Structure, evolution and palaeogeography of the West African craton and bordering belts during the Neoproterozoic

Premmbriun Reseurth ELSEVIER

Precambrian Research 69 (1994) 307-326

Structure, evolution and palaeogeography of the West African craton and bordering belts during the Neoproterozoic M. Villeneuve a, J.J. Corn6e b "Institut de G~odynamique, CNRS-UNSA, Sophia-Antipolis L 06560, Valbonne, France bCentre de SOdimentologie-Palkontologie, URA 1208 "Dynamique des platesformes carbonatbes", place VictorHugo, 13331, Marseille, Cedex 03, Case 67, France Received October 28, 1992; revised version accepted January 27, 1994

Abstract The West African craton consists of an Archaean or Palaeoproterozoic basement covered by sediments ranging from the Neoproterozoic to the present time. It is entirely surrounded by several Pan-African or Hercynian belts. The old basement is exposed in two shields, and the sedimentary units filled up three main basins: Tindouf, Taoudeni and Volta. The largest one, the Taoudeni basin, is located in the middle part of the West African craton. The Taoudeni basin is made of six different basins or troughs having a non-similar evolution. The structural and sedimentary evolution largely depends on the geodynamic evolution of the neighbouring belts. We have the intention to connect the sedimentary environments and the tectonics events that occurred around the craton. Five main tectonic events have been distinguished: the BI Anti-Atlas event on the northern margin (~ 685 Ma), the PanAfrican I tectonic event on the western side ( ~ 660 Ma), the Eastern Pan-African tectonic event ( ~ 600 Ma) from Morocco to Brazil, the Pan-African II tectonic event which occurred on the southwestern pan at ~ 550 Ma and, finally, the Hercynian tectonic event (~ 300 ma) on the northwestern margin. Lithostratigraphic correlations between the differents basins take into account this tectonic diachronism. Thus, we distinguished several oceanic, rift, glacial or molassic stages. For example, three glacial events have been recognized from 980 Ma to 500 Ma. Finally, we are proposing here four palaeogeological schemes illustrating four key periods of the Neoproterozoic evolution. This emphasizes the Pan-African structural inheritance.

1. Introduction The West African craton was located on the northwestern margin of the Gondwanaland. By late Precambrian time it was bounded (Fig. 1 ) by the Brazilian craton (southward), by the Benin-Nigeria shield (southeastward) and by the Hoggar shield (eastward). During the Carboniferous it was linked (northwestward) to the North American craton. This West African craton which widely outcrops (2500 × 1000 km),shows a "guitar" shape. Its basement (Ar© 1994 Elsevier Science B.V. All rights reserved SSD10301-9268 (94) 00029-Q

chaean and Birrimian) is considered as tectonically stable since 1700 Ma. This basement, built up during both the Archaean and Palaeoproterozoic orogens (Birrimian orogen, ~ 2000 Ma), outcrops in the Ivory Coast (or Leo) shield (south) and in the Reguibat shield (north). A small part of the Ivory Coast shield is now located in Brazil (Sflo Luis shield).This basement is covered by three main sedimentary basins: the Tindouf basin (in the northern part), the Taoudeni basin (in the central part) and the Volta basin located in the southeastern part. The

308

M. Villeneuve, J.J. Cornde /Precambrian Research 69 (1994) 307-326

Taoudeni basin is filled with both Proterozoic and Palaeozoic sediments, while the Tindouf basin is mainly made of Palaeozoic sediments and the Volta basin is mainly made of Proterozoic sediments. The West African craton is surrounded by both Pan-African and Hercynian belts. Pan-African belts (800-530 Ma) are running all around the craton except for the northwestern part, while the Hercynian belt is located on the northwestern part, from Morocco to Senegal. Large parts of these Pan-African and Hercynian belts are covered by Mesozoic basins particularly in the northwestern Brasil. The West African craton and adjacent belts were studied by many geologists. However, works were focused either on belts or on basins. Consequently, the linkage between sedimentary environment and tectonic events is not yet satisfactory. The aim of this work is to underline some close relationships between the sedimentary stages in the basins (from Neoproterozoic to the early Palaeozoic times) and the tectonic stages in the bordering Pan-African belts. During this period (800-500 Ma) four major Pan-African tectonic events have been distinguished around the West African craton. They probably had a strong influence on the sedimentation in the internal West African craton basins.

2. Geological setting of the West African craton

2.1. Pan-African tectonics at the boundary of the craton 2.1.1. The western margin The Pan-African belts extend over 1700 km, from the Reguibat shield to the southern part of the Ivory Coast shield (Fig. 1 ). A large part of these belts is concealed underneath the Palaeozoic or Meso-Cenozoic basins. On this western side, Sougy (1962) recognized two different belts: the Mauritanides in the north and the Rokelides in the south. Later on Villeneuve (1984) distinguished three different belts: the Mauritanides, the Bassarides and the Rokelides, from the north to the south. The Bassarides arose at ~660 Ma and the Rokelides at ~ 550 Ma

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Fig. 1. The main structural features of the West African craton and surroundings belts. Encircled numbers: /=Mauritanide belt; 2=Bassaride belt; 3=Rokelide belt; 4 = t h e Anti-Atlas belt; 5 = t h e Ougarta belt; 6=the Pharuside belt; 7=the Dahomeyide belt; 8 = t h e Borborema belt; 9=the Paraguay belt; 10= the Araguay-Tocatins belt; / l = the Gurupi belt. TfB=Tindoufbasin; Td.B=Taoudeni basin; CWA = West African craton; GS= Guyana shield; SIC:= S~o Francisco craton; SLC=S~o Luis craton; ICS=Ivory Coast shield. Legend. 1 = crystalline basement; 2= main Hercynian belts; 3 = Pan-African and Braziliano belts; 4 = foreland basins; 5 = thrust directions; 6 = thrusts, 7= basin boundaries.

(Villeneuve and DaUmeyer, 1987). The Mauritanides appeared at the same time as the Bassarides belt but they were largely reworked by the Hercynian orogen. Many papers point out the thrusting of this belts onto the West African craton. In the northern part (Mauritanides), the external nappes are thrusted over the Palaeozoic formations of the Taoudeni basin, while in

M. Villeneuve, J.J. Cornde / Precambrian Research 69 (1994) 307--326

Guinea, on the contrary, the Cambro-Ordovician Bov6 basin rests upon both the Bassarides and Rokelides (Sougy, 1962; Bassot, 1966; Villeneuve, 1984). In Sierra Leone and Liberia, the internal nappes of the Rokelide belt are thrusted over the Cambrian Rokelide formations (Thorman, 1976; Culver and Williams, 1979 ). Taking into account these observations, the western belts are considered as polyphased by various authors. According to new structural data, Villeneuve (1984) distinguished three different tectonics events, namely: (a) the Panafrican I tectonic event occuring in the Bassaride belt (Bassaride orogen); this belt is covered unconformably by the Cambrian shales of the Mali Group; (b) the Panafrican H tectonic event which deformed the shales of the Mali Group along the Rokelide trough (Rokelide orogen); (c) the Hercynian tectonic event responsible for the internal nappes emplacement in the Mauritanide belt (Hercynian orogen). Later on, these three different tectonic events were dated by the 4°/Ar39Ar radiometric method (Dallmeyer and Villeneuve, 1987; DaUmeyer and Lecorch6, 1989): 660-640 Ma for the PanAfrican I tectonic event, 550-530 Ma for the PanAfrican II tectonic event, and 320-270 Ma for the Hercynian tectonic event. Considering apart the Hercynian event which is linked to the Palaeozoic events, the main geodynamical stages occuring on this western margin during the late Precambrian can be summarized as follows (Fig. 2): (a) opening of a rift in the Bassaride area and probably of a small ocean (Villeneuve, 1984; Remy, 1987 ) in the Mauritanide area before 700 Ma; (b) westward-dipping subduction producing a calc-alkaline magmatism on the western margin (680-660 Ma); (c) collision between the West African craton (CWA) and the Senegalese block (SMB) at ~660 Ma (Pan-African I tectonic event) and deposition of molassic and glacial sediments between 660 and 640 Ma; the remnants of this sedimentation are now located in the Taban and Bakoye basins; (d) intracontinental rifting (Faleme and Rokelide troughs) and deposition of glacial and flysch type sediments before 550 Ma; (e) an intracontinental tectonic event producing a cryptic suture on the

309

southwestern margin of the West African craton at ~ 550 Ma (Pan-African II tectonic event), this event being followed by deposition of molassetype sediments mainly in the Youkounkoun basin; (f) the whole area was then covered by Palaeozoic epicontinental sediments starting with the lower Cambrian transgression (Villeneuve and Corn6e, 1991 ). The Pan-African I orogen is located in the Mauritanides and Bassarides and the Pan-African H orogen, which was produced by the closure of an intracontinental graben (Allen, 1969; ViUeneuve et al., 1990; Culver et al., 1991 ) is located in the Rokelide belt. 2.1.2. The eastern margin The belts on this margin are more than 3000 km long between Morocco and Benin. Four segments or belts can be distinguished from north to south: the Anti-Atlas belt, the Ougarta belt, the Western Pharusian belt (including the western Hoggar and the Adrar des Iforas) and the Dahomeyide belt. Except for the Anti-Atlas, these belts occurred at ~600 Ma (Caby et al., 1991; Affaton et al., 1991 ). (a) The Anti-Atlas belt (685-615 Ma), located in southern Morocco, was mainly studied by Choubert (1963), Leblanc (1975, 1981), Clauer (1976), Chariot, (1976, 1982), Jeannette and Tisserand (1977 ), Leblanc and Lancelot (1980), Jeannette et al. ( 1981 ), Mifdal and Peucat ( 1985 ), Schemerhorn et al. (1986), Hassenforder (1987), Bassias et al. (1988), Saquaque et al. (1989a,b, 1992). The foreland corresponds to the northern edge of the Tindouf basin on which is obducted the Bou Azzer-E1 Graara ophiolitic complex (Fig. 3). Northward pieces of a Pan-African subduction complex are outcropping but the northern block (NMC) that collided with the West African craton is concealed underneath the Palaeozoic and Mesozoic sequences of the Moroccan Meseta. The main geodynamic stages are deduced from radiometric data. Different geodynamic models have been proposed by Leblanc and Lancelot (1980), Schermerhorn et al. (1986) and Saquaque et al. (1992). Taking into account the different geodynamical interpretations and the geo-

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Fig. 2. Lithostratigraphic sequences of the southern Taoudeni and Volta basins. Diagrammes illustrating the geodynamical evolution of the Western (Mauritanides, Bassarides and Rokelides) and eastern (Pharusides, Dahomeyides) Pan-African belts. (A) Geodyl~amic interpretation of the western belts: / =intraexmtine~tal basins filled up with Supergronp 1 (Proterozoic sediments deposited between the Late Precambrian tillites); 2=opening of the Bassaride rift and Mauritanide sea; 3 = closure of the Manritanide sea (Pan-African I tectonic event); 4 = opening of the Early Cambrian troughs; 5 = closure of the Rokelide troughs (Pan-African II tectonic event); 6 = deposition of the Ordovician sandstones blanket; WAC= West African craton; SMB=Se~egalese block; Bs=Bassaride rift; Mk=Madina-Konta basin; Tb=Taban basin; Bk=Bakoye basin; Yk=Youkounkoun basin; PI=Panafrican I tectonic event; PII= Panafrican II tectonic event. (B) Lithostratigraphic sections of the southern Taoudeni basin: I = lithostratigraphic sequence of the Madina-Kouta and Tambaoura basins; IV= lithostratigraphic sequence of the Nara and Bamako areas; V= lithostratigraphie sequence of the Mopti trough; VI-lithostratigraphic sequence of the "s~rie pourpr~e" (Timimoun area); VI/=lithostratigraphic sequence of the Volta basin. Pan-African I and Pharusman pre-teetonic groups (mainly platform deposits ): SG = Segou Group; Mk = Madina-Kouta Group, Kl= Kolombine Group; St = Sotuba Group; Irm =Irma Group; Ou/Sn = Oualo Sarnyere Group; Pdj= Pendjari Group; Bel= Bambouaka Group. Pan-African I molassie groups: Bkg= Bakoye Group. Pan-African I] pre tectonic groups: M/= Mall Group; Tr="triade'; N a = N i o r o and Nara Group. Pharusian molassic groups: Bdg=Bandiag~ra Group; Spp=serie pourpr~e; Tml=lower part of the Tamale Group; Tml sup. =upper part of the Tamale Group. Pan-African II molassic groups: Yk = Youkounkoun Group; Pt = Cambro-Ordovician sandstones ( Pita Group); Eo = Cambro-Ordovician To, = Ordovician tillite. ! = First hypothesis for correlation of the Bandiagara Group. 2 = Second hypothesis for correlation of the Bandiagara group (see in text). (C) Geodynamic interpretation of the Pharusides/Dahomeyides belt: 1 = deposition of the lower part of Supergroup 1; 2 = opening of an eastern ocean (around 800 Ma ) and building of the Tilemsi Island Arc. 3 =internal deformations into the Tilemsi Island Arc (collision with the Hoggar shield); 4 = Eastern Pan-African tectonic event and possibly deposition of the Bandiagara Group; 5 = deposition of the Tamale Group and the "serie pourpr6e, g':4C = West African craton: ESC= Eastern Saharian continent: Tlc= Tilemsi Island Arc: 7)n/= Tamale Group; Slop= s0rie pourpr&: Bde= Bandiagara Group.

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Fig. 3. Lithostratigraphic sequences of the northern Taoudeni and northern Tindouf basins. Diagram illustrating the geodynamic evolution of the AntiAtlas Pan-African belt. (A) Lithostratigraphic sequences in: / = t h e Madina-Kouta and Tambaoura basins; H = A d r a r basin; III=Hank basin; VIII= northern part of the Tindouf basin; Br= Birrimian basement. Pre-phase B 1 groups (Anti-Atlas): PI= Precambrian I; PIE = lower part of Precambrian II. Pan-African I and Pharusian pre-tectonic groups (mainly platform deposits; SG = Supergroup ): Sg= Segou Group; Mk. = Madina-Kouta Group; Ch. =Char Group; At. =Atar Group; Mr. = E l Mreiti group; Ah. = Assabet el Hassiane Group. Post-BI tectonic deposits (Anti-Atlas): PlIs=Upper part of Precambrian II (Tidiline, Siroua and Anezi Groups). Pan-African I molassic groups: Bky. =Bakoye Group. Pan-African II pre-tectonic groups. M/.=Mali Group; Tr.="triade"; Be.=Bhtat Ergil Group; KN.=Kreb en Naga Group; Fa.=Fal6m6 Group. Post-phase B2 (Anti-Atlas): PIII= Precambrian III (Ouarzazate Group); Ad. = Adoudounian Group; E. = Middle and Upper Cambrian; O. = Ordovician. Pan-African II molassic and post molassic groups: Yk. = Youkounkoun Group; Ouj. = Oujeft Group; Co/Pt. = Cambro-Ordovician sandstones; To. = Ordovician tillite; S. = Silurian. (B) Geodynamic evolution of the Anti-Atlas belt: / = o c e a n opening in the northern part of the West African craton (788 Ma); 2=collision in the Siroua/Sahro Island Arc ( 7 4 0 - 7 2 0 Ma); 3=ophiolites obduction onto the West African craton (B1 tectonic event at 685 Ma); 4 =collisionnal stage in the Anti-Atlas (B2 tectonic event at 615 Ma); 5 = molassic stage. CWA = West African craton; CNM= North Moroccan continent; SSi= Siroua/Sahro Island Arc; TZT= Tizi'n Test fault zone.

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M. Villeneuve, J.J. Cornbe / Precambrian Research 69 (1994) 307-326

chemical data from the Bou Azzer ophiolites (Bodinier et al., 1984) we propose here the following stages (Fig. 3), modified from those of Schermerhorn et al. (1986 ). ( 1 ) According to Clauer (1976) there is an accretion of oceanic crust (MORB type) to the north of the West African craton at ~ 788_+ l0 Ma. An island arc is postulated by Schermerhorn et al. (1986) and a back-arc basin identified in the Bou Azzer ophiolites by Bodinier et al. (1984). (2) According to Saquaque et al. (1989b), to the north ofBou Azzer (in the Sahro inlier), volcanics and volcaniclastics assumed to be the remnants of a talc-alkaline arc were strongly deformed (ductile in type) at 754 and 722 Ma. This might be produced by a local collision between an island arc and a supposed northern isolated block. (3) According to Leblanc and Lancelot (1980), the Bou Azzer ophiolitic complex was obducted onto the West African craton at ~ 685 Ma. This ophiolitic complex was associated to an andesitic island arc and a northern fore-arc (Schermerhorn et al., 1986). The suturing zone of Bou Azzer is thought to have been created by an initial north subduction zone and should be the site of the collision between the West African craton and a northernmost island arc (Saquaque et al., 1989b). Obduction and local collision are thought to be a tectonic event associated to a greenschist syntectectonic metamorphism dated at 688 + 15 Ma by Clauer ( 1976 ). This first folding episode is the B 1 Pan-African tectonic event of the Anti-Atlas belt (Leblanc, 1975). In this same episode, several ductile shear-zones were evidenced (Wallbrecher, 1988; Saquaque et al., 1989a). (4) Deposition of clastic sediments including the Tidiline tillites (Siroua-Saghro series, Tiddiline series, Anezi series) was associated with calc-alkaline magmatism which underlines an active margin environment. On the western part, the Anezi series was deposited before a granitic intrusion dated at ~ 660 -+20 Ma (Clauer et al., 1982). (5) The final collision stage associated to the B2 tectonic episode (Leblanc, 1975), occurred

between 615 Ma and 578 Ma (Ducrot, 1979; Jeannette et al., 1981 ). This collisional stage could be related to a collision with a northern Moroccan craton then incorporated in the Hercynian Moroccan Meseta. (6) Thick continental molasses with effusive acid volcanism and alkaline and calc-alkaline intrusions (between 615 and 563 Ma) were then deposited into graben or pull-apart basins (Ouarzazate and Tanalt series) during a postcollisional stage governed by a regional E-Wtrending extension (Hassenforder, 1987; Azizi et al., 1990; Saquaque et al., 1992). (7) During the latest Proterozoic and the Cambrian marine transgressions, sediments from shallow-marine environments progressively invaded the Tindouf and northern Moroccan basins (Destombes et al., 1985; Bernardin et al., 1988; Villeneuve and Cornre, 1991 ). (b) The Ougarta belt, located in the Western part of the Algerian Sahara, was recently studied by Kurek and Priedl (1987). The most important part of the Ougarta belt is concealed underneath the Palaeozoic or the Meso-Cenozoic sedimentary formations. Despite a lack of information, and according to Kurek and Priedl (1987), a subduction stage is supposed to be more recent than 685 Ma, and the final collisional stage occurred at ~ 600 Ma. Conglomerates and volcanic intrusions including rhyolites, dacites and andesites were emplaced between 600 and 500 Ma. (c) The Western Pharusian belt was mainly studied by Caby (1970), Bertrand et al. (1978) and Boullier (1991). The whole Hoggar shield emphasizes a polyphase history from the Palaeoproterozoic to the early Palaeozoic. We will focus our attention on the western part of this belt which is thrusted onto the West African craton. This part shows a wide system of external nappes thrusted over the Gourma aulacogen or over the West African craton basement, from northeast to southwest (Fig. I). The Pan-African suture (Amalaoulaou suture) was apparently eastward dipping. According to Caby (1989) and Caby et al. ( 1991 ), the geodynamic evolution of the eastern margin can be summarized as follows (Fig. 2):

M. Villeneuve, J.J. Corn~e / Precambrian Research 69 (1994) 307-326

( 1 ) deposition of clastic sediments and stromatolitic formations on the Tassendjanet platform between 1100 and 680 Ma; (2) building of an oceanic crust with ultramafic rocks and possible ophiolites ( ~ 800 Ma); (3) eastward-dipping subduction zone and building of an island arc (Tilemsi Island Arc) with calc-alkaline intrusions (mainly granodiorites) associated with greywackes and flysch facies rocks (green series)

313

between 730 and 620 Ma; (4) oblique collision between the Tilemsi Arc and the West African craton at ~ 620-600 Ma and deformation of the foreland sedimentary sequences; (5) deposition of molassic formations (serie pourprre) in several large intracontinental basins with a N-S trend, between 600 and 550 Ma; these molassic formations including glacial sediments (tillites) were associated with rhyolites and alkaline granitic intrusions. (d) The Dahomeyide belt (or Beninian belt), studied successively by Bessoles and Trompette (1980), Affaton (1987), Caby (1989) and Affaton et al. (1991), shows a very similar structural model as the Pharusian one (Fig. 1 ). The nappe complex is widely thrusted over the Volta basin which is folded beneath the foreland nappes. This foreland nappe complex is almost 100 km wide and moved from east to west. According to Affaton et al. ( 1991 ), a similar geodynamic evolution can be considered regarding the Dahomeyide fold belt. The opening of an oceanic domain probably occurred between 1100 Ma and 700 Ma, followed by a convergent stage with eastward-dipping subduction. The collision with the West African craton is estimated at ~ 600 Ma. Molassic deposits are mainly deposited in the Tamale basin.

2.1.3. The southern margin (northeastern Brazilian shield)

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Fig. 4. Palaegeographical sketch map at ~ 700 Ma: 1 = marine domain; 2 = island arc domain; 3 = epicratonic and continental domain; 4 = basement; 5 = limit of basins; 6 = subduction zones; 7=oceanic ridges; 8 = d i r e c t i o n of transgressions. Rs=Reguibat uplift; IC.S= Ivory Coast craton; SNB = Senegalese block; GS= Guyana shield; SFC= S~o Francisco craton; SR/SH=Siroua/Sahro Island Arc; B a = B o u Azzer inlier; Ki= Kerdous inlier; li= Ifni inlier; Zb. = Zemmour basin; A m = M a u r i t a n i a n Adrar; R t = R i c h a t trough; H b = H a n k basin; BS=Bassarides rift; M K = M a d i a n Kouta basin; T b = T a m b a o u r a basin; A f H = Affol6 high; GA = Gourma aulacogen; Vb.=Volta basin; Ach.H. =Achkaikar high; Tim = Tilemsi Island Arc.

According to several authors, the southern part of the West African craton can be observed in northeastern Brazil (S~o Luis craton) where it is surrounded by the Gurupi (west) and the Borborema (east) belts. This part of Gondwana was largely studied by Brazilian geologists (Almeida et al., 1981; Cordani et al., 1984) as well as by other geologists (Trompette, 1988; Caby, 1989; Caby et al., 1991 ). This area is largely covered by three Palaeozoic and Mesozoic basins (Amazonian basin, Parnaiba-Maranhao basin and Parana basin.). The old basement can be divided in two main units: ( 1 ) Archaean and Palaeo- to Mesoproterozoic shields, namely the Guyana shield, the Amazonian shield, the S~o Francisco craton and the S~o Luis craton. Some of them, like the Sao Fran-

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M. Villeneu ve, J.J. CornOe / Precambrian Research 69 (1994) 307-326

cisco craton are blanketed by a Neoproterozoic sedimentary cover (Bambui Group). (2) Neoproterozoic mobile belts made of Neoproterozoic sediments and Archaean and Proterozoic metamorphic rocks reworked during the later Brazilian ( = Pan-African) orogen (700-600 Ma). Several belts can be linked to this Brazilian (=Pan-African) orogen, namely: the Borborema belt of northeastern Brazil, the Gurupi belt (southwest of the Silo Luis craton), the Araguay-Tocatins belt and the Paraguay belt, both located on the eastern margin of the Amazonian craton and, finally, the Brazilian belt on the western side of the Silo Francisco craton. This paper only concerns the Brazilian belt which extends southwards the West African Pan-African belt. So we will give a short overview of the Borborema, Gurupi, and Araguay-Paraguay belts. ( a ) The Borborema belt is composed of Archaean and Palaeoproterozoic rocks strongly reworked by the Braziliano/Pan-African tectonometamorphic event (Caby et al., 1991). The nappes with a westward or a southwestward displacement are emplaced between 620 and 580 Ma. They are crosscut by WSW-ENE or SW-NE lineaments as the SW-NE Transbraziliano lineament (TBFZ) which corresponds to the southern extension of the African Kandi fault zone (Caby, 1989 ). These lineaments reflect a dextral strike-slip motion (Corsini et al., t 991 ). Molassic basins or troughs located along these fault zones, are dated by granitic intrusions at ~ 550 Ma (Nascimento et al., 1981 ). (b) The Gurupi belt is composed of schists, biotite-schists and gneisses outcropping south of S~o Luis city. They display a biotite age of 504 and 520 Ma (Cordani et al., 1984). (c) The Araguay-Tocatins belt, recently studied by Herz et al. (1989 ), is a Mesoproterozoic suture (Urua~uano tectonic event dated at 1100 Ma) reactivated at 550 Ma. However, Neoproterozoic sediments like the conglomerates of the Rio das Barreiras formation were deposited along this suture. (d) The Paraguay belt was well studied by AIvarenga (1990). It is considered as a continental trough filled with glacio-marine sediments, do-

lomites and arkosic sandstones. The fold vergence is rather to the east but not towards the Amazonian craton. According to Alvarenga (1990) the glaciogenic sediments were deposited before 600 or 630 Ma and suffered a thermotectonic event at ~ 547 + 05 Ma (diagenesis of argilites). Post tectonic granites intruded these formations at ,,, 500 Ma. Despite a large number of papers on the Brazilian geology, very few are published concerning the geodynamic model of the Braziliano ( = Pan-African) belts. However, two main tectonic events can be distinguished in these belts. The first one (Borborema belt) corresponds to the Brazilian orogen occurring at ~600 Ma (620-580 Ma) on the eastern side. The second one corresponds to an early Cambrian orogen (550-530 Ma) which occurs mainly on the western side (Paraguay, Araguay and Gurupi belts ).

2.1.4. The main Pan-African (Brazilian) orogens Around the West African craton, four separate tectonic events corresponding to different orogens and belts are distinguishable, namely: (1) The Anti-Atlas B1 tectonic event (685 Ma) which is related to an ophiolite obduction along the Bou Azzer fracture. (2) The Pan-African I tectonic event (660640 Ma) which corresponds to a rift closure on the western side of the West African craton (Bassarides). (3) The Eastern Pan-African tectonic event (620-600 Ma) which corresponds to the ocean closure on the eastern side of the West African craton (Pharusides, Dahomeyides and Borborema). The final collision (Anti-Atlasides ?) between the northern part of the West African eraton and an unknown northern Moroccan craton located north of the Anti-Atlas is contemporary (615-585 Ma). (4) The Pan-African II tectonic event (550 Ma) which corresponds to an intracontinental trough deformation along the southwestern side of the West African craton. This tectonic event includes the Rokelide, the Gurupi and the Paraguay belts. This list does not include other tectonic events

M. Villeneuve, J.J. Corn~e / Precambrian Research 69 (1994) 307-326

3l 5

occurring in the inner part of the surrounding belts like, for example, the different tectonic events of the Hoggar shield. 2.2. Location and stratigraphy of the main basins on the craton 2.2.1. General overview on the the West African craton structure The West African craton (Fig. 5) includes three main basins (Tindouf, Taoudeni and Volta) separated by two old Precambrian shields (Reguibat and Ivory coast). According to gravimetrical (Lesquer et al., 1984) and geological interpretations, the Taoudeni basin, located in the central part of the West African craton can be divided into several sub-basins or troughs. A synthetic view of the basement has already been done by Bessoles ( 1977 ) and Rocci et al. ( 1991 ). So, we will focus the present study on the sedimentary basins or troughs in Fig. 5. 2.2.2. The Tindoufbasin This is a deep (7 to 8000 m) and asymmetric basin which extends over 150,000 km 2 as an open syncline. Deformations are increasing to the north and to the west (Sougy, 1962; Hassenforder, 1987). The Neoproterozoic and Cambrian formations outcrop essentially on the northern edge which is also the foreland of the Anti-Atlas Pan-African fold belt. Proterozoic carbonates occur also in the Zemmour area (Sougy, 1964) underneath the Hercynian thrusts. On the southern part of the Anti-Atlas, Leblanc ( 1975 ), Jeannette et al. (1981) and Hassenforder (1987) distinguished three Proterozoic series (Fig. 3, log VIII) overlain by shallow-marine sediments (Adoudounian carbonates) which are mostly early Cambrian in age (Destombes et al., 1985 ). To the base they pointed out a schisto-quartzitic formation with interbedded ultrabasic rocks. This formation, interpreted as a passive margin deposit, is capped by tillites, argilites and calcalkaline volcanic intrusions. These volcaniclastic sediments (Precambrian I1-III or Tidiline series) are interpreted as active-margin-derived materials. The last Proterozoic series corresponds to a molassic deposit with alkaline rhyo-

Fig. 5. Structural sketch map of the West African basins. NM=northern Meseta; TZT=Tizi'n Test fault zone; Ou = Ougarta belt; RS = Reguibat shield; HS= Hoggar shield; ICS= Ivory Coast shield; GS= Guyana shield; AS=Amazonian shield; SLC=S~o Luis craton; SFC=S~o Francisco craton; CAC=Central African craton; Bch b. = Bechar basin; MM b. = Mahon basin; Ah b. = Ahnet basin; Aj b. = Ajjer basin; Ul b. = Iullemeden basin; Ben b. = Benou6 basin; Ab = Accra basin; Mh b. = Parnaiba-Maranhao basin; Am b.=Amazonian basin; SMb=Senegalo-Mauritanian basin; Tf b.=Tindouf basin; Rg b.=Reggan basin; HK b.=Hank basin; A=Adrar basin; Rt=Richat trough; H b . = H o d h basin; Ft=Faleme trough; BTF=Bissau Tombouctou fault zone; Tbb. = Tambaoura basin (Bakoye basin); MAb.=Madina Kouta basin; Yb=Youkounkoun basin; Bb.=Bov6 basin; Rk. =Rokelide trough; Nt=Nara trough; M t = M o p t i trough; GA=Gourma aulacogen; Vb.=Volta basin; Ma= Achkaikar high. 1 = sedimentary rocks; 2=Variscan orogen in North Africa and Faleme and Rokelide troughs in West Africa; 3=Pan-African orogens; 4 = Precambrian shields; 5 = thrusts and location of logs shown in Figs. 3 and 4; 6=limit of basins.

316

M. Villeneuve, J.J. Corn~e /Precambrian Research 69 (I 994) 307-326

lites and granites interpreted as post-orogenic deposits (Precambrian III or Ouarzazate series). Other Proterozoic rocks occur in the Zemmour (western part of this Tindouf basin ) area (Sougy, 1964). To the east, the Tindouf basin extends through the Reggane basin where the Palaeozoic sediments are capped by Mesozoic sediments.

2.2.3. The Taoudeni basin Firstly, this name was assigned to a tiny basin located in the northwestern part of the central syneclise around the village Taoudeni, in the Hank area (Villemur, 1967). The whole central depression, which extends over 2,000,000 km 2 was then called "Taoudeni basin". Now, we propose the name "Hank basin" for the small basin around Taoudeni, and to use the name "Taoudeni basin" for the whole central cratonic depression. Nowadays, the Taoudeni basin can be divided into eight different sub-basins with different structures and sedimentological evolutions. From north to south we distinguish (Fig. 5 ) the Adrar basin (NW), the Hank basin (NE), the Gourma aulacogen and the Mopti trough (SE), the Nara trough (south-central), the Tambaoura and Madina-Kouta basins (SW) and the Hodh basin (W). Each one can be well distinguished and is bounded by arches or highs like the Achkaikar high between the Hank and Gourma basins. The subsidence rates were not similar for all basins. Many lithostratigraphic correlations were carded out by Deynoux ( 1971 ), Trompette ( 1973 ), Sougy et al. (1985) and Bertrand-Sarfaty et al. ( 1991 ). Most of them took into account the four main lithostratigraphic sequences identified in the Mauritanian Adrar by Deynoux ( 1971 ) and Trompette (1973). From the top to the bottom they distinguished (log II, Fig. 3): sequence 4 (Devono-Carboniferous), sequence 3 (Ordovician and Silurian), sequence 2 (Cambro-Ordovician ), sequence 1 (Neoproterozoic). These sequences are separated by regional unconformities (Fig. 2) and glaciogenic levels (late Precambian tiUites separating sequences 1 and 2 and late Ordovician tillites separating sequences 2 and 3 ). Radiometric data by Clauer et al. (1982)

indicate the time of diagenesis of these different lithological units (see ages on log II, Fig. 3 ). This lithostratigraphic sequence was then extended to the whole Taoudeni syneclise. However, new structural and sedimentological studies (Moussine-Pouchkine and Bertrand-Sarfaty, 1978; Villeneuve, 1989; Deynoux et al.. 1989) pointed out many changes in the sedimentological environments of these basins: (a) The Adrar basin (log II, Fig. 3). According to Trompette (1973) the lithostratigraphic sequence is composed of the four main units previously described. Sequence 1 may be divided into two parts. The lower part (Char and Atar Groups) is made up by argilites, carbonates with Riphean stromatolites and sandstones from a marine epicontinental platform environment. The upper part (Assabet el Hassiane Group) is made up by marine siliclastic rocks which could be thicker than 3000 m in the Richat trough (Bronner et al., 1980). Ages of 998 ___32 for the base of the Char Group and 694 _+20 Ma for the base of the Hassabet el Hassiane Group are supported by Clauer et al. ( 1982 ). Higher up, the tillites and argilites of the lower part of sequence 2 could represent a marine lower Cambrian transgression which followed a glacial period. (b) The Hank basin. To the east, this basin exhibits a sequence more or less similar to the Adrar one (log III, Fig. 3 ). However, the equivalent of the Ass,abet al Hassiane Group is strongly reduced in thickness. (c) The Gourma basin. This basin includes the Gourma, the Mopti and the Nara troughs (logs IV and V, Fig. 2). The Gourrna trough itself is interpreted as an aulocogen connected to the East Pan-African sea (Moussine-Pouchkine and Bertrand-Sarfaty, 1978).The sequence includes three groups: the Lower Group (Irma Group), deposited on a stable cratonic platform; the Middle Group (Hombori or Oualo-Sarnyere Group), which is very thick (more than 6000 m) in the aulacogen centre and is evidence of the detrital in filling of this aulacogen (Vendian to Cambrian); the Upper Group (Bandiagara Group ), poorly deformed by the Pan-African tectonic event and considered by Bertrand-Sarfati et al.

M. Villeneuve, J.J. Cornbe/ Precambrian Research 69 (1994) 307-326

( 1991 ) as an erosive stage of the East Pan-African belt. However, fluviatile current directions indicate a transportation from the southwest to the north east, but not from the east to the west like molassic deposits should do. The two first groups were deformed during the Pan-African orogen. (d) The Tambaoura and Madina Kouta basins (Fig. 2). Both are located in the southwestern part of the Taoudeni basin. Five units have been distinguished (Deynoux et al., 1989; Villeneuve, 1989): unit I (Surokoto 1 and Segou Groups), which represents a platform deposit similar to the lower part of the Adrar basin sequence I; unit 2 (Surokoto 2 and Madina-Kouta Groups), which corresponds to a platform sequence; unit 3 (Bakoye and Taban Groups) supposed to be of molassic origin (ViUeneuve, 1989); unit 4 (Nioro and Mali Groups), which corresponds to a marine transgression (Early Cambrian transgression ) over the West African craton, unit 5 (Youkounkoun Group), reputed from molassic origin. These molassic sediments mainly occur beneath the Palaeozoic Bov6 basin. 2.2.4. The Volta basin This foreland basin (log VII, Fig. 2 ) is located in the eastern part of the Ivory Coast shield and it extends over 450,000 km 2. Affaton (1987) distinguished two superimposed basins: ( 1 ) A lower basin filled with clastic sediments deposited between ~ 1000 and 600 Ma. The lower part (Bombuaka Group), deposited before 660 Ma, mainly consists of sandstones and shales. The upper part (Pendjari Group), including glacial sediments ( ~ 660 Ma old), consists of tillites, shales, siltstones, limestones, greywackes and phosphatic siltstones. Affaton (1987) thought that this basin represented a passive-margin environment. (2) An upper basin (Tamale Supergroup), filled with shales, sandstones and conglomerates including several levels of tillites. According to Affaton ( 1987 ) it could be a molassic basin. Its age is still not known precisely, but it could be younger than the lower part of the "srrie pourprre" (Affaton, 1987). To summarize this section devoted to the in-

317

tracratonic basins we will point out some lithostratigraphic and environmental differences (rift sequence, aulacogen sequence, platform sequence, glacial and molassic environments, etc. ) between the basins which overlie the West African craton. That probably reflects the influence of local tectonosedimentary environments related to the closest Pan-African belts.

3. Palaeogeographical reconstructions of the West African craton and its borders during the Neoproterozoic 3.1. Lithostratigraphic correlations Previously, the correlations were based upon lithologic reference levels like the Neoproterozoic tillite (620-600 Ma) or the late Ordovician tillites (440 Ma). Unfortunately, it is difficult to check the duration of a glacial event. However, other glacial events occurred like the Volta basin glacial event (660 Ma). Another solution is to correlate the sedimentary environments with the geodynamic stage of the adjacent belts. This has already been done on the eastern side (BertrandSarfaty et al., 1977) as well as on the western side (Villeneuve, 1988) of the West African craton. An attempt has been made to correlate both sides of the West African craton (Bertrand-Sarfaty et al., 1991 ) but the geodynamic evolution of the western belts was not yet clear enough. Figs. 2 and 3 present new correlations based upon the geodynamical constraints as described above. The southwestern log (log I, Figs. 2 and 3 ) is used as a lithostratigraphical reference for the entire Taoudeni basin. (a) The Madina-Kouta Group (log I) is correlated to the Oualo Sarnyere Group (log V), the Assabet el Hassianne Group (log II) and the Cheikia Bir Amrane Group (log III). These groups are linked to the opening of oceans and troughs which occurred before 660 Ma on both (east and west) sides of the West African craton. The Hassabet el Hassiane group, beginning at ~694 Ma (Clauer, 1976) is 3000 m deep in the Richat trough and it could be linked to the PanAfrican I rift in the Mauritanides. According to

318

M. Villeneuve, J.J. Cornke / Precambrian Research 69 (1994) 307-326

I

Fig. 6. Palaeogcographical sketch map at ~650 Ma: 1 = marine domain; 2 = island arc domain; 3 = marine epicontinental domain; 4= Bassarides molasses; 5=tfllites ( T l ) ; 6 = Pan-African belts; 7=cratonic domains; 8 = supposed block displacements; 9=subduction zone; lO--Pan-African thrusts; / / = b a s i n boundaries; 12=age of tectonic e v e n t , Bky= Bakoye Group; Tbn = Taban Group; Tb= Tambaoura basin; ESC=eastern Saharan continent. For further explanation see Fig. 4.

Bronner et al. (1980), the average subsidence rate in the Richat trough (Assabet el Hassiane group) was about 10 m/year, while it was less than 1 m/year during the previous periods (1000-800 Ma). This indicates the importance of the tectonic subsidence during this period. (b) The Bakoye Group (log I, Fig. 2), including several levels of tillites, has been considered as the molassic stage of the Pan-African t orogen (Bassaride belt) by Villeneuve ( t 988). It has not been observed in the Adrar area (tog II). The

Fig. 7. Palaeogcographical sketch map, from 620 to 580 Ma: l = volcanic rocks; 2 = basement; 3 = Mali Group tillite ( T2 ); 4 = marine epicontinental domain; 5 = Pharusides, Dahomeyides and Borborema molasses; 6=marine domain; 7=PanAfrican thrusts; 8 = main strike slip faults; 9= supposed mass displacements; 10=troughs boundaries. NMC=north Moroccan craton; Ft=Faleme trough; Rt=Rokelide trough; ARb=Araguay Tocatins trough; Gt=Gurupi trough; TBFZ=transbraziliano lineamant; K F Z = K a n d i fault zone (Trans-saharan lineament); PpFZ=Pompeu lineament: PFZ= Patos lineament; PnbFZ= Pernambuco lineament: AF= Amadaoua lineament: ICS= Ivory Coast shield,

Bandiagara Group (log VI ) has not yet been correlated with another group. Either it is the eastern extension of the Bakoye Group but without glacial markers (solution 1 in Fig. 6), or it represents continental sediments deposited in the Mopti and Gourma troughs, after the deposition of the Bakoye Group and during the arising of the Pharuside belt (solution 2 in Fig. 6 ). Northwestward sediment transportation is against a

M. Villeneuve, J.J. CornOe/ Precambrian Research 69 (1994) 307-326

319

bonates, jaspers and sometimes phosphatic deposits. This reference level occurs at the base of the Mali group. But this "triade" has not yet been observed in the eastern basins. So, a correlation between this Mali Group and the base of the "sede pourprre" (log VI) or the Tamale Group (log VII) is proposed, considering the geodynamic environment in the Pharuside belt. Fossils from the carbonates associated with the "triade" of eastern Senegal indicate an early Cambrian age (Culver et al., 1988). (d) The Youkounkoun Group (log I) which represents the molassic stage of the Rokelide belt (ViUeneuve, 1988) is correlated with the lower part of the Oujeft Group (log II). To the east it can be correlated with the upper part of the "srrie pourprre" (Deynoux, 1980; Fabre, 1982). Correlations between the Taoudeni basin and the northern part of the Tindouf basin are still inaccurate, due to the specific evolution of the AntiAtlas belt.

3.2. Sedimentary environments Regarding the above-mentioned geodynamical evolution, several sedimentary environments can be distinguished. Fig. 8. Palaeogeographical sketch map at ~520 Ma: 1 =marine domain; 2 = marine epicontinental domain (scolithic sandstones); 3 = sandstones and siltstones; 4 = molassic deposits; 5 = Tamale Group tillites (T3); 6 = Rokelides, Gurupi and Araguay-Tocatins belt; 7=basement; 8--sediment transportation; 9= deltaic formations; 10 = major strike-slip faults; l l=Rokelide thrust; 12=grabens or oceanic hinge zone; 13=supposed continental mass displacements direction. RS=Reguibat shield; ICS=lvory Coast craton; GC= Guyana craton; AA = Anti-Atlas area; Yb = Youkounkoun basin; Rk.b = Rokelide belt; Gb. = Gurupi belt; Pb. = Paraguay, Araguay, Tocatins belt; Tm= Tamale group; Al--Iforas Adrar; H = H o g g a r shield; Ou=Ougarta trough; TBFZ=Transbraziliano lineament (=Sobral lineament); TSFZ=Kandi lineament (=Trans-saharan lineament); SPEZ= Borborema fault zone.

molassic interpretation (Pharuside molasses). (c) The Mali Group (log I) is correlated with the Nara Group (log IV), the Bthat Ergil Group (log II) and the Kreb en Naga Group (log III). These correlations are based upon the "triade" which is a reference level including tillites, car-

3.2.1. Epicontinental platform sedimentation environment Epicontinental and platform sedimentation have been emphasized all over the Taoudeni basin between 1000 and 800 Ma (Trompette, 1972; Bertrand-Sarfati et al., 1991). BertrandSarfati et al. (1991) distinguished a siliclastic platform (Segou Group, Char Group, etc. ) and a carbonate platform with biostromes (stromatolites), including the Atar Group. The Mali and correlated groups deposited during the Neoproterozoic or early Cambrian transgression indicate another silicastic epicontinental platform environment. Several troughs like the Faleme and Rokelide troughs were created in this platform. 3.2.2. Marine passive margin and rift environment Marine passive-margin sedimentation was evidenced on the northern and eastern margins of the West African craton between 800 and 600

M. Villeneuve, J.J. Corn~e /Precambrian Research 69 (1994) 307-326

320

I a p e t u s ocean

Saharian sandstones

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M. Villeneuve, J.J. Corn?e /Precambrian Research 69 (1994) 307-326

321

Ma. The Pendjari Group of the Volta basin is the best example. Rift sedimentary series were argued in the Bassaris ridge and parts of the Mauritanide belt (Dupont et al., 1983; Villeneuve, 1984; Remy, 1987).

the tillites of the upper part of the "serie pourprre" (Moussine Pouchkine et al., 1988 ) and of the Tamale Group (Affaton, 1987 ) (T3), occurring between 520 and 480 Ma (comtemporary with the Youkounkoun Group).

3.2.3. Island-arc deposits Sedimentary rocks (andesites, greywackes, cherts, etc.) were recorded from the Tilemsi (Pharusides) and Sahro (Anti-Atlasides) areas.

3.2.6. Molassic deposits Different molassic stages have been evidenced. They are linked to the main tectonic events mentioned above: (a) the Tidiline and Anezi series representing the erosional stage of the B 1 tectonic event in the Anti-Atlas area (between 685 and 660 Ma); (b) the Taban and Bakoye Groups representing the erosional stage of the Pan-African I tectonic event on the western margin (between 660 and 640 Ma); (c) the "serie pourprre", the Tamale Group (lower part) and the Precambrian III (Ouarzazate serie), respectively representing the molassic stages of the Pharusides, Dahomeyides and Anti-Atlas belts (between 600 and 550 Ma); (d) the Youkounkoun Group, the Upper Tamale Group, the upper part of the "serie pourpr6e" (between 550 and 480 Ma).

3.2.4. Aulacogen deposits A typical aulacogen sequence was studied in the Gourma basin (Moussine-Pouchkine and Bertrand-Sarfaty, 1978 ). The Richat trough and the Madina-Kouta basin which are in a similar geodynamic context, do not present such a sequence. 3.2.5. Glacial-deposition records At least, three glacial periods can be distinguished during the Late Precambrian or Cambrian times: (a) the Bakoye Group Tillites (TI), deposited at ~ 660 Ma (680-640 Ma ?); this glacial episode was in a molassic context on the western part (Villeneuve, 1988; Deynoux et al., 1989 ), in a passive-margin context in the Volta basin (Affaton, 1987), and in an active-margin context in the Anti-Atlas area; (b) the Mali Group tillite (T2) which occurred between 620 and 570 Ma was in a marine epicontinental context on the western part (associated with jaspers and carbonates) and in an intramontanous context on the eastern part (Caby and Moussu, 1967; Fabre, 1982); according to Alvarenga (1990), the glacial sediments of the Paraguay belt (Brazil) are in a marine epicontinental context; (c)

3.3. Main geodynamical stages According to these correlations we here propose five palaeogeographic sketch maps, showing some of the main important geodynamical steps of the West African craton evolution.

3.3.1. Palaeogeographical sketch map: ~ 700 Ma (Fig. 4) The West African craton was surrounded by three main oceanic domains, on the eastern, northern and western parts. On the southwestern

Fig. 9. Cartoon reflecting the geodynamic evolution of the West African craton. From the bottom to the top we can see eight different stages: intracratonic stage, opening of the eastern oceanic domain, opening of the Bassaride rift, Pan-African I tectonic event (Bassarides and older Mauritanides belts), Eastern Pan-African belts, Lower Cambrian transgression, Pan-African II tectonic event (Rokelides), deposition of the Cambro-Ordovician sandstone blanket and connection with the lapetus Ocean. 1 = island arc domain; 2 = siliclastic marine deposits; 3 = molassic deposits; 4 = marine epicontinental sandstones with Scolithos; 5 = tillites or glacial deposits; 6 = calk-alcaline intrusions; 7= oceanic crust; 8 = continental crust. Mk. = Madina-Kouta basin; Tb. = Tarnbaoura basin; Ba. = Bamako area; Gm. = Gourma area; Bs. = Bassaris area; IF. = Iforas area; SMB. = Senegalese block; Mauri. =Mauritanian domain; Mp. = M o p t i trough; Tim. =Tilemsi Island Arc; Tb.n. =Taban basin; Bky. =Bakoye basin; Bg. = Bandiagara sandstones; H S = Hoggar shield; Ft. = Falrm6 trough; Hd. = H o d h basin; Sp. = srrie pourprre; Yk. = Youkounkoun basin.

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part it was connected to the Guyana shield. Orogenic island arcs were demonstrated on the northern and eastern margins. But the Anti-Atlas presented a southward-dipping subduction zone, while the eastern margin presented an eastward-dipping subduction zone. The West African craton itself was largely covered by sediments, particularly in its central part. Several types ofgeodynamic contexts can be distinguished: marine passive-margin platforms in the vicinity of the Anti-Atlas or Volta basin, aulacogen or continental trough in the Gourma, the Richat and the Mopti basins, rift in the Bassaride ridge, sag basins in the Hank, Tambaoura and Madina-Kouta basins. Marine transgressions were running from the west in the Madina-Kouta basin (Villeneuve, 1984) and from the east in the Gourma aulacogen (Moussine-Pouehkine and Bertrand-Saffati, 1978). The global tectonic regime was largely "extensive"; this being in contrast with a previous stable tectonic-period mainly represented by a thin carbonate platform (Bronner et al., 1980).

3.3.2. Palaeogeographical sketch map: ~ 650 Ma (Fig. 6) Two separate orogens were demonstrated in this part of Africa: the southern Anti-Atlas orogen before 685 Ma and the Pan-African I orosen in the Bassaride (and Mauritanide belt) at ~ 650 Ma. In these two parts, some molassic-type sediments were deposited either in pull apart basins (Anti-Atlas or south Guinea) or in continental troughs (Senegal, north Guinea and Mali). We notice a large occurrence of volcanic rocks in the Anti-Atlas area. An oceanic domain still remains in the northern part of the Anti-Atlas area and it separated the Anti-Atlas Island Arc (SirouaSahro Island Arc) from a hypothetic northern continent (north Moroccan continent). The southward subduction produced the cale-alkaline rocks which fed the island arc. On the eastern margin, sedimentation on the Iforas and Volta passive margin was still active. The Tilemsi Island Arc was still operating with deposition of tale-alkaline volcano-sedimentary rocks. Glacial deposits were identified in the

Volta basin, in the Anti-Atlas area, in southeast Mali and in north Guinea. The glacial formations of the Volta basin presenting the so-called "triade" association (tillites, silexites and carbonates rocks) were in a submarine environment, while those of the Anti-Atlas and Mauritania areas (without this typical association) were in a continental environment.

3.3.3. Palaeogeographical sketch map." 620-580 Ma (Fig. 7) During this period, a Pan-African tectonic event occurred around 600 Ma (620-580 Ma) on the eastern margin. This event stopped the marine sedimentation (mainly in the Gourma aulacogen). Basins with molassic-type deposits occurred along this new belt particularly in the north of the Hoggar area and along the Transbraziliano lineament (TBFZ) in northeastern Brazil. TiUites were pointed out by Caby and Moussu (1967) in the northwestern Hoggar (srrie pourprre). At the same time a new tectonic event was recorded in the Anti-Atlas belt with a NE-SW compressional constraint on the eastern part and a N-S compressional constraint on the western part. This tectonic event seems to be related to the collision between the north Moroccan continent (CNM) and the Siroua-Sahro Island Arc. The talc-alkaline volcanic rocks related to the southward subduction turned into alkaline intrusions related to a crustal fusion process. Molassic sediments were deposited in "pull-apart basins". On the western margin, on the contrary, we observed the deposition of glaciogenic sediments then covered by argilites and sandstones ascribed to the Early Cambrian transgression. A N-S network of troughs occurred from the Mauritanide to Brazil. Glacial sediments from this period were also discovered in Liberia (Culver and Williams, 1979). So, the tectonic regime and consequently the behaviour of the western and eastern margins were completely different: compressive in the eastern and northern margins, extensive in the western margin.

M. Villeneuve, J.J. Corn~e / Precambrian Research 69 (1994) 307-326

3.3.4. Palaeogeographical sketch map: ~ 520 M a (Fig. 8) A new belt occurred on the southwestern part, due to the northeastward moving of the Guyana and Amazonian shields. These belts are related to the closure of the Rokelide, Gurupi, Paraguay and Araguay intracontinental troughs. The thrusting of the Rokelide belt over the West African craton is well demonstrated in the Liberian Gibi Mountain klippen (Thorman, 1976 ) and in the central part of Guinea (Villeneuve, 1987). Geochronological investigations (Hurley et al., 1971; Cordani et al., 1984) pointed out an EarlyMiddle Cambrian orogen (550-520 Ma) along the Rokelide belt and along the Araguay belt and at about 520-504 Ma along the Gurupi belt. Molassic formations from these belts were there deposited specially in the northern part (Youkounkoun Group) where the Youkounkoun basin is filled up with 3000 m of red conglomeratic sandstones. South of the Reguibate shield, epicontinental scolitic sandstones were deposited. North of the Reguibat shield, the Adoudounian sea settled from the Zemmour area to the Ougarta area. A graben structure in the western part of the AntiAtlas was reported by Villeneuve and Corn6e (1991 ). Many rhyolitic and granitic intrusions were pointed out in the Adoudounian formations. The Jbel Boho syenite, interbedded in the Adoudounian limestones, displays an age of 534_+ 10 Ma. Molassic deposits with tillitic conglomerates were observed along the main N-S lineament in the Kandi fault zone (eastern side of the West African craton). These conglomeratic basins are linked to the Cambrian activity (565-535 Ma) of the main Pan-African lineaments. They frequently are associated with alkaline volcanics and granitic rocks (Nigerian and Cameroun young granites) emplaced at ~ 550 to 480 Ma. A foreland molassic basin exists on the top of the Volta basin (Tamale basin).

4. Conclusions

Comparisons between the evolution of basins and adjacent belts allow us to propose four pa-

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laeogeographical schematic maps reflecting the evolution of the West African craton. Fig. 9 shows the diachronism between the Late Precambrian tectonic events occurring on both sides of the West African craton. These Pan-African events did not occur at the same time all around this craton. For example, the Bassarides and Rokelides located on the western margin are respectively older and younger than the Pharusides and Dahomeyides located on the eastern margin. Consequently, the sedimentary environments (platform, aulacogen or molassic environments) of the foreland basins cannot be similar in the same period on both sides. So the correlation based upon lithologic similarities should be replaced by correlations supported by sedimentological investigations and geochronological data. In the same period (at ~ 650 Ma) molassic sediments were deposited on the western side, while a platform environment is still present on the eastern side of the West African craton. However, between 620 and 580 Ma, platform sediments were deposited on the western side, while some small molassic basins occurred along the eastern Pan-African belt. Later on (at ~ 520 Ma), a lot of molassic deposits covered both the eastern and western sides, while a marine epicontinental platform invaded the northern part of this West African craton. The palaeogeographical sketch maps also underline the structural inheritance from the basement. )

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