DURICRUST DEVELOPMENT AND VALLEY EVOLUTION - PROCESS-LANDFORM LINKS IN THE KALAHARI

Duricrusts are an important landscape component of the Kalahari region of central southern Africa. Their exposures within the dry valleys (m ekgacha) of the Kalahari provide some of the most widespread surface o utcrops of the terrestrial Jurassic to Holocene Kalahari Group sedimen ts. Exposures have been extensively used in the construction of lithos tratigraphic sequences, on the assumption that valley systems have inc ised their courses through a pre-existing duricrust sequence. Recent w ork, however, has identified the role of groundwater erosion processes in valley development, which may have influenced duricrust formation. Studies of duricrusts from boreholes drilled within two mekgacha show that duricrust type is intrinsically related to the presence of a val ley. Analyses of calcretes and silcretes in a series of profiles and t hin sections from the Letlhakeng area of Botswana also indicate extens ive alteration and diagenesis in association with former higher water tables. Sedimentary sequences within duricrust host materials can be i dentified but there is no evidence for correlation of duricrust cement s between exposures. Profile studies from the Auob Valley in Namibia, however, suggest that this valley has incised through a sequence of du ricrusts. Caution is advised in future attempts to correlate duricrust types on the basis of valley exposures, with the recommendation that where such exposures are used in a lithostratigraphic context, only du ricrust host material characteristics and not cementing materials shou ld be considered.