Geochemistry of weathering profile silcretes, southern Cape Province, South Africa

SUMMARY: Silcrete of Cenozoic age associated with deep weathering profiles occurs on residual surfaces along the coastal belt of southern Cape Province. Petrographic and geochemical evidence indicates loss of aluminium and enrichment of silica and titanium during silcrete formation. Silica released locally within the weathering profile was appar-ently precipitated in a zone of restricted drainage close to the water-table where a low pH environment allowed theremoval of aluminium and the migration and concentration of titanium. Silcrete formation probably occurred in a humid tropical or subtropical envi-ronment with minimal local relief. The numerous reports of silcrete now available in the literature indicate the wide range of sedimentological and environmental settings with which it is associated (Langford-Smith 1978; Summerfield 1983). Work in southern Africa and north-west Europe has indicated that silcretes associated with kaolinitic weathering profiles possess a typical suite of petrographic and geochemical characteristics, including authigenic glaebules (Brewer 1964, pp. 259-60), colloform features and relatively high concentrations of TiO a (>0.2%), which are not present in non-weathering profile occurrences (Summerfield 1978, 1979, 1982). In southern Africa, non-weathering profile silcretes, including silicified sands, pan sedi-ments, calcrete and bedrock, occur predomin-antly in the Kalahari Basin in Botswana, north-ern Cape Province and eastern Namibia (Sum-merfield, 1982). This paper describes the weathering profile silcretes of southern Africa, which are confined to a relatively narrow coastal belt (Cape coastal zone) extending from the Oliphants River valley in the west to the Transkei n the east (Fig. 1). A limited number of non-weathering profile silcretes also occur within this area but these are considered else-where (Summerfield 1981). There have been a number of previous studies of silcrete in the Cape coastal zone, but none of these have pro-vided detailed geochemical data on associated weathering profile materials (Bosazza 1936