Carbonate microfacies and paleoenvironmental evolution of the Romualdo Formation (late Aptian/early Albian), Araripe Basin (NE, Brazil)

The Romualdo Formation, located in the Araripe Basin (NE, Brazil), represents an important sedimentary record of the late Aptian/early Albian transition, evidencing the establishment of a mixed siliciclastic–carbonate platform influenced by eustatic oscillations and marine ingressions associated with the breakup of Gondwana. Detailed analysis of thin sections from the Santo Antônio, Cedro, and Sobradinho sections revealed eight main microfacies: Packstone with ostracod (Po), Bioclastic Packstone with intraclasts (BPi), Bioclastic Wackestone with foraminifera (BWf), Wackestone with bioclasts and peloids (Wbp), Microbialite (Mb), Shale with ostracods (So), Massive sandstone with glauconite (Smg), and Lithic Sandstone (LS) which record the transition from restricted inner shelf environments to open marine conditions. Micropaleontological data described in thin sections reveal a high diversity of microfossils, including foraminiferal genera (Spirosigmoilina sp., Quinqueloculina sp., Neoconorbina sp., Sigmoilinita sp., Microhedbergella sp., among others) and ostracod genera (Pattersoncypris sp., Damonella sp., Mongolianella sp., and Dicrorygma sp.). The dasycladacean green algae Neomeris cretacea and Brasiliporella sp. were also identified, representing new records for the south-central margin of the basin and expanding the paleogeographic distribution of these organisms. Biostratigraphic data and the Oceanic Anoxic Event (OAE1b) was used as a chronostratigraphic datum for correlation between the studied sections, marking the upper Aptian/lower Albian boundary. The event OAE1b corresponds to a major transgressive pulse, enabling the integration of local facies successions into the global paleoceanographic framework. Overall, the integration of microfacies, microfossil, and geochemical data reveals the development of a heterogeneous shallow-marine system, composed of third-order transgressive–regressive cycles controlled by eustatic oscillations and marine connections of Tethyan affinity during the late Aptian/early Albian.