U-PB AND AR/AR DATING OF THE CRETACEOUS-PALEOGENE BOUNDARY INTERVAL IN EASTERN MONTANA

Numerous tuffs interbedded in lignites through the fossiliferous terrestrial Cretaceous-Paleogene boundary (KPB) in the Fort Peck region of northeastern Montana offer the potential for high-resolution chronostratigraphy, which in turn can provide unique insights into the nature and tempo of environmental changes occurring at this critical interval in Earth history. The tuffs are variably suited to U-Pb and Ar/Ar dating, which offers both the opportunity to compare the two systems and the need to do so. Preliminary U-Pb (zircon) and Ar/Ar (sanidine) results have been obtained for a previously undated tuff in the uppermost Cretaceous “null coal” beneath the infamous fluvial channels at the Bug Creek locality, for tuffs in the IrZ and HFZ coals at Hauso Flat, and for tuffs in the Z coal at the Bug Creek and Hell Creek localities. Preliminary results thus far from BGC indicate that the KPB is slightly younger than the age of 66.24 Ma recalculated by Renne et al. (2010) from Swisher et al. (1993), and probably lies between 65.7 and 66.1 Ma according to either radioisotopic system using the Ar/Ar system calibration of Renne et al. (2011) or Kuiper et al. (2008). For all of the abovementioned units, the preliminary results suggest a short duration of the dated interval, consistent with a rapid transition from Cretaceous to Paleogene paleoenvironments, fauna and flora. These results will also serve to constrain the age of the paleomagnetic C30n to C29r polarity reversal that predates the boundary. This work is performed under a collaboration between Berkeley and Amsterdam groups as part of the GTSNext program.