Journal of African Earth Sciences 96 (2014) 168–179

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Journal of African Earth Sciences

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Facies analysis, palaeoenvironmental reconstruction and stratigraphicdevelopment of the Early Cretaceous sediments (Lower Bima Member)in the Yola Sub-basin, Northern Benue Trough, NE Nigeria

http://dx.doi.org/10.1016/j.jafrearsci.2014.04.0071464-343X/� 2014 Elsevier Ltd. All rights reserved.

⇑ Corresponding author at: Department of Geology, University of Malaya, 50603Kuala Lumpur, Malaysia. Tel.: +60 166737410.

E-mail addresses: bmsydgombe@yahoo.com, babangida@siswa.um.edu.my (B.M.Sarki Yandoka).

Babangida M. Sarki Yandoka a,b,⇑, M.B. Abubakar b, Wan Hasiah Abdullah a, M.H. Amir Hassan a,Bappah U. Adamu b, John S. Jitong b, Abdulkarim H. Aliyu b, Adebanji Kayode Adegoke a,c

a Department of Geology, University of Malaya, 50603 Kuala Lumpur, Malaysiab National Centre for Petroleum Research and Development, A.T.B.U., P.M.B., 0248 Bauchi, Nigeriac Department of Geology, Ekiti State University, P.M.B., 5363 Ado-Ekiti, Nigeria

a r t i c l e i n f o

Article history:Received 2 October 2013Received in revised form 9 April 2014Accepted 14 April 2014Available online 24 April 2014

Keywords:Northern Benue TroughBima FormationFaciesPaleocurrentFluvial systemLacustrine system

a b s t r a c t

The Benue Trough of Nigeria is a major rift basin formed from the tension generated by the separation ofAfrican and South American plates in the Early Cretaceous. It is geographically sub-divided into Southern,Central and Northern Benue portions. The Northern Benue Trough comprises two sub-basins; the N–Strending Gongola Sub-basin and the E–W trending Yola Sub-basin. The Bima Formation is the oldestlithogenetic unit occupying the base of the Cretaceous successions in the Northern Benue Trough. It isdifferentiated into three members; the Lower Bima (B1), the Middle Bima (B2) and the Upper Bima(B3). Facies and their stratigraphical distribution analyses were conducted on the Lower Bima Memberexposed mainly at the core of the NE–SW axially trending Lamurde Anticline in the Yola Sub-basin, withan objective to interpret the paleodepositional environments, and to reconstruct the depositional modeland the stratigraphical architecture. Ten (10) lithofacies were identified on the basis of lithology, grainsize, sedimentary structures and paleocurrent analysis. The facies constitute three (3) major facies asso-ciations; the gravelly dominated, the sandy dominated and the fine grain dominated. These facies andfacies associations were interpreted and three facies successions were recognized; the alluvial–proximalbraided river, the braided river and the lacustrine–marginal lacustrine. The stratigraphic architectureindicates a rifted (?pull-apart) origin as the facies distribution shows a progradational succession froma shallow lacustrine/marginal lacustrine (at the axial part of the basin) to alluvial fan (sediment gravityflow)–proximal braided river (gravel bed braided river) and braided river (channel and overbank) depo-sitional systems. The facies stacking patterns depict sedimentation mainly controlled by allogenic factorsof climate and tectonism.

� 2014 Elsevier Ltd. All rights reserved.

1. Introduction

The opening of the Atlantic Ocean which started at the begin-ning of the Mesozoic triggered the crustal fragmentation of theWest and Central African craton into rift systems. The BenueTrough is one of the major rift basins formed from the tension gen-erated by the separation of the African and South American plates(Fig. 1). It is a NE–SW trending, intra-continental, Cretaceous sed-imentary basin in Nigeria that extends about 1000 km in lengthand 50 km in width (Fig. 2). It extends from the Niger Delta in

the southwest to the Chad (Bornu) Basin in the northeast(Maurin et al., 1985).

Several authors have presented tectonic models for the genesisof the Benue Trough (Abubakar, 2014). King (1950) proposed ten-sional movement resulting in a rift while Stoneley (1966) proposeda graben-like structure. The RRF triple junction model leading toplate dilation and opening of the Gulf of Guinea was proposed byGrant (1971). Olade (1975) considered the Benue Trough as thethird failed arm or aulocogen of a three armed rift system relatedto the development of hotspots. Benkhelil (1982, 1989) andGuiraud and Maurin (1992) considered wrench faulting as thedominant tectonic process during the Benue Trough evolutionand defined it as a set of juxtaposed pull-apart basins generatedalong the pre-existing N60�E strike-slip faults.

Fig. 1. West and Central African Rift System showing the Nigerian Benue Trough modified (from United Reef Limited Report, 2004).

Fig. 2. Generalized geological map of Nigeria showing the study area represented as open square (from Abubakar et al., 2008).

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The Benue Trough is geographically sub-divided into Southern,Central and Northern portions (Nwajide, 2013). The NorthernBenue Trough is made up of two major sub-basins; the N–S trend-ing Gongola Sub-basin and the E–W trending Yola Sub-basin.Carter et al. (1963), Offodile (1976), Benkhelil (1989), Zaborskiet al. (1997), Obaje et al. (2000), and Abubakar (2006) havedescribed in detail the geology and stratigraphy of the NorthernBenue Trough. The stratigraphic succession in the Yola Sub-basinof the Northern Benue Trough (Fig. 3) comprises the continentalLower Cretaceous Bima Formation, the Cenomanian transitionalmarine Yolde Formation and the marine late Cenomanian–Santo-nian Dukul, Jessu, Sekuliye formations, Numanha Shales and LamjaSandstones (Carter et al., 1963; Abubakar, 2006).

This article describes and analyses the lithofacies of the EarlyCretaceous continental sediments of the Bima Formation (LowerBima Member) and their stratigraphical affinities with an objectiveof identifying palaeodepositional environments, reconstruction ofstratigraphical architecture and depositional model so as to pro-vide an impetus to research on the viability of Early Cretaceouspetroleum system in the Yola Sub-basin of the Northern BenueTrough. This study is important for the tectonosedimentary under-standing of Early Cretaceous sedimentation in the Bima Formationin its relation to similar formations within the West and CentralAfrican Rift System (WCARS) proven as good petroleum systemsin the Muglad Basin of Sudan and in the Termit Basin of Nigerand Chad Republics (Abubakar, 2006).

Fig. 3. Stratigraphic sequence of the Northern Benue Trough, Nigeria (from Abubakar, 2006).

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The study area is located mainly at the Lamurde Anticline(Fig. 4) in the Yola Sub-basin (Fig. 2). The Lamurde Anticline is animpressive feature in the Northern Benue Trough of Nigeria. ThisNE–SW axially trending anticline formed as a result of NW–SEcompression within the Benue Trough during the Santonian toprobably Maastrichian age (Fig. 5). It exposes in spectacular scen-ery most features of the Lower Bima Formation that permits itsdetailed outcrop study.

2. Geological setting

The Bima Formation is the oldest lithologic unit occupying thebase of the Cretaceous successions in the Northern Benue Trough(Fig. 3). The sediments were mainly derived from juxtaposed base-ment rocks of older granites and gneisses and were deposited in acontinental environment under widely varying conditions of allu-vial – fluvial systems (Braide, 1990; Guiraud, 1990). The sedimen-tary structures include large scale trough cross bedding, planarcross bedding, groove marks and soft sediment deformation struc-tures (Guiraud, 1990; Samaila et al., 2006; Braide, 1990). Carteret al. (1963) and Guiraud (1990) differentiated the formation intothree members; the Lower Bima (B1), the Middle Bima (B2) and

the Upper Bima (B3). These three lithologic units were also identi-fied in the seismic section of the Nigerian sector of the Chad Basin(Avbovbo et al., 1986).

The Lower Bima (B1), the subject of this study, is the oldestmember (?Late Jurassic–Berremian–Aptian) and has beendescribed as consisting of fault controlled conglomerates, sandsand gravels with poorly defined internal structures characterizedby well-defined fining upward successions (Guiraud, 1990). Troughcross beds are common in association with minor tabular units.Red, gray and purple clays with fine–medium grained sandstoneswere reported but controversy exists on their origin as lacustrine(Kogbe, 1976; Allix et al., 1981) or river floodplain deposits(Guiraud, 1990).

The Late Aptian–Albian Middle Bima (B2) unconformably over-lies the Lower Bima Member. It is composed of medium to verycoarse grained feldspathic sandstones with trough and tabularcross beddings interbedded with clays (Offodile, 1976; Zaborskiet al., 1997). It is considered to have been deposited in a deeplyentrenched braided river system (Carter et al., 1963; Abubakar,2006). The Upper Bima (B3) conformably overlies the Middle Bima(B2). It has a relatively homogeneous appearance consisting of pla-nar crossbedded medium–coarse grained sandstone associatedwith soft sediment deformation structures Samaila et al. (2006).

Fig. 4. Geological map of the study area showing the locations of sedimentology logs.

Fig. 5. Schematic outcrops photo of the asymmetric Lamurde Anticline with older beds exposed to the surface.

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Volcanic plugs of Tertiary age have being reported as intrusions insome of the Cretaceous formations in the Northern Benue Trough(Carter et al., 1963; Wright, 1976, 1989).

3. Data sets and methods

Traverses were made mainly on the exposed core of the Lam-urde Anticline around Lafia-Lamurde and Maitunku areas in theYola Sub-basin. Additional study was done at the foot of theBurashika fault located some few kilometers toward the northeastof the Lamurde Anticline. Measurements and description of theLower Bima outcrops, documenting thicknesses, lithology, bed,attitudes and contact relationship were carried out. The bestexposed stratigraphic sections (ten in total) were studied for faciesand stratigraphical distribution. Paleocurrent measurements werealso done and corrected for tilt effects based on the method ofTucker (1996). This provides inference to provenance.

4. Facies, facies associations and successions

4.1. Facies and facies associations

The outcrops of the Lower Bima Member around the LamurdeAnticline in the Yola Sub-basin contain conglomerates to verycoarse grained, medium grained and fine grained sandstones con-taining cobbles and pebbles of mainly rock-fragments (gneissose,granitic, volcanic and occasionally sedimentary), quartz and feld-spars. The color of the sandstones and siltstones is white, buff,brown and milky to gray. Shales are dark gray, green, red, purpleto black in color.

The sediments in the study area are grouped into three faciesassociations; the gravelly-dominated, the sandy-dominated andthe fine grain-dominated. These associations constitute two ormore facies identified on the basis sedimentary structures and tex-tures. The facies provide an insight into depositional processes andwere interpreted following Simon et al. (1965), Postma (1990),Miall (1977, 1978, 1996, 2010), Bohacs et al. (2000), Nichols(2009) amongst many others. Table 1 provides a summary on thedescription, classification and interpretation of the facies.

Table 1Summary of facies description and interpretation.

Lithofacies Description

Massive matrixsupported gravel(Gmm)

Massive matrix supported gravels, poorly to moderately ssizes, units up to 3 m thick

Massive clast supportedgravel (Gcm)

Massive clast supported gravels, angular to sub-roundedrarely imbricated, gravels upto 8.5 diameter, erosive base

Trough cross beddedsandstone (St)

Medium to coarse grained sandstone, moderately to wellclast, beds may reach up to 4 m thick, basal contacts are

Planar cross beddedsandstone (Sp)

Medium to fine grained sandstone, moderately sorted, bucoarse to medium sands, thickness of beds up to 2 m thi

Horizontally beddedsandstone (Sh)

Medium to fine grained sandstone, moderately to moderaup to 2 m, display horizontal bedding and occasionally lo

Ripple cross–parallellaminated sandstone(Sr)

Medium to fine grained sandstone, moderately to moderato 1 m, rippled cross laminated, parallel laminations pres

Convoluted sandstone(Sc)

Medium to fine grained sandstone, convolute bedded, th

Mud and silts (Fm) Mud drapes occurred between gravel and sandstone bedsto 1 m

Shale/mudstone (Fl1) Laminated dark to gray shale, thickness of beds up to 15and body fossils

Ripple laminatedsandstone (Fl2)

Fine grained sandstone and siltstone, rippled, up to 1.5 m

Facies codes are slightly modified from Miall (1978, 1996, 2010).

4.1.1. Association I: gravelly-dominatedThe gravelly dominated facies association consists of the two

lithofacies (Gmm and Gcm) (Table 1) as described below. Thesefacies contain gravels that are matrix to clast supported with var-ious sizes and nature (granite, gneiss, quartz, clay and feldspars).

4.1.1.1. Facies Gcm: Massive clast-supported conglomerates (Fig. 6iand ii). This is characterized by massive clast-supported conglom-erates that range in thickness from 1.5 to 4 m. Clasts range from4 mm to 8.5 cm in diameter composed of quartz, (Fig. 6) feldsparsand rock fragments with very minor coarse to very coarse-grainedsand matrix (15–20% of the total). The clasts are angular torounded, poorly sorted and rarely imbricated. This facies isungraded and its basal boundary may be sharp or erosional. Itresembles the lithofacies Gcm of Miall (1977) and interpreted aspseudoplastic debris flow deposit based on its clast-supportedframework and lack of internal organization (Rust, 1978; Miall,1978, 1996).

4.1.1.2. Facies Gmm: Massive matrix-supported conglomerates (Fig. 6iand ii). This is characterized by massive matrix-supported con-glomerates that are poorly sorted with sub-angular clasts that varyin size from granules to cobbles. The clasts occur within medium tocoarse grained sand matrix which comprises between 20% and 40%of the total. It may reach up to 3 m and is mostly overlain by verylarge scale trough cross beds of gravelly to coarse grained sand-stone. The facies corresponds to the Gmm lithofacies of Miall(1977) and interpreted as plastic debris flow (high strength vis-cous) deposits (Rust, 1978; Miall, 1977). Blair and McPherson(1994) described similar facies as alluvial fan sheetflood deposits.

4.1.2. Association II: sandy-dominatedThis facies association is composed of fine to medium and

coarse to very coarse grained sandstones containing pebbles andrare cobbles. Maximum size of the pebbles is 1 cm. It consist of fivelithofacies described as: the trough cross bedded sandstone (St),the planar cross bedded sandstone (Sp), the horizontally beddedsandstone (Sh), the ripple cross to parallel laminated sandstone(Sr) and the convoluted sandstone (Sc) facies (Table 1). These arefurther described below.

Interpretation

orted, sub-angular clasts of various Plastic debris flow (high strength viscous)or sheetflood deposits

clasts, ungraded, poorly sorted, Pseudoplastic debris flow or longitudinalbars, sheet flood to channel deposits

sorted, sub-angular to sub-roundedsharp or erosional

Linguoid and lobate bars (High flowregime), 3-D dunes

ff colored, planar beds form inck

Transverse bars and sand waves (Lowerflow regime), 2-D dunes

tely well sorted, buff colored, bedsw angle cross beds

Planar bed flow (sheet flood to channeldeposits) (Upper flow regime)

tely well sorted, beds thickness upent

Ripples (Lower flow regime)

ickness of beds up to 0.5 to 1 m Post-depositional soft sedimentdeformation deposit

, thickness of beds occasionally up Overbank or drape deposits

m, absence of bioturbations, trace Suspension settling from standing watersor shallow lacustrine deposits

thick Suspension settling from weak currents orshallow lacustrine deposits

Fig. 6. (i–ii) Clast and matrix supported gravels facies (Gcm and Gmm), (iii and iv) trough cross bedded sandstone facies (St), (v) planar and trough cross bedded sandstone(Sp, St) facies, (vi) planar cross bedded sandstones facies (Sp).

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4.1.2.1. Facies St: trough crossbedded sandstone (Fig. 6iii and iv). Thetrough crossbedded sandstone facies comprises medium to verycoarse grained sandstone, moderately sorted with scattered sub-angular to sub-rounded pebbles. Beds may reach up to 4 m inthickness. Large scale trough cross beds are well developed in somemeasured sections (thickness ranges from 2 to 3 m). This facies isassociated with planar cross bedded sandstone (Sp), horizontallybedded sandstone (Sh) and mud and silts facies (Fm). The largescale trough cross beds may overlie the clast and matrix supportedconglomerates facies. Basal boundaries are mostly sharp or ero-sional and occasionally with pebble lag. Similar lithofacies wereinterpreted as migrating sinuous 3-D dunes that stack up to gener-ate bar forms in a channel (Miall, 1978, 1996, 2010; Plint, 1983).

4.1.2.2. Facies Sp: Planar crossbedded sandstone (Figs. 6v and vi and 7iand ii). This facies is characterized by planar crossbedded coarse tomedium grained, moderate to moderately well sorted sandstoneswith thicknesses of up to 4 m (Fig. 7). This facies is associated withconvulated sandstone (Sc), trough cross bedded sandstone (St) andripple laminated sandstone (Sr). Crossbedded foresets vary inthickness from 0.30 to 2 m. Individual beds have been traced fortens of meters parallel to the bedding plane. This facies corre-sponds to Sp lithofacies of Miall (1977), produced by migrationof 2-D dunes or sheetflooding (Feary, 1984). Similar facies areinterpreted as transverse bars under lower flow regime (Miall,1978, 1996, 2010).

4.1.2.3. Facies Sh: horizontally bedded sandstone (Fig. 7i, ii, iv andv). This facies is composed of medium to fine grained, moderatelysorted to moderately well sorted, predominantly buff coloredsandstones. Thickness of beds may be up to 2 m. Some beds havecombination of horizontal lamination and low angle cross-beds.The bed contacts are mostly sharp. This facies is interpreted asplane bed sand of upper flow regime deposited in section of rivervalley as channel fill developed on top of sand bars (Miall, 1978;Harms et al., 1995).

4.1.2.4. Facies (Sr): rippled cross to parallel laminated sandstone. Thisfacies is characterized by fine to medium grained, moderate tomoderately well-sorted sandstone with current ripples. It may beparallel laminated occasionally. Thickness of bed may be up to1 m. This facies is overlain by the mud and silts (Fm) facies andunderlain by planar cross bedded sandstone (Sp), trough cross bed-ded sandstone (St) and horizontally stratified sandstone (Sh) facies.It is interpreted as migrating currents ripple deposits under lowerflow regime (Miall, 1978). Similar beds were also interpreted aswaning flow sheetflood deposits (Handford, 1982; Rust, 1984).

4.1.2.5. Facies Sc: convulated sandstone facies (Fig. 7vi). This facies ischaracterized by convulated medium to fine grained, moderatelysorted sandstones with thickness of about 0.5–2 m. Similar faciesfrom the Upper Bima Member were interpreted as post deposi-tional fluidization structure triggered by seismic shocks (Samaila

Fig. 7. (i–ii) Relationship between horizontally bedded sandstone facies (Sh) and St and Sp, (iii) planar cross bedded sandstone facies (Sp), (iv and v) horizontally beddedsandstone facies (Sh), (vi) convolute bedded sandstone facies (Sc).

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et al., 2006). Soft sediments deformation structures formed due todisturbance of sediments during or after deposition, consolidationand burial (Boggs, 1987; Owen, 1995, 1996) are common in sand-stones that are loosely packed and rapidly deposited (related forexample to braided river deposits – Bristow, 1983).

4.1.3. Association III: fine grain dominatedThe fine grain dominated facies association is composed of

three lithofacies; the mud and silts (Fm), the shale/mudstone(Fl1) and the rippled laminated sandstone (Fl2) facies (Table 1)as described below.

4.1.3.1. Facies (Fm): Mud and silts. This facies is characterized bylaminated mud and silts. The mudstone occurs in between gravelsand sandstone of St, Sp or Sr facies with thickness of about 0.4–1 m. Miall (1977) proposed this lithofacies for mud drapes thatoccur within gravelly and sandy braided sediments, representingdeposits from standing pools of water during low stage of channelabandonment. It is interpreted as overbank or drape deposits, thusit may represent the most distal floodplain (Miall, 1977, 1978;Rust, 1978).

4.1.3.2. Facies (Fl1): shale/mudstone (Fig. 8). This facies is character-ized by dark gray shale with thin beds of silts. The facies lack mar-

ine shells, body and trace fossils (Fig. 8). Beds range in thicknessfrom 5 to 15 m. The facies is interbedded with rippled sand andsilts (Fl2) facies. It is interpreted as suspension settling depositsfrom standing waters (Miall, 1977, 1978; Friend, 1966). Similarfacies were interpreted as deposition in poorly drained (water-logged) floodplain and ponded waters (shallow lacustrine) of lowenergy (Retallack, 1997; Mark and James, 1992). A shallow lacus-trine environment was suggested for similar facies (Ntamak-Nidaet al., 2008; Nichols, 2009).

4.1.3.3. Facies (Fl2): Rippled laminated sandstone (Fig. 8). This ischaracterized by rippled laminated fine grained sand and siltstone.Small amount of mud is associated with the silts. This facies is int-erbedded with the shale/mudstone (Fl1) facies. Individual bedsrange in thickness from 1 to 3 m. It is interpreted as suspensiondeposit settling from weak currents (Miall, 1977) and representingsand sheets or distal splays formed by supply of sand on flood-plains (Retallack, 1997) or shallow lacustrine deposits (lake deltasandstones, Nichols, 2009).

4.2. Facies successions

Facies successions represent genetically related lithofaciesthat occur in combination and typically represent individual

Fig. 8. (i–vi) Shale/mudstone facies (Fl1) and ripple laminated sandstone facies (Fl2).

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depositional environments (Dalrymple, 2010). From the descrip-tions and associations of facies (Fig. 9) of the Lower Bima Memberexposed at the core of the Lamurde Anticline in the Yola Sub-basin,three types of facies successions are identified; alluvial fan to prox-imal braided river, braided river and lacustrine to marginal lacus-trine facies successions.

4.2.1. Alluvial fan to proximal braided river facies successionThis facies succession consists of the gravelly dominated facies

association (Section 4.1.1), Gcm and Gmm facies, with a thicknessof about 5–8 m (Fig. 9). The poorly sorted nature of this facies withoutsized clasts, fining upward packages and rarity of sedimentarystructures suggest that sediment gravity flow mainly associatedwith alluvial fans and proximal part of braided river played a sig-nificant role in their deposition (Miall, 1977, 1978; Rust, 1978).The gravelly dominated facies association exhibit vertical and lat-eral evolution toward the sandy dominated facies association.

4.2.2. Braided river facies successionThis facies succession is made up of sandy dominated (Sec-

tion 4.1.2) facies association (St, Sp, Sh, Sr and Sc) composed ofpebbly very coarse grained, coarse to medium grained and mediumto fine grained sandstones and the mud and silts (Fm) facies fromthe fine grain dominated facies association with thickness of about30–40 m (Fig. 9). Fining upward packages of this facies successionmay represent waning flow in braided river system. The sandydominated facies association is interpreted as braided river chan-

nel deposits while the mud and silts facies represent overbank(floodplain) deposits (Fig. 9). It is inferred that the convolute bed-ded sandstones are indication of rapid deposition under water sat-urated condition probably triggered by seismicity.

4.2.3. Lacustrine to marginal lacustrine facies successionThis succession is composed of fine grain dominated facies

association (Fl1 and Fl2). It is represented by coarsening upwardmotifs (mostly 20–25 m thick). The facies succession is interpretedto represent shallow lacustrine to lacustrine delta depositional sys-tem (Fig. 9) based on its coarsening upward nature and earlierinterpretation (Section 4.1.3) of its associated facies.

5. Paleocurrent analysis

About sixty seven palaeocurrent readings were recorded fromthe Lower Bima Member, determined mainly from sedimentarystructures that include crossbeds, current ripples and local occur-rence of imbrications that indicate direction of transport. Thepaleocurrent and paleoslope trends in the outcrops are generallyfrom NNW but with significant number of readings also towardSW direction (Fig. 10).

6. Stratigraphic development

The outcrops of the Lower Bima Member exposed at theLamurde Anticline comprise more than 500 m of conglomerates,

Fig. 9. Lithologic logs of the Lower Bima Member outcropped at the core of Lamurde Anticline showing the identified lithofacies.

Fig. 10. Paleocurrent flow direction at different localities in the Lower Bima Member exposed at the Lamurde Anticline.

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pebbly coarse, medium and fine grained sandstones with mud-stone, shale and siltstones. The three facies associations describedabove are closely related and their inter-relationship supports theirspatial and temporal evolution from a lacustrine to marginal lacus-trine (at the axial region of the basin) overlain progradationally byproximal braided river/alluvial fans and subsequently braided riverdeposits (Fig. 11).

7. Depositional model

Several depositional models have been proposed to account forthe larger scale architecture of the Bima Formation. Most workers(Carter et al., 1963; Guiraud, 1990; Offodile, 1976; Zaborski et al.,1997; Abubakar, 2006) agreed that the Lower Bima may be alluvialfan-braided river (Scott type), the Middle Bima may be braidedriver dominated by linguoid dunes associated with deeplyentrenched braided river channels and the Upper Bima may bebraided river (Platte type) related to channel aggradations andsand flat formation.

However, the controversial issue alluded to on the lacustrinedeposits in the Lower Bima Member (Section 2) still persist. Thepresent study therefore developed a depositional model of theLower Bima Member of the Bima Formation in the Yola Sub-basinof the Northern Benue Trough (Fig. 12). This model was developedfrom the lithofacies, lithofacies association studies and facies suc-cession stratigraphic relationships. The model involves three majordepositional systems of lacustrine to marginal lacustrine (deltaic),alluvial fan and braided river. The alluvial fan and braided riversystems are inferred to have fed the lacustrine to marginal lacus-trine system with sediment.

The lacustrine facies assemblage shows a large terrigenousinfluence. It is constituted mainly by massive and finely laminatedto thinly stratified shales with rhythmic couplets and parallel toripple laminated sandstones. This assemblage might have beendeposited in shallow littoral to sub-littoral lacustrine environment

Fig. 11. Summarized composite lithologic logs from the studied area showing

(Fig. 12). Strata sets are hierarchically organized within alluvialand braided river systems such that; conglomerates and pebblyvery coarse sandstones make up the alluvial fan–proximal braidedriver deposits. Pebbly coarse, coarse to medium trough crossbeds(from dunes), planar crossbeds (from bed load sheets) and small-scale cross strata (from ripples) make up channel-belt deposits ofbraided river systems.

8. Discussion

Based on the lithofacies, paleocurrent and stratigraphical stud-ies, a generalized depositional model of the Lower Bima Member ofthe Bima Formation in the Yola Sub-basin is proposed (Fig. 12). Thestratigraphical analysis has permitted the generalized determina-tion of depositional patterns as progradational successions. Conti-nental depositional processes of alluvial, braided and lacustrineenvironments were responsible for the deposition of the LowerBima Member. Deposition started with gravity driven and mudflow sedimentation considered as syn-rift at locations closer tofaulted basement margins. Toward the basin axis, textural changesoccur from coarse–medium grained sandstone to very fine grainedsandstone and shale at the central part of the basin. The finegrained facies formed the lacustrine deposit, the gravel facies con-stitute typical alluvial fan trending into braided river depositswhile the sandstone facies are channel bar deposits marked by lon-gitudinal (trough cross-bedded sandstone) and transverse (planarcross-bedded sandstones) inferred as braided stream sedimentswhose high discharge was by lateral and vertical accretion. Thevarious channels sandstones were occasionally deformed fromseismic shocks generated by rapid sedimentation occasioned byhydrodynamic pressures. A similar depositional system was ana-lyzed in South Korea for the pull-apart Cretaceous Eumsung Basin(Ryang and Chough, 1999), Kribo – Campo Sub-basin in the south-ern Cameroun (Ntamak-Nida et al., 2010) and in the Muglad Basinof Sudan (Schull, 1988).

facies, facies successions and their inferred depositional environments.

Fig. 12. Generalized depositional model of the Lower Bima Member in the Yola Sub-basin showing the inferred relationship with the facies successions.

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9. Conclusions

Facies analysis of the Lower Bima Member of the Bima Forma-tion in the Yola Sub-basin outcropping at the core of Lamurde Anti-cline allows identification of depositional environments and madepossible proposal on the depositional model for the successions.The Early Cretaceous (pre-Aptian to Aptian) exhibits vertical andlateral facies transition from alluvial fan at the faulted basin mar-gin to lacustrine environment along the basin axis. Stratigraphicalstudy indicated progradation of the alluvial and braided riverdepositional systems on to the lacustrine caused most probablyby active faulting along the basin margin, climatic changes or com-bination of both. Mass flow processes dominated the deposition ofthe alluvial fan facies suggesting that tectonic activities continuedthroughout the period of deposition of most of the Lower BimaMember. The abundance of debris flow deposits in the alluvialfan association, the poor sorting and the sub-angularity of mostof the clasts indicate source from nearby area and a significantslope on the fan. The palaeocurrent measurements indicate thatthe sediment gravity flows were generally toward the NW. Thissuggests that the paleoreliefs were in the S and SE and perhapswith NE–SW orientation trend.

The proposed depositional model of the Lower Bima Member issimilar to that of the Abu Gabra Formation in the Muglad Basin ofSudan and other Lower Cretaceous formations in the Termit, Dobaand Doseo Basins of the WCARS. Barremian to Aptian lacustrinesediments are proven source rocks for the Early Cretaceous petro-leum system in these basins. Therefore, the presence of the lacus-trine sediments in the Lower Bima Member may also suggest thepresence of the Early Cretaceous petroleum system in the YolaSub-basin.

Acknowledgements

The authors are grateful to the National Centre for PetroleumResearch and Development, Abubakar Tafawa Balewa UniversityBauchi, Nigeria for funding the field trip and data collection. Thestudy was also supported by IPPP Postgraduate Research Fund(No: PG140-2012B) of the University Malaya. Special thanks goto Dr. Ahmed Isa Haruna, Dr. Abubakar Sadik Maigari and Dr. NuhuKadai Samaila for their assistance and inputs. We appreciate mostgratefully the constructive criticism of the anonymous reviewers ofthe manuscript.

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