66-8 A Neritic Record of Upper Cenomanian-Lower Turonian Oceanic Anoxic Event 2 from Coastal Utah

Session: Environmental Instability During Greenhouse Periods: Impact on Terrestrial and Marine Ecosystems (Posters)

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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 Oceanic Anoxic Event 2 OAE2. This study focuses on planktic and benthic foraminifera 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 composite 35-m outcrop and core section near Big Water UT spanning ~1.09 myr. 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 were 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. Other rare species of calcareous benthics above Bentonite A, including Hoeglundina charlottae and Buliminella fabilis, 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 (downwelling and mixing) 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 as the oceanic front waned. 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 in the WIS. Dynamic changes in relative sea level and productivity, and mixing or stratification of water masses characterize the OAE2 interval.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-9089

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