An integrated remote sensing and GIS analysis of the Kufrah Paleoriver, Eastern Sahara
Highlights
► The Kufrah Basin was mapped through the integration of Radarsat, SRTM and Landsat. ► The basin is the largest drainage system yet mapped in the Eastern Sahara. ► It might have served as a spillway from Megalake Chad to the Mediterranean Sea. ► It may have acted as a corridor used by human and animals to cross the Sahara. ► The delta and its vicinity might hold vast reservoirs of groundwater, oil and gas.
Introduction
The Great Sahara of North Africa is known to have undergone major hydrological fluctuations and was vegetated at times in the past. During wet phases, the region presently occupied by the Sahara was green, contained forests, grasslands, and permanent rivers and lakes. When the wet periods ended at about 5.5 ka, the Sahara was transformed into a hyperarid desert (De Menocal et al., 2000) and its original surface and alluvium-filled valleys and lake basins were totally covered with windblown sand sheets and dune fields. Over the past three decades scientists have used both passive and active remote sensing to reveal previously unknown buried river courses, large lakes, geologic structures, and possible Stone Age occupation sites (e.g., Roth and Elachi, 1975, McCauley et al., 1982, El-Baz et al., 2000, Robinson et al., 2006, Ghoneim et al., 2007, Paillou et al., 2009). The present study is a continuation of such efforts, where it is particularly concerned with the past fluvial landscape in the eastern part of the Great Sahara.
The study area covers large portions of eastern Libya, western Egypt, northeastern Chad and northwestern Sudan. The subsurface geology of the area is not well known, but the region as a whole is mapped as continental sandstones of the Cambro-Ordovician to early Cretaceous (Hesse et al., 1987) overlain by Quaternary aeolian sands (Fig. 1). The Nubian Sandstone of this basin is made of continental Mesozoic sediments that rest uncomfortably over Paleozoic rocks (Bellini and Massa, 1980). The Paleozoic sequence has a thickness of 1300–2000 m; the Lower Cretaceous sediments are some 1000–1500 m thick and form the surface bedrock of most of the study area.
During the Late Miocene, rainfall was abundant over North Africa as a result of the northward migration of the Intertropical Convergence Zone (ITCZ) (Gladstone et al., 2007). Runoff from the study area reached the Eastern Mediterranean via rivers that no longer run today, but whose channel courses are relatively well preserved (Griffin, 2002, Griffin, 2006). During the late Messinian age of the Miocene epoch, the Mediterranean experienced a phase of severe desiccation and salinity fluctuations known as the Messinian Salinity Crisis (MSC) (Hsu et al., 1973). The desiccation is thought to have been triggered by restriction of the flow between the Mediterranean and the Atlantic due to tectonic closure of the Strait of Gibraltar (Hsu et al., 1977). The Messinian desiccation caused a lowering of the sea level that led to down-cutting and extension of Libyan river systems during humid periods (Barr and Walker, 1973).
A combination of seismic investigations and shallow drilling near the Gulf of Sirt (Barr and Walker, 1973) revealed a canyon some 396 m deep and ~ 5 km wide cut into Miocene limestone. This canyon, named in this study the Sahabi Canyon, is thought to have been incised at the time of the drawdown of the Mediterranean. The tremendously rich vertebrate fauna (mammals, reptiles, birds, and fishes; Boaz, 1987) and the very large crocodiles found in the paleochannel confirms the existence of a large freshwater riparian corridor in the past (Hecht, 1987). The presence of this large incised channel adjacent to the Gulf of Sirt strongly suggests it was the lowermost reach of a major river system. This paleoriver has been named the Sahabi River (Griffin, 2002); it drained most and perhaps all of Libya during the late Miocene (Drake et al., 2008).
The width and depth of the Sahabi Canyon suggest a large contributing drainage basin. Since there is no obvious surface linkage to the ancestral Niger River to the west, or the Ethiopian highlands to the east, the Sahabi River was proposed to have been an outflow channel of Megalake Chad (Griffin, 2002, Griffin, 2006), which occupied a large Neogene subsidence basin (Genik, 1993) to the south during humid periods of the Late Miocene (Griffin, 2002). The proposed link between the Sahabi River and Megalake Chad is supported by geomorphological evidence for an ancient river (the Amatinga Valley) that flowed northward away from the megalake (Griffin, 2002, Griffin, 2006). The presence of fossils of similar semi-aquatic anthracotheres in both the Sahabi Canyon in the Sirt Basin and Megalake Chad deposits (Lihoreau et al., 2006) also supports the connection.
Intense neotectonic activity in the Tibesti Volcanic Province (TVP) began perhaps as early as the Lower Miocene and continued until the Quaternary (Furon, 1963, Gourgaud and Vincent, 2004). Uplift of the eastern flank of the TVP caused the course of the Sahabi River to shift east toward the sinking Kufrah basin. Several studies (e.g., Pachur, 1993, McCauley et al., 1995, El-Baz, 1998, Ghoneim et al., 2007, Drake et al., 2008, Paillou et al., 2009) have identified large relict channels draining northward toward the Kufrah Oasis. It has been suggested since the late 1970s that the groundwater well fields in the Kufrah Oasis area are located in former river channels that are being recharged by rainfall in the Tibesti Mountains (Ahmad and Goad, 1978). Pachur (1993) traced this north-trending river course for hundreds of kilometers south to 22°N, where it debouchs from the eastern flank of the Tibesti Mountains. In 1994, SIR-C radar imagery revealed evidence for two large branches of this river that drain toward the Kufrah Oasis (McCauley et al., 1995, El-Baz, 1998). Robinson et al. (2006) delineated the two main branches farther south using Radarsat-1 imagery and suggested that the western branch extends as far south as 20°N in Chad. Using Shuttle Radar Topography Mission (SRTM) data, Ghoneim et al. (2007) delineated part of this river course and compared it with previous Synthetic Aperture Radar (SAR) images for data validation. Recent studies using the same data set have confirmed that the Kufrah River was an extensive fluvial network that drained most of eastern Libya and ended in a large terminal inland delta (Drake et al., 2008, Ghoneim and El-Baz, 2008a, Paillou et al., 2009).
To date, none of these studies have determined the full extent of the Kufrah Paleoriver and its drainage network. The objective of this study is to present a comprehensive picture of the entire Kufrah fluvial system by integrating different types of geospatial data in a GIS. The study also investigates possible linkages between this drainage system and the Neogene waterway that connected Megalake Chad with the ancestral Mediterranean Sea.
Section snippets
Data and methodology
Ancient geomorphologic features in the Great Sahara are commonly well preserved due to the hyper-arid climate and the absence of vegetation cover, thus can be easily distinguished using satellite data. In this study, such data were acquired from three different remote sensing platforms, i.e., Radarsat-1, SRTM and Landsat ETM+.
Kufrah Paleobasin dimensions
Based on the SRTM-derived basin, the ancient Kufrah River had an extensive drainage area of about 235,500 km2 and a length of 950 km. The river flowed north with a very gentle gradient of about 0.6 m km− 1 comparable to that of the adjacent modern Egyptian Nile. As is evident from the radar-mapped drainage (Fig. 3), the Kufrah was a massive river system with the main valley width exceeding 30 km in some sectors, particularly in its southern reaches. Robinson et al. (2006) have even indicated the
Conclusion
The entire drainage basin of the Kufrah Paleoriver has been mapped here for the first time by integrating 78 Radarsat-1 scenes with SRTM data and Landsat ETM + images. With a length of at least 950 km and an area of at least 236,000 km2, the Kufrah Paleobasin is the largest paleoriver system yet identified and mapped in the Eastern Sahara of North Africa. It is much larger than the Tushka River (150,000 km2) (Ghoneim and El-Baz, 2007a), the Northern Darfur basin (129,230 km2) (Ghoneim and El-Baz,
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