Die Geologie des südlichen Rand des Warmbad Beckens - Tephrostratigraphie, Alter, Fossilien und Ablagerungsraum der karbon-permischen glazigenen Ablagerungen der Dwyka Gruppe, Zwartbas, südliches Namibia | The Geology of the southern Warmbad Basin Margin - Tephrostratigraphy, Age, Fossil Record and Sedimentary Environment of Carboniferous-Permian Glacigenic Deposits of the Dwyka Group, Zwartbas, southern Namibia

At Zwartbas, about 10 km west of Vioolsdrif, southern Namibia, the Dwyka succession is composed of tillites and distal fossiliferous dropstone-bearing glacio-marine shales. The completely exposed Dwyka succession is interbedded with thin bentonites, altered distal pyroclastic deposits, which werederived from the magmatic arc at the southern rim of Gondwana. Dropstone-bearing and dropstonefree sequences intercalate with four diamictites, of which the two lowest were certainly recognised as tillites. Four events of deglaciation were proven at Zwartbas and thus consist with correlative depositsin southern Africa.Numerous fossilised fishes, trace fossils, and plant fragments appear frequently within the lower half of the Dwyka succession whereas trace fossils were principally found in the complete succession.Although the environmental determination is quite problematic, the fossil assemblage rather implies proximal, shallow water conditions with temporary restricted oxygenation. The hinterland was covered with considerable vegetation, which points to a moderate climate. Water salinity determinations based on shale geochemistry rectify contrary palaeontological results and point to rather brackish or non-marine conditions in comparison to present-day salinites.Geochemical analyses of the bentonites relate the pyroclastic deposits with acid to intermediate source magmas, as they are known from the magmatic arc in present-day Patagonia. Tectono-magmatic comparisons furthermore emphasise a syn-collision or volcanic-arc situation of the magma source.However, significant cyclicity in the production of the pyroclastic deposits was not observed.Radiometric age determinations of two tuff beds clearly date the onset of glacial activity into the Late Carboniferous.