Lateritização e sedimentação cenozoica na evolução da paisagem da Serra dos Carajás

Abstract

The Serra dos Carajás, located in the southwestern portion of the Brazilian Amazon, is one example of lateritic terrains with crusts or “cangas” sustaining plateaus with up to ~ 700 m. Two distinct duricrusts are recognized in this region: 1) an older one, developed during the Paleogene over the Precambrian banded iron formation (BIF) and volcanic rocks; and 2) related to a succession of30 m-thick of Pleistocene ferruginized deposits previously considered as supergenic and reinterpreted here as sediments. The facies, paleomagnetic, and geochemical analyses of these ferruginized deposits in outcrops and drill cores formed by breccia and ironstone found that they register recurrent planation phases associated with the dismantling of lateritic profiles. The breccia is cemented by goethite, and iron and aluminum oxyhydroxides are representative of subaerial debris flows related to colluvial fans adjacent to shallow lake systems comprising the chemical deposition of goethite and hematite. The goethite-bearing beds with even parallel lamination alternate with pisolites / oncoids laminae indicate microbial activity. Petrographic and X-ray diffraction data indicated mainly hematite (50%), goethite (47%) and gibbsite + Al-goethite + magnetite (3%). The high magnetic susceptibility, alternating fields, and thermal demagnetization of these deposits indicated a remaining magnetization provided mainly by hematite and goethite. The high concentration of carbon present in these minerals suggests that the strong magnetization detected may result from old forest fires. The oblate graphic pattern of the magnetic susceptibility anisotropy and the dispersion of the poles of the remaining natural magnetization indicates sin and post-depositional variables related to the inherent magnetization present in the BIF fragments. Geochemical analysis of these deposits shows that the little mobile elements have a limited relationship with the BIF and volcanic substrate. The high titanium values also corroborate concentration during sedimentation not related to lateritization. The progradation of colluvial fans on shallow chemical lakes implanted in the valleys was recurrent in the Serra dos Carajás landscape evolution, attesting to the climatic variations since the Pleistocene. This new insight about the Serra dos Carajás duricrusts initiates a new geological reading that allows a better understanding of the weather and sedimentary processes related to the Cenozoic Amazonian evolution landscape.