EXPLICIT TREATMENT OF INHERITANCE IN DATING DEPOSITIONAL SURFACES USING IN-SITU BE-10 AND AL-26

We describe a new strategy for dating depositional landscape surfaces using in situ-produced cosmogenic radionuclides (CRNs) that removes th e complication of nuclide inheritance by clasts prior to deposition. T wo amalgamated samples, each consisting of 30 clasts, one from the sur face and one from a fixed depth in the subsurface, constrain this CRN inheritance and date the surface. The inheritance may be used to estim ate minimum exhumation rates and maximum transport times within the ge omorphic system, We test the technique using Be-10 and Al-26 to date t he third (FR3) of five terraces along the Fremont River, Utah, and the third (WR3) of 15 along the Wind River, Wyoming. Whereas effective ag es based solely upon the surface samples yield 118-138 ka (FR3) and 93 ka (WR3), the subsurface samples reveal that inheritance accounts for 22% (WR3) to 43% (FR3) of the total CRN concentration, Taking this in to account yields terrace ages of similar to 61-81 ha for FR3 and simi lar to 67-76 ha for WR3. We explore the dependence of age estimates on the accumulation history of the terrace silt caps.