Late Cretaceous alkaline saline lake complexes of the Kalahari Group in northern Botswana

Abstract

Block faulting in northern Botswana during the Late Cretaceous created grabens which were subsequently filled in by erosion of the nearby horsts. The resultant sedimentary succession forms the Mmashoro Formation at the base of the Kalahari Group. The graben sub-basins are partly interconnected, with younger sediments overstepping the horsts in response to expanding lacustrine and marginal lacustrine environments. The basal deposits consist of alluvial fan conglomerates eroded from the fault scarps, grading downslope and upward into ephemeral stream floodplain, sandflat and dry mudflat deposits. In some of the grabens sandflat deposits grade into perennial saline lake sandstones and finally layered cherts precipitated from hypersaline brines under increasingly arid conditions. The successions thus constitute classical alkaline saline lake complexes.