Bayewu, O. O., Oloruntola, M. O. and Mosuro, G.O.
A geophysical study involving Electrical Resistivity Imaging (ERI) survey was carried out to delineate the unconformity between the Sedimentary and Basement rocks in Ijesha-Ijebu, Southwestern Nigeria. This area marks the contact zone between the basement rock of southwestern Nigeria and the sedimentary rocks of Dahomey Basin. Three ERI were carried out using Wenner array of first levels (i.e.n=1) of a=10m, 20m, 30m and the subsequent levels were chosen as the product of the level and the current separation of the first level, a total of 5-6 levels were probed for the traverse. This makes the traverse lines to cover between 0.5 to 1km in distance. Inversion method was carried out on the acquired pseudosection using DIPRO software with percentage error of less than 0.5. The lithologic section in the area revealed a conglomeratic layer of 5-10m thick lying uncomformably on the banded gneiss, overlain by ferruginous sandstone of 7-10m thickness intercalated by oolitic ironstone of 0.3-0.5m thick. This is overlain by sandy topsoil of 0.8-2m. The result of the ERI showed a considerable agreement with the lithological section as it revealed three geoelectric layers; the sandy topsoil (500- 1300m), the highly resistive conglomeratic layer (>1350 m) and the weathered basement rock (400-1350 m). Also, it showed that the conglomeratic layers which mark the basal part of the Ise Formation varied in depth from the surface to a depth of greater than 150m. The study has shown the relevance of ERI in mapping rock-contact, most especially in areas with few or no exposure.
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