New age constraints on the Late Cretaceous lower Williams Fork Formation, Coal Canyon, Colorado

Abstract

The precise age of terrestrial sediments in the Late Cretaceous Williams Fork Formation of western Colorado is poorly constrained due to a paucity of radiometric data. Sanidine and zircon dating of a volcanic ash encased in coal (i.e., the Coal Canyon ash) within the Cameo-Wheeler coal zone of the lower Williams Fork Formation in Coal Canyon, Colorado provides an important new age constraint for the southwestern Piceance Basin. A 10-30 cm thick, light gray, clayey mudstone encased in coal was sampled for both zircon U-Pb and sanidine 40Ar/39Ar geochronology. The presence of numerous euhedral zircon crystals, a lenticular geometry, and a clayey texture suggest that the mudstone is a minimally reworked and slightly altered volcanic ash. Analysis of the euhedral zircon crystals (n=108) in the ash produced a statistically robust U-Pb date with 93 grains yielding a weighed mean age of 74.52 ±0.11 Ma (1σ analytical uncertainty). 40Ar/39Ar sanidine analyses yielded a younger weighted mean age of 73.10 ±0.12 Ma (1σ analytical uncertainty) based on 6 of the 36 grains analyzed. Our preferred age is given by the weighted mean age of the sanidine as it is based on higher precision analyses that can better discriminate older inherited grains that are likely included in the zircon mean-age calculation. Isotopic data for the Coal Canyon ash overlap in age with a K-Ar date of 72.5 ±5.1 Ma for a widespread Williams Fork Formation tonstein, known as the Yampa Bed, found in coal-bearing outcrops and mine workings throughout the northern Piceance and Sand Wash basins and Axial Basin Uplift. Based on the similarity in isotopic age, sedimentologic context and stratigraphic position, we suggest that the Coal Canyon ash and the regionally extensive Yampa Bed are coeval. Additionally, this correlation corroborates that the Cameo-Wheeler coal zone of the Williams Fork Formation in the southwestern Piceance Basin is correlative with the Middle coal zone of the Danforth Hills and Yampa regions. Lastly, this proposed correlation may suggest that the Coal Canyon ash, like the Yampa Bed, correlates with the Baculites reesidei ammonite zone, which is associated regionally with a bentonite dated to 72.94 ±0.45 Ma. Detrital sanidine geochronology of two lower Williams Fork sandstone units that overly the Coal Canyon ash did not produce grains younger than the ash and thus do not quantitatively improve the chronostratigraphy of these specific units. Lastly, the Coal Canyon ash date serves as a basis for future evaluations of the diachroneity of non-marine strata of the Williams Fork Formation.

Effects of contemporaneous orogenesis on sedimentation in the Late Cretaceous Western Interior Basin, northern Utah and southwestern Wyoming

“…Distal equivalents show increasing thickness, but lower net‐to‐gross ratio and limited connectivity of channels (López & Steel, 2015). In the Sand Wash Basin in northwestern Colorado, the Canyon Creek Member is thought to be equivalent to the Williams Fork Formation, based on ammonite control, as well as sanidine and zircon dating (Cobban et al., 2006; Minor et al., 2021; Walker et al., 2021). The Williams Fork Formation contains coastal plain deposits as well as the shoreline sandstones of the Trout Creek and Twentymile members (López & Steel, 2015).…”

Acta Geologica Polonica

“…The vertebrate fauna of the Williams Fork Formation is considered as stratigraphically equivalent to (Fowler 2017;Fowler and Freedman Fowler 2020) or slightly younger than (cf. Walker et al 2021) the Hunter Wash Member of the Kirtland Formation, New Mexico. A sister taxon of Tyrannosauridae, Bistahieversor sealeyi, occurs in this member (Carr and Williamson 2010), but UCM 87636 is distinguishable from this taxon by the aforementioned morphology of the proximal articular surface (Carr 2005).…”

“…The ammonite Didymoceras cheyennense Meek and Hayden, 1856 (74.60-74.21 Ma) occurs in the lower part of the Williams Fork Formation (e.g., Fowler 2017), and Diem and Archibald (2005) 39 Ar age of 73.10±0.12 Ma for the Coal Canyon ash of the lower Williams Fork Formation, which they interpreted as coeval to the Yampa bed. Walker et al (2021) noted that the result of 40 Ar/ 39 Ar sanidine analyses is preferable as this is based on higher precision analyses, and suggested the Coal Canyon ash may correlate with the Baculites reesidei ammonite Zone (72.94±0.45 Ma). These data suggest the vertebrate fauna of the Williams Fork Formation, including the tyrannosaurid taxon represented by UCM 87636, likely dates to the latest Campanian.…”

14th Symposium on Mesozoic Terrestrial Ecosystems and Biota

“…The Late Cretaceous Western Interior Basin (WIB) of North America possesses the highest diversity of hadrosaurids in the fossil record, and the past decade has provided discoveries that continue to refine the spatiotemporal resolution of hadrosaurid biodiversity and clarify the phylogenetic relationships of this clade. We present a preliminary cranial osteology and phylogenetic placement of a new hadrosaurid dinosaur from the Late Campanian Williams Fork Formation ($75-72 Myr Walker et al, 2021]) of northwestern Colorado. The specimen (CNFM-001) was recovered from a sand-rich environment, with localized sections that range from loosely compacted sand grains to hardened concretions and pebble-sized rip-up clasts.…”