Implicações geomorfológicas e paleoambientais de registros sedimentares fluviais do quadrilátero ferrífero - Minas Gerais

Abstract

The Quadrilátero Ferrífero (QF) is one of the most remarkable mountainous areas in the Minas Gerais State, Southeastern Brazil. The morphology of this area can be set as result of long-term evolution of a warped geological substratum. Tectonic inputs and the interleaving of lithologies with different resistance degrees to the weathering led to a great dissection through drainage and to the reversal of the relief. Because of that, nowadays can be found synclines relatively suspended in relation to dissected anticlines. Given this major role of the river work on shaping the relief, and aiming to improve the understanding of regional geomorphology during the Late Cenozoic, various researches of river levels and fluvial successions were undertaken in recent decades. However, these local studies have investigated only one or two river valleys. Starting from new researches and from a rereading of data from literature about the area, this term paper aims to investigate the action of fluvial processes on the Quaternary morphogenesis in QF, focusing on the modeling of the river landscape and on the surface formations. The results reveal clues and evidence of a young fluvial landscape and of a dynamic relief. Seven regional phases of development of fluvial levels were identified and dated between 1-83 ka using Optically Stimulated Luminescence (OSL). Most of these phases are followed by periods of fluvial downcutting in response to the regional uplift, leaving river levels as staircases. The neotectonics is also expressed by the displacement and deformation of fluvial deposits and levels. The OSL dating allowed the association of the fluvial regional phases with climatic oscillations stablished in the literature. These climatic oscillations may have influenced the rhythms of the regional epeirogeny due to isostatic answers to the most erosive periods. The drier/coller periods were marked by the formation of depositional successions with duricrusts which are of great importance to correlate deposits of different valleys. In the Conceição River valley, the mineralogical and geochemical analysis undertaken in these materials allowed verifying changes in sediment source areas, probably as a response of a stream capture. Together with a differential tectonics and local base levels, conglomeratic duricrusts played an important role controlling the drainage downcutting in some valleys. Thus, an interesting geomorphological arrangement arises, with correlated levels differing tens of meters in hight. Thus, as the correlation of erosive surfaces, the correlation of river levels based only on elevation and/or height data is unsuccessful. Phytolith and carbon isotopes analyses in representative deposits allowed verifying that even the transition to different types of cerrado (savanna-like vegetation) can influence significantly the sediment yelding for drainage lines. In the valley bottoms, iron enriched solutions may have percolate easily amid the thick deposits of gravel and sand, stressing that the formation of the duricrusts is result of sedimentary and lateritic processes.