US EPA

1284176 

Journal Article 

Dating fluvial terraces with (super 10) Be and (super 26) Al profiles; application to the Wind River, Wyoming 

Hancock, GS; Anderson, RS; Chadwick, OA; Finkel, RC 

1999 

Yes 

Geomorphology
ISSN: 0169-555X 

Elsevier 

Netherlands (NLD) 

27 

1-2 (February 1999) 

41-60 

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 (super 14) C, however, is difficult. We employ a cosmogenic radionuclide (CRN) profile technique that addresses a major problem of CRN dating on such surfaces: nuclide inheritance. By measuring (super 10) Be and (super 26) 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 sites along the Bull Lake-glacial correlative terrace (WR-3) are approximately 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 approximately 13-130 m/My for the source rocks. Alternatively, assuming the CRNs are inherited during clast transport, the time of fluvial transport from source to terrace is > approximately 10 ka; it increases downstream and is lower for sand than cobbles. The CRN ages for older terraces (WR-7 = approximately 300 ka and WR-15 = approximately 510 ka) are lower by approximately 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 approximately 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 inheritance-correction technique. 

strath terraces; upper Pleistocene; metals; North America; glaciation; Cenozoic; transport; cosmogenic elements; exhumation; upper Quaternary; clasts; Pleistocene; Wyoming; framework silicates; alkaline earth metals; beryllium; isotopes; quartz; Wind River; Bull Lake Glaciation; exposure age; terraces; fluvial features; corrections; silica minerals; glacial geology; geochronology; United States; Be-10; Al-26; aluminum; relative age; Wind River basin; silicates; Quaternary; geomorphology; radioactive isotopes