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Eocene clocks agree; coeval (super 40) Ar/ (super 39) Ar, U-Pb, and astronomical ages from the Green River Formation

M. Elliot Smith, K. R. Chamberlain, B. S. Singer and A. R. Carroll
Eocene clocks agree; coeval (super 40) Ar/ (super 39) Ar, U-Pb, and astronomical ages from the Green River Formation
Geology (Boulder) (June 2010) 38 (6): 527-530

Abstract

U-Pb ages of zircon from the Firehole and Analcite ash beds in the Eocene Green River Formation (Wyoming, United States) are indistinguishable from (super 40) Ar/ (super 39) Ar ages of sanidine after adjusting the latter to the astronomically calibrated age of 28.201 Ma for the Fish Canyon sanidine standard. Six of nine zircon analyses from the Firehole ash yield concordant ages in an overlapping cluster and give a weighted mean (super 238) U- (super 206) Pb age of 51.66+ or -0.20 Ma (full external 2sigma uncertainty), indistinguishable from the recalibrated (super 40) Ar/ (super 39) Ar age of 51.40+ or -0.25 Ma (full external 2sigma uncertainty), which is adjusted to eliminate samples with low radiogenic (super 40) Ar (super *) . Significant (super 238) U- (super 206) Pb age scatter likely reflects some combination of minor inheritance, Pb loss, and possibly magma chamber residence. Seven of eight analyses of Analcite ash zircon yield a weighted mean (super 238) U- (super 206) Pb age of 49.23+ or -0.13 Ma (full external 2sigma uncertainty), whereas the sanidine (super 40) Ar/ (super 39) Ar age is 49.24+ or -0.18 Ma (full external 2sigma uncertainty). Calibrating Green River Formation (super 40) Ar/ (super 39) Ar ages to the 28.201 Ma age for Fish Canyon sanidine permits the first direct comparison of specific Green River Formation strata to the astronomical solution for Early Eocene insolation. This comparison supports the hypothesis that periods of fluvial deposition coincided with minima in long and short eccentricity, and that periods of lake expansion and evaporite deposition correspond to eccentricity maxima.


ISSN: 0091-7613
EISSN: 1943-2682
Coden: GLGYBA
Serial Title: Geology (Boulder)
Serial Volume: 38
Serial Issue: 6
Title: Eocene clocks agree; coeval (super 40) Ar/ (super 39) Ar, U-Pb, and astronomical ages from the Green River Formation
Affiliation: Sonoma State University, Department of Geology, Rohnert Park, CA, United States
Pages: 527-530
Published: 201006
Text Language: English
Publisher: Geological Society of America (GSA), Boulder, CO, United States
References: 36
Accession Number: 2010-050215
Categories: Geochronology
Document Type: Serial
Bibliographic Level: Analytic
Annotation: With GSA Data Repository Item 2010151
Illustration Description: illus. incl. 1 table
N41°00'00" - N45°00'00", W111°04'60" - W104°04'60"
Secondary Affiliation: University of Wyoming, USA, United StatesUniversity of Wisconsin, USA, United States
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Reference includes data from GeoScienceWorld, Alexandria, VA, United States. Reference includes data supplied by the Geological Society of America, Boulder, CO, United States
Update Code: 201027
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