Thin Section Microfossil Analysis: Unlocking Paleoenvironmental Secrets of the Romualdo Formation, Araripe Basin

This study aimed to investigate microfacies to infer paleoenvironmental interpretations of the Romualdo Formation during the Aptian/Albian of the Araripe Basin, Brazil. Samples were collected from three outcrops located in the eastern region and in the central south region of the Araripe Basin. The interpretations in this work were established by evaluating the stratigraphic sections and the subsequent preparation and analysis of 54 petrographic slides. Nine microfacies were recognized: Bioclastic Packstone/Grainstone with ostracods (BPGo); Peloidal bioclastic Grainstone/Packstone with intraclasts (BGPi); Impure Bioclastic Peloidal Packstone with foraminifera (BPfi); Impure Bioclastic Peloidal Packstone with organic matter (BPmoi); Laminated peloidal bioclastic Packstone (BLP); Feldspathic Immature Sandstone (FIS); Laminated boundstone (LB); Parallel-laminated Shale (So); Fine to Medium Sandstone with glauconite (FMSg). In addition, several groups of microfossils were identified on slides: ostracods (eg. Pattersoncypris , Damonella , Mongolianella , and Dicrorygma ), benctonic and planktonic foraminifera ( Spirosigmoilina , Quinqueloculina , Neoconorbina , Sigmoilinita tenuis , Microhedbergella ) and green algae ( Neomeris sp. and Brasiliporella sp.). These microfacies are related to the marine ingressions in the basin during the Aptian/Albian transition, indicating environments ranging from proximal to open marine, according to the microfauna and mineralogical elements described in the microfacies.