Depositional Environment of the Maastrichtian Mamu Formation, Anambra Basin; Integrated Sedimentological and Foraminiferal Analysis

This study integrates sedimentological and foraminiferal analyses to investigate the depositional environment of a part of the Maastrichtian Mamu Formation, exposed in the Ibagba quarry (Owukpa) and Onyeama road-cut sections, Anambra Basin, southeastern Nigeria. Detailed field mapping, stratigraphic logging, and representative sample collection facilitated subsequent laboratory investigations. Lithological units are characterized by interbedded fine to medium-grained sandstones, shales, silty shales, and coaly units. Sedimentological analysis of the sandstones reveals moderate to poor sorting, leptokurtic to very platykurtic kurtosis, and strongly coarse to fine-skewed grain size distributions, indicative of a fluvially influenced paralic depositional setting with rapid deposition and varying hydrodynamic conditions, evidenced by grain size parameters and sedimentary structures (massive bedding and diverse laminations). Petrographic studies of the sandstone reveal an assemblage of quartz 92-93% and rock fragments of almost 4-5%, with no occurrence of feldspar, suggesting the grains are mineralogically mature and have been transported over a significant distance from their source. The low-diversity benthic foraminiferal assemblage, recovered from the shale units and dominated by agglutinated species (Ammobaculites, Ammotium, Haplophragmoides, Reophax globosus, Ammobaculoides, and Lituola senoniensis), suggests a dysoxic to suboxic paleoenvironment within a harsh, shallow, marine paralic setting characterized by fluctuating salinity (brackish to outer-shelf). The juxtaposition of these sedimentological and micropaleontological findings indicates a complex fluvio-marine transitional environment within the Anambra Basin during the Maastrichtian, marked by episodic marine incursions into a predominantly terrestrial, fluvial-dominated system.