Estudo paleoambiental dos carbonatos pensilvanianos da borda norte da Bacia do Amazonas – região do Rio Jatapú

Abstract

In the Amazon Basin the sedimentary record of the Tapajós Group comprises rocks of economic interest in the building industry and oil exploration. Specifically the carbonate rocks are used as the main raw material in the cement manufacturing, but they also might be interesting traps for oil. The limestones located in the Jatapú river, northern edge of the Amazon Basin, Urucará/AM was the object of this study and they are currently being explored by the João Santos Group for cement production. The multi-approach study, including faciology, microfaciology, mineralogy, chemical and stable isotopes data, was undertaken to understand the geological history of these rocks. A total of 46 samples were taken every 0.50 m along a 15-m high stratigraphic section that was divided in three main levels. The lower level consists of pure carbonate rocks with calcite and quartz (high content of CaO), followed by a middle level of shale formed by kaolinite, illite/muscovite, pyrite, ankerite and dolomite with the highest concentrations of SiO, MgO, AlO, FeO, KO, trace 223232 elements (Sc, V, Ni, Cu, Zn, Ga, Rb, Zr, Nb, Mo, Cs, Hf, Pb, Th) and rare earth elements among the levels. Finally, the upper level corresponds to a limestone and consists of calcite and dolomite intercalated by kaolinitic material. In these three levels were identified eight facies and 21 microfacies. The microfaciology characterization allowed us to group them into three facies associations, which are representative of lagoon (AF1), bioclastic bank (AF2) and shallow marine (AF3) environments. Furthermore, the integration of microfaciology and carbon and oxygen isotopic data led us to identify the presence of diagenetic meteoric fluids and B- and C-types dolomites. The lagoon environment comprises shales, dolomites and fossiliferous mudstones foraminifera, echinoderms, brachiopods, gastropods, pelecypods, ostracods, bryozoans and peloids. Based on this association and the carbon and oxygen isotopic signatures we may suggest the existence of an effective connection with a marine environment. The bioclastic bank, which the transition from the lagoon environment is marked by the presence of ooids, is characterized by bioclastic fragments and disjointed shell suggesting an intertidal environment. The marine environment in the subtidal conditions is marked by large amounts of bioclastic fragments and almost lack of lime mud. These conditions are consistent with a depositional model of shallow carbonate ramp developed in a hot and humid environment