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

The existence of erosion surfaces has long been recognised as a macroscale feature of the southern African landscape. Evidence is presented here to de monstrate that laterisation as a soil process affected the calcareous Terti ary 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 limes tones have been assumed to be the coastal equivalent of the inland African Surface. Through field mapping of the study locality discussed below, and t hrough sedimentological and geochemical analysis (X-ray fluorescence spectr ometry) of selected samples, it was possible to demonstrate that the lateri sation process also affected these older Tertiary Limestones. However, the evidence is rarely preserved, and nowhere have complete, intact laterised p rofiles 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 exten ded from the Mid Tertiary Period until the early Pleistocene Period within this particular sub-region of southern Africa. The significance of this loc ality within the broader context of the African Surface remnants which occu r from the Cape Peninsula in the west to the Knysna area in the east, as we ll as the palaeoenvironmental significance of laterisation on a substrate w hich is not conducive to this type of weathering, are also examined. (C) 19 99 Elsevier Science B.V. All rights reserved.