Paleoclimate Implications from Stable Isotope Analysis of Sedimentary Organic Carbon and Vertebrate Fossils from the Cedar Mountain Formation, UT, U.S.A.

Oxygen and carbon isotopic compositions of fossilized vertebrate teeth and bone were analyzed to determine isotopic values of vertebrate faunal diet from the early Cretaceous Cedar Mountain Formation of Utah. Results for δ18O of PO4 (δ18Op) (Suarez et al., 2014) from the same data set have been compared to the δ18O of CO3 (δ18Oc) portion of teeth and turtle shell to determine if diagenetic alteration of the isotopes has occurred by plotting the line of best fit equation that models the relationship between unaltered δ18OP and δ18OC of modern mammals from Iacumin et al., 1996. Results indicate slight diagenesis of some specimens has occurred. Mean annual precipitation (MAP) estimates, using the equations from Kohn (2010) and Diefendorf et al., (2010), were determined for each sampled stratigraphic member using carbon isotopic values derived from vertebrate faunal diet estimations from bioapatite, based on the observation that δ13C of modern C3 plants increases with decreasing MAP (Kohn, 2010; and others). Values indicate a prolonged period of semi-arid to arid climate in the Ruby Ranch and Yellow Cat members of the CMF likely caused by a rain-shadow (Suarez et al., 2014) on the eastern, leeward side of the uplifting Sevier Fold and Thrust belt (SFTB). The upper Mussentuchit Member becomes much more humid as the Western Interior foreland basin, continues to subside, eventually becoming inundated by the Western Interior Seaway. The basal Cretaceous CMF sits unconformably atop the Late Jurassic Morrison Formation, with a hiatus of ~20 Ma (Kirkland and Madsen, 2006). Chemostratigraphic curves of the basal Yellow Cat member (YCM) were constructed using δ13C of bulk organics of sediment, and δ13C and δ18O of inorganic carbonate of sediment in an attempt to correlate with the early Cretaceous chemostratigraphic record. Correlating positive and negative carbon isotope excursions (PCIEs and NCIEs) from the YCM to stratigraphically well-constrained marine sections lends evidence for an age determination of the basal CMF. Correlations indicate the YCM to be Barremian to Aptian (~130-120 Ma) in age and detect several distinct CIEs of this period, specifically the B5-B8 excursions of the Barremian and the A1-C5 excursions of the Aptian

Berriasian–Valanginian Geochronology and Carbon-Isotope Stratigraphy of the Yellow Cat Member, Cedar Mountain Formation, Eastern Utah, USA

The Early Cretaceous Yellow Cat Member of the terrestrial Cedar Mountain Formation in Utah, USA. has been interpreted as a “time-rich” unit because of its dinosaur fossils, prominent paleosols, and the results of preliminary chemostratigraphic and geochronologic studies. Herein, we refine prior interpretations with: (1) a new composite C-isotope chemostratigraphic profile from the well-known Utahraptor Ridge dinosaur site, which exhibits δ13C features tentatively interpreted as the Valanginian double-peak carbon isotope excursion (the so-called “Weissert Event”) and some unnamed Berriasian features; and (2) a new cryptotephra zircon eruption age of 135.10 ± 0.30/0.31/0.34 Ma (2σ) derived from the CA-ID-TIMS U-Pb analyses of zircons from a paleosol cryptotephra. Our interpretations of δ13C features on our chemostratigraphic profile, in the context of our new radiometric age, are compatible with at least one prior age model for the “Weissert Event” and the most recent revision of the Cretaceous time scale. Our results also support the interpretation that the Yellow Cat Member records a significant part of Early Cretaceous time

Berriasian–Valanginian Geochronology and Carbon-Isotope Stratigraphy of the Yellow Cat Member, Cedar Mountain Formation, Eastern Utah, USA

The Early Cretaceous Yellow Cat Member of the terrestrial Cedar Mountain Formation in Utah, USA. has been interpreted as a “time-rich” unit because of its dinosaur fossils, prominent paleosols, and the results of preliminary chemostratigraphic and geochronologic studies. Herein, we refine prior interpretations with: (1) a new composite C-isotope chemostratigraphic profile from the well-known Utahraptor Ridge dinosaur site, which exhibits δ13C features tentatively interpreted as the Valanginian double-peak carbon isotope excursion (the so-called “Weissert Event”) and some unnamed Berriasian features; and (2) a new cryptotephra zircon eruption age of 135.10 ± 0.30/0.31/0.34 Ma (2σ) derived from the CA-ID-TIMS U-Pb analyses of zircons from a paleosol cryptotephra. Our interpretations of δ13C features on our chemostratigraphic profile, in the context of our new radiometric age, are compatible with at least one prior age model for the “Weissert Event” and the most recent revision of the Cretaceous time scale. Our results also support the interpretation that the Yellow Cat Member records a significant part of Early Cretaceous time