Southeastern Section - 64th Annual Meeting (19–20 March 2015)

Paper No. 13
Presentation Time: 1:00 PM-5:00 PM


COUNTS, Ronald, U.S. Geological Survey, MS926A, National Center, Reston, VA 20192, CARTER, Mark W., U.S. Geological Survey, MS 926A, National Center, Reston, VA 20192 and HARRISON, Richard W., U.S. Geological Survey, MS. 926A, National Center, Reston, VA 20192,

A new trench was opened in a hillside at Roundabout Farm, near the confluence of Harris Creek and the South Anna River, in the epicentral area of the 2011 Mineral, VA earthquake. It reveals a history of orgenesis, neotectonic deformation, and landscape evolution from the Paleozoic through the Quaternary. Saprolitic rocks exposed in the trench include ~470 Ma metasedimentary schist and gneiss of the Chopawamsic Formation, ~440 Ma Ellisville granodiorite, and younger aplite intrusions and quartz-tourmaline veins of unknown age. Compositional layering in Chopawamsic rocks is tightly to isoclinally folded; foliation axial-planar to these folds is likely Paleozoic. Ellisville plutonic rocks contain xenoliths of the Chopawamsic Formation. Contacts between the Chopawamsic and Ellisville are both intrusive and faulted. Intrusive aplite cuts a contact between Chopawamsic and Ellisville, contains xenoliths of Ellisville, and is cut by a younger quartz-tourmaline vein. All bedrock units in the trench are cut by numerous long, clay-filled, planar fractures and by brittle faults in various orientations. These faults offset the Paleozoic units by a few cm to several dm. Many brittle fault surfaces are coated by manganese-oxides, some of which preserve slickenlines.

Saprolite in the trench is overlain by a thin (< 1 meter) red colluvial gravel deposit, and the contact between the two units is abrupt and planar. Gravels exposed in the lower end of the new trench may correlate with 23-28 Ka gravels exposed in trenches opened in 2012. At the lower end of the new trench, a meter-wide graben, bounded by mm- to cm-thick seams of clay marking brittle faults, offsets layering in the Chopawamsic Formation, and preserves a wedge of alluvial sand and gravel in its core. A sample of the gravel did not contain sufficient sand for OSL dating. Preliminary GPR transects were done along hilltops and hillsides in the vicinity of the trenches, but clay in the gravels prevented imaging depths below ~1m.

The absence of residuum, the abrupt contact with colluvium, and lack of any red translocated clay in the saprolite exposed in the new trench suggest the landscape is young and dynamic. These observations are consistent with observations and OSL ages from other trenches in the epicentral region.