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

U-series dating of pedogenic carbonateclast coatings provides a reliable, precise minimum age of 45.1 [+ or -] 0.6 ka (2[sigma]) 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 [sup.238]U-[sup.234]U-[sup.230]Th system has remained closed since carbonate formation. The U-series minimum age is used to assess previously published [sup.10]Be exposure ages of cobbles and boulders. All but one cobble age and some boulder [sup.10]Be 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 [sup.10]Be 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 [sup.10]Be-derived age of the T2 fan surface. Dating of the T2 fan surface via U-series on pedogenic carbonate (minimum age, 45.1 [+ or -] 0.6 ka) and [sup.10]Be on boulder-top samples using forward modeling (preferred age, 50 [+ or -] 5 ka) provides broadly consistent constraints on the age of the fan surface and helps to elucidate its postdepositional development. doi: 10.1130/B30018.1