THE TERRESTRIAL PALEOCLIMATIC RECORD OF THE APTIAN-ALBIAN C9-C11 CARBON ISOTOPE EXCURSION IN THE CRETACEOUS NORTH AMERICAN FORELAND BASIN OF EASTERN UTAH, USA

Aptian-Albian siliciclastic strata of the Cedar Mountain Formation in the area of the San Rafael Swell contain a calcrete succession that captures the C9-C10-C11 C-isotope features of the Cretaceous global carbon isotope chemostratigraphy. This long-ranging positive carbon isotope excursion (CIE) spanned from about 115 to 112 Ma, and coincided with a peak in Cretaceous volcanism. Baseline carbonate δ¹³C values in the 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 and organic matter 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. The C10 stratigraphic interval also contains sedimentary evidence for an aridification event in the leeward rain shadow of the Sevier orogenic highlands. These include a shift from baseline carbonate δ¹⁸O values of -8‰ VPDB up to peak δ¹⁸O values of -4‰ VPDB in the C10 interval, and a prominent reddening of strata in the C10 interval in comparison to bounding gleyed strata. Diagenetic studies of selected calcretes in the Cedar Mountain Formation consistently yield Meteoric Calcite Line values with δ¹⁸O values of -8‰ VPDB, showing that the δ¹⁸O of 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—thus explaining the chemostratigraphic shift to higher carbonate δ¹⁸O values. While there is strong evidence for continental aridification in the North American proximal foreland basin (rain shadow) during the Aptian-Albian C10 event, the zonal continental paleoclimatic expression of this CIE remains uncertain at this time.