Aspects of geological and sedimentological evolution of the Pantanal plain

Studies of the geological and sedimentological evolution of the Pantanal plain during the Pleistocene have shown that all major surfaces of the Pantanal, except the inselbergs, are of fluvial/lacustrine origin. As seen at the alluvial fan of the Taquari River, about 20 % of the fan is influenced by the river’s water and sediments. The other areas are paleo-fans whose surfaces are of different ages and modifi ed to different degrees by siltation processes. Aeolian processes, formerly considered to be important determinants of landscape by the formation of white-sand dunes, could not be substantiated by our field studies. The entire surface of the Pantanal is dominated by fine-grained sand and silt from the surrounding Paleozoic/Mesozoic uplands, with small amounts of clays, dominated by kaolinite but lacking in chlorite. The large amount of fine sand and the low amount of clay minerals in the sediment load of the tributaries explains the very peculiar surface structures of the Pantanal. Studies were carried out additionally about the artificial earth mounds (aterros), which were erected by the pre-Columbian population for fl ood protection several centuries ago. These mounds largely consist of shells of freshwater molluscs and are highly fertile. Mineral analysis showed large amounts of beidellite, a phyllosilicate with high ion-exchange capacity. We postulate that this clay mineral developed during the last few centuries, under conditions of a tropical climate combined with the presence of calcium-carbonate derived from the shells, and contributes to the long-lasting fertility of the aterro soils