Southeastern Section - 67th Annual Meeting - 2018

Paper No. 22-2
Presentation Time: 8:20 AM


WITT, Anne C.1, CARTER, Mark W.2, HELLER, Matthew J.1, SPEARS, David B.1 and HANCOCK, Gregory3, (1)Division of Geology and Mineral Resources, Virginia Department of Mines, Minerals, and Energy, 900 Natural Resources Drive, Suite 500, Charlottesville, VA 22903, (2)U.S. Geological Survey, 926A National Center, Reston, VA 20192, (3)Department of Geology, College of William & Mary, Williamsburg, VA 23187

In 2016, three trenches were excavated across two prominent NE-trending Paleozoic faults, the Lakeside Fault (LF) and the Little Fork Church Fault (LFCF), to identify potential evidence of Cenozoic fault reactivation in the Central Virginia Seismic Zone (CVSZ). This work was a collaborative effort between the Virginia Department of Mines, Minerals, and Energy and the U.S. Geological Survey, funded by a grant from the National Earthquake Hazard Reduction Program. The area of investigation is located near the center of the CVSZ, 30 km southwest of the 2011 5.8 Mw epicenter, and near the location of several moderately-sized historic earthquakes (>1.0-4.5 Mag). The LF and LFCF also show evidence of Mesozoic brittle reactivation as normal faults. Ground penetrating radar was used to identify potential discontinuities in upper and lower fluvial terrace sediment of the James River (JR) and select trench sites. Trenching did not show any observable evidence of paleoseismic activity or modern faulting along either fault. However, sediment was collected to constrain the age of the terraces using optically simulated luminescence (OSL), terrestrial cosmogenic nuclides (TCN), and radiocarbon. The geochronology adds to our understanding of Quaternary terrace development and incision history along the JR. The oldest, and topographically highest terrace material was age-dated to 935,000 ± 10% using TCN; results are consistent with terraces upstream and at similar elevations along the JR. Age-dates from the other two trench sites on lower elevation terraces of the JR could generally be correlated by depth, and ranged from 920 ± 20 years (radiocarbon) near the earth’s surface to 7710 ± 660 years (OSL) at <3 meters depth. Native American pottery sherds found in one of the trenches also helped constrain the dates. The age dates, weak horizonation and steady accumulation of sediment in the soil layers, in addition to evidence of long-term human occupation along the lower terraces, suggest a period of seismic quiescence in the Holocene in the study area. This contrasts with evidence of neotectonic uplift and paleoseismicity that has been found in some terraces in the 2011 epicentral area. Future studies should be completed to further constrain the rate of neotectonic deformation and uplift across the CVSZ, outside of the 2011 epicentral area.