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Using In Situ 10Be Soil Profiles to Date Fluvial Terraces of the South Fork, Shenandoah River, VA

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Author: Garber, Jonathan L.
Advisor: Hancock, Gregory S.
Committee Members: Kaste, James M.; Hamilton, Stuart
Issued Date: 2010-05-17
Subjects: Geomorphology
Cosmogenic Radionuclides
Fluvial Terraces
South Fork, Shenandoah River
URI: http://hdl.handle.net/10288/2042
Description: I determined ages and on fluvial terraces of the South Fork of the Shenandoah River, Virginia, obtained from depth profiles of in situ 10Be. These dates represent the first numerically constrained ages on fluvial terraces in the Shenandoah system, and allow us to assess the timing of perturbations to the river system and to estimate longterm river incision rates. I sampled three terrace treads near Lynnwood, Virginia, originally identified in mapping completed by King (1950) and Bell (1985). King (1950) suggested a Pleistocene age for the higher terraces, but the terraces are otherwise undated. The lowest terrace level (T2) is an ~7 m above river level (ARL), and has been inundated by historic flooding. The higher terraces are ~11 m ARL (T3) and ~16 m ARL (T4). On each surface, I excavated a ~2.5 m deep by ~10 m long trench to identify soil properties and to collect ~5 kg bulk samples at 0.25 m depth intervals from 0.25 m to 1.75 m depths. The 250-500 μm quartz sand fraction from the soil was extracted to analyze for 10Be concentration. To estimate terrace age corrected for inherited 10Be, A MATLAB script iteratively solved for age and inheritance through a least squares fit of modeled 10Be concentration profiles to measured profiles on each terrace. T3 showed an exponential profile, which yielded minimum age of 162 kyr and inheritance of 4.62 *105 atoms per gram of quartz. The maximum age was taken to be the age and terrace erosion rate that corresponded to ~2 m of denudation. This assumption was made because of field observations of ~2 m of fine overbank deposit sediments in the active T2 deposit that were not in the upper deposits, suggesting that these layers had been eroded on the older terraces. The maximum age and terrace erosion rate was 388 kyr and 4.7 m/Myr. The T4 deposit showed mixing in the top three data points which 5 corresponded to a paler layer in the profile. To date these terraces a new Inventory method that took into account both radioactive decay of the isotope as well as allow for the input of terrace erosion rates to the model. The age was then checked by iteratively solving for the exponential curve using the inheritance derived from the Inventory calculations. The minimum age assuming no surface erosion was 300 "� 340 kyr and the maximum age assuming ~2 m denudation was 640 - 1000 kyr.These ages give us a range of incision rates between 17 and 65 m/Myr which are statistically higher than the range of bare bedrock summit erosion rates in the Blue Ridge (Whitten, 2009) and suggest a net increase in relief in the Shenandoah Valley over 105 time scales. The incision rates were also comparable to other Appalachian rivers, which suggest disequilibrium across the Central Appalachian landscape. Possible cause of this landscape disequilibrium include (1) the differential response of the landscape to increases in the amplitude and frequency of climate fluctuations at the onset of the Pleistocene, and (2) isostatic flexural uplift caused by loading on the Atlantic Continental shelf and lightening of the Appalachian Plateau from increased erosion of Atlantic draining streams.
Degree: Bachelors of Arts in Geology


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