Foraminifera Biostratigraphy and Depositional Environment of Sediments in SILE-Well, offshore Dahomey Basin, Benin Republic

Transcription

1 Foraminifera Biostratigraphy and Depositional Environment of Sediments in SILE-Well, offshore Dahomey Basin, Benin Republic Akinsile Oladimeji, Department of Geology and Mineral sciences, University of Ilorin, Ilorin Nigeria, Solomon Adeola Adeyinka, Department of Geology and Mineral sciences, University of Ilorin, Ilorin Nigeria, Olabisi Adeleye Adekeye, Department of Geology and Mineral sciences, University of Ilorin, Ilorin Nigeria Olatinpo Olusegun, Department of Geology and Mineral sciences, University of Ilorin, Ilorin Nigeria, Oluyemi Faseki Emmanuel, Department of Geology and Mineral sciences, University of Ilorin, Ilorin Nigeria Abstract- The sedimentary succession penetrated by the SILE Well offshore Benin republic in the Dahomey basin has been investigated for biostratigraphy and paleoenvironment of deposition. This section consists from the base to the top of sandstone overlying by shale measuring a total thickness of approximately 2000m. Twenty samples of shale were selected and processed for Foraminiferal study. The result of micropaleontological analysis reveals moderately rich and diverse microfauna of planktic and benthic foraminifera totalling 78 species altogether. 64 species (82.6%) are calcareous and 14 species (17.9%) are arenaceous. Of the calcareous forms, benthics accounted for 45 species (70.3%) while planktics accounted for 19 species (29.7%). Two major Planktic biostratigraphic zones were identified: The Morozovella subbotinnae zone, Morozovella vellascoensis and an Undiagnostic zone suggesting a Late Maastrichtian to Early Eocene interval. Based on the recovered benthic foraminifera, Haplophragmoides sp., Bulimina marginata, Eponides pseudoelevatus, Karreriella bradyi, Hopkinsina hourqi, Cibicidoides pseudoungerianus, Gyroidinoides girardanus, Lenticulina grandis, Oridorsalis umbonatus, Karreriella bradyi and Bathysiphon sp.suggests a depositional environment ranging between outer neritic and upper bathyal. Integration of biostratigraphy and depositional environment suggests that the sediment were deposited within Late Maastritchtian to Early Eocene interval with 3 major biostratigraphic zones in generally shallow marine environment ranging from outer shelf to upper bathyal. 1

2 I. INTRODUCTION The Dahomey (Benin embayment) Basin is an extensive basin located in West Africa; it covers much of the continental margin of the Gulf of Guinea, extending from Volta-delta in Ghana through Togo and Republic of Benin to southwestern Nigeria, where it is separated from and cut off by stratigraphically younger Niger Delta. Dahomey Basin is a marginal pull apart basin or marginal sag basin which was developed as the African and South American lithospheric plates separated in the Mesozoic and continental margin was formed [1].The area of study is a newly drilled borehole within the Seme Basin, Benin republic. The aim is to investigate the biostratigraphy and paleoenvironment of deposition. From this aim, the Occurrence, distribution and diversity of the assessment of different fossils, Construction of the different biozonations of the fossils recovered from the sediments and assessment of the ecology of the different fossils forms in the sediments. A. Geology of the study area The Seme Field is located south of Benin Republic (Figure 1, 2). The study area, SILE Well which penetrates Cretaceous-Tertiary sediments is located within Block 1, shallow offshore Benin Basin on its border with Nigeria at water-depth of up to 750m. Figure 1. Regional Geology of Dahomey basin [2] 2

3 Figure 2. Regional map of Dahomey Basin showing location of SILE Well [3]. B Stratigraphic Setting of Dahomey Basin The stratigraphic setting of the Dahomey basin has been described in detail in the works of [2], [4], [5], [6], [7], [8], [9]. These authors described five lithostratigraphic formations covering the Cretaceous to Tertiary ages. The formations from the oldest to the youngest include Abeokuta Group (Cretaceous), Ewekoro Formation (Paleocene), Akinbo Formation (late Paleocene), Oshosun Formation (early Eocene) and Ilaro Formation (late Eocene) (Figure 3). 3

4 Figure 3. Geological setting of the Dahomey Basin [11] ]. C. Previous work done in the study area [10] investigated the organic geochemistry of the shales and limestones in the eastern Dahomey Basin. Thereby reporting the prevalent abundance of terrestrially derived organic matter indicates continental environment of deposition which is characterized by b oxic conditions. [11] reported that the IETM (Initiall Eocene Thermal Maximum) that is the Paleocene-Eocene boundary of the Oshosun Formation is defined by the onset of a pronounced negative carbon isotope excursion in the benthic foraminifera species, this is shown by the falll in the population of recovered planktic foraminifera species. [12] identified six (6) biozones in the t Paleocenee Early Eocene sediments of the Basin. II. METHODOLOGY 4

5 Twenty ditch cutting samples ranging from interval m were used for the biostratigraphic studies. The SILE well reached a total depth of 2000m (Figure 4). It comprises four lithologies, which include sandstone, shale, limestone and shaly limestone. Figure 4. Lithologic section of SILE Well D. Foraminifera Preparation and Analysis Samples of approximately 50 grams at each sampled depth were treated with sodium carbonate, boiled for an hour on hot plate to disintegrate the clay, shale and free the fossils from the matrix. They were washed through a 75 micro mesh screen and the residue dried in an oven. A close study of the samples using binocular microscope helped in producing a lithologic chart. The identifications were made from available literature, and in all cases previously described specimens 5

6 have been compared with the type figures and type faunal based on the different general and species were made to determine the relative abundance. III. RESULTS AND DISCUSSION 1) Occurrence and Distribution The 20 ditch cutting samples of the SILE WELL provided were analyzed at 20 meters intervals for the foraminifera and accessory microfauna. The well yielded moderately rich and diverse assemblages of planktic and benthic foraminifera within the studied section with 78 species recorded. Of these, 64 species (82.6%) are calcareous, while 14 species (17.9%) are arenaceous. Of the calcareous forms, benthics accounted for 45 species (70.3%) while the remaining 19 species (29.7%) are planktics. The foraminifera checklist, the plots of the peaks of species diversity and population abundance are shown in (Table 1) Table 1: Foraminifera Specie Distribution CALCAREOUS ARENACEOUS TOTAL 64 (82.1%) 14(17.9%) 78 Benthics Planktics 45 (70.3%) 19 (29.7%) 2) Planktonic Species The 19 Planktic species include: Acarinina nitida, Globigerina daubjergensis, Globigerina specie, Globigerina triloculinoides, Acarinina pentacaramerata, Morozovella aegua, Morozovella inconstons, Morozovella subbstinae, Planktic indeterminate, Pseudohastigerina wilcoxensis, Turborotalia boweri, Turborotalia centralis, Turborotalia praecentralis, Turborotalia sp, 3) Benthonics Species Benthonic foraminifera relatively have narrow ecologic adaptation depending upon lithotopes and have also wide geographic distribution that makes them ideal for study of regional biogeographic and paleoecologic reconstructions. The proposed grouping of the benthonic foraminiferal content followed here is according to the suprageneric taxonomy of [13]. Generally, the arenaceous forms indicate littoral and shallow marine environments. The benthics include Anomalinoides Sp, Bolivina tenuicostata, Calcareous indeterminates, Eponides peudoelevatus, Eponides Sp, Hopkinsina horgui, Lenticulina cultrata, Lenticulina grantis, Uvigerina sp, Valvulineria martinezensis, Valvulineria sp, Bolivina sp, Bulimina marginata, Bulimina sp, Calcareous indeterminate, Cibicides sp, Cristellaria sp, Gyroisinoides girardanus, Lenticulina grantis, Lentica retulata, Nodosaria sp, Stilostomella sp, Valvuleroria sp, Bolivina sp, Bulimina jarvisi, Bulimina marginata, Bulimina sp, Calcareous indeterminate, Cibicides sp, Cristellaria sp, Gyroisinoides girardanus, Lenticulina grantis, Lentica retulata, Nodosaria sp, Stilostomella sp, 6

7 Valvuleroria sp, Bulimina palmarae, Cristallaria sp, Lenticulina inormata, Lenticulina rotulata, Anormalinoides sp, Bolivina sp, Bulimina minima, Hopkinsina hourgi. The agglutinated foraminifera includes Haplophragmoides sp, Vermeulinia sp, Gravelina narivaensis, Haplophragmoides obliguicamerata,haplophragmoides sp, Haplophragmoides obliguicamerata, Martinotella communis, Alvcolophragmium subglobosum, Bathysiphen sp, Eggerella bradyi, Haplophragmoides sp, Trochamonia sp, Vemeulina sp. 1.Morozovella aequa 2.Acarinina pentacamerata 3.Turborotalia sp 4.Acarinina nitida 5.Acarinina soldadoensis angulosa 6.Morozovella subbotinae 7.Globigerina daubjergensis 8.Acarinina primitive 9.Morozovella inconstans 10.Pseudohastigerinawilconhenxis 11.Turboratalia griffinae 12.Turborotalia sp 13.Turborotalia centralis 14. Globigerina triloculinoids Plate 1 :Diagnostic Planktic Foraminifera species recovered from shale sediments 7

8 1.Anomalina sp 2.Bolivina sp 3.Cibicides sp 4.Cibicides pseudoungera 5.Eponides sp 6.Florilus atlanticus 7.Gyroidinoides girardanus 8.Lenticulina cultrate 9.Lenticulina inomata 10.Lenticulina rotulata 11.Marginulina sp 12.Nodosaria raphanistrum 13.Nodosaria sp Plate 2: Diagnostic benthics Foraminifera species recovered from shale sediments 8

9 Possible Probable Confident Unconformable Fault Cutting Semi-quantitative, (Default Abundance Scheme) Core Sidewall core Present ( 1 ) Rare ( 2 ) Common ( 5 ) Abundant ( 15 ) Super Abundant ( 50 ) Semi-quantitative, (Default Abundance Scheme) + Present outside count *1 Turborotalia cerroazulensis pomeroli cerroazulensis transition *2 PALEOWATER DEPTH Semi-quantitative, (Default Abundance Scheme) *2 WELL A Depth 1350m 1375m 1400m 1425m 1450m 1475m 1500m 1525m 1550m 1575m 1600m 1625m 1650m 1675m 1700m Chronostratigraphy Period/Epoch EARLY EOCENE?PALEOCENE LATE MAASTRICHTIAN Base Lithology Lithology Qualifiers Lithology Accessories IGD Boundary Key FORAMINIFERA UNDIAGNOSTIC P7 - P8 Zone ZONE Sample depth is BASE of depth range Samples (metres) Barren Acarinina nitida Acarinina pentacamerata Acarinina primitiva Acarinina soldadoensis angulosa Acarinina sp Globigerina daubjergensis Globigerina sp Globigerina triloculinoides Morozovella aequa Morozovella inconstans Morozovella pseudobulloides Morozovella subbotinae Planktic indeterminate Pseudohastigerina wilcoxensis Turborotalia boweri Turborotalia centralis Turborotalia griffinae Turborotalia praecentralis Turborotalia sp *1 Anomalina sp Anomalinoides alazanensis Anomalinoides cicatricosus Anomalinoides sp Bolivina dertonensis Bolivina sp Bolivina tenuicostata Bulimina fusiformis Bulimina inflata Bulimina jarvisi Bulimina marginata Bulimina minima Bulimina palmarae Bulimina sp Calcareous indeterminate Cassidulina neocarinata Cibicides sp Cibicidoides pseudoungerianus Cristellaria sp Eponides ornatus Eponides pseudoelevatus Eponides sp Fissurina sp Florilus atlanticus Gyroidinoides girardanus Heterolepa floridana Hopkinsina hourqi Hopkinsina semiornata Lenticulina cultrata Lenticulina curvisepta Lenticulina grandis Lenticulina inornata Lenticulina rotulata Marginulina sp Nodosaria raphanistrum Nodosaria sp Oridorsalis umbonatus Praebulimina lata Praebulimina robusta Praeglobobulimina pupoides Stilostomella sp Uvigerina sp Valvulineria martinezensis Valvulineria sp Virgulina sp Alveolophragmium subglobosum Bathysiphon sp Eggerella bradyi Glomospira charoides Gravelina narivaensis Haplophragmoides obliquicameratus Haplophragmoides sp Karreriella bradyi Karreriella sp Martinottiela communis Recurvoides deformis Textularia sp Trochammina sp Verneuilina sp ??Unconformable f?f?fault Sampling FOP (FORAMINIFERA PLANKTONIC) Default Abundance Scheme Text Keys FOBC (FORAMINIFERA BENTHIC) FOBA (FORAMINIFERA AGGLUTINATING) Micropalaeontology Figure 5.Microfauna distribution chart for SILE Well Diversity: Micropalaeontology Micropalaeontology Total count: Micropalaeontology Samples Palaeoenvironment Outer Neritic Upper Bathyal REMARKS 1480 CU : Ma MFS; Gradstein et al FDO: Acarinina nitida 1560 CU : LDO: Globigerina daubjergensis Lone occurrence of Acarinina soldadoensis angulosa 1580 CU : LDO: Acarinina pentacamerata 1600 CU : LDO: Globigerina triloculinoides Depth 1350m 1375m 1400m 1425m 1450m 1475m 1500m 1525m 1550m 1575m 1600m 1625m 1650m 1675m 1700m IV. BIOSTRATIGRAPHY AND PALEOENVIRONMENTS E. Foraminifera Biostratigraphy Three (3) major foraminiferal Zones were recognized in the analyzed portion of SILE Well. The Cenozoic chronostratigraphic scheme of [14]. and the Global Sequence Cycle Chart of [15]. were adopted for this study. The zonal names used conform to the delta-wide foraminiferal zonal scheme developed for the oil industry. The zones are characterized briefly below. 1) Haplophragmoides excavata/ Morozovella subbotinae Zone Stratigraphic Interval: meters Equivalent Planktic Foraminiferal Zone: Lower P6 zone. Age: Lower Eocene (54.64Ma and younger) Diagnosis: This is the first zone encountered in the studied part of SILE Well. The base of this zone is marked by the 54.64Ma MFS [15]. recognized at 1480 meters while the zonal top is tentatively placed at 1340 meters, the depth of the first sample analyzed. The 53.65Ma MFS of [15]. that defines the top of the zone was not recognized in the studied section of the well. The zone correlates with the Lower P6 Planktic foraminiferal zone of [14]. and [15]. The age is Lower Eocene. 2) Eponides pseudoelevatus/morozovella vellascoensis Zone. Stratigraphic Interval: meters Equivalent Planktic Foraminiferal Zone: Lower P6 zone. Age: Lower Eocene (54.64Ma and Older) 9

10 The base of the zone coincides c withh the Cretaceous Tertiary boundary which is marked by the extinction of majority of the Maastrichtian species. The top is also defined by the disappearance of the typical Paleocene species. 3) Undiagnostic Zone. Stratigraphic Interval: meters Age:? Maastrichtian Paleocene The planktic foraminiferal preservation in the well is poor. Some S stratigraphically important taxa (index planktic forms) were not identifiable to generic/species levels. Therefore,, they are placed in the planktic indeterminate group. The zone is characterized/defined by the LDO (Last Downholee Occurrence) of Globigerina triloculinoides at the top. Table. 2 :Foraminiferal zones for SILE Well F. Paleobathymetry The ratio between planktonic and benthonic foraminifera (P/BB ratio, frequently expressed as a percentage of planktonic foraminifera, Tablee 3) is one of the most reliable proxies to estimate palaeo- in water depths. It has been known for a long time that the percentagee of planktonic foraminifera modern sediments increases with water depth e.g. [16], [17], [18]. [17] assumedd that the relative difference between the higher rate of reproduction of planktonic species in open ocean areas and the 10

11 higher rate of reproduction (density) of benthonic species in neritic areas is the main cause for the distribution observed. High proportions of agglutinated taxa in sample 4, 6 and 7 seem to represent shallower environments or reduced paleoproductivity as the ratio correlates directly with foraminiferal abundance (Table 4) Table 3 : Paleobathymetry checklist for SILE Well Inner shelf In n e r s h e lf In n e r s h e lf In n e r s h e lf Inner shelf Inner shelf In n e r s h e lf Middle shelf Upper co ntinental s lo pe Middle shelf Inner shelf M idd le s h e lf Middle shelf Outer s helf M idd le s h e lf In n e r s h e lf In n e r s h e lf In n e r s h e lf In n e r s h e lf Inner s helf Based on P/B ratio, [19] characterised various depositional environment of sediments. Upper continental slopes is characterised by high P/B ratio >2.33 (>70 : <30)%, Outer shelf (open sea) by P/B ratio of (40-70 : 60-30)%, Middle shelf (open sea) by P/B ratio of (10 60 : 90 40)% and Inner shelf (open sea) by P/B ratio <0.25 (<20 : >80)%. Applying these to the studied well, it ranges from inner shelf to upper continental slope. G. Oxygenation In the same way, the ratio of arenaceous and calcareous species reflects the oxygen content of the bottom water since low oxygen content may cause difficulties for foraminiferal calcite secretion 11

12 e.g. [20]. Furthermore, epifaunal genera dominate the entire section (Table 5). Due to this faunal composition (strong dominance of opportunistic species with high tolerance against oxygen deficiency for the entire section), dysoxic conditions are assumed throughout. The dominance of platycopid (filter feeding) ostracods supports this interpretation [21]. The presence of these large shelled organisms indicates no severe dissolution effects for the SILE Well. Thus, the ratio of epifaunal to infaunal genera may reflect the degree of oxygenation (Table 5). In short, and related to the investigated section, high numbers of calcareous foraminifera, large individuals and an abundance of epifaunal forms all point to high oxygen levels. Conversely, the strong dominance of arenaceous foraminifera, small individuals, and an abundance of infaunal forms indicate oxygen depletion. Applying the above information to the well section. The samples represent oxic to dysoxic conditions as they contain abundant epifaunal forms (mainly large Eponides sp, Bulimina sp, Morrozovella subbotinae) and numerous calcareous foraminifera. Samples seven contain rare or no epifaunal forms and rare or no calcareous tests and calcareous foraminifera. These samples are interpreted to indicate an intermediate, dysoxic environment. Sample fourteen to sixteen, on the contrary, contains small individuals with rare calcareous foraminifera. Therefore, it is also interpreted to represent an oxic to dysoxic environment. High proportions of agglutinated taxa seem to represent shallower environments at sample 4, 6 and 7 (Figure 4) or due to reduced paleoproductivity as this ratio correlates negatively with foraminiferal abundance or shows low oxygen as a result of increasing input of organic matter from the incoming fluvial systems [21] while at m shows high oxygen with reduced input of organic matter. A similar methodology to determine planktonic/benthonic ratio (P/B) is followed to establish the arenaceous/calcareous ratio (Table 4). The calculated ratios for each studied ecozone in SILE Well were in the order of low A/C ratio= 0 25%, moderate =26 50% and high > 50%. The distribution patterns of the arenaceous/calcareous foraminiferal ratios are revealed in [22] who stated that the dominance of calcareous foraminifers indicates deposition largely above the calcium carbonate compensation depth CCD line, in an area high in calcium carbonate, well oxygenated and characterized by normal salinity and/or high temperature as applied to this study (Table 4). Table 4: Arenaceous/Calcareous Foraminifera ratio Depth Planktic/Benthic Calcareous Agglutinating A/C Ratio

13 Table.5:Oxygenation in SILE Well with the diagnostic infaunal and epifaunal species. Thus the ratio of epifaunal to infaunal which also reflects the degree of oxygenation which point to high epifaunal for sample 1,2,3,5,7,8 to 20 suggests high oxygen level and large sized shells (oxic to dyoxic) while the sample 4,6,7,8 and 11 on the contrary represent anoxic to dyoxic environment(oxygen depletion) and small sized shells. H. Alpha diversity Index Species diversity is also informative for bathymetry [23]. The values of Fisher-index increase as the depth increases. Outer shelf is characterized by α=5-19, the slope by α=1-5. The highest values of α demonstrates the lowermost slope [24]. Species diversity is also informative for the dissolved oxygen levels. Assemblages, characteristic for low oxygenated environment demonstrate low species diversity (α<7). Hyposaline and nearshore shelf seas, shelf seas of normal salinity and hypersaline 13

14 (Figure 6). Therefore the Fisher index curve also shows that the species are existence of hyposaline to normal saline environment (not deeper than upper bathyal) Figure 6. Graph illustrating the calculation of the diversity index α For SILE WELL [19]. V. CONCLUSION Biostratigraphic studies was carried out on 20 drill cutting samples retrieved from SILE Well, deep offshore seme basin, Benin republic. They were subjected to Foraminifera analysis to determine their biozonation, and depositional paleoenvironments. This section consists from the base to the top of sandstone overlying by shale measuring a total thickness of approximately 2000m. The studied intervals ranged between 1340 and 1720 metres and were sampled at 20-metres intervals for the Foraminifera The result of micropaleontological analysis reveals moderately rich and diverse microfauna of planktic and benthic foraminifera totalling 78 species altogether. 64 species (82.6%) are calcareous and 14 species (17.9%) are arenaceous. Of the calcareous forms, benthics accounted for 45 species (70.3%) while planktics accounted for 19 species (29.7%). Thus investigation gave a two planktic biostratigraphic zones were identified: The Haplophragmoides excavata/morozovella subbotinae zone(early Eocene), Eponides pseudoelevatus zone/morozovella velascoensis zone(latest Paleocene early Eocene )and an undiagnostic zone suggesting Maanstritchtian-Paleocene. 14

15 Based on the recovered benthic foraminifera, Haplophragmoides sp., Bulimina marginata,eponides pseudoelevatus, Karreriella bradyi, Hopkinsina hourqi, Cibicidoides pseudoungerianus, Gyroidinoides girardanus, Lenticulina grandis, Oridorsalis umbonatus, Karreriella bradyi and Bathysiphon sp suggests a depositional environment ranging between outer neritic and upper bathyal. Integration of biostratigraphy and depositional environment suggests that the sediment were deposited within Late Maastritchtian to Early Eocene interval with two major biostratigraphic zones in generally shallow marine environment ranging from inner shelf to upper bathyal. This paper contributes to the understanding of the biostratigraphy and Paleoenvironment of Dahomey Basin, Benin Republic which is currently the focus of intensive hydrocarbon exploration activities. REFERENCES [1] A. Whiteman, Nigeria: Its Petroleum Geology, Resources and Potential, Vols. I & II, Graham & Trotman Ltd., London, 1982, 394p. [2] H.G. Billman, Offshore stratigraphy and paleontology of the Dahomey embayment. Proceedings of the 7th African Micropaleontological Colloquium, March 16-28, 1976, Ile-Ife, Nigeria, 1976, pp: [3] A. Kjemperud, W. Agbesinyale, T. Agdestein, C. Gustafsson and A. Yükler, Tectono-stratigraphic history of the Keta Basin, Ghana with emphasis on late erosional episodes, in Curnelle, R., ed., Géologie Africaine 1er colloques de stratigraphie et de paléogéographie des bassins sédimentaires ouest-africains, 2e Colloque Africain de Micropaléontologie, Libreville, Gabon, May 6-8, 1991: Elf Aquitaine, Mémoire 13, pp [4] P. Lehner and P.A.C. Ruiter, Structural history of Atlantic margin of Africa. Amer. Assoc. Pet. Geol. Bull., Vol. 61, No. 7, 1977, pp [5] M.E. Omatsola and O.S. Adegoke, Tectonic evolution and Cretaceous Stratigraphy of the Dahomey Basin, Nigeria. Jour. Min. Geol. vol. 18, 1981, pp [6] S.A. Adediran and O.S. Adegoke, Evolution of the sedimentary basins of the Gulf of Guinea. In: Matheis and Schandeimeir (eds), Current research in Africa earth sciences, Balkema, Rotterdam, 1987, pp [7] J.O. Idowu, S.A. Ajiboye, M.A. Ilesanmi and A. Tanimola, Origin and significance of organic matter of Oshosun Formation, southwestern Dahomey basin, Nigeria. Journal of Mining and Geology, Vol 29 (1), 1993, pp 9-17 [8] O.A. Adekeye, Aspects of Sedimentology, geochemistry and hydrocarbon potentials of Cretaceous-Tertiary sediments in Dahomey Basin of Southwestern Nigeria., Unpublished PhD Thesis, University of Ilorin, Ilorin, [9] O.A. Adekeye and S.O. Akande, Depositional environments and reservoir potential assessment of the Ewekoro Formation, eastern Dahomey basin, Southwestern Nigeria. Journal of Mining and Geology, Vol 42(1), 2006, pp [10] A.A. Elueze and M.E. Nton, Organic geochemical appraisal of limestones and shales in part of eastern Dahomey basin, southwestern Nigeria. Journal of Mining and Geology, vol. 40, no. 1, 2004, pp [11] H. Gebhardt, O.A. Adekeye and S.O. Akande, Late Paleocene to initial Eocene Dahomey Basin, Southwestern Nigeria. Jb. Geol. B., 2010, pp [12] E.A. Okosun and Y.B. Alkali, Paleocene-early Eocene Foraminiferal Biostratigraphy of the Eastern Dahomey Basin, SW Nigeria. International Journal of Scientific & Engineering Research, vol.3 (7), 2012, [13] A.R. Loeblich and H. Tappan, Foraminiferal genera and their classification: Van Nostrand Reinhold Company, New York, 1988, text v. 970 p., plates v. 212 p., 847 pl. [14] W.A. Berggren and M.P. Aubry, A late Paleocene-Early Eocene NW European and North Sea magneto-biostratigraphical correlation network. Geological society Special Pubication 101, 1998, pp [15] F. Gradstein, J.G. Ogg and A.G. Smith, Cenozoic Time Scale (Mesozoic and Cenozoic Sequence Stratigraphy of European Basins, SEPM Special Publication 60, Cambridge University Press, [16] E. Boltovskoy, D.B. Scott and F.S. Medioli, Morphological variations of benthonic foraminiferal tests in respons to changes ecological parameters: a review.- J. Paleont., 65,(2),1991, pp

16 [17] T.G. Gibson, Planktonic benthonic foraminiferal ratios: modern patterns and Tertiary applicability: Marine Micropaleontology, v. 15, 1989, pp [18] G.J. Van der Zwaan, F.J. Jorissen and H.C. de Stigter, The depth dependency of planktonic/benthic foraminiferal ratios: Con-straints and applications Marine Geology, 95, 1990, [19] J.W. Murray, Ecology and Palaeoecology of Benthic Foraminifera, John Wiley & Sons Inc., New York, 1991, pp 397. [20] F.B. Phleger and A. Soutar, Production of benthic foraminifera in three east Pacific oxygen minima: Micropaleontology, v. 19, no. 1, 1973, pp [21] H. Gebhardt, Benthic foraminifera from the Maastrichtian lower Mamu Formation near Leru (southern Nigeria): Paleoecology and paleogeographic significance. J. Foraminiferal Res. 28, 1998, pp [22] P. Saint-Marc and W.A Berggren, A quantitative analysis of paleocene benthic foraminiferal assemblages in central Tunisia. Journal of Foraminiferal Research, 18, 1988, pp [23] R.A. Fisher, A.S. Corbet and C.B. Williams, The relationship between the number of species and the number of individuals in a random sample of an animal population. Journal of Animal Ecology 12, 1943, pp [24] C.A. Wright, Foraminiferids from the London Clay at Lower Swanwick and their paleoecological interpretation.- Proc. Geol. Ass., 83, 3, 1972, pp