the continental terminal in west africa

Journal of African Earth Sciences, Vol. 10, No. 1/2, pp. 79 - 99, 1990 0899 - 5362/90 $3.00 + 0.00 Printed in Great Britain Pergamon Press plc

The Continental Terminal in West Africa

J. L ~ c ~, C. KOCmE 2, S. ALmOU 3, K. A. ALZOUMA 4, G. BELLION s, D. Dusots e, A. ~ ~, R. Gun~uD 7, A. HouEssou 3, I. DE KLASZ s E. ROMAm~ 9 M. S~J.7,RD-CI-I~OLD~ ~0 J. TmCHEr n /*

~Centre des Sciences de la Terre et U.A. 157 du CNRS, 6 Boulevard Gabriel 21100 Dijon (France) 2 Rock View International, 10 rue Vandrezarme, 75013 Paris Cedex (France)

3Equipo de Recherche G6ologique b6ninoise, B2. 526 Cotonou (B~dn) 4Equipe de Recherche G6ologique nig6rienne, B.P. 10662 Niamey ('Niger)

3Laboratoire de G~.ologie, 04 B,P. 322 Abidjan 04 (C6te d'Ivoire) SLaboratoire de G6ologie, 40 Avw. Recteur Pineau, 86022 Poitiers Cedex (France)

Laboratoire de G~logie, 33 rue Louis Pasteur, 84000 Avignon (France) t"La Verdiane" 74. avenue du Mont Alban, 06300 Nice (France)

9Geomineraria Italiane, Via Bores 21, Borgos d'Almazzo Cueno (Italie) ~°Laboratoire de Pal6obotanique et Palynologie, 12 rue Cuvier, 75005 Paris (France)

t ILaboratoire de G~logie, 45060 Orl6ans Cedex (France)

Abstract- The original definition of the "Continental Terminal" is ambiguous with regard to the nature and age of the deposits in the Saharian basins and their margins. No stratigraphical or palaeogeographical significance was attached to the term "Continental Terminal." During the past few years, an increasing number of studies have analysed the nature of events in West Africa which generated alterites and their subsequent reworking in large detrital deposits (Siderolitic facies) during the Tertiary. These studies have led to more precise definitions of the "Continental Terminal." Kilian ( 1931 ) used the term to define the Cenozoic continental deu'ital formations in the 5aharian basins. The term was later extended to include some Mesozoic facies with ferrallitic weathering evident in them. Thus the Ivory Coast marine series which were identified by Tessier et al. (1975) were included in these facies. In the coastal basins, they were less prominent since a Miocene marine trangression delayed the deposition of this continental regime. The autochtonous alterites, sometimes fossilized under the "Continental Terminal" itself are characteristics of past, often polyphase, climates. The "Continental Terminal" is typical of terminal geodynamical events with variable age between Middle Eocene and Quaternary.

R~sumi- Le concept de Continental Terminal en Afrique de l'Ouest a une double ambiguit~ portant d'une part sur la nature m ~me des d~p6 ts et d' autre part s ur leur ~ge. S i l'on s'en fient ~t la d~ finition origineUe de Kifian (1931) darts les bassins inth'ieurs sahariens et sur leurs bordures, ii s'agit de formations d&ritiques continenmles d'~ge c6nozoi'que. Par une ficheuse d~rive de son sens premier, ce terrne s'est ~largi, notamment darts les bassins c6tiers ouest-africains, ~ des facies c~nozoi'ques, mais 6galement m6sozoi'ques, portant l'empreinte d'une alt&ation ferrallitique. C'est ainsi qu'en C6te d'Ivoire des s6ries marines m6connaissables, et qui n'ont 6t~ reconnues comme miles par Tessier et al. qu'en 1975, on 6t6 incluses dam ces facies. Ceci conduisait ~ d(mier route valeur stratigraphique et mute signification pal6og6ographique hce Continental terminal. Ces demi&es ann,s , les 6tudes se sont multipli6es, en Afrique de l'Ouest, pour analyser la nature des 6v6nements qui ont engendr6 en Afrique, au Tertiarie, des air&ires, puis leur remaniement en vastes 6pandages d6tritiques (faci~:s sid&olithique)-- 6tudes qui conduisent /t revenir h la d~finition originelle du Continental terminal: formations d6tritiques continentales d ' ige c6nozoique. Dam les bassins c6tiers, elles sont d'autant plus r&luites qu'une transgression marine mioc~e ~ retardS, en certaines r6gions, l'apparition de ce m#.me r6gime continental.

Les alt&ites autochtones, parfois fossilis6es sous le Continental terminal lui-m~.me, refl~tent des histoires climatiques anciennes, souvent polyphas6es. Le Continental terminal caract&ise une histoire g6odynamique "terminale" dont l ' ige est variable ~ 1'6chelle de l'Afrique de l'Ouest, enCe l 'Eoc~e moyen et le Quaternaire.

INTRODUCTION

Ki l i an (1931 ) p r o p o s e d t h e t e r m = C o n t i n e n t a l T e r m i n a l " t o d e s i g n a t e t h e t h i r d s e d i m e n t a r y de t r i - t a l c o n t i n e n t a l b o d y w h i c h h e i d e n t i f i e d i n t h e S a h a r i a n b a s i n s . H e c o n s i d e r e d it a s C e n o z o i c , k n o w i n g n e i t h e r t h e p r e c i s e a g e n o r t h e o r i g i n o f t h i s f o r m a t i o n , w h i c h m o r e o v e r p r e s e n t s v a r i o u s f ac ie s .

L a t e r , t h e c o n c e p t o f t h e " C o n t i n e n t a l T e r m i n a l " w a s t h e s u b j e c t o f t w o d i f f e r e n t i n t e r p r e t a t i o n s :

• T h e f i r s t i n t e r p r e t a t i o n r e f e r s t o d e t r l t a l a n d c o n t i n e n t a l f o r m a t i o n s , c o n s t i t u t i n g a T e r t i a r y l i t ho log ic , p a l m o g e o g r a p h i c , a n d g e o m o r p h o l o g i c e n t i t y (Ki l ian 1931) . M o s t a u t h o r s c a l l e d t h e s e S i d e r o l i U c f a c i e s w i t h k a o l i n i t e a n d f e r r u g i n o u s c o n c r e t i o n s a t t h e t o p o f p o s t L a t e C r e t a c e o u s s e d i m e n t a r y s e r i e s i n t h e S a h a r a n a n d s u r r o u n d -

79

80 J. l.a~c et al.

ing c o n t i n e n t a l Complexes , " C on t inen t a l Terminal" (Millot 1964).

• The second interpretation, proposed by Tessier et aL (1975) is based on the autochtonous characteristics of similar facies which were studied in coastal sedimentary basins. Those so-called "Continental Terminal" formations are not considered here as continental detrital deposits despite consistent rubefaction and mottling. They result from a "thick weathering fringe which affects Late Cretaceous to Miocene sediments of marine origin" (Tessier et al. 1975). The thick Tertiary detrital discharges in the West African coastal basins according to these authors actually are autochtonous alterites. They noted, however, that this does not mean that there is no real continental equiva- lent of the marine Tertiary at the edge of the coastal basins.

°The definite existence of autochtonous weathering formations does not exclude the occasional allochtonous detrital formations; these were often assimilated with facies convergence. It was possible to differentiate them on the same vertical profile, the allochtonous overlying the autochtonous (Lang et al. 1982). The objective of Lang et al. (1986) was to distinguish these two fundamental formation types, and to clarify the nature of the events occurring in Africa during the Tertiary, generating alterites and later their reworking in large blanket sand. This, in short, is the Siderolitic problem in West Africa. It seems correct to restrict the term "Continental Terminal" for the later. We are thus adding to the definition proposed by the 127 IGCP-UNESCO project: "Post-Eocene continental formations," excluding the Quaternary.

The present paper takes recent complementary data into account. These include the analysis of Ct2 facies in Niger (Greigert 1966), marine Miocene in the Ivory Coast, probable presence of the Miocene in Benin, publications about other coun- tries (Bellion 1987; Conrad and Lappartient 1987), supplementary information about northern Sahara (R.G.) and the upper limit of the Ct (A.D.).

SAHARA INNER AND BORDERING BASINS: EXAMPLES FAVOURING ALLOCHTONY

(A) IuIIemmeden Bas in The post-Eocene pre-Quaternary Siderolitic

series, the unconformity beneath the Siderolitic series and the sedimentary series beneath the unconformity have already been studied by previous workers in southern Niger (Monciardini 1965; Greigert 1966; Kogbe 1976; Dubois 1979; A1zouma 1982; Boudouresque et o2. 1982; Dubois and Lang 1984; Lang et al. 1982, 1986) and in nor thern Benin (Alidou 1983 and 1987; Alidou et al. 1983 and 1986) (Fig. 1).

• General framework of Tertiary alterite~ e r o j l o u f l n d c lepoJit iont l e v e n t s

Sedtmenta.-~' ser ies and a l ter i tes be low the u n c o n f o r m i t y - The marine sedimentat ion which marked the beginning of the Tertiary with the Upper Paleocene-Lower Eocene transgression corresponds to the following sequence: phosphatic limestones, attapulgite clays, and finally oolitic ferruginous facies and more or less clayey sandstones of the *Ader Doutchi series" in southern Niger. These marine deposits were correlated with a biostasic weathering on continental areas in an equatorial climate progressively becoming a wet tropical one with mangrove settling in the "margino-littoral" domain (Boudouresque et aL 1982) (Fig. 2).

Lang et al. 1986 showed that the "Ader Doutchi series" is a transitional entity, deposited in a marine to "margino-littoral" environment, whose base would have terminated the Paleocene-Eocene marine series, and whose detrital top (Ctl of Greigert 1966) would mark the beginning of the overlying Siderolitic deposit. Furthermore, the alterites preceding this Siderolitic series in the lul lemmeden basin affect the basement crystalline series on the west, the Cambro-Ordovician sandstone facies in north Benin (Alidou et al. 1986) and the Upper Cretaceous sandstone facies in south Niger, as well as the more easterly "Ader Doutchi series" (Fig. 2). They are more or less dismantled and are never higher than 15 m; weathering episo- des are difficult to date accurately. They may have begun in the Upper Cretaceous (interstratified bauxitic levels) and continued later, notably in the Paleocene-Eocene (Greigert 1966; Dubois 1979).

The u n c o n f o r m i t y - Dubois (1979) identified a cartographic unconformitybetween the oolitic levels of the Ader Doutchi series and the overlying formations (4 ° SW gradient). The unique characteristic of these two sedimentary bodies is thus emphasized (Boudouresque et al. 1982).

The over ly ing Siderol i t ic series- This series spreads over nearly 150,000 km 2 in SW Niger; it can attain up to 450 m in thickness.

a) Clayey sandstones and red sandstones with ferruginous oolitic interstratified levels (Ct3 of Greigert 1966) cover the southwestern part of the lul lemmeden basin (Dubois 1979; Boudouresque et al. 1982; Dubois and Lang 1984).

The clayey sandstones, in metric to decametric layers are composed of multi-coloured material from silty clays to fine to med ium sandstones with an argillaceous, essentially kaolinic, matrix. Depo- sited in fluvio-palustrine conditions, these sandstones are perforated by a network of tubulures some of which are probably related to previous root traces or relict burrows.

The red sandstones, in layers that can be 10 m

The Continental Terminal in West Africa 81

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N°~Rr~iJl ~ Fig. 1 Fig. I. Schemat ic map of the great geological uni t s of western and central Africa (after Gulraud e ta / . 1987].

Fig. I. Carte sch6mat lque des grands e n s e m b l e s g6ologiques de l'Afrique de rOuest et du Centre (d'apr~s Gulraud et aL 1987].

thick, are composed of coarse to very coarse sandstones with some reddish argillo-ferruginous cement, with oblique stratifications emphasized by gravel and soft pebble layers. Sometimes they form gullies in the underlying argillaceous sandstones and reflect fluviatfle regimes with a torren- tial trend.

Discontinuous ferruginous oolite layers, sometimes several metres thick, are interstratified at different levels within the previous formations. The oolites are made of cryptocrystalline alumino-goethite and of relics of kaolinite type clayey fabrics;

they are wrapped in an argillo-ferruginous matrix where kaolinite is predominant. The matrix also contains quartz grains, generally angular, in very variable proportions. These oolites are frequently grano-sorted and traces of transport can be observed. Reworking of the underlying "Ader Doutchi Series" oolitic levels is possible; but one may suppose that the oolites, before being reworked, were formed within basins which were contempora- neous with the Siderolitic deposition. The latter would reflect certain Lake Chad landforms where ferruginous ooids with oolithic structure had been

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Fig. 2. Tertiary formations of the western Niger (after Greigert and Pougnet, 1965). 1: Precambrian and Infracambrian. 2: Cretaceous. 3: Pa]eocene-Ypresian: 4: Ader Doutchi

series (Ctl). 5: Continental terminal---clayey sandy series with lignites (Ct2). 6: Continental terminal-undifferentiated series Ct2-Ct 1.7: Continental terminal--Middle Niger argillaceous

sandstones series Ct3). 8: Quaternary. See Fig. 1 for location.

Fig. 2. Formations tertiaires du Niger occidental (d'apr&s Greigert et Pougnet I965): 1: Pr~cambrien et Infracambrlen. 2: Cr&tac~. 3: Palt, oc~ne-Ypr~slen. 4: S6rie de l'Ader

Doutchl (Cfl). 5: Continental terminal--s&-le argilo-sableuse ~ lignites (Ct2). 6: Continental terminal--series indifferenciees Ct2-Ctl. 7: Continental terminal--serie des gres argileux du

Moyen Niger (Ct3). 8: Quaternaire. Voir Fig. 1 pour la localisation.

formed dur ing the last mil lenary (Lemoalle and D u p o n t 1972; Math ieu 1978). One canno t esta- b l ish corre la t ions b e t w e e n the va r ious levels wi th in the b a s i n due to the complexi ty and irregu- larity of f e r rug inous oolite sed imen ta t ion ep i sodes as well as the s imilar i ty of oolitic facies obse rved in the ou tc rops .

b) A =lignite bear ing argf l lo-sandy series" (Ct2 of Greigert 1966), no r th of the argi l laceous s ands to - nes and red s a n d s t o n e s , is observed, b u t outcrops ,

of very poor quali ty, m a k e field conf i rmat ion im- possible.

*Hence the S o u g u e r a drilling, 143 m deep, is impor tan t (Figs. 2 and 3). It i n t e r sec t s main ly quar tzo-kaol in ic levels (Ct3) wh ich con ta in a b u n - dan t organic m a t t e r (Ct2) f rom 65 m depth. F rom pollen analysis , car r ied ou t on drilling w a s t e s from a lignite bea r ing b l ack i sh argfllite at 136-137 m depth, Than ika imoni e t a l . (1987) identified Middle to Uppe r Eocene depos i t s (Fig. 4). Pollen f rom


FORAGE SOUGUERA

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4: Argflite. 5: Fine sand. 6: Argllite with ferruglnous oolites. 7: LiKnlte.

Fig. 3. Lithologle du forage Souguera (sltu~ sur Fig. 2): I: Sable moyen. 2: Gravler ferruglneux. 3: Sable argileuLx. 4: Ax~lite. 5: Sable fin. 6: AJrgillte & oollthes ferrugineuses.

7: Lignite.

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EOCENE NOYEN-SUPERIEUR ( f i g . 4)

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pollen analysis (Fig. 4)

Fig. 5. lnterpr~tatlon du Ct A la lumi~re de ranalyse palynologJque

(Fig. 4) du forage de Souguera.

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Honopoe~ te8 a n n u L o t ~ van dec llammen, 1954

Ctenol.ophonidite8 costatun v. H.-K. emend. S.-CH. ,19"77

Ret£br,ev{tr~eol.p~tes t~£anqu~atus v. H . -K . , 1966

Gr~mada lea p o ~ y g o n a l i s Jan du Ch ine , 1978

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m 1 ~eterocol.p£tes 1.aevi.qatua S a l a r d - C h e b o l d a e f f , 1978

Retitrieo~porites annul~tu~ S a t a r d - C h e b o l d a e f f , 1978

Fig. 4. Po l l en a n a l y s i s o f t h e 136 -137 m level in t h e S o u g u e r a dr lUing (af ter T h a n i k a i m o n i e t a l. 1987). Fig. 4. A n a l y s e p a l y n o l o g i q u e d u n l v e a u 136 -137 m d a n s le fo rage d e S o u g u e m ( d ' a p r ~ T h a n i k a J m o n i e t aL 1987).

mangroves or coastal mar shes similar to those of the Ctl (Boudouresque 1980; Kogbe et cd. 1975) were not observed. Greigert (1966) differentiated 50 m of Ct i overlain by about 300 m of Ct2 (Fig. 5) in that area. Within the Ct3 series, a Ct2 m u s t thus be differentiated which represented 80-180 m thickness at Souguera.

*Other samples were taken in lignitfferous or organic-matter-r ich levels by rural hydraulics

drillings (Figs. 2 and 6). From south to north: GAM (Gamdey); BAB (Baboussey): ZOU (Zougarane): OUR (Oura Tondi); DAN (Dingazi). The sporopolli- nic analysis provides the foUowing results (Fig. 7):

The continental material was well preserved; there were no marine individuals (possibly present in the Ct 1) nor aquatic ferns (Hydropteridales).

Each sample, however, contained some fresh- water algae (Ped~zstru~ BothnjococcaceoJ, as well

84 J. LANG et al.

BABOUSSAY

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Fig. 6. Rural hydraulics bore-holes carried out by the B.R.G.M. (location Fig. 2): Gamdey (GAM}, Baboussay (BAB), Zougarane (ZOU), Oura Tondi (OUR) and Dingazi (DAN). 6a: Summary llthologlcal log from cuttings of two bore-holes as examples. BAaouss^v: I : Fine sandstones, yellowish white with reddish Patches. 2: Argillaceous sandstones, yellowish to ochre. 3: Ochre clay and ferruginous oolites; 4: More-or-less argillaceous sandstones with yellowish white sandy clays. 5: yellowish sandy clays with black grey beds (pollen analysis, Fig. 7). 6: More-or-less clayey sands with llgnites and ferruginous oolites. DmoAz1: I: Fine sands with yellowish silts. 2: Yellowish to white clays. 3,4: More-or-less clayey sands, fine to coarse, wine-coloured, becoming mottled beyond 60 m. 5: Clays with black-grey lignite; 6: Grey plastic clays: 7: More-or-less clayey lignites, black-grey (pollen analysis Fig. 7). 8: Greenish-grey clays. 9: Clays and black grey lignltes. 6b: Location of samples submitted to sporo-pollinic

analysis (cf. Fig. 7).

Fig. 6. Forages d'hydraulique villageoise effectues par le B.R.G.M. (Iocalisation Fig. 2}: Gamdey (GAM), Baboussay (BAB}, Zougarane (ZOU), Oura Tondi (OUR) et Dingazi (DAN]. 6a: Log lithologique sommaire a partlr de cuttings de 2 forages, a titre d'exemple: BABOUSSAV: i: Gr~s fin, b lancjaun~tre & plages rouge~tres. 2: Gr~s argileuxJaun~tre ~ ocre. 3: Argfles ocre et oolithes ferrugineuses. 4: Gr~s plus ou moins argileux ~ argiles sableuses blanc jaun~tres. 5: Argiles sableuses jaunat res avec pass~.'es gris nolr (analyse pollinique, Fig. 7). 6: Sables plus ou moins argileux avec lignites et oollthes ferruglneuses. DI~oAzi: I: sables fins a silts jaunfltres. 2: Argiles Jaun~tres a blanches. 3-4: Sables plus ou moins argileux, fins a grosslers, de teinte lie de vln, devenant barlolas au-dela de 60 m. 5: Argfles/~ lignite de teinte grls noir. 6: Argiles grlaes plastiques. 7: Lignltes plus ou nmins argfleux, grls-noir (analyse pollinique, Fig. 7). 8: Argiles grlses verd~tres. 9: Argiles et llgnites grls noir. 6b: Position des

t-chantillons ayant fa r l'obJet d 'une analyse sporo-pollinique (cf. Fig. 7).

as spores, sometimes abundant, of pteridophytes, a wetland plant. It is to be noted that smectites exist alongside kaolinite in the grey plastic clays.

Except the most southern sample, GAM, which would be of the Lower Eocene, the other samples indicate the Middle to Upper Eocene, or even Oligocene, and can be related to the Ct2 previously described at Souguera. This would mean that Ct2 extends more southwardly than Greigert (1966) indicated (Fig. 2). .Arguments for the age o f th is $iderol i t ic formation in the lu l l emmeden basin

Lower l imit- It is closely linked to the location of

the "Ader Doutchi Series" in the Paleocene-Eocene sequence of the Iullemmeden basin.

From a geodynamical point of view, we mentioned earlier the existence of a light cartographic unconformity between this Lower to Middle Eocene series and the overlying Siderolitic deposits. This unconformity corresponds, in the lullemmeden basin, to a geodynamic event characterized by the uplift of its eastern edge (Faure 1966).

From a sedimentological and palmogeographical point of view, we have concluded that the "Ader Doutchi series" is a transitional entity, closing the marine Paleocene-Middle Eocene sequence.

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However, it mus t be excluded from the over lying Siderolitic, contrary to Greigert (1966).

From a c l imat ic point of view, the warm and wet tropical climate prevalent from the Upper Cretaceous to the Middle Eocene tend to dry up afterwardswith the appearance of a certain number of new plant species (Boudouresque 1980).

Upper l imit- At the top, the Siderolitic deposits are overlain by a planatlon surface, generally hardened. This probably ends the Siderolitic episode or occurred slightly later. It is nevertheless earlier than the beginning of the present valley cutting. With these features one may correlate it with a West African well-spread surface, generally called "Surface interm~diaire" and place it in the last part of the Tertiary (Vogt 1959; Gavaud 1967; Grandin and Delvigne 1969; Michel 1973). It is generally accepted that the hardening of this surface occurred in the Upper Pliocene (Michel 1973 and 1978; Boulet 1974; Grandin 1976) or even at the beginning of the Quaternary (Michel 1977). According to current studies by Durand (Fig. 8) this first incision occurred previous to Quaternary whose limit is now placed at 1.66 M.a. Until recently it would be imperfectly dated in the Air massif between 3.8 and 2.65 M.a. (Morel 1985). It is tempting to correlate it with the major climatic change which occurred near 3.2 M.a., at the boundary between Lower Pliocene and Upper Pliocene (Combourieu-Nebout 1987) and which entailed a considerable increase of terrigenous material towards the continental shelf (Stein 1985).

Conclusion At present, we have no data that would allow the

determination of the age of the base of this Siderolitic set. We only know that it is post-Mid Eocene and pre-Quaternary (Boudouresque et al. 1982). We suppose in reference to other west- African areas that the Oligocene corresponds, in part at least, to a deposition gap (Kogbe e ta l . 1975; Lang et aL 1982; Schlumberger 1983 cf. Figs. 1- I0); Upper Eocene deposits may possibly be found. The upper boundary could be placed in the Pliocene. We think that these large detrital spreadings with a Siderolitic facies, dated as Tertiary, and lying unconformably on all previous formations, correspond exactly to the Continental Terminal as defined by Kilian (1931) (Fig. 10a).

(B) Comparison with the neighbouring areas *Eastern Niger (Faure 1966) The Continental

Terminal in eastern Niger (Agadem and Dolle formations) stretches northwards to the 17th parallel (Agadem) and is comparable to western Niger: Siderolitic facies, with ferruginous oolites interstratified levels near the base of the formation, and included between two geodynamic bound-

aries, There is no evidence of the Paleocene, the Lower and Middle Eocene in the eastern Niger outcrops. Some evidence is found in drillings of the Agadem-Tener~ trenches. The Senonian is represented by fine or medium grained and iron-rich sandstone deposits that can contain ferruginous oolitic levels. Above the unconformity, possibly marking this Eocene gap, more of less coarse detrital terms were found locally. These contained conglomerates at the base where the angular unconformity corresponds to a deformed area being eroded (Termit south). Under the unconformity, alterites (kaolin, alunite, bauxite) are noted.

At the end of the Tertiary, the Niger is subject to erosion, with exceptional deposit accumulat ion in the Chad basin (Fig. 8). The Quaternary rests on a deformed surface. Due to lack of datation, it is difficult to fix an accurate chronostratigraphical position of these two unconformities and to give an age to the Continental Terminal of this area; it could possibly include Oligocene or even Upper Eocene (Faure 1966).

*Northern Nigeria (Kogbe and Sowunmi 1975; Kogbe 1979, 1981; Lang et al. 1986)- In Kalambaina quarries and in several outcrops around Sokoto (Fig. 2), 100 km south of Malbaza, one observes a sequence with a basic chemical pre- dominance, of a Paleocene-Eocene age, comparable to the one previously described: clayey limestones and fossiliferous marls (Kalambaina formation); smectites and attapugite shales, with gra- nule or even nodule levels and gypsum locally present (Gamba formation); ferruginous formation with oolitic levels. It is common to observe lateral petrographical and geochemical variations in these "gulf" deposits whose littoral and "margino- littoral" edges oscillate precisely during the Paleocene-Eocene, in the region situated (approxi- matively) between Malbaza and Sokoto (Fig. 2). For exemple, magnes ium and phosphate contents are higher in clays in the Sokoto quarry than in Malbaza. This "gulf was part of the "northern" sea and had probably connected the Tethys and the Gu If of Guinea during the Paleocene (Kogbe 198 I).

An angular erosion unconformity, clearly visible in the field, separates the Eocene ferruginous oolitic formation from the overlying facies attributed to the Continental Terminal (Gwandu formation). The existence of this unconformity emphasizes a major discontinuity at the regional scale (angular unconformity and gullying) and is not a simple local discontinuity.

The Gwandu formation extends to 22,000 km 2 in NW Nigeria (where it reaches a maximum thickness of 350 m). It was deposited in a continental environment and is composed of massive rube- facted and mottled, or white, beds of ferruginized argflo-sfltites. Concretions and ferruginous oolites

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88 J. LANe et al.

levels reworked in a fluvio-palustrine or lacustrine environment are observed. There are fossil frag- ments related to this reworking in the gangue, which came from the underlying Paleocene series. Locally, concentrat ions of peat are observed. The flora reflects a Sahelian tropical climate (Kogbe 1981) (cf. Fig. 27). From pollen analysis (Kogbe and Sowunmi 1975) this formation may be attributed to the Mio-Pliocene age with a possible Oligocene gap, but the presence of the Upper Eocene is not excluded. We propose a post-Middle Eocene and pre-Quaternary age for the Continental Terminal, ,Chad- The Continental Terminal is well represented in the Chad basin (ferruginous sandstone with oolites, and mottled sandstones) but its upper and lower limits are not known with great precision. In Chad (Mathieu 1978), the Continental Terminal (or paleochadian series) corresponds to the continental Oligo-Miocene, the period during which the Chad basin began to form. We m u s t now add the Pliocene, for a part at least (Fig. 8). The deposits, several hundred metres thick, overlain by the Quaternary ones, rest on the Precambrian in the south and to the nor theas t of the lake, on Upper Cretaceous in the west, and on Paleozoic or Mesozoic sands tones in the north.

.Mall- In the Gao Strait, after a basic chemical marine sedimentat ion Paleocene to Mio-Eocene age, one observes Siderolitic material originated in the Upper Eocene in a continental lacustrine environment (argilites with ferruginous oolites). It became more pronounced after the Oligocene (argilites and ferruginous sandstones). Radier (1957) reported no unconformity between the chemical Eocene series and the terminal Sideroli- tic. Greigert (1966) confirms this fact in the north of the strait, bu t he noted that an unconformity exists in the centre between the Paleocene- Ypresian and Continental Terminal. In the Nara trench, west of the country, and in the Taoudenni synclinorium, the Continental Terminal overlies the Continental Intercalaire (Bassot etal. 1981). In the neighbourhood of Timetrine, it is represented by a gritty conglomeratic formation, clearly unconformable on the Mesozoic (Bellion 1987). *Northern Sahara- The Continental Terminal occupies large areas in the nor thern Sahara, where it was defined by Kilian (1931). From east to west four main basins can be distinguished: Wadi Mya- Lower Sahara, Grand Erg Occidental, Boudenib, and Tindouf (Fig. 1); the first of these basins is the largest and best known.

The review of Bellion (1987) provides a detailed analysis of the n u m e r o u s studies carried out until the present. Only the most important data, already described in part (Guiraud 1978), will be presented hereafter. 1) The Continental Tertninal sustratum- Eocene and

Paleocene sedimentary formations are usually separated by a gap from the Upper Cretaceous. In the eastern regions, the marine transgressive Paleocene formation is mainly represented by marly calcareous formations, containing abundan t and characteristic faunas, in particular, Ranlkhotha//a (Operculinoides) berm~d~z/PALMER and L/nthfa sud_a_nertsis BATH (Busson 1973; Amard et aL 1981). The Lower Eocene corresponds to argilla-

a"

ceous or dolomitic l imestones with Nummulites, among which are N, guettardi, N. fraasi . N. deser t t etc. Gypseous or anhydrit lc formations, with rare argillaceous or carbonate layers, represent the Middle Eocene which is thus marked by a great regression. Detrital intercalations are observed to the north at the approach of the Atlasic domain in the process of being uplifted and emerged (Busson 1970; Guiraud 1973).

In the western regions (Tindouf), the Paleocene and Lower to Middle Eocene correspond to continental detrital formations, with occasional calcareous layers which con ta in Charophytes , Ceratodes and Vertebrates locally (Gevin et al, 1975 and 1978). These various Paleogene formations vary in thick-

ness from 10-100 m. The maximal thickness was observed in the Lower Sahara, at the beginning of the Atlasic domain. Furthermore, the formations are often affected by folds or faults, mainly near the large submeridian fracture bundles or in the proximity of the Sahar ian Atlas or Anti-Atlas. These s t ructures are a product of the intra- Eocene. or Pyreneo-Atlasic tectonics (Guiraud et a/. 1987), a regional event which was followed by an erosional period and the Continental Terminal frequently rests unconformably on its substra- tum.

2) The north Saharian Continental Terminal- a) The most cont inuous and thick formations were observed in the eastern basin (Lower Sahara-Wadi Mya). They are mainly composed of argillo-sandstone deposits whose thickness increases from south (Tademait-Tinrhert) to nor thwhere it reaches nearly 2000 m near the south Aurasian t rench (Guiraud and Ousmane 1980).

The series is well dated only near the Aures, where it becomes more complex and partly marine (Guiraud 1973). Locally, a Mammalian fauna from the Upper Eocene appeared in the base detrital formations (Coiffait et al. 1984). Evidence of the Oligocene is certainly present but has not yet been dated accurately. In the Upper series, the presence of some marine intercalations permits to characterize the Miocene. Lastly, the Plio-Villafranchian, within which the trilogy basal conglomerates and sandstones-sandy clays- conglomerates and sands can be almost regularly recognized, has been dated


on the Algerian-Tunlsian borde r s , Numerous ver- tebrate deposi ts were found (Coque 1962 and recent works of Jaeger and coll.). The complex formed by Upper Eocene to Plio-Villafranchian formations, somet imes m a r k e d by minor disconti- nuities, is highly tectonized (Guiraud 1973; Guiraud et oL 1987). It is overlain by a clear unconformi ty with the th in Qua te rnary s tepped ped imen t s and terraces.

Southwards , in the Tinrhert , the argillo-gritty- conglomeratic series of the Continental Terminal include fossil wood levels with Oligo-Miocene characterist ics, as well as lenses of lacus t r ian l imestones with Charophytes . One of these lenses s i tua ted nea r the base of the series furn ished an Upper Eocene (?)-Oligocene flora (Busson and Grambas t 1965). These formations, locally deformed near the El Biod faults are unconformably overlain by the Quaternary.

b) In the wes te rn regions, between the Mzab ridge and Tindouf, the series are relatively th in and are poorly dated. They were described by var ious au thors , among t h e m Conrad (1969) who described two supe r imposed sets:

• At the base is a reddish grit ty-conglomeratic formation, with argil laceous or calcareous layers; the calcareous layer which tops this uni t and forms a very c o n t i n u o u s surface (Lower Hamada) con- ta ins Gast ropoda (Clavator, Limicolaria) which would at test an Aqui tanian age according to Du Dresnay (1977).

• At the top are a l ternat ing fine argillaceous s ands tones ("torba"), conglomerates and gritty dolomitic l imestones; one of the lacustr ine lime- s tone levels usual ly forms the top of the series (Upper Hamada). Conrad (1969) considered this u ppe r set as Neogene, wi thout paleontological evidence. It m u s t have been slightly deformed; ma in ly upl i f ted, before the first c lass ical Q u a t e m a r y deposi ts were formed (see infra).

The subsequent formations- The end of the Cont inental Terminal series deposit ion was imme- diately followed by a brief episode of deformation which was in tense only at the beginning of the Atlasic domain (Guiraud et al. 1987). This episode occurred dur ing the Villafranchian. The edges of the bas ins were at tha t t ime often uplifted and here the Qua te rnary ped imen t s or terraces, with a th in detrital cover, were shaped af terwards by climatic oscillations. In the centre of the basins , lakes were formed, where Cardium often prol i ferated. Simul taneously , weather ing and pedogenetic phe- n o m e n a occurred which, according to the climate, resul ted in the format ion of calcareo-gypseous c rus t s in the mos t n o r t h e m region or ferruginous cu i rasses at the beginning of the Hoggar. Conclusions In the Northern Sahara, data on the l i thostrat igraphy of the "Continental Terminal" as

89

well as its geodyrtanlical enti ty have been collected since Klllan's investigations (1931). Besides the paleontological conten t (which is unfor tunate ly often insufficient), the facies, the th i ckness of the series and the s t ruc ture usual ly c an be dist inguis- hed from the previous or the s u b s e q u e n t depositio- nal histories.

The analogies with the in t racont inenta l bas ins of the sou th Sahara are numerous .

COASTAL SEDI]~.,NTARY BASINS: M]~RELY AUTOCHTONY OR AUTOCHTONY-

AIJ~CHTONY SUCCESSION

A) Coastal bas lna w i t h alterites: e x a m p l e s of s ing le a u t o c h t o n y

Good examples of format ions which were tradi- tionally called *Continental Terminal" and were proved, after investigation, to be au toch tonous alterites o fmar lne formations, had been described in the Senegalo-Mauri tania bas in (Tessier 1952, 1965; Tessier etal. 1969, 1975; Lappart ient 1985; Conrad and Lappar t lent 1987) and in the Ivory Coast bas in (Tessier eta/. 1975; Charpy and Nahon 1978). These au thor s emphas ize the complete t ransformat ion by weathering, from 1-10 m in depth, of argillo-sandy, somet imes glauconitic sedimentary formations, of var ious ages. The argu- m e n t s are numerous : pers is tence of textures and pat terns; lateral t ransi t ions; identification of fossils; progressive mineral al terat ions (change of glauconite to goethite and clay, and of ca lc ium phospha t e to a l uminous phospha te , corrosion and fine f ragmenta t ion of quartz-grains, leaching of calcite, kaol inisat ion of days , t ransformat ion of pyrite in more of less complex sulphates. . .) ; either oblique or conformable weather ing front, etc.

According to Conrad and Lappart ient (1987) the greater felTuginous crus t ing in the Senegalo- Mauri tanian bas in compared with tha t observed in the Ivory Coast probably resul t s from the different climatic zones in which these bas ins are situated; an equatorial climate for the Ivory Coast, a wet tropical climate with a dry t rend for Senegal.

The convergence led to the confusion of these au toch tonous alterites which had affected strati- fied deposi ts with the *Continental Terminal" detrital s ed imen t s described in the Sahara and its edges. This is the reason why the te rm was tradi- tionally used in the geological mapp ing and voca- bularly in these regions. Tessier etal. (1975) showed tha t this use was erroneous.

(B) The Ben in cos ta l basin: e x a m p l e of a l l o c h t o n y s u p e r i m p o s e d on a u t o c h t o n y

In th is bas in , a success ive Cre taceous- Paleocene-Eocene sed imentary series, al terat ions under lying an unconformi ty and a detrital series

90 J.L.~o et al.

with a Siderolitic facies can be observed (Slansky 1959, 1962; Houessou and Lang 1978) (Fig. lOb).

• Brief post-Middle Eocene his tory o f the Benin coastal basin ( L u g et a/. 1982, 1986).

Sedimentary series a n d alterttes under the unconformity- The Turonian-Coniacian, observed at the nor thern edge of the basin, is made of quartzous sands with gravels and kaolinic clays, ferruginous sandstones, and coal levels. These deposits, 40-60 m thick, have fluviatfle, deltaic and tidal s t ructures (Pereira 1978; report FED 1987). A transgression, with oscillations between the

MaastrichUan and the Ypresian, is related to a fossiliferous argillo-calcareous sedimentation with phosphatic and glaucontitic layers. In these layers, montmorillonite and attapulgite are present; attapulgite is disappearing near the top and kaolinite is increasing (Slansky 1979, 1962).

The Lutetian, regressive with marine oscillations, is formed of argfllo-sandy facies with phosphatic levels at their base. According to Slansky (1959, 1962), argillo-sandy glauconitic undated deposits (Middle or Upper Eocene) lie unconformably on the Lutetian and then give way to continental detrital (quartz, kaolinite) deposits. The marine series, titled some degrees southward and monoclinal, is t runcated by a surface where post-Eocene weathering is evident for thicknesses of more than 10 m. The nor thern alterites modify the Upper Cretaceous sandstones at the basement border: mottled facies, with ferruginous patches and concretions topped by ferruginous cuirasses; persis- tency of sedimentary textures and patterns; identification of fossils: fine quartz fragmentation; clay kaolinization (Slansky 1962; Affaton et al. 1985).

Ferruginous cuirasses are known on the crystalline basement in the north of the coastal basin (Lamouroux, pers. comm.). The southern alterites, of Middle or even Upper Eocene, have the following characterisitics: ferruginous concretions and surface cuirassing of the formation; more or less reduced kaollnic phase; decayed and splitted quartz grains; sedimentary textures and s t rucutres are observed, but no trace of an organism was found (Slansky 1962; Houessou and Lang 1978; report FED 1987).

The unconformity- A detrital series rests unconformably on the marine series and the altered facies. This unconformity is angular, of some degrees, that is often difficult to detect, but it is obvious at the cartographic scale (Slansky 1962; Akpiti eta/ . 1985). The age of this unconformity is u n k n o w n , bu t by ana logy wi th Nigeria (Whitemann 1982) and with neighbouring coastal basins (Ghana, Cameroon, Gabon, etc...), it could be Upper Eocene t o Oligocene (Schlumberger 1983).

The Siderolitic series- The series s tretches over 10,000 km 2 south o f the coastal basin and begins with a pebble formation (10 m thick), related to palaeovalleys. Blunt quartz pebbles are observed in an argfllo-sflto-sandy white or red matrix, to which certain rock elements originating from the crystalline basement in the north of the basin are associated. However, the possibility of reworkings from older detrital series in relation to climatic or even tectonic phenomena (recurrence of Lokossa and Sehoue faults) cannot be excluded. The pebble formation gives way to argfllo-sandy, locally conglomeratic sediments, no more than 40 m thick, and resulting from the reworking of the alterites and the underlying deposits. This Siderolitic set results from sheet flood spreadings and fluviatfle transports; it does not present any interstratified levels of reworked oolites like those in the Saharian basins. It is much th inner than in these basins.

Miocene marine incursion- There is recent evidence of marine incursion south of the basin (report FED 1987) already known in Ghana, Ivory Coast, and Senegalo-Mauritanian basins. In Benin, in the region of Igolo-Sakete, the outcropping deposits were not Middle Eocene (Houessou and Lang 1978), but Miocene with quite comparable weathering facies. This age mus t still be confirmed (by the possible discovery of fossils); marine sedimentary pat terns and s t ruc tures would be present (probable tidalites). An erosional unconformity is visible between these weathered deposits and the overlying Siderolitic. In the Sakete area (Houessou and Lang 1978) e~-osion processes caused Paleocene or Eocene limestone pebbles to be carried along several kilometres to places where pebbles do not usually exist in outcrops. This is indis- putable evidence for allochtony. Lateritic gravels and cuirasse blocks are also observed.

• Discuss ion o f the age o f the Siderol it ic formation in the Benin

Lower limit- Polyphase alterations are super- imposed after deposition. Sandstone facies are related to weathering changes. Dissolutions bring about water draw off phenomena and deformations in the detrital series as well as in the underlying alterites. As a result, the lower limit of the series is altered and sometimes hardly visible. In certain favoured sections, it can be observed that this Siderolitic rests upon Turonian-Coniacian in the north of the basin (report FED 1987), upon Middle or even Upper Eocene deposits more southward (Houessou and Lang 1978), and even upon Miocene deposits near the ocean.

Upper limit- It is very difficult to differentiate the upper limit from the Quaternary, except locally such as in the Se quarry where an erosional unconformity corresponds to that limit (Houessou


and Lang 1978). The question of the origin of the Quaternary is related to the prob~rn of the "Terre de barre" which outcrops south o ~ e basin, 30 m thick, upon the detrital series with a Siderolitic facies. In the north, the "l'erre de barre" is not up to 1 m thick and develops on alterites which modified the Upper Cretaceous sandstones. Numerous authors (Fauck 1972; Dekadjevi 1974) concluded tha t it has resulted from weathering of a sideroliUc or of all sandy-kaollnic material. Quartz was dissolved and the material was enriched with clay and iron. These surface horizons were after- wards bioturbated and could have been reworked by water and wind into sheets which are discor- dant on underlying formations (Houessou and Lang 1979). The "Terre de barre" therefore results from the influence of pedogenetic phenomena and reworkings during the Quaternary.

Conc lus ions - T h e r e are presently no data to de- termine the age precisely of this Siderolitic facies. It is probably post-Middle Eocene (Houessou and Lang 1978) or even, on the basin southern edge, post-Lower Miocene from the age of the marine incursion if one refers to the neighbouring Ghana basin. The facies convergence between the autochtonous alterites and the allochtonous sediments often make the distinction between both origins difficult. One can unders tand why the authors grouped the entire sequence under the term of *Continental Terminal." Nevertheless Slansky (1962) distinguished a lower and an upper compo- nent. Is it simpler to restrict the term *Continental Terminal" to the unconformable, detrital siderolitic geological formation situated between the Middle (or even Upper) Eocene and the Quaternary. This formation is reduced to the Mio-Pliocene when the Lower Miocene marine incursion is evident.

(C) Indicat ions provided by the Ghana, Ivory Coast and Senega l - Mauritanian coastal basins

• The Ghana basin- A marine series which is overlying green-grey argillo-siltites with Middle Eocene fossiliferous calcareous levels outcrops in sou thern Ghana, towards the West (Kesse 1985) and along the East (Smit 1966; Akpatai 1975). The series was dated as Miocene. These deposits, represented by fossiliferous glauconitic sands and shales, beach-sands and estuarian-type fluvioma- rine sandstones, are not present throughout the basin. On this Miocene series and, when it is absent on the Eocene series, continental red more or less argillaceous gravels and sands, which are 25-90 m thick and which are Mio-Pliocene in age are deposited unconformably. They are in tu rn overlain by a detrital series with a siderolitic facies, unconformable on all the previous formations and also overlain unconformably by the Quaternary.

The Q u a t e m a y is represented by coastal to *margino-littoral" deposits of gravels, sands and clays (2~ m). ~ e ~ h a n a basin thus presents a recognizable detrital *Continental Terminal" which, apparently, at the level of the Miocene series, does not overlie autochtonous alterites.

,The Ivory Coast- Br/efhistorg- According to Leneuf and Berthelot

(1966), the Continental Terminal consti tutes all the plateaux bordering the lagoonal system of the Ivorian coast basin; the system is delimited by a large east-west s t ructural feature (Fig. 9). In several sections (Sassandra, Fresco, Mokta, Nzida, Tiagba, Bingervi l le , Ebocco -Ebo inda ) the Continental Terminal rests directly either on the Precambrian basement or on Cretaceous or Paleocene marine series. Drillings in the deep part of the basin permitted the identification of the lithostratigraphic succession. Leneuf (1968), after Guerin-Villeaubrefl (1962, in Leneuf 1968} describes an upper =set" 30 m thick on average, made up of relatively homogeneous lateritic argillaceous sands, ochre coloured, and frequently intersper- sed by hardened ferruginous levels, and a lower "set" of variable thickness, made of an alternation of coarse sands and motley clays.

The upper set is presently called "rerre de barre" by reference to the same formation described in Benin (Houessou and Lang 1979).

North of the basin, the contact with the basement is easily recognized and consist of a white quartz gravelly level "wrapped" in a red brown argillaceous matrix. Berton (1960, in Leneuf 1968) described it as basal conglomerate. Leneuf (1968) is very cautious of this interpretation. He noted that this conglomerate is absent in m a n y places where the contact with the basement exists. The ferruginous levels were carefully examined. Often discon- t inuous, they are mainly located in the upper level ( ' rerre de barre') and may be ferruginous cemen- ted sands tones , concre t ionned fe r rug inous gravels, cutrasse elements or, at the extreme west of the country, t rue iron ores. (Monogaga and Victory sectors) whose outcrops form residual hills located outside the limits of the sedimentary basin. These iron ores, oolitic looking, would be, according to Carozzi (1965) only "glauconitic arenites subst i tuted by iron oxides." The origin of the ferruginous levels in the "Continental Terminal" needs tobe reconsidered, since Millot (1964, 1980) described them as being more detrital than chemical. He describes an erosibve process ofrhexistaslc character on the continent. The continent having been peneplaned and lateritized before the Mio- Pliocene, had concentrated iron in =surface reserve." According to Le Bourdiec (1958) and Leneuf (1968), the genesis of this formation is a mat te r of =deposits of fluviatile origin in which

92 J. Lhr~a et al.

sheet wash would have played a primordial role." These sheets occurred in a semi-arid context.

Tastet (1979), describing the Quaternary sedimentary and s t ructural environments of the Gulf of Guinea, studied the "Continental Terminal" which forms the subs t ra tum of a part of these deposits. He described the geomorphological and lithostraUgraphical aspects of the formation, and then discussed the genesis and the age. The "Continental Terminal" is what he called "the high plateau unit" of more recent age, but pre-Holocene.

The lithostratigraphy that he proposes, is based on the interpretation or the reinterpretation of 100 boreholes. From the 10 that he personally sam- pled, which attained or sometimes penetrated the "ContinentalTerminal," the following sections, from top to base, can be established:

• Level 4: homogeneous ochre sandy clays "Terre de barre" (0-70 m);

• Level 3: sands, often coarse, with mottled clay intercalations (0-90 m);

• Level 2: argillaceous sands and fetid black clays (0- I0 m):

• Level 1: argillaceous sands, more-or-less coarse, with clay intercalations (0-20 m).

Tastet (1979) nevertheless specifies that level 2 was found only in the center of the basin (Abidjan area) and on the east of the lagoon, and that only levels 4 and 3 are well known from a lithological point of view. He interpreted the origin of the "Continental Terminal" as follows: the first Mio- Pliocene sediments (level 1) were placed on an already strongly weathered basement; it does not seem (after reinterpretation of the oil drilling) that the continental Mio-Pliocene would have gone past the large "lagoonal" s tructural feature which would have acted as a palaeogeographical limit at that time. Marine and lagoonal incursions (level 2) were nevertheless observed (Dufaure in Tastet 1979; Bacchiana et al. 1982; Dufaure and Tastet 1984). Aridification provided the opportunity for the spreading, by high energy sheet-wash or fluviatfle action, of detrital material in which are intercalated sporadic elutriaUon deposits (clayey levels) which characterize level 3. At the maximum of aridification and seasonal contrast, the deposits became more homogeneous and windworn. Level 4 may be an eolian reworking of the previous deposits. Level 4 often presents a hardpan. The age of the hardpan as well as the age of the level 4 itself mat extend into the Quatemary.

Paradis (1982) explained the deformation of gravelly intercalations within the "Continental Terminal" and the "Terre de Barre" by draw off caused by groundwater flow provoking dissolution and collapsing.

The onshore oil drillings- Thirty oil drillings (i00 m depth maximum) carried out by the" Direction

des Hydrocarbures de C6te d'Ivoire" between 1977 and 1981, completed the data on three zones of the basin: a western zone (Yocoboue-Nzida area), a central zone {Bingervflle area) and a central- eastern zone (Bonoua area):

West of the basin, Simon and Amakou (1983) reported in five boreholes near Yocoboue, the presence of an ochre-yellow sandy series, with clayey (kaolinite) levels intercalated. This series is 50 m in the drilling F3, but has an average thickness of 20 m. This detrital formation rests on Paleocene deposits dated by foraminifera (de Klasz, in Simon and Amakou 1983) or on Eocene recognized by means of palynology (Charpy in Simon and Amakou 1983).

An exploratory survey, followed by eleven drillings at the center of the basin, were reported by Bacchiana (1981). The "ConUnental Terminal" as it appears in the s tudy area, is made of the "Terre de Barre" which typically presents two stone-lines, one at the top (at nearly -2 m), the other at the base. The latter is rich in pebbles and ferruginous frag- ments which can even form cont inuous decimetric levels. Below, there are more or less coarse sands with oblique stratifications, separated from an underlying mainly clayey material by a ferruginous level. This level emphasizes an unconformity. These mottled clays rest on a glauconite and phosphate-grain level, whose base is dated, further southward by the presence of the foraminifer Epon~:les pseudoelevatus GRAHAM, DE ICJ_ASZ et MERAT, characteristic of the Paleocene. Dating for these formations is still hypothetical. It is known that the *Terre de Barre," at least for the most part, could be Quaternary after the discovery of h u m a n tool assemblages within the upper stone-line. Bacchiana (1981) proposes that the sands which are situated between the lower stone-line of the "Terre de Barre" and the ferruginous level which separates these sands from the clays, are both of Pliocene age. The presence of unconformities at the base and at the top of the Pliocene, detected in the first off-shore drillings (IVCO1 and IVCO2 Brancart, in Bacchiana 1981), can be related to both precited ferruginous levels, The underlying mottled clays may belong to the Miocene. In Bingerville and surroundings, due to hand dug holes for local clay exploitation, Bachiana et oi. (1982) identified a Foraminifer in black clays and palynomorph association characterizing a marine Lower Miocene. Bacchiana (1981) extrapolated this result by proposing to attribute the mottled clays situated under the "Pliocene" sands to the Miocene.

In the eastern part of the basin, ten 50 m deep drillings were carried out not far from the lagoonal fault zone. Simon (1983) presented the lithological succession as follows: after 20 m maximum of


"Terre de Barre," nearly 30 m of versicoloured medium-to-coarse sands, often badly sorted, pre- senting thin intercalations of white or purplish clays and of ferruginous sandstone gravel beds, were observed. The morphoscopy of these sands reveals a fluviatfle origin. They rest locally on the marine deposits which were dated, in two drillings, as Paleocene and Eocene. Simon (1983) attr ibuted these sands to the Mio-Pliocene. Like other authors he proposed a Lower Quaternary age for the "Terre de Barre."

Conclusion on Ivory Coast bas in It is evident in this basin that, besides partly

weathered autochtonous marine series (Fresco), formations exist which are detrital of continental origin and of sandy-clayey nature. They contain quartz, kaolinite and iron hydroxides, and present a typical Siderolitic facies.

The "Continental Terminal" is more restricted than previously considered, as shown by I. and S. de Klasz (in press) who described marine incursions north of the lagoonal fault. However, these authors do not agree with Bacchiana (1981) who reduces the "Continental Terminal" to the only sands si tuated above mottled clays (which he considered as marine Miocene) and for which a Pliocene age was proposed. I. and S. de Klasz are uncer ta in about the limit of the marine Miocene from the previously recognized palaeogulf and far north of the large fault. Nevertheless a certain number of questions remain unsolved:

a) Dating of the "Terre de Barre. ' - Numerous authors consider that it belongs to an ancient Qua- temary(Tastet 1979; Bacchiana 1981; Simon 1983) due to the discoveries of h u m a n tool assemblage remains within its stone-lines (Paradis 1980; Chernokian et al. 1982). In that case, this formation mus t be dissociated from the "Continental Terminal" and confined within its new definition. The "Terre de Barre" would be a product of a later sedimentary cycle (Early Quaternary) in a climatic lateritizing context, responsible for the observed cuirasse levels.

b) The lithostratigraphical succession- The base of the formation lies unconformably either on the Birrimian basement or on Eocene. It was described as conglomeratic by the previous authors relying on outcrops of quartz gravels lying on the basement in the north of the basin. Leneuf (1968) already suspected an influence of drainage axes in their distribution, a hypothesis confirmed by Tas- tet (1979) who did not f n d these conglomerates in drilling to the south. This hypothesis was confirmed by Bacchiana (1981) who established that these pebbles (always overlain by some "Terre de Barre,) were systematically located in the vicinity of rivers (alluvial terraces). The age of these terra-

ces is most probably Quaternary. The base is not Characterized by a particular

bench-mark level; but a lower unit with a clayey- sandy dominance and an upper unit with a sandy- clayey dominance can be recognized. This frame however may not be everywhere; numerous lateral facies shifts can be observed; important variations in thickness related to active tectonics (the series which is extremely reduced in Fresco area, is thicker by 100 m) and possible marine incursions make any at tempt for accurate correlation uncer- tain. However, the base of the formation lies on series not younger than Middle Eocene. Its top, which lacks precise dating, is well delimited by the occurrence of the "I'erre de Barre."

• T h e S e n e g a l o - M a u r i t a n i a n b a s i n - From the Cenozoic history of this basin (BeUion and Guiraud 1984; Bellion 1987), the following can be established:

• the existence of numerous transgressions, which are responsible for the interpenetraUon or for the alternation of marine and continental formations;

• the occasional recurrence of a large number of faults; the existence of an alkaline magmat ism which begins at the end of the Eocene and conti- nues until the Quaternary;

• the importance of weathering. At the geodynamic level, the important event

corresponds to the well dated terminal-Lutetian tectonogenic phase, which is accompanied by the almost total regression of the sea which previously covered the whole basin beyond the present shore- lines. The newly emerged land weathering begins and is interrupted only temporarily and locally, when other marine t ransgressions occur.

The post-Lutetian allochtonous continental detrital formations, corresponding to the t rue "Continental Terminal," are restricted to the eastern edges of the basin, with relatively small th ickness; (Lappartient 1985; Conrad and Lappartient 1987). For a long time Miocene sands and clays, of marine origin, strongly weathered, which outcrop over a large area (cf. supra) were erroneously considered as the "Continental Terminal." In the Bignona (Lower Casamance) borehole penetrating the Miocene, which is about 100 m thick, the upper 50 m is weathered. Accord- ing to Tessier et al. (1975), alteration appears in a progressive m a n n e r from the fresh rock, with con- servation of the sedimentary s t ruc tures to modifi- cation of the components and individualization of kaolinite and iron sesquioxides. Moreover, a ferruginous crust, lying on various terms of the Senonian and Tertiary, covers large areas and seems to have been formed principally in the Pliocene. As for the Quaternary, it essentially corresponds to stepped terraces in the Senegal and Gambia rivers watersheds, to marine beach sands

94 J. L~G et al.

and to eolian spreadin4~s. It is however interesting to note that in the surroundings of Dakar there exists a volcano-sedimentary series which shows the superposit ion of a strongly weathered volcanic unit (dated to 1.5 -+ 0.10 M.a., overlying the Plio- cene cuirasse) and a much less weathered set dated to about 1 M.a. These facts reinforce the idea that important climatic changes occurred during the Plio-Pleistocene. From observations made in the Algero-Tunisian Sahara borders (see supra) and in north ~:frica as well, it can be argued that it is an intra Vfllafranchian geodynamical event.

*Conclusion about the coastal sed imentary basins o f West Africa

These basins provide a great variety of examples: au tochtonous facies; au tochtonous facies overlain by an allochtonous detrital series with a Siderolitic character and extremelyvariable thickness (from a few metres to 50, or even 100 m in some parts of Ivory Coast or Ghana); and allochtonous facies unconformable on non-weathered marine series. This Siderolitic or "Continental Terminal" In the strict sense is often reduced to the Mio-Pliocene in places where the Lower Miocene marine incursion occurred. But in the regions where no marine incursions existed, it could be older. Whatever the origin, it has a post-Middle Eocene age.

CONCLUSIONS

The term "Continental Terminal" was proposed by Kflian (1931) to refer to the third detrital continental unit in the Sahara, which he considered as Cenozoic.

It became widespread in West Africa to give this name to the red to ochre sandy-clayey formations, characterized by the association of quartz- kaolinite with iron oxides, and containing occasional ferruginous accumulat ions and sandstone lithificated horizons.

In 1975, Tessier e t al. showed that many of these formations in the coastal bas ins in fact were thick autochtonous alterites developed from outcropping marine series.

The term "Continental Terminal" can be viewed from the different perspectives:

* The s tudy of the *Continental Terminal" series in the Sahara and its borders permitted the re- definition of the term proposed by Kflian, not only from the facies point of view, but from the geodynamical point of view. For Kilian the "Continental Terminal" was, in these regions, a geological formation which corresponded to thick detrital spreadings removed from the strongly weathered uplands of Africa, and accumulated into the downstream basins. This formation is on the whole Tertiary, but the dating is often not precise. In

several places, it overlies the Mid-Eocene. The lower and upper limits of the "Continental Terminal" are determined by geodynamical events under the influence of Tertiary tectonic pulsations. Even if the l i t te r are not strictly synchronous at the scale of North and West Africa, a supra regional value may nevertheless be at t r ibuted to them. The "Continental Terminal" actually lies with angular unconformity (of some degrees) on underlying series. The upper limit is marked by a p l ina t ion surface, often cuirassed and deformed, plana- tion which would date from the end of the Tertiary.

*The s tudies of Tessier e t al. (1975) showed clearly that certain formations with a "Continental Terminal" facies in the Ivory Coast and Senegalo- Mauritanian bas ins were thick au toch tonous alterites. They are of palaeoclimatic interest and their history is polyphase. The pali~oalterites would belong to the immense family of au toch tonous laterites which affected all the geological formations of West Africa since the Cretaceous. This immense group of African au toch tonous alterites was never included within the "Continental Terminal." It is in a fortuitous way, by convergence and by superposition, that the problem arose in West Africa.

It is hereby proposed to return to Kflian's definition (1931) and to reserve the term "Continental Terminal" to the detrital quartzo-kaolinic formation with Siderolitic facies, lying in stratigraphical unconformity on any subs t ra tum. Its age can be- si tuated between the Mid-Eocene and the Late Pliocene.

In the Saharian region, this Siderolitic facies, which attains a thickness greater than 400 m, overlies scattered, well-preserved, au tochtonous palaeoalterites inherited from a lengthly weathering history (Fig. 10a). The s tudy of the coastal basins also shows that au toch tonous alterites and a Siderolitic series can be super imposed (Fig. 10b). This Siderolitic series are similar, although less thick and without ferruginous oolitihic levels, to that of the bas ins of the Sahara and its borders, and fits the Kilian definition as the terminal detrital formation in West Africa. Moreover, in these coastal basins, the "Continental Terminal* is reduced to the Mio-Pliocene in those sectors where the Lower Miocene marine incursion is to be felt (Fig. 10c1.

For the geological mapping of West Africa, the distinction between au toch tonous alterites and spreadings of the "Continental Terminal" wiU often be difficult because of the weathering of sediments. Moreover, when "Continental Terminal" and alterites are superposed, a common over-weathering may have affected them, with dissolutions and disorders in both facies. Accurate analysis at different scales may allow the separation of the

The Continental Termiral in West Africa 95

" " b (

.. . ~ - ~ . ~ . ~ ~,.~u ~ \ ~ . ~ : ~ ,,. -- Fmult N 0 1 ~ A ~ ~ ~. ",'"~'" .,~ .... f , o :

SASSANDRA ....--/ G O L F E O E

Fig. 9. Ivory Coast sedimentary basin. Fig. 9. Le bass ln sedlmentalre de C6te d'Ivoire.

.<.<<<':<<<':<':':<.<:.~........... ! ~ : : . . . . . . . . . . . v . ' . . 1 . : . : . : . : < < < . 1 ~ . : , , ~ . : . . . . . . . . . . , : . : . : . . / . 1 1 : : : . . . . . . . . . . . • . . . . . . . . . . . + I ~ I I ~ l l l l l l l i l l

""''''" :::i:::L .:.:.:.:.:.:.:.:.:.-.:: • :':':':':':':':':':':':E: :E:i s .,*,%;..- :-:: * • ::::::::::::::::::::::::::::::::::::: v . ' . : . : . : . : . : . : . : . : . : . : v . - . . v . . ' : " : v . ' v , ' . ' : . v . ' . v . ' . v . .

~ -- SOUTH

I_

Fig. lOb Pllo-0uaternary Ferruginous cuirasse

/ t:Zi7 r'

(a r ly Quaternary +÷ + Pl [o~uaternary

Paleocene-Eocene~ligocene? Hamadian Ante-Continental

l E N I N ~0" ]0

~" #e -b ÷

weathering ~. I I / / I I / I I I ,

~Alterites on the , edge of the +

marine deposits

t ,

Easement ,t

C~ {<4Sos) | Oeigoce.m ?

I Upper Cretaceous-Oanian

S Fig. 10c Ouaterna~

Benin Gulf I C~ e n ~ s t r~ to

( flarlne or ~\\\\'.E~,,, - ) contlnentll )

O@ series having l] ;uffered Pedogenesis .~

.ea ther ing • e a s i n g Post- E o c e ~ A Ite~otoon }

Poleoc~,~e

@- #. ÷ ÷ ÷

NIGER

I Part icu lar cases I Spreading

of CT on the marine Miocene

9

+vvU"er Cretaceous + " + " + + " " +

Cuirassed ferruginous 0oIites

e Miocene Fig. ]Od

4- ÷ ÷

15"

::::::

NORTH

• ° °

Alterites on any +

parent rock C T

~ i ! l Marine or continental ~ series

Fig. lOa a

t CoasteI baslna t inner basins

CT L i t t l e thick Very thick

I Scattered Often thick llttle thick

Anyones Marines series Basnent-mar]

/or Contlnenta

:.:.:.:.:.:.:.:.:.:.:.;.:.:

Fig. I0. A: Schematic south-north section in West Africa showing that inner basins and coastal basins are not different in nature but in volume (and in nature of parent rocks) (After Lang et el. 1986}. B: SW Niger and N Benin (lullemmeden basin]. C: Coastal sedimentary basin without Miocene transgression influence. D: Coastal sedimentary basin with

Miocene transgression Influence. Fig. I0. A: Coupe sch~matique Sud-Nord en Afrlque de l'Ouest montrant qu'il n'y a pas de difference de nature entre

bassins internes et basslns c6tlers. El y a diff6rcncc de volume {ct de nature des roches meres) {d'apr~s Lang et oL 1986). B: mud-Guest du Niger et Benin Septentrional {Bassin des lullernmeden). C: Bassln s6dlmentaire c6Uer sans Influence

de la transgression miocene. D: Bassin s6dimentalrc c6ticr avcc influence de la transgression miocene.

96 J.I.,ANG et al.

a u t o c h t o n o u s f rom the a l lochtonous . The a u t o c h t o n o u s al teri tes, som e t i mes fossilized

u n d e r the "Cont inenta l Terminal" itself, cha rac te - rize anc ien t cl imatic his tor ies , of ten po lyphase and difficult to date. The "Cont inenta l Terminal" charac te r i zes a "terminal" geodynamica l his tory, as ind ica ted by its name , a n d whose age probab ly var ies here a n d the re wi th in the Cenozoic. The Cenozoic was d i s t r ibu ted over vas t a r eas of Africa, somet imes in coas ta l b a s i n s b u t mos t ly in i nn l and basins .

A c k n a w l c d g e m e n t , ~ The IGCP 210 project s u c c e e d e d t h e IGCP 127 p ro j ec t on t he "Cont inenta l Terminal ." It was originally p roposed by Professor F. Tess ie r to w h o m we are indebted. We t h a n k the geological ins i tu t ions of Benin, Niger and Nigeria, a nd the Direct ion des H y d r o c a r b u r e s de Cete d'Ivoire for helping in ou r invest igat ions. We ha d frui tful d i s cus s ions with M. G. Millot a n d M. N. Leneuf, to w h o m we express ou r gra t i tude.

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the continental terminal in west africa

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