Relação entre o capeamento pedológico e o substrato rochoso-arenoso no oeste do estado do Rio Grande do Sul

Abstract

The western part of Rio Grande do Sul, which corresponds to geomorphological sector Cuesta de Haedo, is almost entirely composed by volcanic rocks from the Serra Geral Formation, and by sandy sediments from the upper part of the Paraná basin. Because the landscape of the region is susceptible to erosive processes, it is thus necessary, from geological point of view, to know the rocky substratum and the pedologic conditions imposed, when considering the occupation and usage of region. In this way, two profiles in the arenite of the Botucatu formation and three in the arenite of the Guará formation, were taken. In each profile all the layers that form the rocky substratum and the superposed pedogenic horizons were sampled, and physical, mineralogical/petrographical, and chemical analysis were carried after. The physical analysis corresponded to the size separation of the granules, and to the morphological and texture description of them. The mineralogical analysis corresponded to the identification of the mineral associations present, with emphasis on the distribution of heavy minerals along the profiles, and the petrographical analysis to the classification of rocky substratum through the usage polarization and scanning electron microscope. The mineralogy of the clay fraction was determined trough the usage of X-ray. The chemical analysis, which was done trough inductively coupled plasma atomic emission spectroscopy (ICP), tried to identify twelve chemical elements present in every layer and horizon, however only the concentrations of Si, Al, Fe, Ti, Mg, and Mn were used. In all five profiles the rocky substratum was classified as quartz-arenite, because it was almost completely composed by quartz. In one of them, rare altered feldspar was found and was on the limit to subarkose. All the superposed pedogenic horizons showed variable relation with the underneath rocky substratum, however in four out five profiles the external participation of geological environment was responsible for final product. Theoretically, the quartz arenite rocky substratum similar petrografically, should generate similar soils, like quartz-arenite neosoils, however the influence of the geological environment was determinant for such thing not to happen, and to impose clay characteristics to the horizon B in four out five profiles, acting then as a sixth variable in the pedologic generation.