AMMONITES, SEEPS, ASH FALLS, AND OAE 2: PALEOECOLOGY OF COLD METHANE SEEP DEPOSITS IN THE CENOMANIAN-TURONIAN TROPIC SHALE OF SOUTHERN UTAH

Cold methane seeps are unique chemosynthetic ecosystems characterized by dense faunal communities. Modern seeps exist across all latitudes of the global ocean and have a rich fossil history as well. Seep deposits have been documented in the Upper Cretaceous (upper Cenomanian- middle Turonian) Tropic Shale, which is well exposed in the Kaiparowits Plateau region of southern Utah. The Tropic Shale consists of organic-rich mudstones that record offshore sedimentation along the western margin of the Western Interior Seaway (WIS). Bentonites are also a common feature within the Tropic Shale; radiometric dating of bentonites coupled with ammonite biostratigraphy allow for a high-resolution, temporal framework for WIS deposits. Seep-associated carbonates within the Tropic Shale are coeval with Oceanic Anoxic Event 2 (OAE 2), a globally recognized, major carbon cycle perturbation. Studies suggest enhanced volcanic activity and increased atmospheric CO₂ concentrations resulted in elevated temperatures, changes in marine biogeochemistry, widespread black shale deposition, and a significant biotic turnover across the Cenomanian-Turonian boundary. We studied three seep-bearing localities near Grand Staircase-Escalante National Monument (Alton Sink, North and South Cottonwood Canyon) to reveal how OAE 2 impacted the paleoecology of Late Cretaceous seep communities. The source of these seep hydrocarbons appears to originate from the underlying terrestrial coal deposits of the Naturita Formation. In contrast, all previously recorded WIS seeps are thought to originate from underlying, organic-rich marine shales. At North Cottonwood Canyon, four distinct seeps were identified, spanning at least three ammonite biozones (Euomphaloceras irregulare to Nigericeras scotti) and are coeval with OAE 2. One seep deposit crosses the Cenomanian-Turonian boundary. In this study, we document changes in seep fauna as conditions associated with OAE 2 worsened through time. Preliminary results suggest that seep carbonates preserved a rich ammonite fauna throughout the extinction event, including Euomphaloceras, Placenticeras cumminsi, Sciponoceras gracile, and Neocardioceras juddi. This potentially supports previous studies that suggested that seeps in the WIS were a preferred ammonite habitat.