Geologic units in Powhatan county, Virginia

Porphyroblastic Garnet-Biotite Gneiss (Proterozoic Y) at surface, covers 45 % of this area

Heterogeneous layered sequence is dominantly garnetiferous biotite gneiss and porphyroblastic gneiss, migmatitic in part, with subordinate interlayered amphibolite and amphibole gneiss (Ya), pelitic-composition gneiss, calcsilicate gneiss, biotite hornblende-quartz-plagioclase gneiss, and garnetiferous leucogneiss. These lithologies contain amphibolite-facies metamorphic mineral assemblages consistent with rock chemistry. Farrar (1984) reports relict granulite-facies assemblages in some rocks. This unit underlies a wide area that surrounds the State Farm antiform (Poland, 1976; Reilly, 1980; Farrar, 1984) and two subsidiary antiforms to the northeast; the unit includes the Maidens gneiss and portions of the Sabot amphibolite of Poland (1976), the eastern gneiss complex and Boscobel granodiorite gneiss of Bobyarchick (1976), and the Po River Metamorphic Suite of Pavlides (1980). Poland (1976) and Reilly (1980) proposed that the Maidens gneiss and Sabot amphibolite were a Late Precambrian- to Early Paleozoic-age volcanic-sedimentary cover sequence unconformably overlying the State Farm gneiss. Farrar (1984) interpreted relict granulite-facies mineral assemblages to have equilibrated during Grenville-age regional metamorphism; this contributed to his conclusion that the Sabot and Maidens, in addition to the State Farm, are Grenville or pre-Grenville in age. Porphyroblastic garnet-biotite gneiss (Ymd) is intruded by rocks of the Carboniferous-age Falmouth Intrusive Suite (Pavlides, 1980).

Porphyroblastic Biotite Gneiss (Proterozoic) at surface, covers 18 % of this area

Light-gray, medium grained, segregation-layered gneiss, contains prominent potassium feldspar porphyroblasts. Mineralogy: quartz + biotite + plagioclase + potassium feldspar + muscovite ± hornblende; accessory minerals include epidote, apatite, and opaque minerals.

Migmatitic Paragneiss (Proterozoic) at surface, covers 12 % of this area

Leucocratic to mesocratic, medium- to coarse-grained layered gneiss contains interlayered biotite-rich and quartzofeldspathic zones, locally migmatitic; includes lesser amounts of biotite schist, muscovite schist, and thin lenticular amphibolite bodies. Mineralogy: biotite + muscovite + plagioclase + potassium feldspar + garnet ± hornblende.

State Farm Gneiss (Proterozoic Y) at surface, covers 10 % of this area

State Farm Gneiss (Brown, 1937). Leucocratic to mesocratic, medium- to coarse-grained, massive to moderately layered, locally migmatitic gneiss ranges in composition from granodiorite to tonalite. Quartzofeldspathic segregations and discordant pegmatites are common; leucocratic biotite-garnet quartz-plagioclase gneiss, metagabbro, and thin pelitic schist interlayers are minor constituents (Poland, 1976; Reilly, 1980; Farrar, 1984). Mineralogy: quartz + plagioclase + microcline + biotite + garnet + hornblende ± clinopyroxene; titanite is a ubiquitous accessory; other accessory minerals include magnetite and zircon. Farrar (1984) reports that the least deformed portions of the State Farm contain relict granulite-facies mineral assemblages. Geophysical signature: characterized by negative magnetic and positive radiometric anomalies. The age of the State Farm is reported as 1031±94 Ma (Rb- Sr whole-rock; Glover and others, 1982). The titanium-rich chemistry of the State Farm, as indicated by the abundance of titanite, is also characteristic of Grenville-age gneisses in the Virginia Blue Ridge.

Newark Supergroup; Triassic Sandstone, Siltstone, and Shale (Upper Triassic) at surface, covers 7 % of this area

Sandstone, very fine- to coarse-grained, reddish-brown to gray, micaceous, minor conglomerate beds. Siltstone, reddish-brown to gray, micaceous. Shale, reddish-brown, greenish-gray, gray, yellowish-brown, laminated, fossiliferous. Upward-fining sequences, discontinuous vertically and horizontally.

Mylonite, Mylonite Gneiss, and Cataclastic Rocks (Proterozoic - Paleozoic ?) at surface, covers 4 % of this area

Mylonite. Includes protomylonite, mylonite, ultramylonite, and cataclastic rocks. Lithology highly variable, depending on the nature of the parent rock, and on intensive parameters and history of deformation. In most mapped belts of mylonite and cataclastic rock (my), tectonized rocks anastomose around lenses of less-deformed or undeformed rock. In the Blue Ridge, some of these lenses are large enough to show at 1:500,000 scale. In many places mylonitic and cataclastic rocks are gradational into less deformed or undeformed adjacent rocks, and location of contacts between tectonized rocks (my) and adjacent units is approximate or arbitrary. These boundaries are indicated on the map by color-color joins with superimposed shear pattern. Most mapped belts of mylonite represent fault zones with multiple movement histories. In the Blue Ridge, Paleozoic age contractional deformation fabrics are superimposed on Late Precambrian extensional fabrics (Simpson and Kalaghan, 1989; Bailey and Simpson, 1993). Many Piedmont mylonite zones contain dextral-transpressional kinematic indicators that formed during Late Paleozoic collision al tectonics (Bobyarchick and Glover, 1979; Gates and others, 1986). Paleozoic and older faults were reactivated in many places to form extensional faults during the Mesozoic (Bobyarchick and Glover, 1979).

Fine Creek Mills Granite (Proterozoic) at surface, covers 1 % of this area

Fine Creek Mills granite (Poland, 1976; Reilly, 1980). Light-gray, medium- to coarse-grained, homogenous, foliated, flow-banded granite. Mineralogy: microcline + plagioclase + quartz + biotite; accessory minerals include apatite, garnet, fluorite, and zircon. The Fine Creek Mills intrudes the State Farm Gneiss (Ysf).

Newark Supergroup; Triassic Sandstone, Siltstone, Shale, and Coal (Triassic) at surface, covers 0.9 % of this area

Sandstone, fine-to coarse-grained, reddish-brown to gray, arkosic in places, micaceous, displays channel-type primary features. Siltstone light- to dark-gray, micaceous. Shale, light- to dark-gray, carbonaceous, micaceous, fossiliferous. Coal, bituminous, banded, moderate- to well-developed, fine- to medium-cleat, partings and inclusions of shale, siltstone, and sandstone; high methane concentrations recorded in the Richmond and Taylorsville basins. This lithologic unit occurs in the Richmond, Taylorsville, Farmville, Briery Creek, and Danville basins.

Granite Gneiss (Proterozoic Y-Pennsylvanian) at surface, covers 0.8 % of this area

Granite gneiss (Pavlides, 1990). Fine- to medium grained, light-gray to white granite to tonalite gneiss; composed of biotite, oligoclase, quartz, and porphyroblastic microcline, with accessory muscovite, epidote, titanite, and magnetite; hornblende occurs locally within diffuse compositional layering. Inclusions of biotite gneiss and amphibolite are present locally. Unit occurs as irregular lenticular to tabular masses within porphyroblastic biotite gneiss (Ymd).

Flat Rock Granite (Proterozoic) at surface, covers 0.4 % of this area

Flat Rock granite (Reilly, 1980). Light-gray, medium to coarse-grained, homogenous, foliated granite. Mineralogy: microcline + plagioclase + quartz + biotite; accessory minerals include garnet, apatite, fluorite, and zircon. The Flat Rock intrudes the State Farm Gneiss (Ysf).

Amphibolite, Amphibole Gneiss, and Schist (Proterozoic Y) at surface, covers 0.3 % of this area

Melanocratic, fine- to coarse-grained, weakly to strongly foliated, irregularly layered amphibole-rich gneiss and schist. Mineralogy: hornblende + clinopyroxene + plagioclase + magnetite + biotite ± scapolite ± garnet ± quartz ± epidote. Geophysical signature: narrow, strike-elongate, positive magnetic anomaly. Lenses and layers of amphibolite and amphibole gneiss are interlayered with porphyroblastic garnet-biotite gneiss (Ymd). The mafic rocks constitute 50 percent or more of the section in a zone about 0.62 mile wide surrounding outcrop areas of State Farm gneiss (Ysf); farther away from the State Farm contact, lenses and layers of amphibolite and amphibole gneiss are more widely scattered, but are laterally persistent and outline map-scale structures (Marr, 1985). Amphibolite and interlayered biotite gneiss adjacent to the State Farm gneiss were named the Sabot amphibolite by Poland (1976), who characterized the formation as a tabular sheet 0.7 to 1.0 km thick. He and Goodwin (1970) interpreted these amphibolites as metamorphosed mafic volcanic or pyroclastic rocks. Glover and others (1989 and references therein) report a low-angle regional discordance between the base of the Sabot and the compositional layering in the underlying State Farm Gneiss.

Newark Supergroup; Arkosic Sandstone (Upper Triassic) at surface, covers < 0.1 % of this area

Light-gray to light-reddish-brown, medium- to coarse-grained, micaceous.