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2.1 Regional Geology

The Middle Benue Trough forms
an important part of the Benue Trough which links the Lower and the Upper Benue
Troughs. The whole Benue Trough is a linear Cretaceous sedimentary basin which
extends northeast wards for about 800km from the Nigerian continental margin
(Obaje, 2009). The formation of the trough was closely associated with the
separation of Africa from South America and the opening of the South Atlantic

As shown by geologic studies,
Benue Trough was subjected to different cycles of deposition that resulted in
deposition of different sedimentary rocks of varied composition age (Nwachukwu,
1972? Offodile, 1976? Kogbe).

stratigraphic succession of the Middle Benue Trough comprise six Upper
Cretaceous lithogenic formations; the Asu River Group, Ezeaku Formation, Keana
Formation, Awe Formation, Awgu Formation and Lafia sandstone as shown in Fig.
1.6 below. The Asu River Group which predominantly outcrops in the Keana Anticline
and areas around Gboko consists of Albian Arufu, Uombaand Gboko Formations. Its
lithological composition is made up of shales, micaceous siltstone, limestone,
mudstone and clays (Offodile, 1976; Nwajide, 1990). These are overlain by the
Cenomanian-Turonian Ezeaku Formation which shares common boundary with the Konshisha River Group and the Wadata
Limestone in the Makurdi area, followed by
the Keana and Awe Formations.The coal-bearing Awgu Formation
of Late Turonian-Early Santonian lies conformably on the Awe Formation. In the
Makurdi area, the Makurdi Sandstone
inter-fingers the Awgu Formation. The mid-Santonian was a period of folding throughout the Benue Trough. The post-folding
Campano-MaastrichtianLafia Formation ended
the sedimentation in the Middle Benue Trough, after which widespread volcanic activities took over in the Tertiary.

The Asu River Group mainly outcrops east of Keana
town and south of Azara, and in the area around Gboko with a typical section in
the Quarry of the Benue Cement Company near Yandev (Obaje, 2009). The
lithologic composition of the Asu River Group comprises limestones, shales,
micacous siltstones, mudstones and clays. Its average thickness is estimated to
be about 1,800m (Offodile, 1976; Obaje et. al., 1994).

The deposition
of the Ezeaku Formation marked the beginning of marine transgression in the
Late Cenomanian. This took place in a presumably shallow marine coastal
environment with sediments mainly of calcareous shales, micaceous fine to
medium friable sandstones and beds of limestones. Outcrops of the Ezeaku
Formation include those at Ortese, about 4 km east of Jangerigeri, where the
sediments are composed mainly of shelly limestones; almost entirely of oyster
shell. In the bank of River Tokura, about 20 km east of Keana Town, on the
Chikinye- Awe Road, a typical section of the Ezeaku Formation occurs,
consisting mainly of intercalations of shelly limestones and black shales, with
brownish fine to coarse grained feldspartic sandstones at the top (Obaje, 2009)

The Keana Formation resulted from the Cenomanian
regression which deposited fluviodeltaic sediments. The Formation consists of
cross-bedded, coarse grained feldsparthic sandstones, occasional conglomerates,
and bands of shales and limestones towards the top. Massive outcrops occur at
Keana, Noku, Chikinye, Jangerigeri, Azara, and Daudu. The Awe Formation was
deposited as passage (transitional) beds during the Late Albian Early
Cenomanism regression. Its typical sections occur around the town of Awe, where
Offodile (1976) estimated the thickness to be about 100m. The formation
consists of flagy, whitish, medium to coarse grained calcareous sandstones,
carbonaceous shales and clays. The deposition of the Awgu Formation marked the
end of marine sedimentation in this part of the Benue Trough. The formation is
made up of bluish-grey to dark-black carbonaceous shales, calcareous shales,
shaley limestones, limestones, sandstones, siltstones, and coal seams. The
major outcrop of the coal-bearing Awgu Formation is at the bank of River Dep in
Shankodi, 7 km to the west of the village of Jangwa. Along the bank of this
river, the coal seams can be traced laterally for about 500 m. The borehole
cores of the Steel Raw Materials Exploration Agency (formerly National Steel
Council) stock-piled at the Obi camp contain coal seams and coal bands at
various depths within the Awgu Formation.

The Lafia Formation is the youngest formation in
this area. The Formation was deposited under continental condition (fluviatile)
in the Maastrichtian and lies uncomfortably on the Awgu Formation. It is
lithologically characterized by ferruginized sandstones, red, loose sands,
flaggy mudstones, clays and claystones. Outcrops and sections of the Lafia
Formation occur in and around the town of Lafia, and along the bank of River
Amba on the Lafia-Doma Road (Obaje, 2009).

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