Structures Deduced From Gravity Data In The Lower Niger Benue Rift (Confluence) Area, Nigeria

Abstract

Based on a recent gravity data collected over the area, the Lower Niger and Lower Benue basins are interpreted to comprise five major structural zones. The locations, trends, extent and relationships between most previously known structures in the area are confirmed and detailed by the data. However, the structure mapped in the Ankpa area that lies between Abejukolo, Ayangba, Ankpa and Otukpo is a broad gravity minimum that suggests a sedimentary basin hereby referred to as “Ankpa Basin”. On the other hand, the gravity high to the northeast of this anomaly which is centred around Umaisha in the Lower Niger, and which is referred to as the “Umaisha high” is interpreted as an intrabasement basic intrusion existing at shallow depth (2 km). Similarly, the gravity high around Abaji hereby known as “Abaji high” is interpreted as having a depth extent of about 2.0-4.2 km. Another broad linear positive gravity anomaly trending NE and located to the northeast of the Ankpa basin is interpreted as a basic intrusive ridge lying under the northern margin of the Lower Benue. Another previously unrecognized broad positive anomaly is located around Idah area. The anomaly is interpreted as a ridge structure made up of known highly metamorphosed or folded basement. To the southeast of the basin, the broad positive gravity anomaly hereby referred to as “Oju high” and which is located around Igumale, Agila and Oju areas is interpreted as part of the northern edge of the previously known Abakaliki anticlinorium. Thus, in detail, the study area from this work is interpreted to be a structural junction evolved by faulting, block subsidence, uplift, folding and intrusions.

Keywords: Gravity, Niger-Benue (Confluence) basin, rift, structures, evolution.

Nigerian Journal of Physics Vol. 19 (2) 2007: pp. 285-302