Tectonics: The structural framework of the Lower Benue includes two main units: the Anambra Syncline and the Abakaliki Anticlinorium. The Anambra Syncline is a vast sedimentary basin trending N30oE and mainly filled by upper Cretaceous and Tertiary sediments. Tectonic deformations, rare in this basin, are restricted to normal faults most of them being of syn-sedimentary origin.
Located to the southeastern edge of the Anambra Syncline, the Abakaliki Anticlinorium is a 50km wide structure trending northeast-southwest for more than 200km, from the Niger Delta hinge to the Gboko area. The core of the Anticlinorium is occupied by the oldest sediments known in this part of the Benue Trough, the shales of the Asu River Group of Albian age.
The Abakaliki Anticlinorium may be divided into three structural domains according to the degree and nature of deformation (Figures 8, 9 and 10). A narrow band, including the southeastern edge of the basin, displays monoclinal strata gently dipping toward the northwest.
This domain includes sets of strike-slip faults, rare normal faults and joints. Northwestward, the Cretaceous strata increase to form a 2500 metres thick basin (Benkhelil etal., 1986). The sedimentary sequence is gently folded, but faulting is well indicated due to dominance of shales.
The northwestern edge of this domain displays an incipient fracture cleavage. Bounded to the northwest by the Anambra Syncline, the third domain occupies a central position in the anticlinorium. The sediments are tightly folded, cleaved, and intruded by igneous rocks. They are also slightly metamorphosed (Benkhelil,
Albian In Southeastern Nigeria
The oldest sediments in Southern Nigeria are around Abakaliki in Southeastern Nigeria. These sediments are unnamed and undifferentiated. They constitute the “Asu River Group” (Table 4). The type area of the group is along the Asu River (Reyment 1965). The sediments have been thrown into series of folding, lying en enchelon and pitching to the west through to southwest, within Abakaliki and Afikpo respectively.
The sediments consist of rather poorly bedded sandy shales known as the Abakaliki shales with sandstone and sandy limestone lenses. These beds have been recorded to be associated with lead-zinc mineralization (Figure 11a and 11b). Paleontologically, the shale is characterized by species of Mortoniceras and Elobiceras.
The deeply weathered shales contain Radiolaria, Echinoids, and some pelecypods and gastropods. The sediments are folded particularly in the south of Abakaliki, with the fold axes stretching Northeast-Southwest (Reyment, 1965).
Deposits of this age are restricted to the Odukpani Formation of the southeastern corner of the Nigeria Coastal Basin around Calabar (Figure 9), though Cenomenian age has been assigned to Muri Sandstone in the Middle Benue region (McConnell, GSN 752).
The beds consist of arkosic sandstone, limestone and alternating limestones and shales which become predominantly shally in the uppermost part (Reyment, 1965). These sediments are of shallow water origin and they may be about 600 metres thick. The type locality is at the village of Odukpani near Calabar (Kogbe, 1989).
Deposits of this age mainly belong to the Eze-Aku Formation (Eze-Aku Shale, Simpson 1955), deposited in the second transgressive phase in Nigeria. The type locality is the Eze-Aku River valley in Southeastern Nigeria. The formation consists of hard grey to black shales and siltstones with frequent facies changes, to sandstones or sandy shales. The thickness varies, but may attain 100 metres in places (Reyment 1964).
Locally, the outcrops of this sequence are seen at Amasiri “Amasiri Sandstone”, Egedde-Oju (south of Otukpo), Nkalagu, Ezillo, with its lateral equivalence in the Markurdi Formation, and Awgu-Ndiabor Shale Formation of Senonian age. The Eze-Aku Formation is of shallow water deposit. The fossil consists mainly ofvascoceratids, pelecypods, gastropods, echinoids, fish teeth, etc. which indicate a basal Turonian age (Kogbe, 1989).