LATE CRETACEOUS ALKALINE SALINE LAKE COMPLEXES OF THE KALAHARI GROUP IN NORTHERN BOTSWANA

Block faulting in northern Botswana during the Late Cretaceous created grabens which were subsequently fined in by erosion of the nearby hor sts. The resultant sedimentary succession forms the Mmashoro Formation at the base of the Kalahari Group. The graben sub-basins are partly i nterconnected, with younger sediments overstepping the horsts in respo nse to expanding lacustrine and marginal lacustrine environments. The basal deposits consist of alluvial fan conglomerates eroded from the f ault scarps, grading downslope and upward into ephemeral stream floodp lain, sandflat and dry mudflat deposits. In some of the grabens sandfl at deposits grade into perennial saline lake sandstones and finally la yered cherts precipitated from hypersaline brines under increasingly a rid conditions. The successions thus constitute classical alkaline sal ine lake complexes.