CHARACTERISTICS OF PALEOLIQUEFACTION FEATURES IN THE CENTRAL VIRGINIA SEISMIC ZONE (CVSZ)

Paleoliquefaction features, including sand dikes, sills, and soft-sediment deformation features are more widespread than previously thought in the meizoseismal area of the 2011 Mw 5.8 Mineral, Virginia, earthquake. In June 2015, nineteen liquefaction sites were documented in river cut banks across the CVSZ. Several of these sites were found during a 2011 post-earthquake survey but the origin of the liquefaction features was uncertain due to high river levels and limited exposure of only the bioturbated upper 10-40 cm of the sand dikes. When river banks were better exposed, it was possible to see that sandy features high in the cut banks were sand dikes with vertical and lateral continuity lower in the cut banks. Several sand dikes were traced downward to their source beds.

Characteristics of two sand dikes at Horseshoe Farm (HF) near the 2011 epicenter are typical of many of the other sand dikes in the CVSZ. These two dikes, exposed in the cut bank of the South Anna River, are 4 cm wide, filled with fine sand, and extend ~ 1 m above their source bed of unconsolidated fine- to medium sand through iron oxide-stained silty loam. One dike pinches out near the top of the silty loam, and the other is truncated by younger sediment near the top of the exposure. Both dikes utilized pre-existing polygonal cracks and root casts within the silty loam. The HF site occurs in the upland Piedmont where flood plains are narrow and liquefiable sediments are limited in distribution. Therefore, paleoliquefaction features are sparse and difficult to find in this setting.

Dating of sediment in which the paleoliquefaction features formed has been limited to a few sites, but the results suggest that most, if not all, of the features formed during the Holocene. An OSL age of ~8.7 ka for host sediment was obtained from an exposure 25 km southeast and downstream of the HF site, in contrast to an OSL age of ~9.0 ka for source sand at a site on the Rivanna River 26 km west of HF. Two ¹⁴C samples from alluvium along the South Anna and Pamunkey Rivers yielded mid-Holocene radiocarbon ages (calibrated BP 5595 to 4290). Other samples have yielded historical radiocarbon ages, possibly due to bioturbation at those sites.