Abstract
A few widely scattered exposures of silicified sedimentary rocks occur in the central Depressao Periferica along the edge of the Parana Basin in Rio Grande do Sul State, southern Brazil. This paper presents mineralogical aspects of these silicified materials and compare 2 sections: one from Santa Maria and other from Cachoeira do Sul. Silicification occurs as lenses, duricrusts, nodules and pedotubules in the sedimentary rocks of Mesozoic age. Mineralogical and micromorphological features were studied using optic microscopy and Scanning Electron Microscopy (SEM/ EDS). The powder X-ray diffraction was used for the analysis of clay minerals and also to determinate the Cristallinity Index (C.I.) of the silica minerals. Microquartz predominates in the matrix followed by fibrous chalcedony and megaquartz. The main type of silcrete is Massive that has 2 sub-types: with and without glaebules. The mineralogy and the microfeatures suggests that silicification occurred near the surface under pedogenic conditions.
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