Abstract
Silcrete has been widely cited as an indicator of arid or semi-arid environments, such a palaeoclimatic inference deriving from a number of specific studies of Cenozoic silcretes in southern Africa, Australia and elsewhere. Recent investigations of Cenozoic silcretes in southern Africa reveal, however, the existence of two petrographically and geochemically distinct silcrete types, one of which has apparently formed under humid conditions. Silcretes in the Kalahari Basin (Botswana) have developed through silicification of a variety of host materials, especially playa sediments and calcrete. Petrographic characteristics (including length-slow chalcedony vugh-fills), lack of associated deep westhering profiles and limited independent palaeoclimatic evidence, suggest silcrete genesis in an alkaline, arid/semi-arid environment. In contrast, silcretes in the Cape coastal zone (South Africa) are almost invariably associated with deeply weathered bedrock, and are TiO 2-rich in comparison with Kalahari silcretes. Local mobilisation and co-precipitation of silica and titanium is indicated by the presence of authigenic anatase in colloform features and glaebules in silcrete. This implies a low pH (< 4) during silcrete formation by silica replacement of weathering profile clays. Such a highly acidic weathering environment suggests abundant vegetation and a humid tropical or subtropical climate. Other recent reports of occurrences in Australia and France support the view that TiO 2-rich weathering profile silcretes form under a humid climate. A semi-arid/arid palaeoclimatic interpretation remains valid for non-weathering profile silcretes, and for silcretes occurring within, but not genetically related to, deep weathering profiles.
Published Version
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call