OCEANIC ANOXIA EVENT 2 (~94 MA) IN THE U.S. WESTERN INTERIOR SEA: HIGH RESOLUTION FORAMINIFERAL RECORD OF THE DEVELOPMENT OF ANOXIA IN A SHALLOW EPICONTINENTAL SEA

The Upper Cretaceous Tropic Shale of southern Utah is a thick deposit of dark mudrock and shale facies that captures critical oceanographic changes that occurred during Oceanic Anoxic Event 2 (OAE 2) and record environmental perturbations during the transgression of the Greenhorn Sea during latest Cenomanian and early Turonian time. We investigated the response of planktic and benthic foraminifera in a shallow (<100 m) marine environment stressed by the onset of OAE 2 during the Cenomanian-Turonian boundary interval (CTB; 93.9 Ma) to determine the oceanographic mechanisms controlling the observed shifts and turnovers in the foraminiferal record. This study is based on high-resolution quantitative foraminifera population counts and isotope paleoecology (δ¹⁸O and δ¹³C) from a 30-m outcrop and composite 11-m core of the Tropic Shale near Big Water, Utah. The OAE 2 interval is identified by a distinctive δ¹³C_org signature and by correlation of bentonites and carbonate-rich units across the seaway.

Results of assemblage analyses indicate discrete intervals of environmental perturbations across the CTB interval. The basal 6 m of the Tropic Shale are sandy to silty and contain sparse assemblages of agglutinated benthics and very rare specimens of planktic foraminifera. At the onset of OAE 2, surface waters were dominated by planktic Guembelitra cenomana with minor species of Heterohelix. Benthic abundances increase at the same time as the planktics and were initially dominated by oxygen tolerant infaunal Neobulimina albertensis. Epifaunal Gavelinella dakotaensis briefly proliferated during the core of the OAE 2 and coinciding with the planktic “Heterohelix shift” and increasing accumulation of organic matter. The peak of OAE 2 at ~17 m is marked by a rapid shift in benthic assemblages to infaunal Neobulimina dominance. We suspect incursion of oxygen-poor Tethyan intermediate waters with approach of peak transgression during the early Turonian, coupled with high productivity in surface waters resulted in the rapid depletion of benthic oxygen. The foraminiferal record reveals strong cyclicity in planktic/benthic ratio resembling parasequences that can be correlated to the GSSP in central Colorado.