Geoquímica orgânica e isotópica das formações: Piçarras, Itapema e Barra Velha no campo de Lula, Bacia de Santos

Abstract

The new data presented here, stable isotopes (δ13Ccarb and δ18Ocarb), elemental composition by x-ray fluorescence, and organic geochemistry (TOC, pyrolysis, biomarkers, and δ13CTOC) from core samples, provide clues that may help to understand climate variations, assumed to have been contemporary in the lake evolution for the whole basin during the rift and post-rift. This study proposes a high-resolution stratigraphy based on chemostratigraphic units. The Piçarras Formation (Fm.) (unit A) was deposited under fresh-brackish water conditions. Biomarkers showed a high abundance of 4-methyl sterane and dinosterane can be explained by algal input, diatoms, and dinoflagellates in a reducing environment. Followed by a lake-level fall, Unit B, positive value δ13Ocarb (+9‰) and the occurrence of magnesian clays suggest high evaporation rates predominantly oxidizing lake period with warmer, hypersaline, and alkaline waters. The passage from the Piçarras Fm. to the Itapema Fm. is characterized by the depletion of isotopic values of δ13Ccarb and δ18Ocarb, which represented freshening of the lacustrine environment intensified terrigenous inputs and organic productivity (TOC>3.3% and HI>500 mg HC/g TOC). In addition, high hopane/sterane, and TPP/Dia support freshwater input with increasing bacterial and green algae assemblage under anoxic conditions. The alternating cycles of high and low salinity are related to a semi-arid climate with periods of higher humidity, represented by the rhythmicity pattern in the logs of SiO2, CaO, Al2O3, MgO, δ13Ccarb and δ18Ocarb in subunits C2 and C3, respectively. The lake progressively was drowned by episodes of freshwater influx promoted by subsidence; favorable conditions in water chemistry allowed the deposition of the bioclastic carbonates (coquinas) along interval D. After a regional regression, Pre-Alagoas unconformity marked by an excursion with positive shifted and towards positive δ13Ccarb and δ18Ocarb suggest a more stressful environment in the geological record of the Barra Velha Fm. The isotopic distribution, mineralogical and chemical markers (Sr/Ca; Mg/Ca; Mn/Sr) allowed defining four chemostratigraphic intervals. In BV-1 and BV-2 intervals, the δ13Ccarb and δ18Ocarb values vary according to shallowing-upward cycles, in general becoming lighter in their bases, appear to be related to periods of relative humidity, which resulted in less saline water input with higher extrabasinal contribution. In contrast, heavier isotope values are related to conditions of progressive evaporation of the lake. The unit BV-3 remarks the sag phase; throughout this interval, there are cycles of maximum aridity, and reduction of the water table, favoring the nucleation of these carbonates, which have higher δ13Ccarb and δ18Ocarb values and predominance of shrub facies. The BV-04 interval is characterized by relatively negative δ13Ccarb and δ18Ocarb values, which suggests lake-level fluctuations in a semi-arid climate with a lower evaporation rate in shallow lake conditions. In addition, chemozones with the most negative δ18Ocarb values (-9‰) and high Mg supported post-depositional hydrothermal alterations in units BV3 and BV4. This study shows that climatic condition is the primary driver of the lake level variations and, consequently, the water chemistry.