The geomorphologic evolution of the Wilderness dune cordons, South Africa

The Wilderness dune cordons consist of steep-sided ridges up to 207 m high separated by coastal lakes. They probably formed during the sea-level highstands of the Pleistocene interglacials. Three major dune cordons are developed. There is an additional huge coalesced mass of older fossil dunes landward of these cordons, forming hills up to 340 m high, in places lying on the coastal platform (150-260 m generally; locally 210-260 m). The dunesand reaches thicknesses in excess of 300 m. The cordons form by accretion of transgressive dunefields and large compound imbricate parabolic dunes. An individual cordon can result from numerous phases of dune formation during periods of sea-level highstand. The dunes of the present interglacial are mostly accreting on top of the seaward cordon which was initiated during the last interglacial. During the complex sea-level history there have been numerous phases of dune-building followed by coastal erosion that formed seacliffs in the dune cordons; in some areas almost all the cordon was subsequently eroded away. These fossil seacliffs form large, steep seaward-facing slopes parallel to the present coastline and form a major element in the morphology of the cordons. Trailing arms of parabolic dunes form another major topographic element, oriented at 15-30 degrees to the shoreline. River erosion has altered the cordon morphology in places. The geomorphology of the cordons is thus a function primarily of dune-cordon construction accentuated by coastal erosion in the form of cliff-cutting, secondarily of dune morphology, and thirdly of fluvial erosion. The evolution of the cordons can be dated tentatively using palaeontologic, stratigraphic and sediment dynamic data. The seaward cordon formed during the Eem and Holocene (a fairly reliable dating), the middle cordon probably formed between 200-500 ka, and the landward cordon probably formed between 600-900 ka. The coalesced dune deposits lie inland of the cordons and are thus older. This would place the minimum age of the younger coalesced dune deposits in the Middle Pleistocene, about 1000 ka. Early Miocene fossil plant remains in the Knysna Lignites which underlie the older coalesced dunes indicate their maximum possible age. Westerly winds dominated in the area for the past few million years. Copyright (C) 1996 INQUA/Elsevier Science Ltd