HIGH-RESOLUTION CHRONOSTRATIGRAPHY AND MAGNETOSTRATIGRAPHY OF THE TERRESTRIAL CRETACEOUS-PALEOGENE BOUNDARY TRANSITION IN THE HELL CREEK REGION, MONTANA

Constraints on tempo, timing, and mode of latest Cretaceous ecological instability and post-extinction terrestrial faunal recovery in the early Paleocene are poor. To better understand the timing and tempo of terrestrial faunal change across the Cretaceous-Paleogene boundary (KPB), we have developed a high-precision chronostratigraphic framework for fluvial and floodplain sediments within the Hell Creek region, MT, using both ⁴⁰Ar/³⁹Ar geochronology and paleomagnetism. The Hell Creek region is an ideal location for this study as it is one of the best-studied terrestrial KPB sites globally: it has a well-recorded succession of faunal change across the KPB, its sediments contain reliable paleomagnetic recorders, and ash layers amenable to high-precision ⁴⁰Ar/³⁹Ar dating are interbedded throughout the stratigraphy. Over 60 distinct volcanic ash deposits have been identified. We have dated over 30 of these deposits, 15 unpublished, with ~ ±30 ka precision, ranging in age from ~300 ka before the KPB to ~1 Ma after (Sprain et al., 2015). We have further collected new magnetostratigraphic sections at 17 sites throughout the Hell Creek region, allowing for not only better regional correlation of faunal localities but also better correlation throughout North America. Integrating our results into the extensive paleontological framework for this region suggests that pre-KPB ecological decline occurred within the last 200 ka of the Cretaceous, and that mammalian recovery transpired less than ~1 Ma after the KPB. Further, by tying our high-precision ⁴⁰Ar/³⁹Ar ages to our magnetostratigraphic framework, the timing and duration of circum-KPB polarity chrons (C28n-C30r) are constrained with unprecedented precision. These chrons are used for age control in numerous KPB studies, including many marine sections and studies on climate change, however current the calibration suffers from multiple drawbacks making it unsuitable for high-precision study. This work allows for detailed analysis of the rates of terrestrial faunal change across the KPB, will aid comparison between terrestrial and marine KPB sites, and will ultimately provide the means for integration of faunal records with records of climate change and Deccan volcanism.