RAPID ENVIRONMENTAL CHANGES DURING THE EARLY JURASSIC (PLIENSBACHIAN TO TOARCIAN STAGES) IN WESTERN NORTH AMERICA RECORDED IN THE GEOCHEMISTRY OF ORGANIC-RICH MUD ROCKS

Organic-rich facies are known to be archives of both local and global environmental conditions during their deposition. Many Mesozoic successions contain black shales deposited during intervals of widespread ocean anoxia known as Oceanic Anoxic Events or OAEs. These blacks shales are both hydrocarbon source rocks and recorders of rapidly changing oceanic and atmospheric chemistries. Specifically, their chemistry can reflect major perturbations to the carbon (δ¹³C), sulfur (δ³⁴S), and other redox-sensitive element (Mo, Fe, Re) cycles of ancient oceans. The blacks shales of the Toarcian OAE of the Early Jurassic (~183 million years ago) have been shown to contain a prominent negative excursion in δ¹³C, a positive excursion in δ³⁴S, as well as, excursions in the isotope ratios of redox-sensitive trace metals. Shallow epeiric seas, large-scale volcanism, and a subsequent rise in eustatic sea level conspired to promote the deposition of these organic-rich facies. However, the vast majority of T-OAE geochemical studies have been conducted on European sedimentary successions, casting doubt upon the global nature of these changes in these geochemical cycles.

We will present a high-resolution geochemical dataset from the organic-rich Pliensbachian to Toarcian interval of the Fernie Formation (Red Deer, Gordondale and Poker Chip Shale Members) in the Western Canadian Sedimentary Basin of Western Alberta. There, in the highest-resolution record generated to date outside of Europe, we have identified the negative carbon isotope excursion associated with the T-OAE in all of our drill core and outcrop study sites. Importantly, this shows that this isotope excursion represents a perturbation to the global carbon cycle. Additionally, we will present δ³⁴S and iron speciation analyses in order to reconstruct the changes in redox conditions during the T-OAE in the Western Canadian Sedimentary Basin to better constrain the geographic reach of anoxia in the ocean across the event.