COSMOGENIC 26AL/10BE ISOCHRON BURIAL DATING REVEALS MIDDLE PLEISTOCENE SEDIMENTATION IN THE BLUE RIDGE OF NORTH CAROLINA AND VIRGINIA

Geologic mapping following the 9 August 2020 Mw 5.1 earthquake in Sparta, North Carolina identified numerous brittle faults and several potentially associated gravel deposits. These gravels typically appear to be fluvial in origin or reworked by colluvial processes, and may be tied to regional paleoseismic and/or paleoclimatic phenomena. Previously, we burial dated three such deposits: a buried fluvial cutbank three kilometers ENE of Jefferson, NC (0.89 ± 0.07 Ma), a colluvial gravel overlying a fault immediately NW of downtown Sparta (0.46 ± 0.13 Ma), and a fluvial terrace in downtown Sparta (1.66 ± 1.24 Ma). Here, we expand upon this existing dataset with new ²⁶Al/¹⁰Be isochron burial ages from two gravel deposits: a site along the Little River, 11 kilometers NE of Sparta and a site 16 kilometers NW of Sparta, along the New River.

All mineral separation and chemical preparation were performed at the USGS Reston Cosmogenic Nuclide (RECON) Lab, and accelerator mass spectrometry measurements were performed at the Purdue Rare Isotope Measurement (PRIME) Lab. Isochron calculations were performed using a recently developed, in-house Python code that directly fits isochron lines to Monte Carlo samplings of ²⁶Al and ¹⁰Be measurements using orthogonal distance regression. Our analysis indicated that two cobbles from the Little River isochron were previously buried; removal of these samples yielded a likely isochron burial age of 0.47 ± 0.09 Ma. The New River site, on the other hand, featured no reworked samples and produced an isochron burial age of 0.43 ± 0.14 Ma. Paleo-erosion rates of individual samples (cobbles and sands) ranged from 1–12 m/My, consistent with ¹⁰Be-derived modern and paleo-erosion rates measured in the New River and nearby drainages.

These new burial ages overlap with each other and the age of the colluvial gravel overlying a fault immediately NW of downtown Sparta, within 1σ analytical uncertainty. These deposits are located 10-15 km apart from one another, indicating that non-local drivers may be the cause of deposition. Potential regional drivers of sedimentation during this time may include seismic activity, but are more likely climate-driven changes in weathering processes and/or incision of the New River and its tributaries.