Cosmogenic Al-26 and Be-10 exposure ages of blockslide deposits in Craig County, Virginia

Craig County, Virginia hosts large (>500 m wide) blockslides and rock slumps along the southeastern slopes of several mountains. These features differ from Virginian boulder fields in several aspects, most significantly in soil development, foliage, and clast size. In contrast with freshly exposed, likely periglacial, boulder fields that commonly contain centimeter to meter-sized boulders, these blockslides and slumps are heavily vegetated and consist of boulders ranging in size from centimeters to tens of meters hosted in well-developed soils. Prior workers studied the kinematics of these features and hypothesized Quaternary ages (Schultz and Southworth, 1989), but a lack of robust dating techniques precluded testing this hypothesis. Ongoing USGS mapping in the New Castle and Potts Creek 7.5’ quadrangles includes blockslides on Bald Mountain and Nutters Mountain, respectively. Both slides are > 650 meters wide at their head scarps, have runouts that extend > 600 meters, and have elevation differences of > 260 meters from their head scarps to their toes. Sandstone blocks that are house-sized or larger are ubiquitous within both slide masses, and provide useful targets for cosmogenic nuclide exposure dating in order to place absolute ages on these features. We sampled four boulders from the Bald Mountain blockslide and three boulders from the Nutters Mountain blockslide for Al-26 and Be-10 exposure dating. Mineral separation and chemical isolation of Al and Be were performed at the Reston Cosmogenic Nuclide (RECON) Laboratory. Accelerator mass spectrometry measurements were performed at the Purdue Rare Isotope Measurement (PRIME) Laboratory. Topographic shielding from cosmic radiation was calculated following Li (2018) and exposure age modeling was performed following Balco et al. (2008) using the LSDn scaling scheme (Lifton et al., 2014). Both sites featured generally upslope younging trends and concurrent Al-26 and Be-10 exposure ages. Two samples, 23AGNEW-L001 from Bald Mountain and 23AGPOT-L003 from Nutters Mountain, featured evidence of complex exposure histories (nonconcurrent Al-26 and Be-10 exposure ages, deviation from the age-elevation relationship). Excluding these outliers, Bald Mountain yielded exposure ages ranging from ~112 ka to ~59 ka and Nutters Mountain yielded exposure ages ranging from ~102 ka to ~29 ka. Applying a York regression to these Al-26 and Be-10 exposure ages reveals an estimated upslope younging trend of 1.0 ± 0.1 m/ky, which is generally consistent with age-elevation trends seen on Virginian boulder fields. It is faster than the ~0.5 m/ky upslope younging trend of the Paddy Mountain boulder field (Odom and Doctor, 2024), yet slower than the ~4 m/ky upslope younging trend of the Devil’s Marbleyard boulder field (Fame et al., 2023).

References

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