Geologic units in Bristol City, Virginia

Additional scientific data in this geographic area

Beekmantown Group (Ordovician) at surface, covers 41 % of this area

Includes the Pinesburg Station Dolomite, the Rockdale Run Formation, and the Stonehenge Limestone (northern Virginia only) or the Beekmantown Formation and Stonehenge Limestone (central and southwestern Virginia). Pinesburg Station Dolomite (Sando, 1956). Dolostone, dark- to light-gray, fine- to medium-grained, medium- to thick bedded with minor nodular white chert. It ranges from 0 to 400 feet in thickness and is equivalent to beds in the upper Beekmantown Formation. Present only in Clarke and Frederick counties and is conformable with the underlying Rockdale Run Formation and unconformable with the overlying New Market or Lincolnshire Limestones. Rockdale Run Formation (Sando, 1958). Dominantly limestone and dolomitic limestone, lesser dolostone with minor chert throughout. Limestone, light- to medium-gray, fine-grained generally, but coarse, bioclastic limestone locally, medium- to thick-bedded. Dolostone, light-gray, fine- to medium- grained, thick-bedded with "butcher block" weathering and minor nodular or bedded chert in both limestone and dolostone. Unconformably overlain by the New Market Limestone where the Pinesburg Station Dolomite is absent. It is laterally equivalent to the Beekmantown Formation and conformably overlies the Stonehenge Limestone. The formation is about 2700 feet thick. Beekmantown Formation (Clarke and Schuchert, 1899). Dominantly dolostone and chert-bearing dolostone with lesser limestone. Dolostone, light- to very-dark-gray, fine- to coarse grained, mottled light- and dark-gray, with crystalline beds locally contains nodular, dark-brown or black chert and thick, hill forming, lenticular chert beds in lower part. Limestone, very-light- to medium-gray, fine-grained, medium- to thick bedded, locally dolomitic and locally fossiliferous. The formation is present from Page and Shenandoah counties southwestward in the easternmost exposures of the Lower Ordovician rocks. It and the underlying Stonehenge Limestone, are equivalent to the Mas cot and Kingsport Dolomites of the upper part of the Knox Group. It is unconformably overlain by Middle Ordovician limestones and conformably overlies the Stonehenge Limestone. Erosion, related to the unconformity at the top of the Beekmantown Group and Knox Group, has produced erosional breccias, local topographic relief, and paleokarst topography as well as significant regional thinning of the rock units. The Beekmantown Group thins from about 3000 feet in Page County to less than 700 feet in Washington County, largely because of post-Beekmantown erosion. Stonehenge Limestone (Sando, 1956). Limestone with interbedded dolostone in north western Virginia. Limestone, dark-gray, fine-grained, laminated to massive, with black nodular chert. Dolostone, light-gray, fine-to very-coarse-grained, as thin- to medium-interbeds or as coarse- grained, massive, reefoidal bodies. Reefoidal bodies are restricted to the middle portion of the formation. The formation conformably overlies the Conococheague Formation and thins northwestward from 400 or 500 feet in the southeasternmost exposures (Page County) to a few tens of feet in the north western exposures (western Rockingham County) and is not recognizable or included in the lower Beekmantown or upper Conococheague in much of southwestern or western Virginia. It is equivalent to the lower part of the Kingsport Dolomite.

Conococheague Formation (Cambrian-Ordovician) at surface, covers 30 % of this area

Conococheague Formation (Stose, 1908). Dominantly limestone with significant dolostone and sandstone beds in lower part and locally in upper part. Limestone, medium- to very-dark-gray, fine-grained, thin-bedded with wavy siliceous partings that weather out in relief. Vertically repetitious primary sedimentary features such as sharpstone conglomerate, laminated bedding, and algal structures indicate cyclic sedimentation. Dolostone, medium-gray, fine- to medium-grained, laminated to massive-bedded with primary features similar to those in the limestones. Sandstone, medium-gray, brown weathering, cross-laminated, medium to thin-bedded, forms linear ridges, largely associated with dolostone beds but quartz sand common in most lithologies. Formation is present throughout the Valley of Virginia southeast of the Pulaski and North Mountain faults. It ranges in thickness from about 2200 feet in northern Virginia to 1,700 feet near Abingdon. The Conococheague is approximately equivalent to the Copper Ridge and Chepultepec Formations and conformably overlies the Elbrook Formation.

Elbrook Formation (Cambrian) at surface, covers 21 % of this area

Elbrook Formation (Stose, 1906). Dolostone and limestone with lesser shale and siltstone. Dolostone, medium-to dark-gray, fine- to medium-grained, laminated to thick-bedded. Limestone, dark-gray, fine-grained, thin- to medium-bedded, with algal structures and sharpstone conglomerate. Shale and siltstone, light- to dark-gray, dolomitic, platy weathering, with minor grayish-red or olive-green shales. Interbedded limestone and dolostone dominate the upper part of the formation; dolomitic siltstone and shale and thin- bedded argillaceous limestone dominate the lower part. The formation ranges be tween 1500 and 2900 feet in thickness in the southeasternmost exposures but is incomplete elsewhere due to faulting. The Elbrook of northern Virginia is transitional with the Nolichucky and Honaker Formations (locally the limestone facies of the Nolichucky has been differentiated from the Elbrook by Bartlett and Biggs (1980). It is also approximately equivalent to the rock sequence comprised of the Nolichucky and Maryville Formations, the Rogersville Shale, and the Rutledge Formation. Farther southwest the Conasauga Shale is the Elbrook equivalent. The Elbrook appears to be conformable and gradational with the underlying Waynesboro or Rome Formations. From Washington County to Augusta County much of the Elbrook Formation adjacent to the Pulaski and Staunton faults is a breccia of the "Max Meadows tecontic breccia type" (Cooper and Haff, 1940). These breccias are composed of crushed rock clasts that range from sand size to blocks many feet long, derived almost entirely from the lower part of the Elbrook Formation. The breccia commonly forms low lands characterized by karst features.

Knobs Formation, Paperville Shale, Lenoir and Mosheim Limestone (Ordovician) at surface, covers 8 % of this area

Knobs formation (Cooper, 1961). Shale, siltstone, sandstone, and conglomerate. Shale and siltstone, brown. Sandstone, lithic, greenish-brown, fine- to coarse-grained. Conglomerate, polymictic (rounded to subrounded clasts of limestone, dolomite, sandstone, quartzite, vein quartz, shale, chert, and feldspar in calcareous matrix). Some interbeds of calcareous siltstone and sandstone. The Knobs formation ranges from 800 to 3400+ feet in thick ness (the upper part of the unit is eroded) (Bartlett and Biggs, 1980). The Knobs formation corresponds to the upper member of the Athens Shale of Butts (1933) as described by Bartlett and Biggs (1980). Fincastle Conglomerate Member of the Martinsburg Formation. Conglomerate, sandstone, shale, and siltstone (Rader and Gathright, 1986). Conglomerate (type 1), poorly sorted, clast-supported, pebble to boulder clasts of limestone, dolomite, quartzite, sandstone, chert, vein quartz, granite gneiss, quartz pebble conglomerate, greenstone, and shale, subangular to subrounded. Conglomerate (type 2), poorly-sorted, matrix supported clasts of quartzite, vein quartz, limestone, and chert, subrounded to well-rounded. Matrix framework grains in both types are sand-size quartz, limestone, and dolomite with minor chlorite and sericite. The cement is calcite. The conglomerate fines upward from a scoured base to sandstone. Sandstone, lithic, medium- to very-coarse-grained, brownish-gray, cross stratification rare. Shale and siltstone, gray, convolute bedding common. This member is restricted to the Fincastle area of Botetourt County. Paperville Shale (Cooper, 1956). Shale, olive-gray to dark-gray, fissile, thin-bedded; with minor gray, argillaceous siltstone, fossiliferous in lower part. The Paperville Shale ranges from 200 to 2300 feet in thickness (Bartlett and Biggs, 1980). The Paperville Shale corresponds to the lower member of the Athens Shale of Butts (1933) as described by Bartlett and Biggs (1980). Lenoir Limestone (Safford and Killibrew, 1876). Limestone, argillaceous, gray to dark-gray, fine-grained, medium bedded, silty laminations, fossiliferous. Lower contact is unconformable. The Lenoir Lime stone ranges from 0 to 70 feet in thickness (Bartlett and Biggs, 1980). Mosheim Limestone (Ulrich, 1911). Limestone, aphanic, medium-bedded with calcite crystal clusters, sparsely fossiliferous; limestone-dolomite-chert clasts in aphanic limestone matrix common at base of unit; rare thin interbedded dolomite. Unconformable with underlying unit. The Mosheim Limestone ranges from 0 to 150 feet in thickness (Bartlett and Biggs, 1980). The Lenoir and Mosheim Limestones have a combined thickness up to 270 feet in southwestern Washington County (Bartlett and Webb, 1971). In the Fincastle Valley the nomenclature Lincolnshire and New Market Limestones replaces Lenoir and Mosheim Limestones of older reports.