ABSTRACT

The Hulett Sandstone Member in the northeastern Bighorn Basin is a mixed carbonate-clastic sequence representing two regressions of nearshore and shallow marine environments. Quartz-bearing oolitic skeletal packstones and grainstones represent the onset of Hulett deposition as a prograding series of southward-migrating barrier bars deposited onto shelf muds of the Stockade Beaver Shale Member. Pelleted, silt-bearing laminated lime mudstones and nodular lime mudstones were deposited in a shallow, quiet to tidally influenced, back-barrier lagoon and tidal flat landward of the barrier bar complex. These mudstones also stratigraphically overly the high-energy bar sediments. The laminated lime mudstones contain numerous individuals of a single species of fish suggesting a stressed, possibly hypersaline, environment. Burrowed nodular lime mudstones represent a period of deepening of the lagoon and less tidal influence. Tidal-flat sands to the north, and oolitic grainstones to the south, overly the lagoonal mudstones and represent a second episode of progradation and seaward shoaling. The Hulett is unconformably overlain by marine shales of the Redwater Shale Member.

Although thin (generally 10 to 30 feet), the sandstones, packstones, and grainstones of the Hulett Sandstone Member are potential hydrocarbon reservoir facies contained within the thicker marine shales of the Sundance Formation.