HR: 0800h
AN: T51D-1368    [Abstracts]
TI: The use of Cosmogenic Radionuclides in Stratigraphy: Application to the Nenana Gravel, Alaska Range
AU: * Ward, D
EM: dylan.ward@colorado.edu
AF: University of Colorado Dept. of Geological Sciences and INSTAAR, UCB 399, Boulder, CO 80309 United States
AU: Anderson, R
EM: robert.s.anderson@colorado.edu
AF: University of Colorado Dept. of Geological Sciences and INSTAAR, UCB 399, Boulder, CO 80309 United States
AB: We use a modification of the cosmogenic radionuclide (CRN) burial dating technique (differential decay of ²⁶Al and ¹⁰Be) to constrain the age range and deposition history of the Mio-Pliocene Nenana Fm. gravel in the central Alaska Range, AK. The Nenana Fm. consists of coarse fluvial gravel and debris flow deposits sourced in the newly emergent Alaska Range. Deposits of the Nenana Gravel up to ~1200 m thick record the exhumation of specific Alaska Range lithologies beginning south of the Denali Fault and progressing northward. The period of deposition of the formation is bracketed by an underlying tephra dated using ³⁹Ar/⁴⁰Ar to 6.7 Ma and the emplacement of 2.79 Ma Jumbo Dome into the Nenana Gravel. That the latter relation indicates an end of deposition by this time is disputed, however, and no direct dating has been performed on the gravel itself. Standard CRN burial dating is inapplicable to the Nenana Gravel because the requirement for instantaneous burial is not satisfied. The bracketing ages for the formation imply a mean deposition rate of 0.3 mm/yr, which, while rapid, does not instantaneously cut off buried sediment from the zone of CRN production (~35 ka are spent within 10 m of the surface). We thus modify this method to accommodate the effects of CRN production and decay during progressive burial of sediments. Burial rates are derived by using a 1D numerical model to fit a depth profile of ²⁶Al, ¹⁰Be, and their ratio to measured isotope concentrations, assuming incoming sediment is initially buried with a cosmogenic production ratio of Al/Be between 6.0 and 6.1. Absolute ages can be calculated once burial rates are constrained. Importantly, this method allows us to date very old sediments in this setting, as the extra time spent by the sediment in the CRN production zone increases its overall CRN concentrations above the values inherited from exhumation and transport. Eight sand and quartz-rich pebble samples were collected in exposures along Suntrana and Alaska Creeks, which allowed access to ~1200 m of section of the Nenana Gravel. We also collected three samples from a 30 m section of Nenana Gravel equivalent exposed at Honolulu Creek south of the Alaska Range. Two samples of modern Nenana R. sediment were collected to constrain initial inherited values of the Al/Be ratio in the modern setting. Samples were processed at the University of Colorado Cosmogenic Isotope Lab for analysis at the Center for Accelerator Mass Spectrometry at Lawrence Livermore National Laboratory. This suite of samples should allow reconstruction of deposition rate history within the Nenana Gravel, helping to constrain the early history of exhumation in the core of the Alaska Range.
DE: 1130 Geomorphological geochronology
DE: 1165 Sedimentary geochronology
SC: Tectonophysics [T]
MN: Fall Meeting 2005