Palynological Age Control and Paleoenvironments of the Paleogene Strata in Eastern Dahomey Basin, Southwestern Nigeria

Paleogene deposits are extensively exposed in the Eastern Dahomey (Benin) sedimentary Basin in southwestern Nigeria. Outcrop logging and sampling of lithological sections were systematically carried out at the Ibese Quarry, in order to re-establish the age of sediments and reconstruct their depositional environment using samples rich in organic-walled microfossils. Two formations were recognized; the Ewekoro and Akinbo Formations. Two main lithological units were identified; limestones and carbonaceous shales. Minor lithologies include ferrugineous sandstones and glauconite. Results from the palynological examination show that terrestrial palynomorphs (spores and pollen) dominate over the marine dinoflagellates cyst species in the samples recovered from the Ewekoro Formation, while samples from the Akinbo Formation recorded high abundance with less diversity of the marine dinoflagellates cysts over the terrestrial sporomorph. Age determination/correlation was achieved based on selected stratigraphic index taxa recovered. The samples from Ewekoro Formation were dated as late Early Paleocene to early Middle Paleocene, based on the presence of the following pollen key-taxa: Proxapertites operculatus, Retidiporites magdalenensis, Spinizonocolpites baculatus, Mauritidiites crassiexinus, Scabratriporites simpliformis, and Echitriporites trianguliformis. The samples from the Akinbo Formation were assigned late Middle Paleocene to late Paleocene, based on the presence of the following pollen key-taxa: Proxapertites operculatus/cursus, Grimsdalea polygonalis, Retibrevitricolpites triangulatus, Psilatricolporites operculatus, Retistephanocolpites williamsi, Bombacidites sp., Apectodinium homomorphum, and Apectodinium quinquelatum. The environmentally significant palynomorph species indicated differences in paleodepositional environments, ranging from shallow marine in the Ewekoro Formation at the base, up the stratigraphic sequence, to marginal marine (estuarine) depositional environment in the overlying Akinbo Formation.