A NERITIC RECORD OF OCEANIC ANOXIC EVENT 2 FROM COASTAL UTAH: NEW INSIGHTS INTO U.S. WESTERN INTERIOR SEAWAY PALEOCEANOGRAPHY AND FORAMINIFERAL PALEOECOLOGY

The Upper Cretaceous Tropic Shale of southern Utah captures oceanographic changes that occurred along the western margin of the U.S. Western Interior Seaway (WIS) during Oceanic Anoxic Event 2 (OAE2). This study focuses on the response of planktic and benthic foraminifera in a shallow (<80-100 m) marine environment as informed by high-resolution (1.5 - 5.0 ka) population counts from a 40-m composite outcrop and core section of the lower Tropic Shale. The OAE2 interval is identified by a distinctive d¹³C_org signature, and by correlation of bentonites and carbonate-rich units across the seaway.

The onset of OAE2 coincided with a very rapid transgression. Surface waters were initially dominated by the tiny triserial planktic Guembelitra cenomana. The benthic assemblage was initially dominated by the infaunal species Neobulimina albertensis, suggesting low oxygen conditions in these coastal waters at the onset of OAE2 just below Bentonite A. Epifaunal Gavelinella dakotaensis proliferated in the interval embracing Bentonite B during the latest Cenomanian. The "Gavelinella acme" coincides closely with the widespread “Heterohelix shift” and marks the plateau phase of OAE2. Biomarker data suggest that the “Heterohelix shift” was triggered by photic zone euxinia. By contrast, G. dakotaensis likely records higher seafloor oxygen levels, proposed to be a function of caballing along a Boreal-Tethyan oceanographic front.

The peak of OAE2 in Utah is marked by an abrupt shift back to Neobulimina dominance in benthic assemblages of the uppermost Cenomanian. We suspect incursion of oxygen-poor Tethyan waters with approach of peak transgression in the early Turonian, coupled with water column stratification. Caballing, the mixing of two water masses to create a third denser water mass, was a process that may have ventilated the seafloor. The "Gavelinella acme" was a longer-lived bioevent along the western margin of the WIS and shorter-lived in New Mexico and central Colorado where it occurred just below Bentonite B. At an outcrop in Billings Montana, the "Gavelinella acme" occurs above Bentonite B, and the change to Neobuliminadominance does not occur until the Cenomanian-Turonian boundary suggesting that these bioevents, driven by the position of the ocean front and then stratification of Tethyan and Boreal waters, were diachronous from southwest to northeast in the seaway.