Petrological and Geochemical Evaluation of the Paleocene-Eocene Lithofacies in Dahomey Embayment, Southwestern Nigeria

For a geologist, his main work is to study Earth processes, Earth materials and Earth history, which will help people to understand our planet better. And there are many more detailed fields in geology that different geologists focus on in the same place.

Dahomey Basin is an arcuate coastal basin, the onshore parts of which underlie the coastal plains of southwest Nigeria, Benin and Togo. In this paper, the study area is located in Ibese, southwestern Nigeria about 40 km southwest of Abeokuta. And the authors focused on the distribution of diverse lithofacies, genetic description of the Ewekoro Formation with emphasis on the limestone and what characterize the environment of its deposition, the extent of diagenesis, biological influence on the chemistry of the units, major elemental composition, post depositional processes and their viability in determining its economic values.

The field mapping was carried out in order to determine different rock units and their association within the area. The collected samples were disaggregated and subjected to lithological description and petrography in the laboratory while the samples for the XRF analysis were pulverized into a fine powder. The authors analyzed the lithostratigraphy, geochemistry, carbonate petrography and depositional palaeoenvironment of the study area via the samples.

The findings indicated that the total thickness of the limestone ranged from 12.0 m to 15.0 m. And the limestone consists of milky to grey and nodular to massive fossiliferous limestone units while the sand content increases with depth. The composition of the major oxides in the thirty-seven samples from six exploratory wells revealed that CaO, SiO₂, Fe₂O₃ , MgOand Al₂O₃  were the most abundant elements. The trend of the geochemical indices showed increase in the percentage composition of CaO, and MgO. And the percentage composition of Al₂O₃, Fe₂O₃ and K₂O were significant indicating continental influence during diagenesis and diverse depositional episodes. The three different microfacies recognized based on depositional texture are shelly biomicrite, shelly biosparmicrite and alga biosparmicrite. The predominance of micrite as the cementing matrix revealed that the rock was deposited in a quiet shallow inner shelf environment occasioned by storm waves.

Article by Olugbenga A. Boboye and Olusegun J. Omotosho, from University of Ibadan, Ibadan, Nigeria.

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