2010 GSA Denver Annual Meeting (31 October 3 November 2010)
Paper No. 160-7
Presentation Time: 9:30 AM-9:45 AM

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

SIEWERT, Sarah E., Geoscience, University of Wisconsin-Madison, 1215 W. Dayton St, Madison, WI 53706, sesiewert@wisc.edu, SINGER, Brad S., Geoscience, University of Wisconsin-Madison, 1215 W. Dayton St, Madison, WI 53076, CONDON, Daniel, British Geological Survey, Keyworth, Nottingham, NG12 5GG, United Kingdom, OBRADOVICH, John D., U.S. Geological Survey, MS 980, Box 25046, Denver, CO 80225, SAGEMAN, Bradley B., Earth and Planetary Sciences, Northwestern University, 1850 Campus Drive, Evanston, IL 60208, MEYERS, Stephen R., Department of Geoscience, University of Wisconsin, 1215 West Dayton St, Madison, WI 53076, JICHA, Brian R., Geoscience, University of Wisconsin-Madison, and SAWYER, David A., USGS, MS 980, Box 25046, Denver, CO 80225

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.

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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