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Middle Jurassic eolian carbonates, Bighorn Basin, Wyoming
ETD collection for University of Nebraska - Lincoln
  • Zoran Kilibarda, University of Nebraska - Lincoln
Although eolian limestones are widespread in the Quaternary record and have been identified in outcrops and cores of late Paleozoic strata, they have not been previously reported from the Mesozoic. Numerous lenticular bodies of cross-stratified oolite lie near the contact between the upper and lower members of the mudstone-dominated Sundance Formation (Middle and Upper Jurassic) in the Bighorn Basin of north-central Wyoming. The lenses, up to 10 m thick, contain sedimentary structures diagnostic of eolian deposition. Inversely-graded laminae within thick sets of cross-strata were deposited by climbing wind ripples. Adhesion ripples and evenly-spaced lag granules are present in flat-bedded strata at the base of several of the oolite bodies. Thin sections reveal abundant intergranular micrite--a common vadose precipitate in coastal Quaternary eolianites deposited during arid intervals. The lenses represent either: (1) virtually-intact, isolated eolian bedforms that migrated across a nearly sand-free deflation surface; or (2) erosional remnants of a once-continuous sheet of eolian sand. When the Sundance Sea transgressed the eolian sand, a thin ($<$1 m thick), wave rippled, spar-cemented oolite veneer formed on the upper surface of the eolianite. Previous workers, primarily on the basis of the 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 the lower Sundance strata and localized development of ooid shoals. Dips of cross-strata within the dunes are polymodal, but northward dips dominate in most lenses. Ooids that formed in shoals on the windward (southern) side of the paleo-high were exposed and deflated during sea-level lowstand. Lenses of ooid grainstone that crop out south of Greybull bear sedimentary structures indicative of subaqueous deposition and represent the remnants of the marine beds that sourced the eolianites. Most of the eolian dunes prograded over a laminated silty-mudstone. This silty-mudstone represents a dust deposit that accumulated rapidly in a lagoon and broad leeward mudflat in the initial stage of the exposure. ^
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Zoran Kilibarda. "Middle Jurassic eolian carbonates, Bighorn Basin, Wyoming" (1994) p. 1 - 148
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