DELINEATION OF LIMESTONE DEPOSIT AT OKELUSE, SOUTHWESTERN NIGERIA USING INTEGRATED GEOPHYSICAL AND GIS APPROACH

Abstract

Geophysical methods and remote sensing methods have been used in order to delineate the limestone deposit at Okeluse town in the Eastern part of Ondo state, southwestern Nigeria. Aeromagnetic data and Landsat 8 OLI image were utilized to explore for any magnetic signatures and lineaments that are indicative of structures and features that can affect or displace the limestone formation and ground truthing was carried out using electrical resistivity method (dipole-dipole profiling and Vertical Electrical Sounding technique) to delineate the deposit. The magnetic anomaly pattern reflects the geomorphology of the area as depicted by terrain map of the area, which is characterised by hills and valleys. The aeromagnetic results indicates that the limestone was deposited in low magnetic intensity region in form of a graben with residual magnetic intensity values of -0.11 to – 8.89 nT surrounded by regions in form of hills with relatively high magnetic intensity values of + 0.08 to + 7.35 nT. Euler deconvolution technique using the structural index of 0 and 0.5 revealed the depth of the delineated geologic contact to range between 50 m and 300 m. Landsat 8 OLI False Colour Composite (FFC) image of bands 257 in RGB was used to emphasize the limestone region, shape and trend. The observed lineaments occur majorly in the NE to SW direction indicative of sedimentary trough of down warped and/or faulted block. Four traverses were established at the two locations where the limestone outcrops and a total of sixteen (16) VES stations were occupied. The VES results delineated three geologic layers comprising the topsoil, limestone and shale units. The thickness of the limestone layer at the two locations varies from 3 to 30 m with an average thickness of 19.5 m. The resistivity of the limestone ranges from 6 to 48 Ohm-m. From the observed dimensions of the limestone deposit, the estimated probable reserve of the limestone is 843,112,481 metric tonnes. The limestone deposits in Okeluse are not majorly affected by structures therefore cannot pose any risk during mining as they occur majorly as overburden materials and can yield an economic profit, except at areas close to an artesian well and surface water flows.