19-2 The Oregon Basin Section of the Middle Campanian-aged "Mesaverde" Formation, Park County, Wyoming, Fills a Key Gap in Laramidian Biogeography

Session: The Campanian Crucible: A synthesis of vertebrate paleobiogeography and ecosystem dynamics in Laramidia

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Testing biogeographic patterns in Laramidia requires robust chronostratigraphic controls as well as broad geographic coverage. One key geographic knowledge gap in Campanian continental deposits occurs across Wyoming, in the latitudinal zone between the well sampled deposits of Montana and Utah. The “Mesaverde” Formation, best exposed in the Wind River and Bighorn basins, offers significant potential for addressing this gap. Published work to date has focused on vertebrate microfossils, particularly in compiling faunal assemblage lists for sites in the eastern and southern Bighorn Basin and the southern Wind River Basin. Since 2019, the Raymond M. Alf Museum of Paleontology has explored the “Mesaverde” Formation of Oregon Basin, a topographic basin in the northwestern Bighorn Basin near Cody, Park County, Wyoming. Although fragmentary vertebrate fossils were noted in 1926, the Alf Museum’s work represents the first comprehensive surveys, yielding dozens of new vertebrate fossil localities throughout the exposed section. At Oregon Basin, the “Mesaverde” Formation is 350–420 meters thick, underlain by the Cody Shale and overlain by the Meeteetse Formation. The age of the base of the “Mesaverde” Formation in this area is constrained biostratigraphically by ammonites to ~81.75 Ma, and the top is constrained to >78.6 Ma by the presence of helochelydrid turtles. Thus, the Oregon Basin section is penecontemporaneous with portions of the Foremost (Alberta), lower Two Medicine and lower Judith River (Montana), Wahweap (Utah), Menefee (New Mexico), and upper Aguja (Texas) formations. The vertebrate faunal assemblage is consistent with an inland environment, lacking taxa (e.g., Enchodus and Cretalamna) typical of marginal marine deposits within the formation in the eastern Bighorn Basin. Vidalamiin fish, the hybodont Lonchidion, and orectolobiform sharks are common in vertebrate microfossil sites, and champsosaurs are absent. This assemblage is more similar to coeval sites in the southern part of Laramidia (e.g., Wahweap Formation) than to the north (e.g., Foremost Formation), possibly reflecting a latitudinal climate gradient. In order to test this hypothesis, more precise radiometric dates are an urgent need for the Oregon Basin section, but volcanic ashes remain elusive. Ongoing work focuses on further characterizing the overall faunal assemblage and tracking faunal changes through the section.

Geological Society of America Abstracts with Programs. Vol. 58, No. 4, 2026

doi: 10.1130/abs/2026RM-14778

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