Laterisation on limestones of the Tertiary Wankoe Formation and its relationship to the African Surface, southern Cape, South Africa

Abstract

The existence of erosion surfaces has long been recognised as a macroscale feature of the southern African landscape. Evidence is presented here to demonstrate that laterisation as a soil process affected the calcareous Tertiary Wankoe Formation of the southern Cape. Remnants of the Tertiary African Surface along the southern Cape Coastal Plateau are characterised by deep weathering mantles capped by duricrusts of laterite and silcrete. The limestones have been assumed to be the coastal equivalent of the inland African Surface. Through field mapping of the study locality discussed below, and through sedimentological and geochemical analysis (X-ray fluorescence spectrometry) of selected samples, it was possible to demonstrate that the laterisation process also affected these older Tertiary limestones. However, the evidence is rarely preserved, and nowhere have complete, intact laterised profiles survived. More often, strong weathering resulted in case-hardening of the topography and the formation of dense calcrete. The implication from this coastal locality is that laterisation as a soil forming process extended from the Mid Tertiary Period until the early Pleistocene Period within this particular sub-region of southern Africa. The significance of this locality within the broader context of the African Surface remnants which occur from the Cape Peninsula in the west to the Knysna area in the east, as well as the palaeoenvironmental significance of laterisation on a substrate which is not conducive to this type of weathering, are also examined.