Geologic units in Cumberland county, Virginia

Migmatitic Paragneiss (Proterozoic) at surface, covers 31 % 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.

Porphyroblastic Biotite Gneiss (Proterozoic) at surface, covers 25 % 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.

Interlayered Mafic and Felsic Metavolcanic Rocks (Cambrian) at surface, covers 17 % of this area

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). Quartzofeldspathic biotite gneiss. Heterogeneous layered sequence consists of salt-and-pepper and segregation layered biotite granite gneiss interlayered with biotite schist; dark-gray to black, fine- to coarse-grained, thin- to thickly-laminated hornblende gneiss and schist; lesser quartz-muscovite schist; and, locally, gray to green, medium-grained, calcareous gneiss and calc-silicate granofels (Tobish and Glover, 1969). This unit includes the upper and lower felsic gneiss units and intermediate volcanic rocks in the Hyco Formation as used by Baird (1989, 1991); and biotite gneiss and interlayered gneiss of Kreisa (1980), correlative with the biotite gneiss unit of Marr (1980a; 1980b). Mineralogy: (quartzofeldspathic rocks), (1) quartz + albite + potassium feldspar + muscovite + chlorite + actinolite + epidote + calcite + magnetite + zircon; (2) quartz + oligoclase + muscovite + biotite + garnet + hornblende + magnetite + epidote + rutile + calcite + zircon; (mafic rocks), (1) quartz + albite + chlorite + epidote + actinolite + titanite + magnetite ilmenite. (2) quartz + oligoclase + andesine + hornblende + microcline + biotite + garnet + cordierite + magnetite + rutile + titanite + scapolite; (pelitic rocks), (1) quartz + albite + muscovite + chlorite + epidote + magnetite-ilmenite; (2) quartz + muscovite + biotite + kyanite + oligoclase + potassium feldspar + epidote + magnetite-ilmenite + garnet; (3) quartz + muscovite + sillimanite + magnetite-ilmenite; (calcareous rocks), (1) quartz + calcite + biotite + epidote + chlorite + tremolite + ilmenite; (2) calcite+ quartz + epidote + hornblende + pyroxene + scapolite. Geophysical signature: felsic rocks are delineated by strike-elongate positive radiometric anomalies (Henika and Johnson, 1980); mafic metavolcanic rocks and metasedimentary units are characterized by closed strike-elongate radiometric lows and closed strike-elongate aeromagnetic highs.

Newark Supergroup; Triassic Sandstone, Siltstone, and Shale (Upper Triassic) at surface, covers 8 % 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.

Columbia Pluton (Ordovician) at surface, covers 6 % of this area

Light-gray, medium- to coarse grained, foliated. Includes biotite-muscovite granite, granodiorite, tonalite, and granitic pegmatite; contains xenoliths of biotite gneiss, amphibolite, and felsic metavolcanic rocks. Mineralogy: plagioclase + quartz + microcline; common accessories include biotite, muscovite, epidote, zircon, apatite, garnet, magnetite, and pyrite (Bourland and Glover, 1979). Geophysical signature: diffuse magnetic lows and radiometric highs. The pluton was originally named Columbia Granite by Jonas (1928); this name was objected to by later workers because of the relatively small percentage of true granite present. The pluton includes the southeastern portion of the granodiorite unit of Smith and others (1964). Granitic rocks in the Carysbrook area of Fluvanna County are here considered part of a separate Carysbrook pluton, following the usage of Stose and Stose (1948). The Columbia includes, in part, the Hatcher complex of Brown (1969). Bourland and Glover (1979) refer to the pluton as the Columbia metagranite. Given the heterogeneous nature of the pluton, multiple intrusive phases are likely present. Tonalite in the eastern part of the pluton has yielded ages of 590+/-80 Ma, (Rb-Sr whole-rock; Fullagar, 1971). Mose and Nagel (1982) report a Rb-Sr whole-rock age for the Columbia of 454±9 Ma. Because samples for this age are described as coming from the western portion of the Columbia, it is possible that the rocks dated were taken from what is herein mapped as the Carysbrook pluton (grc).

Polygenetic Melange (Proterozoic) at surface, covers 3 % of this area

Pebbles, cobbles, and boulders of granite, gabbro, amphibolite, biotite gneiss, and quartzite in a poly-deformed, mylonitic, schistose metagraywacke matrix. Includes interbedded garnetiferous meta-argillite, feldspathic metagraywacke conglomerate, and meta-arkose. Mineralogy (matrix): quartz + plagioclase + biotite + epidote + apatite + titanite + muscovite ± garnet ± allanite. This unit was named the Ca Ira melange by Marr (1991).

Mylonite, Mylonite Gneiss, and Cataclastic Rocks (Proterozoic - Paleozoic ?) at surface, covers 2 % 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).

Interlayered Mafic and Felsic Metavolcanic Rocks - Amphibolite, hornblende-biotite gneiss, and schist. (Cambrian) at surface, covers 2 % 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.

Newark Supergroup; Triassic Sandstone, Siltstone, Shale, and Coal (Triassic) at surface, covers 1 % 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.

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

Angular to subangular pebbles, cobbles, and boulders of mixed lithologies in a reddish-brown matrix of indurated medium- to coarse-grained sandstone.

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

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

Newark Supergroup; Conglomerate, arkosic maatrix (Upper Triassic) at surface, covers 0.9 % of this area

Round to subround pebbles, cobbles, and boulders of mixed lithologies in a matrix of medium- to very-coarse-grained arkosic sandstone.

Carysbrook Pluton (Proterozoic) at surface, covers 0.5 % of this area

Carysbrook pluton. Light-gray, medium- to coarse grained, massive to indistinctly foliated biotite granite. Mineralogy: quartz + potassium feldspar + plagioclase + biotite + chlorite + muscovite + epidote. Geophysical signature: diffuse pattern of elevated radiometric anomalies. Although Smith and others (1964) included the Carysbrook pluton in a granodiorite unit with granitoid rocks in the vicinity of Columbia, Stose and Stose (1948) recognized that the granite at Carysbrook was different in texture and composition from the granodiorite at Columbia. Our mapping affirms that these are separate plutons. The Carysbrook is unconformably overlain by the Arvonia Formation; this relation is well-exposed in an abandoned railroad cut south of Carysbrook (Smith and others, 1964). The pluton intrudes the Chopawamsic Formation.

Biotite Granite (Proterozoic) at surface, covers 0.2 % of this area

Gray to white, medium-grained, massive to layered granite and granite gneiss range in composition from granite to quartz diorite. Quartz diorite dikes and sills are locally abundant. Mineralogy: quartz + potassium feldspar + plagioclase + biotite + epidote + muscovite ± clinopyroxene + hornblende. This unit intrudes biotite gneiss unit (Cbg).

Biotite Granite Gneiss (Proterozoic) at surface, covers 0.2 % of this area

Light-gray, medium-grained, equigranular, broadly-layered, locally migmatitic. Mineralogy: quartz + plagioclase + microcline + biotite + muscovite + hornblende; apatite and zircon are accessory minerals. Geophysical signature: diffuse pattern of positive radiometric anomalies.