Minnelusa Depositional Cycles and Erosional Topography, Rozet Fields Area, Powder River Basin, Wyoming: ABSTRACT

Abstract

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) supratidal 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, intertidal, beach, and eolian. A Minnelusa cycle and its facies have close counterparts in the Holocene sabkhas in Abu Dhabi. In addition, mapping of Minnelusa sabkhas reveals that they are very narrow, trend in the same direction, and are separated by quartz sandstone buildups that may represent eolian dune trends. A Holocene analog of the Minnelusa sabkha-eolian dune complex is seen in the area southeast of Abu Dhabi where narrow sabkhas are separated by eolian dune ridges. 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 hydrocarbon in the Rozet fields and, apparently, in many other fields that produce from the Minnelusa. End_of_Article - Last_Page 1329------------