Electrical resistivity sounding was employed to delineate different water bearing layers and their architectural parameters. Sixty Vertical Electrical Soundings (VES) were obtained in the built-up area of Olabisi Onabanjo University campus to map out variations in subsurface resistivity, which was used to determine aquifer parameters favourable for groundwater development. Interpretation of electrical sounding data helps in determining the resistivity and thickness of aquifers, aquifer and overburden thicknesses and depth to the basement. We employ a lithology-based hydrogeological model with definite boundary to classify the apparent resistivity. In this model, range of electrical resistivity values were assigned to different layers based on field observation and knowledge of the geology of the area. In all, five litho-units were delineated besides the topsoil, which are: clay with resistivity range of 25–53 ; clayey sand/sandy clay with resistivity range of 122–440 ; weathered layer with 62–119 ; fractured basement with 208–667 and resistivity above 720 were taken as fresh basement. From the initial lithologic-based model, we developed 2D model that conceptualized aquifer architecture and bedrock topography along the major traverses obtained in the area. Also, Isopac map reveals that the overburden is thicker in the western half while the 2D model and structural map clearly show undulating bedrock topography made up of bedrock ridge with elevations of 127.0–170.0 m and bedrock depressions with elevation of 97.0–123 m. Derived Geo-electric parameters revealed that aquifer quality increases as we move northward, though recommended aquifers cut across all the area survey. Based on the study, we recommended VES stations with good overall qualities to be developed to productive water-supply tube wells.
The Turonian Nkalagu Formation exposed at NIGERCEM (Nigerian Cement Company) within the Lower Benue Trough was studied in this work in order to determine and establish the lithostratigraphic successions and reconstruct the paleoenvironment of the deposition. Two major litho-facies were delineated. The first is matrix-supported bioclastic shell lag wackestone facies which account for about 70% of the carbonates. The second is a massive fossil-poor micritic limestone. However, using the fossil contents, five distinct micro-biofacies were recognized namely: massive lithoclastic floatstone, bioclastic wakestone, siliceous fossil-poor limestone, bioclastic (bivalve debris) wackestone and bioclastic packstone. The litho-clast consists of probably shallow water transported materials while still in various degrees of lithification from shelf into deeper basin. Many of the lithoclasts are sub-angular whereas others have rounded shape. Textural gradient was upward fining and algae were completely absent which is typical of shallow calcareous marine sediments of the photic zone. The trends of the textural characteristics fall in pattern correlatable with the different facies outlined in the study. Thus, the evidence from this study has put Nkalagu Formation as a deposition in shallow marine environment but later displaced into relatively deeper water, probably that of the off-shelf zone, by some sedimentary flow mechanisms.
