High-resolution calibration of Eocene strata: 40Ar/39Ar geochronology of biotite in the Green River Formation
- 1University of Wisconsin-Madison, Department of Geology and Geophysics, 1215 W. Dayton St., Madison, Wisconsin 53706, USA
Abstract
The Sixth tuff of the Wilkins Peak Member of the Green River Formation is an important potential calibration point for magnetic stratigraphy, mammalian evolution, and lacustrine processes. However, two recent 40Ar/39Ar ages based on laser fusion of biotite from this important bed are not in agreement with one other. Most large, euhedral biotite from the basal 1 cm give concordant age spectra when incrementally heated. However, some crystals yield discordant age spectra and have plateau and integrated ages that are scattered toward both older and younger apparent ages. Electron-probe microanalysis indicates that intergrown alteration phases occur in a small fraction of Sixth tuff biotite crystals, and likely contributed to open-system processes that led to the observed discordance and age scatter. A new age of 49.62 ± 0.10 Ma is obtained from 22 concordant incremental-heating experiments. This age agrees with a new 49.78 ± 0.08 Ma age for the sanidine from the Layered tuff, which is stratigraphically 24 m below the Sixth tuff.
Footnotes
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↵*msmith{at}geology.wisc.edu
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↵GSA Data Repository item 2006078, Tables DR1–2 and Figures DR1–2: a table showing locations and descriptions of sampled tuff beds, a figure showing inverse isochron and two-step age spectra diagrams, a table containing the full 40Ar/39Ar data set, a diagram illustrating the distribution of measured neutron fluence (J) across the irradiation discs, and a table summarizing major element compositions and backscattered electron images of biotite phenocrysts from the Sixth tuff, is available online at www.geosociety.org/pubs/ft2006.htm, or on request from editing{at}geosociety.org or Documents Secretary, GSA, P.O. Box 9140, Boulder, CO 80301-9140, USA.
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- Accepted 3 January 2006.
- Received 20 September 2005.
- Revision received 3 January 2006.
- Geological Society of America
