Tectono-sedimentary settings and controls of the Karoo Basin Permian coals, South Africa

Abstract

The Karoo Sequence spans an age ranging from Carboniferous to Jurassic and occurs in intra-cratonic and graben-rift basins throughout southern Africa. All of the subcontinent's bituminous, anthracite and coking coal reserves are contained in Early Permian to Triassic Formations of the Karoo Sequence. The most complete assemblage of these upper Paleozoic strata occurs in the Karoo Basin, an asymmetric, epicratonic depository now covering over half of the surface area of the Republic of South Africa. The large aerial extent of these strata belies the potential coal resources as the coals are confined to specific geographic localities, determined by the coal seams confinement to within particular stratigraphic intervals. Two major geographic coal regions are described: (1) the Witbank/Highveld/Eastern Transvaal coalfield in the extreme northern basin areas; (2) the Natal coal fields located southeast of the Witbank region and also more distal to the proximal northern basin coal deposits (Fig. 1). In the Witbank region, coal seams occur in a 80–180-m clastic sequence of the Vryheid Formation. Basal coals overlie post-glacial tillite, glaciofluvial outwash braid-plain conglomerates and sandstones, and minor glaciolacustrine and glaciodeltaic sequences. The lowermost coals often lie directly on pre-Karoo basement with no other intervening strata. The coals overlying these lowermost glaciogenic strata are interbedded with lobate delta and bed-load fluvial sequences. In addition, fine- to coarse-grained anastomosed river deposits are contained within some coal seams and these paleochannel lithologies now locally deleteriously affect coal distribution and quality, i.e., coal beneath paleochannel axes, which were dip-parallel, is thinned due to channel incision and erosion. The coal contains a high percentage of ash immediately above paleochannel axes and up to several kilometers adjacent to paleochannel margins (Cairncross and Cadle, 1988; Cairncross et al., 1989). Paleotopography directly controlled sedimentation patterns in the Witbank coalfield regions. Permo-Carboniferous glaciers scoured valleys into the pre-Karoo basement and the Vryheid Formation sediments and peat initially infilled these paleovalleys. Consequently, coals thin, pinch-out and shale-out against relatively steep-sided valley flanks. Directly above paleotopographic highs, coal and associated strata are attenuated due to compaction, or may be absent due to non-deposition. In the Highveld coalfield, paleotopographic relief was so severe that the total coal-bearing succession is accommodated in paleovalleys, leaving prominent basement highs projecting through the entire sequence (Winter et al., 1987). Paleovalleys were, moreover, the initial sites of clay and silt deposition. These argillaceous sediments infilled depressions and were subjected to differential compaction, thereby providing favorable, low-relief sites for peat accumulation. However, superimposed on these depressions and paleolows, were syn- and post-deposition low-sinuosity, bedload fluvial systems. These originated in the northern granitic source terrain and flowed southward down the paleovalleys, channeling arkosic sediment through the peat swamps. Peat accumulation was therefore interrupted by clastic contamination, channel scouring and erosion. Post-depositional compaction of these paleochannel sandstones was negligible and coal deposited in sequences above paleochannels is thinner than the stratigraphic equivalent coal overlying adjacent fine-grained flood plain sediments. In contrast to the Witbank region, the coal-bearing succession in the Natal coalfields overlies substantial thicknesses of glacial moraine (Dwayka Formation) and distal shelf shale (Pietermaritzburg Formation). The paleotopographic controls evident in the Transvaal coalfields are therefore insignificant and dampened by these thick underlying strata. The fluviodeltaic coals which formed in these areas developed in broad featureless plains and are therefore laterally persistent (Roberts, 1986). Tectonic subsidence was however more rapid in the eastern Natal regions than in the more stable northern platform localities. As a result, the Vryheid Formation is considerably thicker in Natal and in places exceeds 500 m. Greater tectonic instability produced higher geothermal gradients and a general increase in coal rank from the stable cratonic platform in the west to the Natal region in the east is evident. Locally, anthracite has resulted from increased thermal gradients produced by Jurassic dolerite dykes and still intrusions. The southern limit of the Vryheid Formation is defined by the southerly basinward shale-out of the clastic sequence, several hundred kilometers south of the Transvaal coalfields. This distal extent of the coal-bearing succession is, in places, defined by clastic paleoshoreline sequences which fronted the landward positioned fluviodeltaic systems. Breaching of this paleoshoreline did however periodically take place and basin waters transgressed over the swamps terminating peat accumulation and depositing glauconite-rich sediment now preserved as transgressive siltstone and sandstone overlying the coal seams.