Sedimentação coluvial pleistocênica na região de Presidente Figueiredo, nordeste do estado do Amazonas

Abstract

This paper describe Upper Pleistocene colluvial deposits in the Presidente Figueiredo region, northestern State of Amazonas, Brazil. Outcrop-based sedimentologic and stratigraphic study, integrated with geomorphologic analysis and optically stimulated luminescence dating, allowed to characterize the architecture and lithofacies of these sedimentary successions and provided information about the denudation history and landscape changes in the central Amazonian region during the Pleistocene. The colluvial deposits consist of sands and mainly matrix-suppported gravels with massive beddding, locally with inverse grading, suggestive of deposition by gravitaty and torrential flows. Two types of colluvial deposits were identified: Colluvial deposits 1, dated at 57.000±5.000 years BP, constituted of gravels and sands with fragments of pelite, lateritic crusts and ferruginous sandstone, overlying Early Paleozoic rocks; and Colluvial deposits 2, yielding 22.100±2.600 years BP, consisting of gravels with fragments of semi-flint kaolin and lateritic crust, overlying Cretaceous kaolinic silciclastic deposits of the Alter do Chão Formation. The types of fragments indicate as sources Phanerozoic weathered rocks and lateritic bauxitic and ferruginous laterites which were removed during denudation of plateau. The two colluvial events described here seem to confirm that the main phases of geomorphogenesis are correlated with periods of forest retreat related to the two dry climatic events recorded during the Upper Pleistocene in Amazonia.