Formação e evolução de lateritas ferruginosas em Tabuleiros Costeiros do Grupo Barreiras

Abstract

The Barreiras Group is a detrital, siliciclastic, sandy-clayey sedimentary cover, with variegated colors, variably ferruginous and of kaolinitic matrix, which extends along the Brazilian coast, with records in the interior of the continent, having great heterogeneity throughout its exposure, sometimes more ferruginous, sometimes less. Several soil classes are recognized over the Barreiras Group; among them, Petric Plinthosols, on a local scale, in the Coastal Tablelands domain of the Barreiras Group, with expressive laterites. The objective of this research was to investigate laterites from Petric Plinthosols of the Barreiras Group in Coastal Tablelands, aiming understanding the factors and processes involved in their formation, stability and, or, degradation. To do so, geochemical analysis were carried out, using X-ray fluorescence and selective extraction of Fe oxides, mineralogical analysis, using X-ray diffractometry to identify the mineralogical assemblage and define crystallographic parameters, magnetic susceptibility, and micromorphological analysis, using optical microscopy associated with scanning electron microscopy coupled with energy dispersive spectroscopy. The results show clear distinctions between the laterites, resulting from specific local paleoenvironmental, geological and temporal conditions. Laterites found in profiles in the southeast were formed under drier conditions, having greater contribution of vertical iron input from more ferruginous source material, being in a lesser stage of degradation, and more recent. On the other hand, those in the northeast are from more humid conditions, without large contribution of iron from the source material, but having evidence of lateral detrital contribution of iron from geological contexts other than Barreiras, in a more advanced stage of degradation, and more old. Furthermore, the predominant process of iron accumulation in all profiles is residual, although having indications of absolute accumulation contribution in the northeast profiles. Neotectonic structural control seems to be a mechanism related to the occurrence and distribution of Petric Plinthosols with continuous lateritic layers along the Coastal Tablelands of the Barreiras Group.