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Desert Dunefields |
Department of Geological Sciences, University of Texas, Austin, Texas 78713, USA
Wet aeolian systems are those in which the water table is shallow and the floors of the interdune flats are within the capillary fringe. Accumulation within a wet aeolian system occurs because of a relative rise in the water table, and these accumulations are characterized by the presence of both dune and interdune-flat deposits. The Jurassic Entrada Sandstone, studied along a 2.7 km traverse in NE Utah, is interpreted as having been formed in a wet aeolian system because of a preponderance of features indicating a shallow water table throughout accumulation of the unit, and regional correlation to marine-sabkha units that vertically stack, then onlap the aeolian-dominated strata. Features indicating a shallow water table and/or evaporites include subaqueous ripple deposits, contorted strata, breccias, collapse features, wavy bedding, foundered sets, loaded set bases, corrugated surfaces, polygonal fractures, ball-and-pillow structures, and trains of small dunes ‘frozen’ in place. Along the traverse, oriented roughly parallel to the predicted palaeoflow from the NNE, sets of cross-strata, bounding surfaces and ‘flat’-bedded strata climb to the SW, while other surfaces and ‘flat’-bedded strata are horizontal. Climb is interpreted to result from the migration of dunes and interdune areas during periods of a rising water table, whereas horizontal surfaces represent super-bounding surfaces formed during periods of static or falling water table, and horizontal ‘flat’ strata represent vertical sabkha accretion. The cross-stratified units consist of compound sets with two scales of cyclicity. The larger, most prominent cyclicity is formed of scalloped bounding surfaces systematically truncating foresets to the SW, and results from superimposed dunes migrating NW and SE along-slope larger bedforms that migrated toward the SW. The smaller scale of cyclicity consists of packages of wind-ripple laminae and grainflow strata truncated by bounding surfaces, and these cycles are thought to represent annual variations in wind direction. Some cross-stratified units are co-sets in which the foresets of the main set can be traced downward into bottomsets consisting of small sets separated by corrugated surfaces. The basal sets are thought to represent small satellite dunes moving along the base of the larger bedforms and across damp interdune floors. The recognition of super surfaces within the Entrada allow the identification of four genetic packages that comprise the formation. Within genetic packages the available sand supply can be estimated, and is considered as a function of the rates of sediment supply and water table fluctuations. These relative rates, coupled with close proximity of the study area to the Jurassic palaeoshoreline and the regional stratigraphic relationships, suggest that fluctuation in relative sea-level controlled the facies architecture seen in the Entrada Sandstone of NE Utah.
