THE LITHOLOGICAL CHARACTERIZATION OF A MIDDLE PERMIAN DELTAIC DISTRIBUTARY CHANNEL, IN THE KAROO BASIN, SOUTH AFRICA

The Waterford Formation is a Middle Permian unit considered to be transitional between open water deposits of the Ecca Group and fully terrestrial deposits known as the Beaufort Group. It is a deltaic sequence that formed in response to Gondwanan deglaciation. The exposure occurs near Sutherland, Northern Cape Province, South Africa (S32° 37.6’ E20° 21.5’) as a river cut and can be traced approximately 250 m along strike. The present study focuses on lithofacies characterization, sediment-body geometries and architectural elements, and thin section analysis to interpret the depositional history of the channel.

Channel deposits up to 28 m thick eroded into an underlying greenish-gray siltstone facies in which ball-and-pillow structures are common. The channel is subdivided into 6 lithofacies that represent an overall fining upwards sequence. These include; (1) fine to coarse grained, poorly sorted wacke; (2) silty sandstone that fines upwards to sandy siltstone in sheet geometries; (3) very fine, poorly sorted massive sandstone; (4) ripple laminated fine sandstone interbedded with siltstone drapes; (5) coarse siltstone with mm-scale sandstone stringers; and (6) fine, sub-rounded to sub-angular, poorly sorted lenticular sandstone.

At least 2 sedimentary cycles can be identified. Each begins with a sheet sandstone, representing high discharge events, that are overlain by ripple laminated, trough cross bedded, interbedded sandstone and siltstone. This latter sequence can be up to 8 m thick and represents shallow water, low energy deposition. The rippled intervals preserve horizontal trace fossils and the channel fill is capped by a coarse siltstone. This channel sequence provides insight into the fluvio-deltaic processes that operated in the southern Karoo Basin in response to climate change and continental deglaciation.