Erosion, sedimentation and pedogenesis in a polygenetic oxisol sequence in Minas Gerais, Brazil

Abstract

The geomorphic evolution of the south-eastern Brazilian landscape is attributed to climatic changes coupled with tectonic activity. Soils developed on the resulting surfaces are mainly deep polygenetic Oxisols. The combination of stable landscapes (with continuously exposed soils) and neo-Cenozoic graben zones (episodically filled with sediments that may have undergone soil formation) offers the possibility to unravel the history of the soils that are found at the present day surface. Soil-sediment sequences in the southern part of the state of Minas Gerais, Brazil were investigated by micromorphology and mineralogy in order to understand the genesis of the soils. Erosion (profile thinning), weathering, biological activity, clay accumulation and iron translocation are the main soil forming processes imprinted in the various pedosedimentary layers. Strong weathering is shown by dissolution of quartz and weathering of ilmenite. Bioturbation is ubiquitous even in older and deeper buried layers. Clay accumulation is observed as illuviation and precipitation features which were micromorphologically differentiated. Accumulation and movement of iron are represented by gley and pseudogley features, related to groundwater and surface-water saturation processes. These features and the layering of sediments and soils play key roles in understanding the evolutionary phases undergone by the soils and sediments. As main soil formation episodes, the graben fillings show two phases of clay formation and illuviation, separated by a phase of ferralitization, interrupted by various episodes of erosion. The whole material was eventually overprinted by a phase of ferralitization and plinthite formation. Apart from water saturation, these processes also appear to have acted on the soils of the stable areas (not affected by tectonic activity), but overprinting and erosion prevents recognition of the separate evolutionary phases.