Sand sheets of the eastern Sahara and ripple blankets on Mars

Abstract

Summary Landsat images of the eastern Sahara (North Africa) show widespread sandy plains covered by broad, gentle undulations (not dunes) that are interpreted as giant aeolian ripples. This regionally extensive bedform pattern embodies the entire upper unit of the Selima Sand Sheet. This unit, which we interpret as modern and presently active, contains very coarse sand, granules, and small pebbles, medium to very fine sand, and silt in horizontally laminated pairs that form a tabular deposit 1 cm to 10 m thick. It is commonly underlain by a stabilized sand sheet unit, and one or both units are disconformable on an erosion surface of truncated alluvium, mixed alluvial-colluvial-aeolian deposits, or bedrock. Sediment in the sand sheet is derived from alluvium, eroded mostly from sandstones in the Nubia Formation and widely distributed in dry, relict valleys. The upper surface of the sand sheet is a layer of pebbles and granules identical to coarse particles in the internal layers and in the truncated alluvial valley fill. Elongate, thin ripples composed of small pebbles are driven across this surface by prevailing north winds. Fine to medium sand is widely dispersed across the sand sheet from isolated dune fields. Silt particles are added seasonally by dusty winds (Khamsins) from the S. The Selima Sand Sheet may provide an analogue for some thin, horizontally-bedded aeolian sandstones. It offers an explanation for ripple blankets on Mars, where locally-derived coarse particles transported by surface creep and saltation are mixed with globally-derived fine particles deposited from dust storms.