Paleoecology and Depositional Environments of the Upper Cretaceous Igumale Formation, Southern Benue Trough, Nigeria

Analysis of foraminifera from the Igumale Formation of the southern Benue Trough was undertaken to determine the paleoecology and depositional environment of the sedimentary successions. The studied sections span a total stratigraphic thickness of 86.44 m comprising 85.34 m of drill core and 1.10 m outcrop section. The lithologies of the subsurface section are composed of massive to laminated grey shale with subordinate grey to cream limestone in the upper part of the section. The recorded foraminiferal microfauna consists mainly of benthic forms with few planktic taxa. A total of 17 species comprising 12 benthic and five planktic and representing 84 and 16%, respectively, of total foraminiferal abundance were identified. The benthic group is composed of 11 agglutinated species of Ammobaculites, Ammotium, and Haplophragmoides genera and one calcareous benthic species belonging to the genus Anomalinoides. The benthic microfauna is dominated by Ammobaculites and Ammotium. The planktic taxa are represented by Planoheterohelix, Muricohedbergella, and Pseudotextularia. The assemblages are characterized by low diversity and high dominance. Based on taxonomic composition, diversity indices, planktic/benthic foraminifera (P/B) ratio, and test morphotypes alongside sedimentological characteristics, brackish to shelf depositional environment with paleodepths ranging from shallow inner to middle neritic is deduced for the studied section of the formation. Bottom water oxygenation conditions varied intermittently between oxic and dysoxic with possible more oxic conditions towards the top of the borehole section.