BIOSTRATIGRAPHY AND PALEOENVIRONMENT OF DEPOSITION OF NSUKKA FORMATION, ANAMBRA BASIN, SOUTHEASTERN NIGERIA

Biostratigraphy investigation of Nsukka Formation was carried out by subjecting well cutting samples obtained from Nzam-1 well to palynological studies with the view towards determining the relative geologic age and paleoenvironment of deposition of the sediments. Samples were treated in the laboratory through digestion with hydrofluoric acid, sieving with 5µ, oxidation with Nitric acid and mounting of the slides into sections with DPX mountant. A fairly high abundance and diversity of miospores were recovered, while the interval (847-1372m) contains distinctive and diagnostic palynomorphs which are stratigraphically important.

A Spinizonocolpites baculatus assemblage zone was established based on the basal occurrence of Spinizonocolpites baculatus, S. echinatus, Constructipollenites ineffectus, Periretisyncolpites sp. Periretisyncolpites gigantues, Monocolpites sp 1, Foveotriletes margaritae, Syncolporites marginatus, and Longapertites marginatus. The upper part of the interval is marked by association of Anacolosidites luidonisis (at top), Mauritiidites crassibaculatus, Retistephanocolpites williamsi, Proteacidites dehaani, Echitriporites trianguliformis, Proxapertites cursus, Retidiporites magdalenensis, Sinizonocolpites baculatus, Retitricolpites gigeonetti, Foveotriletes margaritae and Araucariacites sp. The paleoenvironment of deposition varies from continental at the base through marginal marine to open marine at the middle section to continental environment at the upper part of the interval based on gonyaulacacea/peridinacean ratio, present of microforaminiferal wall lining and algae. The Nsukka Formation is dated Late Maastrichtian based on the co-occurrence of recovered index fossils. Importantly, Cretaceous-Tertiary (K/T) boundary is marked by high fossil content in the Maastrichtian sediments compared to paucity in palynomorph that characterize the overlying Paleocene facies.