Seismic stratigraphy and development of Avon canyon in Benin (Dahomey) basin, southwestern Nigeria

Seismic stratigraphy and development of Avon canyon in Benin (Dahomey) basin, southwestern Nigeria

Available online at www.sciencedirect.com Journal of African Earth Sciences 50 (2008) 286–304 www.elsevier.com/locate/jafrearsci Seismic stratigraph...

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Available online at www.sciencedirect.com

Journal of African Earth Sciences 50 (2008) 286–304 www.elsevier.com/locate/jafrearsci

Seismic stratigraphy and development of Avon canyon in Benin (Dahomey) basin, southwestern Nigeria S.O. Olabode *, J.A. Adekoya Department of Applied Geology, Federal University of Technology, P.M.B. 704, Ondo State, Nigeria Received 21 February 2007; received in revised form 20 September 2007; accepted 1 October 2007 Available online 6 October 2007

Abstract Interpretation of a grid of high resolution seismic profiles from the offshore eastern part of the Benin (Dahomey) basin in southwestern Nigeria area permitted the identification of cyclic events of cut and fill associated with the Avon canyon. Seismic stratigraphic analysis was carried out to evaluate the canyon morphology, origin and evolution. At least three generations of ancient submarine canyons and a newly formed submarine canyon have been identified. Seismic reflection parameters of the ancient canyons are characterized by transparent to slightly transparent, continuous to slightly discontinuous, high to moderate amplitude and parallel to sub-parallel reflections. Locally, high amplitude and chaotic reflections were observed. The reflection configurations consist of regular oblique, chaotic oblique, progradational and parallel to sub-parallel types. These seismic reflection characteristics are probably due to variable sedimentation processes within the canyons, which were affected by mass wasting. Canyon morphological features include step-wise and spoonshaped wall development, deep valley incision, a V-shaped valley, similar orientation in the southeast direction, and simple to complex erosion features in the axial floor. The canyons have a composite origin, caused partly by lowering of the sea level probably associated with the formation of the Antarctic Ice Sheet about 30 Ma ago and partly by complex sedimentary processes. Regional correlation with geological ages using the reflectors show that the canyons cut through the Cretaceous and lower Tertiary sediments while the sedimentary infill of the canyon is predominantly Miocene and younger. Gravity-driven depositional processes, downward excavation by down slope sediment flows, mass wasting from the canyon walls and variation in terrigenous sediment supply have played significant roles in maintaining the canyons. These canyons were probably conduits for sediment transport to deep-waters in the Gulf of Guinea during their period of formation. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Canyons; Seismic facies and evolution

1. Introduction Submarine canyons are continental slope features that are characterized by deep erosion. Their origins have been linked to river incision, subaerial erosion, turbidity current erosion, faulting and activity of benthic fauna in marine environments (Shepard and Dill, 1966; Shepard, 1981; Belderson and Kenyon, 1976; Twichell and Roberts, 1982). May et al. (1983) emphasized that several factors played *

Corresponding author. Tel.: +234 8033 783 498. E-mail addresses: [email protected] (S.O. Olabode), [email protected] (J.A. Adekoya). 1464-343X/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.jafrearsci.2007.10.002

a part in originating, altering and maintaining the canyons. These factors include tectonics, eustacy and sedimentary processes operating over a long period of time. Recent studies have shown that submarine canyons are of composite origin. Some are related to halokinesis (Lee et al., 1992), while others such as the large canyons in the north western Gulf of Mexico and Mauritania have been attributed to complex faulting, folding, active diapirism and coalescing of salt domes (Bouma et al., 1972; Kenyon et al., 1978; Antobreh and Krastel, 2006). Submarine canyons have been recognized in both convergent and divergent continental margins to serve as pathways for downslope mass sediment transport as in the case of

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slides, slumps, debris flows and turbidity currents that transport sediments from shallow marine to deep marine environments (Laursen and Normark, 2002; McHugh et al., 2002; Antobreh and Krastel, 2006). The study of submarine canyons is important because they not only preserve the sedimentation history of an area but also serve as modern analogues of deepwater hydrocarbon reservoirs with sand rich turbidities (Stow and Mayall, 2000; Posementier, 2003). The development of submarine canyons along the offshore continental margin of Nigeria and the role they have played in the sediment deposition dynamics of the area appear to have received very little attention. However, few studies have revealed that ancient and present-day canyons occur in the Benin (Dahomey) basin and the neighbouring Niger Delta basin. Burke (1972) reported the existence of the Avon, Mahin and present-day canyons in the offshore part of the Benin basin in southwestern Nigeria. Billman (1992) attributed the complications of stratigraphic relations in this part of the basin to submarine deep channel cutting into the Paleogene and older rocks. Ancient submarine canyons of Oligocene–Miocene age have been studied in the nearby Niger delta (Petters, 1984). The area covered by the present study is located between longitude 6°10 0 and 6°25 0 N, and latitude 3°48 0 and 4°05 0 E offshore of the Atlantic Ocean (Fig. 1A). The Avon canyon identified by Burke (1972) falls within the present study area, which is about 82 km and 94 km from Lagos and Okitipupa, respectively. Bathymetrically, the study area covers the continental shelf up to the upper part of the continental slope (Fig. 1A). Water depth ranges from 10 m to 570 m with the shallow water portion corresponding to the upper part of the canyon. To the best of our knowledge there have been no studies directly investigating the origin, evolution and geological significance of Avon submarine canyon. The aim of this paper is to describe the seismic stratigraphy of different generations of cut and fill episodes associated with the Avon canyon in the area within the limit of available data (Fig. 1B). The seismic characteristics will be related to the possible origin and evolution of the canyon, as well as the influence of the depositional processes on the canyon. 2. General geology The Benin (Dahomey) basin is a very extensive sedimentary basin that extends from southeastern Ghana in the west to the western flank of Niger Delta in Nigeria. It is bounded to the west by the Ghana ridge, which is an extension of the Romanche Fracture Zone; and on the east by the Benin Hinge line, a basement escarpment which separates the Okitipupa Structure from the Niger Delta Basin and also marks the continental extension of the Chain Fracture Zone (Wilson and Willians, 1979). The Nigeria portion of the basin extends from the boundary between Nigeria and Republic of Benin to the Benin Hinge Line. Detailed geology, evolution, stratigraphy and hydrocarbon occurrence of the basin are contained in the works of

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Jones and Hockey (1964), Reyment (1965), Adegoke (1969), Omatsola and Adegoke (1981), Coker and Ejedawe (1987), Billman (1992), Okosun (1996), and Hack et al. (2000). Most of these authors have described the Benin (Dahomey) basin and the Okitipupa structure that partly controlled sedimentation in the basin. Coker and Ejedawe (1987) identified three geoblocks, namely, the onshore geoblock (Bodashe – Ileppa – Ojo geoblock), the Okitipupa structure (Union – Gbekebo geoblock) and offshore geoblock. They emphasized that these three geoblocks have gone through three main stages of basin evolution. These stages are initial graben (pre-drift) phase, prolonged transitional stage and open marine (drift) phase. The stratigraphy of the sediments in the Nigerian sector of the Benin basin is controversial. This is primarily because different stratigraphic names have been proposed for the same formation in different localities in the basin. This situation can be partly blamed on the lack of good borehole coverage and adequate outcrops for detailed stratigraphic studies. Billman (1992) divided the stratigraphy of the entire basin into three chronostratigraphic packages. They are pre-lower Cretaceous folded sediments, Cretaceous sediments and Tertiary sediments (Fig. 2). In the Nigerian portion of the basin the Cretaceous sequence, as compiled from outcrop and borehole records, consists of the Abeokuta Group sub-divided into three formational units, namely, Ise, Afowo, and Araromi Formations (Omatsola and Adegoke, 1981). Ise Formation overlies the basement complex unconformably and comprises coarse conglomeratic sediments. Afowo Formation is composed of transitional to marine sands and sandstone with variable but thick interbedded shales and siltstone. Araromi is the uppermost formation and is made up of shales and siltstone with interbeds of limestone and sands (Fig. 2). The Tertiary sediments consist of Ewekoro, Akinbo, Oshosun, Ilaro and Benin (coastal plain sand) Formations. Ewekoro Formation is made up of fossiliferous well-bedded limestone while Akinbo and Oshosun Formations are made up of flaggy grey and black shales. Glauconitic rock bands and phosphatic beds define the boundary between Ewekoro and Akinbo Formations. Ilaro and Benin Formations are predominantly coarse sandy estuarine, deltaic and continental beds (Kogbe, 1975). The Benin Basin has a high hydrocarbon potential. This assertion is supported by the occurrence of large deposits of tar sand in the Nigerian sector of the basin. Also, there have been reported cases of hydrocarbon production in the Cretaceous and Tertiary sedimentary rocks in the Seme Field offshore in the Benin Republic (Coker and Ejedawe, 1987). 3. Materials and methods The seismic data used in this study consist of 54 migrated seismic reflection profiles covering an entire Oil Mining Lease (OML) located offshore of the Atlantic

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Fig. 1. (A) Regional bathymetric map of Nigeria continental margin showing the locations of Avon, Mahin and Calabar canyons, and their corresponding fans (modified after Burke, 1972; Mascle, 1976 and Damuth, 1994). Avon canyon (study area) is located in Dahomey (Benin) basin the western part of Niger Delta. B1 is the seismic lines covering the entire study area and well locations used for age determination on the seismic. Avon canyon is located in the area shown with dotted lines. The expanded section and interpreted lines are shown in B2. Lines trending SE are located down canyon towards the direction of sediment transport within the canyon, while lines trending SW are located perpendicular to the canyon.

Ocean bordering southwestern Nigeria. (Fig. 1B1). The reflection profiles that pass through the northeastern part of the OML where the canyons are located were selected for detailed analysis and interpretation (Fig. 1B2). The data sets were acquired from ChevronTexaco Nigeria. The seismo-stratigraphic interpretation for this study follows the general procedure of Vail et al. (1977), Sangree and Widmier (1979) and Mann et al. (1992). Special atten-

tion was paid to the seismic facies analysis and environmental interpretation. The parameters analysed include reflection configuration, amplitude, continuity and frequency. Each of the different generations of the canyons was characterized by constant reflection discontinuity and change in amplitude, frequency and configurations. Time values were digitized in relation to the shot points on the seismic data and related to geographic coordinates on the

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Fig. 2. Simplified geological map of the Nigerian sector of Dahomey (Benin) Basin and the location of Avon canyon. The stratigraphy are composed of both the Cretaceous and Tertiary sediments as shown. The third well used for age determination and calibration on the seismic is named after the canyon as shown. Note the directions of longshore drift and the prevailing wind (compiled from Jones and Hockey, 1964; Ako et al., 1980; Burke, 1972; Whiteman, 1982).

location map. These values were loaded into computer software to generate various maps. Very few drilling data were available for the area where the canyon was identified. However detailed biostratigraphic data of a well named Avon-1 and located very close to the canyon were provided (Fig. 2). Biostratigraphic information from two other wells (Epiya-1 and Baba-1) drilled in the southeastern part of the OML was also available for study (Fig. 3). The biostratigraphic information facilitated age determination on the seismic sections and regional correlation between the wells (Figs. 1–3). 4. Results and discussion Four generations of submarine canyons were identified from the seismic data. Three ancient canyons have been filled with sediments, and the last is probably active at present. They are numbered I, II, III and IV in chronological order. These canyons are all prominent on the seismic profiles (profiles 01–07) that run both across the width and longitudinal axes, except canyon I whose remnant could only be identified on the profiles. The various features of the canyons are shown on the seismic profiles. Similar series of old and modern canyons have been reported in the Cenozoic strata of New Jersey continental slope (Pratson et al., 2004).

4.1. Seismic facies analysis 4.1.1. Canyon I This represents the first generation of ancient submarine canyon in the area. Only the remnant (Martinsen, 2003) of this canyon could be identified on profiles 01, 03 and 04 due to the effect of erosion caused by subsequent canyons. This remnant is characterized by very deep incision on all the profiles where it was delineated. Because the canyon is preserved as small remnants in the seismic profiles, it was not possible to give a detailed description of it seismic facies. However, the seismic characteristics of the canyon remnant consist of moderate to high amplitude, variable frequency and fairly chaotic reflections (Figs. 4a and b to 7a and b). It is difficult to make the estimate of the twoway time (TWT) values with the contemporary seabed, because of the erosion occasioned by the formation of younger second generation canyon (canyon II). On profile 04 the incision is up to 1.75 s TWT compared to the present day seabed. The maximum measurable width of the canyon remnant from the seismic distance is 1.25 km (position of Figs. 4a and b to 7a and b). 4.1.1.1. Interpretation. It is difficult to deduce the continuity of the seismic facies and the continuity of the sedimentary infill of the canyon. The seismic facies parameters

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Fig. 3. Lithostratigraphic and age correlation of the three wells used to calibrate the seismic sections. The sedimentary section of Avon-1 shows the absence of lower Tertiary sediments interpreted to have been eroded by Avon canyon. Unconformities represented by erosional features are identified in the other two wells.

Fig. 4a. Un-interpreted seismic line 01.

suggest that the sedimentary infill is probably composed of moderately thick alternating beds of shales, sandstones and siltstones. The reflections are truncated at the point of incision of the canyon within the older rocks. This shows the effect of deep channeling erosion during the formation of canyon I.

4.1.2. Canyon II Canyon II was identified on all the seismic profiles in spite of being affected by later erosion. Significant amounts of canyon fill are preserved on both the SW and NE sides of the canyon ( Figs. 4a and b to 8a and b). There are sharp differences in the seismic parameters of the sedimentary

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Fig. 4b. Seismic profile 01 crossing the upper part of the canyons. Canyon I was identified using the remnants of preserved sediments. This is shown as deep valley incision. Canyon II is shown as distinct discontinuous reflections between older and younger sediments as a result of erosion. Canyon III shows discordant reflections against canyon II. These reflections are chaotic on the NE and slightly parallel and gently dipping on the SW. Note the newly generated canyon IV with both V-shaped and U-shaped valleys.

Fig. 5a. Un-interpreted seismic line 02.

Fig. 5b. Seismic profile 02. There is a change in the wall structure of canyon II from stepwise to spoon shaped. Note the different reflection characteristics in the SW and SE parts of canyon III. The two valleys in canyon IV have almost merged completely causing width expansion and depth increase.

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Fig. 6a. Un-interpreted seismic line 03.

Fig. 6b. Seismic profiles 03 showing the continuity transparent facies in canyon II and the contrast between the wall morphology both on the SW and NE sides. Canyon IV has merged completely into a single channel. The reflection characteristics in canyon III shows that sediments are prograding into the canyon from the two sides, suggesting that canyon III was essentially fed from the sides.

infill of canyon II and the pre-existing rocks in the area. They exhibit distinct reflection discontinuity, frequency and configurations as well as change in reflection amplitude. Canyon II is better preserved than canyon I and is composed of transparent to semi-transparent and rare high amplitude reflections in the proximal part as shown on the majority of the dip lines (Figs. 4a and b to 8a and b). On profile 07 (Figs. 8a and b), which occurs at the distal part, the reflections are of moderate to high amplitude and low to moderate frequency. Reflection continuity is not uniform both across and along the canyon. Seismic profiles across the canyon show that the reflections are fairly chaotic and discontinuous. Along the canyon, they are fairly continuous and non-chaotic (Fig. 9). At the boundary of canyon II and III high amplitude reflections are easily observable. On profiles 01 and 03, the uneroded part of the canyon showed that its two walls attained a horizontal width of approximately 5.4 km and 3.2 km (Figs. 4a and b; 6a and b). Seismic evidence shows that the total width of canyon II (before the subsequent

erosion attributable to canyon III) varies from 7.5 km to 11 km. 4.1.2.1. Interpretation. Low amplitude and transparent reflections observed in canyon II could have been caused by the occurrence of any of the following: (i) beds too thin to be resolved in the seismic section; and (ii) a zone of one predominant lithology, which could be sand prone or shale prone (Mitchum et al., 1977; Sangree and Widmier, 1979). Although well information was not available to identify the correct lithology, the proximal transparent reflections are probably a sand-prone facies. This facies grades imperceptibly in the distal portion to siltstone and, possibly, shale. The chaotic facies observed across the canyon could have been caused by the sediments transported by mass-transport processes from the side of the canyon to its centre. 4.1.3. Canyon III This is the last generation of ancient submarine canyons recognized in the seismic profiles 01, 02, 03 and 04 that cut through the entire width of the canyon. The effect of cut

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Fig. 7a. Un-interpreted seismic line 04.

Fig. 7b. Seismic profile 04 shows the prominence of canyon I valley incision, spoon shaped and stepwise wall morphology of canyon II and II. Note the difference in the reflection characteristics of canyon III in the NE and SW. Infilling of sediments was probably uniform in the SW while in the NE it was affected by mass wasting.

Fig. 8a. Un-interpreted seismic line 07.

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Fig. 8b. Seismic profile 07 shows the SE side of canyons II and III. The seismic in canyon II shows a combination of transparent and few high amplitude reflections. Canyon III is characterized by regular oblique progradational and chaotic seismic patterns.

Fig. 9a. Un-interpreted seismic line 56.

Fig. 9b. Seismic line 56 located in the down canyon direction displays the prominence of canyon II and III. The floor of canyon II as seen on the profile is made up of irregular, curved and undulating surface. This was interpreted as complex floor development caused by differential erosion. The chaotic reflections in canyon III was interpreted as gravity induced sediments during canyon filling while the topmost regular reflections could be related to later younger sediments after the canyon has been filled.

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and fill sedimentary process on the canyon is easily observable on all the seismic profiles. The width of the canyon on profile 01 is about 10 km (Fig. 4a and b) and this increases offshore towards the southeast to about 15 km on profile 04 (Fig. 7a and b) suggesting that this canyon is much wider in the offshore area. Profile 03 shows a portion of canyon III characterized by deep incision up to 1.2 s TWT. The seismic section across the canyon revealed both oblique progradational and parallel to sub-parallel reflection configurations. In all the seismic sections, the oblique progradational configurations show prominently on the sides of the canyon. The southwestern side of canyon III is typically less chaotic and shows parallel oblique configurations on some of the seismic lines (Figs. 4a and b; 5a and b). At a more distal part as shown on profile 07 (Fig. 8a and b), the reflections are characterized by complex sigmoid to oblique configurations. Towards the boundary between canyon II and III the reflections are slightly chaotic and become continuous farther away from the boundary. The oblique progradational facies was separated from the facies on top by toplap terminations (Figs. 4a and b; 5a and b; 8a and b). Parallel to sub-parallel reflections occur at the topmost part of canyon III. They occur on all the sections cutting across the canyon, but they are well displayed on the profiles where the erosive activity of canyon IV is less pronounced (Figs. 4a and b; 5a and b). Reflections exhibit moderate to high amplitude, moderate to high frequency, as well as fairly continuous and regular configurations. 4.1.3.1. Interpretation. Oblique progradational configurations have been suggested to denote deposits associated with high sediment supply and high energy sedimentary regime (Mitchum et al., 1977; Sangree and Widmier, 1979). Based on the occurrence of similar reflection configurations, the canyon III sedimentary infill can be interpreted to indicate a sediment supply in a high energy regime. The variation observed in the reflection configurations can be ascribed to variable depositional processes in the canyon. The southwestern side of canyon III with oblique parallel reflections suggests a regular outbuilding of sediment into the centre of the canyon with less mass-transport influence. The sedimentation in the northeastern side was probably affected by sliding and slumping, which is reflected in the chaotic reflections. Sigmoid facies are usually associated with rising sea level or subsiding land level creating accommodation space (Sangree and Widmier, 1979). The complex sigmoid to oblique configurations on profile 07 (Figs. 8a and b) are interpreted as alternating upbuilding and depositional bypass within a high energy depositional regime as sediments were fed into the canyon centre from the sides. This implies that the sediments were deposited in the centre of the canyon from its sides as the relative sea level rose. As a result of the development of steep margins of the canyon, the sediments were probably transported by sliding and slumping. The fairly chaotic sigmoid to oblique reflection configurations of canyon III sed-

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imentary infill support this interpretation. Sub-parallel to parallel reflection configurations delineated at the topmost part of canyon III implies uniform rates of deposition. Reflection amplitude, continuity and cycle breadth suggest alternating layers of sandstones, siltstones and shales that are laterally continuous. The toplap observed in the seismic sections represents a top discordant relation whereby reflections terminate against an overlying surface as a result of non-deposition and minor erosion (Mitchum et al., 1977). This implies that the toplap termination represents the period when canyon III was completely filled and that period was followed by another period of non-deposition or minor erosion. This surface probably represents upper surface of canyon III before later infill under uniform rates of deposition. The toplap termination that separates the two reflection configurations delineated in canyon III suggests that at a particular time the canyon infill was at the same level with the surrounding sediments. Subsequent rise in sea level possibly created accommodation space (Myers and Milton, 1998) both in the canyon and the surrounding areas. The accommodation space was later filled with sediments, which made canyon III and the surrounding areas to be at same level. 4.1.4. Canyon IV Bathymetric contour and surface perspective maps from Figs. 10a and b obtained from computer plots combined with seismic profiles show that another canyon is probably being formed in the area. On all the profiles where the new canyon axis is visible, the depth of the water column to seabed increases towards the offshore area. The V-shape morphology of the canyon becomes more pronounced from profile 01 to 04 (Figs. 4b, 5b, 6b and 7b). The maps presented in Figs. 10a and b show that the presently active Avon canyon is characterized by a major valley branch that goes to the east and is possibly oriented towards the nearby Mahin canyon. This provides an initial indication that there is a link between the two canyons particularly in the distal offshore part. 4.2. Well log correlation with seismic The lithostratigraphy as derived from the three wells drilled in the study area (Baba-1, Epiya-1 and Avon-1), consists of shale, siltstone, sand and sandstone deposited in a variety of environments. Avon-1 shown in Fig. 2 located very close to the canyon was drilled to a total depth of 1463 m where it bottomed on the basement. Baba-1 and Epiya-1 wells were drilled to depths of 3272 m and 2931 m, respectively. In two (Avon-1 and Epiya-1) of the three wells on which detailed biostratigraphic analysis was done, reference microfossils such as foraminifera, ostracods and palynomorphs were employed for the analysis. In Avon-1, unlike the other well (Epiya-1), the lithologic section penetrated by the well was characterized by scarce and poorly preserved fauna that probably led to crude estimations of

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Fig. 10a. Bathymetric contour of the study area derived from water depth on the seismic. It shows the active canyon (IV) in the area. The orientation is similar to the ancient canyons (II and III) towards the SE direction.

Fig. 10b. Bathymetric orthographic surface map for canyon IV. Canyon size and V-shape morphology become more pronounced towards the SE direction.

chronostratigraphic age. However, the analysis showed that Avon-1 penetrated 384 m of Maastrichtian and 1099 m of Miocene to Recent sediments. In this well, lower Tertiary sediments of Paleocene, Eocene and Oligocene

were not recognized. The Cretaceous section observed in Epiya-1 consists of Cenomanian to Lower Maastrichtian sediments, which are approximately 869 m thick. The Tertiary sediments range in age from Paleocene to Early

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Pliocene and are approximately 1966 m thick. The upper section of the well could not be dated as a result of poor preservation of fossils. The oldest sediments recognized in Baba-1 were dated Maastrichtian with an approximate thickness of 380 m. Sediments observed in the Paleogene section are Paleocene in age and are approximately 482 m in thickness. Eocene and Oligocene sediments were not observed in the Paleogene section of this well. All the Neogene sections from Miocene to Pleistocene are present and their total thickness is approximately 1654 m. The upper section of the well which represents Quaternary sediments was not analysed as a result of scarce and poorly preserved microfossils. The gaps that were recognized in the age of the sequence penetrated by the wells are interpreted as unconformities, which were caused by erosional activities. Although there is insufficient information on the erosional processes in the area, Billman (1992) pointed out that subsurface stratigraphic interpretation within the Benin (Dahomey) basin was complicated by numerous cut-and-fill features which made interpretation difficult. The occurrence of unconformities at different levels in the three wells supports this fact. The absence of Lower Tertiary sediments from Paleocene to Oligocene in Avon-1 can be linked with the erosional activities caused by the canyon. It is most likely that the missing section of the sequence as observed in the seismic sections, represents the eroded portion in the lithostratigraphic section of Avon-1 (Fig. 3). If this is true, the early erosional activity that occurred in the canyon probably predates Miocene while the initial infilling took place during Miocene. This is supported by the biostratigraphic information in Avon-1, which showed the oldest Tertiary sediment overlying the Cretaceous section to be Miocene.

notches and small linear troughs (Chuang and Yu, 2002; Yu and Chang, 2002) with steep walls have not been identified. The head segments of canyon II and III may be present in the proximal landward direction outside the oilmining lease (OML) being studied at present. Broad troughs with varying widths and irregular walls are observed in the seismic reflections for canyons II and III. These features are interpreted to be signatures of the middle segments and mouth of the two canyons. Two distinct parts were identified in canyon IV (active canyon at present): the head and the middle segments. Available data did not cover the canyon mouth, which is in deeper offshore waters. The main canyon is V-shaped; it is flanked by small gullies on both sides. The gullies are flat bottomed and U-shaped as shown on seismic profiles 01 and 02 (Figs. 4a and b; 5a and b). The shape of these gullies contrasts with the main canyon shape; the gullies probably serve as feeder channels to the main canyon in this head region. The head segment consists of a V-shaped notch, and small linear troughs, steep walls and relatively shallow water depths. Further down towards the middle segment the feeder gullies have disappeared merging with the bigger main canyon as shown on the seismic profiles 03 and 04 as the canyon loses its topographic expression (Figs. 6a and b; 7a and b). Such changes in canyon profile down slope have been reported for a number of modern day canyons. These changes can be explained by two processes; these are small-scale processes such as bioerosion and large-scale processes such as slumping or turbidity current (Stanley and Kelling, 1978; Malahoff et al., 1982; Chow et al., 2001).

5. Canyon morphological features

The morphology of canyons I and II is very difficult to determine, because they have been intensely eroded during the formation of canyon III. Nevertheless, seismic evidence shows that canyon I and III were characterized by very deep channel cutting. Although the channel cutting is present on profiles 01, 02 and 03, it is conspicuous and more easily identified in profile 04 (Fig. 7a and b). The only trace of canyon I is the channel cutting, which has preserved remnants of the sediments which originally filled canyon I before the commencement of canyon II erosional activities. On the same axis, canyon III shows channel shallow cutting. From the profiles, there is not enough evidence to identify similar features in canyon II, except on profiles 03, where a minor deep channel cutting occurs in the SSW of the canyon. Time value contour and surface orthographic perspective maps for canyons II and III relative to the contemporary sea beds generally show that canyon III experienced deep channel incision when compared with canyon II, but the width of the eroded area is larger in canyon II (Figs. 11a and b). Seismic lines that run through the canyons showed the form of the canyon floors. They revealed a combination of simple and complex axial floor plans for canyon II, while the initial shape of canyon II

Although it was difficult to identify the general features of canyons I and II due to erosion caused by canyon III, the following features were recognized based on the seismic facies analysis and maps generated from the available data: (a) canyons plan view (b) deep channeling/valley incision, (c) step-wise and spoon-shaped canyon wall development (d) similar canyon orientations. 5.1. Plan view shape Contour and perspective orthographic maps generated for the basal erosional surfaces of canyons II and III (Figs. 11a and b; 12a and b), and bathymetric maps generated for canyon IV (Figs. 10a and b) were examined to determine the plan view of these canyons. Contour maps could not be generated for canyon I because of the intense erosion caused by canyon II. The main course of all the submarine canyons extends in the same southeast direction. Available seismic sections did not cover the head segment of canyons II and III. Features that mark the heads of the canyons such as V-shaped

5.2. Valley incision

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Fig. 11a. Time contour from seismic sections for canyon II relative to the contemporary sea bed. Contour values are in seconds. Negative time values are used to reflect the true morphology of the canyon.

Fig. 11b. Time orthographic surface map of canyon II with the contemporary sea bed. High time values at the canyon axis represent area where the sediments of the canyon have been completely eroded. The degree of incision is lower but the canyon width is higher when compared with canyon III.

determines the floor of canyon III. Canyon I floor could not be discerned because of erosion. The simple floor shape of canyon II is composed of flat uniform floor as observed on one of the cross profiles sited near the centre of the canyon. Profile 56 located towards the southern wall of the canyon reveals a complex floor pattern (Figs. 9a and b). The floor of the canyon varies from slightly flat to curved and undulating shape. The depth of erosion increases from the NW to the SE part that is from the proximal to the distal offshore parts. Therefore, the truncated reflections observed on the seismic sections and the occurrence of cut-and-fill features are evidence of erosion of both consol-

idated and less indurated material of older rocks during the development of the canyon. Similar features are present in the canyons identified in the USA Atlantic Continental Slope (Mitchell, 2005). 5.3. Canyon wall development The walls of canyon II show a complex morphology as observed from the various seismic profiles. Generally, the boundary on the SW side shows a relatively steep step-wise wall. Three different levels have been identified when the canyon is viewed from the SW direction on profile 01.

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Fig. 12a. This figure shows the time contour of canyon III relative to the contemporary sea bed.

Fig. 12b. Time orthographic surface map of canyon III relative to the contemporary sea bed. Degree of incision is higher and width is smaller when compared with canyon II.

However, similar but less distinct feature is present on the opposite side, i.e. the NE direction of the canyon (Figs. 4a and b). The inclination of the steps decreases towards the canyon axis, while the canyon wall terrace, formed as a result of erosion, increases in width downwards; the width of the topmost terrace is approximately 1 km, the middle 2 km and the lowest 9 km. On profile 02 the wall morphology has changed slightly with the preservation of the lowest platform on the SW, while the step-wise feature on the NE is irregular. Profiles 03 and 04 show that, although the stepwise wall has been completely changed to a spoon-shaped

wall on the SW flank, the step-wise feature is still preserved on the NE side wall with reduced number of steps (Figs. 6a and b). The seismic profiles did not pass through the head and the mouth of canyon III, so that their precise location cannot be defined. However, there is evidence that profile 01 is closer to the head, while profile 04 is closer to the mouth based on the widening of the canyon observed on the profile. The seismic expression shows two subtle canyon morphological features as follows: (a) a relatively U-shaped valley with flat bottom; and (b) a gradual change to a gen-

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tle step-wise wall. The first feature is present on profiles 01 and 02 and is characterized by a gentle slope wall (Figs. 4a and b; 5a and b). On profile 02, the step-wise wall of the canyon has started developing, although it is not distinct. Profiles 03 and 04 show clearly the full development of the canyon wall morphology. Two steps can be recognized above the valley incision at the base of the canyon (Figs. 6a and b; 7a and b). Here, the slope angle reduces downward just as the platform length increases in the same direction. Four profiles show the wall development of canyon IV at its proximal end. The distal section of the canyon was not covered by the lines. On profile 01 two relatively shallow valleys were identified. Their water depths vary from 176 m to 353 m. The two valleys gradually merge as observed in profiles 02–04 with the valley depth increasing to about 485 m. The initial two valleys are U and V shaped. Their walls are simple and almost uniform, with very steep wall in the V-valley when compared with the U-valley. On profiles 03 and 04 where the valleys have completely merged (Figs. 6a and b; 7a and b), the canyon has changed from a combination of U and V valleys to a single V valley with fairly deep incision. The walls of the single V-shaped valley are equally simple and uniform with almost similar angles. Different wall morphologies are common canyon features for which different genetic models have also been proposed in the literature. The formation of walls and terraces in submarine canyons documents the history of canyon incisions and evolution (Hagen et al., 1994; Deptuck et al., 2003). Antobreh and Krastel (2006) described the wall of Cap Timiris Canyon, offshore Mauritania, and postulated that it developed through a combination of processes including those involved in the formation of slide and slump structures, ‘point bar’ through meanders, ‘inner’ or ‘confined’ levees and deep seated faults. In canyon II of the present study area, which has well developed walls with terraces, we propose that these terraces, which may be appropriately described as ‘slide’ or ‘slump’ terraces, probably formed during the following 3stage event: (i) the process of down-cutting, incision and subsequent erosion that prompted instability in the vicinity of the canyon walls as the sea level lowstand commenced, (ii) failure of the canyon walls both in the NE and SW ends which followed the instability in stage (i); and (iii) a second phase of intense erosion, which modified the failed canyon walls. It has been shown that canyon terraces, formed by the slide and slump mechanisms, are characterized by disturbed seismic facies (Friedman, 2000; Deptuck et al., 2003). However, this type of disturbed facies is not present in the seismic profile of canyon II. The absence of such facies along the walls of canyon II is probably related to the effect of subsequent erosion, which had eroded the disturbed sediments from the canyon walls. The variation in the canyon wall morphology as observed along the seismic profiles is attributable to the effect of cycles of erosion that shaped the canyon walls over time. Erosional activities were probably more intense in the

proximal end of the canyon in the SW as revealed by a rapid change in the canyon II morphology in the area. 5.4. Orientation of the canyons The contour and perspective maps generated for the canyons show that two generations (II and III) of the canyons are oriented in the southeast direction (Figs. 11 and 12). Canyon IV (newly formed) is oriented towards the same direction. To illustrate further the canyon morphology, the computer bathymetric perspective plot shows a relative uniform seabed except for the canyon axis. The canyon axis reveals that the proximal end increases in width progressively towards the southeast offshore. This newly formed canyon is characterized by deep channel giving rise to a V-shaped valley (Fig. 7a and b). 6. Canyon evolution and development The seismic reflection profiles taken across the canyons and along the canyon axis were examined to decipher the evolution and development of the canyons. The factors that were likely to be responsible for initiation and maintenance of these canyons include changes in relative sea level, mass-flow depositional processes, downward excavation by down slope sediment flows and variation in sediment input from land. These depositional controls influence the initiation and maintenance of the canyons and there was no evidence of structural control on the evolution of the canyons observed on the seismic lines. 6.1. Sea level change control This section addresses the possible role of the sea level change on the evolution and development of the different generations of submarine canyons that have been observed in the area of study. As stated by Vail et al. (1977, 1991), relative sea level change is the major factor responsible for the formation of sequence boundaries along continental margins, especially passive ones. A major fall in sea level will result in the exposure of the continental shelf and its subsequent erosion. Intensive erosion on the shelf and upper slope leads to incision of submarine canyons and development of regional unconformities, resulting in the formation of sequence boundaries (Yu and Hong, 2006). During the lowering of the sea level, canyon channels are generally active causing significant erosion and deep down-cutting into the sediment below. Rising sea level will lead to a reversal of such depositional phenomena, causing active canyon channel down-cutting to shift to the landward part. Relative changes in the sea level cause variations in the quantity and types of terrigenous materials to be transported to the basin. If this model is applied to the area of study, the older sediments truncated by younger sediments were probably eroded as a result of lowering of the relative sea level. Sea level lowering subsequently led to active canyon channels,

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which eventually culminated in erosion and deep sediment cutting. Mann et al. (1992) proposed a similar model for the Northern Green Canyon area in the Gulf of Mexico. Regional correlation (Fig. 3) shows that the Avon canyon cut through Cretaceous and lower Tertiary sediments. Significant erosion of canyon II and III started during the Miocene as shown on the regional age correlation of the reflectors (Figs. 4–8). It is difficult to compare canyon I with the contemporary seabed because it was the first canyon formed in the area. Previous studies have shown that a high increase in submarine canyon development off the coast of Africa took place about 30 Ma. This was the period when the sea level was lowered in response to Antarctic Ice sheet formation and about the time the present swell topography of Africa began to develop (Burke, 1996; Burke et al., 2003). The combination of both sea level lowering and uplift of the African continent at this time must have contributed substantially to the formation of canyons along the African coast. Other well studied canyon along the African coast is the Congo canyon which cuts through evaporites of Cretaceous age (Shepard and Emery, 1973). Since significant erosion and infilling of canyon II probably started in Miocene, one can infer that the development of canyon I commenced earlier than Miocene. This period could have coincided with the 30 Ma Oligocene age suggested by various workers. Detailed biostratigraphic information obtained from Epiya-1 and Avon-1 assisted in the determination of paleobathymetry and geologic ages of the sediments recognized in the two wells. Avon-1 well (Fig. 3), which is very close to the canyon shows that the Cretaceous sediments were deposited in the neritic environments before the formation of the canyon. Subsequently, the deposition of other sediments took place in the marginal marine to estuarine environments. This signifies a period of regression, which probably led to the sea level lowstand mentioned earlier. The absence of Paleogene sediments in the well probably as a result of erosion in the canyon, made correlation difficult. However, the age inference of 30 Ma deduced from a combination of this and previous studies show a period of falling relative sea level based on the correlation with the Global Cycle Chart of Haq et al. (1988). Lowstand periods in the geologic history of sea level movement are usually characterized by erosion and non-deposition on the shelves, as well as deposition of deep marine fans in the basins (Vail et al., 1977). Therefore, the earlier development of Avon canyon possibly coincided with a period in the global sea level history, when the sea level was at a lowstand. This probably facilitated erosion activities along the shelf that led to the formation of the canyon. 6.2. Sedimentary processes In the earliest studies of submarine canyons, they were interpreted as submerged river beds during rising sea level at the end of glaciations (Spencer, 1903; Shepard, 1934). As new techniques became available the origin of canyons was

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fundamentally re-evaluated (detailed review is contained in the work of Pratson et al., 1994). Popescu et al. (2006) postulated two ideas as being responsible for the formation of canyons. These are: (i) sediment flow driven retrogressive failure usually related to a sediment source on land; and/ or (ii) retrogressive slide caused by slope destabilization associated with a variety of processes. The complex morphology exhibited by the canyon walls, deep channeling, valley incision and the complex pattern of the canyon fill reflectors were used to interpret the influence of sedimentary processes on the development of the canyons. The steep and curved morphology of the walls observed on canyons II and III could have resulted from slumping or sliding. The asymmetrical V-shaped small valley observed in canyon IV on profile 02 could have resulted from sliding on the SW part of the wall. Slumping and sliding of canyon walls and sediment spill-over are volumetrically important erosive agents for canyon formation (May et al., 1983; Chuang and Yu, 2002). This feature observed on profile 02, which is in the proximal landward direction probably enhanced the effect of downslope erosion and resulted in widening the canyon on profiles 03 and 04 in the distal direction. Canyons II and III show cut and fill features in the flat layers at the bottom of the canyon walls. The flat stepwise layers of the canyon walls are interpreted as eroded layers that were later infilled by sediments transported from the upper segments or by collapsed materials from the upper canyon walls. Similar cut and fill features have been recognized in modern and ancient canyons. Yu and Chiang (1996) identified cut and fill features in Kaohsiung canyon and McGregor (1981) recognized the same features in the Miocene Wilmington Canyon in the England Shelf. In Canyons I and II, deep channeling shows as Ushaped troughs. This reveals the lower segment of the two canyons where intense erosion had completely obliterated the V-shape of the upper segment. The U-shaped troughs were formed on sediments derived from collapsed canyon walls. Canyon IV displays V-shaped and U-shaped troughs in several cross-sections that also show gentle canyon walls (profiles 01–05). The presence of V-shaped and U-shaped troughs suggests significant down-cutting of the upper slopes immediately below the shelf edge that resulted in the initiation of canyon IV. It has been pointed out that over-steeping of the upper slope near shelf edge results in sediment failure, which is a common process in the initiation and development of submarine canyons (Fare et al., 1983; May et al., 1983; Chuang and Yu, 2002). The complex fills in canyon III are well shown on the seismic profiles and this enhances good interpretation. It was difficult to delineate the fill pattern in canyon I, because only a remnant is preserved. Canyon II displays fill patterns on profiles 01, 02, 03 and 04 at lesser degree as a result of erosion that produced canyon III. Canyon IV is still active and has no sediment infill. High to moderate amplitude, chaotic and hummocky seismic facies in canyon I are possibly indicative of mass movement deposits in the

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form of slumps, slides, and debris flow that are derived from both the sediment failure of the canyons walls and the transported sediment from the upper canyon segment. Seismic facies of the sediments in canyon II are generally uniform and continuous on the profiles. Reflections are transparent to semitransparent with rare high amplitudes and are also barren of chaotic and hummocky seismic facies. This suggests sedimentation under uniform conditions, possibly from terrigenous sources in the absence of gravity-driven deposits. Three types of sedimentation process can be interpreted from the complex pattern of the canyon fill reflectors in canyon III. These reflectors exhibit moderate to high amplitude and thick frequency top lap facies with a gently dipping reflection configuration on the SSW part of profiles 02, 03, 04 and NNE part of 07. This suggests a stable canyon wall with progressive continuous building of sediments into the canyon axis. The canyon wall on the NNE part of profiles 01, 02 03 and 04 is of high to moderate amplitude with semi-continuous, chaotic and distorted seismic reflections. This was interpreted as representing unstable canyon wall that experienced a sediment failure. The sediments were transported by a down-slope mass movement and they accumulated rapidly. Mass-transport processes such as slumping, sliding and debris flow could also have contributed significantly to the deposits. The third category of sedimentation pattern occurs in the topmost portion of canyon III. The reflections are parallel continuous and slightly even. They are highly discordant with the underlying dipping reflection facies. The boundary between them exhibits toplap terminations that developed on the canyon walls (Figs. 6a and b; 7a and b; 8a and b). This facies suggests uniform deposits formed after the canyon had been filled. They probably represent pelagic to hemipelagic deposits formed during a rise in the relative sea level. The present day active canyon (canyon IV) exhibits intense erosion. The erosive activity increases in the offshore direction. The head of the canyon shows fairly Ushaped troughs (Figs. 4a and d; 5a and b) but in the distal offshore area; these U-shaped troughs have been modified to slightly V-shaped troughs (Figs. 6–8). This gradual modification in the trough shape suggests that a significant down cutting has taken place as the intensity increases towards the offshore area.

race formation (Figs. 4a and b; 5a and b; 6a and b) can be attributed to major surges in flow discharges. This suggests that the evolution of Avon canyon has been characterized by large episodic sediment input. We associate such large intermittent sediment supply to the canyon with periods of sea level lowstands. In addition, the canyon could have been coupled to the river systems during the sea level lowstands, and were thus likely kept very active (Antobreh and Krastel, 2006). A fall in the sea level would have given rise to an increase in river gradient (Babonneanu et al., 2002), thus rejuvenating the river systems to transport large quantities of sediments to the canyon head for onward transfer to the deep sea. Burke (1972) suggested that four (Ogun, Osun, Ona, and Shasha) rivers may have fed the Avon canyon with sediments during the Pleistocene sea level lowering. The stages of development of the Avon canyon walls and terraces must have, therefore, been strongly influenced by the flow characteristics of these rivers. The eastern and western re-entrants of the modern delta are areas where opposing longshore drifts converge and generate turbidity currents which produce submarine canyons (Burke, 1972; Petters, 1984). Both Burke (1972) and Petters (1984) suggested that the absence of beach sand ridges along the shore of Avon and Mahin canyons (Fig. 1) indicates that the two canyons are located in appropriate positions to channel the sand down to the fans at the delta complex foot. The valley branch recognized in the bathymetric maps in Fig. 10 supports the fact that sand materials carried from the northward longshore drift on the Niger Delta shore (Burke, 1972) through Mahin canyon are merged with that of eastward longshore drift of Avon canyon where they are probably channeled to deep offshore waters. Seismic facies analysis has substantiated the removal of sediments from canyons I, II and canyon III. Since the long axes of all the four generations of the canyons (I–IV) are oriented towards the same southeast direction, most of these sediments would have been transported and deposited in this direction. In addition, canyons have been known to be conduits for sediment transport into deeper waters (Lee et al., 1996; Poulsen et al., 1998; Pratson et al., 2004). Therefore, these canyons would have served as channel ways for transporting continental clastics during their different developmental stages to the deep waters of the Atlantic Ocean.

7. Sediment transport

8. Summary and conclusions

Although it is difficult to estimate the quantity of sediments that would have been eroded and transported to deeper water in this area, it is clear that a very large volume of sediments must have been removed and deposited somewhere in the southeast offshore deeper waters. These sediments were probably derived from two main sources, namely, (a) localized clastics derived from where the canyons were formed; and (b) terrigenous sediments from the continental areas. The different stages and morphology identified in the wall development, canyon incisions and ter-

Seismic facies analysis in this part of the Benin (Dahomey) Basin in southwestern Nigeria has revealed the existence of three generations of ancient submarine canyons (I, II and III), the last (canyon IV) being currently developed. These generations of canyon show that Avon canyon has undergone several episodes of cut and fill. Canyon I is composed of moderate to thick amplitude, low to high frequency, and fairly chaotic seismic facies. Canyon II is dominated by transparent to semi-transparent seismic facies with rare high amplitude reflections. The seismic facies

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parameters recognized in canyon III are the most variable. Their configurations are oblique to progradational and parallel to sub-parallel. These two facies of canyon III are separated by toplap terminations, suggesting a period of non-deposition. Generally, these reflections are of moderate to high amplitude, moderate to thick frequency, fairly continuous and regular. Interpretation of the seismic facies suggests that the infills of the canyons are composed essentially of terrigenous sediments, which include uniformly thick or alternating beds of shales, sandstones and siltstones. In addition, sediment outbuilding into the centre of the canyons has occurred especially in canyon III with simultaneous rise in the sea level. This probably caused the formation of sigmoid oblique configurations. Subsequent modifications by gravity-driven processes as a result of sediment failure along the canyon walls explain the chaotic sigmoid configurations of the seismic facies. The direction of progradation of sediments suggests that a substantial sediment outbuilding occurred from the sides (NE and SW direction) of the canyon into the centre. These canyons probably had developed as result of variation in the sea level and were maintained by: (i) gravity-driven depositional processes and/or (ii) downward excavation by down slope sediment flows. The widening of the canyons was probably caused by mass wasting of the walls both at the NE and SW parts. Seismic reflection characteristics reveal that canyon III experienced periods of mass wasting (chaotic reflections) and other periods when mass wasting was insignificant (regular reflections). Bathymetric contours, surface maps and seismic profiles show the morphological features of the canyons including deep channel/valley incision, V-shaped valleys, step-wise and spoon-shaped wall development, and canyon orientation in a persistent direction. The axial floor of the canyon displays both simple and complex erosional surfaces. Ages of the reflectors from regional correlation show that truncated sediments include both Cretaceous and lower Tertiary rocks. It was difficult to determine the age of commencement of canyon I (the first generation canyon) as a result of intense erosion, but significant erosion and infilling of canyon II started during the Miocene. Hence, the sedimentary infill of canyon II and III is probably Miocene and represents the younger sediments. Although it was almost an impossibility to infer the sedimentary infilling pattern for canyons I and II because of erosion, the seismic facies expression in canyon III has revealed complex sedimentary infill in the canyon. Gravity-driven deposits have occurred as a result of sediment failure on the walls of the canyons. Generally, the SW and NE walls of canyon III have experienced slight variation in their development. The SW wall was not seriously affected by gravity-driven processes and maintained a slightly stable wall, while the NE was affected, resulting in collapsed walls. The orientation of all the canyons is towards the southeastern offshore part of the basin and these canyons were most probably conduits for sediment supply to deepwater areas of the Gulf of Guinea.

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