w

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The continental m'argin fro a Pålmas to Lagos: bottom sedi and submarine morphology

L. Martin

. .

, .

ORSTONI Fonds Documentaire PI0 E &!I

.- I , ..- . -

cote 9 D

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CONTENTS

I

ZENERAL

kJ.O~PHOLOGY

The Coa&

The Continental Shelf

The Continental Slope

The Trou Sans Fond 3EDIMENTATION

Continental Factors,

Oceanic Factors Sedimentation in the Trou Sans Fond

Late Quaternary Palaeogeography and Palaeoclimatology

Sediments of the Ivory Coast Continental Shelf

' ,

t

Page

Sediments of the Continental SheLf off Accra

Sediments of the Continental Shelf of Togo and Dahomey

,

CONCLUSION

REFERENCES

'ig. 1

'ig. .? 'ig. 3

'ig. 4

'ig. 5

'ig; 6

'ig. 7

'ig. 8

1LL;USTRATIONS .

83

83

8 3. 83

' 82

84 86

86

86

86

86

88

92

93 93

93

Coastal morphology

Continental shelf from Cape Palmas to Lagos

Continental shelf and slope off Abidjan

Longitudinal profile and transverse profiles of the Trou sans fond

Currents along the Gulf of Guinea coast

Location of silty areas on the Ivory Coast continental shelf

Influence of the undercurrent on the present day sedimentation

Distribution of pellets on the Ivorian continental shelf

e-

84

85 85

87

89

89

91

9f

81

SUMMARY

. From Cape Palmas to Lagos the morphology of the continental shelf is the only feature known with some accuracy. slope has been studied in the Ivory Coast from Sassandra to Grand Bamam. Chemical, granulometric and mineralogical analysis of eight hundred grab samples and a hundred cores, four radioearboa dathgs on peat and calcareous algae nodules, and a study of mineralised faecal pellets have been executed on the Ivory Coast continental shelf. In other areas only a few short studies have been made. slope have not been studied.

The continental

The sediments of the continental

SOMMAIRE

~e setfi trait connu avec quelque pre'cision entre Cap Palmas et Lagos est la morphologic de la plate-forme continentale. On a Gudie'la pente continentale de la Côte d'Ivoire de Sassandra à Grand Bassam. L'analyse chimique granulome'trique et minéralogique de cent carottes et de huit cent écliantillons obtenus en se servant des bennes, quatre ddterminations dfâges de tourbes et de nodules dtalgues calcaires effectue'es par la méthode radiocarbong: et une &ude de coprolithes min6rdise's ont éte' realisés sur la plate-forme continentale de la C a e d'Ivoire. D'autre p5d on n'a fait que peu d'&des de courte durée. On nra pas étudiéles sediments de la pente continentale.

ZZTSAMDJXNFASSUNG

Von Kap Palmas bis zu Lagos ist die.Morphologie die einzige Eigentümlichkeit, die man einigermassen genau kennt. Kontinentalschelf bei der Elfenbeinküste von Sassandra zu Gross Bassam studiert. granulometrische und mineralogische Analyse Über 800 Greifproben und hundert Herne, vier Radiokarbondatierungen auf Torf u n c i Kalkalgenknollen gemacht, zusammen mit Analyse Über versteinerte Koprolithe. In anderen Gebieten hat man nur einige kurze Untersuchungen gemacht. Die Sedimente des Kontinentalschelfes sind nicht untersucht worden.

Man hat den

Man hat auf dem ElfenbeinkGstekontinentalscheLf chemische,

82

The continental margin from Cape Palmas to Lagos: bottom sedimerits and submarine morphology

L. MARTIN '

GENERAL

The area studied is located on the West African coast in the Gulf of Guinea. It is bordered by the Ivory Coast, Ghana, Togo, Dahomey and a part of Nigeria.

the continental shelf off the Ivory Coast are now well known. known from Sassandra to Grand Bassam, but no studies on sediments have been made.

Short studies have been undertaken on the continental shelf off Accra in Ghana with a view to studying the benthonic fauna.

describes the continental shelf off Togo and Dahomey .

T(e continental margin off Nigeria, from the Dahomey t o Lagos, has not been considered in this report.

Both the sedimentology and morpholpgy of

The continental slope is

A map has been constructed which briefly

MDRPIIOLOGT c' The major part of the contiguous contineat

is formed of Precambrian rocks. cpast Palaeozoic rocks occur only in Ghana, in two small areas, Mesozqic and Caenozoic rocks occur in the Ivory Coast and Nigerian coastal basins. investigations have shown the existence of a very narrow basin which contains a sedimentary series. over 5000 m thick and is c,ut by an east-west fault the throw of which reaches several thousand metres. rocks of Upper Jurassic to Lower Cretaceous age lie directly on Precambrian basement; the top of the seqhence consists of Quaternary sands which cover the basin almost entirely {De Spengler and Delteil, 1966).

The history OP thB Nigerian sedimeritary basin begins with Middle Albian,deposited in a subsiding basin {Reyment, 1966).

On the Ì-

In the Ivory Coast petroleum

This series is probably

In the sedimentary sequence

The Coast

The coast from Cape Palmas to Fresco consists of a Precambrian metamorphic series with predominant gneiss and some granitic and basic intrusions (Fig. 2 ) . consists of a plateau dissected by Plio-Quaternary erosion. The coast cuts across these different units and seems to follow recent faults, as seen at Grand Bereby {Tricart, 1957). The coast is

,

The hinterland

.rj

a3 T+

. . formed of successive rectilinear west-souih- west trending sections, separated by capes jutting out on north-south axes. Precambrian hills a re frequently separated from the shore by a sniall coastal plain, the width of which seldom reaches even a Pew hundred medres.

subsiding area which corresponds io the Ivory Coast sedimentary basin. and sandy with lagoons.

From Asim to the Volta River mouth the coast cut8 the granitic o r schistose Precambrhn metamorphic series and two small Palaeozoic outcrops of sandstone and schists.

From the Volta River mouth to Lagos the coast l ies on the western part of the Nigerian yedinientary b a s h lagoons.

The

F r o m Fresco to Axim the coast lies in a

The coast is low

It is low and sandy, with

The Continenla1 Shclf Along the Ivory Coast the break between the

continental shelf and the continental slope OCCUTS between 120 and 130 m, and in Togo and Dahomey between 85 and 110 m (see 'Fig. 2). The continental shelf between Cape Palmas and Sassdadra increases in width from 1 2 to 1 9 nautical miles, then decreases to 10 miles off Abidjan. Here the continental shelf is broken by a large canyon, the Trou sans foiad. The width of the continental shelf increases to 45 miles off Cape Coast and decreases to 10 SnYles off the Volta River mouth. mouth of the Volta River is a large submarine delta where the 20 m line reaches the edge of ,

the continental shelf. River mouth and Lagos the continental shelf ranges in width from 12 to 15 miles.

At the

Between the Volta

Except for the Trou sans fond, and the submaririe Volta delta, the continental shelf is generally smooth and featureless., there a r e narrow rocky banlrs which a r e unbroken fo r considerable stretches. always parallel to the coast and in function of their depths can be classed into three groups.

The first group of banks occurs at a depth of 55 m. ln the Ivory Coast it ex%ts between Sassandra and Grand Lahnu, disappearing at either end beneath recent muddy sediments. In Togo and Dahomey there is a similar reef.

The second group occurs at depths of

However,

They a r e

I I i rocky comt j, low and m d y Coast with l a p o o ~ rocky coasi iar and sandy Most with

lagoon$

Fig . 1. Coastal morphology i

80-30 m., In the Ivory Coast it is 5-ti m high and is practically unbroken from Abidjan to Cape Palmas. It is sometimes covered by recent muddy sediments.

The third group occurs at the edge of the continental shelf. Allen (1965) has described analogous banks in Nigeria.

These batiks a re constituted of a coarse shelly sandstone and a r e probably beach rocks eormed during a regression.

In the lvory Coast the transverse profiles a r e convex in silty areas and rectilinear and even concave in areas where there has been no deposition.

The Continental Slope The only area where the Continental slope

is known in detail is the lvoryoCoast (Fi,. 3). Here soundings made from 03 40' to O4 30'W in 1964-65 by the Service Hydrographique d e la Marine Francaise and from 04O30' to 06°00'W in 1968 by the ERAI?> oil company. Zan be distinguished.

:haracterised by the presence of the Trou s'ans Sond. There a re also several canyons which start on the edge of the continental shelf and Zontinue beyond 2000 m. These canyons a r e mobably satellites of the Trou sans fond.

The second area from 04'35' to 0fi0OOLW, :ontains no canyons comparable with the 2 àans fond.. The upper part of theí continental slope is very irregular and transverse profiles

Two areas

The first, from U3°408 to 04°351W, is

I_ .

84

cut many small canyons. The gradient ranges from 02°30' tQ 14'30'. show a break at depths of.1400-1600 m $n the

'east, and at a depth of I200 m in the west. In the lower part, the gradient ranges from 2' to so. In this zone tl1e transverse profiles cut n? canyons. The continental slope gyadient is higher in the wtrst than in the centre and in the east, and the width of the continental slope (between the I00 and 1500 m isobaths) ranges from 9 k n ta '20-23 km. The continental slope is narrowest where the continental shelf .is widest.

The Trou Sans Fond

The radial profiles

This originates in the neighbourhood of the shoreline at a depth of 10 u i and deeply cuts

'

the continental shelf. Its head has two branches. a principal branch with a north-north-east direction and a secondary branch with a north- west direction. The canyon axis successively folZows these directions which a re those of faults in the sedimentary basin. At the canyon head the talweg declivity attains 12 per cent but quicMy.decreases to 3 per cent. A steepening to 8 per cent occurs near the shelf break followed by a decrease to 3.5 p,er cent across the upper continental slope. *Although 'it is deep at the beginning, the canyon rapidly widens and on'the shelf-break ï$s width and relief a r e 12 km and 900 m. The transverse profiles a re V-shaped; the walls are: symmetrical and their gradients range from 30 to 35 per cent. When the valley widens: the gradient dec rea~es ; at depths of 2700 m, icis 3 . 5 per cent (Fig, 4).

l i P

. .

. .

6"Od W. 4"OO'W. . 2 ~ 0 0 ~ V ~ . 2 2*'00'E. I

Fig. 2. Continental shelf from Cape Palmas to Lagos : . .

s'3cíw. se1oO'w. 4"30'W. 4"OO'W.

"..

SEDIYEN TATION

Continental Factors This countxy has a low relief .which seldom

reach& 500. m. high throughout and the temperature is always high. chemical and biochemical processes cause a very strong weathering of all rocks. ' But the dense tropical forest which covers the .entire south of the' country, excepting the,,,,cen+al eastern párt where the forest-sai.rar&ah mosaic grows, prevents the drainage of the coarse weathering products. Only the disfiolyed materials and fine particles, carried in suspension by rivers, reach. the sea. However, the Volta River which has built a submarine delta must carry important quantities of sertiment.

The mean annual rainfall is

The climate is hot and humid' and

Oceanic Factors The tidal fall is very small. and the tide is

in phase on the Gulf of Guinea. This explains the weakness of the tidai currents. Ivory Coast and Ghana, on the continental shelf, there is a system of currents with opposing directions (Lemasfion and Rebert, 1968)

In the

(Fig. 5) .

A SuperficialJayer flov7s from west to east along the coast of Africa. partly from the Canary and riorth equatorial currents and partly from the tropical counter current. and its velocity is O. 5 knots.

midercurrent flowing from east to west. Its mean velocity is 0..1 knots in depths of 30 m atid O. 5 knots in depths of 40-50 m,

Waves approach the share from a soutli- westerly direction and are refracted and translated into a heavy surf near the shore. In theory the longshore transport should be important from Cape Palmas, but the beach morphology teiids to prevent transport taking place. At S n Pedro'it has been estimated at 200 O00 myyear. The greatest transport (ma o00 m3/year) it; from Fresco t o .the Trou 6ans fond. Then it decreases and there is no transport at Cape Three Points. Then the cycle begin6 again: there is a very little ,

transport at Takoradi, a moderate amount a;t Sekondi and a large amount from Accra to Lagos (Varlet, 1958);

It is derived

Its thickness varies froin 10 to 20 m

Under this superficial current ,there is an

SedimentaLion in the Trou Sans Fond The. canyon is located at a point where there

is a change in the coast line cliGectj.on (Fig. 4). \IE7est of the canyon, the angle that deep-\.vatep waves m.&e with beach i s 32O and this produces a transport of !O0 O00 m3/year {the longshore

86

transport was determined when the Vridi sea-wall was ,built). the angle i s only 1 4 transport is 400 000 mS/year. of sediment trapped in the canyon is 400 O00 m3/year (Varlet, 1958). These conditions ' .' favour both the mas6 movement of sediment and turbidity currents. There is evidence o f slumping at the head of the canyon (two wharvetj were destroyed in 1905 and 1909). Cores taken on the canyon axis show an alternation OP sandy and silty beds.

In thei east and the lorigshore

The volume

Late Quaternary Palaeogeography and Palaeoclimatology Radiocarbon dating and pollen analysis

o f two peats have enabled the sea Level and climate during their formation to be determined. The stratigraphic setting of the first, terres trial peat shows that it was formed during a regressivn when the sea level was 65 m lower than at present: it lies directly on a marine gravelly sediment containing shallow water shells. This peat was dated 23 000 k 1000 years B. P. (GIF, 114C). It contains a savannah flora which is typical of a dry climate with a rainy season. Clayey sand materials have covered a large part of the continental shelf (Martin and others, 1970). The maximum of the regression, which probably went beyond 100 m, has left traces on the edge of the ivory Coast continental shelf. 70 m and the edge of the continental shelf there is an organic facies containing coral in the process of silting; this was probably developed at the beginning of the last transgression in relation with the rocky banks. This formation is distinguished by the presence

In fact between

of calcareous algal nodules and Amphistegina. This is a large foraminifera, the protoplasm of which has symbiotic zooxaithelkae. present Amphistegina live in shallow waters of coral areas. calcareous algae but Amphisteffina i s found in the shallow depths of the western rocky Ivory Coast continental shelf. taken at depths of 100 m were dated at 1 3 O00 k 250 years B. P. (GIF. 1449) and 1 2 900 250 years (GW.1509). It is likely that they* were formed under 20 10 m of water. We can assume that 13 O00 years ago the sea level was 80 2.10 m below present sea level. Moreover, we can assume that the physical conditions, particularly water transparency, ìvere different from at present.

it to be a mangrove peai. shallow marine sediment, formed, during a transgression when the sean level was 62 m below the present one. It has,

At

Today there are no

Two nodules

Pollen analysis of the second peat shows It is covered by a This peat was

.c

b

, 3 b O

O I 2 3 4 6 Nautical milas -

. .

I

Fig. 4. Longitudinal profile and transverse profiles of lhe Trou sans fond ,

87

. .

been dated at 11 900 IE contains Rhizophora pollens, many fern spores and some pollens of the present forest. can assume that at this time there was a damp climate with a forest less dense than at present. Ages and depths a re attuned to those found in many parts of the.world and particularly in Senegai (Faur& and Elouard, 1967).

250 years €3. P. (GIF. 1146).

We

Sediments of lhe Irory Coast Continentat Shelf The most recent sediments, that is, those

later than the last transgression, lie on a substratum of clayey sand between 0 and 70 m and on a coral substratum beyond thdt depth. The Ivory G0as.t continental shelf is characterised by the presence o f silty areas overlying this substratum and areas where the substratum is only slightly silty o r not at all. 'Sand and coral formations have been found under several. metres of silt in many cores taken on the periphery of silty areas.

Tlic Substru.tuni

:\ledium yellotp-ed smid

Its median ranges between 650 and 2 5 0 ~ ~ with quartz grains covered with iron oxides, sonie ovoid pellets and shell fragments. Generally$ occurs from O to 30-40 m, but it is covered from 10 to 20 m by a fine, dark sand. Beyond 40 m, it is covered by silty sand, sandy silt or silt,

The coral ionvation.

continental shelf. It i s formed essentially of dead species -. Amphistegina, bryozoa, coral branches, calcareous algae and shell fragments - and the CaC03 content of the sediment may reach 70 per cent. some areas where the formation is covered by several metres of silt.

This stretches from 70 m to the edge of the

Silting is in progress in

' The Ovetlying Formations

Si l ty areas

30 and 70 m from Cape Palmas to Sassandra (Fig. 6). San Pedro, Cavally and other small ,rivers which empty into the sea in this part of the

.

coast. The undercurrent, flowing westwards, plays a large part in the distribution o4 these sediments which do not occur at 'all east of the Sassandra river mouth.

Associated with the Bandama River there is a large silty area ivhich stretches from , . 20 m to the edge of the continental shelf. Its thickness is greater than 15 m. i n shallow water where the undercurrent 'does not occur the sediments cawied by the. r iver settle at the ~

east of the mouth. Beyond 3'0 m, where the undercurrent occurs, the sediments settle

Silts less than 10 m thick occur between

They a6e connected with the Sassandra,

westwards (Fig, 7). A further large silty area is related to the Comoe River. The greater part of the sediments lie west of the rives mouth. Its thickness is close to 20.m.

Ftne dark sond

Sometimes it is slightly silty. It is extremely well sorted and its median is generally 9 0 ~ . Its components a re very small quartz grains with some micas and brown ovoid pellets. It forms a strip from 10 to 20 m depth. and overlies the middle-size sand.

The silty samis and sandy silts The sands are limited'by silty sands and

sandy silts. The distribution of these two formations depends on whether the area is silty o r not.' The coarse portton of the sediment is formed of quartz grains, ovoid pellets and shell fragments .

Formations with Ovoid Pellets

Ovoid pellets are present in nearly all the sediment samples taken on the Ivory Coast continental sNelf, in concentrations ranging from less than 1 per cent to almost 300 per cent of the sediment. They occur in a variety of colours and degrees of induration, from grey and soft to dark green or dark brown and hard. These pellets may be considered as the faecal production of bottom burrowing o r filter feeding o'rganisms (polychaetes, gastropods, echiurids) according to their s,imilarity in size and shape with fresh faecal pellets. It is likely that these pellets originally contained clayey material consumed and secreted by animals and agglutinated by organic mattes. Such early, practically unaltered, material is likely to be represented by numerous grey and soft pellets found in. some cores. I f sufficient oxygen is available in sea water the organic material is oxidised and disappears from pellets. If less oxygen is available locally a reducing environment may be maintained in which mineralisation may occur. Mineralised pellets above, and grey soft non-mineralised pellets below, can be seen in the sanie care. Mineralisation of pellets is favoured by a moderate rate of clay deposition (Cloud, 1955); also, i f mineralised pellets are formed in situ, we must suppose that the conditions of sedimentation have changed during deposition of the same Silty layer. This modidcation m.ay be caused by the removal of the sediment source during the last transgression.

'-

-

D;stribution We can distinguish two large ,proups (see

Fig. 8). Shiny brown and,byownish green pellets, smaller than 2506, occur in shallow depths between 20 and 30:m. 'At greater depths there are pale green and earthy or

'

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u

601

5

4

m O s -I

,

I . up welting orsa I UP wsninparm. : i

. ,

__c. Guinean currmt

4 I50 Nautlcd milrm

Fig. 5 Currents along the Gulf of Guinea coast

w . 6 O .

. .

40 30

rondy allt . . . 1..

I '

' I J

Fig. 6 Location of silty areas on the Ivory Coast continental $her€ ' C d

. I

89

dark green glassy pellets larger than 250~.

Pellets of the first type a re numerous (more than 50 per cent of the sediment) to the west of Assinie, off Grand Bassam, Grand Lahou and Sassandra. Pe1let;s of the second type *

occur in large numbers from 70 m to the edge of the continental shelf off Grand Bassam and Grand. Lahou, Except in very silty and coral areas they occur in a l l sediments in verying amounts.

Composition of peflets Pellets of a similar nature @ve been

studied in Gabon by Giresse (L965-1966), in Nigeria by Posrenga (1967) and in Guinea by 7,7011 Eaertner and Schellman (1965). In Gabon and Nigeria sediment with goethite pellets occurs at shallow depths. In Gabon, @iresse has sl>own that the green pellets taken at a depth of 60 m are made of chamosite and a

mixed layered mfneral (v&smiculite- chlorite), In Nigeria, to about 60 m depth, the pellets a r e constituted of chamosite. At greater depths the green pellets are constituted of a highly expandable montmorillonite- like variety of glauconite which is thought to represent a very early stage iii its diagenetic development to the non-expandable micaceous type. Upon glycolation the basal spacing of the Nigeria ‘pso$o- glauconite‘ component shifts .. ,,

to about 17 A indicating full expandability (Porrenga, 196?).

resembles that of iron-bearing chlorite, The poorly ordered chainosite ha.s a fairly high fer rous iron ahd magnesium content and a low aluminium content. The Fe0 content of the Niger chamoaita is about 17 per cent. ferr ic i ron may be present as goethite with which chamosite is often associated.

Chemically the composition QE the chamosite

’

Some

TABLE 2 . CREJMICAL CO$$POSITION OB PELLETS FROM MGERLA. AND LVORY COAST

A B c D E F G

si02 43.87 49 .65 5 2 22.68 27,70 34.40 44.70

25.36 14-04 8 9.14 7.50 13.00 11.00

9.36 17.30 3 $6.32 42.90 33 .00 ’ 27.00 , A1203

Fe203 FeO 2.87 ’ 17 1.00 1.87 ’ 0.53 o. 7 7

Ca0 O. 55 ,1.46 O. 5 , 1.98 3 . 3 3 ’ 1.71 1.29

nxgo 1.40 3.38 8.4 2.92 4. 39 2.55 2.34

O. 28 0.21 o. 3 O. 24 o. 30 o. 33 O, 59 Na20,>*

520. T i a Z 1.09 0.65 0 . 4 0. 79 ‘ O . 66 u. 70

1 .29 2 .62 i 0 . 5 o. 58 o. 41 o. 76 2.29

0.70 ;

MnO o. o5 0.05 0 .2 o. 13, O. 51 II. 21 O, 16

O. 24 o. 18 2.64 o. 2 0 O. 16 o. 20 p2°5 Ignition loss J,5.?3 8.36 11.4 12.25 11.70 10.60 6.30

. TotaJl, 101.2 100.78 101.7 101.15 101.5’7 108.03 99.24

A. Sample of the matrix containing pellets (Porrenga, 1967).

B. Sample of.Niger delta ‘proto-glauconite‘ {Porrenga, 1967),

C. ‘:Sample of Niger delta chamosite (Porrenga, 1961).

D. Saniple of Niger delta brown pellets (Porrenga, 1967).

B. Sample of Ivory Coast brown pellets t a L p at 30 m. ,

F. Sample of Ivory Coast gre& peLlets taken at 6 0 m.

G . Sample of Ivory Coast green pellets taken ai: 120 m. . s

90

loo e'- . .

Fig. 7 . Influence of the undercurrent on the present day sedi&entation

P-

n

I '

I

ABUJA

lj

# .

mors thon SO.%

batwen 50 and 26 O/'

bstwrsn 25 and 5 YO

'

$%& I I

Fîg. 8. Distribution of pel'lets on the Ivoriali continental shelf

93. .

" I

I . .

Chemically, the difference between the pellets and the clay matrix is striking: a much higher iron content, a higher potassium and

'These results suggest that it consists of kaolinite, illite, and a mixed layer (vermiculite- Chlorit e) structure.

magnesium content and a lower aluqlinium content characterise pellets. In the Ivo ry Coast and in Nigeria the brown pellets (D and E in Table 1) bave the same composition. In the Ivory Coast we do not find samples with a high Fe0 content such as the sample of

X-ray analysis of sample I?, after Mehra- Jackson tseatment, gave results which indicate that it consists of kaolinite and a mixed Layer vermic culi te-chlorite) structure.

chamosite from the Niger delta, Sample B {in Table 2), which has a IC20 content greater a composition like that of sample F. than 2 per cent, has a similar composition to the Niger delta 'proto-glauconite'.

break the 7.14 A reflexion shows that there is X-ray unalysis of samples no chamosite but kaolkite. After glycolation

the basal spacing does not shift to about 17 .& the matrix, consists of kaolinite, montmorillonite as in N i g e r delta samples. and illite. Scdimcnis of the Continental Shelf off Accra

Results of analysis of sample G suggest

The treatm$nt with 2N HC1, which does not - '

This showed that Sample A (in Table 1) '

Sample D was formed of goethite and in some cases chamdsite,

'rhe texture of sediments found off Accra are of €our main types and can be described briefly as follows (Buchanan, 1958). Starting - .

Mehra- Jackson treatment, gave the results shown in Table 2,

X-ray analysis of sample E, zifter from the inshore sediments and passing to deeper water, there is first'of all a narrow strip of very fine clean sand which is extremely

TABLE 2. X-ray analysis of samples E, F add G after Mehra- Jackson ti-eatment

. Untreated

Å 13.50

9 .96

7 . 1 4 Sample E 4 .91

3.55

3 .33

12.21

7 ,17

Sample l!' 4 . 2 4

B. 57

3 . 3 3

, 11.79

7. 1.6 Sample G

3 .56

. 3.52

~ g c 1 ~ . ~ g C i ~ + - ~ i p

A Å 14 ,65 14+65

9. 9F 1 0 . 0 6

7. 1.2 7:14

4 . 9 1 3. 55

3: 32

14 .65

7 . 1 4

4: 23

3 . 5 7

3 . 3 3 '

14.24 7 . 1 2

3 . 5 5

3 . 3 2

14.65

7 . 1 6

4. 24

3 .55

3. 33

1 3 . 5 0

1 2 . 9 8

9 . 9 1

7 . 1 4

92

1y KC1

Å 14 .05

9. 96

7 .16

. 3 . 5 5

3 .33

12 .66

7 . 1 4

1 2 . 6 6

7 . 1 4

3 .56

3 . 3 3

490° 2NHC1 o A

9.93 The 7.14 Å 3. 31 reflexion

is not

broken

9 . 9 3 The 7:14 A 4 . 2 3 reflexion 1 .

3.'32 is not

broken

9 . 9 3 The 7.14 A -

4 . 9 3 'reflexion

3. 56 is not

broken

. _ .

4.

* t

7r

well sorted and that stretches from the shore to a depth of about 15 m. As wave action becomes weaker, the sediment becomes progressively silty and a s t r ip between 15 and 40 m deep consists of sandy silt, less sorted "chan the inshore s w d . material begin$ to be apparent in the sediments, so that between 40 and 50 m the deposits can be termed sil ty sands. Below 50 in the sediment becomes increasingly coarse. By d efinitio n i t is sand but it is very different from the shallow water fine sand and is relatively poorly sorted. Towards depths of 100 m th? proportion of coarse material becomes greater and rock fragments appear in most sediment samples. This progressive coarsening of the deposit is accompanied by a. rapid rise in carbonate content. An examination of rock samples from the ouler shelf has shown that it cobsists of a sandy marine limestone, made up largely of shell fragments, foraminifera,l echinoderm spinec. and other ca l ca~eous elements together wLtli quartz grains.

Sediments of tlie Goniinenta1 Siiclf ot Togo and Dahomey In shallow depths there a r e three types of

Below 40 h coarse

formations: to the east OP Grand Popo and Cotonou the bottoni is rocky; off lagoon mouths the sediment is a silty sand; elsewhere there is a coarse and even gravelly sand which extends f r o ~ n irhe beach to 35 m depth.

a stfip of silty sand. of the continental. shell in the east there is a silty bottom and in the west a sandy silt bottom. Between 52 and 55 m deep there is a coral reef which is cut between Grand Popo and Ouidah. Beyond this reef many rock outcrops emerge from the silt (Crosnier and Berrit, 1966).

At greater depths, Ïrom 35 to 55 m there is From 55 m to the edge

COYGLUSIOY Prom Cape Palmas to Lagos the cdntinental

shelf ranges in width f r o m 10 to 45 nautical miles. and 130 m. off Abidjan and th9 submarine Volta delta, t h e continental-shelf is generally smooth and featureless. The continental slope is only fenown in the Ivory Coast. Close to the Trou sans fond several canyons which s tar t on the edge of the continental shelf, coQtinue beyond 2000 m. They are probably satellites of the Trou sans fond, In the west, the upper part of Lhe slope is very irregular arid the transverse profiles cm many small canyons. There is a break on the slope at depths of 1400-1600 m in the east and 1.200 m in the west.

In the Ivory Coast the present sediments settle from O to li0 m on a sandy substratum,. and beyond on a forrnation containing coral. The late recession and the subsequent

The shelf break occurs between 100 Except for the submarine canyon

-

transgression have left some traces on the continental shelf. Ivory Coast mineralised pellets analysed do not contain ohamosite a s the Nigerian samples,

and Dahomey continental margins.

ALbEN, J. 'R. b. lg62. .

The first samples of

There is little data on the Ghana, Togo,

R El7 ER ESC ES I-Io'locene coral banks

and subsidence in tlie Niger delta. Jnl Geol., V O X . 70, NO. 4, pp. 381-397,

1964. The Nigerian continental margjn: bot.tom sediments, suhmarine , morphology and geologic al evolution. R h r . Geol. ; VOLI , No. 4, pp. 289-332.

and adjacent areas: sedimentary environments 19 6 5 . Late Quaternary Niger delta

and lithofacies. =Am. Ass. Petrol.Geol. Vol. 49, NO. 5 , pp. 547-600.

BLACHE, J. 1941. Wn foss4 sous-marin le "Trou sans fond" Abidjan. Revue Ggogr. ale. -. VOL. 29, NO. 4, pp. 721-125.

LE-BOURDIEC, P. 1958a. Contribution 3 1' &tude gebmorphologiyue du bassin s&irnentaire et ,des regions littdrales de la Côte d'Ivoire.

plio-quaternaire en basse C a e dlIvoire. Rev,. Ge'amorph. dyn., Vol. 10: pp. 33-42.

BUCHANAN, J. B. 1938. The bottom fauna communities. across the continental shelf

Etud. &burn., Vol. 8, pp. 7-96.

1958b. Aspect de la 'moxphoge/n&e

-

off Accra (Ghana). Proc. eoolog. Soc. Lo*, Vol. 130, pp. 1-57.

BURST, J. F. 1958a. "Glauconite" pellets: their mineral nature and applications to stratigraphic ititerprutatîon. . EulZ. Am. As s. Petrol. Geol., Vol. 42, No. 2, pp. 310-327,

glzuconite pellets. 1.958b. Mineral heterogeneity in

Jnl Min. Soc, Am. , Vol. 43, pp. 481-487.

CAILLERE, S. -and GIRESSE, P. 1966. Etude min&*alogique de diverses glauconies actuelles. dea minerais de f e r se2imentaire (Gabon,

Nouvelle contribution à la genèse

Gallice, Bretagne)'. C. r. hebd. Se'anc.Acad. Sci., Paris, Vol. 263, No.23, pp. 1804-1807.

LE CALVEZ, Y. 1963, Contribution 'a l 'dude des forams de la re'gíon d'Abidjan. Rev. Micro. F r . G . , No. 1, pp. 41-50. '

I_

CILOTJD, P. E. 1955. Physical limits of glauconite formation. Geol. , - Vol. 39, No. 4, ppr98;Q-492.

Bull, Am. ASS. Petrol.

CROSNIER, A. BERRIT, RIT, G. B. 1.966. Les fonds de pêche le long des côtes des a.

'3.

4 93

. . _._ .

rdpubliques du Dahomey et du Togo. C 2 . océanogr. , Vol. 4, No. 1. Supplement. ) 144 pp,

fora.minif&res, s u r l e rebord du plateau continental 'au large de la Côte d'Ivoire. D. E. s. No. 55 Laboratoire d e gehlogie, Faculte' des sciences, Dijon, France. 75 pp.

Le bassin secondaire-tertiaire de Côte d'Ivoire, Afrique Occidentale, Pg. 96- 99 j33

Symposium s u r les bassins se'dimêntaires7u littoral africain; le littoral Atlantique. (Ed.: D. Reyre. ) I. U. C . S. - A. S.G.A.

(ORSTOM

DESBROSSES, J. ' 1966. Les associations de

DE SPENGLER, A. and DELTEIL, J . R. 1966.

DIETZ, R. S., KNEBEL, N. J . and SOMERS, L. H. 1968. Cayar submarine canyon. Bull. geol. Soc.Am. a Vol. 79, pp. 3821-1828.

FAURE, H. andELOUARD, P. 1967. Sch&na des variations du niveau d e ltoc&n Atlantique sur la c6le de l'ouest africain depuis 40000 ans. C. r. hebd. Se'anc. Acad. Sci., Par is , Sér. D, voi. 265, pp. 784-785.

GIRESSE, P. 1985a. Oolithes ferrugineuses e n

C . r. hebd. Se'anc.Acad. Sci., Paris,

196513. Observations sur la pr&ìence

voie de formation au large du Cap-Lopez (Grabon). Ser .D . , Vol.260, pp. 2550-2552.

d e "glauconie" actuelle dans les sgdiments ferrugineux peu profonds du bassin gabonais. C. r. hebd. Séanc. Acad. Scî. , Paris., Se"r. D, Vol. 260,. pp. 5597-5600,

GUILCHER, 6. ,1961. Ocebnographie c6tik-e

HOWER, J . 1961. Some factors concernbig

en Côte d'Ivoire. Norois, No. 29, pp. 103-105,

the nature and origin of glauconite. Am. Miner., VOl.46; NO. 3/4, pp. 313-334.

LEmSSON, L, andREBERT, J .P. 1968. Observations de courants s u r 1: plateau continental ivoirien. Mise en evidence d'un . sous-courant. Docum. scient. prov. No. 022, Cent. Rech. oc6asogr.Abidjan. 66 pp.

LElVEOP, N. 1962. Les pseudo-Oolithes ferrugineuses des plages de C?Ìte'dtIvoire. c l . , No.5, pp. 142-146.

R;LPIR.TIN, L,. 1969a.' Introduction l'Aude ggclogique du plateau Continental ivoirien., Premiers r&kats. ~ o c u m . scient. prov. I No. 054. Cent. Rech. oc6anogr.Abidjan. 163 pp.

\

1969b. Datation d e deus tourbes quaternaires du plateau continFntal ivoirien. Communication au VI11 congres de 1'INQU.A Paris. C. r. hebd;Sdanc. Acad. Sci., Pa.ris. Vol. 2fi9, pp. 1935-1921.

1970. Premières investigations sur l'origine du "Trou sans fond", canyon sous-marin de la CÕt6 d'Ivoire. C. r . hebd. S6anc .&cad. Sci. , Paris, Sgr. D,- Vol. 270, pp. 32-35.

TASTET, J,'-P., ASSEMIEN, P. and FILLERUN, J. -C. 1970. Le quaternaire d e la zone littorale de la Côte d'Ivoire. Communication au VI11 Congr& de 1IINQUA Paris . BulL..ASEQUA, No.25, pp. 1-14,

relation lo marine sediments. in 11. Symposium on recent marinle sediments. (Ed. : P. D. Trask. ) (American Association of Petroleum Geologists: )

NIZERY. 19 49. Etude hydrographique et ocganographique du "Trou sans fond". Com. Cent. occ'anogr. ClÕtes Bull. %s., Vol. 1,

PELLISIER, L. 1942. Note sommaire SUI' les

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MOORE, II. B. 1939. Faecal pellets in Pp. 51 6- 524

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NO. 7, pp. 13-19.

travaux exe'cut6s dans la r6gion du "Trou sans fond". Instruct. tech. No. 204, Sem. Hydro. Marine, Paris, Annexe 2.

and geochemistry of recent marine sediments in tropical areas. Drusk. Stolk-Dordt. )

floor of Southern California. Pp. 97-119 i n Essays in marine geology in honor o f K. O. EMERY. (Los Angeles: University of Southern California Press. )

1938. La gsologie de la région d'Abidjan. Bull. Con. Etud. hist. scient.Afr. ,

occid.fr., Vol. 21, No.1, pp.1-19.

RANCUREL, P. 1968. Topographie generale du plateau continental de la Côte d'Ivoire et du Liberia, 8 pp. (Paris: ORSTON. 1

of the Nigerian coastal basin. Les bassins sédimentaires du 1itl:oral afrigaTn. , (Ed. : D. Reyre.) .I. U. C Í , S, -A.S.G.A.

1957.. Aspect et problkmes géomorphologiques du littoral occidental

PORRENGA, D. H. 1967. Clay mineralogy

145 pp. , (Amsterdam:

PRATT, W. L. 1963. Glauconites of the sea

I

PRUNET, J.

/ /

RET*'MENT, R.A. 1966. Sedimentary sequence Pp. 115-142'i.n

TRICART, J .

de la Côte d'Ivoire. Bul1,Inst. fr. Ah.. noire, S&.A, Vol. 29, No. I , pp.1-20,

1962. Etude g6ne'rale d e la desserte "

portuaire d e la "Sassca", portuaires, leurs caracte'ristiyuen' morpho- dynamiques et leur possibilite* d'am&agement.

Les sites

' Bull. C. O.E. C., VOL. 14, NO. 3, pp. 146-161.

NO. 2, pp. 88-97;

94

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VARLET, F. 1958a. Les traits essentiels der Halbinsel ICaloum, Guinea. Tschermaks du régime côtier de l'Atlantique prês d'Abidjan.

pp. 1089-1102. '

près d'Abidjan. Etud. éburn., Vol. 7,

miner.petrogr. Mitt., III, 10, No. 1-4, Bull. Inst. fr. Afr. noire, S6r. A, Vol. 20, No. 4, pp. 349-367.

I

1958b. L? rggime de l'Atlantique

Centre de Recherches Oce'anographiques, ORSTOM, B. P. V 18 - Abidjan (Ivory Coast).

pp. 101-220.

VON GAERTNER, H, R. and SCmLLWNN, W. 1965. Rezente Sedimente im Kiistenbereich

'i;

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. I

I /

,

. .

95

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f .

. . 8

NATURAL ENVIRONMENT RESEARCH COUNCIL

I '

I'NSTITUTE OF GEOLOGICAL SCIENCES

.\, .

Report No. 7011 6.

ICSU/SCOR Working Party 31 Symposiurri, Cambridge 1970

The Geology'of the East Atlantic Continental Margin

4 AFRICA .. .

. . . .+. . .

Edited by F. M. Delany, Dipl. Geol., F.G.S. '

., . .

' .

' , i

: i . .

L ndon: Her Majesty . .