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Basemap, modified U.S. Geological Survey DRG 1967 Lexington Quadrangle, Virginia
Projection: UTM zone 17NAD 1927 Datum
Digital Cartography by Aaron Cross
MNGN
61 11
o
o '107 MILS21 MILS
1980 MAGNETIC NORTHDECLINATION AT CENTER OF SHEET Geology mapped from September 2005 to May 2006
SCALE 1:240001
1 0.5 0 1 KILOMETER
1000 1000 2000 3000 4000 5000 6000 7000 FEET0
00.5 1 MILE
CONTOUR INTERVAL 20 FEETNATIONAL GEODETIC VERTICAL DATUM OF 1929
ANNUAL VARIATION - 0° 2’
VirginiaDepartment of
Mines, Mineralsand Energy
BEDROCK GEOLOGIC MAP OF THE LEXINGTON 7.5-MINUTE QUADRANGLE, VIRGINIAGerald P. Wilkes
2007
Open-File Report 07-05Bedrock Geologic Map of the
Lexington 7.5-minute Quadrangle, VirginiaVirginia Division of Geology and Mineral Reources
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DESCRIPTION OF MAP UNITS
BRECCIAHighly fractured Elbrook Formation. Fractures are filled with calcite.
MARTINSBURG FORMATIONOnly the lowest few hundred feet of the Martinsburg Formation is present. It is limestone, light- to dark-gray, argillaceous, and thin- to thick-bedded, with minor amounts of light-gray calcareous shale. It contains cephalopod, brachiopod, bryozoan, and coral fossils. The Martinsburg weathers to a yellow chippy soil that is variable in thickness. In outcrop, the Martinsburg almost always displays strong foliation, which often obscures bedding features. Contact with the under-lying Edinburg Formation is conformable, grading through an approximately 1000-feet-thick interval of interbedded Martinsburg and Edinburg lithologies. These interbeds may be a few feet to tens of feet thick and represent the shifting facies relationship of the two formations through time: the gradational relation-ship is well represented in the pastures east of U. S. Route 11 approximately 0.75 miles north of the I-64/U.S. 11 interchange. The Martinsburg has use as fill mate-rial.
EDINBURG FORMATIONThe uppermost limestone of the Edinburg Formation is the Collierstown Member, which is traditionally considered the marker bed separating the Edinburg and the underlying Martinsburg Formations. This limestone is black, fine- to coarse-grained and conglomeratic at the base. Notable fossils include large coral heads. Although typically a few tens of feet thick, the Collierstown exceeds 500 feet in thickness south of Lexington. Below the Collierstown, the Edinburg consists of two basic facies. The Liberty Hall facies is limestone, black to dark-gray, fine grained, thick-bedded, with black shale partings that are typically less than an inch thick. In addition, there are thick black shale intervals (greater than 100 feet) well exposed in the quadrangle. In contrast, the Lantz Mill facies is limestone, black to dark-gray, medium- to coarse-grained, with distinctive cobbly weather-ing. Both the Liberty Hall and Lantz Mills facies are only sparsely fossiliferous, but graptolites are reported in the black shale unit. At many places in the quad-rangle the Edinburg has been subject to structural deformation as attested by slickensides, gouge veins filled with calcite, and deformed bedding. Zones of intense deformation suggest intraformational faulting. The Edinburg typically forms a moderately deeply weathered soil. Contact with the underlying Lincoln-shire Limestone is sharp and conformable. Thickness estimate of the Edinburg Formation is indeterminate because of structural deformation, but it is probably 800 to 1250 feet thick. The Edinburg is currently being quarried and is used primarily for road stone and gravel fill. There may be potential for high-calcium limestone from the Collierstown Member. Many of the stone walls that are seen throughout Lexington, including those at Washington and Lee University and Virginia Military Institute are constructed of limestone from the Edinburg Forma-tion.
LINCONSHIRE LIMESTONE AND NEW MARKET LIMESTONEThe Lincolnshire Limestone and the New Market Limestone are mapped as a single unit because the New Market is too thin to map at 1:24,000 scale (it is less than 50 feet thick). Also included in this map unit is a locally occurring limestone bed known as the Whistle Creek Limestone. The Whistle Creek is limestone, medium- to dark-gray, irregularly bedded, and very cherty. Its type section is west of Lexington between Whistle Creek and State Route 850 (old U. S. Route 60). The Lincolnshire Limestone consists of limestone, light- to dark-gray to black, very-fine to coarse-grained, with medium to thick beds. Thick intervals of grain-stone, packstone, and micrite comprise the Lincolnshire. It is well exposed at Beans Bottom. The New Market is limestone, dove-gray, micro-grained, and thick-bedded. Local areas of massive biostromal limestone, light- to medium-gray, and coarse-grained, represent the Murat facies of the Lincolnshire Lime-stone. It is well exposed in the cliffs at Beans Bottom. This map unit forms shallow- to moderately-deep soil. Contact with the underlying Beekmantown Formation is unconformable. Because of this unconformity and the varying local facies, total thickness for the Lincolnshire/New Market is highly variable and can range between 100 to at least 450 feet. This rock unit has potential for aggregate and high-calcium limestone.
BEEKMANTOWN FORMATIONThe Beekmantown Formation is dominantly dolostone, light- to medium-gray, fine-grained, thick-bedded, containing beds and nodules of black chert with distinctive, massive, white chert beds in the upper part of the unit. There are interbeds of limestone, light- to medium-gray, fine-grained, and thin- to medium-bedded. The dolostone sometimes weathers to a “butcher-block” surface. The underlying Stonehenge Limestone is not of mapable thickness on the Lexington quadrangle (typically 20 feet thick) and is therefore included with the Beekman-town. The Stonehenge is limestone, dark-gray to black, fine-grained, thick-bedded, with minor amounts of black chert. The Beekmantown forms a deep soil profile and is often conspicuous by its absence of outcrop. Contact with the underlying Conococheague Formation is conformable and the Beekmantown is approximately 1500 to 2000 feet thick. Parts of the Beekmantown have use as aggregate and possibly high-magnesium dolostone.
CONOCOCHEAGUE FORMATIONThe Conococheague Formation is limestone, light- to medium-gray, fine-grained, thin- to very thick-bedded, with cyclically interbedded dolostone, light-gray, fine- to medium-grained, and thin- to medium-bedded. This unit contains distinctive undulating stringers of sand and silt that stand out in relief on weathered surfaces. Also diagnostic are quartz sandstone beds that are light-yellow on weathered surfaces. These sandstones have rounded, medium to coarse grains, occur as beds up to five feet thick, and are carbonate-cemented. Minor amounts of black, bedded chert are present throughout the Conococheague. The Conococheague forms moderately deep soil and the contact with the underlying Elbrook Forma-tion is conformable. The Conococheague is 2000 to 2500 feet thick and may have potential use as aggregate.
ELBROOK FORMATIONThe Elbrook Formation consists of interbeds of mostly dolostone with lesser amounts of limestone. Both rocks are medium-gray, fine- to medium-grained, thin- to thick-bedded, and are interbedded with minor amounts of pink and green shale. The Elbrook forms moderate- to deeply-weathered, yellow chippy soil. Because of the thin beds and shaly nature of the formation, the Elbrook is a zone of structural weakness and often displays bedding-parallel slip in outcrop. In the Lexington quadrangle, the Elbrook is the hanging wall along the Staunton and North Mountain faults and is brecciated or otherwise deformed near the fault traces. Only the upper part of the Elbrook Formation is present in the Lexington quadrangle. The Elbrook could be used as fill material.
ACKNOWLEDGEMENT
This map was prepared in cooperation with the U.S. Geological Surveyunder the National Cooperative Geologic Mapping Program; 2005-2006 STATEMAP component, Grant 05HQAG0088, and 2006-2007 STATEMAP component, Grant 06HQAG0039
Portions of this publication may be quoted or copied if credit is given to the author and the Division of Mineral Resources of the Virginia Department of Mines, Minerals and Energy. Geologic information, concepts, and other ideas gained from the use of this geologic map should be credited as follows:
Wilkes, Gerald P., 2007, Bedrock geologic map of the Lexington 7.5-minute quadrangle, Virginia: Virginia Division of Mineral Resources Open-File Report 07-05.
Thrust fault, teeth on upthrown block
MAP SYMBOLS
b
Omb
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Contact, exposed or approximate
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F Fold axis (anticline)
o Strike and dip of inclined beds
s
Strike and dip of overturned beds
v Vertical beds - showing strike
e Horizontal beds
¹
Strike and dip of inclined foliation
½ Vertical foliation
B Disturbed beds
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ß Quarry (limestone)
Æ Quarry, abandoned
! Sample
A’ANORTH MOUNTAIN
FAULTSTAUNTON FAULT
1000 feet
Sea Level
-1000 feet
1000 feet
Sea Level
-1000 feet
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. Hig
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Interpretive Cross-sectionNo vertical exageration.
Subsurface structures interpreted from surface measurements.
