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Geologic Map of Loudoun County, Virginia

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Geologic Map of Loudoun County, Virginia Geologic Map of Loudoun County, Virginia By Scott Southworth, William C. Burton, J. Stephen Schindler, and Albert J. Froelich Pamphlet to accompany Geologic Investigations Series Map I–2553 2006 U.S. Department of the Interior U.S. Geological Survey U.S. Department of the Interior Dirk Kempthorne, Secretary U.S. Geological Survey P. Patrick Leahy, Acting Director U.S. Geological Survey, Reston, Virginia: 2006 Revised: 2007 Cover: Color-shaded-relief image of Loudoun County, Va. (except the Dulles Airport area), showing high topographic elevations in shades of red and low topographic elevations in shades of blue. The hillshade is illuminated from the east. The highest elevation is 1,920 ft above sea level on Blue Ridge at the southwestern part of the county, and the lowest elevation is 180 ft above sea level along the Potomac River at the easternmost part of the county. The higher elevation region of the Blue Ridge province, in contrast to the lower elevation region of the Piedmont province in the eastern half of the county, is a function of rock hardness and different erosion histories. Image produced by Kerry Langueux and David L. Daniels, USGS, using 5-ft- contour-interval topographic data from the Loudoun County Office of Mapping and Geographic Information, Leesburg, Va. For product and ordering information: World Wide Web: http://www.usgs.gov/pubprod Telephone: 1-888-ASK-USGS For more information on the USGS--the Federal source for science about the Earth, its natural and living resources, natural hazards, and the environment: World Wide Web: http://www.usgs.gov Telephone: 1-888-ASK-USGS Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government. Although this report is in the public domain, permission must be secured from the individual copyright owners to reproduce any copyrighted materials contained within this report. Suggested citation: Southworth, Scott, Burton, W.C., Schindler, J.S., and Froelich, A.J., 2006, Geologic map of Loudoun County, Virginia: U.S. Geological Survey Geologic Investigations Series Map I–2553, scale 1:50,000. ISBN 1-411-30968-5 Printed on recycled paper iii Contents Introduction.....................................................................................................................................................1 Geologic Setting .............................................................................................................................................1 Acknowledgments .........................................................................................................................................1 Blue Ridge Province ......................................................................................................................................1 Mesoproterozoic Basement Rocks ...................................................................................................1 Nongranitic Rocks .......................................................................................................................2 Granitic Gneiss and Metagranite ..............................................................................................3 Eastern Blue Ridge Units ...................................................................................................3 Western Blue Ridge Units .................................................................................................3 Mesoproterozoic Metamorphism and Deformation ..............................................................5 Monazite U-Pb Ages from Mesoproterozoic Rocks ..............................................................5 Summary of Mesoproterozoic Geologic Events .....................................................................7 Late Proterozoic Granite ......................................................................................................................8 Late Proterozoic Dikes .........................................................................................................................8 Dike Composition .........................................................................................................................8 Dike Orientation and Density .....................................................................................................8 Geochemical Relation of Metadiabase Dikes to Metabasalt of the Catoctin Formation ..............................................................................................9 U-Pb Age of Rhyolite Dike ..........................................................................................................9 Late Proterozoic Metasedimentary and Metavolcanic Rocks ......................................................9 Fauquier Formation ......................................................................................................................9 Swift Run Formation ..................................................................................................................22 Catoctin Formation ....................................................................................................................22 Age of the Catoctin Formation ........................................................................................22 Paleozoic Rocks of the Blue Ridge Anticlinorium .........................................................................22 Loudoun Formation ....................................................................................................................23 Weverton Formation ..................................................................................................................23 Harpers Formation .....................................................................................................................23 Antietam Quartzite .....................................................................................................................23 Carbonaceous Phyllite ..............................................................................................................24 Tomstown Formation .................................................................................................................24 Frederick Limestone ..................................................................................................................24 Paleozoic Structure ............................................................................................................................24 F1 Folds and S1 Cleavage ...........................................................................................................24 F2 Folds and S2 Cleavage ...........................................................................................................24 Faults ............................................................................................................................................25 Paleozoic Metamorphism ..................................................................................................................25 40Ar/39Ar Dating of Metamorphic Fabric .................................................................................25 Piedmont Province.......................................................................................................................................26 Potomac Terrane .................................................................................................................................26 iv Mesozoic Rocks of the Lower Culpeper Group .............................................................................26 Manassas Sandstone ...............................................................................................................26 Reston Member .................................................................................................................26 Poolesville Member ..........................................................................................................26 Balls Bluff Siltstone ...................................................................................................................26 Leesburg Member ............................................................................................................26 Fluvial and Deltaic Sandstone and Siltstone Member ...............................................26 Lacustrine Shale and Siltstone Member ......................................................................27 Catharpin Creek Formation ......................................................................................................27 Mesozoic Rocks of the Upper Culpeper Group .............................................................................27 Mount Zion Church Basalt .......................................................................................................27 Midland Formation .....................................................................................................................27 Hickory Grove Basalt.................................................................................................................27 Turkey Run Formation ................................................................................................................27 Sander Basalt .............................................................................................................................28 Thermally Metamorphosed Rocks ..........................................................................................28
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