Evolution of Sediment Wedge, Walvis Shelf, Southwest Africa

The continental margin off Walvis Bay, between 20° and 26° south latitude, is a broad, smooth platform underlain by at least 3 km of sediment. The shelf edge, at 400 m, is unusually deep. A late Pleistocene shoreline at 110 m depth is located near the present coast and forms the approximate seaward limit of recent sedimentation. Beyond this limit, the shelf and upper continental slope are erosional, exhibiting truncated beds and a broad, deep scour trough along most of the outer shelf. Deep currents between 100 and 400 m parallel to the shelf edge rather than Pleistocene transgressions and regressions appear to be responsible for the erosion. The shelf sediment wedge consists of at least five depositional units, each beginning with a trans-gressive onlap over a preexisting, largely erosional surface, followed by depositional outbuilding and a terminal phase of subsidence and deeper-water deposition. The surfaces which separate the units are similar to the present shelf and upper slope and indicate a persistent regime of recurring deep-current scour. The age of the depositional units is unknown; even the youngest is probably older than late Pleistocene and most of them may be of Tertiary age. The cyclic character of the deposits may result from periodic variations in the rate of subsidence, from a fluctuating sediment supply as a function of climatic changes on land, from periodic variations in the transport and erosion capacity of the Benguela Current system, or from a combination of all three. Deep-current observations and drilling on the shelf are needed to solve these problems.