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, comb ined with field evidence, has led to the identification of geomorphological features which are mainly composed of light grey calcareous sandstones (fo rmerly calcretes) overlain by dark grey sands. The literature suggests that palaeolake Makgadikgadi I formed and developed intermittently after initia l downwarping in the early-mid Pleistocene. The calcareous sandstones were formed when calcium carbonate precipitation took place in pre-existing Kala hari sands along the western shoreline of Makgadikgadi I. Field evidence, s upported by X-ray diffraction and SEM analyses, indicates that CaCO3 precip itated mainly in marshy conditions around plant roots and stems and in asso ciation with bacteria in embayments along the lakeshore. The sandstones thi ckened and became partially indurated as a result of increasing palaeolake levels. Deposition was terminated by renewed tectonism which uplifted the s horeline zone relative to the lake basin, leading to falling palaeolake lev els. Post-uplift reworking led to case hardening and pedogenic calcrete for mation in the upper sections of the calcareous sandstones. Sedimentary cond itions altered during the late Pleistocene. Extensive distributaries from t he proto-Okavango system incised the shoreline ridge contributing to the fi lling of Makgadikgadi II. Satellite data suggest that the proto-Okavango ri vers formed a series of fan deltas at this time along the western Makgadikg adi basin. Widespread dispersal of fluvial grey sands took place as a resul t of basin tilting which led to anastomosing channels flowing southward pos sibly around 18,000 B.P. These results, although preliminary in nature, aug ment previous geomorphological analyses by adding some detail in terms of d epositional environments and by providing a tentative age and origin for th e ubiquitous grey sands. (C) 1999 Academic Press.