AMMONITE BIOSTRATIGRAPHY OF THE EARLY JURASSIC (PLIENSBACHIAN–TOARCIAN) SUNRISE FORMATION, GABBS VALLEY AND CLAN ALPINE MOUNTAINS, WEST-CENTRAL NEVADA

Exploring the dynamics and effects of the second-order Pliensbachian–Toarcian mass extinction is an area of considerable interest. Current thought suggests that environmental stresses imposed by the Karoo-Ferrar large igneous province eruption was responsible for initiating a multi-phased global mass extinction of marine organisms; occurring over six short-term intervals that span the Pliensbachian and Toarcian. During this multi-phased extinction event, the Pliensbachian–Toarcian boundary interval (equivalent to the Tenuicostatum / Serpentinum Zone boundary of northwest Europe) appears to have been the most significant – with many taxonomic groups reaching minimum biodiversity levels. Developing a more thorough understanding of ammonites through this critical interval of time in western North America is important, not only in developing hypotheses to better explain extinction mechanisms and responses, but also as a tool for correlating global marine perturbations in the stable isotope record.

Ammonite biostratigraphy is presented herein from two successions of the Early Jurassic Sunrise Formation at New York Canyon (Gabbs Valley Range) and Westgate (Clan Alpine Mountains), west-central Nevada. Data show faunal assemblages that are assigned to the current Pliensbachian and Toarcian zonal schemes of western North America. Assemblages include: 1) Metaderoceras, Dubariceras (Freboldi Zone); 2) Fanninoceras, Arieticeras (Kunae–Carlottense Zones); 3) Lioceratoides, Protogrammoceras, Tiltoniceras (Carlottense Zone); 4) hildoceratids of assemblage 3 with Dactylioceras, Nodicoeloceras? (Kanense Zone); and 5) Peronoceras, Catacoeloceras (Planulata Zone). The succession at Westgate shows assemblages 1–5, while the New York Canyon succession shows assemblages 1, 3, and parts of 4. Work presented herein temporally constrains other preliminary data concerning potential causes of this mass extinction event, which is recorded in Europe and other areas of western North America. Additional posters and presentations, dealing with the sedimentology and isotope stratigraphy in these stratigraphic sections are also presented at this GSA.