EXPLORING THE PALEOCLIMATIC RECORD OF TERRESTRIAL APTIAN-ALBIAN CARBON ISOTOPE EXCURSIONS IN THE EARLY CRETACEOUS CEDAR MOUNTAIN FORMATION OF EASTERN UTAH

Aptian-Albian terrigenous clastic strata of the Cedar Mountain Formation in the area of the San Rafael Swell contain a calcrete succession that captures the Ap7-C11 C-isotope features of the Cretaceous global carbon isotope chemostratigraphy (Ludvigson et al., 2010, JSR 80:955-974). The long-ranging C9-C11 positive carbon isotope excursion (CIE) spanned from about 115 to 112 Ma, and coincided with a late Aptian peak in Cretaceous volcanism. Baseline carbonate δ¹³C values in chemostratigraphic profiles are about -6‰ VPDB, with peak values of about -3‰ VPDB in the C10 interval. Sedimentary organic matter in these profiles has baseline δ¹³C values of about -30‰ VPDB, with peak values of about -27‰ VPDB in the C10 interval. Coordinated carbonate-organic δ¹³C data yield paleobarometric pCO₂ estimates of about 600 ppm in baseline positions, with a buildup of up to about 950 ppm at the peak of the C10 positive CIE. This C10 stratigraphic interval also provides sedimentary evidence for an aridification event in the leeward rain shadow of the Sevier orogenic highlands. This evidence includes a shift from baseline carbonate δ¹⁸O values of -8‰ VPDB up to peak δ¹⁸O values of -4‰ VPDB in the C10 interval, and also a prominent reddening of strata in the C10 interval in comparison to bounding underlying and overlying gleyed strata. Diagenetic studies of selected calcretes in the Cedar Mountain Formation consistently yield Meteoric Calcite Line values with δ¹⁸O values of -8‰ VPDB, indicating that the time-averaged δ¹⁸O values of local Aptian-Albian paleoprecipitation changed little over time. Partly-dolomitized calcretes from the C10 interval contain calcitic components that show evidence for evaporative enrichments of early diagenetic porewater δ¹⁸O values by up to 4 per mil—showing that the chemostratigraphic shift to higher carbonate δ¹⁸O values was related to syndepositional aridification. The global-scale implications of this local continental aridification in the North American proximal foreland basin (rain shadow) are uncertain at this time.