Paleodepositional environments and tectonosedimentary controls of the postglacial Permian coals, Karoo Basin, South Africa

Coal deposits of the Karoo Basin in South Africa are contained within an 80-250-m-thick clastic sequence designated the Vryheid Formation. This formation constitutes part of the Ecca Group of the Karoo Sequence and contains several mineable bituminuos coal and anthracite beds and sporadic coking coal seams. The sedimentary succession was deposited during the Early Permian following the Late Carboniferous Dwyka ice age. Paleoclimate initially influenced the paleodepositional environments which for the lowermost stages, comprised glaciofluvinal outwash braided-plain deposits overlying minor deltaic and glaciolacustrine fill. The Early Permian floral assemblages attest to a cool, temperature climatic regime. In the extreme northern parts of the basin, the lowermost coals frequently directly overlie Dwyka diamictite or pre-Karoo basement. Paleodepositional systems associated with the coals above the glaciogenic succession consisted predominantly of regressive fluviodeltaic facies assemblages. In the northern regions, regressive deltaic lobes prograded rapidly basinward across the relatively shallow intracratonic platform. In the northeast areas, greater subsidence rates were accompanied by multiple stacking of regressive deltaic cycles. In several areas of the coalfields, peat accumulation was associated with the bed-load (braided) fluvial systems. These subsequently modified the underlying deltaic deposits by intense and deep erosion that removed portions of the underlying stratigraphy. Smaller-scale, syndepositional anastomosed rivers, which were contemporaneous with certain peat swamps, locally controlled seam distribution and adversely affected coal-seam quality. Coal distribution was directly controlled: (a) the tectonic setting; (b) the nature of the pre-Karoo basement lithologies; (c) the basement paleotopography; and (d) the paleodepositional systems associated with and following peat formation. The coalfields are restricted to a relatively small area in the northern and northeastern parts of the Karoo Basin where a stable intracratonic shelf provided a suitable tectonic setting for peat accumulation. Resistant basement lithologies (orthoquartzite and felzite) produced positive topographic relief, while easily weatehred. Archean granite basement terrain left residual topographic depressions that were later extensively filled by sediment. Differential compaction of fine sediment (mud and silt) deposited over the diverse basement topography provided further subtle controls on coal-seam thickness. Sediments and coal overlying basement highs compacted significantly causing thinning and a draping effect of the sedimentary cover. In contrast, paleovalleys contained substantial thickness of sediment and host some of the thickest seams. These coals can, however, thin and shale-out against the adjacent elevated valley flanks. The paleovalley coals were further affected by multiple seam splits produced by fluvial channels that contemporaneously occupied the lowland areas.