FLUVIAL FAN DEPOSITIONAL MODEL FOR MIDDLE PROTEROZOIC RED BEDS FROM THE WATERBERG GROUP, SOUTH-AFRICA

The Waterberg Group in South Africa is one of the oldest known red-bed occurrences. These clastic sediments were derived from the Transvaal Supergroup and the acidic and mafic phases of the Bushveld Complex. Sedimentation occurred mainly on proximal to distal reaches of braided alluvial plains under conditions of lateral switching of the active river system associated with prolonged vertical aggradation in a gently subsiding intracratonic depository. Intercalated quartz arenites developed as a result of weak tidal and wave-swash reworking of the fluvial sediments. The red pigmentation, considered to be of authigenic-diagenetic origin, is confined to the immature fluvial sediments and resulted from the liberation of iron from orthoclase and pyroxene grains and felsite rock fragments, followed by its reprecipitation under suitable oxidizing, alkaline conditions. This study has provided evidence that an oxidizing atmosphere existed 1800 Ma ago. © 1979.