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New Radiometric Age for the Lower Williams Fork Formation, Coal Canyon, Colorado


The age of terrestrial sediments in the Upper Cretaceous Williams Fork Formation of western Colorado is poorly constrained due to a paucity of radiometric data. Ages are typically assigned based on correlations with ammonite and Inoceramid biozones established within nearby marine strata. Detrital mineral dating of a tonstein in the Cameo-Wheeler Coal Zone of the Lower Williams Fork Formation provides an important new age constraint on basal Williams Fork deposits in the Coal Canyon area of the western Piceance Basin. We informally refer to this tonstein as the Coal Canyon Bed (CCB).

A 10-30 cm thick, light gray tonstein was sampled for both detrital zircon U-Pb and detrital sanidine 40Ar/39Ar geochronology. Results are consistent among the two dating methods. Analysis of euhedral zircon crystals in the bentonite produced a statistically robust U-Pb date of 72.49 +0.32/-1.32 Ma based on the weighted-mean age of the six youngest zircon grains. The youngest zircon grain is 71.2 ± 1.20 Ma. Detrital sanidine dating yielded a 40Ar/39Ar date of 73.10 ± 0.12 Ma based on the weighted-mean age of the six youngest grains. The youngest sanidine grain is 72.50 ± 0.30 Ma. The euhedral shape of the zircon crystals and the consistency of the dating results suggest that the youngest grains represent volcanic air-fall deposits. These data compare favorably with an age estimate for a basal Williams Fork Formation bentonite, known as the Yampa Bed, located near the Danforth Hills and Craig, Colorado, which produced a K-Ar date from andesine of 72.5 ± 5.1 Ma.

Based on the new detrital mineral ages, we tentatively correlate the CCB with the Yampa Bed, which would extend the areal extent of this bentonite ~100 km southwest of its known distribution. If this correlation is correct, it implies that the Rollins Sandstone of the Iles Formation in the Coal Canyon area could correlate with the Trout Creek Sandstone Member of the Iles Formation in the Danforth Hills-Yampa region since both sandstones underlie the correlated bentonites by ~30 m. Similarly, it seems likely that mudrock-rich and sandstone-poor, coal-bearing strata of the Lower Williams Fork Formation, which overlie the Rollins and Trout Creek sandstones in the Coal Canyon and Danforth Hills areas, respectively, are also correlative. This correlation supports the idea of a broadly coeval episode of Late Cretaceous regression recorded in northern and western Colorado by eastward progradation of the Williams Fork fluvial-paludal depositional system beginning no later than ca. 72.5 Ma.