International Journal of Scientific & Engineering Research Volume 3, Issue 11, November-2012 1
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The Paleocene – Early Eocene Foraminiferal Biostratigraphy of Eastern Dahomey Basin,SW Nigeria
Okosun, E. A. and Alkali, Y. B.
Department of Geology, Federal University of Technology, Minna, Nigeria.
eaokosu@yahoo.com, yalkali@futminnaa.edu.ng
ABSTRACT
The foraminiferal biostratigraphy of the Paleocene – early Eocene strata of SW Nigeria which represents the eastern Dahomey basin has been studied from four boreholes and two outcrops. A fairly diverse planktic and benthic foraminiferal assemblage was recovered.Six planktic biozones comprising Praemurica pseudobulloides, P. inconstans, Morozovella angulata, Globanomalina pseudomenardii, Morozovella velascoensis and M. subbotinae were identified. Two benthic concurrent range zones, Anomalinoides uboniferus – Anomalinoides midwayensis and Planulina oyae and Uvigerina hourcqi have been identified. These biozones will serve as useful correlation tools in the West African coastal and inland sedimentary basins.
INTRODUCTION
The boreholes and quarry section studied are in south western Nigeria (fig. 1) which represents the eastern Dahomey basin. The Gbekebo borehole is located on the Okitipupa ridge on the western flank of the Niger delta. The Araromi and Gbekebo boreholes penetrated the Cretaceous and Tertiary strata while other two boreholes BH No’s 4925 and 1582 only penetrated part of the Tertiary sequence
Several workers have studied the geology and biostratigraphy of the Paleocene sequence in southwestern Nigeria (Berggren, 1960; Jones and
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Hockey, 1964; Kogbe, 1972; Ogbe, 1972, 1976; Adegoke, 1972, 1977; Adegoke et al.,
1971, 1972, 1976; Fayose and Assez, 1972; Petters and Olsson, 1979; Petters, 1982,
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Okosun, 1987, 1989, 1998.And the objective of this study is to undertake a foraminiferal biostratigraphic study of the Paleocene –early Eocene strata of the eastern Dahomey basin.
LITHOSTRATIGRAPHY
The Paleocene – Eocene stratigraphic units in SW Nigeria comprises of Araromi, Ewekoro, Oshosun Formations and Imo Shale (Table 1 below).
A brief description of these units is given below.
Araromi Formation
The formation comprises of dark grey to black shale, shelly shale, sandy black shale and thin intercalations of limestones and sandstone (Okosun 1987, 1990). This formation was found in Araromi – 1 and Gbekebo – 1 borehole at 446 m – 583 m and
880 m – 1093 m respectively. The unit is equivalent to the Araromi Shale of Reyment (1965) and the Nkporo Shale of Billman (1976). The formation has an early Paleocene (Danian) age in SW Nigeria (Okosun, 1998).
Imo Shale
The Imo Shale is composed of grey, dark-grey and black shale with occasional white to brown sands (Okosun, 1998). Glauconite occurs sporadically in the formation. The shale is thinly laminated, and generally fissile and locally calcareous. The black shale facies was not encountered in the Gbekebo – 1 and Araromi – 1 boreholes, it appears to be restricted to the northern (inland) part of the basin. A maximum formation thickness of 178.4 m was reported for the northern part of the basin while 429m (from
421 m to 850 m ) and 240 m ( 212 m to 454 m ) have been reported from Gbekebo – 1 and Araromi – 1 boreholes respectively from the coastal area (Okosun, 1998).
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Table 1:Correlation of early Tertiary Formations in Nigeria
Series | Stage | Inland SW | CoastalSW | SE | NW | NE | |
Q) c Q) u 0 w | c ftl .Q.). =. .... c ftl 'iii Q..). a. > | Oshosun Fm | Oshosun Fm | AmekiFm (Nanka Fm) | |||
Q) c Q) u 0 w | c ftl .Q.). =. .... c ftl 'iii Q..). a. > | Oshosun Fm | lmo Shale | lmo Shale Ebenebe Sandstone Umunna Sandstone lgbalu Sandstone | |||
Q) c Q) u 0 w | c ftl .Q.). =. .... c ftl 'iii Q..). a. > | Oshosun Fm | lmo Shale | lmo Shale Ebenebe Sandstone Umunna Sandstone lgbalu Sandstone | Kalambaina Fm | Kerrikerri Fm | |
Q) c Q) u 0 Q) iii D. | c ftl Q) c ftl ..:.. | lmoShale | lmo Shale | lmo Shale Ebenebe Sandstone Umunna Sandstone lgbalu Sandstone | Kalambaina Fm | Kerrikerri Fm | |
Q) c Q) u 0 Q) iii D. | c ftl Q) c ftl ..:.. | Ewekoro Fm | lmo Shale | lmo Shale Ebenebe Sandstone Umunna Sandstone lgbalu Sandstone | Dange Fm | Kerrikerri Fm | |
Q) c Q) u 0 Q) iii D. | c ftl Q) c ftl ..:.. | lmoShale | AraromiFm | Nsukka Fm | Dange Fm | Kerrikerri Fm |
Ewekoro Formation
The formation is exposed at the Larefag WAPCO quarries at Ewekoro and
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Shagamu. At the Ewekoro quarry, the unit comprises of the following lithofacies: Basal sandy fossiliferous limestone, massive limestone nodular limestone and an uppermost glauconitic shale bed. The exposed section of the formation at the Shagamu quarry is composed of a basal nodular limestone bed that is followed by grey laminated shale, massive limestone, black shale and an uppermost massive fossiliferous limestone.
The formation was only encountered in the inland part of the basin. It was not found in the Araromi-1 and Gbekebo-1 boreholes. A maximum thickness of 47m has been reported for the formation. The base and top of the unit are marked by black and grey shale respectively (Okosun 1999).
Oshosun Formation
The formation comprises of green, greenish- grey or beige clay and shale with interbeds of sand. The shale is usually thickly laminated, calcareous and glauconitic. The associated sand is whitish, light brown or brownish grey in colour; it is predominantly medium to coarse grained with some fine grained horizons. The quartz grains are round, fine and clear, the sand is usually poorly sorted. Vesicular, nodular or compact phosphorites occur sporadically in the formation (Okosun, 1984). Thin limestone or marl beds are locally present in the formation (Russ, 1924; Reyment, 1965; Adegoke, 1969; Kogbe, 1976; Ako et al; 1981; Okosun,1998) the formation becomes arenaceous and calcareous towards the top and base respectively. Vertical and laterals lithofacies variations are common. A maximum thickness if 101.5m was reported for the formation (Okosun, 1998).
MATERIALS AND METHODS
Samples for the study were collected from Araromi—1 borehole (GSN 1131), Gbekebo-1 borehole (GSN 1132), Akinside borehole (GSN 1582) Borehole No 4925, Ewekoro and Shagamu limestone quarries. The samples were collected at 2-3 meter intervals from the borehole core. The representatives’ lithofacies samples were collected from the quarry faces.
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The samples were disaggregated in a solution of 10% H2O2 overnight and boiled in a water with a pinch of soda ash. They were then washed through a 63µm sieve. The washing procedure was repeated until foraminifera with clean surfaces were obtained. All the foraminifera recovered from the residue were counted and studied under the microscope. Generic classification was based on Loeblich and Tappan(1988) Olsson et al (1992) and other relevant foraminifera literature. The species identification was based mainly on Toumarkine and Luterbacher (1985). Since the taxa encountered in this study have been described by some previous workers (Ako et al., 1980; Petters 1979, 1982 and Bolli, 1957) they have not been redescribed. Some of the species have been illustrated in plates 1-3.The illustrated specimens were deposited in the paleontological collections of the Universitatsstadt Tubigen, Germany.
Biostratigraphy
A fairly diverse foraminiferal assemblage of planktic and benthic species was recovered in this study. Their stratigraphic distribution is illustrated in figs 2-5. Majority of the foraminiferal species are illustrated in plates 1-4. From the first and last appearances of diagnostic species, six planktic foraminiferal biozones were recognized form the early Paleocene to the early Eocene (Table 2). The planktic foraminiferal zonal schemes adopted here are those by Tourmakine and Luterbacher (1985) and Berggren et al. (1995) for the early Tertiary. Two benthic foraminiferal biozones were recognized.
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Table 2. Planktic and benthic foraminiferal biostratigraphy of the Eastern Dahomey Basin and correlation with the inter- regional schemes (modified after Obaidalla, 2000)
Inter - Regional Eas1em Dahomey Basin (this study)
Datum events ToumarkJne
& luterbacher. Berggren et al. 1995
Afowo· 1BH
Pal nk:tics tsent ICS
1985
Fayose.1970 Araromi
GSN 1131
1 rtanu"!.'o oyw
M.edgari P6 M.subbotinae
M.subbotinoe &
M. veloscoensis
hourrq.
P. pseudomenardii p
G. G. G.
4 jpseudomenardi ps.udomenardU pseudomenardU
P. pusilla pusilla P3:b
I. albeori
I. albeori
1111111111111111111' AnomolinoidOJ
ubani(erus
M. anguiata P3o M. angulata
::::-lllllilli\
M.unciMta
P. uncinato
A.mi/wensu
a M. trinidadensis Plc P. inGonstons P. trinidadensis P. inconstons
• w
.0
::s P1.b G. compnssa
::s H. pseudolxllhliMs P. pseudobulloides P. pseudobulloih
a: P1.o S. 11iloculinoides
IG. eugubina Pa P. eugubina
Po G. cretaceo
11111111111111111111
The proposed biozones are arranged from base to top.
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Planktic Foraminiferal Zones
Parasubbotina pseudobulloides Zone
P1a + P1b sub zones (Berggren et al., 1995) Age: Earliest Paleocene
Author: Bolli (1966)
The partial range of the index taxon from its FAD to the LAD of Praemurica trinidadensis (Bolli) was used to define the zone (Bolli, 1966). The upper boundary is also correlated to the FAD of Praemurica inconstans (Subbotina) according to Berggren et al., (1995). Praemurica trinidadensis (Bolli) was not found in the study area.
The dominant species in this zone are Subbotina triloculinoides (Plummer), Globanomalina compressa (Plummer), Globoconusa danbjergensis (Bronnimann) and Eoglobigerina trivialis. The P. pseudobulloides zone occurs in the Araromi – 1 and Gbekebo – 1 borehole (Fig. 2 & 3).
Praemurica inconstans Zone
P. trinidadensis Zone (Toumarkine & Luterbacher) Age: Early Paleocene
Author: Berggren et al., (1995)
This zone was defined by Berggren et al., (1995) as sub-zone Pic based on the FAD of P. inconstans (Subbotina) to the FAD of Praemurica ucinata (Bolli). The dominant species in this zone are Parasobbotina pseudobulloides (Plummer), Subbotina triloculinoides (Plummer) and Globanomalina compressa (Plummer). The P. inconstans Zone occurs only in Araromi – 1 borehole. (fig. 2) The P. ucinata Zones of Toumarkine
& Luterbacher (1985) and Berggren et al., (1995) was not encountered in this study.
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PALEOGENE SYSTEM
§ PALEOCENE
I EOCENE
SERIES
., n
DANIAN I
NKPORO '".LEI
THANETlAN
IMO SHALE
IYPRESAI N STAGE
I OSHOSUN FORMATION
c I1 :1 :•:•:"'1:1 1 1 I I I II 111 l ;t• l 1 1
LITHOLOGY
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111 11111 l I I I I I
CORE SAMPLE DEPTH
(met•r•)
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DA NIAN
PALEOCENE EOCENE SERIES
I THANETlAN YPRESIAN STAGE
NKPORO SHALEI IMO SHALE OSHOSUN FORMATION
0::..:.::· ..:.::0 .=:· :' :·.:
I:: :: :: :
=: •'=··wII"IIuI mil:iIl':,l::,,'',:l ::iil 1111 i ill:ilil :ll ai ":1l11l1l ii
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L THOLOGY
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Anomo cnotoes urnoontrera ana ..... mcowO,.Yf!flSlS Planulino oyoae & U.hourcqi
Morozovella angulata Zone
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P3a subzone (Berggren et al., 1995) Age: Late Paleocene
Author: Hillebrandt (1965)
This zone has been defined as the partial range of M. angulata (White) from its FAD to the FAD of Igorina pusilla (Bolli). The top of this zone was defined by Berggren et al, (1995) based on the FAD of Igorina albeari (Cushman and Bermudez) instead of I. pusilla. The dominant species in this zone include Morozovella acuta (Toulmin) M. aqua(Cushman and Renz), Globigerina linaperta (Finlay) Globorotalia haynesis (Fayose) and Acarinina primitiva (Finlay). The M. angulata Zone occurs in Araromi – 1, Gbekebo – 1, Akinside (BH 1582) boreholes.(figs.3,5). The zone also occurs in borehole No 4925 Ewekoro and Shagamu limestone quarries.
Globanomalina pseudomenardii Zone
P4 zone (Berggren et al., 1995) Age: Late Paleocene
Author: Bolli (1957)
The Globanomalina pseudomenardii Zone is a taxon range zone defined by the total range of its nominate species. The characteristic species include
Acarinina primitiva, Morozovella africana, M. costteina, Globigerina occlusa, Acarinina pseudotopilensis, M. angulata (white), M. acuta (Toulmin), M. aqua(Cushman and Renz),
M.Velascoensis(Cushman). The G. pseudomenardii Zone occurs in Araromi – 1 borehole(fig.2) and the Shagamu limestone quarry.
Morozovella velascoensis Zone P5 zone (Berggren et al., 1995) Age: Late Paleocene
Author: Bolli (1957)
This zone was defined as the partial range of the nominate taxon from the last appearance datum (LAD) of Globanomalina pseudomenardii (Bolli) to the LAD of Morozovella velascoensis (Cushman). The characteristic species include Morozovella formosa, M. acuta, M. aequa, Acarinina primtiva.The Morozovella velascoensis zone
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occurs in Araromi-1 (fig.2) borehole and Shagamu limestone quarry.
Morozovella Subbotinae Zone
M. edgari zone (Toumarkine & Luterbacher, 1985) Age: Earliest Eocene
Author: Berggren et al., (1995)
The M. subbotinae zone is an interval zone that was defined between the LAD of Morozovella velascoensis (Cushman) to the FAD of M. aragonensis(Nutal). The Paleocene / Eocene boundary lies near type lower boundary of the M. subbotinae Zone which is placed at the LAD of Morozovella .velascoensis (Cushman). The characteristic species include M. gracilis (Bolli), Globorotalia cerroazulensis(Cole)Acarinina soldadoensis (Bronnimann), A. primitiva (Finlay),Subbotina linaperta(Finlay)
The M. subbotinae zone occurs in Araromi – 1 borehole.(fig.2). Although the nominate taxon M. subbotinae (Morozova) was not encountered in Akinside borehole (BH1582) and BH 4925 the presence of the following species Acarinina pseudotopilensis A. pentacamerata (Subbotina), A.tribulosa (Loeblich and Tappan), Globorotalia wilcoxensis (Cushman and Ponto) and A. broedemani (Cushman and Bermudez), suggest its presence (fig 4-5)
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.5 ftl I>
!a
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...
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ciT ii
0 F
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p
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>.g·
r.;
PALEOGENE SYSTEM
PALEOCENE I EOCENE SERIES
THANETIAN YPRESJAN STAGES
.!I' a.., ID,.
EWEKORO
IMO SHALE
OSHOSUN
FOR1v1ATOI N
SAMPLE DEPTH
l a.,
:t ;;;· 0
'"tl'
•····•····•·· ·· ······ ·····•·· ······ j
! 1
UTHOLOGY Robulus olokuni Quinqueloculina elegbari
Vaginulina sp.
Bolivina koglundi OJ
COO"
':""CI'. 3'
Margulinopsis sp.
Planulina oyae
C1l
:..:.J..
:aa..o...
e:AJ
-.3
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t::tl
·.::· !
Eponides africana
E. c{. pseudoelevatus Sagprina circumspinosa Bolrvina crassicostata
Morozovella angulata
;:r 0
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!lJ
3
::J
:::;;
!-..:...
:;·
•·····
"...'...''.."...'·+· ·····
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M. acuta
A. pseudotopilensis
:::2
::J
C..1.,l
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io
I.
AcariRina tribuloso
A. pentocomeroto loborotolio wilcoxensis
!lJ
!lJ
a::
n
The benthic foraminiferal species recovered in this study are long ranging. Two
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concurrent range zones, one for the Paleocene and the second for the early Eocene have been identified. The concurrent-range-zones comprises of 2 overlapping species ranges which have time significance and possibilities of geographic extensions to several West and Central African basins. This is in consonance with the international stratigraphic guide (Hedberg, 1976).
Anomalinoides umboniferus – Anomalinoides midwayensis zone
Age: Early and late Paleocene
Author: Petters (1982)
This zone was defined by the concurrent range of the nominate taxa Anomalinoides umboniferus (Schwager) and Anomalinoides midwayensis (Schages). The base of the zone coincides with 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 and the nominate taxa (Petters, 1982). The zone embraces the Paleocene and includes the Imo shale (Fig. 2, 3) and Ewekoro Formation in the present study.
The characteristic species include: Lenticulina midwayensis (Plummer), Gavelinella pachysuturalis Chierici (1996) reported the presence of Eponides Pseudoelevatus Graham, de Klasz and Rerat from the Eocene of the Ivory coast and Ghana basins. The species was also found in the Eocene of Akinside Borehole (BH 1582) in this study. The occurrence of Eponides Pseudoelevatus in the Eocene invalidates its usage as a Paleocene index fossil. Anomalinoides midwayensis (Schwager) is hereby proposed as its replacement.
Planulina oyae-Uvigerina hourcqi Zone
Age: Early Eocene
Author: Petters (1982)
The zone is defined by the overlapping ranges of the two nominate taxa Planulina oyae Reyment and Uvigerina hourcqi (Graham,de Klasz and Rerat). Base of the zone is marked by the abrupt appearance of the nominate taxa and other Eocene benthic
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foraminiferal species and the presence of endemic West African Eocene species. The characteristic species include Sagrina circumspinosa (De Klasz & Rerat), Eponides africana De Klasz & Rerat, Hopkinsina danvillensis Howe & Wallace, Uvigerina jacksonensis Cushman. The disappearance of the above taxa characterises the top of the zone.
The zone embraces the Eocene but was found in this study in the Early Eocene. It occurs in the Oshosun Formation in all the wells of the present study.
DISCUSSION AND CONCLUSION
The Paleocene benthic foraminifera from the Eastern Dahomey basin recorded in this study shows strong affinity to the Midway type fauna of the Midway Formation of Gulf coastal plain of North America (Berggren and Aubert,1975). Some species of the assemblage are common to the Paleocene of Tunisia (Aubert and Berggren.1976), Libya and Mali (Berggren,1974) and Alabama (Mancini, 1984). The Paleocene benthic foraminifera from the Sokoto basin (Okosun,1999) shows no similarity to the assemblage recorded in the current study,this is in agreement with Petters(1979). Thus the two coeval assemblages belong to different biogeography provinces. This does not support the view of a union and faunal exchange between the Tethys and the South Atlantic ocean during the Paleocene.
The six planktic foraminiferal zones have be correlated to the interregional planktic foraminifera zones of Toumarkine and Luterbacher (1985) and Berggren et al (1995). Both the planktic and benthic zones will serve as useful correlation tools in the coastal and inland basins of West Africa
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Journal Min. Geol. Nigeria, 13, (1), 1 -5. Okosun, E.A.,1987.Ostracod Biostratigraphy from Dahomey Embayment Niger Delta and the Benue Trough in Nigeria. Geological Survey of Nigeria Bulletin No 4 I. I5I pp., 21 plates.MONOGRAPH
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Okosun, E.A., 1989. Eocene Ostracoda Oshosun Formation Southwestern Nigeria
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of African Earth Sciences. Vol.9. (3 / 4). 669-676 Okosun, E.A., 1990. A Review of the
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Okosun, E. A., 1999. Late PaleoceneBiostratigraphy and Paleoecology (Foraminiferal and Ostracoda) of two boreholes in Sokoto Basin. NW Nigeria-Journal of Mining and Geology vol. 34.(2). 155-170.
Olsson, R. K., Hemleben, C., Berggren, W.A., and Liu, C., 1992. Wall texture
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Premoli-Silva, I., Bolli, H. M., 1973. Late Cretaceous to Eocene planktonic foraminifer and stratigaphy of Leg 15 sites in the Caribbean sea in: Edgar, N.,Saunders,J.etal (Eds),initial Report Deep Sea Drilling Project 15, pp 449 – 547.
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EXPLANATION OF PLATE ONE
Magnification x65
Fig.l: Parasubbotina, pseudodulloides (Plummer).
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EXPLANATION OF PLATE TWO
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Magnification x60
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EXPLANATION OF PLATE THREE
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Magnification x60
Fig. 1, 2: Sagrina, circumspinosa (De Klaz & Rerat). Fig. 3: Hopkisina, ilaroensis (Haynes & Nwabufo-ene). Fig. 4, 8: Dentalina, colei (Cushman & Dusenbury). Fig. 5, 7: Dentalina, alternata.
Fig. 15, 16: Furenkoina, elongata (Petters & Adegoke).
Fig. 18: Lagena sp.
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