Albian/Cenomanian boundary in the Sergipe-Alagoas Basin: Sea-level changes and paleoecology based on ostracods

Abstract

This paper presents an analysis of the paleoenvironmental evolution of the Sergipe-Alagoas Basin, Brazil, at the Albian/Cenomanian boundary, based on ostracods obtained from drill core UFRJ-2-LRJ-1-SE. The variability in the composition of the ostracod fauna allowed for the identification of the stratigraphic cycles throughout the studied sequence. A total of 2292 ostracod specimens were obtained from 44 samples. Ostracod abundance, diversity and richness led to the identification of five ostracod biofacies (A, B, C, D, E), each associated with different paleoenvironmental changes. Combined with available lithological data, we suggest the presence of three 4th order transgressive (T) regressive (R) cycles in the studied section, with the maximum flooding surfaces (MFSs) being delineated by the decrease in the relative abundance of ostracods, and the maximum regression surfaces (MRSs) being characterized by the increase in the relative abundance and diversity of ostracods. Furthermore, the T-R1, T-R2, and T-R3 sequences can be correlated with the global eustatic sea level curve. The stratigraphic distribution of the ostracod biofacies suggests that the studied section was deposited in a middle to outer shelf environment. The transition from a carbonate shelf to a slope environment in the Riachuelo Formation is here suggested as being characterized by the presence of Eucytherura amoriensis, which can be used to track these changes in the Sergipe-Alagoas Basin. The similarity of the paleoenvironments and the ostracod fauna of the Sergipe Group and of the Talme Yafe Group from Israel suggests that there is a close relationship between the depositional environments of these two regions.