THE NEOPROTEROZOIC OF SOUTHWESTERN AFRICA, WITH EMPHASIS ON PLATFORM STRATIGRAPHY AND PALEONTOLOGY

Neoproterozoic sediments in southwestern Africa occur in the Pan-African Kaoko, Khomas, Gariep, Vanrhynsdorp and Saldania Belts. They were deposited in response to rifting, break-up of a supercontinent (formed at similar to 1000 Ma) and subduction and collisions of the Congo, Kalahari, Rio de la Plata and Malvinas plates. These collisions led to the formation of a Neoproterozoic/Cambrian supercontinent. This paper deals with the geology of the platform successions of the Kaoko, Khomas, Gariep and Vanrhynsdorp Belts. Deposition of the sediments occurring in these belts commenced similar to 900 to 1000 Ma ago and probably ended between 500 and 520 Ma. The sediments record Wilson cycles that ended with the deposition of red beds. Palaeontological and carbon isotope studies on the Damara Sequence, Gariep Complex, Nama Group and Vanrhynsdorp Group make intra- and inter-basinal correlations possible. Prior to deposition of the Nama Group two, or perhaps three, glaciations took place over an extensive area in southwestern Africa, these being the Kaigas (Chuos) (similar to 725-700 Ma), Numees (similar to 650-600 Ma or similar to 610-590 Ma) and possibly the Varianto (similar to 745-725 Ma) glaciations. The Numees tillite is overlain by a stratigraphic unit containing pinkish dolomite. During deposition of the Nama Group, the minor Vingerbreek (similar to 600-545 Ma) ''glaciation'' (or cold-waterperiod) occurred. The major Kaigas (Chuos) and Numees and minor Vingerbreek glaciations correspond with low delta(13)C isotope ratios and appear to have taken place at relatively low palaeolatitudes. The Riphean sediments were deposited predominantly during rifting and break-up of a supercontinent. During the late Vendian to early Cambrian, deposition of the Nama Group in southern Namibia and the penecontemporaneous Vanrhynsdorp Group in the Republic of South Africa took place in the Nama foreland basin. Detrital micas indicate that deposition of the Nama Group took place between 635 and 530 Ma. The Nama foreland basin, which was connected to a major ocean to the south, formed as a result of collisions between the Congo, Kalahari and Rio de la Plata plates. With the exception of the Nama Group, little is known about the palaeontology of the Neoproterozoic in southwestern Africa. The lower Nama Group is latest Proterozoic in age. It contains an Ediacaran fauna, a depauperate assemblage of organic-walled microfossils and a relatively well diversified ichnofauna with Brooksella, Diplocraterion and Nereites which are atypical of Vendian strata. Many trace fossils are exceptionally small. The occurrence of the shelly fossil Cloudina and probable calcified metaphytes in the lower Nama Group suggest that skeletalization in animals and metaphytes occurred almost simultaneously towards the end of the Proterozoic. The upper Nama Group is early Cambrian in age as indicated by the occurrence of the trace fossil Phycodes pedum. The upper Nama Group, dominated by red beds, contains a low-diversity ichnofauna with trace fossils of normal size. Bio- and chemostratigraphy combined with sequence stratigraphy will probably establish how the prominent unconformities, which occur in the Neoproterozoic of southwestern Africa, are related to the periods of formation of the Wonoka Canyons in south Australia.