Aspects of palaeo-shoreline silcrete formation, Makgadikgadi Pans, north-central Botswana

Abstract Work was undertaken on unusual littoral deposits along the north-west coast of Sua Pan (Makgadikgadi Pans). These comprise topographically and texturally distinct cal-silcretes and silcrete nodules deposited within shorelines and adjacent foreshores, respectively. Early depositional events include the formation of shoreline silcrete clast precursors that later underwent calcrete cementation during a drying episode evoking shoreline focussed evapo-concentration. Silicification appears to have occurred at a later stage following a wet-humid interval. This occurred when palaeo-lake waters, super-saturated with H4SiO4, were diluted by freshwaters likely emerging from underlying fractures. SiO2-enriched porewaters infiltrated and displaced the authigenic calcrete cement ultimately forming cal-silcrete intergrade conglomerates. Discrete nodule silicification resulted from the down-gradient infiltration of silica-rich pore waters into host sediment aggregates that silicified in-situ.