COASTAL AND NERITIC FORAMINIFERA OF UTAH: A RECORD OF OCEANIC ANOXIC EVENT 2 (~94-93 MA) IN THE US WESTERN INTERIOR SEA (Invited Presentation)

The Upper Cretaceous of southern Utah contains thick deposits of dark mudrock and shale that capture critical oceanographic changes at the onset of Oceanic Anoxic Event 2 (OAE2) and record rapid environmental perturbations during transgression of the Western Interior Sea (WIS). We investigated the response of planktic and benthic foraminifera to OAE2 in a shallow (up to 50-100 m) marine environment, influenced by Boreal and Tethyan water masses, during the Cenomanian-Turonian boundary interval (CTB = 93.9 Ma). Quantitative assemblage data from a composite 40-m outcrop and core section of the Tropic Shale near Big Water, UT, are compared with biomarker data from the SH1 Core. The OAE2 interval is identified by a distinctive δ¹³C_org signature and correlation of ammonite zones, key bentonite beds, and carbonate-rich units across the seaway.

The basal Tropic Shale is sandy and contains sparse assemblages of agglutinated benthics. The onset of OAE2 coincides with rapid transgression; surface waters became dominated by triserial Guembelitra with minor portions of biserial Planoheterohelix. Distal WIS sections show abrupt ventilation at the onset of OAE2, but benthics in the Tropic Shale were dominated by infaunal Neobulimina, suggesting low oxygen conditions in these coastal waters. Biomarkers also show evidence for reducing conditions. Epifaunal Gavelinella briefly proliferated during the early plateau phase of OAE2, but was sensitive to water column redox changes. Shortly after, the widespread planktic “Heterohelix shift” responded to higher productivity as evidenced by increased algal and bacterial biomarkers. The plateau of OAE2 is marked by a diachronous but rapid shift in benthic assemblages to infaunal Neobulimina dominance due to reduced oxygen conditions at the seafloor, while declining relative abundances of Guembelitria and concomitant rising abundances of trochospiral planktic taxa mark a major Tethyan incursion into the western seaway. Periodic productivity events coincide with pulses of Gavelinella after OAE2. The ~800-kyr foraminiferal record reveals strong cyclicity that tracks eccentricity-paced parasequences of the coastal zone. The neritic ecosystem responded to dynamic relationships among rising sea level, shifting water masses, productivity, and water column oxygenation.