Fades distribution and pedogenic evolution of clayey deposits in Caxambu Hill, Quadrilatero Ferrifero, Minas Gerais, Brazil

摘要

Based on stratigraphic relationships, micromorphological and mineralogical studies with X-ray diffraction (XRD), differential thermal and gravimetric analysis (DTA-DGA), optical and scanning electronic microscopy (SEM), four lithofacies were identified in the clayey deposits of Caxambu Hill: fragmentary, nodular, massive and friable. The fragmentary facies is composed of extraclast fragments (3-30 cm) of the local Paleoproterozoic basement and sand size quartz-grains dispersed in a kaolinite-muscovite-goethite-hematite matrix. The nodular facies comprises nodules with recognizable rock and soil fabrics (2-5 cm) dispersed in a matrix similar to the fragmentary facies. The massive facies is characterized by quartz grains dispersed in a matrix of kaolinite, hematite and goethite with minor amounts of muscovite. The friable facies differs from the massive facies by its channel morphology, higher quartz and kaolinite content and the presence of clay-balls millimeter size. These clayey deposits originated by syntectonic sedimentation in a NW/SE oriented graben during the Cenozoic. The differentiation of the facies is related to gravity mass-flow processes. The fragmentary facies was deposited during the early stage of opening of the basin. After that, under sub-arid conditions, the slumping of lateritic materials from the surrounding regolith led to the formation of the nodular and massive facies. The friable facies originated by the action of unidirectional flow that reworked the clayey sediments. The increasing kaolinite content towards the top is related to the chemical weathering action after the deposition of the sediments.