Gênese de formações superficiais oxídicas na superfície geomorfológica de Boa Vista das Missões/RS

Abstract

This present study aims to identify the genesis of oxidic superficial formation on the Geomorphic Surface from Boa Vista das Missões/RS, through the physical, chemical and mineralogical description of the constituent materials. In order to achieve the requested goal, fieldwork in the area of study and laboratory analysis were necessary. The fieldwork aimed at macromorphology description and collect of samples for laboratory tests. The laboratory tests were: granulometric; macronutrients chemistry; sulfuric attack; total chemistry by X – ray fluorescence spectrometry (XRF); mineralogy by X – ray diffraction (XRD) and micromorphology. The results of the macromorphological and textural characteristics enabled to recognize more than 700 centimeters of materials in the study section, individualizing itself into 10 volumes, where: Ap, AB, Bw1, Bw2, Bw3 and Bw4 – corresponding to the solum; BCc – stone line composed of supported clasts; BC and CB – solum/alloalterite transitional horizons; C1 – alloalterite, and C2 and C3 - isoalterite. The macronutrients presented strong acidic materials for the solum, indicating a pedogeochemical environment with basic cations leaching. The CEC highlighted materials consisting of clay minerals of the 1:1 type and a high degree of weathering, allowing the identification of the solum as a Latossolic B horizon. The saturation revealed a dystrophic character, which coincides with highly leached material. The percentage of A1 and the aluminum index determined alytic character. The sulfuric attack confirmed the B horizon classification for latossolic materials, evidencing the predominance of kaolinitic materials. The association of the analysis of macronutrients and sulfuric attack allowed the identification of the class of LATOSOL BRUNO Dystrophic, with an alytic character. The chemical determination of the 10 main oxides indicated the presence of TiO2, Fe2O3 and Al2O3 oxides (the most resistant elements to chemical weathering) and minimum values of CaO, NaO2, MgO, P2O3, MnO, K2O (the most mobile elements). These geochemical values indicate an environment with action by hydrolysis of the monosialitization type. The stone line (420 – 430cm) is individualized by an abrupt transition and made of supported pebbles. The material is oligomitic, composed of approximately 70,40% Fe2O3 oxides, 8,4% Al2O3, 6,2% SiO2 and 1,3% TiO2. However, the presence of the stone line suggests an allochthonous nature for the overlying materials. The mineralogical constitution verified the presence of Phyllosilicates and Oxides: Fe, Ti and Al. In the clay fraction of the solum there is Halloysite, Kaolinite, Goethite, Hematite and Maghemite, and to a lesser extent, Quartz, Ilmenite, Gibbsite and Rutile. Furthermore, the mineralogical composition indicates the action of the monosialitization process (kaolinite, of the 1:1 type), as already verified in the chemical analysis. The association of physical, chemical and mineralogical analysis indicates attributes related to the pedogeochemical phase of ferralitization of superficial deposits. The micromorphology allowed the identification of characteristics of materials of allochthonous and autochthonous genesis. Therefore, the recognition of pedo- and lithorelict, that are usually found in colluvial materials, indicated that above the stone line there is allochthonous genesis, while below it is autochthonous. In this perspective, the materials correspond to LATOSOL BRUNO Dystrophic evolved from different genesis. Thus, the pedostratigraphic interpretation for the horizons highlights: Ap, ABb, BAb, 2Bw1b, 2Bw2b, 2Bw3b, 3Bw4b, 4Bw5b, 5BCcb, BC, CB, C1, C2, and C3.