Paleogene Foraminferal Assemblages and Stratigraphy of the Section Penetrated by AG-1 Borehole, Dahomey (Benin) Basin

Abstract

Biostratigraphic analysis of ditch cutting samples from interval 1095 – 1635 ft (332 – 495 m) of AG-1 borehole, Akinbo Formation within the Benin Basin was carried out.  The study was aimed at determining the age and delineating the well section into planktic foraminiferal biozones. The samples which were analysed using standard biostratigraphic techniques yielded fairly abundant and diverse assemblage of both planktic and benthic foraminiferal species. Typical Paleogene species recovered include planktic forms: Turborotalia cerroazuelensis frontosa, Turborotalia cerroazuelensis pomeroli, Turborotalia boweri, Turborotalia pseudomayeri, Turborotalia rohri, Morozovella subbotinae, Acarinina primitiva, Globigerina velascoensis and Globigerina triloculinoides. The benthic assemblage consists of Eponides pseudoelevatus, Lenticulina midwayensis, Anomalinodes umbonifera, Anomalinoides midwayensis, Nonion oyae, Cibicides succedens and Bolivina afra. The foraminiferal assemblages indicated that the studied section spanned Late Paleocene to Middle Eocene, P4 - ? P12 standard planktic zones. However, the widely recognized P8/P9 boundary index datum – First Appearance Datum (FAD) of Turborotalia cerroazulensis frontosa occurring at 1315 ft (398 m) enabled the delineation of the section into two broad biostratigraphic intervals: P4 – P8 (P4a – E5) and P9 -? P12 (E6 -? E11) zones. The 162-metre thick dark grey shale section of AG-1 borehole spanned about 18.2 Ma, suggesting an average sediment deposition rate of approximately 9 metres per million years.