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

Kathryn E. Fletcher, Warren D. Sharp, Katherine J. Kendrick, Whitney M. Behr, Kenneth W. Hudnut, & Thomas C. Hanks

Published 2009, SCEC Contribution #1290

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 sub-samples 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 published 10Be exposure ages of cobbles (van der Woerd et al, 2006) and boulders (Behr et al, in press). 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. Observations of soil pedons, including variable depths to the tops of horizons of maximum carbonate accumulation, indicate that the T2 surface has been heterogeneously eroded, with up to ~1 meter of differential surface lowering among the described pedons, although it has not been possible to establish the extent of such deep erosion over the surface from soil observations.

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 ka, the uncertainty of the 10Be-derived age of the T2 fan-surface, which arises from uncertainties in the applicable 10Be production rate, the proportion of inherited 10Be, and the significance of residual scatter in the 10Be data set. 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; Behr et al., in press) provide broadly consistent constraints on the age of the fan-surface and help to elucidate its post-depositional development.

Fletcher, K. E., Sharp, W. D., Kendrick, K. J., Behr, W. M., Hudnut, K. W., & Hanks, T. C. (2009). 230Th/U dating of a late Pleistocene alluvial fan along the southern San Andreas fault. Geological Society of America Bulletin,.