Polygenetic oxisols on tertiary surfaces, Minas Gerais, Brazil : soil genesis and landscape development

Unravelling the genesis of polygenetic soils is a complex task because of overprinting and mixing of various phases of soil formation. Large areas of polygenetic Oxisols occur in the state of Minas Gerais, Brazil. They developed on surfaces exposed since the Tertiary or longer, which have been more or less affected by tectonic activities throughout the Cenozoic. The combination of continuously exposed soils in stable landscape compartments with sedimentary deposits in adjacent graben zones offers the possibility of interpreting the history of the soils found at the present day surface. With this aim, soil sequences developed on continuously exposed surfaces and on sedimentary layers were studied by physical, chemical, mineralogical, micromorphological, and submicroscopical means.The study areas are situated near the towns of Sete Lagoas (SL) and São João del Rei (SJR). The main parent materials consist of Tertiary sediments and saprolites of metamorphic and sedimentary rocks. Polygenesis of the soils is indicated by the presence of a large diversity of relict features derived from soils and saprolites that were already strongly weathered. The mineralogy of all sequences reflects a high degree of weathering and homogenization.Two types of kaolinite with different crystallinity occur in the sequences developed on rock-saprolites. Hematite and goethite are the only iron oxides found in the clay and silt fractions. Hematite appears to have been crystallized into micron-sized spherical aggregates in early weathering stages. Goethite is mostly a 'secondary' product that more closely reflects the soil forming environment. The large variation observed in goethite properties within the same horizon in soils on rock-saprolites and on sediments reflects their polygenetic nature. Laser diffraction grain-size distributions showed that Oxisols developed on rock-saprolites do not have strong aggregation due to iron oxides alone. Conversely, aggregation by iron is evident in the Oxisols on sediments, which have secondary iron accumulations related to water saturation phenomena.A number of evolution phases were recognized in both continuously exposed surfaces (5 phases) and graben fillings (10 phases), which could be linked in a relative time frame. The seven oldest phases identified in the various sedimentary layers from the graben fillings are overprinted in a sole profile in the soils formed on stable landscape compartments. Formation of yellow soils and xanthization of the red ones is the most recent phase. The stripping of the saprolites and petroplinthite layers belong to the same erosion event, previous to (sub)recent pedogenesis. The last Oxisol formation phase common to the whole landscape is recognized in both types of sequences. The geological time frame encompassing this evolution extends from the Late Paleogene up to the Holocene, representing some 40 million years.