Drainage-line silcretes of the Middle Kalahari: an analogue for Cenozoic sarsen trains?

This paper describes models put forward to explain the development of silcretes within drainage lines at the distal end of the Okavango Delta system in the Middle Kalahari of Botswana, and proposes that they provide an analogue for the formation of sarsen stones within sarsen trains. The models describe silcrete formation in shallow pans situated within river valleys, with silicification resulting from the accumulation of fluvial inputs of clastic material, silica from groundwater and additions of silica phytoliths from aquatic vegetation in seasonal pools. It is suggested, on the basis of macro- and micromorphological comparisons, that sarsens in the Clatford Bottom area of Wiltshire formed by this mechanism. The sarsens would have originally formed a spatially-limited linear silcrete body and would have then accumulated within contemporary valleys during the course of landscape evolution. The implications of this model for the environmental conditions associated with sarsen formation and the likely timing of silicification are subsequently discussed. It is concluded that the geomorphological setting of sarsen formation may have been more important than climatic conditions at the time(s) of silicification.