Tuffs, tectonism and glacially related sea-level changes, Carboniferous-Permian, southern Namibia

The Carboniferous-Permian glacigene Dwyka Group in southern Africa has been widely regarded as amagmatic. Only in the higher parts of the Permian and subsequent younger strata has evidence of magmatism been recognised, partic ularly in the form of pyroclastic fallout ash beds. The source of these tuf fs has been suggested to be Patagonia and West Antarctica, forming part of an extensive volcanic are which lay some 1500 to 2000 km to the south and w est in their pre-break-up Gondwana positions. Rhyolitic/dacitic and basalti c/andesitic tuff beds within the glaciomarine sediments of the Dwyka Group in Namibia reveal new evidence for an early onset of proximal bimodal volca nic activity in southern Africa. Contemporaneous tectonism is recorded by t ype-1 unconformities and systematic thickness changes across NW-SE trending extensional normal faults. We suggest that this tectonomagmatic period mar ks initial extensional events in southern Namibia and the Carboniferous-Per mian volcanic trend appears to coincide with the position of the eventual N amibian continental margin. Significantly, the stratigraphic positions of t he tuff beds show a distinct association with enclosing marine transgressiv e depositional sequences. Most of them coincide with sharply defined floodi ng surfaces within relatively thick shaly successions, reflecting their mul tiple transgressive nature. Widening the study context to the latest Early Permian in southern Africa, the main Karoo Basin of southern Africa exhibit s a similar coincidence of marine transgressions and extrusion of volcanics . The relationship between tuff beds and transgressive depositional sequenc es is not only the result of enhanced preservational potential during trans gression, but of a genetic coupling between magmatism, extensional tectonic s and basin subsidence, the latter accelerating a rise in relative sea-leve l. This interaction is significant, not only for understanding potential co ntrols on relative sealevel change, but also for understanding the early ge odynamic evolution of the southern South Atlantic continental margin. (C) 2 000 Elsevier Science B.V. All rights reserved.