The geomorphology of the Coastal Platform in the southern Cape

Abstract

The Coastal Platform is the dominant geomorphic macro-feature of the southern Cape south of the Cape Fold Belt. The southern Cape coast has served, on-shore as well as off-shore, as an important sediment sink, since the mid-Cretaceous, and into and throughout the Quaternary. An understanding of the geomorphic evolution of the Coastal Platform since the Cretaceous provides a context for the subsequent Neogene and Quaternary geomorphic palaeoenvironments of the region. Existing evidence for the evolution of the Coastal Platform is reviewed and synthesised, and new evidence supporting the polycyclic nature of the surface is introduced. The Coastal Platform was planed under hot, humid conditions during the Cretaceous, following the break-up of Gondwana. Simultaneously, the Great Escarpment was eroded inland. This resulted in a much modified African Surface which manifests itself as the Coastal Platform extending from Bot River in the west to Port Elizabeth in the east. Evidence based on detailed map analysis is presented to show that the Coastal Platform is polycyclic. Tabular remnants of the African Surface are preserved on the Coastal Platform by laterite or silcrete duricrusts overlying variable depths of saprolite. At least four discrete surfaces can be recognised in the Albertinia–Mossel Bay area, while in the Sedgefield–Knysna area the surface is bimodal. The relationship between the Coastal Platform and two important landscape components of the region, the Bredasdorp Group limestones and the Knysna coversands, is also considered. Finally, the tectonic stability of the Coastal Platform's Cretaceous surface is questioned. Evidence is drawn from zeta bays developed over half-grabens, down-faulted to the west along northwest–southeast trending faults, and from evidence of displacement in the Cape Peninsula, as well as from stripped basal surfaces of differing altitudes. This evidence demonstrates post-Cretaceous lateral and vertical displacement on the Coastal Platform.