Northeastern Section (45th Annual) and Southeastern Section (59th Annual) Joint Meeting (13-16 March 2010)

Paper No. 7
Presentation Time: 3:55 PM


GARBER, Jonathan, HANCOCK, Gregory and REILLY, Charles D., Department of Geology, College of William & Mary, Williamsburg, VA 23187,

We present ages and soil properties on fluvial strath terraces of the South Fork of the Shenandoah River, Virginia, obtained from depth profiles of in situ 10Be. These dates represent the first numerical ages on terraces in the Shenandoah system, and allow us to assess the timing of perturbations to the river system and to estimate long-term river incision rates. We have 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 ~10 m above river level (ARL), and has been inundated by historic flooding. The higher terraces are ~15 m ARL (T3) and ~30 m ARL (T4). On each surface, we excavated a ~2.5 m deep by ~10 m long trench to identify terrace stratigraphy and to collect ~5 kg bulk samples at 0.25 m depth intervals from 0.25 m to 1.75 m depths. We measured soil color, pH, and conductivity on each sample, and extracted the 250-500 μm quartz fraction to analyze for 10Be concentration. To estimate terrace age corrected for inherited 10Be, we iteratively solve for age and inheritance through comparison of modeled 10Be profiles to measured profiles on each terrace. Based on previous terrace ages in the region, we anticipate the highest terrace to have an age of ~1 My once analyses are completed. Numerical ages on terraces will be used to determine river incision rates and to compare soil properties to soil age within the terrace chronosequence. In addition, we will compare Shenandoah River incision rates to previously measured rates of summit lowering and watershed-scale erosion rates obtained from 10Be in the adjacent Blue Ridge, allowing us to assess the direction of relief change in this landscape.