LONGITUDINAL (SEIF) DUNES DOWNWIND OF TAVAN HAR METEOR CRATER, GOBI, MONGOLIA

More than 20 sub-parallel longitudinal (linear) dunes drape eastward from the Tavan Har meteor crater of Mongolia’s Dornogovi province. Each individual dune, approximately 1 km long, decreases in size and elevation as it winds eastward to a low narrow “tail” that seasonally shifts north and south by about 10 m. Maximum dune heights of 10 m and widths of 35 m occur near their upwind or westward ends in the wind shadows of peaks along the crest of the eroded meteor crater. The slip faces shift from north to south in time and space, with steep (35°) slopes opposite gentle (10-20°) slopes. The sinuous crests and north- and south-facing slopes make these dunes similar to the seif variety of longitudinal dunes. Fine (mean grain size of 2.5 phi), well-sorted sand of dune crests indicates moderate wind speeds of 4 m/s. Combined with average orientations of S81°E of sand-abraded grooves found along the crater rim, average dune azimuths of S74°E suggest mean annual wind directions that consist of either (1) a dominant unimodal westerly wind with minor north and south seasonal winds to preserve the dunes’ characteristically sinuous shape, or (2) bimodal wind directions from the northwest and southwest. Both wind models are supported by satellite imagery that shows large areas of unconsolidated sediment to the west and northwest of the dune site; westerly winds transport sand from this source area and deposit it on the leeward side of the crater.