Análise litofaciológica de tufas da Serra do Cândido, município de Itaocara (RJ)

Abstract

Tufa are continental carbonate rocks, usually formed in karst environments. Its fluvial deposits mainly presents beautiful landscapes that are enjoyed by tourism in a lot of places in the world. Besides this tufas deposits can provide valuable information for paleoenvironmental reconstructions. The petrographic and petrophysical study of tufa become fundamental in order to understand the framework of their constituents and the processes that drive the deposition and preservation of calcium carbonate. Tufa deposits occur in the north central region of Rio de Janeiro state, in the municipality of Itaocara, southeastern Brazil, and are associated with karst springs developed on calcitic marbles from São Joaquim Unit, placed in the context of the Ribeira Belt. This work presents the lithofacies description of these deposits, identifying crystal, facies and morphological varieties formed in cascades environment. In total eight lithofacies were identified, divided into two main facies associations. The first corresponds to a cascade environment or a stepped portions of the riverbed, which occurs coating of plants and hanging stems, that change to stalactites at the base of the falls. Also form flow speleothems such as micro-terraces and rimstone dams where different micro-crystalline forms are observed. At waterfall downstream, the water flow with low energy providing phytoclasts deposition and development of phytoherm tufas. The base of inactive deposits have cemented lithoclasts, land gastropod shells or pisolits locally. The hydrochemical analysis of waters associated with the active deposit called Dezoito de Março proved propitious for calcium carbonate precipitation. The waters have pH around 8, high calcium carbonate content and saturation index of calcite with positive values. The circulation of these waters into ancient deposits brings in diagenetic changes, as neomorfism, observed in the form of replacement of acicular micrite into palisadic calcite crystals