Dominantly dark gray to grayish-black, lustrous, very-fine-grained, graphitic slate (northeastern sector); and, medium-grained, porphyroblastic garnetiferous biotite schist (southwestern sector). Discontinuous beds of quartzose muscovite schist, coarse grained to pebbly micaceous quartzite, and conglomeratic schist occur along the margins of the outcrop belt, stratigraphically at the base of the section. Interbeds of dacite metatuff occur in the western portion of the slate outcrop belt. Graded laminated metasiltstone and metasandstone are interbedded with slate in the central and eastern portions of the outcrop belt at the latitude of the James River (Evans and Marr, 1988); these rocks pass into porphyroblastic schists at higher metamorphic grades to the southwest. A distinctive garnet-amphibole-quartz interbed occurs within porphyroblastic schist south of the James River (volcanogenic marker?; Brown, 1969); north of the river, this passes down metamoprphic grade into what is described as an oolitic chlorite schist (Smith and others, 1964). Mineralogy: (slate) chlorite + mus co vite + plagioclase + quartz + magnetite ± biotite ± calcite ± graphite ± pyrite; (porphyroblastic schist) biotite + muscovite + garnet + quartz + plagioclase + magnetite ± kyanite ± calcite; tourmaline and zircon are common accessories. Geophysical signature: The Arvonia is marked by positive magnetic and radiometric anomalies. Originally referred to as the slate in the Arvonia belt by Rogers (1884), the unit was named Arvonia slate by Stose and Stose (1948), and raised to formation status by Brown (1969). An Upper Ordovician age for the Arvonia has been established from fossils collected by Watson and Powell (1911), Stose and Stose (1948), Tillman (1970), and Kolata and Pavlides (1986). The Arvonia has long been considered unconformable on top of adjacent units. Micaceous quartzite, pebbly mus covite schist, and conglomeratic schist are common at the base of the section where that boundary is not faulted (Stose and Stose, 1948; Smith and others, 1964; Brown, 1969; Evans and Marr, 1988). The base of the Arvonia is exposed in an old railroad cut near Carysbrook, Fluvanna County (Smith and others, 1964); there, a micaceous quartzite containing quartz pebbles rests on granite of the Carysbrook pluton. The Stoses (1948) considered the Arvonia a sequence deposited on a post-Taconic-orogeny regional unconformity, and folded and metamorphosed during subsequent orogenies; that interpretation is consistent with geologic constraints as we know them today. The Arvonia is correlated with the Quantico Formation. History of the Arvonia district slate industry is discussed by Brown (1969) and Evans and Marr (1988).
Virginia Division of Mineral Resources, 1993, Geologic Map of Virginia: Virginia Division of Mineral Resources, scale 1:500,000.
Virginia Division of Mineral Resources, 2003, Digital Representation of the 1993 Geologic Map of Virginia, Publication 174, CD ROM (ISO-9660) contains image file, expanded explanation in pdf, and ESRI shapefiles, scale 1:500,000.