CRYPTOTEPHRAS IN SILICICLASTIC MUDSTONE PALEOSOLS AND THE GEOCHRONOLOGIC CALIBRATION OF CRETACEOUS (APTIAN-ALBIAN) C-ISOTOPE STRATIGRAPHY

The Cretaceous C-isotope stratigraphy of Aptian-Albian carbon isotope excursions (CIEs) has been correlated from the original discoveries in marine strata into fully terrestrial strata in the North American continental interior (Ludvigson et al., 2010 Jour. Sed. Res. 80:955-974; Ludvigson et al. 2015, Cret. Res. 56:1-24; and Suarez et al., 2020, Geol. Soc. London SP 507). These studies have shown the CIEs are associated with changes in pCO₂, mean annual precipitation, mean annual temperature, and precipitation-evaporation balances on the continent. While the marine biostratigraphic chronostratigraphy of the Aptian-Albian CIEs is well known, the absolute ages of C-isotope segments of the Aptian-Albian record are poorly constrained (Erba et al., 2015, GSA SP 511:271-303). We are developing a refined time scale of the Aptian-Albian CIEs using cryptotephras contained in paleovertisols of the Cedar Mountain Formation of eastern Utah. Cryptotephras are the mineralogical remnants of discrete volcanic ash fall deposits obscured by pedoturbation in ancient soils in fluvial overbank settings. They are detected by collecting large populations (n ≈ 300) of LA-ICP-MS U-Pb ages of zircons separated from selected paleosol horizons. The cryptotephras are identified by the occurrence of clusters of several tens of U-Pb ages at the young end-member of the zircon age spectra in their respective paleosols. We calculate eruption ages of cryptotephras using the Bayesian modelling approach of Keller et al. (2018, Geochem. Persp. Let. 8:31-35). Our studies of cryptotephras in the Cedar Mountain Formation at the type section of the Ruby Ranch Member and at Dinosaur National Monument indicate age ranges from 118.6±0.7 Ma to 117.4±1.1 Ma for the late Aptian C10 C-isotope segment. Our integration of U-Pb data from cryptotephras and carbonates in the early Aptian C7 C-isotope segment indicate that there still are major uncertainties, with ages ranging from approximately 125 Ma to 119 Ma. Our U-Pb geochronologic refinements of the time scale of Aptian-Albian CIEs can help in clarifying temporal relationships to well-dated oceanic Large Igneous Provinces.