Flow separation and the internal structure of shadow dunes

Abstract

In the sand-undersaturated aeolian terrains of southern Kuwait the only dune bedforms are nebkhas and shadow dunes, both of which are confined to the surfaces of evaporitic pans. Shadow dunes are aerodynamic bedforms with a quasi-pyramid profile and caused by horizontal flow separation to the lee of impermeable barriers (the nebkhas). Their internal structure consists of: (a) bipolar azimuths and bimodal dip distributions of tabular-planar grainfall cross-strata with a chevron-like pattern in the dune crest, and is formed by variable secondary winds, and (b) alternating reactivation surfaces probably representing third-order bounding planes on each slip-face. This structure is developed in an essentially prevailing, unidirectional wind field with minor fluctuations about a vector mean. With a rising water table, causing sand sheets to migrate over and cover the evaporitic pans, these cross-strata could be preserved. In ancient sequences, the internal geometry of shadow dunes could be of potential value in estimating regional palaeowind directions and in palaeogeographic reconstructions of sand-starved desert terrains.