Carbonate Carbon Isotope Chemostratigraphy from the Ruby Ranch Member of the Cedar Mountain Formation in the Western San Rafael Swell

Abstract

The Ruby Ranch Member of the Cedar Mountain Formation in East-Central Utah represents terrestrial deposits in the early Cretaceous Period. This member has been studied at multiple field locations and has been determined to span the Aptian-Albian Boundary based on chemostratigraphic and detrital zircon studies. The study location presented here, Moore CutoffRoad, is further west than previous studies, and thought to be a more expanded section that may contribute more detail to the chemostratigraphic and paleoclimatic record. The Ruby Ranch Member in this area consists mainly of reddish grey and greenish gray silty-sandy mudstones with abundant carbonate nodules suggestive of paleosol development. For this study samples were collected approximately every 25 cm and were used to construct a C-isotope curve using carbonate carbon. These were used to compare to existing bulk sedimentary organic carbon isotope curves. A total of 48 samples were analyzed and values from -7.45 ‰ vs. VPDB to -3.52 ‰ vs. VPDB were observed. The C-isotope data follows the trend of existing organic carbon isotope curves which exhibits the C10 positive isotope excursion near the Aptian-Albian boundary. A set of samples have been analyzed for pCO2 values. These values were calculated using both a high and low value for both S(z) and temperature. pCO2 values range from 795 ppmV to 2850 ppmV over the 4 scenarios presented. A subset of samples have been analyzed for clumped isotope paleothermometry to test a hypothesis that this positive isotope excursion corresponds with a late Aptian “cold snap” in an otherwise greenhouse climate. This data produced values that were not reliable estimates of temperature. Additional studies are needed to confirm a temperature change over the C10 positive isotope excursion and refine the paleoclimate record at this location.