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- Taylor & Francis
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- © 2020 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis Group on behalf of the International Water, Air & Soil Conservation Society(INWASCON).
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- 2474-9508
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Abstract
GEOLOGY, ECOLOGY, AND LANDSCAPES 2021, VOL. 5, NO. 2, 81–93 INWASCON https://doi.org/10.1080/24749508.2020.1833628 RESEARCH ARTICLE Near - surface characterization of sediments of the Sokoto group exposed around Wamakko area, Northwestern Nigeria: an integrated approach a b b Emmanuel Etim Okon , Justus Onyebuchi Ikeh and Chuka Justin Offodile a b Geology Department, University of Calabar, Calabar, Nigeria; Mineral Exploration Department, Mecon Geology and Engineering Services Ltd, Jos, Nigeria ABSTRACT ARTICLE HISTORY Received 20 April 2020 An integrated approach involving sedimentological and geophysical studies was carried out to Accepted 4 October 2020 characterize the surface and subsurface geology of part of the southwestern Iullemmeden basin (Sokoto Basin, North-western Nigeria). The Kalambaina Formation (limestone) of the KEYWORDS Sokoto Group is being investigated. The general objective of this study was to optimize lateral Sokoto basin; electrical and vertical resolution amidst the subsurface lithological heterogeneity and that informed the resistivity tomography; choice of integrating sedimentology and geophysics (involving Vertical Electrical Sounding – lithological heterogeneities; VES and Electrical Resistivity Tomography – ERT). Surface geological mapping complemented facies; resistivity by geophysical (4 VES and 2 ERT) surveys were carried out. Data integration involved geological (outcrop stratigraphic data), geophysics and data from down-hole core descriptions within the area of investigation. Results of 1D resistivity (VES) yielded QH and QHQ curve types with OH being predominant. Based on their contrasting resistivities, four-layer geoelectric sections was erected using VES data (ironstone topsoil, limestone, shale and clay/sandy clay layers). From the ERT modelling, four lithologic sections were also identified. These lithologic discriminations correlated well with few borehole data within the study area. With integration of geophysics with stratigraphic data , a subsurface facies distribution model (fence diagram) was developed having a high vertical and lateral resolution. This study revealed that during reconnaissance mapping, with integrated approach using resistivity and sedimentological data, accurate subsurface imaging could be achieved, at a reduced exploration cost, even in areas of abrupt facies changes (lithological heterogeneities). Introduction distribution to properly guide future expansion pro- This study is an attempt to better understand the grams. Outcrops in the area under investigation are distribution of limestone in the near subsurface using not well exposed and the materials of interest (lime- geophysical prospecting (electrical resistivity) and stone) do not occur at the surface especially in the sedimentological studies. The study is restricted to areas designated for future development. Geophysical the Sokoto Group which represents deposits of an prospecting is known to provide information about epeiric sea that incused into the Nigerian sector of subsurface variations in earth material (rocks) charac- the Iullemmeden Basin during the Paleogene. The teristics, depth to geoelectrical basement (thickness) topography of the area bordered by the basin is gently and aquifer delineation (Olayinka & Olorunfemi, undulating with elevation between 260 and 400 m 1992). This is achievable by careful selection of the above sea level. The most striking topographic feature geophysical approach that best enhances a clear con- of the basin is the Dange Scarp (NNE – SSW structure) trast between the object of interest and its surrounding from which major rivers take their source (Nwajide, materials. This study focusses on the integration of 2013; Obaje, 2009). The Sokoto basin is known to geological mapping (sedimentological) and geophysics possess deposits of industrial and agricultural signifi - (electrical resistivity), to characterize near – surface cance, some of which have been studied appreciably sediments (limestone deposit) around Wamakko area and others only mentioned. Major works carried out where these rocks sparsely outcrop (Figure 1). in the Nigerian sector of the basin have been on the geology and biostratigraphy (Kogbe, 1976, 1976, 1989; Nwajide, 2013; Obaje, 2009; Obiosio et al., 1998; Regional geology and stratigraphic setting Okosun, 1999; Petters, 1979; Raeburn & Tattam, The Iullemmeden Basin is a circular intra-cratonic 1930; Reyment, 1965; Wright et al., 1985). Despite basin in the south-central Saharan region of Africa, current operations of the Cement Company covering an extent of about 700,000 km . It stretches Northern Nigeria (CCNN) on the Kalambaina across parts of Mali, Niger republic, Benin Republic and Formation, it is necessary to study the subsurface CONTACT Emmanuel Etim Okon etyboy911@yahoo.com Geology Department, University of Calabar, Calabar PMB 1115, Nigeria © 2020 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis Group on behalf of the International Water, Air & Soil Conservation Society(INWASCON). This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. 82 E. E. OKON ET AL. Figure 1. Regional geological map of Sokoto Basin (source: United States Dept of Interior Geological survey). north-western Nigeria. To the far north, it is bordered mainly shale, mudstone and limestone as products. by the Adar des Iforas, Hoggar mountains and the Air The “Continental Intercalaire” of Wright et al. (1985) ranges towards the east, and the basement rocks of the represents the basal sediments of the basin are chiefly Benin – Nigeria axis to the south (Petters, 1991). Two continental whereas during the Maastrichtian – notable sedimentary basins in this south Saharan region Palaeocene sedimentation was a result of two trans- to the left and right of the Iullemmeden basin are the gressive-regressive episodes ushered by the advance- Taoudeni and the Chad basin respectively. At the onset ment and retreats of the Tethys sea (Petters, 1979). th of the 20 century, Falconer (1911) described the geol- Figure 2 presents the lithostratigraphic subdivision of ogy and geography of the Nigerian sector of the the sedimentary packages in the Iullemmeden basin, Iullemmeden basin referred to as the Sokoto Basin. emphasizing the terminologies used in its occurrence Other notable studies carried out in this basin are within the Sokoto basin, northwest Nigeria. Notably those of Petters (1978, 1979), Kogbe (1989), and four unconformity bounded sequences are known in Kogbe and Sowunmi (1975), Odunfa (1988); Odibo the entire basin and can be mapped in the Nigerian (1988); Offodile (2002); Ibeh et al. (2007), Obaje et al. sector of the basin. A brief description of the litho- (2013), among others. Nwajide (2013) noted that the logical package is given below, detailed accounts of basin originated probably by rapid subsidence in the the lithologic characteristics are available in pub- late Jurassic (ca. 140 Ma) from its central portion and lished literatures (Kogbe, 1976; Nwajide, 2013; this was accompanied by slow phase in the early Obaje, 2009; Petters, 1979). The first and the oldest Cretaceous – Turonian and then faster again from formations are the non-marine, pre-Maastrichtian about 80 Ma. Stratigraphic evolution in the basin Gundumi and Illo Formations which are believed to dates back to late Jurassic – earliest Cretaceous. be lateral equivalents and overlie the basement Deposition was continuous only from Upper towards the northeast and southwest of the basin Cretaceous to the lower part of the Paleogene with respectively in the Nigerian sector. The beds are marine and estuarine conditions prevailing leaving continental in origin and devoid of body fossils GEOLOGY, ECOLOGY, AND LANDSCAPES 83 Figure 2. Regional stratigraphic subdivision of the Iullemmeden Basin (thickness and depositional environment adapted from Wright et. al., 1985, Petters, 1979). except for some fossil-wood (Kogbe, 1973). The Unconformably overlying the Rima Group is the Gundumi Formation consists of clay, grits, sand- Sokoto Group which is almost entirely marine in stone, and pebbles that are lacustrine and fluviatile origin. This Palaeocene deposits have three main for- in origin, while its lateral equivalent, the Illo mations viz; Dange, Kalambaina and Gamba Formation, consists essentially of cross-bedded peb- Formations. The Dange Formation consists of indu- bly grits, sandstone and pisolitic clay that known to rated bluish-grey shale, interbedded with thin beds of be rich in bauxite. The Gundumi is overlain only by limestone/marl units, while the Kalambaina the Rima Group of Maastrichtian age while the Illo Formation which overlies the Dange Formation is Formation is unconformably overlain successively made up of white, highly fossiliferous limestone and from east to west by Rima Group, Sokoto Group few marl interbeds. The Gamba Formation consists of and Gwandu Formation (Kogbe, 1973). yellowish to brown, slightly fossiliferous “paper” shale. The Rima Group is made up of the Taloka, The limestone deposit of the Kalambaina formation is Dukamaje and Wurno Formations. The Taloka the interest of this investigation. Formation, which is the oldest of the three, consists The Eocene continental Gwandu Formation is the of white, fine-friable sandstone and poorly consoli- fourth phase sedimentary deposit in the Sokoto Basin. dated reddish-purple to brown clayey siltstones and It outcrops in the north-western and Southern part of some carbonaceous mudstone. The Dukamaje the Basin and consists of indurated, interbedded, thick Formation consists predominantly of fossiliferous grey mudstone, quartz and lignite. The Gwandu gypsiferous shale with some marl – mudstone inter- Formation is overlain by the younger deposit, calations (Obaje, 2009) and characteristic bone bed Alluvium, which mostly occurs along the River towards the base (Petters, 1979); while the youngest, Sokoto and its tributaries. Sediments of the Sokoto Wurno Formation, has close similarity in its lithologic Group is the focus of this study. affinities with the Taloka Formation (Kogbe, 1973; The area of interest is located between latitudes 13 ° ° ° Obaje, 2009). It consists of pale, friable, fine-grain 3ˈ15˝ and 13 4ˈ 00˝ N and longitudes 5 10ˈ 00˝ and 5 sandstone, siltstone and intercalated mudstones. 10ˈ 30˝ E (Figure 3). The topography of the area is 84 E. E. OKON ET AL. Figure 3. Map of the study area showing the profile lines for Wenner and VES arrays. gently undulating to almost flat-lying, with elevation studies, including stratigraphic logging were carried ranging from 261 to 264 meters above sea level and out and correlated with data from eight available bore- minor occurrence of outliers in some places princi- holes (core) described for the area. The composite pally consisting of ironstones. The soil profile shows stratigraphic profile (Figure 4(a-c)) erected from field the high resistant laterites/ironstone crust. observations summarizes the lithological characteris- tics of the area. Predominantly, the exposure consists of ferruginized – lateritic ironstones cap, with lime- Methodology stones and shales belonging to the Kalambaina Formation and the Gamba Shales respectively. The This study carried out in north-western Nigeria, shale characteristically thins out towards the east involves the integration of geological (sedimentology) where the ironstone is seen to overlie the limestone and geophysical mapping of a part of the Sokoto in places (Figure 4(d)). This shale unit that overlies the Group in close proximity to the quarry site of limestone is part of the Gamba Formation while the Cement Company Northern Nigeria. Geological field Figure 4. (a) Composite Lithologic description of the exposed section within the study area. (b) Photograph of outcrop exposed at ° ” ° ” ° ” Sokoto Cement Quarry (Lat 13 2ʹ50.30 , Lon 5 10ʹ28.15 ). (c) Photograph of outcrop exposed along the Quarry face (Lat 13 2ʹ42.22 , ° ” ° ” ° ” Lon 5 10ʹ23.72 ). (d) Photograph of outcrop exposure along Kalambaina Village (Lat 13 03ʹ45.11 , Lon 5 10ʹ26.75 ). GEOLOGY, ECOLOGY, AND LANDSCAPES 85 one underlying the Limestone, not exposed in this multi-electrode data measurements was minimized. study area, is the shales of the Dange Formation. Good connectivity between the electrodes and the Geophysical studies were also carried out using connecting cables was ensured, while maintaining Vertical Electrical Sounding (VES) and Electrical effective contact between the ground and the electro- Resistivity Tomography – ERT to further delineate des. The injected current was automatically selected the near subsurface characteristics of sections within from a minimum of 1.0 mA to a maximum of the study area where no outcrop exposure was avail- 200.0 mA by the resistivity meter based on the subsur- able. This was necessary because of the continued face conductivity. Electrical Resistivity Tomography success in the application of electrical resistivity for (ERT), that uses electrical imaging surveys to map characterization of near-surface materials including subsurface areas thought to be characterized by com- groundwater, geotechnical site characterization, litho- plex geology was also carried out according to the logical characterization, etc (Adegbola et al., 2010; method of Griffiths and Barker (1993) to complement Adelekan et al., 2017; Bersezio et al., 2007; Kumar the data generated from the VES. et al., 2014; Olasehinde et al., 2015; Oyedele et al., Borehole core data for three holes were made avail- 2011). able and used for sedimentological description, this The geophysical survey consists of VES and 2D was complemented with outcrop data and used for geoelectrical resistivity. Survey design took into integration with resistivity data for subsurface lithol- account the capabilities of the data acquisition system, ogy characterization. heterogeneity of the subsurface electrical conductivity and the required resolution. Manual data measure- Data processing ment was adopted using an Allied Omega Terrameter for both the resistivity soundings and the The apparent resistivity data generated from the 2D geoelectrical resistivity measurements. The survey soundings were plotted against AB/2 on bi- was designed such that the VES and traverses cover logarithmic sheets. The field curves were then curve- the entire area of interest. It was conducted at the peak matched with Schlumberger master curves to obtain of dry season, and the lack of moisture was potential estimate of the resistivity and thickness of the deli- source of reading error, but was overcame by pouring neated layers. The estimated geoelectric parameters some water on the ground before fastening the elec- were then used as initial models for computer iteration trodes. A total of six VESs were conducted within the on Win-Resist program to obtain model geoelectric area with the aim of delineating the subsurface lithos- parameters for the delineated layers. tratigraphy of the area. Schlumberger array with max- Similarly, the 2D apparent resistivity data sets for imum half current electrode separation (AB/2) each traverse were inverted using RES2DINV inver- ranging from 1.5 m to 95 m was used for data mea- sion code as outlined by Loke and Barker (1996). The surements of the resistivity soundings. The choice of RES2DINV program uses non-linear optimization array was dependent on the geological structures to be technique that automatically determines the inverse delineated, heterogeneities of the subsurface, sensitiv- model of the 2D resistivity distribution of the subsur- ity of the resistivity meter, the background noise level face for the apparent resistivity (Griffiths & Barker, and electromagnetic coupling. Other factors consid- 1993; Loke & Barker, 1996). ered are the sensitivity of the array to vertical and The program subdivides the subsurface into lateral variations in the resistivity of the subsurface, a number of rectangular blocks based on the spread its depth of investigation, and the horizontal data and density of the observed data as well as the survey coverage and signal strength of the array. The 2D parameters (electrode configuration, electrode spa- geoelectrical resistivity was conducted along two tra- cings and positions, and data level). Least squares verses (see Figure 3) along the North-West-South-East inversion technique with standard least-squares con- and Southwest–Northeast directions of the study area; straint (L2-norm), which minimizes the square of the due to proximity of company and its activities, difference between the observed and the computed Wenner array was used for the data measurements. apparent resistivity values, was used for the inversion. Wenner array is preferred for surveys in a noisy site because of its high signal strength. Each of the tra- Results and discussion verses was 180 m in length due to space limitation; the electrode separation used for the measurements The interpreted result for the VES survey is presented ranges from 10.0 to 60.0 m in an interval of 10.0 m. in Table 1. After converting resistance to resistivity by To ensure data quality, the electrode positions were multiplying with appropriate geometrical factors for clearly marked and pegged before the commencement the Schlumberger array, the VES data were plotted on of the data measurements for each traverse as well as log-log graphs with apparent resistivity and half elec- the resistivity soundings. This ensured that electrode trode separation (AB/2) values on the ordinate and positioning error commonly associated with manual abscissa respectively (Figures 5–8). Geoelectric 86 E. E. OKON ET AL. Table 1. Interpreted results for all VES locations/points. VES no. Layer Resistivity (Ωm) Depth (m) Curve type Inferred lithology 1 1 608.2 1.80 QH Ironstone 2 100.7 0.8 Ironestone/Limestone 3 26.0 10.8 Limestone 4 13.1 ?? Shale 3 1 1387.0 2.3 QH Ironstone 2 172.4 12.5 Limestone 3 13.7 27.4 Shale 4 69.1 ?? Sandy clay 6 1 1386.0 2.5 QH Ironstone 2 137.3 14.0 Limestone 3 12.7 30.4 Shale 4 91.4 ?? Sandy clay 7 1 1374.1 2.0 QHQ Ironstone 2 237.4 13.10 Limestone 3 37.6 18.9 Shale 4 38.8 ?? Clay/Sandy clay 8 1 473.4 2.2 QH Topsoil/Ironstone 2 109.5 3.1 Ironstone 3 94.1 7.8 Limestone 4 23.3 ?? Shale/Clay Figure 5. Typical curve for VES station 1. Figure 6. Typical curve for VES 7. GEOLOGY, ECOLOGY, AND LANDSCAPES 87 Figure 7. Typical curve for VES station 8. Figure 8. Typical curve for VES station VES 3. Figure 9. Geoelectric sections of VES 1–3. 88 E. E. OKON ET AL. sections were constructed to visualize the subsurface Ωm and thickness range of 0.0–2.5 m. The high resis- distribution of the various lithologic units along W–E tivity values of the topsoil are attributed to the hard (geologic strike) directions using GeoGraphic 2012 nature of ironstone deposits in the study area. software and the results obtained are shown in The second layer has resistivity and thickness values Figure 9. Superimposed upon the geoelectric sections ranging from 26.0 to 172.4 Ωm, and 2.0–14.0 m are borehole description data that effectively assisted respectively and was inferred to be Limestone. The in the correlation of borehole and geologic data. third layer has resistivity range of 12.7 Ωm to 13.1 1D resistivity model curves of the study area for VES Ωm, was inferred to be Shale, while the fourth layer 1, 3, 4 and 5 are presented (Figures 5–8). resistivities ranged from 23.3 to 91.4 Ωm and sandy The computer-iterated curves showed a smooth clay was inferred. geometry of four layers, characteristic of a typical The 2D Wenner resistance data was converted sedimentary terrain. The curve types identified within to resistivity by multiplying it with the appropriate the study area include QH, and QHQ, type with the geometrical factors of 2πa, where “a” represents QH as the predominant curve type. Four VES data the spacing. The appropriate resistivity values for (Table 1) presented reveal a maximum of four geo- the 2D data set were inverted for true subsurface electric layers which is composed of topsoil/ironstone, resistivity using RES2DINV inversion software Limestone, Shale and Clay/sandy-clay intervals. and the resulting estimated models presented and This interpretation was achieved by combining the interpreted accordingly (Figure 10(a,b)). The ERT resistivity and conductivity values with those of estab- surveys were interpreted by integrating the resis- lished ranges for rocks, soils and water as presented in tivity and conductivity values of rocks, soils and Table 2 and comparing also with the litho-logs from water as shown in Table 2 with the described the study area. The topsoil typically has a relatively lithology (outcrop and drill-hole data) across the high resistivity between the range 1387.0 and 473.4 study area. In survey 1 (DHS3 ERT), the model Table 2. Resistivity and conductivity values of rocks, soils and water (Saad et al., 2012). −1 Material Resistivity (ohm-m) Conductivity (ohm-m) Igneous and metamorphic rocks 3 6 −3 −4 Granite 5 × 10 –10 10 – 2 × 10 3 6 −6 −3 Basalt 10 –10 10 – 10 2 7 −8 −3 Slate 6 × 10 –4 × 10 2.5 × 10 – 1.7 × 10 2 8 −9 −2 Marble 10 –2.5 × 10 4 × 10 – 10 2 8 −9 −2 Quartzite 10 –2 × 10 5 × 10 – 10 3 7 −8 −4 Hornfels 8 × 10 –6 × 10 1.7 × 10 – 1.3 × 10 Sedimentary rocks 3 −4 Sandstone 8–4 × 10 2.5 × 10 – 0.125 3 −4 Shale 20–2 × 10 5 × 10 – 0.05 −2 Marl 3–70 1.4 x 10 – 0.3 2 −3 Limestone 50–4 × 10 2.5 × 10 – 0.02 Soils and water Clay 1–100 0.01–1 −3 Alluvium 10–800 1.25 × 10 – 0.1 Groundwater (fresh) 10–100 0.01–0.1 Sea water 0.15 6.7 Figure 10. Wenner array Pseudo-sections (a) DHS3 and (b) DHS8. GEOLOGY, ECOLOGY, AND LANDSCAPES 89 Figure 11. Lithology description for borehole H-1. resistivity inversion revealed four (4) major geoe- Stratigraphic correlation Within the area of interest, eight (8) exploratory well lectrical layers. The four geoelectrical layers have had been drilled. For the purpose of this study, infor- resistivity range > 72.0 Ωm (Ironstone), mation from three (3) well were used in modelling the 24.1 − 72.3 Ωm (Limestone), 4.66–20.6 Ωm VES and ERT data, while all others were integrated in (Shale unit) and < 5.0 Ωm (Clay unit). In stratigraphic analysis (correlation). Information from the second survey (DHS8 ERT), five (5) geoelec- the boreholes (H-1, H-3 and H-8) previously drilled trical layers were identified. These include the close to the quarry were integrated with some selected topsoil interpreted to be ironstone with resistivity VES locations and in between the wenner profiles; and greater than 70 Ωm and with depth range of 2 m their interpretations revealed that the stratigraphic down to 10 m though thinning out westwards succession within the area was sandy clay, shale, lime- (West – East direction of the survey). The topsoil stone and ironstone from the base to the top. The overlies another layer interpreted to be ironstone- description of borehole data from H-1, H-3 and H-8 limestone intercalations suggesting that the lime- are as presented in Figures 11–13. The Sokoto Group, stone boundary is gradational. The second geoe- within which this study is undertaken, constitutes lectrical layer has the thickness of about 6–7 m a narrow stratigraphic succession that is unevenly with resistivity range 50.3–70 Ωm. The third geoe- distributed in terms of their sediment thickness, lectrical unit is a limestone with thickness ranging about 37 km in the north and up to 280 km towards from 12 m to 26 m and resistivity values ranging the southern part of the basin. For this reason, infor- from 23.5 to 50.3 Ωm. Shale and Clay units form mation about the precise location and distribution of the fourth and fifth layers with resistivity values the limestone deposit is important. range 7.0–18.0 Ωm and 0–7.49 Ωm respectively. 90 E. E. OKON ET AL. Figure 12. Lithology description for borehole H-3. Within the area investigated, indicative features campaign in the area. This character may be attributed and characteristics of ironstone, shale, limestone and to pinching-out of this unit laterally or it may have clay/sandy clay were observed in all the methods experienced rapid erosional activity shortly after adopted. The ironstone, which forms the topsoil mate- deposition during the regression of the Trans- rial averaged about 5.10 m deep. The resistivities Saharan sea. Kogbe (1976) thought that it may have revealed patches of very high readings; and pockets been due to gradual desolution and weathering of the of medium resistivity readings (both with VES and already “folded” formation. Therefore, the overlying Wenner surveys). The high resistivity values may have shale unit is gradually being removed by solution of resulted from some massive rocky boulders common the underlying limestone unit and slumpping of the in the area forming minor scarps and characterized by overling ironstone. Surface mapping revealed some medium resistivity of unconsolidated clayey laterites/ gradational contact between the shale unit and iron- mudstone. Underlying the ironstone is a limestone stone, but the drilled boreholes showed sharp contact unit as observed in the borehole data and geophysics between the ironstone and the underlying limestone at model. But 100 m north of borehole H-3 and 500 m their drilled positions. This was also interpreted from southwest of borehole H-8 of the area, geologic map- the shallow geophysical surveys in the area. The light ping carried out revealed shale unit immediately after grey, whitish grey, chalky limestone unit encounterd the top ironstone (Figure 4(a-d)). This lithostrata in all the drilled holes varies slightly from 11.2 m to (shale) is about 0.50–5.00 m thick. According to 15.70 m in thickness (Figures 11–13) and at the Kogbe (1973), Obaje (2009), and Nwajide (2013), the mapped areas, the bottom of the unit was not expo- shale unit is referred to as Gamba Formation. This sured (see Figure 4(b-d)). It showed gradational con- Formation is not uniformly distributed throughout tact with the underlying shale unit but sharp contact the area based on information from borehole drilling with the overlying ironstone. This shale layer is GEOLOGY, ECOLOGY, AND LANDSCAPES 91 Figure 13. Lithology description for borehole H-8. Figure 14. Fence diagram showing the distribution of the different formation in the area. 92 E. E. OKON ET AL. characterized by yellowish to brownish colour in its significance, and when compared to complementary exposed surfaces and has few fossil (Figure 4(b-d)). approaches, this approach seems to be the most suited, The underlying Kalambaina limestone unit is overlain accurate and less expensive. It is therefore recommended by ironstones in most places (Kogbe, 1973). This suc- in areas deficient in exposed rocks (outcrops) and subsur- cession is not observable in outcrop exposures. face information. A stratigraphic fence diagram was prepared using information from all the drill holes available within Acknowledgments the area of interest and integrated with the ERT to model the subsurface and determine the distribution We acknowledge the management of MECON Geology and of the geobody (limestone) of interest (Figure 14). Engineering Services Ltd for providing the Borehole data and permitting its use in this study. Also, we thankfully Directly underlying the Kalambaina limestone is acknowledge all the authors whose works were referenced a shale Formation known as the Dange Formation. to validate our thoughts in this study. The shale is grey to dark grey in clour while the slightly weathered portion becomes yellowish in col- our. It exhibits high fissility and contains abundant Disclosure statement phosphate nodules. It is gypsiferous towards the base No potential conflict of interest was reported by the authors. and generally constitutes the basal formation of the Sokoko Group. Unconformably underlying the Dange Formation is the Wurno Formation (the youngest ORCID formation of the Rima Group). As revealed from this Emmanuel Etim Okon http://orcid.org/0000-0002-5166- study, the subsurface lithological characterization and lateral extent of part of the Sokoto Group is divisible into four-layer sections. The significance of ERT tech- niques integrated with a good control on the sedimen- References tologically described core data available at point Adegbola, R. B., Oseni, S. O., Sovi, S. T., Oyedele, K. F., & locations has proven to be very useful in the delinea- Adeoti, L. (2010). Subsurface characterization and its tion of the geobody of interest, in this case, the lime- environmental implications using the electrical resistivity stone unit (11.2–15.7 m). survey: Case with LASU foundation programme campus Using the subsurface stratigraphic model developed Badagry, Lagos State, Nigeria. Nature and Science, 8(8), 146–151. http://www.sciencepub.net/nature/ns0808/17_ for the area, sections with varying thickness and espe- 3394_ns0808_146_151.pdf cially towards the NE and central portion of the inves- Adelekan, A., Igbasan, A. O., & Oladunjoye, M. A. (2017). tigated area were identified (Figure 14). Also, the Integration of electrical resistivity and ground penetrat- overburden thickness was obseved to be not more than ing radar methods for site characterization: A case study 5.1 m across the study area and this, in itself, shows how of Ajibode Area, Ibadan, Southwestern Nigeria. International Journal of Environmental Science and the integration of ERT studies will largely reduce Natural Resources, 1(3), 555–564. doi: 10.19080/ exploration cost in the reduction of overall number of IJESNR.2017.01.555564. exploratory boreholes to be drilled if properly deployed. Bersezio, R., Giudici, M., & Mele, M. (2007). Combining sedi- mentological and geophysical data for high-resolution 3D mapping of fluvial architectural elements in the Quaternary Conclusions Po plain (Italy). Sedimentary Geology, 202(1–2), 230–248. https://doi.org/10.1016/j.sedgeo.2007.05.002 Surface geological mapping enhanced through the inte- Falconer, J. D. (1911). The geology and geography of gration of shallow subsurface geophysical mapping lar- Northern Nigeria. Macmillan and Co. Ltd. gely facilitated the delineation of the geometry of Griffiths, D. H., & Barker, R. D. (1993). Two-dimensional limestone resource within the area investigated. Such an resistivity imaging and modelling in areas of complex integrated approach is highly recommended especially in geology. Journal of Applied Geophysics, 29(3–4), areas with limited subsurface/borehole data and where 211–226. https://doi.org/10.1016/0926-9851(93)90005-J Ibeh, E. C., Olatunji, A. S., Yakubu, S., & Okon, E. 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Journal
Geology Ecology and Landscapes – Taylor & Francis
Published: Apr 3, 2021
Keywords: Sokoto basin; electrical resistivity tomography; lithological heterogeneities; facies; resistivity