HR: 16:15h
AN: H52E-02    [PDF]
TI: Determining the Pace of Landscape Evolution in the Sierra Nevada, California, Using Cosmogenic Burial Dating of Cave Sediments
AU: * Stock, G M
EM: gstock@es.ucsc.edu
AF: Department of Earth Sciences, University of California, Santa Cruz, Santa Cruz, CA 95064 United States
AU: Anderson, R S
EM: andersrs@colorado.edu
AF: Institute for Arctic and Alpine Research, University of Colorado, Boulder, CO 80309 United States
AU: Finkel, R C
EM: finkel1@llnl.gov
AF: Center for Accelerator Mass Spectrometry and Geosciences and Environmental Technology, Lawrence Livermore National Laboratory, Livermore, CA 94550 United States
AB: We report cosmogenic nuclide burial ages of Sierra Nevada caves that constrain detailed river incision histories and help clarify models of late Cenozoic topographic evolution. Portions of many Sierra Nevada river canyons are cut into marble bedrock containing numerous caves. These caves represent former river levels etched into the bedrock. Using $^{26Al}$/$^{10}$Be ratios, we determined burial ages for granitic bedload sediments washed into caves once at river level and now perched high in canyon walls. In the South Fork Kings River canyon, where the record is most complete, rapid incision of $\sim$0.2 mm yr$^{-1}$ from 2.7 to $\sim$1.5 Ma slowed markedly to $\sim$0.02 mm yr$^{-1}$ thereafter. Other caves in nearby canyons indicate a similar history. Numerical modeling of river profile evolution indicates that the reduction in incision rate is likely due to a transient response to Pliocene rock uplift. Westward tilting sufficient to drive the proposed 1.5 - 2 km of crestal uplift initiates a pulse of accelerated erosion that begins at the edge of the Central Valley and propagates up the river profile, passing the caves between $\sim$5 and 2 Ma. Incision rates decline markedly after this time. Late Quaternary incision may have been further reduced by periodic mantling of riverbeds with glacially derived sediment. The caves demonstrate that while $\sim$400 m of incision has occurred in the last 2.7 Myr, canyons up to 1.6 km deep existed prior to that time. Much of this paleorelief may be relict from the late Cretaceous. In addition, $^{26}$Al and $^{10}$Be concentrations on granitic interfluve surfaces indicate that these surfaces are eroding at $\sim$0.012 mm yr$^{-1}$ averaged over $\sim$70 kyr. This is more than an order of magnitude slower than late Pliocene and early Quaternary incision rates, and 2 to 3 times slower than late Quaternary incision rates. As slow interfluve erosion likely persisted throughout the Quaternary, local relief increased during the period of rapid incision. Our findings of substantial pre-Quaternary canyon relief and slow interfluve erosion serve to lessen the amount of flexural isostatic rock uplift expected due to late Cenozoic erosion, strengthening the case for geophysically-driven late Cenozoic uplift.
DE: 1035 Geochronology
DE: 1040 Isotopic composition/chemistry
DE: 1815 Erosion and sedimentation
DE: 1824 Geomorphology (1625)
DE: 8109 Continental tectonics--extensional (0905)
SC: Hydrology [H]
MN: 2003 Fall Meeting