The Biostratigraphy of an exposed section of Arimogija-Okeluse shale of the Araromi Formation, Dahomey Basin sequence was carried out on ten samples. A total of five samples each were used for both foraminiferal and palynological analysis. The analysis was done using modern laboratory tools to conduct comprehensive palynological and foraminiferal studies on all samples. The foraminiferal analysis was carried out in this section for the first time to determine the stratigraphic age and paleoenvironment of deposition of the sequence. The lithologic unit comprises of dark to grey fissile shale. The palynomorphs species is more than the foraminiferal species, but in terms of abundance the forams is more abundant than the palynomorphs. Forty-one palynomorph species comprising Pollen, Spores, Dinoflagellate and Diatom frustules were recovered on analysis. The Palynomorphs species is characterized by diagnostic forms such as, Retidiporites magdalenensis, Cingulatisporites ornatus, Constructipollenites ineffectus, Erecipites sp and Monocolpollenites sphaeroidites. Other important features of the shale section are the relatively high frequency of Monosulcites perspinosus, Hafniasphaera hyalospinosa, spiniferites sp, Laevigatosporites sp and cooccurrence of, Longapertites sp, and Operculodinium sp. The section was also rich in forams. The Benthic arenaceous Foraminifera were abundant and diverse with fourteen benthic species while the planktics and calcareous benthic species are absent due to depositional environment which result in the dearth of the species. The interval is characterized by the co-occurrences of Haplophragmoides excavate, Haplophagmoides hausa, Ammobaculites coprolithiforms, Spiroplectammina dentate, Trochammina sewellensis, Trochammina fibstonensis, Trochammina sp, Ammobaculites sp, Trochammina saheliense, Haplophragmoides sp. All these forms are known to depict Maastrichtian age. However, Middle Maastrichtian age is suggested for Arimogija-Okeluse shale based on the fact that it has features such as acme occurrence of Longapertites marginatus which define Middle Maastrichtian ages. The paleoenvironment of deposition of the Arimogija -Okeluse shale is marginal marine characterized by co-occurrence of dinoflagellate such as Adnatosphaaeridium vittatum, Areoligerasenonensis, spiniferites sp Hafniasphaera hylospinosa. A comprehensive study of Arimogija-Okeluse Shale was carried out by adopting both palynological and foraminiferal biostratigraphic studies so as to establish the stratigraphic age and paleoenvironment of the study area as informed by previous workers.
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