Morphogenesis and weathering: colluvia of the Itatiaia Plateau

Abstract

On the lower slopes of the ltatiaia Plateau (2,300 -2,500m) are found two generations of colluvia, which indicate phases of intensification of slope erosion. A generallateritic trend characterizes the regolith and colluvia. Micromorphological analysis of the colluvia shows a mixture of materials at different stages of weathering, especially in C II, which presents stronger contrasts than is usual at any given levei of weathering profiles. C I colluvia, which are clayey, rich in fragments of underlying rocks show little pedogenic development and less weathering contrasts, might have originated in mass movements reaching deep into the regolith. In C II, rock and weathering relicts are less common and skeleton grains smaller, indicating provenance from materiais originally more evolved. The mixture of materiais at extreme degrees of weathering suggests reworking by surficial colluviation processes with the inclusion of poorly weathered rock fragments by downslope transportation. The presence of different processes acting on the slopes allows inference on past environments on the plateau. There would have been a transition from wet, more stable climates, perhaps warmer (favorable to the weathering of the regolith), to a phase of intensification or concentration of rainfall, which would explain the mudflows which deposited C I. In the last 8,000 years, conditions still humid but with stronger oscillations of temperature, as well as phases of more intense gelifraction, would explain the deposition of the weathered materiais of the C II sequences. Such variations, however, were not sufficient to divert the lateritic trend ofthe weathering, that would have persisted after the last glacial maximum.