Geologic Map Of The Buckeystown Quadrangle, Frederick And Montgomery Counties, Maryland, And Loudoun County, Virginia printable pdf download

G O OG

O M

REFERENCES CITED

Brezinski, D.K., 1992, Lithostratigraphy of the western Blue Ridge cover rocks in

Maryland: Maryland Geological Survey Report of Investigations 55, 69 p.

Burton, W.C., Froelich, A.J., Pomeroy, J.S., and Lee, K.Y., 1995, Geologic map of

the Waterford and Virginia portion of the Point of Rocks quadrangles, Virginia:

U.S. Geological Survey Bulletin 2095, 30 p.

Clark, F.W., 1924, The data of geochemistry: U.S. Geological Survey Bulletin 770,

841 p.

Cloos, Ernst, and Cooke, C.W., 1953, Geologic map of Montgomery County and the

District of Columbia: Baltimore, Maryland Department of Geology, Mines, and Wa-

ter Resources, scale 1:62,500.

Drake, A.A., Jr., and Lee, K.Y., 1989, Geologic map of the Vienna quadrangle, Fair-

fax County, Virginia, and Montgomery County, Maryland: U.S. Geological Survey

Geologic Quadrangle Map GQ–1670, scale 1:24,000.

Drake, A.A., Jr., Sinha, A.K., Laird, Jo, and Guy, R.E., 1989, The Taconic orogen,

in Hatcher, R.D., Jr., Thomas, W.A., and Viele, G.W., eds., The Appalachian-Oua-

chita orogen in the United States: Boulder, Colo., Geological Society of America,

The Geology of North America, v. F–2, p. 101–177.

Edwards, Jonathan, 1986, Geologic map of the Union Bridge quadrangle, Carroll and

Frederick Counties, Maryland:

Baltimore, Maryland Geological Survey, scale

1:24,000.

———1988, Geologic map of the Woodsboro quadrangle, Carroll and Frederick

Counties, Maryland: Baltimore, Maryland Geological Survey, scale 1:24,000.

Evans, N.H., and Milici, R.C., 1994, Stratigraphic relations and structural chaos on

the southeastern limb of the Blue Ridge anticlinorium and points east, central Vir-

ginia Piedmont, in Schultz, Art, and Henika, Bill, eds., Fieldguides to southern Ap-

palachian structure, stratigraphy, and engineering geology: Virginia Tech Depart-

ment of Geological Sciences Guidebook Number 10, p. 31–64.

Fauth, J.L., 1968, Geology of the Caledonia Park quadrangle area, South Mountain,

Pennsylvania: Pennsylvania Geological Survey, 4th Series, Atlas 129a, 132 p.

Fisher, G.W., 1978, Geologic map of the New Windsor quadrangle, Carroll County,

Maryland: U.S. Geological Survey Miscellaneous Investigations Series Map I–1037,

scale 1:24,000.

Froelich, A.J., 1975, Bedrock map of Montgomery County, Maryland: U.S. Geologi-

cal Survey Miscellaneous Investigations Series Map I–920–D, scale 1:62,500.

Goddard, E.N., Trask, P.D., DeFord, R.K., Rove, R.N., Singewald, J.T., and Over-

beck, R.M., 1948, Rock-color chart: Washington, D.C., National Research Coun-

cil, 6 p. [reprinted by Geological Society of America, 1951, 1963, 1970].

Hopson, C.A., 1964, The crystalline rocks of Howard and Montgomery Counties, in

The geology of Howard and Montgomery Counties: Baltimore, Maryland Geologi-

cal Survey, p. 27–215.

Horton, J.W., Jr., Drake, A.A., Jr., and Rankin, D.W., 1989, Tectonostratigraphic

terranes and their Paleozoic boundaries in the central and southern Appalachians,

in Dallmeyer, R.D., ed., Terranes in the Circum-Atlantic Paleozoic orogens: Geo-

logical Society of America Special Paper 230, p. 213–245.

Hoy, R.B., and Schumacher, R.L., 1956, Fault in Paleozoic rocks near Frederick,

Maryland: Geological Society of America Bulletin, v. 67, no. 11, p. 1521–1528.

Jonas, A.I., 1924, Pre-Cambrian rocks of the western Piedmont of Maryland: Geo-

logical Society of America Bulletin, v. 35, no. 2 p. 355–363.

———1927, Geologic reconnaissance in the Piedmont of Virginia: Geological Soci-

ety of America Bulletin, v. 38, no. 4, p. 837–846.

Jonas, A.I., and Stose, G.W., 1938a, Geologic map of Frederick County and adjacent

parts of Washington and Carroll Counties: Baltimore, Maryland Geological Survey,

DESCRIPTION OF MAP UNITS

scale 1:62,500.

Antietam Formation of Chilhowee Group (Lower Cambrian)—Light-

———1938b, New formation names used on the geologic map of Frederick County,

[Color designations, in parentheses, are from Goddard and others (1948)]

olive-gray (5 Y 6/1) to olive-gray (5 Y 4/1), medium- to coarse-grained,

Maryland: Washington Academy of Sciences Journal, v. 28, no. 8, p. 345–348.

medium-bedded, locally ferruginous, micaceous, silty metasandstone in-

Keith, Arthur, 1894, Harpers Ferry Folio, Va.-Md.-W. Va.: U.S. Geological Survey

CENOZOIC SURFICIAL DEPOSITS

terbedded with very fine grained, silty metasandstone to sandy metasilt-

Geologic Atlas of the United States, Folio 10, scale 1:125,000, 11 p.

stone. Poorly exposed. Thickness estimated at 300 ft

Alluvium (Holocene)—Unconsolidated mixture of clay, silt, sand, gravel,

Keyes, C.R., 1890, Discovery of fossils in the limestones of Frederick County, Mary-

cobbles, and some boulders underlying flood plains of Potomac and

Harpers Formation of Chilhowee Group (Lower Cambrian)—Brownish-

land: Johns Hopkins University Circular 10, p. 32.

Monocacy Rivers and their tributaries.

Includes alluvial terraces as

gray (5 YR 6/1) to dark-greenish-gray (5 G 4/1), silty, phyllitic metashale

———1891, A geological section across the Piedmont Plateau in Maryland: Geologi-

much as 10 ft above stream channels, and fine colluvial debris from ad-

to highly sheared, phyllitic metasiltstone containing intervals of brownish-

cal Society of America Bulletin, v. 2, p. 319–322.

jacent slopes. Sediments well to poorly stratified, commonly in fining-

gray (5 YR 4/1), medium-grained, silty metasandstone. Poorly exposed.

Knopf, E.F.B., 1931, Retrogressive metamorphism and phyllonitization: American

upward sequences as much as 20 ft thick

Thickness estimated at greater than 900 ft

Journal of Science, 5th Series, v. 21, p. 1–27.

Knopf, E.B., and Jonas, A.I., 1929, Geology of the McCalls Ferry-Quarryville district,

Colluvium (Holocene and Pleistocene)—Coarse cobbles, boulders, and

blocks of quartzite that were transported by gravity and debris flow, and

Pennsylvania: U.S. Geological Survey Bulletin 799, 156 p.

Kunk, M.J., Froelich, A.J., and Gottfried, David, 1992, Timing of emplacement of

subsequently modified by freezing and thawing. Concentrated in hill-

Urbana Formation (Lower Cambrian?)—Predominantly medium-olive-

diabase dikes and sheets in the Culpeper basin and vicinity, Virginia and Mary-

slope depressions and hollows on Sugarloaf Mountain. Thickness rang-

es from thin veneer to greater than 10 ft. Includes subangular to sub-

brown (5 Y 4/4) to light-olive-gray (5 Y 5/2), poorly sorted, graded,

land:

Ar/

Ar age spectrum results from hornblende and K-feldspar in grano-

crossbedded, ripple-marked, calcareous metagraywacke and metasilt-

phyres [abs.]: Geological Society of America Abstracts with Programs, v. 24, no.

rounded pebbles and cobbles of quartzite and vein quartz derived from

rocks of Blue Ridge-South Mountain anticlinorium in fan-like aprons

stone. Contains light-olive-gray (5 Y 6/1), and light-brownish-gray (5

2, p. 125.

Lee, K.Y., 1977, Triassic stratigraphy in the northern part of the Culpeper basin, Vir-

covering strata along western margin of Culpeper basin. Thickness

YR 6/1), fine- to coarse-grained, thin- to medium-bedded, crossbedded,

pitted, friable, lensoidal, discontinuous very calcareous metasandstone

ginia and Maryland: U.S. Geological Survey Bulletin 1422–C, 17 p.

ranges from thin veneer to 3 ft

and quartzite (|

). Interbedded with light-brown (5 YR 5/6) laminated

———1979, Triassic-Jurassic geology of the northern part of the Culpeper basin,

Residuum (Holocene and Pleistocene)—Unconsolidated mixture of

Virginia and Maryland: U.S. Geological Survey Open-File Report 79–1557, scale

metasiltstone. Also contains light-gray (N7) to greenish-gray (5 G 6/1),

moderate reddish brown (10 R 4/6) soil, pebbles, and blocks of grayish

thin-bedded crystalline marble (|

) in laminated beds of indeterminate

1:24,000.

pink (5 R 8/2) to white (N9) angular, locally euhedral quartz derived

thickness marked by seams of sericite and chlorite. Poorly exposed;

Lee, K.Y., and Froelich, A.J., 1989, Triassic-Jurassic stratigraphy of the Culpeper

from in-place weathering of underlying carbonate rocks.

Thickness

produces distinctive reddish-orange (10 R 6/6) soils

and Barboursville basins, Virginia and Maryland: U.S. Geological Survey Professio-

ranges from thin veneer to 10 ft

nal Paper 1472, 52 p.

Sugarloaf Mountain Quartzite (Lower Cambrian?)—Pinkish-gray (5

Terrace deposits (lowest level) (Holocene and Pleistocene)—Sand,

Lindholm, R.C., Hazlett, J.M., and Fagin, S.W., 1979, Petrology of Triassic-Jurassic

YR 8/1) to white (N9), fine- to medium-grained, medium-bedded to

gravel, and boulder deposits 10 to 20 ft thick underlying nearly flat

conglomerates in the Culpeper basin, Virginia: Journal of Sedimentary Petrology,

massive, well-sorted, graded, crossbedded, ripple-marked, granular

benches that are 33 to 80 ft above Potomac and Monocacy Rivers

v. 49, no. 4, p. 1245–1261.

quartzite. Quartzite interbedded with seldomly exposed medium-brown

Muller, P.D., Candela, P.A., and Wylie, A.G., 1989, Liberty Complex; polygenetic

Terrace deposits (highest level) (Pleistocene and late Tertiary)—Gravel

(5 YR 4/4), quartzose metasiltstone and dusky-blue (5 PB 3/2), laminat-

melange in the central Maryland Piedmont, in Horton, J.W., Jr., and Rast, Nicho-

ed metasiltstone (similar to that of the conformably overlying Urbana

and boulder deposits on isolated hillocks as much as 183 ft and 140 ft

las, eds., Melanges and olistostromes of the U.S. Appalachians: Geological Society

Formation) underlies topographic swales. Lower (|

), middle (|

above Potomac and Monocacy Rivers. Clasts of predominantly quartzite

of America Special Paper 228, p. 113–134.

with Skolithos (trace fossil) have thick weathering rinds

and upper (|

) members (informal) separately mapped based on topo-

Nickelsen, R.P., 1956, Geology of the Blue Ridge near Harpers Ferry, West Virginia:

graphic expression of ridge-forming basal units because quartzites are

Geological Society of America Bulletin, v. 67, no. 3, p. 239–269.

INTRUSIVE ROCKS

virtually identical. Total thickness is approximately 2,000 ft

Rankin, D.W., Drake, A.A., Jr., Glover, Lynn, III, Goldsmith, Richard, Hall, L.M.,

Diabase dikes (Early Jurassic)—Medium (N5)- to dark-gray (N3), moder-

Murray, D.P., Ratcliffe, N.M., Read, J.F., Secor, D.T., Jr., and Stanley, R.S., 1989,

METASEDIMENTARY ROCKS OF THE WES

ately to coarsely crystalline, equigranular, massive diabase with charac-

Pre-orogenic terranes, in Hatcher, R.D., Jr., Thomas, W.A., and Viele, G.W., eds.,

teristic light-brown (5 YR 5/6) weathered surface. Discontinuous and

The Appalachian-Ouachita orogen in the United States: Boulder, Colo., Geological

en echelon subvertical tabular bodies

Society of America, The Geology of North America, v. F–2, p. 7–100.

Rankin, D.W., Drake, A.A., Jr., and Ratcliffe, N.M., 1990, Geologic map of the U.S.

SEDIMENTARY ROCKS OF THE CULPEPER BASIN

Appalachians showing the Laurentian margin and the Taconic orogen, plate 2, in

Thermally metamorphosed rocks (Upper Triassic)—Dark-gray (N3) to

Hatcher, R.D., Jr., Thomas, W.A., and Viele, G.W., eds., The Appalachian-Ouachi-

olive-black (5 Y 2/1) cordierite-spotted hornfels in zoned contact aur-

ta orogen in the United States: Boulder, Colo., Geological Society of America, The

eole adjacent to diabase dike at Monocacy Natural Resources Area

Geology of North America, v. F–2, scale 1:2,000,000.

Reinhardt, Juergen, 1974, Stratigraphy, sedimentology, and Cambro-Ordovician pa-

Balls Bluff Siltstone (Upper Triassic)

leogeography of the Frederick Valley, Maryland: Maryland Geological Survey Re-

Leesburg Member—Light-gray-weathering carbonate conglomerate with

port of Investigations 23, 74 p.

subangular to subrounded boulders, cobbles, and pebbles of grayish and

———1977, Cambrian off-shelf sedimentation, central Appalachians: Society of Eco-

reddish lower Paleozoic limestone and dolomite in reddish-brown peb-

nomic Paleontologists and Mineralogists Special Publication 25, p. 83–112.

bly sandstone and calcareous siltstone matrix

Scotford, D.M., 1951, Structure of the Sugarloaf Mountain area, Maryland, as a key

Fluvial sandstone and siltstone member—Dark-reddish-brown (10 R 3/4),

to Piedmont stratigraphy: Geological Society of America Bulletin, v. 62, no. 1, p.

fine- to medium-grained, thin- to medium-bedded, locally crossbedded,

45–75.

Smoot, J.P., 1989, Fluvial and lacustrine facies of the early Mesozoic Culpeper basin,

feldspathic, silty sandstone interbedded with dusky-red (5 R 3/4), thin-

bedded, bioturbated, calcareous, micaceous, feldspathic, clayey and san-

Virginia and Maryland, in Hanshaw, P.M., ed., Field trips for the 28th International

dy siltstone in repetitive sequences 3 to 10 ft thick. Grades down into

Geological Congress; Field Trip Guidebook T213: Washington, D.C., American

O M

M O

Geophysical Union, 15 p.

Poolesville Member of Manassas Sandstone (^

) and intertongues lat-

erally with Leesburg Member (^bl). Composite thickness estimated to

Southworth, Scott, 1996, The Martic fault in Maryland and its tectonic setting in the

central Appalachians, in Brezinski, D.K., and Reger, J.P., eds., Studies in Maryland

exceed 5,000 ft

geology: Maryland Geological Survey Special Publication 3, p. 205–221.

Manassas Sandstone (Upper Triassic)

———1998, Geologic map of the Poolesville quadrangle, Frederick and Montgomery

Poolesville Member—Predominantly medium-gray (N5), pinkish-gray (5

Counties, Maryland, and Loudoun County, Virginia: U.S. Geological Survey Geo-

YR 8/1), and pale-reddish-brown (10 R 5/4), fine- to coarse-grained,

logic Quadrangle Map GQ–1761, scale 1:24,000.

thick-bedded, arkosic and micaceous sandstone; locally pebbly and

———1999, Geologic map of the Urbana quadrangle, Frederick and Montgomery

crossbedded where it fills channels; commonly interbedded with calcare-

Counties, Maryland: U.S. Geological Survey Geologic Quadrangle Map GQ–1768,

ous, dark-reddish-brown (10 R 3/4) siltstone in upward-fining sequen-

scale 1:24,000.

ces in upper part of unit. Grades down into and intertongues with Re-

Southworth, Scott, Burton, W.C., Schindler, J.S., Froelich, A.J., Aleinikoff, J.N., and

ston Member of Manassas Sandstone (^

). Estimated thickness as

Drake, A.A., Jr., in press, Geologic map of Loudoun County, Virginia: U.S. Geo-

much as 3,000 ft

logical Survey Miscellaneous Investigations Series Map I–2533, scale 1:50,000.

Tuscarora Creek Member—Light (N7)- to dark-gray (N3) and light-red (5

Stose, A.I.J., and Stose, G.W., 1946, Geology of Carroll and Frederick Counties,

[Md.], in Carroll and Frederick Counties: Baltimore, Maryland Department of Geol-

R 6/6) conglomerate composed of very fine to very coarse grained, an-

gular to subangular pebbles and cobbles of limestone and dolomite with-

ogy, Mines, and Water Resources, p. 11–131.

———1951, Structure of the Sugarloaf Mountain area, Maryland, as a key to Pied-

in matrix chiefly of limestone and dolomite granules and dusky-red (5 R

mont stratigraphy: Geological Society of America Bulletin, v. 62, p. 697–699.

3/4) to grayish-red (5 R 4/2), clayey sand and silt with calcite cement.

Limestone and dolomite clasts derived from Cambrian and Ordovician

Stose, G.W., 1906, The sedimentary rocks of South Mountain, Pennsylvania: Jour-

nal of Geology, v. 14, p. 201–220.

carbonate strata. Estimated thickness ranges from 0 to 233 ft

Stose, G.W., and Jonas, A.I., 1935, Limestones of Frederick Valley, Maryland:

Reston Member—Light-gray (N7) to pinkish-gray (5 YR 8/1) variegated

Washington Academy of Science Journal, v. 25, no. 12, p. 564–565.

pebble, cobble, and boulder conglomerate containing clasts of phyllite,

Thomas, B.K., 1952, Structural geology and stratigraphy of Sugarloaf anticlinorium

schist, quartzite, and quartz in poorly sorted, coarse-grained, arkosic

and adjacent Piedmont area, Maryland: Baltimore, Md., The Johns Hopkins Uni-

sandstone matrix; locally interbedded with pale-reddish-brown (10 R

versity, unpublished Ph.D. dissertation, 95 p.

5/4) sandstone and siltstone. Basal conglomerate unconformably over-

Walcott, C.D., 1896, The Cambrian rocks of Pennsylvania: U.S. Geological Survey

lies metasedimentary rocks of Westminster terrane. Estimated thickness

Bulletin 134, 43 p.

as much as 70 ft

Whitaker, J.C., 1955, Geology of Catoctin Mountain, Maryland and Virginia: Geo-

logical Society of America Bulletin, v. 66, no. 4, p. 435–462.

Grove Formation (Lower Ordovician and Upper Cambrian)

Upper member (Lower Ordovician)—Medium-light-gray (N6), locally san-

dy, thrombolitic and stromatolitic algal limestone thickly interbedded

with medium-gray (N5), laminated dolomitic limestone and olive-gray (5

YR 4/1) dolomite. Thickness is greater than 450 ft

Lower member (Lower Ordovician and Upper Cambrian)—Medium-light-

gray (N6) to medium-gray (N5), thickly bedded and crossbedded, arena-

ceous limestone and sandy dolomitic limestone containing 1-ft-thick in-

terbeds of medium-light-gray (N6) sandy dolomite.

Thickness is

approximately 150 to 400 ft

Frederick Formation (Upper Cambrian)

Lime Kiln Member—Interbedded, thinly laminated to thinly bedded, dark-

gray (N3), fine-grained limestone; calcareous shale; and fine-grained,

medium-bedded limestone near base. Becomes more thickly interbed-

ded toward the top with medium-dark-gray (N4), fine-grained, wavy-

bedded limestone containing local stromatolitic algal beds. Near top,

member becomes interbedded with medium-light-gray (N6), crossbed-

ded, sandy limestone. Thickness is 600 ft

Adamstown Member—Medium-dark-gray (N4) to dark-gray (N3), fine-

grained, argillaceous limestone thinly interbedded with dusky-yellow (5 Y

6/4) to medium-dark-gray (N4), silty dolomite. Limestone beds range

from .01 to 1.55 in. in thickness. Includes several thin, dark-greenish-

gray (5 G 4/1) to greenish-black (5 G 2/1), light-olive-brown (5 Y 5/6)

weathering, silty, calcareous shale intervals 6 to 16 ft thick throughout

member. Top of member is mapped at base of the lowest medium to

thick bed of sandy or algal limestone. Thickness is approximately 1,600 ft

Rocky Springs Station Member—Dark-gray (N3), nodular to lumpy-bed-

ded, argillaceous, dolomitic limestone at base containing an interval of

grayish-black (N2), platy shale (|

frs

) 45 to 60 ft thick mapped along the

eastern flank of the Frederick Valley synclinorium. Upsection grades in-

to dark-gray (N3), laminated to flaggy-bedded limestone containing dus-

ky-yellow (5 Y 6/4) to light-olive-gray (5 Y 6/1), silty dolomitic partings

and laminations and contains 1- to 32-ft-thick intervals of medium-dark-

gray (N4), polymictic breccia that grade upsection into medium-gray

(N5), planar-bedded, arenaceous limestone. Clast sizes in breccia range

from sand size to 1 ft diameter on western flank of Frederick Valley syn-

clinorium and diminish to less than 1.2 to 2 in. in diameter on eastern

flank of Frederick Valley synclinorium. Top of member is mapped at

top of stratigraphically highest polymictic breccia or sandstone interval.

Thickness ranges from approximately 1,200 to 2,500 ft

Araby Formation (Middle and Lower Cambrian)—Light-olive-gray (5 Y

5/2), pale-brown (5 YR 5/2), and moderate-brown (5 YR 4/4), mottled

metasiltstone containing sandy intervals. Pervasively cleaved bedding

typically obscure

Tomstown Formation (Lower Cambrian)—Medium-light-gray (N6) to

medium-gray (N5), saccharoidal dolomite containing thin (0.04 in.) lay-

ers of sericite. Found in northwest corner of quadrangle where com-

pletely covered by colluvium (

). Thickness of 150 ft measured in drill

core by Hoy and Schumacher (1956)

DISCUSSION

detrital sphene, tourmaline, zircon, and ilmenite, and characteristic clots of hematite

Lime Kiln Member

Thermally Metamorphosed Rocks

after magnetite(?). Dark-blue to black, laminated metasiltstone and phyllite are inter-

The Lime Kiln Member (|

) (Reinhardt, 1974), the uppermost member of the

Siltstone, shale, and minor sandstone were baked by metamorphism to hornfels

INTRODUCTION AND GEOLOGIC SETTING

bedded with the quartzite. Erosion of the rocks forms recessive topographic swales.

Frederick Formation, consists of interbedded, thinly bedded limestone and algal lime-

) adjacent to the diabase dike at the southern edge of the map. These brittle

The laminated metasiltstone is lithologically similar to metasiltstone of the Urbana For-

The Buckeystown quadrangle is underlain mostly by rocks of the western Piedmont

stone. The member records the aggradation from basinal deposition to shallow shelf

rocks are poorly exposed along the bluff of the Monocacy River. Metasiltstone of the

mation that conformably overlies the highest quartzite.

province and a portion of the eastern Blue Ridge province. The western Piedmont

deposition (Reinhardt, 1974). The Lime Kiln is best exposed along the quarry roads

Urbana Formation was locally baked to hornfels by contact metamorphism

The Sugarloaf Mountain Quartzite is divided into informal lower (|

), middle

province is underlain by Late Proterozoic(?) and Lower Cambrian(?) metasedimentary

within the Essroc quarry at Lime Kiln. The trilobite Olenellus, brachiopods, and echi-

), and upper (|

) members that were mapped according to their ridge- and

rocks of the Westminster terrane, Lower and Middle Cambrian metasedimentary rocks

noderms are found near Buckeystown Station and cephalopods are found west of

Diabase Dikes

ledge-forming habit; otherwise, the rocks are virtually indistinguishable. Stereoscopic

and Upper Cambrian to Lower Ordovician carbonate rocks of the Frederick Valley

Lime Kiln as fossil debris (Reinhardt, 1974).

Near vertical, en echelon, north-, northeast-, and northwest-trending diabase dikes

aerial photographs and an orthophotoquadrangle map were used to support field

synclinorium, Upper Triassic sedimentary rocks of the Mesozoic Culpeper basin, and

mapping of the quartzite units. The lower member (|

) is poorly exposed in the core

(

) were emplaced at about 200 Ma based on

Ar/

Ar data (Kunk and others,

Grove Formation

Early Jurassic dikes; Lower Cambrian metasedimentary rocks underlie the Blue Ridge

1992) during continental rifting that led to the opening of the Atlantic Ocean. The di-

of the Sugarloaf Mountain anticlinorium.

The rocks are well exposed along the

province. Within the western Piedmont, Lower Cambrian(?) metasedimentary rocks

The Grove Formation (Stose and Jonas, 1935; Jonas and Stose, 1938b; Rein-

abase is dense, hard, and weathers to large spheroidal boulders aligned along ridges.

Northern Peaks trail northwest of the Mountain Loop trail. Crossbedded quartzite

of the Sugarloaf Mountain anticlinorium are interpreted to be exposed in a tectonic

hardt, 1974) is an interval of Upper Cambrian and Lower Ordovician carbonate rock

The diabase dike swarm that transects the eastern part of this quadrangle has been

can be seen immediately east of Furnace Branch Road north of Bells Chapel. The

window (A.A. Drake, Jr., U.S. Geological Survey, oral commun., 1989; Horton and

in the core of the Frederick Valley synclinorium. Two informal members were map-

traced over 106 mi from Fauquier Co., Va., to Emittsburg, Md. (J.P. Smoot and oth-

base of the lower member is not exposed; therefore, the 600 ft thickness is approxi-

others, 1989; Rankin and others, 1989) through the complexly deformed allochtho-

ped within the formation in the Buckeystown quadrangle. Rocks of the Grove Forma-

mate. The middle member (|

) forms a prominent ridge that is followed by the

ers, U.S. Geological Survey, unpub. data, 1994).

nous rocks of the Westminster terrane (Muller and others, 1989). The undated rocks

tion contain trilobites, brachiopods, cephalopods, and conodonts of Late Cambrian

Northern Peaks trail north of the summit where the quartzite is approximately 800 ft

of the Westminster terrane are interpreted to be rise-slope deep-water deposits of the

CENOZOIC SURFICIAL DEPOSITS

and early Early Ordovician age (Reinhardt, 1974, 1977).

thick. Quartzite of the middle member defines the anticline-syncline-anticline triplet

Iapetus Ocean that were transported westward onto the Laurentian margin (ancestral

of folds on the northern nose of the anticlinorium. These rocks are well exposed be-

High-level terrace deposits (

QTt

) of the ancestral Potomac River are preserved 183

North America) along the Martic thrust fault during the Ordovician Taconic orogeny

Lower member

neath the road and trail north of the west view parking lot. Ripple-marked quartzite

ft above the Potomac River north of Maryland Route 28 near Tuscarora. Cobbles of

(Horton and others, 1989). Continental margin strata, which underlie the Sugarloaf

can be seen south of the stone building at the west view. The upper member (|

) of

The basal strata of the Grove Formation consist of crossbedded calcareous sand-

rounded quartzite have armoured the underlying limestone of the Frederick Forma-

Mountain anticlinorium and continental margin-slope strata which underlie the Freder-

the Sugarloaf Mountain Quartzite is best seen at the summit of Sugarloaf Mountain

stone here informally called the lower member (

). Because of its marked compo-

tion. The original deposit may have been preserved in a sinkhole. Later preferential

ick Valley synclinorium, are here correlated with the Lower Cambrian Chilhowee

where approximately 300 ft of quartzite has been folded into a series of anticlines and

sitional difference from underlying and overlying strata, this member makes an excel-

erosion formed a hillock by topographic inversion. In the Poolesville quadrangle to

Group and overlying carbonate rocks on the limbs of the Blue Ridge-South Mountain

lent marker for mapping purposes. The lower member is well exposed within the

synclines. Gently folded rocks of the upper member can be seen west of Sugarloaf

the south, similar deposits are as much as 288 ft above the Potomac River (South-

anticlinorium to the west. The relation of the Sugarloaf Mountain Quartzite (fig. 1)

Road where they plunge beneath rocks of the Urbana Formation. The upper mem-

abandoned quarry east of the main Essroc quarry at Lime Kiln.

worth, 1998). High-level terrace deposits of the ancestral Monocacy River are pre-

(Jonas and Stose, 1938b) to surrounding rocks is controversial (Scotford, 1951; Stose

ber can also be seen where Bear Branch breaches “west ridge,” especially north of

served 140 ft above the Monocacy River at the Monocacy National Battlefield and

and Stose, 1951; Thomas, 1952). The Martic thrust fault (fig. 1) (Jonas, 1924,

the water gap at “White Rocks” (quotation marks refer to local names of features

Upper member

west of Monocacy Natural Resources Area. These terraces are undated but may be as

1927; Knopf and Jonas, 1929) and the interpretation that the Sugarloaf Mountain

shown on accompanying trail map).

old as 5 Ma and as young as 1 Ma.

anticlinorium (Scotford, 1951; Thomas, 1952) is a tectonic window through the Mart-

Above the basal sandstone of the Grove Formation are medium- to thick-bedded

The Sugarloaf Mountain Quartzite is correlated with the Lower Cambrian Weverton

Low-level terrace deposits (

) of the Potomac and Monocacy Rivers and Tuscarora

ic thrust sheet (A.A. Drake, Jr., U.S. Geological Survey, oral commun., 1989) further

limestone and dolomite of the informal upper member (

Ogu

). The strata are arranged

Formation of the Chilhowee Group which crops out on the limbs of the Blue Ridge-

in cycles consisting of thrombolitic and stromatolitic limestone and laminated dolo-

Creek are as much as 80 ft above the present channels. Terraces of the Monocacy

complicates the stratigraphic correlation of these rocks. Upper Triassic sedimentary

South Mountain anticlinorium (fig. 1). The Sugarloaf Mountain Quartzite is about

River are well preserved east of Buckeystown adjacent to broad meanders.

rocks of the Culpeper basin consist of westward-dipping conglomerate, sandstone,

mite. This member is well exposed in pastures north of Adamstown Road and west of

1,800 ft thick.

Maryland Route 85 and within several abandoned quarries near the large lime kilns

Residuum (

) superficially resembles the terrace deposits but instead is character-

and siltstone. These rocks, as well as intrusive Early Jurassic diabase dikes, accumulat-

ized by pebble- to boulder-size, angular to euhedral, white quartz. The residuum re-

along the railroad tracks east of the Essroc quarry.

ed during an early Mesozoic rifting event that resulted in the opening of the Atlantic

Urbana Formation

sulted from in-place weathering of the underlying carbonate rocks.

The original

Ocean. Contractional faults of Paleozoic orogenesis and extensional faults related to

The Urbana Formation (|

) (Edwards, 1986; Urbana Phyllite of Jonas and Stose,

LOWER CAMBRIAN METASEDIMENTARY ROCKS OF THE BLUE RIDGE-

quartz filled veins and cavities in the limestone. Some of the cobbles are geodes con-

Mesozoic rifting indicate a complex tectonic history for this region.

SOUTH MOUNTAIN ANTICLINORIUM

taining quartz crystals.

Cenozoic deposits, which overlie the bedrock, include high- and low-level alluvial

1938b) conformably overlies the Sugarloaf Mountain Quartzite (|

). The contact

Colluvium (

) is found on Sugarloaf Mountain and along the western margin of

terraces, residual gravel, colluvium, and alluvium. Terrace deposits of the ancestral

may be seen along a tributary to Bennett Creek at the north end of the Sugarloaf

Harpers Formation

Mountain anticlinorium and on the west side of “west ridge” south of Bear Branch.

the Culpeper basin in the northwest part of the map area. Coarse colluvium derived

Potomac River and the Monocacy River are as much as 183 ft and 140 ft, respec-

from quartzite is concentrated in hillslope depressions on Sugarloaf Mountain. Large

tively, above the present river levels. Isolated residual gravel deposits that form in

The Urbana Formation contains a wide variety of metasedimentary rocks that include,

The Lower Cambrian Harpers Formation (|

) (Keith, 1894) of the Chilhowee

colluvial blocks that spalled from the face of the summit can be seen at the parking lot

place from the weathering of the Upper Cambrian Frederick Formation superficially

in decreasing abundance, metasiltstone and metagraywacke (|

), calcareous meta-

Group consists of phyllitic metashale, metasiltstone, and silty metasandstone. The

and trails at the “west view.” Colluvial deposits 10 ft thick choke Bear Branch and

sandstone and quartzite (|

), and marble (|

). Locally, these rocks are poorly ex-

Harpers is poorly exposed in the extreme northwest corner of the map area. The

resemble terrace deposits. Colluvium of quartzite boulders is concentrated in hill-

can be seen at the water gap of “west ridge.” Fine colluvium derived from weathering

slope depressions on Sugarloaf Mountain, and fanlike aprons of colluvial quartz peb-

posed because of a deep regolith of the decomposed calcareous and sandy strata.

type locality is approximately 13 mi to the west at Harpers Ferry, W. Va. On the west

The metasiltstone and metagraywacke (|

) are poorly sorted sediments that con-

of the rocks of the Blue Ridge-South Mountain anticlinorium forms dissected aprons

bles cover the Triassic rocks on the west side of the Culpeper basin. Alluvium was

limb of the Blue Ridge-South Mountain anticlinorium, rocks of the Harpers Formation

that mantle the Paleozoic and Mesozoic bedrock. This colluvium consists predomi-

tain detrital saussurite, calcite, orthoclase, tourmaline, and olivine(?). Bedding is de-

contain Skolithos burrows and Rusophycus traces (Nickelsen, 1956; Brezinski,

mapped along the Potomac and Monocacy Rivers and all their tributaries. Altitude

nantly of subangular to subrounded pebbles and cobbles of vein quartz and minor

ranges from 200 ft along the Potomac River to 1,282 ft on the crest of Sugarloaf

fined by concentrations of heavy minerals. These rocks are best seen north of Ben-

1992).

nett Creek. The calcareous metasandstone and quartzite unit (|

) of the Urbana

quartzite and metasandstone. Locally, the finer colluvium resembles both terrace de-

Mountain. The map area includes the Chesapeake and Ohio Canal National Histori-

Antietam Formation

posits and the residuum unit.

cal Park, and the Monocacy Natural Resources Area. Sugarloaf Mountain is a regis-

Formation is lensoid, discontinuous, and difficult to trace in the field. The calcareous

Alluvium (

Qal

) lies along all drainages, but the best deposits are along the Potomac

tered natural landmark.

metasandstone is medium- to coarse-grained quartz sand in a clay-rich matrix contain-

The Lower Cambrian Antietam Formation (|

) (Keith, 1894) of the Chilhowee

and Monocacy Rivers. The current river channels are incised into bedrock and the al-

ing abundant crystals and seams of calcite. The rock is characteristically friable and

Parts of the Buckeystown quadrangle were mapped by Jonas and Stose (1938a,

Group consists of metasandstone and sandy metasiltstone that underlies the main

luvium crops out on the banks.

O M

scale 1:62,500), Scotford (1951, scale 1:12,500), Thomas (1952, scale 1:25,000),

contains many vugs. The quartzite consists of medium- to coarse-grained quartz and

ridge of the Blue Ridge-South Mountain anticlinorium in the extreme northwest corner

polycrystalline quartz lithic clasts cemented by silica; accessory detrital minerals include

Cloos and Cook (1953, 1:62,500), Reinhardt (1974, scale 1:62,500), Froelich

of the map area. The type locality of the Antietam Formation is approximately 13 mi

STRUCTURAL GEOLOGY

zircon, magnetite, orthoclase, perthite, plagioclase, and ilmenite. The quartzite is best

(1975, scale 1:62,500), and Lee (1979, scale 1:24,000).

to the west at Antietam, Md. The Antietam Formation is the lithic equivalent of the

The map area is subdivided into five domains (fig. 1). Allochthonous rocks of the

seen along Bennett Creek south of Park Mills. Quartzite containing blue-black quartz-

In the Buckeystown quadrangle, polydeformed slope rise to slope prism deposits

Araby Formation although the Antietam Formation is older. On the west limb of the

ite clasts can be seen along the Frederick and Montgomery County line east of Strong-

Westminster terrane are thrust onto rocks of the Frederick Valley synclinorium. With-

Blue Ridge-South Mountain anticlinorium, the Antietam contains the trilobite Olenel-

were thrust over parautochthonous rocks of the Sugarloaf Mountain anticlinorium and

in the Westminster terrane of Muller and others (1989), the Sugarloaf Mountain

hold. Marble (|

) contains sericite, chlorite, graphite, and detrital quartz, zircon, mi-

Frederick Valley synclinorium along the Martic thrust fault during the Ordovician Ta-

lus (Walcott, 1896) and the trace fossils Skolithos, Rusophycus, and Planolites (Bre-

Quartzite and Urbana Formation underlie the parautochthonous Sugarloaf Mountain

crocline, plagioclase, and orthoclase. The marble is well exposed on the bluffs of

conic orogeny (A.A. Drake, Jr., U.S. Geological Survey, oral commun., 1989; Drake

zinski, 1992).

Bennett Creek east of Bear Branch. This impure carbonate rock does not resemble

and others, 1989; Horton and others, 1989; Rankin and others, 1989; Rankin and

anticlinorium. The Lower and Middle Cambrian Araby Formation and Upper Cambri-

an Frederick Formation crop out on the east limb of the Frederick Valley synclinori-

any other carbonate rock within the quadrangle.

Tomstown Formation

others, 1990). In this interpretation, the Martic thrust sheet was folded with rocks of

um. The Frederick Formation and Upper Cambrian and Lower Ordovician Grove

Rocks of the Urbana Formation are correlated with lithologically similar metasilt-

the Sugarloaf Mountain anticlinorium during the late Paleozoic Alleghanian orogeny;

Dolomite of the Lower Cambrian Tomstown Formation (|

) (Stose, 1906) was

stone, metasandstone, and limy metashale of the Lower Cambrian Harpers Formation

subsequent erosion exposed the Sugarloaf Mountain Quartzite and Urbana Formation

Formation lie in the trough of the Frederick Valley synclinorium. Lower Cambrian

encountered in drill core along the Mesozoic basin border fault in the adjacent Point of

Harpers, Antietam, and Tomstown Formations crop out on the east limb of the Blue

of the Chilhowee Group in the Blue Ridge-South Mountain anticlinorium, as earlier

in a tectonic window (fig. 1). Like the Blue Ridge-South Mountain anticlinorium,

Rocks quadrangle by Hoy and Schumacher (1956). The Antietam and Tomstown

proposed by Scotford (1951). Rocks of the Urbana Formation are crossbedded and

Ridge-South Mountain anticlinorium in the extreme northwest corner of the map.

rocks of the Sugarloaf Mountain anticlinorium and Frederick Valley synclinorium are

Formations are roughly time correlative to the Araby Formation exposed to the east.

The Blue Ridge-South Mountain anticlinorium is separated from the Frederick Valley

ripple-marked as first recognized by Hopson (1964) and are interpreted to be conti-

mostly continental-margin deposits of Laurentia and they contain evidence of only

The type locality of the Tomstown Formation is approximately 30 mi to the northwest

synclinorium by Upper Triassic sedimentary rocks in the Culpeper basin half graben.

nental margin deposits.

one phase of folding and axial-planar cleavage development (fig. 2). The overlying

in Pennsylvania. The trilobite Olenellus (Fauth, 1968) and the mollusc Salterella

polydeformed Silver Run Limestone and Ijamsville Phyllite of the Westminster terrane

Figure 1 shows the type localities of rock units.

conulata (Brezinski, 1992) have been recognized in the Tomstown Formation on the

CAMBRIAN AND ORDOVICIAN ROCKS OF THE FREDERICK VALLEY

are allochthonous. Contractional motion of the Martic thrust fault occurred during the

west limb of the Blue Ridge-South Mountain anticlinorium.

ROCKS OF THE WESTMINSTER TERRANE

SYNCLINORIUM

Ordovician with later thrusting and possible dextral strike-slip motion occurring during

the late Paleozoic Alleghanian orogeny. Because the geology is complex and the in-

MESOZOIC ROCKS

Silver Run Limestone

Araby Formation

terpretations controversial, an interpretative sequence of cross sections (fig. 2) por-

The Silver Run Limestone (|

) (Jonas and Stose, 1938b) was named for a karstic

The Lower and Middle Cambrian Araby Formation (|

) (Reinhardt, 1974, 1977)

Culpeper Group of the Newark Supergroup

trays this model.

Structural elements in the rocks include bedding, cleavage, transposition foliation,

valley in the Littlestown quadrangle, Carroll County, Md., but the best exposure is

consists predominantly of argillaceous, burrowed, mottled metasiltstone that contains

considered to be in the Union Bridge quadrangle (Edwards, 1986), Frederick County,

Manassas Sandstone

crenulation cleavage, mineral lineations, intersection lineations, folds, and faults.

sandy intervals and has a phyllitic metashale at the top. The type locality is along

Md. In the Buckeystown quadrangle, thin-bedded, laminated, argillaceous metalime-

Bush Creek south of Frederick Junction. Rocks of the Araby Formation are well ex-

These elements are discussed below by terrane and geographic location. In summary,

The Manassas Sandstone of Late Triassic age was divided into three members by

bedding, cleavage, and bedding-cleavage intersection of the rocks of the Sugarloaf

stone crops out along a tributary north of the Monocacy River northwest of St. Paul

posed along the railroad tracks east of Frederick Junction, along the Monocacy River

Lee and Froelich (1989). The Reston and Tuscarora Creek Members are basal con-

Church. By inference, the metalimestone underlies the linear valley along strike as it

Mountain anticlinorium and Frederick Valley synclinorium support the concept of one

east of Buckeystown, west of Greenfield Mills, and along the road west of Lilypons.

glomerates that are overlain conformably by sandstone of the Poolesville Member.

phase of folding. Cleavage in these parautochthonous rocks is mostly coplanar with

is well exposed along the Monocacy River to the south in the Poolesville quadrangle

The metasiltstone is highly cleaved and jointed and the bedding is commonly obscure.

Reston Member—The Reston Member of the Manassas Sandstone (^

) (Lee,

transposition foliation in the allochthonous rocks. Second-phase folds of foliations

(Southworth, 1998). There, the metalimestone is overlain by Ijamsville Phyllite. In

The rocks underlie ridges that have a thin soil. Classic ridge and valley topography

1977) is a basal conglomerate that unconformably overlies rocks of the Piedmont.

the Buckeystown quadrangle the metalimestone crops out within polydeformed phyl-

formed on the east limb of the Frederick Valley synclinorium where the Araby is tight-

and vein quartz in the allochthonous rocks of the Westminster terrane help to define

The type locality is along the Washington and Old Dominion bike trail near Reston

the Martic thrust sheet.

lite of the Ijamsville Phyllite. Fisher (1978) mapped the Silver Run Limestone within

ly folded with the Frederick Formation.

Parkway in the Vienna 7.5-minute quadrangle in northern Virginia (Drake and Lee,

Ijamsville Phyllite in the New Windsor quadrangle and also interpreted the metalime-

The metasiltstone consists predominantly of quartz sand grains, polycrystalline

1989). The Reston Member is characterized by cobbles and pebbles of vein quartz

Allochthonous Rocks of the Westminster Terrane

stone to be beneath the phyllite. The Silver Run Limestone probably was deposited in

quartz, and opaque minerals that are supported by a clay-rich matrix containing abun-

and subangular phyllite and schist derived from the underlying Ijamsville Phyllite in

deep water prior to the mud (protolith of the phyllite). Alternatively, the metalime-

dant hematite. Sericite, chlorite, magnetite octahedra, sphene, zircon, microcline, sa-

Polydeformed, allochthonous rocks of the Silver Run Limestone and Ijamsville

the map area. Good outcrops can be seen north of Monocacy River, east of St.

stone may constitute blocks (sedimentary olistoliths) deposited in the mud.

nidine, and plagioclase are accessory grains. The Araby Formation probably repre-

Paul Church.

Phyllite of the Westminster terrane constitute the Martic thrust sheet. The Martic

sents the deep-water slope facies of a starved clastic basin that persisted in the

thrust fault (Jonas, 1924; 1927; Knopf and Jonas, 1929; Southworth, 1996) has

Tuscarora Creek Member—The Tuscarora Creek Member (^

) (Lee and Froelich,

Ijamsville Phyllite

Cambrian (Reinhardt, 1977). Rocks of the Araby Formation contain fragments of the

brought the Ijamsville Phyllite onto the Urbana, Araby, and Frederick Formations.

steeply eastward and quartzite of the Urbana Formation (|

) is transected by the

METAMORPHISM

1989) is a basal conglomerate that unconformably overlies carbonate rocks of the Up-

trilobite Olenellus of late Early Cambrian age (Reinhardt, 1974). The Araby Forma-

The type locality of the blue, green, and purple phyllitic slate of the Ijamsville Phyl-

per Cambrian Frederick Formation. It is laterally equivalent to the Reston Member

The trace of the Martic thrust fault is straight from the Potomac River (Southworth,

fault north of Flint Hill. The Urbana Formation was thrust onto the klippe of Ijams-

tion is conformably overlain by carbonate rocks of the Rocky Springs Station Member

1996) north to the Mesozoic Gettysburg basin (Edwards, 1988), suggesting a moder-

ville Phyllite during formation of the Sugarloaf Mountain anticlinorium. The klippe

Westminster Terrane

lite (Jonas and Stose, 1938b) is the town of Ijamsville in the Urbana quadrangle

but differs lithically because of different source rocks. The unit can be best seen at the

(Southworth, 1999). Rocks composing the Ijamsville Phyllite are (in order of decreas-

of the Frederick Formation (|

). The contact can be seen in a tributary south of

ately steep attitude at the surface (fig. 1). Along Bennett Creek within one meter of

masks the relationship between the Urbana Formation and the Araby and Frederick

type locality along Route 28, east of Tuscarora Creek and west of Tuscarora. The

Rocks of the Westminster terrane had a complex metamorphic history under

Greenfield Mills.

Formations. A minor southwestward-directed intraformational thrust fault in the Ur-

ing abundance) hematite-rich muscovite-chlorite-paragonite-chloritoid phyllite, phyllon-

Tuscarora Creek Member is characterized by the presence of limestone and dolomite

the fault, transposition foliation in both the Ijamsville Phyllite (hanging wall) and Fred-

greenschist-facies conditions. The Ijamsville Phyllite contains varying proportions of

erick Formation (footwall) dips 50° to the southeast. The Martic thrust fault was fold-

bana Formation south of Bennett Creek is related to the formation of the northeast-

ite, and slate (undifferentiated) (|

). The Ijamsville Phyllite is characterized by com-

clasts derived from the underlying Frederick Formation. The conglomerate is locally

muscovite-chlorite-paragonite-quartz-magnetite, and calcite, of a first metamorphic

Frederick Formation

posite foliations, abundant and strongly folded vein quartz, and polymetamorphic

lensoidal and discontinuous and was interpreted to be a channel fill and (or) fan depos-

ed and erosion exposed rocks of the Sugarloaf Mountain anticlinorium in a tectonic

plunging anticlinorium.

event. In fault zones, the phyllites have been retrogressively metamorphosed and

window. On the west side of the window, the Martic was reactivated, bringing foot-

features. The unit is commonly poorly exposed, but its presence is indicated by phyl-

The Upper Cambrian Frederick Formation (Frederick Limestone of Keyes, 1890;

its (Lee and Froelich, 1989).

sheared into phyllonitic diaphthorites (Knopf, 1931). Such rocks are characterized by

Frederick Valley Synclinorium

Poolesville Member—The Poolesville Member (^

wall rocks of the Urbana Formation onto the Ijamsville Phyllite during formation of

lite chips and an abundant float of vein quartz that mantles the clay-rich soil of a dis-

Stose and Stose, 1946; Reinhardt, 1974, 1977) is a thick interval of thin- to medium-

) (Lee and Froelich, 1989) is

“sick-looking” (Knopf, 1931) retrograde chlorite and recrystallized quartz. Abundant

sected plateau. The Ijamsville Phyllite is located in a fault zone, the base of which is

bedded limestone, dolomite, and thin intervals of shale and sandstone. Because of the

an arkosic, muscovite-rich sandstone that contains sparse quartz pebbles and fines up-

the anticlinorium.

The asymmetrical Frederick Valley synclinorium contains Cambrian and Ordovician

leucoxene in these rocks also suggests retrogressive metamorphism (Stose and Stose,

The dominant planar structure in the Ijamsville Phyllite is a composite foliation that

the Martic thrust fault.

numerous rock types present in this unit, it is here revised as the Frederick Formation.

ward to siltstone. The unit is transitional with the basal conglomerate members. The

rocks (Reinhardt, 1974). On the east limb of the synclinorium, rocks of the Araby

1946; Scotford, 1951; Hopson, 1964).

Ijamsville Phyllite locally contains static

consists mostly of a transposition foliation overprinted by a phyllonitic foliation and

Most of the Ijamsville in the Buckeystown quadrangle is phyllonite that contains

The three members of Reinhardt (1974 and 1977) were recognized and mapped

Poolesville Member grades up into the Balls Bluff Siltstone; therefore, the contact in

and Frederick Formations were folded into a series of northeast- and southwest-plung-

chloritoid that grew after the deformation or is related to a later prograde metamor-

abundant vein quartz. These rocks are best exposed along Bennett Creek west of

within the Frederick Formation.

this quadrangle is approximate. The type locality is 7 mi to the south at Poolesville,

several sets of cleavage. Vein quartz was sheared and folded into steeply plunging iso-

ing anticlines and synclines (fig. 3). These folds and attendant axial-plane cleavage are

phic event.

clines. The transposition foliation and folded vein quartz were deformed by westward-

Park Mills and along Maryland Route 355 in the northeast part of the map. Slate is

Md. In this quadrangle, the unit is best exposed between Pleasant View and Doubs

best seen north of Bennett Creek. The Araby Formation is well cleaved and highly

well exposed in a quarry west of Hope Hill.

Rocky Springs Station Member

along Route 28 west of Tuscarora Creek.

verging inclined F

folds that plunge steeply to gently northwest or southeast (fig. 3).

Rocks of the Sugarloaf Mountain Anticlinorium, Frederick Valley Synclinorium,

jointed. South of Lilypons and northwest of Hope Hill, antiforms similar to those in

These folds have an axial-planar pressure-solution cleavage that strikes northwest and

The hematite-rich phyllite originally was deep-water mud deposited on the conti-

the Araby Formation and topographic valleys suggest the presence of nonexposed

and Blue Ridge-South Mountain Anticlinorium

The lower member of the Frederick Formation, the Rocky Springs Station Mem-

Balls Bluff Siltstone

dips northeast and can be seen in the immediate hanging wall of the Martic thrust

nental slope (fig. 1). Chemical data (table 1 and fig. 2) of the phyllites shows that

folds of Frederick Formation. For example, the linear valley at Greenfield Mills is a

ber (|

) (Reinhardt, 1974), is characterized by intervals of locally traceable polymictic

The Sugarloaf Mountain Quartzite and Urbana Formation are at chlorite-grade re-

the ratio of K

O to total alkalies differs and may reflect different source areas for the

fault southeast of Lilypons and along Bennett Creek. Bedding is seen only as laminae

breccia. This member resulted from off-shelf deposition at the toe of a slope (Rein-

The type locality of the Balls Bluff Siltstone (Lee and Froelich, 1989) is the Balls

tight syncline in the Frederick Formation.

sulting from a single greenschist-facies metamorphic event. The matrix of the quartz-

in phyllite and slate in the quarry west of Hope Hill. In the Buckeystown quadrangle,

parental muds (Fisher, 1978) or may indicate a volcanic component of the muds

The Lime Kiln Member of the Frederick Formation and the Grove Formation de-

hardt, 1974). The Rocky Springs Station is best exposed along the Monocacy River

Bluff National Cemetery in the adjacent Waterford quadrangle, Va. (Burton and

ites is rich in sericite and contains sparse chlorite porphyroblasts. Cleavage in meta-

rocks of the Ijamsville Phyllite constitute a fault zone. The Silver Run Limestone is

(Clark, 1924).

fine a series of folds in the keel of the westward overturned synclinorium.

within the Monocacy National Battlefield, southwest of Maryland Route 355. Good

others, 1995). Although the unit is predominantly siltstone (^

), it locally contains

graywacke and metasiltstone of the Urbana Formation is marked by aligned minute

poorly exposed in the map area. To the south in the Poolesville quadrangle (South-

outcrops can also be seen along Bennett Creek, north of Lilypons. An interval of

interbedded sandstone. In the map area, the Balls Bluff Siltstone is poorly exposed

The Harpers, Antietam, and Tomstown (subsurface) Formations in the extreme

sericite crystals. The metasiltstone contains abundant chlorite porphyroblasts that lo-

LOWER CAMBRIAN(?) ROCKS OF THE SUGARLOAF MOUNTAIN

worth, 1998), the Silver Run Limestone is folded into F

antiforms that exhibit rod-

northwest corner of the map area constitute a southeast-dipping homoclinal sequence

gray to black shale (|

frs

) occurs on the east limb of the Frederick Valley synclinorium

but can be seen west of Doubs.

cally give them a green hue.

ANTICLINORIUM

ding and contain a strong south-plunging lineation.

and can be seen along Maryland Route 355. The trilobite Olenellus was found near

Leesburg Member—The Leesburg Member (^

that forms the east limb of the Blue Ridge-South Mountain anticlinorium and the west

) of the Balls Bluff Siltstone (Lee

The Harpers, Antietam, Tomstown, Araby, Frederick, and Grove Formations con-

limb of the Frederick Valley synclinorium.

the top of the member in thin-bedded limestone (Reinhardt, 1974).

and Froelich, 1989) is a conspicuous carbonate-clast fanglomerate composed of

tain sparse tiny crystals of sericite and minor chlorite porphyroblasts, both of which

Sugarloaf Mountain Quartzite

Parautochthonous Rocks

subangular to subrounded boulders, cobbles, and pebbles of limestone and dolomite in

are crudely aligned on the cleavage.

Culpeper Basin

The Sugarloaf Mountain Quartzite (Jonas and Stose, 1938b) was named for Sugar-

Adamstown Member

a reddish-brown sandy siltstone matrix. Lindholm and others (1979) suggested that

Rocks of the Sugarloaf Mountain anticlinorium and Frederick Valley synclinorium

loaf Mountain. The Sugarloaf Mountain Quartzite consists of medium-bedded to mas-

have experienced a deformational and metamorphic history similar to that of the Blue

the carbonate clasts were derived from Cambrian and Ordovician rocks of the Grove,

The Culpeper basin in the Buckeystown quadrangle is a half-graben that is bound-

The Adamstown Member (|

) (Reinhardt, 1974), the middle member of the Fred-

sive, medium- to coarse-grained, saccharoidal, white, gray, tan, and maroon quartzite

Ridge-South Mountain anticlinorium to the west (Southworth and others, in press). In

Elbrook, and Conococheague Formations. Isolated outcrops are exposed along the

ed on the west by an east-dipping normal fault that places the Upper Triassic Lees-

erick Formation, consists of thinly bedded limestone and thin intervals of shale. The

cemented by silica and sericite. Crossbedding and sparse ripple marks support the in-

road northwest of Adamstown. The conglomerate was interpreted by Smoot (1989)

the Buckeystown quadrangle, these rocks are structurally and stratigraphically discord-

Adamstown resulted from deposition in a basinal environment (Reinhardt, 1974).

burg Member of the Balls Bluff Siltstone against dolomite of the Lower Cambrian

terpretation that these rocks are continental-margin deposits. The quartzite contains

ant to the allochthonous Silver Run Limestone and Ijamsville Phyllite. In general, the

to be debris flows on an alluvial fan. The conglomerate, locally called “Potomac

Tomstown Formation (Whitaker, 1955). On the east side of the Culpeper basin, the

The Adamstown is well exposed on the south and north faces of the Essroc quarry at

Sugarloaf Mountain anticlinorium and Frederick Valley synclinorium constitute a sys-

Lime Kiln. The trilobite Olenellus, brachiopods, and echinoderms are found as fossil

marble,” was used for the columns in Statuary Hall of the Capitol building in

Upper Triassic Tuscarora Creek Member of the Manassas Sandstone unconformably

Washington, D.C.

tem of parasitic folds to the Blue Ridge-South Mountain anticlinorium (figs. 1 and 2).

overlies the Upper Cambrian Frederick Formation and the Upper Triassic Reston

hash southeast of Doubs (Reinhardt, 1974).

Member of the Manassas Sandstone unconformably overlies the Ijamsville Phyllite.

Sugarloaf Mountain Anticlinorium

Both of the carbonate-clast fanglomerates (Tuscarora Creek and Leesburg Members)

Scotford (1951) and Thomas (1952) demonstrated that the structural geology of

are debris-flow deposits that formed along fault escarpments during early Mesozoic

Sugarloaf Mountain and the surrounding area constitutes a doubly plunging anticlinori-

rifting. Clasts of limestone and dolomite derived from the Elbrook and Conocochea-

um that is overturned to the west. An anticline-syncline-anticline triplet of Sugarloaf

gue Limestone, and the Grove Formation (Lindholm and others, 1979) suggest that

Mountain Quartzite constitutes the north-plunging nose of the anticlinorium south of

these rocks formed a topographically high fault scarp that has since been eroded. Af-

Bennett Creek. The main anticlinorium of Sugarloaf Mountain Quartzite has been

ter deposition and lithification of the fanglomerates, faulting continued, resulting in

west-dipping strata. The greatest thickness of preserved basin fill is along the western

thrust onto quartzite of the middle and upper members that underlie the “west ridge.”

This is the Sugarloaf fault of Thomas (1952) which was first recognized by Keyes

border fault. Smaller down-faulted basins formed by the intersection of northeast and

(1891). The “west ridge” is also an anticlinorium. A south-plunging anticline can be

northwest-striking normal faults. The Triassic rocks between Tuscarora Creek and

seen along Furnace Branch in the Poolesville quadrangle to the south (Southworth,

east of Tuscarora, Md., were down-dropped along such faults. The Araby Formation

1998) at the southernmost exposure of the upper member of the Sugarloaf Mountain

in the footwall of the normal fault east of Tuscarora is truncated by a northwest-strik-

Quartzite. The folded beds of Sugarloaf Mountain Quartzite are cut by normal faults

ing normal fault concealed beneath alluvium of the Potomac River. At the Monocacy

of Mesozoic age. Most of the normal faults strike northwest and are characterized by

Natural Resources Area, the Reston and Poolesville Members of the Manassas Sand-

hematite-cemented breccia and float blocks of iron ore. The normal faults (first descri-

stone were down-faulted against rocks of the Ijamsville Phyllite and Urbana Forma-

tion. The present erosion level provides an opportunity to walk along the Triassic un-

bed by Scotford, 1951) displace the upper member of the Sugarloaf Mountain Quartz-

ite that underlies the “west ridge.”

conformity within and north of the Monocacy River.

These exposures provide

The Sugarloaf Mountain Quartzite and Urbana Formation have a cleavage that is

evidence that Mesozoic faulting and the areal extent of Triassic rocks were more ex-

axial planar to a single phase of folds that constitute the Sugarloaf Mountain anticlino-

tensive. Northwest-trending normal faults that cut the Sugarloaf Mountain Quartzite

rium (fig. 3). The most distinctive features of the Urbana Formation are bedding, one

and Frederick Formation also support the concept that rocks of the Piedmont, Freder-

cleavage, and a conspicuous bedding and cleavage intersection lineation (fig. 3).

ick Valley, and Blue Ridge were also affected by Mesozoic extensional faulting. The

The contact of the Urbana Formation and Ijamsville Phyllite is the Martic thrust

diabase dike swarm that trends northward through the eastern part of the map area

fault, which makes the Ijamsville Phyllite a synformal klippe. This fault contact dips

reflects east-west extension at roughly 200 Ma.

GEOLOG C MAP OF THE BUCKEYSTOWN QUADRANGLE FREDER CK AND MONTGOMERY COUNT ES MARYLAND AND LOUDOUN COUNTY V RG N A

S o Sou hwo h and Da d K B e n k

Geologic Map Of The Buckeystown Quadrangle, Frederick And Montgomery Counties, Maryland, And Loudoun County, Virginia

Related Articles

Related forms

Related Categories