Cosmogenic dating of fluvial terraces, Fremont River, Utah

Absolute dating of river terraces can yield long-term incision rates, clarify the role of climate in setting times of aggradation and incision, and establish the rates of pedogenic processes. While surface exposure dating using cosmogenic 10Be and 26Al would seem to be an ideal dating method, the surfaces are composed of individual clasts, each with its own complex history of exposure and burial. The stochastic nature of burial depth and hence in nuclide production in these clasts during exhumation and fluvial transport, and during post-depositional stirring, results in great variability in clast nuclide concentrations. We present a method for dealing with the problem of pre-depositional inheritance of cosmogenic nuclides. We generate samples by amalgamating many individual clasts in order to average over their widely different exposure histories. Depth profiles of such amalgamated samples allow us to constrain the mean inheritance, to test for the possible importance of stirring, and to estimate the age of the surface. Working with samples from terraces of the Fremont River, we demonstrate that samples amalgamated from 30 clasts represent well the mean concentration. Depth profiles show the expected shifted exponential concentration profile that we attribute to the sum of uniform mean inheritance and depth-dependent post-depositional nuclide production. That the depth-dependent parts of the profiles are exponential argues against significant post-depositional displacement of clasts within the deposit. Our technique yields 10Be age estimates of 60±9, 102±16 and 151±24 ka for the three highest terraces, corresponding to isotope stages 4, 5d and 6, respectively. The mean inheritance is similar from terrace to terrace and would correspond to an error of ∼30–40 ka if not taken into account. The inheritance likely reflects primarily the mean exhumation rates in the headwaters, of order 30 m/Ma.