COSMOGENIC BURIAL DATING AND MAGNETOSTRATIGRAPHY OF EARLY AND MID-PLEISTOCENE MISSOULA FLOOD SEDIMENTS, HANFORD, WA

Previous magnetostratigraphic studies (Bjornstad et al, 2001; Pluhar, 2003) indicate that Pleistocene cataclysmic flood deposits of the Hanford DOE site record events as old at 1.1 Ma. These, and additional new magnetostratigraphic data presented here, derive from drill cores penetrating the Cold Creek flood bar in the Pasco Basin, a depositional sink for much of the material scoured from the Channeled Scabland during Missoula Floods and other similar events. The eastern part of the Cold Creek bar records a magnetostratigraphy characterized by reversed polarity at the base with normal polarity above and reversed polarity on top. We hypothesize that the normal zone is the Jaramillo subchron (0.99-1.07 Ma), or less likely, the Olduvai subchron (1.77-1.95 Ma).

We test these possibilities by cosmogenic burial dating (Granger and Musikar, 2001), using the differential radioactive decay of ²⁶Al and ¹⁰Be. We sampled 2 paleosols from drill cores of eastern Cold Creek bar as well as 14 kyr Missoula Flood sediments from Badger Coulee for cosmogenic burial dating. We chose the paleosols to increase the likelihood that sufficient cosmogenic nuclides would have accumulated in the samples prior to burial to allow application of this technique. We analyzed the 14 kyr sample to provide an estimate of the inherited cosmogenic nuclide ratio. Measurable inherited concentrations in the same proportion as that produced at the surface would indicate that burial dating could be applied throughout the stratigraphy. If not, then only the paleosols should be dated by this technique. Initial results suggest that sufficient cosmogenic nuclides concentrations are present in these sediments to make burial dating feasible.