A NEW LATE TRIASSIC (NORIAN) MARINE STABLE ISOTOPE RECORD FROM WESTERN IDAHO: A PREVIOUSLY UNRECOGNIZED GLOBAL OAE?

Over the past decade, stable isotope stratigraphy, particularly using carbon isotope excursions (CIEs) and trends, has frequently been utilized in stratigraphic correlations with particular importance for global correlations. To date, limited stable isotopic data have been reported from Late Triassic marine rocks outside of the European Tethys. We collected 68 whole-rock Halobia- and ammonite-bearing limestone samples from a 51.5-meter section of the Kurry unit, Doyle Creek Formation in Hells Canyon at Pittsburg Landing, Idaho. The Kurry unit is composed of tan-weathering, dark gray, thinly bedded oolitic grainstones and skeletal packstones. Fossils collected from the upper portions of the Kurry unit are Lower Norian including Halobia cf. H. beyrichi and Halobia aff. H. salinarum. Stable carbon isotope values of bulk carbonates range from -6.0 to 1.0‰ with an overall negative trend from the base to the top of the section. The negative trend is punctuated at 10.5 m by a positive baseline shift of 3.5‰. Above this positive excursion, stable isotope compositions follow a negative trend to ~51.5 m. The long-term negative trend with a positive baseline shift of 3.5‰ in our data matches well-established, long-term negative trends and a positive baseline shift observed in other Norian-age locations in the European Tethys and in our new record from the Martin Bridge Formation, Oregon. In detail, the positive baseline shift is gradual over 20 m, and is punctuated by a large negative CIE. This detailed characteristic profile is similar to data from several Oceanic Anoxic Events (OAEs) in the geologic record, possibly representing an increase in carbon burial interrupted by an input of isotopically light carbon related to the release of methane into the ocean-atmosphere system. Late Triassic, Norian rocks from the European Tethys and western North America may record an isochronous stable isotope signal that represents a previously unrecognized global OAE.