Journal of Geology and Mining Research Vol. 1(1) pp. 008-018 March, 2009 Available online http://www.academicjournals.org/jgmr ©2009 Academic Journals Full Length Research Paper Geoelectric mapping and characterization of limestone deposits of Ewekoro formation, southwestern Nigeria B. S. Badmus1* and O. B. Olatinsu2 1Department of Physics, University of Agriculture, Abeokuta, Nigeria. 2Department of Physics, University of Lagos, Lagos, Nigeria. Accepted 17 February, 2009 62 vertical electrical soundings (VES) were carried out to map limestone deposits of Ewekoro formation, south-western Nigeria. The maximum current electrode spacing AB/2 was 1 km. The results revealed the occurrence of limestone in all the locations within the third and fourth geoelectric layers with resistivity values ranging from 10 to 100 KΩm and thickness range of 15 to 90 m. The resistivity values of the limestone samples collected from the study site were also analyzed by laboratory direct method. This direct method revealed that the resistivity values fall within 6 and 171 KΩm. The two approaches showed a good degree of correlation in the resistivity value of the limestone and their varying qualities. This research work further showed the occurrence of vast deposit of limestone, which can be of economic importance in mining and industrial purposes. Key words: Resistivity, limestone, geoelectric layer, permeability, Ewekoro formation. INTRODUCTION Geophysical exploration was carried out to map the tion of limestone deposits at Ewekoro quarry. Adegoke et limestone deposit of Ewekoro formation using two diffe- al. (1970) subdivided the limestone deposits into three rent approaches: the vertical electrical sounding (VES) units. Omatosola and Adegoke (1981) then proposed a via Schlumberger arrangement and direct laboratory me- fourth unit. These units in stratigraphic order are: sandy thod via direct current power supply. The direct method biomcrosparite (bottom), shelly biomcrite, algal biosparite approach is to complement and compare the study car- and red phosphate biomcrite (top). The sandy biom- ried out by Badmus and Ayolabi (2005) where limestone crosparite forms the base of the formation and consists of of Ewekoro formation was delineated using Schlum- a light brown sandy limestone with very few bioclassic berger electrode array to map both vertical and lateral fragments. Stratification is evident and is accentuated by extents of the deposit (Figure 1). This approach would variations in the quantity and grain size of the interbed- also confirm the accuracy of electrical resistivity method ded quartz and glauconite. The shelly biomcrite consists of geophysical prospecting for mineral exploration as well of pure limestone of about 4.5 to 6.0 m and constitute the as characterizing the various litho-facies of Ewekoro bulk of the Ewekoro formation in the quarry. Limestone limestone (Badmus and Ayolabi, 2005). Balm’s et al. has abundant macrofossil content particularly gastro- (2006) carried out geoelectric evaluation of mica schist pods, pelecypods, echinoderms and corals. The algal deposit of Area J4, southwestern, Nigeria using the direct biosparite limestone overlies the shelly biomcrite unit. laboratory method to characterize the mica schist into Resistivity is the only electrical characteristic of geoelec- different degrees of purity for both economic and com- tric property. Most rock forming minerals have conducti- mercial purposes. This mineral exploration work was car- vities spreading over a wide range and are semiconduc- ried out to analyse both the electrical and physical para- tors. However, these rocks are seen as insulators in their meters of limestone in Ewekoro formation. Several stu- dried form. The amount of moisture contained in the rock dies have also been carried out to evaluate the distribu- depends on the structure especially the amount of pore space and cracks. Rocks and sediments contain space between grains (pore space) in fractures or in dissolved cavities (limestone), which may be filled with water. The *Corresponding author. E-mail: [email protected]. porosity and chemical content of the water filling the pore Badmus and Olatinsu 009 Figure 1. Geological map of Papa-Alanto and its environ (Bulletin: Geological Survery, Nigeria, No. 31, R. D. Hockey, H. A. Jones and J. D. Carter, 1957-61). spaces are more important in governing resistivity than ment quarry and lies between longitude 3° 05´E to 3° conductivity of the mineral grains of which the rock is 15´E and latitudes 6° 40´N to 6° 55´N (Figure 1). composed. Limestone is a raw material for the manufacturing of GEOLOGY OF THE STUDY AREA cement, asphalt filler, ceramics, flux in glass making, fertilizer filler, explosives to mention just a few. Jones and The study area is located in the sedimentary area of Hockey (1964) revealed Ewekoro limestone and the south-western Nigeria. Ewekoro formation belongs to overlying Akinbo shale to be lateral equivalents of the tertiary formed Paleocene and Eocene; and the greater Imo formation of eastern Nigeria. Other authors such as part of the depression is a potential artesian basin where Omatsola and Adegoke (1991) and Oladeji (1992) have ground water can be sourced. Kogbe (1976) and investigated the stratigraphy and depositional character- Adegoke et al. (1972) described the stratigrahpic and ristics of limestone and clay/shale deposits in south- paleography of different sedimentary basins. Adegoke et western Nigeria. The West African Portland Cement al. (1976) outlined the Albran and younger Paleographic Company also conducted extensive geological survey history of Nigeria and summarized the nature and extent and commercial appraisal of Ewekoro limestone and of transgressive, regressive phases as well as the nature shale beds for commercial cement production. of the sediment. The geology of Ogun State comprises sedimentary and basement complex rocks, which underlie the remaining surface area of the state. It also consists of intercalations STUDY AREA of argillaceous sediment. The rock is soft and friable but in some places cement by ferruginous and siliceous The study area lies within Ogun State, which is bounded materials. The sedimentary rock of Ogun State consists in the west by Benin Republic, in the south by Lagos of Abeokuta formation lying directly above the basement State, in the north by Oyo and Osun States, and in the complex (Figure 1). This in turn is overlain by Ewekoro, east by Ondo State. It occupies a total area of 16,400 2 Oshosun and Ilaro formations, which are all overlain by km . Ewekoro is the host to West African Portland Ce- the coastal plain sands (Benin formation). 010 J. Geol. Min. Res. Figure 2. Data acquisition map. METHODS analysed using a simple laboratory experimental set-up (Figure 5). The limestone samples collected were ground and packed into a Electrical resistivity method core sampler of cylindrical shape and saturated hydraulically for 24 h. After which they were dried in the oven to remove completely the The vertical and lateral distribution of limestone in Ewekoro was water content in the samples and were shaped into regular forms investtigated using the electrical resistivity method. 62 vertical so as to make good contact with the pins (connecting wires) inser- electrical soundings (VES) were carried out using the Schlumberger ted into the samples. Direct current source was used to supply electrode configuration (Figure 4). Schlumberger array was used voltage across the two ends of the core sampler. Voltage was because of its greater depth penetration, (Ojelabi et al., 2002). The supplied at 12 and 24 V and the corresponding currents were re- VES stations were arranged based on observable limestone out- corded (Table 2). crops found within the vicinity of the study area and were divided into six profiles (Figure 2). These profiles are: A (VES 01-17); B (VES 18-30); C (VES 31-34); D (VES 35-46); E (VES 47-59); and F (VES 60-62). The maximum current and potential electrodes spac- RESULTS AND DISCUSSIONS ing were 1 km and 30 m respectively, while the VES stations were separated by 150 m. The ABEM Terrameter 300B and its acces- Geoelectric mapping sories were used for data acquisition. The apparent resistivity values obtained from the Terrameter were plotted against half the The results of the VES sounding at each of the profiles current electrode spacing (AB/2) on a log-log graph (Figure 3). Par- tial curve matching (a graphical procedure for semi-empirical inter- showed sounding curves with 4- to 7-layered types as pretation of resistivity sounding data) was adopted to obtain model revealed by the computer iteration techniques. The inter- parameters for quantitative interpretation. preted results revealed the presence of 4 geoelectric layers, while only a few revealed the presence of 5 or 6 Direct laboratory measurement using d.c supply geoelectric layers (Figure 3). The resistivity values from field procedure range between 10-100 km as shown in Resistivity of limestone samples collected from the study area were the work of Badmus and Ayolabi (2005). Badmus and Olatinsu 011 Figure 3. Computer modeling for some selected sounded stations. Profile A (VES 01 – 17): This profile reveals the pre- as shown in Table 1. sence of limestone of 2 different litho-facies mainly in the third and fourth geoelectric layers. The resistivity ranges Profile B (VES 18 – 30): Here, 2 different facies of lime- from 12 to 44 km and thickness between 18.8 and 61.0 m stone were delineated in the third and fourth geoelectric 012 J. Geol. Min. Res. Profile E (VES 47 – 59): Along this profile, there are 4 geoelectric layers beneath 8 VES locations (47, 49, 50, R1 54, 56 - 59) and 5 geoelectric layers beneath the remain- r2 ing 5 VES locations (48, 51, 52, 53, and 55). Limestone r1 R V 2 occurrence was found in 13 of the VES locations in the third and fourth geoelectric layers. The resistivity of these layers range from 10.3 to 96 km and their thickness a from 15 to 81 m (Table 1).