Geologic units in Bland county, Virginia

Additional scientific data in this geographic area

Brallier Formation (Devonian) at surface, covers 24 % of this area

Brallier Formation (Butts, 1918). Shale, sandstone, and siltstone. Shale, partly silty, micaceous, greenish-gray, gray ish-brown and medium- to dark-gray, black, weathers light-olive-gray with light-yellow, brown and purple tints; black shale in thin beds and laminae, sparsely fossiliferous. Sandstone, micaceous, medium-light-gray, very-fine- to fine-grained, thin- to thick-bedded, and light-brown siltstone interbedded with shale. Locally siltstone is in very-thin, nodular, ferruginous lenses (Bartlett, 1974). Lower contact transitional; base at lowest siltstone bed above relatively nonsilty dark-gray shale. Equivalent to part of the Chattanooga Shale. Formation thins southwestward; it ranges from 940 feet in thickness in southwestern Washington County (Bartlett and Webb, 1971) to more than 2200 feet in Augusta County (Rader, 1967).

Knox Group (Cambrian-Ordovician) at surface, covers 15 % of this area

Knox Group (Safford, 1869). Dolostone, limestone, and sandstone. Dolostone, light- to medium-gray, very-fine- to fine-grained, locally with pink streaks in the upper part; and very-light-gray to dark-gray and brownish-gray, medium- to coarse-grained, locally argillaceous dolostone near the base of the unit; greenish-gray shale partings locally present; chert is abundant in some parts of the unit. Limestone, blue gray to dark-blue-gray, very-fine- to coarse-grained, locally sandy. Sandstone, gray to brown, fine- to medium-grained. Limestone is dominant in the eastern thrust belts. The Knox Group ranges from 2000 feet in Southwest Virginia to 3560 feet in thickness to the east in Washington County (Bartlett and Webb, 1971). The Knox includes the Mascot, Kingsport, Chepultepec, and Copper Ridge Dolomites and the Maynardville Formation.

Millboro Shale, Huntersville Chert, and Rocky Gap Sandstone (Devonian) at surface, covers 13 % of this area

Millboro Shale, Huntersville Chert, and Rocky Gap Sandstone. Millboro Shale. Refer to previous description under Dmn. Huntersville Chert (Price, 1929). Chert, white, thin-bedded, iron-stained, blocky, fossiliferous with cherty, glauconitic sandstone and greenish-gray shale. The Huntersville Chert ranges from 10 to 60 feet in thickness (Bartlett and Webb, 1971). Butts (1940, p. 303) states, "The Onondaga [Huntersville Chert] persists to Mendota, Washington County, but 10 miles farther southwest it is absent in a fully exposed section". The Huntersville correlates with the Needmore Formation to the northeast and the upper part of the Wildcat Valley Sandstone in Lee County. Rocky Gap Sandstone (Swartz, 1929): Sandstone, medium- to light-gray, weathers dark-yellowish-orange, coarse-grained, scattered, thin, quartz-pebble conglomerate beds, arenaceous chert in upper ten feet, calcite cement, friable when weathered. Thickness ranges from 0 near McCall Gap, Washington County to about 85 feet in Bland and Giles counties. Equivalent in part to the Wildcat Valley Sandstone of Lee County and the Ridgeley (Oriskany) Sandstone and Helderberg Group north of Craig County. The lower contact is disconformable. The upper contact with the Huntersville Chert appears to be conformable.

Silurian Formations Undivided (Silurian) at surface, covers 13 % of this area

Some landslides with intact stratigraphic units in Giles County area. Includes: Dsu, Skrt, Sm. (Shrc?)

Juniata, Reedsville, Trenton, and Eggleston Formations (Ordovician) at surface, covers 7 % of this area

Juniata Formation (Darton and Taff, 1896). Siltstone, shale, sandstone, and limestone. Siltstone, shale, and sandstone, locally calcareous, grayish-red, locally fossiliferous; with some interbeds of greenish-gray shale, quartzarenite, and argillaceous limestone. Cycles consisting of a basal, crossbedded quartzarenite with a channeled lower contact; a middle unit of interbedded mudstone and burrowed sandstone; and an upper bioturbated mudstone are commonly present north of New River (Diecchio, 1985). The Juniata Formation ranges from less than 200 to more than 800 feet in thickness. In southwestern Virginia the red, unfossiliferous, and argillaceous Juniata Formation is present in the southeastern belts. It is equivalent to the gray, fossiliferous, and limy Sequatchie Formation of western belts (Thompson, 1970; Dennison and Boucot, 1974). Even though the beds along Clinch Mountain, in Scott County, contain minor amounts of carbonate rock (Harris and Miller, 1958) the majority is grayish- red siltstone, which is typical of the Juniata Formation. Reedsville Shale. Refer to description under Ou. Trenton Limestone. Refer to description under Ou. Eggleston Formation. Refer to description under Ou.

Nolichucky and Honaker Formations (Cambrian) at surface, covers 6 % of this area

Nolichucky Formation. Refer to description under [nmrr]. Honaker Formation (Campbell, 1897). Dolostone, limestone, and shale. Dolostone, light- to dark-gray to dark-bluish-gray, aphanic to coarse-grained, thin- to massive-bedded, "butcher-block" weathering; with sparse interbeds of argillaceous limestone, and minor dark-gray chert. Limestone, argillaceous, ribbon-banded in part, light- to medium-gray, very-fine-grained, thick-bedded. Shale, greenish-gray, laminated to thin-bedded. The Honaker Formation is predominantly dolostone with subordinate limestone. The dolostone becomes more dominant in the northeastern part of outcrop belt (Evans and Troensegaard, 1991). Shale is locally present as a 20- to 60-feet-thick unit in the middle of the formation and as thin interbeds with the dolostone and limestone throughout the area. The Honaker Formation ranges from about 1000 to 1100 feet in thickness. It is laterally equivalent to the lower Elbrook to the east.

Moccasin or Bays Formation through Blackford Formation (Ordovician) at surface, covers 6 % of this area

Moccasin Formation, Bays Formation, Unit C, Unit B, and Unit A. Moccasin Formation (Campbell, 1894). Mudstone, shale, imestone, and sandstone. Mudstone and shale, dusky-red to dark-reddish-brown, calcareous, ripple-marks, and mud cracks common. Limestone, light-olive-gray, weathers very-light gray, aphanic with "birds-eyes", locally fossiliferous. The limestone generally is the middle member of the Moccasin southwest of Giles County. In eastern Giles County and northeastward a thin medium-grained, gray sandstone occurs near the base of the Moccasin. The thickness ranges from 0 in northern Alleghany County to about 600 feet in Scott County. Bays Formation (Keith, 1895). Siltstone, sandstone, mudstone, and limestone. Siltstone, grayish-red, olive- to light-olive-gray, locally calcareous, sandy in part. Sandstone, light-gray to yellowish-gray, fine- to very-coarse-grained, locally conglomeratic, calcareous. Mudstone, grayish-red, olive- to light-olive-gray, mudcracks common. Limestone, grayish-red to light-olive-gray, aphanic. Five distinct K-bentonites reported by Hergenroder (1966). Contacts are conformable except perhaps in Botetourt, Roanoke, and Montgomery counties. Thickness ranges from 105 feet north of Wytheville to 890 feet near Daleville in Botetourt County. From Scott and Washington counties to Highland County and northwest of the Pulaski and North Mountain faults, a multitude of stratigraphic names have been applied to the rocks between the Bays or Moccasin (above) and the Beekmantown or Knox (below). The lack of detailed geologic mapping, except in Scott and Giles counties, the restricted area of the two major stratigraphic studies (Cooper and Prouty, 1943; Kay, 1956), and the general disagreement as to mappability and correlation of units makes it impossible to apply specific stratigraphic nomenclature at this time. Therefore, the rocks are described as three packages of lithologies (from youngest to oldest): Unit C, Unit B, and Unit A. Unit C. Limestone, medium- to dark-gray, aphanic to fine-grained with thin, medium- to coarse-grained beds, argillaceous, nodular to planar-bedded, locally very fossiliferous. The following names have been applied to Unit C: Witten, Bowen, Wardell, Gratton, Benbolt, Chatham Hill, Wassum, Rich Valley, Athens, Ottesee, Liberty Hall, Fetzer, and Giesler. Unit B. Limestone, light- to dark-gray, aphanic to coarsegrained, black and gray chert nodules, carbonate mound buildups. This unit is characterized by grainstone with interbedded micrite and chert. The overlying Unit C is very argillaceous and lacks chert. The following names have been applied to Unit B: Wardell, Gratton, Benbolt, Lincolnshire, Big Valley, McGlone, McGraw, Five Oaks, Peery, Ward Cove, Rockdell, Rye Cove, Effna, Whitesburg, Holston, Pearisburg, and Tumbez. Unit A. Dolostone, light- to medium-gray, fine-grained, locally conglomeratic, cherty. Limestone, medium- to dark gray, fine-grained, locally cherty. Shale, light-gray to dusky red. A basal chert-dolomite conglomerate with clasts as much as cobble size is locally present on the unconformity surface. The following names have been applied to Unit A: Blackford, Elway, Tumbez, Lurich (lower part), and "basal clastics".

Chemung Formation (redefined as Foreknobs Formation) (Devonian) at surface, covers 6 % of this area

Chemung Formation (Hall, 1839). Redefined as the Foreknobs Formation (Dennison, 1970). Sandstone and shale, dark-gray and greenish-gray, fine-grained, thin- to thick-bedded, lithic sandstone and interbedded greenish gray, fissile, clay shale. Minor quartz-pebble conglomerate, thin red sandstone, and locally, fossil shell beds. Very thin or absent in southwestern Virginia; thickens to about 2500 feet northeastward in Frederick County. Gradational contact with underlying Brallier Formation and equivalent to part of the Chattanooga Shale to the southwest. Redefined and described as part of the Greenland Gap Group by Dennison (1970).

Lower Devonian and Silurian Formations Undivided (Silurian-Devonian) at surface, covers 5 % of this area

Some landslides with intact stratigraphic units in Craig County area. Includes: Dsu, Skrt, Sm. (Shrc?)

Maccrady Shale and Price Formation (Mississippian) at surface, covers 5 % of this area

Maccrady Shale and Price Formation. Refer to individual units for descriptions.

Tuscarora Sandstone (Silurian) at surface, covers < 0.1 % of this area

Medium- to thick-bedded, white to gray or pinkish sandstone, fine to coarse, quartzitic, ridge-forming. Equivalent to the Clinch Sandstone of Tennessee.

Price Formation (Mississippian) at surface, covers < 0.1 % of this area

Price Formation (Campbell, 1894). Sandstone, quartzarenite, conglomerate, siltstone, shale, limestone, and coal. Sandstone, feldspathic, slightly micaceous, light-gray to medium-gray, weathers olive-gray to greenish-gray, few grayish-red beds, very-fine- to medium-grained, thin- to thick-bedded, cross-laminated in upper part of formation, locally conglomeratic with quartz pebbles and granules. The lowest part of the Price contains quartz pebble conglomerate and quartzarenite, with marine fossils in basal beds. Formation becomes finer grained to the southwest. Sandstone is dominant in the upper half of formation. Siltstone and silty shale, partly calcareous, locally pyritic and glauconitic, greenish-gray, medium-dark-gray to light-olive-gray, locally black and carbonaceous, laminated to medium-bedded, hard, hackly, fissile to platy, fossiliferous; interbedded with sandstone. Limestone, rare, argillaceous, arenaceous, very- thin beds, as much as six inches thick, in interbedded siltstones and shales (Bartlett, 1974, p. 83-84). Coal in upper part of formation (Bartholomew and Brown, 1992; Bartlett, 1974; Cooper, 1944). The Price is a westward thinning clastic wedge (Bartlett, 1974, p. 170) that is equivalent to part of the Grainger Formation in the southwesternmost part of Virginia. It overlies the Chemung Formation from southwestern Washington County to the north east and the Brallier Formation or the Chattanooga Shale to the southwest. Base is conformable, placed at the base of a conglomerate northeast of Lee County (Bartlett, 1974). Thickness is variable; it is 250 feet thick in Lee County, 185 feet thick (Henika, 1988) in Scott County and as much as 1800 feet thick (Campbell and others, 1925) in Montgomery County.

Juniata and Oswego Formations (Ordovician) at surface, covers < 0.1 % of this area

The Juniata is a thin-bedded, blocky, red sandstone and shale. In places it is underlain by the thick-bedded, gray Oswego Sandstone.