SILICA AND CARBONATE RELATIONSHIPS IN SILCRETE-CALCRETE INTERGRADE DURICRUSTS FROM THE KALAHARI OF BOTSWANA AND NAMIBIA

Sitcrete-calcrete intergrade duricrusts (surface or near-surface chemi cally precipitated crusts with a cement comprising a mixture of silica and CaCO3) have been widely identified in the geological, geomorpholo gical and pedological literature, but have not, to date, been systemat ically described or classified. This paper presents a review of previo us definitions of the end members of the silcrete-calcrete spectrum an d subsequently identifies the major silica-carbonate relationships wit hin intergrade duricrust types from the Kalahari of Botswana and Namib ia. Three main intergrade types are identified on the basis of silica- carbonate associations: duricrusts where secondary silica occurs withi n a calcareous matrix; varieties where secondary carbonate occurs with in a siliceous matrix; and materials where silica and carbonate matrix cements appear to have been precipitated contemporaneously or in clos e succession. Within each of these three groups, sub-types are identif ied dependent upon whether secondary materials have replaced or been e mplaced within a pre-existing duricrust. Finally, a practical procedur e for the simple definition of silcrete-calcrete intergrade duricrusts is suggested based upon a combination of bulk chemical and thin-secti on analyses. (C) 1998 Elsevier Science Limited.