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10. Sedimentary and Structural Characteristics of the Cretaceous Along the Côte D’Ivoire-Ghana Transform Margin and in the Benue Trough: a Comparison1

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10. Sedimentary and Structural Characteristics of the Cretaceous Along the Côte D’Ivoire-Ghana Transform Margin and in the Benue Trough: a Comparison1 Mascle, J., Lohmann, G.P., and Moullade, M. (Eds.), 1998 Proceedings of the Ocean Drilling Program, Scientific Results, Vol. 159 10. SEDIMENTARY AND STRUCTURAL CHARACTERISTICS OF THE CRETACEOUS ALONG THE CÔTE D’IVOIRE-GHANA TRANSFORM MARGIN AND IN THE BENUE TROUGH: A COMPARISON1 Jean Benkhelil,2 Jean Mascle,3 and Michel Guiraud4 ABSTRACT The Côte d’Ivoire-Ghana Marginal Ridge (CIGMR) and the Benue Trough were formed during the Early Cretaceous under similar tectonic conditions and continental sedimentary environments. Early stages of sedimentary infilling are characterized by the deposition of subaerial, lacustrine, deltaic, and transitional marine sediments, successively, from the Late Jurassic to the Albian. Intracontinental transcurrent faulting, occurring in the Aptian–Albian, was superimposed on earlier rifting structures, resulting in pull-apart sub-basins. The extensional-transtensional tectonics caused unstable conditions during the sedimenta- tion, resulting in syn-sedimentary deformations such as water-escape structures, listric normal faults, slumps, and intraforma- tional conglomerates. From the Albian to the end of the Cretaceous, the two domains were under marine conditions, and there was deposition of shallow-water carbonates of fine terrigenous sediments. Compressional movements occurred in the late Albian–Cenomanian along the CIGMR and during the Santonian in the Lower Benue Trough resulting in intense deformations in both domains, with folding, cleavage, and shearing. Thermal events, partly related to the compressional phase, were recorded in the sediments of the two basins, including hydrothermal activity and low-grade metamorphic crystallizations. After the compressional event, the two basins have different evolutions; the CIGMR became a passive margin, with open marine sedimentation, while the Benue Trough was almost completely uplifted when a second compressional phase occurred in the Upper Benue Trough at the Cretaceous/Tertiary boundary. INTRODUCTION Tefidet Basin West African Termit-Agadem During the breakup of western Gondwanaland, the American and Basin Craton African continents did not split along their entire lengths simulta- yyyyyy neously. The last areas to be in contact around the South Atlantic yyyyyy were the West African and the Brazilian cratons during the Early Cre- yyyyyy yyyyyy taceous. yyyyyy Benue Trough The Equatorial Atlantic formed as a series of large pull-apart ba- yyyyyy yyyyyy sins, bounded by both rifted and transform segments (Rabinowitz and Deep Ivorian Lagos Basin yyyyyyAccra • Labrecque, 1979; Fig. 1). The deep Côte d’Ivoire-Ghana Basin and Abidjan • Ngaoundéré F yyyyyy• its southern border, the future Côte d’Ivoire-Ghana Marginal Ridge CIGMRRomanche FZ Natal• Chain FZ (CIGMR), is one such basin (Fig. 1). Farther east, within the re- Potiguar • Charcot FZ entrance of the West African coast, a large intracontinental basin, the São Luis Basin Patos F Parnaïba Congo Benue Trough, formed during the Aptian–Albian as a consequence of Basin the general transtensional regime occurring in the region. Pernambuco F Craton Geophysical studies (Basile et al., 1993), a deep dive survey 500 km (Mascle et al., 1993), and Ocean Drilling Program (ODP) Leg 159 Reconcavo Basin drilling (Mascle, Lohmann, Clift, et al., 1996) have now clearly es- Legend tablished that the CIGMR is a thick clastic wedge that has been sub- yy jected to the combined effects of extension and wrench tectonics be- Precambrian craton Proterozoic cover tween the separating African and South American continents. yy The Benue Trough of Nigeria (Fig. 1) is a northeast-trending in- Pan-African mobile zone oceanic crust tracontinental basin >800 km long that connects to the east with the subsident Cretaceous sub-basins African intracontinental rift system through the Bornu basin (Fig. 2). Cretaceous grabens Southward, it reaches the continental margin, where it is concealed transcurrent faults oceanic fracture zones under the thick sedimentary pile of the Niger delta. (active is in black) The Benue Trough may be subdivided into three main structural sinistral motion along the Benue Trough direction of extension responsible during the Aptian-Albian period for the initiation of the Niger basins and sedimentary domains (the Lower, Middle and Upper Benue); each domain has recorded a separate sedimentary and structural his- relative motion of the African and South American continents (the small arrow indicates a slower motion northwest of the tory (Benkhelil, 1988). The Benue Trough includes pull-apart sub- Benue Trough resulting in sinistral movement along it) basins where rifting and wrenching were active concurrently Figure 1. Reconstruction of the Gulf of Guinea area during the Albian, show- 1 Mascle, J., Lohmann, G.P., and Moullade, M. (Eds.), 1998. Proc. ODP, Sci. ing the CIGMR and the Benue Trough. Results, 159: College Station, TX (Ocean Drilling Program). 2URA-CNRS n°715, Sédimentologie et Géochimie Marines, Université de Perpig- nan, 66860 Perpignan, France. [email protected] 3UMR Géosciences Azur, BP n°48, 06230 Villefranche-sur-Mer, France. 4UMR n°5561: Paléontologie Analytique et Géologie Sédimentaire, Centre des Sci- ences de la Terre, Université de Bourgogne, 6 Bd Gabriel, 21000 Dijon, France. 93 J. BENKHELIL, J. MASCLE, M. GUIRAUD Figure 2. Geological sketch map of the Benue Trough and surroundings. Offshore, the traces of the Equatorial Fracture Zones are indicated. (Benkhelil, 1988; Guiraud, 1993). The initiation and evolution of this juvenated during the Pan-African orogeny (–550 Ma; McCurry, large-scale intracontinental system is related to the opening of the 1971). Thick mylonitic zones cut across this basement and are be- Equatorial Atlantic during the Early Cretaceous (Mascle, 1976; Ben- lieved to have helped to localize the emplacement of the future Equa- khelil and Robineau, 1983; Guiraud and Maurin, 1991, 1993). Struc- torial Fracture Zones (Fail et al., 1970; Benkhelil and Robineau, tural patterns result from various depositional environments, and 1983; Maurin and Guiraud, 1993; Figs. 1, 2). from separate and successive tectonic and thermal regimes, as indi- cated in Figure 3. Sedimentary Basins The Côte d’Ivoire-Ghana Domain THE COTE D’IVOIRE-GHANA TRANSFORM MARGIN AND THE BENUE TROUGH: MAIN SEDIMENTARY The oldest rocks that bottom the Côte d’Ivoire-Ghana domain are sandstones and marine shales ranging from the Silurian to the lower AND STRUCTURAL CHARACTERISTICS Carboniferous and are located in the Keta basin along the Ghana Basement coast (Kjemperud et al., 1992; Tucker, 1992). After an early stage of rifting marked by volcanic activity, including doleritic sills during Within the equatorial area, the breakup of Gondwana mainly af- the Triassic and the Middle Jurassic, extensional stresses lead to the fected the old Precambrian West African craton, which was partly re- formation of basins rapidly filled by continental sediments (Guiraud 94 SEDIMENTARY AND STRUCTURAL CHARACTERISTICS Age sedimentation tectonic events magmatism metamorphism (Ma) AB AN MB UB AB AN MB UB AB AN MB UB AB AN MB UB 0 20 40 60 ? ? ? @@@@@PPPPPÀÀÀÀÀ 80 @@@@@PPPPPÀÀÀÀÀ @@@@@PPPPPÀÀÀÀÀ @@@@@PPPPPÀÀÀÀÀ @@@@@PPPPPÀÀÀÀÀ @@@@@PPPPPÀÀÀÀÀ 100 @@@@@PPPPPÀÀÀÀÀ @@@@@PPPPPÀÀÀÀÀ @@@@@PPPPPÀÀÀÀÀ @@@@@PPPPPÀÀÀÀÀ 120 140 continental environment intensity of the tectonic events shallow water extensional regime transitional environment marine environment compressional regime Figure 3. Sequence of geological events in the Benue magmatic activity trantensional regime Trough since the Jurassic. AB = Abakaliki Anticlino- @@@PPPÀÀÀ @@@PPPÀÀÀ intensity of the thermal conditions rium, AN = Anambra Syncline, MB = Middle Benue, @@@PPPÀÀÀ transpressional regime @@@PPPÀÀÀ UB = Upper Benue. @@@PPPÀÀÀ and Maurin, 1991). Reconstructions of the Equatorial Atlantic and which fanglomerates and microconglomerates accumulated the surrounding continental disruption (Sibuet and Mascle, 1978) in- (Guiraud, 1993); a second rifting episode occurred from the Neoco- dicate that a broad rifted basin formed in the area presently occupied mian to the early Albian leading to the deposition of thick continental by the Côte d’Ivoire Margin and the Deep Ivorian Basin (Caprona, and lacustrine deposits in rapidly subsiding depocenters bounded by 1992; Mascle et al., 1993; Masse, 1993). normal and transcurrent faults. In the Upper Benue Trough (Fig. 2), During early infilling in the Early Cretaceous, sedimentation was thick deltaic sediments were deposited during the Albian, while a ma- continental (subaerial, lacustrine, deltaic), and progressively evolv- rine transgression occurred in the southern domain. Finally, incipient ing into transitional marine and then open marine deposition in a half-grabens and wrench-induced structures were submitted to in- deepening basin. This has been clearly demonstrated by deep diving tense folding and faulting at ≈84 Ma (the Santonian; Benkhelil, 1988; observations and sampling (Guiraud et al., 1997a; Benkhelil et al., Guiraud, 1993). 1996) and by Mascle, Lohmann, Clift, et al., (1996). The Albian is The lithologies of the Aptian–Albian deposits are very similar in characterized by fine terrigenous sediments that become carbonates the Lower Benue Trough and along the CIGMR. Albian facies in- in the Turonian. A well-marked erosional unconformity forms a clude, in both domains, finely laminated siltstones and claystones in- sharp boundary between syntransform tectonized siliciclastic sedi- tercalated
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