STRATIGRAPHY AND BIOSTRATIGRAPHY OF UPPER CRETACEOUS STRATA IN CENTRAL WYOMING: IMPLICATIONS FOR CENTRAL LARAMIDIAN PALEOGEOGRAPHY

The Campanian and Maastrichtian deposits of central Wyoming have been a focal point for petroleum exploration for nearly a century, with extensive attention focused on the marine shales, siltstones, and sandstones of the Cody Shale and Lewis Shale. In contrast, the nonmarine deposits of the Mesaverde, Meeteetse, and Lance formations remain largely understudied despite yielding significant fossil vertebrate, invertebrate, and plant assemblages. These deposits provide critical insight into the nonmarine biogeography of Late Cretaceous biotas due to their location at the juncture between the southern (e.g., Utah, New Mexico, Texas) and northern (e.g., Montana, Alberta) regions of Laramidia.

In this study, we combine subsurface data as well as previously published and newly measured outcrop data from Upper Cretaceous strata in the Powder River, Wind River, and Bighorn basins of Wyoming to reconstruct the paleogeography of central Laramidia. Sections are correlated using ammonites, inoceramids, and radiometrically dated ash beds from across the region. Additionally, previously published and newly discovered nonmarine vertebrate fossil localities are georeferenced to determine their precise stratigraphic positions and estimate their depositional ages. This allows for direct comparison to similar nonmarine fossil assemblages in northern and southern Laramidia.

Our analysis reveals six third-order, tectono-eustatic cycles within the lower Campanian to Danian strata of central Wyoming. Each cycle is composed of a basal deltaic to nearshore sandstones, a middle sequence of nonmarine sandstones, carbonaceous shales, and mudstones, and a transgressive marine shale tongue. Nonmarine vertebrate fossil occurrences are currently known from the Parkman Sandstone (“North Platte Tongue”) sequence. Other stratigraphic successions have not been extensively prospected for nonmarine fossils, presenting a promising opportunity for future discoveries of new bonebeds throughout the Upper Cretaceous of central Wyoming.