Degradation of linear dunes in Northwest Ngamiland, Botswana and the implications for luminescence dating of periods of aridity

The long morphodynamic history of the linear dune system of northwest Ngamiland comprises both construction and degradation processes. Here we describe features resulting from the latter. Crest lowering during wet periods is associated with lateral transfer of sand into the straws. Weathering and leaching have mobilised iron to the base of the dune profiles and have corroded the quartz grains. Satellite image interpretation indicates that dune sand has been diluted by bioturbation which transferred Damaran saprolite from below the dunes to the surface. Flooding and waterlogging have modified and in places completely destroyed the original crisply linear morphology. Small linear collapse features across the linear dunes are associated with open fractures or faults where water infiltration is facilitated. The implications for luminescence dating of dune sands, to identify periods of aridity, are discussed. The dunes cannot simply be regarded as providing a stacked record of such periods. They are very old and much of the record is missing as a result or crest lowering. The chronostratigraphy includes degradational components which must be distinguished from those of construction. Bioturbation cannot be disregarded as a complication which overprints the luminescence signals. (c) 2004 Elsevier Ltd and INQUA. All rights reserved.