Lithostratigraphy, Palynostratigraphy and Palynofacies Indications of the Depositional Environments of Upper Cretaceous to Paleogene Sediments, Offshore Eastern Dahomey Basin, SW Nigeria

Abstract

The ditch cuttings from an offshore well, 'JX' were studied for their sedimentology, palynostratigraphy, palynofacies and depositional environment history. Lithologically, poorly sorted sandstones with variable interbeds of shales and siltstones overlain by dark grey shales and interbedded limestones with occasional pyrite occurrence (Abeokuta-Afowo Formations) constituted the basal sections. Overlying is a glauconitic sequence consisting of shale with little clay and lenses of limestones (Araromi Formation). The uppermost section consists of predominantly of shales with some glauconitic materials (Oshosun Formation). Occurrences of Canningia cf. senonica, Nelsoniella; Microdinium ornatum (dinoflagellates), Crybelosporites pannuceus, Cicatricosisporites spp. (spores), and Classopollis spp., Ephedripites (pollen) assemblage suggests a late Cenomanian age for the lowermost section while the occurrences of Lejeunecysta hyaline, Selenopemphix selenoides and Hystrichokolpoma rigudae (dinoflagellates) and Peregrinipollis nigericus, Retitricolporites irregularis and Ctenolophonidites costatus (pollen) and Foveotriletes margaritae (spore) at the uppermost section indicate an Eocene age. Palynological marine index (PMI) curve and the Peridinioid to Gonyaulacoid ratio (P/G ratio) between the different palynological assemblages showed very strong fluctuations. Six broad lithofacies units with characteristic palynofacies assemblages reflecting different paleoenvironment were distinguished, which varied from estuarine through shallow inner neritic (shelf) to middle neritic shelf settings.