NEW RADIOMETRIC AGES FROM THE CEDAR MOUNTAIN FORMATION, UTAH AND THE CLOVERLY FORMATION, WYOMING: IMPLICATIONS FOR CONTAINED DINOSAUR FAUNAS

The Cedar Mountain Formation (CMF) in Utah and Colorado and the Cloverly Formation in Wyoming and Montana preserve several vertebrate faunas that until now have not been dated to anything more precise than the Aptian to Albian. Single-crystal U-Pb dating of zircons from several horizons within the CMF (mostly extracted from clayey siltstones and sandstones) and a single-crystal laser-fusion Ar-Ar age on sanidine from an ashy bed in the Cloverly Formation have provided the first reliable geochronology for these vertebrate faunas. CMF samples dated (with their ages and stratigraphic positions above the Morrison Formation) are: 4 samples from the Price River II quarry in Carbon County, Utah (109 Ma at 40 m, 113 Ma at 15m, 116 Ma at 14 m, and 114 Ma at 1.6 m) and one sample from the Quarry-16 in Dinosaur National Monument (117 Ma at about 30 m above Morrison). An additional sample from 6 m above the CMF/Morrison Formation contact collected near the Dalton Wells Quarry north of Moab, Utah was previously reported by Greenhalgh et al. (2006) at about 124 Ma. The sample from the Cloverly Formation comes from Beaver Creek near Shell, Wyoming at about 75 m above the contact with the underlying Morrison Formation and gives an age of 108.5 ± 0.2 Ma. A published single-crystal laser-probe sanidine age of 98.4 ± 0.1 Ma (Cifelli et al., 1997) from the uppermost CMF comes from an ash about 36 m above the Morrison contact.

There are at least three distinct dinosaur faunas in the CMF. The oldest fauna (~124 Ma) is the Yellowcat Member fauna dominated by sauropods, Gastonia, and Utahraptor. A middle fauna (~113-117 Ma) from the CMF is dominated by a brachiosaurid sauropod and Sauropelta. Sauropelta is also known from the Cloverly Formation. The youngest fauna in the CMF (~100 Ma) is characterized by Eolambia and Animantarx. The new dates presented here provide the first framework for better correlation of these faunas with others worldwide and for better understanding evolutionary rates and paths.