Probable Record of Paleocene - Eocene Thermal Maximum in Southwestern Nigeria: Indications from Nannofossils of FA-1 Borehole, Eastern Benin Basin.

The Paleocene – Eocene Thermal Maximum (PETM) interval has been reported in several calcareous nannofossil studies from Paleogene basins around world including the Southern Tethys basins of North Africa. The Southern Tethys basins are believed to be connected to the Eastern Benin Basin through the Trans – Saharan Seaway during the Paleogene. Based on these reports, we carried out a biostratigraphic and paleoclimatic analyses of the calcareous nannofossils recovered from 25 ditch cutting samples in the FA-1 borehole to investigate possible record of the PETM in the Eastern Benin Basin, Southwestern Nigeria. The samples yielded characteristic Paleocene – Eocene calcareous nannofossils species (Coccolithus pelagicus, Coccolithus formosus, Chiasmolithus solitus, Sphenolithus moriformis, Pontosphaera plana, Pontosphaera multipora, Lophodolithus mochioporus, Lophodolithus nanscens, Neochiastozygus perfectus and Zeugrhabdotus sigmoides) and the Maastrichtian species, Arkhangeskiella cymbiformis. The assemblages allow the recognition of CC 22 (Maastrichtian) and NP 4 – NP 6 (early – late Paleocene) nannofossil zones. The relatively abundant occurrences of the genus Coccolithus and Sphenolithus in the carbonate interval of borehole suggest prevailing oligotrophic, warm water condition. The barren interval succeeding the peak nannofossil abundance at sample 159 m coincides with the onset of continuous clastic sedimentation in the FA-1 borehole. This is believed to have resulted from the shoaling of the Calcite Compensation Depth (CCD) and increased clastic input occasioned by the climatic and ocean water perturbations of the PETM interval. The emergence of the genus Pontosphaera (Pontosphaera plana and Pontosphaera multipora) at depth 87 m above the barren interval suggest a return to cooler, mesotrophic to eutrophic water condition after the drastic temperature rise and oligotrophic waters of the PETM. Key-words - Calcareous nannofossils, Paleocene - Eocene, Benin Basin, Thermal Maximum.