Édipo Henrique Cremon

Fossil megafans evidenced by remote sensing in the Amazonian wetlands

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.

Classification of Vegetation over a Residual Megafan Landform in the Amazonian Lowland Based on Optical and SAR Imagery

The origin of large areas dominated by pristine open vegetation that is in sharp contrast with surrounding dense forest within the Amazonian lowland has generally been related to past arid climates, but this is still an issue open for debate. In this paper, we characterize a large open vegetation patch over a residual megafan located in the northern Amazonia. The main goal was to investigate the relationship between this paleolandform and vegetation classes mapped based on the integration of optical and SAR data using the decision tree. Our remote sensing dataset includes PALSAR and TM/Landsat images. Five classes were identified: rainforest; flooded forest; wooded open vegetation; grassy-shrubby open vegetation; and water body. The output map resulting from the integration of PALSAR and TM/Landsat images showed an overall accuracy of 94%. Narrow, elongated and sinuous belts of forest within the open vegetation areas progressively bifurcate into others revealing paleochannels arranged into distributary pattern. Such characteristics, integrated with pre-existing geological information, led us to propose that the distribution of vegetation classes highlight a morphology attributed to a Quaternary megafan developed previous to the modern fluvial tributary system. The characterization of such megafan is important for reconstructing landscape changes associated with the evolution of the Amazon drainage basin.