Isovolumetric weathering and silcrete formation,southern Cape Province,South Africa

Calculations of absolute elemental gains and losses based on isovolumetric bedrock alteration are applied to deep weathering profiles and silcrete developed on Dwyka Tillite and Bokkeveld Shale. Bedrock weathering to kaolinite and illite yields sufficient SiO₂ to account for observed thicknesses of associated silcrete. Absolute accumulation of SiO₂ in silcrete is complemented by substantial absolute depletion of Al₂O₃. Absolute accumulation of TiO₂ in both silcrete and associated weathered material together with authigenic anatase concentrations in silctete recorded during micromorphological investigations indicate TiO₂ mobilization in a low pH environment (pH < 3·75) during silcrete formation. Under these low pH conditions increased Al₂O₃ solubility led to replacement of kaolinite and illite by a proportion of SiO₂ retained in the weathering profile through restricted drainage.