Composição dos arenitos da formação Serra do Segredo, Caçapava do Sul - RS

Abstract

The Serra do Segredo Formation, belonging to Santa Bárbara Group, is in the south-west of Caçapava do Sul town, composing part of a series of sedimentary units disposed of in the central-south portion of Rio Grande do Sul state. These units are included in the Camaquã Supergroup, deposited between 600 Ma and 530 Ma. Sandstones and conglomerates of Serra do Segredo Formation are related to the progradation of intra-estuarine deltas and alluvial fans. Classification of sedimentary rocks of Serra do Segredo Formation have been published by several authors. However, there are few works that describe the sandstone composition, texture, and diagenetic features in high-resolution. In this work, the composition and texture of sandstones from Serra do Segredo Formation are presented by point counting using thin sections. Nine samples were collected in a vertical log. Sandstones are texturally sub-mature to mature and are classified as feldspathic, feldspathic-lithic, lithic-feldspathic, subfeldspathic and sub-lithic arenites. From base to the top, quartz content is reduced and biotite, muscovite, and chlorite content increase. Plagioclase is higher at the base, and K-feldspar content increase to the top. Diagenetic features include oxide/hydroxide iron coatings, quartz and plagioclase overgrowths. Partial dissolution of some framework grains and cement, and deformation of less competent grains, which indicates a mechanical compaction mechanism. Porosity in the sedimentary rocks is intergranular, intragranular, fracture, and channel. Dissolution processes favor the porosity increase. Results indicate that a gradual change in source areas, by the reduction of quartz and plagioclase content and increase in K-feldspar and micas to the top. This may indicate the source area denudation, by rocks containing plagioclase and quartz to rocks composed by K-feldspar and micas.