The palaeogeography of part of southwestern Gondwana during the Permo-Carboniferous glaciation
Abstract The elevation of the base of the glacial deposits, areas where glaciated bedrock is overlapped by post-glacial sediments, palaeoice-flow directions, thickness of the glacial sequence and the lithology (lithofacies and clast composition) were used as integrated parameters to reconstruct the Permo-Carboniferous glaciated land surface over southern Africa. The palaeography is depicted by pre-glaciation (Westphalian?), maximum glaciation (end of Carboniferous) and post-glaciation (Sakmarian) maps. The pre-glaciation palaeogeography was dominated by a fluvially dissected highland surrounded in the south and southwest by flat-lying lower Palaeozoic marine sediments. This surrounding platform, in part covered by shallow seas and in the south and southwest bounded by a possible island arc system, was the main depocentre for the glacial debris. The palaeogeography during maximum glaciation consisted of a platform region (Karoo basin), overdeepened troughs (e.g. Kalahari basin), highlands (Windhoek, Cargonian and Southern) with extensive glacial valleys incised into these highlands, and a shallow palaeo-Pacific sea arm reaching into the interior. The alpine-type Southern highlands formed by late subduction from the south. The post-glaciation palaeogeography was controlled by isostatic crustal movements (ratio of crustal depression by the weight of the ice and sediments to post-glacial rebound) and sea-level rises. Large areas, including major valleys, were inundated by shallow seas (Irati and Whitehill seas and fjords) so that the land surfaces represent remnants, including islands, of the former highlands. Local high rebound rates caused erosion of the glacial sediments and the deposition of fluvial and deltaic beds in fjord heads and embayments. Favourable conditions for the formation of fossil fuels and base mineral and metal deposits occured in parts of the inundatedbasins.