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New insight on paleoriver development in the Nile basin of the eastern Sahara
Journal of African Earth Sciences 62 (2012) 35–40
Contents lists available at SciVerse ScienceDirect
Journal of African Earth Sciences journal homepage: www.elsevier.com/locate/jafrearsci
New insight on paleoriver development in the Nile basin of the eastern Sahara Mohamed Abdelkareem a,b,⇑, Eman Ghoneim c, Farouk El-Baz a, Mohamed Askalany b a
Center for Remote Sensing, Boston University, 725 Commonwealth Ave., Boston, MA 02215-1401, USA Geology Department, South Valley University, Qena 83523, Egypt c Department of Geography and Geology, University of North Carolina, Wilmington, 601 S. College Road, 28403, USA b
a r t i c l e
i n f o
Article history: Received 19 April 2011 Received in revised form 3 September 2011 Accepted 5 September 2011 Available online 23 September 2011 Keywords: Paleodrainage Eastern Sahara Nile African drift
a b s t r a c t Since the first detection of paleochannels beneath sand sheets and sand dunes in the Sahara using Shuttle Imaging Radar (SIR-A) data, key advances in the understanding of these features have been made. The Sahara is currently the largest and driest region on Earth. However, it was drained by numerous rivers that are now dry channels beneath sand sheets and sand dunes. The present Sahara reflects past pluvial conditions, and the transitions from heavy rainfall to arid or hyperarid conditions reveal major climate shifts. Here, we propose that the evolution of the Sahara occurred in response to the stages of the location of the African Plate relative to the Earth’s equator, i.e., as a result of the northward drift of Africa in space and time. For instance, it is probable that during the late Eocene or Oligocene the Earth’s equator was located at the current-day latitudes of Chad and Sudan. This geometry would have produced pluvial conditions throughout North Africa. With increasing drift in space and time during the Miocene, Pliocene, and Pleistocene, the source points shifted. New valleys were formed and old ones were abandoned, and the length of the main stream of the Nile increased. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction Since the first discovery of paleorivers in the Sahara by radar imaging systems (McCauley et al., 1982), several studies (El-Baz and Mainguet, 1981; McCauley et al., 1986; Schaber et al., 1997; El-Baz, 1988, 1998; Robinson et al., 1999, 2000; El-Baz et al., 2001; Ghoneim et al., 2007; Ghoneim and El-Baz, 2007) have shown increasing details of the paleochannels and their flow directions. However, these findings have not produced strong correlations with the major factors that influenced the development of the paleorivers. These factors were intimately related to the general northward slope of Africa, the nature of the East African Rift and its effects on facilitating the northward flow, previous global sea-level and climate changes, and the position of the African Plate relative to the Earth’s equator. The eastern Sahara is the location of the Nile River, the longest channel system in the world. The Nile crosses 35° of latitude (4°S–31°N) through a wide variety of climates. From the Great Lakes at the Equator and the White Nile, which joins the Blue Nile in central Sudan and the Atbara in northern Sudan, the Nile River flows to the Mediterranean Sea without any present evidence of water supply. In fact, the Nile River appears to have had a complex geological history.
⇑ Corresponding author at: Center for Remote Sensing, Boston University, 725 Commonwealth Ave., Boston, MA 02215-1401, USA. E-mail address: [email protected] (M. Abdelkareem). 1464-343X/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.jafrearsci.2011.09.001
A recent hypothesis for Nile evolution suggests that from the Late Miocene to the present, the main river in Egypt had a northward flow through Egypt’s desert to the Mediterranean Sea (Said, 1981). This river was assigned numerous names and developed in successive stages as follows: the Eonile (Late Miocene), Paleonile (Pliocene), and Protonile, Prenile, and Neonile of the Pleistocene (Said, 1981). Here, we propose a new approach to the evolution of the eastern Sahara’s paleorivers based on the position of the African Plate relative to the Earth’s equator. Our hypothesis would explain major shifts in paleoclimate. It would also explain why the Great Sahara is the driest region on Earth, although it was drained by numerous rivers in the past. Our hypothesis suggests why paleolakes and paleorivers were abandoned in the Sahara and why the Nile River is the largest river in the world. It proposes answers to additional questions: Did the Nile develop in stages (Williams, 2009) to reach its current length? Furthermore, did an earlier stage of the Nile River start at the latitudes of today’s Chad and Sudan? Did the Nile flow through Egypt’s Qena Valley to the Tethys Sea, where its former delta was located?
2. Data and methods Views of the Earth from space provide fast and effective means to complement classical observation methods in field geology in the study of surface and near-surface features. Landsat Enhanced Thematic Mapper (ETM+) and Shuttle Radar Topography Mission
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M. Abdelkareem et al. / Journal of African Earth Sciences 62 (2012) 35–40
Fig. 1. Landsat composite image of Africa. The green areas display the vegetated belt, the blue-black color indicates water, and the pale-yellow to creamy color signifies arid desert. The numbers refer to the site locations mentioned in the text. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
(SRTM) images have been used to interpret the landscape and suggest plausible scenarios for its evolution in space and time. Sand-buried channels were revealed and mapped using radar images. Unlike optical images, radar observations allow representation of the backscattering of the active radar waves. This process reveals the surface and near-surface physical properties of the terrain (Ford et al., 1989). Robinson et al. (2006) and Ghoneim et al. (2007) indicated the similarity of the SRTM data to the Radarsat-1 and SIR-C C-band images. The SRTM images are based on microwave signals (k = 5.7 cm). These signals would be expected to penetrate the sand cover, as do the Radarsat-1 and SIR-C C-band images. The depth of subsurface penetration of the
C-band used for imaging is approximately 0.5 m (Schaber et al., 1997). The Enhanced Thematic Mapper images covering the African continent were used to prepare a base map. The six ETM+ bands 1–5 and 7 were used for the map. The panchromatic band 8 and infrared thermal band 6 were not included. The Environment for Visualizing Images (ENVI) v 4.8 software package was used to furnish digital image pre-processing and processing techniques. A mosaic was prepared and calibrated based on reflectance. Digital image processing using the Landsat ETM+ RGB color combination was selected based on the reflectance spectra of the dominant rock types. False 742 RGB color combinations were
M. Abdelkareem et al. / Journal of African Earth Sciences 62 (2012) 35–40
utilized to define the variations in arid and vegetated areas and define the boundary of the desert (Fig. 1). The Shuttle Radar Topography Mission (SRTM) data covering Africa were obtained from the NASA-Department of Defense National Imagery and Mapping Agency (NIMA) joint project. The data are available internationally at a horizontal resolution of approximately 90 m and a vertical accuracy of 16 m with a 90% confidence level (Rodriguez et al., 2005). These data were processed to construct a continuous Digital Elevation Models (DEMs) of the study area and project it to the Universal Transverse Mercator (UTM) and WGS84 datum. The terrain model was produced with the Environment for Visualizing Images (ENVI) 4.8 and ARC GIS 9.3.1 software packages. The data mentioned above were saved in GIS layers to allow the superposition and correlation of surface and subsurface features. Each layer added significant information, thus facilitating the investigation of the study area. The drainage pattern of Africa and its watershed was extracted using SRTM data. This analysis was performed by employing the surface flow routing based on the 8 D flow direction algorithm (Jenson and Domingue, 1988), which has been widely used in the literature (Ghoneim and El-Baz, 2007; Ghoneim et al., 2007). This step was followed by the derivation of the surface flow accumulation, which was used to delineate the channel network of the basin automatically by specifying a threshold of 5000 cells. The entire Nile basin was extracted by using a mask of the derived channels.
3. Results and discussion A Landsat ETM+ mosaic shows a densely vegetated green belt (Fig. 1), which reflects modern-day tropical rainfall along the Earth’s equator across central Africa. This view offers no indication of the existence of the paleodrainage networks. The desert belt appears as featureless sand and masks most of the older surface features. Furthermore, the Nile River is the only large channel that straddles the arid-to-hyperarid Sahara. The Shuttle Radar Topography Mission (SRTM) data (Fig. 2) show that the Sahara is bounded on the east by the Red Sea highlands and on the northwest by the Atlas Mountains. In the middle are a few less elevated areas, including those in Darfur, Tibesti and Hogar. The results from the automatic extraction of drainage channels reveal that the African continent was veined by paleorivers, particularly in the Sahara. Today, most of these paleodrainages are dry valleys totally obscured by sand deposits. Based on the northward and counterclockwise drift of Africa, the position of the Sahara during the late Eocene and Oligocene was approximately 6–8° of latitude south of its present-day location (Smith et al., 1994; Lotfy and Van der Voo, 2007; Swezey, 2008) (Fig. 3a). Accordingly, the climate in the Sahara during that period is thought to have been humid and warm. Egypt’s climate during that time was dominated by heavy rainfall, averaging 1200–1500 mm/year (Issawi and Osman, 2008, p. 43). It is probable that this climate was similar to that of present-day Rwanda and Uganda, i.e., an equatorial climate. Thus, we believe that the earlier Nile collected its water from the Red Sea highlands, such as the now-arid valleys of Allaqi, Kharit, Qena and El-Hammamat in the east and Tushka in the west. These conditions of heavy rainfall facilitated the formation and transportation of vast amounts of fluvial sediments, consisting primarily of sand and gravel, to the northern Sahara during past pluvial episodes. These deposits are the oldest non-marine Cenozoic strata of Eocene/Oligocene age in the Sahara (in Faiyum in Egypt, and in Algeria and Niger) (Swezey, 2008). The well-preserved strata of Oligocene age (fluvial sand and conglomerate) reveal that north-flowing fluvial systems became a main feature of parts of
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North Africa during that time, i.e., in Faiyum (Egypt), and in the Mauritania–Senegal basin (Swezey, 2008). The presence and magnitude of these fluvial sands in the northern part of the Sahara require thoughtful consideration. The sand accumulations required sufficient rainfall to support the development of extensive fluvial systems. El-Baz hypothesized that the northward flow of paleorivers transported the sand (El-Baz, 1982). This hypothesis supports our idea and suggests that the source of the sand was the rocks of the Nubian Sandstone. This formation has extensive exposures throughout the southeastern Sahara. The exposed rocks north of the sand dunes were primarily limestone. This limestone could not have been the source of the vast amounts of quartz sand. The fluvial deposits might have been transported by multiple ancient rivers during past pluvial episodes along the general northward slope of Africa (Robinson et al., 1999). It has been hypothesized that the modern Nile canyon is of Late Miocene age (Said, 1981). If so, what was the source of the Late Eocene/Oligocene fluvial deposits of the old delta in Faiyum? According to our results, these deposits were transported along the earlier path of the Nile. This route is clearly distinct from the present Nile canyon that leads to the Mediterranean Sea. It has been hypothesized that the Nile flowed through the Qena Valley to the Tethys Sea shoreline at the former location of the Nile delta (Abdelkareem et al., 2010). The northward drift of Africa continued through the Miocene (Fig. 3b) and proceeded in a counterclockwise direction during the late Miocene, approximately 4° of latitude south of its present-day position (Smith et al., 1994; Swezey, 2008). We believe that this movement caused the formation of new valleys and the abandonment of old valleys in the Sahara, including Tushka, Howar, El-Malik, Allaqi, Kharit, El-Hammamat and Qena. For example, the extinct Nile tributary of the Tushka megawatershed (Ghoneim and El-Baz, 2007) is today an arid valley. However, it was most likely a major tributary of the earlier Nile River and drained northeasterly toward the Tushka depression west of the present Lake Nasser in Egypt. This drainage basin would have collected its water from an area of more than 150,000 square km during past pluvial events. This information explains the presence of Nilotic fish and crocodile remains in the western desert, now 300 km west of the present Nile (Issawi and Osman, 2008). Wadi Howar, presently dry, is an extinct tributary of an earlier stage of the Nile River (Pachur and Kropelin, 1987; Ghoneim and El-Baz, 2008). Its upper streams are located in Chad. It flows eastward in Sudan to the Nile near the town of Old Dongola at the Nile bend. The present dry course of Wadi El-Malik runs northeasterly toward the Nile River. Thus, Bowen and Jux (1987) believed that the Nile and Lake Chad were connected owing to the similarity in fish fauna and the probable outlet of Lake Chad to the Nile basin. During that period, tectonic disturbances along the Red Sea throughout Egypt and Sudan produced the great bend of the Nile in Sudan (Gabgaba) and the Qena bend in Egypt. Furthermore, the elevation of the north Qena Valley, adjacent to the Red Sea, began during the late Oligocene and/or Early Miocene (Garfunkel, 1988; Omar et al., 1989) or the Middle–Late Miocene (Bohannon et al., 1989). These events roughly coincided with a major change in fluvial systems (Swezey, 2008) and produced a change from deposits rich in quartz to deposits rich in silt and kaolinite. It is probable that this change coincided with the opening of the Red Sea and the East African Rift. It also caused a change of the Nile pathway from the Qena Valley at the Qena bend to today’s Nile canyon leading to the Mediterranean Sea (Eonile; Late Miocene) as a result of the westward movement related to the tectonics of East Africa.
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The change from the Oligocene to the Miocene was also associated with a gradual rise in sea level and a progressively warmer and more humid climate. Therefore, Egypt’s climate was tropical to subtropical during the late Eocene and through the Oligocene and the Miocene, with 1200–1500 mm/year precipitation (Issawi and Osman, 2008). It is probable that this climate was similar to the present climate at the Earth’s equator, with a mean annual precipitation of 875–1915 mm (Woodward et al., 2007). With the increasing northward drift, the rainfall in the Sahara became less intense during the Pliocene and the Pleistocene (400–600 mm/year, compared with 1200–1500 mm/year in the late Eocene–Miocene) (Issawi and Osman, 2008, p. 43). It is probable that the vast and widespread paleosols during the Pliocene imply the advent of arid desert conditions (Swezey, 2008). With the increasing northward drift (Fig. 3c), the source points changed and were located nearer to today’s Equatorial lakes. However, the absence of central African faunal elements in Late Pliocene sediments suggests that these sediments were not derived from central
Africa (Said, 1981). Thus, we hypothesize that no connection then existed with the Great Lakes of equatorial Africa. However, it is probable that the Blue Nile and Atbara linked the Nile channel system to the Mediterranean Sea. During the Early Pleistocene, it is probable that Egypt’s climate was highly arid and that Egypt developed into a desert (Said, 1981). These changes were intimately connected with the northward drift and the change of the climate in the Sahara. Accordingly, new tributaries and sediments were formed. The mineralogical and paleontological content of these Middle Pleistocene sediments suggest that they are derived from distant sources. They are distinguished from old deposits by their rich pyroxene content. It is probable that these sources are associated with the modern tributaries of the Nile, the Atbara and Blue Nile, in Ethiopia, despite their smaller content of epidote than that found in modern sediments (Shukri and Azer, 1952). This evidence indicates that the Atbara and Blue Nile first began to flow into the Egyptian Nile during this period (Shukri and Azer, 1952), prior to the Nile’s connection with the modern equatorial
Fig. 2. Drainage network of Africa derived from Shuttle Radar Topography Mission (SRTM) data. Drainage lines are shown in white. Solid blue lines specify the drainage lines of the Nile within its watershed. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
M. Abdelkareem et al. / Journal of African Earth Sciences 62 (2012) 35–40
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Fig. 3. Stages of evolution of the Nile River. Solid white lines denote the drainage network of the Nile. Dotted white lines trace former Nile tributaries that became extinct as a result of the northward drift of the African continent. The dashed blue line indicates Earth’s equator relative to the drifting continent. The dashed dark lines trace the East African Rift. (a) Paleogeographic position of Africa during Eocene/Oligocene taken from Smith et al. (1994), Lotfy and Van der Voo (2007) and Swezey (2008). (b) Paleogeographic position of Africa during Miocene taken from Smith et al. (1994) and Swezey (2008). (c) Paleogeographic position of Africa during Pliocene to present taken from Smith et al. (1994) and Swezey (2008). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
lakes. However, it has been hypothesized that the White Nile drained from the equatorial lakes of central Africa and had no real connection with the Egyptian Nile until the latest Pleistocene (Bowen and Jux, 1987). Moreover, the age of the present Nile is approximately 0.5 Ma (Issawi and Osman, 2008). The present Nile became connected with central Africa and the Mediterranean in the Middle Pleistocene, as demonstrated by evidence from oxygen isotope stages (OIS 7–OIS 5) (Maxwell et al., 2010, p. 1135). This connection was facilitated by the structure of the East African Rift and the characteristics of the eastern Sahara. These features maintained the Nile Valley along a pathway bounded by the Red Sea highlands in the east and by the elevated plateaus to the west, particularly in Egypt. These hypotheses about the evolution of the present drainage network from paleorivers and about its fluvial geomorphology reflect long-term tectonic and sea-level changes (Said, 1981) and major shifts in climate. It is probable that these shifts in climate did not result from one stage but from several successive stages. The changes of stratigraphy from early Cenozoic carbonate (a deep marine environment) to the late Cenozoic silicic clastics reflected
a ‘‘long-term eustatic fall in sea level since the middle Cretaceous with a global climate transition from a Late Cretaceous–Early Eocene warm conditions to a late Eocene–Quaternary cool conditions’’ (Swezey, 2008). It is probable that this process coincided with the subsequent northward drift of Africa.
4. Conclusions It is reasonable to assume that the northward drift of Africa initiated a major shift in climate that was responsible for today’s arid valleys and lake basins. Moreover, the heavy rainfall in the Late Eocene/Oligocene and the gradual decrease of rainfall through the Miocene, Pliocene and Pleistocene leading to the present hyperarid climate support the notion that the positioning of Africa relative to the Earth’s equator was responsible for the major shifts in climate. This insight would resolve the scientific argument regarding the origin and evolution of the paleorivers and lakes in the Sahara. Moreover, it resolves the debate about the history of the Nile and the origin of the Qena Valley as a master river. It also provides a
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reasonable explanation of the successive stages of the Nile and the change in its deposits from silica-rich and from pure sandstone in Egypt and Sudan to the present-day sediments from Ethiopia and the central African highlands that are rich in kaolinite. Acknowledgements This research was conducted under the Egyptian Fellowships and sponsored by the Center for Remote Sensing, Boston University. The authors gratefully acknowledge the support of Dr. Abbas Mansour and Dr Ahmed Akawy, South Valley University, Egypt and also we thank an anonymous reviewer for the comments on this manuscript. References Abdelkareem, M., Ghoneim, E., El-Baz, F., Askalany, M., 2010. Did Egypt’s Wadi Qena drain northward during its earlier stages? Geological Society of America Abstracts 42 (5), 645. Bohannon, R.G., Naeser, C.W., Schmidt, D.L., Zimmermann, R.A., 1989. The timing of uplift, volcanism and rifting peripheral to the Red Sea: a case for passive rifting? Journal of Geophysical Research 94 (B4), 1683–1701. Bowen, R., Jux, U., 1987. Afro-Arabian Geology: A Kinematic View. Chapman and Hall, London, pp. 295. El-Baz, F., 1982. Genesis of the Great Sand Sea, Western Desert of Egypt. In: International Association of Sedimentology, 11th International Conference, Hamilton, Ontario, Canada, p. 68. El-Baz, F., 1988. Origin and evolution of the desert. Interdisciplinary Science Reviews 13, 331–347. El-Baz, F., 1998. Sand accumulation and groundwater in the Eastern Sahara. Episodes 21, 147–151. El-Baz, F., Mainguet, M., 1981. Dune forms in the Great Sand Sea and applications to Mars: reports of planetary geology program-1981. NASA Tech. Memo. 84211, 244–246. El-Baz, F., Robinson, C.A., Mainguet, M.M., Said, M., Nabih, M., Himida, H., El-Etr, H.A., 2001. Distribution and morphology of paleo-channels in southeastern Egypt and northwestern Sudan. In: Klaus Heine (Ed.), Palaeoecology of Africa, vol. 27, pp. 239–258. Ford, J.P., Blom, R.G., Crisp, J.A., Elachi, C., Farr, T.G., Saundwrs, R.S., Theilig, E.E., Wall, S.D., Yewell, S.B., 1989. Spaceborn Radar Observations: A Guide for Magellan Radar-image Analysis, vol. 89, no. 41. JPL Publication, pp. 1–126. Garfunkel, Z., 1988. Relation between continental rifting and uplifting – evidence from the Suez rift and northern Red-Sea. Techtonophysics 150 (1–2), 33–49. Ghoneim, E., El-Baz, F., 2007. The application of radar topographic data to mapping of a mega-paleodrainage in the Eastern Sahara. Journal of Arid Environments 69, 658–675. Ghoneim, E., El-Baz, F., 2008. Mapping water basins in the eastern Sahara by SRTM data. IEEE International Geoscience and Remote Sensing Symposium, July 6–11, 2008, Boston, Massachusetts, USA, vol. 1, pp. 1–4. Ghoneim, E., Robinson, C.A., El-Baz, F., 2007. Radar topography data reveal drainage relics in the eastern Sahara. International Journal of Remote Sensing 28, 1759– 1772.
Issawi, B., Osman, R., 2008. Egypt during the Cenozoic: geological history of the Nile River. Bulletin of the Tethys Geological Society, Cairo 3, 43–62. Jenson, S.K., Domingue, J.O., 1988. Extracting topographic structure from digital elevation model data for geographic information system analysis. Photogrammetric Engineering and Remote Sensing 54, 1593–1600. Lotfy, H., Van der Voo, R., 2007. Tropical northeast Africa in middle–late Eocene: paleomagnetism of the marine-mammals sites and basalts in Fayum province, Egypt. Journal of African Earth Sciences 47, 135–152. Maxwell, T.A., Issawi, B., Haynes, C.V., 2010. Evidence for Pleistocene lakes in the Tushka region, south Egypt. Geological Society of America 38 (12), 1135– 1138. McCauley, J.F., Breed, C.S., Schaber, G.G., Breed, C., Hynes, C.V., Grolier, J., Issawi, B., Elachi, C., Blom, R., 1982. Subsurface valleys and geoarchaeology of Egypt and Sudan revealed by 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-african drainage system). IEEE Transactions on Geosciences and Remote Sensing 24 (4), 624– 648. Omar, G.I., Steckler, M.S., Buck, W.R., Kohn, B.P., 1989. Fission-track analysis of basement apatites at the western margin of the Gulf of Suez rift, Egypt: evidence for synchroneity of uplift and subsidence. Earth and Planetary Science Letters 94, 316–328. Pachur, H.J., Kropelin, S., 1987. Wadi Howar – palaeoclimatic evidence from an extinct river system in the southeastern Sahara. Science 237, 298–300. Robinson, C.A., El-Baz, F., Singhroy, V., 1999. Subsurface imaging by Radarsat: comparison with Landsat TM data and implications to groundwater in Selima area, Northwestern Sudan, Canada. Journal of Remote Sensing 25 (3), 268– 277. Robinson, C.A., El-Baz, F., Ozdogan, M., Ledwith, M., Blanco, D., Oakley, S., Inzana, J., 2000. Use of radar data to delineate paleodrainage flow direction in the Selima sand sheet, Eastern Sahara. Photogrammetric Engineering and Remote Sensing 66, 745–753. Robinson, C.A., El-Baz, F., Al-Saud, T.S., Jeon, S.B., 2006. Use of radar data to delineate paleodrainage leading to the Kufra Oasis in the eastern Sahara. Journal of African Earth Sciences 44, 229–240. Rodriguez, E., Morris, C.S., Belz, J.E., Chapin, E.C., Martin, J.M., Dafeer, W., Hensley, S., 2005. An Assessment of the SRTM Topographic Products, Technical Report JPL D-31639. Jet Propulsion Laboratory, Pasadena, California, pp. 143. Said, R., 1981. The Geological Evolution of the River Nile. Springer-Verlag, New York, pp. 151. Schaber, G.G., McCauley, J.F., Breed, C.S., 1997. The use of multifrequency and polarimetric SIR–C/X–SAR data in geologic studies of Bir Safsaf, Egypt. Remote Sensing of Environment 59, 337–363. Shukri, N.M., Azer, N., 1952. The mineralogy of Pliocene and more recent sediments in the Fayium. Bulletin of the Desert Institute of Egypt, Cairo 2 (1), 10–53. Smith, A.G., Smith, D.G., Funnell, B.M. (Eds.), 1994. Atlas of Mesozoic and Cenozoic Coastlines. Cambridge University Press, Cambridge, UK, pp. 99. Swezey, C.S., 2008. Cenozoic stratigraphy of the Sahara, Northern Africa. Journal of African Earth Sciences 53 (3), 89–121. Williams, M., 2009. Late Pleistocene and Holocene environments in the Nile basin. Global and Planetary Change 69, 1–15. Woodward, J.C., Macklin, M.G., Krom, M.D., Williams, M.A.J., 2007. The Nile: evolution, quaternary river environments and material fluxes. In: Gupta, A. (Ed.), Large Rivers: Geomorphology and Management, vol. 13. John Wiley & Sons, Ltd., Chichester, p. 712.
Contents lists available at SciVerse ScienceDirect
Journal of African Earth Sciences journal homepage: www.elsevier.com/locate/jafrearsci
New insight on paleoriver development in the Nile basin of the eastern Sahara Mohamed Abdelkareem a,b,⇑, Eman Ghoneim c, Farouk El-Baz a, Mohamed Askalany b a
Center for Remote Sensing, Boston University, 725 Commonwealth Ave., Boston, MA 02215-1401, USA Geology Department, South Valley University, Qena 83523, Egypt c Department of Geography and Geology, University of North Carolina, Wilmington, 601 S. College Road, 28403, USA b
a r t i c l e
i n f o
Article history: Received 19 April 2011 Received in revised form 3 September 2011 Accepted 5 September 2011 Available online 23 September 2011 Keywords: Paleodrainage Eastern Sahara Nile African drift
a b s t r a c t Since the first detection of paleochannels beneath sand sheets and sand dunes in the Sahara using Shuttle Imaging Radar (SIR-A) data, key advances in the understanding of these features have been made. The Sahara is currently the largest and driest region on Earth. However, it was drained by numerous rivers that are now dry channels beneath sand sheets and sand dunes. The present Sahara reflects past pluvial conditions, and the transitions from heavy rainfall to arid or hyperarid conditions reveal major climate shifts. Here, we propose that the evolution of the Sahara occurred in response to the stages of the location of the African Plate relative to the Earth’s equator, i.e., as a result of the northward drift of Africa in space and time. For instance, it is probable that during the late Eocene or Oligocene the Earth’s equator was located at the current-day latitudes of Chad and Sudan. This geometry would have produced pluvial conditions throughout North Africa. With increasing drift in space and time during the Miocene, Pliocene, and Pleistocene, the source points shifted. New valleys were formed and old ones were abandoned, and the length of the main stream of the Nile increased. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction Since the first discovery of paleorivers in the Sahara by radar imaging systems (McCauley et al., 1982), several studies (El-Baz and Mainguet, 1981; McCauley et al., 1986; Schaber et al., 1997; El-Baz, 1988, 1998; Robinson et al., 1999, 2000; El-Baz et al., 2001; Ghoneim et al., 2007; Ghoneim and El-Baz, 2007) have shown increasing details of the paleochannels and their flow directions. However, these findings have not produced strong correlations with the major factors that influenced the development of the paleorivers. These factors were intimately related to the general northward slope of Africa, the nature of the East African Rift and its effects on facilitating the northward flow, previous global sea-level and climate changes, and the position of the African Plate relative to the Earth’s equator. The eastern Sahara is the location of the Nile River, the longest channel system in the world. The Nile crosses 35° of latitude (4°S–31°N) through a wide variety of climates. From the Great Lakes at the Equator and the White Nile, which joins the Blue Nile in central Sudan and the Atbara in northern Sudan, the Nile River flows to the Mediterranean Sea without any present evidence of water supply. In fact, the Nile River appears to have had a complex geological history.
⇑ Corresponding author at: Center for Remote Sensing, Boston University, 725 Commonwealth Ave., Boston, MA 02215-1401, USA. E-mail address: [email protected] (M. Abdelkareem). 1464-343X/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.jafrearsci.2011.09.001
A recent hypothesis for Nile evolution suggests that from the Late Miocene to the present, the main river in Egypt had a northward flow through Egypt’s desert to the Mediterranean Sea (Said, 1981). This river was assigned numerous names and developed in successive stages as follows: the Eonile (Late Miocene), Paleonile (Pliocene), and Protonile, Prenile, and Neonile of the Pleistocene (Said, 1981). Here, we propose a new approach to the evolution of the eastern Sahara’s paleorivers based on the position of the African Plate relative to the Earth’s equator. Our hypothesis would explain major shifts in paleoclimate. It would also explain why the Great Sahara is the driest region on Earth, although it was drained by numerous rivers in the past. Our hypothesis suggests why paleolakes and paleorivers were abandoned in the Sahara and why the Nile River is the largest river in the world. It proposes answers to additional questions: Did the Nile develop in stages (Williams, 2009) to reach its current length? Furthermore, did an earlier stage of the Nile River start at the latitudes of today’s Chad and Sudan? Did the Nile flow through Egypt’s Qena Valley to the Tethys Sea, where its former delta was located?
2. Data and methods Views of the Earth from space provide fast and effective means to complement classical observation methods in field geology in the study of surface and near-surface features. Landsat Enhanced Thematic Mapper (ETM+) and Shuttle Radar Topography Mission
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M. Abdelkareem et al. / Journal of African Earth Sciences 62 (2012) 35–40
Fig. 1. Landsat composite image of Africa. The green areas display the vegetated belt, the blue-black color indicates water, and the pale-yellow to creamy color signifies arid desert. The numbers refer to the site locations mentioned in the text. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
(SRTM) images have been used to interpret the landscape and suggest plausible scenarios for its evolution in space and time. Sand-buried channels were revealed and mapped using radar images. Unlike optical images, radar observations allow representation of the backscattering of the active radar waves. This process reveals the surface and near-surface physical properties of the terrain (Ford et al., 1989). Robinson et al. (2006) and Ghoneim et al. (2007) indicated the similarity of the SRTM data to the Radarsat-1 and SIR-C C-band images. The SRTM images are based on microwave signals (k = 5.7 cm). These signals would be expected to penetrate the sand cover, as do the Radarsat-1 and SIR-C C-band images. The depth of subsurface penetration of the
C-band used for imaging is approximately 0.5 m (Schaber et al., 1997). The Enhanced Thematic Mapper images covering the African continent were used to prepare a base map. The six ETM+ bands 1–5 and 7 were used for the map. The panchromatic band 8 and infrared thermal band 6 were not included. The Environment for Visualizing Images (ENVI) v 4.8 software package was used to furnish digital image pre-processing and processing techniques. A mosaic was prepared and calibrated based on reflectance. Digital image processing using the Landsat ETM+ RGB color combination was selected based on the reflectance spectra of the dominant rock types. False 742 RGB color combinations were
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utilized to define the variations in arid and vegetated areas and define the boundary of the desert (Fig. 1). The Shuttle Radar Topography Mission (SRTM) data covering Africa were obtained from the NASA-Department of Defense National Imagery and Mapping Agency (NIMA) joint project. The data are available internationally at a horizontal resolution of approximately 90 m and a vertical accuracy of 16 m with a 90% confidence level (Rodriguez et al., 2005). These data were processed to construct a continuous Digital Elevation Models (DEMs) of the study area and project it to the Universal Transverse Mercator (UTM) and WGS84 datum. The terrain model was produced with the Environment for Visualizing Images (ENVI) 4.8 and ARC GIS 9.3.1 software packages. The data mentioned above were saved in GIS layers to allow the superposition and correlation of surface and subsurface features. Each layer added significant information, thus facilitating the investigation of the study area. The drainage pattern of Africa and its watershed was extracted using SRTM data. This analysis was performed by employing the surface flow routing based on the 8 D flow direction algorithm (Jenson and Domingue, 1988), which has been widely used in the literature (Ghoneim and El-Baz, 2007; Ghoneim et al., 2007). This step was followed by the derivation of the surface flow accumulation, which was used to delineate the channel network of the basin automatically by specifying a threshold of 5000 cells. The entire Nile basin was extracted by using a mask of the derived channels.
3. Results and discussion A Landsat ETM+ mosaic shows a densely vegetated green belt (Fig. 1), which reflects modern-day tropical rainfall along the Earth’s equator across central Africa. This view offers no indication of the existence of the paleodrainage networks. The desert belt appears as featureless sand and masks most of the older surface features. Furthermore, the Nile River is the only large channel that straddles the arid-to-hyperarid Sahara. The Shuttle Radar Topography Mission (SRTM) data (Fig. 2) show that the Sahara is bounded on the east by the Red Sea highlands and on the northwest by the Atlas Mountains. In the middle are a few less elevated areas, including those in Darfur, Tibesti and Hogar. The results from the automatic extraction of drainage channels reveal that the African continent was veined by paleorivers, particularly in the Sahara. Today, most of these paleodrainages are dry valleys totally obscured by sand deposits. Based on the northward and counterclockwise drift of Africa, the position of the Sahara during the late Eocene and Oligocene was approximately 6–8° of latitude south of its present-day location (Smith et al., 1994; Lotfy and Van der Voo, 2007; Swezey, 2008) (Fig. 3a). Accordingly, the climate in the Sahara during that period is thought to have been humid and warm. Egypt’s climate during that time was dominated by heavy rainfall, averaging 1200–1500 mm/year (Issawi and Osman, 2008, p. 43). It is probable that this climate was similar to that of present-day Rwanda and Uganda, i.e., an equatorial climate. Thus, we believe that the earlier Nile collected its water from the Red Sea highlands, such as the now-arid valleys of Allaqi, Kharit, Qena and El-Hammamat in the east and Tushka in the west. These conditions of heavy rainfall facilitated the formation and transportation of vast amounts of fluvial sediments, consisting primarily of sand and gravel, to the northern Sahara during past pluvial episodes. These deposits are the oldest non-marine Cenozoic strata of Eocene/Oligocene age in the Sahara (in Faiyum in Egypt, and in Algeria and Niger) (Swezey, 2008). The well-preserved strata of Oligocene age (fluvial sand and conglomerate) reveal that north-flowing fluvial systems became a main feature of parts of
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North Africa during that time, i.e., in Faiyum (Egypt), and in the Mauritania–Senegal basin (Swezey, 2008). The presence and magnitude of these fluvial sands in the northern part of the Sahara require thoughtful consideration. The sand accumulations required sufficient rainfall to support the development of extensive fluvial systems. El-Baz hypothesized that the northward flow of paleorivers transported the sand (El-Baz, 1982). This hypothesis supports our idea and suggests that the source of the sand was the rocks of the Nubian Sandstone. This formation has extensive exposures throughout the southeastern Sahara. The exposed rocks north of the sand dunes were primarily limestone. This limestone could not have been the source of the vast amounts of quartz sand. The fluvial deposits might have been transported by multiple ancient rivers during past pluvial episodes along the general northward slope of Africa (Robinson et al., 1999). It has been hypothesized that the modern Nile canyon is of Late Miocene age (Said, 1981). If so, what was the source of the Late Eocene/Oligocene fluvial deposits of the old delta in Faiyum? According to our results, these deposits were transported along the earlier path of the Nile. This route is clearly distinct from the present Nile canyon that leads to the Mediterranean Sea. It has been hypothesized that the Nile flowed through the Qena Valley to the Tethys Sea shoreline at the former location of the Nile delta (Abdelkareem et al., 2010). The northward drift of Africa continued through the Miocene (Fig. 3b) and proceeded in a counterclockwise direction during the late Miocene, approximately 4° of latitude south of its present-day position (Smith et al., 1994; Swezey, 2008). We believe that this movement caused the formation of new valleys and the abandonment of old valleys in the Sahara, including Tushka, Howar, El-Malik, Allaqi, Kharit, El-Hammamat and Qena. For example, the extinct Nile tributary of the Tushka megawatershed (Ghoneim and El-Baz, 2007) is today an arid valley. However, it was most likely a major tributary of the earlier Nile River and drained northeasterly toward the Tushka depression west of the present Lake Nasser in Egypt. This drainage basin would have collected its water from an area of more than 150,000 square km during past pluvial events. This information explains the presence of Nilotic fish and crocodile remains in the western desert, now 300 km west of the present Nile (Issawi and Osman, 2008). Wadi Howar, presently dry, is an extinct tributary of an earlier stage of the Nile River (Pachur and Kropelin, 1987; Ghoneim and El-Baz, 2008). Its upper streams are located in Chad. It flows eastward in Sudan to the Nile near the town of Old Dongola at the Nile bend. The present dry course of Wadi El-Malik runs northeasterly toward the Nile River. Thus, Bowen and Jux (1987) believed that the Nile and Lake Chad were connected owing to the similarity in fish fauna and the probable outlet of Lake Chad to the Nile basin. During that period, tectonic disturbances along the Red Sea throughout Egypt and Sudan produced the great bend of the Nile in Sudan (Gabgaba) and the Qena bend in Egypt. Furthermore, the elevation of the north Qena Valley, adjacent to the Red Sea, began during the late Oligocene and/or Early Miocene (Garfunkel, 1988; Omar et al., 1989) or the Middle–Late Miocene (Bohannon et al., 1989). These events roughly coincided with a major change in fluvial systems (Swezey, 2008) and produced a change from deposits rich in quartz to deposits rich in silt and kaolinite. It is probable that this change coincided with the opening of the Red Sea and the East African Rift. It also caused a change of the Nile pathway from the Qena Valley at the Qena bend to today’s Nile canyon leading to the Mediterranean Sea (Eonile; Late Miocene) as a result of the westward movement related to the tectonics of East Africa.
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The change from the Oligocene to the Miocene was also associated with a gradual rise in sea level and a progressively warmer and more humid climate. Therefore, Egypt’s climate was tropical to subtropical during the late Eocene and through the Oligocene and the Miocene, with 1200–1500 mm/year precipitation (Issawi and Osman, 2008). It is probable that this climate was similar to the present climate at the Earth’s equator, with a mean annual precipitation of 875–1915 mm (Woodward et al., 2007). With the increasing northward drift, the rainfall in the Sahara became less intense during the Pliocene and the Pleistocene (400–600 mm/year, compared with 1200–1500 mm/year in the late Eocene–Miocene) (Issawi and Osman, 2008, p. 43). It is probable that the vast and widespread paleosols during the Pliocene imply the advent of arid desert conditions (Swezey, 2008). With the increasing northward drift (Fig. 3c), the source points changed and were located nearer to today’s Equatorial lakes. However, the absence of central African faunal elements in Late Pliocene sediments suggests that these sediments were not derived from central
Africa (Said, 1981). Thus, we hypothesize that no connection then existed with the Great Lakes of equatorial Africa. However, it is probable that the Blue Nile and Atbara linked the Nile channel system to the Mediterranean Sea. During the Early Pleistocene, it is probable that Egypt’s climate was highly arid and that Egypt developed into a desert (Said, 1981). These changes were intimately connected with the northward drift and the change of the climate in the Sahara. Accordingly, new tributaries and sediments were formed. The mineralogical and paleontological content of these Middle Pleistocene sediments suggest that they are derived from distant sources. They are distinguished from old deposits by their rich pyroxene content. It is probable that these sources are associated with the modern tributaries of the Nile, the Atbara and Blue Nile, in Ethiopia, despite their smaller content of epidote than that found in modern sediments (Shukri and Azer, 1952). This evidence indicates that the Atbara and Blue Nile first began to flow into the Egyptian Nile during this period (Shukri and Azer, 1952), prior to the Nile’s connection with the modern equatorial
Fig. 2. Drainage network of Africa derived from Shuttle Radar Topography Mission (SRTM) data. Drainage lines are shown in white. Solid blue lines specify the drainage lines of the Nile within its watershed. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Fig. 3. Stages of evolution of the Nile River. Solid white lines denote the drainage network of the Nile. Dotted white lines trace former Nile tributaries that became extinct as a result of the northward drift of the African continent. The dashed blue line indicates Earth’s equator relative to the drifting continent. The dashed dark lines trace the East African Rift. (a) Paleogeographic position of Africa during Eocene/Oligocene taken from Smith et al. (1994), Lotfy and Van der Voo (2007) and Swezey (2008). (b) Paleogeographic position of Africa during Miocene taken from Smith et al. (1994) and Swezey (2008). (c) Paleogeographic position of Africa during Pliocene to present taken from Smith et al. (1994) and Swezey (2008). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
lakes. However, it has been hypothesized that the White Nile drained from the equatorial lakes of central Africa and had no real connection with the Egyptian Nile until the latest Pleistocene (Bowen and Jux, 1987). Moreover, the age of the present Nile is approximately 0.5 Ma (Issawi and Osman, 2008). The present Nile became connected with central Africa and the Mediterranean in the Middle Pleistocene, as demonstrated by evidence from oxygen isotope stages (OIS 7–OIS 5) (Maxwell et al., 2010, p. 1135). This connection was facilitated by the structure of the East African Rift and the characteristics of the eastern Sahara. These features maintained the Nile Valley along a pathway bounded by the Red Sea highlands in the east and by the elevated plateaus to the west, particularly in Egypt. These hypotheses about the evolution of the present drainage network from paleorivers and about its fluvial geomorphology reflect long-term tectonic and sea-level changes (Said, 1981) and major shifts in climate. It is probable that these shifts in climate did not result from one stage but from several successive stages. The changes of stratigraphy from early Cenozoic carbonate (a deep marine environment) to the late Cenozoic silicic clastics reflected
a ‘‘long-term eustatic fall in sea level since the middle Cretaceous with a global climate transition from a Late Cretaceous–Early Eocene warm conditions to a late Eocene–Quaternary cool conditions’’ (Swezey, 2008). It is probable that this process coincided with the subsequent northward drift of Africa.
4. Conclusions It is reasonable to assume that the northward drift of Africa initiated a major shift in climate that was responsible for today’s arid valleys and lake basins. Moreover, the heavy rainfall in the Late Eocene/Oligocene and the gradual decrease of rainfall through the Miocene, Pliocene and Pleistocene leading to the present hyperarid climate support the notion that the positioning of Africa relative to the Earth’s equator was responsible for the major shifts in climate. This insight would resolve the scientific argument regarding the origin and evolution of the paleorivers and lakes in the Sahara. Moreover, it resolves the debate about the history of the Nile and the origin of the Qena Valley as a master river. It also provides a
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reasonable explanation of the successive stages of the Nile and the change in its deposits from silica-rich and from pure sandstone in Egypt and Sudan to the present-day sediments from Ethiopia and the central African highlands that are rich in kaolinite. Acknowledgements This research was conducted under the Egyptian Fellowships and sponsored by the Center for Remote Sensing, Boston University. The authors gratefully acknowledge the support of Dr. Abbas Mansour and Dr Ahmed Akawy, South Valley University, Egypt and also we thank an anonymous reviewer for the comments on this manuscript. References Abdelkareem, M., Ghoneim, E., El-Baz, F., Askalany, M., 2010. Did Egypt’s Wadi Qena drain northward during its earlier stages? Geological Society of America Abstracts 42 (5), 645. Bohannon, R.G., Naeser, C.W., Schmidt, D.L., Zimmermann, R.A., 1989. The timing of uplift, volcanism and rifting peripheral to the Red Sea: a case for passive rifting? Journal of Geophysical Research 94 (B4), 1683–1701. Bowen, R., Jux, U., 1987. Afro-Arabian Geology: A Kinematic View. Chapman and Hall, London, pp. 295. El-Baz, F., 1982. Genesis of the Great Sand Sea, Western Desert of Egypt. In: International Association of Sedimentology, 11th International Conference, Hamilton, Ontario, Canada, p. 68. El-Baz, F., 1988. Origin and evolution of the desert. Interdisciplinary Science Reviews 13, 331–347. El-Baz, F., 1998. Sand accumulation and groundwater in the Eastern Sahara. Episodes 21, 147–151. El-Baz, F., Mainguet, M., 1981. Dune forms in the Great Sand Sea and applications to Mars: reports of planetary geology program-1981. NASA Tech. Memo. 84211, 244–246. El-Baz, F., Robinson, C.A., Mainguet, M.M., Said, M., Nabih, M., Himida, H., El-Etr, H.A., 2001. Distribution and morphology of paleo-channels in southeastern Egypt and northwestern Sudan. In: Klaus Heine (Ed.), Palaeoecology of Africa, vol. 27, pp. 239–258. Ford, J.P., Blom, R.G., Crisp, J.A., Elachi, C., Farr, T.G., Saundwrs, R.S., Theilig, E.E., Wall, S.D., Yewell, S.B., 1989. Spaceborn Radar Observations: A Guide for Magellan Radar-image Analysis, vol. 89, no. 41. JPL Publication, pp. 1–126. Garfunkel, Z., 1988. Relation between continental rifting and uplifting – evidence from the Suez rift and northern Red-Sea. Techtonophysics 150 (1–2), 33–49. Ghoneim, E., El-Baz, F., 2007. The application of radar topographic data to mapping of a mega-paleodrainage in the Eastern Sahara. Journal of Arid Environments 69, 658–675. Ghoneim, E., El-Baz, F., 2008. Mapping water basins in the eastern Sahara by SRTM data. IEEE International Geoscience and Remote Sensing Symposium, July 6–11, 2008, Boston, Massachusetts, USA, vol. 1, pp. 1–4. Ghoneim, E., Robinson, C.A., El-Baz, F., 2007. Radar topography data reveal drainage relics in the eastern Sahara. International Journal of Remote Sensing 28, 1759– 1772.
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