Geologic units in Danville City, Virginia

Interlayered Mafic and Felsic Metavolcanic Rocks - Amphibolite, hornblende-biotite gneiss, and schist. (Cambrian) at surface, covers 45 % of this area

Amphibolite, hornblende-biotite gneiss, and schist.; Heterogenous layered assemblage correlates with the Chopawamsic Formation and Ta River Metamorphic Suite, on strike to the northeast, and in traceable into the Milton belt in North Carolina (Geologic Map of North Carolina, 1985). Amphibolite, hornblende-biotite gneiss and schist. Black to moderate-olive-brown, medium- to coarse-grained, lineated and foliated; light-greenish-gray quartz-epidote stringers are common. Mineralogy: hornblende + tremolite-actinolite + oligoclase + biotite + epidote + garnet. Includes Blackwater Creek Gneiss and Catawba Creek amphibolite member of Hyco Formation of Baird (1989), hornblende gneiss of LeGrand (1960), gneiss unit of Kreisa (1980), and dominantly mafic-composition units mapped by Nelson (1992). Amphibolite is interlayered with biotite gneiss, as discussed above.

Interlayered Mafic and Felsic Metavolcanic Rocks - Foliated felsite (Cambrian) at surface, covers 38 % of this area

Foliated felsite; Heterogenous layered assemblage correlates with the Chopawamsic Formation and Ta River Metamorphic Suite, on strike to the northeast, and in traceable into the Milton belt in North Carolina (Geologic Map of North Carolina, 1985). Foliated felsite. Grayish-orange-pink to white, fine- to medium-grained, foliated to granular metavolcanic rocks range in composition from rhyolite to dacite. Includes muscovite feldspar- quartz schist, gneiss and granofels; massive crystal metatuff; welded ashflow tuff; and, inequigranular metavolcanic breccia. Relict primary volcanic textures are recognizable where metamorphic grade is low (Henika, 1975; 1977). This unit includes felsic gneiss with less common mafic and rare calcareous gneiss mapped by Tobisch (1972), in part the metamorphosed volcanic sequence of Gates (1981), and dominantly felsic-composition units mapped by Nelson (1992). The unit contains numerous granitic dikes, sills, and lit-par-lit injections where it occurs in close proximity to Shelton Formation (Ost). Felsites occur interlayered with amphibolite, amphibole gneiss and schist (Cmv), quartzofeldspathic biotite gneiss (Cbg), sillimanite-quartz-muscovite schist and gneiss (Csg), and ferruginous quartzite (Cfq).

Shelton Formation (Ordovician) at surface, covers 9 % of this area

Pink to gray, coarse-grained, massive, strongly-lineated gneiss ranging in composition from granite to quartz monzonite (Henika, 1977), composed of microcline and perthite augen enclosed by biotite in a quartz-plagioclase matrix. A characteristic linear fabric is produced by rod-like feldspar porphyroblasts and crystalline aggregates. Thin veins of purple fluorite are common. Mineralogy: quartz + potassium feldspar + plagioclase + biotite + muscovite ± pyrite ± fluorite. Geophysical signature: pronounced positive radiometric signature, flat magnetic signature. Originally named Shelton granite (Jonas, 1928), the unit was renamed Shelton Formation (Henika, 1977). Simple linear fabric was cited as evidence for an intrusive origin by Henika (1980). The Shelton has been dated at 424±7 Ma (Rb-Sr whole-rock; Kish, 1983) and 463 Ma (U-Pb; Hund, 1987).

Interlayered Mafic and Felsic Metavolcanic Rocks - Quartz-muscovite schist and gneiss. (Cambrian) at surface, covers 6 % of this area

Quartz-muscovite schist and gneiss; Heterogenous layered assemblage correlates with the Chopawamsic Formation and Ta River Metamorphic Suite, on strike to the northeast, and in traceable into the Milton belt in North Carolina (Geologic Map of North Carolina, 1985). Quartz muscovite schist and gneiss. Very-light-gray to light-bluish-gray, fine- to medium-grained, layered kyanite mica schist, kyanite and sillimanite quartzite, and interlayered biotite-garnet schist. Mineralogy: quartz + muscovite + plagioclase ± biotite ± garnet ± sillimanite ± kyanite ± magnetite. Includes the schist and gneiss unit of Tobisch (1972), and muscovite-quartz schist of Baird (1989, 1991).

Newark Supergroup; Conglomerate, mixed clasts (Upper Triassic) at surface, covers 1 % of this area

Rounded to subangular pebbles, cobbles, and boulders of mixed lithologies including quartz, phyllite, quartzite, gneiss, schist, greenstone, and marble in a matrix of medium- to very-coarse-grained, reddish-brown to gray, locally arkosic, sandstone.

Newark Supergroup; Sandstone, undifferentiated (Upper Triassic) at surface, covers 1 % of this area

Fine- to coarse-grained, reddish-brown to gray, primary bedding features such as cross-beds, channel lags, and ripple marks , minor conglomerate, siltstone, and shale beds.

Biotite Gneiss and Schist (Cambrian/Late Proterozoic) at surface, covers 0.4 % of this area

Inequigranular and megacrystic; abundant potassic feldspar and garnet; interlayered and gradational with calc-silicate rock, sillimanite-mica schist, mica schist, and amphibolite. Contains small masses of granitic rock.

Shelton Granite Gneiss (Silurian) at surface, covers 0.2 % of this area

Poorly foliated; lineated granitic to quartz monzonitic gneiss.

Mylonite, Mylonite Gneiss, and Cataclastic Rocks (Proterozoic - Paleozoic ?) at surface, covers < 0.1 % 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).