A DETAILED GRAVITY SURVEY OF THE MIDDLE BENUE, NIGERIA

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Date
1979-05
Authors
Ajayi, Clement Olatunde
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Abstract
The result of a gravity survey carried out in the Middle Benue and discussed in this work, in dictate the existence of aright of normal width (about 40 km), buried under Cretaceous cover . Thirsts is axially located between the axial-ridge of the trough end t he northern boundary of through . The gravity low (about - 37 to - 15 meals) which occur between the Aldridge and the southern boundary of t h e trough has a ls so been ascribed to another rift . The interpretations further suggest t he existence of buried , deeply - rooted (about 7 intermediate and local is intrusiveness, whose tops accelerated at about 200 to 500 m below the ground surface, just beyond t he present margins of the trough and along the earwax trough retrospectively . The above f acts suggest t h at the Benue trough did not only originate through refitting but that it may presently consist which a re-separated by an Aldridge Further more , the gravity data suggests that the Middle Benue can be divided of destructive region which includes precariously geologically unknown ( and/or geophysical y ) region . Within t h e four regoins which contain known structures, the trends and locations of the structures were confirmed by the gravity data. However, features such as the trend of the Giza sync-line between Radarko and Obi and the location of the axis of the Keana anticline in the project area, as deduced from the residual gravity map, did not agree with those on the geological map. The new structures revealed by the gravity data include a broad anticline about 30 km south' of the River Benue, which is axially placed within the trough as the Keana anticline about 30 to 40 km north of the river, and a normal fault which separates the two axial a n t i c l i n e s . The new structural region deduced from the gravity data is characterized by low gravity (values - 20 to 36 meals)and it is a southeast trending zone which might be a zone in which subsidence is accompanied with block faulting. Finally, the r e s u l t s of the work also suggest possible reasons for the geological p e c u l i a r i t i e s of the trough such as the abnormal width of the trough, the folding of the sediments in the trough and the apparent lack of surface, indications of rifiting at the margins of the trough. The results of a gravity survey carried out in the Middle Benue and discussed in this work, in abdicate the existence of Arafat of normal width (about 40 km), buried under Cretaceous cover. This refit is axial y located between the axial-ridge of the trough end t he Northern boundary of the trough. The gravity low (about - 37 to - 15 mgals) which occur between t h Aldridge and the southern boundary of the trough has also been ascribed to another rift . The interpretations further suggest the assistance of buried , deeply - rooted (about 7 km) intermediate and local is deconstructionism , whose tops eallocated at about 200 to 500 m below the ground surface, just beyond the present margins of the trough and along the axial-ridge of the trough retrospectively. The above facts suggest that the Benue trough did not only originate through refitting but that it may presently consist of two parallelisms which a respected by an axial ridge. Further more , the gravity data suggest shat the Middle benue can be divide-int of divest cultural regions which includes a previously unknown ( geologically and/or geophysical y ) region . Within t h e four region which contain known structures, the trends and locations of the structures were confirmed by the gravity data. However, features such as the trend of the Giza syncline between Radarko and Obi and the location of the axis of the Keana anticline in the project area, as deduced from the residual gravity map, did not agree with those on the geological map. The new structures revealed by the gravity data include a broad anticline about 30 km south' of the River Benue, which is axially placed within the trough as the Keana anticline about 30 to 40 km north of the river, and a normal fault which separates the two axial anticilline . The new structural region deduced from the gravity data is characterised by low gravity (values - 20 to 36 mgals)and it is a southeast trending zone which might be a zone in which subsidence is accompanied with block faulting. Finally, the results of the work also suggest possible reasons for the geological peculiertist of the trough such as the abnormal width of the trough, the folding of the sediments in the trough and the apparent lack of surface, indications of rifting at the margins of the trough. The results of a gravity survey carried out in the Middle Benue and discussed in this work, in dedicate the existence of a rift of normal width (about 40 km), buried under Cretaceous cover . This rift is axially located between the axial ridge of the trough end t he northern boundary of the trough. The gravity low (about - 37 to - 15 mgals) which occur between t h axial ridge and the southern boundary of the trough has also been a scribed to another refit . The interpretation Hesther suggest the existence of buried , deeply - rooted (about 7 km) intermediate and local is volcanically rusions , whose tops are located at about 200 to 500 m below the ground surface, just beyond t he p r e s e n t margins of t h e trough and along the a x i a l r i d ge of t h e trough r e s p e c t i v e l y . The above f a c t s suggest t h at t h e Benue trough did not only o r i g i n a t e through r i f t i ng but t h a t it may p r e s e n t l y c o n s i s t of two p a r a l l e l r i f ts which a r e s e p a r a t e d by an a x i a l r i d g e. F u r t h e r m o r e , the g r a v i t y d a t a s u g g e s t s t h a t t he Middle benue can be d i v i d e d i n t o f i v e s t r u c t u r a l r e g i o ns which i n c l u d e s a p r e v i o u s l y unknown ( g e o l o g i c a l l y and/or g e o p h y s i c a l l y ) r e g i o n . Within t h e four r e g i o n s which ( i) contain known structures, the trends and locations of the structures were confirmed by the gravity data. However, features such as the trend of the Giza syncline between Radarko and Obi and the location of the axis of the Keana anticline in the project area, as deduced from the residual gravity map, did not agree with those on the geological map. The new structures revealed by the gravity data include a broad anticline about 30 km south' of the River Benue, which is axially placed within the trough as the Keana anticline about 30 to 40 km north of the river, and a normal fault which separates the two axial a n t i c l i n e s . The new structural region deduced from the gravity data is characterised by low gravity (values - 20 to 36 mgals)and it is a southeast trending zone which might be a zone in which subsidence is accompanied with block faulting. Finally, the r e s u l t s of the work also suggest possible reasons for the geological p e c u l i a r i t i e s of the trough such as the abnormal width of the trough, the folding of the sediments in the trough and the apparent lack of surface, indications of r i f t i ng at the margins of the trough.
Description
A Dissertation submitted to theAhmadu Bello U n i v e r s i t y , for t he degree of Doctor of Philosophy in Applied Geophysics Zaria, Nigeria May, 1979
Keywords
DETAILED,, GRAVITY,, SURVEY,
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