HIGH RESOLUTION CHRONOSTRATIGRAPHY OF THE TERRESTRIAL K/P BOUNDARY IN NORTHEASTERN MONTANA: IMPLICATIONS FOR EXTINCTION AND RECOVERY

Terrestrial sediments of the Lancian-Puercan (latest Maastrichtian through early Danian; ~67-64 Ma) Hell Creek and Tullock formations in northeastern Montana, provide a well documented record of biostratigraphic and paleoenvironmental change through the Cretaceous-Paleogene boundary (KPB). At least 20 distinct tephras are interbedded in these sediments, providing a unique opportunity for high-resolution age calibration of events before and after the boundary. Regional correlation of the tephras enables paleocological reconstruction at the scale of 10–10² km with up to ca. 10 ka temporal resolution. ⁴⁰Ar/³⁹Ar (sanidine) and U-Pb (zircon) dating show best consistency when the calibration of Renne et al. (2011) is used for the former. Results show that sediment accumulation rates averaged over ~10 m were essentially uniform over ~120 m of section spanning the KPB, contra the implications of previous work (Swisher et al., 1993). The most significant turnover in mammalian fauna occurred early in the Puercan (e.g., Clemens, 2002), within the first 54 ± 34 ka of the Paleogene. A 2-3 permil negative δ¹³C anomaly (Arens & Jahren, 2000) coincident with an Ir spike up to 12 ppb (Smit & Van der Kaars, 1984) occurs in the bottom half of a short stratigraphic interval now constrained to be 25 ±24 ka in duration. The KPB itself is dated to 66.044 ± 0.045 Ma (Renne et al., in press) and is constrained to be synchronous with the Chicxulub impact at a resolution of 6 ± 27 ka. Major climate oscillations and mammalian faunal turnover occurred < 100 ka before the KPB (e.g., Wilf et al., 2003; Wilson, 2005). Thus while high resolution geochronology strongly links the KPB to the Chicxulub impact, precursory events suggest that Chicxulub’s role in the associated extinctions was as a coup de grâce rather than a sole cause. Improved chronology of the Deccan Traps’ extrusion history is needed to allow assessment of its role in KPB phenomena.