Sedimentology of cross-stratified sandstones in Arikaree group, Miocene, southeastern Wyoming

Abstract

The Arikaree Sandstone of Early Miocene age in southeastern Wyoming contains large-scale cross-stratification thought by previous investigators to have formed in a fluvial environment. Detailed study of the cross-strata, however, suggests an aeolian origin. Average angle of dip of foreset beds is 24° with angles as high as 34°. Thickness of sets of cross-strata is commonly 3–4 m with 11 m measured in one location. Settling velocities in water of hornblende and quartz grains from these deposits yield cumulative curves indicative of deposition from aeolian suspension. Petrographic study of the suspected aeolian sandstones shows them to contain smaller percentages of volcanic air-fall shards than associated fluvial sediments. This probably reflects the high winnowing susceptibility of these very fine-grained, low-sphericity shards during aeolian transportation and deposition. Scanning electron microscopy revealed polygonal plate structures on the surfaces of quartz grains which are generally associated with aeolian abrasion. The evidence suggests that these cross-strata represent remnants of aeolian dunes, possibly barchan and transverse types, that migrated across the aggrading High Plains surface during the Early Miocene. They consequently became intercalated with fluvial sediments producing complex facies relationships.