Nature of sedimentary deposits in the western Makgadikgadi basin, Botswana

Quaternary sedimentation in the western Makgadikgadi basin of north central Botswana is evaluated on the basis of new evidence from satellite imagery and sedimentological analyses. Thematic Mapper imagery interpretation, combined with field evidence, has led to the identification of geomorphological features which are mainly composed of light grey calcareous sandstones (formerly calcretes) overlain by dark grey sands. The literature suggests that palaeolake Makgadikgadi I formed and developed intermittently after initial downwarping in the early–mid Pleistocene. The calcareous sandstones were formed when calcium carbonate precipitation took place in pre-existing Kalahari sands along the western shoreline of Makgadikgadi I. Field evidence, supported by X-ray diffraction and SEM analyses, indicates that CaCO 3 precipitated mainly in marshy conditions around plant roots and stems and in association with bacteria in embayments along the lakeshore. The sandstones thickened and became partially indurated as a result of increasing palaeolake levels. Deposition was terminated by renewed tectonism which uplifted the shoreline zone relative to the lake basin, leading to falling palaeolake levels. Post-uplift reworking led to case hardening and pedogenic calcrete formation in the upper sections of the calcareous sandstones. Sedimentary conditions altered during the late Pleistocene. Extensive distributaries from the proto-Okavango system incised the shoreline ridge contributing to the filling of Makgadikgadi II. Satellite data suggest that the proto-Okavango rivers formed a series of fan deltas at this time along the western Makgadikgadi basin. Widespread dispersal of fluvial grey sands took place as a result of basin tilting which led to anastomosing channels flowing southward possibly around 18,000 B.P. These results, although preliminary in nature, augment previous geomorphological analyses by adding some detail in terms of depositional environments and by providing a tentative age and origin for the ubiquitous grey sands.