EARLY JURASSIC PARALIC TO SHALLOW MARINE MICROBIAL ENVIRONMENTS OF NEVADA: A UNIQUE RECORD OF THE TOARCIAN OCEANIC ANOXIC EVENT

The Pliensbachian and early Toarcian oceanic anoxic event of the Early Jurassic can be locally developed. Research suggests that this time interval is associated with significant isotopic excursions (e.g., C, O, S, Sr, Os), marine extinctions, warmer climates, increased carbon dioxide concentrations, marine anoxia and sea level changes. However, most of this research is based on stratigraphic sections from the European Neotethyan realm, and primarily on shale and/or carbonate sequences. Two stratigraphic successions of the Early Jurassic Sunrise Formation of west-central Nevada have been investigated for ammonite biostratigraphy, isotope and chemical stratigraphy, sedimentology and stratigraphy. Ammonite biostratigraphy from the Westgate and New York Canyon sections show faunal assemblages that are assigned to the current Pliensbachian and Toarcian zonal schemes of western North America (Carlottense and Kanense zones). The stratigraphic successions have a wide range of sedimentary environments that contain both carbonate and siliciclastic facies. Sedimentary facies include mudstones, cross-bedded sandstones, conglomerates, black cherts, carbonate mudstones, coquinas, pseudomorphs of gypsum, thrombolites and algal mats. The two stratigraphic sections presented here are each approximately 100 m in thickness and samples have been taken every metre (on average) for stable isotope and elemental, sedimentary petrography (polished hand samples and thin sections), and grain size analysis. The Pliensbachian–Toarcian facies associations in west-central Nevada suggest sedimentation in paralic to shallow marine environments dominated by microbial activity and later by the progradation of fan deltas into the basin. These environments appear to have been restricted at times resulting in anoxic and/or hypersaline conditions. The Nevada successions provide an additional perspective for understanding the environmental response to major perturbations in global biogeochemical cycles during the Early Jurassic.