Journal of African Earth Sciences 61 (2011) 82–93
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Journal of African Earth Sciences
journal homepage: www.elsevier.com/locate/jafrearsci
Geology and geophysics of the West Nubian Paleolake and the Northern Darfur
Megalake (WNPL–NDML): Implication for groundwater resources in Darfur,
northwestern Sudan
Ahmed Elsheikh a, Mohamed G. Abdelsalam a,⇑, Kevin Mickus b
a
b
Department of Geological Sciences and Engineering, Missouri University of Science and Technology, Rolla, MO 65409, United States
Department of Geology, Geography and Planning, Missouri State University, Springfield, MO 65401, United States
a r t i c l e
i n f o
Article history:
Received 12 May 2010
Received in revised form 13 April 2011
Accepted 19 May 2011
Available online 30 May 2011
Keywords:
Groundwater
Sudan
Darfur
Paleolake
a b s t r a c t
The recent delineation of a vastly expanded Holocene paleo-lake (the Northern Darfur Megalake which
was originally mapped as the West Nubian Paleolake and here will be referred to as WNPL–NDML) in
Darfur in northwestern Sudan has renewed hopes for the presence of an appreciable groundwater
resource in this hyper-arid region of Eastern Sahara. This paleolake which existed within a closed basin
paleo-drainage system might have allowed for the collection of surface water which was subsequently
infiltrated to recharge the Paleozoic–Mesozoic Nubian Aquifer. However, the presence of surface exposures of Precambrian crystalline rocks in the vicinity of the paleolake has been taken as indicating the
absence of a thick Paleozoic–Mesozoic sedimentary section capable of holding any meaningful quantity
of groundwater. This work integrates surface geology and gravity data to show that WNPL–NDML is
underlain by NE-trending grabens forming potential local Paleozoic–Mesozoic aquifers that can hold as
much as 1120 km3 of groundwater if the sedimentary rocks are completely saturated. Nevertheless, it
is advised here that recharge of the Nubian aquifer under WNPL–NDML is insignificant and that much
of the groundwater is fossil water which was accumulated during different geological times much wetter
than today’s hyper-arid climate in Eastern Sahara. Excessive extraction will lead to quick depletion of this
groundwater resource. This will result in lowering of the water table which in turn might lead to the drying out of the oases in the region which provide important habitats for humans, animals and plants in
northern Darfur.
Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction
Eastern Sahara is one of the driest places on Earth. Today, this
region receives a meager annual precipitation of less than
50 mm/year. However, unlike the hyper-arid climate that dominates Eastern Sahara today, this region experienced wetter climate
9000 years ago during the early Holocene time and also earlier
geologic times during the Pleistocene. This wetter climate in Eastern Sahara has been documented from sedimentological, paleontological, archeological, remote sensing, and isotopic studies
(McCauley et al., 1982, 1986; Gabriel and Kröpelin, 1984; Heinl
and Brinkmann, 1989; Pachur et al., 1990; Wendorf et al., 1994;
Szabo et al., 1995; Abdelsalam et al., 2000; Hoelzmann et al.,
2000, 2001; Pachur and Hoelzmann, 2000; Abell and Hoelzmann,
2000; Rodrigues et al., 2000; Kuper, 2006; Kuper and Kröpelin,
2006; Ghoneim and El-Baz, 2007; Robinson et al., 2007; Paillou
et al., 2009).
⇑ Corresponding author. Tel.: +1 573 341 4100; fax: +1 573 341 6935.
E-mail address: [email protected] (M.G. Abdelsalam).
1464-343X/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.jafrearsci.2011.05.004
In Darfur in northwestern Sudan, the wetter Holocene period is
reflected in the presence of a paleolake (Fig. 1), the presence of
which was first suggested by Gabriel and Kröpelin (1984) who
reported the presence of Holocene lake sediments in northwestern
Sudan. Subsequently, Pachur and Rottinger (1997) presented
evidence for the presence of the paleolake from the analysis of
the Shuttle Imaging Radar (SIR)-C/X-band Synthetic Aperture Radar (SAR) data and called it the West Nubian Paleolake (Fig. 1).
Abell and Hoelzmann (2000) and Hoelzmann et al. (2000, 2001)
used stable isotopic studies to conclude that the paleolake was
filled with fresh water provided by 500–900 mm/year precipitation resulting from the 800 km northward shift of the Inner Tropical Convergence Zone (ITCZ) in Africa. Hoelzmann et al. (2001)
used radiocarbon dating and archeological studies to show that
the paleolake had existed between 9400 and 3800 years with an
intensive human inhabitation between 6300 and 3500 years.
Recently, Ghoneim and El-Baz (2007) used Digital Elevation
Models (DEMs) extracted from the Shuttle Radar Topography Mission (SRTM) data to significantly expand the boundaries of the
paleolake and renamed it the Northern Darfur Megalake (Fig. 1).
A. Elsheikh et al. / Journal of African Earth Sciences 61 (2011) 82–93
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Fig. 1. Digital Elevation Models (DEMs) extracted from the Shuttle Radar Topography Mission (SRTM) data showing the location (A) and the extent (B) of the West Nubian
Lake as mapped by Abell and Hoelzmann (2000) and Hoelzmann et al. (2000, 2001) and the Northern Darfur Megalake as mapped by Ghoneim and El-Baz (2007).
The identification of this much larger paleolake led to the notion
that there must once have been a large body of surface water that
seeped to recharge the Nubian aquifer, therefore, the hyper-arid
Darfur region may be sitting on a huge supply of groundwater
(Ghoneim and El-Baz, 2007). The implication of this possibility is
significant since the ability to extract groundwater from beneath
Darfur could contribute to providing adequate water resource.
The drilling of 1000 wells is thought to be key in providing water
for the Darfuris (Friedman, 2007). However, the presence of such
groundwater resource has been challenged by many scientists,
and their positions are summarized in Butler (2007). The argument
is that the Nubian aquifer is not thick enough beneath Darfur to
hold any significant amount of groundwater due to tectonic uplift,
which resulted in the exposure of Precambrian crystalline rocks to
the surface. For the rest of this article the West Nubian Paleolake
and the Northern Darfur Megalake will be referred to as WNPL–
NDML.
This work recognizes the need to understand the structural
complexity of the region covered by WNPL–NDML. Since northern
Africa has been affected by a number of Phanerozoic extensional
tectonic events (Schandelmeier and Reynolds, 1997), it is anticipated that this region is underlain by extensional structures filled
with Paleozoic and Mesozoic sedimentary rocks that can serve as
localized aquifers. Given the scarcity of surface and sub-surface
geological and geophysical data, this work relied on the integration
of geological maps by Wycik et al. (1990) and gravity data from the
Earth Gravitational Model (EGM) 2008 gravity model (Pavlis et al.,
2008). These data are used to construct E–W trending geological
cross sections for the region covered by WNPL–NDML. The cross
sections are then used to construct an isopach map to the depth
of the Precambrian crystalline rocks and to estimate the potential
groundwater storage capacity of the Paleozoic–Mesozoic sedimentary section. Results of this work are subsequently discussed in
relation to rechargability of groundwater, and environmental
impact of groundwater extraction.
2. The West Nubian Paleolake and the Northern Darfur
Megalake
The region occupied by WNPL–NDML extends in the northwestern corner of Sudan, and it is bounded by longitudes 24°E–27°E and
latitudes 17°300 N–20°300 N (Fig. 1). The central part of this region is
relatively flat with a mean elevation of 550 m. However, this flat
region is surrounded by up to 1500 m elevated regions represented
by the southern extension of the Uweinat uplift in the north, the
northern extension of the Darfur dome in the south, and the Ennedi
and Erdi Ma mountains in the west (Fig. 1). The low-lying WNPL–
NDML region is separated from the Nile basin drainage system by a
N-trending elevated terrain referred to here as the Rahib ranges
(Fig. 1). The Uweinat uplift and the Rahib ranges expose Precambrian crystalline rocks, whereas the Darfur uplift is dominated by
Cenozoic volcanic rocks. The Ennedi and Erdi Ma mountains are
dominated by Paleozoic sedimentary rocks. Mesozoic sedimentary
rocks are exposed within the WNPL–NDML low-lying terrain.
Various methods were employed to calculate the area of WNPL–
NDML and the volume of water it contained. Analyzing SIR-C/
X-SAR data, Pachur and Rottinger (1997) calculated the size of
WNPL at 17,864 km2. Hoelzmann et al. (2000) analyzed Landsat
Thematic Mapper (TM) data and estimated that the size of WNPL
varies between 1100 and 7000 km2. In another study, Hoelzmann
et al. (2001) used a series of differential Global Positioning System
(GPS) surveys to produce the first precise measurement of WNPL
area at 5330 km2 (using a lake level of 550 m). Ghoneim and El-Baz
(2007) used SRTM data to propose new estimates for the area of
WNPL and the volume of water it contained. Using a lake level
of 550 m, Ghoneim and El-Baz (2007) calculated an area of
8133 km2 for the paleolake and a volume of 372 km3 for the water
it contained. In a different scenario, Ghoneim and El-Baz (2007)
adopted a lake level of 555 m to show that the area of the lake will
increase to 11,230 km2 allowing for 547 km3 of water to be
contained within it. Additionally, Ghoneim and El-Baz (2007)
proposed a new paleo-shoreline for the paleolake (NDML) at an
elevation of 573 ± 3 m. The new shoreline resulted in the expansion of the paleolake to cover an area of 30,750 km2 and a water
volume of 2530 km3.
Sedimentological, paleontological and isotopic studies indicate
that WNPL–NDML existed during the late Holocene (between
9400 and 3800 years). However, other absolute age determination
argues for a relatively greater ages of other Saharan paleolakes.
Based on radiocarbon dating of carbonate samples from some
Eastern Sahara paleolakes, Wendorf et al. (1994) suggested that
many of these lakes existed during the late Pleistocene, 20,000–
40,000 years ago. Szabo et al. (1995) also used radiocarbon dating
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to determine five different paleolake events developed in Eastern
Sahara during the Pleistocene and Holocene periods. These
paleolakes span the periods 320,000–250,000, 240,000–190,000,
155–120,000, 90,000–65000, and 10,000–5,000 years ago.
The region surrounding WNPL–NDML was dominated by three
paleo-drainage systems, with the paleolake drainage system
formed within a closed basin (Fig. 2). The region to the northeast,
in southwestern Egypt and northwestern Sudan was dominated
by a NE- and SE-flowing drainage system originating from the
Uweinat uplift and the Gilf Kebir mountains (Fig. 2; Abdelsalam
et al., 2000; Robinson et al., 2007). Northwest of WNPL–NDML,
N-flowing drainage system dominated (Fig. 2; Paillou et al.,
2009). This drainage system originated from El Fayoud and El
Akdamin mountains in northeastern Tibesiti, and northern
Uweinat-Gilf Kebir-Abu Ras mountains (Fig. 2; Paillou et al.,
2009). The NDPL-WNPL is characterized by a closed basin paleodrainage system in which the drainage originating from the Enndi
mountains in the southeast were flowing NE-ward and that originating from the Erdi Ma mountains and Uweinat Uplift in the
northeast and north were flowing SE-ward and S-ward, respectively (Fig. 2; Hoelzmann et al., 2001; Ghoneim and El-Baz,
2007). The drainage system originating from the Rahib ranges in
the east was flowing W-ward (Fig. 2).
reactivations of these shear zones between the Cambrian and Carboniferous and the formation of sedimentary basins to the northeast and southwest of them, no major extensional structures
developed within the WNPL–NDML region.
At the end of the Carboniferous much of the region around
WNPL–NDML was under an erosion or non-depositional regime,
and the Precambrian crystalline rocks were exposed to the surface.
Extensional structures may have formed within the WNPL–NDML
region and the surrounding areas during the Permian to middle
Jurassic where NE- and ENE-trending grabens were formed
(Fig. 3). These grabens received sediments from the north and
northeast in the form of continental sandstones and conglomerates
(Fig. 3).
The WNPL–NDML region continued to be under an erosion or
non-depositional regime throughout the late Triassic and Jurassic.
However, this changed when continental sandstone began to be
deposited throughout northeastern Africa in the early Cretaceous.
Deposition of continental sandstone continued throughout the Cretaceous, and in the WNPL–NDML region it might have been accompanied by the development of NE-trending grabens. Throughout
the Tertiary, the WNPL–NDML region was once again under an erosion or non-depositional regime.
4. Stratigraphy
3. Regional tectonic setting
Northeastern Africa has a long Phanerozoic history of intraplate deformation as summarized by Schandelmeier and Reynolds
(1997), a synthesis that will be used here to provide regional tectonic context to the geology of WNPL–NDML. The oldest of these
deformation events may be the reactivation of Precambrian structures during the early Cambrian, resulting in the development of
the NE-trending Central African Shear Zone south of WNPL–NDML
and the Trans-Africa Lineament to the north (Fig. 3; Schandelmeier
et al., 1987a,b; Schandelmeier, 1988; Schandelmeier and Pudlo,
1990; Schandelmeier and Reynolds, 1997). Despite the multiple
The WNPL–NDML region is dominated by Paleozoic and Mesozoic sedimentary rocks overlying Precambrian crystalline rocks.
Figs. 4 and 5 are modified from Wycik et al. (1990) to illustrate surface exposure of various formations and to show a generalized
stratigraphic column of the WNPL–NDML region, respectively.
4.1. The Precambrian crystalline rocks
The WNPL–NDML region lies within the Saharan Metacraton,
which is dominated by Precambrian crystalline rocks that span
much of the region west of the Nile in western Sudan and
Fig. 2. Paleo-drainage systems in the eastern Sahara as mapped by Abdelsalam et al. (2000), Ghoneim and El-Baz (2007), Robinson et al. (2007), and Paillou et al. (2009).
A. Elsheikh et al. / Journal of African Earth Sciences 61 (2011) 82–93
85
Fig. 3. Tectonic history of the West Nubian Paleolake and the Northern Darfur Megalake and surroundings between 250 and 220 Ma. After Schandelmeier and Reynolds
(1997).
southwestern Egypt (Abdelsalam et al., 2002). This region is made
up of medium- to high-grade gneisses and migmatites ranging in
age from Archean to Neoproterozoic (Abdelsalam et al., 2002).
North of WNPL–NDML, the Precambrian crystalline rocks are exposed in the Uweinat uplift (Fig. 1). The Precambrian crystalline
rocks are also exposed immediately east of WNPL–NDML. These
rocks constitute the N- to NE-trending Jebel Rahib fold and thrust
belt (Fig. 4) which is made up of low-grade green-schist facies mafic and ultra-mafic rocks interpreted as a Precambrian ophiolite
formed between 860 and 740 Ma (Abdel-Rahman et al., 1990;
Schandelmeier et al., 1990).
The Precambrian crystalline rocks extend eastward from Jebel
Rahib fold and thrust belt until the Red Sea Hills (Fig. 1). This indicates the lack of a thick Paleozoic–Mesozoic sedimentary cover
east of WNPL–NDML. The region west of WNPL–NDML is dominated by exposures of Paleozoic sedimentary rocks, but no exposures of Precambrian crystalline rocks are encountered until the
Tibesti and then Tuareg Shield (Fig. 1). However, the Precambrian
crystalline rocks are exposed north and south of WNPL–NDML,
indicating the lack of a thick Paleozoic–Mesozoic sedimentary cover in these regions as well.
4.2. The Paleozoic sedimentary section
The Paleozoic sedimentary rocks crop out in the eastern and
western sides of WNPL–NDML, but they are rarely encountered
within the paleolake area itself (Fig. 4), suggesting that they may
be buried under the Mesozoic sedimentary section. This sedimentary section, which is 400 m thick, is divided by Wycik et al.
(1990) into several formations starting with a 50 m thick
Cambrian to Ordovician formation that sits uncomfortably on the
Precambrian crystalline rocks and contains basal conglomerate
and coarse sandstone (Fig. 5). The Cambrian–Ordovician formation
is overlain by a number of other Paleozoic formations that are
Silurian to Carboniferous in age (Fig. 5). The lower part of the
Paleozoic section, above the Cambrian to Ordovician formation,
consists primarily of well-sorted coarse-grained sandstone with
minor shale beds. This part of the section is 100 m thick and is
thought to have been deposited within braided stream systems.
This is followed by a 50 m thick formation composed of fluviatile
sandstone. The upper part of the section is 200 m thick consists of
well-bedded lacustrine and fluvial sandstone with some silt and
shale beds followed by quartz sandstone and conglomerate beds
(Fig. 5).
4.3. The Mesozoic sedimentary section
The Mesozoic sedimentary section is exposed within WNPL–
NDML and also to the northwest of it (Fig. 4). It has been divided
by Wycik et al. (1990) into a number of formations spanning the
Permian and the Cretaceous period with a total thickness of
2200 m (Fig. 5). Much of the section is dominated by medium- to
coarse-grained sandstone deposited in braided and low-sinuosity
stream systems. However, the upper part of the Mesozoic sedimentary section, which is Campanian–Maastrichtian in age, is composed
of fine-grained sandstone with silt and clay beds (Fig. 5). The thickest
formation in this section is Turonian–Santonian in age (the Aptian,
Cenomanian and Coniacian are missing), composed of medium to
coarse-grained fluvial sandstone and reaches 1500 m (Fig. 5).
5. Methods
To aid in determining the sub-surface structure of the WNPL–
NDML area, a Bouguer gravity anomaly map is made from the
EGM2008 gravity model (Pavlis et al., 2008). The EGM2008 gravity
model was constructed from land, marine and airborne gravity
data, marine gravity anomalies derived from satellite altimetry,
and long wavelength gravity variations from the Gravity Recovery
And Climate Experiment (GRACE) satellite data. The resultant gravity model is estimated to spherical harmonic degree 2160. Free-air
gravity and Bouguer gravity anomalies using the WGS84 Geodetic
Reference System as a datum were computed from the spherical
harmonic coefficients. The resultant grid (3 km spatial resolution) is used in this analysis.
Since this work is interested in the geology of the upper crust, a
residual gravity anomaly map is created (Fig. 6A) to emphasize
anomalies caused by these sources. There are a number of techniques that can determine a residual gravity field including the
wavelength filtering, the polynomial trend surface, and the isostatic residual anomalies methods. Since there are no constraints
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Fig. 4. Geological map of the West Nubian Paleolake and the Northern Darfur Megalake and surroundings. After Wycik et al. (1990).
to help in defining the deeper structure (such as seismic refraction
models, seismic broadband analysis, or bore holes data) other than
the surface geology, the selection of which technique to be used is
ubiquitous. In this work both polynomial trend surfaces and wavelength filtering are used since other methods such as the isostatic
residual method requires knowing the crustal thickness. Additionally, a variety of bandpass filters and higher order polynomial surfaces are used in this study where similar anomaly patterns are
produced. As a representative residual gravity anomaly map, a
bandpass filter where wavelengths between 250 and 5 km are used
is shown in Fig. 6A.
The residual gravity anomaly map (Fig. 6A) shows a number of
gravity minima (labeled A–G) and gravity maxima (labeled 1–7).
Some of these gravity anomalies can be correlated with the surface
geology (Fig. 6B) and others cannot be correlated. To further analyze the sub-surface structure, four E–W trending geological cross
sections are constructed along latitudes 20°N, 19°N, 18°N, and
17°300 N (Fig. 7). The cross sections are constructed by first determining preliminary geologically-based cross sections by projecting
the surface exposures of various formations within WNPL–NDML
(Fig. 4; Wycik et al., 1990) onto topographic profiles extracted from
SRTM DEMs. Uniform average thicknesses, as shown in Fig. 5, are
adopted for all formations. Then the cross sections are modified
based on gravity models constructed using a two and one-half
dimensional forward modeling algorithm using the surface geology as constraints. Given the lack of a noticeable regional gravity
A. Elsheikh et al. / Journal of African Earth Sciences 61 (2011) 82–93
87
Fig. 5. Generalized stratigraphic column of the West Nubian Paleolake and the Northern Darfur Megalake and surroundings. Compiled from Wycik et al. (1990).
anomaly, a zeroth-order polynomial is used as a regional gravity
anomaly which is removed from each profile before modeling.
The gravity models are generated using the following densities:
2.57 g/cm3 for the Mesozoic sedimentary rocks, 2.63 g/cm3 for
the Paleozoic sedimentary rocks, 2.72 g/cm3 for ‘‘normal’’ Precambrian crystalline rocks, 2.87 g/cm3 for mafic Precambrian crystalline rocks, and 2.57 g/cm3 for felsic Precambrian crystalline rocks.
Since no samples or seismic p-wave velocity data are available to
determine different densities, the above densities are determined
using average density values from representative samples available in the literature (e.g., Telford et al., 1990). The density values
are modified during the modeling process and the final density values represent only one possible solution.
The final step in constructing the cross sections in Fig. 7 is to
compare the preliminary gravity models to the geological cross
sections with new gravity models being generated to reconcile
the differences between the geologically- and gravity-based models. In particular, the gravity models are modified whenever they
suggest that the Paleozoic and Mesozoic sedimentary section is
thicker than would be supported by the surface geology. One possible solution is to include less dense (2.57 g/cm3) felsic Precam-
brian igneous bodies (granitic intrusions) where the surface
geology suggests thin Paleozoic and Mesozoic sedimentary section,
but the gravity data show large amplitude gravity minima. Similarly, more dense (2.87 g/cm3) mafic Precambrian igneous bodies
are used to model the presence of large amplitude gravity maxima.
The presence of such sub-surface mafic Precambrian igneous
bodies is supported by the presence of the Jebel Rahib ophiolite
east of WNPL–NDML region (Fig. 4; Abdel-Rahman et al., 1990;
Schandelmeier et al., 1990).
6. Results
Below is the description of each final gravity model.
6.1. Geological cross section 20°N
The gravity profile along 20°N (Fig. 7A) shows a gravity low of
about 5 mGal interrupted by two gravity highs with amplitudes
of +5 mGal in the west and +10 mGal in the east. These peaks
correspond to the gravity high regions labeled 1 and 2 in Fig. 6A.
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A. Elsheikh et al. / Journal of African Earth Sciences 61 (2011) 82–93
Fig. 6. (A) Residual gravity anomaly map of the West Nubian Paleolake and the Northern Darfur Megalake and surroundings produced from the EGM2008 gravity model
(Pavlis et al., 2008). Numbers 1–7 highlight relative gravity maxima and letters A–G highlight relative gravity minima. Outlined areas show the extent of the above anomalies.
(B) Geological map of the West Nubian Paleolake and the Northern Darfur Megalake and surroundings. After Wycik et al. (1990).
The magnitudes of these gravity highs are too large to be explained
by the presence of near-surface normal Precambrian crystalline
rocks overlain by a thin Paleozoic and Mesozoic sedimentary section. Hence, these gravity highs are modeled as mafic Precambrian
igneous bodies intruding the Precambrian crystalline rocks
(Fig. 7A). Additionally, the 5 mGal magnitude of the gravity lows
(labeled A, B, and C in Fig. 6A) suggests the presence of a relatively
thick Paleozoic and Mesozoic sedimentary section. However, the
lower part of the Mesozoic sedimentary section is exposed at the
surface in this region, suggesting that the Paleozoic and Mesozoic
sedimentary section is only 500 m thick. So, the gravity low is
modeled by a 500 m thick Paleozoic and Mesozoic sedimentary
section overlying Precambrian crystalline basement intruded by a
less dense felsic Precambrian igneous body (Fig. 7A).
6.2. Geological cross section 19°N
The gravity profile along 19°N (Fig. 7B) is characterized by a
central gravity low that has a minimum amplitude of 5 mGal,
but there is a gravity high in the middle of the profile. These gravity
anomalies correspond to the region labeled B in Fig. 6A. Additionally, there is several smaller wavelength gravity highs correspond
to the regions labeled 3, 4, and 5 in Fig. 6A. The central gravity
low lies in a region where the uppermost part of the Mesozoic sedimentary section is exposed suggesting the presence of a thick
Paleozoic and Mesozoic sedimentary section. This part of the profile is modeled with two, 1.6 km deep grabens that are separated
by a horst structure (Fig. 7B). East of this section, is a prominent
gravity high (labeled 5 in Fig. 6A) over a region where the Precambrian crystalline rocks and the lower part of the Paleozoic sedimentary section are exposed, suggesting the presence of a
relatively thin Paleozoic and Mesozoic sedimentary section. This
gravity high has an amplitude of +10 mGal (Fig. 7B), which is larger
than would be expected from an exposure of normal Precambrian
crystalline rocks. This anomaly is modeled as a narrow mafic
Precambrian igneous body (Fig. 7B) which may represent the
northward continuation of the Jebel Rahib ophiolite (Fig. 4).
Immediately east and west of this gravity high there are two small
A. Elsheikh et al. / Journal of African Earth Sciences 61 (2011) 82–93
89
Fig. 7. Gravity profile and geological cross section across the West Nubian Paleolake and the Northern Darfur Megalake along latitudes 20°, 19°, 18°, and 17°300 N. Legand
same as in Fig. 4.
wavelength gravity lows ranging between zero and 5 mGal.
These gravity lows are modeled as felsic Precambrian igneous
bodies (Fig. 7B) since they cannot be reconciled with those expected for gravity anomalies resulting from normal Precambrian
crystalline rocks. The western part of gravity profile 19°N is
dominated by a gravity low that is slightly lower than zero mGals
(Fig. 7B). The gravity anomaly corresponds to the region labeled B
in Fig. 6A. However, within this region the upper part of the
Paleozoic and the lower part of the Mesozoic sedimentary sections
are exposed. This suggests that the Paleozoic and Mesozoic section
is only 500 m thick. Hence, the gravity low is modeled by a
500 m thick Paleozoic and Mesozoic sedimentary section underlain by Precambrian crystalline rocks that are intruded by a felsic
Precambrian igneous body (Fig. 7B).
6.3. Geological cross section 18°N
The gravity profile along 18°N (Fig. 7C) has two gravity lows
(labeled F and G in Fig. 6A) and two gravity high (labeled 6 and 7
in Fig. 6A). The easternmost gravity low of 5 mGal (labeled G in
Fig. 8A), coincides with a region where both the Precambrian
crystalline rocks and the lower part of the Paleozoic sedimentary
section are exposed. This gravity low is modeled with a thin Paleozoic sedimentary section overlying Precambrian crystalline rocks
that are intruded by a felsic Precambrian igneous body (Fig. 7C).
West of this gravity low is a gravity high (labeled 7 in Fig. 6A) with
an amplitude of +10 mGal (Fig. 7C). Nevertheless, this gravity high
is over a region where the middle-upper part of the Mesozoic
sedimentary section is exposed, suggesting that the Paleozoic
and Mesozoic sedimentary section is at least 1 km thick. Hence,
this anomaly is modeled by a 1 km thick Mesozoic and Paleozoic
sedimentary section underlain by Precambrian crystalline rocks
intruded by a mafic Precambrian igneous body (Fig. 7C). The westernmost part of the gravity profile 18°N shows a gravity low with
an amplitude of 7 mGal (labeled 6 in Fig. 6A). This is also where
the lower part of the Paleozoic sedimentary section is exposed,
indicating the presence of only a thin (100–200 m) Paleozoic sedimentary section. Hence, this gravity low is modeled by a thin
Paleozoic sedimentary section overlying Precambrian crystalline
rocks that are intruded by a felsic Precambrian igneous body
(Fig. 7C).
6.4. Geological cross-section 17°300 N
The gravity profile along 17°300 N (Fig. 7D) shows a significantly
higher amplitude of +30 mGal gravity high along its easternmost
part. This gravity high slopes sharply to the west to a gravity low
that reaches 10 mGal (Fig. 7D). West of this anomaly are broad
gravity high and lows that fluctuate between 0 and +10 mGal.
The gravity lows correspond to the ones which are labeled E and
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Fig. 8. Three-dimensional perspective view of the West Nubian Paleolake and the Northern Darfur Megalake and possible underlying geologic structures. Legand same as in
Fig. 4.
F in Fig. 6A, whereas the gravity high is labeled 7 in the same figure. Precambrian crystalline rocks are exposed within the region
defined by the gravity high. This gravity high is expected because
this is where the Jebel Rahib ophiolite is exposed to the surface.
Hence, the gravity high is modeled by a mafic Precambrian igneous
body representing the root of the Jebel Rahib ophiolite (Fig. 7D).
The short wavelength gravity high as well as the long wavelength
gravity lows and highs to the west are modeled by a series of grabens as deep as 2.5 km (Fig. 7D). The gravity high in the westernmost part of gravity profile 17°300 N (labeled E in Fig. 6A) cannot be
explained as due to the presence of a thick Paleozoic and Mesozoic
sedimentary section. This is because the lowermost part of the
Paleozoic sedimentary section is exposed to the surface in this region. Hence, the gravity low is attributed to a felsic Precambrian
igneous body intruding the Precambrian crystalline rocks underlying a thin (100 m) Paleozoic sedimentary section (Fig. 7D).
6.5. Potential groundwater storage
Fig. 8 shows the distribution of graben structures in relation to
the surface expression of WNPL–NDML. It shows that the central
part of the paleolake is underlain by prominent graben structures.
Similarly, the region to the south of WNPL–NDML is underlain by
potentially deep grabens. Additionally, reconstruction of a structural map of the paleolake region from the surface geology (mostly
geological contacts that juxtapose formations of vastly different
ages) and the gravity data (especially the gravity lows labeled B
and C in Fig. 6A and gravity highs labeled 2 and 5 in the same figure) indicate the domination of NE-trending graben and horst
structures (Fig. 9A). However, NW-trending faults are not uncommon (Fig. 9A).
To evaluate groundwater storage capacity of the Nubian aquifer
under WNPL–NDML, an isopach map to the depth of the Precambrian crystalline rocks is created (Fig. 9B). This is achieved by
measuring the depth to the Precambrian crystalline rocks at
specific points along profile 20°N, 19°N, 18°N, and 17°30N using
the geological cross sections shown in Fig. 7. These are subsequently contoured at interval of 100 m. This map suggests the
presence of a major basin under WNPL–NDML, especially in its
central part where the depth to the Precambrian crystalline rocks
is as deep as 1.6 km. The depth to the Precambrian crystalline basement rocks becomes shallow to the east where the Precambrian
rocks are exposed to the surface in many locations (Fig. 9B).
The groundwater storage capacity of the basin under WNPL–
NDML is calculated through the equation V = A AT EP. Where V
is the volume of groundwater, A is the surface area of the basin,
AT is the average thickness of the Paleozoic and Mesozoic sedimentary section, and EP is the effective porosity. The effective porosity
in Northern Darfur under saturated conditions is estimated by the
Food and Agricultural Organization (FAO, 2003) to be 7%. Additionally, the depth of water table in the region is estimated to be
50 m (Gossel et al., 2004). Hence, this basin which has an area
of 13,160 km2 and a Paleozoic and Mesozoic sedimentary section
of 1.6 km is capable of holding 1120 km3 of groundwater if the
sedimentary rocks are completely saturated. However, considering
an economically feasible depth of 350 m to extract groundwater,
only 275 km3 of groundwater will be available for use. It is worth
mentioning here that estimates of groundwater storage capacity of
the Nubian Aquifer in northwestern Sudan vary significantly. Bakhbakhi (2006) has estimated that the Nubian aquifer in northwestern Sudan holds up to 33,570 km3 of groundwater. Similarly, FAO
(2003) estimated that the Nubian aquifer in the Sudan contains
33,880 km3 of groundwater but only 2610 km3 of this resource
is recoverable. In contrast, Ibrahim (1995) estimated the total volume of groundwater in the Nubian aquifer in northwestern Sudan
to be only 6 km3.
7. Discussion
Results of this work are discussed in relation to groundwater
rechargability and migration, and potential environmental impact
of the extraction of groundwater.
A. Elsheikh et al. / Journal of African Earth Sciences 61 (2011) 82–93
91
Fig. 9. (A) Structural map of the West Nubian Paleolake and the Northern Darfur Megalake. (B) Isopach map of the West Nubian Paleolake and the Northern Darfur Megalake
showing the depth to the Precambrian crystalline rocks.
7.1. Groundwater rechargability
The WNPL–NDML is underlain by the Paleozoic–Mesozoic
Nubian aquifer which is estimated to have a reservoir capacity of
75,000 km3 (Hess et al., 1987). There might be some presentday recharge of the aquifer by surface water around the Ennedi
and Tibesti mountains in Chad and Libya, and the Erdi Ma mountains in Sudan and Chad (Ball, 1927; Sandford, 1935; Hellström,
1940; Pachur et al., 1990; Pachur and Hoelzmann, 2000;
Hoelzmann et al., 2000; Robinson et al., 2007; Ghoneim and
El-Baz, 2007; Paillou et al., 2009). The River Nile does not provide
recharge for the aquifer, except in low rate around limited areas
along the river in northern Sudan and southern Egypt (Heinl and
Brinkmann, 1989; Kim and Sultan, 2002; Gossel et al., 2004).
As early as the 1940s and 1950s, it is proposed that the Nubian
aquifer has not been in a hydrological steady-state condition for
thousands of years. It is asserted that groundwater in the aquifer
is fossil water, and its extraction would amount to mining an unrenewable water resource (Hellström, 1940). This point of view
has been reinforced by research in southwestern Egypt, where
the extraction of significant amounts of groundwater began in
the 1960s (Pallas, 1980; Heinl and Brinkmann, 1989; Heinl and
Thorweihe, 1993; Nour, 1996; Ebraheem et al., 2002; Sabed and
Zeid, 2003; Gossel et al., 2004).
The lack of recent recharge of the Nubian aquifer is also supported by isotopic studies. Heinl and Brinkmann (1989) used Carbon-14 (14C) dating of samples from wells in Egypt and Libya to
show that the majority of the groundwater is 25,000–40,000 years
old and that only 5% of the analyzed samples yield evidence of recharge at 8000–6000 years ago. Abdelghafour (1993) concluded
that the age of the groundwater in the aquifer varies between
10,000 and 33,000 years. Dabous and Osmond (2001) used
uranium isotopes to confirm that there is no meaningful recent
recharge of the Nubian Aquifer in Egypt and that a significant portion of the groundwater is deep artesian inflow rather than local
pluvial recharge. Sturchio et al. (2004) used Krypton-81 (81Kr/Kr)
and Chlorine-36 (36Cl/Cl) age dating for groundwater samples from
southwestern Egypt to show that water in the Nubian aquifer
ranges between 200,000 years and 1 Ma.
7.2. Environmental impact of the extraction of groundwater
It is feared hear that one direct impact of excessive extraction of
the groundwater under WNPL–NDML is the drying-out of natural
oases present in northern Darfur. Two oases (Nukheila and Atrun)
exist in the eastern side of WNPL–NDML (Fig. 1). The presence of
the Nukheila oasis was first reported in the scientific literature
by Newbold and Shaw (1926) and Shaw (1928) who described it
as a lake with a water depth ranging between 1 and 2 m. The
groundwater table in the Atrun oasis ranges between 3 and 6 m
and it varies between 1 and 5 m in the Nukheila oasis (Haynes
et al., 1979; Abdelghafour, 1993). Haynes et al. (1979) determined
the extent of this lake to be 5 by 10 km, with a water depth of
about 3–4 m. Haynes et al. (1979) suggested that the lake is fed
by groundwater trapped by sand dunes. Hoelzmann et al. (2001)
stated that the Atrun oasis was connected to WNPL in periods of
flooding during the wetter early Holocene time. Abdelghafour
(1993) proposed that the Nubian aquifer beneath the Nukheila
and Atrun oases is confined due to the thickness of the aquifer. This
suggestion agrees with the cross sections constructed for WNPL–
NDML which show deep basins at the local level (Fig. 9). In addition, the location of these oases is aligned with normal faults
mapped in this study from geological and geophysical data
(Fig. 9A).
The Nukheila and Atrun oases in this hyper-arid region provide
a significant amount of surface water that supports life in northern
92
A. Elsheikh et al. / Journal of African Earth Sciences 61 (2011) 82–93
Darfur. These oases exist because of recharge from the groundwater of the Nubian aquifer under WNPL–NDML. It is unlikely that the
15 mm per year precipitation in this region will result in the formation of standing water bodies, especially that the evaporation
rate is 2.5–3.0 m/year (Hoelzmann et al., 2000, 2001; Pachur and
Hoelzmann, 2000). Hence, the extraction of large amounts of
groundwater from the Nubian Aquifer under WNPL–NDML would
likely depress the groundwater table beneath these oases, causing
them to dry out completely.
8. Conclusions
1. There are a number of NE-trending grabens under WNPL–NDML
within which the Nubian aquifer can be as thick as 1.6 km. The
basin constituting these grabens has groundwater storage
capacity of 1120 km3 if the Paleozoic–Mesozoic sedimentary
rocks are completely saturated.
2. Present-day recharge of the Nubian aquifer under WNPL–NDML
is unlikely and the local aquifer under the paleolake will not be
in a steady-state condition because any extraction of groundwater will not be met with equal recharge. This groundwater,
therefore, must be considered fossil water.
3. Excessive extraction of groundwater from under WNPL–NDPL
will likely lead to lowering of the groundwater table which will
lead to the drying-out of oases in northern Darfur.
Acknowledgements
The authors would like to thank Drs. F. El-Baz and E. Ghoneim
for providing the shapefile of NDML and for valuable discussion
during the preparation of this manuscript. We thank two anonymous reviewers for their constructive comments. This is Missouri
University of Science and Technology, Geology and Geophysics
Program Contribution Number 30.
References
Abdelghafour, O.H., 1993. A Three Dimensional Groundwater Flow Model for Wadi
Howar Area, Darfur Province, Western Sudan. M.S. Thesis, Ohio University
(unpublished), 127p.
Abdel-Rahman, E.M., Harmes, U., Schandelmeier, H., Franz, G., Darbyshire, D.P.F.,
Horn, P., Muller-Sohnius, D., 1990. A new ophiolite occurrence in NW Sudan:
constraints on late Proterozoic tectonism. Terra Nova 2, 363–376.
Abdelsalam, M.G., Robinson, C., El-Baz, F., Stern, R.J., 2000. Applications of orbital
imaging radar for geologic studies in arid regions: the Saharan testimony.
Photogrammetric Engineering Remote Sensing 66, 717–726.
Abdelsalam, M.G., Liegeois, J., Stern, R.J., 2002. The Saharan Metacraton. Journal of
African Earth Sciences 34, 119–136.
Abell, P.I., Hoelzmann, P., 2000. Holocene palaeoclimates in northwestern Sudan:
stable isotope studies on molluscs. Global Planetary Change 26, 1–12.
Bakhbakhi, M., 2006. Nubian sandstone aquifer system. In: Foster, S., Loucks, D.P.
(Eds.), Non-Renewable Groundwater Resources: A Guidebook on Socially
Sustainable Management for Water-Policy Makers. United Nations
Educational, Scientific and Cultural Organization (IHP-VI Series on
Groundwater 10), Paris, pp. 75–81.
Ball, J., 1927. Problems of the Libyan desert. Journal of Geography 70, 21–512.
Butler, D., 2007. Darfur lake is a mirage. Nature 448, 394–395.
Dabous, A.A., Osmond, J.K., 2001. Uranium isotopic study of artesian pluvial
contributions to the Nubian aquifer western desert Egypt. Journal of Hydrology
243, 242–253.
Ebraheem, A.M., Riad, S., Wycisk, P., Seif El Nasr, A.M., 2002. Simulation of impact of
present and future groundwater extraction from the non-replenished Nubian
sandstone aquifer in SW Egypt. Environmental Geology 43, 188–196.
Food and Agriculture Organization (FAO), 2003. Groundwater Management – The
Search for Practical Approaches. Water Report No. 25, 54 p.
Friedman, B., 2007. Can Water Flow Peace in Sudan. American Association of
Petroleum Geologists Explorer, November 18, 2007, p. 20.
Gabriel, B., Kröpelin, S., 1984. Holocene lake deposits in northwest Sudan.
Palaeoecology of Africa 16, 295–299.
Ghoneim, E., El-Baz, F., 2007. DEM optical radar data integration for
palaeohydrological mapping in the northern Darfur Sudan: implication for
groundwater exploration. International Journal of Remote Sensing 28, 5001–
5018.
Gossel, W., Ebraheem, A.M., Wycisk, P., 2004. A very large scale GIS based
groundwater flow model for the Nubian sandstone aquifer in eastern
Sahara Egypt, northern Sudan, and eastern Libya. Hydrology Journal 12,
698–713.
Haynes, C.V., Mehringer, P.J., Zaghloul, E.S.A., 1979. Pluvial lakes of northwestern
Sudan. The Geographical Journal 145, 437–445.
Heinl, M., Brinkmann, P.J., 1989. A groundwater model of the Nubian aquifer
system. Hydrological Sciences 34, 425–447.
Heinl, M., Thorweihe, U., 1993. Groundwater resources management in SW Egypt.
In: Wycisk, P., Meissner, B. (Eds.), Geopotential Ecology, vol. 6. Catena Verlag,
Cremlingen-Destedt, pp. 99–121.
Hess, K.H., Hissene, A., Kheir, O., Schnacker, E., Schneider, M., Thorweihe, U., 1987.
Hydrogeological investigation in the Nubian aquifer system, eastern Sahara.
Berliner Geowissenschaftliche Abhandlungen (A) 75.2, 397–464.
Hellström, B., 1940. The subterranean water in the Libyan desert. Geografiska
Annaler Series A – Physical Geography 21, 206–239.
Hoelzmann, P., Kruse, H., Rottinger, F., 2000. Precipitation estimates for the eastern
Saharan palaeomonsoon based on water balance model of the west Nubian
palaeolake basin. Global Planetary Change 26, 105–120.
Hoelzmann, P., Keding, B., Berke, H., Kröpelin, S., Kruse, H., 2001. Environmental
change archaeology: lake evolution human occupation in the eastern Saharan
during the Holocene. Palaeogeography, Palaeoclimatology, Palaeoecology 169,
193–217.
Ibrahim, F.N., 1995. Sustainable water utilization in northern Darfur, Sudan. Applied
Geography Development 45–46, 41–145.
Kim, J., Sultan, M., 2002. Assessment of the long term hydrologic impacts of the Lake
Nasser related irrigation projects in southern Egypt. Journal of Hydrology 262,
68–83.
Kuper, R., 2006. After 5000 BC: the Libyan desert in transition. Palevol 5,
409–419.
Kuper, R., Kröpelin, S., 2006. Climate controlled Holocene occupation in the Sahara:
motor of Africa’s evolution. Science 313, 803–807.
McCauley, J.F., Schaber, G.G., Breed, C.S., Grolier, M.J., Haynes, C.V., Issawi, B., Elachi,
C., Blom, R., 1982. Subsurface valleys geoarchaeology of the eastern Sahara
revealed by shuttle radar. Science 218, 1004–1020.
McCauley, J.F., Breed, C.S., Schaber, G.G., McHugh, W.P., Issawi, B., Haynes, C.V.,
Grolier, M.J., El Kilani, A., 1986. Paleodrainages of the eastern Sahara the radar
rivers revisited SIR-A/B implications for a mid-Tertiary trans-Africa drainage
system. Transaction on Geosciences and Remote Sensing 24, 624–648.
Newbold, D., Shaw, W.B., 1926. An exploration in the south Libyan desert. Sudan
Notes and Records 11, 103–194.
Nour, S., 1996. Groundwater potential for irrigation in the east Oweinat area,
Western Desert of Egypt. Environmental Geology 27, 143–154.
Pallas, P., 1980. Water resources of the Socialist People’s Libyan Arab Jamahiriya. In:
Busrewil, M.T. (Ed.), The Geology of Libya III. Academic Press, London, pp. 539–
594.
Pachur, H.J., Kropelin, S., Hoelzmann, P., Goschin, M., Altmann, N., 1990. Late
Quaternary fluvio lacustrine environment of western Nubia. Berliner
Geowissenschaftliche Abhlungen (A) 102.1, 203–260.
Pachur, H.J., Rottinger, F., 1997. Evidence for a large extended paleolake in the
eastern Sahara as revealed by spaceborne radar lab images. Remote Sensing of
the Environment 61, 437–440.
Pachur, H.J., Hoelzmann, P., 2000. Late Quaternary palaeoecology of the eastern
Sahara. Journal of African Earth Sciences 30, 929–939.
Paillou, P., Schuster, M., Tooth, S., Farr, T., Rosenqvist, A., Lopez, S.,
Malezieux, J., 2009. Mapping of a major paleodrainage system in
eastern Libya using orbital imaging radar: the Kufrah river. Earth
Planetary Science Letters 277, 327–333.
Pavlis, N., Kenyon, S., Factor, J., 2008. Earth gravitational model 2008. Society of
Exploration Geophysicists Expanded Abstracts 27, 761.
Robinson, C.A., Werwer, A., El-Shazly, F., El-Shazly, M., 2007. The Nubian aquifer in
southwest Egypt. Hydrogeology Journal 15, 33–45.
Rodrigues, D., Abell, P.I., Kröpelin, S., 2000. Seasonality in the early Holocene climate
of northwest Sudan: interpretation of Etheria elliptica shell isotopic data. Global
Planetary Change 26, 181–187.
Sabed, S.S., Zeid, A.M., 2003. Contributions to the hydrogeology of Nubian sandstone
aquifer in east El-Oweinat area southwestern desert Egypt. Isotope and
Radiation Research 33, 161–177.
Sandford, K.S., 1935. Sources of groundwater in northwestern Sudan. Journal of
Geography 85, 412–431.
Schandelmeier, H., 1988. Pre-Cretaceous intraplate basins of NE Africa. Episodes 11,
270–274.
Schandelmeier, H., Klitzsch, E., Hendriks, F., Wycisk, P., 1987a. Structural
development of northeast Africa since Precambrian times. Berliner
Geowissenschaftliche Abhlungen (A) 75, 5–24.
Schandelmeier, H., Richter, A., Harms, U., 1987b. Proterozoic deformation of the east
Saharan craton in southeast Libya, south Egypt, and north Sudan.
Tectonophysics 140, 233–246.
Schandelmeier, H., Richter, A., Harms, U., Abdel Rahman, E.M., 1990. Lithology
structure of the late Proterozoic Jebel Rahib fold-and-thrust belt (NW Sudan).
Berliner Geowissenschaftliche Abhlungen (A) 102.1, 15–30.
Schandelmeier, H., Pudlo, D., 1990. The central African fault zone (CAFZ) in Sudan a
possible continental transform fault. Berliner Geowissenschaftliche Abhlungen
(A) 102.1, 31–44.
Schandelmeier, H., Reynolds, P.O., 1997. Palaeogeographic palaeotectonic atlas of
north eastern Africa, Arabia, and adjacent areas. In: Schandelmeier, H.,
A. Elsheikh et al. / Journal of African Earth Sciences 61 (2011) 82–93
Reynolds, P.O. (Eds.), Palaeogeographic palaeotectonic atlas of north eastern
Africa, Arabia, and Adjacent Areas. Balkerma, Rotterdam, p. 160.
Shaw, W.B., 1928. Darb el Arab’in: the forty day’s road. Sudan Notes and Records 12,
63–71.
Sturchio, N.C., Du, X., Purtschert, R., Lehmann, B.E., Sultan, M., Patterson, L.J., Lu, Z.T.,
Mueller, P., Bigler, T., Bailey, K., O’Connor, T.P., Young, L., Lorenzo, R., Becker, R.,
El Alfy, Z., El Kaliouby, B., Dawood, Y., Abdallah, A.M.A., 2004. One million year
old groundwater in the Sahara revealed by Krypton-81 Chlorine-36.
Geophysical Research Letters 31, 1–4.
Szabo, B.J., Haynes, C.V., Maxwell, T.A., 1995. Ages of Quaternary pluvial episodes
determined by uranium series radiocarbon dating of lacustrine deposits of
eastern Sahara. Paleogeography, Paleoclimatology, Paleoecology 113, 227–242.
93
Telford, W., Geldart, L., Sheriff, R., 1990. Applied Geophysics. Cambridge University
Press, 771p.
Wendorf, F., Schild, R., Close, A.E., Schwarcz, H.P., Miller, G.H., Grun, R., Bluszcz, A.,
Stokes, S., Morawska, L., Huxtable, J., Lundberg, J., Hill, C.L., McKinney, C., 1994.
A chronology for the middle late Pleistocene wet episodes in the eastern Sahara.
In: Bar-Yosef, O., Kra, R.S. (Eds.), Late Quaternary Chronology Paleoclimates of
the Eastern Mediterranean. Radiocarbon in Association with American School of
Prehistoric Research, Tucson, pp. 147–168.
Wycik, P., Klitzsch, E., Jas, C., Reynolds, O., 1990. Intracrational sequences
development structural control of Phanerozoic strata in Sudan. Berliner
Geowissenschaftliche Abhlungen (A) 102.1, 31–44.