Araguaia River Floodplain: Size, Age, and Mineral Composition of a Large Tropical Savanna Wetland

The Araguaia River floodplain is a large wetland in the tropical savanna belt ( cerrado ) in the southern Amazon basin. Studies using multitemporal satellite Landsat 5 TM images with a spatial resolution of 30 m indicate a surface area at maximum flood level of 88,119 km 2 . During the low-water period, only 3.3 % of the area is covered by water. Flooding is the result of the annual rise in the water level of the Araguaia River and of local rainfall and insufficient drainage during the rainy season. Sedimentology studies have distinguished between an active recent and sub-recent floodplain, which covers ~20 % of the area, and a paleo-floodplain probably several hundreds of thousand years in age. Paleo-floodplain sediments are strongly weathered and marked by the clay mineral association of kaolinite, gibbsite, goethite, and Al-chlorite, predominantly formed from feldspars and micas. The active paleo-floodplain participates in the hydrological cycle but does not receive recent sediments from the river. Higher-lying, not flooded areas (inactive paleo-floodplain) are probably the remnants of paleo-levees, now in an advanced stage of erosion. A hypothesis to explain the genesis of the floodplain is proposed herein.