1 Regional GeologyThe Middle Benue Trough formsan important part of the Benue Trough which links the Lower and the Upper BenueTroughs. The whole Benue Trough is a linear Cretaceous sedimentary basin whichextends northeast wards for about 800km from the Nigerian continental margin(Obaje, 2009). The formation of the trough was closely associated with theseparation of Africa from South America and the opening of the South AtlanticOcean. As shown by geologic studies,Benue Trough was subjected to different cycles of deposition that resulted indeposition of different sedimentary rocks of varied composition age (Nwachukwu,1972? Offodile, 1976? Kogbe).
Thestratigraphic succession of the Middle Benue Trough comprise six UpperCretaceous lithogenic formations; the Asu River Group, Ezeaku Formation, KeanaFormation, Awe Formation, Awgu Formation and Lafia sandstone as shown in Fig.1.6 below. The Asu River Group which predominantly outcrops in the Keana Anticlineand areas around Gboko consists of Albian Arufu, Uombaand Gboko Formations. Itslithological composition is made up of shales, micaceous siltstone, limestone,mudstone and clays (Offodile, 1976; Nwajide, 1990). These are overlain by theCenomanian-Turonian Ezeaku Formation which shares common boundary with the Konshisha River Group and the WadataLimestone in the Makurdi area, followed bythe Keana and Awe Formations.The coal-bearing Awgu Formationof Late Turonian-Early Santonian lies conformably on the Awe Formation. In theMakurdi area, the Makurdi Sandstoneinter-fingers the Awgu Formation.
The mid-Santonian was a period of folding throughout the Benue Trough. The post-foldingCampano-MaastrichtianLafia Formation endedthe sedimentation in the Middle Benue Trough, after which widespread volcanic activities took over in the Tertiary.The Asu River Group mainly outcrops east of Keanatown and south of Azara, and in the area around Gboko with a typical section inthe Quarry of the Benue Cement Company near Yandev (Obaje, 2009). Thelithologic composition of the Asu River Group comprises limestones, shales,micacous siltstones, mudstones and clays. Its average thickness is estimated tobe about 1,800m (Offodile, 1976; Obaje et.
al., 1994). The depositionof the Ezeaku Formation marked the beginning of marine transgression in theLate Cenomanian. This took place in a presumably shallow marine coastalenvironment with sediments mainly of calcareous shales, micaceous fine tomedium friable sandstones and beds of limestones. Outcrops of the EzeakuFormation include those at Ortese, about 4 km east of Jangerigeri, where thesediments are composed mainly of shelly limestones; almost entirely of oystershell. In the bank of River Tokura, about 20 km east of Keana Town, on theChikinye- Awe Road, a typical section of the Ezeaku Formation occurs,consisting mainly of intercalations of shelly limestones and black shales, withbrownish fine to coarse grained feldspartic sandstones at the top (Obaje, 2009)The Keana Formation resulted from the Cenomanianregression which deposited fluviodeltaic sediments. The Formation consists ofcross-bedded, coarse grained feldsparthic sandstones, occasional conglomerates,and bands of shales and limestones towards the top. Massive outcrops occur atKeana, Noku, Chikinye, Jangerigeri, Azara, and Daudu.
The Awe Formation wasdeposited as passage (transitional) beds during the Late Albian EarlyCenomanism regression. Its typical sections occur around the town of Awe, whereOffodile (1976) estimated the thickness to be about 100m. The formationconsists of flagy, whitish, medium to coarse grained calcareous sandstones,carbonaceous shales and clays. The deposition of the Awgu Formation marked theend of marine sedimentation in this part of the Benue Trough.
The formation ismade up of bluish-grey to dark-black carbonaceous shales, calcareous shales,shaley limestones, limestones, sandstones, siltstones, and coal seams. Themajor outcrop of the coal-bearing Awgu Formation is at the bank of River Dep inShankodi, 7 km to the west of the village of Jangwa. Along the bank of thisriver, the coal seams can be traced laterally for about 500 m. The boreholecores of the Steel Raw Materials Exploration Agency (formerly National SteelCouncil) stock-piled at the Obi camp contain coal seams and coal bands atvarious depths within the Awgu Formation.The Lafia Formation is the youngest formation inthis area. The Formation was deposited under continental condition (fluviatile)in the Maastrichtian and lies uncomfortably on the Awgu Formation.
It islithologically characterized by ferruginized sandstones, red, loose sands,flaggy mudstones, clays and claystones. Outcrops and sections of the LafiaFormation occur in and around the town of Lafia, and along the bank of RiverAmba on the Lafia-Doma Road (Obaje, 2009).