THE BIOSTRATIGRAPHY AND PALEOENVIRONMENT STUDIES OF MAMU, AJALI, AND NSUKKA FORMATIONS, ANAMBRA BASIN, OUTCROPPED AT IHUBE, IMO STATE, SOUTHEASTERN NIGERIA

Biostratigraphy and paleoenvironment of deposition studies were carried out on the Mamu, Ajali, and Nsukka Formations of Anambra Basin, Southeastern Nigeria. The outcrops are located at Ihube and Uturu. The three lithologic units include: Unit A, bluish-brown sandy-shale which is the Mamu Formation; Unit B, whitish friable sandstone which is the Ajali Sandstone; and Unit C, brownish lateritic sandy-shale which is the Nsukka Formation. The result of grain-size distribution analysis carried out on the samples collected from unit B showed that the unit was deposited in a fluvial environment. The area shows a moderately well sorted to poorly sorted unit and lack of carbonate cement and thus a continental origin is suggested. Also the area shows a fining upward sequence and this indicates a transgressive phase and suggests the depositional environment to be a fluvial continental environment. The unimodal paleocurrent pattern with a mean vector azimuth of 2440 indicates a Southwesternly direction and provenance probably from the Benue Trough. The results of the bio-stratigraphical analysis carried out on the shale samples showed that unit A is of Lower-Upper Maastrichtian age, on the basis of the following age diagnostic palynomorphs species such as Cretacaeiporitesscabratus, Cretacaeisporitesmulleri, Droseriditessenonicus, Monocolpopollenitessphaeroidites, Tricolporopollenites sp. SCI. 141, Ephedripites sp., Galaecorneaclavis and Tricolporopollenites sp. and unit C is of Upper-Maastrichtian age, with the following index species: Longapertitesmarginatus, Spinizonocolpitesbaculatus, Retidiporitesmagdalenensis, Buttineaandreevi, Echitriporitestrianguliformis, Longapertitesmicrofoveolatus, Rugulatisporitescaperatus, Gleicheniditessenonicus and Monocolpitesmarginatus. Its depositional environment indicated shallow marine for unit A, because of the presence of marine micro-plankton such as, Spiniferites sp. and Oligosphaeridium complex, with corresponding absence of Longapertites marginatus and Spinizonocolpites baculatus, and mangrove swampy environment for unit C, because of the preponderance occurrence of Longapertites marginatus and Spinizonocolpites baculatus species, along with Laevigatosporitesovatus