INTEGRATING RADIOISOTOPIC AND ASTROCHRONOLOGIC TIME SCALES FOR THE CRETACEOUS

The chronostratigraphy pioneered by John Obradovich and Bill Cobban on the basis of ⁴⁰Ar/³⁹Ar dating of ash beds within a framework of ammonite biozones established a Cretaceous time scale with ~0.5% resolution. To build from this toward the EARTHTIME goal of 0.1% resolution, we are undertaking new ⁴⁰Ar/³⁹Ar dating of these and previously undated ash beds coupled with U-Pb dating of zircons and newly developed astrochronologic age models for key intervals. Initial focus is on the Cenomanian to Santonian stages which: span 37 ammonite (31 inoceramid) zones, include a GSSP associated with OAE2, and yield, through spectral analysis of cores, floating astrochronologies for ~7 Ma of time. ⁴⁰Ar/³⁹Ar ages of sanidine from newly collected samples are identical to those obtained from legacy samples of Obradovich (1993). Between 33 and 103 concordant age determinations, from as many as three separate sites for a single ash bed, yield analytical uncertainties of between 100 and 170 ka, i.e., <0.2%. Whereas this level of precision can directly test floating astrochronologies that rely on identification of eccentricity forcing of sedimentation, it also highlights the need to place each dated ash bed precisely within a taxon range zone and to understand how biozones are defined, before assigning new ages to stage boundaries. Owing to the high precision of single crystal CA-TIMS ²⁰⁶Pb/²³⁸U ages, between 3 and 11 zircons measured in subsamples from each of 8 ash beds span 0.5 to 1.0 Ma, reflecting inheritance or recycling of some zircon during protracted evolution of the volcanic sources. Using the ET535 tracer, the youngest group of zircons from ash beds in six ammonite zones are indistinguishable from the ⁴⁰Ar/³⁹Ar ages, provided the latter are calculated relative to a 28.201 Ma age for Fish Canyon sanidine (FCs). The traditional age of 28.02 Ma for FCs results in ⁴⁰Ar/³⁹Ar ages 0.5 Ma younger than the youngest concordant zircons. The recently proposed age of 28.305 Ma for FCs results in ⁴⁰Ar/³⁹Ar ages for these ash beds that are significantly older than the youngest concordant zircons, a scenario difficult to reconcile given the higher T_closure of zircon relative to sanidine. The radioisotopic dating confirms astrochronologic durations of ~600 ka for Cenomanian-Turonian OAE2, and 6.7 Ma for the Coniacian-Santonian Niobrara Formation.