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Geology; February 2004; v. 32; no. 2; p. 137-140; DOI: 10.1130/G20091.1
© 2004 Geological Society of America
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Eocene calibration of geomagnetic polarity time scale reevaluated: Evidence from the Green River Formation of Wyoming

Malka Machlus1, Sidney R. Hemming1, Paul E. Olsen1 and Nicholas Christie-Blick1

1 Department of Earth and Environmental Sciences and Lamont-Doherty Earth Observatory of Columbia University, Palisades, New York 10964, USA

We reevaluate the Eocene geomagnetic polarity time scale on the basis of single-crystal 40Ar/39Ar ages for air-fall tuffs from the Wilkins Peak Member of the Green River Formation of Wyoming. Tuff 6 is dated as 49.1 ± 0.2 Ma, and tuff 3 is dated as 50.4 ± 0.3 Ma (maximum estimate). When combined with published magnetostratigraphic constraints, these age determinations suggest that the currently accepted age of chron C22r is 1.5–2.5 m.y. too old, which supports a significantly longer duration for the early Eocene, for the early Eocene climatic optimum, and the Wasatchian North American Land Mammal Age.

Key Words: Green River Formation • Eocene • Ar/Ar • lacustrine • Wasatchian-Bridgerian • early Eocene climatic optimum




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