Abstract
Laser-fusion 40Ar/ 39Ar analysis of four bentonite horizons produces the first absolute ages for the 860-m-thick Kaiparowits Formation and resolves previous age uncertainty caused by ambiguous biostratigraphy. A late Campanian (Judithian) age of ca. 76.1–74.0 Ma is determined, resulting in a high-resolution temporal framework for the richly fossiliferous formation. Detailed stratigraphic correlation reveals that the Kaiparowits Formation is contemporaneous with many of the most important vertebrate fossil-bearing formations in the Western Interior Basin, and with other well-studied strata across Utah and southeastern Wyoming, including portions of the Book Cliffs sequence. The Judithian age determination and correlations for the Kaiparowits Formation presented here provide a new chronological basis for addressing questions relating to mammal biostratigraphy, vertebrate evolution, biodiversity and paleobiogeography (e.g., dinosaur provincialism) in the Cretaceous Western Interior Basin.
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