THE BENUE TROUGH: Investigation on Benue sedimentary trough has always been a course for alarm to most young and advanced geologists, as its complex and complicating nature have resulted to inconclusive theories on its origin.
Geologically, the Benue Trough consist of a linear stretch of sedimentary basin running from about the present confluence of the Niger and the Benue rivers to the northeast, and bounded roughly by the Basement Complex areas in the north and south of the River Benue. This elongated trough like basin is continuous with the coastal basin, and infact, has been correctly described as the long arm of the Nigeria Coastal Basin
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On the origin, a report has been put forward to describing Benue Trough as a rift depression of up to 600 metres; composed of marine and fluvio-deltaic sediments that have undergone distortion by compressional folding (Carter et al., 1963, Cratchley and Jones, 1965, e.t.c.). While some traced the origin of the trough to be triple junction; one arm of which gave rise to Benue Basin (Wright, 1976), In (1970) and with the emergence of the Spreading sea floor and Plate Tectonics hypotheses, Burke, Dessauvagie and Whiteman came up with a new theory for the origin of the trough. The authors contended that the Benue rift first opened, in the Cretaceous, due to the spreading of a crustal ridge in the region of the present trough. This spreading, according to Burke and others, seized by Late Cretaceous and then was followed by a closing episode of the North Atlantic and South Atlantic African plates, in the Santonian. The resultant differential motion of the two parts of the African plate, in their view, resulted in the Santonian folds and gave them their unique parallel and sub-parallel structure along the trough. Nwachukwu (1972), apparently, disagreeing slightly, suggested that while the tectonic evolution of the Benue Trough may be reconstructed in terms of disruptive and convergent interactions of the two continental plates, underthrusting was not likely and crustal spreading therefore minimal.
Nevertheless, the continuity of the sediments of the Benue Trough with the Nigeria Coastal Basin is not disputed as has been shown (Cratchley and Jones, 1965) to be marked by:
1. The continuity of the paleontological zonation with the coastal marine formations.
2. The series of long narrow folds with East- northeast- West- southwest
trend linked with the southwestern folds of the Abakaliki sedimentary area.
3. The narrow Lead-Zinc mineralization belt running from the Abakaliki area to the north east part of the basin.
Stratigraphically, the Benue Trough is subdivided into three regions: the Upper or North-east regions; Middle Benue region or the Lafia-Muri area; and the Lower or Southern Benue Trough; which is the area South and West of Markurdi.
The Southwestern end of the Benue Trough is concealed by the Niger Delta.The Mamfe basin connected to the Lower Benue is a Lower Cretaceous structure stretching in a southeasterly direction. On the other edge of the Trough, the Niger Basin (Bida Basin) is a northwest trending sedimentary basin of Maastrichtian age. In the northwest direction, the Lower Benue Trough connects with the Upper Benue Trough.
The study area seen to be underlain by the Abakaliki Shales of the Asu River Group falls within the Cretaceous Lower (Southern) Benue Trough