High precision U/Pb age dating and correlation of Maastrichtian bentonite horizons in the North American Western Interior for insight on provenance and volcanic migration

The Hell Creek Formation is world-renowned as one of the most significant terminal-Cretaceous continental sedimentary units. This is due in large to its voluminous preservation of plant, invertebrate, and vertebrate fossil material, which paint a vibrant picture of North American ecosystems leading up to the K/Pg mass extinction. A century of exploration has enabled the discovery and dating of numerous bentonite (devitrified ash) horizons on both sides of the boundary interval. Despite this, few bentonites have been identified or dated from the lower 2/3rds of this terrestrial fossil-bearing unit, representing nearly 70m of chronologically unconstrained sediment. As a result, this project is focused on providing comparable, high-precision U/Pb age control for the rest of this formation, in addition to correlative early- to middle-Maastrichtian strata across the Rocky Mountain region. Here we present the discovery of over a dozen previously unrecognized bentonites in the lower to middle Hell Creek Formation of Montana and North Dakota, as well as half a dozen new Maastrichtian-aged bentonites from units in Utah, Colorado, and Wyoming. These bentonites provide valuable marker horizons for further Maastrichtian correlation across the greater Western Interior Basin. A select suite of these are notably under preparation for the CA-ID-TIMS dating technique, a method which presents unrivaled precision in U/Pb age determination. The preliminary results from bentonite horizons in North Dakota and Montana provide novel insight on depositional timing, basin partitioning, volcanic migration, and faunal evolution.