Depositional and diagenetic conditions during early South Atlantic opening: Evidence from the upper Aptian Oiteirinhos Member, Muribeca Formation, Sergipe-Alagoas Basin, NE Brazil

Abstract

The Muribeca Formation from the Sergipe-Alagoas Basin, northeastern Brazil, records the transition from continental to marine conditions during the initial South Atlantic opening. The formation is constituted by the basal coarse clastics of the Carmópolis Member, covered by the Ibura Member evaporites, and the Oiteirinhos Member. The Oiteirinhos Member presents a wide lithological variation, including siliciclastic, carbonate, and hybrid deposits. Considering the scientific importance of understanding the conditions during this key time in South Atlantic evolution, very few sedimentological studies were performed on the Oiteirinhos Member. This article presents a petrological characterization, integrated with sedimentological data from five onshore wells. The studied rocks include siliciclastic sandstones, siliciclastic-carbonate hybrid arenites, marls, microbialites, calcarenites, and calcilutites. The composition of the sandstones is rich in low-grade metasedimentary fragments from the erosion of upper Proterozoic supracrustal orogenic terrains. Intrabasinal carbonate constituents include ooids, peloids, oncoids, intraclasts, bioclasts of ostracodes, benthic foraminifers, and echinoderms. Carbonate microbialites occur associated with laminated organic mud deposits, commonly rich in carbonaceous and phosphate fragments. The main diagenetic process occurred near-surface, under the influence of an evaporitic, dry climate under continental and marine conditions. The depositional and diagenetic aspects reveal progressively less evaporitic conditions during the deposition of the Oiteirinhos Member, with anhydrite followed by siliciclastic and hybrid deposits and finally by marine carbonates.