Geologic mapping and basement–sediment contact delineation along Profile X, Igarra–Auchi area, Southern Nigeria using ground magnetic and electromagnetic methods

Outcrop mapping as well as electromagnetic and ground magnetic surveys was carried out within Auchi and Igarra localities in order to attempt an interpretation of the geology of the areas and to delineate the boundary between basement and sedimentary terrains. Geologic mapping was done by collecting samples of outcrops at five different locations within the areas. Three lithofacies were identified within Auchi area and they are the basal shale unit, tabular cross-bedded sandstone unit and ferruginized sandstone unit. The pebbly shale is greyish black in colour; the cross-bedded sandstone unit is greyish white, coarse-grained at the base and finer at the top with pockets of clay, while the ferruginized sandstone is dark red. Rocks of the Precambrian basement complex underlie Igarra area. The area is underlain by metasediments that have been intruded by igneous rocks. Results show the presence of three major groups of igneous and metamorphic rocks within the area, and they are the migmatite–gneiss complex, metasediments and porphyritic granites. The electromagnetic and ground magnetic data acquired along Profile X located along Auchi–Igarra–Ibillo road were processed using Microsoft Excel Software and the resulting plots delineated areas with lower electrical conductivities and higher magnetic susceptibilities, as well as areas with higher electrical conductivities and lower magnetic susceptibilities. The areas with lower electrical conductivities and higher magnetic susceptibilities are interpreted to be underlain by basement rocks, while the areas with higher electrical conductivities and lower magnetic susceptibilities are underlain by sedimentary rocks. The plots also delineated the most likely basement–sedimentary boundary in the area.


Introduction
The Auchi/Igarra area of Edo State situated within the southern region of Nigeria is an area that is geologically significant. Its geologic significance stems from the fact that Auchi lies within the western part of Anambra Basin which is located in the Lower Benue Trough and as such is underlain by rocks of purely sedimentary origin, while Igarra area is an area underlain by rocks of the Precambrian basement, the area consists of metasediments intruded by Pan-African igneous rocks (Egbuniwe and Ocan 2009).
Several authors including Umeji and Nwajide (2007), Adekoya et al. (2011) have studied the sediments of Auchi area using methods like geologic mapping, textural and mineralogical characterization and diagenetic studies among others. The makeup, structural elements and nature of the Precambrian basement rocks underlying Igarra area have also been studied by authors including Egbuniwe and Ocan (2009), Odeyemi and Rahaman (1992) and Oluseyi (2006). None of these authors was, however, able to delineate and establish the approximate contact or boundary between the basement terrain of Igarra area and the sedimentary environment of Auchi area.
The methods of investigation in this study involved field geologic mapping, electromagnetic survey using the very low-frequency (VLF) method and ground magnetic survey. Field geologic mapping is a method which entails studying rocks and geologic structures where they are best exposed and most easily interpreted. Electromagnetic method, on the other hand, is a geophysical survey technique that applies the principle of electromagnetic induction to measure the electrical conductivity of the subsurface (Kearey et al. 2002). The magnetic method is a geophysical survey technique which is used to investigate subsurface geology based on anomalies in the Earth's magnetic field originating from the magnetic properties of the underlying rocks (Kearey et al. 2002). The electromagnetic method and the magnetic method are the most important and commonly used technique for studying the electrical conductivity and magnetic susceptibility of underlying rocks. Field geologic mapping of outcrops in this area will validate the fact that Auchi area is underlain by rocks of purely sedimentary origin and Igarra area is underlain by rocks of the Precambrian basement. Delineating the boundary between these geologic terrains by studying the electrical and magnetic properties of the underlying materials will enable solid mineral miners and geologists involved in quarrying activities within Igarra area to have a good knowledge of the termination point of basement environment in the area. Knowledge of the boundary between these two terrains will also serve as a guide for drilling activities within these environments.
This work therefore presents a detailed account of the electrical conductivities and magnetic susceptibilities of the rocks underlying Auchi and Igarra areas leading to a better delineation of the boundary between the sedimentary and basement environments.

Geology of the area
Auchi and Igarra area, situated in Edo State Southern Nigeria, lies within latitude 7°04′00″N and Longitude 6°16′00″E. Figure 1 shows the location map of the study area.

Auchi
Auchi area is located within the western portion of Anambra Basin. The shape of this basin is nearly triangular as shown in Fig. 3 and covers an area of about 3000 km 2 with a sedimentary pile of approximate thickness of 9 km (Olubayo 2016;Iheanacho 2016). This basin is situated west of the lower Benue Trough as shown in Fig. 2 and according to Obaje (2009); it forms part of the Lower Benue Trough that contains sediments of Campanian-Maastrichtian to Eocene ages. Three lithofacies belonging to two different lithostratigraphic units are recognized in Auchi area, namely shale unit overlain by cross-bedded, burrowed sandstone which is then overlain by ferruginized sandstone (Adekoya et al. 2011). The shale unit is said to likely belong to Mamu Formation while the sandstone units are most likely the extension of Ajali Formation (Geologic map of Nigeria, GSN 1994). The depositional environment of the shale is believed to be shallow marine while that of the sandstone units is fluviodeltaic (Nwajide 1990). According to Ilegieuno et al. 2020, the sediments of Auchi area are fine-to coarse-grained and also poorly to moderately sorted. Figure 2 shows the sedimentary basins of Nigeria after Obaje (2009) , Fig. 3 shows the geologic map of Anambra Basin indicating the extension of Ajali sandstone within Auchi environs while Fig. 4 shows the stratigraphy and likely depositional environment of Anambra Basin modified from Tijani et al. (2010).

Igarra
Precambrian basement rocks, mainly metasediments intruded by igneous rocks of mostly pan-African age, underlie the entire Igarra area (Egbuniwe and Ocan 2009). According to Egbuniwe and Ocan (2009), this area is characterized by structural complexity due mainly to the different Underlying this area are metamorphic and igneous rocks classified into three major groups by Odeyemi and Rahaman (1992), namely the migmatite-gneiss complex, a sequence of upper Proterozoic metasediments and syn-to late-tectonic porphyritic granites shown in Fig. 5. Granodiorites and syenites also exist in this area (Odeyemi and Rahaman 1992). From extensive study of the rocks of Igarra area, Odeyemi and Rahaman (1992) reported the existence of a discordance in structure between the older migmatite and both metasediment and older granite. This was due to the fact that most structural elements including folds and lineaments existing in this area have a common central axis with a dominant axis trending NW-SE in the metasediments and the older granites while the major structural element in the older migmatite trends E-W. This implies that there is no uniformity in the trend of structural elements in the rocks of this area. Marble, calc-gneiss, metaconglomerate, granites, pure quartz and kaolin occur in large quantities at different localities within Igarra area Oluseyi (2006). Figure 6 shows the geologic map of Igarra area after Anifowose et al. (2006).

Field geologic mapping
The field geologic mapping covered a total of five localities within Auchi and Igarra areas from where samples were taken and examined. The materials used for this study include the Brunton Compass used to determine the direction of dip and strike of beds, sledge hammer to pick rock samples where they outcrop, hand lens to ascertain whether there are minerals within the rock, camera used for photographs and a small field knife for scratching the rock. At 1 3 every location, the name of the locality, the geographical coordinates, the strike and dip of the beds at outcrop as well as the physical characteristics of the sediments visible at outcrop were determined and recorded. The field observations of outcrops at 5 different locations within Auchi and Igarra are provided in Table 1.

Electromagnetic surveying
In carrying out the electromagnetic survey along the profile of interest (profile X shown in Fig. 1), the very lowfrequency (VLF) method of electromagnetic surveying which involves a stationary source and a moving receiver was employed. The name of the equipment used as receiver is the ABEM WADI EM receiver. The equipment has an antenna unit which contains horizontal and vertical bars. These bars contain the transmitting and the receiving coils. The horizontal bar measured horizontal variations while the vertical bar measured vertical variations in the subsurface. The power supply unit was used to power the antenna and the receiver, while the receiver unit received the resultant field. The starting point of the survey was at about 120 m from SMAC Quarry Mills, along Auchi-Igarra-Ibillo road, Ikpeshi-Igarra, Akoko-Edo LGA and was run toward Auchi. A total of 155 data points was covered, and data were collected every 20 m along the profile. At every data point, the raw real (%), the filtered real (%), the raw imaginary (%) and the filtered imaginary (%) values were read and recorded.
The VLF electromagnetic data consists of two separate components: the raw real otherwise called the in-phase data and the raw imaginary otherwise known as the quadrature data. These carry very important complex information about the geology of the subsurface of that environment. Because of the complexity of the information, it cannot be seamlessly related to the body causing the anomaly. In addition, these data carry high noise. To correct for these, filtering operators are applied, among other things, to reduce the nonlinear and harmonic noise, to transform the raw real into current density and to adequately position the amplitude of the anomalous body over its source. Hence, the filtered real and filtered imaginary data are respectively the raw real and raw imaginary data which have undergone filtering to correct the defects mentioned above. Microsoft Excel software was used to plot the survey data. The acquired survey data are shown in Table 2.

Magnetic surveying
In carrying out the magnetic survey along the same profile X, a set of field equipment which includes the magnetometer and the sensor unit was employed. As in electromagnetic survey, the starting point of the ground magnetic survey was at about 120 m from SMAC Quarry Mills, along Auchi-Igarra-Ibillo road, Ikpeshi-Igarra, Akoko-Edo LGA and was run toward Auchi. A total of 44 data points was covered, and data were collected every 50 m for the first 1 km and every 100 m for the remaining 2.3 km along the profile. At every data point, the peak value (µT) was read and recorded twice, and the field value (µT) was also read and recorded twice. Their average values were later calculated. This was done to reduce errors in the readings. Also, the time at which data were acquired at every location was read and recorded. Microsoft Excel software was used to plot the survey data. The acquired survey data are shown in Table 3.

Results
The results of the geologic field investigations of outcrops carried out at five different locations within Auchi and Igarra areas are presented below.  Location 1 (Ekpesa, Auchi): At this location, an outcrop of ferruginized sandstone underlain by greyish black pebbly shale was seen. This shows a coarsening upward sequence.
Location 2 (Ipeme, Auchi): This location has an exposure of tabular cross-bedded, greyish white sandstone with ripple marks, coarse-grained at the base and finer at the top with pockets of clay. This outcrop shows a fining upward sequence (Fig. 7).
Location 3 (Ikpeshi-Igarra): Here, an outcrop of light brown, shiny quartzite was observed. By visualization, the quartzite shows evidence of shearing (faulting) which indicates mylonitization. The shiny nature may indicate the presence of mica. It also has veins with quartz. The quartzite has been tilted (Fig. 8).
Location 4 (Akoko-Igarra 1) This location is at Akoko-Igarra and it is an older granite complex. The biotite granites are foliated, medium-to-coarse-grained, porphyritic and greyish black in colour. They have areas of jointing which helps to determine their direction of weakness (Fig. 9).
Location 5 (Akoko-Igarra 2): This location has an exposure of granite gneiss. This granitic gneiss is dark grey in colour, feldspathic and invaded by pegmatitic materials (muscovite). Metasomatism was evident in the location due to metamorphic reaction. There is also the presence of augengneiss which is as a result of tectonism. The gneiss is specially banded (Figs. 10, 11).

Electromagnetic survey
The data in Table 2 were obtained from the electromagnetic survey carried out along profile X within the study area.
The above data were plotted using Microsoft Excel software. The filtered real (%) and raw real (%) were plotted against distance (metres) on the same graph (Fig. 12); the raw real (%) was plotted against distance (Fig. 13); and finally, the filtered real (%) was also plotted against distance (Fig. 14).

Magnetic survey
The data in Table 3 were obtained from the ground magnetic survey carried out along profile X within the study area.
The above data were also plotted using Microsoft Excel software. The variation in peak measured in MicroTesla (µT) was plotted against distance (metres); the variation in field (µT) was plotted against distance (metres), and finally, the variations in peak and field (µT) were plotted against distance (m) on the same graph. The plots are shown in Figs.15, 16 and 17 respectively.

Discussion
From the field observations, three different lithofacies belonging to two lithostratigraphic units were observed within Auchi area studied (locations 1 and 2). These are: shale unit, cross-bedded sandstone unit and ferruginized sandstone unit. The shale unit is overlain by the cross-bedded sandstone and the ferruginized sandstone units which are in agreement with previous studies carried out in the area. The outcrops observed within Igarra area are rocks of the Precambrian basement complex, a metasedimentary suite which has been intruded by igneous rocks of mostly Pan-African age.
From the results of the electromagnetic survey carried out, it can be seen that from a distance of 0 m (starting point) to a distance of about 2000 m, the conductivity was much lower when compared to the conductivity from a distance of about 2120 m up to 3000 m. This shows that the rocks underlying the area between 0 m to about 2000 m have lower electrical conductivities when compared to the rocks underlying the area between about 2120 m up to 3000 m. From theoretical knowledge, basement rocks have very low electrical conductivities because of their lack of porosity, saturation and possibly salinity, while sedimentary rocks have higher conductivities because of these properties. Hence, since the study profile is underlain by basement terrain on one section and sedimentary terrain on the other, it can be deduced from the plots that the distance between 0 m to about 2000 m is underlain by basement rocks because of their lower electrical conductivity values, while the area between a distance of about 2120 m up to 3000 m is underlain by sedimentary rocks because of their higher electrical conductivity values. Also, the area between a distance of 2000 and 2120 m (about 120 m) can be said to be the transition zone and boundary between basement and sedimentary environments.
From the results of the ground magnetic survey carried out, variations in magnetic susceptibility are higher from a distance of 0 m to a distance of about 1900 m than from a distance of about 2100 m up to 3000 m. Considering the linear variation in peak (µT) and the linear variation in field (µT) in the plots, it can be seen that the average magnetic susceptibility decreased when moving from a distance of 0 m up to a distance of 3000 m. The magnetic susceptibility of most rocks is directly proportional to their content of ferromagnetic minerals like magnetite, ilmenite, pyrrhotite and franklinite (Kearey et al. 2002). Basement rocks contain more of these magnetic minerals than sedimentary rocks.
It can be deduced from the plots that the distance between 0 m to about 1900 m is underlain by basement rocks because of their higher magnetic susceptibility values, while the area between a distance of about 2100 m up to 3000 m is underlain by sedimentary rocks because of their lower magnetic susceptibility values. The area between a distance of 1900 m to 2100 m (about 200 m) can be said to be the transition zone and boundary between basement terrain and sedimentary terrain. These are in agreement with the information obtained from the electromagnetic survey analysed above.
A schematic model of the basement-sedimentary geometry along profile X is shown in Fig. 18.

Conclusion
The field geologic investigation of some outcrops within Auchi and Igarra area of Edo State, Southern Nigeria, was carried out. Also, an electromagnetic and ground magnetic survey was carried out along profile X within Auchi-Igarra area. From the results of these geologic and geophysical investigations carried out, it can be concluded that the sediments of Auchi areas studied (locations 1 and 2) are made up of three major lithofacies: the basal shale unit, the tabular cross-bedded sandstone unit and the ferruginized sandstone unit. The shale unit is overlain by the  Rocks of the Precambrian basement complex underlie Igarra area. The area is underlain by metasediments that have been intruded by igneous rocks. Results show the presence of three major groups of igneous and metamorphic rocks within the area and they are the migmatite-gneiss group, metasediments and porphyritic granites.
From the two geophysical surveys carried out, the boundary between basement terrain and sedimentary terrain in this area lies at approximately between a distance of 2000 and 2120 m from SMAC Quarry Mills, along Auchi-Igarra-Ibillo road, Ikpeshi-Igarra, Akoko-Edo Local Government Area, Edo State, Southern Nigeria (moving toward Auchi from Igarra).
From the entire investigations carried out, it is recommended that more outcrop locations should be accessed and investigated within the study area. This might give additional information about the geology of the area. In future, data or sampling points on the acquisition profile should be established at a much closer interval, particularly for the magnetic survey, in order to obtain a far better subsurface electrical conductivity and magnetic susceptibility information about the area.
It is also suggested that electrical resistivity method should be carried out around the suggested boundary between basement and sedimentary terrains to validate the results of the geophysical surveys utilized in this study.
Funding The authors received no special funding for this research.

Conflict of interest
There are no competing interests to declare.
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