EVIDENCE FOR FLUVIAL SEDIMENT TRANSPORT OF KALAHARI SANDS IN CENTRAL BOTSWANA

A textural and mineralogical study has been made of Kalahari cover san d, collected on a regular (approximately 13 km x 13 km) grid, over an area of approximately 78 500 km(2), covering much of the central Kalah ari region of Botswana. Sand textural characteristics vary systematica lly over the area sampled, but are not readily explained in terms of t he wind patterns which have prevailed from at least the Quaternary to the present day. Rather, they indicate that deposition was dominated b y fluvial processes. Zones of coarse sand associated with relatively h igh concentrations of heavy minerals are interpreted as lags associate d with the basin margins. Ephemeral streams and sheetwash transported finer material into the distal portions of the basin, which is charact erized by relatively low numbers of heavy minerals. The variation in s and texture and the distribution of heavy minerals indicate that the g round sampled covers the major portion of a subsidiary central Kalahar i Basin. This is bounded by the Bakalahari Schwelle in the south, and the Ghanzi ridge in the northwest. The divide between the Limpopo and endoreic fossil drainage lines, which formerly emptied into the Makgad igadi, is inferred to form the eastern margin of this central Kalahari Basin. The mineralogical and textural data presented here have import ant implications for kimberlite prospecting in the sandveld of the wes tern two thirds of Botswana.