Basin analysis of the Soutpansberg and Tuli coalfields, Limpopo province of South Africa

Abstract

The Soutpansberg and Tuli Coalfields are both hosted in the Karoo Basin, Limpopo Province of South Africa. The Soutpansberg Coalfield is situated north of the Soutpansberg Mountain Range and has a strike length of about 200 km and width of about 80 km which is fault controlled and extends from Waterpoort in the west to the Kruger National Park in the east. The Tuli Coalfield occurs in a small intracratonic, east-west trending fault-controlled sedimentary basin with a preserved width of 80 km and length of 120 km. The east to west trend of the Tuli Coalfield parallels that of the Soutpansberg Coalfield further east, and the two coalfields link up with the north-south trending Lebombo Basin. The Tuli Coalfield occurs in the Tuli Basin, while the Soutpansberg Coalfield occurs in the Soutpansberg Basin. The two basins preserve a heterogeneous succession of the Upper Paleozoic to Lower Mesozoic sedimentary and volcanic rocks of the Karoo Supergroup. Because the area is largely covered by the Quaternary Kalahari Group sands, the stratigraphy of the succession is not as well understood as the Main Karoo Basin in South Africa. This study deals with the intra-basinal stratigraphic correlation, facies and depositional environments, petrography, geochemistry, provenance, geophysics, structural geology, diagenesis of sandstone, subsidence history and coal quality in the Soutpansberg and Tuli Coalfields. Based on the field work and detailed sedimentological analyses of over 2000 borehole data, seven facies associations (FAs) comprising sixteen major lithofacies were identified. The facies associations are: Glacial diamictite and sandstone (FA 1), Clast supported conglomerate and sandstone (FA 2), Tabular cross-bedded sandstone (FA 3), Trough and planar cross-bedded sandstone (FA 4), Fine calcareous and micaceous siltstone and mudstone (FA 5), Sandy shale/mudstone (FA 6), Laminated or thin-bedded Carbonaceous shale/mudstone and coal (FA 7). The facies associations (FA 1 to FA 7) correspond to the lithostratigraphic sub-divisions of the Tshidzi, Madzaringwe and Mikambeni Formations. The Madzaringwe Formation in this study is informally sub-divided into the Lower, Middle and Upper Members while the Mikambeni Formation is informally sub-divided into the Lower and Upper Members. Sedimentological characteristics of the identified facies associations indicate the following depositional environments: Fluvioglacial (braided streams) depositional environment (FA 1, Tshidzi Diamictite Formation); Floodplain ponds, lakes, marshes and backswamps (FA 6 and FA 7, Lower Member of the Madzaringwe Formation); Meandering and braided channels, crevasse splays, levees and crevasse channels (FA 2, FA 3, FA 4 and FA 5, Middle Member of the Madzaringwe Formation); Floodplain ponds, lakes and backswamps (FA 6 and FA 7, Upper Member of the Madzaringwe Formation); Meandering and braided channels, crevasse splays, swamps and shallow lakes (FA 5, FA 6 and FA 7, Lower Member of the Mikambeni Formation) and lastly braided channels, meandering channels, levees and crevasse channels (FA 2, FA 3, FA 4 and FA 5, Upper Member of the Mikambeni Formation). Paleocurrent directions were measured using directional structures (cross-bedding and asymmetric ripple marks). The paleocurrent analysis shows that the direction of the channels was from south-west to north-east in both coalfields. Based on the structural study and geophysical interpretations, the structural and tectonic settings of the two coalfields have been revealed, both coalfields are normal fault-bounded. The geological evolution of the Karoo strata, at least since the Upper Carboniferous, essentially follows the type model for passive continental margin terrain. Paleostress inversion techniques have been employed to interpret the stress regime of the two coalfields. The Soutpansberg Basin is characterised by W-E to ENE-WSW extension and N-S to NNW-SSE compression. The Tuli Basin is characterised by N-S to NNW-SSE compression and W-E to ENE-WSE extension. This stress field reflects the established structural trend of the two shear belts (the Tshipise and Siloam shear zones) bounding the Central Zone of the Limpopo Mobile Belt. The geophysical interpretations were focused on outlining structures, contacts and on the delineation of gravity, magnetic and radiometric signatures in areas defined as anomalous. The magnetic, gravity and radiometric data showed low amplitudes in the sedimentary strata compared to the surrounding and basement geological bodies. The E-N-E fault system has a notable signature, defining two magnetic domains on both southern and northern sides of the Soutpansberg Coalfield. The intrusive emplacements are mainly fault controlled and they trend in the same direction as the two fault systems. Jurassic volcanics (Letaba and Jozini Formations) follow a SW-NE trend, outcropping in the east (Soutpansberg Basin), producing a strong magnetic response in this area, and partly buried in the west, where magnetic intensity tends to be reduced. Petrographic and geochemical analyses of the Soutpansberg sandstones revealed immature sub-litharenite, sub-arkose and minor arkosic arenites in nature, dominated by sub-angular to rounded detrital grains, sourced from recycled orogens, craton interior to transitional continental. The sandstones of the Tuli Coalfield are classified as sub-arkoses and minor sub-litharenites and sourced from the craton interior and recycled orogen provenances. Both petrographic and geochemical results suggest a passive continental margin source. Petrographic and geochemical results of the samples of the Soutpansberg Coalfield suggest uplifted basement source areas dominated by sedimentary rocks with minor granite-gneiss rocks. The petrography and geochemistry of the Tuli sandstones suggest source areas dominantly composed of plutonic (granites) and metamorphic (gneisses and schists) rocks with a component from a sedimentary (quartz-arenites, quartzites, shales, arkoses and meta-arkoses) rocks. Diagenetic features of Mikambeni and Madzaringwe sandstones are subdivided into early, middle and late stages. Time is relative with the earliest diagenetic event occurring shortly after deposition and the latest occurring up until present time. The main diagenetic processes that have affected the sandstones include mechanical compaction, cementation and the dissolution of framework grains and cements. Early diagenetic processes include mechanical compaction, silica and calcite cementation, clay minerals (pore lining and pore-filling kaolinite, illite and smectite), feldspar authigenesis and the formation of hematite cements and coatings. Late diagenesis includes quartz and feldspar overgrowths, seritisation, chlorite alteration, grain deformation, pressure-solution and fracturing and albitisation. The subsidence of the basins is believed to be initiated and thermally controlled by tectonics (i.e. faults of basements blocks) rather than sedimentary burial. The subsidence within the basins supports the primary graben system which must have been centered within the present basins, and later became a region of major faulting. This gave way to the Late Carboniferous rapid subsidence, with areas of greater extension subsiding more rapidly. The Early Permian (last phase) is characterised by a slow subsidence representing the post-rift thermal subsidence. The rift flanks were gradually uplifted and, and then generally subsided as a results of thermal contraction after the extension terminated. Based on the coal analysis, both coalfields are characterised by coking bituminous coal. The study has revealed that the eastern Soutpansberg Coalfield is likely to present better opportunities for identification of potentially exploitable coal deposits as compared to the Tuli Coalfield.