Anoxic conditions and water circulation restriction degree during deposition of black shales of the Romualdo Formation (Araripe Basin)

The Romualdo Formation, upper strata of the Santana Group in the Araripe Basin, presents clear evidence of Aptian marine oscillation events in the interior of Northeastern Brazil. Evidence of this oscillation occurs in the transition of the base to the intermediary portion of this unit, where intercalations of siltstones and sandstones (coastal facies) are superimposed by a thick interval of dark gray to black shales (shelf facies). Those black shales are characterized by an intermittent presence of fossiliferous (especially an extent ichthyofauna) carbonate concretions. More specifically at the Sobradinho outcrop, one of the thickest sections of this unit, more recent studies have interpreted a variation in non-marine and brackish salinity within its black shale interval through geochemical analyzes (V/Cr, Sr/Ba and S/COT). Furthermore, zones of anoxia were also identified in this outcrop at the most basal portions of the black shales using TOC and δ¹³C analyzes of its organic matter. In the present work, the presence of an anoxic event and the degree of water circulation restriction of two outcrops were evaluated: Serra do Mãozinha and Sobradinho (NE and SE regions of the Araripe Basin respectively). d¹³C isotopic data from organic matter and associated carbonates, and geochemical data on TOC, Hg and trace elements from black shales from these sections were used in the intended assessment. In general, it is observed that the TOC and d¹³C data point out to anoxic conditions at the base of the shales of the Romualdo outcrop, as has already been identified by previous studies, but they do not present signs of anoxia for those of the Serra da Mãozinha outcrop. The trace element data allowed the creation of graphs using TOC and enrichment parameters in Ni, Mo, Cu, U and V, which indicate the degree of anoxia and water circulation restriction during sedimentary deposition. The graphs indicate an anoxic environment, which may eventually present particle shuttle conditions, and a low to moderate degree of restriction (Saanich Inlet and Cariaco Basin likely) for the basal black shales of the Sobradinho outcrop. This restriction becomes practically non-existent in the intermediate portion of the black shale interval. On the other hand, samples from the Serra do Mãozinha outcrop were deposited in a more suboxic environment where no significant water circulation restriction was observed. Therefore, our data indicate that water anoxic conditions and restrictions in the eastern part of the basin during the deposition of black shales from the Romualdo Formation were limited to southeast regions.