SEQUENCE-STRATIGRAPHIC ARCHITECTURE AND FACIES OF THE MIDDLE TO UPPER JURASSIC PREUSS AND STUMP FORMATIONS, WYOMING AND IDAHO, USA

This study tests the principles of sequence stratigraphy in foreland basins where rates of accommodation and sedimentation increase from the slowly subsiding distal portions of the basin into the rapidly subsiding foredeep. Specifically this study focuses on the Middle to Upper Jurassic Preuss and Stump formations of the Wyoming Range, Wyoming and Idaho, USA, which were deposited in the foredeep of the Sundance Seaway, a retroarc foreland basin. The Preuss and Stump formations are interpreted to lie within the foredeep because of significant westward thickening of the formations: the Preuss Formation is ~ 400 m thick in parts of Idaho, over seventeen times thicker than equivalent rocks in central Wyoming. A facies and sequence-stratigraphic analysis was undertaken to understand how the sequence architecture of the foredeep correlates to the previously established architecture of Middle and Upper Jurassic strata in the Bighorn Basin. The Giraffe Creek Member of the Twin Creek Formation, the Preuss Formation, and Stump Formation record three distinct facies associations. The Giraffe Creek Member was deposited on a carbonate ramp, the Preuss Formation was deposited on a siliciclastic, tidally dominated coast, and the Stump Formation was deposited on a low-energy, wave-dominated shelf. The Giraffe Creek Member, the Preuss Formation, and the Stump Formation represent one partially complete depositional sequence. This depositional sequence is equivalent to the J-4 sequence in the Bighorn Basin as described by McMullen et al. (2014) and Danise and Holland (2018). The lowstand systems tract is equivalent to the J-4 unconformity updip in the distal portion of the basin. The transgressive and highstand systems tracts are equivalent to the Redwater Shale and Windy Hill Members of the Sundance Formation. The results of this study support for the theoretical framework for foreland basin sequence stratigraphy proposed by Posamentier and Allen (1993). The sequence architecture of the foredeep suggests that the rate of tectonic subsidence and the position of the shoreline in the high or low-accommodation zone are the primary controls on the sequence architecture of a foreland basin.