Early Jurassic (Sinemurian) Biostratigraphy, Chemostratigraphy and Eustatic Sea Level Changes in Southwestern British Columbia and Nevada

Abstract

The Early Jurassic was a time of significant changes in the Earth system and witnessed the continuing fragmentation of Pangaea, marine transgressions and widespread black shale deposition. The current understanding of the Sinemurian Stage (199–191 Ma) is limited. This study is the first attempt to integrate high-resolution biostratigraphy, chemostratigrapy and their relationships with eustatic sea level changes of the Sinemurian in North America based on two of the most complete and fossiliferous sections: the Last Creek Formation in Last Creek, Taseko Lakes map area, British Columbia and the Sunrise Formation in Five Card Draw, Gabbs Valley Range, Nevada. Ammonites provide the best resolution and are the zone fossils for the Mesozoic. The current Sinemurian zonation for North America (Taylor et al., 2001) was tested and revised based on 38 ammonite species distributing among 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 (Porter et al., 2014) corresponds with a coeval CIE reported from England (Jenkyns and Weedon, 2013), which may suggest a global increase in primary productivity. The values of radiogenic Osmium suggest a restricted environment with significant continental influence at Five Card Draw versus an open ocean environment at Last Creek during the Sinemurian (Porter et al., 2014). 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, and 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 turnover also suggests significant differences in depositional environments of the study areas.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015