Continental carbonates from Itaboraí Formation in southeastern, Brazil
Introduction
The continental carbonates in the last decade has became an object of major interest in the scientific and industrial community due to the discovery of the pre-salt plays in South Atlantic. It represent a challenge mainly because those carbonates have received minor attention comparing to the marine carbonates and also, only few reservoirs are known. In Brazil, besides the recently discovered reservoirs, only few examples of continental carbonates are documented, the occurrence in Itaboraí Basin is the only one that could be recognized, so far as continental thermal carbonate or travertine. The occurrence is part of the sedimentary sequence of the Itaboraí Formation, deposited in the São José de Itaboraí Basin (SJIB), located in Rio de Janeiro state, about 60 km from the city of Rio de Janeiro (Riccomini et al., 2004).
It is considered continental carbonate any type of rock formed by precipitating calcium carbonate and calcium/magnesium under the strictly influence of continental water (Alonso-Zarza and Tanner, 2009). Continental carbonates are separated informally in travertines, precipitated from hot springs, tufas, precipitated from low temperature sources, such as lakes and rivers (meteogene travertine), and speleothems precipitated from water in low to high temperature in caves or karst fracture systems (Pentecost, 1995, Ford and Pedley, 1996).
In general terms, Itaboraí Formation deposits are composed of travertine, pisolites, carbonate breccias, marl and lake/marsh eocenic deposits. In association with the precipitation of carbonates, amorphous silica (chalcedony) can also be observed (Sant'Anna et al., 2004).
Travertine occurs mainly as fine laminated deposits, crystalline crust deposits and veins in the context of hotspring carbonates. In areas near the border of the basin, carbonate breccias can be found, while in the regions less affected by tectonic influence finely laminated lacustrine deposits are found. Carbonates are present as crystalline crusts and veins from hotspring affiliation and marls as massive deposits with high fossil content of mollusks and gastropods (Sant'Anna et al., 2004, Bergqvist et al., 2008). SJIB is the single Brazilian deposit to register the first mammal's irradiation after KT extinction around 65 Ma.
In this paper, the term travertine will be exclusively used as a reference to thermal deposits described in different sites as Cambazlı, a fissure-ridge travertine in Gediz Graben, Turkey (Selim and Yanik, 2009), Lianchangping hot springs in Yunnan, China (Wang et al., 2015) and Süttő Travertine Complex, Hungary (Pazonyi, 2014). Some authors also use the term to describe precipitated deposits in ambient temperature usually associated with macrophytes, invertebrates and bacteria, which preserve isotopic signature compatible with the thermal water source (Pentecost and Viles, 1994, Özkul et al., 2014). The term tufa (Pedley, 1990, Capezzuoli et al., 2014) will be applied to precipitated carbonates at low temperatures with macrophytes, invertebrates and bacteria association, whereas the term travertine will be used to describe deposits without macro and microbiological traces associated with high temperature water sources.
The article focuses in the study of travertine occurrences in southeast of Brazil, where are deposited inside a graben system and has its formation conditioned by fault reactivation processes. The purpose of this study was to understand the basin carbonates development supported with facies and isotopes analysis and obtain better comprehension of the diagenetic processes, as well.
Section snippets
Regional setting
During late Paleocene the southeastern margin of Brazil was widely passive. At the time, São José de Itaboraí Basin (SJIB) was starting to be formed as a result of the settlement of southeastern Brazil's Continental Rift - CRSB (Riccomini et al., 1989, Ferrari, 2001). CRSB consists of a series of cenozoic grabens in southeastern Brazil from Paraná to the north of Rio de Janeiro and coinciding with the extension of Serra do Mar and, partially, of Serra da Mantiqueira mountain range with a
Material and methods
A set of 90 samples were collected from different carbonate facies in 8 outcrops in order to build a lithostratigraphic profile and allow the analyses of the sedimentary, diagenetic and stable isotopes features. Of those 90 samples, 72 were selected to produce thin polished sections to be studied under an optical microscope. For carbon and oxygen stable isotopes (total sample) analyses, 53 samples were sorted out, processed by maceration and sieved in grain size of 80 meshes. Later they were
Facies and facies associations
The carbonate deposits studied were grouped into six sedimentary facies (Table 1), including: Bedded Carbonate - Bc (Fig. 5A); Carbonate enriched with Silica – Cs (Fig. 5B.); Cemented Carbonate Conglomerate (Wackestone matrix with terrigenous sediments) – Cc (Fig. 5C.); Calcretized Fossiliferous Marly Limestone (Wackestone/Packstone) – Fml (Fig. 5D.1 e D.2); Clayey Limestone with Fossils (Mudstone/Wackestone) – Cl (Fig. 5E); and Reworked Fossiliferous Argillaceous Carbonate
Origin and depositional settings
According to Sant'Anna et al. (2004) the settings of the deposits were highly variable and the travertine generation was the result of a combination of Cenozoic reactivation of an old tectonic structure in the basement and groundwater hydrology. Tectonic activation processes would be responsible for the instability required to start the movement of alluvial sediments (B), while travertine (A) would be deposited due to the presence of thermal sources associated with faulting. Hydrothermal
Conclusions
SJIB carbonates are consistent with deposition under continental conditions within a Paleocene hydrothermal system associated with alluvial, marsh, lake and fissure deposits. The facies associations are genetically related to the tectonic basin.
The presence of conglomerates associated with travertine deposits on the active border of the basin shows episodes of major tectonic activity associated with less active ones, while travertine deposits show genesis associated with the reactivation of
Acknowledgements
The authors thank the Chemistry Stratigraphy and Organic Geochemistry Laboratory (LGQM) of the Faculty of Geology - UERJ (FGEL) for carrying out the isotopic analysis, and the Sample Preparation Geological Laboratory (LGPA), also from FGEL – UERJ, for the production of thin section carbonates. The authors also thank FAPERJ for providing financial assistance by means of Project E26/111 565/2013 and Petroleum National Agency (ANP) (process number is 4810.008925/99) for providing the project
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