Timing of the Greenhorn Transgression and OAE2 in Central Utah Using CA-TIMS U-Pb Zircon Dating

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Contextualizing subbasinal influences on accommodation and pace of the rapid landscape evolution during the base-level rise of the Upper Cretaceous (upper Cenomanian–lower Turonian) Greenhorn Cycle within the Western Interior of North America requires refined stratigraphic controls of key sediment successions. Herein, a blended analysis of laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), chemical abrasion thermal ionization mass spectrometry (CA-TIMS), and δ13C isotopic data is utilized to identify two regionally significant ash-fall marker beds southeast of the Wasatch Plateau, specifically the TT1 and TT4, which bracket the Cenomanian–Turonian boundary and the Greenhorn Cycle transgression in the Western Interior Seaway (WIS). Based on our analysis of the recovered ash-fall zircons, a meaningful depositional age (DA) for the TT1 is 94.616 Ma ± 0.027 and 94.010 Ma ± 0.017 for the TT4 bentonite of the Tununk Shale.

When coupled with the pre-existing biostratigraphic framework, correlation potential greatly increases with the (TT1) Naturita falling within the Sciponoceras gracile Assemblage Zone. Common molluscs associated with this biozone within the southern and central regions of Utah include Inoceramus pictus, Euomphaloceras spp., and Pycnodonte newberryi. The (TT4) Tununk within the upper portion of the Watinoceras coloradoense Assemblage Zone is commonly associated with the following mollusc genus within the areas to the south and north of the study area: Mytiloides and Morrowites. Furthermore, these dates constrain the Cenomanian–Turonian boundary, provide accurate chronostratigraphic markers for intra and interbasinal correlation and serve to strengthen global linkages to Ocean Anoxic Event 2 (OAE2) during the Greenhorn Cycle transgression.