Foraminifera and Ostracoda from the Lower Cretaceous (Aptian–lower Albian) romualdo formation, Araripe basin, northeast Brazil: Paleoenvironmental inferences

Abstract

The goal of this study is to investigate the Aptian–lower Albian microfossil association present in the Romualdo Formation, Araripe Basin, Pernambuco State, northeastern Brazil. Fifty-six samples of calcarenites and claystones were collected from the level where marine invertebrates are concentrated and in the coquinoid limestones, which compose the transgressive strata of the Romualdo Formation. The samples were from five outcrops (Cedro, Santo Antônio, Arrojado, Canastra and Torre Grande) located in the south-central region of the Araripe Basin. Nine species of foraminifera and five species of ostracods were found. The foraminifera species were: Agathammina cf. pusilla, Agathammina sp., Spiroloculina sp. 1. Spiroloculina sp. 2, Quinqueloculina sp. Astacolus sp., Gaudrynopsis sp., Conorboides minutissima and Lingulogavelinela sp. The recorded foraminifera associations are representative of different environments, from shallow marine settings to hypersaline lagoons. The five species of ostracods were: Pattersoncypris salitrensis, Pattersoncypris symetrica, Pattersoncypris sinuata, Pattersoncypris cf. micropapillosa and Damonella grandiensis. The ostracods inhabited a myxohyaline environment and tolerated the variation of salinity occurred during the deposition of the Romualdo Formation. Three assemblages were identified and their stratigraphic position was determined for each outcrop, suggesting that the south-central portion of the Araripe Basin presents clearer evidence of marine influence than the southwestern portion. Although a precise reconstruction of the route of marine ingression would require additional studies, the presence of the foraminifera genus Agathammina indicates an affinity with the Tethyan fauna. Therefore, we can conclude that the marine ingression during the Aptian–Albian followed a north-northeast direction, reaching initially the south-central part of the basin.