Eolian Sabkha Sandstones in the Nugget Sandstone (Jurassic), Vernal Area, Utah
SCHENK, C. J., and F. PETERSON, U.S. Geological Survey, Denver, CO
The Jurassic Nugget Sandstone in the Vernal, Utah, area is characterized by thick (up to 25 m) sets of cross-stratified eolian dune sandstone separated by either erosional planar bounding surfaces or thin (mostly < 3 m) sandstones interpreted as sabkha sandstones. Structures in Nugget sabkha sandstones are predominantly wavy or irregular bedding and thin, remnant sets of dune cross-strata consisting of eolian ripple and avalanche strata. The types of sedimentary structures and erosional features in Nugget sabkha sandstones indicate a close relationship between sand deposition and erosion and fluctuations in the local water table. Irregular or wavy bedding can form by displacive salt growth, infilling of thin, crumbled algal mats, or a combination of the two processes. Salt growth i displacive and typically buckles up the sand. This small-scale topography fills in with sand blown across the sabkha, forming irregular bedding. The same process of infilling occurs on the small-scale topography on thin algal mats. Aerosol-derived silt and clay can adhere to the damp surfaces of sabkhas, increasing the finer grained component in these beds.
Thin, remnant eolian dune sets are common in Nugget sabkha sandstones. The remnant sets form when dunes migrating across a sabkha are partially wetted as the water table rises slightly (on a scale of tens of centimeters); the lower part of the dune with wetted sand remains on the sabkha as the rest of the dune continues to migrate. Typically, ripple strata of the dune apron and the toes of avalanche strata are preserved in dune remnants. The avalanche strata, being slightly coarser grained, are preferentially deflated, leaving microtopography. This topography is commonly filled in with ripple strata that form as dry sand again blows across the sabkha. Stacked sets of remnant dunes separated by erosional surfaces illustrate the control of sand deposition on eolian sabkhas by the local ater table.
AAPG Search and Discovery Article #91010©1991 AAPG Rocky Mountain Section Meeting, Billings, Montana, July 28-31, 1991 (2009)