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 incisi on, and establish the rates of pedogenic processes. While surface expo sure dating using cosmogenic Be-10 and Al-26 would Seem to be an ideal dating method, the surfaces are composed of individual clasts, each w ith its own complex history of exposure and burial. The stochastic nat ure of burial depth and hence in nuclide production in these clasts du ring exhumation and fluvial transport, and during post-depositional st irring, results in great variability in clast nuclide concentrations. We present a method for dealing with the problem of pre-depositional i nheritance of cosmogenic nuclides. We generate samples by amalgamating many individual clasts in order to average over their widely differen t exposure histories. Depth profiles of such amalgamated samples allow us to constrain the mean inheritance, to test for the possible import ance of stirring, and to estimate the age of the surface. Working with samples from terraces of the Fremont River, we demonstrate that sampl es amalgamated from 30 clasts represent well the mean concentration. D epth profiles show the expected shifted exponential concentration prof ile that we attribute to the sum of uniform mean inheritance and depth -dependent post-depositional nuclide production. That the depth-depend ent parts of the profiles are exponential argues against significant p ost-depositional displacement of clasts within the deposit. Our techni que yields Be-10 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 t o terrace and would correspond to an error of similar to 30-40 ka if n ot taken into account. The inheritance likely reflects primarily the m ean exhumation rates in the headwaters, of order 30 m/Ma. (C) 1997 Els evier Science B.V.