Continental carbonates from Itaboraí Formation in southeastern, Brazil

Since the discovery of the massive South Atlantic hydrocarbon reservoirs (Pre-Salt), the interest in these terrestrial carbonates has increased. In Brazil few examples of continental carbonates are known, and solely the occurrence described in this study can be recognized as continental thermal carbonate or travertine. The occurrence is part of the sedimentary sequence of the Itaboraí Formation, deposited in Itaboraí Basin, located in Rio de Janeiro state, about 60 km from the city of Rio de Janeiro. The basin dates back to the Paleocene due to its vertebrate fossil content, which can be found in a small half-graben basin (1.5 km × 0.5 km). Despite the small size of the basin, it represents a huge source of geological information for its diverse fossil assemblage, the occurrence of travertine and the exposition of alkaline dyke. Most of the studies regarding the basin are related to the extensive fossil community from the lake-fill facies succession, which are not directly related to the travertine deposition. The present study conducted a sedimentological investigation applying petrography, facies association and isotope analysis as its methodology. As a result, four distinct facies associations were described: Thermal Springs, Alluvial, Palustrine/Lacustrine and Fissural. Sedimentation succession allowed the study to recognize sedimentation and tectonic relationship. The origin of the thermal activity is probably related to the tectonic pulses of the hemi-graben system, so in these conditions two different contexts for travertine deposition associated to the flexural and active borders of the basin were established. The petrography exhibited shrubs textures with remarking crystalline quality indicating a major abiotic influence during the process of precipitation. The stable isotope ratios provided contrasting results against those expected for a thermal carbonate signature, but not so different from those expected for meteoric carbonate signature, indicating a mixture of both processes.