Original paper

An African surface weathering profile in the Kalahari of North West Ngamiland, Botswana: processes and products

McFarlane, M.J.; Eckardt, F.D.; Coetzee, S.H.; Ringrose, S.


A 128 m profile in the Kalahari of northwest Ngamiland, Botswana, is described. Textural, chemical and mineral characteristics of profile units were determined by XRF and ICP, EM, with EDAX, and XRD.Fifteen metres of aeolian sand overlies 14 m of residual sand, this overlying a saprolite profile. The parent bedrock was not reached but surviving primary minerals, quartz, feldspars and mica, indicate a crystalline rock. Within the saprolite there is up-profile depletion of Al and Fe and relative enrichment of carbonates, with maximum values directly below the residual sand.A model of profile development is presented. Quartz becomes rounded in situ by a process of progressive spalling of a thin layer of an (hydrated?) alteration product and is a relative accumulation. The carbonate-rich horizon is progressively enriched by repeated solution at the top of this profile unit and precipitation at the base, a process comparable to the relative accumulation of the silica-boxwork/garnierite horizon in Ni-laterite profiles.Fluvio-lacustrine Kalahari Sediments were not found, consistent with earlier deductions, based on remote sensing, that saprolite directly underlies the linear dune sand in North West Ngamiland. It is suggested that a shift in paradigm would resolve many outstanding problems regarding the origin of the Kalahari Sediments, i.e. a major part are autochthonous rather than allochthonous


botswanakalahariafrican surfaceweathering profiledunes