The Aptian in the Onshore Basins of Northeastern Brazil: Pre-Salt Analogues?
Aptian (late Early Cretaceous) strata in the brazilian sedimentary basins are known by the local Alagoas Stage (pro parte), defined in the Sergipe-Alagoas basin, but occurring along all other brazilian marginal basins, where they stand generally for the “Transitional” (tectonic) or the “Gulf”/“Saliferous” (paleoenvironmental/ paleogeographical) episodes of these basins’ evolution. Recently, the “Pre-salt” interval, belonging to this stage, was shown highly attractive for petroleum exploration, particularly in the Santos Basin. However, many challenges from the geological point of view are posed for this exploratory frontier, including its facies analysis and paleodepositional models. Analogs for this study occur (and outcrop) in the Araripe and Parnaíba (or Grajaú) onshore basins, in the Northeast of Brazil. Some of their characteristic lithofacies are organic shales, carbonates and evaporites (sufate). Detailed analysis of these facies are carried by the present study on well-corings (lithofacies) and thin sections (microfacies), complemented by organic matter data (mostly palynofacies), for these basins’ interval. Results point out that many of these facies are microbially mediated (truly microbialites), deposited in lacustrine (embayment) to coastal sabkha paleoenvironments, with episodical marine floodings that preceeded the final Albian atlantic transgression. Despite possible (paleo )geographical and fine tuned age constraints, the anoxic and hypersaline character of the shaly facies deposition is also discussed under the OAE1a event perspective. Stratigraphically, tectonosequences could be also defined by clastic input changes, including sandstone provenance, also raising discussions on the geotectonic evolution of the Araripe and Parnaíba basins, and on the own marginal basins’ evolution. These pieces of information highlight the regional Pre-salt paleogeography.