Gênese e evolução da superfície quaternária no médio São Francisco: fatos e evidências na bacia do Rio Grande - Bahia

Abstract

The region of medium São Francisco river holds a lot of evidences of the Quaternary landscape genesis and evolution, that can be observed in the areas where there are the formation of agradation reliefs forms, by means of extensive sand sheets, as occur in the low course of Grande river, one of its mainly tributary. The low course of this river is located in a relief unit called of São Francisco Depression, rounded by degradation relief forms, called of São Francisco Plateau, São Francisco/Tocantins Landing Plateau, Tabatinga Plateau and Espinhaço Mountains. A part of the material eroded in degradation zones, was deposited in research place, which holds an area of the 7.504Km2, that correspond to 1,2% of the São Francisco basin. The research area is located in Bahia northwest. The sediments where transported and deposited by colluvial and alluvial action, which had originated sediment deposits, that came from a lot of sedimentation fronts, forming the Grand River Alluvial Flat Land, Preto River Alluvial Fan, Curralim River Alluvial Flat Land and Mansidão Alluvial Flat Land. The sand sheet was formed by the coalescence of sedimentation fronts. It is necessary to mention that this sediments where reworked by wind action during dry climate periods and more intensity of winds. The formation of these unconsolidated deposits occurred according to a climatic fluctuation during the Quaternary, when the semi-arid climate had predominated, in the time of last glaciations event. But other climates fluctuations had occurred in Pleistocene and Holocene, both before and after this deposition, with some time more cold or hot and dry or wet. In the landscape, the response of the fluctuations where printed as different soil classes, kind of relief forms and characteristics of the sediments deposits. The dating work allowed identification of ages between 21Ky BP (±2,7) and 0,88Ky BP (±0,04), that corresponds to the paleo-geographic interval occurred since the Last Glacial Maximum until the Short Ice Age. During the phase that corresponds to Superior Pleistocene and Inferior Holocene, there were deposits expansions by means of alluvial action, and at the end of the Pleistocene there were the begin of erosion process intensification and a consequent formation of the mainly fluvial channels, caused by a shift climate, which were more wet, and had reflections in fluvial patterns, ending the time of intensive braided channels. The following phase, that corresponds the Medium and Superior Holocene, was marked by climatic fluctuations of short duration, and the climate use to variety between dry and more wet. The response was phases with aeolian work, culminating in the formation of deflation process (pans), and phases when the erosion was intensified. Part of the soil that composes the research area has its genesis linked to older process then the interval ages identified in this research, because they are classified as allochthonous Oxisols and Ferrasols, with genesis associated to physical and chemical changes in the deposits. But the most part of the soils are represented by Arenossols, formed during the last glacial period. At the end, it is necessary to detach that the research area is formed by approximately 67.680x106m3 of sediments, and it is necessary to have a lot of care in the local soil management, what will keeps the deposits stabilized. In the other hand, the siltation in the São Francisco River will be intensified.