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A simple 3D visualisation of joints in a Migmatized Gneiss, AgoIwoye NE, SW Nigeria.

1. INTRODUCTION

Joints are extension fractures that show very small displacement normal to their surfaces and no, very little, displacement parallel to their surfaces [13]. The study of Joints in area is important to underground water development and engineering works; they provide information on fracture porosity/permeability useful for hydraulic hydrologic modelling, failure plane (slope stability, dam stability, tunnel stability, strength anisotropy), mass wasting surface, mineralization, important geomorphic control, trellis drainage, lineaments, hydrocarbon migration, and interpretation of paleostress system [10].

One of the most common means of studying joints includes the determination of their preferred orientation which is represented as their attitude (Strike and Dip). Because the joints surfaces are curved, twisted and uneven, the contact methods of measuring strike is inappropriate to provide information on their orientation otherwise the measurement is taken using the former with the assumption that the joint surface is straight. Because joints show no discernable displacement, the measurement of the dip is often overlooked by most researchers; it has been a major challenge for students of structural geology to establish such attitude on the field; not only is this, professionals alike (especially in the academic) rapidly underestimates them. A good estimation of the dip angle requires a skill of observing the structure from different angles/percept, and also the ability to constrain the Geology of the area in 3D. The thrust of this paper is to present a simple way of visualising joints in 3dimensions for accurate estimation of dip angle and intersection geometries at depth during geological field mapping exercise.

1.1 PREVIOUS RELATED WORK

Previous observation of geological structures in 3D relied on quantitative structural analysis of geologic surfaces using stereoscopic remote sensing imagery [3]. They used stereoscopic methods with satellite imagery to obtain surface bedding attitude measurements. The data were used to constrain geologic structures in three dimensions using quantitative structural model, the approach involved numerical method for measuring the strike and dip of bedding using stereoscopic pairs of air photos, as well as Landsat Thematic MapperTM and SPOTTM images. The measurement from the North American and Andean cordilleras, proved consistent with direct surface control and subsurface structures imaged in seismic reflection profiles.

The above method allows the calculation of the strike and dip of bedding, which can be used to supplement field mapping [1]. Fernandez et al. [6] also generated 3D reconstructions of geologic surfaces by integrating geologic mapping and field data.

Damien Dhont et. al [5] used surface information from published geologic maps, remote sensing data, and a digital elevation model (DEM) to produce 3D Geologic Maps of the Beirut watershed (Lebanon). Their work presented a way to produce mathematically and geometrically correct three-dimensional (3D) geologic maps consisting of the volume and shape of all geologic features of a given area based on a modeling algorithm that only uses surfaces calculated from scattered data points and that intersects them following a series of geologically sound rules.

Omosanya et al [10] also studied joints in a Migmatized-Gneiss at Ago-Sunmonu, SW Nigeria, they measured the length and attitude (Strike and Dip) of forty five (45) joints on the outcrop and forty-three (43) others in surrounding exposures in order to understand their origin, architecture and regional connection. Their work identified branching, Y-shaped, gradual dying out, T-joint, and conjugate tip and intersection geometries and was able to classify the joints as extensional and shear joints produced by dominant NW-SE, NE-SW, and E-W tectonic forces. A shortcoming of the work was that there was no attention 3D orientation of joints which could have enhanced the classification of the joints.

This work provides a simple and layman approach of estimating the intersection geometries of joints and their 3D orientation in space. It is simple in the sense that, it only requires measurements to be taken on the field and the results plotted or drawn on the field and later digitized using any graphic package; in verifying the integrity of the data collected on field, this technique provides a platform upon which further software models can be built, and subsequently probes the data collected on the field with a view to predicting how well they will perform on softwares. It is different from previous work because it presents a simplified approach upon which the other sophisticated techniques can be established.

1.2 GEOLOGIC SETTING.

Nigeria is cover in equal proportion by both crystalline and sedimentary rock. The crystalline rocks are distributed a circular area in the North central, a triangular area in the west which runs into the Benin republic and a rectangular area broken into three parts by sedimentary rocks on the eastern border of Nigeria with Cameroun Republic. The crystalline rocks are divided into three main groups: the Basement complex, the younger Granites, and Tertiary- Recent Volcanics. The sedimentary rocks are distributed over eleven (11) sedimentary basin. The lower Benue, Gongola Basin, Middle Benue Basin, Upper Benue, The Yola Basin, The Anambra Basin, The Dahomey Basin, The Nigerian Chad Basin, The Bida Basin, The sokoto Basin and the Niger Delta Basin. The sokoto and Chad basin are part of the Illumenden and Taudenni Basins respectively in central Africa. Most of the remaining sedimentary basins appear to have been initiated in the cretaceous and are related to the Gulf of Guinea, and Mesozoic separation of South America and Africa plates.

The Basement complex is exposed over half of the entire country; it extends in the west into the Benin (Dahomey) basin and in the East into the Cameroon Republic. In the central part the basement has been intruded by high level granites and porphyries of Jurassic age as ring-dykes. Tertiary volcanic rocks, mostly basalt, cover small areas of the Basement complex mostly in the North-East. In all, nearly 90% of the crystalline rocks exposed in Nigeria belong to the Basement complex. The Nigerian Basement complex is believed to be Precambrian in Age. However, it probably contains a number of intrusive of Palaeozoic age.

The outcrop of the Migmatised Gneiss of the study area belongs to the Migmatised Gneiss complex of the Basement complex in Southwestern Nigeria; this rock unit is polycyclic in nature. The Migmatised Gneiss Complex polycyclic events are due to the Liberian, Eburnean, Kibaran and Pan-African orogenies, which modified the Precambrian geology of Nigeria. This rock unit is characterised by grey foliated Biotite acid/Biotite Hornblende quartz feldspathic gneiss of tonalitic to granodioritic composition [12];Mafic to ultramafic component which outcrops as discontinuous boudinaged lenses or concordant sheet of amplibolites with minor amount of biotite-rich ultramafite; and Felsic component, a varied group comprised of pegmatite, aplite quartz-oligoclase veins, fine-grained granite gneiss, and porphyritic granite.

Grant [7], Grant et al. [8] Oversby [11] assigned Archean to Palaeoproterozoic ages (2700-2000 Ma) to the MGC in Nigeria. Burke et al (1976) were of the opinion that the metamorphic rocks of the lbadan area were emplaced before 2000 Ma. On the basis of Rb-Sr dating [7] suggested that the lbadan granite gneiss of the MGC was emplaced around 2205 + 70 Ma and that the metamorphic deformation age of about 500 Ma corresponds to the waning stage of the Pan-African tectonic event while Rahaman [12] using U-Pb zircon ages gave 2500 + 200 Ma for the lbadan grey granite gneiss and 1875 Ma to the pink granite gneiss at Ile-lfe.

2. MATERIALS AND METHODS

The joints were systematically mapped on the outcrop, during this exercise it was discovered that the joints were located on the southern western part of the outcrop. This informed the creation of a grid of 12 x 12m at the south western part of the outcrop, the grid is located within Latitude 06056133.911- 06056134.211N and longitude 003054103.211- 003054103.511E; the grid was further divided into nine (9) smaller grid of 4x4m with grid 1 & 2 having the highest population of joints.

In mapping the joints, outcrop data were collected along joints surfaces which include their attitude (Strike & Dip) was measured using a silva compass clinometers, a Garmin GPS was used to establish a (x,y) coordinates over each of the joints i.e. x represents the perceived starting point while y is the limit of the joints (NB: the y coordinate in this distance does not represent the usual vertical direction while z was determined from the altitude read by the GPS). The length of the joints were measured across joints where the tip geometry is straight; measuring the (x,y) coordinates was not possible for most of the joints and as such just a single coordinates of the midpoints was taken for joints in this category. In measuring the orientation, the surfaces of the joints were assumed to be straight, the results obtained from the contact method; which is the most commonest technique of measuring strike and dip along surfaces, a technique that assumed that the surface being measured is smooth were compared with standing/kneeling method of measuring strike and dip with the measurement taken in the same manner as bedding [2].

The contact method is done in such a way that the edge of the compass clinometers is placed on the surface of the joints whilst the compass is held horizontal and parallel to the strike, the value of the strike is read by rotating clinometers till it reads zero dip while the standing/kneeling down method entails standing over the surface with the compass opened and held parallel at waist height; this technique is most suitable on large uneven planes of relatively low dip while using this method, the azimuth technique of reading the strike was adopted which involved the non-rotation of the clinometers but the reading of the value of the compass and subtracting it from 3600.

The joints were described by assigning them letter J and the number described their position from Grid 1-3, 4-6, & 7-9. The average perpendicular distance was measured by taking three (3) spacing a, b, c between the joints and finding the average, this was done in order to establish the different joints sets present in the outcrop.

[FIGURE 1 OMITTED]

After establishing the sub-grids, a sketch geological map of the entire grid was drawn on the field in order to understand the relationship of the joints to other structures on the outcrop; this exercise was also done for the most joint-populated grid. The joints relationship was established and their 3D relationship shown by drawing their orientation and attitude as egg within two beds, sometimes enclosed in a cube using Corel draw 8. In visualising the joints in 3D our bed-egg-cube model, this entails the assumption of the joints surface to be a egg, like normal faults the displacement along the joints surface is assumed to be maximum at the centre and 0 at the tips. Based on the angle of the dip, the egg tip (North) is projected under a flat surface (the Top bed), and beneath another bed (Base Bed), such that the egg is sandwiched between the two beds. When projecting the angle below the top bed, the egg is made to trend along strike and inclined by the amount of dip.

3. RESULTS AND DISCUSSION

The joints are restricted to Grid 1, 2, 3, 4, & 5; generally they can be described as oblique joints because they run oblique to the strike of the outcrop. The joints range in length from 24 cm to 290 cm with Joint 19 being the longest and J3 the shortest (Fig.5). The average perpendicular distance between the joints was determined in order to classify them as systematic or non-systematic joints. Because the measurement of such distance is prone to error as a result of the curved and non-linear nature of the surfaces, error in the premises of + 3cm was entertained, with this, J9 & J8, J11 & J12, J16 & J18, and J20 & J21 were categorised as systematic joints with average perpendicular distance of ~62cm between them others are non-systematic joints (Table. 1). Apart from the J1 that is characterised by curved tip geometry others generally dies out at their tips. J5 represents an example of non-linear joints that propagates in a zigzag manner at the termination J9, J10, and J11.

Using their intersection geometry, all the joints intersected virtually all every other structure on the outcrop, the lineation, foliation and veins strike North westerly (Fig. 1 & 2) while the joints are striking in E-W direction (fig. 3) and dips North easterly, they are therefore oblique joints produced by NW-SE tectonic stresses; the implication for timing is that the joints are younger than other structures on the outcrop. The veins are evidence of previous fracturing and were produced during an extension regime with subsequent mineral precipitation, the intrusion give credence to the last magmatic event experienced in the outcrop and precursory extensional deformation, the other intrusions labelled I1 & I2, this episode may have been succeeded by shearing of the rock as shown by the twisting of the tension gashes in a right lateral manner, the tension gashes are extension fractures that are mineral filled, that form en echelon sets along zones of ductile shear, their orientation or twisting are important tool in deciphering the direction of shearing; the badinages (fig. 1) are typical pinch and swell type are evidence of lengthening of the rock. The last tectonic events may be extension which is evinced by orientation of the joints seen in the outcrop. By using the principle of crosscutting relationship "the cause of disruption is younger than the disrupted pattern", the joints could be said to be younger than every other structure found with the grid.

The sketch map of the entire grip shows that the mid grip is cut into two halves by two (2) Quartzo-feldspathic (I1 & I2) intrusions which were surrounded by joints oriented obliquely to them. These intrusions are separated by a zone that is characterised by J5 and some other minor veins that resemble tension gashes.

[FIGURE 2 OMITTED]

In demonstrating the Bed-Egg-Cube technique, the J1 & J2 were projected on a surface (datum of 35m at the base of the outcrop) and the egg were produced in such a manner that the maximum displacement was recorded at the centre and 0 at the tips, the results prove effective for this joints set, with the two layers at the top and base of the two eggs, and the egg oriented at an angle of 200 E, the picture shows that the joints will not intersect at depth and that they are parallel joints. Whenever it becomes difficult to picture it into the sub surface, the orientation of the egg should be projected above the top bed and this picture will become clearer in 3D. This technique proves ineffective when the tip geometry of the joints is considered, an attempt to make the tip of J1 curved (Fig 6.ii) shows that the egg become distorted and it bend towards and may intersect J2 at depth. In applying the method to other sets J11, J13 & J14 and also J19, J20 & J21 (fig.6), it was easier to see the joints in three (3) dimensions and effectively inferred their intersection geometries as being null at depth.

When the zigzag orientation of J5 is considered the egg becomes totally distorted but with J9, J10 and J11 nearly perpendicular to J5, the egg and 0 tip geometry shows that this systematic joints do not intersect with J5 at any point (fig 6.iii). Overall, the projection of the grid 2 (fig. 7.) using this technique shows that most of the joints abuts near the intrusion 2 (I2) and do not intersects with one another with an unequal but parallel distance between themselves.

[FIGURE 3 OMITTED]

The economic implication of these geometries as observed from the bed-eggcube model is that the joints do not intersect at any point in the sub-surface and as such no significant input to the fluid conductivity (permeability) through the joints; where present Intersection geometries may entail cross Joints and T- joints which will aid the free flow of fluid through the joints. Because they are weakness zone, they should be given priority during engineering and construction works.

[FIGURE 4 OMITTED]

[FIGURE 5 OMITTED]

[FIGURE 6 OMITTED]

[FIGURE 7 OMITTED]

4. CONCLUSION

It is quite simple to picture a structure in 3 dimension insofar a bed-egg-cube technique can be applied, this technique will assist student in training and greatly influence the judgments of professionals, except otherwise present the surfaces markings of a joints can provide the needed data to determine the mode and direction of propagation of a joint. This technique is elementary and it is intended to assist geologists in training, it does not override the other techniques of DEM and satellite imagery mapping; the estimation of the orientation or dip angle of the joints is error prone. Other problems inherent is the ability to think about the subsurface, dying out geometries provides an estimation of the maximum displacement at the centre but with other geometries it is quite difficult to estimate the maximum displacement at the centre except where the limit of the joint can be determined. Other than these, joints are mapped in outcrop or rock exposures, it is not clear whereby they are deep seated like faults or are restricted to the surface exposure.

The bed-egg-cube model effectively shows the 3 dimension characteristics of the joints in the study area; it has accounted for their intersection geometries at depth. This technique of visualising in 3D can also be applied to the study of veins, faults, and intrusions with the premonition that background knowledge of the behaviour of these structures will greatly enhance the mapping exercise, and collection of data for use on different modelling software. Though the method found application in the visualisation of parallel joints, there is need to test its efficiency with different joints types (oblique, Cross Joints etc), tip and intersection geometries, it has provided the impetus for investigating/testing this method on other joints and planar geological structures. An improvement/advancement of this research will involve the projection of the joints surfaces and drawing of structural cross sections across the area of interest using the third factor depth/height(z) recorded at the (x,y) points.

CITATION

Omosanya, K.O, M.Sc Structural Geology with Geophysics (Leeds), B.Sc Geology (OOU), a Lecturer in the Department of Earth Sciences, Olabisi Onabanjo University. He is a Structural Geologist, Basin analyst, and a Seismic interpreter, His research interests include field and/ laboratory-based project with emphasis on structural studies of sedimentary, metamorphic, igneous rocks and geological structures.[1]

Akinmosin Adewale. A, is a Lecturer at the University of Lagos, Nigeria. He is a registered member of many professional associations among which are the Nigerian Council of Mining Engineering (NMGS) and Geosciences and Council of Mining Engineers and Geoscientists (COMEG). He holds a Ph.D. degree in environmental geology from University of Ibadan, Nigeria. His research area is in the geology of tar sands [2]

Adio, N. A, B. Sc Geology (OOU), a graduate of Geology from Olabisi Onabanjo University. He is currently studying for his postgraduate degree at School of Engineering, Faculty of Physical Sciences, The University of Aberdeen, United Kingdom [3]

Omosanya, H.O, Geology (OOU), a graduate of Geology from Olabisi Onabanjo University. She is currently working with a geotechnical firm in Nigeria. [4]

Akinbodewa A.E, A geologist in training, she is j currently working on the structural framework of rocks in Ago-Iwoye, NE, SW Nigeria.: Evidence from Outcrop and Satellite Imageries [5]

Lawal, M.A, A geologist in training, currently working on the structural framework of rocks in Ago-Iwoye, NE, SW Nigeria [6].

ACKNOWLEDGEMENT

We appreciate the assistance of the department of Earth Sciences, Olabisi Onabanjo University, Falana Louis and Ogunleye Dolapo in the initial reconnaissance survey.

REFERENCES

[1.] Banerjee. S and Mitra. S, "Remote Surface Mapping using orthophotos and geologic maps draped over digital elevation models: Application to the Sheep Mountain anticline, Wyoming" AAPG Bulletin, September 1, 2004; 88(9): 1227-1237

[2.] Barnes, J.W. & Lisle, R.W, Basic Geological Mapping, fourth Edition, John Wiley and Sons Limited, United Kingdom 2008.

[3.] Bilotti., F, John;, H.S and Peter, A. Brennan., "Quantitative Structural Analysis with stereoscopic Analysis with stereographic Remote Sensing Imagery" American Association of Petroleum Geologist; 2000, V.84; No.6; P 727-740

[4.] Burke, K., Freeth, S. J. and Grant, N. K. "The structure and sequence of geological events in the basement complex of the lbadan area, western Nigeria". Precambrian Research 1976; 3, 537-545.

[5.] Damien Dhont, Pascal Luxey, and Jean Chorowicz; "3-D Modelling Of Geologic Maps From Surface Data". AAPG Bulletin; November 2006; V 89; No. 11. 1465-1474.

[6.] Fernandez Bellon, O. "Reconstruction of geological structures in 3D. An example from the Southern Pyrenees. PhD Dissertation, Universitat de Barcelona, Barcelona", 2004, 377pp

[7.] Grant, N. K. "Geochronology of the Precambrian basement rocks from lbadan southwestern Nigeria". Earth Planetary Science Letters 1970, 10, 29-38.

[8.] Grant, N. K., Hickman, M., Burkhotder, F. Ft. and Powell, J. L. "Kibaran metamorphic belt in Pan-African domain of West Africa". Nature Physical Science 1972, 238, 90-91.

[9.] Jones, H. A. and Hockey, R. D "The Geology of part of south-western Nigeria". Bulletin Geological Survey,Nigeria 1964, 31, 101 p.

[10.] Omosanya, K., O. Ajibade, and A. Akintola.. "Architecture of Joints at Ago-Sunmonu, South Western Nigeria". Pacific Journal of Science and Technology. 2010, 11(2):625-635.

[11.] Oversby, V. M. "Lead isotopic studies from the Precambrian basement near Ibadan, south-western Nigeria". Earth Planetary Science Lerrers 1975, 27, 177-l 80.

[12.] Rahaman, M.A " Recent advances in the study of the basement complex of Nigeria". First symposium on the Precambrian Geology of Nigeria, Summary 1981.

[13.] Twiss, R.J and Moores, E.M., "Structural Geology" 2nd edition, Freeman and Company Newyork, 2007,364.

Omosanya, K. O (1) *, Akimosin, A (2), Adio, N. A (3), Omosanya, H. O (1), Akinbodewa A. E (1),and Lawal, M. A (1)

(1) Department of Earth Sciences, Olabisi Onabanjo University, Ago-Iwoye. (2) Geosciences Department, University of Lagos, Akoka, Lagos (3) School of Engineering, Faculty of Physical Sciences, University of Aberdeen, United Kingdom

kamaloomosanya@yahoo.com, waleakinmosin2001@yahoo.com, fathernas@yahoo.com, htomosanya@yahoo.com, ennyakins@yahoo.com, lawal2b@yahoo.com.
Table 1: Average perpendicular
distance between the joints

S/N   Related Joints    A     B     C       Average
                                         Perpendicular
                                             Dist.

1        J1 & J2       102   101   101        101
2        J4 & J6        6     7    7.2         7
3        J8 & J5       38    39    39         39
4        J9 & J8       58    62    70         63
5        J9 &J10       37    34    31         34
6       J10 & J11      23    26    26         25
7       J11 & J12      62    57    63         61
8       J12 & J15      128   117   118        121
9       J16 & J17      50    51    51         51
10      J16 & J18      65    66    65         65
11      J19 & J20      38    35    36         36
12      J20 & J21      60    60    60         60
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Author:Omosanya, K.O.; Akimosin, A.; Adio, N.A.; Omosanya, H.O.; Akinbodewa, A.E.; Lawal, M.A.
Publication:International Journal of Emerging Sciences
Article Type:Report
Geographic Code:6NIGR
Date:Sep 1, 2011
Words:3790
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