Descrição e significado ambiental dos morfotipos estromatolíticos do Grupo Vazante na Fazenda da Barra, Coromandel, MG,

Abstract

In Brazil, stromatolite records frequently occur in Proterozoic units associated with folding belts, such as in the Brasília, Ribeira and Araçuaí belts. In the Vazante Group, stromatolites were identified in the 1970s, with studies carried out mainly from the north to the center south of the occurrence range of the unit. The Vazante Group comprises a narrow strip with a N-S direction, composed of successions of carbonate-siliciclastic metasedimentary rocks, with ages ranging from the end of the Mesoproterozoic to the beginning of the Neoproterozoic. The absence of guide fossils and the history of regional metamorphism results in important consequences for studies of associated units, mainly in relation to correlation, ages and paleogeography. The municipality of Coromandel is located in the extreme south of the unit, in a karst context where several exokarst and endokarst features develop. Some of these caves preserve stromatolites in dolomitic limestone, which are exposed on the internal walls of the caves, as well as in several outcrops in the external areas of the cavities. Although studied in other locations of occurrence, the stromatolites of the Vazante Group have never been studied in detail in the southernmost portion, especially in caves, whose karst process results in good quality of preservation of these microbial organisms. This project aims to describe the stromatolitic morphotypes of the Vazante Group (Lagamar Formation, Sumidouro Member) that occur in the caves and nearby Fazenda Barra, Coromandel, MG, positioning them in stratigraphic sections to interpret the paleoenvironmental dynamics of these levels of stromatolite occurrence. The morphotypes are mainly stratiform and turbinate that grade to columnar with a parabolic top. This pattern, together with the absence of coniform stromatolites, suggests that conditions were shallower in Coromandel than in Lagamar, the northernmost region where morphotypes have been studied. The succession of stromatolitic morphotypes from the Lagamar Formation in Coromandel suggests that this occurrence represents a different stratigraphic level from those already described with stromatolites for the Vazante Group.