EVIDENCES OF LATE QUATERNARY PALEOHYDROLOGICAL CHANGES IN THE SOUTHERN PORTION OF PANTANAL WETLAND, BRAZIL

The Pantanal is an active Cenozoic sedimentary basin in the central-west Brazil. The depositional tract is dominantly alluvial, being the Paraguay River collector of the waters from several megafans formed by rivers that drain the surrounding plateaus. Three major alluvial systems compose the depositional tract in the southern portion of the Pantanal basin: the Negro and the Miranda fluvial plains and the Aquidauana fluvial megafan. The three systems are genetically related and they coalesce in their distal portions, where there are many evidences of paleohydrological changes, such as channel abandonment and river shifting, changes in fluvial style, presence of paleochannels wider than modern channels, crossing paleochannels and reoccupation. The entrenchment at the entrance into the Pantanal suggests incision followed by fluvial aggradation in all the studied systems. The Negro and Aquidauna rivers meanders in confined belts incised on Pleistocene alluvial deposits at the entrance to the wetland. It is noteworthy that after running about 70 km confined, the Aquidauana River suffered a remarkable avulsion and the course changed towards west and the river stile acquired a distributary pattern with channel-levee belts. Superimposed belts with distinct meander-bend morphology were mapped in the middle Miranda River as well a low-sinuosity distributary paleochannels in its lower plain. An outstanding wide Negro River paleomeander belt over 80 km long and now truncated by the modern Miranda River meander belt, was recognized near the confluence with the Paraguay River. The features recognized in these changing rivers evidence paleohydrological changes controlled by avulsion processes, fluvial discharge fluctuations and regional base level up and down, that has been occurring in the Pantanal wetland since the late Pleistocene.