The lithostratigraphy of the Riverton Formation, South Africa

The Riverton Formation occurs intermittently along the banks of the lower Harts, Vaal, Riet and middle Orange Rivers in the Northern Cape Province. These alluvial terrace deposits represent a complex history of sedimentary sequences that are stacked upon older deposits with crosscutting or onlapping relationships and display varying degrees of pedogenic overprinting. These cut-and-fill alluvial and colluvial sands and silts include subordinate interbedded gravel layers and lenses and buried palaeosols with vertic pedogenic characteristics. The deposits occur lower than 17 m above the present river bed or in erosional gullies/dongas up to 20 m above the present river channel, far below the older and higher Wedburg and Proksch Koppie terraces. The formation reaches a maximum thickness of 37 m at the Riverton type area southeast of Barkly West. Middle and Late Stone Age artefacts indicate an age range from the Late Pleistocene to Holocene. Alternating alluvial deposition and minor incision with pedogenesis of the Riverton Formation indicates complex environmental fluctuations over a long time period after deposition of the older, underlying Rietputs Formation gravels. This period of aggradation within alluvial channels and floodplains indicates a general reduction in catchment discharge of rivers in the summer rainfall region and conforms to the progressive drying of the southern African climate similar to 1.2 Ma (Gibbon, 2009).