ENVIRONMENTAL PERTURBATIONS DURING THE EARLY JURASSIC (PLIENSBACHIAN TO TOARCIAN) IN WESTERN CANADIAN SEDIMENTARY BASIN INDICATED BY INORGANIC AND ORGANIC GEOCHEMISTRY

Several discrete intervals characterized by enhanced deposition of organic-rich marine sediments, carbon isotope excursions and biomass turnover occurred during the Mesozoic. These intervals are known as oceanic anoxic events (OAEs). Unlike the Cretaceous OAEs, the Early Jurassic Toarcian OAE (T-OAE, ~183 Ma) lacks any open sea records and the current geochemical data is limited to epicontinental depositional settings in Europe and the Mediterranean region. This leads to difficulty in rectifying the global versus local extent of the T-OAE. The well-preserved lower Jurassic Fernie Formation (Pliensbachian and Toarcian) in the Western Canadian Sedimentary Basin of North America provides an ideal target to constrain questions regarding this. The Early Jurassic portion of the Fernie Formation was deposited off the northwestern coast of Pangea, which at the time was connected to the Panthalassic Ocean and therefore paleogeographically distinct from the European records, which were confined to the Western Tethys and Boreal Oceans. We collected samples from two outcrops at Shunda Creek and East Tributary of Bighorn Creek, and two drilling wells at the Peace River and Athabasca River area to generate high-resolution chemostratigraphic profiles to identify and better understand the depositional controls on the OAE. This study focuses on integrating the inorganic geochemical (sulfur and carbon isotopes, redox-sensitive trace elements, and sequential iron speciation) and organic molecular records (biomarkers). Preliminary results indicate increased total organic carbon content and a negative δ¹³C_org excursion during the T-OAE. Gas chromatography-mass spectrometry (GC-MS) analyses of outcrop sample extracts yield low pristane/phytane ratios (generally <1.0), high Ts/Tm, high C₂₉/C₃₀ hopane and a predominance of hopanes over steranes, indicating shallow depositional environments which appear to have been restricted at times resulting in anoxic and/or hypersaline conditions. We will additionally present Rock-Eval and biomarker analyses of core samples at the GSA South-Central Section Meeting. In summary, rapid environmental changes occurred during the Early Jurassic in western North America, and that the extent of environmental perturbation during the T-OAE was global.