Journal of Mining and Geology Vol. 48(2) 2012, pp. 167–175© Nigerian Mining and Geosciences Society (NMGS) - Printed in Nigeria

1116-2775

Microfossils and Paleoenvironment of the Numanha Shale of the Upper Benue Trough, Northeastern Nigeria

Saka Adelayo OpeloyeDepartment of Applied Geology, Federal University of Technology, Akure

Abstract

The paleoenvironment of the Numanha Shale was analysed from the study of limestones as well as the microfossil content of the shale units. The cyclicity of the interbedded lithounits is attributed to depth fluctuation. The limestone beds are intramicrite and biosparmicrite while the microfossils indicate a shallow marine, brackish-water environment. The foraminiferal assemblage is dominated by species of Ammobaculites, Haplophragmoides and Heterohelix while the ostracods are composed of the smooth cyprideids and cytherideids of shoreline habitat with Ovocytheridea in abundance. The age of the Numanha Shale is deduced as Turonian-Coniacian based on the occurrences of Heterohelix species.

IntroductionThe mid-Cretaceous marine sediments of the Benue Trough are rich in foraminiferal and ostracod assemblages which are of paleoenvironmental and paleoecological significance. The Upper Benue Trough, which is the northern section of the linear basin, bifurcates into the north-south trending Gongola Arm and the east-west Yola Arm. Previous studies on the foraminifers and ostracods of the Gongola Arm (Petters, 1982; Gebhardt, 1997; 1999; Okosun, 1987; 1994) have often been generalized for the entire Upper Benue Trough while relatively little is known of the micro-fauna of

the Yola Arm where the marine sediments are limited in occurrence to the western half of the basin (Fig. 1).

The Numanha Shale is one of the formations that marked prominent thick outcrops of interbedded limestone and shale along the stream channels at Gwolitse, Numanha and Ayatse (Fig. 2). The purpose of this paper is to document the distribution of the foraminifers and ostracods in the outcrop sections of the Numanha Shale and their significance in regard to the depositional environment.

O NUMANHA

O GWOLITSE

LEGEND

LAMJA SANDSTONE

VOLCANICS

NUMANHA SHALE

SUKULIYE FORMATION

JESSU FORMATION

DUKUL FORMATION

YOLDE FORMATION

CRYSTALLINE BASEMENT

BIMA 3

BIMA 2

BIMA 1

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Fig. 1 Geological Map of Yola Arm of Upper Benue Trough

Geologic settingThe geodynamic events leading to the network of rifted basins in central - West Africa have been associated with the fragmentation and dispersal of Gondwana (Petters, 1982) either through the process of plate divergence (Evans, 1978), rifting (Burke, 1976; Olade, 1975; Wright, 1981), or reactivation of an existing shear zone (Benkhelil, 1982). The Benue Trough was formed as a major strike-slip influenced aulacogen forming an integral part of the West African Rift System (Guiraud, 1990).

The Bima Group was the earliest sedimentary deposit in the Upper Benue Trough and was discussed by Guiraud, (1990) and Samaila et al., (2005). The basal lithostratigraphic unit, referred to as 'Lower Bima Formation' unconformably overlies the basement directly and bears the imprint of syn-rift sedimentation. The sediments are in form of facies changes from conglomeratic alluvial fan and debris flow adjacent to the basins marginal fault and grading

into fining upward fluvial sequences. It is succeeded by 'Middle Bima Formation' which is composed of fine to coarse grained trough cross bedded sandstone interpreted as proximal braided river and palustrine deposits. The 'Upper Bima Formation' with medium to coarse grained tabular cross bedded sandstone overlies the Middle Bima Formation' and syn-depositional overturned and convolute bedding are occasionally encountered among its beds.

The Yolde Formation succeeds the Bima Group. It is of Cenomanian age and represents transitional stages between the fluvial sedimentation of the lower Bima Group and the upper Cretaceous marine formations (Lawal and Moullade, 1986). In the Yola arm the latter are the Dukul, Jessu, Sukuliye, Numanha and Lamja Formations. They are formed during eustatic cycles of sedimentation in which the peaks of Late Cenomanian to Early Turonian and Coniacian transgressions are marked by fossiliferous Dukul and Numanha Formations respectively.

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NUMANHA SHALELONGUDA PLATEAU

BIMA SANDSTONE YOLDE FORMATION

DUKUL FORMATION

JESSU FORMATION

SUKULIYE FORMATION

LAMJA SANDSTONE

O GWOLITSE

O AYATSE

O NUMANHA

GO

NG

OL

AR

IV

ER

O Km 4.5Km O GWOLITSE O AYATSE O NUMANHA

OUTCROP LOCALITIES

Fig. 2: Lithounits in the Yola Arm showing the outcrop localities of Numanha Shale along the slope of Longuda Plateau

MethodsThe outcrop of the Numanha Shale was mapped and samples of limestone and shale from three prominent sections of the formation were derived along the stream channels at Gwolitse, Numanha and Ayatse. Thin sections of the limestone samples were made for petrographic studies of microtextural characteristics while samples derived from the shale interbeds were used for microfossil analysis.

Sample preparation for microfossil analysis was accomplished by soaking about 100g of each sampled shale in water with 0.5g of Na CO and leaving for two 2

days to allow total dissagregation of shale and mud. After heating for about 15 minutes, washing over a 230 mesh (63 m) screen was carried out. The residue was air-dried before ostracods and foraminifers were picked using an OLYMPUS binocular microscope. Identification of the microfossils follows earlier descriptions of morphotypes by Petters (1982) Ramanathan and Nair (1984) and Okosun (2001).

Lithologic relationship Sections of the Numanha Shale occur along the

3

Numanha, Gwolitse and Ayatse streams on the east-facing side of the Longuda basalt plateau. The formation consists of fissile dark grey to black shale with thin to medium interbedded limestones. The beds generally lack macrofossils except the uppermost limestone bed of each of the sections which contain numerous pelecypod shells.

The Numanha stream section is about 30m thick comprising of five grey shale with limestone horizons. The base of the outcrop section consists of a 1m thick indurated clay which marks the boundary between Sukuliye and Numanha Formations. The limestone beds are generally micritic and contain well preserved gastropod shells (Fig. 3) towards the base of the section.

The Gwolitse section is 35m thick and consists of seven beds of fissile shale units whose thickness range from 2m to 3m. The lower beds are black, hard and brittle while the rest are fissile grey shale. Maximum thickness of each of the seven interbedded limestone is 20cm. The limestones content are bryozoans and pelecypods (Fig. 4).

The Ayatse section is about 60m thick with the shale bed thickness ranging from 2m to 8m in

Fig 3; Numanha Shale exposed along Numanha Stream. The insets are photomicrographs of the limestone textural characteristics: a) Biomicrite of upper limestone beds. b) Microgastropod shell

in the micritic limestone of the basal beds

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Fig 4 Numanha Formation along Gwolitse stream near Gundanyu village. The Insets show photomicrographs of textural characteristics of

the limestone: (a) biosparite of upper limestone bed with pelecypods and bryozoans allochems; (b) ostracods and pelecypods as

allochems of the limestone of middle section (c) Intraclast of the basallimestone beds

Fig 5: Numanha Shale showing large thickness of shale beds, The \\ Insets are the photomicrographs of limestone bioclasts with we ll preserved Bryozoans and ostracods microfossils

along ayatse stream channel

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(1) Ammobaculites bauchensis (Petters) 2) Ammobaculites benuensis (Petters) (3) Ammomaginulina sp. (Petters) (4) Haplophragmoides bauchensis (Petters) (5) Ammobaculites irregulariformis (Petters) (6) Ammobaculites pindigensis

(Petters) (7) Ammotium nwalium (Petters) (8) Ammotium nkalagum (Petters) (9) Haplophragmoides (10) Talokaense (Petters) 10) Miliammina sp. (Petters) (11) Heterohelix reussi (Cushman) (12) Heterohelix moremani (Cushman)

Fig 6 Foraminifera of the Numanha Shale

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FO

RA

MIN

IFE

RA

AN

D O

ST

RA

CO

D C

OU

NT

S F

OR

NJM

AN

HA

FO

RM

AT

ION

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1) Bythocypris sp. 2) Bythoceratina sp. 3) Cythereis vitiliginosa 6) Dumontina sp. 4 &10) Ovocytheridea reniformis 5 & 12 ?Ovocytheridea sp. 7)Dolocytheridea sp. 8) Cytherella sp. 9) Ovocytheridea ashakaensis 11) Ovocytheridea symmetrica

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thickness. The lower beds are thicker, darker and more compact than the succeeding ones which are mainly fissile. Thin to medium beds of biomicrite limestone (wackestone) terminates each of the shale with sharp boundaries (Fig. 5).

Microfossil compositionForaminifera and ostracods of the Numanha Shale as encountered in the Numanha, Gwolitse and Ayatse stratigraphic sections are fairly rich and diverse (Figs 6 and 7). A total of 437 specimens of foraminifera belonging to six genera as well as 120 ostracod specimens belonging to seven genera were recovered from the studied samples (Table 1). The Gwolitse section provides the highest number of species with observed maximum diversity and abundance.

Of the foraminifers four genera are arenaceous and are found in almost every bed in the sections: they are monospecific of Haplophragmoides Miliammina Ammobaculites and Ammotium species. Planktonic foraminifers were recovered only from the Gwolitse and Ayatse sections where they are limited to black shale beds. The genera Hedbergella and Heterohelix are well represented with Heterohelix reussi (Cushman) being the most abundant. The ostracods are smooth forms, most commonly the Cytherellids, with Ovocytheridea being the most abundant.

DiscussionLimestone and shale alternations in the Numanha Shale are the result of paleosea level fluctuation. The interpretation of the microtextural attributes, of the thin to medium bedded limestone bands, provide information on the paleoenvironmental condition of the paleosea. Some of the basal beds contain intraclasts with pelecypod fragments. The texture reflects the process whereby sediments of the basin floor were reworked, transported and finally deposited to form limestone beds. The upper beds are however dominated by fragmental bioclasts cemented with sparite crystals suggestive of high current energy deposition. The gastropods, pelecypods and bryozoan contents of the limestone, on account of being benthic forms, clearly reveal near-shore confined shallow marine environment.

The adjoining shale layers were deposited in deeper water. They contain a large number of arenaceous benthics and smooth ostracods which suggests a shallow marginal marine environment. The abundance of Haplophragmoides talokaense and the different forms of the Ammobaculites are related to

their successful exploitation of the environment as detritivores. Haplophragmoides talokaense which constitutes the most abundant forms among the species is much reduced in size. The reduction is probably due to a shortened reproductive cycle on account of abundant nutrients in the substrate which allows such opportunistic detritivore to reach maturity at an earlier ontogenic stage. Species of Ammotium are also typical of brackish habitats below 10m depth while Miliammina can tolerate condition up to 50%o salinity (Murray, 1991). The planktonics indicates an increase in water depth. However, the preponderance of arenaceous benthics as well as smooth ostracods in the shale beds indicates a dominantly brackish shoreline environment.

The arenaceous are the forms found within the Planulina beadnelli - Ammoastuta nigeriana biozone which is Turonian at its base (Petters, 1982 p.17). The Heterohelicids are age diagnostic according to Gebhardt (1997). Heterohelix fayosei (Petters) is Lower Turonian, Heterohelix moremani (Cushman) ranges from Albian to Turonian while Heterohelix reussi (Cushman) ranges from Turonian to Campanian. Since the entire sedimentary succession of the Yola Arm is believed to be involved in the Santonian deformation (Odebode, 1987), Numanha Shale therefore was deposited between Turonian and Coniacian.

ConclusionsThe Numanha Shale lithounits comprises dark grey to black fissile shale with thin interbedded limestone. Microtextural attributes of the limestone indicates high energy current of the shallow environment which might have reworked carbonate sediments of the basin floor. The shales yielded foraminifers and ostracods belonging to the six and seven genera respectively. The identified Heterohelix planktonics indicates Turonian age which is in line with the assertions of both Petters (1982) and Gebhardt (1997) on the ages of other marine formations in the Upper enue Trough. The dominant arenaceous benthic foraminifers and the smooth ostracods suggest a mainly shallow marginal marine brackish water environment with an increase in water depth represented by horizons with planktonic foraminifers. Sea level flunctuation due probably to intermittent subsidence was responsible for the alternation of shale and limestone in the formation.

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References

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Received 30th June 2009, Revision accepted 8th September 2011

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