REFINING THE CHRONOSTRATIGRAPHY OF THE CRETACEOUS CEDAR MOUNTAIN FORMATION, UTAH USING VOLCANOGENIC ZIRCON FROM MATURE PALEOSOLS

Geochronological constraints on terrestrial strata are crucial for the context of paleoclimatic records. However, sections consisting of stacked paleosols mostly lack rocks typically using for dating. Zircons have traditionally not been extracted from mudrocks for U-Pb dating on the assumption that heavy minerals would be too small, due to hydraulic equivalence. However, based on the hypothesis that exposed continental surfaces can receive zircons directly from volcanic airfall deposits during times of volcanism, hydraulic equivalence is not relevant. Since volcanogenic zircons are not restricted by equivalent grain sizes of hydraulic sorting, large zircons may be deposited in fine-grained sediments (Möller et al., 2019). This study uses maximum depositional ages (MDAs) from zircons in mature paleosols from the Ruby Ranch Member and Yellow Cat Member of the Cretaceous Cedar Mountain Formation (CMF) in Utah. These are compared to results from intervening fluvial sandstones of the Poison Strip Member of the CMF. We propose that zircon MDAs from mature paleosols are likely closer to the true depositional age than those of fluvial sandstone deposits due to the sandstone’s dominance by detrital zircon populations.

Ten samples were collected from a section at the type locality Ruby Ranch Road (RRR), as well as three samples from a section at Dinosaur National Monument. These sites have been previously studied using chemostratigraphy, ensuing the identification of prominent carbon isotope excursions (CIEs; e.g. Ludvigson et al. 2010; 2015). U-Pb data collected from this study will help refine the positions and ages of these CIEs to better understand the global Cretaceous paleoclimate, and to compare continental and marine records. Initial results from the RRR section indicate that the sampled section covers the time between ca. 130 Ma and ca. 105 Ma, which spans across several major CIEs. Constrained by this time range is the Aptian/Albian boundary, which is associated with an oceanic anoxic event distinguished by a prominent CIE (e.g. Ludvigson et al., 2010; Leckie et al., 2002). U-Pb data collected thus far, supports our hypothesis that mature paleosols can be important and reliable tools to establish the chronostratigraphy of continental strata.

Leckie et al., 2002, Paleoceangraphy., 17. doi:10.1029/2001PA000623

Ludvigson et al., 2010, J. Sed. Res., 80, 955-974. doi:10.2110/jsr2010.086

Ludvigson et al., 2015, Cret. Res., 56, 1-24. doi:10.1016/j.cretres2014.11.008

Möller et al., 2019, Goldschmidt Conference Abstracts, #2314.