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Evidence for the evolution of the Kalahari dunes from the Auob River, southeastern Namibia

The 45-m deep Auob River valley occurs within an end-Kalahari erg consisting of unconsolidated aeolian sheet sands overlain by southeast-orientated linear dunes. The depth of the valley decreases to 15 m downstream. Younger gravel terraces occur within the valley at 5 m and 2 m above the present river level. Fluvial, pre-dune, calcrete-cemented Kalahari sediments form exposed lateral terraces on either side of the valley. These terminate in vertical cliffs up to 25 m high along the top of the valley margins. Numerous embayments give the cliffs jagged outlines. Valley incision post-dates the linear dunes. Upstream there are no accumulations of aeolian sand within the jagged terrace edges but downstream dunes and aeolian sheet sands cover the calcrete cliffs of the windward margin of the river valley, indicating a progressive increase in the intensity of aeolian reworking towards the southeast subsequent to incision of the Auob River valley. Regional geological, palaeontological, marine and lacustrine evidence and palaeo-climatological proxies suggest that the Kalahari dune system formed during periods of extreme aridity in the Plio-Pleistocene. Acheulean and Middle Stone Age implements record occupation of the Auob valley after 40 m of post-dune incision by the river. Luminescence ages of 186 ka and younger for the dunes largely represent aeolian reworking. This reworking becomes more obvious downstream towards the South African border.