Stratigraphy, Sedimentology, and Ichnology of the Upper Cretaceous Frontier Formation in the Alkali Anticline Region, Bighorn County, Wyoming

The Upper Cretaceous Frontier Formation was studied along two strike-parallel cliff-lines in the Alkali Anticline region of the northeastern Bighorn Basin, Bighorn County, Wyoming. The unit comprises up to 145 m of mudrock, sandstone, conglomerate, and volcanic fallout sediments deposited along the western margin of the Cretaceous Western Interior Seaway (KWIS) in the mid- to late-Cenomanian. Eighteen facies, comprising six facies associations are identified from physical and biogenic sedimentary features. Sediments were deposited in open marine offshore to shoreface and subaqueous deltaic to delta platform environments. The observed trace fossil suites record departures from the archetypal ichnofacies. Such departures record environmental stresses associated with nearshore deltaic settings. Resolving the ichnological signature of these stressed nearshore settings was crucial to reconstructing the depositional environment. The Frontier Formation consists of multiple progradational and retrogradational sequences deposited during a low-frequency (high magnitude) lowstand characterized by lower-magnitude, higher- frequency fluctuations. This study reveals a complex succession of parasequences and deltaic coarsening upward successions deposited under low-accommodation conditions. Parasequence boundaries were the most useful for sub-regional correlation. Two sequence boundary candidates are identified in the Peay and Torchlight Members but they are not useful for correlating across the study area. This investigation provides new insights into the recognition and interpretation of the facies and stratigraphic architecture of nearshore sediments deposited in low accommodation settings, and provides a framework for future evaluations of similar deposits in the Western Interior Seaway