Geologic units in Floyd county, Virginia

Additional scientific data in this geographic area

Ashe Formation - Biotite gneiss (Proterozoic Z) at surface, covers 55 % of this area

Medium- to light-gray, massive, conglomeratic biotite schist and gneiss, with feldspar, quartz, and granitic clasts; grades upwards into medium- to fine-grained, salt-and-pepper-textured two-mica plagioclase gneiss with very-light-gray mica schist interbeds. Quartzite, impure marble, calcareous gneiss and amphibolite occur locally. Some dark-gray to black, pyrite-bearing mica schist occurs at tops of thick, fining-upwards graded sequences. Mineralogy: (1) quartz + plagioclase + potassium feldspar + biotite + muscovite + chlorite + epidote + ilmenite; (2) quartz + plagioclase + biotite + muscovite + epidote-allanite + garnet + titanite + ilmenite; (3) quartz + calcite + plagioclase + biotite + muscovite + epidote + ilmenite + titanite; chlorite occurs as a secondary mineral. Unit is unconformable on Grenville basement and cut by Late Precambrian mafic and felsic dikes.

Ashe Formation - Amphibolite (Proterozoic Z) at surface, covers 10 % of this area

Dark-greenish-gray to black, coarse to fine-grained amphibolite, hornblende gneiss, and schist, with interlayered biotite-muscovite gneiss and mica schist. Coarse garnetiferous amphibolite, pink and white marble, and pyrite-chalcopyrite-calcite veins are common near the top of the Ashe. Mineralogy: (1) quartz + actinolite + epidote + chlorite; (2) quartz + hornblende + plagioclase + epidote + garnet + magnetite. Geophysical signature: amphibolite, and hornblende gneiss and schist give positive linear magnetic anomalies. Relict amygdaloidal textures and hyaloclastic (pillow) structures indicate massive to thick-bedded amphibolite and hornblende gneiss were derived from basaltic flows or shallow sills. Some thin-bedded hornblende gneiss and schist units that commonly contain interbedded micaceous and feldspathic layers may be derived from volcaniclastic sedimentary rocks.

Porphyoblastic Biotite-Plagioclase Augen Gneiss (Proterozoic Y) at surface, covers 8 % 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).

Ashe Formation - Mica schist or phyllite (Proterozoic Z) at surface, covers 7 % of this area

Medium- to dark-gray, medium- to fine-grained mica schist, phyllite, and slate. Mineralogy: (1) quartz + muscovite + magnetite; (2) quartz + muscovite + chlorite + stilpnomelane; (3) quartz + plagioclase + biotite + garnet + magnetite; (4) quartz + biotite + stau ro lite + garnet + magnetite; (5) quartz + biotite + kyanite + garnet + staurolite; chlorite occurs as a secondary mineral. Geophysical signature: isolated magnetic peaks resulting from concentrations of magnetite in the wider belts of mica schist. Although these rocks have been grouped as a single unit following Espenshade and others (1975), mapping in Floyd County (Dietrich, 1959) suggests that the unit includes rocks stratigraphically at the base of the Ashe as well as rocks stratigraphically above the Ashe, coeval in part with the Alligator Back Formation.

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).

Alligator Back Formation - Feldspathic metagraywacke (Proterozoic Z-Cambrian) at surface, covers 3 % of this area

Heterogeneous assemblage of rock-types includes medium- to light-gray, laminated quartzofeldspathic to calcareous gneiss with thin mica schist partings; white and gray, fine- to coarse-grained, generally laminated marble; gray to greenish-gray fine-grained graphitic mica schist and quartzite; light-gray, medium- to fine-grained mica schist; massive quartzite and micaceous blue quartz granule metasandstone; and, dark-greenish-black actinolite schist. Mineralogy: (1) quartz + potassium feldspar + pla ioclase + biotite + muscovite + calcite + epidote + titanite + magnetite- ilmenite; (2) quartz + muscovite + chlorite + graphite + titanite + ilmenite; (3) quartz + albite + muscovite + biotite + titanite + ilmenite; (4) quartz + mus co vite + garnet + kyanite; (5) chlorite + tremolite + magnetite-ilmenite; (6) chlorite + actinolite-tremolite + talc + dolomite + magnetite-ilmenite; (7) quartz + albite + actinolite + biotite + epidote + magnetite. Units here mapped as Alligator Back Formation were previously mapped as the Evington Group (Espenshade, 1954; Brown, 1958; Redden, 1963; Gates, 1986; Patterson, 1987) and considered to be younger than the Lynchburg Group. Regional mapping by Henika (1991) and Scheible (1975) indicates that rocks assigned to Alligator Back Formation by Rankin and others (1973) are continuous with the upper part of the Lynchburg Group in the type section along the James River at Lynchburg (Jonas, 1927) and that the Alligator Back consistently dips southeast beneath the overlying Candler Formation from the Virginia-North Carolina border to the James River at Lynchburg. Sedimentary and structural facing criteria indicate that rock units immediately southeast of the Candler Formation in an outcrop belt from Stapleton on the James River, southwest to Leesville Dam on the Roanoke River, are older than the Candler (Henika, 1992). Although previously mapped as upper Evington Group (Espenshade, 1954; Brown, 1958; Redden, 1963; Patterson, 1987), these rocks are herein correlated with the Alligator Back Formation (upper Lynchburg Group), having been uplifted against the Candler Formation to the northwest along the Bowens Creek fault (Henika, 1992). Rocks in the same outcrop belt along strike to the southwest of the Leesville Reservoir were previously correlated with the Alligator Back Formation by Conley (1985). The sequence of lithologic units within the Alligator Back Formation southeast of the Bowens Creek fault is the same as that proposed by Brown (1951; 1958), and Espenshade (1954) for the formations in the Evington Group, that are structurally above the Candler Formation. The sequence is based on the detailed structural and stratigraphic relationships first established by Brown (1958) in the Lynchburg 15-minute quadrangle.

Layered Quartzofeldspathic Augen Gneiss and Flaser Gneiss (Proterozoic Y) at surface, covers 2 % of this area

Leucocratic to mesocratic, mesoscopically-layered coarse-grained quartzofeldspathic biotite gneiss contains prominent polycrystalline quartz-feldspar augen within an anastomosing, mica-rich, schistose matrix. Major mineralogy includes quartz, plagioclase, microcline, muscovite, biotite, epidote, titanite, and ilmenite; apatite and zircon are accessory minerals. This unit is gradational into biotite granulite and gneiss (Ygb), and is at least in part derived from that unit by superimposition of cataclastic to mylonitic fabric. Includes in part Stage Road layered gneiss (Sinha and Bartholomew, 1984; U-Pb discordia from 915 Ma to 1860 Ma).

Leucocratic Granulite and Gneiss (Proterozoic Y) at surface, covers 2 % 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.

Charnockite (Proterozoic Y) at surface, covers 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).

Alligator Back Formation - Actinolite schist (Proterozoic Z-Cambrian) at surface, covers 1 % of this area

Dark-grayish-green chlorite-actinolite schist metabasalt. Mineralogy: actinolite + epidote + chlorite ± biotite + albite + quartz + magnetite-ilmenite. Geophysical signature: linear, positive magnetic anomaly. Schist commonly contains recognizable flow structures, deformed and mineralized pillow basalts, pyroclastic breccia, pink and white marble, and laminated metatuff. Massive to thin beds are interlayered with metamorphosed sedimentary and mafic to ultramafic rocks. This unit was previously mapped as the Catoctin Formation or the Slippery Creek Greenstone in the Lynchburg quadrangle (Brown 1958).

Erwin and Hampton Formations (Cambrian) at surface, covers 0.9 % of this area

Erwin Formation (Keith, 1903,1907). Quartzite, sandstone, and shale. Quartzite, light-gray to white, medium- to fine-grained, thick-bedded, cross-laminated, quartz cemented, and very resistant. Sandstone, ferruginous, dark-gray to bluish- black, medium- to coarse-grained, locally conglomeratic, and with various amounts of hematite cement, in medium- to thick-beds. Shale, silty and sandy, drab-greenish-gray, thin- to medium-bedded, non-resistant, comprises much of the formation but is poorly exposed. The Erwin is less than 1000 feet thick and is equivalent to the Antietam Formation and possibly the upper part of the Harpers Formation in northern Virginia. Hampton Formation (Keith, 1903). Shale, sandstone, and quartzite. Shale, dark-gray or dark-greenish-gray, fissile, very argillaceous, silty laminae common, with interbeds of siltstone and fine-grained, lithic sandstone. Sandstone, feldspathic, greenish-gray, vitreous, medium- to coarse-grained, pebbly, cross-laminated. Quartzite, white to light-brown, vitreous, fine-grained, medium- to thin-bedded, resistant, restricted to the upper part of the formation. The Hampton is largely equivalent to the Harpers Formation to the northeast and ranges in thickness from more than 1500 feet to about 1200 feet with the thinner sequence in the northwesternmost exposures.

Layered Porphyrobliastic Pyroxene Granulite (Proterozoic Y) at surface, covers 0.9 % of this area

Leucocratic to mesocratic, segregation-layered quartzofeld-spathic granulite contains prominent potassium feldspar porphyroblasts; major mineralogy, quartz, plagioclase, K- feldspar, orthopyroxene or clinopyroxene, and magnetite-ilmenite; hornblende, reddish-brown biotite, and garnet are widespread minor constituents. Accessory minerals include apatite and zircon. Segregation layering is defined by millimeter- to centimeter scale quartz-feldspar- and pyoxene-rich domains; migmatitic leucosomes of alkali feldspar and blue quartz are common. This rock-type is considered to be pre-Grenville-age country rock, although no radiometric data is available.

Chilhowee Group (Cambrian) at surface, covers 0.6 % 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.

Porphyritic Leucocharnockite (Proterozoic Y) at surface, covers 0.2 % of this area

Leucocratic, very coarse-grained, porphyritic pyroxene-bearing granite with euhedral Potassium feldspar megacrysts, interstitial plagioclase and blue quartz; clinopyroxene and/or orthopyroxene are thoroughly uralitized; hornblende, titaniferous biotite, and garnet may be present; accessory minerals include magnetite ilmenite, apatite, and zircon. Rocks within this map unit were dated at 1075 Ma (UPb zircon; Sinha and Bartholomew, 1984), and 1021±36 Ma (Sm-Nd whole rock; Pettingill and others, 1984).

Ultramafic Rocks (Proterozoic Z-Cambrian) at surface, covers 0.2 % 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.

Unicoi Formation (Cambrian) at surface, covers < 0.1 % of this area

Unicoi Formation (Keith, 1903,1907). Sandstone and quartzite with phyllite, tuffaceous phyllite, conglomerate, and minor basalt. Sandstone, lithic or feldspathic, pinkish-gray to dark-greenish-gray, fine- to coarse-grained, angular, poorly sorted, locally conglomeratic. Quartzite, largely in upper part of the unit, white, pale-green, or gray, vitreous, medium- to coarse-grained, locally feldspathic, medium- to very-thick bedded, very resistant to weathering and erosion. Phyllite, reddish-, purplish-, or greenish-gray, as thin, sparse interbeds throughout, with purple tuffaceous phyllites in lower part. Conglomerate, fine- to coarse-polymictic-pebble conglomerate, medium- to thick-bedded, with lithic clasts and quartz pebbles. Basalt, very-dark-grayish-green, aphanitic, locally amygdaloidal; in one to three beds a few feet thick in the lower part only. Upper part has more quartzite and contains phyllite beds similar to the overlying Hampton Formation. Lower part is very feldspathic, contains most of the conglomerate beds and all of the volcanic rocks. The Unicoi is present from Augusta County to Tennessee and is laterally equivalent, at least in part, to the Weverton Formation to the northeast (King and Ferguson, 1960; Brown and Spencer, 1981; Rankin, 1993). The formation unconformably overlies the rocks of the Blue Ridge basement complex and possibly the Catoctin Formation in western Amherst County and is disconformable with the underlying Konnarock Formation in Grayson County. The upper unit is generally 600 to 1000 feet thick and the lower unit ranges from less than 100 feet to more than 1500 feet.

Rome Formation (Cambrian) at surface, covers < 0.1 % of this area

Pumpkin Valley Shale and Rome Formation. Pumpkin Valley Shale (Bridge, 1945). Shale, light-greenish-gray to dark-greenish-gray, grayish-brown, and maroon; a few beds of similar colored siltstone; sparse beds of limestone and dolostone. The Pumpkin Valley Shale conformably overlies the Rome Formation. The formation is approximately 350 feet thick. Harris (1964) identified the Pumpkin Valley Shale of Southwest Virginia as a formation within the Conasauga Group; however, because of similar lithologies it is often indistinguishable from the Rome Formation and the two formations commonly are mapped together. Rome Formation (Hayes, 1891). Siltstone, shale, sandstone, dolostone, and limestone. Siltstone and shale, greenish-gray and grayish-red, laminated to thin-bedded. Sandstone, micaceous, locally glauconitic, greenish-gray and reddish-gray, very-fine- to medium-grained, thin-bedded. Dolostone, light- to dark-gray, aphanic to medium-grained, thin-to massive-bedded, with ripple marks and mudcracks. Lime stone, argillaceous, very-light-gray to dark-gray, thin- to medium- bedded. Carbonate rocks range from sparse 1- to 2- feet-thick beds in western Scott County to discontinuous units as much as 50 feet thick which comprise 30 to 40 percent of the formation in western Russell and Washington counties (Evans and Troensegaard, 1991; Bartlett and Webb, 1971). Maximum recorded thickness is 1500 feet in the Clinchport area (Brent, 1963); although this may have included the Pumpkin Valley Shale. A complete thickness has not been determined because the lowermost part of the Rome Formation is normally absent due to faulting.

Shady Dolomite (Cambrian) at surface, covers < 0.1 % of this area

Shady Dolomite (Keith, 1903). Dolostone with minor limestone and shale divided into three members: Ivanhoe (upper) Member; Austinville (middle) Member, and Patterson (lower) Member. Ivanhoe Member, dark-gray, fine-grained limestone and minor interbedded black shale; 100 to 500 feeet thick. Austinville Member, very-light-gray to cream colored, fine- to medium-grained, crystalline or saccharoidal, massive-bedded dolostone with several sequences of interbedded limestone, very-dark-gray dolostone or mottled dolostone and shale; 1000 feet thick. Patterson Member, medium- to dark-gray, fine-grained, thin-bedded dolostone or limestone with siliceous partings and intraformational brec ia beds; 800 feet thick. The Shady Dolomite is gradational with the underlying Erwin Formation and the upper two members grade southeastward into shaly dolostone with biohermal mounds, intraformational limestone or dolostone breccias, oolitic limestone, and arenaceous limestone and dolostone. This upper,southeastern facies, is in part equivalent to beds in the lower Rome Formation (Pfi el and Read, 1980). The Shady is very poorly exposed except near New River in Wythe and Smyth counties where it is at least 2100 feet thick and where major lead and zinc deposits were mined from the upper members (Currier, 1935).