VALANGINIAN CARBON-ISOTOPE EXCURSION IDENTIFIED AND DATED IN PALEOSOL-BEARING YELLOW CAT MEMBER, CEDAR MOUNTAIN FORMATION, EASTERN UTAH, USA

Earlier work by some of us identified Aptian–Albian carbon-isotope excursions in chemostratigraphic profiles of the continental strata of the Cedar Mountain Formation (CMF) in eastern Utah. We now identify the Valanginian C-isotope excursion (aka "Weissert Event"; WE)—the earliest, global, positive C-isotope excursion during the Cretaceous Period—in the Yellow Cat Member of the CMF. Paleosols, many of them with well-developed vertic features, are common in the alluvial mudstones of the Yellow Cat Member. Some of these paleosols contain cryptotephras with dateable volcanogenic zircons. Thus, have also attained an in-situ radiometric date constraining the timing of the Valanginian C-isotope excursion.

Organic δ¹³C values from our composite chemostratigraphic profile of the Yellow Cat Member at Utahraptor Ridge have a range of approximately -30‰ VPDB to -23.5‰ VPDB. Two positive δ¹³C excursions appear in our profile between 33.74 and 40 m above the upper contact of the Brushy Basin member of the Morrison Formation. The stratigraphically lower of these two excursions has a magnitude of +3.6 per mil and the upper one, some 6.25 m higher, has a magnitude of of +2.3 per mil higher than the baseline value of -27‰ VPDB. We selected six zircons from a population of 600 zircon crystals from a paleosol mudstone within the same stratigraphic interval as the two positive δ¹³C peaks. That population was first screened by LA-ICP-MS U-Pb ages to identify the youngest part of the age spectrum, and those six youngest grains were then analyzed through CA-ID-TIMS, attaining concordant ²³⁰Th-corrected U-Pb ages between 136.4 Ma and 135 Ma. Our Bayesian estimate of the eruption age from these volcanogenic zircons is 135.10 ± 0.30/0.31/0.34 Ma (2σ). Therefore, we interpret the two δ¹³C peaks at Utahraptor Ridge as the double-peak of the Valanginian C-isotope excursion, as identified in marine strata around the world by others over a period of more than three decades. Our results imply an earlier onset of the WE than some age models have suggested. Two other excursions lower in our δ¹³C chemostratigraphic profile appear to represent Berriasian events.