Silica and carbonate relationships in silcrete-calcrete intergrade duricrusts from the Kalahari of Botswana and Namibia

Silcrete-calcrete intergrade duricrusts (surface or near0surface chemically precipitated crusts with a cement comprising a mixture of silica and CaCO₃) have been widely identified in the geological, geomorphological and pedological literature, but have not, to date, been systematically described or classified. This paper presents a review of previous definitions of the end members of the silcrete-calcrete spectrum and subsequently identifies the major silica-carbonate relationships within intergrade duricrust types from the Kalahari of Botswana and Namibia. three main intergrade types are identified on the basis of silica-carbonate associations: duricrusts where secondary silica occurs within a calcareous matrix; varieties where secondary carbonate occurs within a siliceous matrix; and materials where silica and carbonate matrix cements appear to have been precipitated contemporaneously or in close succession. Within each of these three groups, sub-tupes are identified dependent upon whether secondary materials have replaced or been emplaced witin a pre-existing duricrust. Finally, a practical procedure for the simple definition of silcrete-calcrete intergrade duricrusts is suggested based upon a combination of bulk chemical and thin-section analyses.