Implications on Gravity Anomaly Measurements Associated with Different Lithologies in Turkana South Subcounty

Daniel Mogaka Nyaberi

Department of Environmental Earth Sciences, University of Eldoret, Uasin Gishu, Kenya.

DOI: 10.4236/gep.2023.111006                

Abstract

The use of gravity data has demonstrated capability for monitoring lithological changes on a large scale as a consequence of differentiating basement and sedimentary of buried valleys. Gravity anomalies are associated with lateral contrasts in density and therefore deformation by faulting or folding will be manifested if accompanied by lateral density changes, otherwise, the vice versa is true. The study’s objective is to evaluate the effectiveness of gravity method in establishing different lithologies in an area. The study has revealed that regional anomaly gravity map presents high anomalies in the Northern region in the NW-SE trend and low anomalies in the southern trend in NW-SE, while the residual anomaly gravity map shows different trends for the low and high gravity anomalies. The gravity anomalies are well interpreted in line with the lithologies of the study area rather than the deformation of the same lithologies. There are observed high values of gravity anomaly values (ranging from -880.2 to -501.2 g.u.) where there are eolian unconsolidated rocks overlying the basement compared to low gravity anomaly values (ranging from -1338.9 to -1088.7 g.u.) where the andesites, trachytes and phonolites overly the basement. The different regional gravity anomalies relate well with different rock densities in the study area along the line profile for radially averaged power spectrum. The gravity highs are noted in the eastern point and are associated with andesites, trachytes, basalts and igneous rocks, while the gravity lows are associated with sandstone, greywacke, arkose, and eolian unconsolidated rock. The utilization of the information from the Power spectrum analysis demonstrates that the depth to the deepest basement rock is 12.8 km which is in the eastern flank, while the shallowest to the basement of 1.1 km to the western flank.

Keywords

Regional Gravity Anomalies, Power Spectrum Analysis, Density Contrasts, Lithologies

1. Introduction

There subsist variations on the Earth materials’ densities and the gravity technique measurements by yields deviations in the measured gravity field help pick where the differences in densities of subsurface rocks exist (Hakim, 2018). Gravity technique helps manifest the subsurface geologic structures (Balsubramanian, 2007) and has the capability of detecting the structural trends including mapping fractures and intrusions, determining the binding interfaces of bedrock, therefore becoming dominant method in geophysical surveys. Gravity application is greatly applied in investigating geological structures, ascertaining existence of geothermal reservoirs, detecting volcanic activity and hydrothermal movement beneath volcanoes, examination of CO₂ movement, locating active faults associated with earthquakes and detection of confined cavities (Hakim, 2018). The gravity method helps determine the configuration for the bedrock surface over an area of recent sediment cover (Idris et al., 2015), show the relationship between deeply buried basin and (Handayani et al., 2018) and find mineral resources and groundwater in sedimentary terrain (Balsubramanian, 2007; Chandler, 1994). Residual gravity anomalies’ application in detection of the trends of the faults is tested in study areas by distinguishing between high and low gravity anomalies and the downthrown of faults towards low anomalies (Sultan et al., 2015).

The research deals with the southern part of Turkana County considering data from the gravity survey carried out from 1955 to 1975 (Figure 1). The objective of the study is to use the gravity data to advance geological interpretation in the region, especially looking at the subsurface structure of the basement and the relation of gravity in relation to known geology of the area.

2. Geology

The geology of Turkana South Subcounty (Figure 2) encompasses the basement (The Neoproterozoic belt), the tertiary volcanics and the quaternary sediments with similarities across border in eastern Uganda, and counties of West Pokot, Baringo, Samburu and area and Marsabit. The present-day basin has its origins in Pliocene tectonic developments. The Subcounty is traversed by the extensive of the modern rift, with subsidence making room for more than one kilometer of Plio-Pleistocene (Feibel, 2011). It is within this structure and deep basins exists manifestations of prospects of oil and gas deposits within sedimentary.

The metamorphic basement in the Turkana South Subcounty mainly of crystalline nature formed as part of the Mozambique Belt during a Neoproterozoic to Cambrian age mountain-building episode (Schlϋter, 1997). The lithologies in the basement encompass schists, gneisses and marbles that originate from metamorphism of sandstones, sediments-grits, limestones and shales due from heat, pressure or through impregnation by permeating subsurface fluids. Igneous rock formations are comparatively rare and are made up of granitic masses and dykes, and sills of epidioritic and amphibolitic nature resultant from originally dolerite material or associated rocks, and more or less ultrabasic lithologic formations. Some of the formations are seemingly younger as compared to the metamorphism that affected the sedimentary host rocks, though all are undoubtedly of Precambrian in age.

Figure 1. Gravity data locations of in study area.

Figure 2. The geological map of Turkana South Subcounty.

The older sediments accumulated within components of the Central African Rift System or in isolated depositional basins (Feibel, 2011). These sedimentary strata offer a limited opening unto understanding earlier stages in the geological history of the region as revealed from some outcroping, faulting, or uncertain relationships to broader depositional systems, which preserve vital paleontological records and are important openings towards deciphering evolutionary patterns across Africa (Feibel, 2011).

The Late Eocene to Miocene volcanics dominated by extensive basalts volcanic rocks cover a large proportion of the study area, and attain considerable thickness. These are the oldest volcanic rocks which are generally fine-grained basalts which overlie the Turkana Grits. This volcanic series of basalts tend to weather fairly rapidly by mechanical breakdown on joints and fractures, which reduce much of the mass of the exposed rock to small boulders and cobble which in their turn suffer surface weathering and alteration. For this reason, the topography of outcrops of the lower basalts is generally a series of low rolling hills. The volcanics shape much of the landscape, and are significant contributors to sediment flux and solutes. Volcanic raw materials are also the dominant sources used in manufacture of most of the Turkana basin’s archaeological record (Roche et al., 2004). Volcanism is of a “cyclic” nature, and six cycles of plateau basalt eruption followed by salic, usually trachyte central volcano activity are recognised (Truckle, 1977). The dominant mechanism of deformation is noted particularly from the morphology of multi-centred plateau basalt formations and dykes and their relationship to progressive easterly tilting (Truckle, 1977).

There is an emerging understanding of the link between extensional pulses and magmatic episodes especially modern magmatism located within the Turkana Depression and its relationship with the distribution of extensional strain and find that the magmas are derived from sub-lithospheric sources equivalent to magmatism in the more mature sectors of the rift (Rooney et al., 2022).

These Quaternary are sediments of the Lake Turkana basin and according to Walsh and Dodson (1969) the formation comprises a sequence of lacustrine deposits of Pleistocene age and whose accumulation and formation has continued through to the recent period. Evidently, the geology of Turkana South Subcounty is to a larger way affected by geological structures which are cross cutting the metamorphic and igneous formations in the region (Figure 3).

3. Materials and Methods of Research

3.1. Gravity Method Theory and Application

Gravity method is referred to as a potential field method because the measurements involve a function of the potential of the observed field of force, i.e. terrestrial gravity, at the observation site (Hinze et al., 2013). The gravity method operates on basis that depends on two laws derived by Newton, namely, the Universal Law of gravitation, and the Second Law of Motion. Using Kepler’s empirical third Law (Equation (1)) which relates the period (T) and the semi-major axis (a) of the orbit of the satellite to the mass (M) of the parent body, Newton deduced that the force of attraction between a planet and the Sun varied with the “quantities of solid matter that they contain” (i.e., their masses) and with the inverse square of the distance between them (Lowrie, 2007) whose application to two particles or point masses m and M separated by a distance r gives relationship for the gravitational attraction F exerted by M on m (Equation (2)).

$GM=\frac{4{\pi }^2}{T^2}{a}^3$ (1)

$F=-G\frac{mM}{r^2}\underset{\dot{}}{\bar{r}}$ (2)

Figure 3. Structural geology map of Turkana South Subcounty.

In this equation $\underset{\dot{}}{\bar{r}}$ is a unit vector in the direction of increase in coordinate r, which is directed away from the center of reference at the mass M. The negative sign in the equation indicates that the force F acts in the opposite direction, toward the attracting mass M. The constant G, which converts the physical law to an equation, is the constant of universal gravitation. The second law of motion states that the rate of change of momentum of a mass is proportional to the force acting upon it and takes place in the direction of the force (Lowrie, 2007) which defines force (F) in terms of the acceleration (a) given to a mass (m) as Equation (3).

$F=ma$ (3)

According to Hinze et al. (2013), gravity applications include micro-scale surveys in mapping physical property variations of the upper meter or two of the subsurface, or larger-scale applications for regional to global surveys designed to image the deeper variations of the Earth’s crust, mantle, and core. The crust exhibits highly complex structural and compositional properties attributed to erosion, sedimentation, metamorphism, tectonics, and igneous activity, and the plastic movement of the mobile asthenosphere underlying the lithosphere occurred over time span of 4600-Myr. These processes have led to the differentiation of chemical elements, deposition of a variety of sediments, vertical and horizontal movements, zones of crustal weakness, and the focusing of geological processes, such as volcanism, which variations in the lithosphere control the formation and distribution of the Earth’s resources, and volcanic, earthquake, and other natural hazards. The gravity survey investigates variation (gravity anomalies) in Earth’s gravitational field generated by differences in density whose variations are induced by the presence of a causative body such as salt domes, granite plutons, sedimentary basins, heavy minerals like chromite and manganese, and faults and folds within the surrounding subsurface rocks (Haldar, 2018). The variations measured by gravity are dependent on Newton’s universal law of gravitation, which takes into account the differential mass and the distance between the source and observation point (Hinze et al., 2013). The size of the anomalies primarily depends on the difference in density between host rocks and causative body, their geometrical form, and depth of occurrence (Haldar, 2018). The gravity anomalies are the differences between the observed and the theoretical field based on planetary considerations and the assumption of radial symmetry of the Earth layers (Hinze et al., 2013).

3.2. Gravity Data Collection

The gravity data used is secondary data collected in Kenya from 1955 to 1975 using La Coste & Romberg gravimeter G-16 with the reference point being Nairobi Pendulum Station (IGSN71 STATION “35716 A NAIROBI” VALUE. 9775260.7 g.u.). The calibration of the gravimeter was checked by measuring the gravity difference between Nairobi and Mombasa airport (STATION “35749 JII”, VALUE 9780346.1 g.u.) and found correct to 1 in 5000.

The station elevations for the pre-1971 data were all measured using the “modified leapfrog” method (Searle, 1969) with two “Baromec” aneroid barometers supplemented later by two “Paulin” altimeters. The normal “leapfrog” is in principle the better method as index correction errors tend to cancel). Networks were constructed from interconnecting traverses and control points “tied into” whenever possible. Full terrain corrections were computed automatically for all stations for which adequate topographic maps, taking the earth’s curvature into account and using a digital model of the East African topography based on the five-minute square (approx. 9 km.). Local terrain corrections (within 2 km. of the station) were included in the automatic computation by an interpolation method but where these were larger than 10 g.u. were always checked by zone chart. The accuracy of some of these methods has been studied by Stacey and Stephens (1970) and an overall standard deviation of 10% of the total correction was expected.

3.3. Gravity Data Presentation

The gravity data used originates from the gravity measurements in Kenya done from 1955 to 1975. The data tables presented in this research contain three columns: 1) and 2) are the geographic co-ordinates in decimal degrees, and 3) is the observed absolute gravity.

The observed gravity data is presented as format of the Table 1 and the entire data in Appendix Table A1 and gravity units (g.u.) have been used throughout. (1 g.u. = 0.1 mgal = 10⁻⁶ m∙s⁻²). There is about 632 gravity readings measured and plotted in the area map (Figure 1) which have been reduced pursuant to the International Gravity Standardization Net 1971 and the National Gravity Reference Net of 1973.

3.4. Gravity Data Processing

The datum used in reducing the gravity observations is defined by the International Gravity Standardization Net adopted in 1971 (IGSN 71) (Morelli et al., 1971) together with the 1967 gravity formula (I.A.G. 1967) as recommended by the I.U.G.G. (Morelli, 1976). This required the subtraction of 137.5 g.u. from values that are referred to the Original Gravity Station (O.G.S.) primary net of Masson and Andrew (1962)—taking Nairobi “A” as the primary base. The tidal corrections to all the post 1971 data was done with the Base station readings made at intervals of up to 4 weeks, during the surveys, and indicated low long-term drift rates < 3 g.u. per month. The pre-1971 data were less accurate, perhaps ±4 g.u. It would have been possible to use the repeat readings during surveying to adjust the gravity values but as the average error in the elevations (±3 m) gives rise to an error of ±6 g.u. in the Bouguer anomalies this was not justified.

Table 1. Observed gravity data in the collected format.

The considered theoretical sea-level gravity, ϒ, at latitude Ø, was calculated from the 1967 gravity formula after the Geodetic Reference System 1967 (International Association of Geodesy, 1971) with the approximation (Equation (4)) whose accuracy is up to 0.04 g.u., being used.

ϒ = 9780318.5(1 + 0.005278895sin²Ø + 0.000023462sin⁴Ø) g.u. (4)

The correction −171.0 ± 0.5 g.u. can be applied to values of ϒ calculated with Cassinis (1930) formula within 5˚ of the equator.

The free air anomalies (F.A.A.) were calculated by use of the continuing upward of the sea level theoretical gravity ϒ to the measured station height by applying the free air correction factor which is then subtracted from the observed gravity g_o thus (Equation (5)):

$\text{F}.\text{A}.\text{A}.=g_o-\left(\Upsilon -3.086h\right)\text{g}.\text{u}.\left(h\text{in}\text{m}\right)$ (5)

The conventional constant factor was used throughout with the variation of the factor with latitude and elevation given as shown (Equation (6)) by Vyskocil (1960):

F.A, Correction (g.u.) = −(3.08772 + 0.00439sin²Ø − 0.72 × 10⁻⁶h)h (6)

which implies an error at the equator, through using the constant quoted, of −1.0 g.u. at 1000 m. and +4.6 g.u. at 4000 m. These errors are unchanged upto 5˚ from the equator.

The “simple Bouguer anomalies” (S.B.A.) were determined by subtracting a calculated value from the F.A.A. (Equation (7)), which represents the effect of an slab considered to be infinite and horizontal of thickness equal to the station height h and density p (2.67 × 10³ kg∙m⁻³).

$\text{S}.\text{B}.\text{A}.=\text{F}.\text{A}.\text{A}.-\text{2}\Pi Gph=\text{F}.\text{A}.\text{A}.-1.119h\text{g}.\text{u}.$ (7)

The “complete Bouguer anomaly” (C.B.A.) should in principle be calculated by subtracting from the F.A.A. the effect at the observation point, of all masses outside the geoid. However, the attraction of topography beyond Hayford’s Zone 0 is inconvenient to calculate (Bullard, 1936) and the “indirect effect” of the topography, causing the geoid to depart from the spheroid, becomes relatively large for such distant masses. Except where extreme elevation changes occur in relatively short distances, as occurs in New Guinea for example, (St. John & Green, 1967) the effect of the more distant topography will show slow spatial variation. For these reasons the convention of Heiskanen and Veninq (1958) was followed and an attempt was made to correct for those masses outside the geoid and within the outer limit of Hayford Zone 0 (166.7 km).

A terrain correction T was computed for stations, as the difference in gravity effect between the actual topography within 166.7 km. of the station and a disc of thickness equal to the station elevation, radius 166.7 km. and curved to the radius of the earth. A correction B was made to all stations for the gravity difference between the curved disc, just defined, and the infinite slab of the simple Bouguer correction (Bullard, 1936).

The valueB (Equation (8)) was computed for elevations using different approximate methods including that of Takin and Talwani (1966) and finally the resultant in Equation (9) adopted.

$B=0.0134h-3.5\times {10}^{-6}{h}^{\text{2}}\text{g}.\text{u}.\left(h\text{in}\text{m}\right)$ (8)

Thus

$\text{C}.\text{B}.\text{A}.=\text{S}.\text{B}.\text{A}.+T-B$ (9)

It should be noted that both T and B depend on the density. With this method of reduction, a constant density must be used and 2.67 × 10³ kg∙m³ was used throughout. The Processed data presented in this research (Table 2) contain four columns: 1) and 2) are the geographic co-ordinates in decimal degrees, 3) is the observed absolute gravity, and 4) complete Bouguer anomalies. The data is presented in format as in the Table 2 and the entire data in Appendix Table A1.

3.5. Gravity Data Analysis and Interpretation

Secondary gravity data was used where standard gravity corrections, including removal of the effect of instrument drift, tide, elevation and latitude were applied to the data. These corrections were carried out within the gravity processing package of Oasis Montaj. The corrected gravity values were contoured to produce anomaly maps. Gravity data analysis and interpretation was used to measure the differences in density on the earth’s surface that indicate the underlying geologic structures.

The corrected gravity values were contoured to produce anomaly maps. Gravity data analysis and interpretation was used to measure the differences in density on the earth’s surface that indicate the underlying geologic structures. The observed gravity data sets were reduced to complete bouguer anomaly data and they were gridded and contoured by using Geosoft Program Oasis Montaj to produce several maps. The corrected data (complete bouguer anomaly data) were plotted into bouguer anomaly maps which were filtered into regional (Figure 6) and residual (Figure 7) gravity anomaly maps. These maps show

Table 2. Processed gravity data as complete Bouguer anomaly.

different high and low anomalous values through the survey area. The minimum curvature method was applied to determine the residual and the regional gravity anomalies. Algorithm was used where minimum-curvature gridding technique uses a two-dimensional (2-D) differential equation for the displacement of a thin sheet under the influence of point forces. The algorithm used after Webring (1981) was a keen in the minimization of aliasing in large gaps between adjacent data points. The minimum-curvature gridding algorithm was applied to the complete bouguer anomaly data to define the regional gravity field. The contours of the minimum-curvature regional gravity map were matched in the contouring as that of the complete bouguer anomaly and were as influenced by the features of interest.

The deployment of complex attributes (that is analytic signal and tilt derivative) of filtered Bouguer Gravity signal helped to extract properties of the source of the anomalous fields in the study area and thus the gridded analytic signal gravity maps (Figure 8). In processing of the complex attributes, a filter of passing the wavelengths of not more than 1 km was used on the complete bouguer anomaly gravity data resulting unto several maps (Figures 6-8) with the regional bouguer anomaly map (Figure 4) considered for determination of depths. The Figure 5 shows a power spectrum curve of a line running in the E-W direction of the study area. The power spectrum curve is further divided into components as are related to gravity anomalies starting the deepest, to the shallow and later handling the noise sources.

The depth calculation by using Power Spectrum Curve Numbers (Table 3) were achieved by using the Equation (10) after Nadiah (2016), where the subsurface depth was calculated by the basis of difference of power spectrum. In the equation the depth value of positive ᶁ indicates the mean depth along the gravity data profile.

|ᶁ| $=\frac{-1}{4\pi }\left(\frac{\log E_1-\log E_2}{k_1-k_2}\right)$ (10)

Whereby the E₁ and E₂ stand for the power spectrum, k₁ and k₂ standing for the wave numbers. Equation (10) provides the depths, ᶁ that are derived by getting the difference of the power spectrum curve slopes divided by −4π. Using the Equation (10) and reading the values of the Power Spectrum Curve Numbers from Figure 5, Figure 9 and Figure 10, the values in Table 3 were determined.

4. Results

Several outcomes of data analysis is presented under the gravity consideration in this study. The data presented include the regional bouguer anomaly maps (Figure 6), the residual bouguer gravity anomaly maps (Figure 7), and the analytic signal maps (Figure 8).

Equally power spectrum numbers were established and Figure 9 displays the power spectrum curves as interpreted for a line along the E-W direction along latitude 2.261814 degrees, and the correlation of curvatures in the interpreted power spectrum and the depth estimate curves is given in Figure 10.

Figure 4. Regional bouguer anomaly map key in establishing of the power spectrum.

Table 3. Calculation for depths using power spectrum curve numbers.

5. Discussion

The use of gravity data has demonstrated capability for monitoring lithological changes in large-scale in as a consequence differentiating basement and sedimentary of buried valleys. Gravity anomalies are associated with lateral contrasts in density and therefore deformation by faulting or folding will be manifested if

Figure 5. Complete bouguer anomaly spectrum.

Figure 6. The regional bouguer anomaly map with contours.

Figure 7. The residual bouguer gravity anomaly map with contours.

accompanied with lateral density changes, otherwise the vice versa is true. The area presents an overall range of Bouguer anomaly of −1756.8 g.u to −501.2 g.u, all negative, and the descriptions are enumerated in Table 4 in the different zones presented in the map (Figure 6). The regional anomaly gravity map presents high anomalies in the Northern region in the NW-SE trend and low anomalies in the southern trending in NW-SE, while the residual anomaly gravity map shows different trends for the low and high gravity anomalies.

The major rock groups in the study area have varying densities (Table 5) ranging from Sandstone beds with a density of 2050 kg/m³ to Archean basement with densities in excess of 3200 kg/m³.

Figure 8. The analytic signal gravity map with contours.

Consequently, the gravity anomalies are well interpreted in line with the lithologies of the study area rather than the deformation of the same lithologies. Meaning the deformation in the area isn’t accompanied with changes in densities of deformed lithologies. Therefore, analysis of Figure 6 (regional bouguer gravity anomaly map), and Figure 2 (lithology map) shows a good correlation. Zone A with pyroclastics overlying basement rocks with ~−1756.8 g.u to −1445.6 g.u. anomalous amplitude and Zone B with basalt, igneous rock, granite and acidic metamorphic rocks overlying basement with anomalous amplitude of ~−994.3 g.u. to −680.2 g.u. The pyroclastics, andesites, trachytes, phonolites, sandstone, greywacke and Eolian unconsolidated rock all overlying the basement dominate Zone C having low amplitude (~−1396.8 g.u to −1102.2 g.u). Zones D,

Figure 9. The interpreted power spectrum curves of a line along the E-W direction.

Table 4. Bouguer anomaly zone descriptions based on gravity pattern.

Figure 10. The correlation of curvatures in the interpreted power spectrum and the depth estimate curves.

Table 5. Densities of Common Rocks in the study area.

E, F portents alternating moderate to high amplitude (~−994.3 g.u to −501.2 g.u) representing geology of granite and acidic metamorphic rocks, andesites, basalts, and pyroclastics overlying the basement.

The residual anomaly gravity map indicates that the area is affected with faulting with faults running in the NE-SW and N-S trends. This is seen in residual anomaly gravity contour map in that the edges of contoured map (Figure 7) matches well with points of high faulting in the structural geology map (Figure 3). The technique of analytical signal calls for its interpretation in conjunction with other geological and geophysical information to maximize on its results. Thus the calibration of the analytic signal map (Figure 8) is done where points of high anomaly values matches well with areas which in geological map (Figure 2) are covered by igneous intrusive rocks considered to be of high density or fluvial deposits considered to be deposits of heavy minerals. This is evident also in the analytical signal map for these formations poses high density in comparison to surrounding rocks.

The Figure 9 displays the power spectrum curves as interpreted for a line along the E-W direction along latitude 2.261814 degrees. Accordingly, the depths related to the gravity sources have been calculated using the curve slope obtained from Figure 10. The Figure 10 shows comparison between the graphs of radially averaged power spectrum and the depth estimate graph.

Fundamentally, the power spectrum curves are separated into three components interrelated with gravity anomalies originating from the deepest formations, the shallow formations and structures and finally from the noise sources. By application of Equation (10), the depths to gravity anomaly sources have been calculated using the three slope curves. The radially averaged power spectrum curve is presented as In(Power) in the Y-axis and the Wave Number (1/K_Unit) in the X-axis. Equally the depth estimate has been given as Depth (K_Unit) in the Y-axis and the Number (1/K_Unit) in the X-axis. The Table 6 presents estimated and calculated depths t subsurface sources of gravity anomalies.

Based on the research, the different anomalies relate well with different rock densities in the study area along the line profile. The gravity highs are noted in the eastern point and are associated with andesites, trachytes, basalts and igneous rocks, while the gravity lows are associated with sandstone, greywacke, arkose, and eolian unconsolidated rock. By the use of the Power spectrum analysis the showing of the depth of the deepest basement rock is 12.8 km which is in the eastern flank, while the shallowest to the basement of 1.1 km to the western flank.

The bouguer anomaly maps happen to always acts as the best and indeed display the best subsurface density changes of the bedrock in the areas of interest. The variations of the bouguer anomaly have been enhanced by calculating 2^nd derivative and horizontal gradient (tilt derivative) relief maps. These maps have helped in recognizing, discovering, and categorizing formations and structures affecting gravity. Local variation of gravity is well observed in the residual bouguer anomaly, tilt derivative and analytic signal maps where, the sources are aligned to quantities relating to position, shape and structure of geological formations.

Table 6. Depth estimation for unconformity bedrock.

6. Conclusion

The gravity method used in this study has been instrumental and the conclusion of results of interpretation is that the area is affected by different fault elements trending NE-SW, N-S and minor in the E-W reflecting the orientation of different lithologies, which can be tied to past tectonic activities. The gravity anomalies are well interpreted in line with the lithologies of the study area rather than the deformation of the same lithologies. There is observed high values of gravity anomaly values (ranging from −880.2 to −501.2 g.u.) where there are eolian unconsolidated rocks overlying the basement as compared to low gravity anomaly values (ranging from −1338.9 to −1088.7 g.u.) where the andesites, trachytes and phonolites overly the basement. The different regional gravity anomalies relate well with different rock densities in the study area along the line profile for radially averaged power spectrum. The gravity highs are noted in the eastern point and are associated with andesites, trachytes, basalts and igneous rocks, while the gravity lows are associated with sandstone, greywacke, arkose, and eolian unconsolidated rock. The utilization of the information from the power spectrum analysis demonstrates that the depth to the deepest basement rock is 12.8 km which is in the eastern flank, while the shallowest to the basement of 1.1 km to the western flank.

Acknowledgements

Special appreciations go to the Ministry of Petroleum and Mining, Kenya from whom geological and topographic maps were obtained, which were useful in the production of the final geological map of the area. To the survey of Kenya which was instrumental in provision of data used, I am indebted. I also wish to recognize the prayers, efforts, encouragement, humour and cooperation received from my wife Lydia Gesare and our Children (Amygrace Kerubo Tsitsi, Merrybell Meita, and Madiba Mandela) during the entire period of this research. They allowed me to use the meagre family resources for the research and am dearly indebted.

Appendix

Table A1. Gravity observed and processed gravity data as complete Bouguer anomaly.

Conflicts of Interest

The authors declare no conflicts of interest regarding the publication of this paper.

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