Geologic units in Albemarle county, Virginia

Additional scientific data in this geographic area

Catoctin Formation - Metabasalt (Proterozoic Z-Cambrian) at surface, covers 19 % 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).

Porphyoblastic Biotite-Plagioclase Augen Gneiss (Proterozoic Y) at surface, covers 17 % 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).

Candler Formation - Phyllite and schist (Cambrian) at surface, covers 11 % 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).

Lynchburg Group; Charlottesville Formation (Proterozoic Z) at surface, covers 7 % 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.

Layered Pyroxene Granulite (Proterozoic Y) at surface, covers 6 % 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).

Metagraywacke, Quartzose Schist, and Melange (Proterozoic Z-Cambrian) at surface, covers 5 % 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).

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).

Mylonite, Mylonite Gneiss, and Cataclastic Rocks (Proterozoic - Paleozoic ?) at surface, covers 4 % 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).

Charnockite (Proterozoic Y) at surface, covers 4 % 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).

Layered Biotite Granulite and Gneiss (Proterozoic Y) at surface, covers 4 % 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; Triassic Sandstone, Siltstone, and Shale (Upper Triassic) at surface, covers 4 % 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.

Charnockite Gneiss (Proterozoic Y) at surface, covers 3 % of this area

Leucocratic to mesocratic, coarse grained, porphyritic, pyroxene-bearing granite gneiss; well-developed shear foliation is superimposed on segregation layering defined by quartz-feldspar and mafic-rich domains; includes mylonitic augen gneiss.

Biotite-Muscovite Leucogranite Gneiss (Proterozoic Y) at surface, covers 2 % of this area

Leucocratic, coarse- to very-coarse-grained granite gneiss contains subhedral to euhedral Potassium feldspar megacrysts, and interstitial quartz and plagioclase; muscovite, biotite, epidote, titanite, and magnetite-ilmenite make up 5 to 15 percent of the mode; hornblende and garnet are locally present. Plagioclase is largely replaced by felty masses of epidote, muscovite, and titanite; clusters of reen-brown, biotite laths contain subhedral to euhedral epidote prisms; magnetite-ilmenite is rimmed by anhedral masses of titanite. The rock contains a penetrative schistosty defined by biotite and muscovite.

Swift Run Formation (Proterozoic Z) at surface, covers 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 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.

Lynchburg Group - Fanglomerate (Proterozoic Z) at surface, covers 0.8 % of this area

Matrix-supported, poorly-sorted pebbly to cobbly lithic conglomerate occurs at the base of the Lynchburg Group; includes in part the Rockfish conglomerate formation of Nelson (1962).

Biotite Granite (Proterozoic Y) at surface, covers 0.8 % of this area

Leucocratic, medium- to coarse grained, gneissic granite; dominantly perthitic Potassium feldspar, plagioclase, and quartz, with minor biotite, muscovite, and ilmenite; accessories include apatite and zircon. Feldspars show alteration to epidote and sericite; gneissic fabric is defined by flatten quartz and feldspars.

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.

Robertson River Igneous Suite (Proterozoic Z) at surface, covers 0.7 % of this area

White Oak Alkali Feldspar Granite. Light-gray to gray, coarse-grained, inequigranular alkali feldspar granite composed of microcline microperthite, quartz, and plagioclase, with hastingsitic amphibole, allanite, fluorite, stilpnomelane, zircon, chlorite, and calcite. Unaltered surfaces display pronounced vitreous luster. Granite typically occurs intermixed with light-gray, fine-grained, mineralogically identical alkali feldspar granite that is locally younger. The unit has been dated at 725± 8 Ma (U-Pb zircon; Tollo and Aleinikoff, in press).

Two-Mica Granite (Proterozoic Y) at surface, covers 0.7 % of this area

Two-mica granite (Conley, 1989). Coarse-grained, inequigranular muscovite-biotite two-feldspar granite gneiss contains 2-cm clots of biotite and relict orthopyroxene; accessory minerals are epidote, rutile, actinolite, hornblende, and zircon. Unit is considered intrusive into biotite augen gneiss.

Lynchburg Group - Graphitic phyllite and metasiltstone (Proterozoic Z) at surface, covers 0.6 % 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.

Newark Supergroup; Breccia, mudstone clasts (Upper Triassic) at surface, covers 0.6 % of this area

Angular to subangular pebbles and cobbles of pre dominantly mudstone in a siliceous cemented sandstone. Occurs only in the Scottsville basin.

Biotite Granodiorite and Biotite Granite (Proterozoic Z) at surface, covers 0.6 % of this area

Leucocratic to mesocratic, medium-grained, equigranular, vaguely foliated, epidote-, garnet-, fluorite- and/or allanite-bearing, two-feldspar muscovite-biotite granodiorite to granite; salt-and-pepper appearance is characteristic; associated aplite and pegmatite dikes common. Geophysical signature: weak positive radiometric anomaly. The plutons of this suite are widespread in the southeastern portion of the basement complex, and are correlated with Rockfish River, Mobley Mountain, Striped Rock plutons, and with Robertson River Igneous Suite.

Fauquier Formation - Metasiltstone and phyllite (Proterozoic Z) at surface, covers 0.5 % 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.

Candler Formation - Metasiltstone (Cambrian) at surface, covers 0.5 % 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).

Catoctin Formation - Hyaloclastite pillow breccia (Proterozoic Z-Cambrian) at surface, covers 0.5 % 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.

Mafic Igneous Complex Undivided (Proterozoic Z-Cambrian) at surface, covers 0.5 % of this area

Metamorphosed stratiform mafic and ultramafic rocks include: greenish-gray, locally layered, coarse-grained metagabbro; dark-greenish-black schistose metabasalt; and, gray to grayish-green talc-chlorite-tremolite schist. Mineralogy: (1) chlorite + epidote + plagioclase + quartz + titanite + ilmenite; (2) chlorite + actinolite + biotite + epidote + titanite + plagioclase + quartz; (3) chlorite + actinolite + talc + dolomite + magnetite-ilmenite; (4) tremolite + chlorite + magnetite-ilmenite; (5) serpentine + talc + chlorite + actinolite ± olivine ± augite. Geophysical signature: strike-elongate positive magnetic anomaly. Metamorphosed mafic and ultramafic complexes are generally sheet-like bodies, concordant near the base of the Alligator Back and Charlottesville Formations. In Nelson County, these rocks are cut by metagabbroic dikes (CZmd) that are likely related to the Catoctin. Hess (1933) reports a stratified association of ultramafic, mafic, and minor silicic lithologies at Schuyler which he attributes to in situ differentiation of a sheet-like concordant intrusion. Glover and others (1989) report a non-tectonized intrusive contact between Charlottesville Formation metasiltstone and ultramafic schist at Schulyer. In contrast, Conley (1985) presents evidence that ultramafic-mafic complexes in Franklin County are tectonicly-emplaced slices of oceanic crust (ophiolites). Tectonic setting and mode of emplacement for these rock assemblages remain enigmatic; correlation of complexes in the southwestern Piedmont with those in the central Blue Ridge may ultimately prove invalid.

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

Crozet granite (Nelson, 1962). Leucocratic, coarse grained, porphyritic alkali feldspar granite.

Robertson River Igneous Suite - Rivanna granite (Proterozoic Z) at surface, covers 0.2 % of this area

White, medium-grained, biotite-, allanite-, fluorite-, and stilpnomelane-bearing granite to alkali feldspar granite; color index less than 5; locally displays miarolitic cavities containing quartz and pyrite. The granite has been dated at 735± 4 Ma (U-Pb zircon; Tollo and Aleinikoff, in press).

Rockfish River Pluton (Proterozoic Z) at surface, covers 0.2 % of this area

Rockfish River pluton (Sinha and Bartholomew, 1984). Medium-gray, equigranular medium-grained, vaguely-foliated, fluorite-, epidote-, and garnet-bearing muscovite-biotite granodiorite; aplite and pegmatite dikes cut pluton and surrounding country rock. The pluton has been dated at 730 Ma (U-Pb zircon; Sinha and Bartholomew, 1984, recalculated from Davis, 1974); and 573±50 Ma (Rb-Sr whole-rock; Smith and others, 1981).

Alkali Feldspar Leucogranite (Proterozoic Y) at surface, covers < 0.1 % of this area

Leucocratic, coarse grained to megacrystic, equigranular to porphyritic granite contains white alkali feldspar phenocrysts and interstitial blue quartz, with accessory biotite, pyroxene, and garnet; primary flow-banding is locally delineated by aligned feldspar phenocrysts. Geophysical signature: positive radiometric anomaly. This lithology occurs as dikes and discrete plutons, comprises migmatitic leucosomes within early or pre-Grenville age layered gneisses, and occurs as xenoliths with in some Grenville-age plutonic rocks. This is a lithologic unit that likely includes rocks spanning a range of ages.

Newark Supergroup; Conglomerate, carbonate clasts (Upper Triassic) at surface, covers < 0.1 % of this area

Occurs only in the Culpeper basin. Rounded to subrounded pebbles, cobbles, and boulders of predominantly Cambrian and Ordovician limestone and dolostone in a matrix of fine- to coarse-grained, calcite-cemented, light-gray, silty sandstone. Occurs only in the Culpeper basin.

Newark Supergroup; Conglomerate, mixed clasts (Upper Triassic) at surface, covers < 0.1 % 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.

Leucocratic Granulite and Gneiss (Proterozoic Y) at surface, covers < 0.1 % of this area

Leucocratic, fine- to medium-grained layered gneiss contains predominantly alkali feldspar and blue quartz; ferromagnesian minerals including pyroxene, ilmenite, horn blende, reddish-brown biotite, or garnet constitute less than one percent of the mode. Quartz and feldspar are granoblastic; gneissic fabric is defined by discontinuous quartz-rich domains.

Newark Supergroup; Sandstone, undifferentiated (Upper Triassic) at surface, covers < 0.1 % 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.

Everona Limestone (Cambrian) at surface, covers < 0.1 % 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.