Rjeas Research Journal in Engineering and Applied Sciences 1(1) (2012) 7-11 Rjeas © Emerging Academy Resources www.emergingresource.org 7 DEPOSITIONAL ENVIRONMENT, GEOPHYSICAL MAPPING AND RESERVE ESTIMATION OF LIMESTONE DEPOSIT IN ARIMOGIJA - OKELUSE AREA, SOUTH-WESTERN NIGERIA O.A Ehinola 1 , A.Oluwajana 2 and C.O. Nwabueze 1 1 Energy and Environmental Research Group, Department of Geology, University of Ibadan, Nigeria. 2 Department of Geology, Adekunle Ajasin University, Akungba Akoko, Ondo State, Nigeria Corresponding Author: O.A Ehinola ___________________________________________________________________________ ABSTRACT The shale – limestone sequence at Arimogija – Okeluse belong to the Cretaceous Abeokuta Formation deposited during the first marine transgressive cycle in the Dahomey basin. Petrographic study was carried out to identify skeletal and non – skeletal grains for the determination of depositional environment. Detailed geophysical mapping of the area using Vertical Electrical Sounding (VES) was applied to establish the thickness of the different subsurface layers. The limestone horizons were delineated and reserve estimated. The skeletal grains identified include: brachiopods, bivalves, gastropods, ostracods, echinoids and broken shell fragments while non-skeletal grains are ooids and pisoids. Micrite and sparry calcite are found to be abundant and are the dominant microfacies. The VES results reveal 3 to 4 geo-electric layers which include: the clay / lateritic top soil, limestone and shale. The reserve volume of the limestone in the Arimogija area is approximately 1.3 x 10 8 tonnes while Okeluse area stands at approximately 3.04 x 10 8 tonnes for areas of 10.4 x 10 6 m 2 and 4.7 x 10 6 m 2 respectively. This study indicates that the limestone reserves in the Okeluse area exceeded that of the Arimogija area. It will be cost - effective to set - up a cement factory in the Okeluse axis due to its possession of a substantially larger portion of the limestone reserves. Petrographic features observed in the limestone microfacies suggest quiet water, low energy, and open shelf environment. © Emerging Academy Resources KEYWORDS: Geoelectric Sections, Reserve, Overburden, Limestone, Micrite __________________________________________________________________________________________ INTRODUCTION Limestone is a fundamental raw material of the construction industry. Limestone and its derivatives are used as fluxes, glass refractory fillers, abrasives, fertilizers (soil conditioners) and as ingredients in several chemical processes. A substantial number of large limestone occurrences in Nigeria have been reported (Bell, 1963; Ola, 1977; Gwosdz, 1996; and Ehinola et al., 2010). Knowledge of the estimated reserves of limestone in this area will guide prospective investors and stakeholders in making decisions. This will especially guide the relevant parties in the establishment of a cement manufacturing plant in the area which will reduce the dependence of manufacturing companies on the few cement manufacturing companies presently operating in the country. The increase in the availability of the product might even go some distance in reducing the current price of cement in the market. The study area is located at the easternmost part of the Dahomey Basin. The study area lies within longitude 5°33’E - 5°43’E and latitude 6°46’N - 6°51’N with elevation range between 50 and 80 meters (Fig. 1). The mapped area is part the Cretaceous Abeokuta Group (Figs. 2 and 3). Geophysical techniques have gained wide application in the determination of different subsurface layers. It has been used in the reserve estimation of clay deposit from parts of Southwestern Nigeria (Ehinola et al., 2009). This study is thus aimed at defining the thickness of the various subsurface layers in the area using Vertical Electrical Sounding (VES), delineating the limestone horizon, estimating the reserves and determining the depositional environment of the limestone. FIGURE 1: Topographical map of the study area showing geographical location of VES points