International Journal of Coal Geology 61 (2005) 97 – 118 www.elsevier.com/locate/ijcoalgeo The Lower Karoo coal (k2–3) of the Mid-Zambezi basin, Zimbabwe: depositional analysis, coal genesis and palaeogeographic implications P.M. Oesterlena,*, J. Lepperb b Girlitzpark 49, 30627 Hannover, Germany Nieders7chsisches Landesamt fur Bodenforschung, Stilleweg 2, 30655 Hannover, Germany ¨ Received 17 March 2004; accepted 19 July 2004 Available online 13 October 2004 a Abstract Intensive lithological study and correlation of borehole records from the k2–3 coalfields or coal occurrences in the MidZambezi basin led to the identification of two sedimentological types of coal: the Alluvial plain coal and the freshwater-lake shoreline coal. The Alluvial plain coal was found only at Gokwe and in the Nyamandlovu area. Its depth of more than 200 m below surface, the thinness and discontinuous nature of the seams, and the high ash content of the coal make the economic significance extremely small. In clear contrast, lithologically and economically, stand the lake shoreline coal fields at Wankie, Lubimbi, Lusulu, Lubu, Busi, and Sengwa. The pay-zone is the basal Main Seam, up to 17-m thick. The shoreline coal is either more or less massive (Wankie, Lusulu-Lubu) or is thin coal bands alternating with carbonaceous mudstone (Lubimbi, Sengwa). The clearest evidence for a lake shoreline environment comes from the lateral lithofacies change of the coal, e.g., at Wankie where it turns down-dip into sapropelic mudstone of the lake, and up-dip into terrestrial sediments of the coastal plain. The lake shoreline interpretation results finally in the delineation of a 20- to 40-km-wide coal-belt stretching from Wankie in the W to Sengwa in the E. The new model also opens up new perspectives for more coal within and between the coalfields. The study of quality and petrography of the shoreline coal supports the above depositional environment and reveals a standard maceral profile characterized by a basal vitrinite-rich coal passing upwards into inertinite-rich coal forming the major upper part of the sequence (typical Gondwana coal). The profile reflects an initial swamp phase generating a wetforest swamp with Glossopteris trees, but this turned soon to a dry-forest swamp, with oxidation and decomposition of the vegetation, before it was finally overlain by fluviodeltaic sandstones of k4. The paludification is referred to an eustatic rise of the water-table caused by post-ice-age meltwater, but soon the water level dropped, due to the warmer climate. The local and regional controls of the peatswamp formation were considered, as well as the autochthonous and diachronous nature of the coal. The two coal types led to a new palaeogeographic setting for the Mid-Zambezi basin which is in agreement with the new rift concept. It was more of a trough having a SW–NE trend axis which was in the centre filled by a shallow freshwater lake. The * Corresponding author. E-mail addresses:
[email protected] (P.M. Oesterlen)8
[email protected] (J. Lepper). 0166-5162/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.coal.2004.07.002 98 P.M. Oesterlen, J. Lepper / International Journal of Coal Geology 61 (2005) 97–118 above coal-belt was formed out of a peatswamp zone along its palaeo-shoreline. South of this stretched a ca. 100-km-wide shallow alluvial plain drained towards the NW by some meandering rivers, with adjacent flood plains temporarily occupied by local swamps. The alluvial plain was bounded on the SE by crystalline highlands representing the source of clastic sediments for the basin. D 2004 Elsevier B.V. All rights reserved. Keywords: Lower Karoo coal; Mid-Zambezi basin; Depositional environment; Coalfields; Coal petrography; Palaeogeography 1. Introduction Coals are delicate indicators for a certain environment in sedimentation and regional tectonics: the water-table must remain at or near the ground surface of the swamp, the area must subside to keep pace with the vertical growth of the swamp mat, and the site has to be protected from detrital input. These special conditions were fulfilled during the Lower Karoo in the Mid-Zambezi basin, as manifested by a number of coalfields and occurrences in the MidZambezi basin (Fig. 1). The main fields, such as the active coal mine of Wankie and the coalfields Sengwa-South, Lubimbi, etc., occur in the Black Shale and Coal Member of the Lower Permian Wankie Formation (k2–3, see Table 1). Some occurrences, however, such as Marowa and Nebiri, are now known to belong to the Upper Permian Madumabisa Formation (k5). This paper focuses on the k2–3 coal sequence, as this coal represents the prime energy resource for Zimbabwe and, secondly, its main conditions of formation, the depositional environment and palaeogeographic setting, are, to date, poorly understood. The knowledge on the Mid-Zambezi k2 coal is rather limited, the most recent publications being from Duguid (1986). The general Lower Karoo lithostratigraphy of the Mid-Zambezi was covered by Lepper (1992). But since then, new ideas have been put forward on the formation and evolution of the Zambezi basin and rift in general, as well as on the Mid-Zambezi basin which shed a new light on the Lower Karoo coal of Zimbabwe (e.g., Oesterlen and Blenkinsop, 1994; Oesterlen, 1998, 1999). The previous models of k2 coal genesis and palaeogeographic setting are summarized below and are discussed in the light of the new ideas on the Mid-Zambezi basin formation and evolution. Then, the lithology of the main k2 coalfields and occurrences is reviewed applying sedimentological criteria, and depositional environments and trends are established. Subsequently, these results are viewed in the light of coal quality and maceral data from the individual coalfields so far published. Finally, a new model for the coal formation and the palaeogeographic setting for the k2–3 sequence of the Wankie Formation is proposed. For the stratigraphic subdivision of the MidZambezi basin, the new classification of Oesterlen (1999) is used (Table 1). 2. Previous models of coal origin and their palaeogeographic settings—a summary Lightfoot, mapping the Wankie coalfield in 1912 and again in 1923, was the first to suggest a detrital origin for the Wankie Main Seam (Lightfoot, 1914, 1929). He used as main arguments the lack of underclay for the Main Seam, its common compositional alternation of bright and dull bands, the high ash content of the coal (compared to the coals of the UK), the exclusive clastic nature of organic remains in the coal and also borehole results (Watson, 1960, p. 15 ff.). Watson (1960), following a mapping survey of the Wankie coalfield between 1950 and 1956, opposed all these arguments in detail and instead came to the conclusion that the coal was formed din situT, dat the shoreline of a lakeT occupying much the same area as the Middle Zambezi Valley of today (Fig. 2). Bond (1967) accepted, in general, the arguments of Watson (1960) for the in situ origin of the coal, and pointed out the dfundamental differences between Wankie coal and Northern Hemisphere (Europe– USA) Upper Carboniferous coalsT (p. 185). 3). allowing him to apply the dk2 shoreline modelT not only for Wankie. and even for the k2 coal intersected in the boreholes at Gokwe and Sawmills. Zimbabwe (I=Insuza. Hosking (1981). T=Tjolotjo). J. Tjolotjo. Location of coalfields and coal-occurrences in the Mid-Zambezi basin. he subdivided the Lower Karoo basin into a northern or dWankie intrabasinT and a southern or dLusulu intrabasinT (Fig. Lusulu. in studies of the stratigraphy and sedimentation of the Karoo Supergroup in the Mid-Zambezi Valley. S=Sawmills. Sengwa-South. However. Insuza (Fig. agreed with the palaeogeographic setting of Duguid (1977).M. 1986) changed the generally accepted palaeogeographic picture of the basin by introducing the dKamativi–Sijarira inlierT as a dpalaeo-islandT during Lower Karoo time. 3).P. mainly on grounds of palaeocurrent data. 1. Lepper / International Journal of Coal Geology 61 (2005) 97–118 99 Fig. Sebungwe and Sengwa-North but also for Lubimbi. Thus. he . Duguid (1977. Oesterlen. 1999): the nondeformational phase of a sag basin originated by . Gokwe Calcareous M. J. Oesterlen.. Black Shale and Coal.100 P. Zimbabwe changed the existing stratigraphy of Bond (1967) for the Lower Karoo of the Mid-Zambezi Valley by defining the Wankie Formation as constituted by the three members Lower Wankie Sandstone. 1999) Period Cretaceous Late Middle Early Late Middle Early Late Group Post Karoo Gair (1959) Bond (1967) Gokwe Sutton (1979) White sandstone M. Zimbawe Madumabisa Madumabisa (k 5) Lower Madumabisa mudstone Upper Wankie sandstone Black shale and coal Lower Wankie sandstone Tillites and varved shales Gokwe area. His argument was that these three units replace each other laterally. the palaeocurrent directions. from Oesterlen. Hosking (1981) Gokwe Oesterlen (1999) Gokwe Jurassic Upper Karoo Triassic Batoka basalt Red sandstone Sandstone and Interbedded mudstone Escarpment grit Middle Early Late Batoka basalt Forest sandstone Pebbly arkose Fine red marly sandstone Ripple-marked flagstone Escarpment grit Batoka basalt Forest sandstone Pebbly arkose Fine red sandstone Batoka basalt Forest sandstone Tashinga Batoka basalt Forest sandstone Pebbly arkose Escarpment grit Escarpment Escarpment Permian Lower Karoo Madumabisa mudstone Early Gwembe coal Red mudstone and Basal sandstone Basal beds Gwembe area/ Zambia Carbonifer. From the isopach pattern of the various units and subunits of the Lower Karoo Group. and the pebble size distribution pattern Lepper (1992) concluded a postsedimentary (post-Lower Karoo) uplift of the central Kamativi–Sijarira horst. The introduction of the Wankie Formation was accepted by Oesterlen (1999) and is used in this paper. Oesterlen and Blenkinsop.M. Region Late Upper Madumabisa mudstone Middle Madumabisa mudstone Lower Madumabisa mudstone Upper Wankie sandstone Black shale and coal Lower Wankie sandstone Tillites and varved shales MZB. The new rift concept of the Mid-Zambezi basin The kinematic–thermal models of McKenzie (1978) and others for rift basins were applied to the intracratonic Lower Zambezi Basin (Orpen et al. Zimbabwe Upper Wankie sandstone (k 4) Wankie Black shale and coal (k 2–3) Lower Wankie sandstone (k 1) Dwyka (k 0) MZB. but did not review the palaeogeographic setting for the k2–3 member in detail. Lepper / International Journal of Coal Geology 61 (2005) 97–118 Table 1 Stratigraphic correlation of the Karoo System for the Mid-Zambezi basin (MZB. and Upper Wankie Sandstone (Table 2). 1989. 1994) and to the Mid-Zambezi basin (Oesterlen. Zimbabwe Wankie Dwyka MZB. 3. the lack of any coarse clastic marginal facies adjacent to the inlier. J. .P. Lepper / International Journal of Coal Geology 61 (2005) 97–118 101 Fig. The Lower Karoo Mid-Zambezi basin (from Duguid. 1993). Apparent shape of the Early Karoo Mid-Zambezi basin (from Watson.M. Fig. 1960). 2. 3. Oesterlen. in particular.3 13.93 13.2 28.08 8. which resulted in the socalled dTexas longhorn cross-sectionT of a typical rift basin.69 9.0 8.3 18.08 8. both calculated by Palloks (1984).13 11.9 18. p. For the Lower Karoo time.46 15.5 Roof 3 Cumulative AshN25% Ash (%) Volatile matter (%) Subseam Definition of Subseam of the graben which happened only in Upper Karoo time.4 8.3 11.24 13.7 12. Lusulu. see Oesterlen (1999). Lusulu.5 5. Lubu.6 13.3 32.2 35.03 5. the new model contradicts substantially the palaeogeographic settings described by Duguid (1977.98 58. Harrison. This model is supported by a number of arguments: (1) The inliers consisting of Precambrian rocks of various ages still carry a number of Lower Karoo erosional remnants. and Gokwe) done by Lepper (1985) who had selected and reinterpreted 480 .52 11.85 14.42 5. and Insuza (Thompson.46 15.102 P.9 21.42 5.86 7. Sengwa-South and -North.32 12.03 5. Wankie formation All the available literature and selected borehole records of the k2–3 coalfields from the Mid-Zambezi basin were studied concerning lithology.0 7.13 11.9 12. J.58 To 4.91 10.47 8. Humphreys. 1987) Depth (m) From 3.1 23. as the palaeo-shoreline model of Duguid (1986) is no longer suitable.52 11.0 22. The coal seams at Sengwa.0 21. being witnesses of the original Lower Karoo roof sediments overlying the Precambrian base (Chappell.91 10.25 6.2 7.96 4.9 16.4 21. 23–24).93 13.32 12.4 16. accompanied by rising rift shoulders during the Upper Karoo (syn-rift phase during Triassic time).07 15. For further details on the evolution of the MidZambezi basin. due to crustal breakdown.74 12.3 20. and coal quality and were tentatively interpreted with regard to the environment of deposition.64 6.25 6. for the k2 coal– mudstone member of the Wankie Formation. was recognized in the Mid-Zambezi basin.4 20.3 26. 1969).30 9.3 16. (2) 2 Cumulative Ashb20% (3) 1 Cumulative Ash up to 15% Floor kinematic stretching of crust and lithosphere during the Lower Karoo (Permian. 1067.8 25.9 10. 1977.9 13.M. and Wankie Concession show a distinct increase in thickness towards the uplifted blocks (Lepper.30 9. Lithology and coal quality related to the environment of deposition of the Black Shale and Coal member (k2–3). 4. Lepper / International Journal of Coal Geology 61 (2005) 97–118 Table 2 Subseam development of borehole No. 1969.3 29.19 4. the phase of formation of a subsiding central rift zone along boundary faults. pre-rift phase). 1978).9 13.7 18. Sawmills.1 21.85 14.4 32.6 6. do not reflect the boundary faults of the inliers. The most substantial document for this paper was the borehole correlation of the main coalfields and coal occurrences (Wankie Concession with Entuba and Western Areas. This new picture has also to consider the k2 coal findings in the boreholes of Tjolotjo. The answer to all these questions comes only from the k2– 3 coal–mudstone sequence and from the sediments below and above the Wankie Formation.4 8. Oesterlen.86 7. The Kamativi–Sijarira inliers were uplifted in conjunction with the rifting and subsidence The new model of a large shallow Lower Karoo basin calls urgently for a new palaeogeographic and depositional picture. 1992. Isopach trends for the k2 Main Seam from Wankie to Sengwa coalfields and also the ash content isolines.3 30.24 13. Evidence for all these processes. lithofacies.64 6.07 15. thickness.5 26. 1986) and Hosking (1981) mentioned above.1 24.47 8.69 9.19 4. Busi.5 30.9 22. Lubimbi.68 15.74 12. Wankie Concession (from Palloks.9 13. from Lower Triassic onwards. and the phase of widening of the Mid-Zambezi basin in Jurassic time. due to thermal subsidence on the margins of the basin (post-rift phase). as it describes a large shallow sag basin which was not subdivided by an intrabasinal high.7 22. as Duguid has already pointed out (Duguid. having only siltstone and fireclay resting on the k1 sandstone (borehole M 92. 1. Lepper / International Journal of Coal Geology 61 (2005) 97–118 103 key boreholes out of approximately 5000 drilling records. 1986. 1992). 4. thus. Seam No. 5). The boreholes of the coalfields revealed striking lithological differences. Towards the east. Oesterlen. 20-m carbonaceous mudstone succession.P. the database for sedimentological interpretation is excellent. The Wankie Concession k2–3 sequence typically consists of the Main Seam at the base. J. This pelite–coal lithology changes in the Western Areas gradationally replacing the coal by clastic intercalations in the Main Seam and in the hanging mudstones and fireclay. 4). and Fig. The coalfields of Lusulu and Wankie Concession contain medium volatile bituminous coal. Western Areas. in places intersected in the upper part by a thin coal seam. until finally the Main Seam tapers out. and Entuba) More than 3000 boreholes have been drilled altogether in the fields since the beginning of mining activities in 1903. with its up-dip Fig. This general trend from sand–silt facies in the far WSWpart via a peatswamp–mud facies in the centre towards a pure mud facies in the far ENE was recognized by Duguid (1986) and interpreted as a lake shoreline peatswamp environment. Wankie coalfields (Wankie Concession.M. 1993. The coalfields consist of the Wankie Concession in the centre. Fig. 5). all located south of Hwange town and stretching over 40 km in WSW–ENE direction (Fig.1. and a 6-m-thick fireclay horizon (see hole W 1539. . 1987). and the Western Areas to the west. the dblack shale and coalT lithology is replaced increasingly by carbonaceous mudstone (borehole SE 33. 5). 5). Entuba to the east. which is overlain by a ca. up to 14-m thick. Fig. Fig. The coal localities of Wankie coalfields (from Palloks. whereas the rank of all the other fields ranges from subbituminous to high-volatile bituminous coal (Lepper. 4. is the striking high ash content of about 20% in the first 50 cm at the footwall contact of the Main Seam. The roof of subseam No. Table 1) towards the edges of distribution. 2b20%. Oesterlen. 5. before this was replaced by a shoreline peatswamp–mud environment of k2–3.104 P. The same picture was given by Thompson et al. by vertical quality variations in ash content and volatile matter. Another characteristic found often. and its down-dip lacustrine facies in the other direction. Palloks (1987) found out. subseam No. Palloks (1987. 3N25% (see Table 2). that apparently the Main Seam of Wankie Concession consists of three subseams: subseam No. and the down-dip edge at Entuba. margin on one side representing the shore of the MidZambezi lake. The k2-3 lithofacies changes at Wankie coalfields including lithological explanation (borehole data from Lepper. the up-dip margin at the Western Areas. and subseam No. J. Watson (1960) described for the Main Seam of Wankie Concession the increase of ash contents upwards. This is considered as evidence of the previous fluviodeltaic clastic depositional environment of k1. 1993. Table 5). 1 revealing a cumulative ash yield of maximum 15%. Lepper / International Journal of Coal Geology 61 (2005) 97–118 Fig.M. from 5–7% at the base to 30% at the top of the Main Seam. 10) explained these djumpsT by either a sudden subsidence of the floor or rise of the water level. 1987. before it drops abruptly to b10% (Palloks. This is also in full agreement with the general lithofacies trend of the k2–3 sequence . 1985). p. However. 3 is again marked by a sudden increase in ash content (N30%). (1982b) for the Entuba Coalfield and Palloks (1984) for the Western Areas. Duguid (1993) pointed out that the Main Seam in the Wankie district becomes progressively higher in ash and lower in basal coking coal (Duguid. and not only at Wankie coalfields. P. . 1981).M. carbonaceous mudstone. Above the Grey Fig. Lepper / International Journal of Coal Geology 61 (2005) 97–118 105 described above. and Lubimbi-East (Fig. The main publications are from Taupitz (1976) and Thompson (1981). The coal localities of Lubimbi coalfields. mudstone. D. Fig. The 40. which is the database available for the authors. arranged from SW to NE. The lithology of the k2–3 succession at Lubimbi coalfields is rather different from that of Wankie coalfields. Oesterlen. forming mudstone partings and resulting in higher ash contents of the raw coal. The basal Bhorizon consists of dull coal with a higher ash content. They altogether cover an area at least 40-km long and 5-km wide at Dahlia and 25-km wide at LubimbiEast. where the coal was replaced by clastic or pelitic sediments. and the dGrey Shale MarkerT is traditionally subdivided into the six coal-bearing horizons B. The D-horizon is composed not of coal but of stratified carbonaceous mudstone with bright coal bands. is predominantly a lowash bright-banded coal alternating with dull coal and carbonaceous mudstone layers. thus representing its eastern extension. 7 displays the lithology. ubiquitously occurs between the E. The E-horizon is an alternation of bright coal with bituminous mudstone.and F-horizon.2. 6). More than 300 boreholes were drilled up to 1975. E. Lubimbi coalfields The Lubimbi coalfields are located about 100 km east of Hwange town and consist of the four individual fields Dahlia. 1 Seam with the E–F-horizons. and an ash content of N50%. Hankano. and the sequence was called dBira Coal MeasuresT (Thompson. Lubimbi. rarely bearing bright coal band. The Grey Shale Marker. of bright and dull coal. C. 4. in general. the main dpay-zoneT. 6. The A-coal-horizon occurs interbedded in the k1 sequence. The overlying Chorizon. coal horizons and thickness of one typical borehole from each of the four coalfields arranged from SW to NE.to 50-m-thick succession consisting. These environments brought along a large amount of inorganic material deposited in the peatswamp. F. and the A. J. Comparing the Lubimbi lithology with Wankie the Main Seam of Wankie can be correlated with the Band C-coal horizons from Lubimbi. and G.or No. on average is 4-m thick and is petrographically the same as the Fireclay of Wankie. 5. from 32 m at Dahlia to 53. the alternation of coal with bituminous mudstone layers. as suggested by Thompson (1981). a bright coal with mudstone partings and the upper. it can be stated (1) that the ash content increases upwards . the latter suggestion appears more probable. the lower. the relatively high amount of silty mineral matter of the coal. p. The k2-3 lithofacies changes at Lubimbi coalfields (data from Lepper. looks more like the change from shoreline swamp into sapropelic lacustrine sediment. 1985. but to a lacustrine shoreline–swamp environment of deposition. however. 60) compiled some figures for all horizons of each coalfield (Table 3). Shale marker occur the F. in contrast to the Wankie coal. modified). the thickness decrease of the coal layers of horizons C and D to the north. All the lithological criteria cited above do not point to a deltaic environment. carbonaceous mudstone was found which could have come from a sapropelic lake environment or an alluvial plain environment.106 P.M. For palaeogeographic reasons. and the considerable increase in thickness of the k2–3 member. the common stratification of the coal (dbanded coalT) with bands displaying a lenticular shape on a larger scale (Thompson. Oesterlen. Lepper / International Journal of Coal Geology 61 (2005) 97–118 Fig. The Hankano lithology. see Fig. Another interesting hint to the palaeogeographic setting is the fact that the Lubimbi coalfields did not give any evidence for shore-face sediments in the southern part (Dahlia-South). This depositional environment is also found in the northern part of Dahlia coalfield which is composed completely of carbonaceous mudstone (boreholes 504 and 509. The higher thickness accounts for Lubimbi and LubimbiEast. Lepper. 7. The greater thickness of this sequence towards the northeast might indicate the growing influence of a delta system nearby delivering higher mineral input into the adjacent swamp in times of flooding.to G-horizons) is best explained by a long period of water-level fluctuation.5 m at Lubimbi-East (Fig. 1981). but Thompson (1981.and G-horizons. The coal quality data for Lubimbi are scarce. In general. 1985). where they are replaced by carbonaceous mudstone. are the alternation of bright coal sequences with dull coal horizons. again a carbonaceous mudstone. The typical features of the Lubimbi coal. Instead. J. but not for the Dahlia sequence. leading to a temporary drowning of the swamp and deposition of sapropelic mud. For explanation. The thick alternation of coal with mudstones (D. 7). which compares better with the Wankie Concession lithology and thickness. such as in the Wankie coalfields. appears to be a transition between the Dahlia type in the SW and the Lubimbi types in the NE. However. 9 28. towards the top the ash content increases (Palloks. .5 73.6 36. occur higher up in the succession. which is 368-m deep and located about 5 km south of the southwestern corner of Lusulu coalfield (Falcon. Lusulu coalfield The Lusulu coalfield is situated ca. 1984. 4. a shoreline swamp environment which.4 36. 8) has probably an origin from a nearby delta system.and E-Seams (Fig.0 m in the SE and SW. and the A-Seam. J. The Lusulu coal is classified as a high-ash coal on average (Palloks.3 61. as with the Lubimbi coalfields.9 26.4 23.5-m thick and up to 3. in agreement with the above explanation that the shoreline sediments grade into sediments of an alluvial plain. All the coal seams above the Main Seam are of inferior quality. In borehole MT 1. The best quality coal (ash content b20%) occurs in the lower portion of the Main Seam.) Ltd. The same trend was also found for the A-Seam.e.5. 8 displays four borehole records.6 34. p. C. up to 10-m thick.9 30.9 71.to 4.2 48. as in Wankie coalfields and (2) that the amount of mineral matter or ash content for the Lubimbi coal is higher than for Wankie—which again is in line with the hypothesis of a delta system not far to the northeast of the Lubimbi coalfields. and in borehole 256. as in borehole MT 1 (Fig. Although lithologically the same.to 60-m thick. the quality of the Main Seam becomes poorer towards the south.5 40.0 69.5 65.3 43. 1983).2 37. Palloks (1984) demonstrated that the thickness of the Main Seam decreased from 10.3 26. 1985. 1984).7 56. Taupitz (1976) recognized the lithological similarity of the so-called Lusulu dLower coal shaleT to the Lubimbi k2–3 lithology. The k2–3 sequence at Lusulu is 40. and E.4 63.3 28.2 33. of lesser quality. Oesterlen. Lepper / International Journal of Coal Geology 61 (2005) 97–118 Table 3 Averages of raw coal ash-content for the k2–3 coal horizons. 50 km southwest of SengwaSouth.9 61. The sequence starts with the two main coal horizons. and borehole 256. 8). The northwestern continuation of the Lusulu coalfield might be seen in the Lubu coalfield (see below).0 29. 2. the total thickness is greater due to a thick pile of carbonaceous mudstone overlying the E-Seam. It stretches 45 km in a NE–SW direction and has an average width of 5 km (Fig. the carbonaceous mudstone of the uppermost sequence towards the NE (C-section from Lepper.8 33. Fig.0 19.1 61. interbedded with carbonaceous mudstone.8 24. The younger coal seams B. 90 km northeast of Lubimbi and ca.7 32. towards the south. the coal being replaced by carbonaceous mudstone. D. The main source of data is Palloks (1984) who interpreted 185 boreholes and other results from exploration by Shell Developments Zimbabwe and others during 1975– 1976 and 1980–1982 (Shell Developments Zimbabwe (Pvt. the Main Seam.9 30. Fig. 26).M. grades rapidly into the carbonaceous floodplain sediments of the adjacent alluvial plain (borehole MT 1).5 m above the Main Seam.3 throughout the horizons of k2–3.9 27..0 20. the coal seams are widely replaced by carbonaceous mudstone.4 55. The Grey Shale Marker horizon separates the D.3 55. Laterally. 1).P.0 15. Further information was obtained from the research borehole MT 1. 1981) Horizon of coalfield Bright banded coal fraction (Ash %) Dull coal/ carbonaceous mudstone fraction (Ash %) 62.9 24.0 m in the N to 4.0 60. i. where the two logs of boreholes 201 and 250 represent the typical lithology of central Lusulu.2 66.3.4 107 Lubimbi F or F/G E or E/F D or D/E C or C/D B or B/D Lubimbi East F or F/G E or E/F D or D/E C or C/D B or B/C Hankano F or F/G E or E/F D or D/E C or C/D B or B/C (one borehole) Dahlia F or F/G (two boreholes) E or E/F D or D/E C or C/D B or B/D 37. 8). the alternation of lenticular coal seams with carbonaceous mudstone indicates.. The typical lithology of the dLower coal shaleT. 1973). Lubimbi coalfields (from Thompson.9 65. 3% for raw coal). as well as the A. 10 km to the W of borehole 6. most probably are of lacustrine origin resulting from a transgression of the lake. the very thick sapropelic mudstones. 20 km . see Fig.and B-coal seams (Palloks. ca. but a sapropelic origin is suggested.5. For explanation.5 m. the Main Seam is high in ash content (27. like at Wankie Concession. has much more mudstone in the succession than the other boreholes. but near its centre. probably indicating the lateral influence of another fluvial delta towards the west.0 m thickness average. is the expression of a persistent shoreline swamp environment which was replaced twice by thick nonorganic mud intervals.5 m thickness (maximum of 18 m). see Fig. and B-Seam. The k2–3 lithology is similar to that of central Lusulu.M. Lepper / International Journal of Coal Geology 61 (2005) 97–118 Fig. A-. and the unusual thickness of the total succession (Fig. 5. The k2-3 lithofacies changes at Lusulu coalfield (data from Lepper. Borehole 13. J. and the Sengwa coalfields. 9). Oesterlen.108 P. The lack of information does not allow a sedimentological classification of the coal. However. p. 9). It has an extent of only 15 km in a SW–NE direction and a maximum 8 km width. Lubu coalfield Lubu coalfield is located about 35 km in the northwestern continuation of Lusulu (Fig. 19). but no sampling or drilling results were reported. 9). 4. located about 25 km west of Lubu (see Fig. This explanation is supported by the high ash content of the Main Seam (see below) and the considerable thickening of the Main Seam towards the SE (Palloks. The lower portion. about 40 km towards the SW. the bottom and upper portions being of inferior quality. as it occurs on a down-faulted block within the Sijarira Inlier. which was found also in most of the other boreholes. 1). On average. Busi coalfield Busi is situated between the Lusulu. with few sandstone tongues. Palloks (1984) compiled the results of the coal exploration done by Messina–Transval Development (MTD) in 1982. The best quality coal is not found at the base of the Main Seam as typical. There is no fireclay or Grey Shale marker horizon intersected in the sequence. Thompson (1980) described a 1.2-m thick coal seam at Sebungu. 4. a basal Main Seam averaging 12. 1984. B-seam with 2. which included the drilling of 13 boreholes. 1985). 8. ca. overlain by mudstone and carbonaceous mudstone with two smaller intercalated coal seams (A-seam with 2. at the expense of coal.4. 1984: from 6 m in the NW to 18 m in the SE). of the upper portion. 1). The Lubu k2–3 sequence represents the northwestern down-dip extension of the Lusulu shoreline sediments. including the Main Seam. representing the lateral input of a nearby delta system (Fig. and Oesterlen (1999). Fig. The other two boreholes (TB 4 and TB 5). Fig.) Ltd. Lepper (1985). the latter on the northern side and 15 km NNW of Sengwa-South (Fig. sometimes with pebble bands and in places with a few coal bands in the k2–3 sequence (see borehole TB 1. which consist of an alternation of carbonaceous mudstone with coal bands. but is not wide enough to affect the Lusulu coalfield.. Sengwa coalfields The Sengwa coalfields consist of Sengwa-South and Sengwa-North. Oesterlen. Nevertheless. and then . 9). The data is very limited (Lepper. 1995). in places with a Main Seam at the base (see borehole TB 5. leading to temporary swamps before they were terminated by suspended sediment load. Unfortunately. the former on the southeastern side. 4.P. here at least 40-m thick. Rio Tinto Rhodesia carried out coal exploration in this region. 5. The delta obviously follows the SE–NW depositional trend. Some k2-3 lithofacies records of Lubu and Busi coalfields (from Lepper. of which 13 boreholes of each area were recorded by Palloks (1984) and Lepper (1985). see Fig. The lithologies of both coalfields are nearly identical. represent a shoreline environment affected by frequent flooding of the adjacent delta system. overlain by the Lower Carbonaceous Shale (LCS).6. Rio Tinto Zimbabwe drilled another 178 boreholes in Sengwa-South. the only economic seam of the area. 1). at the base. 9) represent a delta environment of a river system. 15 km in the SW–NE direction and 4 km in the perpendicular direction. For explanation. with a pure sand-facies. Lepper / International Journal of Coal Geology 61 (2005) 97–118 109 Fig. both situated along the northeastern end of the Sijarira Inlier. but only very limited data were available for the authors (Falcon Research Laboratory (Pty. 1). no coal quality data are available. In 1994. This facies resembles the one found in the Lubimbi coalfields. From 1973 to 1980. 9. The k2–3 sequence starts with the Main Coal Seam (MCL). The two boreholes TB 1 and TB 3. 1). 1985) and consists only of six borehole-logs stretching over 18 km in a SW–NE direction.M. only four of them having intersected the k2–3 sequence of the Wankie Formation. towards the NE (Fig. J. as expected. 1985). the Upper Coal Seam (UCL). The main source of information is again Palloks (1984). the four litho-logs display two distinct different sedimentological environments. Sengwa-South area extends 7 km in the N–S direction and 2 km in the E–W direction (Fig. The area drilled at Sengwa-North stretches ca. 7. This concept is supported by the shaling-out of the Main Seam in Sengwa-North towards the east. 4. The best coal quality is found in the lower portion of the Main Seam (b10% ash). The k2-3 lithofacies changes at Sengwa-North and Sengwa-South coalfields (data from Lepper. 5. As in the other coalfields. Palloks. where the hinterland provided large amounts of detrital load (see Fig. The Main Coal Seam of Sengwa-North averages 14. 10). it deteriorates towards the top of the seam (N20% ash. The fireclay is developed in Sengwa-North. Both the LCS and UCS are in reality carbonaceous mudstones intercalated with thin coal bands.110 P. 10. towards the south (Palloks. with its down-dip side passing into the lake to the NW (Sengwa-North) and the up-dip side into the alluvial floodplain towards the SE (Sengwa-South. The central part was eroded by the uplift of the Sijarira Inlier. either an episodic flooding of the lake or a rhythmic influx of suspension material from an adjacent delta system.1 m thickness (17 m maximum). Lepper / International Journal of Coal Geology 61 (2005) 97–118 the Upper Carbonaceous Shale (UCS). is probably explained by the particular palaeogeographic position of Sengwa in a large bay of the eastern margin of the lake. 1984. Oesterlen.e. The interpretation of this portion could consider. 25-km wide SW–NE-trending shoreline peatswamp. typical for an alluvial plain environment.M. see also borehole 4. but from 2 m above the footwall. J.5 m thickness (maximum 15 m) in Sengwa-South. The Gokwe coal occurrences Sessami and Kaongo are the two subsurface coal occurrences of the Gokwe area.)— results which were already reported from the other coalfields. . 14 km apart in a northeasterly direction. ca. p. Explanation see Fig. Subsequently. and both located ca. the UCL is often missing in Sengwa-North (see Fig. and in Sengwa-South. as described already for Lubimbi and Busi coalfields. the swamp vegetation was periodically terminated by the deposition of suspension load. Another argument is the synchronous Bari coal occurrence. This concept is also supported by the high ash contents of coals in both coalfields. and 12. The overall configuration suggests the two coalfields were originally part of one ca. which displayed only four thin coal bands within a 50-m-thick carbonaceous mudstone succession of k2–3. i. 1984. Fig. about 22. 65–80 m at Sengwa-South and 40–100 m at Sengwa-North. p. the lithology of the entire sequence and the high thickness of the Main Seam suggest a shoreline swamp environment. 12). but not in SengwaSouth. 60 km Fig. 10). 30 km to the east (see Fig. 1985). 34 ff. 1). 35).5% on average for both. The unusually great thickness of the k2–3 sequence at Sengwa. the higher proportion of silt. were correlated by Lepper (1985). The litho-logs of the k2–3 sequence intersected at about 300 m depth in all three holes are similar in all boreholes: one or several thin coal seams are interbedded in an alternation of carbonaceous mudstone. J. the dominant lithology. 5. Sometimes the Main Seam occurs at the base.M. The results were compiled by Sutton (1979). 1). 1985). with a maximum of 9 m thickness. 1977) and the increasing thickness for the Lower Karoo succession towards the NW.P. thus. as well as a progressive decrease of organic material in the mudstone. 1985. and Insuza. The boreholes were correlated and interpreted by Harrison (1978). covering an area of 70 km in an W–E direction and 30 km in a N–S direction. The k2-3 lithofacies changes at Gokwe coal occurrences (data from Lepper. all located in the Nyamandlovu district and arranged on a 50-kmlong NE–SW-trending line in an area situated ca. sandstone. This environmental classification is supported by the very high ash content of N30% for the coal seam at the base (Thompson. with siltstone or. and 13 borehole logs. with about 20% ash content at the base of the Main Seam and N30% in the upper portion (Sutton. Section B) and a stable thickness of ca. Oesterlen.8. sometimes the sequence is represented only by carbonaceous mudstone (borehole SY 1) or siltstone or sandstone (boreholes G 12. Explanation see Fig. 15 m in E–W direction (see Fig. The coal quality is not good. or is completely missing (boreholes G 11. see Sutton. Lepper / International Journal of Coal Geology 61 (2005) 97–118 111 southeast of Sengwa-South and 30 km north of Gokwe town (Fig. borehole G 7 and G 10. overlain by siltstone (e. It is composed of various lithologies changing rapidly in a lateral direction (see Fig. 1). Its total thickness is reduced to 15 m average (maximum 40 m). 12). rarely. 11) point to drainage from a source area in the SE towards the lake in the NW (Fig. Boreholes of Tjolotjo. Sawmills. Fig. and Insuza Further information on the coal-bearing k2–3 succession comes from the three deep research boreholes at Tjolotjo.g. G 4). 11. The rapid lateral change of lithology. 4. 1979). 1974). A general coarsening-up of grain size is recognizable. 1979). The lithofacies and the small thickness of 26–30 m resemble much the Gokwe logs. Sawmills.and sandstone in the sequence and the higher ash content in the coal all indicate an alluvial plain environment for the coal. . 200 km southeast of Wankie (Fig. The k2–3 sequence intersected in depths of 200– 300 m below surface looks very different from the ones described above. with the entire sequence composed of river systems and adjacent flood plains. 11). 11). The thickness increase of the succession towards the NW (Lepper. They were investigated by reconnaissance drilling first in 1951–1952 and later in 1971–1974 by Rio Tinto Rhodesia (Boehmke and Duncan. the sediments are also considered typical sediments of a flood plain on an alluvial plain. Fig. 3% inertinite (I).2% other components). 5. on the geological background (I= Insuza. the data is scarce and in parts incomplete. The k2-3 coal-belt and adjacent sedimentological boundaries in the Mid-Zambezi Lower Karoo basin. 44. Gokwe occurrence.7% I. Thompson et al. The same principal lithostratigraphic subdivision of the Wankie Formation also provides evidence.2% liptinite (L) and 1. Lusulu.M. The one sample is a bi-maceral coal (54. J.5% L and 3.112 P. Oesterlen. the other a typical Gondwana coal (61. However.3% vitrinite (V). presumably sampled from the lower portion of the Main Seam. without giving any details of sampling. with no information at all from Busi and Lubu coalfields. Lepper / International Journal of Coal Geology 61 (2005) 97–118 Fig. Watson (1960) reported on two maceral analyses from the Main Seam coal of Wankie Concession No. and Sengwa. 0. while Lubimbi coalfields present a different one. 2 colliery. 34.2% other components) which comes certainly from the upper Main Seam. 0. that the area of the boreholes is still a part of the same Lower Karoo basin of the Mid-Zambezi terrain. it appears that there exists one dominant maceral model for the coalfields of Wankie. (1982b) . 12. S=Sawmills. and the deep boreholes of Nyamandlovu area.6% V. Information from maceral analysis of the coal The petrography of the k2–3 coal is not wellstudied yet. T=Tjolotjo). that is. classified as steam coal. 40% V.5–12% L) was subordinate occurring only at the base of the Main Seam and in the Upper Seam. The Main Seam coal was determined to be dominantly an inertinite-rich coal (51–96% I. The second stage was followed at Wankie by the deposition of carbonaceous mudstone. Similar maceral values were given for the Main Seam 2.P. and 5% minerals. having a high water-table (vitrinite-dominated coal). The first stage of coal formation changed gradationally by the fall of the water level. of 1. while its carbonaceous mudstone coals are typical inertiniterich coals (Gondwana coal). but it is noted that the bright coal changes to a dull coal accompanied by an abrupt drop of the vitrinite content to 0% (Thompson et al. The dull coals of the B-horizon are inertiniterich coals (N70% I) which are overlain by a vitriniterich coal of the C-horizon in the south (51% V. The banded coals of the E/F-horizons. and 0. which again is a vitrinite-rich coal (71.2% I. 50% I. Lepper / International Journal of Coal Geology 61 (2005) 97–118 113 produced some data from Entuba coalfield. This second stage was more stable than the first and produced the inertinite-dominated coal of the major part of the Main Seam. 1 Seam above the Main Seam.. and 13% minerals). whereas the overlying 1. 30–45% I. 28% I.5–35% V. 10% L. Falcon Research Laboratory (Pty.3% not reported)). The vitrinite-rich coal (50–60% V.0% not reported). 33% I. either by higher subsidence of the ground or a rise of the lake water level (transgression of the shoreline towards the south). One result is known for the No. 2. a typical Gondwana coal. are vitrinite-rich (64% V. No maceral analyses were given for the main upper portion of the Main Seam. 1981) Bright banded coal (vol. The maceral profile of the entire k2–3 succession at Lubimbi does not fit to the above standard maceral Table 4 Maceral analyses from boreholes No. 20. as suggested by the authors). Lubimbi coalfields (from Thompson. The main two coal lithologies within the k2–3 sequence. Similar results are available from Lusulu coalfield (Shell Developments Zimbabwe (Pvt.3% L (and 23. 11% L. classified as coking coal. 6% L. The brightbanded coal is mainly a vitrinite-rich coal. 27.0% I and 2. The maceral data of the individual coalfields described above are the result of depositional processes. The base of the Main Seam is a vitrinite-rich coal (64% V. from washed coal with specific gravity of 1. equivalent to the No. and Sengwa coal was. see Table 5).M. The swamp formation started as a paludification process. (1995) reported the same results from Sengwa-South coalfield based on the analysis of three boreholes. Oesterlen. the vitrinite-rich coal of the No.%) Vitrinite Liptinite Inertinite Minerals 45–59 8–11 23–31 10–13 Dull coal and carbonaceous mudstone (vol.3 m coal. 101 and 107. and 1. and 4. 1982a). Fig.7% V. The above-described standard maceral model for the Wankie.3% V. 7. due to a rising groundwater-table.1% V. for the lower ca. with a higher mineral content than the vitrinite-rich coal (Table 4).%) 13–23 5–7 41–61 19–31 All data refer to a float at S.4% minerals. i. 1 Seam at Wankie and equivalent coal horizons in Lusulu and Sengwa demonstrate that the first stage of a dwet forest swampT could be reestablished by a transient regression of the lake . and 10% L) grading into the less-common. strongly inertinitic coal (73% I. characterized by a wet forest swamp with a Glossopteris–Gangamopteris flora.7% L (the remaining 36.4: the Main Seam lowermost 1.4 m of coal classified as blend coking coal revealed only 29.4% L (the remaining 17.9 model.. the fluviodeltaic sandstones of the k1 sequence were replaced by a swamp. 7% L. 1982a.5% not reported).G. 12% V. the bright banded coal and the dull coal differ in the maceral composition and the ash or mineral content (see Tables 3 and 4).) Ltd.. and 5% minerals). 1983). and 2% minerals). J. Thompson et al. 2). Lusulu.3–13% L). is composed of 47. However. but the most common coal type is a Gondwana-type inertinite-rich coal (ca. 1 Seam of Wankie. 16% I. 18. 1.) Ltd.0% I and 1. leading to oxidation and decomposition of the plants in the swamp (dry forest swamp). with mudstone intercalations and minor inertinite-rich coal bands.3% L (the remaining 51. The banded coal of the D-horizon revealed similar results.6% V. 1981). 2 m of the Main Seam.e.2% I. when the shoreline swamp was drowned.3 m bottom coal (coking coal) of the Western Area coalfield (40. while the dull coal and carbonaceous mudstone coal are dominantly inertinite-rich. The maceral analyses of the Lubimbi coalfields were carried out on seven boreholes (Thompson. the dull coal at the base is due to a local. These irregularities were most probably caused by a temporary low water level for these areas—which was already suggested by Palloks All data refer to a float at S. and a characteristic microlithotype in Gondwana coals as well.1. and the increase in mineral content of the inertinite-rich coals. or further coal seams in the upper sequence at Lusulu. were explained by palaeotopographic conditions (Palloks.8 C South 51 11 33 5 14. Lepper / International Journal of Coal Geology 61 (2005) 97–118 Table 5 Maceral analyses of the coal horizons from Lubimbi coalfields (from Thompson. sometimes with detrital inertiniteT. i. i.e. the favourable conditions for a water-covered shoreline–swamp were established again. Discussion 6.. certainly due to local controls on deposition.%) Liptinite (vol. the most dominant coal microlithotype of k2–3 in this report. This facies regression is also indicated by the alternation of mudstone with coal bands.G. J. where the lack of bright coal at the base of the Main Seam. The maceral profile of the Lubimbi coal (see above) differs from the standard model mainly in the bottom part of the k2–3 sequence. Lusulu and Sengwa-South as well. the swamp was drowned by a high water level or by suspension load from a sedimentary source nearby. With regard to the vitrinertite. where inertinite coal was found either all over or in parts at the base of the Main Seam or equivalent. Oesterlen. 1987). It means that at least three different depositional sub-environments were present along the shoreline of the k2–3 Mid-Zambezi lake.6 14 7 53(?) 26 41. 1. temporary topographic high. Peatswamp formation and controlling parameters It is well known that swamps form only under certain special conditions. the vitrinite coal of the No. The flooding was deduced from the higher liptinite content of the inertinite-rich coal.9 (?) Data suggested by the authors. The maceral composition of the overlying carbonaceous mudstones and mudstone–coal band alternation of the horizons D to G is similar to the maceral development of the standard model including the vitrinite-rich coal seams in the upper k2–3 sequence. that the liptinite macerals were dtypically concentrated in layers which are sometimes associated with high mineral contents. but for Lubimbi.7 D 51 10 27 12 25.9 North 37 9 44 10 22.%) Inertinite (vol. typical for the upper portion of k2–3 at Sengwa. more or less synchronously: sapropelic lacustrine mudstone was deposited in Wankie Concession. 1 Seam formed. Irregularities of this type were already reported by Palloks (1987) from the Wankie Concession. the subseam No.114 P.. and a lack of clastic contamination. the dull coal of mainly inertinite macerals was dominant. Styan and Bustin (1983) came to the conclusion . at about the same time.5 Dull coals B Carbonaceous mudstones E/F D that it has formed from herbaceous peats after flooding with oxygenated water and subsequent desiccation. where the swamp started to develop as a dry forest swamp. In Lubimbi. 1981) Banded coals Horizon Vitrinite (vol. both horizons together being the equivalent of the Main Seam of Wankie. In Entuba. and Sengwa-South. the main coalfields presented a number of different lithologies. in its report on the Sengwa-South coal. a subsiding floor. the deposition of organic and/or suspension material was abruptly replaced by the fluviodeltaic environment of the k4 sandstones (regression again). the oxygen-rich dry forest–swamp was still active.7 35 6 33(?) 26 42. Falcon Research Laboratories (Pty. This explanation corresponds well with the conclusions drawn by the authors for the sedimentological environments of the individual coalfields. which is an inertinite-rich coal.) Ltd. Finally. a stable water level. Local controls were also the reason for the unusual maceral development at Lubimbi and Wankie Concession. 6.4 6 10 71(?) 13 28.%) Minerals (vol. while in Entuba.%) E/F 64 7 16 13 23. whereas in Wankie Concession. Most probably.e. of 1. During the time span of the upper k2–3 sequence. The latter grades into vitrinite-coal of the Chorizon. Lusulu. (1995) stated.M.%) Ash (wt. shoreline. and vice versa. clarain. in principle. The width of the coal-belt is. from k1 onwards. stretches from Wankie coalfields in the west in a lobe over more than 300 km towards Sengwa coalfields in the northeast. the authors consider the shoreline coal of Wankie and the other coalfields to be a hypautochthonous coal. became the sedimentary source area for the Lower Karoo basin of the Mid-Zambezi. In the lateral direction.P. the general trend of the maceral development for all the coalfields. a process which slowly came to a standstill at the end of k2–3.. characterized by high precipitation in the rainy season followed by a cold dry season. high ash content and fragmentary nature of the coal) lost their value. In the light of these criteria. as seen in the Busi coalfield. allochthonous coals are in general much richer in mineral content. The dwet forestT zone with its vitrain. 6. Lepper / International Journal of Coal Geology 61 (2005) 97–118 115 (1987). for example. as coal petrography has investigated and understood the substantial differences between the Carboniferous coals in Europe and North America and the Permian coals of Gondwana. J.g. when the water level rose. the Wankie coal-belt attains a width of at least 30 km. On the other side. and bituminous mudstone intercalated with clastic sediments of an alluvial floodplain on the up-dip side. and Sengwa (North– South) about 25 km. with meltwater causing an eustatic rise of the water level from k 1 onwards. 1929) had decided for the drift coal theory. the vertical change from a low-ash vitrinite coal at the base of the sequence to the high-ash inertinite coal (Gondwana coal) in the upper portion is referred to a regional control. 12). however. Lusulu and Lubu coalfields about 40 km.M. after Lightfoot (1914. whereas Watson (1960) disagreed and opted clearly for the insitu origin of the coal (see Section 2). which led to periodic or episodic flooding and then drying up of lake waters. Evidently. Diachronous coal Duguid (1986) was the first to consider the diachronous nature of the k2–3 coal. may have been another parameter. i. Based on the drilling data available.e. small reworking of plant remains or of peat took place repeatedly within the peatswamps of k2–3 during times of flooding. this migration of the peatswamp on the shoreline of the lake led eventually to an unusually broad zone for a coal-belt. However. Another regional factor was the annual rhythm of precipitation. when the water level dropped. approximately at the Permian/Triassic boundary. Therefore. Lubimbi belt at least 20 km. the inertinite coal consists mainly of decomposed and detrital macerals. which. The most common coal type of the k2–3 sequence.. as described above. Crustal rebound following melting of ice in the highlands to the southeast. Autochthonous or allochthonous origin of the coal This discussion was opened. were generated by aerobic or subaerobic decay during the process of oxidation of the swamp vegetation in situ and were not necessarily of detrital origin. Oesterlen. the coalbelt having a thickness of maximum 18 m at Lubu. defined by the coal seams which grade laterally into a sapropelic mudstone of lacustrine origin on the down-dip side. inertodetrinite. Since then. A substantial controlling factor was slow post-ice-age climatic warming. The lithological facies changes in vertical and lateral direction are the result of changes in the sedimentary environment. the vitrain-dominated peat replaced by the durain-dominated peat. which. Of course. 6. especially with respect to the individual maceral types of the coal and their origin. The main arguments of Lightfoot (1929) for a detrital coal (composition. the general topographic relief of the area was very low. . With the beginning of k5. and led to the term of dhypautochthonous coalT. it is interbedded with several wide clastic zones of delta systems. the isostatic control became the dominant factor—the crust of the basin began to subside and formed a sag-basin. and duroclarain lithotypes moved up-dip. The migration affected also the maceral types of the peat. e. coal petrography in general has developed considerably. and led of course to a lateral migration of the shoreline peatswamp. until it broke in the central zone and generated the Zambezi-graben.2. reflecting the palaeo-shoreline of the k2–3 lake (Fig. This factor could have played a role in the onlap of the k4 fluviodeltaic clastic sediments over the k2–3 shoreline coals and mudstones.3. and down-dip into the lake. it can be seen that in particular the Lubimbi coalfields still have an interesting potential for more coal towards the up-dip side in the southeast. On the other hand. Wet forest swamps of mainly Glossopteris trees established themselves along the shoreline which had a high water-table (vitrinite-rich coal). similar in all coalfields. the new sedimentological model for the coalfields opens new potential for more coal within and between the coalfields explored so far. the coal contains high ash contents and is encountered only at depths of more than 200 m below the surface. Even the Lubimbi coalfields show good indications of the same facies change. The respective coal seams are in general thin and discontinuous. e. over its course. These new results are in full agreement with the new rift concept for the MidZambezi basin of Oesterlen (1999). Only one . where Sengwa-North represents the down-dip extension of the Sengwa-South shoreline coal (Fig. the wet forests changed slowly to dry forests where the swamp vegetation was prone to oxidation and decomposition. the wet forest swamp returning only at the end temporarily in some coalfields. and the presence of suspension load sediments as the main country rock. In some cases. and continues upwards with up to five thinner coal seams intercalated in carbonaceous mudstone. In other places. Oesterlen. J. and at Sengwa. The drainage system of the gentle alluvial plain had its catchment area in the southeastern highlands and was running towards the NW where the channels emptied into the Mid-Zambezi lake with delta systems (Fig. 12). The belt’s width points to a synsedimentary migration of the shoreline peatswamps. Wankie Formation) were deposited over the shoreline peatswamps. due to a change of the water level. The coal-belt is. This type of coal was formed for most of the time span of the k2–3 sequence. the coal consists of thick alternations of carbonaceous mudstone with thin coal bands. like the coals of Wankie or Lusulu–Lubu coalfields. due to a regression of the lake (typical Gondwana inertinite-rich coal. the coal is massive and is not split-up by mudstone partings.. which represents the main dpay-zoneT. Other ones are the lack of coarser-grained clastic contamination. with higher ash-content). Lusulu and Lubu. The Alluvial plain coal was found only at Gokwe and in the Nyamandlovu area (see Sections 4. the economic significance of this type of coal in the Mid-Zambezi basin is extremely low. at Busi (Fig. where drilling has proven the shoreline coal changes down-dip into sapropelic mudstone of lacustrine origin and up-dip into terrestrial sediments of the coastal plain of the lake. Subsequently. e. suggesting continuous peat formation without episodes of clastic influx. manifested at Lubimbi and Sengwa coalfields. 12).g. up to 17-m thick. It suggests an initial paludification (term from Diessel. This is in contrast to the other main type of coal. This interpretation was supported by the finding in the Busi coalfield of delta sandstones laterally interfingering with the shoreline coal. The interpretation results in the delineation of a 20. Busi to Sengwa (see Section 4). The thickness of the Main Seam is one indicator of a shoreline environment of the coal.8) and occurs embedded in fine-grained flood plain sediments deposited along meandering river channels. most probably coming from nearby delta systems. A similar. at Wankie coalfields. the lake shoreline coal. In other words. 1992) of the shoreline of the newly formed lake—due to a transgression on the fluviodeltaic k1 sediments caused by post-ice-age meltwater.116 P. but less clear dfacies cross-sectionT of the shoreline coal has been found at Lusulu.M. Conclusions—the new concept of k2–3 coal genesis and related palaeogeographic setting Intensive lithology and lithofacies interpretation of the coal-bearing k2–3 sequences from the individual coalfields and occurrences of the Mid-Zambezi Karoo basin and of their coal quality and coal petrology has led to the identification of two different sedimentological types of coal and also a new palaeogeographic setting for the coals. 12). from Wankie over Lubimbi. the strongest argument derives from the lateral lithofacies and environment change. Finally. the water-table began to drop. reveals some more information on the coal genesis. interrupted by some delta systems related to river channels draining from the SE. However. The maceral profile of the shoreline coal which is. where the peatland was contaminated by the deposition of suspension load. with Lubu as the down-dip continuation.g.7 and 4. The coal-bearing sequence starts almost in all fields with the Main Seam at the base.to 40-km-wide coal-belt along the palaeo-shoreline of the lake which stretches in Zimbabwe from the Wankie coalfields in the west to the Sengwa coalfields in the east. before the final regression of the lake took place and fluviodeltaic sandstones of k4 (Upper Wankie Sandstone. in its main trend. of all the coalfields in the NW. Lepper / International Journal of Coal Geology 61 (2005) 97–118 7. mainly in the rainy seasons. 378 (Gokwe).. Lepper / International Journal of Coal Geology 61 (2005) 97–118 117 exception to this standard maceral profile was found at Lubimbi coalfields where the paludification phase did not start with a wet. the basin was rather a trough elongated along a SW–NE axis. Probably. E. Bartholomew. The shoreline follows a SW–NE trend parallel to the basin axis. A review of Karoo sedimentation and lithology in southern Rhodesia. but in the NE it turns sharply towards the NW and meets the lakeshore. Consequently. but often. having one central depositional centre which was filled with a shallow freshwater lake—located more or less coincident with the modern Lake Kariba (Fig.P. an idea previously suggested by Oesterlen (2001) in his report on the Mana Pools basin. Accordingly. there was apparently no direct connection to the k2–3 deposits of Botswana. The depositional classification of the k2–3 MidZambezi coal results in a new palaeogeographic model for the Karro basin—a model which is in contrast to the hypothesis of Duguid (1986). Duncan. G. New Galloway/Scotland. flooding episodes brought a clastic splay covering the peatground. also for reviewing the manuscript. by bringing the original figures into a digitized format. This leads to the question of the controls for the lake–shoreline peatswamp formation. Bond. Hiltmann and Th. but conforms to the rift concept of Oesterlen (1999). but some drill records of the Western Areas of Wankie coalfields revealed terrestrial sediments replacing the coal–mudstone sequence.O.C. G. 12). The southern boundary runs in general parallel to the lake shoreline. Thielemann. Hannover/Germany. but a dry forest–swamp (inertinite-rich coal). 18 pp.. and at Sengwa towards the N. 173 – 195. 12). References Boehmke. and the final reading by Dr. R. F. The interfluvial flood plains were in some places temporarily occupied by swamps. 1970). when all the ice was melted and the warming climate resulted in higher evaporation.G. This may have been the consequence of palaeo-topography. Final Report on Exploration for Coal and Natural Gas. by meltwater production following the Dwyka ice age (Latest Carboniferous–Earliest Permian). it reflects the northeastern end of the MidZambezi basin. representing the southern source area for the Karoo basin (Fig. W. 463 – 472. Federal Institute for Geosciences and Natural Resources (BGR).. . The lithofacies of the various coalfields indicates that its palaeo-shoreline coincided with the southern boundary of the coal-belt (Fig.. that is. ProceedingsGeological Association 81 (3).P. but turns at Wankie towards the NW. which offered him accommodation at Sengwa Mine during June–July 1996. Southwards stretched an approximately 100-km-wide shallow alluvial plain comprised of meandering rivers with accompanying flood plains. The origin of the alluvial plain so far is unknown. where the k5 mudstones overlap on the k1 sequence. firstly. the eustatic rise of the groundwater-table. The manuscript has been greatly improved by the comments from the editor and two reviewers of IJCG. J. The gradational change from the wet forest to the longer-lasting dry forest–swamp and the final cessation of peat formation with the deposition of the k4–fluviodeltaic sandstones is explained mainly by the slow fall of the water level. An essential precondition for the regional paludification phase was. Review First Symposium Gondwana Stratigraphy. BGR supported also the publication substantially. but it was generated most probably by erosion of glaciers flowing downwards from the southeastern highlands during the Dwyka ice age (Bond. pp. Acknowledgements The authors thank Drs. Harare. The senior author is grateful to Rio Tinto of Zimbabwe. Lepper (1992) suggested that the k2–3 succession tapered out towards the west. Unpublished report Rio Tinto (Rhodesia) Ltd. 1967.. the crust subsided by extension to form a sagbasin—again setting the depositional environment for another paludification phase which produced the coal of the lower k5 Madumabisa Formation (Table 1). D. 12). Oesterlen. 1974.M. such as a local shallow floor of the lake shoreline. for discussions and advice. The alluvial plain of the Mid-Zambezi Karoo basin was bounded in the south by the crystalline highlands. The alluvial plain was drained towards the lake in the NW. The southwestern end of the basin at Wankie is less intensely studied. Only from k5 onwards did isostatic controls take over. The Dwyka series in Rhodesia. Bond. 1970. K. Wankie and Lupane districts. T. Broderick. 1981. 1969.118 P. Dahlia and Hankano coalfields.. Thompson. archive Bundesanst Geowiss. 1978. Rhodesia. Thompson.P. 721 pp. Reihe B. A cross-section through the Lusulu coalfield.O. Broderick. Southern Rhodesia. Records of Zimbabwe Coalfields V. Rohstoffe.-H.. 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