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.