Geologic units in Culpeper county, Virginia

Additional scientific data in this geographic area

Diabase (Jurassic) at surface, covers 12 % 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.

Newark Supergroup; Triassic shale and siltstone (Triassic) at surface, covers 12 % 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.

Lynchburg Group; Charlottesville Formation (Proterozoic Z) at surface, covers 11 % 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.

Catoctin Formation - Metabasalt (Proterozoic Z-Cambrian) at surface, covers 10 % 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).

Layered Biotite Granulite and Gneiss (Proterozoic Y) at surface, covers 7 % of this area

Leucocratic to mesocratic, segregation-layered quartzofeldspathic granulite and gneiss contain quartz, plagioclase (albite), microcline (includes assemblages with one alkali feldspar), biotite, ilmenite, and titanite; garnet and horn blende are commonly present. Accessory minerals include apatite and zircon. Epidote and white mica are ubiquitous secondary minerals. Relict pyroxene, largely replaced by actinolitic amphibole, occurs locally. Segregation layering is defined by alternating quartzofeldspathic and biotite-rich domains on the order of a few millimeters to centimeters thick. Quartz and feldspar are granoblastic; biotite defines a penetrative schistosity that crosscuts segregation layering. Migmatitic leucosomes composed of alkali feldspar and blue quartz cut segregation layering, and locally define attenuated isoclinal folds. This unit surrounds pods of layered pyroxene granulite (Ypg), and is cut by Grenville-age metaplutonic rocks including porphyroblastic biotite-plagioclase augen gneiss (Ybg) and alkali feldspar granite (Yal). The unit has been correlated with Flint Hill Gneiss (Yfh) (Evans, 1991), and may correlate with Stage Road layered gneiss of Sinha and Bartholomew (1984). These gneisses have been interpreted as derived from layered pyroxene granulite (Ypg) by retrograde hydration reactions (Evans, 1991).

Newark Supergroup; Conglomerate, greenstone clasts (Upper Triassic) at surface, covers 7 % 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.

Mine Run Complex - Melange Zone III (Proterozoic Z-Ordovician) at surface, covers 7 % 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).

Lynchburg Group - Metagraywacke (Proterozoic Z) at surface, covers 5 % 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).

Porphyroblastic Granite Gneiss (Proterozoic Y) at surface, covers 4 % 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.

Lynchburg Group; Monumental Mills Formation (Proterozoic Z) at surface, covers 3 % 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).

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.

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; Triassic Sandstone, Siltstone, and Shale (Upper Triassic) at surface, covers 2 % 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.

Robertson River Igneous Suite - Arrington Mountain alkali feldspar granite (Proterozoic Z) at surface, covers 2 % of this area

Light gray, medium-grained, equigranular alkali feldspar granite composed of microcline microperthite, quartz, and plagioclase, with hastingsitic amphibole, biotite, stilpnomelane, allanite, fluorite, zircon, epidote, apatite, rare garnet, and muscovite. The granite is locally intruded by light-gray, fine-grained dikes of mineralogically identical alkali feldspar granite. The unit has been dated at 730± 4 Ma (U-Pb zircon; Tollo and Aleinikoff, in press).

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

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.

Metagabbro (Proterozoic Z-Cambrian) at surface, covers 1.0 % 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 0.9 % 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).

Plagiogranite Tonalite (Cambrian-Ordovician) at surface, covers 0.8 % 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.

Mine Run Complex - Melange Zone II (Proterozoic Z-Ordovician) at surface, covers 0.8 % 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).

Porphyoblastic Biotite-Plagioclase Augen Gneiss (Proterozoic Y) at surface, covers 0.7 % 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).

Robertson River Igneous Suite - Battle Mountain Alkali Feldspar Granite - felsite (Proterozoic Z) at surface, covers 0.7 % of this area

Light-gray to gray, fine-grained to aphanitic, equigranular to sparsely porphyritic (mesoperthite phenocrysts) felsite composed of alkali feldspar, quartz, and rare plagioclase with minor biotite, aegirine, fluorite, and basnäsite; locally displays flow banding, miarolitic cavities containing quartz, and probable lithophysae; intrudes associated alkali feldspar granite; dated at 704±4 Ma (U-Pb zircon; Tollo and Aleinikoff, in press).

Goldvein Pluton (Cambrian-Ordovician) at surface, covers 0.6 % 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.

Newark Supergroup; Jurassic Interbedded Sandstone and Siltstone (Lower Jurassic) at surface, covers 0.4 % 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.

Lynchburg Group - Conglomerate and metagraywacke (Proterozoic Z) at surface, covers 0.4 % of this area

Lynchburg Group - Conglomerate and metagraywacke.

Chopawamsic Formation - Interlayered felsic and mafic metavolcanic rocks (Cambrian) at surface, covers 0.3 % of this area

Chopawamsic Formation, undivided, (Pavlides, 1981). Includes laterally discontinuous lenses and tongues of metamorphosed felsic, intermediate, and mafic volcanic flows and volcanoclastic rocks, with interlayered quartzite, quartzose graywacke, schist, and phyllite. Volcanic flows are locally highly vesicular; fragmental breccia and tuff are common. Felsic flows are typically light-gray aphanitic rocks with phenocrysts of quartz and feldspar; intermediate flows are dark-green amphibole-bearing rocks with fine-grained quartz-feldspar matrix; greenstone metabasalts contain blue green amphibole, chlorite, albitic plagioclase, and quartz. Geophysical signature: linear strike-elongate pattern of elevated magnetic anomalies. The Chopawamsic is correlated with the James Run Formation in Maryland; the James Run has been dated at 570 to 530 Ma (U-Pb zircon; Tilton and others 1970). The Chopawamsic is unconformably overlain by the Late Ordovician Arvonia and Quantico Formations. Pavlides (1981 and subsequent works) has made the interpretation on the basis of geologic and geochemical data that the Chopawamsic and related plutons represent an ancient is land-arc sequence.

Robertson River Igneous Suite - Hitt Mountain alkali feldspar syenite (Proterozoic Z) at surface, covers 0.3 % of this area

Light-gray, coarse-grained to locally pegmatitic, inequigranular alkali feldspar syenite composed of microcline mesoperthite, quartz, and saussuritized plagioclase, with lesser hastingsitic amphibole, biotite, allanite, zircon, stilpnomelane, apatite, and rare garnet; locally displays cumulate and pseudocumulate texture. The syenite has been dated at 706± 2 Ma (U-Pb zircon; Tollo and Aleinikoff, in press).

Catoctin Formation - Metabasalt breccia (low titanium) (Proterozoic Z-Cambrian) at surface, covers 0.3 % 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.

Marshall Metagranite (Proterozoic Y) at surface, covers 0.3 % 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.

Ultramafic Rocks (Proterozoic Z-Cambrian) at surface, covers 0.3 % 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.

Newark Supergroup; Jurassic Basalt (Lower Jurassic) at surface, covers 0.3 % 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.

Metasandstone and Metasiltstone (Proterozoic Z-Cambrian) at surface, covers 0.3 % 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.

Metavolcanic and Metavolcanic Clastic Phyllite (Proterozoic - Cambrian) at surface, covers 0.2 % 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).

Robertson River Igneous Suite - Battle Mountain granite (Proterozoic Z) at surface, covers 0.2 % 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).

Micaceous Quartzite (Cambrian) at surface, covers 0.2 % of this area

Coarse-grained micaceous quartzite and feldspathic metasandstone with interbedded metasiltstone and phyllite; occurs above Catoctin greenstone (CZc) in an outlier surrounded by Mesozoic rocks east of Culpeper. These rocks are lithologically similar to, and probably correlative with micaceous quartzites in the True Blue formation of Pavlides (1989, 1990), and with feldspathic metasandstone lenses in the Candler Formation.

Lynchburg Group - Graphitic phyllite and metasiltstone (Proterozoic Z) at surface, covers 0.2 % 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.

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).

Fauquier Formation - Metasiltstone and phyllite (Proterozoic Z) at surface, covers 0.1 % 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.

Lynchburg Group - Quartzite (Proterozoic Z) at surface, covers < 0.1 % of this area

Lynchburg Group - Quartzite

Catoctin Formation - Metasedimentary rocks (Proterozoic Z-Cambrian) at surface, covers < 0.1 % 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).

Candler Formation - Phyllite and schist (Cambrian) at surface, covers < 0.1 % 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).

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

Includes white and light-gray to grayish-blue, fine-grained dolomitic marble and siliceous marble, dolomitic meta-arkose, dolomitic quartz-muscovite schist, and calcitic marble; may contain quartz, feldspar, muscovite, phlogopite, and tremolite. Marble is poorly exposed in discontinuous lenses either just below the top of the Fauquier Formation laminated metasiltstone. (Zfl ) or just above the base of the Catoctin (CZc); a lense of dolomitic marble occurs within the Swift Run Formation in Loudoun County. Along the Hazel River, a marble clast conglomerate with a biotite-rich feldspathic matrix occurs just below the base of the Catoctin Formation.

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

Mafic plutonic rocks as exotic blocks within melange units

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).

Robertson River Igneous Suite - Amissville alkali feldspar granite (Proterozoic Z) at surface, covers < 0.1 % of this area

Light- to dark gray, medium-grained, inequigranular to porphyritic (quartz phenocrysts) alkali feldspar granite composed of microcline mesoperthite, quartz, and plagioclase, with aegirine, riebeckite, biotite, zircon, fluorite, and stilpnomelane. Euhedral quartz phenocrysts are diagnostic; locally displays miarolitic cavities containing quartz; intruded by fine-grained dikes of identical mineralogic composition. Field relations indicate the Amissville is older than Battle Mountain granitoid; geochemistry indicates these units are comagmatic.

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

Felsic/mafic volcanic rocks as exotic blocks within melange units