Geomorfologia e dinâmica quaternária do planalto vulcânico riograndense

Abstract

The quaternary dynamics of the South Brazil Volcanic Plateau (PV) landscape is still a topic under construction. The present research is justified in an attempt to fill the gap of studies focused on the geomorphological evolution of the Riograndense Volcanic Plateau (PVRG) in the Quaternary period, with the central objective of reconstructing the geomorphological dynamics of this sector, from the study of the morphostratigraphic relationships between the geomorphic surfaces and their surface formations. Specifically, we sought to: a) individualize geomorphic surfaces/geomorphic compartments on a mesolocal scale; b) verify if there are lithological and structural controls in the spatial distribution of geomorphic surfaces/geomorphic compartments; c) individualize erosion surfaces and their corresponding deposits; d) determine the genesis of the orthomorphic surface formations that cover the PVRG; and e) verify if the quaternary dynamics of the PVRG landscape follows the same trend verified for other sectors of the Volcanic Plateau in southern Brazil. The methodology used integrated both office and field work, as well as laboratory procedures. In the office, the identification of surfaces and delimitations of geomorphological compartments was carried out, where it was decided to divide the study area into six (06) compartments: Central Plateau; Plateau of Missions; Southwest Plateau; Dissected Plateau of the Uruguay River; Dissected Plateau of the Jacuí-Taquari River and Atlantic Dissected Plateau. In the field, a survey of substrates and surface formations was carried out in the individual compartments, with 487 points being observed. The points were grouped into six (06) classes: 1) Thick soil; 2) Thin soil; 3) Alterite; 4) Rock; 5) Colluvium with Crust and 6) Undifferentiated Colluvium. Afterwards, five (5) stratigraphic sections were selected corresponding to the locations observed for the stratigraphic description of their materials, two of the sections refer to outcrops of materials from remnants of colluvial ramps; one corresponds to 2nd Order valley paleobottom; one the draining paleoheadwater and one the partially clogged drainage headwater that still integrates the modern landscape. In general, the subdivision made it possible to understand that the compartments are the result of the differential erosion of the current tributary system of the Uruguay River, adding control of local base levels, lithological variations of the flows and tectonic-structural controls. The stratigraphic materials described led us to believe that the area went through successive phases of weathering profiles formation followed by erosion resumption, where morphogenesis had a strong presence in the last 65,000 years BP. However, it is evident that the Riograndense Volcanic Plateau constitutes an erosion surface that can be called a degraded pediplan and retouched etchplanado.