THE LIMNOGEOLOGY OF LAKE UBERABA: FLUVIO-LACUSTRINE SEDIMENTARY PROCESSES ALONG THE DISTAL PARAGUAY MEGAFAN (PANTANAL WETLANDS, BRAZIL)

The pristine landscape of the Pantanal wetlands (Brazil) is conducive to broadening our understanding of lake basins associated with tropical fluvial megafans. Here we present a limnogeological study of Lake Uberaba, which is the Pantanal’s largest floodplain lake situated on the distal fringe of the Paraguay fluvial megafan. We used a multi-level integrated approach based on remote sensing, water and sediment chemistry data, and sedimentary indicators to assess how Lake Uberaba functions as a modern depositional system. The results of the study show that Lake Uberaba is hydrologically open, even in unusually dry years. Water chemistry data reveal a dilute, circumneutral Ca⁺²-Na⁺-HCO₃^- system derived from mixing of the water column by wind-driven waves. The lake surface area is ~230 km² with a maximum depth of 3.5 m near its southern outlet. Massive brown sandy silts with minimal organic carbon and low to moderate biogenic silica predominate in the lake floor sediments. Sediment facies reflect the strong influence of numerous inflowing megafan streams on modern sedimentary processes. Sediment cores contain recent lake deposits underlain by oxidized clayey silts and sands, suggesting that a subaerially-exposed floodplain existed at coring sites prior to the most recent lake transgression. This transition is consistent with recent increases in decadal and secular trends of precipitation in the Pantanal. These findings improve our understanding of fluvio-lacustrine sedimentary processes and landforms in the Pantanal, as well as the history of recent environmental change along the Paraguay fluvial megafan.