BIOSTRATIGRAPHY, CHEMOSTRATIGRAPHY AND PALEOECOLOGY OF PALEOCENE TO MIOCENE SEQUENCES OF THE BENIN BASIN, SOUTHWESTERN NIGERIA.

Abstract

The Paleocene – Miocene of the Benin Basin has paucity of information, when compared with the contiguous oil producing Niger Delta Basin. The available information is attended with ambiguities, particularly, the stratigraphy as well as the microfauna and microflora in the sequences. There is therefore the need for further study to improve on the knowledge of the basin, which will aid hydrocarbon exploration. Detailed palynological, foraminifera, nannofossil and geochemical studies were carried out on surface and subsurface sediments (BH, IL, EW, AJ and XY wells) of the Paleocene – Miocene sequences of the Benin Basin, in order to identify the biozones and reconstruct environment of deposition. 263 samples were subjected to standard palynological preparation, using HCl, HF, HNO3 and ZnBr and were studied under the transmitted light microscope for occurrences of the acid insoluble microfossils. 290 samples, were prepared and analyzed for the nannofossil content. These samples were observed under the transmitted light microscope at about 1000x magnification. 47 samples were also analyzed for foraminifera occurrences. Major, trace and rare earth elemental composition analysis were carried out using the multi-acid, ultra- trace ICP-ES/MS (MA250, 30 samples) and whole rock Lithium Fusion ICP finish (LF300, 30 samples) methods. The measurement of variations in the stable isotopes of carbon and oxygen of 15 samples, were carried out at the University of Wyoming Stable Isotope Facility, USA. XRD analysis was carried out on 13 limestone samples and 17 clay samples, so as to ascertain the mineralogical contents. The palynological samples yielded fairly abundant palynomorphs. Based on the marker palynomorphs recovered from studied samples, five palynological Zones were identified in the Paleocene to Miocene sequences of the basin and the zones were labelled, PfI – PfV Zones. About 128 species of calcareous nannofossils belonging to 59 genera of about 24 families were identified in BH, IL, AJ and wells. Five calcareous nannofossil zones (CI – CV) were delineated using marker species. The zones correspond to NP1 – NN6 Zones in the Paleocene – Late Miocene Epochs. The absence of typical Late Oligocene to earliest Miocene forms (Discoaster druggii, Helicosphaera recta and Sphenolithus ciperoensis) indicated stratigraphic gaps just below the middle Miocene. So also, an unconformity was identified above the Early Eocene, as typical Late Eocene assemblage was missing (Chiasmolithus oomaruensis, Rhabdolithus gladius,Nannoterina fulgens and Discoaster saipanensis). At least, four recognizable major and trace element chemostratigraphic boundaries were established in the study column. The estimated paleotemperature from the stable isotopes of carbon and oxygen in the limestone samples was between 190C and 390C. The Paleocene Eocene Thermal Maximum event was identified at around 11m in the EW core. The relative low occurrence of herbaceous species, savanna and montane species in the Early Paleocene indicates warm tropical climate, while the preponderance of rain forest elements and spores can be associated with humid and wet conditions, as well as increased fluvial activities. The presence of Braarudosphaera bigelowii, Broinsina, Nannoconus spp., and also the presence of some species of the Calyptrosphaeraceae family, Holodiscolithus solidus and Calculites ovalis, indicates shallow marine or near shore deposition. Overall, a warm, wet tropical climate was inferred for the Early Paleocene and cooling conditions in the Miocene.