Organic geochemical evidence for the transition of Aptian-Albian hypersaline environments into marine restricted seas: The South Atlantic oceanic northern gateway and its implications for the pre-salt deposits

Abstract

The transition between Aptian and Albian stages of the Lower Cretaceous (∼113 Ma) was a critical moment in the development of the proto–South Atlantic Ocean. At this time, marine ingressions in Brazilian sedimentary basins occurred in the northeastern region and deposited organic-rich mudstones and sulfate evaporites. However, the paleogeographic configuration that controlled sedimentation and biota dynamics remains poorly understood. This study presents new paleogeographic insights based on tectonic reconstructed Gplate models and a high-resolution chemostratigraphic framework. We explore new isotopic data of bulk organic carbon and nitrogen as well as saturated fraction biomarkers collected from a well-preserved section in the eastern part of the Brazilian northeastern Parnaíba Basin (Codó Formation). We also present new biomarker data for samples from the Barra Velha Formation from a well in the Santos Basin, which belongs to the same local stage of the Codó Formation (Alagoas Stage). For the Parnaíba Basin section, geochemical proxies subdivide the environmental evolution into five stages. These stages reflect a transition from a highly saline environment (stage A) that experienced transient marine influxes and eventually transitioned into a closed evaporitic system (stage B). A major marine ingression established an epicontinental anoxic sea (Stage C) that subsequently experienced an increase in water circulation and water depth; turning into an oxic marine environment (Stage D). Interruption of the connection with the sea and constant terrestrial input led to a continentalization process establishing a lacustrine/continental environment (Stage E). For the Santos Basin section (Barra Velha Formation), we measured the occurrence of the C30 tetracyclic polyprenoids which are consistent with a dominant lacustrine saline environment. Our paleogeographic model suggests that marine ingressions towards the southern basins, during the first stages of Pangea breakup, occurred through the northern portion of the Borborema Province and produced an interior seaway that possibly connected the Brazilian northeastern with the southeastern basins through the Recôncavo/Tucano rift system. Our findings and the environmental interpretations for stratigraphic correlated units from the southern continental basins infers that the greater marine influence in the northeastern areas shifted from hypersaline environments into restricted marine environments that resulted in the deposition of shales and mesohaline/penesaline salts. Concomitantly, the existence of a Brazilian-African landbridge acted as a barrier point for the first marine inflows towards the south. The restricted marine connection transformed south alkaline environments into extensive areas of superhaline conditions that resulted in giant thick salt successions of the South Atlantic basins. Finally, further tectonic evolution and complete breakup of Pangea terminated the Brazilian-African landbridge and shutoff interior marine routes that later became terrestrial environments. As the continents steadily became rifted-segments, the proto-South Atlantic Ocean expanded leaving a marine record in the southern basins.