Abstract

The Upper Cretaceous Frontier Formation in northwestern Wyoming consists of alluvial-plain to offshore-marine lithofacies deposited near the western edge of the Cretaceous Interior seaway. Excellent outcrops along the Gros Ventre River, northeast of Jackson, provide an opportunity to observe vertical and lateral facies changes and to determine paleoenvironments of deposition. The Frontier Formation in this area is composed of two major clastic wedges, the lower of earliest Cenomanian to early Turonian and the upper of middle Turonian to middle Coniacian age. Recognizable genetic units in the Frontier outcrop along the Gros Ventre River are organized into five members that conform to the member-level nomenclature developed for the Frontier in southwestern Wyoming and north-central Utah.

The Frontier Formation conformably overlies the upper Albian Aspen Shale. At its base are a series of three upward-coarsening, shoreface sequences that are assigned to the Longwall Sandstone Member. This unit is overlain by fluvial sandstones and shales of the Chalk Creek Member which, on outcrop, is characterized by numerous colorful ashfall marker beds. The Longwall and Chalk Creek members constitute the lower of the two clastic wedges. Nonmarine deposition was terminated by transgression of the shoreline near the peak of the Greenhorn transgressive-regressive cycle, which culminated in deposition of the Allen Hollow Shale Member.

The upper clastic wedge of the Frontier Formation is also represented by two members. The lower of these, the Oyster Ridge Member, is a thick, wave-dominated shore-face sandstone that gradationally overlies offshore-marine shale of the Allen Hollow. The Oyster Ridge Member is overlain by alluvial-plain to bayfill facies assigned to the lower part of the Dry Hollow Member. The upper part of the Dry Hollow includes a series of shoreface to tidal channel sandstones. They are believed to interfinger eastward with the lower part of the Cody Shale and record westward shifting of facies during the Niobrara transgression. Frontier deposition in northwestern Wyoming was terminated by transgression of the Cody Sea in middle Coniacian time.