Siliciclastic slope deposits from the Cretaceous Abeokuta Group, Dahomey (Benin) Basin, southwestern...

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Journal of African Earth Sciences 46 (2006) 187–200

Siliciclastic slope deposits from the Cretaceous AbeokutaGroup, Dahomey (Benin) Basin, southwestern Nigeria

Solomon O. Olabode *

Department of Applied Geology, Federal University of Technology, Akure, P.M.B. 704, Akure, Ondo State, Nigeria

Received 10 June 2005; received in revised form 15 April 2006; accepted 20 April 2006Available online 14 July 2006

Abstract

Outcrops in the Cretaceous Abeokuta Group of the Dahomey (Benin) Basin in southwestern Nigeria are described. Based on detailedsedimentological logging, 13 lithofacies were delineated. They are: massive and loose sand; cross-bedded loose sand; normally gradedsandstone (upward fining sandstone); laminated silt; clay; laminated sandstone; clay with sandstone injections and clasts (sandstoneboulders); clayey sand with sandstone boulders; silty clay with intraformational folds and breccias; highly contorted conglomerate/clayeysilt; and channel infills. The association of the lithofacies and their sedimentary structures support that these lithologies were depositedby turbidity current, grain flow, debris flow, liquefied flow and fluidized flow. Since these depositional features are diagnostic of slopeenvironments, the Cretaceous Abeokuta Group was concluded to have been deposited in slope environments.� 2006 Elsevier Ltd. All rights reserved.

Keywords: Cretaceous; Slope; Mass transport; Depositional processes

1. Introduction

Cretaceous rocks outcrop in the region commonlyreferred to as the southwestern part of the Nigeria sedimen-tary basin, which is predominantly the Dahomey (Benin)Basin. Since the discovery of tar sand and other mineraldeposits such as limestone in large quantities, the regionhas attracted numerous studies (Jones and Hockey, 1964;Reyment, 1965; Omatsola and Adegoke, 1981; Whiteman,1982; Billman, 1992).

The objectives of this paper are to: (1) carry out adetailed outcrop study of the Cretaceous Abeokuta Group,(2) interpret the field evidence in terms of depositionalmechanisms and depositional environments, and (3) estab-lish a model for the stratigraphic relationships and thedepositional mechanisms.

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2. Geological setting

The Benin (Dahomey) Basin forms one of a series ofWest African Atlantic Margin basins that were initiatedduring the period of rifting in the late Jurassic to early Cre-taceous, (Omatsola and Adegoke, 1981; Weber and Dau-koru, 1975; Whiteman, 1982). The basin stretches alongthe coast of Nigeria, Benin Republic, Togo and Ghanaalong the margin of the Gulf of Guinea (Fig. 1). It is sep-arated from the Niger Delta in the Eastern section by theBenin Hinge Line and Okitipupa Ridge and marks the con-tinental extension of the chain fracture zone (Wilson andWilliams, 1979; Coker and Ejedawe, 1987; Onuoha,1999). It is bounded on the west by the Ghana Ridge,and has been interpreted as the Romanche fracture zone(Burke et al., 1971; Whiteman, 1982).

Tectonically, two structural elements have been recog-nized in what was originally defined as Dahomey Embay-ment (Reyment, 1965; Jones and Hockey, 1964; Cokerand Ejedawe, 1987; Coker, 2002): (i) the Benin Basinproper, which is defined within the confines of the eastern

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Fig. 1. Simplified regional map of Dahomey (Benin) basin from Ghana in the west to Nigeria in the east. The Nigerian part shows both the Cretaceousand Tertiary rocks. Note the structural elements as mentioned in the text (modified after Slansky, 1962; and Jones and Hockey, 1964).

Fig. 2. Simplified Cretaceous and Tertiary stratigraphy of Nigeria part ofDahomey Basin (compiled after Reyment, 1965; Jones and Hockey, 1964;Omatsola and Adegoke, 1981; Billman, 1992).

188 S.O. Olabode / Journal of African Earth Sciences 46 (2006) 187–200

Ghana ridge extending through Orimedu Ijebu Ode andthe Western flank of Ilesha spur. Along strike (E–W), thebasin shows a steady drop in elevation of the basementfloor, ranging from about 1990 m (eastern margin) to2200 m along the basin axis (Coker, 2002). (ii) The Okiti-pupa structure, a submarine extension of the Ilesha spur(Fig. 1). It stretches from the Ijebu–Ode–Ilorin axis tothe Benin Hinge Line. Along strike, the structure showsconsiderably shallower basement topography; (1000–1400 m) dissected by isolated grabens (1800–2200 m). Thebasin floor tectonics and the facies distribution have beencorrelated to three major basement blocks. They areperipheral blocks of the pre-drift basin, the central graben(active pre-drift basin) and the Okitipupa structure. Thebasin-fill covers a broad arc-shaped profile, attaining about13 km maximum width in the onshore at the basin axisalong the Nigerian Benin boundary. This narrows west-wards and eastwards to about 5 km (Coker and Ejedawe,1987; Coker, 2002).

The stratigraphy of the Cretaceous and Tertiary Forma-tions in the Nigerian sector of the basin is controversial.This is due primarily to different stratigraphic names thathave been proposed for the same Formation in differentlocalities in the basin (Billman, 1992; Coker, 2002). Thissituation can be partly blamed on the lack of good bore-hole coverage and adequate outcrops for detailed strati-graphic studies. Earlier studies on the basin stratigraphyby Jones and Hockey (1964) recognized both Cretaceousand Tertiary sediments (Figs. 1 and 2). Other, subsequentworkers recognized three chronostratigraphic units: (i)pre-lower Cretaceous folded sequence, (ii) Cretaceoussequence and (iii) Tertiary sequence (Omatsola and Ade-goke, 1981; Billman, 1992). The Cretaceous stratigraphy

as compiled from outcrop and borehole records consistsof the Abeokuta Group, sub-divided into three formationalunits, namely Ise, Afowo, and Araromi (Omatsola andAdegoke, 1981). The Tertiary sediments comprise theEwekoro, Akinbo, Oshosun, Ilaro, and Benin formations(Fig. 2).

S.O. Olabode / Journal of African Earth Sciences 46 (2006) 187–200 189

3. Methods

Field mapping was conducted in the eastern part of theDahomey (Benin) Basin, southwestern Nigeria (Fig. 3).The objective of the work was to map both the Cretaceousand Tertiary sedimentary successions in the basin. Thework included reconnaissance mapping and detailed mea-surement of stratigraphic sections. During both the recon-naissance and measured section work, outcrops wereexamined for bedding contacts, bed thickness variations,grain-size variations, composition, primary physical sedi-mentary structures and post-depositional sedimentarystructures. In general, road cuts where construction workshave been carried out provide the most reliable outcropsused for the mapping. The mapping was carried out forover 18 km and nine outcrops were studied (Fig. 3b).

4. Facies associations

Thirteen lithofacies were identified in the CretaceousAbeokuta Group: massive and loose sand; cross-beddedloose sand; normally graded sandstone (upward finingsandstone); laminated silt; clay; laminated sandstone; claywith sandstone injections and clasts (sandstone boulders);clayey sand with sandstone boulders; silty clay with intra-formational folds and breccias; highly contorted conglom-erate/clayey silt; and channel infills (Tables 1–3).

4.1. Facies assemblage 1: distal lower shoreface to upper

slope

The lithofacies delineated within this assemblage consistof clay, sand, sandstone and silt (Fig. 4). Table 1 containsdetailed descriptions and interpretations of lithofacies pres-ent in the assemblage. The massive loose sand to cross-bed-ded loose sand with lateral facies change to very fine sand

Fig. 3. Location of the study area shown in relation to (A) simplified map ofThe outcrops that are well exposed and studied in detail are shown with arro

and silt exhibit unidirectional, western paleocurrent direc-tions (Fig. 5a). The association of the sands with accessoryminerals such as mica flakes and carbonaceous debris sug-gests poorly winnowed sediments, such as distal or submar-ine environments. The occurrence of skolithos ichnofaciesand horizontal ophiomorpha burrows support distal lowershoreface environments (Maceachern and Pemberton,1994) (Fig. 5b and c). Both sharp upper and lower contactsof the lithofacies suggest a submarine channel, which actedas a feeder for a submarine fan at its basinward side (Selley,1985). Mutti et al. (1999) have pointed out that turbiditesare composed of highly efficient and poorly efficient flows,depending on their abilities to segregate their grain popula-tions to relatively well-sorted facies types. The coarsegrained moderately well sorted cross-bedded sand suggestshighly efficient turbidity current deposition (Lowe, 1982).Further down-current deposition was predominantly well-laminated fine-grained sand to silt, which probably resultedfrom a moderate rate of sediment fallout from an overlyingsuspended load cf. Middleton and Hampton (1976). Gallo-way (1998) showed that a turbidite lobe at the base of slopefalls into two end-members: mounded lobe and sheet lobe.Mounded lobe facies are sand- and gravel-rich and consistof amalgamated coarse grained turbidites. Sheet lobe faciesare associated with mixed to muddy systems and consist ofwell bedded coarse to heterolithic turbidites. Therefore, thetextural and sedimentary characteristics recognized in thislithofacies favour sheet lobe turbidite deposition. Normallygraded sands characterized by mm-thick traction carpets ofcoarse sand with upward-fining medium and fine sandcould be interpreted as the product of a mixed type of sed-imentation (Mutti et al., 1999). In such mixed sedimenta-tion, traction of coarse sand at the base of the flow tookplace concomitantly with fall out of medium and fine sandfrom the overlying suspended material. These sedimenta-tion patterns are characteristic of highly efficient turbidity

part of Southwestern Nigeria and (B) map showing the outcrop locations.ws.

Table 1Facies assemblage 1: distal lower shoreface to upper slope facies

Lithofacies Descriptions Interpretations

Massive andloose sand

Coarse to medium, reddish brown to light brown, moderately well sorted,sharp and indurated upper boundary, no recognizable sedimentary structures.Sharp upper boundary, lower boundary not exposed, structures.

Highly efficient deposition from lowdensity turbidite. Structures have beenobliterated because of the very loose texture.

Cross beddedloose sand

Fine to medium, moderately well sorted, sub-rounded to rounded,unidirectional and clay draped, accessories minerals are abundant mica andcarbonaceous detritus. Sharp upper boundary, lower boundary not exposed.It is fairly bioturbated with very straight and vertical skolithos burrows.The tubes are approximately 5 mm in diameter, 45.5 cm long.

Highly efficient deposition from lowdensity turbidite.

Normally gradedsandstone

Milky white stained with reddish brown, medium to coarse, sub-rounded torounded, rich in clay, crude laminations, contain load structures and few sandinjections. Upward fining intervals are arranged in cycles. Sharp upper andlower boundaries.

Low density turbidity current depositionaffected by slumping.

Laminated silt White, friable, very loose and moderately well sorted, contain abundant quartzgrains, and few accessory minerals such as mica flakes and carbonaceous debris,essentially horizontally (planar) bedded and well laminated, well bedded in thecentimetric scale and clay drape and contain horizontal ophiomorpha burrows.Sharp lower and upper boundaries.

Undisturbed suspended load depositionfrom low density turbidity current.

Clay The clay is smooth, slippery, laminated and compact. Its colour is generallygray with occasionally red and brown. This facies is separated from the facieson top by thin bands of indurated siltstone.

Pelagic to hemipelagic deposition in offshore.

Laminatedsandstone

It is dark grey to brown, medium to very coarse, angular to sub-angular andpoorly sorted. The sand shows horizontal bedding with parallel laminations andthickly bedded. Laterally it changes to very fine, dark gray to black, laminated,and contains nodules. Nodules are very small and they have millimetric scalethicknesses. It is moderately sorted, angular to subangular and highly indurated.This lithofacies is flaggy and exhibit distinct planar horizontal bedding.

Foreshore to lower shoreface deposition.

Table 2Facies assemblage 2: upper slope gravity flow deposits

Lithofacies Description Interpretation

Clay with sandinjections andsandstone boulders

Background colour is light grey to milky white, vary from red to purple,massive, fairly laminated and moderately hard. Pervasiveness of clasticinjections. Clastic injections are irregularly arranged, cross-cutting and showvarious textural characteristics. Injected sands are purple to dark brown,and limonitic (yellow ochre). The widths of the dykes vary form 2 to5 cm and cut across the entire clay unit.

Slump and debris flow deposits inupper slope environment.

Clayey sand withsandstone clasts

Brown to dark yellow, medium to fine grained, slightly sticky. Abundantclasts of ferruginous sandstone are present. Clasts are predominantlyrounded to sub-rounded cobbles to boulders. Angular clasts are also present.

Debris flow deposits.

Silty clay associatedwith intraformationalfoldsand brecciationi

Composed of white, slightly sticky, very fine and smooth clayey silt. Micaflakes and carbonaceous detritus are present. Broken fragments offerruginised sandstone form intraformational breccias. Soft sedimentdeformational structures (intraformational folds) are present separated bymillimetric thin bands. It contains siltstone pseudo-nodules- purple tolight yellow, indurated and closely packed. Thickness is 13–35 cm. Brecciatedsandstone clasts show variable clast-matrix arrangement and good tangentialcontacts. Very angular to angular with dimension range from few millimetersto 4 cm. Grains of quartz are in rare association with the sandstone breccias.

Slump deformation in upper slopeenvironment.

Highly contortedconglomerate andclayey silt

Contorted layers are composed of clayey silt with injected sands andconglomerate. Clayey silt is light brown to yellow, slightly sticky and veryfine. Conglomerate is poorly sorted, angular to rounded and matrix supported.Sharp erosional boundary separates the conglomerate from the underlyingclayey silt. Ferruginised sandstone boulders are present in the erosional surfacesand scattered on the layers. These two layers are strongly contorted and rotated.

Debris flow deposit affected by slumpdeformation in upper environment.


currents. Therefore, the normal graded sand can be inter-preted as a product of turbidites. Load casts present inthe lithofacies are composed of a similar sandstone(Fig. 5d). On some of the exposures where the sandstone

exhibits textural changes to mudstone the load structuresare still sandstone. Soft sediment deformation structureshave been interpreted as diagnostic of slope and rise envi-ronments (Shanmugam et al., 1995; Prothero and Schwab,

Fig. 4. Lithology section for distal lower shoreface to upper slope facies.

Table 3Facies assemblage 3: upper slope channel fills

Lithofacies Description Interpretation

Complex channelinfills 1

Five channels were identified-channels 1–5; some younger channels bifurcateinto older ones. Channels infills are variable; Channel 1 infill—cobbles andpebbles of fairly hard sandstone; Channel 2 infill—clay; channel 3 infill—thinlayer of sandstone and clay; Channel 4 infill—highly contorted clay withclastic injections; channel 5 infill—clayey sand.

Upper slope channels filled with gravityflow deposits.

Complex channelinfills 2

Two channels were identified. First channel has lag composed of brokenfragments of sandstone, siltstone and quartz. Channel infill is predominantlypoorly sorted sandstone, medium to coarse, poorly sorted, subangular tosub-rounded and poorly bioturbated. Spiral rolls were delineated. Secondchannel contains lag which are rounded pebbles of sandstone and siltstone.The lag changes to boulders of ferruginous sandstone. The dimensions of theboulders measured in this channel are variable and can be as large as 120 cmby 20 cm. Channel infill is inversely graded sandstone. Angular to sub-rounded,pebbly with clayey silt matrix and poorly sorted.

Upper slope channels filled with gravityflow deposits predominantly debris and slump.

Regional channelinfills

The channel lag consists of rock fragments (cobbles to boulders of mudstone)and quartz granules and contains fluid escape structures. They are sub-angular torounded, slightly elongate and matrix supported. The overlying section is aconglomerate. It is dark brown to dark red and strongly lateritized. The clastsof the conglomerate are composed of very coarse quartz grains, while the matrixcould have been sandy clay before it was lateritized. The clasts are subangularto angular, and poorly sorted. Boulders of ferruginised sandstone are welldistributed in the facies.

Deposits of regional erosion as a result ofrelative sea level fall below the shelf edgeand the upper slope.


Fig. 5. Photographs of important sedimentary features that are related to descriptions in the text. (a) Order of occurrence of lithofacies associationspresented in the litholog. (b) Close-up view of the cross bedding in the loose sand. Paleo-current direction is generally towards the west. Black arrows pointto skolithos biogenic structures. (c) Lateral facies as observed in the loose sand. Cross bedding has changed to planar (horizontal) bed while the size hasreduced to silt. Dark colours indicate the horizontal burrows. The upper section above the 56 cm long matchet is composed of cycles of upward fininginterval. b and c photographs were taken at an approximate distance of 1.2 km from each other. (d) Close-up photo of load structures in the upward finingsandstone.


1996). The clay unit was interpreted as offshore deposits inquiet water environments. Stratigraphically, this assem-blage dominantly occupies the lowest part of the outcropsthat are exposed in the studied area.

4.2. Facies assemblage 2: upper slope gravity flow deposits

Internal gravity-flow deposits of upper slope environ-ments are well preserved in the outcrops (Fig. 6). Thelithofacies that were delineated occur differently in the out-crops. In the clayey sand, the presence of sandstones clastscan be used to subdivide the lithofacies into two sub-units,upper and lower. The upper sub-lithofacies contains boul-ders of ferruginised sandstone, whose dimensions varyfrom 64.5 cm · 60 cm to 3 cm by 2 cm. The lower sub-lithofacies passes gradually into the upper one, thus theirboundary could not be sharply defined. The lower faciesis made up of clasts of smaller sizes essentially granulesto pebbles. An association of such extremely poorly sorted,cobbles, pebbles, silt and clay is commonly referred to as adebris-flow deposit, which is diagnostic of the upper slopeenvironment (Cook et al., 1982; Prothero and Schwab,1996; Galloway, 1998). Clay in association with clastic

injections (sandstone dykes) is pervasive in most of the out-crops (Shanmugam et al., 1994) (Fig. 7a). Lithologies withsimilar evidence for soft sediment deformation have com-monly been originated from slumps and debris-flow (Shan-mugam et al., 1995) in a slope environment. Rapid burialof mass transported sediment often occurs as a result offailures on the upper slope. Subsequently, the result willbe sealing and early overpressuring. Such overpressuringcan cause dilation, which produces fracture or tensiongashes within the affected lithology (Hiscott, 1979; Kimuraet al., 1989; Shanmugam et al., 1995). Also, the presence ofsandstone clasts made up of pebbles to boulders in the clay(Fig. 7b), suggests debris-flow deposits. The occurrence ofintraformational folds and intraformational brecciation(Fairbridge, 1946), was identified on one of the outcrops(Fig. 7c and d). Intraformational folds are interstratifiedbetween the clayey silt and composed of ferruginous veryfine-grained sandstone and mudstone. The thickness of thislayer is between 31 cm and 43 cm and it is characterised bypseudonodules and exhibits both sharp upper and lowercontacts within the clayey silt. Two attributes of the foldingwas recognized; some folds show minor amplitude in thecentimetric scale while others exhibit amplitude in the

Fig. 6. Lithology section for upper slope gravity flow deposits. Thecontorted and rotated layers are composed clayey silt, clay with sandinjections and debris deposits.


meter scale. Fold types observable on this outcrop are iso-clinal or ‘concertina’ folds (Fairbridge, 1946) and recum-bently folded stratified sediment. The mode of occurrenceof the layer characterized by intraformational brecciationis similar to the intraformational folds. Fig. 7d shows thebreccias of fine- to medium-grained sandstone arrangedin a matrix of clayey silt. Intraformational folds and brec-cias are products of slump deformation, common in the inslope environment (Fairbridge, 1946; Prothero andSchwab, 1996).

Only the lithological units exposed at the upper part ofthe outcrop where a high degree of slump deformationresulted into contorted and rotated units are affected. Thelower two lithologies (laminated very fine sand and clayeysilt) were not deformed. Both the clayey with clastic injec-tions and conglomerate, and the associated boulders offerruginised sandstone were strongly deformed. The mud-rock and the conglomerate are intensely deformed and cha-otic in arrangement (Fig. 7e and f). Boulders of ferruginoussandstone predominantly display rotational slides in thisoutcrop. Rotational slide has been described as shear fail-ure along concave-up discrete planes accompanied by rota-tion of the slide (Cook et al., 1982; Mutti and Ricci Lucchi,1972). Also, it is a slumping process that represents rota-tional movements along shear surfaces (Dingle, 1977;

Shanmugam et al., 1995). Diamictite was also recognizedin the outcrop. The first type of diamictite is conglomeraticand poorly clast supported, in which the matrix content ishigher. Clasts are poorly sorted quartz grains, they rangefrom very fine to pebble size. The matrix is clayey siltand rich in limonite (yellow ochre). Grains of quartz arefloating in the clayey silt and in a few cases they occur veryclose to the ferruginous sandstone (Fig. 7 g). The secondtype of diamictite occurs as a poorly circular to elongatesedimentary feature with an outer band. This category ofdiamictite can be divided into two, normal and inverse.‘‘Normal’’ diamictite is ferruginous sandstone encirclingfine grained silt. The hammer in Fig. 7h is resting on siltsurrounded by coarse sandstone. ‘‘Inverse’’ diamictite ischaracterized by fine grained sediments surrounding coarsegrained ones. The dimensions of the inverse diamictitemeasured are 1.85 m by 0.90 m and 3.2 m by 5.7 m(Fig. 7i). The associations of clayey silt matrix and con-glomerate with ferruginised sandstone boulders suggestdeposition by either high density turbidity currents or veryhigh fluid-mud debris flow. It was possible for a muddymatrix to have created enough buoyancy to enable theclasts to be carried within the flow. The debris depositwas later modified by slumping, which resulted in rotationand contortion of the strata. Diamictites have beendescribed as products of gravity-flow deposits, which areusually associated to slope environments (Prothero andSchwab, 1996; King, 1996; Raymond, 1995).

4.3. Facies assemblage 3: upper slope channel fills

Three types of channel infills identified on the outcropsshow some similarities in their infills. The infills in channelcomplex 1 (Fig. 5a), which include cobbles to pebbles ofpre-existing rock, clayey sand, chaotic clay with clasticinjections, all point to gravity flow related deposition(Fig. 8a). Channel complex 2 was characterized by snow-ball/spiral structures and clasts as channel lags. Snow-ball/spiral structure was identified as broken fragments offerruginous sandstones with rolled-up shape and a centercore. Two main types were identified on the outcrop andoccur within the clayey silt; they are isolated rolls andspiral rolls. Isolated rolls are made up of closely packed fer-ruginous mudstone/siltstone showing load structures(Fig. 8b). The clasts delineated in the channel lags areessentially debris flow deposits composed of pebbles tocobbles and very large continuous boulders of ferruginisedsandstone (Fig. 8c and d). The regionally developed chan-nel equally exhibits similar types of debris flow deposits.Infills identified are sandstone to conglomerate in associa-tion with very large boulders of ferruginised sandstone(Fig. 8d–f).

5. Depositional model

The outcrop data obtained in this study indicate that theoutcrop section of the Abeokuta Group comprises three

Fig. 7. Photographs of sedimentary features associated with the contorted layers in the upper slope gravity flow deposits. (a) Sandstone dykes (clasticinjections) in the clay. Clastic injections occur as vertical, inclined, cross-cutting and irregular (black arrows). (b) Boulders of ferruginous sandstone(marked with black line) within the upper part of the clay. The dimensions of the boulders vary from 3 cm by 2 cm to 64.5 cm by 60 cm and are essentiallydark purple in colour. (c) Intraformational fold caused by mega-slump. The amplitude of the fold is indicated with black arrows. Folded materials areessentially very fine grained sandstone, siltstone and mudstone with pseudo nodules. (d) Intraformational breccias associated with the clayey silt. (e)Contorted ferruginous sandstone boulders and clay with sandstone dykes. Ferruginous sandstone boulders was probably a debris deposit in a slopechannel while the sandstone dykes could have been a product of slumping before the last phase of slump deformation. (f) Rotational slide as exemplifiedby boulder of sandstone (hammer) that is nearly perpendicular to the bed. The cutlass is resting on another sandstone body, linked to the boulder butdeformed. (g) Diamictite in association with boulder of ferruginous sandstone. (h) Normal diamictite in the outcrop. The hammer is resting on clayey siltsurrounded by medium to coarse grained sandstone. (i) Inverse diamictite occur as roughly circular and elongate structures. The dark brown outer layer isessentially ferruginous sandstones while the light brown part is pebbly sandstone. These sandstones are surrounded by contorted mudstone containingsandstone dyke. (For interpretation of the references in colour in this figure legend, the reader is referred to the web version of this article.)


environmental facies assemblages, which are: distal lowershoreface to upper slope, upper slope gravity flow depositsand upper slope channel infills. It is envisaged that the dis-tal lower shoreface to upper slope started with pelagic to

hemipelagic deposition, which caused clay deposition(Fig. 9a). The lateral facies change from cross-beddedcoarse grained sand to planar-bedded silt and clayey siltreflect a general transition in flow velocities (Shepard,

Fig. 7 (continued)


1973; Howell and Normark, 1982). Turbidity currentspassing through submarine channels in the distal lowershoreface to upper slope debouch their sediments load todeposit these sands, silts and clayey silts (Selley, 1985;Shanmugam et al., 1995). The coarse sand is the proximaldeposit while the silt and clayey silt are the distal deposits(Fig. 9b). They compare with facies A and B of Mutti andRicci Lucchi (1972). Additionally the sandstone with cyclesof normal grading exhibits an inter-tongueing geometrywith the clay characterized by sandstone injection. This isprobably a product of turbidity flow generated fromslumped sediment on the slope (Fig. 9b). The most diag-nostic feature of turbidites is vertical grading (Selley,

1985), which was observed in the sandstone. (Figs. 4 and5a). This sandstone characterized with upward finingcycles, which could have been deposited in a submarinechannel. This is exemplified by the distinct erosive lowerboundary.

As the deposition by turbidity current ceased, there wasresumption of transgression, which occurred contempora-neously with slumping (Fig. 9c). This deposited the claywith pervasive sandstone dykes. In all the outcrops wherethe clay is exposed sandstone dykes occur as regular orcrosscutting sedimentary features (Figs. 4 and 7a). Theseassociations indicate sediments induced by slumping(Shanmugam et al., 1995). Slumping was also evident on

N ��Fig. 8. (a) Complex channels present on the outcrop. The sequence of develoSpiral rolls in the clayey silt white arrows in the photo are showing areas with c(c) Pebbly to cobble sized ferruginous sandstone deposited on the channel lags.(e) Slope channel incising into clay. Black arrows are pointing to boulders ofview of dislodged ferruginous sandstone boulders found at the top of the outc

(A)

(B)

(C)

(D)

(E)

Sea Level

Sea Level

Sea Level

Sea Level

Sea Level

Fig. 9. (a–e) Schematic two dimensional model illustrating the deposi-tional processes that occurred in the area of study. This model is based onthe information obtained from outcrop studies. (A) Early phase of deepmarine process dominated by pelagic to hemipelagic sedimentation. (B)The early phase was followed by sediment gravity flow processes. Thisphase was dominated by sand deposition as a result of turbidity currentsThe latter part of this phase was affected by slumping. (C) Another phaseof pelagic to hemipelagic sedimentation resumed with contemporaneousslumping. (D) This phase of slope development was characterised byerosion and backfilling. This caused channel development and channelinfill. Infilling of channel was predominantly through sediment gravityflows. (E) This last phase was predominantly a period of slumpdeformation (rotational slide) accompanied by deposition in quiet waterenvironment.


the outcrops which also exhibited the following: ball-and-pillow structures, pseudo-nodules, intraformational fold-ing, and intraformational brecciation (Fairbridge, 1946;Prothero and Schwab, 1996). A more plausible explanationfor the origin of rotational slide (slumping) is the highdegree of contortion recognized in one of the outcrops.In this outcrop, the mudstone with sandstone dykes, boul-ders of ferruginous sandstone of debris origin and infill ofthe channel were all contorted (Fig. 7e–g).

The channel forms that cut across most of the outcropsstudied in the entire area are filled with debris deposits. Inall the outcrops, clasts are pebbles to boulders of ferrugi-nous sandstone, which are different from their matrix,while in others they are deformed. This indicates that theclasts are extraformational in origin, possibly transportedas slurries and have undergone syn-sedimentary deforma-tion by slumping (Prothero and Schwab, 1996). Slopechannelisation as recognized in the outcrops falls intotwo distinct morphologies; composite paleo-channels andmajor paleo-channels (Fig. 7d). Composite paleo-channelswere recognized in two outcrops as channel complex 1 and2. These channels are filled with debris and slump deposits.The descriptions made above are considered generaliza-tions regarding the slope depositional processes that tookplace during and after the sediments were deposited. Sedi-ment gravity flow processes appear to have occurred morethan once in this area, as revealed from the schematicmodel diagrams developed from the outcrops studied inthe area (Fig. 9a–e). In chronological order, it is envisagedthat the depositional settings were as follows:

(a) An early stage of deep marine processes wherebypelagic to hemipelagic and turbidity currents weredominant. In this early stage, the system consistedof mudstone, sand, silt and clayey silt. Overall, thisstage resulted in the deposition of clay and turbiditesands (Fig. 9a and b).

(b) This was accompanied by the first phase of slumping,which induced the formation of intraformationalfolds and intraformational brecciation associatedwith the lithofacies deposited by the turbidity cur-rents (Fig. 9b).

(c) This stage was followed by pelagic and hemipelagicsedimentation combined with contemporaneousslumping. Overpressure was created, which producedfractures and tension gashes within the mudstone.Subsequently, the mudstone traveled over the buriedsand. Thereafter, the sand was forcefully injected intothe mudstone to form sandstone dykes (Fig. 9c).

��

pment of the channels is numbered 1–6. Sixth is the regional erosion. (b)enter core while the black arrows show the area extent of the roll structure.(d) Large and continuous boulders of ferruginous sandstone in the channel.ferruginous sandstone which originated from debris deposits. (f) Close-uprop. Its dimension is 97.5 cm by 40 cm by 27 cm.


(d) The next stage experienced a series of channelisationevents characterized by a network of paleo-channelsand major channels. The networks of paleo-channelsare filled with debris deposits, sandstones, clayeysand and clay. The major channels are predominantlyfilled with debris deposits induced by slumping(Fig. 9d).

(e) Finally, slope channels and their infill were affectedby slump deformation (rotational slide). This rota-tional slide resulted in the high degree of contortionthat affected the mudstone associated with sandstonedykes and the infill of the major channel. Pelagic andhemipelagic sedimentation capped the entire succes-sion in this area during the Cretaceous as revealedfrom the outcrops (Fig. 9e).

6. Discussion

In the area studied, the slope depositional systems thatwere recognized within the Abeokuta Group can be cate-gorised into a turbidite system, rotational slide (slump),debris flows, channeling and pelagic to hemipelagic sedi-mentation. This analysis supports the deductions on slopeenvironments and facies building blocks of Galloway(1998). Similarly, it compares favourably with outcroppingslope and deep-sea sedimentary features recognized byother workers (Selley, 1985; Hiscott and Aksu, 1994; Har-rison and Graham, 1999; Peter, 2002; Browne and Slatt,2002; Beabouef, 2004; Mosher et al., 2004). The results ofthose studies are compared with this study.

The basal loose sand unit with lateral facies changefrom coarse to silt and clayey silt is consistent with grad-ual reduction in the energy of the depositing medium (tur-bidity currents). The coarse grained sands and the clayeysilt were deposited under high and low energy conditions,respectively. This depositional process is in accordancewith turbidite sequences, which accumulate as broadsheets and lobes of sediment fed from a submarine fan.The proximal and the distal parts of the fan are character-ized by coarse and fine sediments respectively (Protheroand Schwab, 1996). Fig. 5c in this study is similar tothe classical example of facies B of Mutti and RicciLucchi (1972) based on outcrops of submarine fan depos-its. Additionally, the overlying normally graded section isa classical Bouma sequence, although it is incomplete.Selley (1985) underlines the incompleteness of structuresin any one turbidite. Collectively the basal lithofaciesassociations observed on these outcrops provide signifi-cant examples of sand deposition on the slope settingand/or normally graded bed by turbidity currents. Turbi-dites are known to be diagnostic sedimentary features atthe downstream end-of-slope channels (Mutti et al.,1987; Mutti, 1992).

Slump related sedimentary features recognized in theseoutcrops appear analogous to other examples. The giantintraformational fold shown in Fig. 7c of this study is sim-

ilar to a giant submarine slump fold within the CastaicFormation (Prothero and Schwab, 1996) and slump foldsfrom the Brushy Canyon Formation (Howell andNormark, 1982). Intraformational brecciation associatedwith the clayey silt in Fig. 7d is a typical feature of minorsubmarine land slides (Fairbridge, 1946). More impor-tantly, brecciated shale clasts interstratified between mas-sive sandstone in an outer submarine fan deposit of theTanqua-Kario basin, have been interpreted as a productof disruption of muddy deposits in a more proximal partof the basin, caused by sediment gravity (Rozman, 2000).The brecciated sandstone interval interstratified with theclayey silt in the studied area appears analogous to thisouter submarine fan deposit. Therefore, submarine slidein the proximal part of this basin combined with gravityflow processes were responsible for the formation of thebrecciated sandstone. In addition to this, the spiral rollsassociated with the upper layer are inferred to be productsof similar sedimentary processes. But traction and highenergy flow could have been involved. The Shanmugamet al. (1995) model on the origin of mudstone with sand-stone dykes appears to be related to mudstones that under-lie thick units of sandstone of slump and debris flow origin.In the present study, the mudstone overlies the sandstoneand no other sandstone lithofacies directly succeeded themudstone. The possible relationship of these lithofaciesassociations to the model is that excess pore-fluid pressurewas created in the mudstone. The result of the excess pres-sure is generation of fractures and tension gashes within themudstone. Subsequently, the sand was forcefully injectedinto the mudstone, forming sandstone dykes.

Debris flows in the outcrops are composed of pebbles toboulders of ferruginised sandstone, and inversely gradedbeds. Generally the textural characteristics of ferruginisedsandstone are different from the matrix. This is a pointerto the fact that the clasts are extraformational in origin.These observations support the work of Cook (1979),Cook et al. (1982), and Wach et al. (2000), with very fewdifferences. The similarity of this study and the work ofCook (1979) is the occurrence of large clasts of meter scalewithin sediments (Fig. 8d). Modern surface examples ofdebris flows have shown that these flows have the capabil-ity to carry very large boulders and even cars. An exampleis the La Crescenta, California debris flow of February1978 (Prothero and Schwab, 1996). The contrast is the rela-tionship between the clasts and the matrix. From Cook(1979), both the clasts and the matrix are slope-derivedand composed of limestone. But the Cretaceous debris flowin this area consists of ferruginised sandstone clasts that arecompletely different from their matrix. Ferruginous sand-stone clasts vary in texture from very fine to conglomeratic,while the matrix varies from clayey silt to clayey sandstone.In addition, large and continuous boulders of ferruginoussandstone in Fig. 8d and e are similar to the anomalouscoarse grained deposits of Aschoff and Giles (2005) recog-nized in the la Popa Basin. This deposit was interpretedas being of debris flow origin. This supports the earlier


interpretation of large boulders of ferruginous sandstoneidentified in the present study area as debris deposits.

Contortion in sedimentary rocks has been used exten-sively to interpret slides and rotational slumping. Thissyn-sedimentary deformation process has been well docu-mented in other sedimentary basins (Shanmugam et al.,1995; Cook et al., 1982; Ritchards, 1998). The similarlyhigh degree of contortion that affected the mudstone withsandstone dykes and ferruginised sandstone is convincingevidence of syn-sedimentary deformation. It is importantto note that this deformation represents the last gravityflow process that occurred in the study area, after the initialslumping and deposition from debris flow. Cores recoveredfrom slope channel-fills of the Nile Delta equally demon-strated high a degree of contortion and this was interpretedas a product of slumping (Samuel et al., 2003). This is inagreement with the earlier interpretation of this deforma-tion in the present outcrops.

Slope-channeling in this area was predominantly charac-terized by erosional and backfilling processes. Channelmorphologies vary from composite bodies comprisingnumerous smaller scale channel elements to large scalechannel elements. Similar slope channel elements have beenwell documented in the Cerro Toro Formation, upper Cre-taceous and in the Miocene slope fan system in the Mt.Messenger Formation (Beabouef, 2004; Browne and Slatt,2002). Channels have been described as prominent featureson the slope and base of slope settings and exhibit similarcharacteristics with their subaerial counterparts (Schumm,1993). The morphologies of the channels delineated onsome of the present outcrops are similar to smaller can-yons, gullies and upper slope escarpments of Mosheret al. (2004), recognized on the Scotian slope.

Finally the analogies and comparisons drawn from thiswork provide for some general acceptance of the occur-rence of slope depositional systems in the Abeokuta Groupduring the Cretaceous. Additionally, these slope elementscompare favourably with similar elements in other sedi-mentary basins around the world.

7. Conclusions

This paper presents outcrop documentation of field rela-tionships observed within the Cretaceous Abeokuta Groupof Dahomey (Benin) basin, southwestern Nigeria. Outcropdata indicated 13 lithofacies, which are: massive and loosesand; cross-bedded loose sand; normally graded sandstone(upward fining sandstone); laminated silt; clay; laminatedsandstone; clay with sandstone injections and clasts (sand-stone boulders); clayey sand with sandstone boulders; siltyclay with intraformational folds and breccias; highly con-torted conglomerate/clayey silt; and channel-infills. Thelithofacies were interpreted to have originated from threedepositional environments: distal lower shoreface to upperslope, upper slope gravity flow deposits and upper slopechannel infills. These facies associations can be related toa variety of depositional processes directly linked to slide

and gravity flows. Sedimentary features that are diagnosticof slide, channeling, turbidites, slump and debris flow wererecognized in the lithofacies. Based on these observations,slope depositional mechanisms were proposed for the depo-sition of these sediments. Comparisons between the resultsof this study and other slope environments of deposition inother sedimentary basins point out broad similarities. Inconclusion, these outcrops of the Cretaceous AbeokutaGroup contain excellent exposed sedimentary features thatare diagnostic of slope depositional environments.

Acknowledgements

I sincerely thank the management of ChevronTexaco forproviding fund for the fieldwork. I acknowledge the assis-tance of Mr. Bayo. Akinpelu, Mr. J. Pryor and Ms. LouiseLinden all of ChevronTexaco Nigeria.

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