PEDOGENIC PALYGORSKITE AND DOLOMITE IN A LATE NEOGENE SEDIMENTARY SUCCESSION, NORTHWESTERN TRANSVAAL, SOUTH-AFRICA

A detailed field survey identified a late Neogene sedimentary succession which accumulated in ephemeral palaeolakes on the edge of the Kalahari Basin in the northwest of Transvaal Province, South Africa. Palygorskite and dolomite formed along lake margins which fluctuated between lacustrine and dry-land conditions. The sedimentary sequence presented an opportunity to investigate the interrelationships between these minerals and study their possible modes of formation, either by lacustrine sedimentation or pedogenic processes. Clay mineralogy, transmission and scanning electron microscopy and geochemical analysis revealed an increase in magnesium-enriched minerals, palygorskite and dolomite, within the pedogenically altered marginal lacustrine sediments. Field relationships showed that the dolomite is a replacive phase within the decomposing palygorskite, although the scanning electron micrographs do not display such unequivocal evidence for a direct replacive relationship. They suggest rather that the dolomite forms by neoformation from soil solutions, which included decomposition products released by the breakdown of the palygorskite. Seen in a regional context, subaerial pedogenic processes acted on marginal lacustrine sediments and resulted in the decomposition of smectite and neoformation of palygorskite. A change in climate to drier conditions encouraged the formation of calcrete in surrounding areas which increased the magnesium/calcium ratio in groundwaters. This increase was supplemented by evaporative concentration during the desiccation of the lacustrine environment. The high magnesium concentration led to the formation of the dolomite-rich dolocrete. This interpretive model is a means by which existing genetic models for calcareous pedocretes and those models that define the concentration of magnesium in groundwater and saline lacustrine environments can be combined. The pedogenic mineral relationships described are suggested as useful palaeoenvironmental indicators for the late Neogene/early Pleistocene of southern Africa.