Red Sea rifting controls on aquifer distribution: Constraints from geochemical, geophysical, and remote sensing data M. Sultan1,†, A.F. Yousef2, S.E. Metwally2, R. Becker3, A. Milewski1, W. Sauck1, N.C. Sturchio4, A.M.M. Mohamed5, A. Wagdy6, Z. El Alfy7, F. Soliman8, M. Rashed8, D. Becker1, Z. Sagintayev1, M. Ahmed1, and B. Welton1 1Department of Geosciences, Western Michigan University, Kalamazoo, Michigan 49008-5200, USA 2Desert Research Center, El Matariya, Cairo, Egypt 3Department of Environmental Sciences, University of Toledo, Toledo, Ohio 43606, USA 4Department of Earth and Environmental Sciences, University of Illinois at Chicago, Chicago, Illinois 60607-7059, USA 5Department of Geology, South Valley University, Qena, Egypt 6Irrigation and Hydraulics Engineering Department, Cairo University, Giza, Egypt 7Hammash Misr for Gold Mines, Cairo, Egypt 8Department of Geology, Suez Canal University, Ismalia, Egypt ABSTRACT the Red Sea–Gulf of Suez system, and in rift referred to as “fossil” aquifers. Fossil aquifers systems elsewhere. An understanding of the are believed to have been recharged under pre- Highly productive wells in the Central distribution of Red Sea rift–related aquifers vious wet climatic periods (e.g., Sturchio et al., Eastern Desert of Egypt are tapping ground- and modern recharge contributions to these 2004) but may have also received local meteoric water in subsided blocks of Jurassic to aquifers could assist in addressing the rising contributions in intervening dry climatic periods Cretaceous sandstone (Taref Formation of demands for fresh water supplies and water (such as at present; Sultan et al., 2007, 2008a). the Nubian Sandstone Group) and Oligocene scarcity issues in the region. These aquifers are of extreme importance to Miocene sandstone (Nakheil Formation), to the general population of these countries given now occurring beneath the Red Sea coastal INTRODUCTION the prevailing arid conditions and the increasing plain and within the proximal basement com- need for fresh-water supply in these areas. The plex. Aquifer development is related to Red Countries in the arid and semiarid parts of the identifi cation of the hydrologic and geologic Sea rifting: (1) rifting was accommodated by world are facing shortages in their fresh water settings of these groundwater resources, the vertical extensional displacement on preexist- supplies because of their increasing populations assess ment of their areal distribution, and ing NW-SE– to N-S–trending faults forming and the absence of a comprehensive understand- the understanding of the modern contributions a complex array of half-grabens and asym- ing of the geologic and hydrogeologic controls to these systems could assist in addressing water metric horsts; and (2) subsided blocks escaped on the development and preservation of these shortage problems in these areas. In this paper, erosion accompanying the Red Sea–related resources. Such an understanding is needed to we examine one such setting, namely aquifers in uplift. Subsided blocks were identifi ed and enable sustainable utilization of these resources. the Red Sea rift system. verifi ed using satellite data, geologic maps, One such area is the Saharan Africa and the An integrated approach, involving data from and fi eld and geophysical investigations. In- Middle Eastern countries. A vital resource for fi eld, geochemical, geophysical, and remote terpretations of very low frequency (VLF) the populations of some of these countries is the sensing and geographic information system measurements suggest the faults acted as extensive system of rivers that collect precipita- (GIS) studies, was applied to identify the distri- conduits for ascending groundwater from the tion from distant mountainous areas and channel bution of the Red Sea–related extensional struc- subsided aquifers. Stable isotopic composi- it from regions where precipitation is abundant tures and aquifers and to assess their groundwater tions (δD: –19.3‰ to –53.9‰; δ18O: –2.7‰ to to the much drier climatic regions downstream. potential on a regional scale. The role of these –7.1‰) of groundwater samples from these One such system is the Nile River watershed structures in aquifer development, groundwater aquifers are interpreted as mixtures of fossil that collects precipitation from the highlands transport, and groundwater exchange between (up to 70%) and modern (up to 65%) pre- in subtropical Africa and channels it to Saharan aquifers is examined. We also identify the origin cipitation. Groundwater volumes in subsided Africa. Unfortunately, the majority of the of groundwater in these structurally controlled blocks are large; within the Central Eastern world’s arid and semiarid regions lack extensive aquifers and examine potential mechanisms for Desert basement complex alone, they are esti- river systems and must resort to other fresh- groundwater recharge. mated at 3 × 109 m3 and 10 × 109 m3 for the water resources. Many of the arid and semi- Nakheil and Taref Formations, respectively. arid parts of the world are fortunate in having GEOLOGIC SETTING Results highlight the potential for identify- large amounts of fresh water stored in extensive ing similar rift-related aquifer systems along aquifers that stretch for hundreds to thousands The Red Sea began to open about 20 million of kilometers across political boundaries; these years ago during the late Oligocene to early Mio- †E-mail: [email protected] are largely nonrenewable aquifers, commonly cene as the Red Sea rift propagated northward GSA Bulletin; May/June 2011; v. 123; no. 5/6; p. 911–924; doi: 10.1130/B30146.1; 8 fi gures; 2 tables. For permission to copy, contact [email protected] 911 © 2011 Geological Society of America Sultan et al. and separated the Arabian plate from the once- line rock and the overlying thick (up to 2.5 km) uplift devastated the thick sedimentary succes- contiguous Arabian-Nubian plate (Bosworth sedimentary successions to extensive erosion sions (largely Eocene limestone and Cretaceous et al., 2005) (Fig. 1). Rifting was associated (Garfunkel and Bartov, 1977). The rifting was sandstone), the extensional faults preserved with uplift; the shoulders of the rift along largely accommodated by extensional normal these successions as subsided blocks under the the Red Sea and the Gulf of Suez and along the faults that strike north and northwest forming a Red Sea and Gulf of Suez coastal plains and Dead Sea transform fault were elevated by as complex array of half-grabens and asymmetric the Red Sea trough. Fossil groundwater from much as 4 km, exposing the underlying crystal- horsts (Pivnik et al., 2003) (Fig. 1). Whereas the subsided Cretaceous Nubian Sandstone blocks A Figure 4 Figure 3 Figure 1. (A) Location map Figure 2 showing the distribution of Neo- proterozoic outcrops along the Red Sea margins, the Red Sea coastal plain, and the lithotec- tonic subdivisions of the Eastern Desert (Stern, 1985). Also shown are the locations of our ground- water samples, geoelectric cross sections in Wadis Queih and Al Nakheil, and very low frequency (VLF) transects in Wadis Esl and Ambagi, Gona North-1, and Gona North-2 areas. (B) Simpli- fi ed cross section along line A–A′ in Figure 1A modified from Pivnik et al. (2003). Areas cov- km ered by Figures 2, 3, and 4 are outlined by boxes. Neoproterozoic Sample locations Geoelectric locations (Red Sea Hills) VLF Eastern Desert divisions Coastal plain B ′ A Rift Rift A boundary boundary Shoreline Shoreline 0 km 0 km 2 2 4 4 6 6 8 8 Neoproterozoic Eocene-Cambrian Lower-middle Middle Miocene (pre-rift) Miocene (synrift) to Holocene (late rift) 912 Geological Society of America Bulletin, May/June 2011 Red Sea rifting controls on aquifer distribution: Constraints from geochemical, geophysical, and remote sensing data (up to 4 km deep) discharges along the exten- lift was followed by a period of intense exten- (e.g., gabbros and mafi c volcanics) appear in sional faults bounding the Gulf of Suez at high sion and faulting along axes trending NW-SE, shades of blue, and rocks poor in Fe-bearing temperature (up to 70 °C) within the coastal plain NNW-SSE, and N-S. alumino silicates, hydroxyl-bearing minerals , of the Gulf of Suez and along its coastline (Stur- and opaque phases (e.g., granites) appear green. chio et al., 1996). In Sinai, these fossil waters VERTICAL DISPLACEMENT ON Carbonate-rich rocks (e.g., Eocene limestone originated as precipitation over the mountains PREEXISTING ACCRETIONARY and marl) appear in shades of yellow, and of southern Sinai, and recharged the aquifers AND POSTACCRETIONARY PLANES quartz-rich Nubian Sandstone appears in shades cropping out at the foothills. The groundwater OF WEAKNESS of green. An interpretation map was constructed fl ow directions are northward, toward the Medi- (Fig. 2B) using our fi eld data and published fi eld terranean Sea, and westward toward the Gulf of Studies have shown that for the Red Sea rift relations (Abuzeid, 1994) together with the spa- Suez (west) where discharge occurs along the system and surrounding areas in the Afar region, tial distribution of lithologic units displayed in faults defi ning the Gulf (Issar, 1979). These ex- it is easier to propagate extensional displace- Figure 2A. tensional faults are not restricted to the Red Sea ment along preexisting faults than to develop We adopted the following satellite-based cri- and Gulf of Suez coastal plains and trough but new faults, especially for those oriented at near teria and fi eld observations (Sultan et al., 1988) extend