Geologic units in Greene county, Virginia

Additional scientific data in this geographic area

Layered Pyroxene Granulite (Proterozoic Y) at surface, covers 21 % 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).

Catoctin Formation - Metabasalt (Proterozoic Z-Cambrian) at surface, covers 18 % 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).

Charnockite (Proterozoic Y) at surface, covers 13 % 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).

Charnockite Gneiss (Proterozoic Y) at surface, covers 8 % 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.

Lynchburg Group - Metagraywacke (Proterozoic Z) at surface, covers 7 % 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 5 % 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).

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).

Fauquier Formation - Metasiltstone and phyllite (Proterozoic Z) at surface, covers 2 % 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.

Robertson River Igneous Suite (Proterozoic Z) at surface, covers 1 % 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 1 % 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.

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.

Robertson River Igneous Suite - Hitt Mountain alkali feldspar syenite (Proterozoic Z) at surface, covers 1 % 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).

Alkali Feldspar Leucogranite (Proterozoic Y) at surface, covers 0.8 % 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.

Chilhowee Group (Cambrian) at surface, covers 0.2 % 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.

Biotite Granite (Proterozoic Y) at surface, covers 0.1 % 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.

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.

Fauquier Formation - Arkosic metasandstone (Proterozoic Z) at surface, covers < 0.1 % 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).