Wyoming Geological Association: 1982 Luncheon Meetings Casper, Wyoming: Abstracts of Papers

Marine cyclic deposition is very apparent in cores of the upper Minnelusa Formation in the Rozet Fields area, Powder River Basin, Wyoming. A complete cycle from bottom to top is comprised of: (1) subtidal facies of dolomicrite or dolomitized packstone that sometimes contains fusulinids or crinoid fragments, (2) intertidal facies of algal-laminated or layered dolomicrite which is, sometimes, desiccated and slightly brecciated, and (3) supra-tidal facies of anhydrite marked by “chicken-wire” structure. However, complete cycles are interrupted by exceptionally clean, well-sorted quartz sandstone units that may have been deposited in a variety of closely related, coastal environments including very shallow subtidal (foreshore), beach, and eolian.

A Minnelusa cycle and its facies have close counterparts in the recent sabkhas near Abu Dhabi on the Trucial Coast, Persian Gulf. In addition, mapping of Minnelusa sabkhas reveals that they are very narrow, are trending in the same direction, and are separated by quartz sandstones that make up linear dune trends of possible eolian origin. A recent analog of the Minnelusa sabkhas-eolian dune complex is seen in the area southeast of Abu Dhabi where narrow sabkhas are separated by longitudinal, eolian dunes. Thus, it appears that Minnelusa anhydrites formed in a sabkha setting very similar to the modern sabkha-eolian dune complex southeast of Abu Dhabi.

Erosional topography at the top of the Minnelusa Formation plays a prime role in the entrapment of hydrocarbons in the Rozet Fields and in many other fields that produce from the Minnelusa.