Evolução da paisagem da porção centro-oriental da Amazônia do Cretáceo ao Paleógeno.

Abstract

This study presents a new perspective for the evolution of the relief in Central and Eastern Amazonia based in geomorphologic and stratigraphic analysis of two areas: the first one at the surroundings of Paragominas and the second includes territories from Juruti, Belterra and Santarém counties, both at the state of Pará. The results of the relief analysis demonstrate a diversity of morphological modellings that indicate a polycyclic evolution involving periods of development of flat plateaus, associated with the development of lateritic profiles, alternated with intervals that tend to erosional activity from unstable and collapsing slopes, being the planation reliefs representative of the Paleocene-Eocene and Early-Middle Miocene, while the dissection modellings mark ages from the Neoeocene-Neooligocene and Neomiocene-Pliocene. The ages of the planation modellings are confirmed by age estimations of Cenozoic paleosurfaces that occur along the Amazon region (with which they are correlatable) and by global eustatic and climatic tendencies. The sedimentological and stratigraphic analysis identified two typical profiles for these study areas – PWH (profiles with well-defined horizons) and PMC (profiles with massive conglomerates), distinguished by their in situ/reworking characteristics. Regarding to the PMC profiles, there were observed two facies associations: FA1: an association of conglomerates of diverse supportings and arenites assumed to be formed by proximal fan deposits generated through debris flows; and FA2: packs of clay displaying mostly a massive framework, interpreted as distal fan deposits generated through sheet flow and mudflow. Both associations have subaerial exposure evidence and erosive surfaces in their bases, that allow to conclude a deposition at embedded valleys through rapid pulses of detritus, and hence were interpreted as colluvial and alluvial fan deposits, that formed along the most erosive periods of the regions, having been initiated in the vicinities of the main drainages of the analyzed areas (Capim and Amazonas rivers). The results of this study demonstrate a more intense sedimentary and tectonic activity than that generally accepted for the Amazon region during the Cenozoic Era, showing that this region’s landscape was more affected than it was believed to be by another tectonic remarkable events in the South America continent, such as: the Andes Uplift, the deposition of the Amazonas and Marajó basins and Bragantina Platform neogene formations, the Tortonian Regression and the install and evolution of the Amazonas River.