Document Type

Article

Publication Date

2001

Abstract

The Selima Sand Sheel o upies more than 120,000 km² of the hyperarid, uninhabited Darb el-Arbain 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 felds and giant ripples of varying height and wavelength. Monitoring of the largest of these chevron-shaped ripples using repcat orbital images and field surveys indicates migration rates of: 500-1000m/yr, accompanied by 0-2.0cm erosion or deposition ol the youngest sand shect 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 i*C and Uranium-series: dating indicate that the younger strata ofthe o t "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 Plcistocene pluvials, resulting in an initial undulatory surface that set the Stage for later accumulation of sand i Sheet. Below thesc Quaternary sediments lies irregular topography dissected by channels Of mid-Tertiary drainage.' The Selima Sand n Sheet is neither the resuli of net aggradation nor degradation, but results from inheritance of an initial fluvial landscape increasingly modified during climatic cyeles. Wet periods led to local drainage and deposition, whilc the increasingly severe arid periods of the late Pleistocene and Holocene resulted in deposition of the blanketing bimodal sediments of the sand: sheet.

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