the stratigraphic trapping outer-ramp buildups were again predominantly of petroleum. as in the Sunniland Basin in South facies and carbonate sandbody types (see e. many ramps are also the petroleum reservoirs. 15). Devonian of the U. mid. In exceptional cases. and New Mexico. Droste. 18) by subtle depositional or secondary topography in are widespread and occur in a number of configu- an underlying ramp sequence (e. 1986). + mounds) and the Illinois Basin (Coburn. 1979). dard. Silurian of the northern Michigan reef belt (400 Where buildups are stacked one upon the other. Wer- vonian of the West Canada Basin (Chevron Stan. depending on the age and the location of the tonic. 1974) and in the Dc. However. 1986).46 T. 1989) and Pennsylvanian (Strawn Michigan Basin (Mesolella. 1975. sary. Finnmark area. a general characteristic of many ramp grain- develop during or following ramp drowning (e. Buildups in tract when vertical growth is dominant. Shoreline reservoirs may also pinch . Barents Shelf (Gerard and Reservoirs may occur in individual shoreline Buhrig.g. if numerous. stone reservoirs is that they are relatively thin or Swan Hills and Leduc reefs in the West Canada layered. In a few docu. Hopkins and Ahr. the Urals—Caspian area (Ulmishek. 1990). 1990).g. particularly in a transgressive systems source rocks in the same fairway. 1988) or evaporites (e. the with drowning of carbonate ramps are known small isolated buildups associated with many from the Late Devonian of the West Canada ramps have limited “footprint” areas. migration seems to have been fo- cussed into large isolated buildups. Neverthe- tres across. amalgamate to form a single larger buildups. and corn- seen on seismic lines from the Permian of the monly of wide lateral extent. Grainstone reservoirs in ramp settings (Fig. and buildups in the Mississippian (see list in umented examples occur in the Silurian of the Kosters et a!. Doc. shoal complexes over offshore highs. the Watts. BURCHETFE AND V. 1985). 1983). giant fields. may restrict Isolated buildups lie ideally situated in an in. The composition of the grainstone sedi- Creaney. Examples of such fairways occur in: the tion of offshore deposits and the amount of layer- Frasnian Nisku Pembina trend of the West ing increases. 1979). terrigenous or carbonate mudstones (Figs. such trends can represent large gross re. although they individually constitute small reser. Germany (Meyer and Schmidt-Ka!er. Stoakes and rations.. and the Late Jurassic Ma!m of southern thereby facilitate reservoir filling may be neces. oil equivalent in over 50 producing mounds). several small mounds may metres of reservoir facies in any one zone (e. so that in Basin (Stoakes and Creaney. Reservoirs in grainstone-dominated ramps mented cases. The ramp (e. Europe situations where they are charged indirectly some (Krebs. may less. Buildup shape and location may reflect tec. regional or global oceanic anoxia. Ramp organic buildups commonly beaches. barriers. but larger buildups several tens of kilome. petroleum reservoirs may be “multistorey”. or bars sealed by outer-ramp form “swarms” numbering tens to hundreds and. topographic or hydrographic control. Reser- thickness of ramp buildup reservoirs is seldom voir facies range from shoreline carbonate sand- greater than 100—200 m (not necessarily sea-floor bodies to major detached shoal complexes or relief).8 billion barrels of Nisku Formation in the West Alberta Basin. oolitic or bioclastic). as Thamama Formation.g. whether windward or leeward). Formation) of north-central Texas (e.g. 1985). circulation at the sea floor and contribute to the termediate location which will be on any updip development and preservation of organic-rich migration path for petroleum generated from sediments. of the order of only 5—50 million barrels of servoir facies generally decreases basinwards over oil. 18H). mund. 18G. several to a few tens of ki!ometres as the propor- serves.P.g.g. WRIGHT dal facies (e. The proportion of reservoir to non-re- voirs. 18C. This is assisted by their development on topographic or tectonic highs. In places. This phe- outer-ramp settings may lie in juxtaposition with nomenon will be accentuated during times of petroleum source rocks developed in transgres.g. seldom with more than a few tens of Basin).g.P. ments may also vary (e. Examples of sive systems tracts and receive their hydrocarbon organic-rich deposits which occur in association charge directly by lateral migration. mechanism to focus petroleum migration and 1988). Organic buildups. forming giant oil and gas fields. Arabian Gulf). the Wolf Lake Member of the Canada Basin (gross reserves 1. at the top of such coarsening-upwards fourth. and the Cenomanian Mishrif Formation of Storm. and the Early Cretaceous of cated or karstified updip at major sequence the Neuquén Basin in Argentina (Mitchum and boundaries or regional tectonic unconformities Uliana. More significant mudstone interca. tens of metres thick and several tens of kilome. 1985. any depositional sequence may generate laterally Genevieve Limestone of the Illinois Basin extensive. . 1985).or tide-dominated shoal-belt prograda. For similar reasons. will depend on the duration and magnitude of Carbonate shoal-belts may comprise up to 80% exposure. 1979). 18E. 18G. the distal and proxi. 1985). Thick reservoir units develop mostly as a result of 181). Se. packstone-prone intervals (lower shoreface and queira and Ahr. and isolated from those in the previous highstand base-level behaviour control the thickness and systems tract and sealed by onlapping and down- character of potential reservoir facies developed lapping outer-ramp and/or basinal deposits (Fig. the composition (e. 1984). and on climate. Hunter et a!. cies form an effective seal.. clearly the ma! margins of the sheets and wedges which they geologic age of the sediments will have an impor- generate may subdivide into decimeter packstone tant bearing in this respect. tent) of the sediments. stones/grainstones (60—70% of sandbody thick. However. Druckinan stone (upper shoreface) interbedded with mud. for example in foreland basins Several sandbody geometries are important in or on passive margins. the distances prograded (Burchette et al. England (Sellwood et a!. the Arabian Gulf which is overlain uncon- tion. 1986) and the Paris Inner-ramp grainstone reservoirs may be trun- Basin (Purser. 1987). Lindsay shoreline transgression and progradation within and Kendall. was deposited during the transgressive systems lations may break up the vertical continuity of the tract or the highstand systems tract. 18C. 1985). 1985). It is important in this or grainstone units intercalated variably with context to determine whether or not a sandbody muddy facies. 181). gradational) shorelines (cf. lowstand carbonate sandbodies may be zones.. 1987. All the sandbody types may extend more oric diagenesis. or local stacking of shoreface sands. the Jurassic of southern offshore).g. 1982) and locally packstones and wackestones to grainstones on a the Mississippian and Triassic of the West Canada c. 18C). 4). 18E). Tidal range. Examples of reservoirs in shoal or stacking of discrete sha!lowing-upwards (pro- shoreline-dominated ramps occur in: the Missis. the Jurassic (Figs. wave and current regime. 1987). in foreland basin settings occur in the Frasnian mentary sequences coarsening upwards from Grosmont Formation (Rand.. with units of grain- Smackover Formation (Feazel.. leaching in this location may be limited. However. (Figs. 10 m scale (Fig. but vertically subdivided sandbodies (Choquette and Steinen. although the opportunity for meteoric fifth-order sequences (cf. 1990).or 18F). 1987). tude and duration at major sequence boundaries. sippian Mission Canyon Formation of the Willis. This relationship can gen- grainstone-dominated ramps because they may crate sub-unconformity plays if the overlying fa- influence reservoir heterogeneity. Where base-level fall has been of sufficient mag- ness) represent the most likely potential reservoir nitude.to and Moore. Examples of reservoirs ed shoreline sandbodies produce vertical sedi. metres along depositiona! strike. 1985. Meendsen et al.CARBONATE RAMP DEPOSITIONAL SYSTEMS 47 out into lagoonal mudstones or evaporites (Fig. Fig. and or less continuously for tens of hundreds of kilo. but varying in geometry and Basin. aragonite con- grainstone facies. where they are truncated updip by the width from string-like to sheet-like depending on Cretaceous unconformity (Jardine and Wilshart. therefore the consequences for reservoir quality. produce formably by the Turonian Laffan Formation grainstone wedges or “tongues” up to several (Harris et a!. Wave-dominat. 18D. since expo- shoal sandbodies (Burchette. the Mississippian St. In prograding sure-related leaching will be of greatest magni- strandlines well-sorted shoreface carbonate pack.. The effects of this process. inner-ramp grainstones tres across with a range of transitions to encasing may also experience repeated episodes of mete- facies. 1960. the internal heterogeneity produced by repeated ton Basin (Thomas and Glaister. mid-. sequences of the shallow ramp.and outer-ramp environ. quences comprise lowstand. Ramp stacks may show gross ver- thick. and foreland basins. forming seldom more than 2 km in diameter and 200 m ramp “stacks”. while on a flat-topped rimmed continental margins. posed. wave-. due to tectonism. and tide-dominated carbonate windward versus leeward locations on ramp se- ramps can be recognized and this forms the most quence geometry has yet to be properly investi- appropriate basis for ramp classification. transgressive and ditional slope environment. Lowstand deposits may consist of evapor- quence geometries and are similar in this respect ites in restricted basins. thinning base to normal storm wave base. WRIGHT Conclusions gesting that ramps are actually seldom “homocli- na!” but posses subtle slope geometries which (11) Carbonate ramps can be subdivided into reflect the ramp depositional environments. and outer-ramp environments. The zone of great. with wave-dominated shorelines. they were the dominant location of isolated buildups is governed by tec- carbonate platform style at times when reef-con. cies will shift basinwards and only the inner ramp These are mostly cratonic-interior basins. BURCHETTE AND V. a distally steepened ramp or rimmed shelf. since the outer-ramp margin.and and basin. The all geological periods. Steepening of the is different to that of rimmed shelves. tonism (e. sedimentary basin. or by However. the whole of the platform interior (6) The thin.g. ments. tecedent topography (e. 48 T. sequence. while distally steepened ramps and low-angle sigmoida! or shingled clinoforms. slope inher- contrast in productivity across the shoreline to itance. presence or absence of reefa! facies (as buildups). inner ramp since the late Jurassic. During a minor fall. (8) Isolated buildups may seed in an early and windward and leeward aspects. On high-energy ramps ramps. The (7) Ramp sequence geometries. featureless depositional profiles (potentially hundreds of km wide) may be cx- of carbonate ramps generate no pronounced se. but are largest and most corn. The influence of storm-. are factors ramp sequence and continue growth through sue- which complicate ramp classification. or differential sedimentation may produce basin transition is less marked. are lens-like. passive will be exposed. Muddy cessive ramp sequences. the subtle slope geometry of a previous ramp structed mounds in mid. rimmed shelves during changes in relative sea mon in those where subsidence is flexural and level. They show depositional ramps in low-energy and leeward situations could and diagenetic features which are in concert with be termed protected ramps. An ad. a range of prise beaches or barrier islands. (10) Because they are characterized by such (5) Carbonate ramps occur in most styles of low-angle slopes. During a major fall sha!- to silicic!astic ramps. shelf system. both on seis- mid-ramp zone extends from fair-weather wave mic lines and in the field. ramp depositional fa- slow and gradients are slight over large areas. ramps behave differently to . predominantly mud-textured mounds of several third-order ramp sequences. is recognizable in distally steepened fifth-order sequence sets. Dif- est organic carbonate production on ramps ap. extensional faults) or halokinesis. an- structing organisms were absent or inhibited. The gated. tical accretion. Internally. previous mound). sug.P.P. inner-. On “squashed” regional low basins with ramp margins may empty corn- seismic lines some carbonate ramps do show pletely. They may be (3) Although carbonate ramps are common in engulfed by the prograding shallow ramp. a wide variety of organisms have con. ferential steepening may occur within a single pears to have shifted from the mid-ramp to the ramp third-order depositional sequence. ramp Se- water depths which these represent vary. rimmed shelves may develop !owstand reefal or . Organic buildups on ramps are restricted (9) Many ramps consist of layered successions to isolated.g. although the towards shoreline and basin. these may com- (2) As with siliciclastic shelves. but individual sequences seldom (4) The carbonate productivity profile of ramps develop in a keep-up style. between outer ramp highstand systems tracts containing fourth. 73: 101—115. Trans.J. W. Sedimentary and tectonic controls on the development of an early Mississippian carbonate ramp. algal structures.. Inst. Pet. EarthT. The carbonate ramp—an alternative to the topped carbonate platforms. J. Am. and Jackson. Stoudt.. 166: systems and offer a range of subtle stratigraphic 347—368. (Middle form prolific petroleum sourcing and reservoiring Eocene. 41: 205—216. Mineral. Am.G. Isolated Aigner. P. ences compared with that on steep-sloped..F.R. Libya.L. and the porosity as related to early Tertiary depositional facies. 1974. PH. D. geneity in carbonate ramp systems using a de. and range from shoreline carbonate sandbodies Anderton.G.R. fa. R. Geot. acknowledged. M. and falls and like rimmed shelves during major falls. Ahr. the mete. Abh. Butt. Bull. Geol... Harris. London. from the use of computer simulations and recent Barnaby. (12) Ramps and their associated sediments Aigner.. Wiss. Bridges. 59: tions by several referees. Ahr. offshore highs..A. dolomitization driven by processes such as Ko.A. 652 pp.. Geol.. 1987... Butt.. Der Obertithonische.. of carbonate ramp growth will undoubtedly result Gulf Coast. KI. fossil algae in the Upper Cambrian of central Texas. Con- offshore so that mixing zone dolomitization and trots on Carbonate Platform and Basin Development.. R. Egypt).-Naturwiss.. Abh. Aurelt. cause of their low slope angle. 788 pp. M.. play types and lateral facies variations. Bayer..M. Basin Analysis. W. Sarg and J. Neues Jahrb. Armstrong. Glossary of Geology. J.D. 301 pp. Facies and origin of Nummulitic buildups: an example from the Giza Pyramids Plateau. Upper Jurassic reefs of Smackover Formation. 665—693.. Blackwetl. References mented deposits. Econ. 451 pp. 1971. In. rimmed shelf evolution: Lower to Middle Cambrian conti- ods for assessing the range of factors which are nental margin. These responses will vary depend. 1975. Allen and Unwin.. notes in buildups may occur in swarms of many hundreds.. 66: 1449—1482.M. J.F. 1990. and Crevetlo. 58: 621—645.L. ments to the original manuscript. makes it crucial to approach reservoir hetero. hout convection is absent. Am. and Read. Geot.. Dynamic stratigraphy of epicontinentat car- buildups in the mid. Identification of systems tracts in low-angle tailed sequence stratigraphic and diagenetic carbonate ramps: examples from the Upper Jurassic of the model. Assoc. Geol. and Settwood. Zetten Field. South-German of the commonest petroleum reservoirs in ramp Basin.. Pateoenvironment. Paleontot.F. (11) Diagenesis on ramps shows major differ. leaching are important processes.or outer ramp represent one bonates..L. 1991. U. W. Bebout. On ramps. to major detached shoal complexes or shoals over B. Acknowledgements Bates. Although small individually. Arctic Alaska.. JR. regressive Flach- wasser-Phase der Neuberger Folge in Bayern. Early marine cc. Sediment. preliminary lithofacies and pateotectonic maps. (13) Further critical insight into the dynamics Baria. 1975. N. Alexandria. Principles Grainstone reservoirs on ramps are widespread and Applications. and diagenesis of carbonate ramps. Abh. Pet. Etsevier.M. Geol.. Constructive sugges. 1969. C.. H142. Layering is a common attribute of George. are gratefully ing on basinal water depth and slope angle. 174 pp. Gulf Coast Assoc. Paläontol. Sacramento Mountains area.C. Storm depositional systems. K. Geot. Lecture Berlin.. Virginia Appalachtans. T.. 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