GEOPHYSICAL MAPPING OF BASEMENT STRUCTURES AND ITS GEOLOGIC IMPLICATIONS AROUND ILOKO–IJESA, SOUTHWESTERN NIGERIA.

Abstract

An integrated Geophysical study was conducted in Iloko-Ijesha within the schist belt of southwestern Nigeria in order to map the lithological boundaries and other basement structures across different rock types for better understanding of the subsurface geology. The area consists principally of Gneiss and Migmatite undifferentiated, Quartzite and Quartz Schist and Schist Amphibolite complex. The subsurface geology of the area is yet to be fully established; existing records of work in the area are mostly based on extrapolation of surface geology; consequently, there is a need to establish the subsurface true or appropriate positions of faults/fractures and lithological contacts within the schist belt in the study area. The integrated geophysical techniques employed are Aeromagnetic, Ground Magnetic, Very Low Frequency (VLF-EM), Vertical Electrical Sounding (VES) and Dipole-Dipole methods. A total of nineteen (19) traverses were established in the study area, along which both Very Low Frequency (VLF-EM) and ground magnetic were run concurrently along eleven (11) traverses. The ABEM Wadi VLF instrument and GEM- Proton magnetometer equipment were used for the data collection along all the traverses at a station interval of 10 m. Also, Ohmega resistivity equipment was used for the electrical resistivity survey. Forty-nine (49) sounding stations using Schlumberger electrode configuration were occupied along the established traverses and the current electrode spacing (AB/2) was varied from 1 to 225 m. The Dipole-Dipole data were acquired along the traverses with inter-stations separation of a = 20 m with an expansion factor (n) which varies from 1 to 5. The aeromagnetic and ground magnetic data were interpreted qualitatively and quantitatively aided by the manual half slope method and automated 3D Euler deconvolution technique which reveal varying magnetic intensities and existence of linear features from near surface to a depth of up to 120 m. The structural index of 0.5 confirmed lithological contact on Quartz-Schist/ Amphibolite complex which most times coincides with Iwaraja fault trending approximately NE – SW direction and other minor fractures. The VLF-EM data further confirmed the existence of these linear features. The VES data obtained were used to characterize the subsurface and to generate geoelectric sections. Three to five subsurface layers were delineated within the study area which is the topsoil, weathered layer, Laterite, weathered/fractured basement and the fresh basement. The layers have resistivity ranging from 33 to 504 ohm – m, 62 to 731 ohm-m, 791 to 945 ohm-m, 86 to 285 ohm-m and 510 ohm-m to 10927 respectively. The thickness ranges from 0.4 to 1.4 m, 2.4 to 22 m 4.3 to 8.6 m and 4.7 to 106.2 m for topsoil, weathered layer, Laterite, weathered/fractured basement respectively. The Dipole-Dipole data were presented as pseudosections which also showed the linear features. Based on the integrated geophysical methods results, structural and modified geology maps were also generated for the study area