4.2 Topographic, free air and complete Bouguer anomaly maps
The Sokoto Basin and its surrounding areas have topography that is unevenly undulating and are located in the southeasterly portion of the Iullemmeden Basin (Fig. 4a). The southern and eastern rift margins of the interior basin flanks, which may be seen at a cursory glance in Fig. 4a, are the study area's highest elevation sites. The rift floor of the Gwandu formation can be seen in the lower elevated location that is aligned to the northwest. This floor is covered in quaternary alluvium, and the peak appears like an elongated escarpment ridge. The rift's edges are defined by the change from lowlands on the rift floor, which are only 200 meters above sea level, to higher elevations, which are 350 meters above sea level. Since humid, thick materials from the mantle have lower magnetism, high gravity and reduced magnetic susceptibility in these regions may suggest thermal heat sources.
Using a mathematical model (Eq. 2b), the term "free air anomaly" refers to the gravitational acceleration after the impact of height has been eliminated. The vertical variation in gravity between the reference datum and the observation height, as well as theoretical gravity, are both taken into account by the free-air gravity anomaly. According to Hinze et al. (2013), it is presumable that the space in between the observation point and the height datum is devoid of any mass and gravitational effects. The most fundamental anomalies employed in geologic investigations, unlike other anomalies, make no assumptions about the mass of the Earth.
The theoretical gravity and height above sea level were taken into consideration when creating the free air anomaly map (Fig. 4b) for this study using the Geosoft oasis montaj software. Eq. 2b provides a mathematical analysis of the relationship between free air anomaly and elevation. Comparing the elevation (Fig. 4a) and free air anomaly (Fig. 4b) maps from the current investigation also clearly demonstrates this. Looking at these maps reveals that Gulma (escarpment features), higher elevated locations in the northwest and southeast, as well as other locations, are characterized by negative free air anomaly. On the other hand, the rift floor's lower elevated locales are characterized by substantially negative free air anomalies that can be as low as -55.84 mGal. The Sokoto Basin's undulating topography produces a high range value (-2.0 mGal) of free air anomaly, which cannot be disregarded. The Free Air Anomaly cannot be used to analyze local gravity studies because it ignores the impact of mass differences between the observation point and the reference geoid (Hinze et al., 2013).Using the Bouguer plate adjustment (Eq. 4a,b), the influence of the attraction of rock units between the observation site and reference geoid can be eliminated. After accounting for elevation effects, theoretical gravity, and the mass difference between the observation point and the geoid, the residual gravity value is a straightforward Bouguer anomaly (Telford et al., 1990).
After gravity corrections were applied to the acquired raw gravity data, the complete Bouguer anomaly (CBA) was determined. The density of rocks in the Earth's crust is not constant; therefore the entire Bouguer anomaly would be zero. We may assess how elevation affects gravity survey by carefully reviewing the mathematical equations for CBA (Eq. 4a,b). In order to assess the subsurface geology based on gravity measurements, it is preferable to take into account the spatial variation in elevation of the area (Fig. 4c). Looking briefly at it suggests that elevation is inversely connected with the spatial variance in the Bouguer anomaly (Fig. 4a). The observed gravity anomaly map (Fig. 4c), which is a result of lower to upper crust density heterogeneities, can be explained by the area's relative small size. The southwestern Sokoto inland basin (the research area) is depicted in full on Fig. 4c's Bouguer anomaly map. The CBA map (Fig. 4c) can be thoroughly examined to reveal three regions with gravity field signatures: high (-27.38 mGal to -39.29 mGal), intermediate (-54.02 mGal to -41.47 mGal), and low (-74.88 mGal to -55.26 mGal). Gravity maxima are present on the floor of the Iullemmeden Basin, which follows the northern terminus of the Sokoto Basin, and its surrounds. The Earth's gravitational field changes very little as a result of variations in the density of the crustal rocks beneath it. As a result, the Earth's gravitational field changes, causing a gravity anomaly (Kearey et al., 2002).
Between the highest density and lowest density causative materials in this study area, there is a -20.67 mGal gravity anomaly change in the Earth's gravitational field. The CBA map in Fig. 4c was used to try and interpret the data qualitatively, which required a visual study of the map to spot gravity highs and lows. The variety in density of the underlying masses is revealed by this. Gravity high suggests denser subsurface masses, while gravity low suggests that the Earth's crust contains less dense subsurface masses. Pre-Maastrichtian sediments are found in the research area's northernmost regions, where there are minor patches of high gravity. A high gravity area may be found in the southern portion of the research area, which is defined by the coordinates (128000 N, 400000 E) and (144000 N, 400000 E). It has a CBA of -51.13 mGal on average and is directed Northeast-South. According to Kogbe (1981), this gravity high separates a gravity low and may be an indication of a problem. Both a low and a high anomaly may be seen to the west in the map's central region. With a few points of high gravity, the region to the South is typically characterized by low gravity and is defined by the coordinates (132000 N, 80000 E). Dykes in the form of high-density minerals penetrating from the mantle could be the cause of these gravity highs. The map shows gravity highs with minimal gravity lows to the northeast. According to Kearey et al. (2002), an intruding material from the mantle has a density range of 2.24 gcm3 to 2.67 gcm3, which is a negative anomaly. As a result, areas with a high gravity anomaly suggest that the Earth's crust contains denser materials that could act as heat sources.
4.3 Upward continuation as regional-residual separation of gravity field
The gravitational impacts of substantial deeply seated mass distributions and smaller, localized mass distributions close to the observation site are added to determine the anomalous value of the gravity field at a given position. Relative gravity anomalies, or anomalies of importance, are frequently isolated when interpreting Bouguer gravity anomalies (Mickus et al., 1991; Linser, 1967). A regional and residual gravity anomaly field makes up the observed Bouguer gravity anomaly field. Gravity data set was subjected to the regional-residual separation technique in order to determine the magnitude of the regional background. Using upward continuation, a regional gravity anomaly originating from deep sources can be separated from the observed gravity (Kebede et al., 2020; Mammo, 2004; Linser, 1967). This maintains a level at a constant height above the surface of the earth (or the measurement plane). It is used to determine the broad or local (low frequency or long wave length) trends of the data. Since the sedimentary in-fill is the intended depth and it is approximately undulating between 1 km and 5 km, the data is extended upward at 10 km to eliminate wavelength deviations. If a potential field moves upward to a given height z, as demonstrated by Jacobsen (1987) and Lyngsie et al. (2006) and Mammo (2004), sources located at a depth greater than z = 2 can be focused on. When the entire Bouguer gravity anomaly (Fig. 4c) is subtracted from it, the residual gravity anomaly (Fig. 6) is calculated.
4.3.1 The regional gravity anomaly map compiled from upward continuation of the complete Bouguer anomaly
Depending on the level of inquiry we are interested in, we have different interests in characterizing the causal bodies of gravity anomaly. Here, the regional anomaly is taken into account in order to examine the entire crust under the Sokoto Basin and its surroundings. This study's regional gravity anomaly map (Fig. 5) was created by extending the CBA map upward (Fig. 4c). The CBA map (Fig. 4c) has been extended for various continuation distances (Fig. 5) in an effort to observe the impact of deep seated entities. In this sense, Fig. 5 offers significant insight into comprehending the regional gravity field pattern at various distances. To see the gravity effect as if we were to examine the subsurface at 500 m, 2.5 km, 3.5 km, 5 km, and 7 km depths from the surface, respectively, the CBA map has been continued upward to 1 km, 5 km, 7 km, 10 km, and 14 km. Figure 4.4c illustrates the suppression and subsequent enhancement of shallow and deep seated bodies, respectively.
The impact of shallow seated others layered on deep seated bodies can be seen on the CBA map at ground level (0 km) in Fig. 5. However, the removal of the short wavelength and the augmentation in effect of deep seated bodies are revealed when the CBA map is continued upward to 5 km to see the anomaly sources buried beneath 2.5 km. On this map, four gravity areas can be quickly identified: (1) The Sokoto Group's flanks (Dange and Gwandu) show very low gravity responses from deep-seated entities. (2) The rift's margins and floor after the southern termination of the West Africa merging area show low gravity responses (3) intermediate to high gravity responses represent the rift's floor and the Gundumi-Illo formation. After the southern terminus of the Basement complex, a very high gravity field response is seen moving northward. This gravity anomaly zones are further intensified (5 km, 7 km, and 10 km upward continuation), and shallow seated bodies are afterwards suppressed (Fig. 5). On the regional Bouguer anomaly map (Fig. 5), the Sokoto inland basin's broad regional gravity trend is depicted, with a northward gravity increment following the axis of the profiles. The impact of border faults and structural highs is seen at shorter upward continuation lengths as a dominant northeast-southwest striking feature. Northwest of the Rima group, a sharp increase in the local gravity anomaly is seen (Figs. 5). The hypothesis put forth by earlier geology and geophysical investigations (Umego and Osazuwa, 2001; Wolfenden et al., 2004; Osazuwa, 2011) that the Sokoto and Rima group is more advanced than the western part is supported by this gravity anomaly signal.
4.3.2 The residual gravity anomaly map
Both positive and negative anomalies may be found on the residual map (Fig. 6), which generally follows the same NW–SE trend as the equivalent Bouguer anomalies. The anomalies are situated in the same locations. Similar to Bouguer, the residual anomalies additionally confirmed the Rima group (RG), Sokoto group (SG), Continental intercalair (CI), and Continental terminal (CT), which were respectively located in Kaloye (near the Isah-Sabon Birni structures), Sokoto Dogon-daji, Gundumi/Illo formation, and the vicinity of the Argungu-Gwandu formation. While the anomaly RG in Sokoto-Dogon-daji is gravity high with amplitude of + 9.2 mGal, the anomaly Sokoto group (SG) is gravity low with amplitude of -2.7 mGal. The anomalies of Gwandu Continental terminal (CT) have gravity amplitudes of -14.2 mGal and + 2.6 mGal, respectively, and are close to Argungu-Gulma settlements. A minor positive anomaly with amplitude of + 16.2 mGal is present at the Gundumi and Illo formation in anomaly (CI). When compared to the nearby (host) rocks or basement, it may be claimed that the gravity high near the Gundumi and Illo formation is directly over a subsurface with a positive density contrast. The existence of thickened sediments from the Paleocene Sokoto group or the thickened sediments of the Gwandu formations match to the gravity lows that have been reported in the area. As a result, the residual anomaly map and the geological map have a moderately good correlation.
The Gundumi and Illo formation of rocks, which is entirely beyond the zone where Pre-Maastrichtian sediments spread, is the site of the most discernable gravity high Continental intercalair (CI), reflected in the Bouguer map simultaneously. The high density intrusion in the basement beneath could be the cause of the relative gravity. Although one should not entirely rule out the possibility that the explanation could be the presence of a low density intrusive bodies like red molted ironstone, the relative gravity low could be a sign of abundance of sediments. Around Duku-Tarasa in Birnin Kebbi, the escarpments are visible. Over the years, Kebbi has documented both local and mechanized mining of various economically important rocks and minerals that have their origins in the Gwandu formation (Kogbe, 1979; Obaje et al., 2020).
The abundance of Eocene sediments may be related to the Continental Terminal of Gwandu's gravity low. Naturally, it is clear from the geological map since the anomaly is almost at the point where a Rima group formation starts to enlarge eastward. The anomaly Continental Intercalair shows a very tiny gradient of roughly 1 mGal/km; this steep gradient either indicates a strong geologic contact or could perhaps be indicative of faulting. Around Dutsin-Bardawa and Gamba, there is also gravity low. Given that there are less Gamba formations in certain places as shown on the geological map, this might be a faint sign of granitic intrusion. The mid-southern pan of the image shows a steady gravity gradient of roughly 1 mGal/km, which may also be indicative of faulting. All of the preceding hypotheses about the study area are completely based on the findings.
4.4 Enhancements Filtering Utilization
The technique parts go over the mathematical formulations for the following concepts: analytical signal, tilt of angle derivative, directional filtering, power spectrum analysis, 2D Werner De-convolution, 2D joint forward modeling, and 3D structural flipping. The first set approaches are automatic methods that do not require prior geological information as opposed to the last two. The first five techniques are the edge detection filters for qualitative assessment whereas the subsequent groups of three are depth estimation filters for quantitative assessment. Figure 6 depicts the estimated depths of the gravitational and magnetic sources along and across the rift direction of the three profiles. A profile line (black color) is shown where the residual gravity anomalies are retrieved and examined.
4.4.1 Analytic signal gravity map
As a result, the geological map (Fig. 2) of the study area has been used in conjunction with the analytic signal map (Fig. 7). The source body and geologic contacts are depicted by the map's highest gradient (Fig. 7). The impact of shallow seated persons is synthesized in the Analytical signal map (Fig. 7).
The analytical signal map shows a cumulative of maximum gradients that define the Eocene sediments of the Gwandu formation, which are included in the centre and western portions of Fig. 7. This is so because the quaternary deposit that covers the Argungu, Gwandu, and Bunza is denser than the corresponding quaternary sediments of the nearby rift floor. The lithological discontinuities are clearly mirrored by gravity signal discontinuities, therefore in this regard, the AS maps (Fig. 7) and the geological map (Fig. 2) of the area is reasonably in agreement. The majority of the geological map is widely covered with lacustrine sediments (shales, limestone), particularly near the western edge of the rift, which follows the eastern portion of the Sokoto groups and includes the Kalambaina and Dange formations. As shown in Fig. 7, the sedimentary rocks in the Eocene, Paleocene, and Maastrichtian deposits are characterized by low gradients, in contrast to the area covered by this lithology, which has low AS values. In the southeast flank and eastward to the southern borders of (Fig. 2), intrusive masses and welded crystalline granite are reflected by a maximum gradient in the AS map.
4.4.2 Tilt of angle derivative gravity map
Finding both a lithological and a structural discontinuity at once is difficult with horizontal derivative and AS (Fig. 7) gravity map; this is another advantage of the TDR technique of interpretation (Fig. 8). As a result, the TDR map (Fig. 8) is more detailed than the residual map (Fig. 6) and the AS map (Fig. 7), as the former increases both short and long wavelength anomalies concurrently. The legend offered by the color scale helps to identify the edge (border) of source bodies in such a way that the source body, edge, and outside source bodies are represented by the colors pink, red, yellow, green, and blue, respectively.
The peak of gravity anomalies are sharpened and the weak anomalies signal is made wider by TDR, according to Putri et al. (2019), which is a useful method for locating deep sources. A maxima in the AS (Fig. 7) and a minimum in the TDR (Fig. 8) map characterize the region at the Continental Terminal of Eocene deposits of the Gwandu formation in the southern section of Argungu. Because the latter way improves both deep and shallow seated bodies, it improves both types of seated bodies more than the earlier method does. NE-SW, E-W, and NW-SE striking faults, which represent the direction of boundary faults, are depicted in Fig. 8 at their respective locations (black colors).
4.4.3 Directional filtering gravity map
The gridded Bouguer anomaly and residual gravity data are first transformed using the Fourier method as part of the approach's implementation process. To obtain the directionally filtered data, the Fourier transformed data was multiplied in the frequency domain with the necessary directional filters functions. The resulting product was then inverse Fourier transformed (into the space domain). Figures 9a,b show the outcomes of this study's application of a directed filter to the residual gravity data, which creates a pie-slice at a 45° angle pointing northwest. Onyedim and Ogunkoya (2002) also noted that linear characteristics that show up on topographic maps may be found to identify locations of vertical movements, tilting, or horizontal displacements that are frequently interpreted as faults or lineaments. As a result, the directionally filtered maps created for this study were colored to clearly indicate zones showing the offsets, as seen in Figs. 9a,b. This enhanced the fault structures on the maps created by the directionally filtered method. As seen in Figs. 9a,b, the improved fault structures were superimposed on the residual gravity field as well as the directionally filtered maps for additional analysis.
4.4.4 Spectral depth analysis
The NE-SW axis, NW-SE, and N-S axes are used to obtain the residual gravity anomaly profile (Fig. 6, along black line). The residual gravity anomaly profile with 25 data points is subjected to the one-dimensional power spectrum approach. Figure 10 shows the calculated average power spectrum curve as a logarithm of spectral energy against the wave number. The slope (gradient) of the power spectrum curve is used to calculate the elevation above the gravity source (density interfaces). Based on a piecewise least-squares approach, the linear curves (from which slope values are read) are fitted to the power spectrum data (Fig. 10). The fitted lines, which display the averaged energy of the source origin from low to medium and extensive frequency values, allow us to read the gradients (slopes). This energy is largely derived from low frequency deep sources (Fig. 10). Table 2 lists the outcomes of the power spectral analysis.
Depending on the wave number (frequencies) and slope (gradient) variations, source depth interfaces are classified as shallow, middle, and deep. According to residual gravity anomalies, the distance to intermediate sources along the rift axis and rift bottom is around 1.5 km, but the distance to deep gravity sources is 2.87 km. In Fig. 10, the third slope almost perfectly captures the data noise. Table 2, column 2, reads three gradient values from the fitted trend lines (Fig. 10), and column 3 of Table 2 displays the depths that were determined using Eq. (3). Two clearly visible interfaces are detected and classified as deep and middle source depths (Table 2, column 4) as the method is utilized to calculate mean depths to the various density contrast interfaces. The third interface's depth is estimated to be roughly 23 m, with r2 value of 0.002. Accepting 23 m as the source depth with a negative r-square value may result in incorrect interpretation. It would therefore be prudent to regard it as a noise component.
By comparing the gravity source depth results with the geologic stratigraphy (Table 1, Fig. 2) generated using well-log data, the estimated top of density differences (Table 2, column 4) could be validated (Kogbe, 1979; 1981, Bonde et al., 2014 & Obaje et al., 2013). In the northern region of Niger Republic, which is close to southern part of Sokoto, this source depth estimation method was utilized to estimate density horizons (Nwanko and Shehu, 2015).
Table 3
Estimates of the subsurface anomalous source's depths in terms of its deep (regional), shallow (residual), and noise components
|
S/N
|
Layers
gradients
|
Depth to Regional-residual
(Components)
(m)
|
Causative source
(Categories)
|
Lithostratigraphic approximation from Table 1
|
|
1
|
-23450
|
-2870
|
Deep
|
Intrusive bodies layers
|
|
2
|
-119.76
|
-1500
|
Shallow
|
Lacustrine shale layers
|
|
3
|
-11786
|
-2036
|
Deep
|
Quaternary layers
|
|
4
|
-2098
|
-1380
|
Shallow
|
Lacustrine shale layers
|
|
5
6
|
-1325
-2278
|
-2544
-1158
|
Deep
Shallow
|
Top of Gravel layers
Variegated shale layers
|
|
7
|
-1921
|
-2536
|
Deep
|
Quaternary layers
|
|
8
|
-1043
|
-1009
|
Shallow
|
Lacustrine shale layers
|
|
9
|
-2798
|
-2106
|
Deep
|
Top of Gravel layers
|
|
10
|
-978.0
|
-14.26
|
Noise
|
Sandstone layers
|
|
11
|
-1248
|
-2709
|
Deep
|
Intrusive bodies layers
|
|
12
|
-1089
|
-908.1
|
Shallow
|
Lacustrine shale layers
|
4.4.5 Werner depth analysis
The gravity source depth and position along the rift axis of the profiles are calculated automatically using Werner depth solution. This procedure determines the depths and positions of the vertical gravity and magnetic sources using an iterative 2D inversion methodology. The equation that is used to estimate this depth is given in the subsection above. The residual gravity anomaly along the selected profiles is first extracted using a residual gravity anomaly map (Fig. 6). The positions of the gravity source vertical contact depth are revealed by applying the Werner analysis to this profile anomaly, as illustrated in Table 3. Additionally, the fault trends of the contact and dyke solution faults (Figs. 6 and 7) that were discovered from residual gravity anomaly are almost exact replicas of the fault patterns of tilt angle derivative (Fig. 7).
Table 3
Statistical analysis of Werner depth anomaly along & across the profiles
|
Trend
|
Numbers
|
Parameter
|
Depth
(minimum)
(m)
|
Depth
(maximum)
(m)
|
Depth
(mean)
|
Standard
deviation
(m)
|
Remarks
|
|
N-S
|
1
|
Z_Contacts
Z_Dykes
|
-4210.46
-4507.12
|
-0.25
-0.23
|
-125.39
-189.20
|
130.25
|
Estimates vertical Sources
|
|
NE-SW
|
2
|
Z_Contacts
Z_Dykes
|
-3245.51
-3450.04
|
-0.18
-0.14
|
-110.28
|
143.23
|
//
|
|
NW-SE
|
2
|
Z_Contacts
Z_Dykes
|
-3756.98
-3865.90
|
-1.34
-1.13
|
-140.32
|
167.21
|
//
|
|
E-W
|
4
|
Z_Contacts
Z_Dykes
|
-3989.54
4032.54
|
-0.21
-0.18
|
165.89
|
156.98
|
//
|
|
Total
|
9
|
|
|
|
|
|
|
4.5 Gravitational field of the Sokoto Basin
It is discovered that the variation in the gravitational field of the area that makes up the study a location can be used as a beneficial tool for gathering pertinent information about a study site in a local setting. The Nigerian National Petroleum Company Limited, NNPCL, contributed 1095 gravity data points for this study, covering the Sokoto Basin in northwest Nigeria, which covers the study area. The full Bouguer anomaly (CBA) map (Fig. 4c) and its improved parts (Figs. 5a–e and 6), which were produced using this regional gravity data set, were analyzed. For the purpose of sampling all structural features, two-dimensional (2D) gravity models have been built along three profiles (A-A′, B-B′, and C-C′) that run along and across the rift axis and intersect at the continental terminal of the Gwandu formation (Fig. 6).
4.5.1 2D Gravity models within the framework of Sokoto Inland basin
As shallow subsurface investigations are a result of important geologic processes that began in a deeper environment, geophysical understanding of the subsurface at a regional scale has a significant impact on those studies. Two 2D gravity and magnetic models are built utilizing residual gravity anomaly map within the context of this understanding and are limited by geologic data (Figs. 6, & 2). The measured anomaly values are retrieved along profile A - A′ (Fig. 11a) that crosses the rift axis and along profile B - B′ (Fig. 11b) that runs along the rift axis, both of which are purposefully chosen to intersect at the Gwandu continental terminal (CT). For the objective of sampling all structural aspects in the research region, 2D models of the area are built.
4.5.2 Restrictions imposed by earlier researchers
The non-uniqueness of models produced, particularly when integrating potential field data (e.g. gravity and magnetic data), is a typical issue in a majority of mathematically-based geological models incorporating geophysical data. This gives the impression that no single geophysical model can be considered to be more correct in terms of geology than the other models (Saltus and Blakely, 2011). To minimize the non-uniqueness of the potential field gravity models, Saltus and Blakely, (2011) emphasized the significance of incorporating existing knowledge and the application of various methodologies. The 2D model produced for this research area and its environs was constrained using various geological and geophysical data. (a) The surface geology was used as a first-order along the model's profile, particularly where the Quaternary formation and the sediment filling were exposed. As a result, the thicknesses of the Quaternary alluvium units were also estimated from geological observations and used as a first constraint for the model. (b) According to Adamu and Likkason (2022), the average density values in a few chosen research locations and their environs were determined by direct density measurement in some selected areas of study and its environs were used, and the average density values for various units in the 2D model were found to range from ∼1.87 to ∼2.99 kg/m3 (limestone), ∼1.46 to ∼3.62 kg/m3 (clay), ∼1.76 to ∼5.30 kg/m3 (shales), ∼1.97 to ∼3.50 kg/m3 (ironstone-fine grained) and ∼1.67 to ∼2.01 kg/m3 (ironstone-coarse grained). (c) The outcome of the spectrum analysis of the gravity data, which were used to limit the depth of the Causal structures and edge boundaries in the 2D model. The basement depth determined by the spectrum analysis of the gravity data was discovered to be in good agreement with that determined by Nwonko and Shehu, (2015); Bonde et al., (2014), and Reed et al., (2014).
4.6 Quantitative Interpretation of the Residual anomalies
When interpreting gravity data quantitatively, the goal is to determine a subsurface structure whose computed gravity effect approximations of the observable gravity field recorded on the surface are satisfactory. Any structure's volume-density contrast determines how much of a gravity anomaly it produces. Furthermore, the magnitude of the anomaly grows as the underlying structure that is producing it deepens. The observed gravity is predicted to be less intense and smaller in size if the structure has an uneven or diffuse shape. However, a structure's own gravity anomaly cannot be distinguished from those of other structures if it is not sufficiently well separated from other structures, whether or not they are identical to it (Hay, 1976).
Since quantitative interpretation always relies on geology interpretations, it rarely is particularly original or accurate. As a result, for a meaningful interpretation, adequate and sufficient knowledge of the geology of the studied area is required. The study location is located within the Northwestern Sedimentary Basin of this nation, and the specific sediments discovered there are of the Eocene age and are part of the Gwandu Formation. They have an average density of around 2.35 x 103 kg/m3 when compared to the lithologic sequence that extends down to a depth of about 80 m. The formations underneath it, which are of Paleocene and Maastrichtian age and have average densities of around 2.40 x 103 kgm3 and 2.55 x 103 kgm3, have a small unconformity. These are the Kalambaina, Dange, Gamba, Wurno, Dukamaje, and Taloka formations. According to Kogbe (1979), these deposits go down approximately 270 meters. Continental deposits (fluvial) with a lower cretaceous or pre-Maastrichtian age are found below this (Chukwuike, I975) and Grant, (1978) age range. The Illo and Gundumi formations, which are located at this depth (about 700m), have an average density of 2.64 x 103 kg/m3 (Burk and Dewey, 1972). The author calculated the mean density of all the sediments overlaying the basement rocks to be 2.35 x 103 kg/m3, taking into account the ages of the deposits above. When modeling the profiles, the sediment density was determined using this density value. In comparison to the used average basement density (2.67 x 103 kg/m3), the calculated density value exhibits a negative density difference of -0.20 x 103 kg/m3. As a result, the thickening of the sediments can be used to explain practically all of the gravity lows in the studied area. Any postulated initial model's gravitational influence is estimated as part of the interpretational methods and compared to the observed effect. The presumed model is modified as required to achieve a better fit. Contrasts in volume, shape, and density are frequently present in common alterations. Within geologically acceptable limits, this process is repeated until a new structure is found whose calculated effect matches the observed effect the best. Forward modeling is the term used to describe this strategy (Patterson and Reeves, 1985). Based on the requirement that each profile intersect at least one of the significant anomalies previously indicated for interpretation, three profiles A-A', B-B', and C-C' were selected (Figs. 4.18a, 4.18b & 4.18c). Additionally, it was made sure that each profile had data points nearby or surrounding it and that it was picked to be at a right angle with the anomalies' strikes.
Although the approach of interpretation utilized in this work is susceptible to all conceivable ambiguities in field data, it is nevertheless advantageous because there are some geological and geophysical limitations to rely on that will keep most ambiguities within acceptable bounds. To simulate the profiles, Mike et al., (1991) created a dimensional gravity/magnetic software called Geosoft Montaj (GMSYS).
In the northeastern region of Argungu sheet 48 and Birnin Kebbi sheet 49 of the airborne geophysical data, a prominent magnetic low negative value is indicated by the profile created from the Bouguer anomaly and exactly matches the location of the prominent Duku-Tarasa and Gulma magnetic anomaly. This confirms the precision of the aircraft geophysical data regarding the position and size of this anomaly. The profile and profile B-B' of the gravity data are practically parallel. According to this hypothesis, the well-known Duku-Tarasa and Gulma gravity and magnetic anomalies share a same origin.
4.6.1 2D gravity model across Profile A-A' of Sokoto basin
The anomalies of the Gwandu formations are crossed by this profile, which extends north to south (Fig. 11a). In terms of distance from the southern end of the profile, it is distinguished by two gravity highs at around 2.0 km and 6.0 km and a gravity low at approximately 4.5 km. Although the density contrast in the sedimentary strata was − 0.20, the low gravity and the intermediate gravity were accounted for by the introduction of two escarpment structures called Gulma and Duku-Tarasa (Fig. 11a). With a density contrast of 0.30, the causal body is located at the SW end of the profile. On this profile, the body stretches for nearly 3.0 kilometers. It has a depth extent of approximately 6.0 km and a small inward dip with an angle of about 35o at each of its flanks.
The ‘Gore’ in Fokku Gundumi formation's intrusive bodies, which are nearby, is what causes the gravity to be high along the profile. Due to their similarity in density and method of emplacement, as well as the fact that they both exist in the same location, it may be inferred that they represent the same intrusion as the Gore Illo formation (Fig. 11a & Fig. 2). Both profiles revealed that the maximum depth extend was 9 km below ground level. The depth extent and density contrast of the intrusion, which are 3 km and 6.0 km, respectively, as shown in Fig. 11a, are the same as those of the intrusion 16. The whole ranges of depths to the top of this basement along this profile are about 9.0 km and 2.5 km respectively.
4.6.2 2D gravity model along profile B-B′ crossing the Sokoto basin
Figure 11b depicts this profile running NE-SW through the Illo formation and Rima group. The basement intrusions along this profile were classified as Gwamba scarp in Illo formation from the southern half of the profile, which is defined by a gravity low at 3 km distance from the southwestern end of the profile and two gravity highs at 4.0 km and 10.0 km, respectively (Figs. 2, 6). There is a 2.0 x 10− 3 kg density differential in the ore masses that are thought to have caused the incursion. It explains the gravity low in the eastern region (Taloka formation) of the Rima group and can be seen to have dipped toward the southwest region of the profile at an angle of roughly 20o to the vertical; its maximum depth is about 5.3 km. An effort has been made to model the subsurface both across (Fig. 6) and along (Fig. 2) the rift axis in order to describe the characteristics of the upper lithospheric beneath the Sokoto basin and its surroundings. Where the Sokoto basin lies, the two modeled parts cross. The interfaces of the respective density layers in the two models have similar density values assigned to them, but their depths vary. This finding is generally consistent with Kogbe's (1981) report, according to which the northern and central segments of the Sokoto basin are separated by the northwestern structural high just south of the Gwandu formation.
4.6.3 2D gravity model of the shallow structures along profile C-C′
Figure 11c displays the architecture of the shallow crust constructed using the residual gravity anomaly map (Fig. 6) compiled for the study area constrained by the existing geologic data. The residual gravity values extracted along profile C - C′ are used as an input to construct the 2D model of profile C (Fig. 11c). The profile is selected to run beginning from northwest of the Rima group mount towards the southwest of Gundumi formation and crosses the Wurno-Rabah trench and the Sokoto-Bodinga - Tambuwal trench which is claimed to be the most promising hydrocarbon prospective site in the study area. The aim of constructing this model is to characterize structure of the shallow features beneath the magmatic segment interms of geologic structures suitable for geodynamic studies and oil and gas resource exploration. Interpretation of the fault patterns from gravity signal response (Fig. 11c) has been assisted and constrained by geological studies (Obaje 2009; Obaje et al., 2013).
Visualizing the model gives a clear picture of the lateral and vertical discontinuity of geological formations and structures including buried weak zones and faults in addition to mapping the intrusive bodies. The top density layer is the quaternary rift sediment (ρ = 2.38 kg/m3) which covers the surface of most of the rift floor as can be observed from the geological map (Fig. 2). The second layer, consists of (Eocene sediment of Gwandu formation) and the Sokoto group (ρ = 2.6 kgm− 3) (Paleocene) (Kogbe, 1981). Although there is no sharp knowledge on their thickness within the northern sector of Iullemmeden basin, the presence of Maastrichtian sediments is reported by different authors (e.g. Kogbe, 1981; Obaje et al., 2013). Gamba shale and clay stone (ρ = 2.49 kgm− 3) overly the Dange formation (ρ = 2.42 kgm− 3) is underlie by the Kalambaina formation (ρ = 2.17 kgm− 3). The Kalambaina formation uncomformably overly the Wurno-Rima group (ρ = 2.24 kgm− 3) (Fig. 11c).
4.6.4 Profile intersection points
The points where the profiles intersect demonstrate that the relevant models produced outcomes that were similar. At these crossing points, modeling was restricted to ensure identical outcomes. Figure 6 depicts three intersecting places where profiles A-A′, B-B′, and C-C′ cross one another. At 40000 m east and 132000 m north, profile A crosses profile C. Intrusive bodies with a density of 3.2 kg/m3 and zero magnetic susceptibility are now depicted in their respective models to be located 5.2 km beneath the surface. Figure 6 also depicts the intersection of profiles A-A′ and B-B′ at a distance of 136,000 meters north and 40000 meters west. The Maastrichtian silt of the Rima group, with a density of 2.0 kg/m3 and zero magnetic susceptibility, is depicted by their respective models to be located at a depth of 5.2 km. At an easting of 40000 meters and a northing of 116000 meters, profile B-B′ crosses profile C-C′. They currently exhibit intrusive bodies with a density of 2.64 kgm3 and a magnetic susceptibility of 0.302 SI at a depth of 8.0 km.
4.6.5. The Sokoto basin's shallow structural features
Due to the predominance of mechanical deformation type, where fractures and faults are common, the shallow subsurface of an extensional environment is distinguished by its fragile nature. A strong tool to find buried structures in such a geologic context is the gravity method combined with magnetic survey. In this regard, residual anomaly maps (Fig. 6) were used to generate two-dimensional models (Figs. 11b, 11c) utilizing the observed values recovered along the profiles A-A′, B-B′, and C-C′. The 2D gravity models are built to comprehend a 3D image (Fig. 11d) of the shallow structural characteristics beneath the Sokoto basin. In this study, an effort is made to estimate the shallow subsurface density layer within the bounds of the available geologic information. This is intended to have an impact on identifying the lithostratigraphic sequence as geologic features (prospective areas) such as troughs, trenches, ditches, sinks, and holes, ferruginous materials, and shallow magnetic intrusive bodies. By comparing the findings of this study with other prior work in establishing stratigraphic columns of the southeast sector of the Iullemmeden basin, emphasis is placed on filling and strengthening this information gap (Table 3; Kogbe, 1979, 1981; Obaje et al., 2013; Ali et al., 2016). Table 3 summarizes the initial density model for the various lithologic units in addition to the depth information for the lithologic units.
Table 3
density and susceptibility of common rocks and minerals (Telford et al., 1990)
|
Rock/mineral type
|
Density range (Kgm− 3)
|
Average density (Kgm− 3)
|
Susceptibility range (SI)
|
Average Susc.
|
|
Overburden Soil
|
1.24–2.4
|
1.92
|
-
|
-
|
|
Sandstone
|
1.61–2.72
|
2.35
|
0–20
|
0.4
|
|
Sand
|
1.7–2.3
|
2.0
|
-
|
-
|
|
Gravel
|
1.7–2.4
|
2.0
|
-
|
-
|
|
Clay
|
1.63–2.6
|
2.21
|
-
|
-
|
|
Shale
|
1.77–3.20
|
2.40
|
0.01–15
|
0.6
|
|
Limestone
|
1.93–2.90
|
2.55
|
0–3
|
0.3
|
|
Basalt
|
2.70–3.30
|
2.99
|
0.2–175
|
70
|
|
Acidic igneous rocks
|
2.30–3.17
|
2.61
|
0–80
|
8
|
|
Metamorphic
|
2.4–3.1
|
2.74
|
0–70
|
4.2
|
|
Basic igneous rocks
|
2.09–3.17
|
2.79
|
0.5–97
|
25
|
|
Mafic intrusions
|
2.50–2.81
|
2.64
|
0–50
|
2.5
|
|
Schist’s
|
2.39–2.90
|
2.64
|
0.3–3
|
1.4
|
The following density layers were created in the final model of the shallow structural features beneath the Sokoto basin and its surroundings after the initial model had information on the density and susceptibility of common rocks and minerals (Table 3), as well as other pertinent information from previous studies:
-
Sediments from the Quaternary Period, having a density of around 2.38 kg/m3
-
Clay and gravel that range in thickness from 8 meters at the rift's flank to 12 meters at the bottom, with an average density of 2.21 kg/m3.
-
Variegated shale with an average density of 2.40 kg/m3.
-
Kalambaina limestone has an average density of 42 kg/m3 and an approximate thickness of up to 200 m.
-
Sandstone has an average density of 2.35 kg/m3 and an average thickness of 200 m.
-
Mafic intrusive bodies of density 2.64 kg/m3
4.6.6 3D model of Sokoto basin shallow structural features
It is necessary to have access to the 2D gravity models of the upper lithospheric structures (Fig. 11d), which cross at the southern edge of the Yerimawa-Sabon-Birni-Isah trough (the Sokoto group) and Koko-Giro sinks (the Eocene deposits) (Obaje et al., 2013). The upper lithospheric characteristics beneath the Sokoto basin can be clearly visualized in three dimensions using this geophysical model (Fig. 11d). It is anticipated that more knowledge will eventually be gained about the surface, subsurface, and associated geological features of the Sokoto basin and surrounding areas. Due to the extreme deformation in this area, which is a component of the rift floor, more Tureta-Bakura ditch faults are present than would be expected based on Fig. 11d.
It is discernible that the resolution of subsurface imaging decreases as the studied area grows. However, the geophysical model presented here is sufficient to represent the nature and structure of the subsurface at a semi-regional scale, with an emphasis on the shallow and deep plate origins of top lithospheric structures in the Sokoto basin and its environs. The complexity of geologic structure and its relationship to subsurface geology are clear. The most notable feature of Fig. 11d is that it offers a thorough overview of the potential exploration locations with their lithologic classification and an up-dip section around Tureta-Bakura, Sabon-Birni, and Isah, all of which are located in the Gundumi, Illo, and Taloka Formations, which illustrates the corresponding response of the shallow structures fetched on by diking effect.
The interpreted gravity map (prospective areas) shows a number of ridges, trenches, ditches, sinks, and holes. By adding ferruginous rocks from the Gwandu Eocene and Sokoto groups, the shallow plate sources' fractures and fault planes are opened up. The migration of the intrasedimentary unit to the deep subsurface plate sources is responsible for the formation of the shallow structural characteristics of the Sokoto basin. The discontinuity or lack of linkages between zone supplies due to underlying anomalies is what leads to the different structural designs of the Sokoto inland basin.
4.6.7 Subsurface structural features of the Sokoto basin
From a geodynamic and structural position, it is extremely valuable to understand the subsurface framework of such a geologically changing terrain. With an emphasis on the upper lithospheric structures, the gravity maps, 2D gravity models with profiles, and 3D geophysical models of this study all present structural elements of the studied area in various forms. Gravimetric (Umego et al., 2007) and geologic (Kogbe, 1979; Kogbe, 1981) studies both show the interior layout of the uppermost levels of Sokoto inland basin structures in the residual (Fig. 6), upward continued (Figs. 5), and directional filtered (Figs. 9a,b) gravity maps. Gravity anomaly maps of the Sokoto inland basin, including the Alluvium, Paleocene Sokoto groups, and Maastrichtian sediments of the Rima groups, show subsurface reflections of Quaternary sediments of the Gwandu formation in the area (Figs. 2, 4a,b,c, and 6). Nwanko (2000) thought about using a gravity survey to identify the structure of the Dange aeromagnetic anomaly's causal body in the Sokoto basin. In contrast to the findings of these earlier studies, the current effort is the first to map the full extent of the southern Sokoto inland basin formations, which can be distinguished from one another by their gravity signatures (Fig. 7). The residual gravity anomaly map (Figs. 6), the upward continued gravity map (Fig. 5a,b,c,d), the edge detector filters, and the direct filtered gravity maps (Figs. 7 and 8) can all be used to determine the lateral extent of the Sokoto inland basin structures. The lateral extent of the lithostratigraphic sequence is estimated to be 5 km wide and 12 km long based on the gravity signature of this map. This figure is almost identical to earlier research's estimates of the Sokoto basin's Curie point depth (Nwanko et al., 2015). In the current analysis, direct filtered gravity maps (Figs. 9a,b) and quantitative interpretation by gravity modeling (Figs. 11a,b, & c) were confirmed by upward continuation (Fig. 5a,b,c & d) and demonstrated that the lithostratigraphic units occur at approximately 4–8 km of depth. This conclusion is consistent with earlier studies that map the intrusive bodies at depths between 12 and 25 km (Obaje et al., 2013; Bonde et al., 2014), as well as with Curie point depth estimation investigations (Nwanko et al., 2015). But according to Adamu et al., (2023) aeromagnetic investigation, the basement configuration beneath the Sokoto basin started at a depth of just 3 km and persisted all the way to 5 km. If this hypothesis is true, the regional map of the study's upward trend (Figs. 5a,b,c & d) and the overall gradient and tilt of angle filtered gravity map (Fig. 8) should both show the basement intrusion's signal response. Gravity data study inside the Sokoto basin supports the latter hypothesis, where an ongoing dyke intrusion to the shallow subsurface can occur via quaternary formation, aiding the ongoing process towards Eocene sediments of Gwandu formation.
4.6.8 The Upper lithospheric structures
The underlying structural characteristics along and across the Sokoto inland basin structures and its surroundings are shown in the 2D gravity models (Figs. 11a,b & c). In accordance with earlier studies (Bonde et al., 2014; 2015; Obaje et al., 2013; Nwanko and Shehu, 2015; Egwuonyi, 2000), the sediment thickness in the north-central part of the Iullemmeden basin decreases to the rift axis (up to 3.9 km) from its adjacent flanks (up to 8.4 km). The N to S ward decrease near the interface of shallow features in Fig. 6.6 suggested that the deformation strength increased northward. The regional gravity anomaly pattern (Fig. 5a,b,c & d), which shows an increase in gravity anomaly maxima to the rift axis and towards the southeastern part of the Iullemmeden basin, and the CBA map (Fig. 4c) both reflect this outcome.
By limiting the gravity modeling (Fig. 11a & b) to just the Sokoto basin and its surroundings, a high resolution and more illuminating depiction of the shallow crust structure in the region is produced. A complete 3D representation of the upper crustal structure of the Sokoto basin is obtained by fusing the geophysical data from this study with the body of knowledge in surface geology (Fig. 11d). It is clear that the vast Cenozoic-Recent Gulf of Guinea seawater regression and the Palaeogene Petroleum System of the West and Central African Rift System (WCARS) are associated with Palaeogene Rift Phase III and the shift from mechanical to magma-enhanced rifting (Genik, 1993). The complete 3D model view (Fig. 11d) and the 2D gravity models (Figs. 11a,b & c), together with the subsurface data, support the idea that the WCARS is not only in the stage of magmatic segmentation but is also in the stage of lithostratigraphic segmentation (Genik, 1993). The SSW-NNE-oriented quaternary faults and the NE - SW -oriented boundary faults are confirmed by the AS and TDR gravity maps (Figs. 7, & 8).
4.6.9 Hydrocarbon and other mineralized potential implications
Although it is a crucial component in interpreting the formation of the West and Central African Rift System (WCARS), the Sokoto inland basin is currently being evaluated within the framework of a tectonic and geodynamic setting (Obaje et al., 2009). It can be challenging to understand the geodynamic setting of a place because the majority of geologic activities are not only obscured from human direct observation but also extremely slow-moving processes that do not finish during the lifespan of people. A study of the density distribution from gravity measurements, however, supports information from geophysical, geological, and geodetic evidence.