PALYNOZONATION, CHRONOSTRATIGRAPHY AND PALEOENVIRONMENT OF DEPOSITION OF THE ALBIAN TO PLIOCENE SEDIMENTS OF THE NZAM- 1, UMUNA- 1 AND AKUKWA- 2 WELLS ANAMBRA BASIN, SOUTHEASTERN NIGERIA

Abstract

Anambra Basin is an inland sedimentary basin with oil, gas and coal deposits. Previous palynological studies from the basin were mostly from outcrop samples which are insufficient for detailed palynozonation and paleoenvironmental reconstructions. The aim of this study was to investigatedetailed lithostratigraphy, palynozonation and paleoenvironments of the sediments from Anambra Basin. Ditch cutting samples from Nzam-1 (390-3672 m), Umuna-1 (24-2310 m) and Akukwa-2 wells (2935-3653 m) were collected for lithological reconstruction and palynological analyses.Two hundred and twenty-six samples were prepared for palynological analysis by treating samples with sixty percent hydrofluoric acid, sieved with 5 µm nylon mesh and the residue separated with zinc bromide. The treated residues were then oxidised with nitric acid and neutralised with potassium hydroxide to form macerals. The macerals were subsequently mounted on glass slides. The prepared slides were then studied under stereoscopic microscope for pollen, spores, dinoflagellates, algae, fungal spores and microforaminiferal wall linings. Palynomorph abundance and diversity were used to identify maximum flooding surfaces (MFS) and sequence boundaries (SB). Shale, sandstone and siltstone with their heteroliths, intercalated gypsum and strongly diagenised iron shale were the identified lithological units. The lithogical units varied across the wells except the lower partswith correlatable shaly units. Ten correlatable palynological zones were established including Forma PO 304 Lawal zone 1, (Albian-Lower Cenomanian); Cretacaeiporites spp. acme zone 2, (Upper Cenomanian-Turonian); Zlivisporitesblanensis zone 3, (Coniacian) and Milfordia spp. acme zone 4, (Campanian). Other recognised biozones were Foveotriletes margaritae zone 5, (Lower Maastrichtian); Longapertites margaritae zone 6, (Middle Maastrichtian); Spinizonocolpites bacculatus zone 7, (Upper Maastrichtian); Mauritiidites crassibaculatus zone 8, (Lower Paleocene); Monoporites annulatus zone 9, (Middle Eocene) and Echitricolporites spinosus zone 10, (Upper Miocene-Pliocene age). In Umuna-1 well, palynological zones 3, 5 and 6 were missing; suggestive of more than one episode of faulting and folding associated with tectonic activity in the basin. Lithostratigraphy sequences varied in the basin and were non-correlatable except at the Albian age stratigraphic interval.This suggested widespread marine inundation in the basin duringAlbian period. The palynological zones ranged from Albian to Pliocene and the sediments were deposited in fluviatile to marine environments.Sequence stratigraphy analysis indicated six sequences of events characterised by five SB and eight MFS. The Anambra Basin sediments have varied lithology with distinct palynozones that could be used for paleoenvironmental reconstructions for the oil and gas exploration.