RADIOMETRIC CALIBRATION OF THE STABLE ISOTOPE CHEMOSTRATIGRAPHY OF THE EARLY CRETACEOUS CEDAR MOUNTAIN FORMATION

The early Barremian-early Cenomanian Cedar Mountain Formation (CMF) is a dinosaur-bearing deposit that accumulated in a semi-arid terrestrial setting in the proximal foreland basin of eastern Utah, USA. In ascending order, it is comprised of the Yellow Cat, Poison Strip Ss, Ruby Ranch, and Mussentuchit Mbrs. All but the Poison Strip are dominated by mudstone facies. Since 2003, we have reported that the CMF captures the δ¹³C chemostratigraphic record of the Aptian-Albian OAE1a, OAE1b, and OAE1d events (Ludvigson et al., 2003, GSA Abst. Prog. 35:289), but until now, lack of independent dates to confirm marine-terrestrial correlations has restrained formal publication. The Yellow Cat and Ruby Ranch Mbrs contain numerous calcretes and nodular carbonate horizons, while the Poison Strip is a coarse-grained unit. Volcanic ash beds have not been found in these units, and they don't yield palynomorphs, making their ages difficult to constrain. Burton et al. (2006, GSA Abst. Prog. 38:7:52) reported single crystal U-Pb dates from detrital zircons in the Yellow Cat and Ruby Ranch ranging between 124 to 109 Ma (Barremian-Albian). Recently, a palustrine carbonate bed in the uppermost Yellow Cat was dated by U-Pb methods at 119.4±3.4 Ma (early Aptian) by Gulbranson working with Rasbury. Moreover, Hunt is compiling U-Pb age spectra from detrital zircons in the Poison Strip Ss (early Aptian?) and Dakota Ss (early Cenomanian?) bracketing the reference section (designated RRR) with the carbonate U-Pb date. Carbonaceous strata of the Mussentuchit Mbr accumulated in a more humid setting, and have yielded Cenomanian palynomorphs. This unit also contains volcanic ash beds that have been dated by Ar-Ar methods at 98.39±0.07 Ma (Cifelli et al. 1997, PNAS 97:11163-11167), and from 98.2±0.6 Ma to 96.7±0.5 Ma (Garrison et al., 2007, Cret. Res. 28:461-494), all early Cenomanian in age. Recently, an ash bed from near the base of the Mussentuchit was dated at 104±2.7 Ma (late Albian) by single crystal U-Pb analyses of zircon phenocrysts in the laboratory of Stockli. The RRR section has carbonate baseline δ¹³C values of -6‰, with early Aptian (OAE1a) and late Aptian (OAE1b) peak δ¹³C values of -3‰ VPDB, a pattern recognized elsewhere in the basin. Our δ¹⁸O data suggest that OAE1a and OAE1b resulted in continental aridification.