A NEW SPATIAL AND TEMPORAL CORRELATION OF WYOMING’S UPPER CRETACEOUS STRATIGRAPHY

The stratigraphy of Wyoming has been investigated since the mid-1800s, when the first geologic surveys were sent to explore and document the western United States. These early surveys interpreted stratigraphic relationships and developed the nomenclature in use today, which has been further refined over the decades. After more than 150 years of study, some of Wyoming’s most notoriously difficult strata to interpret have been the carbonates and siliciclastics deposited in the Cretaceous Western Interior Foreland Basin and early Laramide basins. Correlation of these strata have been improved by three major developments, including: 1. high-resolution biostratigraphic frameworks developed by W.A. Cobban and others, 2. the advent of sequence stratigraphy in the mid-1970s, and 3. subsurface exploration and the invention of geophysical well-logging techniques.

These advancements, when coupled with enhanced precision in geochronology, provide the framework for a new temporal and spatial correlation of Wyoming’s Upper Cretaceous strata. Here, we correlate the lithostratigraphy of 25 generalized sections across the state, spanning the uppermost Albian to lower Danian (105 to 63 Ma). Lithostratigraphic units are correlated to stages, polarity chrons, U.S. Western Interior biostratigraphic zonations (molluscan, palynostratigraphic, and land vertebrate biozones), as well as radiometric dates of ash beds from Wyoming and throughout the Western Interior.

Our correlation highlights four state-wide marine transgressions, four regressions (including the final transition to non-marine deposition during the Maastrichtian and Paleocene), and two significant hiatuses. These stratigraphic patterns reflect the interplay between eustasy and local to regional sedimentation and tectonics.