Ostracod , Diatom and Radiolarian Biostratigraphy of the Niger Delta , Nigeria

Ditch cuttings from seven wells and foraminiferal slides of three wells have been analyzed for diatoms, ostracods and radiolaria. The wells penetrated Eocene to Miocene strata in the Niger delta. A taxonomic study of the ostracod assemblages has been undertaken. The paleobiogeography of the ostracods indicates their affinity to the coeval assemblages from Sierra Leone, Benin Republic and Gabon. Paleoecological interpretations of the ostracod assemblages suggest inner to middle neritic depositional environments. Diatoms and radiolarians were identified, with the majority of them being placed in open nomenclature. This was a pilot study to determine the stratigraphical and paleoenvironmental potential of the three fossil groups in the Niger delta.


The Microfosils Group in This Study
Diatoms, ostracods and radiolaria (Figure 1) are microfossils which are usually recovered from the preparation of rock samples for the analysis of foraminifera.They are usually grouped as miscellaneous microfossils (MM) in the Shell Petroleum Development Company (SPDC) data-bases.Radiolarians and diatoms have siliceous skeletons and are of the size ranges of 30-200 µm and 0.5 µm -2 mm respectively.Ostracods on the other hand are small crustaceans composed of calcareous bivalve shells.Both diatoms and ostracods inhabit all aquatic environments while radiolarians are restricted to the marine realm.

Previous Work
Very limited published information is available on the three microfossil groups from the Niger Delta.Omatsola (1969) described three ostracod species from the basin while Jan du Chene et al. (1978) investigated the palynomorphs and ostracods of the Nsukwa-1 well.Okosun (1987Okosun ( , 2000) ) studied the ostracods of the Akata-1 well.Okosun and Petters (unpublished report) described some ostracods from both the Meren-1 and Opukeba-1 wells.Reyment (1963) carried out a taxonomic study of ostracods from the northern fringe of the Niger Delta.Ostracods, diatoms and radiolarians though occasionally recovered in microfossil sample preparations were usually grouped under miscellaneous without any detailed study and interpretations.

Aims and Objectives
The aims and objectives of this study were to document the biostratigraphy of the diatoms, ostracods and radiolaria from selected wells across the depobelts of the Niger Delta.It also includes biozonations and paleobiogeographic distribution of the fossils.The study was in essence a pilot study to determine the stratigraphic and palaeoenvironmental potential of the three microfossil groups.

Materials and Methods
The present study was based on the diatom, ostracod and radiolarian content of SPDC foraminiferal slides and ditch cuttings.The selection of the wells was guided by the recorded occurrences of the three microfossil groups in the SPDC StrataBugs database.Some of the microfossils were identified to the species level while others were only identified to the generic level (Coscinodiscus sp) and the rest of them as indeterminate in the database.The selected wells covered all the depobelts of the Niger Delta (Figure 2).Ostracod specimens from the Awaizombe-1, Akata-1, Imo River-8 and radiolaria from Ikono-1 wells were studied from the foraminiferal slides of these wells.Ditch cuttings were collected from Ihuo-1(59), Awoba-1(40), Alo-1 (15), Bosi-6 (20), KK-1 (11) and Zarama-11 (10) wells.The sampling intervals were not uniform because some well intervals were not available.About 25 grammes of the samples were heated on a hot plate, allowed to cool and soaked in kerosene overnight.The kerosene was used to aid disaggregation of clays and shales.The samples were washed over a 45 µm sieve.An initial use of 63 micron sieve in the washing of samples did not yield diatom and radiolarians.The residues were dried and stored in well-labeled packets.The diatoms, ostracods and radiolarians were picked from the residues with the aid of a Wild Heerbrugg binocular microscope which has a maximum magnification of x 500.The taxonomy of the three microfossil groups was undertaken with references to several specialist publications.Details of these publications are listed under Taxonomic Remarks section.
Aglaiocypris schweyeri occurs in the well from 4240 ft to 7600 ft and the species displays morphological variations.The material presently referred to Aglaiocypris cf.schweyeri and Aglaiocypris aff.schweyeri may actually be conspecific with Aglaiocypris schweyeri.The morphological variation is probably a polymorphic trait in the species, it is anticipated that further detailed study of these species with the Scanning Electron Microscope (SEM) will clarify the taxonomy.The genus Benisymmetricythere which was first described from the Oligocene of the Benin Republic (Carbonnel et al., 1996) has its first reported occurrence in the Niger Delta from this study.The species has not been reported from pre-Oligocene strata.
The age of the studied interval is Eocene-Oligocene.The Eocene/Oligocene boundary is tentatively placed at 6750 ft in the well.This is the point of origination of the genus Benisyymmetricythere in the Awaizombe-1 well, thus the upper part of the well with Benisyymmetricythere species is of Oligocene age while the lower part is Eocene in age.This agrees with the F7200-F5700 age (Late Eocene-Early Miocene) obtained for the well based on benthic foraminifera.Only 13 out of the 59 samples were fossiliferous in ostracods.The 13 samples contained poorly diversified assemblages (Figure 5).

Diatom and Radiolaria Biostratigraphy
Diatoms and raidolarians were recovered from some of the samples from Zarama-11, Ikono-1 and Bosi-6 wells.
All the samples from KK-1 well yielded diatoms and some radiolaria.Due to the low magnification microscope (x 500 maximum) that was used for the study, only two (2) species could be given full identification, others were left in open nomenclature, which means they were identified only to the genus level.

Ihuo-1 Well
The presence of the diatom Actinocyclus cf.cuavatulus was noted in this well.Coscinodiscus spp.which could belong to 2 or 3 species, Cenosphaera spp.and indeterminate radiolaria were accorded for this well.The diatoms and radiolaria occurrences in this well were very sparse consisting of 2-3 specimens in the majority of samples (Figure 5).

Ikono-1 Well
Radiolaria were found in this well from 2560 to 2960 ft.One species Dictyomitra ex gr.multicostata dominates the assemblages.Dictyomitra sp. 1 and Dictyomitra sp. 2 and indeterminate radiolaria were also recorded for this well.Diatoms were not found in the slides (Figure 7).

Alo-1 Well
Seven ostracod species and 1 species of diatom were recovered from this well.Asymmetricythere aff.monciardinii, which was first described from the Eocene by Carbonnel (1978) was recorded from this well.Stephanopyxis sp.Stephanopyxis sp.

Taxonomic Notes
The systematics employed in this study follows that used by previous workers in the coastal basins of West Africa (Van den Bold, 1966;Omatsola, 1969Omatsola, , 1972;;Carbonnel, 1988;Carbonnel et al., 1996;Keen, 1996;Okosun, 1987Okosun, , 2000;;Petters & Okosun, 1995).The taxonomy also benefited from Moore (1961).A total of 47 species which belonged to 21 general was recorded.Majority of them were identified to the species and subspecies levels.Some of the species were compared to, or referred to existing species (cf.and aff.).Few species were grouped as indeterminate (indet.)because of poor preservation.Some species were left in open nomenclature due to insufficient material.The species have been described by previous workers.Remarks with taxonomic comments are provided for the species to aid their identification.The well preserved species were studied using the scanning electron microscope (SEM).The figured materials have been deposited at the Paleontological Museum of the Department of Geology, Federal University of Technology, Minna, Nigeria.
Remarks: The oblique narrow depression in the anterior half of the species is well expressed in many of the specimens.
Remarks: The oblique narrow depression in the anterior half of the carapace characteristic of the species is not very well expressed.
Remarks: The carapace is oval with faint reticulations.
Remarks: Specimens have a more drawn out posterior.
Remarks: Species are elongate ovoid, anterior margin bluntly rounded while posterior margin is narrowly but broadly rounded with a broad marginal zone.Maximum height is in the posterior one third.Broad inner lamella which is narrow ventrally and posteriorly.
Remarks: Specimens with faintly bluntly rounded anterior margins.
Remarks: The carapace is elongate ovoid with straight dorsal and convex ventral margins.The anterior and posterior margins are broadly and narrowly rounded respectively.The right valve is overlapped by the larger left valve at the ventral, anterodorsal and posterodorsal margins.Short spines/denticles may be present at the posterior and ventral margins.Carapace is usually smooth.
Asymmetricythere aff.sp Carbonnel 1986 Pl.1, Figure 10 Occurrence: Alo-1 well The carapace is ovoid with straight dorsal and weakly convex ventral margins.A weak burge is noticeable at the ventrolateral area; three lateral ribs take their origin from it.The ribs terminate before the posterior-ventral corner.The centrolateral area of the valve bears four short ribs.An eye tubercle may be on each valve.
Remarks: Carapace is elongate and subrectangular with a strongly reticulate ornament, anterior and posterior margins are thickened.The species shows some faint resemblance to Benina postsehouensis Carbonnel et al. (1996).
Remarks: Carapace has curved and slightly straight ventral and dorsal margins.One dorsal rib which joins the median rib also joins the anterior marginal rib.A ventral rib runs near the ventral margin and is joined to the anterior marginal rib.Intercostal areas have faint reticulations.
Remarks: Carapace is roughly rectangular with straight dorsal and ventral margins.The anterior margin is more broadly rounded than the posterior margin.Three lateral ridges run across the dorsal, median and ventral areas of the carapace.Rows of reticulations occur in the intercostals areas.There is a prominent anterior marginal rib.The anterior marginal surface of the carapace is smooth.The median rib joins the anterior marginal rib.11 Occurrence: Awaizombe-1 well.
Remarks: Smooth, robust carapace with fairly straight dorsal and much curved ventral margins.Few short spines may be present at the posterior end of carapace.There may be irregular depressions at the anterior and posterior sides.There seems to be a slight resemblance between the species and Togoina (a possible evolutionary relationship).
Remarks: Carapace is robust with slight depression at the anterior area.Traces of weakly developed ribs may be present at the medioposterior part of the carapace.One or two short spines are present at the posterior margin.

Genus Soudanella Apostolescu, 1961
Soudanella fusa (Van den Bold), 1966 Pl.2, Figures 4, 5 Occurrence: Akata-1 well.inner to middle neritic paleoenvironments indicated for the two wells in this study (Figure 12).The ostracod assemblages contain species which are indicative of normal marine salinity (Neal, 1988).Cytherellids (Platycopids) which are filter feeders and thus better adapted to reduced oxygen conditions (Whatley, 1995; Boomer & Whatley, 1992) were poorly represented in the assemblages.This indicates that the strata were deposited in oxic to suboxic conditions.The presence of large arenaceous foraminiferal species and macroinvertebrates support this interpretation.

Recommendations
A high magnification binocular microscope (up to x 1000) should be provided for taxonomic work on diatoms and smaller radiolarians.The use of 38 µm sieve is more adequate in the washing of samples for diatom analysis.Further efforts on diatom studies should be directed to the Niger Delta deep water.

Figure 2 .
Figure 2. Map of Niger Delta showing the locations of the study wells

Figure 8 . 2 R ad i ol a ri a * 1 O s traco d i nde term i nat e 5 Os tra cod s 2 CFigure 9 . 1 KK- 1
Figure 8. Ostracod, diatom and radiolariandistribution chart for the Bosi-6 well

Figure 11 .
Figure 11.Triangle diagram for palaeobathymetric reconstruction (modified after Dingle, 1981) where the points are Cytheracea, Cypridacea + Baidiacea and Cytherellidae.The assemblage fields used to depict palaeodepth are as follows: 1-3: depth < 100 m; 4a: 100-200 m depth; 4b: -200 m depth; 5a: 200-300 m depth; 5b: 300-500 m depth; 6 and 7: > 500 m depth Our knowledge of the deep sea ostracods shows that species of the following genera: Heryhowella, Bradleya, Krithe and Poseidonamicus have pandemic distribution trends(Bergue et al., 2006; Yosuharo et al., 2009).This has been supported by the work of Guernet (1998) on the Neogene and Pleistocene ostracods of Sites 959 and 960 in the Gulf of Guinea.The two sites are located in the Ivorian deep water acreage.Species of the genera mentioned above were recorded by Guernet (1998) with Krithe spp.dominating the assemblages.None of the Niger Delta assemblages from this study are comparable to the deep sea assemblages of the Gulf of Guinea.This further confirms the shallow water paleoenvironment of the Niger Delta ostracods.

Table 2 .
Percentage of ostracod species from the Awaizombe-1 well