Correlation of aptian-albian carbon isotope excursions in continental strata of the cretaceous Foreland Basin, Eastern Utah, U.S.A.

摘要

Nodular carbonates ("calcretes") in continental foreland-basin strata of the Early Cretaceous Cedar Mountain Formation (CMF) in eastern Utah yield δ~(13)C and δO records of changes in the exogenic carbon cycle related to oceanic anoxic events (OAEs), and terrestrial paleoclimate. Chemostratigraphic profiles of both forebulge and foredeep sections show two prominent positive δ~(13)C excursions, each with a peak value of -3% VPDB, and having background δ~(13)C values of about -6% VPDB. These excursions correlate with the global early Aptian (Ap7) and late Aptian-early Albian (Apl2-All) carbon isotope excursions. Aptian-Albian positive δ~(13)C excursions in the CMF also correspond to 3-4 per mil increases in carbonate δ~(18)O. These phenomena record local aridification events. The chemostratigraphic profile on the thinner forebulge section of the CMF is calibrated, for the first time, by a radiogenic U-Pb date of 119.4 ± 2.6 Ma on a carbonate bed, and by detrital zircon U-Pb dates on two bounding sandstone units (maximum depositional ages of 146 Ma and 112 Ma). Pétrographie observations and diagenetic analyses of micritic to microsparitic carbonates from nodules indicate palustrine origins and demonstrate that they crystallized in shallow early meteoric phreatic environments. Meteoric calcite lines derived from CMF carbonates have δ~(18)O values ranging between -8.1 to -7.5%o VPDB, supporting an estimate of zonal mean groundwater δ~(18)O of -6% VSMOW for an Aptian-Albian paleolatitude of 34° N. Furthermore, our two chemostratigraphic profiles exhibit a generally proportionate thinning of correlative strata from the foredeep on to the forebulge, suggesting that there were consistently lower rates of accumulation on the forebulge during the Aptian-Albian. Identification of the global Aptian-Albian δ~(13)C excursions in purely continental strata, as demonstrated in this paper, opens a new avenue of research by identifying specific stratigraphie intervals that record the terrestrial paleoclimatic impacts of perturbations of the global carbon cycle.