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

Prior to the onset of OAE2, the foram assemblages are dominated by rare agglutinated taxa. 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. Other rare species of calcareous benthics just above Bentonite A, including Hoeglundina charlottae, demonstrate that this interval is correlative with the "Benthonic Zone" elsewhere in the WIS.

Epifaunal Gavelinella dakotaensis proliferated as OAE2 intensified in the interval below Bentonite B during the latest Cenomanian. The "Gavelinella acme" coincides closely with the widespread “Heterohelix shift”, likely triggered by photic zone euxinia. Planoheterohelix globulosa dominated the planktic foram assemblages when productivity was high. By contrast, G. dakotaensis likely records higher seafloor oxygen levels, proposed to be a function of caballing along a Boreal-Tethyan oceanographic front, alternating with euxinic conditions dominated by Planoheterohelix. The peak of OAE2 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 during the early Turonian, coupled with water column stratification.

The "Gavelinella acme" was longer-lived along the western margin of the WIS, and the change to Neobulimina dominance did not occur until the C/T boundary at Billings MT suggesting that these bioevents were diachronous from SW to NE in the WIS.