GEOLOGIC MAP OF THE FERNCLIFF AND LOUISA 7.5’ QUADRANGLES, CENTRAL VIRGINIA

The area encompassed by the geologic map of the Ferncliff and Louisa, Va., 7.5-minute quadrangles includes the hypothetical surface projection of the Quail fault, which is the subsurface fault responsible for the 2011 M5.8 Mineral, Va., earthquake. The mapping shows that the Quail fault appears to have reactivated the Harris Creek fault, a SE-dipping Paleozoic fault that has been mapped and named in the study area and marks the boundary between the Ordovician-Silurian Ellisville pluton and Ordovician Chopawamsic Formation. The Harris Creek fault was also possibly reactivated in the early Mesozoic as a normal fault, as evidenced by slight offsets on subvertical Jurassic diabase dikes and the local occurrence of quartz-filled breccia.

The map also delineates a southwest to northeast-trending, narrow zone of metagraywacke and ultramafic rocks interpreted to mark the closure of a small ocean basin during the accretion onto Laurentia of the Ordovician Chopawamsic volcanic arc, which has been locally dated at 468- to 460-Ma by U-Pb TIMS and SHRIMP zircon crystallization ages. The accretion zone is intruded by the 444 Ma (U-Pb TIMS) Ellisville pluton; the 444-Ma age of the pluton therefore constrains the minimum age of the accretionary zone and indicates likely closure of the ocean basin and accretion of the Chopawamsic volcanic arc during the Taconic orogeny. Across the map area, the metamorphic grade ranges from lower greenschist facies in the northwest to amphibolite facies in the southeast. ⁴⁰Ar/³⁹Ar age-dating across this metamorphic gradient indicates that Taconic metamorphism was overprinted by late-Paleozoic Alleghanian metamorphism that was accompanied by refolding and faulting of Taconic structures. Quaternary gravel terraces mapped along the South Anna River record a long history of incision and downcutting. A continuing question is how much of this downcutting was a result of neotectonic uplift in the central Virginia seismic zone.