CHRONOSTRATIGRAPHY OF THE GABBS FORMATION (LATE TRIASSIC) IN NEW YORK CANYON, NEVADA

Published ages for the Late Triassic Gabbs Formation in New York Canyon, Nevada are conflicting. This shallow marine section has been attributed to either the Norian or Rhaetian stage, or some combination of the two. Based on a distinct Sr isotope excursion recorded in well preserved brachiopods from Late Triassic Tethyan strata, we use secular ⁸⁷Sr/⁸⁶Sr isotope trends of shelly material in the Gabbs Formation, which bordered on the Panthalassic Ocean, to clarify its age. Strontium isotope values begin around 0.7076, rise to as high as 0.7082, and then fall back to near 0.7077, very similar to the composite Tethyan curve across the late Norian and early Rhaetian stages. The excursion to higher ⁸⁷Sr/⁸⁶Sr may be the result of enhanced continental inputs of Sr to the world ocean, which might reflect either tectonic or climatic events at this time. Supporting the chemostratigraphic age model, conodonts observed in samples from the lower Gabbs Formation are consistent with a middle-Late Norian age, while those from the upper Gabbs Formation were of Rhaetian aspect. The matching strontium isotopic profiles from different ocean basins provide a useful tool for correlating index fossils between Tethys and Panthalassa. Furthermore, if correct, the refined age designation for the Gabbs Formation allows for a revision of the Panthalassic paleoecological timeline. Norian faunal dynamics exhibited in the Gabbs Formation include an overall decline of stationary epifaunal taxa, and a concurrent diversification and increasing dominance of mobile infauna. These paleoecological events are in line with those recorded in benthic Tethyan paleocommunities, and were formerly attributed to paleoecological changes at or near the Norian-Rhaetian transition.