High-resolution calibration of Eocene strata: 40Ar/39Ar geochronology of biotite in the Green River Formation

M. Elliot Smith, Brad S. Singer, Alan R. Carroll, John H. Fournelle

Research output: Contribution to journalArticlepeer-review

36 Scopus citations


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/39 Ar 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.

Original languageEnglish (US)
Pages (from-to)393-396
Number of pages4
Issue number5
StatePublished - May 2006
Externally publishedYes


  • Ar-Ar
  • Biotite
  • Geomagnetic polarity time scale
  • Green River Formation
  • Sanidine

ASJC Scopus subject areas

  • Geology


Dive into the research topics of 'High-resolution calibration of Eocene strata: 40Ar/39Ar geochronology of biotite in the Green River Formation'. Together they form a unique fingerprint.

Cite this