T183 Soils and Paleosols across Space and Time II: A Celebration of Steven G. Driese

Tuesday, 11 October 2022
1:30 PM - 5:30 PM
207

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

Paper number: T183-2
Program: Technical Sessions
Time: 1:35 PM – 1:50 PM

Authors

Abstract

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.

Geological Society of America Abstracts with Programs. Vol. 54, No. 6

doi: 10.1130/abs/2022AM-377052

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