The Casper Formation of Pennsylvanian and Permian age in the southernmost Laramie Basin contains large trough cross-stratification for which both eolian and subaqueous origins have been proposed. To determine which is the most reasonable interpretation of origin, information concerning depositional processes was obtained from a study of associated sedimentary structures and settling velocities of light and heavy minerals.
Two important characteristics of sandstone units in the Casper Formation strongly suggest an eolian origin: (1) The cross-stratified units typically contain low-crested, locally truncated ripple marks that nearly everywhere are oriented with their crests parallel to the dip of the cross-strata, and (2) there are evenly spaced lag grains along bedding surfaces.
Other characteristics of the sandstone units are compatible with an eolian environment, although their origin is more ambiguous. For example, low-dipping cross-strata, often cited as evidence against an eolian origin, are similar to the internal stratification of eolian dunes where moisture prohibits development of large avalanche faces. Moreover, large-scale contemporaneous deformation of sand, although commonly attributed to subaqueous gliding, probably requires more cohesion than would be present in saturated subaqueous sand. Moist eolian conditions may provide this cohesion. Finally, settling velocities of associated light and heavy minerals indicate deposition from eolian suspension. Therefore, I conclude that sandstone units in the Casper Formation of the southernmost Laramie Basin were deposited as a coastal dune field that bordered the ancestral Front Range uplift and migrated laterally in response to a fluctuating sea level.