Subsurface Valleys and Geoarcheology of the Eastern Sahara Revealed by Shuttle Radar

Abstract

The shuttle imaging radar (SIR-A) carried on the space shuttle Columbia in November 1981 penetrated the extremely dry Selima Sand Sheet, dunes, and drift sand of the eastern Sahara, revealing previously unknown buried valleys, geologic structures, and possible Stone Age occupation sites. Radar responses from bedrock and gravel surfaces beneath windblown sand several centimeters to possibly meters thick delineate sand- and alluvium-filled valleys, some nearly as wide as the Nile Valley and perhaps as old as middle Tertiary. The now-vanished major river systems that carved these large valleys probably accomplished most of the erosional stripping of this extraordinarily flat, hyperarid region. Underfit and incised dry wadis, many superimposed on the large valleys, represent erosion by intermittent running water, probably during Quaternary pluvials. Stone Age artifacts associated with soils in the alluvium suggest that areas near the wadis may have been sites of early human occupation. The presence of old drainage networks beneath the sand sheet provides a geologic explanation for the locations of many playas and present-day oases which have been centers of episodic human habitation. Radar penetration of dry sand and soils varies with the wavelength of the incident signals (24 centimeters for the SIR-A system), incidence angle, and the electrical properties of the materials, which are largely determined by moisture content. The calculated depth of radar penetration of dry sand and granules, based on laboratory measurements of the electrical properties of samples from the Selima Sand Sheet, is at least 5 meters. Recent (September 1982) field studies in Egypt verified SIR-A signal penetration depths of at least 1 meter in the Selima Sand Sheet and in drift sand and 2 or more meters in sand dunes.

Sand sheet dynamics and Quaternary landscape evolution of the Selima Sand Sheet, southern Egypt

“…Said (1980Said ( ,1983 pointed to the occurrence of gravel spreads north of the sand sheet and a hypothetical distribution of Eocene-Oligocene drainage that debouched to the north as evidence that the mid-Tertiary stream systems were responsible for much of the denudation of the southern part of the desert. McCauley et al (1982) documented the occurrence of drainage in this region revealed by the SIR-A experiment, which showed NE-SW aligned stream channels at the border of Egypt and Sudan interpreted to represent fluvial denudation as old as mid-Tertiary. The direction of drainage beneath the sand sheet led McCauley et al (1986) to postulate the occurrence of a "Trans-African drainage system" or "TADS," composed of mid-Tertiary drainage that flowed from east to west across what is now southern Egypt and northern Sudan.…”

Laboratory and field measurements of the modification of radar backscatter by sand

“…An early study (Dellwig, 1969) showed that P-band (m = 0.5 GHz; k = 60 cm) radar had potential for penetrating loose deposits. Subsurface imaging by radar gained widespread attention when images from the 1981 NASA Shuttle Imaging Radar mission (SIR-A) revealed buried river channels beneath the sand of the Sahara (McCauley et al, 1982). River channels appeared as dark, dendritic patterns in L-band (m = 1.25 GHz; k = 24 cm) images, and sand-covered bedrock returned a bright radar signal (McCauley et al, 1982.…”

“…Related field studies revealed that the radar signals were able to penetrate up to 1 -2 m of very dry sand to return a signal from buried bedrock and calcium carbonate-cemented nodules Schaber, McCauley, Breed, & Olhoeft, 1986). The dark radar response from the channels was caused by reflection of the radar signal from smoother channel fill material and by attenuation of the signal by that material (Breed et al, 1983;Davis, Breed, McCauley, & Schaber, 1993;Elachi, Roth, & Schaber, 1984;McCauley et al, 1982McCauley et al, , 1986Schaber et al, 1986). Schaber et al (1986) note that the modern sand sheet in the Eastern Sahara now covers much of the region, but the L-band radar of SIR-A was able to penetrate sand cover to expose previously obscured information about past fluvial activity.…”

“…Subsurface imaging by radar gained widespread attention when images from the 1981 NASA Shuttle Imaging Radar mission (SIR-A) revealed buried river channels beneath the sand of the Sahara (McCauley et al, 1982). River channels appeared as dark, dendritic patterns in L-band (m = 1.25 GHz; k = 24 cm) images, and sand-covered bedrock returned a bright radar signal (McCauley et al, 1982. Related field studies revealed that the radar signals were able to penetrate up to 1 -2 m of very dry sand to return a signal from buried bedrock and calcium carbonate-cemented nodules Schaber, McCauley, Breed, & Olhoeft, 1986).…”

Topographic data reveal a buried fluvial landscape in the Simpson Desert, Australia

“…Because Australian paleovalleys have the ability to retain moisture and often interact with playas and pans on the surface (Beard 1973) they also represent ecologically important refugia (Morton et al 1995). In some instances buried paleodrainage, such as those found in the Sahara, are also dramatic reminders of climatic changes that have taken place (McCauley et al 1982).…”