Petrologia e análise faciológica das rochas sedimentares do grupo guaritas na região do Arroio Torrinhas, Pinheiro Machado, RS

Abstract

Geological structures influence the formation and evolution of sedimentary rocks. The Minas do Camaquã fault zone is an important structure of the Camaquã Basin because it deforms the sedimentary units in the central sub-basin, influencing its geological evolution. In this work, a faciologic analysis was carried out together with the petrographic analysis of the sedimentary rocks in a part of the Minas do Camaquã fault zone. The main objective of the work is to understand the geological evolution of the region, mainly seeking to establish the tectonicstratigraphic-petrological relationship between the deformed units. Faciologic analysis was accomplished using high resolution orthophotomosaics and field facies surveys. The data were integrated into a set of facies vertical profile, map and geological section. Thin sandstone sections were made for analysis of composition, texture, structure and diagenesis. In the studied area exists a fault in the Minas do Camaquã fault zone, which places a unit inclined to the west with a horizontal unit to the east. Seven sedimentary facies were defined, grouped into three facies associations. The facies show a succession of depositional environments influenced by climate. The transition occurs from a fluvial environment (CAF facies association) to a eolian environment (AR facies association), succeeded at the top by a high energy environment characterized by amalgamated gravel and sandy bodies, probably corresponding to an alluvial environment (CAL facies association). The CAF facies association has a detrital composition corresponding to litho-feldspatho-quartzose and quartz-feldspathic sandstones, and its main diagenetic processes were: cuticle formation and iron oxide substitutions, illite formation, quartz overgrowth, substitution of clay minerals by kaolinite, and calcite occurring in some samples in a subordinate manner. The AR facies association presents a detrital composition corresponding to litho-quartzose and litho-feldspatho-quartzose sandstones and its main diagenetic constituents are: cuticles of iron oxide, vermicular ilite and kaolinite, quartz overgrowths and poikilotopic calcite. The CAL facies association presents a detrital composition corresponding to litho-feldspatho-quartzose and feldspatho-litho-quartzose sandstones and the main diagenetic processes were: cementation of macrocrystalline and poikylotopic calcite, replacement of detrital grains by calcite, illite formation, grain replacement detrital by iron oxides and pseudomatrix formation. Despite the relative compositional variation, the analyzed sandstones are derived from recycled orogens, indicating a granite source area and composed of supracrustal rocks. The calcite volume increases in the CAL facies association, and there are no quartz overgrowths or have been completely dissolved and/or replaced by calcite. The western facies associations of the fault (CAF and AR) occur largely cemented by iron oxides whose source was the dissolution of heavy minerals, biotite and volcanic lithoclasts. It is concluded that diagenetic patterns are similar in the three facies associations, suggesting a single burial history for both units at west and east of the fault. Therefore, the fault must have acted as a barrier to the hydrological flow that provided the precipitation of carbonates in the eastern portion of the fault, limiting its action in the western portion.