NEW DEPOSITIONAL AGE CONSTRAINTS FOR THE CRETACEOUS-PALEOGENE TRANSITION (HELL CREEK REGION, WILLISTON BASIN)

The Hell Creek region of the northwestern Williston Basin contains Late Cretaceous through Paleocene terrestrial strata which encompass the Cretaceous-Paleogene Boundary (KPB) mass extinction. Cretaceous units record ~300ka of pre-KPG paleoenvironmental changes, extinctions, ecological decline, and climate change, though the tempo of these processes remain ambiguous due to a paucity of interbedded tephra. The Paleogene Fort Union Fm. contains abundant tephra which have been used to construct a high-resolution chronologic framework, however, this is difficult to attain within the energetic fluvial strata of the Cretaceous Hell Creek Fm. Here we refine the chronostratigraphic framework for the Hell Creek region using coupled single crystal U-Pb zircon CA-TIMS and Ar-Ar sanidine geochronology of detrital crystals recovered from sandstones. Zircons were separated and screened using LA-ICP-MS to identify juvenile crystals. The youngest zircon crystals, including those which experienced obvious post-depositional Pb-loss, were further analyzed using CA-TIMS. Sanidines from the same depositional horizons were separated and analyzed in single grain total fusion experiments. Results show that a robust population of Fort Union sanidine fall within expected depositional ages. A Hell Creek Fm. sandstone ~40m below the KPB contains overlapping populations of volcanic zircon and sanidine that fall within expected depositional ages indicating that volcanic sources were active during the late Cretaceous. Both Cretaceous and Paleogene sandstones express similar modes of detrital sanidine which provide insight into paleo-incision depths and could suggest the presence of older preserved tephra layers. This work stands as a crucial test for furthering applications of detrital mineral chronology within Cretaceous units of the Hell Creek region, and for the extraction of depositional ages from redeposited crystals in sedimentary layers.