PLEISTOCENE COSMOGENIC 26AL/10BE ISOCHRON BURIAL AGES OF SURFICIAL DEPOSITS IN THE BLUE RIDGE, NORTH CAROLINA: POSSIBLE CONNECTIONS WITH PALEOCLIMATE AND NEOTECTONICS

Mapping efforts to characterize the geologic setting of the Mw 5.1 earthquake that occurred in the Blue Ridge near Sparta, North Carolina on August 9, 2020, have identified numerous shallow faults outside of the primary surface rupture and several fluvial/colluvial gravel deposits ranging from 4-20 m above nearby modern stream channels. Together, these observations raise questions about the long-term stability of the landscape and the relative roles of paleoclimate and neotectonics in driving recent geomorphic change in the Blue Ridge.

We have been working to date surficial deposits, some of which directly overlie shallow faults, in order to constrain the Late Cenozoic history of crustal deformation, erosion, and deposition in the Sparta area. This study sampled three fluvial/colluvial gravel deposits for cosmogenic nuclide burial dating: a deposit 3 km ENE of Jefferson, NC, and two deposits along Bledsoe Creek in Sparta. These deposits are all located along tributaries of the New River.

All sample preparation for accelerator mass spectrometry measurements was performed at the USGS Reston Cosmogenic Nuclide Laboratory. Initial geochronologic results reveal early to middle Pleistocene depositional ages for these deposits. The fluvial terrace near Jefferson was buried at 1.01 ± 0.09 Ma, while the alluvial gravels near Sparta were deposited at 0.49 ± 0.14 Ma and 1.85 ± 1.47 Ma; the latter age is not discussed further. The two aforementioned deposits date to interglacials, respectively preceding major glaciations at ~0.8 Ma and ~0.2 Ma. Paleo-erosion rates derived from sand fractions range from ~7 m/My in the Sparta deposit to ~12 m/My in the Jefferson deposit, both of which are consistent with modern erosion rates measured from cosmogenic ¹⁰Be elsewhere in the Blue Ridge Mountains. Paleo-erosion rates from mixed gravels (<5 m/My) are likewise consistent with Pleistocene paleo-erosion rates from New River cave deposits near Pearisburg, Virginia. These preliminary results suggest that despite faulting and modern seismicity in the Sparta region, regional erosion rates are not anomalous when compared to other areas in the Blue Ridge. Currently, we are working to date additional deposits in the area to improve the framework surficial geology chronology, and further identify any paleoclimatic or neotectonic signatures.