VOLCANOGENIC ZIRCONS FROM MUDROCKS AND CHRONOSTRATIGRAPHIC REFINEMENT OF THE CRETACEOUS CEDAR MOUNTAIN FORMATION, UTAH

The chronostratigraphy of continental strata is important for paleoclimate and paleontological studies but challenging in the absence of discrete volcanic ash beds. For example, absolute ages within the Cedar Mountain Formation are not well constrained, especially within the Ruby Ranch Member, a unit composed primarily of stacked paleosols. Paleosols have conventionally not been targets for heavy mineral (zircon) extraction for U-Pb dating, because their hydraulic equivalent size compared to quartz is regarded as too small for analysis. However, because volcanogenic zircons may be deposited onto soil horizons directly from ash fall events, hydraulic size equivalence is irrelevant. This allows large zircons to be deposited in fine-grained sediments (e.g. Joeckel et al. 2019; Möller et al., 2019). We propose that mature paleosols have the capability of preserving volcanogenic zircons within the soil via biotic and abiotic pedoturbation, effectively making them cryptotephra deposits. The goal of this study is to better refine the ages of deposition throughout the Cedar Mountain Formation to provide a temporal framework for important carbon isotope excursions (CIEs). Maximum depositional ages (MDAs) were interpreted from zircons in mature paleosols within the Ruby Ranch Member and Yellow Cat Member and a fluvial sandstone from the Poison Strip member using the Bayesian Model expressed in Keller et al. (2018).

Thirteen samples were collected from the Cedar Mountain Formation, ten samples collected from the type Ruby Ranch Member locality, and three samples from two sections at Dinosaur National Monument. Chemostratigraphic studies have identified two prominent CIEs within these sections, known as the Aptian C10 and C7 intervals (e.g. Ludvigson et al., 2010; 2015). The chronostratigraphic data collected from this study allows refinement of the ages and stratigraphic positions of these CIEs to acquire a better understanding of the global Cretaceous greenhouse climate and correlation with the marine record (e.g. Leckie et al., 2002, Erba et al., 2015). Initial results indicate that the sampled Cedar Mountain Formationat the Ruby Ranch Road section range from the late Hauterivian-early Barremian to the late Albian and spans across several prominent CIEs.