Entrada Sandstone: an example of a wet aeolian system

Abstract

Wet aeolian systems are those in which the water table is shallow and the floors of the interdune fiats 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 'fiat'-bedded strata climb to the SW, while other surfaces and 'fiat'-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.

Groundwater Controls and Processes

“…The presence of replaced gypsum sand crystals in a number of horizons provides further supporting evidence that the water table was sufficiently close to the surface to allow evaporite formation (Loope, 1988). The Jurassic Entrada Sandstone, studied along a detailed 2.7 km traverse in northeast Utah by Crabaugh and Kocurek (1993), is also interpreted as having formed in a wet aeolian system. The sandstone architecture exhibits 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, all indicating a near-surface water table with occasional shallow flooding of interdune areas.…”

“…Bromley, 1992;Chan and Kocurek, 1988;Gaylord, 1990;Crabaugh and Kocurek, 1993;Loope, 1985Loope, , 1988. Units of the Permian age Cedar Mesa Sandstone in southeast Utah, USA, for example, represent an ancient wet aeolian system (Mountney and Jagger, 2004), Carboniferous) and Lower Permian rocks exposed in Canyonlands National Park, Utah (after Loope, 1985).…”

Aeolian Sediment Transport Integration in General Stratigraphic Forward Modeling

“…It may be either absolute, resulting from a climatic shift to wetter conditions, or relative, whereby the sediment accumulation subsides through a constant water-table level. A shallow water table, either at the depositional surface or within its capillary fringe, ensures that moisture at the interdune surface is at least sufficient to raise erosional threshold values to the point at which the surface is largely protected from deflation, thus increasing preservation potential [61]. Wet aeolian systems characterized by migrating dunes and a rising water table will therefore preserve climbing sets of dune-interdune strata.…”

Scientific drilling and downhole fluid sampling of a natural CO<sub>2</sub> reservoir, Green River, Utah

“…encountered marine and lacustrine red siltstones facies of the "earthy" Entrada Sandstone member, that grade into the 125 m-thick bleached aeolian dune deposits of the lower Entrada Sandstone, with intercalated marginal marine and sabkha influences throughout (see Crabaugh and Kocurek, 1993). Below the Entrada Sandstone lies the Carmel Formation (top at 149 m b.s.…”