Geologic units in Fauquier county, Virginia

Additional scientific data in this geographic area

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).

Newark Supergroup; Triassic shale and siltstone (Triassic) at surface, covers 9 % 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.

Marshall Metagranite (Proterozoic Y) at surface, covers 5 % of this area

Medium-to dark-gray, fine- to medium-grained, mostly equigranular, but rarely inequigranular granite. Principal minerals are bluish-gray quartz, oligoclase, microcline, and biotite, with lesser amounts of muscovite, opaque minerals, epidote, chlorite, and rare garnet. Gneissic layering, commonly absent, is well developed locally. This unit commonly occurs as dikes intruding porphyroblastic granite gneiss (Ybp), and contains xenoliths of Ybp. U-Pb zircon data indicate crystallization ages of 1110±4 Ma and 1112±3 Ma (Aleinikoff and others, 1993). Single crystals of monazite give an age of 1051±3 Ma, interpreted to be a metamorphic age related to intrusion of adjacent granites that yield ages ranging from 1055 to 1070 Ma.

Newark Supergroup; Jurassic Basalt (Lower Jurassic) at surface, covers 5 % of this area

Fine- to medium-crystalline, equigranular, porphyritic, vesicular, or amygdaloidal; medium- to dark-gray subophitic intergrowths of plagioclase laths and clinopyroxene with amygdules of calcite, zeolites, and prehnite. Occurs only in the Culpeper basin as three principle basalt flows separated by sedimentary rocks.

Porphyoblastic Biotite-Plagioclase Augen Gneiss (Proterozoic Y) at surface, covers 5 % of this area

Mesocratic, medium- to coarse-grained, biotite-rich quartzofeldspathic gneiss con tains prominent subhedral to euhedral monocrystalline feldspar augen. The ratio plagioclase: potassium feldspar may be as high as 10:1; color index ranges from 30 to 50. Apatite, epidote, muscovite, ilmenite, and titanite are ubiquitous accessories. Plagioclase contains abundant prismatic epidote and white mica; ilmenite is rimmed with masses of anhedral titanite; subhedral hornblende and subhedral to euhedral almandine-grossular garnet occur locally. In the vicinity of adjacent charnockite, anhedral actinolitic amphibole pseudomorphs after pyroxene or rims thoroughly uralitized relict pyroxene. Rock fabric is gradational from granofels to mylonite gneiss. Geophysical signature: negative magnetic signature relative to adjacent charnockite. In northern Virginia, this unit strongly resembles prophyroblastic granite gneiss (Ybp); however, the augen in Ybp are more commonly polycrystalline aggregates rather than single-crystal porphyroblasts. This unit is widespread in the central and southeastern Blue Ridge, encompassing a number of lithologically similar metaplutonic entities: the "biotitic facies"of the Roses Mill and Turkey Mountain ferrodiorites of Herz and Force (1987), the Archer Mountain quartz monzonite of Bartholomew and others (1981), biotite granofels and augen gneiss of Evans (1984, 1991), biotite augen gneiss of Conley (1989), and augen-bearing gneiss of Lukert and Halladay (1980), and Lukert and Nuckols (1976). Historically, most workers have interpreted these rocks as Grenville-age plutons in which the present-day biotite-rich mineral assemblage is a primary igneous assemblage that crystallized from a melt (for example, Bartholomew and others, 1981). Herz and Force (1987) and Evans (1991) presented evidence that these biotite gneisses were derived from charnockite plutons by retrograde hydration reactions. Pettingill and others (1984) reported ages of 1009±26 Ma (Rb-Sr whole-rock) and 1004±36 Ma (Sm-Nd whole-rock) for ferrodiorite to quartzmonzonite in the Roseland district. Where this unit has been mapped in the Upperville quadrangle (A.E. Nelson, unpublished data), U-Pb zircon data suggest a crystallization age of 1055±2 Ma (Aleinikoff and others, 1993).

Porphyroblastic Granite Gneiss (Proterozoic Y) at surface, covers 5 % of this area

Dark-yellowish brown to moderate-yellowish-brown, medium-grained, granoblastic to megacrystic, mafic-rich monzogranite composed of 27 to 38 percent quartz, 28 to 39 percent orthoclase, rod and bleb perthite, microcline, and myrmekite, and 33 to 40 percent oligoclase and andesine. Porphyroblasts of potassium feldspar range from 1 to 10 cm while plagioclase and quartz are generally 2 cm or less in maximum dimension. Mafic minerals, including almandine, biotite, chlorite, hornblende, and opaque minerals, are, in places, concentrated in layers. Almandine (up to 1 cm in diameter) makes up as much as 3 percent of the mode, while hornblende, commonly 0.5 to 0.75 cm in length, constitutes less than 1 percent. The rock breaks along cleavage surfaces that are commonly rich in chlorite, giving the whole rock a light green color. A U-Pb zircon age from this rock is 1144±2 Ma; two different populations of monazite give ages of 1106±1 Ma and 1063±1 Ma, respectively (Aleinikoff and others, 1993). This unit is the oldest dated granitic rock in the northern Virginia Blue Ridge, and is very commonly intruded by dikes of the Marshall Metagranite (Ym) and garnetiferous leucocratic metagranite (Ygt), and less commonly by leucocratic metagranite (Yg). These field relations suggest that the monazite ages are not cooling ages but represent the times of metamorphic growth during subsequent intrusive events.

Mine Run Complex - Melange Zone III (Proterozoic Z-Ordovician) at surface, covers 5 % 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).

Robertson River Igneous Suite - Cobbler Mountain alkali feldspar quartz syenite (Proterozoic Z) at surface, covers 4 % of this area

Light- to dark-gray, medium- to coarse-grained, porphyritic (mesoperthite phenocrysts) to seriate-equigranular alkali feldspar-quartz syenite composed of microcline mesoperthite, quartz, and plagioclase, with hastingsitic amphibole, biotite, stilpnomelane, zircon, allanite, fluorite, and rare aegirine-augite. Euhedral to subhedral feldspar phe nocrysts are diagnostic. Syenite locally displays miarolitic cavities contain ing quartz. The rock has been dated at 722± 3 Ma (U-Pb zircon; Tollo and Aleinikoff, in press).

Robertson River Igneous Suite - Laurel Mills granite (Proterozoic Z) at surface, covers 4 % of this area

Gray, coarse-grained, inequigranular granite composed of alkali feldspar mesoperthite, quartz, and plagioclase, with hastingsitic amphibole, biotite, stilpnomelane, zircon, apatite, ilmenite, magnetite, and titanite. Amphiboles typically are entirely to partially replaced by fine-grained intergrowths of quartz, biotite, stilpnomelane, magnetite, and titanite. Pale blue color of the quartz is locally diagnostic. The granite typically displays numerous, anastomosing, mesoscopic zones of deformation. The unit has been dated at 728±2 Ma (U-Pb zircon; Tollo and Aleinikoff, in press).

Goldvein Pluton (Cambrian-Ordovician) at surface, covers 4 % of this area

Goldvein pluton (Pavlides, 1990). Mesocratic, coarse- to medium-grained, weakly- to strongly-foliated metamonzogranite. Altered feldspars commonly impart pink and green colors to the rock. Mineralogy includes perthite and plagioclase feldspars, each locally megacrystic; granoblastic quartz, muscovite, and sparsely distributed garnet.

Diabase (Jurassic) at surface, covers 4 % of this area

Fine- to coarsely-crystalline, subaphanitic or porphyritic with aphanitic margins; dark-gray mosaic of plagioclase laths and clinopyroxene, with some masses characterized by olivine or bronzite, others granophyric. Also occurs as dikes and sills in the Valley and Ridge, Piedmont, and Blue Ridge physiographic provinces.

Fauquier Formation - Arkosic metasandstone (Proterozoic Z) at surface, covers 3 % of this area

Dark-gray, medium- to coarse-grained metasandstone contains quartz, plagioclase, perthitic potassium feldspar, and sericite, with minor biotite and epidote; thin beds of pebble conglomerate occur with coarse-grained metasandstone; commonly cross-bedded. Unit comprises the lowest part of the Fauquier; thickness is extremely variable. In the vicinity of Castleton, fine-grained volcanogenic rocks geochemically indistinguishable from nearby Battle Mountain Felsite (Zrbf) are interbedded with the basal Fauquier (Hutson, 1990).

Newark Supergroup; Jurassic Interbedded Sandstone and Siltstone (Lower Jurassic) at surface, covers 3 % of this area

Interbedded fine- to coarse grained, pebbly, reddish-brown, and arkosic sandstone and reddish-brown siltstone; rhythmically interbedded with siltstone and shale unit (sh). Occurs only in the Culpeper basin.

Newark Supergroup; Triassic Sandstone, Siltstone, and Shale (Upper Triassic) at surface, covers 3 % 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.

Newark Supergroup; Sandstone, undifferentiated (Upper Triassic) at surface, covers 3 % 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.

Layered Leucocratic Granite Gneiss (Proterozoic Y) at surface, covers 2 % of this area

Pale-red, pinkish- gray to light-gray leucocratic syenogranite with medium light- gray to greenish-gray melanocratic layers (0.25 to 1 cm thick) that are commonly migmatitic. Mineralogy consists of 26 to 39 percent quartz, 49 to 51 percent rod and bleb perthite, microcline, and orthoclase, 12 to 23 percent oligoclase, and minor garnet and biotite. Layering consists of 0.5- to 2-cm thick segregations of alkali feldspar, plagioclase, and quartz. Garnet and biotite are commonly restricted to melanocratic layers containing plagioclase and quartz. Isotopic data from multigrain fractions and single zircons are scattered, with Pb-Pb ages ranging from 1092 to 1139 Ma (Aleinikoff and others, 1993). Field relations are complex and suggest that the protolith for this unit may have been a composite of Ybp and younger granite(s) that was highly tectonized and homogenized during the Grenville orogeny.

Flint Hill Gneiss (Proterozoic Y) at surface, covers 2 % of this area

Flint Hill Gneiss (Lukert and others, 1977). Segregation-layered quartzofeldspathic biotite gneiss contains quartz, plagioclase, microcline, green biotite, ilmenite, and titanite; accessories include epidote, apatite, and zircon. Segregation layering is defined by quartz-feldspar- and biotite-rich domains on the order of a few millimeters thick; migmatitic leucosomes of quartz and alkali feldspar cut segregation layering in places; veins of blue quartz are com mon. This unit is considered correlative with layered biotite granulite and gneiss (Ygb); the Flint Hill has been dated at 1081 Ma (U-Pb zircon; Lukert and others, 1977).

Catoctin Formation - Metasedimentary rocks (Proterozoic Z-Cambrian) at surface, covers 2 % of this area

Quartzite, feldspathic metasandstone, metasiltstone, and phyllite; occurs in discontinuous beds that are generally less than 50 feet thick, interbedded with Catoctin metabasalt (CZc).

Layered Pyroxene Granulite (Proterozoic Y) at surface, covers 2 % of this area

Medium- to dark greenish-gray, fine- to medium-grained, segregation-layered quartzofeldspathic granulite. Major minerals are quartz, plagioclase, potassium feldspar (includes assemblages with one alkali feldspar), orthopyroxene and clinopyroxene, and magnetite-ilmenite; garnet, hornblende, and reddish-brown biotite are widespread minor constituents. Apatite and zircon are accessory minerals. Color index ranges from 15 to 35. Quartz and feldspars are granoblastic; ferromagnesian minerals define dark layers on the order of 1 to 3 mm thick, giving the rock a characteristic pinstriped appearance. Migmatitic leucosomes locally cut segregation layering. Geophysical signature: positive magnetic anomalies relative to adjacent biotite granulite and layered gneiss (Ygb). This unit pre-dates charnockite, alkali feldspar leucogranite, and other plutonic rocks on basis of cross-cutting relations, and is generally considered pre-Grenville-age country rock that was metamorphosed under granulite-facies metamorphic conditions and intruded by plutonic rocks during the Grenville orogeny. The unit includes Lady Slipper granulite gneiss (1130 Ma, U-Pb zircon, Sinha and Bartholomew, 1984), and Nellysford and Hills Mountain granulite gneisses of Bartholomew and others (1981).

Fauquier Formation - Meta-arkose and metasiltstone (Proterozoic Z) at surface, covers 1 % of this area

Alternating beds of dark-gray, very-fine-grained meta-arkose and metasiltstone; composed dominantly of angular quartz grains, with lesser plagioclase and potassium feldspar, and minor biotite. Crossbedding and graded bedding are present; thickness ranges from 300 to 500 meters.

Lynchburg Group; Monumental Mills Formation (Proterozoic Z) at surface, covers 1 % of this area

Monumental Mills Formation (Wehr, 1985). Lightgray, fine- to very-fine-grained metasandstone characterized by thin planar beds separated by biotite-rich silty partings; and dark-gray laminated siltstone and mudstone containing abundant synsedimentary deformation features including folds, faults, convolute bedding, and erosional-depositional discordances. Mineralogy: quartz + albite + mu covite + biotite + epidote + calcite + chlorite + titanite + magnetite ± garnet ± pyrite; porphyroblastic garnet and biotite are common. The Monumental Mills has been interpreted to represent deposition in a delta front-slope environment (Wehr, 1985).

Catoctin Formation - Metabasalt breccia (low titanium) (Proterozoic Z-Cambrian) at surface, covers 1 % of this area

Metabasalt breccia (low-titanium) (Espenshade, 1986). Blocky or angular fragments of grayish-green, fine grained rock in a fine-grained, ge erally schistose matrix. Breccia fragments range from 1 to 20 cm long, and form rough, knobby surfaces on weathered outcrops. Where present, this breccia occurs at or near the base of Catoctin metabasalt (CZc) on the southeast limb of the Blue Ridge anticlinorium; the unit is as much as 3000 feet thick. Furcron (1939) mapped both high- and low-titanium breccias as Warrenton agglomerate; R.L. Smith (personal communication in Espenshade, 1986) interpreted the breccias as subareal agglutinates. Kline and others (1990) presented evidence that the breccias are hyaloclastite pillow breccias, erupted in a subaqueous environment. Reed (1955) recognized a mappable stratigraphy within the Catoctin, and made the interpretation that the greenstones were metamorphosed tholeiitic flood basalts that originated in a "nonorogenic" setting. Rankin (1975) considered the Catoctin to have originated during continental volcanism associated with the opening of a proto-Atlantic Iapetus ocean in Latest Precambrian time, and included the Catoctin in the Crossnore volcanic-plutonic group. Badger and Sinha (1988) studied chemical stratigraphy within individual fl ows and dated Catoctin metabasalts at 570 ± 36 Ma using Rb-Sr systematics on samples showing little evidence of elemental mobility during metamorphism. This date is consistent with radiometric age data from Catoctin metarhyolite, discussed above, and with the occurrence of Early Cambrian-age Rusophycus stratigraphically above the Catoctin near the base of the Chilhowee Group (Simpson and Sundberg, 1987). These constraints suggest that some portion of the Catoctin may be as young as Cambrian in age. Regional mapping suggests that metabasalts and amphibole gneisses within the Ashe (Zaa), Alligator Back (CZas), and Bassett Formations (CZba) are volcanogenic rocks that are correlative with the Catoctin in a lithostratigraphic sense and in terms of tectonic setting, but not necessarily in a strict time sense. Catoctin volcanism likely spanned a considerable time period; correlative units to the southwest were probably time-transgressive to a degree.

Leucocratic Metagranite (Proterozoic Y) at surface, covers 1.0 % of this area

Leucocratic metagranite. White to light-olive-gray, to pink, fine- to medium-grained, massive monzogranite composed of 22 to 38 percent quartz (white, clear, or blue), 33 to 34 percent orthoclase, microcline, and rod and bleb microperthite, and 38 to 44 percent oligoclase and albite, and minor biotite. Locally, potassium feldspar porphyroblasts are 1 to 2 cm in diameter. In Loudoun County this unit becomes coarser-grained and richer in biotite from west to east across the outcrop belt. U-Pb zircon data from two localities in Loudoun County indicate crystallization ages of 1058±3 Ma and 1060±2 Ma (Aleinikoff and others, 1993).

Chilhowee Group (Cambrian) at surface, covers 0.8 % of this area

Chilhowee Group (Keith, 1903). The Chilhowee Group includes the Antietam, Harpers, and Weverton Formations in the northeastern portion of the Blue Ridge Province and the Erwin, Hampton, and Unicoi Formations in the southwestern portion of the Blue Ridge Province. Antietam Formation (Williams and Clark, 1893). Quartzite, medium-gray to pale-yellowish-white, fine- to medium grained, locally with very minor quartz-pebble conglomerate, cross-laminated, medium- to very-thick-bedded, very resistant, forms prominent cliffs and ledges, contains a few thin interbeds of light-gray phyllite, has calcareous quartz sandstone at the top that is transitional with the overlying Tomstown Dolomite, and many beds contain Skolithos linearras. It is laterally equivalent to the Erwin Formation to the southwest. The formation interfingers with the underlying Harpers Formation and ranges in thickness from less than 500 feet in Clarke County to nearly 1000 feet in Rockingham County (Gathright and Nystrom, 1974; Gathright, 1976). Harpers Formation (Keith, 1894). Metasandstone, metasiltstone, and phyllite. Metasandstone, dark-greenish gray to brownish-gray, fine-grained, sericitic, thin- to medium-planar bedded, locally bioturbated, Skolithos-bearing litharenite; dark-gray, fine-grained, cross-laminated, thickbedded, laterally extensive bodies of quartzite; and very-dark gray, medium- to coarse-grained, thick-bedded, ferruginous, very resistant, quartzitic sandstone. These beds were extensively mined for iron ore north of Roanoke (Henika, 1981). Metasiltstone, dark-greenish-gray, thin, even bedded, sericitic, and locally bioturbated. Phyllite, medium- to light-greenish gray, bronze weathering, laminated, sericitic. The Harpers is laterally equivalent to the Hampton Formation to the southwest and they are so similar that the names have been used interchaneably in the northern Blue Ridge (Gathright, 1976; Brown and Spencer, 1981). The Harpers conformably overlies the Weverton or Unicoi Formations, thickens northeastward from about 1500 feet north of Roanoke to about 2500 feet in Clarke County. The thicker sections are dominated by phyllite and metasiltstone and the thinner sections by metasandstone and quartzite. Weverton Formation (Williams and Clark, 1893). Quartzite, metasandstone, and phyllite. Quartzite, medium- to very dark-gray, weathers light-gray, fine- to coarse-grained, well rounded quartz-pebble conglomerate beds locally, medium- to thick-bedded, cross-bedded, very resistant, with interbedded metasandstone, dark-greenish- gray, feldspathic, thick-bedded, with ferruginous cement in some beds. Phyllite, light- to dark-greenish-gray or dark-reddish-gray, laminated, sericitic, with coarse sand grains and quartz-pebble conglomerate in a few thin beds, generally in lower part. Formation ranges in thickness from more than 600 feet in Clarke County to less than 200 feet in Augusta County (Gathright and Nystrom, 1974; Gathright and others, 1977). The Weverton is lithologically very similar to strata in the upper portion of the Unicoi Formation to the south to which it may be equivalent. The Weverton appears to unconformably overlie the Catoctin and Swift Run Formations and the Blue Ridge basement complex and is present northeast of Augusta County.

Newark Supergroup; Jurassic Siltstone and Shale (Jurassic) at surface, covers 0.6 % of this area

Interbedded reddish-brown siltstone and reddish-brown, greenish-gray, dark-gray, fossiliferous shale. Occurs only in the Culpeper basin.

Lynchburg Group; Charlottesville Formation (Proterozoic Z) at surface, covers 0.5 % 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.

Fauquier Formation - Metasiltstone and phyllite (Proterozoic Z) at surface, covers 0.4 % of this area

Medium- to dark-gray (fresh), very-pale-orange (weathered), very-fine grained, laminated metasiltstone, composed of alternating silty and micaceous layers on the order of a millimeter to several millimeters thick, and phyllite without discernable layers; major minerals are silt-size quartz and sericite; chlorite, biotite, and magnetite occur locally. Thiesmeyer (1939) described these rocks as "varved slates" interpreted as lacustrine deposits. Espenshade (1986) called this unit metarhythmite. The unit is on strike with, and in part equivalent to the Monumental Mills Formation of Wehr (1985), interpreted as de pos it ed in a delta front-slope environment.

Marshall Metagranite (Proterozoic Y) at surface, covers 0.4 % of this area

Medium-gray to brownish-gray, medium- to coarse-grained monzogranite composed of 30 percent quartz (clear or blue), 28 percent rod and bleb perthite, microcline, and orthoclase, and 42 percent saussuritized oligoclase. Sheared rock commonly has as much as 20 percent biotite. Porphyroblastic augen, commonly 1 to 2 cm in length, consist of aggregates of potassium feldspar, plagioclase, and quartz. U-Pb zircon data indicate a crystallization age of 1127±7 Ma (Aleinikoff and others, 1993).

Purcell Branch Formation (Cambrian) at surface, covers 0.3 % of this area

Purcell Branch Formation (Pavlides, 1989; 1990). Metadiamictite, characterized by a dark-gray, micaceous quartzofeldspathic matrix with a pervasive, anastomosing foliation. Mineralogy: quartz + plagioclase + muscovite + biotite (commonly retrograded to chlorite) + epidote + magnetite. Lumps of milky quartz, sparsely distributed, have undergone tectonic flattening and recrystallization; phyllitic rock chips are sparse. Exotic blocks on the order of several meters to several hundreds of meters in length include felsite, greenstone and greenschist, mafic rocks, and amphibolite; these are interpreted as derived from the Chopawamsic Formation (Pavlides, 1989).

Newark Supergroup; Conglomerate, mixed clasts (Upper Triassic) at surface, covers 0.3 % 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.

Phyllite (Cambrian-Ordovician) at surface, covers 0.3 % of this area

Phyllite (Pavlides, 1990): Mostly gray-to-green phyllite with lesser metasiltstone. Mylonitic rocks composed commonly of schist or phyllite with elongate granules of quartz, occur in the southern part of the outcrop belt. These are interpreted as highly tectonized parts of this formation. Alternatively, these rocks may be part of a separate terrane.

Mine Run Complex - Melange Zone II (Proterozoic Z-Ordovician) at surface, covers 0.3 % 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).

Catoctin Formation - Metabasalt breccia (high titanium) (Proterozoic Z-Cambrian) at surface, covers 0.3 % of this area

Metabasalt breccia (high-titanium) (Espenshade, 1986). Dark-green, amygdular ellipsoids ranging from about 5 to 40 cm in length in a matrix of dark-green, dense angular fragments about 0.5 to 5 cm across; epidote is commonly abundant in the matrix; ellipsoidal amygdules contain quartz and epidote. Titanite constitutes several percent of the mode. This unit, where present, may be as thick as 2800 feet, and occurs stratigraphically above the low-titanium breccia (CZlb).

Metasedimentary Rocks Undivided (Cambrian-Ordovician) at surface, covers 0.2 % of this area

Metasedimentary rocks, undivided (Pavlides, 1990). Gray to green phyllite, gray to white metasiltstone and fine grained quartzite, fine-grained mica schist, green slate and phyllite, and sparse granule quartzite and graywacke; may be coeval in part with Old Mill Branch Metasiltstone Member of the Popes Head Formation (OCpo).

Mylonite, Mylonite Gneiss, and Cataclastic Rocks (Proterozoic - Paleozoic ?) at surface, covers 0.2 % 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).

Newark Supergroup; Jurassic Conglomerate (Lower Jurassic) at surface, covers 0.2 % of this area

Rounded pebbles, cobbles, and boulders of quartz, gneiss, schist, basalt, minor greenstone, and marble in a matrix of medium- to very-coarse-grained, reddish-brown to light-gray sandstone. Occurs only in the Culpeper basin.

Swift Run Formation (Proterozoic Z) at surface, covers 0.1 % of this area

Swift Run Formation (Jonas and Stose, 1939; King, 1950; Gathright, 1976). Heterogeneous assemblage includes: pebbly to cobbly quartzite and feldspathic metaconglomerate; gray, grayish-pink, or grayish-green, feldspathic quartzite and metasandstone, locally crossbedded; greenish-gray, silvery quartz-sericite-chlorite sandy schist; and, greenish-gray to grayish-red-purple chlorite-sericite tuffaceous phyllite and slate. In Loudoun County, contains pinkish-gray and yellowish-gray to light brownish-gray, fine-grained dolomitic marble (Southworth, 1991). Individual lithologies are laterally discontinuous; formation ranges up to 350 feet in total thickness, but is locally very thin or absent (Gathright, 1976). The Swift Run was originally defined on the northwest limb of the Blue Ridge anticlinorium (Stose and Stose, 1946), where the unit rests unconformably on Grenville-age rocks, and is overlain conformably by the Catoctin Formation; the upper contact is mapped at the bottom of the lowest massive metabasalt. In places Swift Run lithologies are interbedded with Catoctin metabasalts, and the contact between the two units is gradational (Gathright, 1976). Swift Run metasedimentary rocks on the northwest limb have been interpreted as deposited in alluvial fan, floodplain, and lacustrine environments (Schwab, 1986); these are interbedded with metamorphosed tuffaceous and volcanoclastic units (Gathright, 1976; Bartholomew, 1977). Although the Swift Run has been interpreted as a thin western equivalent of the Lynchburg Group in the southeastern Blue Ridge (Stose and Stose, 1946; Brown, 1970), some workers have correlated the Swift Run with discontinuous lenses of feldspathic sandstone interbedded with felsic metatuff that occur immediately below the Catoctin on the southeast limb of the anticlinorium (Nelson, 1962; Conley, 1978; 1989; Wehr, 1985). On the Geologic Map of Virginia (1993), the Swift Run is terminated along an east-west-trending normal fault just west of Leesburg, and is not mapped farther southwest on the southeast limb of the Blue Ridge anticlinorium.

Metagabbro (Proterozoic Z-Cambrian) at surface, covers 0.1 % 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.

Metagraywacke, Quartzose Schist, and Melange (Proterozoic Z-Cambrian) at surface, covers 0.1 % 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 I (Proterozoic Z-Ordovician) at surface, covers 0.1 % 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.

Robertson River Igneous Suite - Battle Mountain granite (Proterozoic Z) at surface, covers 0.1 % of this area

Granitoid. Light-gray to gray, medium-grained, inequigranular alkali feldspar granite composed of alkali feldspar mesoperthite, albite, and quartz with minor biotite, aegirine, zircon, fluorite, and basnäsite; locally displays miarolitic cavities containing quartz; dated at 705±2 Ma (U-Pb zircon; Tollo and Aleinikoff, in press).

Fauquier Formation - Metaconglomerate (Proterozoic Z) at surface, covers 0.1 % of this area

Pebbles, cobbles, and occasional boulders of quartz, several varieties of granite, and feldspar, in a meta-arkosic matrix. Discontinuous lenses occur at or near the base of the Fauquier. Meta-arkose, metasiltstone, and metaconglomerate of the Fauquier (Zfs, Zfa, Zfc) are interpreted as non-marine, fluvial sediments, deposited unconformably on Grenville-age basement (Espenshade, 1986). These units have been mapped on a lithologic basis in metasedimentary outliers west of the principal Fauquier strike-belt, and include some rocks previously mapped as Mechums River Formation (Gooch, 1958). Stratigraphic and facies relations between Fauquier lithologic units and laterally equivalent Monumental Mills and other Lynchburg Group units are discussed by Wehr (1985), Wehr and Glover (1985), Conley (1989), and Kasselas (1993).

Biotite Granite Gneiss (Proterozoic Y) at surface, covers 0.1 % of this area

Pink to gray, medium-grained, well-foliated or lineated biotite-plagioclase-quartz-microcline gneiss.

Garnetiferous Leucocratic Metagranite (Proterozoic Y) at surface, covers < 0.1 % of this area

Leucocratic, medium- to fine-grained, equigranular to granoblastic monzogranite contains very-light-gray to light-gray feldspar, medium-gray quartz as much as 0.5 cm in diameter, and dusky-red euhedral to anhedral almandine garnet as much as 1 cm in diameter. Mineralogy includes quartz, microperthite, microcline, myrmekite, plagioclase, symplectitc biotite and minor chlorite, ilmenite, zircon, titanite, epidote, leucoxene, and clinozoisite. Modal composition ranges from 28 to 29 percent quartz, 35 to 40 percent potassium feldspar, 28 to 32 percent plagioclase feldspar, 2 to 5 percent almandine garnet. U-Pb zircon data suggest a crystallization age of approximately 1070 Ma (Aleinikoff and others, 1993). In northern Virginia there are numerous localities where dikes of this unit cut porphyroblastic granite gneiss (Ybp).

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

Includes dusky-green, mesocratic, coarse- to very-coarse-grained, equigranular to porphyritic, massive to vaguely foliated pyroxene-bearing granite to granodiorite; contains clinopyroxene and orthopyroxene, intermediate-composition plagioclase, potassium feldspar, and blue quartz. Reddish-brown biotite, hornblende, and poikilitic garnet are present locally; accessory minerals include apatite, magnetite-ilmenite, rutile, and zircon. Geophysical signature: charnockite pods in the southeastern Blue Ridge produce a moderate positive magnetic anomaly relative to adjacent biotite gneisses, resulting in spotty magnetic highs. This unit includes a host of plutons that are grouped on the basis of lithology, but are not necessarily consanguineous. These include Pedlar charnockite, dated at 1075 Ma (U-Pb zircon, Sinha and Bartholomew, 1984) and Roses Mill charnockite (Herz and Force, 1987), dated at 1027±101 Ma (Sm-Nd, Pettingill and others, 1984).

Mafic Plutonic Rocks (Proterozoic Z-Cambrian) at surface, covers < 0.1 % of this area

Mafic plutonic rocks as exotic blocks within melange units

Ultramafic Rocks (Proterozoic Z-Cambrian) at surface, covers < 0.1 % of this area

Grayish-green to light-gray talc chlorite-actinolite or talc-tremolite schist. Mineralogy: (1) chlorite + actinolite + talc + dolomite + ilmenite + magnetite; (2) serpentine (antigorite) + talc + chlorite ± olivine ± augite; (3) tremolite + cummingtonite + chlorite + talc + magnetite-ilmenite ± quartz. Geophysical signature: elongate positive magnetic anomaly. Elongate, lenticular bodies generally trend parallel to schistosity of enclosing rocks and are concordant at variable stratigraphic levels within the Lynchburg Group.

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

Felsic/mafic volcanic rocks as exotic blocks within melange units

Garnet-Graphite Paragneiss (Proterozoic Y) at surface, covers < 0.1 % of this area

Light-brown-weathering, medium- to fine-grained graphite-biotite-garnet-plagioclase quartz paragneiss; includes quartz-chlorite-magnetite schist and carbonaceous phyllonite; graphite makes up 10 percent of the rock in places and garnet, up to 25 percent locally. The protolith of this unit is intepreted to be metasedimentary country rock that Grenville-age plutonic rocks intruded. The rock is similar in appearance to parts of the border gneiss (Ybr).

Felsic Volcanic and Volcaniclastic Rocks (Proterozoic Z) at surface, covers < 0.1 % of this area

Dark-gray to black, biotite- and pyrite-rich volcaniclastic rock interbedded with medium-gray, fine-grained rocks with numerous quartzfilled vesicles; upper part of the unit consists of medium dark-gray, fine-grained felsic rock with numerous clasts of fine-grained, white-weathering, vesiculated felsite interlayered with fine-grained, clast-free felsic rock. Dikes of clast-free felsic rock cut nearby Middle Proterozoic granitic gneiss (Ybp); conformably overlain by feldspathic metasandstones of the Fauquier Formation.

Plagiogranite Tonalite (Cambrian-Ordovician) at surface, covers < 0.1 % of this area

Plagiogranite tonalite (Pavlides, 1990). Includes leucocratic to mesocratic plagioclase- and quartz-rich metamorphosed intrusive rocks containing little or no potassium feldspar. Plagioclase is variably altered to epidote, white mica, and chlorite. Quartz, generally blue, forms granoblastic aggregates that locally have cores of coarse-grained quartz with wavy extinction. Garnet is present locally. Hornblende, generally a minor constituent, is particularly abundant in the southwest portion of the pluton. Many of the plagiogranitic rocks have undergone cataclasis and are protomylonitic to mylonitic.