| Summary: | São José de Itaboraí Basin (SJIB) is located in the city of Itaboraí, Rio de Janeiro State (Brazil), at about 60 km from the Rio de Janeiro city. Despite the small size area, about 1.5 km2, this basin represents an important source of scientific knowledge, due to the occurrence of banded carbonates and exuberant fossil assemblages. In the last decade, following the discovery of massive carbonates in South Atlantic Ocean, this basin has attracted the attention due to the presence of continental carbonates, in the form of travertines which are included the Itaboraí Formation. The SJIB carbonate rocks have been recognized as possible analogues of the pre-salt reservoir rocks or at least, similar in their depositional processes. The SJIB travertines were submitted to some diagenetic influence due to the presence of silica rich fluids, similar to that found in pre-salt carbonates. The present research aims to perform a petrographic analysis regarding the silica-rich travertine facies, and to recognize textures and structures in order to provide a better understanding of the relationship between silica and carbonate. The deposits associated with silica are easily characterized by their reddish/brown coloring and their higher resistance, due to the presence of iron oxide and silica replacing the carbonate. In some extreme cases can occur the complete substitution of the carbonate fabric generating silex materials. The relationship between silica and iron oxide/carbonate allowed the development of different structures as the banding formation, probably due to mineralogical and rheological differences during recrystallization. In some regions where the diagenetic process was more intense it could be noticed a complete replacement of the primary carbonate material. While, in other cases, the diagenesis preserved structures and past textures, such as iron oxide mouldic features.
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