CRETACEOUS SOUTHERN BENUE TROUGH: The Southern part of the Benue Trough is separated from the Atlantic Ocean by the important Niger delta which covers the relationship between the sedimentary basin, lying on a continental crust, and the Atlantic oceanic basin. Knowledge of the geological history of the southern part of the Benue Trough helps in reconstructing the first stage of its formation in relation to the opening of the Gulf of Guinea.
The particular feature of the sedimentary basin is its final evolution into a small folded chain. The geology of the Lower (Southern) Benue Trough is relatively well known from surveys of economic interest (lead-zinc, coal, petroleum) carried out since the beginning of the 20th century.
The biostratigraphy of the area was established by sFarrington 1952. From 1960 onward, the first syntheses were published (Reyment 1965; Cratchley and Jones 1965; Murat 1972; Whiteman 1982) and models based on the global tectonic concept were proposed Stoneley 1966; Wright 1968; Burke etal., 1971). The deformation, the metamorphism and magmatic events in the Lower Benue Trough, during Cretaceous times is given by Benkhelil (1989).
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The Cretaceous successions in the Southern Benue Trough were grouped into: the lower Cretaceous and upper Cretaceous. In the lower Cretaceous, sedimentation was said to have started during the Middle Albian and ended about the Maastrichtian. This Middle Albian sedimentation was proved to be responsible for the deposition of very thick marine, dark, grey shales, siltstone andsubordinate limestones. The shales in most places lie uncomfortably on the Basement Complex.
In the upper Cretaceous, there were no clear evidence on Cenomenian deposition within this region, as there was no geological evidence on its deposition except in the Odukpani area of the southeastern corner of the Nigerian Coastal Basin; though Cenomasnian age have been assigned to Muri Sandstone in the Middle Benue region (McConnell, GSN 752). And these sandstones were reported to wedge out in the southwest and have not been evidenced in the South.
Investigations in the Southern Benue Trough came up with a conclusion that the Cretaceous lithologic units occur in the east and north of the region forming the western limb of the Okigwe-Abakaliki Anticlinorium. These were seen possessing a low regional dip to the West or Southwest regardless of the sharp folding exhibited by the shale formations in the core of the structure.
These Cretaceous Formations outcrop in a general North-South direction. Close to the axis of the anticlinorium near Okigwe, the regional dip veers to the Southwest.
Approaching the axis of the structure in the highest formations, remarkable variation in lithology is experienced, with the shale giving place to sandstone as evidenced in Okposi. Over the axis of the anticlinorium, the outcrops narrow considerably due to part to a steeping of the dip, and also due to the marked thinning of the Formations.
The basement underlying the Lower Benue Trough is dominated by granite and migmatitic rocks including rare relics of meta-sedimentary rocks.
All these basement rocks have been affected by the Pan-African Orogeny around 600 ± 700 Ma (McCurry 1971). Post-Pan-African geological units lying near the Lower Benue include the Jurassic Younger Granites of Jos to the North and the Tertiary Cameroon Volcanic Line to the East.