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

The Upper Cretaceous of southern Utah contains thick deposits of dark mudrock and shale facies that capture critical oceanographic changes that occurred at the onset of Oceanic Anoxic Event 2 (OAE 2) and record environmental perturbations that occurred during the transgression of the Greenhorn Sea. We investigated the response of planktic and benthic foraminifera in a shallow (<50 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. Our work is based on quantitative foraminifera population counts and isotope paleoecology (d¹⁸O) from a 29-m outcrop of the Tropic Shale near Big Water, Utah. The OAE 2 interval is identified by a distinctive d¹³C_org signature and 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 species of Guembelitra with minor portions 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 (early Turonian), coupled with high productivity in surface waters resulted in the rapid depletion of benthic oxygen. These correlations result from an intricate relationship among rising sea level, changing water masses, flux of organic matter, reduced benthic oxygenation, and consequent biotic turnover. The foraminiferal record reveals strong cyclicity in planktic/benthic ratio and in species dominance resembling parasequences that can be correlated to the GSSP in central Colorado.