Reassessing southern African silcrete geochemistry: Implications for silcrete origin and sourcing of silcrete artefacts

A synthesis of the geochemistry of silcretes and their host sediments in the Kalahari Desert and Cape coastal zone, using isocon comparisons, shows that silcretes in the two regions are very different. Kalahari Desert silcretes outcrop along drainage-lines and within pans, and formed by groundwater silicification of near-surface Kalahari Group sands. Silicification was approximately isovolumetric. Few elements were lost; silicon (Si) and potassium (K) were gained as microquartz precipitated in the sediment porosity and glauconite formed in the sub-oxic groundwater conditions. The low titanium (Ti) content reflects the composition of the host sands. Additional elements in the Kalahari Desert silcretes were supplied in river water and derived from weathering of silicates in basement rocks. Evaporation under an arid climate produced high-pH groundwater that mobilized and precipitated Si; this process is still occurring. In the Cape coastal zone, pedogenic silcretes cap hills and plateaus, overlying deeply weathered argillaceous bedrock. Silicification resulted from intensive weathering that destroyed the bedrock silicates, almost completely removing most elements and causing a substantial volume decrease. Some of the silica released formed a microcrystalline quartz matrix, and most Ti precipitated as anatase, so the Cape silcretes contain relatively high Ti levels. The intense weathering that formed the Cape silcretes could have occurred in the Eocene, during and after the Palaeocene-Eocene Thermal Maximum, when more acidic rainfall and high temperatures resulted in intensified silicate weathering worldwide. This could have been responsible for widespread formation of pedogenic silcretes elsewhere in Africa and around the globe. Trace element sourcing of silcrete artefacts to particular outcrops has most potential in the Cape, where differences between separate bedrock areas are reflected in the silcrete composition. In the Kalahari Desert, gains of some elements can override compositional differences of the parent material, and sourcing should be based on elements that show the least change during silicification.