Robert J. Stern and Mohamed Gamal Abdelsalam,
Center for Lithospheric Studies, University of Texas at Dallas,
Box 830688, Richardson TX 75083-0688 (Published in Science)
Abstract The course of the Nile in northern Sudan follows a contorted path through Precambrian basement rocks. Data from shuttle imaging radar show that basement structures control the course of the Nile in this region. Northward-flowing segments of the Nile follow Precambrian basement fabrics, whereas east-west segments follow faults of much younger age. These faults may reflect recent uplift of the Nubian Swell and deflection of the river to the SW to form the great bend of the Nile.
The Nile (6825 km long) (1) transports water from high rainfall regions in Ethiopia and Equatorial Africa across the Sahara Desert to Egypt. In northern Sudan the Nile forms a great bend, first flowing north from Khartoum (where the White and Blue Niles unite), then southwest for over 300 km before it resumes its northward course (Fig. 1). Bedrock fabrics are an important control on the course of the Nile in this region (2, 3), although relationships between crustal movements and the river's course are poorly understood. In this article we present remote sensing data over this part of the Nile, acquired using the SIR-C/X-SAR imaging radar system (4) during two flights of the NASA space shuttle Endeavor in 1994. These data reveal how basement structures of different age control much of the Nile's course. Many of these structures could be mapped by geologists on the ground, but such studies have been inhibited by the size and harsh climate of the study area. Other structures are revealed for the first time by the radar images (5).
The evolution of the Nile can be traced from the late Miocene desiccation of the Mediterranean Sea (6). Lowering of hydrologic base level led to rapid erosion of sedimentary rocks and carving of a deep canyon [2.5 km deep beneath Cairo 1)] and vigorous stream piracy upstream from Aswan. Reflooding of the Mediterranean basin at the end of the Miocene drowned this canyon, and sedimentary filling of the estuary produced the broad and fertile Nile floodplains north of Aswan ['Egyptian region' of (7)]. The Cataract region of the Nile extends 1850 km south from the first cataract at Aswan to the sixth cataract just north of Khartoum. Although parts of this region are occupied by broad floodplains, the Nile is mostly incising its channel into Precambrian basement (8) (Fig. 1).
We focused on the third, fourth, and fifth cataract stretches. The third cataract stretch extends north over basement exposures from about 19”45'N to Lake Nasser, as the river traverses the Nubian swell (Fig. 2B) (9). The fourth cataract stretch extends SW from the bend at Abu Hamed, where the Nile flows over basement rocks for about 200 km before Cretaceous sandstones are encountered (Fig. 1). The fifth cataract stretch extends NNW from Atbara for over 200 km to the bend at Abu Hamed (Fig. 2B) and is entirely over Precambrian basement rocks.
Cataract region Precambrian granitic and metamorphic rocks (Fig. 1) (10-12) were covered by late Mesozoic sandstones (13) affected by 47 to 81 Ma old igneous intrusions and lava flows (14). Volcanic fields younger than 15 Ma are scattered across the Bayuda Desert (15). Pleistocene and younger alluvial cover occurs discontinuously along the riverÕs course. Cataract region rocks preserve three sets of planar structures. The early structures formed at about 700 Ma (10, 12) and have NE-SW to E-W orientations. These structures are localized along sutures and fold-and-thrust belts, as exemplified by the Atmur suture and the Abu Hamed and Dam et Tor fold-and-thrust belts (Fig. 2B), and rarely control the course of the Nile, probably because they generally do not dip steeply. These structures are overprinted by younger zones of ductile deformation, including the NNW-trending sinistral Abu Hamed and Abu Dis shear zones in the east (Fig. 2B) (12) and the N-S trending Akasha and Kosha shear zones to the west (Fig. 2A). These formed about 600 Ma (16) and strongly control the river along the third and fifth cataract stretches. There is no evidence for NW-SE rift structures of Mesozoic age that control much of the course of the Nile in the central Sudan region (17). E-W steep, normal or strike-slip faults of late Cretaceous or younger age are common along the third cataract stretch. Faults with this orientation are rarely reported from the Sudan (18), but are common in southern Egypt (19). Because they affect Cretaceous sedimentary rocks, these faults must be Cretaceous and younger in age. An intrusion is truncated by an E-W fault (Fig. 2A), giving an apparent up to the south displacement. A similar intrusion at J. Sheikh (Fig. 2A) yields a K-Ar isochron age of 90±2 Ma (20).
At the start of the fourth cataract stretch downstream from Abu Hamed, the river follows the northern margin of the Abu Hamed fold-and-thrust belt. Southwest of this, no controlling Precambrian structures can be identified on the radar images. Instead, the river follows multiple channels controlled by NNE and E-W oriented fractures (Fig. 3). The E-W portion of the Nile south of Us island is controlled by a zone of 'highly crushed granite', whereas the NNE-SSW portion of the Nile west of Us island follows easily eroded porphyritic dykes and 'splintered granite' (21). A recent change in the riverÕs course is demonstrated by a previously unknown paleochannel, 25 km long (Fig. 3)lying as much as 10 km north of the Nile. A shuttle photograph of the region SW of Fig. 2 shows a north-flowing stream course, now crossed by the Nile. These observations indicate that the course of the Nile along the fourth cataract stretch has recently shifted to the south due to relative uplift of adjacent regions of the Nubian swell. NNE and E-W structural controls cannot be inferred from the course of the paleochannel, and we infer that fracturing accompanied or followed tectonic uplift. The Nile in Egypt has had dramatic changes in flow regime during Quaternary time (1, 22). These are generally ascribed to climatic changes in Ethiopia and equatorial Africa (23). Our data suggest that tectonic activity in the cataract region may also have affected the hydrologic regime of the Nile during the Quaternary.
The radar data and field studies reported for the fifth cataract stretch (12) indicate that this part of the Nile follows an important, older structural grain, defined by the boundary between the Nile craton and the Arabian-Nubian Shield (Fig. 4A). These structures continue north of Abu Hamed (24), yet the Nile turns away to form the great bend. The course of the Nile in Egypt also approximates the Late Precambrian continental margin (25, and this structure might be expected to control the Nile north from Atbara all the way to the Nile.
The great bend must be due to the influence of the Bayuda uplift and Nubian swell on the Nile, and it is generally thought that uplift of the Bayuda is responsible (3, 26). If so, the Nile should have been diverted away from the rising region, to the east or north. The Nile follows a well-established course through the fifth cataract region to the east, and the diversions in the fourth cataract area are to the south. The Nubian swell is interpreted as a late Paleozoic uplift (27) although the E-W faults reported here require a much younger age for some tectonic activity. Furthermore, the presence of marine sediments of Lower Tertiary age in northern Sudan south of the Nubian swell indicates that this was not a positive tectonic element at that time (28). We thus suggest that recent uplift of the Nubian swell diverted the Nile to form the great bend. This is consistent with the sequence of diversions indicated from the fourth cataract region. In this case, E-W faults in northern Sudan and southern Egypt may be of Late Cenozoic age and reflect Nubian swell uplift. This hypothesis also explains the proximity of the Gabgaba-Nile drainage divide to the Nile (Fig. 1): We suggest that before Nubian swell uplift, the drainage now flowing along the fifth cataract stretch continued north through what is now Wadi Gabgaba (Fig. 4A). Quaternary uplift diverted the Nile to the west, through the fourth cataract region, perhaps to join a tributary of the Nile to the west (Fig. 4B), an interpretation that is consistent with the inference that the fourth cataract stretch of the Nile has only been established since the early Pleistocene (2). Continued uplift of the Nubian swell continued to deflect the Nile to the south along the fourth cataract stretch.
REFERENCES AND NOTES
REFERENCES AND NOTES
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5. The SIR-C/X-SAR radar system has advantages over visible and near infrared imagery in that radar maps subtle changes in surface roughness and orientation and has limited capabilities for imaging beneath sand. Hence, the SIR-C/X-SAR imagery is more sensitive to geologic structures, especially those that dip steeply, and is especially useful for geologic studies of hyperarid and poorly known regions such as the Sahara.
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9. The Nubian swell is a region of shallowly buried and exposed basement that extends E-W along the Egypt-Sudan border as far as J. Uweinat, 600 km west of the Nile. Only the eastern part of it is shown in Fig. 1.
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29. We thank Dave Amesbury for bringing our attention to the shuttle photograph over the fourth cataract stretch, and to K. Burke and W. Bosworth for their ideas on the problem. Our research is supported by NASA through subcontracts from JPL. This is UTD Programs in Geosciences Contribution # 846