Sedimentology and foraminiferal paleoecology of the exposed Oligocene-Miocene Ogwashi-Asaba Formation in Issele-Uku area, Anambra Basin, southern Nigeria: A paleoenvironmental reconstruction

Sedimentological and foraminiferal paleoecological studies were carried out for the Oligocene-Miocene Ogwashi-Asaba Formation of the Anambra Basin at the Issele-Uku area in southern Nigeria, to reconstruct paleoenvironments. The pattern of rock successions and textural characteristics suggest the prevalence of fluvio-deltaic setting during the formation of lignites and clastic rocks of Ogwashi-Asaba Formation. Agglutinated benthic foraminiferal species such as Ammotium sp., Textularia sp., Gavelinella sp., Milliamina sp., Reophax sp., Bolivina sp., Eponides sp., Pseudobolivina sp. and Lenticulina sp. dominate the shale units whereas the planktonic forms are absent. All these species have very low population and diversity in the shales, which could be due to the presence of harsh conditions such as low oxygen level and scarcity of food, leading to poor survival. The assessments of benthic foraminifera's habitat suggest middle-to-inner neritic marine environment for the shales. Granulometric results suggest fluvial environment for the sandstone and deltaic for the lignite. The Ogwashi-Asaba Formation in Issele-Uku area of Anambra Basin, southern Nigeria, therefore, have records of shallow marine, and fluvio-deltaic paleoenvironments. (c) 2022 The Author(s). Published by Elsevier B.V. on behalf of China University of Petroleum (Beijing). This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Automated summary

Sedimentological and foraminiferal paleoecological studies of the Ogwashi-Asaba Formation in the Issele-Uku area of the Anambra Basin, southern Nigeria, indicate the presence of a fluvio-deltaic setting during the formation of lignites and clastic rocks. Benthic foraminiferal species dominate the shale units, with low population and diversity likely due to harsh conditions. The benthic foraminifera habitat suggests a middle-to-inner neritic marine environment for the shales, while the granulometric results indicate a fluvial environment for the sandstone and a deltaic environment for the lignite.