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Assessment of the Geographic Extent of the Toarcian Oceanic Anoxic Event: Implications for Early Jurassic Hydrocarbon Source Rock Deposition

Abstract

The onset of deposition of organic-rich black shale during the Toarcian (Early Jurassic, ∼183 million years ago) has been suggested to be a globally widespread event. This interval, known as the Toarcian Oceanic Anoxic Event (T-OAE), is characterized by enrichments in total organic carbon (TOC) (up to 25 wt% in some regions) appearing apparently synchronously in geographically distinct sedimentary basins. Very large and consistent carbon and sulfur isotopic perturbations (d13C and d34S, respectively) have also been identified in sedimentary successions that span this event, indicating past changes in these geochemical cycles during the T-OAE. While recent geochemical evidence from the T-OAE interval supports the argument that marine anoxia was in fact geographically widespread, an ongoing debate still exists concerning the global extent of the event due to the focus of the majority of the previous studies on the records from European sedimentary basins. We have conducted a high-resolution chemostratigraphic analysis of the Lower Jurassic Fernie Formation from the Western Canadian Sedimentary Basin (WCSB) in western Alberta. The Fernie Formation is known to contain Toarcian-age hydrocarbon source rocks; however, their precise correlation to the T-OAE interval is presently unresolved. Both drill cores and outcrops of the Fernie Formation were sampled from several locations within the WCSB in order to generate d13C and %TOC data. Relative ages are constrained by biostratigraphic positions of ammonite fauna within each section. %TOC values are relatively high during the Pliensbachian-Toarcian and approach 20% in the drill cores and 8% from the outcrop sections of the Fernie. Further, a negative 3.5 ‰ shift recorded in d13C at all the study sites during the Early Toarcian is consistent with the excursion associated with the onset of the T-OAE in European geochemical datasets. Importantly, these data suggest the geographic extent of the T-OAE was global and other sedimentary basins with known Early Jurassic aged strata may contain significant deposits of hydrocarbons and Toarcian source rocks.