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

Fluvial strath terraces provide a record of river incision and the timing o f climatic perturbations to the fluvial system. Dating depositional surface s like terraces that are older than the range of C-14, however, is difficul t. We employ a cosmogenic radionuclide (CRN) profile technique that address es a major problem of CRN dating on such surfaces: nuclide inheritance. By measuring Be-10 and Al-26 profiles, we constrain the exposure age and the m ean CRN inheritance for the deposit. The CRN profile also yields a self-che ck on the assumptions underlying the method. Pie report our attempts to dat e terraces along the Wind River, WY. Like many sequences of western North A merican 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 mak es this a unique location to compare dating methods. Dates from five sites along the Bull Lake-glacial correlative terrace (WR-3) are similar to 118-1 25 ka, which agrees with dates on Bull Lake-age moraines and independent ag e estimates on the terrace, and is consistent with the model of terrace-gla cial relationship. CRN inheritance is significant and highly variable, requ iring it be considered despite the additional sampling complexity. Assuming all inheritance in WR-3 deposits arises during exhumation in the headwater s, we obtain minimum mean rates of exhumation of similar to 13-130 m/My for the source rocks. Alternatively, assuming the CRNs are inherited during cl ast transport, the time of fluvial transport from source to terrace is > si milar to 10 ka; it increases downstream and is lower for sand than cobbles. The CRN ages for older terraces (WR-7 = similar to 300 ka and WR-15 = simi lar to 510 ka) are lower by similar to 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 similar to 0.5-1.5 m are required to reconcile these concentrations of CRN with the previous estimates of age. Difficulty in dating the older terr aces emphasizes that geologic caution, independent estimates of age, and mu ltiple sample sites should still be part of dating depositional surfaces wi th CRNs, even when employing the inheritance-correction technique. (C) 1999 Elsevier Science B.V. All rights reserved.