Journal of African Earth Sciences 100 (2014) 179–190

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Journal of African Earth Sciences

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Lithofacies and depositional environment of the Amasiri Sandstone,southern Benue Trough, Nigeria

http://dx.doi.org/10.1016/j.jafrearsci.2014.05.0091464-343X/� 2014 Elsevier Ltd. All rights reserved.

⇑ Corresponding author. Tel.: +234 8065751244.E-mail address: ezekieloigwe@yahoo.com (E.O. Igwe).

A.U. Okoro a, E.O. Igwe b,⇑a Department of Geological Sciences, Nnamdi Azikiwe University, Awka, Nigeriab Department of Geology, Ebonyi State University, P.M.B., 053, Abakaliki, Nigeria

a r t i c l e i n f o a b s t r a c t

Article history:Received 28 June 2013Received in revised form 9 May 2014Accepted 13 May 2014Available online 27 June 2014

Keywords:Amasiri SandstoneLithofaciesDepositional systemsTidally influenced shorelineDeep water

Eight lithofacies typical of tidally-influenced shelf, mass flow and turbidity current processes characterizethe Amasiri Sandstone (Cenomanian – Turonian) in the southern Benue Trough, Nigeria. The cross beddedsandstone lithofacies (Sxm) in Afikpo area were deposited in tidally influenced, shallow sandy shorelineenvironment while similar lithofacies associated with the conglomeratic lithofacies (Sfc) in Akpoha areproximal canyon–fill deposits. The conglomeratic lithofacies with rip-up clasts together with the massive,horizontal-bedded lithofacies (Smm) and parallel-laminated sandstone lithofacies (Sfl) in Akpoha weredeposited in confined channels in proximal submarine canyon setting. The wavy/ripple-laminated sand-stone lithofacies (Sfw) and very fine grained bioturbated sandstones lithofacies (Sfb) represent weaklyconfined distributary splay and unconfined associations in proximal to distal submarine canyon settings.The bioturbated mudstone lithofacies (Msb) and parallel-laminated mudstones lithofacies (Msl) com-prise the bypass/levees association in the inner to outer shelf and in the distal canyon settings. Overall,these lithofacies indicate deposition in shelf to deep water depositional environments.

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1. Introduction

The depositional model for the Amasiri Sandstone of theEze-Aku Formation, southern Benue Trough has been controversial.Reyment (1965) interpreted the Eze-Aku Formation as shallowwater deposits based on ammonite contents while Murat (1972)regards the shale as transgressive deposits in shallow shelf setting.Petters (1978) and Iwobi (1991) also consider the Eze-Aku Forma-tion as shallow marine on the basis of foramineferal content.

In the study area, the dominantly shaly Eze-Aku Formationchanged facies to a sequence of sandstones interbedded withshales and minor limestones referred to as the Amasiri Sandstoneby Shell D’arcy geologists (Whiteman, 1982, p. 42), (Table 1). Thissandstone represents the uppermost part of the Eze-Aku Formationon the eastern limb of the Abakaliki Anticlinorium and occurs asparallel ridges trending in a NE–SW direction (Fig. 1). The sand-stones have been variously interpreted as tidal/subtidal shallowmarine deposits (Banerjee, 1980); storm-dominated shallow shelfdeposits (Amajor, 1987), marginal marine sand deposits (Ojoh,1990). Recently Odigi (2012) described the Amasiri Sandstonewhich clearly belongs to the pre-Santonian Eze-Aku Formation

(Simpson, 1954; Reyment, 1965; Banerjee, 1980, Whiteman,1982; Amajor, 1987), as part of the Campanian–MaastrichtianNkporo Formation. He interpreted the rocks as deposited in alluvialfan, braided river, point bar and deltaic environments.

This study has carried out a detailed outcrop and lithologicdescription of the Amasiri Sandstone from Amasiri to Akpoha(Fig. 1) in order to identify the rocks and their critical descriptivefeatures, particularly, the composition and sedimentary structures.These have been used to review the depositional model for theAmasiri Sandstone in the light of the above discussed controversies.

2. Regional tectonic and stratigraphic setting

The Amasiri Sandstone was deposited in the southern BenueTrough during the late Turonian (Reyment, 1965). The Benue Riftwas installed as the failed arm of a trilate rift system during thebreakup of the Gondwana supercontinent and the opening up ofthe southern Atlantic and Indian Oceans in the Jurassic (Burkeet al., 1972; Benkhelil, 1982, 1989; Hoque and Nwajide, 1984).Genik (1993) showed that the Benue Rift is part of the West andCentral African Rift System which originated during the splittingapart of the Gondwana supercontinent.

The stratigraphy of the southern Benue Trough was describedby Murat (1972) and Hoque (1977) in the concept of three tectonic

Table 1Stratigraphic Framework of southern Benue Trough and Anambra Basin showing the position of Eze-Aku Group andAmasiri Sandstone relative to other formations in southeastern Nigeria (Modified from Hoque, 1977).

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– sedimentary cycles. Sedimentation in this rift structure startedwith deposition of alluvial fans and lacustrine sediments in theAptian – Albian period. These sediments belong to the Mamfe For-mation (Reyment, 1965) in the southern Benue Trough. Two cyclesof marine transgressions and regressions occurred from the middleAlbian to the Coniacian culminating in the deposition of marinemudrocks, sandstones and limestones as well as pyroclastic flowsand basic intrusives with an estimated thickness of 3500 m(Murat, 1972; Hoque, 1977). These sediments belong to the AsuRiver Group (Middle Albian), the Odukpani Formation (Cenoma-nian), the Eze-Aku Formation (Turonian) and the Awgu Formation(Coniacian).

A relaxation of the causative mantle plume during the Santonianperiod set up a stress regime that resulted in sub-crustal compres-sional folding and uplift of these earlier synrift (pre-Santonian) sed-iments into the Abakaliki Anticlinorium and the simultaneoussubsidence of the western margin into the Anambra Basin and theeastern platform into the Afikpo Sub-basin (Murat, 1972; Burkeet al., 1972; Hoque and Nwajide, 1984). The Amasiri Sandstonebelongs to the Eze-Aku Formation and outcrops within the AfikpoSynclinorium on the southeast margin of the Abakaliki Anticlinori-um. Table 1 shows the stratigraphic framework of the southernBenue Trough and Anambra Basin (Murat, 1972; Reyment, 1965)and the stratigraphic position of the Amasiri Sandstone.

3. Methodology

Out of the several outcrops mapped, nine (9) outcrop sections ofthe Amasiri Sandstone were described from Akpoha to Afikpo,using critical descriptive lithologic characteristics such as

lithology, texture, physical and biogenic sedimentary structuresand fossil contents. The lithologic and sedimentological character-istics of the described outcrop sections were captured using alogging format designed to provide an insight into the vertical var-iation in lithology, texture (grain size, sorting, roundedness), phys-ical and biogenic sedimentary structures, fossil content, bedthickness, and contact types. Lithofacies were also recognizedusing the descriptive above listed lithologic and sedimentologicalcharacteristics of the rocks. Outcrop photographs of some of thedescribed sections were taken and the characteristic features ofthe lithofacies were correlated with their lithologic logs. Thevertical lithofacies transition patterns were used to interpret thepaleoenvironment of deposition following the method ofPemberton et al. (1992) and Maill (2000).

4. Results and interpretation

4.1. Outcrop description

4.1.1. Outcrop 1 – Behind Government Technical College (GTC),Akpoha

This outcrop occurs on a ridge trending in a NE–SW directioncovering a lateral extent of about 100 m along the ridge (Fig. 2a).The basal section of the outcrop consists of a sandy conglomeratecomprising of a structureless (massive), thickly bedded, fine tomedium grained sandstone, containing irregular-shaped limestonerip-up clasts, floating in a sandy matrix (Fig. 2b). The rip-up clastsare randomly distributed in the rock and vary from 5 to 40 cm indiameter. Some of the clasts have dissolved out leaving open holesin the rock.

13

14

Fig. 1. Geological map of the area showing alternating sequence of sandstones and shales in the Eze-Aku group.

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Very fine to medium grained sandstone with horizontal lamina-tions and double mud layers overlies the sandy conglomerates. Thebasal contact of this lithofacies is erosional and burrowed withload and flute casts. The beds are characterized by numeroussub-horizontal planar laminations organized into unevenly lami-nated multiple lamina sets and normally graded. Very fine grainedripple/wavy laminated sandstones and siltstones with ripplelaminations and climbing ripples overlie the horizontal beddedsandstone. The bedding planes appear planar but undulating witherosional (flame structures). Faintly laminated siltstones andintensely bioturbated mudstone overlies the rippled/wavy lami-nated sandstone beds. The mudstone facies appears massive dueto bioturbation.

4.1.2. Outcrop 2 – Section in premises and opposite GovernmentTechnical College, Akpoha

This section is located 400 m northwest of outcrop 1 in the pre-mises of Government Technical College, Akpoha. The outcropextends across the road outside the premises of the College. Thelower part of the outcrop section is composed of light grey biotur-bated mudstones overlain by very fine grained, highly induratedcalcareous sandstone. The bioturbated mudstone is characterizedby cryptic bioturbation due to intense burrowing while the overly-ing fine grained sandstone is also bioturbated and structurelesswith occasional ophiomorpha and skolithos isp burrows as well asdispersed carbonaceous and shell debris. The middle and uppersections of this outcrop are characterized by several broad chan-nels filled with medium to very coarse grained, poorly sortedaranaceous sandstones with planar cross beds overlying coarsegrained, planar cross bedded calcareous sandstone with occasionalrip-up clasts (Fig. 3a and 3c). Sigmoidal cross stratified foresetscharacterize some of the cross stratified sets and co-sets in thisoutcrop (Fig. 3b). The channel-fills exhibit characteristic fining

upward profiles with pebble/cobble-size limestone clasts concen-trated on the base of the channel.

4.1.3. Outcrop 3: Section 300 m east of outcrops 1 behind GovernmentTechnical College, Akpoha (Location 3)

This outcrop consists of a 5.50 m thick section of massive sand-stone (Fig. 4a). The sequence is horizontal bedded with regular bedthicknesses and laterally continuous beds. The contacts are undu-lating and planar. It is structureless with occasional fluid escapeand flame structures (Fig. 4b).

4.1.4. Outcrop 4: Abandoned local quarry along Akpoha – Afikpo Road(Location 4)

The lower section of this quarry consists of fine – mediumgrained, massive sandstone containing cobble-sized limestoneclasts. The sandstone is bioturbated and structureless with occa-sional ophiomorpha and skolithos isp burrows as well as dispersedcarbonaceous and shell debris (Fig. 5). This section is overlain bya dark grey bioturbated mudstone unit.

4.1.5. Outcrop 5: Julius Berger Quarry Section 2Km after Akpoha onAbakaliki – Afikpo Road

This is a 4.40 m quarry section comprising massive and biotur-bated sequence of fine to medium grained calcareous sandstones,shelly limestone lag and mudstone lithofacies (Fig. 6) shows thelithologic section for part of the quarry section.

4.1.6. Outcrop 6: Abandoned local quarry section 400 m southwest ofIbii Junction

A 2.50 m section of intensely bioturbated, very fine to finegrained sandstone is exposed in this quarry (Fig. 7). The sandstoneis highly calcareous with distinct burrows of Ophiomorpha,

Fig. 2a. Section showing lateral and vertical extent of outcrop. The lower part shows the horizontal laminated facies; the middle is the ripple laminated fine grainedsandstone facies while the upper part is the bioturbated siltstone and mudstone facies. (Hammer is 30 cm long).

Fig. 2b. Chaotic/sandy conglomerate overlying medium to coarse grained siliceoussandstone with limestone rip-up clasts on an erosional contact. Red arrow points toopen vugs due to limestone dissolution in the sandy conglomerate bed. (Forinterpretation of the references to colour in this figure legend, the reader is referredto the web version of this article.)

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Rhizocorralium and Skolithos isp burrows obliterating any physicalsedimentary structures (Fig. 7).

4.2. Crush stone quarry outcrop along Amasiri – Okposi Road

The Crush stone quarry exposed massive, very fine to finegrained, bioturbated sandstone containing rip up clasts of crystal-line limestone, mudstone and recrystallized bivalve and gastropodshells scattered in the rock (Fig. 8). No distinct burrow occurs butthe sandstone exhibits characteristic cryptic bioturbation. It is lightgrey color and very calcareous.

4.3. Ozaraukwu ridge outcrop, 1 Km to Amasiri Junction from AmasiriTown

The Ozaraukwu ridge runs in a NE–SW trend and exposes ara-naceous sandstone which vary in grain size from fine to very coarsegrained and often pebbly. They are strongly cross bedded, exhibit-ing thick beds of planar and trough cross stratified beds with clay-draped foresets (Fig. 9a). The beds dip steeply at angles varyingfrom 40� to 60� to the southeast.

4.4. Road cut 500 m from Amasiri Junction on Mcgregor Hill, Amasiri –Afikpo Road

This outcrop consists of an exposed section of 4.02 m of crossbedded sandstone. The sandstone varies in grain size from fine tovery coarse and exhibits an upward fining grain size profile. Theoutcrop expose characteristic channels 5–10 m wide and 1–3 mdeep (Fig. 9b).

5. Lithofacies

Eight lithofacies were identified in the Amasiri Sandstonedescribed in Akpoha, Amasiri and Afikpo areas. These are (1) themassive conglomeritic sandstone lithofacies with rip-up clasts(Sfc), (2) horizontal bedded medium to very coarse grained andmassive sandstone lithofacies (Smm) (3) parallel laminated, finegrained sandstone lithofacies (Sfl), (4) wavy/ripple laminated sand-stone lithofacies (Sfw) (5) bioturbated mudstone lithofacies (Msb)(6) very fine grained bioturbated sandstone lithofacies (Sfb). (7)fine-very coarse grained cross bedded sandstone lithofacies(Sxm) and (8) Parallel laminated siltstone (stp) lithofacies. Theselithofacies are discussed as follows.

5.1. Massive conglomeritic Sandstone Lithofacies with rip-up Clasts(Sfc)

This lithofacies is composed of massive, fine to medium grained,poorly to moderately sorted, siliceous sandstone with floating

Fig. 3a. Outcrop of cross bedded facies of the Amasiri Sandstone in the premises of Government Technical College, Akpoha correlated with the lithogic section.

Fig. 3b. Broad angular discordance (marked by red arrows) in the AmasiriSandstone outcrop in GTC Akpoha showing channel-fill beds. (For interpretationof the references to colour in this figure legend, the reader is referred to the webversion of this article.)

Fig. 3c. Sigmoidal cross beds with mud-draped foresets; overlain by very coarsegrained sandstone horizontally bedded over an erosional scour – channel marginand onlapping channel-fill beds – possibly a slope fan or tidal current deposits insubmarine canyon.

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irregular shaped limestone and carbonaceous (wood) rip-up clasts(see Fig. 2b). The clasts are randomly distributed but sometimesare aligned on the bedding planes. The sizes of the clasts varybut most range from 10 to 40 cm in diameter. The shapes vary fromelliptical to oval or sub-rounded. Hydrodynamically generatedsedimentary structures are absent in this lithofacies. Generally thislithofacies is associated with the medium to very coarse, poorlysorted sandstone lithofacies (Smm) and together may representthe Ta division of the Bouma sequence and facies association Fof Mutti and Lucchi (Bouma, 1962; Mutti and Ricchi, 1975,Pickering et al., 1986). They are interpreted as high density massflow deposits (debris flows) in proximal submarine canyon setting(Lowe, 1982; Shanmugam, 2003; Nichols, 2009). This belongs tothe confined channel lithofacies association of Sprague et al. (2005)

5.2. Massive, medium – very coarse grained, sandstone Lithofacies(Smm)

This lithofacies comprises medium to very coarse but domi-nantly coarse grained, poorly sorted sandstone that lacks internal

sedimentary structures. This lithofacies is horizontally beddedand composed dominantly of dirty white to brownish sandstones.The sandstone beds are laterally continuous and typically the bed-ding contacts are planar and undulating with flame structures (seeFig. 4). Bed thickness generally ranges from 9 cm to 25 cm. Thelithofacies appears to exhibit a subtle coarsening upward grain sizeprofile that is uncertain. The logged section is about 6.5 m. Thislithofacies is similar to Facies B2 of Mutti and Lucchi (1975) andrepresents product of grain flows and high density turbidity flowsin distal shelf to proximal (slope) submarine canyon setting (Lowe,1982; Maill, 2000; Catuneanu, 2006; Nichols, 2009). This lithofa-cies is part of the confined channel lithofacies association ofSprague et al. (2005).

5.3. Parallel laminated Sandstone Lithofacies (Sfl)

This lithofacies is composed of fine grained, well sorted sand-stone characterized by parallel mudstone laminations. The lithofa-cies is thin bedded with thickness ranging from 10 to 20 cm (seeFigs. 2a and 10). The sand/shale ratio is high (about 80%). The basal

Fig. 4a. Horizontal bedded but structureless sandstone lithofacies showing regularbed thicknesses, and laterally continuous beds behind Government TechnicalCollege.

Fig. 4b. Flame structures (F) in undulating contact of outcrop section behindGovernment Technical College.

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contact is interlaminated (with double mud layers) and character-ized by various forms of load and tool casts as well as Lorenziniaisp burrows of the Zoophycos ichnofacies (Pemberton et al.,1992). Generally, this lithofacies overlies the massive conglomeriticsandstone lithofacies (Sfc) wherever it outcrops. It represents

Fig. 5. Parallel laminated shale and mudstone lithofacies (Msl) at location 4 – an ab

products of high density flows and tidal bottom currents depositedduring late stage of forced regression in distal slope/ submarinecanyon setting (Maill, 2000; Shanmugam, 2003; Catuneanu,2006). The lithofacies resembles the Tb division of the Boumasequence and the B2 facies association of Mutti and Lucchi(Bouma, 1962; Pickering et al., 1986). This lithofacies belongs tothe weakly confined and distributary splay associations/environ-ments of Sprague et al. (2005).

5.4. Wavy/ripple Laminated Sandstone Lithofacies (Sfw)

This lithofacies is composed of light to grey sandstone withmudstone interlamination/interbedding (Fig. 11a and b). The sand-stones are very fine to fine and moderately to well sorted. Thethickness of the beds varies from 8 to 28 cm. The sand/shale ratiois relatively high (up to 70%). The prevalent sedimentary structuresare wavy climbing ripples and ripple cross laminations sometimesdraped with mudstone interlaminations and flaser beds. There is avertical grading from parallel laminated sandstone lithofacies towavy/ripple drift laminated sandstones. The wavy/rippled lithofa-cies is analogous to the Tc division of the Bouma sequence (Bouma,1962; Catuneanu, 2006) and the C2 facies association of Mutti andLucchi (1975) and here interpreted as products of intermediateenergy turbidity flows and tidal currents reworking in mid to outersubmarine fan setting (Bouma and Stone, 2000; Shanmugam,2003; Nichols, 2009). The lithofacies was deposited during latestage of forced regression when the shore line was near the shelfedge (Catuneanu, 2006). The lithofacies is part of the weakly con-fined and distributary splay associations/environments (Spragueet al. (2005).

5.5. Bioturbated Mudstone Lithofacies (Msb)

This lithofacies consists dominantly of mudstone and siltstone.Primary sedimentary structures are lacking due to intense biotur-bation. This lithofacies is represented in the upper section ofoutcrop section presented in (see Fig. 2a). The lithofacies is charac-terized by presence of discrete burrows of Zoophycos isp and Lore-nzinia isp burrows (Fig. 12a and b) though generally thebioturbation is (cryptic). This lithofacies is interpreted as low den-sity muddy debris flow/turbidite deposits in distal submarine fansetting (Bouma, 1962; Bouma and Stone, 2000). It is analogous tothe Te division of the Bouma and Facies association D1 of Muttiand Lucchi (1975). This lithofacies belongs to the bypass and lev-eed lithofacies association (Sprague et al. (2005).

andoned local quarry 1Km to Julius Berger Quarry along Akpoha – Afikpo Road.

Fig. 6. Lithologic log of quarry section in Julius Berger Quarry Akpoha. Note the shelly limestone lag.

Fig. 7. Intensely bioturbated sandstone with distinct burrows in abandoned local quarry 400 m south of Ibii Junction, Amasiri. Logged section correlated to Photograph.

Fig. 8. Recrystallized mollusk Shell in bioturbated calcareous sandstone in Crushstone quarry, Amasiri.

A.U. Okoro, E.O. Igwe / Journal of African Earth Sciences 100 (2014) 179–190 185

5.6. Very fine grained bioturbated sandstones Lithofacies (Sfb)

This lithofacies consists of very fine grained siliceous sandstonewith few distinct burrows characteristic of the skolithos and cruzi-ana ichnofacies, but intensely bioturbated. The bed thicknessesvary between 0.10 m and 0.30 m thick. The lithofacies lacks phys-ical sedimentary structures, possibly due to the intense bioturba-tion. Carbonaceous/coal fragments and wisps occur within thissandstone while irregularly shaped limestone clasts line the bed-ding planes or float within the sandstone in low concentration.Fig. 13a shows outcrops of this sandstone lithofacies near the Amo-ha Development centre along the Akpoha – Amasiri Road. Monoc-raterion isp burrows of the skolithos ichnofacies are mixed withdeep water Crossopodia and Lorenzinia isp burrows of the zoophy-cos ichnofacies. The lithofacies is interpreted as product of weaklyunconfined high density turbidity current flow on low gradientslopes in distal submarine fan setting. Deposition occurred duringwaning flow in mid to distal fan environment. The lithofacies ispart of the weakly confined and distributary splay associations/environments (Sprague et al., 2005).

Fig. 9. Outcrops of cross bedded sandstone in the Amasiri Sandstone (a) Planar cross beds in a steeply dipping sandstone bed on Ozara ukwu ridge 1Km to Amasiri Junction(b) Outcrop of road cut on Mcgregor Hill, Amasiri – Afikpo Road, with the logged section correlated with the outcrop photograph. howing channel-fill sandstones with planarcross beds.

Double mud layers

Fig. 10. Parallel laminations lithofacies with double mud layers in AmasiriSandstone on an outcrop behind the Government Technical College, Akpoha.

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5.7. Parallel laminated shale/silty mudstones lithofacies (Msl)

This lithofacies comprises very thinly bedded shales interlami-nated/interbedded with cm – thick beds of mudstone beds (seeFig. 5). It is dark grey in color and composed of clay and silt-sizequartz. This facies outcrop at the road cut section near Julius BergerQuarry, Akpoha; on the Ebonyi River Channel along the Abakaliki –Afikpo road, and at the Marlum Quarry along the Amasiri – Ibii road.The interstratifications of thin deposits of silty mud and shale is evi-dence of waning flow typical of storm or turbidity currents (Amajor,1987). Based on associated lithofacies, this lithofacies could either

be of shallow shelf environment or low density turbidity currentmud deposited during the early transgressive stage in distal/lowerfan setting (Maill, 2000; Catuneanu, 2006). The thick, continuousthinly laminated dark grey to black shale outcrops in the EbonyiRiver channel are thought to represent deposits of fairly low energyin shallow marine to outer shelf water depths (Reyment, 1965;Ehinola et al., 2003). Beyond the shelf, the shales with interbeddedand interlaminated siltstones are similar to Facies D2 of Mutti andLucchi (1972) and deposited as muddy turbidite (Shanmugam,2003). The association with bioturbated fine grained sandstonewith re-sedimented limestone rip-up clasts and siltstone interlam-inations lends credence to this interpretation.

The Msl lithofacies therefore represents shallow marine muddyshelf lithofacies association as well as bypass and leveed deepwater lithofacies association (Sprague et al. (2005) with abundantsilty intercalations.

5.8. Medium – very coarse grained cross bedded Sandstone Lithofacies(Sxm)

This lithofacies comprises of light grey to whitish sandstone,fine to very coarse and pebbly but dominantly medium to coarsegrained and pebbly in places; moderately to poorly sorted. Thesandstones displays prominent sigmoidal planar cross beddingswith mud draped foresets and tidal bundles. Trough cross bedsare also prominent in this lithofacies together with ophiomorphaisp burrows in the lower parts of the succession. The bedding con-tacts are erosional (see Fig. 9). The bed thicknesses vary from

Fig. 11. (a and b) Ripple laminations and wavy beds facies in the Amasiri Sandstone outcrops behind Government Technical college, Akpoha (a) ripple laminated sandstonewith climbing ripple cross laminations (b) wavy bedded and laminated sandstone, with low amplitude and high wavelengths.

Fig. 12. Bioturbated mudstone lithofacies with (a) Zoophycos isp burrows and (b) Lorenzinia isp burrows in outcrop 1, behind GTC, Akpoha.

Fig. 13. Cryptic bioturbated very fine grained sandstone lithofacies (Sfb) with (a) dissolved out limestone rip-up-clasts lining the bedding planes and (b) Ophiomorpha andMonocraterion isp and (c) Crossopodia isp burrows within the sandstone in outcrop near the Amoha Development Center along Akpoha – Afikpo Road.

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0.60 cm to 1.50 cm. This lithofacies represents the uppermost partof several upward coarsening cycles of deposition on the AmasiriSandstone ridges from Akpoha to Afikpo. It is interpreted as prod-ucts of tidal processes in shallow upper shoreface – foreshore set-ting (Anderton, 1976; Shanmugam, 2003; Nichols, 2009). Thislithofacies represents a tidally influenced, shallow sandy shorelinelithofacies association.

6. Depositional model

Based on the lithofacies and their depositional environments, ashelf to deep water depositional model is proposed for the AmasiriSandstone (Figs. 14 and 15 and Table 2). The late Cenomanian–earlyTuronian transgression (Nyong, 1995) deposited the Lokpanta OilShale and the mainstream Eze-Aku Shale in shelfal environmental

Fig. 14. Shelf to deep water depositional model showing the various lithofacies of the Amasiri Sandstone studied.

Fig. 15. Outcrop of Amasiri Sandstone and correlation with typical Bouma Sequence (Divisions Ta to Tep) in location 1 (Behind GTC, Akpoha).

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setting (Iwobi, 1991 and Ehinola et al., 2003). This environmentalsetting changed in the late Turonian when sea level fall and shore-line progradation initiated shelfal erosion creating submarine can-yons through which sediments were transported into deeper waterenvironments. Sediment-laden water masses triggered turbiditycurrents at the shelf edge and deep water with the deposition ofthe Amasiri Sandstone.

The exotic limestone, carbonaceous and shale clasts of pebble tocobble sizes scattered in different orientations and floating withinfine-grained matrix of the conglomeritic lithofacies interpreted as

debris flows are believed to have been eroded from the AlbianAsu River Group sequence and the Mfamosing Limestone east ofthe study area and re-sedimented during the Turonian within theAmasiri Sandstone.

Sprague et al. (2005) recognized deep water channel associa-tions which include bypass/levees, confined channel, weaklyconfined and distributary splay associations/environments. Thelithofacies Sfc, Sfl, and Smm described in the Amasiri Sandstonebelong to the confined channel lithofacies association in the uppersubmarine canyon setting; lithofacies Sfw and Sfb belong to the

Table 2Summary table of lithofacies and depositional environments of the Amasiri Sandstone.

Lithology and sedimentary structure (including tracefossils)

Lithofacies Depositional process Depositional environment

1 Fine – very fine – grained sandstone with floatingexotic limestone rip-up clasts, poorly sorted; massivewith siliceous matrix

Choatic(Conglomeratic)lithofacies (Sfc)

Debris flow/mass flow Proximal submarine canyonsetting (Shanmugam, 2003;Nichols, 2009)

2 Massive, medium – very coarse grained, horizontalbedded

Massive SandstoneLithofacies (Smm)

Grain flows and high density turbiditycurrents (Mutti and Lucchi, 1975;Shanmugam, 2003)

Proximal (slope) submarinecanyon setting (Lowe, 1982;Maill, 2000; Catuneanu, 2006;Nichols, 2009)

3 Sandstone, calcareous, fine to medium grained, wellsorted, thin bedded sandstone with horizontallylaminated muds and erosional contacts; Tool casts,load and fluid escape structures occur, occasionalburrows

Parallel laminatedSandstone Lithofacies(Sfl)

High density flows and tidal bottomcurrents deposited during late stage offorced regression (Shanmugam, 2003;Catuneanu, 2006)

Outer shelf to proximal slopesubmarine canyon fill setting(Maill, 2000; Shanmugam, 2003)

4 Sandstone, with wavy/ripple laminations/crosslaminations; very fine to fine grained, poorly sorteddue to 10–20% mud content

Wavy/ripple laminatedlithofacies (Sfw)

Short-lived traction processes due toturbidity current and tidal-currentreworking (Bouma and Stone, 2000;Shanmugam, 2003)

Mid to outer submarine fansetting (Bouma and Stone, 2000;Nichols, 2009)

5 Mudstone, intensely burrowed and bioturbatedDominantly silts and clays Lorenzinia isp and Zoophycosisp burrows of Zoophycos ichnofacies Massive due tobioturbations

Bioturbated mudstonelithofacies (Msb)

Low density turbidity currents (Bouma,1962; Bouma and Stone, 2000)

Distal shelf and lower fan settings(Bouma, 1962; Bouma and Stone,2000)

6 Very fine to fine grained bioturbated sandstones;contains carbonaceous debris and clasts with irregularand imbricate limestone clasts lining the beddingplanes; massive with tubular burrows of the skolithosichnogenus

Bioturbated Sandstonelithofacies (Sfb)

Unconfined high density turbidity currentflow on low (Sprague et al, 2005)

Distal submarine fan setting(Sprague et al., 2005)

7 Siltstone, dark to light grey, well laminated; mudstoneinterbeds are common Bedding planes are planar andsharp

Parallel laminatedSiltstone lithofacies(Msl)

Low density turbidity current depositsduring early transgressive stage (Arnott,2007)

Distal shelf and lower fan settings(Maill, 2000; Catuneanu, 2006)

8 Light grey to whitish, dominantly medium to verycoarse but dominantly coarse and pebbly sandstonewith sigmoidal planar cross beddings with mud drapedforesets and tidal bundles The bedding contacts areerosional

Medium – very coarsegrained cross-beddedsandstone lithofacies(Sxm)

Tide-dominated subaqueous flows inshallow water

Shallow foreshore to uppershoreface environments(Anderton, 1976; Shanmugam,2003; Nichols, 2009)

A.U. Okoro, E.O. Igwe / Journal of African Earth Sciences 100 (2014) 179–190 189

weakly confined distributary splay lithofacies association in thedistal/lower fan settings; Lithofacies Msb and Msl belongs to thebypass/levees lithofacies association in the shelf and distal sub-marine canyon setting while the Sxm represents tidally influenced,shallow sandy shoreline lithofacies association.

Fig. 14 is a representation of the various lithofacies and associa-tions in a depositional model while Fig 15 shows a well-developedBouma sequence in the distributary splay environment furtherdemonstrating a deep water model for the Amasiri Sandstone.

7. Discussion

The Amasiri Sandstone occurs as parallel to near-parallel ridgesexhibiting an upward coarsening profile from the shales at theswales or low-lying areas to the top of the ridges with mediumto very coarse cross stratified sandstones. The Sandstone has beenparticularly studied by Banerjee (1980) and Amajor (1987) bothwith different interpretations of the depositional environments ofthe Formation. While Banerjee (1980) suggested subtidal and tidedominated shallow marine depositional environment, Amajor(1987) suggested deposition in storm dominated, shallow shelfenvironment. Ojoh (1990) interpreted the Amasiri Sandstone asmarginal marine sand deposits. Recently, Odigi (2012) interpretedthe rocks as alluvial fans, braided river, point bar and deltaicdeposits. He ignored the fossiliferous interbedded shales, shelldebris and distinct cruziana–zoophycos ichnospecies burrows inthe sequence.

Based on this study however, the paleoenviromental scenariodepicts environments ranging from tidally-influenced sandy silici-clastic shoreline setting to deep water setting. The presence of fineto very coarse and pebbly but dominantly medium to coarse grained

and moderately to poorly sorted sandstones with prominent sig-moidal planar cross beddings, mud draped foresets and tidal bun-dles (Lithofacies Sxm support shallow subtidal to tidal origin forthe upper part of the Amasiri Sandstone ridges (Anderton, 1976;Compton, 1985, p.180; Kreisa and Moiola, 1986; Shanmugam,2003; Nichols, 2009). This is in agreement with Banerjee (1980)who may have interpreted facies of the upper part of these ridgesin the localized outcrops he studied. It also corroborates the opinionof Ojoh (1990) that the sandstones are of marginal marine origin.

The absence of unequivocal structures typical of storm sedi-mentation such as hummocky cross stratifications precludes stormsedimentation although the debris flows may suggest so. Accord-ing to Einsele (2000) the difference between turbidite and stormdeposits is that storm deposits contain hummocky cross-stratifica-tions. Even though storm and turbidity currents are unpredictableand catastrophic events that usually produce similar sedimentcharacteristics, the evidences in favor of a deep water turbiditemodel over a shelfal storm model are overwhelming. The AmasiriSandstone was likely deposited during a lowstand of sea levelabove fair weather wave base such that hummocky cross stratifica-tions could not be preserved. Karim (2007) described storm depos-its and noted that they comprise an association of normal graded,hummocky cross stratified, parallel laminated and cross stratifiedsandstone lithofacies. The absence of hummocky cross stratifiedsandstone lithofacies in the Amasiri Sandstone studied is notewor-thy and precludes the preservation of storm deposits in shallowshelf depths though tidal and transgressive mudstone depositswere deposited. It is possible that the shelfal sands and muds werereworked during the storm events and re-deposited in the deeperwater environments as turbidites (Shanmugam, 2002). The ubiqui-tous presence of debris flows and grain flows as well as outcropsections exhibiting pronounced Bouma sequences in the lower

190 A.U. Okoro, E.O. Igwe / Journal of African Earth Sciences 100 (2014) 179–190

and middle parts of the ridges support deeper water sedimentationranging from outer shelf (canyon mouths) to deep marine canyonsetting (Bouma, 1962; Mutti and Lucchi, 1975; Walker, 1992;Bouma and Stone, 2000; Maill, 2000; Shanmugam, 2002, 2003;Sprague et al., 2005; Karim, 2007; Arnott, 2007; Nichols, 2009).

A tidal shallow shelf to deep marine turbidite depositionalmodel (Fig. 14) best describes the depositional scenario of theAmasiri Sandstone. The paleo-depositional environments rangefrom sandy tidal shoreline environment to deep marine environ-ments. The presence of double mud drapes and wavy/ripple lami-nations/cross laminations indicate short-lived traction processesdue to turbidity current and tidal-current reworking in submarinecanyon setting (Shanmugam, 2003).

8. Conclusion

Lithofacies and sedimentary structures have provided usefuldata for interpreting the depositional environments and modelfor the Amasiri Sandstone outcropping in Akpoha, Amasiri and Afi-kpo areas in the southern Benue Trough. Eight (8) lithofaciesbelonging to four lithofacies associations were described and inter-preted. The rocks were deposited in a wide range of depositionalenvironments varying from shallow shoreline (tidal lithofaciesassociation) through shelf to submarine canyon/upper slope(bypassed and leveed lithofacies associations) to proximal to midfan (confined channel lithofacies association) and submarine can-yon environments (weakly confined distributary splay distal/lowerfan lithofacies association).

References

Amajor, L.C., 1987. The Eze-Aku Sandstone ridge (Turonian) of southeastern Nigeria:a re-interpretation of their depositional origin. J. Min. Geol. 23, 17–26.

Anderton, R., 1976. Tidal-shelf sedimentation: an example from the ScottishDaldradian. Sedimentology 23 (4), 429–458.

Arnott, R.W.C., 2007. Character of Deposition from Shallow-and Deep-MarineSuspended-Sediment-LadenGravity Flows, Let it Flow – CSPG CSEG Convention,pp. 160.

Banerjee, I., 1980. A subtidal bar model for the Eze-Aku Sandstones, Nigeria. J.Sediment. Geol. 30, 133–147.

Benkhelil, J., 1982. Benue Trough and Benue Chain. Geol. Mag. 119, 155–168.Benkhelil, J., 1989. The evolution of the Cretaceous Benue Trough, Nigeria. J. African

Earth Sci. 8, 251–282.Bouma, A.H., 1962. Sedimentology of some flysch deposits: a graphic approach to

facies interpretation. Elsevier, Amsterdam, pp. 168.Bouma, A.H., Stone, C.G., 2000. Fine-grained turbidite systems. AAPG Memoir 72

and SEPM Special. Publication 68, 342.Burke K.C., Dessauvagie, T.F.J, Whiteman, A.J., 1972. Geological history of the Benue

Valley and its adjacent areas. In Dessauvagie, T.F.J, Whiteman, A.J., (Eds). AfricanGeology, University of Ibadan Press, pp. 187–205.

Catuneanu, O., 2006. Principles of sequence stratigraphy. Elsevier BV, Amsterdam,Netherlands, pp. 375.

Compton, R.R., 1985. Geology in the Field. John Wiley and sons, New York, 392p.Ehinola, O.A., Bassey, C.E., Ekweozor, C.M., 2003. Preliminary studies on the

Lithostratigraphy and depositional environment of the oil shale deposits ofAbakaliki Anticlinorium, southeastern Nigeria. J. Min. Geol. 39, 85–94.

Einsele, G., 2000. Sedimentary Basin: Evolution, Facies and Sediment Budget. 2ed.Springer, pp. 792.

Genik, G.J., 1993. Petroleum geology of Cretaceous – Tertiary basins in Niger, Chadand Central African Republic. AAPG Bull. 77, 1405–1434.

Hoque, M., 1977. Petrographic differentiation of tectonically controlled Cretaceoussedimentary cycles southeastern Nigeria. Sediment. Geol. 17, 235–245.

Hoque, M., Nwajide, C.S., 1984. Tectono-sedimentological evolution of an elongateintracratonic basin (aulagogen): a case of the Benue Trough of Nigeria. J. Min.Geol. 21, 19–26.

Iwobi, O.C., 1991. Foraminiferal ages in the Southern Benue Trough. Nigerian Assoc.Petrol. Exp. Bull. 6, 39–47.

Karim, K.H., 2007. Possible effect of storm on sediments of upper CretaceousForeland Basin: A case study for Tempestite in Tanjero Formation, SulaimanyiaArea, NE-Iraq. Iraq J. Earth Sci. 7 (2), 1–10.

Kreisa, R.D., Moiola, R.J., 1986. Sigmoidal tidal bundles and other tide generatedsedimentary structures of the Curtis Formation Utah. Geol. Soc. Am., 381–387.

Lowe, D.R., 1982. Sediment gravity flows, Il. Depositional models with specialreference to the deposits of high-density turbidity currents. J. Sediment. Petrol.52, 279–297.

Maill, A.D., 2000. Principles of sedimentary Basin Analysis, 3rd ed., Springer–VerlagBerlin Heidelberg, pp. 616p.

Murat, R.C., 1972. Stratigraphy and paleogeography of the Cretaceous and lowerTertiary in southern Nigeria. In: Dessauvagie, T.F.J., Whiteman, A.J. (Eds.),African Geology, University of Ibadan Press, Nigeria, pp. 251–266.

Mutti, E., Ricci Lucchi, F., 1975. Turbidite facies and facies associations. In examplesof turbidite facies and facies associations from selected formations of thenorthern Apennines. In: Field Trip Guidebook A-11, InternationalSedimentologic Congress IX Nice, pp. 21–36.

Nichols. G., 2009. Sedimentology and stratigraphy. 2nd ed, Wiley-BlackwellPublications, pp. 419.

Nyong, E.E, 1995. Cretaceous SEDiments in the Calabar Flank. In: Ekwueme, B.N.,Nyong, E.E, Petters S.W., (Eds). Geological excursion guidebook to Oban Massif,Calabar Flank and Mamfe Embayment, southeastern Nigeria, pp. 14–25.

Odigi, M.I., 2012. Sedimentology of the Nkporo Campanian – Maastrichtianconglomeratic Formation, Afikpo Sub-basin, southeastern Benue Trough,Nigeria. J. Min. Geol. 48, 45–55.

Ojoh, K.A., 1990. Cretaceous geodynamic evolution of the southern part of theBenue Trough (Nigeria) in the equitorial domain of the south Atlantic.Stratigraphy, basin analysis and paleo-oceanography. Bull. Exp. Prod. Elf-Aquitaine 14, 419–442.

Pemberton, S.G., MacEacham, J.A., Frey, R.W. 1992. Trace fossil facies models:environmental and allostratigraphic significance. In: Walker, R.G, James, N.P.(Eds.). Facies Models: Response to sea level change. Geological Association ofCanada, pp. 47–72.

Petters, S.W., 1978. Stratigraphic evolution of the Benue Trough and its implicationsfor the upper Cretaceous paleogeography of West Africa. Journal of Geol. 86,311–322.

Pickering, K.T., Stow, D.A.V., Watson, M.P., Hiscott, R.N., 1986. Deep water facies,processes and models: a review and classification scheme for modern andancient sedments: Earth Science. Review 23, 75–174.

Reyment, R.A., 1965. Aspect of the Geology of Nigeria. University of Ibadan press,Nigeria, pp. 145.

Shanmugam, G., 2002. Ten turbidite myths. Earth-Sci. Rev. 58, 311–341.Shanmugam, G., 2003. Deep-marine tidal bottom currents and their reworked sands

in modern and ancient submarine canyons. Mar. Petrol. Geol. 20, 471–491.Sprague, A.R.G., Garfield, T.R., Goulding, F.J., Beaubouef, R.T., Sullivan, M.D.,

Rossen,C., Campion, K.M., Sickafoose, D.K., Abreu, V., Schellpeper, M. E.,Jensen, G.N.,Jennette, D.C., Pirmez, C., Dixon, B.T., Ying, D., Ardill, J., Mohrig,D.C.,Porter, M.L., Farrell, M.E., and Mellere, D., 2005. Integrated slope channeldepositional models: the key to successful prediction of reservoir presence andquality in offshore West Africa. CIPM. Este artículo fue preparado para supresentación en el cuarto E-Exitep.

Simpson, A., 1954. The Nigerian coalfield. The geology of parts of Onitsha, Owerriand Benue provinces. Bull. Geol. Survey Nigeria 24, 85.

Walker, R.G., 1992a. Turbidites and submarine fans. In: Walker, R.G., James, N.P.(Eds.), Facies Models: Response to Sea Level Change, GEO text 1, GeologicalAssociation of Canada, pp. 239–263.

Whiteman, A.J., 1982. Nigeria: Its petroleum geology, resources and potential.Graham and Trotman, London, pp. 166.