Aptian shell beds from the Romualdo Formation (Araripe Basin): Implications for paleoenvironment and paleogeographical reconstruction of the Northeast of Brazil

Abstract

The Aptian Romualdo Formation in the Araripe Basin represents the establishment of an epeiric sea in the Brazilian Northeast during the Early Cretaceous. This unit is composed by a succession of limestones, shales, sandstones, and coquinas, which constitute shell beds near the top. This last interval strongly suggests a marine influence in the whole basin based on the fossil assemblage. However, even though the establishment of marine waters during the deposition of the Romualdo Formation is a consensus, paleogeographical and paleoenvironmental aspects still need to be better understood. This study aims to perform a sedimentological analysis of the coquina interval, based on petrographic and taphonomic analysis of field data and in a new fully recovered well-core (2-AB-1-CE) drilled in the Serra do Mãozinha at Abaiara county in the Ceará State. The described rocks were subdivided into eight facies constituted by bivalves, gastropods, ostracods, peloids, micrite, siliciclastic grains, punctual foraminifers, and plant debris. Based on facies' vertical stacking at well 2-AB-1-CE, a main regressive pattern was observed. It was also possible to subdivide the coquina interval into high-frequency transgressive–regressive cycles. The regional distribution of the facies permitted the identification of a shallow epeiric sea throughout the basin, with variations in the paleoenvironment energy and siliciclastic input. These variations were mainly controlled by paleotopography, fluvial discharge, and sea-level oscillations. The data allowed the identification of two subsequent paleogeographical configurations controlled by sea-level change. The first one, related to a sea-level high, was characterized by a protected bay on the western border and a gulf in the rest of the basin. Then a sea-level fall led to the formation of closed and open lagoons associated with higher terrigenous supply. As such, this study adds new data for a better understanding of the Araripe Basin and the impacts of sea-level fluctuations in South America during the Aptian.