40Ar/39Ar AND U-PB DATING OF THE CRETACEOUS NIOBRARA FORMATION

The Niobrara Fm. spans 12 ammonite zones of the Coniacian, Santonian, and Campanian stages and its 85 m of section is sampled by the Libsack and Aristocrat Angus cores. Spectral analysis of resistivity logs from these cores led Locklair and Sageman (2008) to propose an astronomical time scale for the Niobrara Fm. with a duration of 6.1-6.7 Ma. ⁴⁰Ar/³⁹Ar ages of 7 ash beds (Obradovich, 1993) were used as tie points for this orbital time scale. To further test the astrochronology and refine this part of the Cretaceous time scale we determined new ⁴⁰Ar/³⁹Ar and U-Pb ages from recently collected ash beds, as well as legacy samples of Obradovich (1993). For each of the 7 ash beds, which span the S. hippocrepis II through P. macombi biozones, ⁴⁰Ar/³⁹Ar sanidine ages are based on 40 to 90 fusion analyses. Full uncertainties (decay constant, standard age, analytical sources) are between 0.14 and 0.30% and ages range from 81.86 to 91.10 Ma. Using the ET535 tracer CA-TIMS analyses of single zircons from 4 ash beds yield ²⁰⁶Pb/²³⁸U ages that span 0.6 to 1.4 Ma for each bed, indicating that some crystals contain inherited ²⁰⁶Pb. However, weighted mean ages of the youngest group of zircons from ash beds within the P. macombi and D. bassleri zones are 91.37 and 84.67 Ma and these agree with the ⁴⁰Ar/³⁹Ar ages, provided the latter are calculated relative to an age of 28.201 Ma for Fish Canyon sandine. ⁴⁰Ar/³⁹Ar and ²⁰⁶Pb/²³⁸U ages of the P. macombi and D. bassleri ammonite zones, which bracket the Niobrara Fm., give durations of 6.75±0.18 and 6.70±0.13 Ma, respectively, with uncertainties that are far smaller than a 404 ka cycle, and both support the orbital time scale of Locklair and Sageman (2008). The Coniacian-Santonian boundary is unofficially defined as the base of the C. saxitonianus ammonite zone, which does not have a corresponding radioisotopic age. New ⁴⁰Ar/³⁹Ar ages tied directly to the astrochronologic age model for the C. vermiformis (directly above saxitonianus) and S. depressus zones places this boundary around 86.7±0.2 Ma. The Turonian-Coniacian boundary is loosely bracketed by ash beds for which both ⁴⁰Ar/³⁹Ar and U-Pb ages taken together, suggest an age of 90.3±0.3 Ma, or ~1 Ma older than in GTS2004.