OF DIAMONDS, DINOSAURS AND DIASTROPHISM - 150-MILLION YEARS OF LANDSCAPE EVOLUTION IN SOUTHERN AFRICA

The state of knowledge on the tectonic and geomorphological evolution of southern Africa during the post-Gondwana period is reviewed in the context of Alex du Toit's fundamental contributions to these fields of geology. Basic to an understanding of post-rifting events are, firstl y, the high elevation which much of Africa possessed prior to rifting; secondly, the erosion of one to three kilometres from its surface dur ing the Cretaceous; and thirdly, the role of Neogene uplift in re-esta blishing high elevations, particularly within the eastern half of the subcontinent. This history is traced through the massive denudation of the early Cretaceous, which was followed by the establishment of a de nse, integrated drainage net on a well-planed land surface from the Sa ntonian onwards. The configuration of the Upper Cretaceous river syste m is fundamental to a comprehension of the present distribution of all uvial diamonds and of gems transported into the sea via these conduits . Equally significant for an appreciation of the present macro-geomorp hology of southern Africa is the continent-wide planation surface - kn own as the African Surface - generated by the multi-phase cycle of Cre taceous erosion. This surface forms a readily identifiable datum acros s the high plains because of the widespread preservation of deep weath ering and massive cappings of laterite and silcrete on remnants which have survived later dissection. The African silcretes reflect a world- wide shift to greater aridity at the beginning of the Palaeocene. The evidence for large-scale Neogene uplift, particularly within the easte rn half of the subcontinent, is now beyond question and argues for the late development of at least the southern part of the African Supersw ell. The largest movements post-date the Miocene and have contributed both to the anomalous elevations of the eastern hinterland and to the strong east-west climatic gradient across southern Africa. Controversi es surrounding the mechanisms underlying these recent movements appear to have been resolved in favour of buoyancy forces originating from a massive low-density anomaly in the Earth's mantle below East and sout hern Africa.