Jurassic aeolian oolite on a palaeohigh in the Sundance Sea, Bighorn Basin, Wyoming

Aeolian limestones are widespread in the Quaternary record and have been identified in outcrops and cores of late Palaeozoic strata. These rocks have been interpreted as a low latitude signal of glacio-eustatic sea level fluctuations and have not been previously reported from the Mesozoic or from other episodes of earth history generally believed to have been non-glacial. Numerous lenticular bodies of cross-stratified oolite lie near the contact between the lower and upper members of the mudstone-dominated lower Sundance Formation (Middle and Upper Jurassic) in the Bighorn Basin of north-central Wyoming, USA. The lenses, up to 12 m thick, contain sedimentary structures diagnostic of aeolian deposition. Inversely graded laminae within thick sets of cross-strata were deposited by climbing wind ripples. Adhesion structures and evenly dispersed lag granules are present in flat-bedded strata at the bases of several of the oolite bodies. Thin sections reveal abundant intergranular micrite of vadose origin. The lenses appear to represent virtually intact, isolated aeolian bedforms that migrated across a nearly sand-free deflation surface. When the Sundance Sea transgressed the dunes, a thin (<1 m thick), wave-rippled, oolite veneer formed on the upper surface of the aeolianite. Previous workers, primarily on the basis of sedimentary structures in the veneer, interpreted the oolite lenses as tidal sand bodies. The dunes provide clear evidence of widespread subaerial exposure on the crest and north flank of the Sheridan Arch. This structural high was delineated by previous workers who demonstrated thinning of pre-upper-Sundance Formation strata and localized development of ooid shoals. Ooids that formed in shoals on the windward (southern) side of the palaeohigh were exposed and deflated during lowstand. Thin, scour-filling ooid grainstone lenses that crop out in the southern part of the study area represent remnants of the marine beds that sourced the aeolianites. Farther north (down-wind), oolitic dunes prograded over thinly laminated lagoonal silts. When relative sea level began to rise, the uncemented dunes were buried under fine-grained marine sediment as the lee side of a low-relief island was inundated.