Gênese e classificação das formações superficiais tropicais em paisagem cárstica do topo do Grupo Paranoá

Abstract

Understanding the genesis and behavior of surface formations in karst reliefs is an intriguing problem for the scientific community. The highly soluble nature of calcite and dolomite makes congruent dissolution the dominant weathering mechanism, complicating the task of explaining the presence of thick unconsolidated covers over carbonate rocks. Given this perspective, the question arises: what is the genesis of the surface formations covering the karst of the Legado Verdes do Cerrado (LVC) in Niquelândia-GO? This study sought to understand, from a dynamic point of view, the nature of the soils overlying the geological substrate of the study area and, specifically, to characterize the rocks and surface formations in terms of granulometry, chemical composition, and mineralogy, in order to verify if there is a vertical relationship between surface formations and the underlying rock. Based on bibliographic research, the concept of surface formations, their classification systems, and the main interpretations of their dynamics were studied. Maps of geology, structural lineaments, hypsometry, geomorphological compartments, slope, and soils of the study area were created using the free software QGIS, with vector and raster data acquired from the following platforms: Diretoria de Serviço Geográfico do Exército (DSG), Divisão de Geração de Imagens, do Instituto Nacional de Pesquisas Espaciais (DIDGI/INPE), OpenTopography, IBGE Map Portal, and the Sistema Estadual de Geoinformação do Estado de Goiás (SIEG – GO). Georeferencing and vectorization of maps from bibliographic sources were performed. Block diagrams were created from the SRTM/Topodata DEM using the free software Sketchbook® for Android®. Two field trips were conducted: November 3rd, 4th, and 5th, 2022, for area reconnaissance, and May 3rd, 4th, and 5th, 2023, for soil and rock sampling using auger drilling and trenches. After in loco morphological description, 40 samples were collected from 5 profiles (P-1, P-2, P-3, P-5, and P-6), with 33 soil samples and 7 rock samples. Granulometry was performed on the soil samples by profile. To estimate the chemical composition, X-ray Fluorescence (XRF) was used, performed with the Rayny Shimadzu EDX720 equipment, with a Rh source. The data were organized into graphs and a mass balance was performed, using the fresh or slightly altered rock as a reference and the chemical element Ti as an invariant. X-ray Diffraction (XRD) was obtained using a RIGAKU – Ultima IV diffractometer with CuKα radiation. Mineral identification was performed using Origin Student Version software. The results present a general classification proposal for surface formations, based on structural and dynamic criteria. For the karst depression, it was assumed that the surface formations are residual, that is, they formed under autochthonic and parautochthonic conditions. There is a relationship of vertical affiliation in P-1, P-2, and P-5. In P-5 and P-6, although there were no rock samples, observations were made based on the deepest sample, considering affiliation from a more clayey lithology for P-5 and a sandy lithology for P-6. P-3 does not have vertical affiliation, as its carbonatic rock supports a surface formation comparable to the textural, geochemical, and mineralogical data of P-2. The color is variable, with profiles P-1 and P-5 being more reddish and constant throughout the depth, and P-2, P-3, and P-6 being more yellowish. An increase in clay content was observed from east to west of the area, with P-5 being the most clayey. The mass balance in P-1, P-2 and P-5 it indicates vertical affiliation with the substrate. Very high gains (exceeding 300%) in mass balance reinforce the interpretation that there is no vertical affiliation between the surface formation and the underlying rock in P3. In general, the mineralogy of the surface formations is comparable to that of silicate rocks covering karst terrains at the top of the Paranoá Group, presenting mainly quartz, muscovite, chlorite, kaolinite, vermiculite, hematite, goethite, magnetite, gibbsite, rutile and anatase. In smaller proportions, almost undetectable, there is calcite and dolomite. Finally, an evolutionary sequence was suggested for the surface formations, followed by geomorphological interpretation and classification of the surface formations in the area, based on dynamic and structural criteria. For classification purposes, a map was created at a scale of 1:50,000, highlighting the unconsolidated covers within what was defined as the Felipa-Itambé Karst Border Polje.