EARLY JURASSIC (SINEMURIAN) BIOSTRATIGRAPHY, CHEMOSTRATIGRAPHY AND EUSTATIC CHANGES IN SOUTHWESTERN BRITISH COLUMBIA AND NEVADA

The Early Jurassic was a time of significant changes in the Earth system and witnessed the continuing fragmentation of Pangaea, marine transgressions and widespread of black shale deposition. The current understanding of the Sinemurian Stage (199-191 Ma) is limited in North America. This study is the first to integrate biostratigraphy, chemostratigraphy and eustatic changes of the Sinemurian in North America based on two most complete and fossiliferous successions: the Last Creek Formation in Last Creek, southwestern British Columbia and the Sunrise Formation in Five Card Draw, Nevada.

Ammonoids provide the best resolution and are the zonal fossils for the Mesozoic. The current Sinemurian zonation for North America (Taylor et al., 2001) was tested and revised based on 38 ammonite species, 15 genera, and 5 families identified in this study. In ascending order, the Involutum, Leslei, Carinatum and Harbledownense zones are redefined and their correlations with the primary standard zonation in northwest Europe are updated. This zonation also provides chronologic control for geochemical profiles and correlation with eustatic sea level changes.

Stable carbon and osmium isotopes were utilized in this study. The carbon isotope excursion (CIE) discovered in Last Creek corresponds with a coeval CIE reported from England (Jenkyns and Weedon, 2013), which may suggest a global increase in primary productivity. The results of radiogenic Osmium suggest a restricted marine environment with significant continental influence at Five Card Draw versus an open ocean environment at Last Creek during the Sinemurian.

The transgressive and regressive events in the study areas are calibrated with the revised Sinemurian zonation, and are compared with eustatic sea level changes, ammonite biodiversity, and faunal turnovers. The Early Sinemurian transgression proposed by Hallam (1981, 1988) is well represented in both the Last Creek and Five Card Draw, which co-occurs with ammonite biodiversity maxima and a possible global CIE. The mid-Late Sinemurian regression and Late Sinemurian transgression are represented by lithological and paleobathymetric changes in Five Card Draw. The contrast in ammonite paleobiodiversity and faunal turnovers also suggests significant differences in depositional environments of the study areas.