USGS B 2123 -- Paleozoic Rocks

The Chilhowee Group consists of the Loudoun, Weverton, Harpers, and Antietam Formations. The stratigraphic relation between the Chilhowee Group and the underlying Late Proterozoic Catoctin Formation has been interpreted as either transitional (Nickelsen, 1956) or unconformable (King, 1950; Reed, 1955; Edmundson and Nunan, 1973; Gathright and Nystrom, 1974). Rounded clasts of red jasper and metabasalt (King, 1950; Reed, 1955; and Rader and Biggs, 1975) indicate a period of erosion between the two formations. Simpson and Sundberg (1987) identified Early Cambrian fossils, above a bed of metabasalt, in the Unicoi Formation in southwestern Virginia. The correlative rocks of the Chilhowee Group are interpreted to be a transgressive sequence of a rift-to-drift transitional regime.

Two units occur in the Loudoun Formation: phyllite at the base(-CZlp) and coarse quartz pebble conglomerate (-Clc)at the top. The base of the phyllite is placed at the top of the metabasalt and (or) tuffaceous metasedimentary rocks of the Catoctin Formation. The coarse pebble conglomerate occurs in discontinuous lenses.

Confusion about the use of the name Loudoun Formation has been persistent (Stose and Stose, 1946; Woodward, 1949; Bloomer and Werner, 1950; King, 1950; Cloos, 1951; Furcron, 1969) because it is largely indistinguishable from phyllites in the Swift Run and Catoctin Formations (Nickelsen, 1956). Phyllite in the Loudoun Formation is tuffaceous with elongated amygdules so it may be genetically related to Catoctin volcanism (Stose and Stose, 1946; King, 1950; Reed, 1955; Toewe, 1966; Gathright and Nystrom, 1974). The phyllite has been mapped previously as a unit within the Catoctin Formation, and the conglomerate as a unit within the lower member of the Weverton Formation (King, 1950; Reed, 1955; Freedman, 1967; Gathright and Nystrom, 1974; Rader and Biggs, 1975; Lukert and Nuckols, 1976; Southworth, 1991a).

Phyllite of the Loudoun Formation(-CZlp) is well exposed north of Purcell Knob on the limbs of inverted folds (pl. 1, ref. loc. 10). It is in sharp contact with the overlying conglomerate of the Loudoun or quartzites of the Buzzard Knob Member of the Weverton Formation. The phyllite is gray black and grades downward to light-olive gray and contains pale-red-purple lithic clasts (rhyolite?) in a dusky-blue matrix. Medium- to light-gray phyllite contains pinkish-gray, very light gray, and pale-red-purple amygdules elongated in the slaty cleavage plane, which indicates a volcanic origin. Some of the black phyllite may be an iron-rich paleosol (Reed, 1955; Nickelsen, 1956). At the Loudoun Heights trail, approximately 10 m of phyllite containing very fine quartz pebbles is interbedded with thin layers of metabasalt. Phyllite of the Loudoun is not observed on the east side of Short Hill Mountain.

Conglomerate of the Loudoun Formation(-Clc) is a lensoid, discontinuous, coarse, quartz-pebble conglomerate that may be channel fill of the Weverton Formation cut into the phyllite of the Loudoun. The contact with the phyllite is sharp, and the conglomerate contains rip-up clasts of the phyllite.

Conglomerate of the Loudoun Formation is best exposed north of Purcell Knob (pl. 1, ref. loc. 11) in vertical contact with the phyllite of the Loudoun and quartzite of the Buzzard Knob Member of the Weverton Formation. The quartz-pebble conglomerate contains rounded white and purple quartz clasts as large as 6 cm in diameter in a hematite-rich matrix. The conglomerate is arkosic, and some beds are cross stratified. A 1-m-thick, light-gray, massive cross-stratified quartzite lies between the conglomerate and the phyllite at one locality (fig. 10).

On the Loudoun Heights trail, coarse pebbles to small cobbles of rounded to subrounded quartz and red jasper occur at the base of the Buzzard Knob Member of the Weverton Formation. No conglomerate is observed on the east side of Short Hill Mountain, but it is present on South Mountain, Md., approximately 8 km north of the map area.

The Weverton Formation has traditionally been divided into informal lower, middle, and upper members (Nickelsen, 1956; McDowell and Milton, 1992). The type locality is located along U.S. Route 340 near Weverton, Md. (pl. 1 and fig. 11). Brezinski (1992) named these members the Buzzard Knob, Maryland Heights, and Owens Creek Members, respectively (fig. 1B). The Buzzard Knob Member consists of quartzite interbedded with metasiltstone. In places, this lower member is transitional with the conglomerate of the Loudoun Formation. Where the conglomerate is absent, the base of the Weverton may be unconformable on the phyllite of the Loudoun or the Catoctin Formation. The Maryland Heights Member consists of quartzite, metasiltstone, and metagraywacke. The Owens Creek Member consists of pebble conglomerate, quartzite, and metasiltstone and grades into the thin-bedded metasiltstone, arkose, and fine pebble conglomerate of the overlying Harpers Formation.

Rocks of the Weverton Formation are interpreted as alluvial sediments (Schwab, 1986) deposited at the base of a marine transgressive sequence and reflect a change from a volcaniclastic to a predominately fluvial environment; paleocurrent directions suggest a source from the west (Whitaker, 1955). Individually, the Buzzard Knob, Maryland Heights, and Owens Creek Members are fining-upward sequences, but the Owens Creek Member is coarser and more poorly sorted than the Buzzard Knob and Maryland Heights Members.

The Buzzard Knob Member(-Cwb) consists of two well-sorted, crossbedded, mature quartzite beds that are separated by and interbedded with light-colored sandy metasiltstone. The Buzzard Knob Member is best seen at the type locality of the Weverton Formation (Weverton Cliffs); (pl. 1, ref. loc. 12) (figs. 11 and 12B). At Purcell Knob, the Buzzard Knob Member is arkosic and rests unconformably on hornblende gneiss (Yhg). Stose and Stose (1949) recognized this relation and interpreted doming and erosion between the time of deposition of the Catoctin and Weverton Formations.

The Maryland Heights Member(-Cwm) consists of interbedded, dark-greenish-gray metasiltstone and metagraywacke with dusky-blue to greenish-gray, very coarse grained to granular quartzite. The dark metasiltstone is similar in appearance to metasiltstones in the lower part of the Harpers Formation (King, 1950) and has in the past been confused with it (Woodward, 1949). Quartzite beds vary from 5 to 10 m in thickness and are well exposed at Weverton Cliffs (fig. 11) as well as at the type section of the member (pl. 1, ref. loc. 13) along the railroad tracks at Blue Ridge-Elk Ridge (fig. 12A). The Maryland Heights Member is transitional with the underlying Buzzard Knob Member and overlying Owens Creek Member and is often mapped on the basis of a swale between the topographic ledge of the adjacent quartzite beds.

The Owens Creek Member(-Cwo) is a diagnostic "gun-metal blue" (Nickelsen, 1956) to green-gray coarse-grained sandstone to pebble conglomerate. The Owens Creek Member can be seen on Short Hill Mountain where upright and overturned sections are preserved in the Hillsboro syncline, in the cliffs of Maryland Heights (fig. 12A), and the Weverton Cliffs (fig. 11). The Owens Creek Member is more poorly sorted than the Buzzard Knob and Maryland Heights Members and contains pebbles of blue and red quartz, magnetite, opaque minerals, and blue-green phyllite clasts, 0.5 to 15 cm long, that give it a dark blue color. At the base of the Owens Creek Member is 4 m of clean gray-green conglomeratic quartzite. More than 8 m of gun-metal blue pebble conglomerate grades upward into 20 m of green-gray pebble conglomerate at the top. A total section of 27 to 32 m is in agreement with the 23 to 39 m measured by Nickelsen (1956). Exposures on Blue Ridge at the Potomac River gorge are so deformed by folding and cleavage that the member on Chimney Rock (local usage) was interpreted by Woodward (1949) to be part of the Harpers Formation.

The Harpers Formation(-Ch) is a sequence of phyllitic metasiltstone interbedded with meta-arkose and pebble conglomerate at the base and ferruginous, magnetite-rich sandstones(-Chq; pl. 1, ref. loc. 14) in the upper part. Skolithos tubes (trace fossil) are interpreted to suggest that the Harpers Formation was deposited in marine environments of the transgressive Chilhowee Group (Simpson and Eriksson, 1989).

Strongly developed cleavage typically obscures bedding in outcrops of the Harpers Formation. Bedding-cleavage relations can be seen along the Shenandoah River near the type locality and along the Potomac River on the Sandy Hook Road. Within the Harpers Ferry National Historical Park, the stairs to "Jefferson's rock" are excavated along cleavage and bedding planes. Deformation is less intense west of the thrust fault near the U.S. Route 340 bridge at Bolivar, W.Va., where bedforms occur in light-gray sandstone and metasiltstone. Metasiltstone of the Harpers Formation is phyllonitic in the immediate upper plate of the Short Hill fault near Weverton, Md. Calcareous and sandy metasiltstones, magnetite-rich arkose, thin-bedded metasandstones and a 1-m-thick, dark greenish-gray fine pebble conglomerate are exposed on Short Hill-South Mountain along the Potomac River gorge.

The Antietam Formation is gradational with the underlying Harpers Formation. The lowermost strata consists of thin (2 to 6 cm) very light gray quartzites with numerous Skolithos burrows interbedded with green-gray, sandy metasiltstone. Higher within the formation, the Antietam is characterized by bioturbated, very light gray, medium-bedded, well-sorted, fine- to medium-grained sandstone. The uppermost lithologies are composed of medium-gray, calcareous, crossbedded, coarse-grained sandstone. These rocks are exposed near Dargan, Md. (pl. 1, ref. loc. 15).

The Tomstown Formation is the youngest Paleozoic unit exposed in the map area (pl. 1). Brezinski (1992) sub-divided the Tomstown into the Bolivar Heights, Fort Duncan, Benevola, and Dargan Members (in ascending order); the latter is not exposed in the Harpers Ferry quadrangle (fig. 1B). Carbonate rocks of the Tomstown Formation are indicators that the passive or trailing continental margin was established (Read, 1989). The vertical sequence of lithologies from the Bolivar Heights Member to the Benevola Member suggests that changes from shallow carbonate shelf to deep shelf to bank edge carbonate sand shoal had occurred. The Dargan Member was deposited during shallowing into more peritidal type environments.

The Bolivar Heights Member(-Ctbh) is characterized by thin-bedded, dark-gray, fine-grained limestone with tan, wispy, and rounded dolomitic burrows (pl. 1, ref. loc. 16). Bioturbation becomes more prevalent upsection. At the base of the formation is a 15-m-thick interval of very light gray to tan mylonitic marble that Brezinski (1992, p. 25) termed the Keedysville marble bed (fig. 13). Brezinski and others (1992) proposed that this marble occurs in a stratigraphically restricted fault zone that can be traced for more than 100 km from Berryville, Va., northward into south-central Pennsylvania.

The Fort Duncan Member(-Ctf) is a 70-m-thick interval of dark-gray, burrow-mottled, thick bedded dolomite (pl. 1, ref. loc. 17). The contact with the underlying Bolivar Heights Member is sharp and likely erosional in origin. The cone-shaped fossil Salterella occurs throughout this member. This member is equivalent to the Vintage Dolomite of the Conestoga Valley of Pennsylvania both in stratigraphic position and lithologic character. Facies analysis suggests that the Fort Duncan Member was deposited in the deepest water deposits of the Lower Cambrian of the western Blue Ridge cover rocks.

The Benevola Member(-Ctb) consists of approximately 25 m of very thick bedded to massive, light-gray sugary dolomite. Faint crossbedding is common. The lower contact is gradational with the underlying bioturbated dolomites of the Fort Duncan Member and can usually be placed where burrows are no longer evident. The purity and massive nature of this member make it a favored lithology for quarrying, such as near Millville, W. Va., in the adjacent Charles Town quadrangle.