Stratigraphy and basin modelling of the Gemsbok Sub-Basin (Karoo Supergroup) of Botswana and Namibia

The Gemsbok Sub-basin is situated in the south-western corner of the Kalahari Karoo \nBasin and extends south from the Kgalagadi District of Botswana into the Northern Cape \n(South Africa); and west into the Aranos Basin (southeast Namibia). The Sub-basin \npreserves a heterogeneous succession of Upper Palaeozoic to Lower Mesozoic \nsedimentary and volcanic rocks of the Karoo Supergroup. Because the succession is \nlargely covered by the Cenozoic Kalahari Group, the stratigraphy of the succession is not \nas well understood as the Main Karoo Basin in South Africa. Most research in the \nGemsbok Sub-basin is based on borehole data. This study focuses on the intrabasinal \ncorrelation, depositional environments and provenance of the Karoo Supergroup in the \nGemsbok Sub-basin in Botswana and Namibia. \nBased on detailed sedimentological analyses of 11 borehole cores of the Karoo \nSupergroup in the Gemsbok Sub-basin of Botswana and Namibia, 8 facies associations \n(FAs) comprising 14 lithofacies and 2 trace fossil assemblages (Cruziana and Skolithos \nichnofacies) were identified. The facies associations (FA1 to FA8) correspond to the \nlithostratigraphic subdivisions (the Dwyka Group, Ecca Group, Beaufort equivalent \nGroup, Lebung Group [Mosolotsane and Ntane formations] and Neu Loore Formation) of \nthe Karoo Supergroup. Sedimentological characteristics of the identified facies \nassociations indicate the following depositional environments: glaciomarine or \nglaciolacustrine (FA1, Dwyka Group), deep-water (lake or sea) (FA2, Ecca Group), \nprodelta (FA3, Ecca Group), delta front (FA4, Ecca Group), delta plain (FA5, Ecca \nGroup), floodplain (probably shallow lakes) (FA6, Beaufort Group equivalent), fluvial \n(FA7, Mosolotsane and Neu Loore formations) and aeolian (FA8, Ntane Sandstone \nFormation). \nThe Dwyka Group (FA1) forms the base of the Karoo Supergroup in the Gemsbok Subbasin \nand overlain by the Ecca Group deposits. Three types of deltas exist within the \nEcca Group: fluvial-dominated; fluvial-wave interaction and wave-dominated deltas. The \nGemsbok Sub-basin was characterised by rapid uplift and subsidence and high sediment influx during the deposition of the Ecca Group. Petrographic and geochemical analyses of \nthe Ecca Group sandstones revealed immature arkose and subarkose type sandstones \ndominated by angular to subangular detrital grains, sourced from transitional continental \nand basement uplifted source areas. The sandstones of Ntane Sandstone Formation are \nclassified as subarkoses and sourced from the craton interior provenances.