Forty four (44) different forms of nannofossils recovered from the middle-Miocene to late-Miocene sediments of a section ((8900 - 11,000) ft (2713 - 3353 m)) of a well in the prolific oil producing Niger delta, Nigeria were described and grouped into twelve, though not on phylogenetic relationship, for documentation purpose. The standard strewn-slide technique was used to recover the fossils. The aim is to present the distinguishing morphological features of each of the nannofossils group. Of the recovered forms Genus, Discoaster, was found prominent and divisible into six sub-groups: 6-rayed discoaster with bifurcations, 6-rayed discoasters without bifurcation, 5-rayed discoasters, 5-rayed discoaster with acute bifurcations, 5-rayed nannolith and the indeterminate forms. Genus Sphenolithus recovered is also divisible into two: species without prominent apical spines and species with prominent, monocrystalline apical spines. Other genera found and described are Catinaster, Cyclicargolithus, Coccolithu, Minylitha, Syracosphaera, Helicosphaera, Ponto sphaera, Calcidiscus and Reticulofenestra. All the nannofossils fall within NN11, NN10, NN9, NN8 and NN5 zones belonging to the Middle-Late Miocene age.
Until recently, published biostratigraphic information on the prolific oil producing Niger delta were based mainly on palynomorphs and foraminiferal unlike nannofossils [
This work aims at presenting the morphological features of some nannofossils group recovered from a section of a well in the Niger delta. The morphological groups constitute neither a classification nor an attempt to organize possibly phylogenetically related forms under the same heading. The goal is to point out the distinguishing features of forms within each group for documentation purpose. Although the samples are recovered from the Agbada Formation no emphasis is laid on their biostratigraphic and paleoclimatic significance. Of note is the use of the ratio of D. brouweri and Coccolithus pelagicus, which are common in this study, for paleoclimatic studies [
To date the Niger Delta is perhaps the most important sedimentary basin in Sub-Sahara Africa in terms of petroleum production. It covers an area of about 75,000 square kilometers on the continental margin of the Gulf of Guinea in equatorial West Africa between latitudes 3˚N and 6˚N and longitudes 5˚E and 8˚E (
Three major lithostratigraphic units make up the Cenozoic Niger Delta. They are: a sequence of over-compacted over pressured pro-delta marine shales, named Akata Formation; which is overlain by a paralic sequence of alternating sands and shales, named Agbada Formation; and finally covered by continental sands and gravels of Benin Formation (
Presently the Cenozoic Niger Delta is a large, high-energy arcuate delta of the destructive, wave-dominated type divisible into offshore and onshore (
The changes in the stratigraphy of the Akata shale have been described from inboard to ultra-deepwater as a time-transgressive series of Lower Tertiary prodelta and marine shales that prograded from onshore Nigeria to ultra-deep water areas. The outbuilding and general geology are contained in several publications including [
The technique used to retrieve the forms described in this study follows the standard strewn-slide technique of [
From the section studied, 44 different forms were recovered. Of all these forms, the following are the marker species identified: Discoaster quinqueramus, Discoaster bergrennii, Discoaster bollii, Discoaster hamatus, Discoaster prepentaradiatus, Reticulofenestra pseudoumbilicus (>7 microns), Catinaster coalitus, Sphenolithus abies, Sphenolithus heteromorphus, Minylitha convalis. Absolute abundance of all the forms recovered shows dominance of Sphenolithus abies, Sphenolithus moriformi and Discoaster bollii and their acme events (
Generally the forms recovered from the studied well are herein grouped into twelve genera: Discoaster, Catinaster, Helicosphaera [
The genus fund in this study could further be grouped into six thus:
1) 6-Rayed Discoaster with Bifurcations
This is the predominant group of Neogene discoasters [
a) D. bolli (Plate 1, photograph numbers: 11, 12 and 13)
Description: This species has relatively short arms bifurcating into terminations in different planes. Both sides of the large central area have a stellate stem, one small, one large, a structural element which distinguishes D. bollii from any other species of otherwise somewhat similar appearance. According to [
b) D. variabilis
Description: This form was described by [
c) D. deflandrei (Plate 1, photograph number; 17)
Description: [
2) D. brouweri Group
These are 6-rayed discoasters without bifurcation, which are closely related to D. brouweri with fewer rays [
a) D. braarudii. It is described by [
Plate 1. Photomicrographs of Some of the Forms Recovered in well A1, Niger delta, Nigeria.
N.B. Discoasters with this general morphology also often occur through dissolution or poor development of other species e.g. D. exilis, D. brouweri [
b) D. brouweri
c) Description: This is a 6-rayed discoaster without bifurcation. This form has been to include forms with usually 6 long arms that bend down like umbrella ribs with or without a central knob on the concave side [
It differs from other 6-armed forms by the absence of any bifurcation or sideward bending of the tips of the arms.
3) Symmetric 5-Rayed Discoasters
The followings were identified within this group:
a) D. quinqueramus
Description: According to [
Range: NN11 [
b) D. berggrenii (Plate 1, photograph numbers: 6 and 7)
Description: This form is similar to D. quinqueramus but with a central area that is two times wider. It is a symmetric, 5 armed form with tapering arms and a very prominent knob consisting of the inwardly directed prolongations of the ridges on the arms on one side and of ridges extending from the inter-arm area over a depressed central area to form a central knob (CSEM in [
4) a) D. pentaradiatus (Plate 1; photograph number: 10)
Description: In [
b) D. hamatus (Plate 1, photograph numbers: 14, 15, and 16)
Description: It is a large, symmetric, 5 rayed form with a small knob in the centre. The arms are long, somewhat curved, and turn sharply clockwise and downward near the end, as viewed on the convex side. A much smaller knob is usual as a bifurcation, although it appears to be a continuation of the main part of the arm as it extends in the same direction. D. hamatus differs from D. neohamatus and D. calcaris by having only 5 arms instead of 6. The other two species have tapering and terminally bent arms.
Range: NN9
5) D. prepentaradiatus
D. prepentaradiatus is described as a 5-rayed nannolith with broad bifurcations, it is planar and non-birefringent [
Range: NN9-NN10
6) Indeterminate Forms
Unidentifiable/Broken discoasters fall under this group. They are usually common, and for biostratigraphic precision, it is best to record them as such. In addition, isolated rays and pairs of rays are common; care is sometimes needed to avoid misidentifying these.
As described by [
Range: NN8-NN10
1) C. coalitus: (Plate 1, photograph number: 20)
Description: They have their 6 rays confined to basket and according to [
The Helicosphaera identified in this study include:
1) H. carteri (Plate 1, photograph number: 26)
This is the most common Helicosphaera species. It is medium to large in size and flange ends in wing with two pores in central-area.
Range: NN1-NN21
2) H. intermedia (Plate 1, photograph numbers: 25)
Description: According to [
They are coccoliths whose central areas have variable number of perforations.
i.) P. multipora (Plate 1, photograph numbers: 28)
Description: Its central area protrudes above rim, 3 or more cycles of large pores in other words, its floor are pierced by many perforations or thinned by large depressions.
Range: Paleogene-NN21
2) P. discopora
Description: The rim is elevated with 3 or more cycles of pores.
This genus is responsible for much of the diversity of modern coccolithophores (>50 species) but most species produce small coccoliths with low preservation potential and which are difficult to identify under the microscope [
1) S. pulchra (Plate 1, photograph number: 29)
Description: It is a coccolith with an oblong or irregularly elliptical with bright rim and central-area of radial laths showing tangential c-axis orientation [
Range: Paleogene-NN21
These are forms which have circular to sub-circular central area [
1) C. floridanus. They are common forms with sizes usually less than 10 µm.
They are circular to elliptical in shape with an opened or closed central-area and no special structures.
1) R. pseudoumbilicus (>7 micron): (Plate 1, photograph number: 23)
Description: The size of the central opening is relatively large and greater than 7 micron.
2) R. haqii
Description: Distinguished by the size of the central opening of size 0.1 to 1.5 mm (micron meters).
3) R. minuta
Description: Used for forms smaller than 3 mm (3 micron meter).
Range: Paleogene-NN21 i.e. it ranges from the early Oligocene through the Pliocene.
4) R. minutula
Description: Used to describe forms that range between 3 - 5 mm.
Range: NN15-NN18
1) C. pelagicus (Plate 1, photograph number: 27)
Description: The size ranges between 3 - 16 mm and has a central-area open or with transverse bar.
Range: Paleogene-NN21
Calcidiscus recovered from this study have circular to sub-circular proximal shield and are birefringent and the distal shields are non-birefringent with curved sutures. Shields are easily separated [
1) C. leptoporus (Plate 1, photograph number: 22)
Description: [
Range: NN2-NN21
2) C. macintyrei (Plate 1, photograph number: 21)
Description: Large placoliths with sub elliptical outline and an elliptical tube which has a relatively large opening and usually lacks calcareous central filling. The placoliths have a non-birefringent distal shield and a birefringent proximal one as the extinction bands of the birefringent proximal shield curve sinistrally in proximal view.
Range: NN7-NN19, but usually only distinguished in the Upper Pliocene and Quaternary [
1) M. convallis
Description: This form has a Rhomb-shaped nannoliths with elevated rim [
Range: NN9-NN11A
Two groups of this were recovered:
1) Species without prominent apical spines
a) Sphenolithus abies (Plate 1, photograph numbers: 1 and 2)
Description: A representative of the latter type of the sphenoliths distinguishable by its wide proximal shield. It is slightly conical in shape and can be viewed parallel to crossed nicols and also at 45˚ to crossed nicols. According to [
Range: NN7-NN15
b) S. moriformis (Plate 1, photograph number: 3)
Description: Generalized form with no spine, and upper and lower parts of similar size. It has an obtuse proximal column.
Range: Lower Eocene-NN10
1) Species With Prominent, Monocrystalline Apical Spines [
a) S. heteromorphus (Plate 1, photograph numbers 4 and 5)
Description: This form has well developed apical spine. Both Its lateral elements and proximal elements are large.
Range: NN4-NN5
1) Thoracosphaera spp.
Description: Some of the species assigned to the Thoracosphaera are calcareous dinoflagellates. The width, shape and outline of archaeopyle vary in the various species. Also, the skeletal ultrastructure showing size, shape and arrangement of skeletal elements in the species vary with the test size which is used with some reservations, is spherical and prolate spheroidal.
Crenalithus spp.
Description: This form belongs to the Crenalithus family.
Considering the age range of the recovered taxa from a section of a well in the Coastal depobelt of Niger delta, the following nannofossil zones could be defined: NN11, NN10, NN9, NN8 and NN5. Zone NN11 which is late Miocene belongs to the Messinian stage. Zones NN10 (Late Miocene), NN9 (Late Miocene) belong to the Tortonian stage, NN8 (Middle Miocene) belong to the Serravallian stage, NN5 (Middle Miocene) belong to the Langhian stage based on [
In this study the Genus, Discoaster, was found divisible into six sub-groups: 6-rayed discoaster with bifurcations, 6-rayed discoasters without bifurcation, 5-rayed discoasters, 5-rayed discoaster with acute bifurcations, 5-rayed nannolith and the Indeterminate Forms. Genus Catinaster, Cyclicargolithus, Coccolithu, Minylitha and Syracosphaera have one sub-group each while Helicosphaera, Pontosphaera and Calcidiscus have two and Reticulofenestra four. Two groups of Sphenolithus were recovered. They are species without prominent apical spines and species with prominent, monocrystalline apical spines. All the nannofossils fall within the following zones NN11, NN10, NN9, NN8 and NN5 belonging to the Middle-Late Miocene age.
I am grateful to Global Energy Company Limited, Lagos whose facilities were used for this study. Kayode Aiyemomi and Sola Idris contributed immensely to the processing and interpretation of the data. These are acknowledged with thanks.
The authors declare no conflicts of interest regarding the publication of this paper.
Ola, P.S. (2018) Morphological Description of Calcareous Nannofossils Assemblage of a Middle-Miocene to Late-Miocene Section in the Niger-Delta, Nigeria. Open Journal of Geology, 8, 925-937. https://doi.org/10.4236/ojg.2018.89055