HR: 1340h
AN: H53C-1389    [Abstracts]
TI: Terrace ages, timing of colluvial hollow denudation, and sediment routing in Pancho Rico Valley, Coast Ranges of central California (Monterey County), USA
AU: * Garcia, A F
EM: afgarcia@calpoly.edu
AF: Physics Department, Cal Poly State University, San Luis Obispo, CA 93407, United States
AU: Mahan, S A
EM: smahan@usgs.gov
AF: US Geological Survey, Box 25046, MS 974, Denver Federal Center, Denver, CO 80225, United States
AB: Field observations and mapping, combined with optically stimulated luminescence (OSL) dating reveal the effect of the Pleistocene-Holocene transition on colluvial hollows and stream channels in Pancho Rico Valley (part of the Pancho Rico Creek [PRC] drainage basin). Three samples of PRC fill-terrace sediment yielded OSL ages (14.1 +/- 1.41 ka, 13.8 +/- 1.59 ka, 13.1 +/- 1.30 ka) that are coincident with the Pleistocene-Holocene transition. These PRC fill terraces, which overlie straths, together with PRC-tributary fans and PRC-tributary valley fills comprise a physically continuous geomorphic surface and are collectively mapped as Qta. In the context of the terrace ages, colluvial hollow morphostratigraphy and debris flow deposits within Qta sequences indicate sediment was excavated from hollows and delivered to PRC via slope failures. The slope failures were triggered by relatively high frequency, potent storms during the Pleistocene-Holocene transition. Landslides caused by the stormy climate of this climatic transition may have also led to creation of new hollows in incompetent bedrock. Hollows were completely stripped of sediment sometime during (or after) the Pleistocene-Holocene transition, and drier Holocene climatic conditions have precluded refilling of older hollows or infilling of newly created hollows. Vegetation on Pancho Rico Valley hillslopes is relatively sparse because of relatively low precipitation during Holocene-time, and the predominance of south-facing slopes in the strongly asymmetric Pancho Rico Valley. Holocene-time refilling of colluvial hollows has not been established because of scouring caused by slope-wash erosion on sparsely vegetated hillslopes. Qta morphostratigraphy provides insight regarding Late Quaternary sediment routing and geomorphic processes in Pancho Rico Valley. The absence of Qta lobes in the alluvial fan that exists where PRC flows into the Salinas Valley indicates most sediment discharged from Pancho Rico Valley colluvial hollows is stored in Qta PRC terraces, Qta tributary fans, and Qta valley fills. Qta treads are base level for many Pancho Rico Valley hillslopes, and because Qta is mostly intact, Pancho Rico Valley hillslopes have been isolated from base-level fall caused by ongoing Holocene-time PRC incision. Debris flows like those impinging on the PRC channel during Qta time have been uncommon in the Holocene because there is insufficient sediment within colluvial hollows for slope failures to occur. Instead, sediment generated on hillslopes and mobilized by slope wash is deposited on Qta treads.
DE: 1807 Climate impacts
DE: 1825 Geomorphology: fluvial (1625)
DE: 1826 Geomorphology: hillslope (1625)
SC: Hydrology [H]
MN: 2007 Fall Meeting