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Geologic Map of Loudoun County, Virginia · B U L L R U N S F A U L T Cambrian Weverton Formation...
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NORTHERN VIRGINIA REGIONAL PARK LOWES ISLAND ALGONKIAN REGIONAL PARK STATE ROUTE 7 STATE ROUTE 641 STATE ROUTE 734 STATE ROUTE 643 STATE ROUTE 621 STATE ROUTE 690 U.S. ROUTE 15 HILLSBORO SYNCLINE Black Oak Ridge Belmont Airmont GOOSE CREEK Broad Run North Fork Crooked Run Butchers Branch SHO R T HIL L F A U L T BU L L RUN FAUL T SEA LEVEL 8000 SEA LEVEL 8000 2000 feet=610 meters Quaternary units are not shown NO VERTICAL EXAGGERATION 6000 4000 4000 2000 2000 4000 6000 8000 10000 4000 2000 2000 FEET 8000 6000 6000 10000 FEET A A' Ab An Or tonalite granodiorite quartz monzonite trondhjemite granite Q A P Ybg Yml Yg Ygt Yqp Ym Ypg Yhm Ylg Yc Yn monzogranite syenogranite granodiorite quartz monzodiorite quartz monzonite diorite quartz diorite tonalite Ybg Yml Yg Ygt Ym Ypg Yhm Yn A A B D E C B Figure 2.—A, Quartz-alkali feldspar-plagioclase ternary diagram of Mesoproterozoic gneisses (Streckeisen, 1976); Q, quartz; A, alkali feldspar; P, plagioclase. B, Normative feldspar ternary diagram of Middle Proterozoic gneisses (O’Connor, 1965); An, anorthite; Ab, albite; Or, orthoclase. La Ce Nd Sm Eu Tb Yb Lu 1 10 100 Rock/Chondrites 5 10 MgO 50 55 45 40 60 SiO 2 0.5 1.0 0 0 1 2 3 4 5 TiO 2 P 2 O 5 5 10 0 1 2 3 4 5 MgO TiO 2 A M F calc-alkaline tholeiitic Coarse-grained metadiabase dike, n=6 Fine-grained metadiabase dike, n=30 Porphyritic metadiabase dike, n=10 Low TiO 2 /high MgO metavolcanic flow or flow breccia, n=9 Catoctin Formation metabasalt, n=26 EXPLANATION Figure 4.—A, AFM diagram of Late Proterozoic dikes and metabasalt of the Catoctin Formation showing the tholeiitic–calc-alkaline boundary of Irvine and Baragar (1971). A, Na 2 O+K 2 O; F, total iron as FeO; M, MgO. B–D, Variation diagrams of Late Proterozoic dikes and metabasalt of the Catoctin Formation showing TiO 2 -MgO, TiO 2 -P 2 O 5 , and MgO-SiO 2 ratios, respectively. E, Chondrite-normalized rare earth element diagram of Late Proterozoic metadiabase dikes and metabasalt of the Catoctin Formation. n=number of measurements. N S W E N S W E N S W E N S W E N S W E N S W E N S W E N S W E N S W E N W E N S W E N S W E N S W E N S W E Structural features developed during metamorphism of the Mesoproterozoic rocks Structural features developed during metamorphism of the Mesoproterozoic rocks Orientation of metadiabase dikes emplaced into Mesoproterozoic rocks in the Late Proterozoic Orientation of metadiabase dikes emplaced into Mesoproterozoic rocks in the Late Proterozoic Structural features developed during the Paleozoic in both Proterozoic and Cambrian rocks on opposite sides of the Short Hill fault Structural features developed during the Paleozoic in both Proterozoic and Cambrian rocks on opposite sides of the Short Hill fault n=20 n=20 n=34 n=34 n=76 n=76 n=605 n=605 Mean: N. 21° E., 28° SE. Mean: N. 21° E., 28° SE. n=52 n=52 n=52 n=52 n=806 n=806 n=182 n=182 n=403 n=403 n=216 n=216 n=1,017 n=1,017 n=71 n=71 Mean: N. 18° E., 36° SE. Mean: N. 18° E., 36° SE. Mean: N. 26° E., 37° SE. Mean: N. 26° E., 37° SE. Mean: N. 9° E., 39° SE. Mean: N. 9° E., 39° SE. Orientation of bedding planes in Paleozoic and Mesozoic sedimentary rocks Orientation of bedding planes in Paleozoic and Mesozoic sedimentary rocks Mean: N.10° E., 58° SE. Mean: N.10° E., 58° SE. Strike directions of contacts Strike directions of contacts D 2 mineral lineation, n=15 D 2 fold hinge, n=5 D 3 fold hinge, n=2 D 2 mineral lineation, n=15 D 2 fold hinge, n=5 D 3 fold hinge, n=2 Poles to bedding in Triassic and Jurassic rocks of the Culpeper basin Poles to bedding in Triassic and Jurassic rocks of the Culpeper basin Contoured poles to bedding in Cambrian Weverton Formation on Short Hill Mountain Contoured poles to bedding in Cambrian Weverton Formation on Short Hill Mountain Contoured poles to bedding in Cambrian Weverton Formation on Blue Ridge Contoured poles to bedding in Cambrian Weverton Formation on Blue Ridge Contoured poles to S 1 cleavage Contoured poles to S 1 cleavage Poles to contacts Poles to contacts Contoured poles to D 1 foliation Contoured poles to D 1 foliation Contoured poles to D 2 foliation Contoured poles to D 2 foliation Contoured poles to S 1 cleavage Contoured poles to S 1 cleavage Contoured poles to S 1 cleavage Contoured poles to S 1 cleavage Contoured poles to S 1 cleavage Contoured poles to S 1 cleavage Contoured poles to S 2 cleavage Contoured poles to S 2 cleavage Contoured poles to S 2 cleavage Contoured poles to S 2 cleavage Two generations (S 1 and S 2 ) cleavage exhibited at an outcrop locality Two generations (S 1 and S 2 ) cleavage exhibited at an outcrop locality Single generation (S 1 ) cleavage exhibited at an outcrop locality Single generation (S 1 ) cleavage exhibited at an outcrop locality West West East East Figure 3.—Equal-area projections (Schmidt net) of structural data. Contour intervals are 2 percent per 1 percent area. n=number of measurements. Figure 3.—Equal-area projections (Schmidt net) of structural data. Contour intervals are 2 percent per 1 percent area. n=number of measurements. A B E C F H J G I K L M N D ß 1 2 3 4 5 6 7 9 8 10 11 12 13 15 14 16 17 19 18 Sources of Geologic Mapping Blue Ridge anticlinorium (W.C. Burton, J.S. Schindler, and Scott Southworth, compilers) Culpeper basin (A.J. Froelich and Scott Southworth, compilers) Potomac terrane (A.A. Drake, Jr. and Scott Southworth, compilers) SOURCES OF DATA FOR 7.5-MINUTE QUADRANGLES IN LOUDOUN COUNTY Charles Town: Nickelson (1956); Scott Southworth (unpub. data) Harpers Ferry: Southworth (1991); Southworth and Brezinski (1996) Point of Rocks: Lee (1979); Burton and others (1995); J.P. Smoot (unpub. data) Round Hill: McDowell and Milton (1992); W.C. Burton and Scott Southworth (unpub. data) Purcellville: Southworth (1995); W.C. Burton (unpub. data) Waterford: Lee (1979); Burton and others (1995); J.P. Smoot (unpub. data) Poolesville: Lee (1979); Southworth (1998); A.J. Froelich and J.P. Smoot (unpub. data) Ashby Gap: Gathright and Nystrom (1974); Tollo and Lowe (1994); W.C. Burton and Scott Southworth (unpub. data) Bluemont: Southworth (1994); W.C. Burton (unpub. data) Lincoln: Lee (1979); W.C. Burton, A.J. Froelich, J.S. Schindler, and J.P. Smoot (unpub. data) Leesburg: Toewe (1966); Lee (1979); A.J. Froelich, J.S. Schindler, J.P. Smoot, and R.E. Weems (unpub. data) Sterling: Lee (1979); A.J. Froelich, J.P. Smoot, Scott Southworth, and R.E. Weems (unpub. data) Seneca: Lee (1979); A.A. Drake, Jr., A.J. Froelich, J.P. Smoot, Scott Southworth, and R.E. Weems (unpub. data) Upperville: W.C. Burton, A.E. Nelson, and Scott Southworth (unpub. data) Rectortown: Espenshade (1983); Leo (1990); W.C. Burton and Scott Southworth (unpub. data) Middleburg: Lee (1979); Kline and others (1990) Arcola: Lee (1978); A.J. Froelich, J.P. Smoot, and R.E. Weems (unpub. data) Herndon: Eggleton (1975); Lee (1979); A.J. Froelich, J.P. Smoot, and R.E. Weems (unpub. data) Gainesville: Lee (1979); A.J. Froelich, J.P. Smoot, and R.E. Weems (unpub. data) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 77°45' 77°37'30" 77°52'30" 39°07'30" 39° 0 1 2 MILES 0 1 2 KILOMETERS S H O R T HILL F A U L T 201 119 58 15 -27 -61 -95 -124 -156 -183 -213 -241 -272 -301 -336 -369 -411 -454 -515 -597 NANOTESLAS Zss Ypg Yn Yc Zss Zss Zss Ygt Ylg Ypg Ymc Yml Ym Ym Ym Ymc Ymc Zfa Zfc Zfa Zc Ju Ju Ju |w |w Zc Yq Yq Ylg Yp Yp Yn Ygt Yq Yp Yp Yp Zc Yp Yp Yp Yp Yp Yc Yc Jd Yg Yg Jd Jd Yg Yg Yg Yg Yg Zsp Yg Jd Jd Jd Jd Jd Ylg Ylg Ylg Ypg Ygt Ygt Ygt Ygt Ygt Yp Yp Zss Jd Jd BLUE RIDGE MOUNTAINS BLACK OAK RIDGE B L U E M O N T L I N C O L N Figure 5.—Color-shaded-relief aeromagnetic image of the Lincoln and most of the Bluemont 7.5-min quadrangles with general geologic contacts. Image based on aeromagnetic survey flown in 1989 by the U.S. Geological Survey. Data were processed and interpreted by D.L. Daniels, W.F. Hanna, and R.E. Bracken. Ju, Culpeper Group, undivided. ERA/ERATHEM Cenozoic Mesozoic Jurassic Lower Triassic Cambrian Lower Upper Chilhowee Group Upper Paleozoic Late Proterozoic Mesoproterozoic PERIOD/SYSTEM EPOCH/SERIES LITHOLOGIC UNIT LITHOLOGY AND MAP SYMBOL THICKNESS, IN FEET <1,000 715 <700–1,200 1,250 330 2,500 (J^cg) >3,280 (J^c) >5,000 (^bsh) >5,000 (^bs) 3,510 (^bl) 3,000 (^mp) 70 (^mr) ? <200 <200 500–1,000 105–380 200–240 100–200 0–200 0–5,000 10 (Zsm) 0–400 (Zsp) 300–1,000 (Zss) 0–160 (Zfs) 1,500 (Zfa) 0–200 (Zfc) 100 Quaternary Quaternary or Tertiary Pleistocene or late Tertiary Holocene Alluvium Colluvium, lag gravel, and terraces Terraces Sander Basalt Hickory Grove Basalt Mount Zion Church Basalt Turkey Run Formation Midland Formation Catharpin Creek Formation Balls Bluff Siltstone Manassas Sandstone Frederick Limestone Tomstown Formation Harpers Formation Loudoun Formation Catoctin Formation Robertson River Intrusive Suite Biotite granite gneiss (Ybg) Pink leucocratic metagranite (Yml) Leucocratic metagranite (Yg) Garnetiferous leucocratic metagranite (Ygt) Quartz-plagioclase gneiss (Yqp) Marshall Metagranite (Ym) Coarse-grained metagranite (Ymc) Porphyroblastic metagranite (Ypg) Hornblende monzonite gneiss (Yhm) Layered granitic gneiss (Ylg) Charnockite (Yc) Metanorite and metadiorite (Yn) Paragneiss (Yp) Quartzite and quartz tectonite (Yq) Swift Run Formation and Fauquier Formation upper member middle member lower member Weverton Formation Carbonaceous phyllite Antietam Quartzite Diabase dikes and sheets Holocene and Pleistocene QTt Qlg Qt Jdh Jss Js Jtr Jtrc Jhg Jmz Jdh Jdh J^cg ^bs ^bsh ^bl ^mp |f |a |h |wu |wm |wl |l Zcp Zcp Zcm Zsm Zsm Zrr Ylg Yml Ymc Ymc Ym Ym Ym Yml Yhm Ybg Ybg Zmd Zrd Ygt Ygt Ygt Yqp Yg Yg Yp Yp Yp Yc Yn Yq Ypg Ypg Ypg Ypg Zsp Zss Zfs Zfa Zfc Zcs Zcr Zcb Zc |lc |t |cp ^mr J^c J^tm J^tm Jmc Jm Jhgs Jd Jd Qal Qc Coarse cobbles, boulders, and blocks Conglomerate Sandstone Sandstone and siltstone Siltstone Limestone Quartzite Basalt-diabase Metabasalt-metadiabase Igneous rocks EXPLANATION 50 15 15 28 7 7 9 734 671 287 Leesburg Purcellville Middleburg Sterling Maryland West Virginia Virginia Loudoun County P o t o m a c R i v e r 11 2 4 16 14 (2) 1 12 11 19 6 3 5 (1) (2) (4) (4) 7(5) 10(8) 18 (3) 15(10) 8(6) 13(7) 17(9) 9 WESTMINSTER TERRANE POTOMAC TERRANE PIEDMONT PROVINCE CULPEPER BASIN GETTYSBURG BASIN BLUE RIDGE PROVINCE BLUE RIDGE ANTICLINORIUM PLEASANT GROVE FAULT BULL RUN FAULT MARTIC FAULT B U LL R U N F A ULT 77°30' 39° 0 5 10 MILES 0 5 10 15 KILOMETERS Figure 1.—Map showing general geologic provinces and prominent geologic features of the area of and surrounding Loudoun County, Va. EXPLANATION Normal fault—Bar and ball on downthrown side Thrust fault—Sawteeth on upper plate Location of sample providing radiometric age determination—Number is keyed to samples listed in table 3 of the pamphlet. Note that those sample numbers in parentheses represent the age of metamorphism and those sample numbers without parentheses represent the age of intrusion S H O R T HILL F A U L T S H O R T H I L L F A U L T S H O R T H I L L FAUL T W HI TE RO CK F A U L T DUT C HM AN C REEK S HEAR Z ONE BULL R U N F A UL T B U L L R U N F A U L T FURNA CE MO UNTA IN F AU L T M O R V E N S Y N C L I N E 66 82 60 (3) 15(10) 5 14 (2) (2) (2) (4) 1 13(7) 12 8(6) 16 2 10(8) 18 (4) (2) (1) 4 9 7(5) A' A Qal Qc Qlg Qt QTt Jd Jdl Jdo Jdg Jdh Jdc J^tm Js Jss Jtr Jtrc Jhg Jhgs Jm Jmc Jmz J^c J^cg ^bsh ^bs ^bl ^mp ^mr |f |t |cp |a |h |wu |wm |wl |w |lc |l |Zms |Zmg Zc Zcb Zcr Zcp Zcs Zcm Zmd Zrd Zsm Zsp Zss Zfs Zfa Zfc Zrr Ybg Yml Yg Ygt Yqp Ym Yhm Ymc Ylg Ypg Yc Yn Yp Yq 32 44 6 77 47 60 32 43 80 31 16 12 57 Topographic base scanned from the USGS County Map Series, Loudoun County, Va., 1992. SCALE 1:50 000 1 1/2 0 1 2 3 4 MILES 1 .5 1 0 2 3 4 5 KILOMETERS GEOLOGIC MAP OF LOUDOUN COUNTY, VIRGINIA By Scott Southworth, William C. Burton, J. Stephen Schindler, and Albert J. Froelich 2006 U.S. DEPARTMENT OF THE INTERIOR U.S. GEOLOGICAL SURVEY Prepared in cooperation with the LOUDOUN COUNTY OFFICE OF MAPPING AND GEOGRAPHIC INFORMATION GEOLOGIC INVESTIGATIONS SERIES MAP I–2553 Explanatory pamphlet accompanies map Printed on recycled paper VIRGINIA MAP LOCATION Digital compilation by Remo Nardini, Peter G. Chirico, and James E. Reddy MAGNETIC NORTH APPROXIMATE MEAN DECLINATION, 2006 TRUE NORTH 10 / ° 1 2 This map is available for sale by U.S. Geological Survey, Information Services, Box 25286, Denver Federal Center, Denver, CO 80225 For product and ordering information: World Wide Web: http://www.usgs.gov; Telephone: 1-888-ASK-USGS Any use of trade, product, or firm names in this publication is for descriptive purposes only and does not imply endorsement by the U.S. Government Stratified Intrusive Stratified Intrusive Stratified Intrusive Blue Ridge anticlinorium Culpeper basin Potomac terrane 197 Ma 201.2±1.3 Ma 200 Ma 571.5±5 Ma 600 Ma 722±3 Ma 1,055±4 Ma 1,059±2 Ma 1,060±2 Ma ~1,077±4 Ma 1,077±4 Ma 1,111±2 Ma ~1,140 Ma 1,140±4 Ma 1,149±19 Ma 1,153±6 Ma CENOZOIC MESOZOIC PALEOZOIC PROTEROZOIC Holocene Early Jurassic Late Triassic Chilhowee Group Early Cambrian Late Cambrian Newark Supergroup Culpeper Group, upper part Culpeper Group, lower part Quaternary Jurassic Cambrian Late Proterozoic Mesoproterozoic Triassic Quaternary and Tertiary(?) Holocene and Pleistocene(?) Pleistocene and Pliocene(?) Unconformity Unconformity Unconformity Unconformity Jdg* Jdc Qal Jd* |f |t |cp |a |h Zrd* Zrr* Ybg* Yml* Yg* Ygt* Yqp* Ym* Ymc* Ypg* Yhm* Ylg* Yp Yq Jdl Jdo Qc Qal QTt Jss Jhgs Jhg Jm Jmc Jtr Jtrc |lc |wu |wm |wl ^bl ^bs J^cg J^tm J^c ^mr ^mp ^bsh |w Zcr* Zc Zcp Zcm Zcb Zsp Zss Zfa Zsm Zsm Zfc Zfs Zcs Qlg Qc Qt Yc Yn Jdh* |l |Zms |Zmg Zmd Jd Jdl Qal Jdh Js Jmz CORRELATION OF MAP UNITS (Dashed lines identify gray shale beds. Asterisked units have been age dated; see table 3 in pamphlet) 35 35 45 52 22 64 30 85 12 15 11 30 35 8 36 60 60 21 88 28 20 70 15 16 20 20 30 30 53 36 32 30 20 29 38 48 25 16 23 31 17 49 12 30 45 25 32 78 75 70 45 35 30 18 18 65 32 42 31 20 44 32 32 30 32 32 65 29 5 34 30 21 26 23 13 10 77 24 36 33 26 12 10 10 10 17 14 25 15 18 18 20 17 30 27 24 25 38 24 41 23 37 39 45 50 43 37 32 43 31 25 51 44 40 40 33 8 26 29 23 34 27 22 11 10 7 15 6 6 11 28 28 35 27 31 24 20 22 6 25 23 17 7 15 20 14 20 26 10 23 17 25 22 20 25 22 21 21 6 20 11 12 24 28 38 33 30 24 11 10 65 66 78 38 35 78 15 6 15 22 78 15 16 8 10 12 12 12 12 16 15 14 12 10 10 21 16 23 18 27 22 17 25 16 23 27 12 15 25 18 15 16 15 25 15 20 20 20 39 20 27 37 28 23 18 30 20 34 17 25 15 37 26 22 37 37 45 44 55 30 41 41 17 25 26 23 28 21 10 24 26 26 23 21 20 23 18 25 18 10 23 33 28 30 23 13 4 17 13 24 19 20 8 45 30 75 75 65 45 67 45 74 37 58 57 35 50 49 33 32 60 45 43 57 80 5 30 17 50 25 27 60 25 65 50 50 70 34 15 12 80 8 35 50 30 37 37 60 36 35 10 70 21 20 22 65 69 39 25 18 60 10 15 25 65 46 58 42 58 34 68 34 72 35 47 25 40 20 40 52 30 31 84 80 54 51 35 65 38 22 10 65 36 25 50 61 72 50 27 53 20 33 36 35 35 60 20 47 20 80 47 74 78 41 35 50 50 59 36 32 30 31 56 51 55 54 57 43 34 45 45 35 55 60 33 20 40 39 53 34 30 24 22 25 70 52 47 5 40 70 62 66 52 45 25 24 27 11 59 35 45 45 48 25 30 40 45 51 35 45 45 40 30 45 20 48 45 60 40 65 35 25 40 23 32 28 26 25 52 35 18 28 32 34 46 50 13 44 42 34 40 35 25 70 35 50 30 68 36 43 36 40 43 31 20 45 34 42 24 40 25 28 31 31 23 62 40 45 15 56 20 43 25 27 5 5 25 28 10 17 30 30 65 60 60 45 35 47 40 65 34 35 20 35 80 50 75 74 42 52 27 60 68 50 62 43 44 31 35 2 45 46 58 31 25 20 53 45 46 41 25 30 46 32 35 65 50 35 27 33 20 26 20 15 45 36 35 15 22 25 10 65 62 22 44 45 35 80 8 40 45 35 42 55 33 25 25 22 35 45 35 48 16 52 45 40 25 38 64 60 44 87 12 29 36 38 27 36 58 46 42 30 30 42 42 36 26 37 42 36 3 36 60 40 10 70 30 20 10 70 75 25 50 25 56 22 56 28 70 60 65 62 25 28 56 24 70 60 40 12 50 50 65 45 44 45 36 25 10 25 58 15 62 10 42 38 36 56 56 60 40 35 35 36 30 22 30 50 65 20 48 48 26 30 50 30 70 50 22 25 20 20 20 20 45 32 68 22 72 72 56 52 6 49 32 36 75 50 30 20 55 56 15 15 25 50 37 36 43 8 28 58 40 43 62 40 36 48 51 46 50 60 60 47 43 43 40 80 23 25 16 37 28 28 53 35 35 30 29 32 30 27 21 28 23 34 35 50 60 16 88 28 32 68 39 32 34 46 30 13 62 42 35 27 22 62 32 50 52 25 48 18 39 30 48 25 67 38 38 35 60 45 30 45 30 40 50 27 30 16 17 35 53 62 28 40 53 29 59 25 38 32 22 32 35 35 48 23 50 50 40 49 36 25 20 20 20 35 35 47 54 51 54 16 28 55 10 45 25 32 48 45 10 25 4 45 50 55 40 35 48 68 39 80 30 50 35 46 36 67 44 33 38 65 52 56 51 53 52 53 49 54 10 25 25 30 40 30 38 34 85 80 79 85 60 68 62 65 87 20 15 15 15 34 50 30 62 15 42 70 48 50 50 50 26 62 12 55 35 55 56 53 46 50 30 70 72 71 78 78 80 45 43 57 18 75 28 30 30 40 50 40 45 55 35 47 60 88 65 70 70 20 65 15 20 10 20 86 30 30 28 74 5 22 20 10 47 78 43 43 25 15 28 35 40 46 46 53 53 33 36 48 34 30 12 32 65 33 80 80 80 40 25 67 32 35 30 31 20 36 36 36 26 26 75 16 60 30 25 76 40 45 60 45 20 10 50 78 67 53 47 42 44 45 45 82 42 37 42 20 30 46 5 36 20 48 66 70 46 45 53 47 58 77 70 84 60 50 35 65 33 45 |wl |wl |w |lc |t |t |t |a |a |a |h |h |h |wu |wu |wu |wm |wm |wm |wm |lc |l Jd Jd SURFACE Zc Zc Zc Zcr Zcp Zcp Zcp Zmd Zmd Zmd Zss Zss Zss Zss Yml Yg Yq Yc Ymc Ym Ym Ym Ymc Ylg Ylg Ypg Ypg Yp Yp Zmd Zmd Zmd Zmd Zmd Zmd Zmd Zmd ^mp ^mp ^mp ^mp ^mr ^mr ^mr |Zms |Zmg |Zmg J^tm J^tm J^tm J^tm J^tm J^tm ^bsh ^bs ^bs ^bs ^bs ^bs ^bs ^bs J^c J^c J^c J^c J^c J^cg |h |w Jdg Jdg Jdh Jdh Jdh Js Jss Jhg Jmc SURFACE SURFACE Zc Zcp Zmd Jdl Jdc Jdh ? ? Jdh Jhgs Jhgs Jmc Jm Jmz J^c J^c J^c J^cg J^cg J^cg J^tm J^tm J^tm J^tm J^tm J^tm J^tm J^tm J^tm J^tm J^tm J^tm J^tm J^tm J^tm J^tm J^tm J^tm J^tm J^tm J^tm J^tm J^tm J^tm J^tm J^tm J^tm J^tm J^tm J^tm J^tm J^tm J^tm |h |a |wu |wl |f |t |t |t |cp |t |cp |cp |cp |a |a |a |a |h |h |h Jd Jd |w |w |w |w |w |w |w Qc |w |w Qc Qc Qlg |w |l |h |h |h |h |h |wu |wu |wu |wu |wu |wm |wm |wm |wm |wm |wm |wm |wm |wm |wm |wm |wm |wm |wm |wm |wl |wl |wl |wl |wl |wl |wl Zmd Zmd Zmd Zmd Zmd Zmd Zmd Zmd Zmd Zmd Zmd Zmd Zmd Zmd Zmd Ylg Zmd Zmd Zmd Zmd Zmd Zmd Zmd Zmd Zmd Zmd Zmd Zmd Zmd Zmd Zmd Zmd Zmd Zmd Zmd Zmd Zmd Zmd Zmd Zsp Zmd Zmd Zmd Zmd Zmd Zmd |wl |wl |wl |wl |wl |l |l |l |lc |lc |wu |wm |l |l |wm |wm |wl |wl |wl |wm |l |l |l |wm |wm |wl |wl |wl Qal Qal Qal Qal Qal Qal Qal Qal Qal Qal Qal Qal Qal QTt Jdl QTt QTt QTt Jdl Jdh QTt QTt QTt QTt Qc Qc Qal Qal Qt Qal Qal Qal Qc Qc Qc Qc Qal Qal Qal Qal Qc Qc Qlg Qlg Qlg QTt QTt Qc Qc QTt Qc Qc Qc Qc Qc Qc Qc Qc Qc Qc Qc Qt Qc Qc Jd Qc Qc Qal Qal Qal Qal Qal Qt Qt Qal Qal Qal Qal Qal Qal Qal Qal Qt Qc Qc Zcp Ybg Ybg Yc Zc Zcp Zcp Zcp Zcp Zcr Zcr Zcr Zcr Zcr Zcr Zcr Zcp Zcp Zcp Zcp Zcp Zcp Zcp Zcp Zcp Zrd Zc Ygt Yq Yq Zc Zc Zmd Zmd Zmd Zmd Zmd Zmd Zmd Yg Yg Yg Yg Yg Yg Ybg Ybg Yqp Zrd Yqp Yqp Yqp Zc Zc Jd Jd Jd Jd Zc Zc Zc Zc Zc Qal Qal Qal Qal Qal Qal Qal Qal Qal Qal Qal Qal Qal Qal Qal Qal Qal Qal Qal Qal Qal Qal Qal Qal Qal Qal Qal Qal Qal Qal Qal Qal Qal Qal Qal Qal Qal Qal Qal Qal Qal Qal Qal Qal Qal Qal Qal Qal Jdo Jdo Qal Qal Qal Qal Qal Qal Yg Yg Yg Yg Jd Yg Yg ^bl ^bl ^bl Yp Yp Yp Yp Yp Yp Yp Yp Yp Yp Yp Yp Yp Zc Zsp Zsp Zsp Zcm Zcm Zcp Yqp Yqp Yqp Yqp Yqp Zcr Zc Zmd Zmd Zmd Zmd Zmd Zmd Zmd Zmd Zmd Zmd Ybg Zsp ^mp ^mp Yn Yn Yn ^bs ^bs ^bs ^bs Ybg Ybg Ybg Ybg Ybg Ybg Ybg Zsm Zsm Zss Zsp Zcs Zcs Zcs Zcs Zcp Zcp Zcp Zcp Zcp Zcp Zsp Zsp Yn Yn Yn Yn Yg Yg Ym Yg Yg Jd Yg Yg Yn Yn Yn ^bs ^bs ^bs ^bs ^bs ^bs ^bs ^bs ^bs ^bs ^bs ^bs ^bs ^bl ^bl ^bl ^bl ^bl ^bl ^bl ^bs ^bs ^bl ^bl ^bl ^bl ^bl ^bl ^bl ^bl ^bl ^bl Yg Yg Yg Yg Zcm Yg Zss Zss Zss Zss Zss Zss Zss Zss Zss Zss Zss Zss Zss Zss Zsm Zsm Zsm Zsp Zsp Zsp Zss Ygt Yn Yn ^bs Ym Ypg Ypg Ym Ym Ylg Yml Yml Ym Ymc Ym Zfa Ym Ym Ym Yp Yp Yp Yc Yc Jdh Jdh Jdh Jd Jdh Jdh Ymc Ymc Jd Jd Jd Yn Yp Yp Yp Yp Yp Yp Yp Yp Yp Ylg Ylg Jdc Jdl Jdl Jdl Jdl Jdl Jdl Jdl Jdl Jd Jd Jd Jdl Jd Jd Jd Jdl Jdl Jdl Jdl Jdc Jdc Jdc Yc ^bs ^bs J^c J^c J^c J^c J^c J^c J^c J^c Jmz Jmz Jmz Jdh Jdh Jdg Jdh Jdh Jdh Yq Yq Jm Jm Jmc Jdh Ypg ^bsh ^bs ^bs ^bs ^bs QTt Zfa Zfs Zfs Zfa Zcm Zcm Zcb Zcb Zcb Jdg Jdg Jdg Jdg Jdg Zfa Zfc Zfc Zfc ^bsh ^bsh Zrr Jdc ^bs Ylg Ymc Ymc Ymc Ymc Yn Yn Yn Yc Ylg ^mp Yq Yp ^bsh ^mp ^bs ^bs ^bsh Js Js Js Jss Js Jss ^bsh ^bsh ^bsh ^bsh ^bsh ^bsh J^cg J^cg J^cg J^cg J^cg J^cg J^cg J^cg J^cg Jtr Jtr Jtrc Jtr Jhg Jhg Jhg Jhgs Jhgs Jhg Jm Qlg Jdg Jdg Jdh Jdh Jdh Jdl J^c J^c Jdh Jdh Jdh Jdh Zcp Qal Qal Qal Qal Qal Qal Qal Qal Ygt Jdo Zmd Zmd Zmd Zcp Zcp Zcp Zcp Zcp Zsp Zmd Zmd Zrd Zrd Zrd Zrd Zrd Ygt Zsp Zcp Zcp Zcp Zss Zss Zss Zss Zss Zss Zcr Zcr Yp Zc Zc Zc Zss Zss Zss Zsm Zss Zcr Zss Ygt Ygt Ygt Ygt Ygt Ygt Zc Zc Zc Qc Qc Qc Qc Qc Qc Jd Jd Jdl Zc Jd Jd Jd Zrd Jd Jd Jd Jdl Jd Jd Zrd Jd Jd Jd Jd Jdl Jd Jd Jd Yp Qc Qc Qc Zc Qc Qc Qc Qc Qc Qc Qc Qc Qc Qc Qc Qc Qc Qc Qc Yp Yp Yp Yp Yp Yp Yhm Yhm Yhm Yhm Yhm Yhm Zss Zss Ygt Yn Yhm Ypg Ypg Ypg Ypg Ylg Ygt Ym Qt Qt QTt |Zmg |Zmg |Zmg Jdh |Zms ^mr ^mp ^mp Yp Jdl |l Jd Jd Zcp |a Yg Jtrc Jtr PLEASANT GROVE FAULT DESCRIPTION OF MAP UNITS CENOZOIC SURFICIAL DEPOSITS Alluvium (Holocene)—Mixtures of clay, silt, sand, gravel, and cobbles underlying flood plains of the Potomac River and its tributaries. Includes low level alluvial terraces as much as 10 ft above stream channels and fine colluvial debris from adjacent slopes. Well to poorly stratified, fining-upward sequences as much as 20 ft thick Colluvium (Holocene and Pleistocene?)—Coarse cobbles, boulders, and blocks that were transported by gravity, debris flow, and freeze-thaw and found on mountain slopes. Boulder streams, boulder fields, rock slides, and talus composed of quartzite and metabasalt are concentrated in hillslope depressions in the Blue Ridge province. Gravel, sand, and silt in sheetlike or lensoid aprons that consist of unsorted subangular clasts of quartzite, epidosite, and vein quartz are mapped on the western margin of the Culpeper basin. Ranges from a thin veneer to over 50 ft thick; deposits greater than 10 ft thick are shown Lag gravel (Holocene and Pleistocene?)—Cobbles and gravel of rounded quartzite, greenstone, and quartz with minor sand, silt, and clay; litter upland surface overlying deeply weathered saprolite developed on conglomerate in the Culpeper basin; thickness varies from 3 to 10 ft Terrace deposits, low level (Holocene and Pleistocene?)—Sand, gravel, and boulders underlying nearly flat benches (three or more), 33 to 165 ft above the Potomac River are 10 to 20 ft thick Terrace deposits, intermediate and high level (Pleistocene and Pliocene?)—Sand, gravel, and boulders in isolated hillocks, 200 to 235 ft above Potomac River are 33 to 120 ft thick; clasts of predominantly quartzite and chert, but granite, metabasalt, metarhyolite, and diabase are deeply weathered with thick rinds MESOZOIC ROCKS OF THE CULPEPER BASIN Diabase dikes and sheets (Early Jurassic)—Medium- to dark-gray, medium crystalline and equigranular, massive diabase with characteristic orange-brown weathered surface; locally coarsely crystalline and gabbroic or gabbroic or granophyric; finely crystalline and aphanitic near chilled margins. At least three magma types: (1) dikes of olivine-normative (olivine-plagioclase-pyroxene) tholeiitic diabase ( Jdo); (2) dikes of low-titanium, quartz-normative tholeiitic diabase containing inch-size phenocrystic clusters of calcic plagioclase in a fine-grained groundmass of pyroxene and plagioclase (Jdl); (3) narrow dikes and differentiated sheets of high-titanium, quartz-normative tholeiitic diabase (Jdh). Igneous differentiation produced early, bronzite-bearing cumulates (Jdc) lower in the sheets and late-stage differentiates higher in the sheets such as granophyre (Jdg), ferrogabbro, diorite, syenite, and aplite, which contain pink potassium feldspar, albite, hornblende, biotite, and quartz. Diabase dikes with magma type not determined are shown as Jd. Dikes vary from several feet to more than 500 ft wide and sheets exceed 2,000 ft in thickness Thermally metamorphosed rocks (Lower Jurassic and Upper Triassic)—Hornfels, quartzite, marble, meta-arkose, and metaconglomerate in zoned contact aureoles adjacent to diabase intrusions. Includes dark-gray to olive-black cordierite-spotted hornfels; bluish-gray to mauve epidote and chlorite hornfels; white to pinkish-gray tourmaline granofels or quartzite; greenish to purplish-gray epidote and chlorite meta-arkose; and white, light-gray, and pink crystalline marble. Contact aureoles a few feet thick adjoin narrow dikes. Contact aureoles more than 700 ft thick and as much as 1 mi wide adjacent to thick sheets; rocks are hard, brittle, fractured, and unweathered; sedimentary structures are commonly preserved Culpeper Group Sander Basalt (Lower Jurassic)—Dark-gray to bluish-gray, fine- to medium-crystalline, porphyritic to equigranular basalt with plagioclase, augite, and pigeonite phenocrysts; locally vesicular and amygdaloidal at tops of flows. Lower flows of basalt locally separated from stratigraphically higher flows of basalt by poorly exposed reddish-brown sandstone and siltstone (Jss). Basalts are characterized by distinctive curved columnar joints locally overprinted by closely spaced fractures; apparently paraconformable with underlying and intercalated sedimentary rocks; estimated thickness less than 1,000 ft Turkey Run Formation (Lower Jurassic)—Sandstone, siltstone, conglomerate, and shale interbedded in cyclic sequences; sandstone, reddish-brown and dark-gray, fine- to coarse-grained, locally pebbly and crossbedded, micaceous, poorly sorted; siltstone, reddish-brown, greenish-gray, and dark-gray, micaceous, ripple-laminated; shale, dark-red and dark-gray, fissile, laminated, fossiliferous, and carbonaceous; conglomerate of variegated, subrounded boulders, cobbles, and pebbles of greenstone, quartzite, marble, quartz, and subangular basalt clasts (Jtrc). Cycles consist of alternation of coarser and finer grained layers several feet thick. Deeply weathered and poorly exposed; about 715 ft thick Hickory Grove Basalt (Lower Jurassic)—Medium- to dark-gray, fine- to medium-crystalline, microporphyritic to equigranular high-titanium, high-iron quartz-normative tholeiitic basalt with plagioclase, augite, and pigeonite phenocrysts; locally vesicular and amygdaloidal at tops of flows with vugs filled by zeolite, calcite, and prehnite. Consists of two or three separate flows of basalt, locally separated by poorly exposed, reddish-brown sandstone and siltstone (Jhgs). Locally disconformable but regionally paraconformable with underlying and overlying sedimentary rocks; calculated thickness as much as 1,200 ft but thins southward to less than 700 ft Midland Formation (Lower Jurassic)—Siltstone, sandstone, shale, and conglomerate, interbedded in cyclic sequences; siltstone, reddish-brown and light- to dark-gray, micaceous, commonly ripple-laminated, locally bioturbated, calcareous, carbonaceous, and fossiliferous; sandstone, reddish-brown and gray, fine- to coarse-grained, feldspathic, locally pebbly, crossbedded and ripple-laminated; shale, dark-red, light-greenish-gray, and dark-gray to black, silty, burrowed, with dessication cracks, carbonaceous, pyritic, calcareous, laminated, and fossiliferous; lenticular variegated cobble and pebble conglomerate and conglomeratic, coarse-grained, reddish-brown arkosic sandstone (Jmc) are locally near the Bull Run fault. Poorly exposed but unit is paraconformable with basalt formations above and below; as much as 1,250 ft thick Mount Zion Church Basalt (Lower Jurassic) —Dark-gray to black, aphanitic to very fine crystalline, microporphyritic high-titanium, quartz-normative tholeiitic basalt. Contains phenocrysts of augite and plagioclase; flow tops are vesicular and contain amygdules filled by calcite, zeolite, and prehnite. Weathers to reddish-brown or gray saprolite in uplands. Poorly exposed but probably conformable or paraconformable with underlying and overlying sedimentary rocks. Consists of one or two flows about 330 ft thick Catharpin Creek Formation (Lower Jurassic and Upper Triassic)—Sandstone, siltstone, and conglomerate, interbedded in cyclic sequences about 100 ft thick. Sandstone, very dark red to dusky-red, micaceous, arkosic, and pebbly; overlain by dusky-red and olive-gray, calcareous, micaceous, thin-bedded to ripple-laminated siltstone that is sparsely fossiliferous and laterally persistent. Lenticular conglomerate is reddish brown with rounded cobbles and pebbles of mainly quartzite and greenstone in a fine- to coarse-grained arkosic sandstone matrix. Unit is more than 3,280 ft thick Goose Creek Member—Lenticular conglomerate and interbedded pebbly sandstone; conglomerate, reddish-brown to grayish-green, thick-bedded to massive, with subrounded pebbles and cobbles of mainly quartzite, greenstone, metasiltstone, gneiss, vein quartz, and minor carbonate in medium- to coarse-grained arkosic sandstone matrix; sandstone, very dark red to dusky-red, micaceous, arkosic, pebbly, and medium- to thick-bedded, poorly sorted, silty, fine- to coarse-grained. Deeply weathered to thick orange-brown saprolite mantled by lag gravel. Thickness may locally exceed 2,500 ft; intertongues laterally into main body of Catharpin Creek Formation Balls Bluff Siltstone (Upper Triassic) Lacustrine shale and siltstone member—Light- to dark-gray, light-greenish-gray, and black, thin-bedded to laminated, locally ripple-marked, mud-cracked, calcareous and dolomitic, sparsely fossiliferous, silty and sandy shale; interbedded with dusky-red, thin-bedded, calcareous, bioturbated, micaceous, feldspathic, clayey and sandy siltstone in cyclic sequences 10 to 30 ft thick. Unit grades northward into the fluvial and deltaic sandstone and siltstone member. Maximum thickness estimated to exceed 5,000 ft, but unit is partly repeated by faulting and is laterally equivalent to the fluvial and deltaic sandstone and siltstone member Fluvial and deltaic sandstone and siltstone member—Reddish-brown, thin- to medium-bedded, feldspathic, locally crossbedded, fine- to medium-grained silty sandstone interbedded with dusky-red, thin-bedded, calcareous, bioturbated, micaceous, feldspathic, clayey and sandy siltstone in cyclic sequences, 3 to 10 ft thick. Intertongues laterally with carbonate conglomerate of the Leesburg Member to the northwest and with the lacustrine shale and siltstone member to the south. Thickness estimated to exceed 5,000 ft Leesburg Member—Light-gray-weathering, crudely bedded conspicuous carbonate conglomerate with subangular to subrounded boulders, cobbles, and pebbles of grayish and reddish lower Paleozoic limestone and dolostone in reddish-brown, pebbly sandstone and calcareous sandy siltstone matrix. Intercalations of calcareous sandstone and siltstone thicken to the southeast where they tongue into the main body of sandstone and siltstone of the Balls Bluff. Thickness calculated as 3,510 ft Manassas Sandstone (Upper Triassic) Poolesville Member—Predominantly gray, pinkish-gray, and reddish-brown, fine- to coarse-grained, thick-bedded, arkosic and micaceous sandstone; locally pebbly and crossbedded where it fills channels; commonly interbedded with calcareous, reddish-brown siltstone in upward-fining sequences in upper part of unit; as much as 3,000 ft thick; gradational and intertonguing relation with overlying and underlying units Reston Member—Basal conglomerate with cobbles and pebbles of metagraywacke, schist, and quartz in poorly sorted, coarse-grained, arkosic sandstone matrix; locally interbedded with reddish-brown sandstone and siltstone; as much as 70 ft thick PALEOZOIC ROCKS OF THE BLUE RIDGE ANTICLINORIUM Frederick Limestone (Upper Cambrian)—Gray-weathering, pale-gray to dark-bluish-gray, laminated, thinly bedded limestone of the Rocky Springs Station Member. Found east of Furnace Mountain, just south of the Potomac River; thickness unknown Tomstown Formation (Lower Cambrian)—Buff-weathering, white to bluish-gray, fine- to medium-grained dolostone, exposed near Furnace Mountain. Massive to locally well-bedded, with 0.5-in-thick partings. Locally sandy with dolomitic cement. Thickness less than 200 ft Carbonaceous phyllite (Lower Cambrian)—Medium- to dark-gray, fine-grained muscovite-graphite phyllite. Locally contains 0.04- to 0.4-in-thick alternating light- and dark-gray bedding laminae. Produces distinctive light-gray “ashy” soil. Found only as float or small slumped outcrops; occurs as discontinuous lenses above Antietam Quartzite near Furnace Mountain. Interfingers laterally with dolomite of the Tomstown Formation. Maximum thickness probably less than 200 ft Chilhowee Group Antietam Quartzite (Lower Cambrian)—Gray to buff-weathering, fine-grained, poorly bedded to massive meta-arkose. Poorly exposed on ridges. Near contact with overlying Tomstown Formation or carbonaceous phyllite are local ferruginous horizons with abundant botryoidal hematite and limonite. Found only on east side of Catoctin Mountain. Thickness about 100 ft Harpers Formation (Lower Cambrian)—Dark-greenish-gray to brownish-gray, fine- to very fine grained, laminated to massively bedded biotite-chlorite-muscovite-quartz phyllite and metasiltstone. Magnetite- rich, sandy metasiltstone and fine-pebble conglomerate found locally at the base on Short Hill Mountain. Rocks are fine grained with bedding locally preserved on Blue Ridge and Short Hill Mountain; on Catoctin Mountain bedding is totally transposed, and dominant fabric is metamorphic foliation consisting of alternating 0.04-in-thick quartzose and micaceous laminae. Thickness varies from 1,000 ft on west limb to 500 ft on east limb Weverton Formation (Lower Cambrian) Upper member—Dusky-blue, dark-gray to dark-purplish-gray, very coarse grained quartzite and quartz-pebble conglomerate. Poorly sorted, thick-bedded with graded beds and crossbeds; contains local accumulations of magnetite, ilmenite, and pebbles of red jasper, red and purple quartz, and phyllite. Interbedded with poorly exposed dark-greenish-gray phyllitic metasiltstone. Gradational with overlying Harpers Formation. Thickness ranges from 105 ft on Short Hill Mountain to 380 ft on Blue Ridge Middle member—Greenish-gray to light-gray, massive, medium-grained to granular quartzite in 16- to 32-ft-thick beds interbedded with metasiltstone. Forms topographic ledges but is not well exposed. Thickness ranges from 200 to 240 ft Lower member—Light- to medium-gray, fine- to medium-grained, well-sorted, graded, crossbedded, massive, thick-bedded vitreous quartzite interbedded with light-gray metagraywacke and metasiltstone; locally arkosic; unconformable to basement gneiss on Blue Ridge. Thickness ranges from 100 to 200 ft Weverton Formation, undivided (Lower Cambrian)—White to gray, massive to thick-bedded vitreous quartzite with thin (less than 1 ft thick) interbeds of dark phyllite is restricted to Catoctin Mountain where it is conformable with overlying Harpers Formation and underlying Loudoun Formation; as much as 120 ft thick Loudoun Formation (Lower Cambrian)—On Blue Ridge consists of gray-black, light-olive-gray to dark-purple-gray phyllite containing tuffaceous clasts and elongated amygdules; locally overlain by dark-gray to dusky-blue, very coarse pebble conglomerate (|lc) with pebbles of rounded to subrounded white, blue, and red quartz, gneiss(?), red jasper, and variegated phyllite in grayish-black, iron-rich silty matrix; local interbeds of coarse quartz-pebble conglomerate and graded and crossbedded quartzite. On Catoctin Mountain consists of dark-gray- to greenish-gray-weathering chlorite-quartz-graphite-muscovite phyllite and lesser white- to gray-weathering pebbly metasandstone with thin phyllite interbeds. Base of formation is transitional with the underlying Catoctin Formation; top of phyllite is sharp contact with conglomerate or overlying Weverton Formation; conglomerate is transitional with overlying Weverton Formation. Ranges from 0 to 200 ft thick where present ROCKS OF THE POTOMAC TERRANE IN THE PIEDMONT PROVINCE Mather Gorge Formation (Lower Cambrian and (or) Late Proterozoic) Schist—Lustrous, greenish-gray to gray, reddish-brown-weathering, very fine grained, quartz-muscovite-chlorite-plagioclase-epidote-magnetite- hematite schist. Thin to very thin beds are largely transposed into early cleavage. Contains interbedded metagraywacke and a few lenses of calc-silicate rock. Found only in the extreme eastern corner of Loudoun County. Thickness is unknown Metagraywacke—Light- to medium-gray, yellowish- to reddish-brown weathering, fine- to medium-grained, generally well-bedded metagray- wacke and semi-pelitic schist. Beds range from about 1.2 in to as much as 10 ft thick, averaging about 8 in. Many beds are graded; sole marks and slump features are abundant. Contains interbedded quartzose schist and lenses of calc-silicate rock. Forms bold outcrops along the Potomac River in the extreme eastern corner of Loudoun County. Thickness is unknown LATE PROTEROZOIC ROCKS OF THE BLUE RIDGE ANTICLINORIUM Catoctin Formation (Late Proterozoic)—Dark-green to bluish-gray, fine-grained to aphanitic, massive to schistose, amygdaloidal metabasalt and greenstone, composed of actinolite, chlorite, epidote, albite, and rare quartz. Contains lenses and layers of fine-grained, hard, massive, apple-green epidosite (an epidote-quartz rock). Subunits interlayered with metabasalt include metabasalt breccia (Zcb); lustrous, silvery-white metarhyolite and fine-grained quartz-sericite phyllite interpreted as felsic metatuff (Zcr); light-gray, fine-grained phyllite and metasiltstone and gray to buff, crossbedded feldspathic metasandstone (Zcs); finely laminated, gray, quartz-graphite-muscovite phyllite (Zcp); and 3- to 10-ft-thick lenses of gray- to buff-weathering, fine- to medium-grained white calcite marble (Zcm). Margins of the marble commonly contain coarse-grained actinolite, tremolite, and chlorite. Ranges in thickness from about 2,500 to 5,000 ft on east limb of anticlinorium and from 0 to 1,600 ft on west limbs Metadiabase dikes (Late Proterozoic)—Dark-greenish-gray, fine- to medium-grained, massive to schistose metadiabase (greenstone) composed predominantly of chlorite, epidote, albite, and actinolite. Coarse-grained and porphyritic metadiabase dikes have stubby, 0.08- to 0.16-in-long actinolite pseudomorphs after clinopyroxene that produce distinctive nubbly texture and are shown by a pattern north of Taylorstown. Rare aphanitic metadiabase has relict euhedral plagioclase laths. Dikes range in thickness from a few inches up to 164 ft thick. Compositionally similar to metabasalt of the Catoctin Formation Metarhyolite dikes (Late Proterozoic)—White- to gray-weathering, aphanitic to fine-grained metarhyolite. Ranges in texture and mineralogy from fine-grained quartz sericite schist to very fine grained and flinty rock with plagioclase feldspar phenocrysts in a felty groundmass consisting of quartz, feldspar, biotite, and epidote. Ranges in thickness from 33 to 66 ft; maximum length is about 9 mi Swift Run Formation (Late Proterozoic) Marble—Gray- to buff-weathering, medium- to fine-grained, white calcitic and dolomitic marble. Locally contains thin arenaceous layers. Occurs as discontinuous layers, 3 to 10 ft thick, both near top of section, in phyllite (Zsp) and near bottom of section, in schist (Zss) Marble, slate, and phyllite—Medium-dark-gray slate; grayish-red-purple, grayish-green, finely laminated phyllite; and dark-greenish-gray to brown-gray sandy sericitic phyllite in fining-upward sequence. Pink-gray to light-brownish-gray, fine-grained, dolomitic marble locally found near top. Top of unit is transitional with overlying metabasalt and phyllite of the Catoctin Formation. Thickness ranges from 0 to 400 ft Metagraywacke, schist, metasandstone, quartzite, and meta-arkose— Brownish-green chlorite-sericite-feldspar-quartz metagraywacke; lustrous, silvery quartz-sericite schist; and pinkish- to greenish-gray, very coarse to medium-grained metasandstone and quartzite with crossbeds and quartz pebbles and cobbles in fining-upward sequence. Grayish-brown meta-arkose (paleoregolith?) locally found at basement-cover contact. Thickness ranges from 0 to 300 ft on Blue Ridge and from 0 to 1,000 ft on Catoctin Mountain Fauquier Formation (Late Proterozoic) Metamudstone—Light-gray, light-brown-weathering, thinly laminated (less than 1 in) sericite-quartz metamudstone and lustrous sericite phyllite. Quartz in metamudstone is very fine sand. Upper contact gradational with rocks of the Catoctin Formation. Thickness ranges from 0 to 160 ft Meta-arkose—Light- to medium-gray and olive-gray, light-yellowish-gray- weathering, coarse- to fine-grained sericite-potassium feldspar-quartz meta-arkose. Bedding is of variable thickness (1–6 ft), defined by fining-upward sequences; tangential and trough crossbeds are common. Gravel and isolated cobble sized clasts occur locally. Upper contact gradational with metamudstone; lower contact gradational with metaconglomerate or unconformable on Mesoproterozoic gneiss. Thickest section is 1,500 ft Metaconglomerate and meta-arkose—Light- to dark-gray, cobble conglomerate and meta-arkose with biotite-chlorite-sericite-potassium feldspar-quartz matrix. Bedding is of variable thickness (2–20 ft), defined by fining-upward sequences. Cobbles are locally derived metagranite. Trough crossbeds of coarse- to medium-grained arkose are common. Lower contact is an unconformity that has as much as 328 ft relief and is distinct and abrupt; elsewhere it is obscured within a zone of biotite-chlorite-quartz schist that is possibly metamorphosed paleoregolith. Upper contact is gradational with meta-arkose unit. Thickness ranges from 0 to 200 ft Robertson River Igneous Suite (Late Proterozoic) Cobbler Mountain Alkali Feldspar Quartz Syenite—Gray- to buff- weathering, massive, medium- to coarse-grained alkali feldspar quartz syenite. Consists of stubby, euhedral mesoperthite crystals 0.04 to 0.08 in. in diameter intergrown with anhedral quartz and minor interstitial plagioclase, in addition to a mafic phase (amphibole?) that has been broken down into quartz, plagioclase, and magnetite. Mesoperthite crystals conspicuous on weathered surface. Forms low, rounded outcrops whose surfaces become friable upon weathering. Locally cut by dikes of fine-grained granite of presumably coeval magma source MESOPROTEROZOIC BASEMENT ROCKS OF THE BLUE RIDGE ANTICLINORIUM Biotite granite gneiss—Orange- to gray-weathering, pink, medium- grained, well-foliated and (or) lineated biotite-plagioclase-quartz- microcline gneiss. Biotite content ranges from 10 to 15 percent. Mesoproterozoic foliation expressed by planar aggregates of biotite; locally, where foliation is weak or absent, replaced by a lineation expressed by biotite streaks and rodded quartzofeldspathic grains. Interlayered with leucocratic metagranite (Yg and Ygt) Pink leucocratic metagranite—Pink, medium- to medium-fine-grained, massive to moderately foliated plagioclase-quartz-microcline metagranite. Biotite content ranges from 0 to 10 percent. Foliation defined by flattened quartz and feldspar grains and local thin biotite-rich layers Leucocratic metagranite—White, medium- to medium-fine-grained, massive to moderately foliated plagioclase-quartz-microcline metagranite. Biotite content ranges from 0 to 5 percent. Mesoproterozoic foliation defined by biotite where present and by flattened grains of quartz and feldspar and thin aplite layers Garnetiferous leucocratic metagranite—White, medium- to medium- fine-grained, massive to moderately foliated garnet-plagioclase-quartz- microcline metagranite. Identical to Yg except for the presence of irregularly distributed almandine crystals 0.04 to 0.4 in. in diameter. Garnets commonly dark due to alteration of rim to biotite and chlorite Quartz-plagioclase gneiss—White, medium- to medium-fine-grained, weakly to moderately well-foliated biotite-quartz-plagioclase gneiss. Biotite content ranges from 0 to 5 percent. Foliation defined by flattened quartz and feldspar grains and thin aplite layers and by biotite, where present. Strongly resembles leucocratic metagranite (Yg) but potassium feldspar rare or absent Marshall Metagranite—Pink, medium-grained, weakly to moderately well-foliated and (or) lineated biotite-plagioclase-quartz-microcline metagranite. Biotite content ranges from 10 to 15 percent. Layering locally produced by veins of pink pegmatite parallel to foliation. Resembles biotite granite gneiss (Ybg) in outcrop Coarse-grained metagranite—Gray to pink, medium- to coarse-grained, massive to well-foliated biotite-plagioclase-quartz-microcline metagranite. Characterized by 0.4- to 0.8-in-long white or pink microcline porphyroblasts and aggregates of blue quartz; biotite content ranges from 0 to 10 percent. Commonly contains pronounced augen texture due to overprinting by Paleozoic schistosity Porphyroblastic metagranite—Yellowish-brown-weathering, medium- to coarse-grained garnet-biotite-plagioclase-quartz-microcline metagranite. Characterized by megacrysts of orange to pink microcline or microcline-rich aggregates that are deformed into rounded ovoids 0.4 to 1.2 in. in diameter; garnet, biotite, plagioclase, opaques, and distinctive clots of blue quartz occur interstitially. Flattened ovoids define foliation, which is locally cut by dikes of garnetiferous leucocratic metagranite (Ygt) and pink leucocratic metagranite (Yml) Hornblende monzonite gneiss—Gray-weathering, medium-fine- to fine-grained, well-foliated hornblende-quartz-microcline-plagioclase gneiss. Foliation defined by strongly flattened quartz and feldspar grain aggregates and prismatic hornblende; quartz content 10 to 20 percent, hornblende as much as 30 percent, and microcline as much as 50 percent. Rarely occurs as a more massive, spotted rock. Biotite and orthopyroxene are rare mafic constituents Layered granitic gneiss—White, gray, or pink, medium- to fine-grained, well-layered garnet-biotite-plagioclase-quartz-microcline gneiss. Layer- ing, 0.04 to 0.4 in thick, is defined by concentrations of biotite and aplite; garnets up to 0.4 in. in diameter scattered throughout rock. Complex structures in outcrop include folded layering and swirly, migmatitic texture suggestive of partial melting. Relation to other granitic units unknown Charnockite—Yellow- to brown-weathering, dark-green, medium- to coarse-grained, massive to locally well-foliated quartz-hornblende- orthopyroxene-microcline-plagioclase rock. Mafics compose about 35 percent of the rock. Poorly exposed; forms topographic highs, a bright orange soil, and distinctive float that consists of pitted boulders and cobbles with a 0.4- to 0.8-in-thick, orange-yellow weathering rind. Locally contains garnet and biotite. Occurs as discontinuous lensoid bodies Metanorite and metadiorite—Gray-weathering, medium- to coarse- grained, massive to well-foliated hornblende-orthopyroxene-plagioclase metanorite and medium- to medium-fine-grained biotite-hornblende- plagioclase metadiorite. Brown hornblende and colorless pyroxene commonly altered to pale-green amphibole. Occurs as lenses and thin belts, commonly in proximity to garnet-graphite paragneiss Paragneiss—Rusty-weathering, medium-fine- to fine-grained, well-foliated to layered graphite-biotite-garnet-plagioclase-quartz gneiss and schist. Layering defined by alternating 0.4-in-thick garnet-rich zones containing garnets that are 0.04 to 0.4 in. in diameter and 0.4-in-thick quartzofeldspathic layers. Garnets typically deformed and retrograded to green lensoid clots of fine-grained chlorite and muscovite. Graphite occurs as disseminated, small, rounded flakes. Distinctive rusty stain produced by secondary hematite after accessory magnetite. Retrograded, schistose varieties include quartz-chlorite-magnetite schist and carbonaceous phyllonite. Probable remnant of pregranitic country rock Quartzite and quartz tectonite—Light-gray to white, fine- to medium-grained, massive quartzite and quartz-sericite tectonite. No primary textures recognized such as bedding or early metamorphic foliation; commonly contains strong Paleozoic penetrative cleavage. Contains thin lenses of graphite in outcrop and larger mappable pods of paragneiss (Yp). Considered to be part of metasedimentary suite EXPLANATION OF MAP SYMBOLS Contact—Dotted where concealed Lake bed—Dotted where concealed FAULTS (Dashed where inferred; dotted where concealed) Thrust fault—Sawteeth on upper plate Normal fault—Ball and bar on downthrown side Reactivated fault—Open ball and bar on downthrown side of early normal fault and sawteeth on upper plate of later thrust fault; shown in cross section as a double-headed arrow Strike-slip fault—Arrows show relative movement FOLDS (Folds show trace of axial surface, direction of dip of limbs, and, where known, direction of plunge. Dotted where concealed) Overturned anticline Syncline Overturned syncline Synformal anticline Antiformal syncline PLANAR FEATURES (May be combined with linear features) Strike and dip of bed—Ball indicates top of bed known from sedimentary structures Inclined Overturned Rotated more than 180° Strike and dip of Mesoproterozoic foliation including compositional layering Inclined Vertical Strike and dip of first-generation (S 1 ) Paleozoic cleavage and (or) schistosity Inclined Vertical Strike and dip of second-generation (S 2 ) Paleozoic cleavage Strike and dip of highly crenulated first-generation (S 1 ) Paleozoic cleavage and (or) schistosity Strike and dip of mylonitic and phyllonitic foliation Strike and dip of joint Inclined Vertical Strike and dip of axial plane of fold Mesoproterozoic (first-generation) Paleozoic (second-generation) LINEAR FEATURES (May be combined with planar features) Mesoproterozoic mineral lineation, rodding, and (or) axis of minor fold Paleozoic mineral lineation, rodding, and (or) axis of minor fold OTHER FEATURES Sample site for radiometric date—Refer to figure 1 and table 3 (in pamphlet) Sinkhole 39°15' 39°15' 39°10' 39°10' 39°05' 39°05' 39°00' 39°00' 38°55' 38°55' 77°50' 77°50' 77°45' 77°45' 77°40' 77°40' 77°35' 77°35' 77°30' 77°30' 77°30' 77°30' 77°25' 77°25'
Transcript of Geologic Map of Loudoun County, Virginia · B U L L R U N S F A U L T Cambrian Weverton Formation...
NO
RT
HE
RN
VIR
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IAR
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ION
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LOW
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ISLA
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E 7
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Bla
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idg
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Cro
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ranch
SHORT HILL FAULT
BULL RUN FAULT
SEA LEVEL
8000
SEA LEVEL
80002000 feet=610 metersQuaternary units are not shown
NO VERTICAL EXAGGERATION
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A A'
Ab
An
Or
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lite
gran
odio
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quar
tz m
onzo
nite
trondhjemitegranite
Q
A P
YbgYmlYgYgtYqpYmYpgYhmYlgYcYn
monzogranite
syenogranite granodiorite
quartzmonzodioritequartz monzonite
diorite
quartz diorite
tonalite
YbgYmlYgYgtYmYpgYhmYn
A
A B
DEC
B
Figure 2.—A, Quartz-alkali feldspar-plagioclase ternary diagram of Mesoproterozoic gneisses (Streckeisen, 1976); Q, quartz; A, alkali feldspar; P, plagioclase. B, Normative feldspar ternary diagram of Middle Proterozoic gneisses (O’Connor, 1965); An, anorthite; Ab, albite; Or, orthoclase.
La Ce Nd Sm Eu Tb Yb Lu1
10
100
Ro
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nd
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5
10
MgO
50 554540 60SiO2
0.5 1.00
0
1
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4
5
TiO2
P2O5
5 100
1
2
3
4
5
MgO
TiO2
A M
F
calc-alkaline
tholeiitic
Coarse-grained metadiabase dike, n=6
Fine-grained metadiabase dike, n=30
Porphyritic metadiabase dike, n=10
Low TiO2/high MgO metavolcanic flow or flow breccia, n=9
Catoctin Formation metabasalt, n=26
EXPLANATION
Figure 4.—A, AFM diagram of Late Proterozoic dikes and metabasalt of the Catoctin Formation showing the tholeiitic–calc-alkaline boundary of Irvine and Baragar (1971). A, Na2O+K2O; F, total iron as FeO; M, MgO. B–D, Variation diagrams of Late Proterozoic dikes and metabasalt of the Catoctin Formation showing TiO2-MgO, TiO2-P2O5, and MgO-SiO2 ratios, respectively. E, Chondrite-normalized rare earth element diagram of Late Proterozoic metadiabase dikes and metabasalt of the Catoctin Formation. n=number of measurements.
NN
SS
WW EE
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SS
WW EE
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SS
WW EE
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WW EE
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SS
WW EE
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WW EE
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WW EE
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WW EE
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WW EE
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WW EE
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WW EE
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WW EE
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WW EE
NN
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WW EE
Structural features developedduring metamorphism of the
Mesoproterozoic rocks
Structural features developedduring metamorphism of the
Mesoproterozoic rocks
Orientation of metadiabase dikesemplaced into Mesoproterozoic
rocks in the Late Proterozoic
Orientation of metadiabase dikesemplaced into Mesoproterozoic
rocks in the Late Proterozoic
Structural features developed during the Paleozoicin both Proterozoic and Cambrian rocks on opposite
sides of the Short Hill fault
Structural features developed during the Paleozoicin both Proterozoic and Cambrian rocks on opposite
sides of the Short Hill fault
n=20n=20 n=34n=34
n=76n=76
n=605n=605
Mean:N. 21° E.,28° SE.
Mean:N. 21° E.,28° SE.
n=52n=52
n=52n=52
n=806n=806 n=182n=182
n=403n=403
n=216n=216
n=1,017n=1,017
n=71n=71
Mean:N. 18° E.,36° SE.
Mean:N. 18° E.,36° SE.
Mean:N. 26° E.,37° SE.
Mean:N. 26° E.,37° SE.
Mean:N. 9° E.,39° SE.
Mean:N. 9° E.,39° SE.
Orientation of bedding planesin Paleozoic and Mesozoic
sedimentary rocks
Orientation of bedding planesin Paleozoic and Mesozoic
sedimentary rocks
Mean:N.10° E.,58° SE.
Mean:N.10° E.,58° SE.
Strike directions of contactsStrike directions of contacts
D2 mineral lineation, n=15D2 fold hinge, n=5D3 fold hinge, n=2
D2 mineral lineation, n=15D2 fold hinge, n=5D3 fold hinge, n=2
Poles to bedding in Triassic and Jurassic rocks
of the Culpeper basin
Poles to bedding in Triassic and Jurassic rocks
of the Culpeper basin
Contoured poles to bedding in Cambrian Weverton Formation
on Short Hill Mountain
Contoured poles to bedding in Cambrian Weverton Formation
on Short Hill Mountain
Contoured poles to bedding in Cambrian Weverton Formation
on Blue Ridge
Contoured poles to bedding in Cambrian Weverton Formation
on Blue Ridge
Contoured poles toS1 cleavage
Contoured poles toS1 cleavage
Poles to contactsPoles to contactsContoured poles to D1 foliationContoured poles to D1 foliation
Contoured poles to D2 foliationContoured poles to D2 foliation
Contoured poles toS1 cleavage
Contoured poles toS1 cleavage
Contoured poles toS1 cleavage
Contoured poles toS1 cleavage
Contoured poles toS1 cleavage
Contoured poles toS1 cleavage
Contoured poles toS2 cleavage
Contoured poles toS2 cleavage
Contoured poles toS2 cleavage
Contoured poles toS2 cleavage
Two generations (S1 and S2) cleavage exhibited at an outcrop localityTwo generations (S1 and S2) cleavage exhibited at an outcrop locality
Single generation (S1) cleavage exhibited at an outcrop localitySingle generation (S1) cleavage exhibited at an outcrop localityWestWest EastEast
Figure 3.—Equal-area projections (Schmidt net) of structural data. Contour intervals are 2 percent per 1 percent area. n=number of measurements.Figure 3.—Equal-area projections (Schmidt net) of structural data. Contour intervals are 2 percent per 1 percent area. n=number of measurements.
AA
BB EE
CC
FF
HH
JJ
GG
II
KK
LL
MM
NN
DD
ßß
1 2 3
4 5 6 7
98 10 11 12 13
1514 16 17
19
18
Sources of Geologic Mapping
Blue Ridge anticlinorium (W.C. Burton, J.S. Schindler, and Scott Southworth, compilers)
Culpeper basin (A.J. Froelich and Scott Southworth, compilers)
Potomac terrane (A.A. Drake, Jr. and Scott Southworth, compilers)
SOURCES OF DATA FOR 7.5-MINUTE QUADRANGLES IN LOUDOUN COUNTY
Charles Town: Nickelson (1956); Scott Southworth (unpub. data)Harpers Ferry: Southworth (1991); Southworth and Brezinski (1996)Point of Rocks: Lee (1979); Burton and others (1995); J.P. Smoot (unpub. data)Round Hill: McDowell and Milton (1992); W.C. Burton and Scott Southworth
(unpub. data)Purcellville: Southworth (1995); W.C. Burton (unpub. data)Waterford: Lee (1979); Burton and others (1995); J.P. Smoot (unpub. data)Poolesville: Lee (1979); Southworth (1998); A.J. Froelich and J.P. Smoot
(unpub. data)Ashby Gap: Gathright and Nystrom (1974); Tollo and Lowe (1994); W.C. Burton
and Scott Southworth (unpub. data)Bluemont: Southworth (1994); W.C. Burton (unpub. data)Lincoln: Lee (1979); W.C. Burton, A.J. Froelich, J.S. Schindler, and J.P. Smoot
(unpub. data)Leesburg: Toewe (1966); Lee (1979); A.J. Froelich, J.S. Schindler, J.P. Smoot,
and R.E. Weems (unpub. data)Sterling: Lee (1979); A.J. Froelich, J.P. Smoot, Scott Southworth, and R.E.
Weems (unpub. data) Seneca: Lee (1979); A.A. Drake, Jr., A.J. Froelich, J.P. Smoot, Scott Southworth,
and R.E. Weems (unpub. data)Upperville: W.C. Burton, A.E. Nelson, and Scott Southworth (unpub. data)Rectortown: Espenshade (1983); Leo (1990); W.C. Burton and Scott Southworth
(unpub. data)Middleburg: Lee (1979); Kline and others (1990) Arcola: Lee (1978); A.J. Froelich, J.P. Smoot, and R.E. Weems (unpub. data)Herndon: Eggleton (1975); Lee (1979); A.J. Froelich, J.P. Smoot, and R.E.
Weems (unpub. data)Gainesville: Lee (1979); A.J. Froelich, J.P. Smoot, and R.E. Weems (unpub.
data)
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77°45' 77°37'30"77°52'30"39°07'30"
39°
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2011195815-27-61-95-124-156-183-213-241-272-301-336-369-411-454-515-597
NANOTESLAS
ZssYpg
Yn
Yc
Zss
Zss
Zss
Ygt
Ylg
Ypg
Ymc
Yml
Ym
Ym
Ym
Ymc
Ymc
Zfa
Zfc
Zfa
Zc
Ju
Ju
Ju
|w
|w
Zc
Yq
Yq
Ylg
Yp
YpYn Ygt
Yq
Yp
Yp
Yp
Zc
Yp
Yp
Yp
Yp
Yp
Yc
Yc
Jd
Yg
Yg
Jd
Jd
Yg
Yg
Yg
Yg
Yg
Zsp
YgJd
Jd
Jd
Jd
Jd
Ylg
Ylg
Ylg
Ypg
Ygt
Ygt
YgtYgt
Ygt
Yp
YpZss
Jd
Jd
BLU
E RI
DG
E M
OU
NTA
INS
BLA
CK
OA
K R
IDG
E
BL
UE
MO
NT
LI
NC
OL
N
Figure 5.—Color-shaded-relief aeromagnetic image of the Lincoln and most of the Bluemont 7.5-min quadrangles with general geologic contacts. Image based on aeromagnetic survey flown in 1989 by the U.S. Geological Survey. Data were processed and interpreted by D.L. Daniels, W.F. Hanna, and R.E. Bracken. Ju, Culpeper Group, undivided.
ER
A/E
RA
TH
EM
Cen
ozo
icM
eso
zoic
Jura
ssic
Low
er
Tria
ssic
Cam
bri
an
Low
er
Up
per
Ch
ilho
wee
Gro
up
Up
per
Pal
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icLa
te P
rote
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icM
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PE
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D/S
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OC
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ER
IES
LITHOLOGIC UNITLITHOLOGY AND
MAP SYMBOLTHICKNESS,
IN FEET
<1,000
715
<700–1,200
1,250
330
2,500 (J^cg)>3,280 (J^c)
>5,000 (^bsh)>5,000 (^bs)3,510 (^bl)
3,000 (^mp)70 (^mr)
?<200
<200
500–1,000
105–380
200–240
100–200
0–200
0–5,000
10 (Zsm)0–400 (Zsp)
300–1,000 (Zss)0–160 (Zfs)1,500 (Zfa)0–200 (Zfc)
100
Quaternary
Quaternaryor
Tertiary
Pleistoceneor
late Tertiary
Holocene AlluviumColluvium, lag gravel, and terraces
Terraces
Sander Basalt
Hickory Grove Basalt
Mount Zion Church Basalt
Turkey Run Formation
Midland Formation
Catharpin Creek Formation
Balls Bluff Siltstone
Manassas Sandstone
Frederick LimestoneTomstown Formation
Harpers Formation
Loudoun Formation
Catoctin Formation
Robertson River Intrusive Suite
Biotite granite gneiss (Ybg)Pink leucocratic metagranite (Yml)
Leucocratic metagranite (Yg)Garnetiferous leucocratic metagranite (Ygt)
Quartz-plagioclase gneiss (Yqp)Marshall Metagranite (Ym)
Coarse-grained metagranite (Ymc)Porphyroblastic metagranite (Ypg)
Hornblende monzonite gneiss (Yhm)Layered granitic gneiss (Ylg)
Charnockite (Yc)Metanorite and metadiorite (Yn)
Paragneiss (Yp)Quartzite and quartz tectonite (Yq)
Swift Run Formation andFauquier Formation
upper member
middle member
lower member
WevertonFormation
Carbonaceous phylliteAntietam Quartzite
Diabase dikes and sheets
Holocene andPleistocene
QTtQlg
Qt
Jdh
Jss
Js
JtrJtrc
Jhg
Jmz
Jdh
Jdh
J^cg
^bs
^bsh^bl
^mp
|f
|a
|h
|wu
|wm
|wl
|l
Zcp
Zcp
Zcm
Zsm
Zsm
Zrr
Ylg
YmlYmc
Ymc
Ym
Ym
Ym
Yml
Yhm
Ybg
Ybg
Zmd Zrd
Ygt
Ygt
Ygt
Yqp
Yg
Yg
Yp
YpYp
YcYnYq
Ypg
Ypg
Ypg
Ypg
Zsp
Zss
Zfs
ZfaZfc
Zcs
Zcr
Zcb
Zc
|lc
|t|cp
^mr
J^c
J^tm
J^tm
Jmc Jm
Jhgs
Jd
Jd
Qal
Qc
Coarse cobbles, boulders, and blocks
Conglomerate
Sandstone
Sandstone and siltstone
Siltstone
Limestone
Quartzite
Basalt-diabase
Metabasalt-metadiabase
Igneous rocks
EXPLANATION
50
15
1528
7
7
9
734
671
287
Leesburg
Purcellville
Middleburg
Sterling
M a r y l a n dW e s t
V i r g i n i a
V i r g i n i a
LoudounCounty
Potomac
River
11
2
4
16
14
(2)
1
12
11
19
6
3
5
(1)
(2)
(4)
(4)7(5)
10(8)
18
(3)
15(10)
8(6)13(7)
17(9)
9
WESTMINSTERTERRANE
POTOMACTERRANE
PIEDMONT PROVINCE
CULPEPER BASIN
GETTYSBURGBASIN
BLUE RIDGE PROVINCE
BLUE RIDGE ANTICLINORIUM
PLEA
SAN
T G
ROVE
FAU
LT
BU
LL R
UN
FA
ULT
MA
RTIC
FA
ULT
BU
LL R
UN
FA
ULT
77°30'
39°
0 5 10 MILES
0 5 10 15 KILOMETERS
Figure 1.—Map showing general geologic provinces and prominent geologic features of the area of and surrounding Loudoun County, Va.
EXPLANATION
Normal fault—Bar and ball on downthrown side
Thrust fault—Sawteeth on upper plate
Location of sample providing radiometric age determination—Number is keyed to samples listed in table 3 of the pamphlet. Note that those sample numbers in parentheses represent the age of metamorphism and those sample numbers without parentheses represent the age of intrusion
SH
ORT
HIL
L
FA
ULT
SH
ORT
H
ILL
FAU
LT
S
HO
RT
HIL
L
FAULT
W
HITE
ROCK
FA
ULT
DU
TC
HM
AN
CREEK SHEA
R ZO
NE
BU
LL R
UN
FA
ULT
BU
LL R
UN
FA
ULT
FUR
NA
CE
M
OU
NTA
IN
FA
ULT
MO
RV
EN S
YN
CLI
NE
66
82
60
(3)
15(10)
5
14
(2)
(2)
(2)
(4)
1
13(7)
12
8(6)
16
210(8)
18
(4)
(2)
(1)
4
9
7(5)
A'
A
Qal
Qc
Qlg
Qt
QTt
Jd
Jdl
Jdo
Jdg
Jdh
Jdc
J^tm
Js
Jss
Jtr
Jtrc
Jhg
Jhgs
Jm
Jmc
Jmz
J^c
J^cg
^bsh
^bs
^bl
^mp
^mr
|f
|t
|cp
|a
|h
|wu
|wm
|wl
|w
|lc
|l
|Zms
|Zmg
Zc
Zcb
Zcr
Zcp
Zcs
Zcm
Zmd
Zrd
Zsm
Zsp
Zss
Zfs
Zfa
Zfc
Zrr
Ybg
Yml
Yg
Ygt
Yqp
Ym
Yhm
Ymc
Ylg
Ypg
Yc
Yn
Yp
Yq
32
44
6
77
47
60
32
43
80
31
16
12
57
Topographic base scanned from the USGS County Map Series,Loudoun County, Va., 1992.
SCALE 1:50 0001 1/2 0 1 2 3 4 MILES
1 .5 10 2 3 4 5 KILOMETERS
GEOLOGIC MAP OF LOUDOUN COUNTY, VIRGINIABy
Scott Southworth, William C. Burton, J. Stephen Schindler, and Albert J. Froelich2006
U.S. DEPARTMENT OF THE INTERIORU.S. GEOLOGICAL SURVEY
Prepared in cooperation with theLOUDOUN COUNTY OFFICE OF MAPPING AND GEOGRAPHIC INFORMATION
GEOLOGIC INVESTIGATIONS SERIES MAP I–2553
Explanatory pamphlet accompanies map
Printed on recycled paper
VIRGINIA
MAP LOCATION
Digital compilation by Remo Nardini, Peter G. Chirico, and James E. Reddy
MA
GN
ET
IC N
OR
TH
APPROXIMATE MEANDECLINATION, 2006
TR
UE
NO
RT
H
10 / °1 2
This map is available for sale by U.S. Geological Survey, Information Services, Box 25286, Denver Federal Center, Denver, CO 80225
For product and ordering information:World Wide Web: http://www.usgs.gov; Telephone: 1-888-ASK-USGS
Any use of trade, product, or firm names in this publication is for descriptive purposes only and does not imply endorsement by the U.S. Government
Stratified Intrusive Stratified Intrusive Stratified IntrusiveBlue Ridge anticlinorium Culpeper basin Potomac terrane
197 Ma
201.2±1.3 Ma200 Ma
571.5±5 Ma600 Ma
722±3 Ma
1,055±4 Ma
1,059±2 Ma
1,060±2 Ma
~1,077±4 Ma
1,077±4 Ma
1,111±2 Ma
~1,140 Ma
1,140±4 Ma
1,149±19 Ma
1,153±6 Ma
CENOZOIC
MESOZOIC
PALEOZOIC
PROTEROZOIC
Holocene
Early Jurassic
Late Triassic
ChilhoweeGroup
Early Cambrian
Late Cambrian
NewarkSupergroup
Culpeper Group,upper part
Culpeper Group,lower part
Quaternary
Jurassic
Cambrian
Late Proterozoic
Mesoproterozoic
Triassic
Quaternary andTertiary(?)
Holocene andPleistocene(?)
Pleistocene andPliocene(?)
Unconformity Unconformity
Unconformity
Unconformity
Jdg*Jdc
Qal
Jd*
|f
|t
|cp
|a
|h
Zrd*
Zrr*
Ybg*
Yml*
Yg*
Ygt*
Yqp*
Ym*
Ymc*
Ypg*
Yhm*
Ylg*
Yp
Yq
Jdl Jdo
Qc
Qal
QTt
Jss
JhgsJhg
JmJmc
JtrJtrc
|lc
|wu
|wm
|wl
^bl ^bs
J^cgJ
t̂mJ^c
^mr^mp
^bsh
|w
Zcr*
ZcZcp
ZcmZcb
Zsp
ZssZfa
Zsm
Zsm
Zfc
Zfs
Zcs
QlgQc Qt
Yc Yn
Jdh*
|l|Zms |Zmg
Zmd
Jd Jdl
Qal
Jdh
Js
Jmz
CORRELATION OF MAP UNITS(Dashed lines identify gray shale beds. Asterisked units have been age dated; see table 3 in pamphlet)
35
35
45
52
22
64
30
85
12
15
11
30
35
8
36
60
60
21
88
28
20
70
15
16
20
20
30
30
53
36
32
30
20
29
38
48
25
16
23
31
17
49
12
30
45
25
32
78
75
70
45
35
30
18
18
65
32
42
31
20
44
32
32
30
32
32
65
295
3430
21
26
23
13
10
77
24
36
33
26
12
10
10
10
17
14
25
15
1818
20
17
30
27
24
25
38
24
41
2337
39
45
50
43
37
32
43
31
25
51
44
40
40
33
8
26
29
23
34
27
22
11
107
15
6
6
11
28
28
35
27
31
24
20
22
6
25
23
17
7
15
20
14
20
26
1023
1725
22
20
25
22
21
21
6
20
11
12
24 2838
33
30
24
11
10
65
66
78
38
35
78
15
6
15
22
78
15
16
8
10
12
12
12
12
1615
14
12
10
10
21
16
23
18
27
22
17
25
1623
27
12
15
25
18
15
1615
25
15
20
20
20
39
20
27
37
28
23
18
30
2034
17
25
15
3726
22
37 37
45
44
55
30
41
41
17
25
26
23
28
21
10
24
26
26
23
21
20
23
18
25
18
10
23
33
28
30
23
13
4
17
13
24
19
20
8
45
30
75
75
65 45
67
45
74
37
5857
35
50
49
3332
60
45
43
57
80
5 30
17
50
2527
60
25
65
50
50
70
34
15
12
80
8
35
50
30
37
3760
36
35
10
70
21
20
22
65
69
39
25
18
60
10
15
25
6546
58
42
58
34
68
34
72
35
47
25
40
20
40
52
30
31
84
80
54
51
35
65
38
22
10
65
3625
50
61
7250
27
53
20
33 36
35
35
60
20
47 20
80
47
7478
41
35 50
50
59
36
32
30
31
56
51
55
54
57
43
34
45
45
35
5560
33
20
40
39
53
34
30
24
22
25
70
52 47
5
40
70
62 66
52
45 2524
27
11
59
35
4545
48
25
30
40
45
51
35 45
45
4030 45
20 48
45
60
40
65
35
25
40
23
32
2826
25
52
35
18
28
32
34 46
50
13
44
4234
40
35
25
70
35
5030
68
36
43
36
40
43
31
20
45
34
42
2440
25
28
31
3123 62
40
45
15
56
20
43
25
27
5
5
25
28
10
17
30
30
65
60
60
4535
47
40
65
34
3520
35
80
50
75
74
4252
27
60
6850
62
43
44
31
35
2
4546
5831
25
20
53
45
46 41
25
3046
32
35
6550
35
27
33
20
26
20
15
45
36
35
15
22
25
10
65
62
22
44
45
35
80
8
40
45
35
42
55
3325
25
22
35
45
3548
16
52
45
40
25
38
64
60
44
87
12
29
36
38
27
36
58
46
42
30
30
42
42 36
26
37
42
36
3
36
60
40
10
70
30
20
10
70
75
25
50
25
56
22
56
2870
60
65
62
25
28
56
24
70
60
4012
50
50
65
45
44
45
36
25
10
25
58
15
62
10
42
38
36
56
56
60 40
3535
36
30
22
30
50
65
20
4848
26
30
50
30
70
50
22
25
2020
20
20
45
32
68
22
72
72
56
52
6
49
3236
75
50
30
20
55
56
15
15
25
50
3736
43
8
28
5840
43
62
40
36
48
51
46
50
60 60
47
43
43
40
80
23
2516
37
2828
53
35
35
30
29
32
30
27
21
28
23
3435
5060
1688
28
32
68
39
32
34
46
3013
62
42
35
27
22
62
32
50
52
25
48
18
39
30
48
25
67
38
38
35
60
45
30
45
3040
50
27
30
16
17
3553
62
28
40
53
29
59
25
38
32
2232
35
3548
23
50
50
4049
36
2520
20 20
35
35
4754
51
54
16
28
55
10
45
25
32
48
45
10
25
4
45
50
55
40
35
48
68
39
80
30
50
35
46
36
67
44
33
38
65
52
56
51
53
52
53
49
54
10
25
2530
40
30
38
34
85
80
79
85
6068
62
65
87
20
15
15
15
34
50
3062
15
42
70
48
50
50
50
2662
1255
35
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56
53
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50
30
70
72
71
78
78
8045
4357
18
75
28
30
30
40
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35
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60
88
65
70
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20
65
15
20
10
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86
30
30
28
74
5
22
20
10
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78
43
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2515
28
35
4046
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53
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33
36
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3012
32 65
3380
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40
25
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32
35
30
31
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3636
26
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16
60
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20
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5
36
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66
7046
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53
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58
77
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84
60
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3565
33
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PLEASANTGROVE FAULT
DESCRIPTION OF MAP UNITS
CENOZOIC SURFICIAL DEPOSITS
Alluvium (Holocene)—Mixtures of clay, silt, sand, gravel, and cobbles underlying flood plains of the Potomac River and its tributaries. Includes low level alluvial terraces as much as 10 ft above stream channels and fine colluvial debris from adjacent slopes. Well to poorly stratified, fining-upward sequences as much as 20 ft thick
Colluvium (Holocene and Pleistocene?)—Coarse cobbles, boulders, and blocks that were transported by gravity, debris flow, and freeze-thaw and found on mountain slopes. Boulder streams, boulder fields, rock slides, and talus composed of quartzite and metabasalt are concentrated in hillslope depressions in the Blue Ridge province. Gravel, sand, and silt in sheetlike or lensoid aprons that consist of unsorted subangular clasts of quartzite, epidosite, and vein quartz are mapped on the western margin of the Culpeper basin. Ranges from a thin veneer to over 50 ft thick; deposits greater than 10 ft thick are shown
Lag gravel (Holocene and Pleistocene?)—Cobbles and gravel of rounded quartzite, greenstone, and quartz with minor sand, silt, and clay; litter upland surface overlying deeply weathered saprolite developed on conglomerate in the Culpeper basin; thickness varies from 3 to 10 ft
Terrace deposits, low level (Holocene and Pleistocene?)—Sand, gravel, and boulders underlying nearly flat benches (three or more), 33 to 165 ft above the Potomac River are 10 to 20 ft thick
Terrace deposits, intermediate and high level (Pleistocene and Pliocene?)—Sand, gravel, and boulders in isolated hillocks, 200 to 235 ft above Potomac River are 33 to 120 ft thick; clasts of predominantly quartzite and chert, but granite, metabasalt, metarhyolite, and diabase are deeply weathered with thick rinds
MESOZOIC ROCKS OF THE CULPEPER BASIN
Diabase dikes and sheets (Early Jurassic)—Medium- to dark-gray, medium crystalline and equigranular, massive diabase with characteristic orange-brown weathered surface; locally coarsely crystalline and gabbroic or gabbroic or granophyric; finely crystalline and aphanitic near chilled margins. At least three magma types: (1) dikes of olivine-normative (olivine-plagioclase-pyroxene) tholeiitic diabase (Jdo); (2) dikes of low-titanium, quartz-normative tholeiitic diabase containing inch-size phenocrystic clusters of calcic plagioclase in a fine-grained groundmass of pyroxene and plagioclase (Jdl); (3) narrow dikes and differentiated sheets of high-titanium, quartz-normative tholeiitic diabase (Jdh). Igneous differentiation produced early, bronzite-bearing cumulates (Jdc) lower in the sheets and late-stage differentiates higher in the sheets such as granophyre (Jdg), ferrogabbro, diorite, syenite, and aplite, which contain pink potassium feldspar, albite, hornblende, biotite, and quartz. Diabase dikes with magma type not determined are shown as Jd. Dikes vary from several feet to more than 500 ft wide and sheets exceed 2,000 ft in thickness
Thermally metamorphosed rocks (Lower Jurassic and Upper Triassic)—Hornfels, quartzite, marble, meta-arkose, and metaconglomerate in zoned contact aureoles adjacent to diabase intrusions. Includes dark-gray to olive-black cordierite-spotted hornfels; bluish-gray to mauve epidote and chlorite hornfels; white to pinkish-gray tourmaline granofels or quartzite; greenish to purplish-gray epidote and chlorite meta-arkose; and white, light-gray, and pink crystalline marble. Contact aureoles a few feet thick adjoin narrow dikes. Contact aureoles more than 700 ft thick and as much as 1 mi wide adjacent to thick sheets; rocks are hard, brittle, fractured, and unweathered; sedimentary structures are commonly preserved
Culpeper Group
Sander Basalt (Lower Jurassic)—Dark-gray to bluish-gray, fine- to medium-crystalline, porphyritic to equigranular basalt with plagioclase, augite, and pigeonite phenocrysts; locally vesicular and amygdaloidal at tops of flows. Lower flows of basalt locally separated from stratigraphically higher flows of basalt by poorly exposed reddish-brown sandstone and siltstone (Jss). Basalts are characterized by distinctive curved columnar joints locally overprinted by closely spaced fractures; apparently paraconformable with underlying and intercalated sedimentary rocks; estimated thickness less than 1,000 ft
Turkey Run Formation (Lower Jurassic)—Sandstone, siltstone, conglomerate, and shale interbedded in cyclic sequences; sandstone, reddish-brown and dark-gray, fine- to coarse-grained, locally pebbly and crossbedded, micaceous, poorly sorted; siltstone, reddish-brown, greenish-gray, and dark-gray, micaceous, ripple-laminated; shale, dark-red and dark-gray, fissile, laminated, fossiliferous, and carbonaceous; conglomerate of variegated, subrounded boulders, cobbles, and pebbles of greenstone, quartzite, marble, quartz, and subangular basalt clasts (Jtrc). Cycles consist of alternation of coarser and finer grained layers several feet thick. Deeply weathered and poorly exposed; about 715 ft thick
Hickory Grove Basalt (Lower Jurassic)—Medium- to dark-gray, fine- to medium-crystalline, microporphyritic to equigranular high-titanium, high-iron quartz-normative tholeiitic basalt with plagioclase, augite, and pigeonite phenocrysts; locally vesicular and amygdaloidal at tops of flows with vugs filled by zeolite, calcite, and prehnite. Consists of two or three separate flows of basalt, locally separated by poorly exposed, reddish-brown sandstone and siltstone (Jhgs). Locally disconformable but regionally paraconformable with underlying and overlying sedimentary rocks; calculated thickness as much as 1,200 ft but thins southward to less than 700 ft
Midland Formation (Lower Jurassic)—Siltstone, sandstone, shale, and conglomerate, interbedded in cyclic sequences; siltstone, reddish-brown and light- to dark-gray, micaceous, commonly ripple-laminated, locally bioturbated, calcareous, carbonaceous, and fossiliferous; sandstone, reddish-brown and gray, fine- to coarse-grained, feldspathic, locally pebbly, crossbedded and ripple-laminated; shale, dark-red, light-greenish-gray, and dark-gray to black, silty, burrowed, with dessication cracks, carbonaceous, pyritic, calcareous, laminated, and fossiliferous; lenticular variegated cobble and pebble conglomerate and conglomeratic, coarse-grained, reddish-brown arkosic sandstone (Jmc) are locally near the Bull Run fault. Poorly exposed but unit is paraconformable with basalt formations above and below; as much as 1,250 ft thick
Mount Zion Church Basalt (Lower Jurassic)—Dark-gray to black, aphanitic to very fine crystalline, microporphyritic high-titanium, quartz-normative tholeiitic basalt. Contains phenocrysts of augite and plagioclase; flow tops are vesicular and contain amygdules filled by calcite, zeolite, and prehnite. Weathers to reddish-brown or gray saprolite in uplands. Poorly exposed but probably conformable or paraconformable with underlying and overlying sedimentary rocks. Consists of one or two flows about 330 ft thick
Catharpin Creek Formation (Lower Jurassic and Upper Triassic)—Sandstone, siltstone, and conglomerate, interbedded in cyclic sequences about 100 ft thick. Sandstone, very dark red to dusky-red, micaceous, arkosic, and pebbly; overlain by dusky-red and olive-gray, calcareous, micaceous, thin-bedded to ripple-laminated siltstone that is sparsely fossiliferous and laterally persistent. Lenticular conglomerate is reddish brown with rounded cobbles and pebbles of mainly quartzite and greenstone in a fine- to coarse-grained arkosic sandstone matrix. Unit is more than 3,280 ft thick
Goose Creek Member—Lenticular conglomerate and interbedded pebbly sandstone; conglomerate, reddish-brown to grayish-green, thick-bedded to massive, with subrounded pebbles and cobbles of mainly quartzite, greenstone, metasiltstone, gneiss, vein quartz, and minor carbonate in medium- to coarse-grained arkosic sandstone matrix; sandstone, very dark red to dusky-red, micaceous, arkosic, pebbly, and medium- to thick-bedded, poorly sorted, silty, fine- to coarse-grained. Deeply weathered to thick orange-brown saprolite mantled by lag gravel. Thickness may locally exceed 2,500 ft; intertongues laterally into main body of Catharpin Creek Formation
Balls Bluff Siltstone (Upper Triassic)Lacustrine shale and siltstone member—Light- to dark-gray,
light-greenish-gray, and black, thin-bedded to laminated, locally ripple-marked, mud-cracked, calcareous and dolomitic, sparsely fossiliferous, silty and sandy shale; interbedded with dusky-red, thin-bedded, calcareous, bioturbated, micaceous, feldspathic, clayey and sandy siltstone in cyclic sequences 10 to 30 ft thick. Unit grades northward into the fluvial and deltaic sandstone and siltstone member. Maximum thickness estimated to exceed 5,000 ft, but unit is partly repeated by faulting and is laterally equivalent to the fluvial and deltaic sandstone and siltstone member
Fluvial and deltaic sandstone and siltstone member—Reddish-brown, thin- to medium-bedded, feldspathic, locally crossbedded, fine- to medium-grained silty sandstone interbedded with dusky-red, thin-bedded, calcareous, bioturbated, micaceous, feldspathic, clayey and sandy siltstone in cyclic sequences, 3 to 10 ft thick. Intertongues laterally with carbonate conglomerate of the Leesburg Member to the northwest and with the lacustrine shale and siltstone member to the south. Thickness estimated to exceed 5,000 ft
Leesburg Member—Light-gray-weathering, crudely bedded conspicuous carbonate conglomerate with subangular to subrounded boulders, cobbles, and pebbles of grayish and reddish lower Paleozoic limestone and dolostone in reddish-brown, pebbly sandstone and calcareous sandy siltstone matrix. Intercalations of calcareous sandstone and siltstone thicken to the southeast where they tongue into the main body of sandstone and siltstone of the Balls Bluff. Thickness calculated as 3,510 ft
Manassas Sandstone (Upper Triassic)Poolesville Member—Predominantly gray, pinkish-gray, and reddish-brown,
fine- to coarse-grained, thick-bedded, arkosic and micaceous sandstone; locally pebbly and crossbedded where it fills channels; commonly interbedded with calcareous, reddish-brown siltstone in upward-fining sequences in upper part of unit; as much as 3,000 ft thick; gradational and intertonguing relation with overlying and underlying units
Reston Member—Basal conglomerate with cobbles and pebbles of metagraywacke, schist, and quartz in poorly sorted, coarse-grained, arkosic sandstone matrix; locally interbedded with reddish-brown sandstone and siltstone; as much as 70 ft thick
PALEOZOIC ROCKS OF THE BLUE RIDGE ANTICLINORIUM
Frederick Limestone (Upper Cambrian)—Gray-weathering, pale-gray to dark-bluish-gray, laminated, thinly bedded limestone of the Rocky Springs Station Member. Found east of Furnace Mountain, just south of the Potomac River; thickness unknown
Tomstown Formation (Lower Cambrian)—Buff-weathering, white to bluish-gray, fine- to medium-grained dolostone, exposed near Furnace Mountain. Massive to locally well-bedded, with 0.5-in-thick partings. Locally sandy with dolomitic cement. Thickness less than 200 ft
Carbonaceous phyllite (Lower Cambrian)—Medium- to dark-gray, fine-grained muscovite-graphite phyllite. Locally contains 0.04- to 0.4-in-thick alternating light- and dark-gray bedding laminae. Produces distinctive light-gray “ashy” soil. Found only as float or small slumped outcrops; occurs as discontinuous lenses above Antietam Quartzite near Furnace Mountain. Interfingers laterally with dolomite of the Tomstown Formation. Maximum thickness probably less than 200 ft
Chilhowee Group
Antietam Quartzite (Lower Cambrian)—Gray to buff-weathering, fine-grained, poorly bedded to massive meta-arkose. Poorly exposed on ridges. Near contact with overlying Tomstown Formation or carbonaceous phyllite are local ferruginous horizons with abundant botryoidal hematite and limonite. Found only on east side of Catoctin Mountain. Thickness about 100 ft
Harpers Formation (Lower Cambrian)—Dark-greenish-gray to brownish-gray, fine- to very fine grained, laminated to massively bedded biotite-chlorite-muscovite-quartz phyllite and metasiltstone. Magnetite- rich, sandy metasiltstone and fine-pebble conglomerate found locally at the base on Short Hill Mountain. Rocks are fine grained with bedding locally preserved on Blue Ridge and Short Hill Mountain; on Catoctin Mountain bedding is totally transposed, and dominant fabric is metamorphic foliation consisting of alternating 0.04-in-thick quartzose and micaceous laminae. Thickness varies from 1,000 ft on west limb to 500 ft on east limb
Weverton Formation (Lower Cambrian)Upper member—Dusky-blue, dark-gray to dark-purplish-gray, very coarse
grained quartzite and quartz-pebble conglomerate. Poorly sorted, thick-bedded with graded beds and crossbeds; contains local accumulations of magnetite, ilmenite, and pebbles of red jasper, red and purple quartz, and phyllite. Interbedded with poorly exposed dark-greenish-gray phyllitic metasiltstone. Gradational with overlying Harpers Formation. Thickness ranges from 105 ft on Short Hill Mountain to 380 ft on Blue Ridge
Middle member—Greenish-gray to light-gray, massive, medium-grained to granular quartzite in 16- to 32-ft-thick beds interbedded with metasiltstone. Forms topographic ledges but is not well exposed. Thickness ranges from 200 to 240 ft
Lower member—Light- to medium-gray, fine- to medium-grained, well-sorted, graded, crossbedded, massive, thick-bedded vitreous quartzite interbedded with light-gray metagraywacke and metasiltstone; locally arkosic; unconformable to basement gneiss on Blue Ridge. Thickness ranges from 100 to 200 ft
Weverton Formation, undivided (Lower Cambrian)—White to gray, massive to thick-bedded vitreous quartzite with thin (less than 1 ft thick) interbeds of dark phyllite is restricted to Catoctin Mountain where it is conformable with overlying Harpers Formation and underlying Loudoun Formation; as much as 120 ft thick
Loudoun Formation (Lower Cambrian)—On Blue Ridge consists of gray-black, light-olive-gray to dark-purple-gray phyllite containing tuffaceous clasts and elongated amygdules; locally overlain by dark-gray to dusky-blue, very coarse pebble conglomerate (|lc) with pebbles of rounded to subrounded white, blue, and red quartz, gneiss(?), red jasper, and variegated phyllite in grayish-black, iron-rich silty matrix; local interbeds of coarse quartz-pebble conglomerate and graded and crossbedded quartzite. On Catoctin Mountain consists of dark-gray- to greenish-gray-weathering chlorite-quartz-graphite-muscovite phyllite and lesser white- to gray-weathering pebbly metasandstone with thin phyllite interbeds. Base of formation is transitional with the underlying Catoctin Formation; top of phyllite is sharp contact with conglomerate or overlying Weverton Formation; conglomerate is transitional with overlying Weverton Formation. Ranges from 0 to 200 ft thick where present
ROCKS OF THE POTOMAC TERRANE IN THE PIEDMONT PROVINCE
Mather Gorge Formation (Lower Cambrian and (or) Late Proterozoic)Schist—Lustrous, greenish-gray to gray, reddish-brown-weathering, very
fine grained, quartz-muscovite-chlorite-plagioclase-epidote-magnetite- hematite schist. Thin to very thin beds are largely transposed into early cleavage. Contains interbedded metagraywacke and a few lenses of calc-silicate rock. Found only in the extreme eastern corner of Loudoun County. Thickness is unknown
Metagraywacke—Light- to medium-gray, yellowish- to reddish-brown weathering, fine- to medium-grained, generally well-bedded metagray-wacke and semi-pelitic schist. Beds range from about 1.2 in to as much as 10 ft thick, averaging about 8 in. Many beds are graded; sole marks and slump features are abundant. Contains interbedded quartzose schist
and lenses of calc-silicate rock. Forms bold outcrops along the Potomac River in the extreme eastern corner of Loudoun County. Thickness is unknown
LATE PROTEROZOIC ROCKS OF THE BLUE RIDGE ANTICLINORIUM
Catoctin Formation (Late Proterozoic)—Dark-green to bluish-gray, fine-grained to aphanitic, massive to schistose, amygdaloidal metabasalt and greenstone, composed of actinolite, chlorite, epidote, albite, and rare quartz. Contains lenses and layers of fine-grained, hard, massive, apple-green epidosite (an epidote-quartz rock). Subunits interlayered with metabasalt include metabasalt breccia (Zcb); lustrous, silvery-white metarhyolite and fine-grained quartz-sericite phyllite interpreted as felsic metatuff (Zcr); light-gray, fine-grained phyllite and metasiltstone and gray to buff, crossbedded feldspathic metasandstone (Zcs); finely laminated, gray, quartz-graphite-muscovite phyllite (Zcp); and 3- to 10-ft-thick lenses of gray- to buff-weathering, fine- to medium-grained white calcite marble (Zcm). Margins of the marble commonly contain coarse-grained actinolite, tremolite, and chlorite. Ranges in thickness from about 2,500 to 5,000 ft on east limb of anticlinorium and from 0 to 1,600 ft on west limbs
Metadiabase dikes (Late Proterozoic)—Dark-greenish-gray, fine- to medium-grained, massive to schistose metadiabase (greenstone) composed predominantly of chlorite, epidote, albite, and actinolite. Coarse-grained and porphyritic metadiabase dikes have stubby, 0.08- to 0.16-in-long actinolite pseudomorphs after clinopyroxene that produce distinctive nubbly texture and are shown by a pattern north of Taylorstown. Rare aphanitic metadiabase has relict euhedral plagioclase laths. Dikes range in thickness from a few inches up to 164 ft thick. Compositionally similar to metabasalt of the Catoctin Formation
Metarhyolite dikes (Late Proterozoic)—White- to gray-weathering, aphanitic to fine-grained metarhyolite. Ranges in texture and mineralogy from fine-grained quartz sericite schist to very fine grained and flinty rock with plagioclase feldspar phenocrysts in a felty groundmass consisting of quartz, feldspar, biotite, and epidote. Ranges in thickness from 33 to 66 ft; maximum length is about 9 mi
Swift Run Formation (Late Proterozoic)Marble—Gray- to buff-weathering, medium- to fine-grained, white calcitic
and dolomitic marble. Locally contains thin arenaceous layers. Occurs as discontinuous layers, 3 to 10 ft thick, both near top of section, in phyllite (Zsp) and near bottom of section, in schist (Zss)
Marble, slate, and phyllite—Medium-dark-gray slate; grayish-red-purple, grayish-green, finely laminated phyllite; and dark-greenish-gray to brown-gray sandy sericitic phyllite in fining-upward sequence. Pink-gray to light-brownish-gray, fine-grained, dolomitic marble locally found near top. Top of unit is transitional with overlying metabasalt and phyllite of the Catoctin Formation. Thickness ranges from 0 to 400 ft
Metagraywacke, schist, metasandstone, quartzite, and meta-arkose— Brownish-green chlorite-sericite-feldspar-quartz metagraywacke; lustrous, silvery quartz-sericite schist; and pinkish- to greenish-gray, very coarse to medium-grained metasandstone and quartzite with crossbeds and quartz pebbles and cobbles in fining-upward sequence. Grayish-brown meta-arkose (paleoregolith?) locally found at basement-cover contact. Thickness ranges from 0 to 300 ft on Blue Ridge and from 0 to 1,000 ft on Catoctin Mountain
Fauquier Formation (Late Proterozoic)Metamudstone—Light-gray, light-brown-weathering, thinly laminated (less
than 1 in) sericite-quartz metamudstone and lustrous sericite phyllite. Quartz in metamudstone is very fine sand. Upper contact gradational with rocks of the Catoctin Formation. Thickness ranges from 0 to 160 ft
Meta-arkose—Light- to medium-gray and olive-gray, light-yellowish-gray- weathering, coarse- to fine-grained sericite-potassium feldspar-quartz meta-arkose. Bedding is of variable thickness (1–6 ft), defined by fining-upward sequences; tangential and trough crossbeds are common. Gravel and isolated cobble sized clasts occur locally. Upper contact gradational with metamudstone; lower contact gradational with metaconglomerate or unconformable on Mesoproterozoic gneiss. Thickest section is 1,500 ft
Metaconglomerate and meta-arkose—Light- to dark-gray, cobble conglomerate and meta-arkose with biotite-chlorite-sericite-potassium feldspar-quartz matrix. Bedding is of variable thickness (2–20 ft), defined by fining-upward sequences. Cobbles are locally derived metagranite. Trough crossbeds of coarse- to medium-grained arkose are common. Lower contact is an unconformity that has as much as 328 ft relief and is distinct and abrupt; elsewhere it is obscured within a zone of biotite-chlorite-quartz schist that is possibly metamorphosed paleoregolith. Upper contact is gradational with meta-arkose unit. Thickness ranges from 0 to 200 ft
Robertson River Igneous Suite (Late Proterozoic)Cobbler Mountain Alkali Feldspar Quartz Syenite—Gray- to buff-
weathering, massive, medium- to coarse-grained alkali feldspar quartz syenite. Consists of stubby, euhedral mesoperthite crystals 0.04 to 0.08 in. in diameter intergrown with anhedral quartz and minor interstitial plagioclase, in addition to a mafic phase (amphibole?) that has been broken down into quartz, plagioclase, and magnetite. Mesoperthite crystals conspicuous on weathered surface. Forms low, rounded outcrops whose surfaces become friable upon weathering. Locally cut by dikes of fine-grained granite of presumably coeval magma source
MESOPROTEROZOIC BASEMENT ROCKS OF THEBLUE RIDGE ANTICLINORIUM
Biotite granite gneiss—Orange- to gray-weathering, pink, medium- grained, well-foliated and (or) lineated biotite-plagioclase-quartz- microcline gneiss. Biotite content ranges from 10 to 15 percent. Mesoproterozoic foliation expressed by planar aggregates of biotite; locally, where foliation is weak or absent, replaced by a lineation expressed by biotite streaks and rodded quartzofeldspathic grains. Interlayered with leucocratic metagranite (Yg and Ygt)
Pink leucocratic metagranite—Pink, medium- to medium-fine-grained, massive to moderately foliated plagioclase-quartz-microcline metagranite. Biotite content ranges from 0 to 10 percent. Foliation defined by flattened quartz and feldspar grains and local thin biotite-rich layers
Leucocratic metagranite—White, medium- to medium-fine-grained, massive to moderately foliated plagioclase-quartz-microcline metagranite. Biotite content ranges from 0 to 5 percent. Mesoproterozoic foliation defined by biotite where present and by flattened grains of quartz and feldspar and thin aplite layers
Garnetiferous leucocratic metagranite—White, medium- to medium- fine-grained, massive to moderately foliated garnet-plagioclase-quartz- microcline metagranite. Identical to Yg except for the presence of irregularly distributed almandine crystals 0.04 to 0.4 in. in diameter. Garnets commonly dark due to alteration of rim to biotite and chlorite
Quartz-plagioclase gneiss—White, medium- to medium-fine-grained, weakly to moderately well-foliated biotite-quartz-plagioclase gneiss. Biotite content ranges from 0 to 5 percent. Foliation defined by flattened quartz and feldspar grains and thin aplite layers and by biotite, where present. Strongly resembles leucocratic metagranite (Yg) but potassium feldspar rare or absent
Marshall Metagranite—Pink, medium-grained, weakly to moderately well-foliated and (or) lineated biotite-plagioclase-quartz-microcline metagranite. Biotite content ranges from 10 to 15 percent. Layering locally produced by veins of pink pegmatite parallel to foliation. Resembles biotite granite gneiss (Ybg) in outcrop
Coarse-grained metagranite—Gray to pink, medium- to coarse-grained, massive to well-foliated biotite-plagioclase-quartz-microcline metagranite. Characterized by 0.4- to 0.8-in-long white or pink microcline porphyroblasts and aggregates of blue quartz; biotite content ranges from 0 to 10 percent. Commonly contains pronounced augen texture due to overprinting by Paleozoic schistosity
Porphyroblastic metagranite—Yellowish-brown-weathering, medium- to coarse-grained garnet-biotite-plagioclase-quartz-microcline metagranite. Characterized by megacrysts of orange to pink microcline or microcline-rich aggregates that are deformed into rounded ovoids 0.4 to 1.2 in. in diameter; garnet, biotite, plagioclase, opaques, and distinctive clots of blue quartz occur interstitially. Flattened ovoids define foliation, which is locally cut by dikes of garnetiferous leucocratic metagranite (Ygt) and pink leucocratic metagranite (Yml)
Hornblende monzonite gneiss—Gray-weathering, medium-fine- to fine-grained, well-foliated hornblende-quartz-microcline-plagioclase gneiss. Foliation defined by strongly flattened quartz and feldspar grain aggregates and prismatic hornblende; quartz content 10 to 20 percent, hornblende as much as 30 percent, and microcline as much as 50 percent. Rarely occurs as a more massive, spotted rock. Biotite and orthopyroxene are rare mafic constituents
Layered granitic gneiss—White, gray, or pink, medium- to fine-grained, well-layered garnet-biotite-plagioclase-quartz-microcline gneiss. Layer- ing, 0.04 to 0.4 in thick, is defined by concentrations of biotite and aplite; garnets up to 0.4 in. in diameter scattered throughout rock. Complex structures in outcrop include folded layering and swirly, migmatitic texture suggestive of partial melting. Relation to other granitic units unknown
Charnockite—Yellow- to brown-weathering, dark-green, medium- to coarse-grained, massive to locally well-foliated quartz-hornblende- orthopyroxene-microcline-plagioclase rock. Mafics compose about 35 percent of the rock. Poorly exposed; forms topographic highs, a bright orange soil, and distinctive float that consists of pitted boulders and cobbles with a 0.4- to 0.8-in-thick, orange-yellow weathering rind. Locally contains garnet and biotite. Occurs as discontinuous lensoid bodies
Metanorite and metadiorite—Gray-weathering, medium- to coarse- grained, massive to well-foliated hornblende-orthopyroxene-plagioclase metanorite and medium- to medium-fine-grained biotite-hornblende- plagioclase metadiorite. Brown hornblende and colorless pyroxene commonly altered to pale-green amphibole. Occurs as lenses and thin belts, commonly in proximity to garnet-graphite paragneiss
Paragneiss—Rusty-weathering, medium-fine- to fine-grained, well-foliated to layered graphite-biotite-garnet-plagioclase-quartz gneiss and schist. Layering defined by alternating 0.4-in-thick garnet-rich zones containing garnets that are 0.04 to 0.4 in. in diameter and 0.4-in-thick quartzofeldspathic layers. Garnets typically deformed and retrograded to green lensoid clots of fine-grained chlorite and muscovite. Graphite occurs as disseminated, small, rounded flakes. Distinctive rusty stain produced by secondary hematite after accessory magnetite. Retrograded, schistose varieties include quartz-chlorite-magnetite schist and carbonaceous phyllonite. Probable remnant of pregranitic country rock
Quartzite and quartz tectonite—Light-gray to white, fine- to medium-grained, massive quartzite and quartz-sericite tectonite. No primary textures recognized such as bedding or early metamorphic foliation; commonly contains strong Paleozoic penetrative cleavage. Contains thin lenses of graphite in outcrop and larger mappable pods of paragneiss (Yp). Considered to be part of metasedimentary suite
EXPLANATION OF MAP SYMBOLS
Contact—Dotted where concealed
Lake bed—Dotted where concealed
FAULTS(Dashed where inferred; dotted where concealed)
Thrust fault—Sawteeth on upper plate
Normal fault—Ball and bar on downthrown side
Reactivated fault—Open ball and bar on downthrown side of early normal fault and sawteeth on upper plate of later thrust fault; shown in cross section as a double-headed arrow
Strike-slip fault—Arrows show relative movement
FOLDS (Folds show trace of axial surface, direction of dip of limbs, and, where
known, direction of plunge. Dotted where concealed)
Overturned anticline
Syncline
Overturned syncline
Synformal anticline
Antiformal syncline
PLANAR FEATURES(May be combined with linear features)
Strike and dip of bed—Ball indicates top of bed known from sedimentary structures
Inclined
Overturned
Rotated more than 180°
Strike and dip of Mesoproterozoic foliation including compositional layering
Inclined
Vertical
Strike and dip of first-generation (S1) Paleozoic cleavage and (or) schistosity
Inclined
Vertical
Strike and dip of second-generation (S2) Paleozoic cleavage
Strike and dip of highly crenulated first-generation (S1) Paleozoic cleavage and (or) schistosity
Strike and dip of mylonitic and phyllonitic foliation
Strike and dip of joint
Inclined
Vertical
Strike and dip of axial plane of fold
Mesoproterozoic (first-generation)
Paleozoic (second-generation)
LINEAR FEATURES(May be combined with planar features)
Mesoproterozoic mineral lineation, rodding, and (or) axis of minor fold
Paleozoic mineral lineation, rodding, and (or) axis of minor fold
OTHER FEATURES
Sample site for radiometric date—Refer to figure 1 and table 3(in pamphlet)
Sinkhole
39°15'
39°15'
39°10'
39°10'
39°05'
39°05'
39°00'
39°00'
38°55'
38°55'
77°50'
77°50'
77°45'
77°45'
77°40'
77°40'
77°35'
77°35'
77°30'
77°30'
77°30'
77°30'
77°25'
77°25'
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