Sand Sheet Dynamics and Quaternary Landscape Evolution of the Selima Sand Sheet, Southern Egypt Ted A

PERGAMON Quaternary Science Reviews 20 (2001) 1623-1647 QSR Sand sheet dynamics and Quaternary landscape evolution of the Selima Sand Sheet, southern Egypt Ted A. Maxwella**,C. Vance Haynes ~r.~," 'Center for Earth and Planetary Studies, National Air and Space Museum, Smithsonian Institution, Washington, DC 20560 USA b~epartment.of Anthropology, University of Arizona, Tucson, AZ 85721 USA 'Department of Geosciences. University of Arizona, Tucson, AZ 85721 USA Abstract The Selima Sand Sheet occupies more than 120,000km2 of the hyperarid, uninhabited Darb el-Arba'in Desert centered at the border of Egypt and Sudan at latitude 22" N, and is characterized by a featureless surface of lag granules and fine sand broken only by widely separated dune fields and giant ripples of varying height and wavelength. Monitoring of the largest of these chevron-shaped ripples using repeat orbital images and field surveys indicates migration rates of 500-1000 m/yr, accompanied by 0-2.0 cm erosion or deposition of the youngest sand sket stratigraphic units. Beneath this active surface, several developmental stages of sand sheet sediments have undulatory upper contacts and varying degrees of pedogenic alteration. The younger stages retain their horizontal lamination and have cracking patterns indicative of past wetter conditions, while older stages have lost their laminar structure through pedogenesis. Historical remains in the desert as well as 14C and Uranium-series dating indicate that the younger strata of the sand sheet have a very low accumulation rate, despite the active movement of the surface. The lower strata were extensively modified during mid and late Pleistocene pluvials, resulting in an initial undulatory surface that set the stage for later accumulation of sand sheet. Below these Quaternary sediments lies irregular topography dissected by channels of mid-Tertiary drainage. The Selima Sand Sheet is neither the result of net aggradation nor degradation, but results from inheritance of an initial fluvial landscape increasingly modified during climatic cycles. Wet periods led to local drainage and deposition, while the increasingly severe arid periods of the late Pleistocene and Holocene resulted in deposition of the blanketing bimodal sediments of the sand sheet. 0 2001 Elsevier Science Ltd. All rights reserved. 1. Introduction of the Darb el-Arba'in Desert, and was interpreted as a broad aeolian peneplain by Sandford (1933). The The present-day surface of western Egypt consists of southern portion of the sand sheet was traversed numer- a broad sloping monocline that dips gently to the north, ous times by the Sudan Defense Force, supplying Kufra floored by middle to late mesozoic, continental- to shal- Oasis in Libya from Sudan during World War I1 low-marine sediments in the south and early Tertiary (Wright, 1945), but was not studied during that period marine sequences in the north (Issawi, 1972). This surface because of the bland nature of the surface. Some idea of is broken by south-facing scarps that outline the major the surface characteristics can be inferred from its depressions in the Western desert, the Kharga-Dakhla description by Bagnold (1990, p. 66): oases, and the Bahariya depression 300km north of Kharga (Fig. 1). South of these widely separated oases, A single utterly flat sheet of firm sand, it is now known the mesozoic sequence is covered by active aeolian de- to cover an area of nearly 100,000 square miles. Driving posits and quaternary soils and is known as the Darb at speed for hour after hour, on a compass bearing, often el-Arba'in Desert after the 40-day camel caravan track blindly into a mirage, it became hard to stay awake. On that crosses the eastern part of the desert (Haynes, one occasion both occupants of a car did fall asleep, the 1982a). The Selima Sand Sheet occupies the central part driver's foot remaining hard down on the pedal. They were only retrieved after an anxious chase. *Corresponding author. Tel.: + 1-202-357-2767; fax: + 1-202-633- Both the similarity of the surface to that of Mars 8926. (El-Baz et al., 1980; El-Baz and Maxwell, 1982) and the E-mail address: [email protected] (T.A. Maxwell). discovery of subsurface channels via the Shuttle Imaging 0277-3791/01/$-see front matter 0 2001 Elsevier Science Ltd. All rights reserved. PII: SO277-3791(01)00009-9 1624 T.A. Maxwell, C. V. Haynes Jr. / Quaternary Science Reviews 20 (2001) 1623- 1647 Radar experiments (McCauley et al., 1982) prompted modification of the sand sheet, as well as to provide remote sensing studies of the sand sheet in the past two baseline surficial sedimentologic data for comparison decades. The few scattered, uninhabited oases in the with radar reflectivity. Our investigations of surface region had long been the sites of archaeological expedi- modification have concentrated on three questions. (1) tions, from which has come most of our knowledge of the Does the sand sheet result from erosion or deposition, nature and timing of climate change in southern Egypt and when and at what rates have these occurred? (Wendorf et al., 1976; Wendorf and Schild, 1980; Said, McFadden et al. (1987) have shown that aeolian and 1980; Haynes, 1982b; Pachur et al., 1987). pedogenic processes play a major role in desert pavement Hypotheses for the development of the sand sheet formation, and suggested that such close-spaced angular range from aeolian to fluvial mechanisms. Sandford fragments originate as surface deposits. In contrast to (1933) and Peel (1966) attributed the origin of the sand desert "pavement", the subrounded granule veneer of the sheet to the penultimate stage of erosion of the desert sand sheet may be the opposite endmember of the desert surface, and interpreted the widely scattered relict hills surface, originating in the subsurface and exposed by north and south of the region as outliers left by aeolian aeolian redistribution and erosion. The origin of the sand deflation. Haynes (1982a) proposed that climatic cycling sheet by net deflation (Sandford, 1933) or by climatic was most important, suggesting that climate change al- cycling (Haynes, 1982b) may not be mutually exclusive, ternately weakened the bedrock surface during humid however, and estimates of the present rates of erosion periods, which made them susceptible to aeolian erosion from remote sensing and field observations allow a first during arid cycles. Even before the identification of sub- quantitative estimate of net sediment flux. (2) What are surface channels with spaceborne radar, Breed et al. the relationships among the various scales of large (1982) had proposed the existence of a now-extinct river ripples measured in the field and brightness patterns system that originated in the remaining highlands of the (chevrons) detected by orbital data? Breed et al. (1987) Gilf Kebir Plateau west of the sand sheet and had eroded provided the first detailed description of giant ripples in the surface, now taken over by aeolian sculpturing. this region, which have a wavelength of about 500 m, and The discovery of channels buried beneath the sand are visible on the ground only where dark lag deposits sheet in the Shuttle Imaging Radar A (SIR-A) experiment are exposed in inter-ripple "hollows". In the southern led to renewed interest in the Tertiary history of the part of the Selima Sand Sheet the surface becomes un- region (McCauley et al., 1982), the location of archae- dulatory where giant ripples have amplitudes of up to ological sites (McHugh et al., 1988, 1989), and the evolu- 10 m and wavelengths up to 1 km (Haynes and Johnson, tion of north African drainage. The few channel junctions 1984). However, in our present area of study, the low present in the radar images suggested to McCauley et al. amplitude, giant ripples grade laterally into even larger (1986) a southwestward drainage direction, away from scale chevron-shaped bedforms (Maxwell and Haynes, the present Nile valley although this hypothesis has not 1989) that for several years eluded an exact demarcation been universally accepted (Burke and Wells, 1989). Ac- on the ground because of their extremely low amplitude. cording to Issawi and McCauley (1992), the latest Terti- Whether these features are replenished from local deposi- ary channels were deflected by the uplift of the Gilf Kebir ts, result from long distance transport, or a combination plateau region, and drained into the Mediterranean of sources remains open to question. (3) Finally, the through central Egypt. It was not until the desiccation of origin of this part of the desert has been ascribed to the Mediterranean during the late miocene that the pres- denudation and cliff retreat. If we extrapolate the pro- ent channel of the Nile was deepened in response to this cesses seen today and interpreted from the Quaternary new, lower base level (Said, 1990, p. 506). Whether early history, does this help to explain the low relief surface generations of Nile drainage were supported by channels that forms the Selima Sand Sheet? entering the Nile valley south of Aswan from the west, or In addition to these questions of sand sheet origin and whether those channels of the Darb el-Arba'in Desert sediment transport, the identification of active sand were captured by the newly incised Nile is open to con- allows us to document the location and characteristics of jecture (Said, 1993, p. 37). In either case, it is apparent the mobile component of the sand sheet, which will that several generations of fluvial deposits are (or were) eventually enable the mapping of near-surface stratigra- present in the area of the Selima Sand Sheet (Haynes et phy. In the Safsaf oasis area at the eastern margin of the al., 1993), which are now visible only through radar Selima Sand Sheet, Davis et al.

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