2010 GSA Denver Annual Meeting (31 October 3 November 2010)
Paper No. 160-6
Presentation Time: 9:15 AM-9:30 AM


SINGER, Brad S., Geoscience, University of Wisconsin-Madison, 1215 W. Dayton St, Madison, WI 53076, bsinger@geology.wisc.edu, CONDON, Daniel, NERC Isotope Geoscience Laboratory, British Geological Survey, Keyworth, Nottingham, NG12 5GG, United Kingdom, SIEWERT, Sarah E., Geoscience, University of Wisconsin-Madison, 1215 West Dayton St, Madison, WI 53076, MEYERS, Stephen R., Department of Geoscience, University of Wisconsin, 1215 West Dayton St, Madison, WI 53076, SAGEMAN, Bradley B., Earth and Planetary Sciences, Northwestern University, 1850 Campus Drive, Evanston, IL 60208, OBRADOVICH, John D., U.S. Geological Survey, MS 980, Box 25046, Denver, CO 80225, SAWYER, David A., USGS, MS 980, Box 25046, Denver, CO 80225, and JICHA, Brian R., Geoscience, University of Wisconsin-Madison

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 Tclosure 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.

2010 GSA Denver Annual Meeting (31 October 3 November 2010)
General Information for this Meeting
Session No. 160
Geochronology of the American West: In Honor of the Careers of Bill Cobban and John Obradovich and the Roots of EARTHTIME
Colorado Convention Center: Room 205
8:00 AM-12:00 PM, Tuesday, 2 November 2010

Geological Society of America Abstracts with Programs, Vol. 42, No. 5, p. 394

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