Biomarkers and PAH chemostratigraphy in the study of the Frasnian anoxic event in the Pimenteiras Formation outcrop of the Parnaiba Basin, Brazil

The Late Devonian is characterized by episodes of sea-level rise and expansion of dysoxic to anoxic paleoenvironments, resulting in a global abundance of organic-rich sedimentary rocks. These changes have been well documented in biostratigraphic studies of Euroamerica basins. However, the relationship between sea-level rise and marine anoxia in Western Gondwana sedimentary rocks is not completely understood. Here, biomarker and PAH diagnostic ratios were applied to investigate changing depositional paleoenvironment and organic matter inputs of the Pimenteiras Formation along the western edge of the Parnaiba Basin. First, 24 shale samples collected from 16 regionally distributed outcrops were analysed, revealing variability in depositional paleoenvironment conditions. Secondly, for detailed study a 15-m thick outcrop was chosen, from which 11 shale samples were collected. These samples were submitted to chemostratigraphic and palynostratigraphic analyses and their records were highlighted. Two sample groups were identified, one sample group containing mixed organic matter, predominantly terrigenous and with fresh-brackish water organisms, deposited under dysoxic conditions. The other sample group comprised mixed organic matter containing a high contribution of marine organisms, preserved under anoxic conditions. Results indicated a marine flooding surface in the Pimenteiras Formation Outcrop 1, associated with a Devonian transgressive event related to sea-level rise and probably to global anoxic conditions during the Frasnian in the Parnaiba Basin. In addition, it was possible to correlate the abundance of miospores Verrucosisporites genus with high Retene/Chrysene values, suggesting that these miospores can be associated with a biological precursor of retene.