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The effects of dykes and faults on groundwater flow in an arid land: the Red Sea Hills, Sudan

The effects of dykes and faults on groundwater flow in an arid land: the Red Sea Hills, Sudan
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  The effects of dykes and faults on groundwater flowin an arid land: the Red Sea Hills, Sudan Mohamed Babiker a , Agust Gudmundsson b, * a  Nansen Environmental and Remote Sensing Centre, Edvard Griegsv. 3A, N-5059 Bergen, Norway b  Department of Structural Geology and Geodynamics, Geoscience Centre, University of Go¨ ttingen,Goldschmidtstrasse 3, D-37077 Go¨ ttingen, Germany Received 20 November 2002; revised 31 March 2004; accepted 16 April 2004 Abstract In this study the focus is on a part of the Red Sea Hills of Sudan, an area which suffers from a severe shortage of groundwater.Thisshortageispartlybecausetheprecipitationinthisareaisverysmall,fromamaximumofonly164 mm year 2 1 toaminimumof36 mm year 2 1 .Partly,however,theshortageisrelatedtothegenerallylowpermeabilityofthe(mostlyPrecambrianbutpartlyPhanerozoic) bedrock. The bedrock is, however, dissected by numerous lineaments, mostly faults and basaltic dykes, some of which transport groundwater to the surface in springs and wells. We made field studies of 107 dykes, complemented by LandsatETMandSPOTimagestudiesof1419lineamentsinterpretedasdykes.Additionally,wemadeimagestudiesof1707lineamentsinterpretedasfaults,fracturesandshearzonesmanyofwhichmeetwiththedykesatnearlyrightangles.Manyofthedykesareof dense,low-permeabilitybasaltandrangeinthicknessupto14 mandinlengthuptoseveralkilometres.Thedominantdykestrikeis NNW, roughly parallel with the coast of the Red Sea and perpendicular to the topographic slope and the trends of many of thelineaments interpreted as faults. Using the field and image data, as well as a new digital elevation model of the study area, weproposeaconceptualmodeltoexplaintherelationshipbetweenfaults,dykesandgroundwaterinthearea.InthismodeltheNNW-trending dykes, particularly the long and thick low-permeability dykes, act as barriers for much of the topography-drivengroundwater flow. The groundwater collected by these dykes is transported along their margins towards the topographicdepressions occupied by the (comparatively) high-permeability E–W trending fault zones. Because these fault zones trendparallel with the inferred hydraulic gradient in the area the faults also tend to collect groundwater. In terms of the modelgroundwater is thus driven along both dykes and faults to their near-orthogonal intersections. These intersections normally haverelatively high fracture-related permeability, along which groundwater is transported towards the surface. This model thuspredictsthatwaterwellsandspringswouldbeexpectedatdyke–faultintersections,whichisinagreementwiththeavailabledataindicating that the majority of the springs occur at such intersections. q 2004 Elsevier B.V. All rights reserved. Keywords:  Dykes; Faults; Lineaments; Groundwater; Arid land; Red Sea Hills; Sudan 1. Introduction Recently, many areas in the arid parts of Africahave witnessed acute shortages of water supply. Journal of Hydrology 297 (2004) 256–$ - see front matter q 2004 Elsevier B.V. All rights reserved.doi:10.1016/j.jhydrol.2004.04.018* Corresponding author. Tel.:  þ 49-551-397930; fax:  þ 49-551-399700. E-mail addresses: (A.Gudmundsson), (M. Babiker).  One of these is the study area, the Red Sea Hills of theSudan. The reasons for these shortages are partly aninadequate amount of rain during the last decades andoverutilisation of groundwater from alluvial aquifers.Partly, however, the shortage is due to changes in thesites and distribution of the populations of the areas.These factors are directly related to the question as towhether or not the available water sources in theseareas can support present and future demands.In several developing countries, there is anincreasing need to provide safe drinking water forthe vast population living in bedrock terrains. Thisneed has contributed to the development of efficientand economic methods for groundwater explorationand quality evaluation. Of these methods, integratedgeological and remote sensing methods have provedparticularly successful.Bedrocks with low primary permeability coverlarge parts of the Red Sea area. Parts of the bedrock may, however, have moderate to good secondarypermeability on account offaults, fractures and dykes.Faults, fractures and dykes may play a very importantrole in groundwater recharge and flow in bedrock terrains. In detail, however, the effects of thesestructures on the bedrock permeability depend ontheir distribution, orientation and density. It followsthat in order to evaluate the permeability of areas suchas those associated with the Red Sea, we must makedetailed studies of dykes, faults and other lineaments.Mapping of lineaments using remote sensing datahas been an integral part of many groundwaterexploration programmes in bedrock terrain. Manyworkers have investigated lineament data with respectto their groundwater potential, several with anemphasis on arid or semi-arid hard rock areas(Krishnamurthy et al., 1996; Koch and Mather,1997; Sander et al., 1997; Smith et al., 1997; Druryet al., 2001). Few such studies, however, havecombined detailed field observation with the remotesensing analysis.Thispaperhasthreeprincipleaims.First,todescribethe general trend and attitude of the lineaments(fractures and dykes) of the Red Sea Hills area with aview of improving our knowledge of their generalgeometry and structure. The lineaments were mappedfrom landsat ETM and SPOT panchromatic images.Second, to investigate the spatial and geometricalrelation between the fractures and dykes, and theirinfluencesonthelocationof(potentialandactual)waterresources, wells and springs. In this connection wemadedetailedobservationsandmeasurementsofdykesand related fractures in the field. Third, to develop aconceptualmodelofgroundwaterflowinthestudyarea.Here the focus is on the different effects that dykes andfaultscanhave on thegeneral groundwater transport aswell as the location of springs and water wells. 2. Geological and hydrogeological background The study area lies in the southern part of the RedSea Hills, Sudan, between 18 8 30 0 and 19 8 6 0 N latitudeand 36 8 40 0 E and 37 8 22 0 longitude (Fig. 1). Inphysiographic terms the area can be divided intothree subareas: the coastal plain, the Red Sea Hills,and the western slope. The coastal plain comprises the20–50 km wide strip of low land striking parallel tothe Red Sea. The Red Sea Hills is a range of north–south trending mountains that rise steeply from thecoastal plain and extend to both Egypt and Eritrea.Most of these hills rise to 1000 m above the sea level;some even higher, to over 2000 m. The third subareais the western slope. This subarea comprises the areaswest of the Red Sea Hills, where the landscapechanges from the steep hills to the gently sloping,open landscape towards the river Nile.The location of the research area and its physio-graphic characteristics leads to its receiving precipi-tation at different times during the year from that of the rest of Sudan. Thus, in the study area twocontrasting rainfall zones exist within a relativelysmall area. The zone west of the Red Sea Hillsexperiences maximum rainfall during the summer,while the zone east of the Hills receives its maximumrainfall during the winter. Summit areas such asErkowit (Fig. 1) experience winter as well as thesummer rainfall maxima. The mechanisms thatgovern rainfall in summer and winter are quitedifferent. For the winter rain, which is brought bynortheast and southeast winds, the rain source isevaporation in the Red Sea. Rain falls as the windsrise over the mountains (orographic rain). Summerrain is brought by southwesterly monsoon windswhich srcinate from the Gulf of Guinea. Rain fallseither as the winds rise over the mountains or as aresult of thermal convection. The amount of rainfall  M. Babiker, A. Gudmundsson / Journal of Hydrology 297 (2004) 256–273  257  received in the area is very small; the highest mean(arithmetic average) annual is not more than 164 mmwhile the lowest mean annual is as low as 36 mm(El Tom, 1975, 1991; Musa, 1990).Geologically, the Red Sea Hills are dominated by aPrecambrian crystalline basement complex overlainby a variety of Phanerozoic (cover) rocks, fromTertiary to Holocene in age. Studies in NortheastAfrica and Western Arabia show that the geology of the Red Sea Hills, as a part of the Arabian–NubianShield, can be interpreted in terms of a general plate-tectonic model of the volcano-tectonic evolution of the Red Sea (Coleman, 1993; Shackleton, 1994; Stern,1994). However, the detailed tectonic evolution of theRed Sea Hills is still poorly known.As for Northeast Sudan, Reischmann and Kroner(1994) divide the Nubian–Arabian Shield into fiveintra-oceanic and island arc areas, separated by suturezones and fault zones (Fig. 2). These five areas, fromnorth to south, are those of Gerf, Gabgaba, Gebeit,Haya and Tokar. The study area is located within thatof Haya (Fig. 2).The Haya area, primarily formed as an island arc, islocated in the western part of the Red Sea Hills. In thenorth, the Haya area is separated from the Gebeit areaby the NE-trending Amur Nakasib Shear Zone(AMN Fig. 2), partly an ophiolite fragment, whereasin the south the N-trending Barka Surture Zone (BA),ofserpentinite,separatestheHayaareafromtheTokararea. The NW-trending Oko Shear Zone (Abdelsalam,1994) affects the northwest part of the Haya area(Fig. 2), while the ENE-trending Khor Ashat ShearZone (AS Fig. 2) divides the Haya area itself intosouthern and northern parts (Kroner et al., 1991).Duringthedryseasonsinthestudyarea,therearenonatural surface water resources such as rivers or lakes.Hence, the surface hydrology of the area comprisesmainly surface runoff and drainage during the wetseasonswiththeexceptionofsomesmallspringsfoundin the mountain areas. Groundwater is thus the onlypermanentwaterresourceinthearea.Thegroundwaterresources,however,varyinlocation,yieldandquality,and are highly dependent on rainfall.Groundwater occurs mainly in alluvial depositsand bedrock formations. Of these, the aquifers in thealluvial sediments of the seasonal streams, known as‘khors or wadies’, are those in most common use. Thegroundwater potential of the bedrock is, however,largely unknown.Groundwaterinthealluvialaquifersalongthekhorsis by far the most commonly used water source inthe Red Sea Hills. This source is distributed all over Fig. 1. Maps show of the study area. (A) Sudan map showing the location of the study area. (B) Study area map where the main towns (Sinkat,Gebeit and Erkowit) main roads, railway and main seasonal streams (khors) are indicated.  M. Babiker, A. Gudmundsson / Journal of Hydrology 297 (2004) 256–273 258  the area, but it is often rapidly exhausted by over-utilisation and suffers not only form general evapor-ation but also from erratic recharging. The variousgroundwater resources have different properties. Inarid regions such as the Red Sea Hills, the khors andwadis are dryforseveral monthsofthe year.However,during rainy seasons most of these khors carry surfacewater (runoff) for a short time. Part of this flowinfiltrates into the khor bed so that a subsurfacereservoir or an alluvium aquifer is established.Khor deposits comprise material with a grain sizeranging from clay particles to gravel and boulders.Because of the ever-shifting sediments in the khors,the composition of the deposits in the khor beds isvery heterogeneous. To evaluate the khor bed as anaquifer, the extent, thickness, and location of silt andclay layers must be known because these layersobstruct recharge and reduce storage volume. Thekhor beds thus form mainly unconfined aquifers.The main water supply in the study area comesfrom excavated wells (hand dug) supplemented by afew drilled wells. Wells are located mainly in (or/andalong) khor beds, but a few ones are on the plains.Excavated wells are usually lined with stones or drymasonry. Some wells have a superstructure thoughmost are open at the khor bed level. The character-istics of the wells (depth, water amount and quality)vary considerably within the same khor.Dykes are very common in the study area. Theycan act either as good conductors of, or as barriers to,groundwater flow depending on the intensity of fracturing associated with the dykes. Whether thedykes act as water barriers or conductors, theirstructures, locations and orientations with respect tothe groundwater flow are very important. 3. Dyke structures and trends The regional dyke swarms of the Red Sea Hillshave been known for some time (Whiteman, 1971;Vail and Hughes, 1977; AbuFatima, 1992), but norecent systematic studies have been published on theirstructural and geometrical aspects or their influence Fig. 2. Geological map of the Arabian–Nubian shield (shaded), showing major structures in and around the study area. Abbreviation for theophiolites and/or suture and shear zones are: AF, Afif; Al, Allagi; AM, Al Amar; AMN, Amur-Nakasib; AS, Khor Ashat; BA, Baraka; BU, BirUmq; HAM, Hamisana; NAB, Nabitah; ONSH, Onib-Sol Hamed; OSZ, Oko Shear Zone and YA, Yanbu. Box outline indicates the researcharea. Modified from Kroner et al. (1987), Kroner et al. (1991) and Reichsmann and Kroner (1994).  M. Babiker, A. Gudmundsson / Journal of Hydrology 297 (2004) 256–273  259  on groundwater flow. Data from Landsat ETM andSPOT images are used for a general overview of thestrike and length distributions of the dykes. Inaddition, their main field characteristics are described,focusing on dyke attitude, thickness and structure.The field results are then compared with the resultsobtained from the images. Emphasis is on thosegeometrical aspects that are important for ground-water flow, including dyke strike and thicknessdistributions, as well as dyke lithology. 3.1. Strike Fig. 3 indicates the geographical distribution of lineaments, interpreted as dykes, in the research area.Fig. 4 shows that NNW-trending dykes dominate.In addition there are a number of ENE-trending dykesin the area but most of these are located south of theNNW-trending dykes. The change in dyke trendoccurs across the Khor Ashat Shear Zone and westand southwest of the town of Sinkat (Figs. 2 and 3).The dyke strike, as measured in the field (Fig. 4),generally coincides with the trend of lineaments asmeasured from the images. The field measurements,however, indicate a somewhat more northerly dyketrend. The field measurements, however, are muchfewer (107) than the image measurements (1419).Most dykes are extension fractures as indicated bythe present field observation and supported by studiesworldwide (Gudmundsson and Marinoni, 1999;Gudmundsson, 2002). We can thus safely assumethe dyke trends, in general, to be perpendicular to thedirection of the minimum compressive principal stress s  3  at the time of dyke emplacement. During dyke Fig. 3. Map showing the geographical distribution of 1419 dykes in the research area. Mapped from SPOT panchromatic and Landsat ETMimages. Location of the town Sinkat Gebeit, Erkowit and Summit are indicated.  M. Babiker, A. Gudmundsson / Journal of Hydrology 297 (2004) 256–273 260
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