Geologic units in Amelia county, Virginia

Porphyroblastic Garnet-Biotite Gneiss (Proterozoic Y) at surface, covers 42 % 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).

Migmatitic Paragneiss (Proterozoic) at surface, covers 22 % 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.

Mylonite, Mylonite Gneiss, and Cataclastic Rocks (Proterozoic - Paleozoic ?) at surface, covers 14 % 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).

Mafic and Felsic Volcanic Rocks (Proterozoic) at surface, covers 5 % of this area

Heterogeneous layered metavolcanic sequence includes crystal and lithic tuff, dacite porphyry, chert, phyllite, and greenstone metabasalt; greenschist-facies metamorphic mineral assemblages occur in the various lithologies. This unit correlates with the Roanoke Rapids volcanogenic complex of the eastern slate belt in North Carolina (Farrar, 1985a, 1985b; Geologic Map of North Carolina, 1985; Horton and Stoddard, 1986). To the extent that correlation with lithologically similar Carolina slate belt rocks is valid, mafic and felsic metavolcanic rocks (v) are Late Proterozoic to Cambrian in age.

Gneissic Granite and Granodiorite (Proterozoic) at surface, covers 4 % of this area

Light-gray to white, fine- to medium-grained, massive to foliated, muscovite-biotite gneissic granite to granodiorite containing minor garnet, and xenoliths of biotite gneiss and amphibolite. Several different intrusive phases are present.

Porphyroblastic Biotite Gneiss (Proterozoic) at surface, covers 3 % 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.

Biotite Granite Gneiss (Proterozoic) at surface, covers 3 % 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.

Newark Supergroup; Conglomerate, mixed clasts (Upper Triassic) at surface, covers 2 % 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.

Biotite Gneiss (Proterozoic) at surface, covers 1 % of this area

Light-gray, medium- to coarse grained, compositionally-layered and locally migmatitic rocks, include interlayered biotite gneiss, muscovite-biotite gneiss, muscovite-biotite schist, and sillimanite-mica schist; also includes minor interlayers and lenses of granitic gneiss, biotite-amphibole gneiss, amphibolite, garnet-mica schist, calc-silicate granofels, and rare ultramafic rocks. This unit correlates with Raleigh belt rocks in North Carolina (Parker, 1979; Geologic Map of North Carolina, 1985).

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

Burkeville Pluton (Proterozoic) at surface, covers 1 % of this area

Grayish-blue, fine- to medium grained, massive to faintly foliated; composition ranges from granodiorite to monzonite. Mineralogy: oligoclase + microcline + quartz + biotite + epidote + apatite + zircon. Geophysical signature: diffuse pattern of negative magnetic and circular positive radiometric anomalies. The pluton was originally referred to as the Burkeville granite (Husted, 1942), and is the "granite in the Burkeville granite quarry" of Steidtmann (1945); the pluton intrudes migmatitic paragneiss (mpg).

Granite Gneiss (Proterozoic Y-Pennsylvanian) at surface, covers 0.5 % 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).

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

Light-gray to silvery gray, medium-grained, strongly-foliated quartz-muscovite schist; occurs as discontinuous lenses within porphyroblastic garnet-biotite gneiss (Ymd).

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

Dark-gray to black, medium-grained, strongly foliated and lineated. Mineralogy: hornblende + plagioclase + biotite + quartz + epidote; apatite, titanite, and magnetite are accessory minerals. Geophysical signature: strike-elongate positive magnetic anomalies. These rocks are interlayered with migmatitic paragneiss (mpg).

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; Triassic Sandstone, Siltstone, Shale, and Coal (Triassic) at surface, covers 0.2 % 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 Pegmatite (Proterozoic) at surface, covers 0.2 % of this area

Typically leucocratic and coarse grained; consists of lenticular bodies that are gen rally concordant with the regional foliation. Mineralogy includes microcline, albite, quartz, muscovite, biotite, garnet, and tourmaline.

Ultramafic Rocks (Proterozoic) at surface, covers 0.1 % of this area

Small pods and plutons of gray to greenish-gray, medium- to coarse-grained, locally porphyroblastic schist and granofels. Mineralogy: tremolite-actinolite + chlorite ± talc ± plagioclase ± quartz; locally contains relict olivine.

Pegmatite - Amelia pegmatite district (Permian) at surface, covers 0.1 % of this area

Pegmatite (Amelia district pegmatites). Leucocratic, coarsely crystalline pegmatite, occurs in discordant lens-shaped dikes and pods. Major minerals are quartz, microcline, albite, muscovite, garnet, and minor biotite; locally contains numerous rare and unusual accessory minerals (Pegau, 1932; Dietrich, 1990). Biotite and muscovite from the pegmatites have yielded Rb-Sr ages ranging from about 261 to 289 Ma (Deuser and Herzog, 1962).

Quartzofeldspathic Gneiss (Proterozoic Y-Pennsylvanian) at surface, covers < 0.1 % of this area

Quartzofeldspathic gneiss (Bobyarchick and others, 1981). Light-gray, fine- to coarse-grained, foliated, layered muscovitie-bearing quartzofeldspathic gneiss; contains intercalated quartz-muscovite schist. Mineralogy: quartz + plagioclase + microcline + garnet + muscovite + biotite.

Hornblende-plagioclase Gabbro (Proterozoic) at surface, covers < 0.1 % of this area

Dark-grayish-green, coarse- to medium-grained, massive to foliated metagabbro. Mineralogy: amphibole + plagioclase + clinopyroxene + quartz + biotite + muscovite + epidote ± magnetite. Geophysical signature: small circular positive magnetic anomalies. Plutons of these gabbros intrude interlayered mafic and felsic metavolcanic rocks.