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Reconnaissance Study of the Relationship Between Lineaments
and Fractures in the Southwest Portion of the Lake Chad Basin

By
Solomon A. Isiorho and Tom O. Nkereuwem

ABSTRACT

One of the problems facing drought-stricken parts of the world is the location of potable sources of water. The delineation, verification, and study of the effect of fractures on groundwater resources and movement are essential parts of the hydrologic data set in drought-prone regions. In this reconnaissance study prominent lineaments were mapped from Landsat images of the Lake Chad region. A Wenner array resistivity profile 14 kilometers in length was made perpendicular to a lineament identified on Landsat images. The profile shows a segment with significantly lower resistivity values at the position of the lineament. Low total dissolved solids (TDS) values were also recorded for shallow groundwater samples near the lineament, suggesting it is a recharge zone. Prominent Landsat lineaments in this area are thus probably fractures, as suggested by the resistivity profile and TDS values, and should be investigated through more detailed geophysical and hydrogeological surveys.

Introduction

Locating a potable source of water in arid and semiarid regions requires a knowledge of the local hydrology. The Sahel region of Africa has experienced many droughts including those in the 1960s and 1980s. The shortage of potable water in this region is even more serious with the southward advance of the Sahara Desert. Located to the south of the Sahel region is Lake Chad. Slightly larger than Lake Erie, Lake Chad on occasions has completely, or almost completely, dried as evident from prehistoric sand dunes found on the lake bottom (Isiorho, 1989).

Lake Chad is located in a semiarid region with a low annual rainfall (-30 cm) and a high evaporation rate (-2 in/ year; Roche, 1980; Carmouze, 1983). These facts, coupled with the proximity of the Sahara desert, cause the lake to shrink during periods of low annual rainfall in the active watershed south of the lake. Lake Chad is the main source of water in the region, both at the surface and through recharge of the underlying aquifer through seepage through the lake bed (Isiorho and Matisoff, 1990).

Structural features around Lake Chad have been mapped by geophysical methods (Cratchley, 1960; Cratchley et al., 1984). Some of these structures, although covered by the thick sediments (-549 in) of the Quaternary Chad Formation, may be visible at the land surface (Durand, 1982; Isiorho et al., 199 1). Casual observation of Landsat and shuttle images indicate the presence of linear features. Is there any relationship between these linear features and known structures? Could these linear features affect water movement between the lake and the surrounding aquifers or provide zones of enhanced infiltration? This reconnaissance study is an attempt to answer these questions for the southwest portion of the Lake Chad Basin.

Study Area

The study area encompasses the southwestern portion of the Lake Chad Basin (fig. 1) where annual rainfall is approximately 0.31 in (Eugster and Maglione, 1979; Jaekel, 1984). The average annual rainfall for the past seven years at New Marte within the study area was 0.43 in . The annual high temperature is 30-45C with relative humidity averaging 30%. The southwestern portion of the Lake Chad Basin is a plain that slopes gently towards Lake Chad. It is devoid of rock outcrops and is covered by superficial deposits of sand and clay. All surface drainage is towards the lake.

The climate of the region is semiarid with two seasons: a long dry season from October to April when a dry dusty wind blows off the Sahara desert with daytime temperatures of 30-36C (85-98 F) and nighttime temperatures of 5-11C (40-50F), and a short wet season, from about May to September with daily maximum temperature of 34C (90F) and relative humidity of about 40-70 %. The vegetation in the study area can be described as Savannah woodland which is divided into two zones: Sudan Savannah to the south and Sahel Savannah towards the north.

Geologic History

The Lake Chad Basin is located in a tectonically active area with structural features that extend northwest to the Air Plateau and southwest towards the Benue trough, a failed arm of a triple junction (Burke, 1976; Ajayi and Ajakaiye, 1981). The locations of present day rivers (Lagone, Chari and Kamadogu) and earlier streams are controlled by structural features (Durand, 1982). The western shoreline of the present day lake, and the direction of the perilacustrine ridge, correspond to known structural features (Durand, 1982). A number of positive Bouguer gravity anomalies correspond to neotectonic lineaments near Lake Chad, and the structural history of this area is extremely complex (Cratchley et al., 1984; Avbovbo et al., 1986). In general, the surface morphology is a direct reflection of subsurface structures.

Figure 1. Map showing the study area in the Lake Chad Basin.

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The Lake Chad Basin was formed by extensional tectonic forces during the Cretaceous Period (Burke, 1976). Located within the basin is the Quaternary Chad Formation, which is the youngest geologic formation that contains aquifers. The Chad Formation is of major concern in this study because it is hydraulically connected with Lake Chad. In the southwest part of the basin, the Chad Formation is composed of three aquifers referred to as the Upper, Middle and Lower Aquifers. Although buried in the study area, the Chad Formation is composed of argillaceous (fine grained) sequences with arenaceous (coarse grained) horizons. A generalized section of the Chad Formation is shown in fig. 2. The upper aquifer of the Chad Formation, made up of fine grained sediments approximately 30 m thick, is hydraulically connected to Lake Chad (Carmouze, 1983; Isiorho and Matisoff, 1990), and is separated from the underlying middle aquifer by approximately 100 m of clay-rich sediment.

The Chad Formation is overlain by aeolian sands, fluvial, deltaic and lacustrine deposits approximately 1 to 6 m thick. Most of the fluvial deposits occur along stream valleys which are made up of two units: the old alluvium and the young alluvium (Hammand and Abdou, 1982). The old alluvium consists of deposits of old rivers, while the young alluvium contains recent river beds and flood plains. Field observations indicate the presence of silt and clay-sized sediments approximately 0.6 m thick in some places. Along the New Marte-Kirenowa road, the silt-clay sediment overlying the Chad Formation may be at least 1-1.5 m thick. Figure 3 shows the overlying sediments. The detailed geology of the area is described by Raebum and Jones (1934), Barber and Jones (1960), Carter et al., (1963) and Avbovbo et al., (1986).

Objectives

The objectives of this study are to verify that some lineaments mapped from Landsat images southwest of Lake Chad are fractures, or fracture zones, and to investigate the effect of these lineaments on groundwater quality within the area. This paper describes how electrical resistivity may be used to accomplish these goals and to determine the depth to groundwater immediately southwest of Lake Chad. This reconnaissance survey is intended to provide a basis for planning future detailed surveys in this area, for the purpose of characterizing groundwater resources.

Methods

Lineament Identification

Identification of lineaments has proven useful in hydrologic studies especially where large areas are involved (Salomonson, 1983; Isiorho, 1988) and many lineaments are believed to be directly related to tectonic activity or tectonic features (Rowan and Lathran, 1980). "Lineaments are naturally occurring alignments of soil, topography, stream channels, vegetation, or a combination of these features that are visible on remotely sensed imagery and aerial photographs. The main assumption inherent in performing any lineament analysis is that these alignments represent fracture zones or other discontinuities" (Mabee et al., 1994). In this study Landsat and shuttle images were analyzed and lineaments were delineated on acetate film. Several Landsat photographic images and transparencies covering the same area acquired at different months during the year (1987) and during different years (1972, 1975, 1976, 1985, 1986, 1987) were used in delineating the lineaments. The images were studied using standard photogeologic techniques under transmitted light and reflected light from a light table as described in Isiorho (1985). Lineaments were drawn on acetate film placed over the images. Prevalent lineament orientations were noted and compared to known geologic structural features in the area. Only prominent lineaments (lineaments at least 20 km long that

were clearly visible on all images of the same area were recorded. Some of the images were reexamined some months later to evaluate the observers’ ability to reproduce lineaments at the same geographic location. New lineament maps were made and were compared with the first set and the corresponding lineaments were noted. These prominent lineaments were thus verified by their multiple rendition in several images and at different times. One of these prominent lineaments was then targeted for field verification.

Figure 2. Generalized stratigraphic column of the Chad Formation.

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Figure 3. Photo showing the 1-1.5m clay/silt sized sediments overlying the Chad Formation along the New Marte - Kirenowa road.

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Resistivity Profiling

One prominent lineament was selected for a resistivity profile based on its accessibility. The purpose was to verify that a fracture actually existed where identified from the Landsat images. Such a fracture (or fracture zone) should exhibit an anomalously low resistivity due to the presence of a shallow water table or elevated soil moistures. Over four hundred resistivity measurements were made along a 14 km profile along the Kirenowa-New Marte road which runs nearly perpendicular to the inferred lineament. The Wenner array was adopted (instead of the Schlumberger array) because it involved moving only one probe at a time after each reading. The electrode spacing used at all stations was 30 m, as dictated by the available wire length and the available time for profiling. Electrical soundings were also performed at two locations along the profile, and ten soundings at other places to determine the depth to groundwater. The resistivity soundings were then analyzed using a DC resistivity computer inversion program. The presence of Earthwatch volunteers made it possible to collect a large number of resistivity data points within a two-week period.

Figure 4. Landsat mosaic image of the southwest Chad Basin.

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Figure 5. Map showing the prominent lineaments visible on all the Landsat images, the resistivity profile (indicated as a broken line), and the TDS values of some groundwater and lake water samples.

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Water Sampling

Seventy-five water samples were collected from the lake and open wells (dug wells with diameters of 0.5 to 1.5 meter) for laboratory analysis. Many of these wells occur far away from the Kirenowa - New Marte profile. The depth to water in some open wells were determined by electric probe. Only water table samples with depths less than fifty meters were used in this study. The water samples were collected either in the mornings or evenings when the villagers collected (fetched) water. The water samples were collected in 250 ml plastic bottles, filtered, and acidified. The water samples were then analyzed for major ions, pH, acidity, alkalinity, total dissolved solids, dissolved oxygen, carbonate and sulfate using portable test kits and field instruments (e.g. Hach DR/2000 Spectrophotometer).

Figure 6. Photo showing flat terrain along the profile line.

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Results and Discussion

Lineaments

Analysis of Landsat images indicate a NE-SW trend to the majority of lineaments. Figure 4 shows a part of the Landsat mosaic image from May, 1985 over the southwest Lake Chad Basin where some of the most prominent lineaments occur. Figure 5 shows the prominent lineaments that were identified on all of the Landsat images. Approximately 50% of the 30 most prominent lineaments were observed in all images. Most of the smaller lineaments, close to the lake, have NE-SW to E-W trends. These lineaments appear to be related to sand dunes based on limited field observation. With the exception of a few with a N-S trend, smaller lineaments are generally oriented in the same direction as the prominent lineaments. The sinuous parallel lineaments, especially northwest of Baga, are old lake shorelines. A prominent lineament starts northwest of Baga with a NNW-SSE trend, and assumes an E-W trend -lineament south of Kirenowa; is the longest such feature (-240 km) in the study area. This lineament is labeled Z in fig. 5.

Resistivity Profiling

The Kirenowa New Marte transect was chosen for the study because it was easily accessible through a road that cuts across the lineament. The profile is approximately 14 kilometers long. Several parts of the profile were repeated at different times and days to verify the stability of previous readings. The apparent resistivity readings in ohm-m were plotted with distance without correcting for the topography (elevation) or depth to groundwater levels. Correction was deemed unnecessary because only the general resistivity trend was sought and the topography of the area is relatively flat with a relatively uniform overburden thickness of approximately 1-1.5 in. This estimate is based on

Figure 7. Interpretation of a resistivity sounding station about 2km south of Kirenowa. (The Figure depth to water in a well 5m from sounding station = 24.1m)

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Figure 8. Map showing the water table for the south-west Lake Chad Basin (well=*; resistivity sounding +, resistivity sounding station shown on fig. 7+).

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soil profile exposures observed within the Kirenowa - New Marte canal which also lies along the transect. The relative flatness of the study area is exhibited in fig. 6, with two people at extreme ends of the line approximately 90 m apart. The depth to groundwater increases southwards from approximately 15 m near Kirenowa to 30 m near New Marte. Figure 7 is a Wenner array sounding curve made approximately 2 kilometers south of Kirenowa. The depth to groundwater from the interpretation was 25.0 m, close to the 24.01m measured from a nearby well approximately 5 m from the sounding station. Available lithology well logs of the Chad formation within the study area are similar to the generalized log shown in fig. 2. The spatial distribution of wells, as well as resistivity sounding locations are shown in fig. 8. This figure also shows the groundwater table map of southwest Lake Chad Basin. The general direction of groundwater movement is southwestwards away from Lake Chad (Isiorho and Matisoff, 1990).
Figure 9a. Resistivity profile along the New Marte-Kirenowa road. The lowest resistivity readings correspond toe the lineament (fracture zone) position.

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Figure 9 is the resistivity profile along Kirenowa - New Marte road. The resistivity reading was not uniform from point-to-point, but there was a general reduction in the resistivity readings until about midway between New Marte and Kirenowa where the lowest resistivity readings were obtained in the profile. Initially, change in the personnel was thought to be the reason for the varied readings and as a result some of these areas were profiled again with new personnel, and in a few cases, a different meter was used. Most of the low resistivity values in the 9-12 km segment were below 10 ohm-m (for example, 6.0, 6.0, 2.0, 0. 1, 0. 1, and 4 ohm-m were some of the low resistivity readings recorded) with 0.09 ohm-m being the lowest apparent resistivity reading. Some areas of low apparent resistivity readings were visited the same day or at a later time using the same resistivity meter or another meter. No significant differences in apparent resistivity were recorded.

The segment (approximately 3 km wide between 9 and 12 km) with lowest apparent resistivity corresponds to the inferred prominent lineament from the Landsat images. Figure 9 b is a "blow up" of the area corresponding to the lineament from fig. 9 a. The low resistivity readings are probably due to the presence of shallow moisture in the subsurface materials. The moisture hypothesis is further corroborated by the presence of a high density of trees/shrubs relative to areas outside the fracture zone. In several of the Landsat images, the lineament has a dark tone due to the presence of vegetation. The fracture zone is estimated to be approximately three kilometers wide.

TDS

During the summer of 1991, seventy-five water samples were collected from the southwestern portion of the basin with most samples from the lake and near-shore region. Total dissolved solids (TDS) in the groundwater generally increased with distance from the lake. The TDS value attains a minimum where the inferred fracture is located (fig. 5), but increases again, south of the apparent fracture zone. A 2-sample t-Test (df=11, t-stat=-5.56, alpha=0.05, p=0.0002) rejects the null hypothesis of similarity between the TDS values (from the fracture zone compared to samples taken outside the fracture zone). This seems to corroborate the inference that the lineament is a fracture, and the low TDS values observed in the region can be attributed to the fracture acting as a local recharge zone. Groundwater recharge by direct rainwater through the fracture zone would have lower TDS values because of the short distance traveled by groundwater relative to groundwater in the surrounding area. However, limited chemical data precludes the establishment of a definite relationship between the lineament and water chemistry of the study area. More water samples are needed to fully discover the impact or influence of the lineaments on the water chemistry of the region.

Future Studies

The relationship between lineaments/fractures and water movement can be verified by performing pumping tests around, and near, the lineaments. Detailed resistivity surveys or other geophysical surveys (such as EM) should also be carried out within a fifty kilometer wide region around the lake, together with the installation of monitoring wells for both water sample collection and water level measurements. The relationship of the lineaments to both groundwater production and the fate of the recharge water from the lake should also be examined.

Figure 9b. Resistivity profile segment within the lineament.

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Conclusions

The study has identified a prominent lineament in the southwest portion of the Lake Chad Basin that appears to be a bedrock fracture zone. This prominent lineament was investigated with resistivity profiling and a 3 kilometers wide low apparent resistivity zone was found to coincide with the position of the lineament. Although groundwater TDS tends to increase with distance from the lake, the groundwater TDS is at a minimum along this low resistivity zone. Soil moistures also appear high in this zone. This suggests that groundwater within the fracture zone may be recharged, at least in part, from precipitation.

Acknowledgments

Financial support for this project was received from the Earthwatch Research Corps, the National Geographic Society, and Indiana University - Purdue University Fort Wayne. The Chad Basin Development Authority, the Geology Department of the University of Maiduguri, Karen Wehn, and the Earthwatch "Lake Chad Project" volunteers were instrumental in the completion of the present project. Thanks are due to D. Chowdhury and C. Drummond for reviewing the manuscript. The anonymous reviewer and editor’s comments helped in the clarification of points in this paper.

References

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Avbovbo, A. A., Ayoola, E. O., and Osahon, G. A., 1986, Depositional and structural styles in Chad Basin of northeast Nigeria, Amer. Assoc. of Petrol. Geol. Bull., v. 70, n. 12, p. 1787-1798. Barber, W. and Jones, D.C., 1960, The geology and hydrogeology of the Maiduguri, Borno Province, Geol. Survey Nigeria. p. 520.

Burke, K., 1976, The Chad Basin: an intra-continental basin: Tectonophysics v. 36, p. 192-206.

Carinouze, J.P., 1983, Hydrochemical regulation of the lake: in Lake Chad; ecology and productivity of a shallow tropical system, Carmouze, J.P., Durand, J.R., and Leveque, C., Eds., Dr. W. Junk Publishers, Boston, p. 95-123.

Carter, J.D., Barber, W., Tait, E.A., and Jones, D.G., 1963, The geology of parts of Adamawa, Bauchi and Bornu Provinces in NE Nigeria: Bull. Geol. Survey of Nigeria #30, p. 108.

Cratchley, C.R., 1960, Geophysical survey of the southwestern part of the Chad Basin. Conference on Geology, Kaduna, Northern Nigeria: Kaduna, Nigerian Geological Survey, Pub. 35, p 21. Cratchley, C.R., Louis, P.,

and Ajakaiye, D.E., 1984, Geophysical and geological evidence for the Benue Chad Basin Cretaceous rift valley system and its tectonic implications: Journal of Africa Earth Sciences, v. 2, no. 2, p. 141-150.

Durand, A., 1982, Oscillations of Lake Chad over the past 50,000 years: new data and new hypothesis: Palaeogeogr. Palaeoclim., Palaeoecol., v. 39, p. 37-53.

Eugster, H.P., and Maglione, G., 1979, Brines and evaporates of the Lake Chad Basin, Africa: Geochimica et Cosmochimica Acta, v. 43, p. 973 -98 1.

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Isiorho, S.A., 1988, Fractures in the water budget of Chad Basin, Africa: in Symposium Proceedings of International Conference on Fluid Flow in Fractured rocks, Georgia State University, May 15-18,1988, p. 571-583.

Isiorho, S. A. 1989, Remote sensing applications in Chad Basin (Abstract):Geological Society of America Abstracts with Programs, v. 21, no. 4, p. 16.

Isiorho, S.A., and Matisoff, G., 1990, Groundwater recharge from Lake Chad: Limnology and Oceanography v. 35, n. 4, p. 93 1938.

Isiorho, S.A., Taylor-Wehn, K.S., and Corey, T.W., 1991, Locating groundwater in Chad Basin using remote sensing technique and geophysical method (Abstract): EOS (Transactions of the American Geophysical Union), v. 72, no. 44, p. 220-22 1.

Jaekel, D. 1984, Rainfall patterns and lake level variations at Lake Chad: in Climatic changes on a yearly to millennial basis, Geological, Historical and Instrumental Records, Morner, N., and Karlen, W., Eds., D. Reidel Publ. Co. Dordrecht, Netherlands, p. 191-200.

Mabee, S.B., Hardcastle, K.C., and Wise, D.U., 1994, A method of collecting and analyzing lineaments for regional-scale fracturedbedrock aquifer studies: Ground Water, v. 32, no. 6, p. 884894.

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Rowan, L.C., and Lathran, E.H., 1980, Mineral Exploration: in Remote Sensing in Geology, Siegel, B.S., and Gillespie, A. R.,Eds., Wiley, New York, p. 553-605.

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