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230Th/U dating of a late Pleistocene alluvial fan along the southern San Andreas fault

GSA Bulletin

By:
, , , , ,
DOI: 10.1130/​B30018.1

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Abstract

U-series dating of pedogenic carbonate-clast coatings provides a reliable, precise minimum age of 45.1 ± 0.6 ka (2σ) for the T2 geomorphic surface of the Biskra Palms alluvial fan, Coachella Valley, California. Concordant ages for multiple subsamples from individual carbonate coatings provide evidence that the 238U-234U-230Th system has remained closed since carbonate formation. The U-series minimum age is used to assess previously published 10Be exposure ages of cobbles and boulders. All but one cobble age and some boulder 10Be ages are younger than the U-series minimum age, indicating that surface cobbles and some boulders were partially shielded after deposition of the fan and have been subsequently exhumed by erosion of fine-grained matrix to expose them on the present fan surface. A comparison of U-series and 10Be ages indicates that the interval between final alluvial deposition on the T2 fan surface and accumulation of dateable carbonate is not well resolved at Biskra Palms; however, the “time lag” inherent to dating via U-series on pedogenic carbonate can be no larger than ∼10 k.y., the uncertainty of the 10Be-derived age of the T2 fan surface. Dating of the T2 fan surface via U-series on pedogenic carbonate (minimum age, 45.1 ± 0.6 ka) and 10Be on boulder-top samples using forward modeling (preferred age, 50 ± 5 ka) provides broadly consistent constraints on the age of the fan surface and helps to elucidate its postdepositional development.

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Additional Publication Details

Publication type:
Article
Publication Subtype:
Journal Article
Title:
230Th/U dating of a late Pleistocene alluvial fan along the southern San Andreas fault
Series title:
GSA Bulletin
DOI:
10.1130/​B30018.1
Volume
122
Issue:
9-10
Year Published:
2010
Language:
English
Publisher:
Geological Society of America
Publisher location:
Boulder, CO
Contributing office(s):
Earthquake Science Center
Description:
13 p.
Larger Work Type:
Article
Larger Work Subtype:
Journal Article
Larger Work Title:
GSA Bulletin
First page:
1347
Last page:
1359
Country:
United States
State:
California