Benthic foraminifera from the Campanian-Maastrichtian shales of the Mamu Formation at the Benin flank of the Anambra Basin, southern Nigeria: A paleoenvironmental assessment

This study qualitatively describes foraminiferal assemblages from the Mamu Formation exposed along the Imiegba-Uzebba road in the Benin flank, westem Anambra Basin in southern Nigeria and highlights the assemblage differences from those identified at the eastem sector of the Anambra Basin. The identified assemblage entirely consists of the agglutinated benthic foraminiferal taxa Ammobaculites, Ammotium, Haplophragmoides and Miliammina, and largely represent an inner brackish, hyposaline, estuarine paleoenvironment. The agglutinated benthic assemblage identified in the westem Anambra Basin is distinct from those characterizing the Lower Mamu Formation in the southeastem section of the basin, which was under fully marine influence through the Trans-Sahara seaway. Some of the identified agglutinated taxa have been described from Upper Cretaceous sections of other nearby sedimentary basins. The absence of calcareous foraminifera was probably caused by prevailing brackish conditions at the westem Benin flank, while its eastem portion was more open to marine ingressions. Based on the paleogeographic distribution of foraminiferal assemblages at both sides of the basin, we suggest temporal episodic landward movement of marine water into the paleoriver of the Anambra Basin, with occasional ingression into the Bida valley.