Análise paleoambiental da Formação Teresina, Permiano da Bacia do Paraná (Brasil), e suas implicações paleoclimáticas

Abstract

The Teresina Formation, which composes the middle portion of the Passa Dois Group, Permian of the Paraná Basin, has lithological records subject to a detailed chemostratigraphic study involving isotopic techniques that help to characterize the paleoenvironment and climate of this time interval. Samples from the Teresina Formation were collected from two sedimentary cores from the drill holes FP-04-PR and FP-11-PR, from the project “The Eastern Edge of the Paraná Basin”, for analysis of C and O stable isotopes in carbonate and siliciclastic rocks, total organic carbon (TOC) in shales and for petrographic study. It was observed, in the description of the drill holes, that the Teresina Formation consists predominantly of heterolytic rocks, composed of very fine sandstones, interspersed with siltstones and shales, presenting wavy, flaser and lenticular bedding, with different levels of bioturbation. The carbonate rocks, which occur interspersed with the heterolytic rocks, were analyzed in a petrographic study, with four microfacies being characterized: microbial, including columnar, domal and planar stromatolites, and thrombolites, intraclastic floaststone, bioclastic wackestone and oolitic grainstone. Regarding the diagenetic history of these rocks, it was observed that it includes processes that occurred during eodiagenesis, mesodiagenesis and telodiagenesis. In the drill hole FP-04-PR, the isotopic ratios of C and O (δ13C, δ18O) in bulk rock were obtained from 24 samples of carbonate and heterolytic rocks. In the drill hole FP-11-PR, the isotopic analyzes were performed on 62 samples of carbonate and heterolytic rocks. Based on the values obtained from δ13C and δ18O in the samples, different chemostratigraphic intervals were defined. In both drill holes, negative excursions were observed in the values of δ13C in the interval B, reaching in the drill hole FP-04-PR the value of -4.51 ‰ (62.6 meters), and in the drill hole FP-11-PR, - 9.02 ‰ (181.3 meters). Regarding the δ18O isotopic curve, it was observed that the values are negative during the event and become less negative for the top of the section, which could indicate that the carbonates would have been precipitated in an environment with high salinity. To determine the influence of diagenesis, δ18O versus δ13C plots for the three chemostratigraphic intervals were plotted. It was seen that the isotopic data from the drill hole FP-11-PR would have been more affected during diagenesis. The negative excursions in δ13C recorded in these drill holes may be related to the Mesocapitanian mass extinction event, and would indicate an increase of 12C in the water, due to a severe reduction in primary productivity, resulting from climate change, possibly related to volcanic eruptions. The correlation to the event would be supported by previous biostratigraphic studies and recent radiometric dating carried out in the Teresina Formation.