FUNDAMENTAL CONTROLS ON FLUVIAL SAND PETROLOGY IN A TROPICAL SAVANNA WETLAND (PANTANAL, BRAZIL)

This study is the most comprehensive study of sedimentary source-to-sink processes to date in the Pantanal, the world’s largest wetland in Brazil. The Upper Paraguay River Basin was sampled to identify the control on sediment production and transport. River sands (n = 98) were sampled in 2019-2021, and each sample was point counted to 500 grains following the Gazzi-Dickinson method. Sedimentary lithic grains were commonly counted in Serra do Bodoquena and in the northern Pantanal. Monocrystalline quartz was most comon in the Paraguay River of the central Pantanal, whereas polycrystalline quartz was most prevalent near the parent rocks, along the northern and southern Pantanal region. Chert grains were common near the source rocks (Miranda, MS and San Matías) or in the headwaters (Cuiabá River, Vermelho River, and Taquari River in the planalto). Feldspathic minerals were common occurrences in the Serra da Bodoquena and in the Taquari River fluvial megafan. Hematite and other accessory minerals (clinopyroxene and amphibole) were most present in San Matías, Vermelho River, Cuiabá, and Miranda Rivers. Carbonate sand-sized grains were preserved only near the parent rocks. The warm year-round temperature and greater precipitation in the planalto remove less chemically stable minerals and leave behind more refractory minerals such as quartz and hematite. These compositions indicate that the snapshot of fluvial sand compositions is controlled by hydrodynamic control and parent rock weathering rather than by hydroclimate. Some blocks may be overrepresented, such as the metasedimentary Mesozoic quartzites in the eastern plateau region. The Rio Apa craton is the primary contributor of feldspathic grains. This dataset provides critical information for managing the natural resources across a wide region.