The nature and possible origin of mega-dunes in Liwa, Ar Rub' Al Khali, UAE

The Liwa area is located in the south-central parr. of the UAE along the no rtheastern upwind margin of the Ar Rub' Al Khali sand sea and covers about 3694 km(2). It has one type of dunes: a mega-barchan that is compound and c omplex in form characterized by the superimposition of smaller dunes. Due t o the giant size of barchans, interdune areas appear as closed depressions that vary in shape from irregular, elongate, triangular, to circular or sem i-circular. These depressions are mostly covered with small barchans or sab kha deposits with patches of gypcrete in their upper parts. Scattered flat- top mounds are also observed in interdune areas. The size of dunes in Liwa is directly controlled by Quaternary climatic changes: multiple generations are superimposed on one another separated by calcretic horizons. The clima te during dune formation alternated between relatively arid to humid. The d une sands are unimodal with a modal class of fine sand, moderately to very well sorted, and near symmetrical and mesokurtic. Quartz grains are mainly subrounded to subangular. Based on the heavy-mineral suites, the sand came mostly from the ultrabasic rocks that form the ophiolitic sequence of the O man Mountains, the acidic rocks of Iran. and from the Arabian Shield. (C) 2 000 Elsevier Science B.V. All rights reserved.