Dating fluvial terraces with 10 Be and 26 Al profiles : application to the Wind River , Wyoming

Fluvial strath terraces provide a record of river incision and the timing of climatic perturbations to the fluvial system. Dating depositional surfaces like terraces that are older than the range of C, however, is difficult. We employ a Ž . cosmogenic radionuclide CRN profile technique that addresses a major problem of CRN dating on such surfaces: nuclide inheritance. By measuring Be and Al profiles, we constrain the exposure age and the mean CRN inheritance for the deposit. The CRN profile also yields a self-check on the assumptions underlying the method. We report our attempts to date terraces along the Wind River, WY. Like many sequences of western North American fluvial terraces, these are inferred to reflect oscillation between glacial and interglacial conditions in the headwaters. Previous dating of some of these terraces and the associated terraces and glacial deposits makes this a unique location to compare dating methods. Dates from five Ž . sites along the Bull Lake-glacial correlative terrace WR-3 are ;118–125 ka, which agrees with dates on Bull Lake-age moraines and independent age estimates on the terrace, and is consistent with the model of terrace–glacial relationship. CRN inheritance is significant and highly variable, requiring it be considered despite the additional sampling complexity. Assuming all inheritance in WR-3 deposits arises during exhumation in the headwaters, we obtain minimum mean rates of exhumation of ;13–130 mrMy for the source rocks. Alternatively, assuming the CRNs are inherited during clast transport, the time of fluvial transport from source to terrace is );10 ka; it increases downstream and is lower for sand than cobbles. Ž . The CRN ages for older terraces WR-7s;300 ka and WR-15s;510 ka are lower by ;50% than previous estimates based on tephrochronology; the most plausible explanation is eolian deflation of a once thicker loess cover on the terrace surfaces. Mean thicknesses of loess of ;0.5–1.5 m are required to reconcile these concentrations of CRN with the previous estimates of age. Difficulty in dating the older terraces emphasizes that geologic caution, independent estimates of age, and multiple sample sites should still be part of dating depositional surfaces with CRNs, even when employing the inheritancecorrection technique. q 1999 Elsevier Science B.V. All rights reserved.