Carbon and Sulfur Cycle Changes Across the Toarcian Oceanic Anoxic Event in Western North America

Abstract

The Mesozoic Oceanic Anoxic Events (OAEs) represent important intervals in our planet's history, not only because of the marine extinctions associated with them, but also because of the substantial amounts of hydrocarbons that have been generated from their deposits. The Toarcian OAE (T-OAE) occurred during the Early Jurassic (~183 million years ago) and is associated with the global deposition of organic-rich facies in geographically distinct sedimentary basins. This interval is specifically characterized by enrichments in total organic carbon (TOC) (up to 25 wt% in some regions) and total sulfur, which are a result of more efficient organic carbon burial due to pervasive water column anoxia in marine settings. Carbon and sulfur isotopic perturbations (δ13C and δ34S, respectively) have also been identified in the same sedimentary successions that span the T-OAE, indicating past changes in these geochemical cycles. However, there remains a debate as to how widespread Toarcian anoxia and subsequent source rock deposition were since the majority of published data are derived solely from European stratigraphic successions. We will present the highest resolution chemostratigraphic analysis of the T-OAE outside of Europe from organic-rich facies of the Fernie Formation from the Western Canadian Sedimentary Basin (WCSB). These records (from several outcrop and drill cores) show the prominent negative CIE associated with the T-OAE. At these locations, the rate of δ13Corg change suggests that the onset of the negative excursion was associated with a transgressive systems tract (TST) and drowning of the local and shallower carbonate ramp system. Alternatively, this may be a reflection of very low sedimentation rates during the onset of the CIE. Regardless, this portion of the Toarcian is considered to be an interval of eustatic sea level rise. Enrichments in total sulfur are also correlated with the onset of the negative CIE, but remain high after δ13Corg returns to pre-excursion values. Increased sulfur to carbon ratios (S/C) are also coupled with total sulfur enrichments and suggest anoxic conditions remained pervasive in the WCSB after the T-OAE. These results suggest that western North American Toarcian sedimentary deposits may be ideal locations for unconventional hydrocarbon exploration and correlative sedimentary successions from eastern North America may hold the potential for substantial hydrocarbon deposits.

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