Characterization of the parent materials in agricultural soils in the area of Carlos Reyles, Durazno

Abstract

The objective of this work is to determine the parent material of the area of Carlos Reyles, Durazno, as well as the rest of the area and its relation with the local geology. We studied four different places where geological survey was performed as well as sampling of horizons C. For the horizons C we estimated the size of the mineral particles by using the Bouyoucos method and X Ray Diffraction to quantify the mineral composition and the kind of dominant clay mineral. For the main representative lithologies we made thin sections for a better visualization in the petrographic microscope. These indicate, for the different lithologies, a similar composition and distribution of minerals, having as differences between them that they were affected by subsequent diagenetic processes. With respect to the gilgai microrelief, the parent material is a fine sandstone showing a silty-clayey matrix, which had not been reported in the area so far, associated with sandy levels of the Fray Bentos Formation. The C horizons show similar features to the sampled outcrops, both for the East and West of the study area. In the East, the resulting soil is sandy, grayish brown in color, and exhibits varying depths (70-100 cm), with a dominant mineralogy composed of quartz, plagioclase, potassium feldspar and little to very little clay, where the dominant clay mineral is the kaolinite. For the West zone, where the gilgai microrelief occurs, the resulting soils are deeper (>100 cm), dark, with a higher clay content with respect to the eastern area. The dominant mineralogy in the C-horizon is composed of quartz and plagioclase, with a medium proportion of clay. In the areas of these sites where the microrelief is not recognized, the mineralogy of C horizons is similar to those observed for the southern part of the West zone. The dominant clay minerals of the clay fraction of every C horizon in this area belong mainly to the smectite group.