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

GSA Bulletin
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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