Sedimentary Facies and Palynological Studies of Ajali Sandstones Formation Outcropping in Idah, Northern Anambra Basin, Nigeria

Abstract

The paleo-depositional environment of the Maastrichtian Ajali Sandstone Formation of the Anambra Basin has remained controversial as numerous studies have inferred differing origins of the sedimentary unit. This study presents analytical evidence on the depositional environment of the Formation based on its lithofacies and palynological characteristics by evaluating its depositional mechanisms, paleoclimate, and depositional environment. Microfacies and palynological studies were carried out on sediment samples from outcrop sections along the western flank of River Niger. Nine lithofacies and two facies’ associations were identified with characteristics such as fining and coarsening upward sequences, which indicate subtidal channel and sandflat deposition in a shallow marine environment. Palynological studies reveal the occurrence of significant land-derived palynomorphs such as Tricolporopollenites sp., Cyathidites sp., Distaverrusporites simples, Cingulatisporites ornatus, Psilatricolporites Crassus, Longapertites sp. with freshwater algae Botryococcus braunii of Maastrichtian age. These results confirm that the sediments of the Ajali Sandstone are of a marginal marine system fed by continental facies. These facies have been reworked by tidal processes characteristic of an inner neritic environment deposited during regressive episodes.