RECOGNITION OF THE UPPER ALBIAN OAE 1C CHEMOZONE (AMADEUS EVENT) IN TERRESTRIAL SEDIMENTS OF THE NORTH AMERICAN WESTERN INTERIOR BASIN

A positive carbon isotope excursion (CIE) from a succession of palustrine carbonates in the uppermost Ruby Ranch Member of the Cedar Mountain Formation at the Price River Section of Ludvigson et al. 2010 (https://doi.org/10.2110/jsr.2010.086) has now been identified as the Albian OAE 1c Chemozone . This terrestrial record of the OAE 1c CIE has a δ¹³C magnitude of 2 per mil, with carbonate δ¹³C values ranging from -8 to -6‰ VPDB. Ludvigson et al. 2010 interpreted an Albian age for this CIE because it lies above the Cretaceous C11 carbon isotope segment identified in the same section. Nevertheless, this abstract presents the first independent geochronological data that constrain the age of this CIE. Initial LA-ICP-MS analyses of calcite from sample PRC-31 at the base of the CIE yielded U-Pb discordia ages of 106.4±3.5 Ma and 103.6±1.2 Ma, which closely match the age of OAE 1c given by Gale et al. 2020 (https://doi.org/10.1016/B978-0-12-824360-2.00027-9). In concert with other carbonate U-Pb ages generated from Mesozoic palustrine carbonates, the results from PRC-31 were obtained from laser ablation spots in authigenic calcite veins with oscillatory cathodoluminescence (CL) zonation. This zonation evinces calcite precipitation from shallow meteoric phreatic groundwaters under fluctuating redox conditions. We propose that repeated alternations between oxidizing and reducing phreatic environments likely led to scavenging of the redox-sensitive metals Mn and U from surrounding sediments, and subsequent enrichment of them in the lattice of precipitating calcite.Our work outlines an approach for future chemostratigraphic investigations of OAE 1c in strata of the Albian Kiowa-Skull Creek marine cycle as well as those bounding it.