Quaternary alluvial sedimentation in the Conceição river valley, southeastern Brazil

Abstract

The aim of this paper is to discuss the first results of the analyses concerning indications of influences of litho structures, tectonics, and climate on the pattern and chronology of Quaternary alluvial sedimentation in the Conceicao river valley. This is a mountain tropical area located in the northeastern of Quadrilatero Ferrifero, one of Brazil's most known mountainous domains and one of the world's major iron ore producers. The used methods were based on field research and laboratory analyses of sediments, including optically stimulated luminescence dating. Eight alluvial depositional levels were identified, most of which were morphologically uncharacterized (without the configuration of terraces). Probable river diversion, sediment traps, and tilted blocks indicate the tectonic conditioning on fluvial dynamics, besides the terrace staircases. The formation of cyclic alluvial levels with gravel-bed facies cemented by iron oxides-hydroxides could be related to climatic fluctuations. Although data on luminescence dating offer critical information for the understanding of the Quaternary fluvial evolution of the area, several geological and geomorphological inconsistencies require careful analysis. Furthermore, although some ages are consistent with the specific case of the Conceicao river, these ages are significantly different to those found in other valleys of Quadrilatero Ferrifero.