EXPRESSION OF THE TOARCIAN CARBON ISOTOPE EVENT IN A SILICEOUS BLACK SHALE FROM NEW YORK CANYON, NEVADA

Our understanding of the Early Jurassic Toarcian oceanic anoxic event (T-OAE) has greatly improved over the past decade, with a primary focus on stable isotope geochemistry. Nearly all of the studied stratigraphic successions have involved either black shales and/or marine carbonate sequences. Ammonite biostratigraphy from the Early Jurassic Sunrise Formation of west-central Nevada shows a time interval spanning the Pliensbachian and Toarcian zonal scheme (Carlottense and Kanense zones) of western North America. The stratigraphic succession at New York Canyon (Nevada) is predominantly interpreted as a shallow marine, restricted lagoonal/estuarine succession that is capped by a sandstone-dominated deltaic environment. Within this stratigraphic succession – equivalent to the Kanense ammonite Zone – is a 1.8 m siliceous organic black interval that we interpret as a shallow-marine expression of the T-OAE. Bulk sediment carbon isotope values show the negative excursion (3.5 ‰) that is now recognized globally to occur at the onset of the T-OAE and organic-carbon deposition. This negative carbon isotope excursion occurs over an interval of 50-60 cm after which there is a positive carbon isotope shift on the order of 10 ‰ from the most negative carbon isotope value. The isotopic pattern recorded in Nevada is also recorded from other global locations. Additional information regarding sedimentology and geochemistry from the Pliensbachian and Toarcian sediments in west-central Nevada is also presented at GSA.