Geologic units in Orange county, Virginia

Additional scientific data in this geographic area

Mine Run Complex - Melange Zone III (Proterozoic Z-Ordovician) at surface, covers 21 % of this area

Melange zone III (Pavlides, 1989). Phyllite and schist matrix contains abundant euhedral magnetite; many matrix rocks are highly deformed on a mesoscopic and microscopic scale. Mafic exotic blocks (mf) include amphibolite, ultra mafic rocks, serpentinite, and talc; many mafic and ultra mafic blocks are composite. Biotite gneiss blocks (gn) are also present. Metavolcanic olistoliths (vo) are rare. Geophysical signature: Strong positive magnetic anomaly. This unit is intruded by the Ellisville biotite granodiorite (SOe).

Catoctin Formation - Metabasalt (Proterozoic Z-Cambrian) at surface, covers 20 % of this area

Grayish-green to dark-yellowish-green, fine-grained, schistose chlorite- and actinolite-bearing metabasalt, commonly associated with epidosite segregations. Mineralogy: chlorite + actinolite + albite + epidote + titanite ± quartz + magnetite. Relict clinopyroxene is common; biotite porphyroblasts occur locally in southeastern outcrop belts. Geophysical signature: The Catoctin as a whole has a strong positive magnetic signature. However, between Warrenton and Culpeper the lowest part of the Catoctin, which consists of low-titanium metabasalt and low-titanium metabasalt breccia, is non-magnetic, and displays a strong negative anomaly. Metabasalt (CZc) is by far the most widespread unit comprising 3000 feet or more of section (Gathright and others, 1977). Primary volcanic features are well preserved in many places. In the north west ern outcrop belt, these include vesicles and amygdules, sedimentary dikes, flow-top breccia, and columnar joints (Reed, 1955; Gathright, 1976; Bartholomew, 1977); relict pillow structures have been reported in Catoctin greenstones east of Buena Vista (Spencer and others, 1989). In the southeastern outcrop belt, amygdaloidal metabasalts are common, as are volcanoclastic rocks interbedded with basaltic fl ows (Rossman, 1991). Fragmental zones occur locally between individual lava fl ows; map-scale hyaloclastite pillow breccias occur at three strati raphic levels within the southeastern outcrop belt (CZcb, CZhb, CZlb; Espenshade, 1986; Kline and others, 1990).

Metagraywacke, Quartzose Schist, and Melange (Proterozoic Z-Cambrian) at surface, covers 17 % of this area

Metagraywackes are quartzose chlorite or biotite schists containing very fine to coarse granules of blue quartz; primary graded laminations have been transposed by shearing into elongate lozenges that give the rock a distinctive pin-striped appearance in weathered surfaces perpendicular to schistosity. A mylonitic fabric is superimposed in varying degrees, as are late-stage chevron-style folds. In Buckingham, Appomattox, and Campbell Counties, rocks in this unit are progressively more tectonized from east to west across the outcrop belt; in the western portion, the dominant lithology is a polydeformed, mylonitic mica schist with abundant quartz-rich boudins; transposed pinstriped lamination or segregation layering at a high angle to mylonitic schistosity is characteristic. Metagabbroic blocks ranging in size from 5 cm to 3 m across and larger have been identified at widely scattered locations throughout the outcrop belt. Mineralogy: quartz + albite + epidote + chlorite + muscovite + magnetite ± chloritoid ± calcite; biotite- and staurolite-bearing assemblages occur in Appomattox and Campbell Counties. Detrital minerals identifi ed in thin section include plagioclase, perthite, epidote, magnetite, tourmaline, and titanite. Lithic fragments include dacite tuff, gabbro, and monocrystalline quartz with zircon and biotite inclusions (Evans, 1984). The northern portion of the outcrop belt includes melange zone IV of the Mine Run complex of Pavlides (1989; 1990). In Albemarle and Fluvanna counties, CZpm includes the lower chlorite-muscovite unit of Smith and others (1964) and Hardware metagraywacke of Evans (1984). In Appomattox and Buckingham Counties, polydeformed quartzose mica schists in the western portion of the outcrop belt are lithologically indistinguishable from schists mapped as Fork Mountain Formation in structural blocks that occur to the west; these units are considered to be at least in part correlative. In Campbell County, polydeformed metagraywacke and mica schist is intruded by the Cambrian-age Melrose Granite (Cm).

Mine Run Complex - Melange Zone II (Proterozoic Z-Ordovician) at surface, covers 7 % of this area

Melange zone II (Pavlides, 1989). Schist (Pavlides, 1989). Schist and phyllite matrix is more complexly deformed than the matrix of melange zone I; contains metavolcanic blocks (vo) similar to Chopawamsic Formation rocks (Ccv), in addition to granitoid blocks of altered tonalite and granodiorite (gr); intruded by the Ellisville biotite granodiorite (SOe).

Candler Formation - Phyllite and schist (Cambrian) at surface, covers 5 % of this area

Medium- to dark-gray and greenish- gray mica phyllite and sandy laminated schist. Lenses and pods of feldspathic quartzite, metamorphosed quartzarenite, dolomitic marble, and dark-gray to medium-bluish-gray, laminated marble are common in the upper part. Mineralogy: quartz + albite + muscovite + chlorite + magnetite-ilmenite + epidote ± biotite ± chloritoid ± calcite. Chloritoid and magnetite porphyroblasts are common near the Bowens Creek fault. Geophysical signature: Low amplitude, linear magnetic highs are superimposed on a pronounced southeast-sloping magnetic gradient between Alligator Back units northwest of the Candler and a persistant linear magnetic trough localized along the trend of the Bowens Creek fault zone. Microstructural elements in the upper Candler indicate dextral transpression along a continuous shear zone (Bowens Creek fault zone) within the Candler outcrop belt from the Virginia-North Carolina boundary in Patrick County northeastward to at least the north end of Buffalo Ridge on the Amherst-Campbell County line. Conley and Henika (1970) and Gates (1986) hypothesized that the Bowens Creek fault is part of a major strike slip (wrench) system that is part or a continuation of the Brevard fault zone to the southwest . Northeast of the Scottsville Mesozoic basin, the Candler includes laminated metasiltstone (Ccas), ferruginous metatuff, dolomitic marble, and phyllite that are conformable above Catoctin metabasalt (Evans, 1984; Conley, 1989; Rossman, 1991); in Orange County, the Candler includes the True Blue formation of Pavlides (1989, 1990).

Ellisville Biotite Granodiorite (Ordovician-Silurian) at surface, covers 4 % of this area

Ellisville biotite granodiorite (Pavlides, 1990). Mesocratic, coarse- to medium-grained, equigranular to porphyritic, massive to strongly foliated granodiorite. Mineralogy: quartz + plagioclase + potassium feldspar + biotite; accessories include epidote, allanite, titanite, and apatite. Porphyritic rocks contain potassium feldspar megacrysts up to 1.5 cm across; myrmekite commonly occurs adjacent to potassium feldspar. Brownish-green, strongly pleochroic biotite is associated with, and in places poikilitically encloses epidote, allanite, titanite, and apatite. Subhedral epidote locally encloses euhedral titanite. Pleochroic green amphibole and muscovite are minor constituents locally. The Ellisville has been dated at 440±8 Ma (Rb-Sr whole rock; Pavlides and others, 1982).

Lynchburg Group - Metagraywacke (Proterozoic Z) at surface, covers 3 % of this area

Metagraywacke, quartzose schist, and conglomerate. Graded bedding, cut-and-fill structures, and incomplete Bouma cycles are characteristic; conglomeratic lenses occur throughout the unit. Geophysical signature: Positive magnetic and positive radiometric anomalies. Metagraywacke is interpreted to have been deposited in a series of coalescing submarine fans, with conglomerate deposited in submarine distributary channels developed on the fans (Conley, 1989). The unit as mapped includes in part the Rockfish conglomerate formation, and the Lynchburg gneiss formation (restricted) of Nelson (1962), and is equivalent to Ashe Formation metagraywacke (Zam), on strike to the southwest. The unit has been mapped on a lithologic basis in outliers to the west of the main strike-belt, including parts of the Mechums River formation strike-belt of Gooch (1958) and Nelson (1962).

Lynchburg Group; Charlottesville Formation (Proterozoic Z) at surface, covers 3 % of this area

Charlottesville Formation (Nelson, 1962). Coarse-grained to pebbly quartzose metasandstone and quartzite interbedded with laminated micaceous metasiltstone and graphitic phyllite and slate. Sandstone beds are typically amalgamated and massive; grading, horizontal stratification, and complete Bouma sequences are preserved locally (Wehr, 1985; Conley, 1989). The formation includes cross-bedded quartzite, feldspathic metasandstone, and muscovite schist in the upper portion (Conley, 1989; mapped as Swift Run Formation by Nelson, 1962); in the Culpeper area, includes in part the Ball Mountain Formation of Wehr (1985) and Kasselas (1993). The unit contains numerous apparently concordant mafic and ultramafic sills in the lower portion. Southwest of Nelson County, rocks equivalent to the Charlottesville Formation have been mapped as Alligator Back Formation. Outcrop belt is as much as 3.7 miles wide.

Newark Supergroup; Conglomerate, mixed clasts (Upper Triassic) at surface, covers 3 % 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.

Lahore Complex - Amphibole monzonite (Ordovician) at surface, covers 2 % of this area

Mesocratic, medium-grained, amphibole monzonite and amphibole-quartz monzonite have a foliation defined by alignment of tabular feldspar crystals. Mineralogy includes microcline or orthoclase, intermediate-composition plagioclase, and quartz; mafic minerals are amphibole ranging from common hornblende to edenite, locally rimming pyroxene, subordinate biotite, and magnetite. Epidote, titanite, apatite and allanite are also present.

Granitoid Rocks (Proterozoic) at surface, covers 2 % of this area

Granitoid blocks as exotic blocks within melange units

Newark Supergroup; Sandstone, undifferentiated (Upper Triassic) at surface, covers 2 % 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.

Newark Supergroup; Triassic shale and siltstone (Triassic) at surface, covers 2 % of this area

Shale, light-greenish gray, light- to dark-gray, carbonaceous, and reddish-brown in cyclic sequences, laminated, silty to sandy, fossiliferous. Siltstone, typically reddish-brown to gray, sandy, micaceous, with minor fine-grained sandstone beds.

Mine Run Complex - Melange Zone I (Proterozoic Z-Ordovician) at surface, covers 2 % of this area

Melange zone I (Pavlides, 1989). Fine-grained schist and phyllite matrix encloses coarse-grained metasandstone beds locally; contains exotic blocks of mafic and felsic metavolcanic rocks (vo) similar to metavolcanic rocks of the Chopawamsic Formation (Ccv). Blocks of blastomylonitic tonalite and granodiorite gneiss (gn) are present locally.

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

Rounded to subrounded pebbles, cobbles, and boulders of predominantly metavolcanic Catoctin greenstone in a matrix of fine- to coarse-grained, silicified, ferruginous-cemented, greenish-gray to dark-green, clayey sandstone.

Metasandstone and Metasiltstone (Proterozoic Z-Cambrian) at surface, covers 1 % of this area

Metasandstone and metasiltstone (Pavlides, 1990). Quartz- and plagioclase-bearing metasandstone and metasiltstone are interbedded with chlorite- and muscovite-rich laminae; unit grades into phyllonite and mylonite in the Mountain Run fault zone.

Catoctin Formation - Hyaloclastite pillow breccia (Proterozoic Z-Cambrian) at surface, covers 0.8 % of this area

Hyaloclastite pillow breccia (Kline and others, 1990). Poorly-sorted assemblages of ellipsoidal to angular clasts of metabasalt in a finer-grained matrix; clasts range in size from 1 cm to greater than 1 m across, but generally are on the order of 2 to 15 cm. Mineralogy and textural features in clasts are identical to those in Catoctin metabasalt (CZb). Matrix consists of coarse- to fine-grained material of similar mineralogy to clasts, in a mesostasis that may constitute up to 30 percent of the matrix. Breccias are gen er al ly matrix supported. Pillows and pillow breccias are locally delineated by concentrations of epidote + quartz on rims and in interstices; pillows are flattened and ellipsoidal, and range up to about 3 m in maximum dimension (Kline and others, 1990). Thickness is on the order of 300 m.

Newark Supergroup; Triassic Sandstone, Siltstone, and Shale (Upper Triassic) at surface, covers 0.8 % 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.

Candler Formation - Metasiltstone (Cambrian) at surface, covers 0.8 % of this area

Grayish-green, fine- to very fine-grained, laminated schistose and phyllitic metasiltstone; relict detrital quartz and albite define graded laminations on the order of 0.5 to 1 cm thick. Mineralogy: quartz + chlorite+ muscovite + albite + epidote + magnetite + tourmaline +zircon. Laminated metasiltstone occurs in stratigraphic conformity above the Catoctin Formation; in southeastern Albemarle County, the unit contains discontinuous interbeds of ferruginous metatuff. Laterally discontinuous beds of feldspathic metasandstone, micaceous quartzite, and coarse-grained to pebbly metagraywacke occur within the metasiltstone outcrop belt, and are interbedded with phyllites somewhat higher in the section (Evans, 1984).

Metavolcanic and Metavolcanic Clastic Phyllite (Proterozoic - Cambrian) at surface, covers 0.7 % of this area

Metavolcanic and metavolcanic clastic phyllite (Pavlides, 1990). Chlorite- and chlorite-epidote phyllite with variable amounts of silt-size quartz and plagioclase; in part metafelsite or metatuff. Euhedral magnetite is locally abundant. Unit grades westward into metasandstone and metasiltstone (mss).

Mafic Plutonic Rocks (Proterozoic Z-Cambrian) at surface, covers 0.5 % of this area

Mafic plutonic rocks as exotic blocks within melange units

Lahore Complex - Pyroxene monzonite (Ordovician) at surface, covers 0.5 % of this area

Dark-gray to black, fine- to medium-grained, massive- to indistinctly-foliated; consists of large, locally-twinned augite, poikilitic in part, enclosing apatite, opaque oxide, biotite, and plagioclase. Compositionally-zoned and unzoned plagioclase is also a significant coarse-grained consitituent. This lithology is gradational into amphibole monzonite.

Everona Limestone (Cambrian) at surface, covers 0.5 % of this area

Everona Limestone (Jonas, 1927). Thinly-laminated medium-bluish-gray limestone with graded, graphitic silty partings; includes calcareous graphitic phyllite, and pyrite-bearing graphitic slate. Mineralogy: calcite + quartz + muscovite ± graphite ± chlorite ± albite ± pyrite + tourmaline. The Everona occurs dominantly as lenticular bodies immediately north west of or within the Mountain Run fault zone (Evans, 1984; Conley, 1989; Rossman, 1991). In most outcrops primary laminations are complexly folded due to strain accommodation associated with the Mountain Run fault. Unit thickness and degree of lateral continuity are structurally controlled in many places. Mack (1965) reports thicknesses ranging from about 20 to 1100 feet. Jonas (1927) reports retrieving trilobite fragments from sandy beds adjacent to the Everona, but these were so poorly preserved that E.O. Ulrich could not identify them, even with respect to genus. Recent attempts to extract conodonts from the Everona have so far proven fruitless. The Everona is interpreted to be in stratigraphic continuity with phyllites and metasiltstones that are conformable above the Catoctin Formation (Evans, 1984; Rossman, 1991), and is likely Early Paleozoic in age.

Lahore Complex - Mafic and ultramafic rocks (Ordovician) at surface, covers 0.5 % of this area

Composite mass that consists of partially serpentinized pyroxenite containing diopside rimmed by antigorite or by tremolite.

Lynchburg Group - Graphitic phyllite and metasiltstone (Proterozoic Z) at surface, covers 0.3 % of this area

Black graphite and pyrite-bearing phyllite and slate, with thin interbeds of sericite phyllite, metasiltstone and quartzite. The unit includes the Johnson Mill graphite slate formation of Nelson (1962); thickness is on the order of 100 m.

Metagabbro (Proterozoic Z-Cambrian) at surface, covers 0.2 % of this area

Dusky-green to black, medium- to coarse-grained, massive to vaguely-foliated amphibolite. Mineralogy: (1) actinolite + chlorite + albite + epidote + quartz + magnetite ± titanite; (2) hornblende + pla gio clase + epidote + magnetite + quartz ± titanite. Geophysical signature: strong positive magnetic anomaly. Metagabbro occurs as dikes that cut Grenville basement and the Lynchburg Group, and as sills occurring primarily in the Charlottesville and Alligator Back Formations in association with ultramafiic rocks. Cross cutting relations imply that these rocks are related to the Catoctin in time. Reed and Morgan (1971) demonstrated on the basis of geochemistry that metadiabase dikes cutting Grenville basement in northern Virginia are feeders to the Catoctin. Metagabbroic dikes in the central Virginia Blue Ridge could represent a deeper level of Catoctin feeder system, although that hypothesis has not been substantiated by field or geochemical study.

Felsic/Mafic Volcanic Rocks (Proterozoic) at surface, covers < 0.1 % of this area

Felsic/mafic volcanic rocks as exotic blocks within melange units

Gneiss (Proterozoic) at surface, covers < 0.1 % of this area

Gneiss as exotic blocks within melange units