Road Log and Stop Descriptions

This trip transects the Piedmont beginning with Stops 1 and 2 in the chlorite-grade rocks of the Westminster terrane west of the Pleasant Grove fault. The remainder of the trip will be in several different tectonothermal domains of the Potomac composite terrane east of the Pleasant Grove fault. Stop 3 is in biotite-grade rocks of the Blockhouse Point domain, the westernmost part of the Potomac terrane. Stop 4 and lunch is in kyanite-grade rocks of the western Bear Island domain. Stop 5 is in staurolite-sillimanite-grade rocks of the eastern Bear Island domain. Stop 6 is in sillimanite-grade rocks retrograded to chlorite grade in the Stubblefield Falls domain in the Plummers Island fault zone.

Stop 1. Hyattstown.

Western Westminster terrane above Hyattstown thrust fault, in metasiltstone and phyllite of the Marburg Formation, Urbana, Md., 7.5-minute quadrangle (Southworth, 1999).

The rocks at this stop are chlorite-grade metasiltstone, phyllite, and thin quartzite assigned to the Neoproterozoic and Early Cambrian Marburg Formation (Southworth, 1999). The foliation regionally and in the rocks here strikes northeast and dips steeply to the southeast. This foliation is a composite of nearly coplanar cleavages, bedding, and transposed vein quartz. Where bedding is at a high angle to cleavage it can be conspicuous, as it is on the left side of the knoll here as we climb up from the parking area. Here, graded beds of metasiltstone strike northwest and dip moderately to the southwest. Bedding is made conspicuous by beds that are relatively rich and poor in hematite, giving the rock a strikingly banded appearance (fig. 8A). Also visible here are isoclinally folded quartz veins (fig. 8B). These are significant in that they cut older foliation, but have limbs transposed into the schistosity, and axial planes parallel to the later composite foliation. They thus demonstrate the composite nature of the foliation here. Late quartz veins do exist, however, that are relatively undeformed and cut this cleavage at moderately high angles.

Regionally, similar Marburg Formation rocks comprise a ~12-km-wide lithotectonic belt in the eastern part of the Westminster terrane. The belt is separated from the Potomac composite terrane to the east by the Pleasant Grove fault zone. To the west 4.7 km, the 50-km-long Hyattstown thrust fault places rocks of the Marburg Formation above greenstone and phyllite of the Sams Creek Formation. Rocks of the Marburg Formation are distinctively different from the rocks to the west of the Hyattstown thrust fault, which are a diverse assemblage of phyllite, phyllonite, metasiltstone, quartzite, metagraywacke, greenstone, marble, and metalimestone (Southworth and others, 2002). In contrast to the fine-grained rocks of the Sams Creek Formation and Ijamsville Phyllite immediately west of the fault, the phyllite and metasiltstone of the Marburg Formation locally contain paragonite as well as porphyroblasts of albite and chloritoid. The metasiltstone commonly has a distinctive pinstriped appearance of quartz laminae and ribbons interlayered with chlorite phyllite; a later crenulation cleavage crinkles these ribbons, in many places to produce a characteristic texture. The quartz laminae may represent thin turbidites or metamorphic segregations. Within the 12-km-wide belt of the Marburg Formation there are a few meter-thick beds of metagraywacke and pebbly quartzite, and rare bodies of greenstone. To the northeast about 25 km, rocks of the Prettyboy Schist are in gradational contact with rocks of the Marburg Formation.

Age spectra from three samples from this part of the Westminster terrane (Kunk, unpub. data; Mulvey, 2003) give ages of ~435 to 430 Ma. These samples all contain muscovite preserved in multiple generations of spaced cleavage. Typically, older generations contain muscovite intergrown with chlorite, whereas later generations contain primarily chlorite. We interpret the reproducible early Silurian ages of muscovites from several samples, including a marble, to be a close approximation to the time of S₂ muscovite growth, thus indicating a Silurian cleavage-forming event.

Stop 2. Black Rock Mill.

Metagraywacke and phyllite of the Marburg Formation, eastern part of Westminster terrane, Damascus, Md., 7.5-minute quadrangle (A.A. Drake, Jr., unpub. data; Southworth and others, 2002).

The rocks exposed on the north bank of Great Seneca Creek are metagraywacke interbedded with phyllite and metasiltstone assigned to the Marburg Formation by A.A. Drake, Jr. (unpub. data) and Southworth and others (2002). At this location we are about 1.75 km west of the Pleasant Grove fault zone. Like the rocks at Stop 1, the regional foliation here strikes northeast, dips steeply to the southeast and locally is vertical. Well-cemented metagraywacke beds are relatively resistant, and probably are responsible for the exposure here. They form rapids in the streambed, and are well exposed in a bluff across and downstream from the mill. There the nearly vertical schistosity of the rocks is axial planar to vertical isoclinal folds (fig. 9A). Note also that the phyllitic limbs of these folds contain lozenges or boudins of quartz, undoubtedly representing dismembered lenses of once-continuous quartz veins (fig. 9B)

In thin section (fig. 4B) the phyllites are composed of phyllosilicate-rich layers (~60 percent) separated by quartz microlithons and veins (~40 percent). The phyllosilicate-rich layers contain ~90 percent muscovite flakes up to 80 µm long. Accessory minerals include 10- to 100-µm grains of chlorite (5 percent), rutile (5 percent), and quartz (<1 percent). The quartz microlithons and veins are elongate parallel to compositional layering and range in thickness from 1 to 9 mm. Although several generations of white mica can be identified, by far the dominant and coarsest mica is S₂. Because of its coarse grain size, it is also the mica that is concentrated in the sample analyzed isotopically. Replicate analyses of two different samples both record an age of ~375 Ma, approximately the age of S₂ mica. This age is distinct from the age determined for the samples at Stop 1, and clearly reflects a Middle Devonian cleavage-forming event that was not recorded in the phyllites of Stop 1. This is the basis for interpreting a fault or boundary of a shear zone between Stops 1 and 2.

Stop 3. Seneca Road/Northern Virginia Regional Park.

Chlorite phyllonite of Mather Gorge Formation, western Blockhouse Point domain, Potomac terrane, Seneca, Va.-Md., 7.5-minute quadrangle (Drake and others, 1999).

The rocks exposed here are chlorite phyllite and phyllonite with transposed and folded vein quartz speckled with magnetite. These chlorite-grade rocks were called the Upper Pelitic Schist of the Wissahickon Formation by Fisher (1970), chlorite schist and lesser metasiltstone of the Peters Creek Schist (Drake, 1989), and later were assigned to the metagraywacke and lesser semi-pelitic schist unit of the Mather Gorge Formation by Drake and others (1999). This is the westernmost part of the Blockhouse Point domain in the western part of the Potomac terrane of Kunk and others (in press) and is Stop 1 of Drake (1989).

The regional composite foliation strikes northeast and dips gently to moderately to the southeast. The eastern margin of the Mesozoic Culpeper basin unconformably overlies these rocks about 0.5 km to the west beneath Quaternary alluvium. The southwestward projection of the Pleasant Grove fault is about 1.5 km to the west but is also unconformably overlain by Late Triassic rocks of the Culpeper basin and by alluvium.

At this stop, we can recognize both muddy and gritty protoliths, now phyllites and thin lenses of lithic and arkosic quartzites. These are intruded by several generations of quartz veins, and deformed and folded by several cleavage-forming events. A composite cleavage composed of S₁ and S₂ (at least) gives the overall steep cleavage of the outcrop (fig. 10). This composite cleavage contains transposed bedding, intrafolial folds, and dismembered quartz veins, and shear-band cleavage records a dextral, transpressive fabric. An S₃ cleavage anastomoses across this structure, giving the composite cleavage a wavy fabric. Finally, there is a late, gently dipping discontinuous cleavage, S₄, that locally cuts all previous structures.

In thin section our nearby analyzed sample (Kunk and others, in press) (fig. 4C) is a differentiated quartz-muscovite phyllonite. Most phyllosilicate layers are dominated by muscovite (>90 percent), but contain ~10 percent chlorite, with accessory sphene, magnetite, and albite, and trace amounts of epidote. Most of the muscovite grains that define the foliation in these layers are 200 to 400 µm long and 50 µm thick, but some are over 500 µm long and 100 µm thick.

The age spectrum of this sample is very slightly sigmoidal, but forms an age plateau (Fleck and others, 1977; Haugerud and Kunk, 1988) at 362±2 Ma that contains 70 percent of the ³⁹Ar released from the sample. The vast majority of the gas released from the sample is from muscovite flakes that together with chlorite define S₂. The muscovite-chlorite association is interpreted to reflect below-closure growth, and the plateau age is interpreted to represent the age of this cleavage-forming event. By this analysis, the younger cleavages, S₃ and S₄, must be Carboniferous or younger. This interpretation is completely consistent with our results from the Plummers Island fault (Stop 6).

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

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 to the south.

Stop 4. Great Falls Park.

Metagraywacke and staurolite schist of the Mather Gorge Formation, Bear Island domain, Vienna and Falls Church, Va.-Md., 7.5-minute quadrangles (Drake and Lee, 1989; Drake and Froelich, 1997).

The level ground of the parking area and Visitor Center is a bedrock strath terrace of the Potomac River that may be as young as 30 ka (Bierman and others, this volume). The current position of the Great Falls of the Potomac River is situated at perhaps the thickest section of metagraywacke exposed in the Potomac River valley (fig. 11A). Formerly called the Wissahickon Formation and Peters Creek Schist, these rocks were renamed the Mather Gorge Formation by Drake and Froelich (1997), based on this type locality, and are in the Bear Island domain of Kunk and others (in press). In general, rocks mapped as Mather Gorge Formation are dominantly schist interbedded with metagraywacke; metagraywacke interbedded with schist is subordinate but best exposed here. The metagraywacke consists of well-graded beds of quartz and detrital feldspar (fig 11B). Soft-sedimentary features exposed along Bear Island to the east support the interpretation that these are turbidite deposits (Hopson, 1964; Drake and Morgan, 1981). Isoclinal recumbent folds of metagraywacke and schist are locally refolded upright. About 1 km to the west are bodies of ultramafic rocks, and about 1.5 km to the east are bodies of amphibolite and granodiorite/pegmatite. Immediately east of the overlook along the bluffs is one of the northwest-dipping, undeformed lamprophyre dikes that has conventional K-Ar biotite cooling ages of ~360 Ma (Reed and others, 1970), similar to the dikes shown in figure 11C.

Stop 5. Difficult Run.

Migmatitic schist of the Mather Gorge Formation, amphibolite, and Bear Island Granodiorite, eastern boundary of Bear Island domain, Potomac terrane, Falls Church, Va.-Md., 7.5-minute quadrangle (Drake and Froelich, 1997)

This outcrop of a variety of rocks was scoured clean by the floods of Hurricane Agnes in 1972, and was Virginia stop 8 of Reed and others (1980) and stop 5 of Drake (1989). These sillimanite-grade rocks were called the Upper Pelitic Schist of the Wissahickon Formation (Fisher, 1970) and the metagraywacke and semipelitic schist of the Mather Gorge Formation (Drake and Froelich (1997); they are the Bear Island domain of the Mather Gorge Formation of Kunk and others (in press). The dominant rock is gray coarsely mottled mica schist that contains conspicuous dark-green 1- to 2-cm-diameter crystals of garnet, staurolite, cordierite(?), and shimmer aggregates. The schist also contains segregated leucosomes of quartz and albite or sodic oligoclase, forming a wavy migmatite. Several 2-m-thick layers of dark-green, fine-grained epidote-plagioclase-hornblende amphibolite are found within the schist, and are probably gabbro sills or basalts whose origin may have been either intrusive, extrusive, or as olistoliths. Drake (1989) suggests that the amphibolite intruded the schist prior to deformation. The known distribution of amphibolite is restricted to the sillimanite-grade migmatites of the eastern part of the Bear Island domain.

Both the amphibolite and schist are cut by tabular dikes of light-gray granite and granodiorite called the Bear Island Granodiorite (fig. 12). Streaks of biotite define flow foliation in these dikes. The largest dike here is cut by a pegmatite 5 to 15 cm thick. Small bodies of Bear Island Granodiorite are abundant in the migmatites of the Bear Island domain and in the deformed schists of the Stubblefield Falls domain. The distribution of these dikes seems to be structurally controlled, in that dikes are most common cutting fractures and boudin necks in the amphibolite. It is possible that these dikes were passively injected into dilational sites caused by breakup of the competent amphibolite layer within ductile schist under anatectic conditions. The Bear Island Granodiorite is commonly seen associated with amphibolite, but it is recognized only where metamorphic grade reached at least sillimanite zone, an area that spans both east and west of the known distribution of amphibolite. Most of the dikes are undeformed as they are here, but similar dikes are locally folded. Muscovite from this pegmatite yielded the 469-Ma Rb-Sr cooling ages (Muth and others, 1979). Fisher (1963, 1970), Hopson (1964), and Drake (1989) suggest that the granodiorite had a source at depth, and was emplaced well after the climax of regional metamorphism and deformation.

Amphibole from an outcrop near here yielded a disturbed ⁴⁰Ar/³⁹Ar age spectrum with an isochron age of 475 Ma, which is interpreted as the time of cooling through 500°C. Further cooling below 500°C (staurolite to biotite grade) was complete by the end of the Devonian. With this Early Ordovician amphibole cooling age, the highest metamorphic grade and melting of these rocks (T>650°C) could very possibly be older than Ordovician and thus predate the classic Taconian orogeny.

Stop 6. Plummers Island Shear Zone.

Phyllonitic schist and migmatite, and migmatitic phyllonitic schist, Mather Gorge Formation, eastern boundary of Stubblefield Falls domain of the Mather Gorge Formation, Potomac terrane, Falls Church, Va.-Md., 7.5-minute quadrangle (Drake and Froelich, 1997).

The rocks seen at this stop are very difficult to interpret (Drake, 1989, Stop 6). Fisher (1970) mapped these sillimanite- to staurolite-grade rocks as the Upper Pelitic Schist of the Wissahickon Formation. Beneath the American Legion (I-495) Bridge, he mapped a transitional contact with the diamictite of the Sykesville Formation. Fisher (1963, 1970) and Hopson (1964) describe the transition between Peters Creek Schist and Sykesville diamictite as containing a decreasing amount of olistoliths in the Sykesville Formation. Drake (1989) described these rocks as retrograded chlorite-sericite phyllonite of the quartzose schist of the Peters Creek Schist, and called Fisher's (1963, 1970) contact with the rocks of the Sykesville Formation the Plummers Island fault. He suggested that the Sykesville is choked with olistoliths of Peters Creek Schist near its contact with that unit. Drake and Froelich (1997) later classified rocks at this stop as the upper part of the Sykesville Formation, with the upper part containing 50 percent or more phyllonite olistoliths, interpreted as having been derived from rocks of the Mather Gorge Formation. Drake and Froelich (1997) placed the Plummers Island fault, the contact between phyllonite of the Mather Gorge Formation and the rocks of the upper part of the Sykesville Formation, about 0.5 km to the west of this stop.

We retain the original contact of Fisher (1963, 1970) as the Plummers Island fault of Drake (1989), but suggest that this linear fault was reactivated as a wide Alleghanian shear zone. The major (Alleghanian) foliation in rocks across the fault strikes northeast and dips steeply to the northwest. This steep northwest-dipping to near-vertical foliation begins in the Stubblefield Falls domain and continues with a decreasing amount of muscovite recrystallization for 11 km east, to the Rock Creek shear zone. To the west of the Plummers Island fault of Fisher (1963, 1970), the rocks exposed on scoured outcrops on the south side of the Potomac River appear to be a phyllonitized migmatite that has been later remigmatized and sheared and retrograded. Blocks and rafts of the disrupted phyllonitic migmatite appear as "olistoliths," along with subrounded quartz "cobbles" and rootless quartz veins that are transposed (fig. 13). The blocks and rafts of first-generation phyllonitic migmatite contain foliations that strike in many different directions, and are supported by a matrix of feldspathic second-generation migmatite (fig. 12). The matrix is migmatitic, not granular quartzofeldspathic like the Sykesville. Overprinted on this is a strong north-south foliation that is subparallel to the late (Alleghanian) movement on the Plummers Island fault.

⁴⁰Ar/³⁹Ar age spectra of muscovites from the Stubblefield Falls domain provide estimates of the minimum time of cooling (S₁) through 350°C, decreasing from 375 Ma to the west to 354 Ma here at the Plummers Island fault. ⁴⁰Ar/³⁹Ar age spectra of muscovites from the rocks of the Sykesville Formation to the east give an estimate of the minimum time of cooling through 350°C at 357 Ma. This age is virtually identical to the time of cooling on the eastern side of the Stubblefield Falls domain, and is interpreted as a minimum age for thrusting of the Sykesville Formation over the Mather Gorge Formation in the Devonian (Acadian).

Estimates of the time of below-closure growth of muscovite (S₂) also decrease from west to east in the Stubblefield Falls domain, to a minimum of 328 Ma at the Plummers Island fault. In Sykesville Formation rocks to the east, on the other side of the fault, the below-closure ages increase dramatically from 304 Ma at the fault to 332 Ma near the Rock Creek shear zone. The ~304-Ma age within the fault is a maximum estimate of the time of the last Alleghanian movement on the fault.

Return to vehicles and retrace route back to Georgetown Pike, Va. 193.

U.S. Department of the Interior, U.S. Geological Survey
URL: https:// pubsdata.usgs.gov /pubs/circ/2004/1264/html/trip5/log.html
For more information, contact Michael Kunk
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