Interpretation of high resolution aeromagnetic data over southern Benue Trough, southeastern Nigeria

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1 Interpretation of high resolution aeromagnetic data over southern Benue Trough, southeastern Nigeria IAOha 1,, K M Onuoha 1, A N Nwegbu 2 and A U Abba 2 1 Department of Geology, University of Nigeria, Nsukka, Nigeria. 2 Nigerian Geological Survey Agency, Abuja, Nigeria. Corresponding author. ifeanyi.oha@unn.edu.ng HighresolutionairbornemagneticdataofpartsofthesouthernBenueTroughweredigitallyprocessedand analyzed in order to estimate the depth of magnetic sources and to map the distribution and orientation of subsurface structural features. Enhancement techniques applied include, reduction to pole/equator (RTP/RTE), first and second vertical derivatives, horizontal gradients and analytic signal. Results from these procedures show that at least 40% of the sedimentary basin contain shallow (<200 m) magmatic bodies, which in most cases are intermediate to mafic intrusive and hyperbysal rocks, and may occur as sills, dikes or batholiths. Magnetic lineaments with a predominant NE SW trend appear to be more densely distributed around the basement rocks of the Oban Hills and metamorphosed rocks around the Workum Hills. 3D standard Euler deconvolution and Source Parameter Imaging (SPI TM ) techniques were employed for depth estimation. Results from the two methods show similar depth estimates. The maximum depth to basement values for 3D Euler and SPI are 4.40 and 4.85 km with mean depths of 0.42 and 0.37 km, respectively. Results of 2D modelling of magnetic profiles drawn perpendicular to major anomalies in the study area reveal the existence of deep seated faults which may have controlled the emplacement of intrusive bodies in the basin. The abundance of intrusive bodies in the study area renders this part of the southern Nigerian sedimentary basins unattractive for petroleum exploration. However, the area possesses high potential for large accumulation of base metal mineralization. 1. Introduction Following the release of the first aeromagnetic data collected over most parts of Nigeria by the Geological Survey of Nigeria (GSN) in 1974, attempts were made by early researchers to interpret the data both qualitatively and quantitatively. Studies involving the interpretation of aeromagnetic data over the Benue Trough have revealed the existence of block faulting and numerous intrusive bodies (Osazuwa et al.1981; Ajakaiye 1981; Ofoegbu 1984, 1985; Ofoegbu and Mohan 1990; Ofoegbu and Onuoha 1991). Various depth estimate techniques were applied on the data and results obtained showed that estimates of the thickness of sedimentaryrocksobtainedbydifferentauthorsagreefairly well with each other. For example, Osazuwa et al. (1981) obtained a depth range of km and km in the northern Benue Trough from magnetic and gravity data respectively, while Nur (2000)combiningtwo-dimensionalspectralanalysis and Hilbert transform of magnetic data reported the existence of two main source depths in parts of thenorthernbenuetrough,withthedeepestsource lyingbetween1.5and2.25km.inthesouthernbenue Trough, Ofoegbu (1984) found that the thickness of sediments vary between 0.5 and 7 km. Ofoegbu and Onuoha (1991) from the results derived from Keywords. Airborne magnetic data; magnetic sources; magmatic bodies; depth estimation; 2D modelling; Benue Trough. J. Earth Syst. Sci. 125, No. 2, March 2016, pp c Indian Academy of Sciences 369

2 370 IAOhaetal. 2D spectral analysis identified the existence of two main source depths in parts of the southern Benue Trough, the deeper source lying at a depth of km, while the shallower depths were generally <250 m. Recently, Obi et al. (2010) carried out depth estimates based on SAKI modelling, power spectrum and horizontal gradient magnitude and observed that sediment thickness range between 1.0 and 4.0 km. Between 2005 and 2009, the Federal Ministry of Mines and Steel Development engaged the services of Fugro Airborne Surveys Limited, to acquire high resolution data of 500 m line spacing and 80 m terrain clearance for most parts of Nigeria. The release of these data by the Nigerian Geological Survey Agency (NGSA) in 2010 coincided with the availability of high speed and robust computer programs, making it increasingly possible to generate Table 1. Characteristics of existing aeromagnetic data for the study area. Line Terrain Date spacing clearance Flight Data available (m) (m) line direction Data format Old data E W Hard copy contour maps on a scale of 1:100,000 New data NW SE Digital formats in.gdb,.xls (excel), PDF, JPEG, etc. Figure 1. The study area as part of the West African and Central African rift system.

3 Interpretation of aeromagnetic data over the southern Benue Trough 371 subtler results and more detailed information. This study aims at utilizing newly acquired high resolution airborne magnetic data to generate more detailed information on the structural framework and distribution of igneous bodies in the basin. Table 1 accounts for the improvement in the new digital high resolution data over the existing data in which most of the works earlier cited are based upon. The southern Benue Trough includes the southernmost part of the Benue Trough, which is a major sedimentary basin in Africa, stretching over 1000 km in length with width ranging between 150 and 250 km (figure 1). It is part of the Cretaceous West African Rift System (WARS) which can be traced along a distance of about 4000 km from Nigeria, running northwards into the neighboring Republic of Niger and terminates in Libya (Binks and Fairhead 1992). Thevariousmechanismsthatarebeingproposed for the formation of the Benue Trough, have generated a lot of controversy. However, the most popular theories include: i) Tensional movement resulting in a rift (King 1950; Cratchley and Jones 1965). ii) Horst and graben genesis related to the Cretaceous opening of the Gulf of Guinea (Stoneley 1966). iii) Asthenospheric uplift or mantle plume, block faulting, crustal stretching and thinning and emplacement of igneous bodies in the lithosphere (Olade 1975; Bott 1976; Adighije 1979; Fairhead and Okereke 1986). iv) Large scale wrenching leading to the formation of numerous pull-apart basins (Benkhelil and Robineau 1983; Benkhelil 1986, 1989). The inherent genetic ambiguity, structural complexity, coupled with the existence of igneous bodies and base metal mineralization in the basin has generated a lot of interest amongst geoscientists. The studies have been multi-dimensional involving geophysical, geochemical, structural and petrological studies. As newer data and tools are generated and developed, it becomes increasingly possible to model and characterize the basin. 2. Regional geology The sedimentary fill of the southern Benue Trough was controlled by cycles of transgressions and regressions accompanied by interferences of local tectonics. Figure 2 is a simplified geological map of the southern Benue Trough. Three cycles of basin fill have been recorded in the basin, they include; (i) the Neocomian Cenomanian Asu River Group, (ii) the Early to Late Turonian Eze Aku Group and (iii) the Coniacian Santonian Awgu Group (Ojoh 1992; Umeji 2007). The Asu River Group represents the earliest clastic fill of the Lower Benue Trough and occupies the core of the basin. It consists of about 3000 m thick basal arkosic sandstones and middle and upper marine shales. Overlying the Asu River Group is the Turonian Eze Aku Group which consists of fossiliferous calcareous sandstones, shales and limestones. These sediments are noted as the most extensive marine sedimentation deposited when the Mediterranean or Tethys Sea linked up with the Atlantic across the Sahara (Ojoh 1992). The overlying Awgu Group consists of the greyblue Awgu Ndeaboh shale and the medium-tocoarse grained bioturbated Agbani sandstones. Large scale cross-stratification and herringbone cross-bedding are common primary and sedimentary structures in the Agbani sandstone. Reyment (1965) considered the Agbani sandstone as a time equivalentofthe Awgushale,conversely,Cratchley and Jones (1965) thought that it is a late deposit of thick sandstone beds which accompanied the shallowing of the Coniacian Sea. The Awgu shale is estimated to be about 900 m thick (Benkhelil 1986). The Santonian represented a period of tectonic and igneous activity, when the sediments of the Abakaliki Benue Trough were folded, uplifted and intruded by igneous rocks, leading to lowgrade metamorphism in some cases (Ojoh 1992). The sediments of the Benue Trough were eroded and variously overstepped by the Campanian and Maastrichtian beds of the Anambra Basin. The horst and graben structure in the Calabar Flank of the Benue Trough were initiated at this time (Whiteman 1982; Reijers 1996). The study area also includes the Oban and Bamenda Massifs, which are composed of crystalline metamorphic rocks of Neo-Proterozoic age. These rocks are mainly migmatitic gneisses, banded gneisses, coarse porphyritic granites, pegmatites withthindykesofintermediate basicintrusiveand hyperbysal rocks. They generally underlie the sedimentary fills of the Lower Benue Trough. Younger sediments of Campanian Paleocene age are shown in the southwestern tip of the geological map (figure 2). It is known that they belong to two younger adjacent basins, namely, the Afikpo and Anambra basins. Sedimentation ended in the Lower Benue Trough after the Santonian and this was as a result ofwidespreadupliftconsequent ofapervasivecompressive event. This led to the development of two basins, the Afikpo Syncline to the south and the Anambra Basin to the west. The Campanian representstheoldestsedimentsinboththeafikpoand Anambra basins and sedimentation may have continued until the end of the Cretaceous. The structural history and stratigraphic successions of

4 372 IAOhaetal. Figure 2. Generalized geologic map of the study area. these basins have been well documented elsewhere (Nwajide and Reijers 1996; Akaegbobi and Boboye 1999; Obi and Okogbue 2003; Odigi and Amajor 2009). 3. Data and methods 3.1 Data The data used for this research form part of the new high resolution digital airborne data for most parts of Nigeria acquired between 2005 and They were collected in two phases, with PhaseIdataacquiredbetweenMayandSeptember 2007, while Phase II data was acquired during the period September 2007 August The entire data included 1,930,000 line km of magnetic and radiometric surveys flown at 500 m line spacing and 80 m terrain clearance. Data acquisition was carried out by Fugro Airborne Surveys. A subset of the nationwide grid covering the study area (lat N and long E) was made available by the Nigerian Geological Survey Agency (NGSA). The survey for most of the Benue Trough was flown along the NW SE direction (i.e., perpendicular to the axis of the basin). The geomagnetic gradient was removed from the data using the International Geomagnetic ReferenceField(IGRF) formulafor2005.thisnew data offers a lot of advantages in resolution and format over the old data (acquired in 1974) in whichmostoftheearlierinterpretationsweremade (see table 1). 3.2 Methods Data processing Data processing including editing and initial filtering was performed by the preliminary processing contractors, Paterson Grant and Watson Ltd (PGW). This includes cultural editing to correct for rough effect due to interference from substantial infrastructure in the area. Diurnal variations in the airborne magnetometer data were corrected by subtraction of the filtered and IGRF corrected ground station data. Thereafter, the magnetic data was IGRF corrected using the 2005 model. After

5 Interpretation of aeromagnetic data over the southern Benue Trough 373 the diurnal and IGRF corrections, a levelling procedure was applied to account for a number of effects including data differences at intersections of tie and traverse line recordings. The products were interpolated into a regular grid with cell size of 125 m (one quarter of the flight line spacing) using a minimum curvature algorithm with a constant elevation of 80 m. This final product is the total magnetic intensity (TMI) map Data analysis and interpretation techniques Data analysis and interpretation can be broadly divided into two main procedures which include data enhancement and forward modelling. Enhancement is necessary since the TMI data displays gross interpretation limitation. In order to enhance subtle anomalies which, in many cases, are anomaliesofinterest,anumberoffiltersareappliedtothe raw TMI data. This was done in the spatial frequency domain by the introduction of Fast Fourier Transform (FFT). The enhancement routines performed in this work include, reduction to pole(rtp), reduction to equator (RTE), first and second vertical derivatives (1VD, 2VD), horizontal gradient (HG), analytic signal (AS), standard Euler deconvulation and source parameter imaging (SPI). The last two are essentially depth-estimation filters, while the others enhance or suppress certain anomalies invariably aiding the interpretation process. More elaborate description of the principle, theory, application and formulation of these routines are given elsewhere (Nabighian 1972, 1984; Thompson 1982; Roest et al. 1992; Thurston et al. 1999, 2002; Reeves 2005; Nabighian et al. 2005; Whitehead and Musselman 2008). Only a brief presentation of these routines is outlined here. The reduction to pole (RTP) filter simplifies interpretation of anomalies by reconstructing the magnetic field as if it were at the pole. (i.e., vertical magnetic field and declination of zero). Vertical bodies hence will produce induced magnetic anomalies that are centred on the body and are symmetrical. First and second vertical derivatives emphasize shallower anomalies and can be calculated either in the space or frequency domains. Before the digital age, use of second vertical derivative for delineating and estimating depths to the basement formed the basis of aeromagnetic interpretation (Nabighian et al. 2005). Many modern methods for edge detection and depth to source estimation rely on horizontal and vertical derivatives. For this study, the first and second vertical derivatives were generated in Oasis Montaj using the MAGMAP GX. The amplitude of the analytic signal (total gradient), possesses considerable advantage over the maximum horizontal gradient, due to its lack of dependence on dip and magnetization direction, at least in 2D (Nabighian et al. 2005). Thus, the analytic signal performs well at all magnetic latitudes. Thisnotionisextendedto3DbyRoestet al.(1992) and represented using the expression. ( T A(x,y) = [ x ) 2 ( ) 2 ( ) 2 ] T T + + y z (1) where A(x,y) is the amplitude of the analytic signal at (x, y). T is the observed magnetic field at (x,y). Thompson (1982) expressed the Euler s homogeneity as: (x x 0 ) T x +(y y 0) T y +(z z 0) T z = N(B T) (2) where x 0, y 0, z 0 is the position of the magnetic body.t is total field measured at (x,y,z). N is the degree of homogeneity which can be interpreted as the structural index (SI). B is background value of the TMI. The Standard 3D Euler method is based on Euler s homogeneity equation, which relates the potential field and its gradient components to the location of the sources, by the degree of homogeneity N. The method makes use of a structural index in addition to producing depth estimates. The structural index combined with depth estimates have the potential to identify and compute depth estimates for a variety of geologic structures such as faults, magnetic contacts, dykes, and sills. The source parameter imaging (SPI) technique is based on the principle of complex analytic signal and computes source parameters from gridded magnetic data. It requires first and second order derivatives and is thus susceptible to noise in the data and interference effects (Nabighian et al. 2005). The wavenumber theory is that for vertical contacts, the poles of the local wave number define the inverse of depth. In other words, depth is given by the expression Depth = 1 K max (3) where K max is the peak value of the local wave number K over the step source. It can be shown that ( Tilt ) 2 ( ) 2 ] K max = [ Tilt + (4) x y

6 374 IAOhaetal. and Tilt = arctan (VDR/HGRAD) =arctan ( T/ z) [ ( T/ x) 2 +( T/ y) 2]. (5) Solution grids using the SPI technique show the edge locations, depths, dips and susceptibility contrasts. Hence, the SPI map more closely resembles geology than either the magnetic map or its derivatives. The technique works best for isolated 2D sources such as contacts, thin sheet edges, or horizontal cylinders (Nabighian et al. 2005). The ratio of the vertical gradient to the horizontal derivative shown in equation (4) has been defined as the tilt angle (Miller and Singh 1994). The tilt angle acts as an automatic gain filter and is seen here as an excellent edge detector. In this study, forward modelling of two profiles across the study area were performed using GM- SYS software. The GM-SYS program is based on themethodoftalwaniet al. (1959) and Talwani and Heirtzler (1964). 4. Results 4.1 Edge detection The RTP, vertical and horizontal derivative grids display useful edge information. The RTP transformation has correctly placed the peaks over the source, magnetic highs are observed over the basement Oban Hills, the Abakaliki magmatic area which contains pyroclastics, the metamorphosed and magma invaded Workum Hills (outcropping between Wanakom and Wanakonde) and the Ishiagu area (figure 3). The cause of the magnetic anomalyaroundtheenyigbaareacannotbeclearly ascertained from this image, the absence of intrusives and hyperbysal rocks (based on field and borehole data) further complicates it. Prominent magnetic lows are observed between Afikpo and Aka Eze to the south, around Eha Amufu to the Figure 3. Reduction to pole (RTP) grid with amplitude correction=40.

7 Interpretation of aeromagnetic data over the southern Benue Trough 375 northwest of the study area and Bansara Ogoja axis to the northeast. First and second vertical derivatives (1VD and 2VD) of the TMI were performed in order to enhance shallow sources. The 2VD grid (figure 4) tends to reveal more features, but is noisy. The 2VD grid is upward continued to a height of 300 m in order to smoothen the image. The 1VD grid can be displayed in grey scale so as to enhance linear features, which may be dikes, sills, geologic contacts, faults, fractures etc. When compared with the tilt angle map, it was observed that the latter possesses greater potential for edge detection. The features are digitized onscreen and superimposed on the tilt angle map (figure 5). The horizontal gradient display is characteristically employed in the enhancement of linear features from aeromagnetic data. The magmatic induced patterns in the study area are observed and are accompanied by numerous linear features. The analytic signal grid simplifies the interpretation by placing the anomalypeaksdirectlyabovethesource.theintrusive bodies around Ishiagu, Workum Hills, Igumale, Obubra, Ikom and the basement rocks around the Oban Hills are clearly defined in the analytic signal grid of the study area (figure 6). 4.2 Source depth estimates Depth estimates from the SPI image shows the distribution of deep basins. It is observed in figure 7 that 70 80% of the study area contain sources that are relatively shallow (<1 km). The deep parts of the basin are generally small, isolated pockets and may attain depths of up to 4.6 km. Most of the intrusives, hyperbysal and volcanics are generally less than 300 m deep. The basement areas around k Figure 4. Second vertical derivative (2VD) image.

8 376 IAOhaetal. Figure 5. Tilt angle map (in radians) with interpreted magnetic lineaments overlaid. the Oban Hills are generally exposed and where buried they rarely exceed a depth of 300 m. Four 3D Euler maps are displayed in figure 8 with structural index (S.I) values of 0, 1.0, 2.0 and 3.0. The Euler images are similar to the SPI image (figure 7) except that in some locations the Euler images lack solutions. Table 2 compares SPI depth estimates with Euler (SI = 3.0) for certain deep basin locations in the study area. 4.3 Modelling 2D modelling of profiles AA and BB was carried out in order to verify the distribution of lithologic units and the structural framework of the subsurface. Three main rock types were built into themodel,basedontheircontrastingsusceptibility values.theoverlyingsedimentaryrockswereassigned susceptibility values of zero; the metamorphic

9 Interpretation of aeromagnetic data over the southern Benue Trough 377 k Figure 6. Analytical signal image, revealing the extent and shapes of shallow and deep seated intrusive bodies. (AA and BB are profile lines for the models in figure 9). basement was assigned values ranging from to cgs, while the intermediate to basic intrusives were assigned values ranging from to cgs. The raw TMI image of the study area was used in building the model. ProfileAA isannw SSEtrendingsectionwith an approximate length of 63 km. It starts from a point approximately 18 km north of Onitcha Uburu, through Afikpo and terminates around Ugep (see figure 9a). The 2D model reveals a faulted basement with depth range between 1.8 and 2.5 km and numerous intermediate to mafic sills and dikes, most of them occurring at depths between 0 and 500 m. The outcropping sill north of Afikpoisobservedaroundthe48,000mpointalong the profile and appears to be closely associated to normal faulting at depth (see figure 9a). Profile BB also trends in the NNW SSE direction with an approximate length of 65 km. It starts from a point approximately 12 km northwest of the Workum Hills and passes through Wanikande and Wanakom, terminating close to Bansara (see figure 6). The structure of the area is typically that of an anticline with the oldest sediments (Albian Asu River Group) at the core and younger sediments at the flanks. The underlying basement rocks display a characteristic horst and graben structure with an obvious interplay between tectonism and magmatism. Numerous bodies of intrusive igneous rocks with diameter ranging from 1

10 378 IAOhaetal. Figure 7. Source parameter imaging (SPI) grid, showing approximate depths to intrusive bodies and the underlying basement complex rocks. 1 7 represent positions where the basins are thick and are explained in table 2. to 8 km occupy the core of the anticlinal structure around the Workum Hills and can be traced for close to 30 km along the profile. Emplacement of the intrusive bodies are observed to have been closely related to large dip-slip faulting at depths in excess of 3 km. A thick intermediate to mafic intrusive body (14,000 m along BB )mayhave compensated for the large depth to source (about 2.2 km) observed on the SPI image at that point. The area around Bansara also has depth to basement in excess of 2 km. For other parts of the section, depth to basement is generally below 2 km and most of the intrusive rocks at the core of the local anticlinal feature are outcropping. 5. Discussion and conclusion The 529 magnetic lineaments extracted from the enhanced aeromagnetic image represents deep seated fractures, dykes, sills and vents. It is observedthatthedistribution(figure10a)isclosely controlled by the host lithology, as basement rocks and intrusives are seen to display higher density of these features. Figure 10 highlights the similarities and differences in trend and length between lineaments digitized from enhanced Landsat7 ETM+ data (Oha 2014) with the magnetic lineament generated from this study. The dominant trend observed from the rose diagram (figure 10b, c)

11 Figure 8. 3D standard Euler deconvolution with structural index (SI) = 0, 1, 2 and 3. (1 7 on SI = 3 grid represent positions where the basins are thick and are explained in table 2). Interpretation of aeromagnetic data over the southern Benue Trough 379

12 380 IAOhaetal. Table 2. Comparative depth to basement estimates for selected (deep basin) locations in the study area. Sl. no. Location Latitude Longitude Euler (SI = 3) SPI (km) 1 Southeast of Bansara 6.41 N 8.57 E No solution Afikpo Akaeze 5.82 N 7.76 E 3.37 km Northwest of Oshirigwe 6.80 N 8.31 E 2.88 km South of Enugu 6.43 N 7.67 E No solution West of Ogoja 6.60 N 8.55 E No solution Southwest of Eha Amufu 6.65 N 7.68 E No solution Southwest of Ugep 5.59 N 7.96 E 3.84 km 3.73 is consistent with the major (NE SW) structural trend of the Benue Trough. However, there is a marked deviation in subordinate trends from what is observed on Landsat and on the field. The N S and NW SE subordinate trends reported in the area (Ezepue 1984; Oden 2012; Oha 2014), are not prominent in the filtered aeromagnetic maps. This implies that these trends do not persist at depth. Results from aeromagnetic data interpretation suggests the demarcation of six magmatic centres in the study area (figure 11). They include, Ishiagu, Ugep, Obubra, Ikom, Wanikande and Igumale magmatic centres. At these areas, igneous bodies either outcrop or they are covered by thin overburden of not more than 100 m. Depth to basement estimation using Euler deconvolution and source parameter imaging (SPI) gave depths between 2.2 and 4.8 km for areas where there are no intrusive bodies (see table 2). Spatial distribution of these bodies shown on the 300 m upward continued second vertical derivative map (figure 11) reveal that about 60% of the study area is covered by either basement rocks or intrusives. This includes some parts of Oban and Bamenda Massifs, which also outcrop in the study area. These rocks are observed from the models (figure 9) to include large volumes of intrusive, hyperbysalandvolcanicrocksofhighlyvariedcomposition. These large volumes of igneous bodies combined with their widespread occurrence may be responsible for the observed close spatial association with the Pb Zn Ba mineralizations in the study area (Olade and Morton 1985; Akande and Mucke 1993; Oha 2014). It is inferred that the observed close spatial association between mineralizedveinsandintermediatetomaficintrusiverocks inthestudyareadoesnothavegeneticimplication. Furthermore, it is obvious from field observations that the intrusives predate the mineralization. In some places, barite-dominated veinlets are seen cross-cutting late Cenomanian to Santonian intrusives (Oha 2014). Depth estimates from this study agrees fairly with results from previous works. The works of Osazuwa et al. (1981); Ajakaiye (1981); Ofoegbu (1984, 1985); Ofoegbu and Mohan (1990); Ofoegbu and Onuoha (1991); Nur (2000) and Obi et al. (2010) gave depth to deeper source range in the Benue Trough as km. Whereas, the deepest parts of the basin from this study is about 6 km, the high frequency of shallower bodies (<200 m) as displayed by the powerful visualizing capability of Oasis Montaj software offers a remarkable addition to the existing knowledge on distribution of intrusives in the Lower Benue Trough. Some of the magmatic centres reported in this work were previously reported (Obiora and Charan 2010), but their extent and spatial distribution were not clearly demarcated. The proliferation of near surface intrusives in the study area as shown in the various filters and 2D model is an indication of widespread pervasive magmatism in the Lower Benue Trough. This consequently may have adversely increased temperature ranges in the basin leading to possible overmaturation of potential source rocks. Hence, the abundance of igneous bodies in the Lower Benue Trough may have adversely affected the petroleum potential of the Lower Benue Trough. Notwithstanding, reports of extensive mineralization of Pb, Zn, Cu and Ba mineralization are well documented (Farrington 1952; Orajaka 1965; Olade 1976; Ezepue 1984; Olade and Morton 1985; Akande et al. 1988; Akande and Mucke 1993; Oha 2014). Although, recent work in the basin suggests the deposits may not be genetically related to the igneous bodies (Oha 2014), understanding their presence and disposition presents a useful exploration guide. Based on the results and inferences generated from this work, we conclude that the Lower Benue Troughcontainsnotonlypocketsofmagmaticbodies as earlier thought, but include enormous and widely distributed magmatic rocks. These rocks generally cover approximately 60% of the study areaandareeitheroutcroppingorconcealedbyrelatively thin overburden. Whereas, in most areas, depth to basement range between 2 and 2.5 km, a few isolated portions of the basin show depth estimates in excess of 4 km. The existence of deep

13 Interpretation of aeromagnetic data over the southern Benue Trough 381 Figure 9. 2D model for profiles AA and BB. seated ( m) magmatic lineament is confirmed and tends to deviate in trend from surface linear trends (lineaments) obtained from fieldwork and interpreted Landsat7 ETM+ data. The proliferation of the basin by large scale magmatism renders the basin unattractive for petroleum

14 382 IAOhaetal. Figure 10. (a) Combined lineament map (from aeromagnetic and Landsat 7 ETM+ data) for the study area. (b)roseplot for lineaments from Landsat data. (c) Rose plot for lineaments from aeromagnetic data.

15 Interpretation of aeromagnetic data over the southern Benue Trough 383 k Figure 11. 2VD image upward continued to 300 m. A are basement rocks of the Oban Hills. B represents basaltic rocks around Ikom. C are basement rocks. D are basaltic/doleritic rocks outcropping around Obubra. E represents the metamorphose Workum Hills and the magmatic centres around Wanikande and Wanakom. F represents near surface intrusives associated with barite mineralization around Gabu-Oshina. G represents the Igumale area characterized by numerous near surface intrusives. H represents the Abakaliki area with its pyroclastics and associated rocks. I represents the Ishiagu magmatic area. exploration, but a deeper understanding of the distributionanddispositionoftheseigneousbodies presents a valuable base metal exploration tool. Acknowledgements The authors are grateful to the former DG of NGSA, Prof. S Malamo who granted the first author a 6-month intenship at NGSA Abuja, in Dr O Okunola was on ground to offer useful suggestions and advice during the internship. Ria Tinion of Geosoft South Africa, provided the software (Oasis Montaj) used for data processing and interpretation. The quality of this paper has improved enormously as a result of vital comments and contributions made by two anonymous reviewers. References Adighije C 1979 Gravity field of Benue Trough, Nigeria; Nature, London Ajakaiye D E 1981 Geophysical investigation in the Benue- Trough. A review; Earth Evolution. Arbitrary shape; In:

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