STRATIGRAPHIC FRAMEWORK OF A POTENTIAL CENOMANIAN-TURONIAN BOUNDARY AND ASSOCIATED OCEANIC ANOXIC EVENT 2 AT JAGO RIVER, NORTH SLOPE, ALASKA

Oceanic Anoxic Events (OAEs) in the Mesozoic were representative of large carbon cycle perturbations that corresponded to widespread marine anoxia and the deposition of organic-rich black shale. The OAE2, which occurred ~94 Ma and spans the Cenomanian-Turonian boundary, is globally well documented from a variety of marine settings. This study addresses a potential new OAE2 section from the Hue Shale at the Jago River outcrop, located on the North Slope of Alaska. Rock samples were taken in two consecutive field seasons (2017 and 2018) and were analyzed for total organic carbon (TOC), bulk organic carbon isotopic composition (δ¹³C_org), and fossil content. Bentonite layers were dated using zircon U/Pb and constrained the depth of the Cenomanian-Turonian transition to approximately 42-46 m, where ages of 93.5±0.3-94.5±0.3 Ma and 8-15%TOC were documented.

Lithologically, the Hue Shale at Jago River consists of black mudstones and black shales with mm-scale bedding with occasional gray-white, tan, and ochre bentonite beds of varying thickness. A monospecific Inoceramid-rich layer is present just prior to the Cenomanian-Turonian boundary, from the 14-23 m level. This layer is present in other outcrops on the North Slope and is possibly indicative of either deep (~1000 m), inhospitable water and/or low oxygen conditions. Bentonite ages indicate it is coeval with a sea-level lowstand on adjacent continental shelves. Although an overall approximate -1.5‰ shift inδ¹³C_orgvalues occur across the Cenomanian-Turonian boundary, three separate positive shifts of ~ 1‰ each occur within the OAE layer and correlate with three distinct maxima of %TOC across the same interval. These positive shifts are provisionally correlated to three pulses identified from other outcrops globally. Smear slide and scanning electron microscope analyses revealed the presence of secondary opal-CT in samples from 42.0-44.0 m and 45.8-46.0 m. The presence of this metastable mineral implies one of two things; 1) that the Hue Shale in the Jago River outcrop was subjected to increased temperatures post burial or 2) that the presence of opal-CT in the Hue Shale was due to reworking from the underlying sediments.