Palynofacies and sequence stratigraphy of the Aptian–Albian of the Sergipe Basin, Brazil

Abstract

In order to place information from palynofacies variations in a sequence stratigraphic context, palynofacies analyses for wells GTP-17-SE and GTP-24-SE from the Sergipe Basin, was integrated with sequence stratigraphy. Previous works assigned the succession to two second-order sequences, K50 and K60, the last being subdivided into systems tracts KM1 and KM2 (third-order sequences). By combining lithofacies data and gamma-ray logs from the studied wells with palynofacies and paleoecological data, the succession was subdivided into third-order sequences and correlated with the integrated sequence stratigraphic framework. The K50 sequence shows a slightly transgressive upward trend. The sequence boundaries of K50 are marked by major peaks in abundance of phytoplankton and amorphous organic matter and in the gamma-ray. KM1 sequence starts below of the boundary between Muribeca and Riachuelo Formations. This sequence was subdivided into three parasequences. The first transgressive systems tract is supported by a marked increase in Palynological Marine Index and amorphous organic matter values, and the phytoclast relative abundance shows a progressive decrease. The upper boundaries are marked by maximum flooding surface characterized by high abundances of dinoflagellate cysts. The boundaries are also marked by an abrupt decrease of phytoclasts, which is best observed in well GTP-24-SE. The highstand systems tract is characterized by an increase in amount of phytoclast particles, a clear decrease in amorphous organic matter percentage values and moderate values that tend to increase upwards of the Palynological Marine Index. The upper boundary of this sequence is marked by a peak of dinoflagellate cysts indicating the transgressive surface. In well GTP-24-SE, the boundary is placed at the base of an abrupt lithological change, from calcilutites to dark shales. The second transgressive systems tract is based on a marked increase in Palynological Marine Index and amorphous organic matter percentage values, together with the clear decrease in phytoclast particles. These are observed clearly in well GTP-24-SE, especially the decrease of phytoclasts. The top of these parasequences is distinguished on the basis of the peaks of Palynological Marine Index indicating the maximum flooding surface. In well GTP-24-SE, the lowest abundances of the phytoclast group are recorded at the maximum flooding surface. The KM2 sequence interpreted previously as a highstand systems tract is confirmed herein, on the basis of palynofacies.