Late Quaternary Cuiabá megafan, Brazilian Pantanal: Channel patterns and paleoenvironmental changes

Abstract

The Brazilian Pantanal wetlands comprise some of the largest Quaternary megafans in South America. The origin of these megafans has been linked to climate changes during the Late Quaternary. This study focuses on the Cuiabá megafan, the second largest megafan of the Pantanal, which has not been studied from the perspective of its basic geomorphology, river channel patterns and sediment deposition ages. Analysis of the channel and floodplain morphology as well as optically stimulated luminescence dating enable us to establish the succession of events that built up the Cuiabá megafan since the last glacial period. Abandoned lobes indicate that a phase of fan aggradation occurred between 48 and 19 ka, with a network of distributary paleochannels suggesting that there was relatively low precipitation and reduced vegetation cover. Subsequently, abrupt changes in discharge and sediment supply triggered the channel incision of the upper fan valley, indicating a degradational phase after the Last Glacial Maximum (<19 ka). A new phase of fan aggradation began at the Early Holocene, when meandering and anabranching channels were constrained into the incised valley on the upper fan. In the distal part of the fan, the development of modern depositional lobes has been driven by numerous single-sinuous channels that shift laterally due to successive avulsions. Regional tectonic features suggest that modern Cuiabá lobes are in a subsiding area that provides accommodation space for sediment accumulation and prolonged flooding events. Changes in the channel patterns observed in the Cuiabá megafan are related to changes in the South American monsoon strength, which driven precipitation variation during the Late Quaternary. Periods of monsoon intensification and relatively high precipitation lead to channel incision, while periods of fan aggradation correlate to times of fewer oscillations. Our findings highlighted the diversity of fluvial styles in the Pantanal and its importance as a climate changes archives in the South American lowlands.