Geomorphic evolution of Southern Africa since the Mesozoic

Controversies regarding the correlation of erosion cycles within southern Africa are traced through some earlier contributions; many of these reflect a failure to apply rigorous field criteria to problems of mapping. The analysis of sections linking key coastal deposits to planation remnants in the interior is used to extrapolate field observations over wider areas. The resulting map of erosion surfaces, and contour plans of elevations on the more important of these, form the basis of the paper. A consistent datum is provided by the oldest of the surfaces (the African) through the diagnostic deep weathering profiles and duricrust cappings associated with it. The Great Escarpment was formed following continental rifting, and its survival up to the present as a major topographic feature is, in large measure, a function of the high elevations consequent upon the central position of southern Africa in Gondwanaland prior to this event. There is conclusive evidence for the co-existence of surfaces of the same age at different elevations inland of and below the Great Escarpment. This vertical separation can be ascribed to the existence, from early times, of separate base-levels for the plateau and coastal areas. The onshore evidence of erosion surfaces is correlated with recent data on offshore sedimentation, and reveals that a single cycle of erosion (interrupted by minor tectonic interludes) prevailed from the time of rifting to the early Miocene. By the end of this period a gentle pediplain (the African surface) extended across most of southern Africa at elevations of 500-600 m. Most erosion and scarp recession occurred during the earlier part of this interval and produced thick late Jurassic and Cretaceous sedimentary sequences, but shelf sedimentation declined during the Tertiary and had virtually ceased by the Oligocene, when interior planation had advanced to a stage where sediment supply to most rivers was minimal. Modest renewed uplift of 150-300 m in the Miocene tilted the continent slightly to the west and initiated a new (Post-African J) landscape cycle. This was accompanied by renewed offshore sedimentation, although at lower rates than during the Cretaceous. The cycle was terminated near the end of the Pliocene, and its relatively short duration resulted in imperfect planation in most areas to levels of no more than 100-300 m below the African surface. A second uplift of major proportions at the end of the Pliocene raised the eastern interior of the sub-continent by as much as 900 m, although much smaller movements characterized the western areas and interior axes of uplift. Major monoclinal warping resulted in the southeastern hinterland. The ensuing Post-African II cycle is manifested chiefly in deep incision of the coastal hinterland and downcutting along major rivers of the interior. Earlier surfaces were severely deformed and dissected, especially in the south-east of the country. Planation was limited to a few areas close to the coast. The resulting sedimentation is evident mainly in the offshore deltas of major rivers owing to the late development of vigorous coastal currents, but a major increase is reflected throughout the deeper ocean basins. The successive uplifts to which the continent was subject produced major changes in drainage patterns, although the evidence for pre-Miocene distributions is tenuous in many areas. The preservation of discrete erosion surfaces with separating escarpments over considerable periods not only confirms the episodic nature of these uplifts, but indicates a dominance of backwearing over down wearing in the geomorphic evolution of the sub-continent. The mechanisms controlling the uplift of southern Africa and similar passive continental margins are of considerable importance in the light of an increasing awareness that many elevated areas within the continents can be ascribed to such movements. Some thoughts are offered towards the formulation of a tectonic model which takes account of the evolutionary scheme now established for southern Africa.