Fossil megafans evidenced by remote sensing in the Amazonian wetlands

Abstract

In the present work, remote sensing data are analyzed for investigating if a set of Late Quaternary triangular, fan-shaped morphologies from a large wetland in northern Brazilian Amazonia corresponds to residual megafan deposits. The results revealed main characteristics suggestive of megafans, which include: fan shape; distribution on a low-lying wetland; wide individual areas of several tens or hundreds of km2 in surface; concave-up and convex-up longitudinal and transverse profiles, respectively; very low gradient; axes pointing toward basement rocks and smooth slopes converging into the basin center; and surfaces with distributary drainage networks. The recognition of megafan systems with distributary drainage in this region is an interesting finding, as these megafans record a past landscape that differs from the modern fluvial tributary drainage of the Amazon basin. Characterizing these palaeomorpho logies is of high relevance for reconstructing changes in the Amazonian landscape during the Late Quaternary.