Rejuvenation of Dry Paleochannels in Arid Regions in NE Africa: a Geological and Geomorphological Study

Arab J Geosci (2017) 10:14 DOI 10.1007/s12517-016-2793-z ARABGU2016 Rejuvenation of dry paleochannels in arid regions in NE Africa: a geological and geomorphological study Bahay Issawi1 & Emad S. Sallam2 Received: 20 June 2016 /Accepted: 5 December 2016 # Saudi Society for Geosciences 2016 Abstract Although the River Nile Basin receives annually ca. and west of Aswan. The nearly flat Sahara west of the Nile 1600 billion cubic meters of rainfall, yet some countries within Valley rises gradually westward until it reaches Gebel the Basin are suffering much from lack of water. The great Uweinat in the triple junction between Egypt, Sudan, and changes in the physiography of the Nile Basin are well Libya. Gebel Uweinat has an elevation of 1900 m.a.s.l. sloping displayed on its many high mountains, mostly basement rocks northward towards the Gilf Kebir Plateau, which is that are overlain by clastic sediments and capped by volcanics 1100 m.a.s.l. The high mountains and plateaus in the southern in eastern and western Sudan. The central part of the Nile Basin and western Egypt slope gradually northward where the Qattara is nearly flat including volcanics in the Bayuda Mountains and Depression is located near the Mediterranean coast. The depres- volcanic cones and plateaus in southwestern Egypt. The high sion is −134 m.b.s.l., which is the lowest natural point in Africa. mountains bordering the Nile Basin range in elevation from All these physiographic features in Sudan and Egypt are related 3300 to 4600 m.a.s.l. in the Ethiopian volcanic plateau in the to (i) the separation of South America from Africa, which east to ca. 3070 m.a.s.l. in the western Gebel Marra, and started in the Late Paleozoic and continued up to the 1310 m.a.s.l. in the Ennedi Mountains in northwestern Sudan. Cretaceous, giving rise to several generally E–W-oriented tec- In central Sudan, the Nile Valley rises approximately 200– tonic features inside Africa, (ii) the uplift of the Red Sea 300 m.a.s.l. In Egypt, the River Nile is bounded by the Red Mountains and their continuation inside Africa resulted in the Sea Mountains in the east, assuming ca. 1000–2600 m.a.s.l., East African Rift System (EARS), (iii) the Guinea–Nubia mostly of basement rocks, which are covered to the north of Lineament crossing Africa from the Atlantic to the Red Sea Aswan by Phanerozoic sediments sloping to the west, passing where many havoc trends, mostly E–W-trending faults, and by the Nile Valley and continuing through the Western Desert. uplifted basement features pierce the overlying sediments, (iv) The Phanerozoic cover on both sides of the Nile is known as the parallel and longitudinal structures associated with volcanic Eastern and Western Limestone Plateaus. These plateaus as- plateaus and cones extend from west Sudan (Gebel Marra) to sume elevations varying from 300 to 350 m.a.s.l. near the east- Ethiopian Plateau, passing by volcanics and plume features in ern bank of the Nile to 400–500 m.a.s.l. south Luxor at Esna between and the basins in east Africa were subjected to wrench related inversions, and (v) the Sudd linear E–W area stretching This article is part of the Topical Collection on Current Advances in more than 1000 km between Gebel Marra in the west, passing Geology of North Africa by South Sudan and reaching southwestern Ethiopia. Here, fluviatile and subsurface waters led to ponds, lakes, and wet * Emad S. Sallam areas that are hard to exploit. The impact of these features led to [email protected] the present south to north River Nile, but passing by many Bahay Issawi changes in the direction of its many tributaries and slope rever- [email protected] sal of some of the major extinct rivers, either sectors of the main Nile or the rivers once flowed into the main river. The paleo- 1 Geological Survey of Egypt, Cairo, Egypt climatic changes during the Quaternary period: wet and dry 2 Department of Geology, Faculty of Science, Benha University, have a great effect on the physiographic features and slope Benha, Egypt reversal of the Nile Basin drainage system. 14 Page 2 of 20 Arab J Geosci (2017) 10:14 Keywords The Nile Basin . Geology . Stratigraphy . river channel and the desert beyond. This setting stimulated a Geomorphology . Hydrogeology . NE Africa close co-existence of people and aurochs to become accustomed to each other (Wendorf et al. 1987a, 1987b; Wendorf et al. 2001). The gradual spread of humans and animal domestication Introduction throughout the Nile Valley increased rapidly within an unbeliev- able high percentage. From 10,000 (Early Holocene) to From time immemorial, before the Nile reached Egypt at 600– 5000 years ago when Egypt became under Pharaonic rule, 700 ka ago, Egyptians lived in NE Africa depending on rainfall Egyptians spread on almost every square meter of the Nile water (Wendorf and Schild 1980; Hassan 1981; Wendorf et al. Valley and an exponential increase in human population became 2001). Egypt then was not a desert area as it is now, but it was evident. Over time, the population of Egypt increased by an humid covered by different plants, fruitful trees, and many dif- unexpected rate reaching yearly by more than 2.5% (according ferent species of wild and tamed animals (Wendorf and Schild to the Egyptian Central Agency for Public Mobilization and 1976;Wendorfetal.1993). Rainfall decreases gradually since Statistics; August 2015). the Oligocene, reaching 1500 mm/year (Simons 1972, 1987)to The many radar images reveal the occurrence of many old dry 1200 mm/year during the late Miocene (Pickford et al. 2008)to channels crossing African Sahara from the south to north and 600 mm/year in the early Quaternary (Williams 2009; Williams also rare channels from the north to south. In the work of Issawi et al. 2010). Rainfall built many pools, lakes, and local rivers et al. (2016), the central African waters (Congo River and White mostly trending E–W consequent to the uplift of the Red Sea Nile River) have shown to pass through old dry channels in the Mountains (Priabonian–Burdigalian). The main water stream Western Deserts of Sudan and Egypt. The present study confirms was the north–south Qena River, which drains the northern sector the importance of the two main drainage systems in the Eastern of the Red Sea Mountains. Southwards, the main river was the Desert (Wadi Gabgaba–Allaqi) and in the Western Desert (Wadi Gabgaba–Allaqi, trending north to northwest, draining northeast- Gilf). These two dry wadis were important drainage systems, ern Sudan and southeastern Egypt before joining the Qena River, carrying waters from rainy areas in Ethiopia through the Blue 120 km south of Aswan (Issawi and McCauley 1992, 1993). The Nile and from high mountains in Uweinat and Gilf Kebir. The plants and water were enough to keep a limited number of man- Gilf and Gabgaba–Allaqi are now dry channels since dry condi- kind and animals to live happily together though man avoided to tions dominated over the Gilf area and over all the Western live on the banks of the many rivers traversing old Egypt. The Desert of Egypt. On the other hand, the Gabgaba lost its connec- shrubs and pushes were a good hiding place for dangerous tion with the Blue Nile by the rising of the Bayuda volcanics, beasts. Gradually, over time, rainwater diminished; hence, man which blocked its connections with Ethiopian water. In the pres- had to go further south probably even to what is now Sudan, ent work, it is suggested that these two dry paleochannels can be where water was available. The Egyptian migration to the south rejuvenated, adding a good supply of water to the thirsty Egypt. continued until they encountered the Kush people in north Sudan. Thence, the Egyptians were compelled to dig water wells in the Nabta area west of Abu Simbel (Wendorf et al. 1985; Study area Wendorf et al. 1987a, 1987b; Wendorf and Close 1992; Wendorf et al. 2001) and in many other places in southern The River Nile Basin occupies an area of approximately three Egypt. The present Nile Delta was soft, muddy, and susceptible million square kilometers. The river generally trends S–Nfor for marine invasions during the Neogene and the Quaternary. about 6800 km, passing through tropical to Mediterranean In southern Egypt, the beginning of a permanent society climatic zones (Fig. 1). The Nile Basin displays a broad diver- started to develop, enhanced by the discovery of animal domes- sity in geology, physiography, climate, and fluvial tication (ca. 8000 BP) and probably with cultivation of some lands environments reflected in five discrete landscape sectors. around water wells a bit earlier (Wendorf et al. 1985; Wendorf These sectors have been adopted by Said (1981) and outlined and Close 1992; Wendorf et al. 2001). In the Nabta area, by Woodward et al. (2007)asfollows: archeologists discovered Late Neolithic Tumuli ceremonial com- plex, several megaliths, old houses, and a solar calendar 1. The densely forested equatorial lake region at the southern (Wendorf et al. 1985; Wendorf et al. 1987a,1987b;Wendorf headwaters of the White Nile, with tropical catchments and Close 1992; Wendorf et al. 2001). The close co-existence and perennial flow regimes. of cattle and humans in Nabta Valley was related to what was 2. The swampy areas of Sudd and central Sudan with low- then (during the Holocene) the floodplain of the Nile bordered by gradient floodplains, broad channel belts, and low- low and high deserts, lacking vegetation.

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