Central Appalachian Piedmont and Blue Ridge tectonic ...

135

INTRODUCTION

This fi eld trip highlights over a hundred years of research on the tectonic evolution of the complex rocks of the central Appalachians in the Piedmont and Blue Ridge provinces of

Maryland, Virginia, and Washington, D.C. (Fig. 1) (Keith, 1894; Darton and Keith, 1901; Jonas and Stose, 1938a; Cloos and Broedel, 1940; Stose and Stose, 1946; Cloos and Cooke, 1953; Fisher, 1963; 1970; Hopson, 1964; and Drake, 1986, 1989, 1994, 1995, 1998). Systematic geologic mapping at a scale of

Geological Society of AmericaField Guide 8

2006

Central Appalachian Piedmont and Blue Ridge tectonic transect, Potomac River corridor

Scott SouthworthAvery Ala Drake Jr.

U.S. Geological Survey, Reston, Virginia 20192, USA

David K. BrezinskiMaryland Geological Survey, Baltimore, Maryland 21218, USA

Robert P. WintschDepartment of Geological Sciences, Indiana University, Bloomington, Indiana 47405, USA

Michael J. KunkU.S. Geological Survey, Reston, Virginia 20192, USA

John N. AleinikoffU.S. Geological Survey, Denver, Colorado 80225, USA

Charles W. NaeserNancy D. Naeser

U.S. Geological Survey, Reston, Virginia 20192, USA

ABSTRACT

This fi eld trip highlights the current understanding of the tectonic assemblage of the rocks of the Central Appalachians, which include the Coastal Plain, Piedmont, and Blue Ridge provinces. The age and origin of the rocks, the timing of regional deformation and metamorphism, and the signifi cance of the major faults, provide the framework of the tectonic history which includes the Mesoproterozoic Grenvillian, Ordovician Taconian, Devonian to Mississippian Neoacadian, and Mississippian to Permian Alleghanian orogenies.

Keywords: Piedmont, Blue Ridge, Potomac River, Paleozoic tectonics, geochronology.

Southworth, S., Drake, A.A., Jr., Brezinski, D.K., Wintsch, R.P., Kunk, M.J., Aleinikoff, J.N., Naeser, C.W., and Naeser, N.D., 2006, Central Appalachian Piedmont and Blue Ridge tectonic transect, Potomac River corridor, in Pazzaglia, F.J., ed., Excursions in Geology and History: Field Trips in the Middle Atlantic States: Geological Society of America Field Guide 8, p. 135–167, doi: 10.1130/2006.fl d008(08). For permission to copy, contact [email protected]. ©2006 Geologi-cal Society of America. All rights reserved.

136 Southworth et al.

1:24,000 (Drake, 1986, 1994, 1998; Drake and Froelich, 1997; Fleming et al., 1994; Drake et al., 1999; Southworth, 1994, 1995, 1998, 1999; Southworth and Brezinski, 1996a, 1996b, 2003) and compiled at a scale of 1:100,000 (Davis et al., 2002; Southworth et al., 2002) summarize the latest understanding and distribution of the rocks. Recent analyses of rocks using U-Pb techniques (Aleinikoff et al., 2000; 2002), and 40Ar/39Ar and fi ssion-track methods (Kunk et al., 2005) provide geochronol-ogy of the tectonic assemblage. The focus of the research was largely designed to determine (1) the ages and correlations of bedrock units, (2) the timing of metamorphism and deforma-tion, and (3) the timing of major fault activity.

Modern geochronology throughout the Appalachians defi nes at least fi ve major orogenic events that involved pluto-nism, deformation, and metamorphism (summarized by Rodg-ers, 1970; Higgins et al., 1988; Butler, 1991; Hatcher and Gold-berg, 1991; Thomas et al., 2004; Ratcliffe, 2006; Van Staal and Whalen, 2006): (1) the Mesoproterozoic Grenvillian orogeny (ca. 1200–1000 Ma [million years ago]); (2) the Ordovician Taconian orogeny (502–419 Ma for plutons, 480–460 Ma for metamorphism, and 454–453 Ma for potassium-rich benton-ite beds in Middle Ordovician strata); (3) the Devonian Aca-dian orogeny (421–395 Ma); (4) the Mississippian Neoacadian orogeny (395–360 Ma); and (5) the Carboniferous to Permian Alleghanian orogeny (330–270 Ma for plutons, 316 Ma and 311 Ma for volcanic ashes in strata, and 330–260 Ma for meta-morphism and deformation).

GEOLOGIC SETTING AND STRATIGRAPHY

Introduction

The geology is presented along an east to west transect from the eastern Piedmont province (near its border with the Coastal Plain province in Washington, D.C.), westward across the Pied-mont and Blue Ridge provinces, and into the Great Valley section of the Valley and Ridge province (Figs. 1 and 2). From east to west, the Piedmont province consists mostly of Neoproterozoic to Ordovician rocks of the Potomac and Westminster terranes (Drake, 1989; Horton et al., 1989; 1991), the Sugarloaf Mountain anticlinorium, and the Frederick Valley synclinorium. Mesozoic rocks of the Culpeper basin are in the westernmost Piedmont. The Potomac terrane is a composite terrane (Kunk et al., 2005) that was thrust onto the Westminster terrane along the Pleasant Grove fault. The Westminster terrane was thrust onto continen-tal margin strata of the Sugarloaf Mountain anticlinorium and Frederick Valley synclinorium along the Martic fault. Paleozoic metamorphism and deformation resulted in a large fold train that extends westward from the Piedmont into the Valley and Ridge province (Figs. 1 and 2). All of these rocks are allochthonous above the North Mountain thrust fault (Evans, 1989) that crops out along the west side of the Great Valley. Most of the rocks are Paleozoic except for the Mesoproterozoic and Neoproterozoic rocks that are exposed in the Blue Ridge province and Mesozoic rocks exposed in the western Piedmont.

Blue RidgeGreat Valley Piedmont

Coastal Plain0 4 8 12 162

Miles

Potomac River

Middleburg

Leesburg

Reston

Rockville

MARYLAND

RT-15I-70

I-340

I-270

RT-7

I-495

D.C.

FrederickNew Market

VIRGINIA

WESTVIRGINIA

Harpers Ferry

N

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78O

I-340

1-5

1-6

1-3

1-4

2-5 2-4 2-12-2

2-3

2-6

1-1 & 1-2

Figure 1. Index map of the Potomac River region showing the physiographic provinces and fi eld trip stops.

Central Appalachian Piedmont and Blue Ridge tectonic transect 137

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Mar

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Blue Ridge -South

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Culpeperbasin

FrederickValley

synclinorium

SugarloafMountain

anticlinorium

Westminsterterrane

Potomacterrane

CoastalPlain

Continental extension

Grenvillian orogeny

Mar

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Mar

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Plea

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Taconianorogeny

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Mesoproterozoic

Neoproterozoic

Cambrian

Ordovician

Devonian

Mesozoic

Tertiary

Plummers Islandshear zone

PleasantGrove

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Figure 2. Generalized geologic map (modifi ed from Southworth et al., 2002) and cross section of the Potomac River region showing the index to the geologic maps for the stops. Circled dots are 40Ar/ 39Ar samples.


The rocks refl ect several Wilson cycles (Wilson, 1966) of opening and closing ocean basins. Rocks of the Blue Ridge prov-ince record evidence of the Grenvillian orogeny at 1 Ga (billion years ago). About 500 m.y. (million years) later, the Laurentian continent rifted to create the Iapetus Ocean. Rocks of the cen-tral and western Piedmont province as well as the rocks of the Great Valley of the Valley and Ridge province were deposited at various depths and in different environments within and along the western margin of the Iapetus Ocean. Fine-grained rocks of the Westminster terrane were deposited in deep water on the continental rise; however, to the west, carbonate rocks of the Frederick and Great Valleys were deposited in shallow to deep water on the continental slope and shelf platform, respectively. Rocks of the Potomac terrane are turbidites that were deposited in a deep oceanic trench. The Piedmont rocks were metamor-phosed, deformed, and intruded by plutons in the Ordovician Taconian orogeny (Drake, 1989). The change in sedimentation from carbonate rocks to clastic rocks of the Ordovician Mar-tinsburg Formation in the Great Valley section of the Valley and Ridge province resulted from this orogeny. The rocks were folded, faulted, and metamorphosed in several events through-out the Paleozoic and culminated in the late Paleozoic (Pennsyl-vanian to Permian) Alleghanian orogeny. This orogeny resulted from the collision of Laurentia (North America) and Gondwana (Eurasia) which in turn formed Pangea. The rocks were trans-ported westward above the North Mountain thrust fault. The Alleghanian structural culmination collapsed and continental extension resulted in the formation of the Atlantic Ocean in the Jurassic. Drainage systems that fl owed west into the Appala-chian basin, perhaps since the Ordovician, were captured and fl owed east into the Atlantic Ocean in the Miocene (Pazzaglia and Brandon, 1996; Naeser et al., 2004). Debris shed from the eroding Appalachian Highlands was deposited as thick uncon-solidated sediments that now form the Coastal Plain. Steep, southwest-dipping thrust faults that placed the Laurel Forma-tion onto Pleistocene gravel near Rock Creek Park (Darton and Keith, 1901) demonstrate contractional deformation and active tectonics of the passive margin.

Atlantic Coastal Plain Province

The metamorphic rocks of the Piedmont province are unconformably overlain by Cretaceous to Pleistocene sheets of unconsolidated sediments derived from erosion of the rocks of the Blue Ridge and Piedmont provinces, along with transgressive marine deposits. The oldest such deposits in this region are sand and clay of the Cretaceous Potomac Formation and Tertiary Cal-vert Formation. These units are overlain by at least four levels of upland terrace deposits of the ancestral Potomac River and range in age from middle Miocene (ca. 13.5 Ma) to late Pliocene (2.5–1.8 Ma) (Fleming et al., 1994). Erosion and reworking of sedi-ment produced a complex patchwork of landforms and deposits of various ages. Colluvium derived from the upland deposits is draped on many of the steep slopes.

Piedmont Province

There are at least fi ve sections of the Piedmont province along the Potomac River corridor. From east to west, these are (1) the Potomac terrane, (2) the Westminster terrane, (3) the Sug-arloaf Mountain anticlinorium, (4) the Frederick Valley synclino-rium, and (5) the Culpeper basin.

Potomac Terrane

From east to west, the Potomac terrane consists of the Lau-rel, Sykesville, and Mather Gorge Formations (Drake and Fro-elich, 1997; Drake, 1998); it is bounded on the west by the Pleas-ant Grove fault. The protoliths of these rocks were mélanges and distal turbidite deposits of the continental slope and rise, which contain bodies of ultramafi c rocks. These undated rocks, inter-preted to be Neoproterozoic to Cambrian (Drake, 1989), were intruded by Ordovician tonalitic to granodioritic rocks (Aleini-koff et al., 2002). The terrane contains two major faults: the Rock Creek shear zone (Fleming et al., 1994) and the Plummers Island fault (Drake, 1989).

Ultramafi c Rocks

The oldest rocks are metagabbro, metapyroxenite, amphibo-lite, serpentinite, soapstone, chlorite-tremolite-epidote schist, talc-chlorite schist, and talc-calcite-actinolite schist that are blocks within the Mather Gorge Formation. The Soldiers Delight Ultramafi te consists of serpentinite that once was oceanic pyrox-enite consisting of dunite and harzburgite (Drake, 1994) but was later altered to talc schist and soapstone along shear zones.

Mather Gorge Formation

The Mather Gorge Formation (Drake and Froelich, 1997) consists of graywacke and mudstone that was metamorphosed to become granofelsic metagraywackes and quartz-mica schists. Well-bedded metagraywacke locally preserves graded beds and soft-sed-imentary slump features (Hopson, 1964; Fisher, 1970). Sillimanite-grade migmatites transition both eastward and westward into chlo-rite-grade phyllitic and phyllonitic rocks near the Plummers Island fault and Pleasant Grove fault (Fisher, 1970; Drake, 1989).

The rocks of the Mather Gorge Formation were subdivided into three domains on the basis of lithology, metamorphic history, structure, and geochronology (Kunk et al., 2005); from east to west, these are the Stubblefi eld Falls, Bear Island, and Blockhouse Point domains. The Stubblefi eld Falls domain is characterized by migmatitic schist that was pervasively retrograded to chlorite-sericite phyllonitic schist, and then partially melted. This domain contains a few small bodies of amphibolite, granodiorite, and diamictite. The Bear Island domain is characterized by garnet- to sillimanite-grade migmatitic metagraywacke and schist (similar to that seen at the type locality of the Mather Gorge Formation), ultramafi c rocks, granodiorite, pegmatite, and lamprophyre dikes.


The Blockhouse Point domain is characterized by chlorite-sericite phyllonites and map-scale blocks of ultramafi c rocks.

Sykesville Formation

The Sykesville Formation (Hopson, 1964) crops out between the Rock Creek shear zone and the Plummers Island fault. The Sykesville Formation consists of diamictite, which is a sedimentary mélange that contains clasts of amphibolite, mig-matite, schist, metagraywacke, granofels, phyllonite, metabasalt, and vein quartz supported in a matrix of quartz-feldspar-musco-vite granofels and schist (Muller et al., 1989; Drake and Froelich, 1997). Clasts within the diamictite were deformed and metamor-phosed prior to deposition, and presumably were derived from the Mather Gorge Formation (Drake, 1989; Muller et al., 1989). Large map-scale olistoliths of metagraywacke, schist, migmatite, phyllonite, and ultramafi c rocks are surrounded by the diamic-tite (Drake, 1989; Fleming et al., 1994). The Sykesville Forma-tion structurally underlies rocks of the Mather Gorge Formation beneath the Plummers Island fault, such as along the Potomac River (Drake, 1989) (Fig. 2), but the diamictite may stratigraphi-cally overlie them to the north (Muller et al., 1989).

Laurel Formation

The Laurel Formation (Hopson, 1964) is a sedimentary mélange (diamictite) consisting of a quartzofeldspathic matrix supporting clasts of vein quartz, meta-arenite, biotite schist, actinolite schist, and local amphibolite (Fleming et al., 1994). The diamictite contains a greater number and variety of olisto-liths than the Sykesville and it is restricted to the east side of the Rock Creek shear zone (Drake, 1998).

Ordovician and Devonian Intrusive Rocks

Ordovician intrusive rocks include the Bear Island Grano-diorite, the Georgetown Intrusive Suite, the Dalecarlia Intrusive Suite, and the Kensington Tonalite (Cloos and Cooke, 1953; Drake and Fleming, 1994; Fleming et al., 1994; Aleinikoff et al., 2002). The intrusive suites contain many xenoliths of ultra-mafi c, mafi c, and sedimentary rocks. Sensitive high-resolution ion microprobe (SHRIMP) U-Pb zircon ages (Aleinikoff et al., 2002) of muscovite trondhjemite of the Dalecarlia Intrusive Suite are 478 ± 6 Ma, 472 ± 4 Ma for biotite-hornblende tonalite of the Georgetown Intrusive Suite, and 463 ± 8 Ma for Kensington Tonalite. The Bear Island Granodiorite forms sheets, sills, and dikes in migmatite and phyllonite of the Mather Gorge Forma-tion from the Plummers Island fault west to Bear Island. The Georgetown Intrusive Suite (biotite-hornblende tonalite, quartz gabbro, and meta-pyroxenite), the Dalecarlia Intrusive Suite (biotite monzogranite and granodiorite, leucocratic monzogran-ite, and muscovite trondhjemite), and the Kensington Tonalite intruded rocks of the Sykesville Formation. Nonfoliated thin sheets of Devonian Guilford Granite (Cloos and Broedel, 1940)

and pegmatite crosscut bedding and schistosity in the Sykesville and Laurel formations east of the Rock Creek shear zone. Non-foliated Devonian lamprophyre dikes intruded the Mather Gorge Formation at Great Falls.

Westminster Terrane

The Westminster terrane consists of deepwater deposits of an accretionary wedge on the continental margin and slope and rise. The rocks are interpreted to be Neoproterozoic to Early Cam-brian that were metamorphosed at greenschist facies, assembled as thrust sheets and fi nally folded and contractionally inverted during Paleozoic orogenesis (Jonas and Stose, 1938a; Cloos and Broedel, 1940; Cloos and Cooke, 1953; Hopson, 1964; Fisher, 1978; Edwards, 1986, 1988, 1994; Southworth et al., 2002).

The Marburg Formation (Drake, 1994) (Marburg Schist of Jonas and Stose, 1938b; Wissahickon Formation of Fisher, 1978; and Gillis Group of Edwards, 1986, 1994), consists of phyllite and metasiltstone containing small bodies of metagraywacke, metab-asalt, and quartzite. Quartz-sericite-chlorite phyllite contains chloritoid and albite porphyroblasts, and chlorite-albite metasilt-stone contains thin ribbons and laminae of quartz. The Ijamsville Phyllite (Jonas and Stose, 1938b) consists of phyllite, phyllonite, and slate and locally contains bodies of metabasalt, quartzite, conglomeratic metagraywacke, metalimestone, and marble. The Sams Creek Formation (Jonas and Stose, 1938b; Stose and Stose, 1946; Fisher, 1978) consists mostly of metabasalt and lesser fel-sic schist, phyllite, metasiltstone, metasandstone, quartzite, met-alimestone, and marble. The metabasalt is chemically identical to tholeiitic metabasalt of the Neoproterozoic Catoctin Forma-tion found to the west in the Blue Ridge–South Mountain anti-clinorium (Southworth, 1999). Massive to thin-bedded calcitic and dolomitic marble and metalimestone occur as linear bodies as much as 1 km long and 0.5 km wide in metabasalt, quartzite, and phyllite. The Wakefi eld Marble (Jonas and Stose, 1938b) is a calcitic to dolomitic marble stratigraphically above metabasalt of the Sams Creek Formation (Fisher, 1978).

Sugarloaf Mountain Anticlinorium and Frederick Valley Synclinorium

Structurally beneath the Martic thrust sheet are clastic metasedimentary rocks of the Sugarloaf Mountain Quartzite and Urbana Formation, which are interpreted to be Lower Cam-brian continental margin deposits exposed in at least two tectonic windows (Southworth, 1996). Structurally beneath the western leading edge of the Martic thrust sheet are Cambrian (Araby and Frederick formations) and Ordovician (Grove Limestone) clastic and carbonate rocks of the Frederick Valley synclinorium.

Sugarloaf Mountain Quartzite and Urbana Formation

The Sugarloaf Mountain Quartzite (Jonas and Stose, 1938b) is a medium- to coarse-grained quartzite (characterized by


crossbeds, graded beds, and ripple marks) that is interbedded with laminated metasiltstone. The Urbana Formation (Edwards, 1986) (Urbana Phyllite of Jonas and Stose, 1938b) conform-ably overlies the Sugarloaf Mountain Quartzite and consists of quartzite, calcareous metasandstone, metagraywacke, conglom-eratic quartzite, metasiltstone, and lenses of schistose to massive, sandy, and chloritic marble.

Araby, Cash Smith, Frederick Formations, and Grove Limestone

The Frederick Valley synclinorium is underlain by Cam-brian and Ordovician sedimentary rocks of the Araby, Cash Smith, Frederick formations, and Grove Limestone. The Lower and Middle Cambrian Araby Formation (Reinhardt, 1974, 1977) consists of argillaceous, burrow-mottled sandy metasiltstone and phyllitic metashale. The Middle and Upper Cambrian Cash Smith Formation (Edwards, 1988) consists of black metashale that grades up into thin-bedded, calcareous metashale with limestone nodules. The Upper Cambrian Fred-erick Formation (Southworth and Brezinski, 2003) (Frederick Limestone of Stose and Stose, 1946; Reinhardt, 1974, 1977) is a thick interval of thin- to medium-bedded limestone and dolostone, containing thin intervals of shale, sandstone, and polymictic breccia. The Upper Cambrian and Lower Ordovi-cian Grove Formation (Stose and Jonas, 1935; Jonas and Stose, 1938b; Reinhardt, 1974) consists of crossbedded, dolomitic, sandy limestone and sandstone overlain by medium- to thick-bedded limestone and dolostone.

Culpeper Basin

Triassic and Jurassic sedimentary and Jurassic volcanic rocks unconformably overlie rocks of the Blue Ridge and west-ern Piedmont in a large half graben bounded on the west by the east-dipping Bull Run fault. The Culpeper basin was continuous with the Gettysburg basin to the north. The west-dipping strata are thickest and contain conglomerates on the western margin because the Bull Run normal fault was active during sedimenta-tion. The normal fault dips moderately to the east and truncates rocks as young as Early Jurassic. The minimal displacement is equal to the greatest thickness of basin strata, or ~10 km.

Blue Ridge–South Mountain Anticlinorium

The Blue Ridge–South Mountain anticlinorium is a large, west-verging, complex fault- bend fold that has Mesoproterozoic paragneisses, granitic gneisses, and a swarm of Neoproterozoic metadiabase dikes in its core. Deposited unconformably on the Mesoproterozoic rocks are Neoproterozoic clastic sedimentary rocks and basalt that are overlain by Early Cambrian clastic rocks. This cover rock sequence mostly underlies three ridges that are the three limbs of the Blue Ridge–South Mountain anti-clinorium, which are (from east to west) (1) Catoctin Mountain,

(2) the Black Oak Ridge–Short Hill Mountain-South Mountain, and (3) the Blue Ridge–Elk Ridge.

Mesoproterozoic Rocks

Mesoproterozoic paragneisses occur as xenoliths within granitic gneiss and consist of layered garnet-graphite schist, hornblende-orthopyroxene-plagioclase-amphibolite gneiss, metanorite, metadiorite, hornblende-biotite gneiss, quartzite, quartz tectonite, and carbonaceous phyllonite. Plutonism, meta-morphism, and deformation in the Blue Ridge spanned a period from ca. 1153–1000 Ma (Aleinikoff et al., 2000). SHRIMP U-Pb zircon ages of Mesoproterozoic metaigneous rocks defi ne at least four groups of orthogneisses (Aleinikoff et al., 2000). Group 1 range in age from 1153–1140 Ma, group 2 is ca. 1111.5 Ma, group 3 is ca. 1077 Ma, and group 4 ranges in age from 1060 to 1055 Ma. Layered granitic gneiss, hornblende monzonite gneiss, porphyroblastic metagranite, and coarse-grained metagranite comprise the oldest group, and white leucocratic metagranite, pink leucocratic metagranite, biotite granite gneiss, and megac-rystic metagranite comprise the youngest group.

Neoproterozoic Swift Run Formation

Metasandstone, schist, phyllite, and marble of the Swift Run Formation (Stose and Stose, 1946) unconformably overlie the Mesoproterozoic gneisses. The clastic rocks are crossbedded and fi ne upward from the metasandstone at its base which also contains pebbles and cobbles of quartz, phyllite, and ferruginous sandstone. Quartz-sericite schist and sandy, sericitic phyllite locally contain podiform bodies of calcitic to dolomitic marble.

Neoproterozoic Metadiabase and Metarhyolite Dikes

A swarm of northeast-trending metadiabase and minor metar-hyolite dikes intrude the Mesoproterozoic rocks (Southworth et al., 2006). The metadiabase is fi ne-grained, coarse-grained, and porphyritic. Metarhyolite is fi ne-grained to porphyritic with phe-nocrysts of plagioclase in a matrix of microcrystalline quartz and potassium feldspar. A metarhyolite dike intrudes metabasalt of the lower part of the Catoctin Formation (Fauth and Brezinski, 1994). U-Pb TIMS (thermal ionization mass spectrometer) anal-yses of zircons from another metarhyolite dike yielded an age of 571.5 ± 5 Ma (Aleinikoff et al., 1995).

Neoproterozoic Catoctin Formation

The Catoctin Formation (Keith, 1894) is characterized by green aphanitic, massive to schistose, amygdaloidal metaba-salt and chlorite schist. Locally at or near the base are bodies of white calcitic marble. Interbedded with the metabasalt are thin, discontinuous, fi nely laminated, phyllitic metasiltstone and thin metasandstone; dark, variegated, vesicular tuffaceous phyl-lite; and quartz-muscovite schist. Extrusive metarhyolite of the


Catoctin Formation in Pennsylvania has U-Pb ages that range from 597 ± 18 Ma (Aleinikoff et al., 1995) to 560 Ma (J.N. Ale-inikoff, 2005, unpublished data).

Early Cambrian Chilhowee Group

The Chilhowee Group (Safford, 1856) consists of the Lou-doun, Weverton, Harpers, and Antietam Formations that uncon-formably overlie rocks of the Catoctin Formation. The basal Loudoun Formation (Keith, 1894; Stose and Stose, 1946) is a discontinuous sequence of phyllite and minor quartz-pebble con-glomerate and pebbly metasandstone that occurs locally between the Catoctin and Weverton Formations. The Weverton Formation (Keith, 1894) consists of coarse, clastic rocks (Brezinski, 1992). The overlying Harpers Formation (Keith, 1894) is predominantly a dark-gray, foliated, quartz-laminated metasiltstone that transi-tions upward into the Antietam Formation (Keith, 1894), which consists of metasandstone that contains Skolithos lineariz and trilobites (Walcott, 1896). A lustrous, gray, laminated, unnamed carbonaceous phyllite may be found between rocks of the Antie-tam Formation and Tomstown in Virginia.

Great Valley Section of the Valley and Ridge Province

The Great Valley section of the Valley and Ridge province is underlain mostly by Cambrian and Ordovician carbonate rocks of the Massanutten synclinorium. The western boundary of the Great Valley is the North Mountain thrust fault. The Paleozoic rocks in the area of the fi eld trip consist of the Tomstown Forma-tion, the Waynesboro Formation, and the Elbrook Limestone (Stose, 1906; Brezinski, 1992).

METAMORPHISM, GEOCHRONOLOGY, AND STRUCTURE

Introduction

The rocks along the Potomac River corridor were deformed and metamorphosed at different times, at different grades, and under different conditions, and subsequently were juxtaposed and transported westward during Paleozoic orogenesis. Major faults not only separate distinct bedrock units but subsequent reactiva-tion along and across some of the faults created shear zones that are as much as 13 km wide. The foliations in the rocks include bedding, cleavage, and schistosity (S

1), overprinted by phyllonitic

and mylonitic foliations, and spaced crenulation cleavage (S2 and

S3). This fi eld trip examines foliations that have different charac-

ters and ages (Gradstein et al., 2004) across the region.

Metamorphism and Geochronology

In the Mesoproterozoic rocks of the Blue Ridge, granulite-facies mineral assemblages of orthopyroxene and microcline (in charnockitic granite) and orthopyroxene and brown hornblende

(in amphibolite) have granoblastic texture and triple-junction grain boundaries. Monazite ages (Aleinikoff et al., 2000) and 40Ar/39Ar hornblende-cooling ages of 1000–920 Ma (Kunk and Burton, 1999) indicate that no regional thermal events hotter than 720 °C and 550 °C, respectively, affected the rocks since the Mesoproterozoic. In general, Paleozoic metamorphic grade is highest (sillimanite) in the east and lowest (chlorite-sericite) in the west, but it is not a simple Barrovian sequence related to one event. The metasedimentary rocks in the Potomac terrane expe-rienced at least two prograde amphibolite-facies events, and sev-eral retrograde and secondary prograde greenschist-facies events (Fleming and Drake, 1998). In the sillimanite-grade zone (Fisher, 1970; Drake, 1989), migmatization may have predated the Middle Ordovician (Becker et al., 1993; Kunk et al., 2005) as indicated by amphibole cooling ages of ca. 475 to ca. 455 Ma (Jäeger, 1979) that could refl ect growth ages of 490 Ma (Becker et al., 1993). The metamorphism was complete by the Late Devonian as indicated by the ages of the nonmetamorphosed and undeformed Guilford Granite, and by the cooling ages of amphibole (405, 404, 401, and 398 Ma) (Kunk et al., 2005). Muscovite cooling and growth ages related to retrograde deformation are complex from east to west. Muscovite cooling ages of S

1 range from 422 to 411 Ma and 375

to 354 Ma and S2 growth ages range from 385 to 373 Ma and 350

to 328 Ma (Becker et al., 1993; Kunk et al., 2005). Deformation in the Bear Island domain was complete by 363 ± 13 Ma and 360 ± 13 Ma (2 σ), which are the cooling ages of two biotite samples from an undeformed lamprophyre dike (Reed et al., 1970). These thermal events are also recorded in rocks near Baltimore which yielded SHRIMP U-Pb metamorphic ages for titanite of 374 ± 8, 336 ± 8, and 301 ± 12 Ma (Aleinikoff et al., 2004).

Cooling curves, plots, and cross sections summarize the complex thermal history of the rocks (Fig. 3) (Kunk et al., 2005). The ages and closure temperatures of amphibole, S

1 and

S2 muscovite, and zircon fi ssion-tracks show discontinuities

along faults in the Piedmont. Plots of the age of igneous zircons that crystallized at 700 °C (Aleinikoff et al., 2002) and the age and closure temperatures of amphibole, muscovite, and zircon fi ssion-track yield a convex cooling curve (Kunk et al., 2005). The rocks west of the Plummers Island fault yield a concave cooling curve typical of many metamorphic terranes (Kunk et al., 2005). Extrapolating back in time on the curve, migmatitic conditions of 650–700 °C occurred in the Early Ordovician, and the rocks cooled continuously from the Middle Ordovician to greenschist-facies conditions by the Late Silurian. At this time, the rocks to the east were at upper amphibolite facies and ~3–6 km deeper in the crust than at present. The cooling rate of these rocks (Sykesville) exceeded those to the west (Mather Gorge) from the Middle Silurian through the Pennsylvanian, and at any given time, the Sykesville rocks were hotter. The cooling curves converge between 300 and 280 Ma, when the rocks were uplifted and transported westward to higher crustal levels above the North Mountain thrust fault in the Permian.

The rocks of the Westminster terrane and Blue Ridge have had a complex Paleozoic metamorphic history under


WEST LONGITUDE77.0577.177.1577.277.2577.3

Plu

mm

ers

Is

land

faul

t

M a t h e r G o r g e C o m p l e x Sykesville Fm. and Ord. Plutonic rocks

Zircongrowth

Zirconovergrowth

300

500

400

Laurel Fm.

Roc

k C

reek

shea

r zon

e

AG

E (

Ma)

Range ofamphibole cooling

S1S2

S2

S1

S1

TaconianA

cadianA

lleghanian

Migmatitic

Retrogrademuscovite

growth

Range ofmuscovite

cooling

Range ofamphibole

cooling

Ran

ge o

f

ZFT

ages

Bloc

khou

se fa

ult

Block-housePoint

domain

BearIsland

domain Retrograde muscovite growth

Muscovitecooling

S2

Zirconovergrowth

Biotitecooling

Stubble-fieldFalls

domain

Muscovite.cooling

S2

Resetmigmatitic

schistBiotitegrade

Ple

asan

t Gro

vefa

ult z

one

S2

S1 S1?

S2?

Chloritegrade

Chloritegrade

Marburg West

Marburg East

AExplanat ion

of ages

Rb-Sr mu cool.

Mu growthZFT;

Amph cooling

Mu cooling

Biot Z gr ; ovgr

Parrs

Rid

ge fa

ult

AG

E (

Ma)

300

500

400

North Mountain fault

Pleasant Groveshear zone


Rock Creekshear zone

B

Pleasant Grove fault

Parrs Ridge fault

Hyattstown fault

Barnesville-

Martic fault

Monrovia fault

Zircon Fission Track Cooling Age

Corresponds to Diagram Above

Figure 3. Ages of metamorphic cooling and deformation events. (A) Diagram plotting age against sample location along an east-to-west (right to left) transect for parts of the Potomac and Westminster terranes (modifi ed from Kunk et al., 2005). ZFT—zircon fi ssion track; Amph—amphi-bole; mu—muscovite; biot—biotite; Ord—Ordovician; Fm—Formation. (B) Regional schematic cross section showing the ages of metamorphic cooling and deformation events (black bars).


greenschist-facies conditions. They contain varying proportions of muscovite, chlorite, paragonite, quartz, magnetite, calcite, actinolite, epidote, stilpnomelane, and sericite. Retrograde phyl-lonitic rocks contain multiple generations of white mica (Kunk and Burton, 1999) and chlorite (Knopf, 1931). Chloritoid and albite crystals overgrew foliation in the eastern Westminster ter-rane. The rocks of the Sugarloaf Mountain anticlinorium, Fred-erick Valley synclinorium, and Great Valley contain only sparse sericite and chlorite. Locally along faults in the Great Valley, limestone was recrystallized to marble (Brezinski et al., 1996; Campbell and Anderson, 1996), but most of the rocks here were not metamorphosed. Potassium-rich bentonites in the Middle Ordovician Martinsburg Formation in the Massanutten synclino-rium contain 282 Ma illite that is related to burial during Allegha-nian thrusting (Meyer et al., 2005).

Muscovite yielded the following S2 growth ages: 435–

430 Ma and 382–375 Ma for Marburg Formation (Mulvey, 2003), 441 Ma for the Sams Creek Formation, and 431 Ma for Ijamsville Phyllite. Muscovite grew fi rst in the lower thrust sheets (western area) and later in the higher thrust sheets (eastern area) during a Late Devonian reactivation of the Pleasant Grove shear zone. Muscovite ages of 439 and 437 Ma and 380–360 Ma across the Martic fault suggest that the Martic was emplaced and later accreted onto the Frederick Valley synclinorium.

In the Blue Ridge, 40Ar/39Ar dating of muscovite yielded cooling ages of 430 Ma (Harpers Formation), 370 Ma (Loudoun Formation); 357–334 Ma, 347–337 Ma, and 338–313 Ma (Swift Run Formation); and 340–315 Ma (Catoctin Formation). Phlog-opite from marble (Catoctin Formation) yielded 409 and 405 Ma (Kunk and Burton, 1999).

Fission Track

Twenty-nine samples collected between the eastern Pied-mont and the Appalachian Plateau yielded 19 apatite ages and 21 zircon ages. The degree of fi ssion-track annealing in zircon varies from east to west (Fig. 4), ranging from total annealing in the Piedmont and parts of the Blue Ridge provinces to partial and possibly no annealing in the Valley and Ridge and Appalachian Plateaus provinces. In the Valley and Ridge and Appalachian Pla-teaus, zircon fi ssion-track ages are older than the depositional age of the rocks. In contrast, zircon ages from the metamorphic and Paleozoic sedimentary rocks from the Blue Ridge and Piedmont are all signifi cantly younger than the depositional and metamor-phic ages. Zircons from Triassic sandstone are older than the depositional age and refl ect local sources for the detritus. Zir-con fi ssion-track ages in the eastern Piedmont were totally reset by burial and heating prior to cooling through the zircon closure temperature of ~235 °C at ca. 280 Ma, during the Alleghanian orogeny. Zircon ages show signifi cant scatter along the traverse, refl ecting different maximum temperatures of the rocks.

Apatite ages decrease from ca. 200–180 Ma at the eastern and western ends of the traverse to ca. 115 Ma near Harpers Ferry, West Virginia. The entire region was at temperatures in excess of

90–100 °C before the latest cooling through the apatite closure temperature in Early Jurassic to Early Cretaceous time.

Histograms of individual zircon fi ssion-track ages show the difference in thermal history across the region. No single-grain ages in excess of 800 Ma were observed in samples collected in the Blue Ridge and Piedmont. Grains with ages greater than 800 Ma are a signifi cant part of the zircon population in the Val-ley and Ridge and Appalachian Plateaus. This difference in ages shows up in the detrital zircon populations in outcrop and shal-lowly buried (<411 m) sands on the Atlantic Coastal Plain. Early Miocene and younger sands contain zircon grains older than 800 Ma, but late Oligocene and older sands do not. These data indicate that the proto-Potomac River breached the Blue Ridge and began transporting the eroded rocks in the Valley and Ridge during late Oligocene to Early Miocene time.

Structure

Foliations and FoldsFoliations defi ne at least three episodes of Mesoprotero-

zoic deformation in rocks of the Blue Ridge province (Burton et al., 1995; Aleinikoff et al., 2000; Burton and Southworth, 2004). Northwest-trending foliation (D

1), which is locally par-

allel to unit contacts, is cut by a weaker foliation with mineral lineations and southeast-plunging sheath folds (D

2), which were

later deformed into broad, northwest-trending folds (D3). D

1 is

between 1145 Ma and 1077 Ma. D2 and D

3 occurred after the

latest intrusion at 1055 Ma.Within the Piedmont province, the Potomac and Westmin-

ster terrane rocks contain composite foliations that mostly strike northeasterly. The dominant foliation in the Potomac terrane is S

2, which forms a fan structure along the Potomac River (Drake,

1989); to the west it dips gently eastward and to the east it dips steeply westward. The foliations in the Westminster rocks dip steeply southeast. Shear zones across the region resulted in transposition, phyllonitic, and mylonitic foliations (Fleming and Drake, 1998; Southworth et al., 2002). Transposition foliation containing folded vein quartz is structurally aligned parallel to the faults. Local shear-band cleavage (S

3) is related to faulting.

In the Potomac and Westminster terranes, isoclinal F1 folds

were refolded by northerly-trending upright to steep F2 folds

(Jonas, 1937; Fisher, 1978; Drake, 1989). The F1 isoclinal folds

of bedding contain axial plane S1 schistosity and the F

2 folds have

S2 axial- planar cleavage. Adjacent to faults are steep, tubular

sheath folds and antiforms of foliation that have vertical S2 axial-

planar crenulation cleavage (Southworth et al., 2002).From the western Piedmont west across the Great Valley

section of the Valley and Ridge province is an F1 fold train with

associated axial-planar cleavage. The Sugarloaf Mountain anti-clinorium (Scotford, 1951; Thomas, 1952) and Frederick Valley synclinorium doubly plunge, are overturned to the northwest, and contain northeast-striking, southeast-dipping S

1 axial-planar

cleavage. The rocks are characterized by a conspicuous bedding and cleavage intersection lineation that gently plunges to the


Age

(M

a)

0

100

200

300

400

500

600

700

800

Zircon

Apatite (Metamorphic)

Metamorphic & Mesozoic RocksPaleozoic Sedimentary Rocks

NW SEBLUE RIDGE & PIEDMONTVALLEY & RIDGE and PLATEAU

Apatite

Apatite (Mesozoic)

Zircon (Metamorphic)

Zircon (Mesozoic)

A

Fre

quen

cy

0123456789

1011121314151617181920

0 400 800 1,200 1,600 2,000

Single-crystal age (Ma)

Comparative Zircon Fission-Track Rock Ages

Blue Ridge and Piedmont

Valley and Ridge and Plateau

B

Figure 4. Fission track data from the Potomac River corridor between the eastern Piedmont, Rock Creek Park, Washington, D.C., (right) and the Appalachian Plateau near Frostburg, Maryland (left). (A) Apatite and zircon fi ssion-track ages. (B) Histograms of zircon single-grain fi ssion-track ages of rocks in the Blue Ridge and Piedmont Provinces (black outline only) and Valley and Ridge and Plateau Provinces (gray fi ll).


north or south. The Blue Ridge–South Mountain anticlinorium is a broad, asymmetrical west-verging and gently north-plunging fold complex, with gently plunging folds on the homoclinal east limb, and complexly folded rocks on the overturned west limb. A regional penetrative northeast-striking, southeast-dipping S

1

cleavage (South Mountain cleavage of Cloos, 1951; Mitra and Elliott, 1980) is axial planar to it, and locally there are F

2 folds

and associated S2 crenulation cleavage. To the west, the Massa-

nutten synclinorium consists of tight, upright and inclined, sec-ond-, third-, and fourth-order disharmonic folds that verge up the limbs. A pressure-solution S

1 axial-planar cleavage is best devel-

oped in the shale of the Martinsburg Formation near the axis, and it fans about the mesoscopic folds.

Faults

The Rock Creek fault (Fleming and Drake, 1998) separates rocks of the Laurel Formation, Loch Raven Schist, and North-west Branch Formation on the east from Sykesville Formation and suites of intrusive rocks on the west. The steeply dipping fault strikes north for 25 km, and was reactivated as the 3-km-wide Rock Creek shear zone (Fleming and Drake, 1998). Sinis-tral motion occurred under middle-amphibolite-facies conditions during intrusion of the Ordovician Kensington Tonalite at 463 ± 8 Ma (Aleinikoff et al., 2002). Dextral reactivation occurred under greenschist-facies conditions (Fleming and Drake, 1998) from 354 to 340 Ma. Post-Cretaceous brittle thrust faults locally cut the shear zone.

The Plummers Island fault separates rocks of the Mather Gorge Formation from Sykesville diamictite to the east (Drake, 1989). Rocks of the Mather Gorge Formation are structurally above diamictite of the Sykesville, and the kinematic indicators of the foliated rocks are equivocal (Schoenborn, 2001, 2002). The fault was reactivated to form a complex Plummers Island shear zone that is as much as 6 km wide. The units on both sides of the fault were sheared at different times during reactivation (Kunk et al., 2005). The western part of the shear zone contains magmatic breccia overprinted by foliations containing retrograde musco-vite. The eastern part of the shear zone was reactivated from 332 to 304 Ma, with the youngest motion along the fault.

The Blockhouse fault was recognized by an S1 muscovite age

discontinuity of 60 m.y. (Kunk et al., 2004). A Devonian shear zone between this fault and the Pleasant Grove fault is character-ized by complex aeromagnetic patterns (Southworth et al., 2002) and discontinuous blocks of ultramafi c rocks (Drake, 1994).

The Pleasant Grove fault is the Taconian suture that placed rocks of the Potomac terrane onto rocks of the Westminster ter-rane. The fault is more than 60 km long and as much as 3 km wide. The Pleasant Grove shear zone (Krol et al., 1999)—also referred to as the Pleasant Grove Formation and zone (Muller et al., 1989)—contains dextral strike slip indicators (Krol et al., 1999; Gates et al., 1999). The rocks on either side of the fault are similar and appear to have experienced the same structural and metamorphic history. Muscovite from the Potomac terrane along

the fault yielded a ca. 371 Ma S1 cooling age and S

2 growth age of

362 Ma (Kunk et al., 2005). Muscovite from the shear zone 30–60 km north of the Potomac River indicates that cooling occurred between 364 and 308 Ma, with white mica growth during dextral shearing occurred at 311.4 ± 3.2 Ma (Krol et al., 1999).

The Westminster terrane, from east to west (structurally high-est to lowest), is comprised of a family of thrust faults. Rocks of the Marburg Formation are bounded by the Pleasant Grove fault (roof) and the Hyattstown fault (fl oor); the intraformational Parrs Ridge fault marks a metamorphic discontinuity (Mulvey, 2003; Kunk et al., 2004). The Hyattstown fault thrust rocks of the Mar-burg Formation onto rocks of the Sams Creek Formation, Wake-fi eld Marble, and Silver Run Limestone. The Barnesville-Monro-via fault thrust rocks of the Sams Creek Formation onto Ijamsville Phyllite. Ijamsville Phyllite is thrust onto Cambrian rocks of the Urbana, Araby, and Frederick Formations along the Martic fault.

The Short Hill–South Mountain fault extends 60 km from a shear zone in Mesoproterozoic rocks in the core of the Blue Ridge–South Mountain anticlinorium north to Rohrersville, Mary-land, where limestone of the Middle and Upper Cambrian Elbrook Formation structurally overlies metabasalt of the Neoproterozoic Catoctin Formation. The fault is nearly horizontal; it was folded and transected by cleavage. It is a normal fault that formed after the Late Cambrian and before the Devonian contractional reacti-vation (Southworth and Brezinski, 1996a; Southworth, 2005).

The North Mountain thrust fault transported all of the rocks above and southeast of it onto the rocks of the Valley and Ridge province. Cambrian and Ordovician rocks thrust onto Silurian and Devonian rocks are exposed at the surface within a 0.5-km-wide zone of horse blocks (Orndorff and Epstein, 1994; Orndorff et al., 1999). The fault duplicates the Cambrian and Ordovician rocks that underlie the Great Valley. A displacement estimate of 60 km (Evans, 1989) was based on the position of the footwall ramp beneath the east limb of the Blue Ridge–South Mountain anticlinorium. If the allochthonous Blue Ridge–South Mountain anticlinorium is part of the North Mountain sheet, the total dis-placement was at least 107 km (Evans, 1989). Palinspastic resto-ration placed the Blue Ridge footwall ramp ~10 km east of Rock Creek Park, and the North Mountain footwall ramp beneath Great Falls Park (Evans, 1989).

TECTONIC ASSEMBLAGE

Introduction

The Mesoproterozoic Grenvillian orogeny was followed by several hundred million years of extension that resulted in the opening of the Iapetus Ocean in the early Cambrian. Synde-positional normal faults (Espenshade, 1986; Kline et al., 1991) that juxtaposed Neoproterozoic cover rocks against Mesoprotero-zoic gneiss and the metadiabase dike swarm suggest that signifi -cant extension of the crust occurred during rifting and associated volcanism. The rift-to-drift stage was followed by establishment of a passive continental margin from the Early Cambrian to the


Early Ordovician, when contractional tectonics associated with attempts to close the Iapetus Ocean began (Fig. 5).

Ordovician

By the Early to Middle Ordovician (480–460 Ma), rocks of the Potomac terrane were partly melted and were cooling through ~500 °C. The rocks to the east were at upper amphibolite facies and were being intruded by tonalite (463 Ma) within a sinistral shear zone. The Potomac terrane rocks accreted westward onto the Westminster terrane at this time.

Silurian

By the Early Silurian (441–430 Ma), rocks in the western Westminster terrane (and perhaps the Blue Ridge) were meta-morphosed to lower greenschist facies during S

1 cleavage devel-

opment. The Martic thrust sheet was emplaced on the Urbana Formation. In the Late Silurian, migmatitic rocks of the Potomac terrane had cooled to greenschist-facies conditions. To the east, the Sykesville Formation was at upper amphibolite facies, having suffi cient heat for zircon overgrowths (439 Ma).

Devonian to Pennsylvanian

In the eastern Potomac terrane, S1 cleavage formed in the

Sykesville (357–340 Ma), zircon overgrowths developed in the plutonic rocks (386 Ma), and the Rock Creek shear zone experi-enced dextral strike-slip motion (Fig. 5). In the western Potomac terrane, S

2 developed between 385 and 373 Ma, at ca. 361 Ma,

and between 350 and 328 Ma, about the same time that titanite was growing in rocks to the north near Baltimore, Maryland, (374 and 336 Ma). S

1 cleavage developed in the rocks of the

Frederick Valley synclinorium and Blue Ridge–South Mountain anticlinorium from 380 to 350 Ma, and at 340 Ma in the east-ern Westminster terrane. Titanite crystals formed in the eastern Piedmont at 336 Ma; Sykesville Formation rocks were sheared and developed muscovite between 332 and 304 Ma. Blue Ridge developed cleavage between 347 and 313 Ma, while parts of the Rock Creek, Plummers Island, and Pleasant Grove shear zones were reactivated. The latest events recorded occurred between 304 and 301 Ma with the growth of titanite in rocks near Balti-more and of muscovite along several shear zones.

Permian

By ca. 280 Ma, regional cooling through 235 °C resulted from unroofi ng associated with emplacement of the rocks along the North Mountain thrust fault

Post-Alleghanian

The collapse of the orogenic highlands was followed by continental extension, resulting in half grabens fi lled by

terrestrial sediments. Further extension produced intrusive and extrusive mafi c igneous rocks. Normal faulting continued after emplacement of the Early Jurassic rocks at 200 Ma. Apatite fi s-sion-track data that indicate cooling through ~90–100 °C in the Early Jurassic to Early Cretaceous, are slightly younger to the west, suggesting either kilometer-scale uplift in the Cretaceous and (or) Tertiary or different thermal gradients. This uplift was probably the combined result of motion along the exten-sional Bull Run fault, and loading of Cretaceous and Tertiary sediments in the Atlantic Coastal Plain and the shelf/slope/rise (Pazzaglia and Brandon, 1996).

Summary

The rocks of the Piedmont and Blue Ridge provinces along the Potomac River corridor record a long complex Paleozoic tec-tonic history (Fig. 6). Rocks of the Potomac terrane in the eastern Piedmont preserve Ordovician or earlier amphibolite-facies meta-morphism, deformation, and synkinematic plutonism. They were thrust onto the Westminster terrane during the Taconian orog-eny, but the suture was reactivated and dismembered in the Late Devonian to form a 13-km-wide shear zone. Greenschist facies metamorphism and deformation progressed westward into rocks of the Westminster terrane, and perhaps rocks of the Blue Ridge, in the Silurian. The Silurian thrust faults were reactivated in the Carboniferous. All of the rocks saw major deformation and meta-morphism from the Devonian through the Mississippian (ca. 380–340 Ma), followed by emplacement of the North Mountain thrust sheet in the Permian. The deformation and regional development of cleavage generally was younger to the west, as the North Moun-tain thrust system propagated from the east, but many of the faults were out of sequence. Although the Rock Creek and Pleasant Grove faults were active in the Taconian, the Westminster terrane thrust faults were active in the Silurian, and the Plummers Island and Pleasant Grove shear zones were active in the Neoacadian, all of the faults were contractionally reactivated in the Carboniferous during Alleghanian deformation. The Piedmont rocks were not passively transported west above the North Mountain thrust fault, but they were undergoing deformation with some dextral strike-slip fault displacement. The regional west to east progression of greenschist- to amphibolite-facies metamorphism, the apparent Barrovian metamorphic sequence of chlorite to sillimanite grade eastward in the Potomac terrane, and the foliation fans (Potomac and South Mountain), are a composite of features that resulted from different tectonic events over a period of at least 180 m.y., making previous stratigraphic correlations questionable.

ROAD LOG AND STOP DESCRIPTIONS

This trip transects the Central Appalachian Piedmont and Blue Ridge provinces in the central Appalachians. Day 1 and part of Day 2 examines rocks thought to be the Wissahickon Forma-tion by previous workers. The type locality of the Wissahickon is Wissahickon Creek, near where our trip begins in Philadelphia,


Pennsylvania. Day 1 is devoted to rocks of the Potomac terrane in the eastern Piedmont that constitute the lithotectonic unit of Neoproterozoic to Ordovician multiply tectonized accretionary complex with magmatism (Hibbard et al., 2005) (Figs. 1 and 2).

Stops Geology

1-1 and 1-2 Along the Rock Creek shear zone in Rock Creek Park, Washington, D.C. (Fig. 7).

1-3 Sillimanite-grade rocks retrograded to chlorite grade in the Stubblefi eld Falls domain in the Plummers Island fault zone.

1-4 Lunch in Great Falls Park on kyanite-grade rocks of the western Bear Island domain.

1-5 Sillimanite-grade rocks of the easternmost Bear Island domain.

1-6 Biotite-grade rocks of the Blockhouse Point domain, the westernmost part of the Potomac terrane, within the Pleasant Grove shear zone.

Day 2 will traverse the Westminster terrane of the western Piedmont into the western Blue Ridge (Figs. 1 and 2). These rocks constitute the lithotectonic units of Neoproterozoic to Ordo-vician Iapetus continental slope and rise facies (Stops 2-1 and

T A

A T

A T

North Mountainfault

North Mountainfault

Marticfault

Hyattstownfault

Pleasant Grovefault

PleasantGroveshearzone

Plummers IslandfaultWestminster

thrust system




PleasantGrove

shear zone

WESTMINSTER TERRANE POTOMAC TERRANE

WESTMINSTER TERRANE POTOMAC TERRANE

Blue Ridge

FrederickValley

IjamsvilleSams Creek

Marburg Mather GorgeSykesville-Laurel

plutons

oceanic crust

W E

Cambrian to Ordovician

Silurian

Devonian

Carboniferous

GREAT VALLEY BLUE RIDGE

BLUE RIDGE

A

B

C

D

Figure 5. Schematic illustration showing time slices of the tectonic evolution of the rocks of the Potomac River region. A is motion away from the observer and T is motion toward the observer.


2-2), passive margin sequence (Stops 2-2 and 2-6), drift facies (Stops 2-3 and 2-4) (Fig. 13), Mesoproterozoic Grenvillian base-ment gneiss of Laurentia (Stop 2-5) (Fig. 17), Neoproterozoic metasedimentary rocks (Stop 2-5), and metabasalt of Iapetus rift facies (Stop 2-6) (Fig. 18) (Hibbard et al., 2005).

Stops Geology

2-1 Hyattstown shear zone.2-2 Martic fault.2-3 Sugarloaf Mountain anticlinorium, a tectonic

window through the Martic thrust sheet.2-4 Lunch2-5 Folds on the west limb of the Blue Ridge-South

Mountain anticlinorium.2-6 Short Hill–South Mountain fault, the boundary

between the Blue Ridge and Great Valley prov-inces.

Buckle up for a 2.5 hour drive south!

DAY 1

Mileage (Cumulative)

0 Take I-95 south from Philadelphia, Pennsylvania, to I-495 west, to Connecticut Avenue exit (MD

185; right lane exit), turn left heading south on Connecticut Ave. (MD 185) for about one mile, turn left at the stop light at East-West Highway (EAST MD 410), and follow it about a mile to a right turn at the stop light onto Beach Drive.

2 Follow Beach Drive south into Rock Creek Park. Rock Creek Park was established in 1890 as the fi rst and largest urban National Park. Within or adjacent to Rock Creek Park are the Carter Barron Amphitheater, Meridian Hill Park, Mon-trose Park, Rock Creek and Potomac Parkway, Dumbarton Oaks Park; Fort Totten Park, Fort Stevens Park, Fort DeRussy, and Fort Reno Park of the Civil War Defenses of Washington; Battle-ground Cemetery, Soapstone Valley Park, Pine-hurst Parkway, Normanstone Parkway, the Old Stone House, Pierce Mill, Palisades Park–Bat-tery Kemble Park, Glover Archbold Park, White-haven Park, Melvin Hazen Park, Klingle Valley Park, and Piney Branch Parkway.

4 Restrooms are on the right (or at the Visitor Cen-ter off of Military Road).

7 Turn right at the stop sign onto Broad Branch Road.

8 Continue northwest on Broad Branch Road to the pull offs on the right side of the road.

Outcrops are within the small stream called Broad Branch.

Stop 1-1. Kensington Tonalite; Rock Creek Shear Zone; Eastern Potomac Terrane

Washington West, D.C., Maryland, and Virginia, 7.5 min quad-rangle (Fleming et al., 1994) (Fig. 7).

The Kensington Tonalite is a metamorphosed muscovite-biotite tonalite (Hopson, 1964; Fleming et al., 1994; Fleming and Drake, 1998) that intruded rocks of the Sykesville and Laurel Formations, as well as the Georgetown and Norbeck Intrusive Suites. The Kensington Tonalite yielded a weighted average of 19 SHRIMP analyses for an age of 463 ± 8 Ma (Aleinikoff et al., 2002); it contained an older inheritance age of 519 Ma. Most of the eastern half of the elongate pluton is protomylonitic with multiple subparallel foliations. Quartz and feldspar are sheared into lenses that are offset by shear-band foliations containing muscovite and biotite (±chlorite). At the contact with the Laurel Formation along the Rock Creek fault, it is an ultramylonite that resembles fi ne-grained feldspathic quartzite.

Geochronology helps constrain the timing of the complex structural history (Fleming and Drake, 1998).

1. North-trending, near-vertical, sinistral shear in the Lau-rel Formation was under prograde amphibolite-facies conditions during intrusion of the Kensington Tonalite at 463 Ma (Fig. 8B).

2. Regional middle-amphibolite facies (or higher) metamor-phism appears to have outlasted deformation. Hornblende

X

X

X

X

BlueRidge

NC Blue Ridge PiedmontPiedmontMD-DE

(Ma)

300

400

500Hbl

Hbl

Zr FT

Zr FT

Musc

Musc

Alleghanian

Acadian

Taconian

Zr

sphene

Figure 6. Summary of geochronology of the rocks in the Piedmont and Blue Ridge Provinces. For comparison, data from the Blue Ridge of North Carolina (Southworth et al., 2005) are shown on the left, and data from the Piedmont of Maryland and Delaware (Aleinikoff et al., 2002) are shown on the right.


0.5 km0

A'

A

Ok

Og

KTQ

Cs

CZu

Cl

{Rock Creek shear zone

Rock

Cre

ek

Roc

k C

reek

fau

lt

A A'Ok

Og Og

CZu

Cl

KT

KTQ Cretaceous, Tertiary and Quaternary strata

Ordovician

Ok Kensington Tonalite

Og Georgetown Intrusive Suite

Cambrian

Cs Sykesville Formation

Cl Laurel Formation

CZu Ultramafic rock

Stop 1-1

Stop 1-2

Stop 1-1

Stop 1-2

Explanation

Cl

Sea

leve

l

KTQ

KTQ

KTQ

Og

Og

KTQ

KTQ

Og

Cl

CZu

0 1 KM

0 1 KM

N

39050'

7703'

A

B

C

D

Figure 7. (A) Generalized geologic map and (B) cross section of Rock Creek Park (modifi ed from Fleming et al., 1994). Insets (C) and (D) are areas of the Wash-ington West 7.5 min quadrangle showing the loca-tions of Stops 1-1 and 1-2. Circled dots on the map are argon sample localities.


cooling ages of ca. 400 Ma (Kunk et al., 2005) across the Rock Creek fault, and zircon overgrowths of 439 ± 23 Ma and 386 ± 27 Ma (Aleinikoff et al., 2002) constrain the ages of the metamorphism.

3. A system of west and northwest-trending, west-dipping, ductile, strike-slip and oblique-slip faults indicate sinis-tral motion with the east side thrown upwards, and dex-tral shear under retrograde, lower- to middle-greenschist

conditions. An S1 muscovite cooling age of ca. 340 Ma

and S2 retrograde muscovite growth of ca. 335 Ma indi-

cates Mississippian deformation associated with the Alleghanian orogeny (Fig. 8A). Zircon fi ssion track ages of 270 Ma record late cooling related to emplacement of the North Mountain thrust fault.

4. Near the south end of the shear zone are northwest-trend-ing dextral slip faults and southwest-dipping thrust faults

hammer head

Roc

k C

reek

faul

t

West East

A B

C

Figure 8. Photographs of rocks along the Rock Creek shear zone. (A) Kensington Tonalite and boudin of vein quartz with dextral shear kinematics immediately west of the Rock Creek fault. Time of dextral shearing is constrained by muscovite cooling ages of 340–335 Ma (Stop 1-1). (B) Mylonitic Laurel Formation contains vein quartz and leucosome with sinistral shear immediately east of the Rock Creek fault. Sinistral shear was synkinematic with intrusion of the Kensington Tonalite at 463 Ma. (C) Laurel Formation diamictite immediately east of the Rock Creek fault, in the drainage south of Klingle Road (Stop 1-2).


that place metamorphic rocks on unconsolidated sedi-ments that are at least as young as middle Pleistocene (Fleming and Drake, 1998). These late faults are part of a regional fault system related to post-Cretaceous con-tractional deformation in a zone ~50 km wide across the eastern Piedmont and western Coastal Plain.

The Rock Creek shear zone exemplifi es inherited geologic structures in this region. North-trending faults in the broad shear zone that were active in the Ordovician were reactivated as north-west-trending, southwest-dipping faults in the Mississippian and again after the Cretaceous. The 20-km-long, north-south–oriented channel of the Potomac River from Rock Creek to Mount Vernon may be structurally aligned with the Rock Creek shear zone.


8 Return to the vehicles, turn around, and retrace the route back to Beach Drive.

10 Turn right at the Porter Street exit and follow the left lane under the overpass to the end of Klingle Road (which is closed). Turn around and park on right side of the road. Outcrops of Laurel Forma-tion are along Klingle Road and in the creek.

Stop 1-2. Laurel Formation; Rock Creek Shear Zone; Eastern Potomac Terrane

Washington West, D.C., Maryland, and Virginia, 7.5 min quad-rangle (Fleming et al., 1994) (Fig. 8).

The Potomac terrane contains two unique sedimentary mélanges of diamictite; the Sykesville Formation west of the Rock Creek shear zone, and the Laurel Formation east of the shear zone. At this stop is a good exposure of the diamictite of the Laurel Formation (Fig. 8C), which is coarse grained, locally gneissic, and micaceous, and consists of a quartzofeldspathic matrix containing numerous olistoliths of meta-arenite and bio-tite schist. Several hundred meters of this unit contains abundant olistoliths, and 50–75% of them are similar to meta-arenite of the structurally overlying Loch Raven Formation to the north-east. The diamictite represents deposits resulting from submarine slides; individual slides and slide surfaces are evident elsewhere in Rock Creek Park (Fleming and Drake, 1998).

The outcrops on the north side of Klingle Road and in the creek are the olistolith-rich slide unit of the upper part of the Laurel Formation (Fleming et al., 1994). The clasts are predom-inantly meta-arenite; the largest clast is ~3 m long. The diamic-tite is situated between fault strands that separate it from tonal-ite and gabbro of the Georgetown Intrusive Suite. The matrix is weakly mylonitized, and the clasts of meta-arenite have tails with a sinistral sense of shear. Coarse staurolite pseudomorphs grew across mylonitic foliation and resemble turkey tracks. Garnets were chloritized and staurolite retrograded to shimmer aggregates of fi ne sericite during retrograde conditions (Missis-sippian dextral shearing).


10 Return to vehicles, retrace route back under the overpass to a right exit onto Porter Street.

11 From Porter Street, turn right onto Connecticut Avenue.

12 Follow Connecticut Ave to a left turn onto Yuma Street. Follow Yuma Street and turn left onto Nebraska Avenue near Fort Reno. Follow Nebraska Avenue past American University.

14 From Nebraska Avenue, turn left onto Arizona Avenue and follow it across MacArthur Boule-vard.

15 Turn right onto Clara Barton Parkway and fol-low it to a left turn onto Chain Bridge, which crosses the Potomac River.

16 Turn right at the stop light onto Chain Bridge Road.

18 Turn right at the stop light onto Balls Hill Road before the ramp to I-495. Follow Balls Hill Road and turn left onto Live Oak Drive over I-495. Follow Live Oak Drive to the end.

20 Park on the right side of the road next to the trailhead of the National Park Service’s Potomac Heritage Trail. Follow the trail down until you see the Potomac River. Do not take the foot bridge that crosses the culvert on the right, but take the foot trail into the woods to the river and go west to the large outcrops beneath the cliffs.

Stop 1-3. Phyllonitic Migmatite, Eastern Stubblefi eld Falls Domain, Mather Gorge Formation, and Sykesville Formation Diamictite; Plummers Island Shear Zone; Potomac Terrane

Falls Church, Virginia-Maryland, 7.5 min quadrangle (Drake and Froelich, 1997) (Fig. 9).

Fisher (1970) mapped a transitional contact between sil-limanite- to staurolite-grade “Wissahickon” schist (west) and diamictite of the Sykesville Formation (east) (Fig. 9) beneath the I-495 American Legion Bridge; the schist transitioned into diamictite as the number of olistoliths decreased (Fisher, 1963, 1970; Hopson, 1964). Drake (1989) described these rocks as ret-rograded chlorite-sericite phyllonite derived from schist. Fisher’s (1963, 1970) contact with the rocks of the Sykesville Formation was named the Plummers Island fault by Drake (1989), and he noted that the Sykesville contained abundant olistoliths of the quartzose schist near its contact with that unit. These same rocks were called the upper part of the Sykesville Formation (Drake and Froelich, 1997), as they contained 50% or more of phyllonite olistoliths presumably derived from the Mather Gorge Formation. The placement of the Plummers Island fault was moved ~0.5 km to the west of the original contact by Drake and Froelich (1997).

We retain the original contact of Fisher (1963, 1970) as the Plummers Island fault (Drake, 1989). However, this is a


0 10 KM

Plu

mm

ers I

sland

faul

t

Potomac River

A

A

A'

A'

1-6

CZmbpC

Zmbi

mCZmbi

CZuCZm

Potomac River

CZms

Cst

Cs

O

TR

1-4 1-5 1-3

1-6

1-4

1-5

1-3

Stop 1-3

Stop 1-5

Stop 1-6

Cs

Pleasant Grove shear zone PlummersIsland

shear zone

RT Triassic strata

O Ordovician plutonic rock

Cambrian

Cs Sykesville Formation

Cst Sykesville Tectonite

Neoproterozoic and Cambrian rocks

CZm Marburg Formation

Mather Gorge Formation

CZmbp Blockhouse Point domain

CZmbi Bear Island domain

CZmbim Migmatitic Bear Island domain

CZms Stubblefield Falls domain

CZu Ultramafic rocks

Explanation


Bear Island Domain

0 1 KM

0 1 KM

0 1 KM

N

Foliationcleavage

Ple

asan

t Gro

ve fa

ult

A

B

C

D

E

Figure 9. (A) Generalized geologic map and cross section (B) of the western Potomac terrane and insets showing the locations of the stops (modi-fi ed after Southworth et al., 2002; Kunk et al., 2005). (C) Stop 1-3, Plummers Island fault, Falls Church quadrangle. (D) Stop 1-5, Diffi cult Run, Falls Church quadrangle. (E) Stop 1-6, Pleasant Grove fault zone, Seneca quadrangle. Stop 1-4 is lunch in Great Falls Park.


late planar fault within a broader Plummers Island shear zone. The dominant foliation in the rocks strikes northerly and dips steeply to the northwest across the fault. The attitude of this foliation is consistent from the Stubblefi eld Falls domain east-ward for 11 km to the Rock Creek shear zone. West of the Plum-mers Island fault, the rocks are phyllonitized migmatite that, locally, were further partially melted, sheared, and retrograded (Fisher, 1963, 1970). Cobbles and blocks of phyllonitic mig-matite appear as olistoliths along with subrounded cobbles of quartz and rootless, transposed vein quartz (Fig. 10A). Blocks of foliated, fi rst-generation phyllonitic migmatite have multiple orientations. A strong north-south foliation overprints the ran-domly oriented early foliations in the blocks. The blocks are within a matrix of feldspathic second-generation migmatite, not the granular quartzofeldspathic matrix as seen in the Sykesville diamictite to the east (Fig. 10B), or the xenolith-rich tonalite that intrudes the Sykesville diamictite.

40Ar/39Ar cooling of S1 muscovite in rocks of the Mather

Gorge Formation ranged from 375 Ma to 354 Ma, from Diffi -cult Run east to the Plummers Island fault. 40Ar/39Ar cooling of S

1

muscovite from the Sykesville Formation to the east was 357 Ma. Therefore, the earliest motion occurred in the western part of the Plummers Island shear zone in the Late Devonian (Acadian). The age of S

2 muscovite decreases eastward across the Stubblefi eld

Falls domain to 328 Ma at the Plummers Island fault, suggest-ing reactivation at that time. In the Sykesville Formation, the S

2

muscovite decreases in age from 332 Ma near the Rock Creek shear zone to 304 Ma at the Plummers Island fault to the west. Whether the Mather Gorge was thrust westward over the Sykes-ville Formation and folded (Drake, 1989), or whether the Sykes-ville was thrust westward over the Mather Gorge Formation (Kunk et al., 2005) and subsequently backfolded, or whether the Mather Gorge was thrust eastward above the Sykesville Forma-tion is undetermined.

Return to vehicles and retrace route back to Georgetown Pike (Va. Route 193).


21 Turn right onto Georgetown Pike (Va. Route 193), and drive west about a mile.

22 Turn right at the stop light at the entrance to Great Falls Park.

Lunch Stop. Restrooms are available at the Visitor Center en route to Overlook 2, which affords a view of the Great Falls of the Potomac River. Bag lunches can be eaten at the overlook or at the picnic tables.

Stop 1-4. Metagraywacke and Staurolite Schist, Western Bear Island Domain, Mather Gorge Formation; Potomac Terrane

Vienna and Falls Church, Virginia-Maryland, 7.5 min quadran-gles (Drake and Lee, 1989; Drake and Froelich, 1997) (Fig. 9).

The level ground of the parking area and Visitor Center is a bedrock strath terrace that was incised by the Potomac River about 30,000 years ago (Bierman et al., 2004). The current posi-tion of the Great Falls of the Potomac River is situated at perhaps the thickest section of metagraywacke exposed in the Potomac River valley. Formerly called the Wissahickon Formation and Peters Creek Schist, these rocks were renamed the Mather Gorge Formation by Drake and Froelich (1997), and this is the type locality. The Mather Gorge Formation is dominantly schist inter-bedded with metagraywacke; subordinate metagraywacke inter-bedded with schist is exposed here. The metagraywacke consists of well-graded beds of detrital quartz and feldspar. There are abundant soft-sedimentary features in the metagraywacke that are consistent with turbidite deposition (Hopson, 1964; Drake and Morgan, 1981). Isoclinal recumbent folds of metagraywacke

W

Plu

mm

ers

Isla

nd fa

ult

E

A BFigure 10. Photographs of rocks along the Plummers Island fault, Stop 1-3. (A) Complex second generation migmatite and phyl-lonitic rocks of the Stubblefi eld Falls domain of the Mather Gorge Formation immediately west of the Plummers Island fault. (B) Sykesville Formation diamictite contains clasts of deformed and metamorphosed metasedimentary rocks (outlined).


and schist are locally refolded upright. About 1 km to the west are bodies of ultramafi c rocks, and about 1.5 km to the east are bodies of amphibolite, granodiorite, and pegmatite. Immediately east of the overlook along the bluffs is one of the steep north-west-dipping undeformed lamprophyre dikes that yielded con-ventional K-Ar biotite cooling ages of ca. 360 Ma (Reed et al., 1970); one other dike is exposed just east of the overlook. The planar, nonfoliated dike suggests that there has been no major contractional deformation in this area since its emplacement and crystallization in the Devonian. En echelon, brittle, normal faults and fractures parallel the north-northwest-striking Mather Gorge, and they may be related to Mesozoic extension.


23 Return to the vehicles, retrace the route, and turn left (east) onto Georgetown Pike (Va. Route 193).

24 Turn right into the parking lot at Diffi cult Run. Follow the trail at the south end of the parking area around the meander of Diffi cult Run and under the bridge carrying Va. Route 193. Con-tinue on the trail along Diffi cult Run for ~500 m to a broad outcrop on the bank near a sharp bend in Diffi cult Run.

Stop 1-5. Migmatitic schist, Amphibolite, and Granodiorite, Eastern Bear Island Domain, Mather Gorge Formation; Potomac terrane

Falls Church, Virginia, and Maryland, 7.5 min quadrangle (Drake and Froelich, 1997) (Fig. 9).

The complex rocks in this outcrop became better exposed as a result of the fl oods of Hurricane Agnes in 1972 (Stop 8 in Reed et al., 1980; Stop 5 in Drake, 1989). These sillimanite-grade rocks were called the Upper Pelitic Schist of the Wissahickon Forma-tion (Fisher, 1970) and the metagraywacke and semipelitic schist of the Mather Gorge Formation (Drake and Froelich (1997). The dominant rock is gray coarsely mottled mica schist and gneiss that contains conspicuous, dark-green, 1–2-cm-diameter crystals of garnet, staurolite, cordierite(?), and shimmer aggregates. The schist contains segregated leucosomes of quartz and albite or sodic oligoclase, which form a wavy texture. Several 2-m-thick layers of dark-green, fi ne-grained epidote-plagioclase-hornblende amphibolite within the schist were probably gabbro or basalt. It is not clear whether the amphibolite sills were intrusive, extrusive, or olistoliths. They predate deformation (Drake, 1989) and are restricted to the sillimanite-grade migmatites of the eastern part of the Bear Island and Stubblefi eld Falls domains eastward to the Plummers Island fault.

The amphibolite and schist are cut by tabular dikes of light-gray Bear Island Granodiorite (Fig. 11). The granodiorite has streaks of biotite that defi ne fl ow foliation. The large dike here is cut by a pegmatite 5–15 cm thick. Meter-scale bodies of Bear Island Granodiorite intruded the migmatites of the Bear Island domain and, locally, rocks in the Stubblefi eld Falls domain east to the Plummers Island fault. Commonly the dikes intrude fractures and boudins in amphibolite, suggesting that they were passively injected into dilational structures in rigid amphibolite under anatectic conditions. Most of the dikes are undeformed as they are here, but locally, similar dikes are folded. The granodio-rite was emplaced after the climax of regional metamorphism and deformation (Fisher, 1963, 1970; Hopson, 1964; Drake, 1989).

Figure 11. Photographs of rocks along Diffi cult Run at Stop 1-5. (A) Ordovician Bear Island Granodiorite intruded amphibolite (sill?) and migmatitic schist of the Bear Island domain of the Mather Gorge Formation. Muscovite from pegmatite here yielded the Rb-Sr cooling age of 469 Ma (Muth et al., 1979), and amphibole yielded a disturbed Ar/Ar age spectrum with an isochron of 475 Ma (Kunk et al., 2004). (B) Some of the granodiorite was folded; it has axial planar cleavage.

W E

AA

BB


Muscovite from pegmatite at this outcrop yielded a 469 Ma Rb-Sr cooling age (Muth et al., 1979).

Amphibole from the amphibolite yielded a disturbed 40Ar/39Ar age spectrum with an isochron age of 475 Ma, suggest-ing that the highest metamorphic conditions and partial melting of these rocks (T > 650 °C) is older.


24 Return to vehicles and turn left (east) on George-town Pike (Va. Route 193).

30 Turn right onto Seneca Road (Va. Route 602). 33 At the end of the road, park on the right shoul-

der. Walk north along the paved road (gated) to the bottom of the hill and follow the gravel road to the right (east) for ~10 m. The outcrops are on the forested bluff on the right.

Stop 1-6. Chlorite Phyllonite, Blockhouse Point Domain, Mather Gorge Formation; Pleasant Grove Shear Zone; Potomac Terrane

Seneca, Virginia, and Maryland, 7.5 min quadrangle (Drake et al., 1999) (Fig. 9).

The rocks here are magnetite-speckled chlorite phyllite and phyllonite with transposed and folded vein quartz. Previously they were called the Upper Pelitic Schist of the Wissahickon Formation (Fisher, 1970), chlorite schist and lesser meta-siltstone of the Peters Creek Schist (Drake, 1989), and meta-graywacke and lesser semipelitic schist of the Mather Gorge

Formation (Drake et al., 1999). This is the westernmost part of the Mather Gorge Formation’s Blockhouse Point domain in Virginia. The southwestward projection of the Pleasant Grove fault is ~1.5 km to the west but the rocks are covered by Late Triassic rocks of the Culpeper basin.

The composite foliation here strikes northeast and dips gently to the southeast (Fig. 12). The protoliths of the phyllites and thin arkosic quartzites were mud and sand. The rocks were intruded by several generations of vein quartz and folded with several cleavage-forming events. This composite cleavage (S

1

and S2, at least) contains transposed bedding, intrafolial folds,

and dismembered vein quartz. Dextral shear-band cleavage and anastomosing S

3 cleavage form a wavy fabric (Fig. 12). Locally,

there is discontinuous S4 cleavage.

Quartz-muscovite phyllonite was analyzed for argon (Kunk et al., 2005). Most of the phyllosilicate layers were more than 90% muscovite, ~10% chlorite, with accessory sphene, magne-tite, and albite, and trace amounts of epidote. Most of the mus-covite grains that defi ne the foliation were 200–400 µm long and 50 µm thick, but some were over 500 µm long and 100 µm thick. The age spectrum is slightly sigmoidal, but forms a plateau at 362 ± 2 Ma for S

2 (70 percent of the 39Ar released was from musco-

vite fl akes and chlorite that defi ne S2).

Return to vehicles, retrace the route south on Seneca Road, and turn left (east) on Georgetown Pike (Va. Route 193).


33 Return to vehicles.End of day 1

NE SW

Figure 12. Photograph looking southeast into the foliation of rocks of the Block-house Point domain of the Mather Gorge Formation, within the Pleasant Grove shear zone ~4 km east of the Pleasant Grove fault. Chlorite phyllite with south-west-verging intrafolial folds of thin me-tagraywacke and vein quartz (top right of photo) with dextral shear band cleavage. Tip of hammer is right center.


J Jurassic

T Triassic rocks

Og Ordovician Grove Formation

Cambrian

Cf Frederick Formation

Car Araby Formation

Cu Urbana Formation

Csmq Sugarloaf Mountain Quartzite

Neoproterozoic and Cambrian

CZi Ijamsville Phyllite

CZsc Sams Creek Formation

CZm Marburg Formation

2-1 FIELD TRIP STOP

40Ar- 39Ar sample

2-1

0 10 KM

39015'

77022'30"

Mar

tic fa

ult

Og

JRT

R

Cu

CZi CZsc

Ca

Explanation

2-2

2-3

2-4

CB

Mar

tic fa

ult

Barn

esvi

lle-M

onro

via

faul

t

RT

WESTMINSTER TERRANE

Cu

2-2

2-3

Stop 2-1

Stops 2-2 and 2-3

SMA

CB

A A'

2-22-3 2-4

A

A'

Martic fault

Martic fault

km

1

1SL

2

2CZscCZi

Cu

CuOg

CZi

0 1 KM

0 1 KM

N

NewMarket

2-1

Parrs

Rid

ge f

ault

Hya

ttsto

wn

faul

t

Csmq

CZm

Frederick

CfFVS

CsmqCaCf CZm

2-1

A

B

C

D

Figure 13. (A) Generalized geologic map and (B) cross section of the western Piedmont Province, Sugar-Loaf Mountain anticlinorium (SMA), Frederick Valley synclinorium (FVS), and Culpeper basin (CB), showing the locations of Stops 2-1 through 2-4 (modifi ed from Southworth et al., 2002). Insets (C) and (D) show parts of the 7.5 min quadrangle maps of Stop 2-1 (Urbana quadrangle), and Stops 2-2 and 2-3 (Sugarloaf Mountain quadrangle).


DAY 2


0 Take the Dulles Toll Road (Va. Route 267) east to I-495 North toward Baltimore, Maryland.

12 Exit to I-495 19 Exit to I-270 41 Exit to Md. Route 109 (Exit 22, Hyattstown,

Barnesville). At end of exit ramp, turn left on Md. Route 109 north. Turn right on Md. Route 355. Immediately turn left on Frederick Road–Hyattstown Mill Road.

43 Use parking area by rusty footbridge at junction of Hyattstown Mill Road and Prescott Road to turn around and head back westbound on Hyatt-stown Mill Road. Pull off to the right at trailhead of Dark Branch Trail. Outcrop is on north side of road, on a small hill on the right side (east) of small valley of Dark Branch.

Stop 2-1. Marburg Formation Metasiltstone and Phyllonite; Hyattstown Shear Zone; Westminster Terrane

Urbana, Maryland, 7.5 min quadrangle (Southworth, 1999) (Fig. 13).

The rocks here are chlorite-grade metasiltstone, phyllonite, and thin quartzite of the Neoproterozoic and Lower Cambrian Marburg Formation (Southworth, 1999). The foliation strikes

northeast and dips steeply to the southeast and northwest. This is a composite foliation of nearly coplanar cleavages, bedding, and transposed vein quartz (Fig. 14A). Where bedding is at a high angle to cleavage, it can be conspicuous (Fig. 14B), as it is on the left side of the knoll above the parking area. Graded beds of metasiltstone strike northwest and dip moderately to the south-west. Beds that are alternately rich and poor in hematite give the rock a banded appearance. Isoclinally folded vein quartz cuts older foliation, but the fold limbs were transposed into the cleav-age and axial planes parallel the later foliation. These structures demonstrate the composite nature of the foliation here. Unde-formed vein quartz cuts the cleavage at moderately high angles.

Regionally, similar Marburg Formation rocks comprise a ~12-km-wide lithotectonic belt in the eastern part of the West-minster terrane. The belt is separated from the Potomac terrane to the east by the Pleasant Grove fault. About 4.7 km to the west of this stop is the 50-km-long Hyattstown thrust fault that placed rocks of the Marburg Formation above Sams Creek Formation. Rocks of the Marburg Formation are distinct from the rocks to the west of the Hyattstown thrust fault. Sams Creek Formation rocks are a diverse assemblage of phyllite, phyllonite, metasiltstone, quartzite, metagraywacke, metabasalt, marble, and metalimestone (Southworth et al., 2002). In contrast to the fi ne-grained rocks of the Sams Creek Formation and Ijamsville Phyllite immediately west of the Hyattstown fault, the phyllite and metasiltstone of the Marburg Formation locally contain paragonite and porphyroblasts of albite and chloritoid. The metasiltstone commonly has a dis-tinctive pinstriped appearance because of quartz laminae and rib-bons interlayered with chlorite phyllite. This ribbon texture was crinkled by contraction and crenulation cleavage, which produced

NW SE

A

NE SW

BFigure 14. Photographs of rocks of the Marburg Formation at Stop 2-1. (A) Foliated metasiltstone with transposed vein quartz. View is to the northeast into the foliation. Hammer handle is at the lower right for scale. (B) Laminated bedding (trending upper left to lower right) exposed on the cleavage plane of the outcrop face. Pen is at upper right for scale.


a characteristic fabric. The quartz laminae are probably metamor-phic segregations but they also may be thin turbidite deposits.

Age spectra from three samples of Marburg Formation in this area (Mulvey, 2003) give S

2 ages of ca. 435–430 Ma. Musco-

vite is preserved in multiple generations of spaced cleavage. The older cleavage contains muscovite intergrown with chlorite, and the later cleavage contains primarily chlorite.

Return to the vehicles and retrace the route back to Md. Route 355.


43 Turn right onto Md. Route 355 and follow it to Urbana, go under I-270 and bear right onto Md. Route 80. Turn left onto Park Mills Road, right onto Flint Hill Road, and left onto Monocacy Bottom Road.

52 Turn around and park on the right side of the road at the bottom of the hill.

Stop 2-2. Martic Fault; Ijamsville Phyllite of the Westminster Terrane Thrust on Frederick Formation Limestone of the Frederick Valley Synclinorium

Buckeystown, Maryland, and Virginia, 7.5 min quadrangle (Southworth and Brezinski, 2003) (Fig. 13).

The Martic fault is at the crest of the hill. Polydeformed phyl-lite and phyllonite with vein quartz (both of the Ijamsville Phyl-lite) are exposed on the east (left) side of the road; limestone of

the Frederick Formation is exposed on the west (right) side of the road. Along the fl ood plain are cliffs of limestone of the Frederick Formation and a rare outcrop of an Early Jurassic diabase dike that cuts the fault. The east-dipping dike has closely spaced frac-tures that are perpendicular to it, which resulted from cooling.

The composite foliation in the Ijamsville Phyllite is coplanar to the foliations in the Frederick Formation. Here and along Ben-nett Creek at the southern end of Monocacy Bottom Road, the foliations dip uniformly to the southeast. In the Frederick Forma-tion, northeast-trending, southeast-dipping cleavage and bedding are mostly coplanar, and the cleavage is strong. Away from the fault, the composite foliation in the Ijamsville Phyllite is folded by mesoscopic F

2 folds that plunge moderately southeast and

northwest. The limestone is folded into a series of anticlines and synclines with axial-planar cleavage (Fig. 15).

Along Bennett Creek to the south, a sample of cleaved lime-stone conglomerate in the footwall of the Martic fault yielded an 40Ar/39Ar muscovite age of 380–360 Ma. Opposite the fault in that area, Ijamsville Phyllite in the hanging-wall of the Martic fault yielded an 40Ar/39Ar muscovite age of 431 Ma.

The Martic thrust sheet was emplaced in the Silurian, folded with the footwall rocks during the Devonian, and reactivated in the Carboniferous to truncate the folds in the footwall rocks. The orientation of F

2 folds and crenulation cleavage in the Martic

thrust sheet also supports dextral strike-slip motion during late Alleghanian deformation (Horton et al., 1989; Southworth, 1996) (Fig. 16).

Return to the vehicles and retrace the route back to Flint Hill Road.

bedd

ing

S1 cleavage

W E

Mar

tic fa

ult

A B

Figure 15. Sketches of rocks on either side of the Martic fault il-lustrating the structural discor-dance. (A) Outcrop of anticline of Frederick Formation limestone with axial planar cleavage, foot-wall; hammer for scale. (B) Hand specimen of Ijamsville Phyllite, hanging-wall.


Ijamsville PhylliteF2 antiforms

Frederick Valley synclinoriumBedding-cleavage

intersection lineation

Westminsterterrane

Urbana FormationBedding-cleavage

intersection lineation

Martic fault

Martic faultMarticfault

Marticfault

Marticfault

Bush CreekWindow

Fre

der i

ck

Val

ley

syn

clin

oriu

m

Su

ga

r lo

af

M

ou

nta

in

a

nt i

c lin

or i

um

EXPLANATION

fold axesBedding-cleavage intersectionantiform axesCrenulation cleavage

Figure 16. Structural map of the western Westminster terrane showing the structural discordance between the rocks of the Martic thrust sheet and the rocks of the Sugarloaf Mountain anticlinorium and the footwall rocks of the Frederick Valley synclinorium. The structurally overlying rocks of the Martic thrust sheet have a more complex history (modifi ed after Southworth, 1996).



52 From Monocacy Bottom Road, turn right onto Flint Hill Road. At the stop sign at the top of the hill, turn right onto Park Mills Road.

55 Turn left onto Mount Ephraim Road and proceed to a bridge over Bennett Creek. Park on the left or cross the bridge and park on the right at the small quarry.

Stop 2-3. Urbana Formation Metasandstone and Metasiltstone, Sugarloaf Mountain Anticlinorium


The outcrops at either end of the bridge over Bennett Creek show bedding, cleavage, and bedding-cleavage intersection lin-eations. The mesoscopic F

1 folds and the intersection lineations

plunge gently to the north and south (Southworth, 1996; South-worth and Brezinski, 2003) (Fig. 16).

A sample of metasiltstone from the north side of the bridge yielded an 40Ar/39Ar muscovite age of 439 Ma, which is identical to that from the Ijamsville Phyllite of the last stop. The Ijamsville Phyllite of the Martic thrust sheet between here and the last stop forms a klippe between the Urbana and Frederick Formations. The Urbana Formation rocks experienced a single deformation event in the Silurian, therefore the polydeformed rocks of the Ijamsville Phyllite must have been deformed earlier. They were emplaced onto the Urbana rocks in the Silurian when both were folded with a cleavage-forming event. The east side of the klippe was cut by a later fault that thrust the Urbana Formation onto the klippe during contractional deformation, overturning it to the northwest.

In summary, the Martic thrust sheet was (1) emplaced and folded with the rocks of the Sugarloaf Mountain anticlinorium at ca. 440–430 Ma (Silurian), (2) transported west onto rocks of the Frederick Valley, (3) folded with the Frederick Valley syn-clinorium between 380 and 360 Ma, and (4) later reactivated as a thrust sheet and dextral strike-slip fault, as well as being cut by thrust faults.


55 Return to vehicles and follow the Mount Ephraim Road south. Turn left onto Comus Road and turn left into the entrance to Sugar-loaf Mountain at the intersection in Stronghold, Maryland. Lunch is on top of the mountain at the southwest overlook.

Stop 2-4. Sugarloaf Mountain Quartzite, Sugarloaf Mountain Anticlinorium; Tectonic Window through the Martic Thrust Sheet

Lunch and regional overlook


Sugarloaf Mountain is underlain by three informal members of the Sugarloaf Mountain Quartzite. The 1280-ft summit of Sug-arloaf Mountain is underlain by folded beds of the upper mem-ber. A syncline at the peak led Stose and Stose (1946) to think that the Urbana Formation was beneath the Sugarloaf Mountain Quartzite; however, multiple outcrops along the fl anks of the anticlinorium show right-side-up Sugarloaf Mountain Quartzite dipping beneath right-side-up rocks of the Urbana Formation.

These quartzites have long been correlated with similar quartzites that underlie the ridges to the west in the Blue Ridge–South Mountain anticlinorium. The Sugarloaf Mountain Quartz-ite is at least twice as thick as the Weverton Formation in the Blue Ridge, but the crossbeds and ripple marks support a similar depositional setting. If the correlation is correct, these quartzites are Early Cambrian age. Other lithologic and stratigraphic cor-relations can be made as well. The overlying rocks of the Urbana Formation are correlative with the Lower Cambrian Harpers Formation. The Middle and Lower Cambrian Araby Formation of the Frederick Valley is correlative with the Ollenellus-bear-ing Lower Cambrian Antietam Formation. The Upper Cambrian Frederick Formation limestone at Stop 2-2 is correlative with the Lower Cambrian Tomstown Formation. Lithologically similar rocks are younger to the east (that is, they were time transgres-sive into the developing basin).

To the east, rocks of the Ijamsville Phyllite share the same structures as the Ijamsville Phyllite in the belt along the Martic fault (Stop 2-2), and they structurally overlie rocks of the Urbana Formation. They frame the Sugarloaf Mountain anticlinorium as well as a small antiform to the north. The rocks of the Sugarloaf Mountain Quartzite and Urbana Formation are interpreted to be exposed in at least two tectonic windows through the Martic thrust sheet (Fig. 16). About 2 km to the northeast at Thurston, Mary-land, calcitic marble in the Urbana Formation yielded an 40Ar/39Ar muscovite age of 437 Ma, which is very close to the ages of white mica in the Urbana Formation at Stop 2-3 (439 Ma), the Ijams-ville Phyllite of Stop 2-2 (431 Ma), and the Marburg Formation of Stop 2-1 (430 Ma). These data suggest that the allochthonous Westminster terrane rocks were fi rst emplaced along the Mar-tic fault, and then metamorphosed and deformed along with the rocks of the Sugarloaf Mountain anticlinorium between 439 and 430 Ma.

Return to the vehicles and retrace the route down the moun-tain to the intersection.


Go east on Comus Road and turn left at the stop sign onto MD 109. Get on I-270 west bound, near the exit to Hyattstown.

0 Take I-270 west to Frederick, take the I-70 west exit, and then the I-340 west exit toward Harpers Ferry, West Virginia.


11 U.S. 340 climbs Catoctin Mountain, which forms the east limb of the Blue Ridge–South Mountain anticlinorium.

15 West of the Jefferson exit, U.S. 340 crosses the core of the anticlinorium which is comprised of Mesoproterozoic gneisses intruded by Neo-proterozoic metadiabase dikes, all of which are exposed in the road cuts near the bridge that crosses Catoctin Creek.

20 U.S. 340 crosses South Mountain, one of two west limbs of the anticlinorium that are repeated by the Short Hill–South Mountain fault, which we will see at the last stop.

~22 Cross the bridge over the Potomac River and look right to see Harpers Ferry, West Virginia, through the water gap. Turn left onto Harpers Ferry Road at the stop light.

30 To the right is a low ridge and intervening val-ley in the foreground of the Blue Ridge on the horizon. The rocks that underlie these landforms are the subject of the next stop. Turn right onto Sawmill Lane, and turn right onto Arnold Lane, and park on the right shoulder at the left turn.

Stop 2-5. Mesoproterozoic Hornblende Gneiss and Neoproterozoic Swift Run Formation, Purcell Knob Folds on the West Limb of the Blue Ridge–South Mountain Anticlinorium

Harpers Ferry, West Virginia, Maryland, and Virginia, 7.5 min quadrangle (Southworth and Brezinski, 1996b) (Fig. 17).

The route from U.S. 340 to this stop has been in Mesopro-terozoic gneisses that underlie Loudoun Valley; these same rocks are exposed along Arnold Lane. The low hill to the west is framed by quartzite and metasandstone of the Neoproterozoic Swift Run Formation. A podiform body of calcitic marble between quartzite and phyllite are considered to be related to the snowball earth hypothesis (Hebert, 2003). In the bed of Piney Run in the water gap is Neoproterozoic Catoctin Formation metabasalt exposed beneath the older rocks of the Swift Run Formation. To the west, the quartzite encloses and underlies Mesoproterozoic hornblende gneiss. The quartzite and S

1 cleavage dip beneath the gneiss and

defi ne a synform that underlies the valley. The stratigraphy is inverted in a recumbent fold.

The west-verging Blue Ridge–South Mountain anticlino-rium consists of F

1 isoclinal folds that have axial-planar S

1 cleav-

age and F2 inclined folds that have S

2 axial-planar crenulation

cleavage (Fig. 17). The F1 and F

2 folds plunge gently to the north-

east and southwest. The Purcell Knob inverted folds are refolded folds (Nickelsen, 1956; Southworth and Brezinski, 1996b); an antiformal syncline, a synformal anticline, and several inverted parasitic folds show that the basement rocks were folded with the cover rocks. Cleaved younger rocks dip beneath older rocks. The rocks are thickened in fold hinges and thinned on inverted

limbs, which is consistent with supratenuous folds of passive fl ow. Deformation of the hornblende gneiss is heterogeneous. Mylonitic foliation occurs at the contact with the cover rocks on the inverted limb, but the gneiss is massive in the core of the anticline. This style of folding is unique to the Blue Ridge–South Mountain anticlinorium (Nickelsen, 1956; Elliott, 1970), but similar recumbent folds are recognized along the west limb in this region over a distance of 25 km (Southworth et al., 2006). The later deformation was probably a continuum of the earlier, dominant phase of deformation that produced F

1 and S

1 (Nick-

elsen, 1956).Zircons from the hornblende gneiss yielded a SHRIMP age of

1149 ± 19 Ma, making it one of the oldest dated rocks in the Blue Ridge province of Virginia (Aleinikoff et al., 2000). Hornblende sampled here yielded an 40Ar/39Ar cooling age of 978 ± 2 Ma (Kunk and Burton, 1999). Two rocks of the Swift Run Formation sampled here yielded 40Ar/39Ar muscovite cooling ages that range from 347 to 337 Ma and 338 to 313 Ma (Kunk and Burton, 1999). The collective range in age from 347 to 313 Ma suggests Car-boniferous deformation and metamorphism; therefore, these are Alleghanian folds associated with greenschist-facies conditions.


30 Retrace the route back to U.S. 340, turn right onto U.S. 340, cross the Potomac River bridge, and turn right onto Md. Route 67 to Boonsboro.

38 Turn left onto Millbrook Road at Rohrersville. After about a hundred yards, park on the right shoulder adjacent to a small quarry. Examine exposures in the quarry and in Millbrook Creek.

Stop 2-6. Neoproterozoic Catoctin Formation and Lower Cambrian Tomstown Formation, Short Hill–South Mountain fault

The provincial boundary between the Blue Ridge and Great Valley, and how the Blue Ridge anticlinorium of Virginia becomes the South Mountain anticlinorium of Maryland.

Keedysville, Maryland, 7.5 min quadrangle (Southworth et al., 2002) (Fig. 18).

The boundary between the Blue Ridge province and the Great Valley section of the Valley and Ridge province is usu-ally marked by the contact between the clastic rocks of the Blue Ridge along the lower west slope of the ridges and the adjacent, overlying carbonate rocks that underlie the valley. Locally, espe-cially in central Virginia, the boundary is defi ned by a thrust fault that placed Blue Ridge rocks onto Great Valley rocks. The boundary here is unique. The west limb of the Blue Ridge–South Mountain anticlinorium (BR-SMA) in northern Virginia and southern Maryland is repeated by the Short Hill–South Mountain fault (SH-SMF). The previous stop was on the west limb proper (Blue Ridge–Elk Ridge, visible here to the west), and the ridge to the east is South Mountain. The SH-SMF extends at least 60 km

162 Southworth et al.A A'

B B'

SL

C C'

SLSL

SLZct

Yhg

Cw

Cw

CwYhg

B

B’

A

A'

Zct

ZsCw

Clc

Clc

Zs

Yhg

Zs

Zsp

Zct

YhgClc

Cl

Cw

Cw

C

C'

Zc marble

1000

1000

FT

Zs

Cl

ZsZsp

Zct

Yhg

Zc

Loudoun FormationWeverton Formation

ConglomeratePhyllite

Catoctin Formation

Swift Run Formation

PhylliteMetabasalt

PhylliteMetasandstone

Mesoproterozoic hornblende gneiss

Neoproterozoic

Yhg ZsZsp

ZctCl

Cl Zsp

ClClc

1000

1000

1000

1000

1000

1000

SL

SL

1000

1000

1000

1000

FT

FT

Cambrian Explanation

Stop 2-4

N N N

m

Bedding Cleavage Crenulation cleavage

77 54 54

0 1 KM

0 1 KM

N

0 1 KM

39017'30”

77045'A B

C

D

E

F

Figure 17. (A) Generalized geologic map and (B–D), cross sections of the western Blue Ridge–South Mountain anticlinorium showing the Pur-cell Knob folds of Stop 2-5 (modifi ed after Southworth and Brezinski, 1996a). (E) is the location within the Harpers Ferry quadrangle. (F) Equal area stereonet projections showing poles to bedding, S1 cleavage, and S2 crenulation cleavage (Southworth and Brezinski, 1996a).


from here to a shear zone in Mesoproterozoic basement rocks in the core of the anticlinorium. Younger rocks are structurally above older rocks. Hanging-wall rocks are in a regional syncline that formed during contractional motion on the SH-SMF, and the axis of the syncline was transected by the SH-SMF during con-tractional reactivation. The SH-SMF is an early fault that is paral-lel to the regional S

1 cleavage in greenschist-facies rocks; S

2 pres-

sure-solution crenulation cleavage transects the SH-SMF. When the contractional structures are schematically restored, the geom-etry shows that the SH-SMF is an extensional fault that formed after deposition of the Middle and Upper Cambrian Elbrook Formation. The extensional fault was tectonically inverted dur-ing Paleozoic orogenesis.

This fault has been controversial for more than 100 years. Keith (1894) and Stose and Stose (1946) called it the “Harp-ers Ferry overthrust” and thought that it placed older rocks of the Blue Ridge on younger rocks of the Great Valley. Cloos (1951) was puzzled by the relations of “why the South Mountain sequence is repeated in Elk Ridge to the west and how this area terminates near Rohrersville,” but he portrayed the geology as a series of high-angle normal faults that bounded horsts and gra-bens of Triassic extension. David Elliott (Johns Hopkins Univer-sity, deceased) recognized that the fault was folded (Wojtal, 1989;

Howard, J.L., Wayne State University, oral commun., 1991) and that the rocks on the east side of South Mountain had been over-ridden by a thrust fault. Wojtal’s (1989) schematic reconstruction of Elliott’s down-plunge projection portrays the structure as an Early Cambrian graben. Map patterns and foliation attitudes sug-gest that younger rocks overlie the older rocks along a fl atlying fault. Two drill cores, one on Elk Ridge and one here along Mill Creek, proved this to be the case (Southworth, 2005).

Metabasalt, metarhyolite, quartz-sericite schist, and phyl-lite of the Catoctin Formation (in the footwall) underlie the hilly topography of the Blue Ridge province. The karst lowland to the north is underlain by limestone and dolomite of the Tomstown Formation of the Great Valley section of the Valley and Ridge province. Metabasalt in Mill Creek is gently folded; S

1 dips

northwest and S2 dips southeast (Fig. 19). Near the fault, phyl-

lonitic foliation strikes N. 20° W. and dips 19° NE. The metamor-phic segregation layering of quartz and phyllosilicates in the S

1 of

the Catoctin Formation contains magnetite that has asymmetric quartz pressure shadows of east-over-west motion. Intersection lineations of S

1 and S

2 and microfold axes plunge gently to the

northeast or southwest parallel to the folds of S1. Near the fault in

the creek is a large knocker of dolomite that has abundant veins of calcite and quartz. To the west and north, beds of limestone

A A'

ZcZc

Cw

CaCa

CwCt

Cw

Ch

Ch

Ct Sea Level Sea Level

-1,000

1,000FT

-1,000

1,000FTShort Hill-South Mountain fault

Cambrian

Ct Tomstown Formation

Ca Antietam Formation

Ch Harpers Formation

Chs sandstone bed

Cw Weverton Formation

Neoproterozoic

Zc Catoctin Formation

Zcr metarhyolite

Zs Swift Run Formation

Y Mesoproterozoic gneiss

2-6 FIELD TRIP STOP

Explanation

Y

A

B

A

Zc

Cw

Zs

Zc

SH-SMF

Zcr

Zs Cw

Ct

Ct

Ct Ch

Ch

Chs

Ch

Ca A’2-6

Y

0 2 KM

1 MI0 MI

N

Figure 18. Generalized geologic map (A) and cross section (B) of the northern part of the Blue Ridge–South Mountain anticlinorium (modifi ed from Southworth and Brezinski, 1996a; Southworth, 2005). Index of part of the Keedysville 7.5 min quadrangle shows the location of Stop 2-6, the Short Hill- South Mountain fault.


and dolomite strike east-west and dip 10° N. Strong S2 strikes

N. 30° E. and dips 18° SE. Mesoscopic, west-verging F2 folds

plunge gently to the northeast. The dominant S1 strikes parallel to

and dips away from the fault trace, even where the fault changes strike. Rocks of the Tomstown Formation here are L-S (linea-tion-schistosity) tectonites that contain west-verging intrafolial F

1 folds that have horizontal axial surfaces. Southeast-dipping S

2

transects S1 and the fault. Discrete and zonal S

2 is axial planar to

northwest-verging F2, and is a pressure solution cleavage. F

2 in

the Tomstown Formation folds bedding, S1, and stretching linea-

tions; pavement outcrops defi ne broad domes and basins. Phyl-lonitic foliation (in the footwall) and intrafolial F

1 folds in the L-S

tectonites (in the hanging-wall) are related to thrust motion. The southeast-dipping S

2 transects the SH-SMF and is axial-planar to

northwest-verging F2 that folds the SH-SMF.

Rocks of the Catoctin Formation here yielded an 40Ar/39Ar S1

muscovite cooling age that ranged from 338 to 326 Ma, which is within the range of the rocks of the Swift Run Formation at the previous stop. The post-Upper Cambrian extensional fault was contractionally reactivated as a thrust fault in the Mississippian and was later folded (Fig. 20).

Continue on Mill Creek Road. Turn left onto Hog Maw Road. Turn left onto Main Street.

Optional Stop: Park on the right side of the road on the other side of the bridge, adjacent to the outcrop.

0 1 cm

Short

Hill-Sou

th M

ount

ain

faul

t

Thin section

N

B

N

meanattitude

S2

Early Cambrian Tomstown FormationA

B

C

F2

bedding

F1

Neoproterozoic Catoctin Formation

S1

S2

S2

S1

LENSCAP

hammerhead

S1 S2

SH-SMF

Mississippian

Post-Upper Cambrian

SH-SMF

Figure 19. Sketches of photographs (Southworth, 2005) of the rocks along the Short Hill-South Mountain fault stop 2-6. (A) Hanging-wall rocks. (B) Footwall rocks. (C) Equal area stereonet projections showing poles to cleavage along the fault defi ne a great circle of the north-north-east–plunging Blue Ridge–South Mountain anti-clinorium. S2 and inter-section lineations of S1 and S2 are related to the folding of the fault.

Figure 20. Schematic illustration of the inverted Short Hill–South Mountain fault (SH-SMF) showing the normal fault and the contrac-tional reactivation (Southworth, 2005).


This outcrop is metarhyolitic tuff of the Neoproterozoic Catoctin Formation. S

1 cleavage strikes northeast and dips mod-

erately to the northwest. S2 pressure-solution cleavage strikes

northeast and dips moderately to the southeast. S1 is parallel to

the SH-SMF (formerly about ten’s of meters above your head); kinematic indicators of mica in S

1 show that the top of the tuff

moved to the northwest. S2 is axial-planar to F

2 folds that fold

the fault, which plunges at a low angle to the northeast. The S2

is strongly developed here and may be related to fl exural slip motion during later folding.


Continue on Main Street, turn right onto Md. Route 67 and follow it to U.S. 340. Take U.S. 340 eastbound to Frederick, and take I-70 east to I-695 in Baltimore. Take I-95 north to Philadel-phia, about a 3 hour drive.

REFERENCES CITED

Aleinikoff, J.N., Zartman, R.E., Walter, M., Rankin, D.W., Lyttle, P.T., and Bur-ton, W.C., 1995, U-Pb ages of metarhyolites of the Catoctin and Mount Rogers Formations, central and southern Appalachians; evidence for two pulses of Iapetan rifting: American Journal of Science, v. 295, no. 4, p. 428–454.

Aleinikoff, J.N., Burton, W.C., Lyttle, P.T., Nelson, A., and Southworth, S., 2000, U-Pb geochronology of zircon and monazite from Middle Protero-zoic granitic gneisses of the northern Blue Ridge, Virginia and Maryland: Journal of Precambrian Research, v. 99, p. 113–146, doi: 10.1016/S0301-9268(99)00056-X.

Aleinikoff, J.N., Horton, J.W., Drake, A.A., Jr., and Fanning, C.M., 2002, SHRIMP and conventional U-Pb Ages of Ordovician granites and tonal-ites in the central Appalachian Piedmont; implications for Paleozoic tec-tonic events: American Journal of Science, v. 302, no. 1, p. 50–75, doi: 10.2475/ajs.302.1.50.

Aleinikoff, J.N., Wintsch, R.P., Horton, J.W., Jr., Drake, A.A., Jr., and Fanning, C.M., 2004, Deciphering Mesoproterozoic and Paleozoic events pre-served in the isotopic compositions of zircon and titanite; SEM imaging, SHRIMP U-Pb geochronology, and EMP analysis: Geological Society of America Abstracts with Programs, v. 36, no. 2, p. 80.

Becker, J.L., Kunk, M.J., Wintsch, R.P., and Drake, A.A., Jr., 1993, Evidence for pre-Taconic metamorphism in the Potomac terrane, Maryland and Vir-ginia; hornblende and muscovite 40Ar/39Ar results: Geological Society of America Abstracts with Programs, v. 25, no. 2, p. 5.

Bierman, P., Reusser, L., and Pavich, M., Zen, E-an, Larsen, J., and Finkel, R., 2004, Great Falls is 30,000 years old, Episodic incision along the Potomac River revealed using fi eld mapping and 10-Be analysis: Geological Soci-ety of America Abstracts with Programs, v. 36, no. 2, p. 94.

Brezinski, D.K., 1992, Lithostratigraphy of the western Blue Ridge cover rocks in Maryland: Maryland Geological Survey Report of Investigations 55, 69 p., scale 1:62,500.

Brezinski, D.B., Anderson, T.H., and Campbell, P., 1996, Evidence for a regional detachment at the base of the Great Valley sequence in the cen-tral Appalachians, in Studies in Maryland geology: Maryland Geological Survey Special Publication 3, p. 223–229.

Burton, W.C., and Southworth, S., 2004, Tectonic evolution of the northern Blue Ridge massif, Virginia and Maryland, in Tollo, R.P., Corriveau, L., McLelland, J., and Bartholomew, M.J., eds., Proterozoic tectonic evolu-tion of the Grenville orogen in eastern North America: Geological Society of America Memoir 197, p. 477–493.

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 quad-rangles, Virginia: U.S. Geological Survey Bulletin 2095, 30 p., and map, scale 1:24,000.

Butler, J.R., 1991, Metamorphism, in Horton, J.W., Jr., and Zullo, V.A., eds., The geology of the Carolinas; Carolina Geological Society 50th Anniversary Volume: Knoxville, Tennessee, University of Tennessee Press, p. 11–35.

Campbell, P.A., and Anderson, T.H., 1996, The formation of recumbent fold-nappes and ductile detachments within the Cambrian-Ordovician section of the western Blue Ridge and eastern Great Valley in Maryland in Studies in Maryland geology: Maryland Geological Survey Special Publication, v. 3, p. 231–252.

Cloos, E., 1951, Structural geology of Washington County, in The physical fea-tures of Washington County: Baltimore, Maryland, Maryland Department of Geology, Mines, and Water Resources, p. 124–163.

Cloos, E., and Broedel, C.H., 1940, Geologic map of Howard County and adja-cent parts of Montgomery and Baltimore Counties: Baltimore, Maryland Geological Survey, scale 1:62,500.

Cloos, E., and Cooke, C.W., 1953, Geologic map of Montgomery County and the District of Columbia: Baltimore, Maryland, Maryland Department of Geology, Mines, and Water Resources, scale 1:62,500.

Darton, N.H., and Keith, A., 1901, Washington Folio, District of Columbia, Mary-land, and Virginia: U.S. Geological Survey Folio 70, scale 1:62,500.

Davis, A.M., Southworth, C.S., Reddy, J.E., and Schindler, J.S., 2002, Geologic map database of the Washington, D.C., area featuring data from three 30 × 60 minute quadrangles; Frederick, Washington West, and Fredericksburg: U.S. Geological Survey Open-File Report OFR-01-227, CD-ROM.

Drake, A.A., Jr., 1986, Geologic map of the Fairfax quadrangle, Fairfax County, Virginia: U.S. Geological Survey Geologic Quadrangle Map GQ-1600, scale 1:24,000.

Drake, A.A., Jr., 1989, Metamorphic rocks of the Potomac Terrane in the Potomac Valley of Virginia and Maryland: 28th International Geological Congress Field Trip Guidebook T202, Washington, D.C., American Geo-physical Union, Washington, D.C., 22 p.

Drake, A.A., Jr., 1994, The Soldiers Delight Ultramafi te in the Maryland Pied-mont, in Stratigraphic Notes, 1993: U.S. Geological Survey Bulletin 586, p. 83–86.

Drake, A.A., Jr., 1995, Tectonic studies in the central Appalachian Piedmont—New fi ndings: Geological Society of America Abstracts with Programs, v. 27, no. 1, p. 4.

Drake, A.A., Jr., 1998, Geologic map of the Kensington quadrangle, Mont-gomery County, Maryland: U.S. Geological Survey Geologic Quadrangle Map GQ-1774, scale 1:24,000.

Drake, A.A., Jr., and Fleming, A.H., 1994, The Dalecarlia Intrusive Suite and Clarendon Granite in the Potomac Valley, in Stratigraphic notes, 1991–1992: U.S. Geological Survey Bulletin 2060, p. 25–33.

Drake, A.A., Jr., and Froelich, A.J., 1997, Geologic map of the Falls Church quadrangle, Fairfax and Arlington Counties, and the city of Falls Church, Virginia and Montgomery County, Maryland: U.S. Geological Survey Geologic Quadrangle Map GQ-1734, scale 1:24,000.

Drake, A.A., Jr., and Morgan, B.A., 1981, The Piney Branch Complex—A metamorphosed fragment of the central Appalachian ophiolite in northern Virginia: American Journal of Science, v. 281, no. 4, p. 484–508.

Drake, A.A., Jr., and Lee, K.Y., 1989, Geologic map of the Vienna quadrangle, Fairfax County, Virginia, and Montgomery County, Maryland: U.S. Geo-logical Survey Geologic Quadrangle Map GQ-1670, scale 1:24,000.

Drake, A.A., Jr., Southworth, S., and Lee, K.Y., 1999, Geologic map of the Seneca quadrangle, Montgomery County, Maryland and Fairfax and Lou-doun Counties, Virginia: U.S. Geological Survey Geologic Quadrangle Map GQ-1802, scale 1:24,000.

Edwards, J., Jr., 1986, Geologic map of the Union Bridge quadrangle, Carroll and Frederick Counties, Maryland: Baltimore, Maryland, Maryland Geo-logical Survey, scale 1:24,000.

Edwards, J., Jr., 1988, Geologic map of the Woodsboro quadrangle, Carroll and Frederick Counties, Maryland: Baltimore, Maryland, Maryland Geologi-cal Survey, scale 1:24,000.

Edwards, J., Jr., 1994, Geologic map of the Libertytown quadrangle, Carroll and Frederick Counties, Maryland: Baltimore, Maryland, Maryland Geo-logical Survey, scale 1:24,000.

Elliott, D., 1970, The fl ow of basement into nappes: Eos (Transactions, Ameri-can Geophysical Union), v. 51, no. 4, p. 431.

Espenshade, G.H., 1986, Geology of the Marshall quadrangle, Fauquier County, Virginia: U.S. Geological Survey Bulletin 1560, 60 p.

Evans, M.A., 1989, The structural geometry and evolution of foreland thrust sys-tems, northern Virginia: Geological Society of America Bulletin, v. 101, p. 339–354, doi: 10.1130/0016-7606(1989)101<0339:TSGAEO>2.3.CO;2.


Fauth, J.L., and Brezinski, D.K., 1994, Geologic map of the Middletown quad-rangle, Frederick and Washington Counties, Maryland: Baltimore, Mary-land, Maryland Geological Survey, scale 1:24,000.

Fisher, G.W., 1963, The petrology and structure of crystalline rocks along the Potomac River near Washington, D.C. [unpublished Ph.D. dissertation]: Baltimore, Maryland, The Johns Hopkins University, 241 p.

Fisher, G.W., 1970, The metamorphosed sedimentary rocks along the Potomac River near Washington, D.C., in Fisher, G.W., et al., eds.; Studies of Appalachian geology, central and southern: New York, New York, Inter-science, p. 299–315.

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.

Fleming, A.H., and Drake, A.A., Jr., 1998, Structure, age, and tectonic setting of a multiply reactivated shear zone in the Piedmont in Washington, D.C., and vicinity: Southeastern Geology, v. 37, no. 3, p. 115–140.

Fleming, A.H., Drake, A.A., Jr., and McCartan, L., 1994, Geologic map of the Washington West quadrangle, District of Columbia, Montgomery and Prince Georges Counties, Maryland, and Arlington and Fairfax Counties, Virginia: U.S. Geological Survey Geologic Quadrangle Map GQ-1748, scale 1:24,000.

Gates, A.E., Muller, P.D., and Krol, M.A., 1999, Alleghanian transpressional orogenic fl oat in the Baltimore terrane, central Appalachian Piedmont, in Valentino, D.W., and Gates, A.E., eds., The Mid-Atlantic Piedmont; tectonic missing link of the Appalachians: Geological Society of America Special Paper 330, p. 127–138.

Gradstein, F.M., Ogg, J.G., Smith, A.G., Agterberg, F.P., Bleeker, W., Cooper, R.A., Davydov, V., Gibbard, P., Hinnov, L.A., House, M.R., Lourens, L., Luterbacher, H.P., McArthur, J., Melchin, M.J., Robb, L.J., Shergold, J., Villeneuve, M., Wardlaw, B.R., Ali, J., Brinkhuis, H., Hilgen, F.J., Hooker, J., Howarth, R.J., Knoll, A.H., Laskar, J., Monechi, S., Plumb, K.A., Powell, J., Raffi , I., Röhl, U., Sadler, P., Sanfi lippo, A., Schmitz, B., Shackleton, N.J., Shields, G.A., Strauss, H., Van Dam, J., van Kolf-schoten, T., Veizer, J., and Wilson, D., 2004, A geologic time scale 2004: Cambridge, U.K., Cambridge University Press, 589 p.

Hatcher, R.D., Jr., and Goldberg, S.A., 1991, The Blue Ridge province, in Hor-ton, J.W., Jr., and Zullo, V.A., eds., The geology of the Carolinas; Carolina Geological Society 50th Anniversary Volume: Knoxville, Tennessee, Uni-versity of Tennessee Press, p. 11–35.

Hebert, C.L., 2003, Stratigraphic and geochemical investigations of Neopro-terozoic glaciation in the Appalachian Mountains of Virginia [unpublished M.S. thesis]: College Park, Maryland, University of Maryland, 87 p.

Hibbard, J.P., van Staal, C.R., Rankin, D.W., and Williams, H., 2005, Geol-ogy, lithotectonic map of the Appalachian orogen (south), Canada; United States of America; Geological Survey of Canada, Map 02041A, scale 1:1,500,000.

Higgins, M.W., Atkins, R.L., Crawford, T.J., Crawford, R.F., III, Brooks, R., and Cook, R.B., 1988, The structure, stratigraphy, tectono-stratigraphy, and evolution of the southernmost part of the Appalachian orogen: U.S. Geological Survey Professional Paper 1475, 173 p.

Hopson, C.A., 1964, The crystalline rocks of Howard and Montgomery Coun-ties, Maryland, in The geology of Howard and Montgomery Counties: Baltimore, Maryland, Maryland Geological Survey, p. 27–215.

Horton, J.W., Jr., Drake, A.A., Jr., and Rankin, D.W., 1989, Tectono-stratigraphic terranes and their Paleozoic boundaries in the central and southern Appala-chians, in Dallmeyer, R.D., ed., Terranes in the circum-Atlantic Paleozoic orogens: Geological Society of America Special Paper 230, p. 213–245.

Horton, J.W., Jr., Drake, A.A., Jr., Rankin, D.W., and Dallmeyer, R.D., 1991, Preliminary tectonostratigraphic terrane map of the central and south-ern Appalachians: U.S. Geological Survey Miscellaneous Investigations Series Map I-2163, scale 1:2,500,000.

Jäeger, E., 1979, The Rb-Sr Method, in Jäeger, E., and Hunziker, J.C. eds., Lec-tures in isotope geology: Springer, Berlin Heidelberg New York, p. 13–26.

Jonas, A.I., 1937, Tectonic studies in the crystalline schists of southeastern Pennsylvania and Maryland: American Journal of Science, v. 34, no. 203, p. 364–388.

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, scale 1:62,500.

Jonas, A.I., and Stose, G.W., 1938b, New formation names used on the geologic map of Frederick County, Maryland: Journal. Washington Academy of Sciences, Washington, D.C. v. 28, no. 8, p. 345–348.

Keith, A., 1894, Description of the Harpers Ferry quadrangle [Va.-Md.-W.Va.]: U.S. Geological Survey, Folio 10, 5 p., 4 map sheets, scale 1:125,000.

Kline, S.W., Lyttle, P.T., and Schindler, J.S., 1991, Late Proterozoic sedimen-tation and tectonics in northern Virginia, in Schultz, A., and Compton-Gooding, E., eds., Geologic evolution of the eastern United States: Vir-ginia Museum of Natural History Guidebook No. 2, p. 263–294.

Knopf, E.B., 1931, Retrogressive metamorphism and phyllonitization: Ameri-can Journal of Science, 5th Series, v. 21, p. 1–27.

Krol, M.A., Muller, P.D., and Idleman, B.D., 1999, Late Paleozoic deformation within the Pleasant Grove shear zone, Maryland; results from 40Ar/39Ar dating of white mica, in Valentino, D.W., and Gates, A.E., eds., The mid-Atlantic piedmont; tectonic missing link of the Appalachians: Geological Society of America Special Paper 330, p. 93–111.

Kunk, M.J., and Burton, W.C., 1999, 40Ar/ 39Ar age-spectrum data for amphi-bole, muscovite, biotite, and K-feldspar samples from metamorphic rocks in the Blue Ridge anticlinorium, northern Virginia: U.S. Geological Sur-vey Open-File Report 99-552, 111 p.

Kunk, M.J., Wintsch, R.P., Southworth, S., Mulvey, B., Naeser, C., and Naeser, N., 2004, Multiple Paleozoic metamorphic histories, fabrics, and faulting in the Westminster and Potomac composite terranes, central Appalachian Piedmont, northern Virginia and southern Maryland, in Southworth, S., and Burton, W., eds., Geology of the National Capital Region—fi eld trip guidebook: U.S. Geological Survey Circular 1264, p. 163–188.

Kunk, M.J., Wintsch, R.P., Naeser, C.W., Naeser, N.D., Southworth, C.S., Drake, A.A., Jr., and Becker, J.L., 2005, Contrasting tectonothermal domains and faulting in the Potomac terrane, Va.-Md.,; discrimination by 40Ar/39Ar and fi ssion-track thermochronology: Geological Society of America Bulletin, v. 117, no. 9/10, p. 1347–1366, doi: 10.1130/B25599.1.

Meyer, E.E., Aronson, J., and Haynes, J.T., 2005, K/Ar dating of Blue Ridge thrusting and of its infl uence on sediment input to two Pennsylvanian-age depocenters in the Appalachian basin: Geological Society of America Abstracts with Programs, v. 37, no. 2, p. 20.

Mitra, G., and Elliott, D., 1980, Deformation of basement in the Blue Ridge and the development of the South Mountain cleavage, in Wones, D.R., ed., Proceedings of the Caledonides in the USA, International Geologi-cal Correlation Programme, Project 27—Caledonide orogen: Blacksburg, Virginia, Virginia Polytechnic Institute and State University, Department of Geological Sciences Memoir 2, p. 307–311.

Mulvey, B.K., 2003, Devonian recrystallization of Silurian white mica in the West-minster terrane of Maryland, identifi ed by petrography and 40Ar/39Ar analyses [unpublished M.S. thesis]: Bloomington, Indiana, Indiana University, 55 p.

Muller, P.D., Candela, P.A., and Wylie, A.G., 1989, Liberty Complex; Poly-genetic mélange in the central Maryland Piedmont, in Horton, J.W., Jr., and Rast, N., eds., Mélanges and olistostromes of the U.S. Appalachians: Geological Society of America Special Paper 228, p. 113–134.

Muth, K.G., Arth, J.G., and Reed, J.C., Jr., 1979, A minimum age for high-grade metamorphism and granitic intrusion in the Piedmont of the Potomac River gorge near Washington, D.C: Geology, v. 7, p. 349–350, doi: 10.1130/0091-7613(1979)7<349:AMAFHM>2.0.CO;2.

Naeser, N., Naeser, C., Southworth, S., Morgan, B., and Schultz, A., 2004, Paleozoic to recent tectonic and denudation history of rocks in the Blue Ridge province, central and southern Appalachians—Evidence from fi s-sion-track thermochronology: Geological Society of America Abstracts with Program, v. 36, no. 2, p. 114.

Nickelsen, R.P., 1956, Geology of the Blue Ridge near Harpers Ferry, West Vir-ginia: Geological Society of America Bulletin, v. 67, no. 3, p. 239–269.

Orndorff, R.C., and Epstein, J.B., 1994, A structural and stratigraphic excur-sion through the Shenandoah Valley, northern Virginia: U.S. Geological Survey Open-File Report 94-573, 24 p.

Orndorff, R.C., Epstein, J.B., and McDowell, R.C., 1999, Geologic map of the Mid-dletown quadrangle, Frederick, Shenandoah, and Warren Counties, Virginia: U.S. Geological Survey Geologic Quadrangle Map GQ-1803, scale 1:24,000.

Pazzaglia, F.J., and Brandon, M.T., 1996, Macrogeomorphic evolution of the post-Triassic Appalachian Mountains determined by deconvolution of the offshore basin sedimentary record: Basin Research, v. 8, p. 255–278, doi: 10.1046/j.1365-2117.1996.00274.x.

Ratcliffe, N.M., 2006, Tremadocian to Pridolean accretionary arc, collisional, and post-collisional aspects of the Taconide zone, northeastern US: Geo-logical Society of America Abstracts with Program, v. 38, no. 2, p. 9.

Reed, J.C., Jr., Marvin, R.F., and Mangum, J.H., 1970, K-Ar ages of lampro-phyre dikes near Great Falls, Maryland-Virginia: U.S. Geological Survey Professional Paper 700-C, p. C145–C149.


Reed, J.C., Jr., Sigafoos, R.S., and Fisher, G.W., 1980, The river and the rocks—The geologic story of Great Falls and the Potomac River Gorge: U.S. Geological Survey Bulletin 1471, 75 p.

Reinhardt, J., 1974, Stratigraphy, sedimentology, and Cambrian-Ordovician paleo-geography of the Frederick Valley, Maryland: Maryland Geological Survey Report of Investigations 23, 74 p.

Reinhardt, J., 1977, Cambrian off-shelf sedimentation, central Appalachians: Society of Economic Paleontologists and Mineralogists Special Publica-tion, v. 25, p. 83–112.

Rodgers, J., 1970, The tectonics of the Appalachians: New York, New York, Wiley-Interscience, 271 p.

Safford, J.M., 1856, A geological reconnaissance of the State of Tennessee: Nash-ville, Tenn., Tennessee Geological Survey, 1st Biennial Report, 164 p.

Schoenborn, W.A., 2001, Structural geometry, kinematics, and strain in Pied-mont rocks, southwestern Maryland and northern Virginia: Geological Society of America Abstracts with Programs, v. 33, no. 1, p. A–4.

Schoenborn, W.A., 2002, Conditions of deformation in the Mather Gorge and Sykesville Formations, Potomac River, Maryland and Virginia: Geologi-cal Society of America Abstracts with Programs, v. 34, no. 1, p. A–19.

Scotford, D.M., 1951, Structure of the Sugarloaf Mountain area, Maryland, as a key to Piedmont stratigraphy: Geological Society of America Bulletin, v. 62, p. 45–76.

Southworth, S., 1994, Geologic map of the Bluemont quadrangle, Loudoun and Clarke Counties, Virginia: U.S. Geological Survey Geologic Quadrangle Map GQ-1739, scale 1:24,000.

Southworth, S., 1995, Geologic map of the Purcellville quadrangle, Loudoun County, Virginia: U.S. Geological Survey Geologic Quadrangle Map GQ-1755, scale 1:24,000.

Southworth, S., 1996, The Martic fault in Maryland and its tectonic setting in the central Appalachians, in Studies in Maryland geology: Maryland Geological Survey Special Publication 3, p. 205–221.

Southworth, S., 1998, Geologic map of the Poolesville quadrangle, Frederick and Montgomery Counties, Maryland and Loudoun County, Virginia: U.S. Geological Survey Geologic Quadrangle Map GQ-1761, scale 1:24,000.

Southworth, S., 1999, Geologic map of the Urbana quadrangle, Frederick and Montgomery Counties, Maryland: U.S. Geological Survey Quadrangle GQ-1768, scale 1:24,000.

Southworth, S., 2005, Kinematics of the Short Hill fault; late Paleozoic contrac-tional reactivation of an early Paleozoic extensional fault, Blue Ridge–South Mountain anticlinorium, Northern Virginia and southern Maryland: U.S. Geological Survey Bulletin 2136, 88 p.

Southworth, S., and Brezinski, D.K., 1996a, Geology of the Harpers Ferry quadrangle, Virginia, Maryland, and West Virginia: U.S. Geological Sur-vey Bulletin 2123, 33 p. with map, scale 1:24,000.

Southworth, S., and Brezinski, D.K., 1996b, How the Blue Ridge Anticlino-rium in Virginia becomes the South Mountain Anticlinorium in Maryland: Studies in Maryland Geology: Maryland Geological Survey Special Pub-lication, v. 3, p. 253–275.

Southworth, S., and Brezinski, D.K., 2003, Geologic map of the Buckeystown quadrangle, Frederick and Montgomery Counties, Maryland and Loud-oun County, Virginia: U.S. Geological Survey Geologic Quadrangle Map GQ-1800, scale 1:24,000.

Southworth, S., Aleinikoff, J.N., Kunk, M.J., Naeser, C.W., and Naeser, N.D., 2005, Geochronology of the Great Smoky Mountains National Park region, TN/NC with correlation to the rocks and orogenic events of the Appalachian Blue Ridge; Blue Ridge geology geotraverse east of the Great Smoky Mountains National Park, Western North Carolina, in Hatcher, R.D., Jr., and Merschat, A.J., eds., Carolina Geological Society Annual Field Trip: Knoxville, Tennessee, University of Tennessee, p. 45–56.

Southworth, S., Brezinski, D.B., Drake, A.A., Jr., Burton, W.C., Orndorff, R.C., Froelich, A.J., Reddy, J., and Daniels, D., 2002, Digital Geologic Map of the Frederick 30 by 60 Minute Quadrangle, Maryland, Virginia, and West Virginia. U.S. Geological Survey Open-File Report 02-437, scale 1:100,000.

Southworth, S., Burton, W.C., Schindler, J.S., Froelich, A.J., and Drake, A.A., Jr., 2006, Geologic map of Loudoun County, Virginia: U.S. Geological Survey Geologic Investigations Map GI-2553, scale 1:50,000.

Stose, G.W., 1906, The sedimentary rocks of South Mountain, Pennsylvania: The Journal of Geology, v. 14, p. 201–220.

Stose, G.W., and Jonas, A.I., 1935, Limestones of Frederick Valley, Maryland: Washington Academy of Science Journal, v. 25, no. 12, p. 564–565.

Stose, A.J., and Stose, G.W., 1946, Geology of Carroll and Frederick Coun-ties, in The physical features of Carroll County and Frederick County: Baltimore, Md., Maryland Department of Geology, Mines, and Water Resources, p. 11–131.

Thomas, B.K., 1952, Structural geology and stratigraphy of Sugarloaf anti-clinorium and adjacent Piedmont area, Maryland [unpublished Ph.D. dis-sertation]: Baltimore, Maryland, The Johns Hopkins University, 95 p.

Thomas, W.A., Becker, T.P., Samson, S.D., and Hamilton, M.A., 2004, Detrital zircon evidence of a recycled orogenic foreland provenance for Allegha-nian clastic-wedge sandstones: The Journal of Geology, v. 112, p. 23–37, doi: 10.1086/379690.

Van Staal, C.R., and Whalen, J.B., 2006, Magmatism during the Salinic, Aca-dian, and Neoacadian orogenies: Geological Society of America Abstracts with Programs, v. 38, no. 2, p. 31.

Walcott, C.D., 1896, Cambrian rocks of Pennsylvania: U.S. Geological Survey Bulletin 134, p. 1–43.

Wilson, J.T., 1966, Did the Atlantic close and then re-open?: Nature, v. 211, p. 676–681, doi: 10.1038/211676a0.

Wojtal, S., 1989, Day one—Blue Ridge anticlinorium and Massanutten syn-clinorium in western Maryland, in Woodward, N.B., ed., Geometry and deformation fabrics in the central and southern Appalachians Valley and Ridge and Blue Ridge; 28th International Geological Congress: American Geophysical Union Field Trip Guidebook T357, p. 4–14.

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