CLAY MINERALOGY OF MODERN FLUVIAL SEDIMENTS FROM THE PANTANAL BASIN, BRAZIL

X-ray diffraction analysis of fine-grained sediments provides key insights on the influences of climate, weathering processes, hydraulic sorting, source rock lithology, and recycling on clay mineral suites in tropical sedimentary basins. The Pantanal, the world's largest freshwater wetland, contains diverse source rocks with a tropical savanna climate, making it an excellent natural laboratory to examine which factors control fine sediment production. Sampling locations (n=43) in rivers draining the planalto highlands were selected using pour point analyses to capture the range of lithology, hydroclimate and topography in each sub-watershed. Arenite, granite, shale, and conglomerate source rock lithologies frequently constituted >20% of each watershed with surface areas of 10²-10⁴ km². Modern fluvial sediments were collected along riverbanks, pretreated to remove carbonate and organic matter, and reduced to <2 µm fraction to prepare oriented clay mounts. Kaolinite and illite were commonly present with quartz in the samples, attesting to intense weathering. Most samples were rich in interstratified vermiculite and mica independent of watershed size, whereas some samples contained clearly identifiable diagnostic peaks for goethite, gibbsite, and potassium feldspar. These results combined with relative abundance estimates provide a basis for understanding source-to-sink processes in the current tectonic and geomorphic configuration of the Pantanal Basin.