RECONNAISSANCE CARBON ISOTOPIC CHEMOSTRATIGRAPHY OF PEDOGENIC-PALUSTRINE CARBONATES IN THE EARLY CRETACEOUS CEDAR MOUNTAIN FORMATION, SAN RAFAEL SWELL, EASTERN UTAH

Uppermost strata of the terrestrial Cedar Mountain Formation (CMF) were radiometrically dated by Cifelli et al. (1997, Proc. Natl. Acad. Sci. USA 94:11163-11167) as 98.4 Ma, but the chronostratigraphy of the remainder of the unit remains poorly constrained. Recent studies in other regions show major temporal excursions in the d¹³C values of organic matter from Early Cretaceous C₃ terrestrial plants (Rennison, 1996, M.S., SMU; Jahren et al., 2001, Geology 29:159-162). We are investigating the d¹³C of authigenic carbonate beds in the CMF to test whether they similarly capture this record, following the rationale of coupling between marine, atmospheric, and continental carbon reservoirs outlined in Koch et al. (1992, Nature 358:320-322). Chemostratigraphic profiles from carbonate beds of the Ruby Ranch and Mussentuchit members in stratigraphic sections at Dinosaur National Monument (40 m) and Price River Canyon (135 m), and from the Yellow Cat Member (21 m) (Kirkland et al., 1997, BYU Geol. St. 42:II:69-103) have been developed. All three profiles show organized structure. Sections at Dinosaur National Monument and Price River Canyon both show positive carbon isotope excursions in a 30 m-thick interval of the Ruby Ranch Member, with magnitudes of up to 4 per mil (peak values of –3 ‰ PDB), with the falling limb corresponding in each section to a thick calcrete-bearing interval. The profile in the basal Yellow Cat Member resembles the lower part of the Price River Canyon profile. Carbon isotope values at the base of the Yellow Cat Member begin at –3 ‰ and decrease upward to –7 ‰ PDB at the top, a trend which also occurs in the lower Price River Canyon profile. It appears that internal chemostratigraphic correlations within the CMF can be proposed from these data. Based on general similarity in form, it also appears that tentative correlations can be made to the marine carbon isotope record of OAE 1a and OAE 1b in the Aptian-Albian stages (Weissert et al., 1998, Palaeo³ 137:189-203).