Kinematics of faults and joints at Enugu area of the Anambra basin

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1 Journal of Geology and Mining Research Vol. 2(5), pp , October 2010 Available online ISSN Academic Journals Full Length Research Paper Kinematics of faults and joints at Enugu area of the Anambra basin D.K. Amogu*, A. C. Ekwe and K. M. Onuoha Department of Geology, University of Nigeria, Nsukka, Enugu State, Nigeria. Accepted 23 August, 2010 Structural analysis of joints and fault data obtained from exposures of the Enugu and Mamu formations formed the basis for establishing the paleostress direction and stress field rotation within the study area. Four sets of joints were dominant in the study area. Analysis of two conjugate sets: J 1 and J 2 separated by a dihedral angle of 40 shows that the maximum principal stress σ 1 at the time of formation of these joints was oriented 128 azimuth and 16 plunge. The intermediate stress σ 2 was oriented 262 azimuth and 70 plunge while the minimum stress σ 3 was oriented 35 azimuth and 16 plunge. The joint sets gave a NW-SE orientation of the maximum principal stress. The joint spacing ranges from cm in the Enugu Shale to about 300 cm at the top unit of Mamu formation. The abrupt termination of the siltstone/fine sandstone unit against shale within the Enugu shale is an evidence of the deformation of the unit by several normal faults. Two faults F 1 and F 2 form a conjugate fault pattern. F 1 has a listric fault plane dipping NE, a growth index of 1.35 and is flanked by a smallscale rollover structure. F 2 is oriented 315 azimuth parallel to one of the joint sets and dipping in the NW direction. The paleostress direction deduced from these faults suggest that the maximum stress (σ 1 ) orientation was 275 and plunge 5 ; the intermediate stress (σ 2 ) was orientated 10 azimuth and 40 plunge while the minimum stress (σ 3 ) orientation was 179 azimuth and 52 plunge. This shows a clockwise rotation of the stress fields that formed the joints and the faults respectively. Extrapolation of the results of the orientation analysis of these fractures into the subsurface will enhance the understanding of permeable zone, fluid migration pattern and therefore increase the success rate in the exploration and exploitation of groundwater and even in the management of the mine drainage problems in the Enugu coalfields. Key words: Kinematics, paleostress direction, principal stress, dihedral angle, faults, joints. INTRODUCTION On outcrop scale, the cretaceous sedimentary rocks of the Anambra basin bears some deformation structures which till date, have not been carefully studied using structural method. These structures are rarely mentioned either as supporting evidence to continuous subsidence of the basin or to buttress the tertiary extensional regime within the basin as often stated by some authors. *Corresponding author. dan_amogu@yahoo.com. OBJECTIVES The objectives of this study are to: 1. Establish the spatial distribution of joints and faults in the study area; 2. Map and analyze them using structural methods; 3. Determine the principal stress directions at the time of formation of these structures; and 4. Highlight the implications of these structures as they affect the stress regime in the basin, reservoir potentials

2 102 J. Geol. Min. Res. Figure 1. Kinematics of the Benue Trough after Benkhelil et al. (1988). of the deformed rocks, groundwater prospect, and stability of engineering structures. REVIEW OF THE EVOLUTION AND STRATIGRAPHY OF THE ANAMBRA BASIN The Anambra basin originated as a direct consequence of the stresses generated by the movements along Chain and Charcot fracture zones that formed the Benue Rift valley. Prior to the Santonian episode, the south-western part of this rift valley consisted of a tectonically stable part - the Anambra platform which received little or no sediment in the west and to the east, a subsiding area, the Abakaliki basin, which was receiving large amounts of sediment (Asu River Group and Ezeaku Formation) (Murat, 1972; Petters, 1978; Whiteman, 1982; Benkhelil, 1982; Ojoh, 1992). The Santonian movement which was widely accompanied by magmatism (Maluski, 1995), folding and faulting was probably caused by a transcurrent movement along some of the fracture zones that extends from the oceanic Charcot fracture zone into the Benue trough. Such movements usually result in both extensional and compressional domains as shown in Figure 1. A transcurrent movement in the Santonian resulted in the folding and uplift of the sediments in the Abakaliki domain and a parallel down warping of the Anambra platform to form the Anambra basin (Figure 2). Consequently, the depocentre translated westward from Abakaliki domain to Anambra domain (Murat, 1972; Nwachukwu, 1972; Benkhelil, 1982). All the structures used for this analysis were measured in the Anambra basin between the longitudes and latitudes The stratigraphic evolution can be outlined using subsurface data and correlation along sections connecting several wells in the Anambra basin. Analysis of the sedimentological and stratigraphical characteristics of the deposits (Allix, 1987) shows three main period (Figure 3) in the evolution of the basin. The periods include the pre-cenomanian, post-santonian and the lower tertiary history. The pre-cenomanian history was characterized by a major subsidence in the Abakaliki domain (Ojo, 1992), while the Anambra domain remained a platform where mud was deposited in a shallow restricted marine environment. The deposits of this period in the Anambra basin are essentially the Awgu Shale. During the post- Santonian period that followed the formation of the Basin an east to west prograding delta developed, during which the Nkporo Group, the Mamu Formation (lower coal measures) and the Ajali Sandstone were deposited. At the end of the deltaic system, a major transgression

3 Amogu et al. 103 ANAM BRA SUB -B ASI N B EN UE TR OU GH ABAK ALIKI UPLIFT/ A FIKP O SY NCLINE O G O JA S UB-BAS IN Study area a j Nkalagu. ez a r m a w.en ugu en Aba k alik i. ez a g aw ar m n ez o Afikp o. ez ABAK ALIKI AN TICLINOROIM m am aj Um uahia. c s i cs AF IKPO SYNCLINE n k n ar ar OB AN MAS SI F O duk pa ni. CALABAR FLANK e z O BUD U H ILLS M AMFE E M BAYM ENT cs Coastal plain sands (M iocene-pleistocene ) am Am e ke F orma tio n (Eoce ne ) i Imo Shale (Paleoc ene) n Ns ukka M aastrichtian aj A jali m M am u ag o Agbani Ow elli Enugu Nkporo Form ation nk en (Cam panian -M aast.) aw Awgu (Coniacian) NIG E R DE LTA C alabar. ez ar Ezeaku group (C eno. Turonian) Asu River group (Albian) GULF O F G UINEA B ase m ent com plex Figure 2. Geologic map of Southeastern Nigeria showing the study area. (Modified from Oformata, 1973). reworked the Ajali sandstone and deposited the Nsukka Formation. The progradation of the Anambra axis started in the tertiary while another marine transgression resulted in the deposition of the Imo Shale - an Anambra basin equivalent of the Akata Formation in the Niger delta. The Imo Shale was followed by the Ameki Formation (Nanka Sandstone inclusive) and then, the Benin Sand. The spatial distribution of these rock formations are shown in the geologic map of the south-eastern Nigeria in Figure 2.

4 104 J. Geol. Min. Res. Figure 3. Schematic sedimentological cross-section and well log correlation in the Anambra syncline (Allix, 1987). Study background Fault kinematic analysis is very useful in quantifying brittle deformation based on orientation measurements of mesoscale fractures (faults, joints and associated striations). This analysis commonly determines the stress tensors that is, directions of principal stresses (σ 1 > σ 2 > σ 3 ), or kinematic axes (Ramsey and Huber, 1983; Marrett and Peacock, 1999) and the stress ratio R = (σ 2 - σ 3 /σ 1 - σ 3 ) at any locality. The following assumptions are made when kinematic data analyses are interpreted in terms of stress: (a) The stress tensor is symmetric, that is deformation is coaxial, pure shear. (b) Deformation is homogeneous. (c) Fracture formation is consistent with the Mohr- Coulomb yield criterion (Coulomb, 1773), that is, faults and joints develop parallel to σ 2 and σ 1 irrespectively. In nature, none of these assumptions is realized. However, pure shear and homogeneous deformation (assumptions a and b) may be a realistic simplification at a small (outcrop) scale (Pollard et al., 1993). As a result, inferred stress tensors reflect a local stress state. The third assumption is only valid in isotropic rocks, where faults may develop parallel to σ 2 at a fracture angle varying between 0 (pure tensional fractures that is joints) to 30 (shear fracture that is faults) to σ 1 (Griffith, 1921). However, in anisotropic rocks, pre-existent discontinuities (that is existing fractures, foliations, bedding planes) may be reactivated irrespective of their orientation with respect to σ 1. If the applied differential stress is low and if the pore fluid pressures are sufficiently high to exceed lithostatic pressures, even angles of are observed (Sibson, 1977). METHODOLOGY The study was carried out in two phases, field studies and laboratory analysis. In the field, joints and faults were identified, their positions were recorded using a Garmin global positioning system (GPS), and their orientation were measured. Forty-two joints were identified and measurements of their orientations was also carried out from the outcrops of Enugu Shale and the very fine, clayey sandstone that forms the top unit of the Mamu Formation. Only four faults were identified in the Enugu shale. The relationship of these fractures with other structures like the bedding surface was also noted. In the laboratory, the orientation data were plotted using the stereographic projection technique and rose diagram. The results of the plots were analyzed and interpreted. Data presentation Fault data A total of four normal faults were studied in the area and all of them deformed only the Enugu Shale at different locations. Two of the faults occurred at the Enugu Shale exposure near the Onitsha road

5 Amogu et al. 105 flyover. One of the faults is striking 50 azimuth and dipping 52 with a dip direction of 320 (that is NW). This fault is shown by the displacement of the heterolithic unit of the Enugu Shale at this location. The sediment thickness on downthrown block is 11.4 m while the thickness on the upthrown block is 8.4 m, giving a growth index value of This fault is associated with small-scale rollover structure as shown by the thin fine sandstone unit in Figure 4a. The rollover geometry is typical of the hydrocarbon bearing structures in the subsurface Niger delta. The fault is sealing, implying that fluid migration across the fault is not possible hence, the potential reservoir, if there were any hydrocarbon accumulation, would have been compartmentalized. The peculiar features of this fault are attributes of a growth fault. A second fault (Figure 4b), at the same location with the fault in Figure 4a, is shown by the displacement of a silt stone unit. This fault is striking 315 azimuth, parallel to one of the joint sets in this location, and dips at 45 with a dip direction of 65 (that is NE). The two faults give the impression of a conjugate or antithetic fault pattern. The other two faults occurred at Akagbe Ugwu and at Ozalla junction. At Akagbe Ugwu the fault is inferred from angular difference between materials of the same formation (Figure 5b). One side of the affected unit is tilted at 15 while the other is nearly horizontal. Both are separated by a plane that is oriented 40 azimuth. The material on top of the plane dips into the plane at about 15 while the plane itself has a very low dip (> 12 ). The geometry of this structure gives the impression of intra-formation angular unconformity. However, it is interpreted as a normal, low angle fault formed due to slumping and rotation of the material at the right -hand side of the plane. The Ozalla fault is seen as an abrupt termination of a meter-thick siltstone bed against a shale unit (Plate 5c). The fault zone is oriented 85 azimuth and the dip is 50 in 165 azimuth (SSE direction). A close examination of the fault zone revealed that the shale has been sheared such that its lamina is bent down at the contact with the siltstone (Figure 5d). This implies that the block containing the siltstone (hanging wall) moved down relative to the footwall. The stereographic projections of these faults are shown in Figure 8. Figure 4b. A typical normal fault resulting from the abrupt termination of siltstone against shale. It strikes 315 o azimuth, parallel to one of the joint set in this locality, and dips at 45 o in 65 o direction (that is, NE). Joint data The joints presented in this work were studied at the heterolithic unit at the mid-section of the Enugu Shale exposure near the Onitsha Road flyover along the Enugu-Onitsha express road Figure 5a. Outcrop face of the Enugu Shale near Enugu- Onitsha flyover. Figure 5b. Outcrop of Enugu Shale at Akagbe Ugwu showing angular difference between the same units across a fault plane. Figure 4a. A typical growth fault at an Outcrop of Enugu Shale. Growth index is Observe the rollover geometry of the fine sandstone on both the hanging and footwall blocks. The fault strikes 50 o azimuth and dips at 52 o in 320 o direction (that is, NW). (Figure 6a) and at a fine sandstone unit at the top of the Mamu Formation (Figure 6b). Forty two (42) joints were measured at these

6 106 J. Geol. Min. Res. Figure 5c. Normal fault at Ozalla. Note the termination of the siltstone against Shale. Figure 6b. Vertical to steeply inclined Joints at the top unit of the Mamu formation. locations. The joints at the Enugu Shale form two joint sets which intersect to form a diagonal joint system while those at the Mamu Formation have a dominant NNW to NNE trending joints with length ranging up to 90 m. Another set of joints that are oriented WNW- ESE and ranging from m terminates on the joints with NNW to NNE trend. The orientation data of the joints are shown in Figures 9 and 10. Figure 5d. A close up on the fault zone. The shearing of the shale indicates that the siltstone moved down. Strikes 85 o, dips 50 in 175 direction. Figure 6a. Intersecting joint sets cutting through the heterolithic unit of the Enugu Shale. RESULTS AND INTERPRETATION Figure 8a shows an equal area projection of faults F 1 and F 2 at the Enugu Shale exposure near the Onitsha road flyover. The orientation of the two faults is such that they form a conjugate pattern on the projection. Considering the growth fault-like geometry of fault F 2, F 1 forms a counter regional or antithetic fault to F 2. This fault pattern is common in the subsurface Niger Delta. Analysis of this fault pattern suggests that the line of intersection i of the faults is oriented 10 azimuth and plunge 40, the acute bisector ac is oriented 276 azimuth and plunge 5 and the obtuse bisector ob is oriented 179 azimuth and plunge 52. Both the acute and the obtuse bisectors are contained by a plane that strikes 280 azimuth and dips 54 to the SW. From the figure above the obtuse angle between F 1 and F 2 is 122 while the acute angle is 58. Figure 8b shows the projection of the principal stresses that formed the faults. At the time of initiation of these faults the maximum principal stress σ 1 oriented 276 azimuth and plunge 5, intermediate stress σ 2 is oriented 10 o azimuth and plunge 40 and the minimum principal stress σ 3 is oriented 179 azimuth and plunge 52. This means that the maximum principal stress during the formation of the joint was aligned WNW and the minimum

7 Amogu et al. 107 Figure 7. Lithologic sections from the study area. principal stress was aligned south. Figure 8c is a schematic representation of the arrangements of the principal stresses during the formation of the conjugate fault. The projection of Akagbe Ugwu and Ozalla faults are shown in Figure 8d. These faults, though on the same formation, are kilometres apart and therefore never intersected in reality, despite the fact that they intersected on the projection. Consequently, these faults could not give reasonable paleostress information. The joint data are plotted on equal area projections. The data from the Enugu Shale exposure show two main concentrations (Figure 9), defining two joint sets The average orientation of the two sets are 315 /70 and 285 /80 ; both are dipping in the south western direction. The planes representing the two sets are plotted in Figure 9c as great circle and the density per 1% area is shown in Figure 9d. The joint system has diagonal intersect along a line i that is oriented 262 azimuth and plunges 70. The dihedral angle d, is the angle made between two lines a and b on the two planes that are perpendicular to the line of intersection i of the planes. The dihedral angle, which is equal to the angle between the two poles π 1 and π 2, is 40. This acute angle between the two joint sets J 1 and J 2 is bisected by a line ac which is the acute bisector of the planes. The acute bisector is oriented 128 azimuth and plunge 16. The

8 108 J. Geol. Min. Res. N F 1 F 2 i 276 o W Ac b 90 o 90 o E 1 Ob 2 a Up Down S Figure 8a. Conjugate or antithetic pattern F 1and F 2 at the Enugu Shale exposure near Onitsha flyover, Enugu. Ac = acute bisector of the faults, Ob is the Obtuse bisector, i is the intersection of the planes, π 1 and π 2 are poles to the fault planes, split circle show slip direction. N F 1 F 2 2 W 1 E 3 Figure 8b. The axes σ 1, σ 2 and σ 3 represent the directions of the principal stresses producing the conjugate shears, σ 1, plunge 3, azimuth 276 ; σ 2 plunge 40, azimuth 10 and σ 3 plunge 52, azimuth 179. S obtuse bisector line ob is oriented 35 azimuths and plunge 16. The joint data from the Mamu Formation also shows two concentrations which indicate a dominant NNW to NNE trend and a sub-ordinate NW trends that terminate on the former. The planes of the two sets are represented by 350 /170 strikes, 85 dip towards East and 290 /110 strike and 80 dip towards southwest. The projections of these planes are shown in Figure 8c. The

9 Amogu et al. 109 F 1 σ 1 F 2 58 o σ3 σ 2 Figure 8c. Schematic illustration of the principal stresses that formed the conjugate shear at the Enugu shale. N F 3 3 W 4 F 4 E S Figure 8d. Projection of Akagbe ugwu and Ozalla faults (F 3 and F 4 ) acute bisector of the planes is oriented 320, the obtuse bisector 48 and the acute bisector oriented 150. Analysis of the joint data from the two locations suggests a maximum principal stress that is aligned NW - SE and a minimum principal stress that is aligned NE. DISCUSSION AND CONCLUSION Outcrop studies of exposures of Enugu Shale and Mamu Formation revealed the presence of faults and joints as the tectonic deformation structures in the Enugu area of

10 110 J. Geol. Min. Res. a b North Statistics No. of Data = 15 Sector angle = 10 Scale: tick interval = 10% [3.9 data] Maximum = 46.2% [18 data] Mean Resultant dir'n = [95% Confidence interval = ±9 ] c Statistics No. of Data = 15 Mean Principal Orientation = 76/212 Mean Resultant dir'n = Mean Resultant length = 0.90 (Variance = 0.10) Calculated. girdle: 14/031 Calculated beta axis: d e f Figure 9. Graphical representation of joints from Enugu Shale. (a) Rose (frequency-azimuth) plot of Mamu Fm joints. (b) Joints plotted as points only. (c) Cyclographic great circle plots. (d) Plot contour and gridded density per 1% area. (e) The two dominant joint sets at the Enugu Shale exposure. π J1 and π J2 are poles to joint sets J1 and J2. i = line of intersection, d = dihedral angle, Ac and Ob are acute and obtuse bisectors of the acute and obtuse angles between the sets..(f) The joint data gave paleostress orientation values of 128 for σ1, 262 for σ 2 and 35 for σ 3. the Anambra basin. Structural analysis using projection techniques formed the basis for establishing the paleostress orientation and fault movement reconstruction in the area. Analysis of the faults shows a paleostress

11 Amogu et al. 111 a b North Statistics No. of Data = 23 Sector angle = 10 Scale: tick interval = 5% [2.3 data] Statistics No. of Data = 23 Mean Principal Orientation = 85/254 Mean Resultant dir'n = c d e f Figure 10. Graphical representation of joints from Mamu Formation. (a) Rose (frequencyazimuth) plot of Mamu Fm joints. (b) Joints plotted as points only. (c) Cyclographic great circle plots. (d) Plot contour and gridded density per 1% area. (e) The two dominant joint sets at the Mamu Formation exposure. π J1 and π J2 are poles to joint sets J1 and J2. i = line of intersection, d = dihedral angle, Ac and Ob are acute and obtuse bisectors of the acute and obtuse angles between the sets. (f) The joint data gave paleostress orientation values of 320 for σ 1, 150 for σ 2 and 48 for σ 3. orientation of 10, 276 and 179 for the maximum (σ 1 ), intermediate (σ 2 ) and minimum (σ 3 ) principal stresses respectively. Similar analysis for the joints from the two locations gave σ 1 of 128, σ 2 of 262 and σ 3 of 35 for the Enugu Shale and σ 1 of 320, σ 2 of 150 and σ 3 of 48 for the very fine sandstone unit of Mamu Formation. This implies a σ 1 aligned in the WNW direction and a σ 3 aligned NNE during the formation of the faults and a σ 1 aligned NW - SE and a σ 3 aligned NE during the formation of the joints. This suggests that the principal stresses rotated clock-wise sometime between the Late Campanian and the Middle

12 112 J. Geol. Min. Res. Figure 11. Relationship between Mohr circle and failure envelope at point I shows that joints will form parallel to the maximum principal stress. Figure 12. Correlation of litho logs from Enugu coal field. Displacement of coal seams probably indicates the presence of faults in the surbsurface (Borehole data is from De Swardt et al. (1963). Maastrichtian. The pattern of fault movements observed from the study area indicates an extensional tectonic regime in this part of the basin. That is to say that these joint are tensile fractures, and according to the relationship between the Mohr s circle and the failure envelop in Figure 9, joints are expected to form parallel to the maximum principal stress direction in the study area. The understanding of the stress directions will help in the prediction of the more permeable zones since fractures enhance permeability. The presence of these joints in the very fine sandstone unit of the Mamu Formation suggests

13 Amogu et al. 113 a greater reservoir potential for this unit especially, since the first hydrocarbon bearing Formation in the Anambra basin is the Mamu Formation as shown by Anambra River - 1 well. With respect to groundwater prospect in the area, the fractured heterolithic unit of the Enugu shale is a good prospect. The presence of the extensive fractures that range up to 100 m long, crossing the Enugu-Onitsha expressway constitute problems to the stability of the engineering structure. Fractures form good migration path for fluids especially, when they are not sealing. The presence of fluid reduces the frictional resistance along these weak zones thereby reducing the bearing capacity of the rock and eventually, resulting in the failure of the engineering structures in the affected areas. Correlation of borehole data within the Enugu coal field reveals relative displacement of some of the coal seams from well to well (Figure 12), thereby confirming the presence of these fractures at the subsurface. The orientation of these fractures when projected into the subsurface can be used to interpret joint pattern, extrapolate fracture distribution from borehole fracture data and to study stress concentrations in a fracture system. The ultimate benefit of this technique will be the understanding of permeable zones, fluid migration pattern and greater success in exploration and exploitation of groundwater, hydrocarbon, and even solid minerals in an area. De Swardt AMJ, Casey OP (1963). The coal resources of Nigeria. Bull. Geol. Surv. Nig., 28: 100. With supplement of 17 plates. Griffith AA (1921). The phenomena of rupture and flow in solid. Roy. Soc. (London) Philos. Trans. Ser., A 221: Maluski H, Coulon C, Popoff M, Baudin P (1995). 40Ar/39Ar Chronology, Petrology and geodynamic setting of Mesozoic to Early Cenozoic Magmatism from the Benue Trough, Nigeria. J. Geol. Soc. London, 152: Marrett R, Peacock DCP (1999). Strain and stress. J. Struct. Geol., 21: Murat RC (1972). Stratigraphy and Paleogoegraphy of the Cretaceous and Lower Tertiary in southern Nigeria. In: African geology (Edited bydessauvagie, T.F.J. and Whiteman, A.J.), Ibadan University Press Ibadan, Nigeri, pp Nwachukwu SO (1972). The tectonic evolution of the southern portion of the Benue trough. Geol. Mag., 109: Ofomata GEK (1973). Aspects of geomorphology of the Nsukka- Okigwe cuesta, East central state of Nigeria. Bull., 35: IFAN. Ojoh KA (1992). The southern part of the Benue Trough (Nigeria) Cretaceous stratigraphy, basin analysis, paleogeography, and geodynamic evolution in the equatorial domain of the South Atlantic. NAPE Bull., 7(2): Petters SW (1978). Stratigraphic evolution of the Benue Trough and its implications for the Upper Cretaceous paleogeography of West Africa. J. Geol., 86: Pollard DD, Saltzer SD, Rubin AM, (1993). Stress inversion methods: Are they based on faulty assumptions? J. Struct. Geol., 15: Ramsey JG, Huber MI (1983). The techniques of modern structural geology. Strain Anal., Academic Press Inc., London, 1: 391. Sibson RH (1977). Fault Rocks and Fault Mechanism. J. Geol. Soc. London, 133: Whiteman, A. J., Nigeria: Its Petroleum Geology Resources and Potential: Graham and Trotman, London, 1(1): 350. ACKNOWLEDGEMENTS We appreciate the input of Prof. K.O Uma at the conception stage of this study. We are also grateful to the Holcombe Coughlin Associates, Australia for providing the GEOrient 9.2 which we used for some of the orientation plots. REFERENCES Allix P (1987). Le bassin d Anambra essande caractérisation de l évolution tectono-sedimentaire au crétacé supérieur. Bull. Centres Rech. Explor-Prod Elf-Aquitane, 111: Benkhelil J (1982). Benue Trough and Benue Chain. Geol. Mag., 119: Coulomb CA (1773). Sur une application des regles de maxims et minimis a quelqus problemes de stratique relatifs a l architectie. Acad. Roy. Sci. Memde. Math. Phys., 7: