Microfacies analysis and paleoenvironmental interpretation of the late Aptian–Albian (Early Cretaceous) carbonates of the Riachuelo Formation in the Sergipe Sub-basin, northeastern Brazil

Abstract

The beginning of the drift phase, in the Aptian–Albian transition, marked the development of marine conditions in the South Atlantic Ocean, characterized by the deposition of evaporites, followed by the development of carbonate platforms. The carbonate rocks of this interval were deposited in a passive margin context as part of the Riachuelo Formation in the Sergipe-Alagoas Basin, northeastern Brazil. The objective of present study was to analyze the microfacies of the Aptian–Albian interval in the northern portion of Sergipe State, near the city of Pacatuba. The mainly carbonates of two cores were investigated and five carbonate microfacies were identified: oncoidal rudstone, bioclastic rudstone, oncoidal packstone, peloidal packstone/wackestone, and mudstone with planktonic bioelements that suggest the paleoenvironments were relatively shallow marine. The microfacies were dominated by oncoids in almost every sample, which occurred in association with stromatolites. The biostratigraphic results were based on palynomorphs and calcareous nannofossils. The palynomorphs assemblages indicated a late Aptian age. Paleoecological results of these fossil groups show paleoenvironmental conditions of the shallow waters with proximal characteristics, with the predominance of pollens and spores associated with rare dinoflagellates and ascidians. A striking feature was the presence of authigenic glauconite which filled skeletal cavities and partially replaced the oncoids and their nuclei, mostly in the oncoidal packstone facies. Together, these findings suggest that the samples formed at the basal portion of the Riachuelo Formation in a transgressive systems tract, corresponding to the first depositional stage of the Riachuelo Formation.