Siliceous duricrusts as palaeoclimatic indicators: evidence from the Kalahari desert of Botswana

Abstract

Previous attempts to identify the environmental conditions associated with the development of siliceous duricrusts have used differences in bulk chemical compositions and cement mineralogy amongst other criteria to distinguish between samples. High (up to 3.00%) levels of TiO 2, together with petrographic characteristics, have been used as an indicator of past wetter climatic conditions, often failing to fully emphasise the potential importance of local controls upon micromorphology and chemistry. In southern Africa, silcretes from the Cape Coastal zone associated with deep-weathering profiles have been attributed to formation under wetter conditions whereas Kalahari silcretes are suggested to have formed under more arid, alkaline conditions. Petrographic and geochemical analyses of silcretes from five locations within the Kalahari Desert of Botswana (one site associated with deeply-weathered bedrock) indicate the great micromorphological complexity of Kalahari silcretes. It is suggested that variations in the host material composition of Kalahari silcretes are an important control of their chemistry. Silcretes have developed from unconsolidated Kalahari Sand with variations in the mineral content of the host sediment appearing to cause variation in silcrete geochemistry. Geochemical analysis confirms that TiO 2 provides the main variation between Kalahari and Cape Coastal silcretes but the low levels of TiO 2 in Kalahari samples are attributed to differences in host material as opposed to overriding climatic controls.