High-resolution sequence stratigraphy from well-log patterns and base-level variations of the Paleocene - Early Eocene N’kapa Formation, Douala onshore Basin (SW Cameroon)

Abstract The present study aims to establish the stratigraphic architecture in order to understand the depositional history during the Paleocene- Early Eocene interval in the Douala Onshore Basin. The sequence stratigraphic analysis from gamma-ray logs and well cuttings was used to define stratigraphic surfaces and systems tracts based on the depositional sequence model. The results obtained show that the Paleocene-Early Eocene interval could be subdivided into three lithostratigraphic units (Unit 1, Unit 2, and Unit 3) of Middle Paleocene - Late Paleocene, Late Paleocene - Early Eocene, and Early Eocene age respectively. The sequence stratigraphic interpretation revealed three major stratigraphic surfaces : the sequence boundary (SB1) of the Middle Paleocene top, the transgressive surface (TS2) of the Late Paleocene, and the maximum flooding surface (MFS2) of the Early Eocene. The first highstand systems tracts (HST1) of the Middle Paleocene, the lowstand systems tract (LST2) of the Late Paleocene, the transgressive systems tract (TST2) of the Late Paleocene to Early Eocene, and the second highstand systems tracts (HST2) of the Early Eocene are the depositional sequences observed. The Paleocene-Eocene interval is therefore globally prograding, interrupted by a short transgressive episode of Late Paleocene to Early-Eocene age. Three cycles of base-level have been identified: the first cycle of base-level fall dated Middle Paleocene, the second cycle of rapid base-level rise which occurred between Late Paleocene to Early Eocene, and the third cycle of base-level fall dated Early Eocene. This observation confirms that the main factor which influenced the depositional history of Paleocene- Early Eocene succession in the Douala Onshore Basin was the eustatic sea-level changes due to the Cenozoic post-rift subsidence of the West African margin.