Geologic map of the Ferncliff and Louisa quadrangles, Louisa, Fluvanna, and Goochland Counties, 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 that was responsible for the 2011 magnitude 5.8 (M5.8) Mineral, Va., earthquake. The mapping shows that the Quail fault appears to have reactivated the Harris Creek fault, a Paleozoic fault that has been mapped and named in the study area and marks the boundary between the Ellisville pluton neck and Chopawamsic Formation. The Harris Creek fault was also reactivated in the early Mesozoic. Another result of the mapping is a well-defined, southwest to northeast, narrow zone of metagraywacke and ultramafic rocks (both part of the informal Shores complex) that marks the closure of a small ocean basin and the accretion of the 468- to 460-Ma (mega-annum) Ordovician Chopawamsic volcanic arc (part of the Carolina terrane) onto Laurentia. The accretion zone is truncated by the Ordovician-Silurian (444 Ma) Ellisville pluton; the 444-Ma age of the pluton therefore represents the minimum age of the accretion zone and indicates likely closure of the ocean basin 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 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.

This report consists of a single geologic map sheet and an online geographic information systems database that includes bedrock geologic unit contacts and polygons, surficial geologic polygons, faults, and structural geologic information.