PROGRESS OF USGS BEDROCK GEOLOGIC MAPPING IN THE CENTRAL VIRGINIA SEISMIC ZONE

Detailed geologic mapping in the Central Virginia seismic zone following the August 23, 2011 M5.8 earthquake continues in the Mineral and Lake Anna West 7.5-minute quadrangles. Bedrock comprises, from W to E: (1) Silurian Ellisville Granodiorite; (2) Middle Ordovician Chopawamsic Fm. volcanic arc rocks; (3) a synclinorium of late Ordovician to Devonian (?) Quantico Fm. successor basin rocks; (4) amphibolite assigned to the Ta River Metamorphic Suite that correlates with the Chopawamsic Fm.; and (5) metagranitoids intruding the Ta River amphibolites. ⁴⁰Ar/³⁹Ar analysis of equivalent rocks outside the field area shows early to middle Paleozoic (Taconic orogeny) metamorphic mineral assemblages NW of the Ellisville Granodiorite grading to assemblages that have been thoroughly recrystallized in the late Paleozoic (Alleghanian orogeny) to the SE of the pluton.

From NW to SE, notable features include: (1) an igneous contact between the Ellisville and Chopawamsic Fm. rocks has been overprinted by late Paleozoic ductile deformation and potential Mesozoic to Cenozoic brittle faulting; (2) Chopawamsic Fm. rocks consist mostly of felsic and intermediate volcanic-arc rocks; massive sulfide mines of the Mineral District occur along the contact between felsic gneiss and a distinctive through-going mafic volcanic marker unit; (3) NE–SW trending Paleozoic ductile faults (e.g., Long Branch fault) result in repetition of the Chopawamsic Fm. and NW–SE trending brittle faults offset both Chopawamsic Fm. stratigraphy and ductile faults; (4) an angular unconformity between the Chopawamsic Fm. and overlying Quantico Fm. is marked by an aluminous, garnet-rich horizon interpreted to be a meta-paleosol; a newly dated felsic volcanic gneiss at this contact (~448 Ma U-Pb igneous zircon) is overlain by discontinuous Quantico Fm. orthoquartzite (<427 Ma U-Pb detrital zircon), which is overlain by typical garnet-graphite-mica schist; (5) Ta River Metamorphic Suite amphibolite SE of the Quantico exhibits strong deformational features that we currently attribute to the Alleghanian orogeny based on ~300 Ma ⁴⁰Ar/³⁹Ar amphibole ages; and (6) well-foliated hornblende ± biotite metagranite and granitic gneiss, dated at ~441–450 Ma (U-Pb igneous zircon), intrudes Ta River rocks and underlies the North Anna Nuclear Power Station.