A Record of Bajocian and Early Bathonian Restricted Marine Sedimentation along the Cratonic Margin of the Sundance Sea, Bighorn Basin, WY

This study examines the depositional environments that existed along a portion of the cratonic margin of the Bajocian and Early Bathonian-aged Sundance Sea in the western interior of North America. The hypotheses considered state that the Bajocian and Early Bathonian evaporite-bearing units of the eastern Bighorn Basin, WY, were deposited within a variably restricted marginal marine environment that existed along the cratonic margin during the second, of six, regional marine transgressions. Secondly, if depositional shallowing sequences are recorded within these restricted marine strata, they represent shorter duration relative sea level fluctuations that are the result of either eustatic fluctuations, episodic basin subsidence/uplift, or both.;These hypotheses were addressed through detailed observation of the Bajocian and Early Bathonian stratigraphic record, combined with petrographic analysis of the various lithologies and correlation of measured stratigraphic sections and electric logs to develop regional cross-sections. These methods subsequently led to the identification of seven lithofacies groups within the studied succession, each based on sedimentologic and petrographic similarities. The environment of deposition of each lithofacies group was interpreted based on the similarity of each group's defining characteristics to published characteristics of similar modern and ancient environments. Within these depositional environments, five upward shallowing cycles were interpreted: a basal salina cycle, three coastal sabkha cycles, and a capping carbonate ramp cycle. Thinning of the carbonate ramp cycle and the absence of the subaerial facies of the uppermost coastal sabkha cycle suggests the presence of a paleo-high in the southern portion of the study area, coincident with the suggested location of the Sheridan Arch.;The comparison of shallowing sequences with existing eustatic and regional sea level records for the Bajocian and Early Bathonian led to the conclusion that the eustatic curves were of insufficient resolution to compare with the temporal scale of the observed shallowing cycles. However, the third-order sea level oscillations recorded in the Boreal record did roughly correlate with the shallowing sequences observed, suggesting that regional sea level fluctuations may have been the dominant control on the formation of the coastal sabkha shallowing cycles, as well as the salina and carbonate ramp cycles.