Facies architecture, depositional environments, and organic matter enrichment of the Mamu Formation on the western flank of the Anambra Basin, Nigeria

Abstract

Coal-bearing strata of the Maastrichtian Mamu Formation on the western flank of the Anambra Basin are widely recognised. Yet, the controls on facies variability, organic matter enrichment, and reservoir distribution remain poorly resolved. This study integrates detailed sedimentological logging, agglutinated foraminiferal assemblages, and inorganic geochemical proxies from representative outcrops to reconstruct depositional environments and sequence stratigraphic architecture. Eight lithofacies are grouped into swamp, tidally influenced estuarine, estuarine mudflat, bay-fill delta, and braided fluvial facies associations. Vertical stacking patterns and key stratigraphic surfaces define a complete LST-TST-HST succession comprising lowstand peat-forming coastal mires, retrogradational estuarine deposits capped by an Ammobaculites-rich maximum transgressive interval, and highstand bay-fill delta to braided fluvial progradation. Geochemical proxies indicate brackish estuarine conditions with strong terrigenous influx, predominantly oxygenated bottom waters, and reducing pore-water conditions favourable for organic matter preservation. Organic enrichment reflects estuarine restriction, sediment dilution, and early diagenetic recycling rather than persistent anoxia. By integrating facies architecture, micropalaeontology, and redox-salinity proxies, this study establishes the high-resolution sequence stratigraphic model for the western flank of the basin and clarifies the paleoenvironmental controls on organic matter enrichment. Coal seams and Ammobaculites-bearing shales constitute potential gas-prone source rocks, whereas braided-channel sandstones represent the most prospective reservoirs. The proposed model provides a predictive framework for reservoir and source-rock distribution and reduces exploration uncertainty in this underexplored sector of the Anambra Basin.

HIGHLIGHTS • Integrated facies, geochemistry, and foraminifera refine basin evolution. • LST-TST-HST succession defined for western Anambra Basin. • Coal mires formed during lowstand under freshwater–brackish conditions. • Maximum transgression marked by Ammobaculites-rich estuarine muds. • Coal and shale are gas-prone sources; braided sands are key reservoirs.