Geological Survey Bulletin 1265 Ancient Lavas in Shenandoah National Park Near Luray, Virginia

STRUCTURAL SETTING

The northwestern flank of the Catoctin Mountain-Blue Ridge anticlinorium in the Luray area is marked by a large asymmetrical northeast-trending anticline. The eastern limb of the fold is gently dipping; the western limb is vertical or overturned (geologic map in pocket). Most of the crest of the Blue Ridge is capped by greenstone of the Catoctin Formation in the upper limb of the anticline, but in the area between Thornton Gap and Stony Man, the Catoctin Formation has been removed by erosion and the underlying granitic gneisses form the crest of the ridge. The steep western limb of the fold is marked by a series of low foothills composed of resistant rocks of the Catoctin Formation and Lower Cambrian quartzites in which bedding is vertical or overturned to the east. In the Shenandoah salient, west of Big Meadows (map in pocket) the gentle upper limb of the anticline has smaller folds superimposed and is complicated by the wedging out of the Catoctin beneath the Cambrian sedimentary rocks. The anticline is cut by several high-angle faults. The Stanley fault, which cuts the fold diagonally, brings the Lower Cambrian sedimentary rocks into contact with the plutonic basement rocks, a relationship that in the past has been explained as the result of a major overthrust along the western foot of the Blue Ridge. Probably no such overthrust exists in the Luray area or in the Elkton area to the south (King, 1950).

Between the Shenandoah salient and U.S. Highway 211, granitic basement is exposed over wide areas on the western slopes of the Blue Ridge in the core of the anticline. On the gentle eastern limb of the fold the Catoctin is cut off by steeply dipping faults which have brought the granitic basement to the surface, where it crops out in the peaks of Oventop, Hazel Mountain, and Old Rag Mountain east of the main Blue Ridge.

The pattern of cleavage and lineation in the Catoctin Formation and Chilhowee rocks conforms to the pattern in the rest of the Catoctin Mountain-Blue Ridge anticlinorium. Cleavage strikes north or northeast and dips southeast; dips of 50° or 60° are common on the eastern limb, whereas dips of 30° or 40° predominate on the overturned western limb of the anticline. Local variations in the dip of the cleavage are related to differences in lithology or to fanning in subsidiary folds. Lineation lies in the cleavage plane and plunges down the dip to the east or southeast, normal to the fold axes. In the sedimentary rocks it is marked by the elongation of phyllite fragments, parallel orientation of minerals, or a faint grooving of the cleavage planes. In the volcanic rocks it is marked by parallel orientation of chlorite blebs.

Slickensiding normal to the fold axes was found on bedding planes and flow surfaces in a few places, which indicates that flexure folding, as well as shear folding, played a part in the deformation of the area.

ROCKS UNDERLYING THE CATOCTIN FORMATION

General character

Two major lithologic types form the basement complex underlying the Catoctin Formation in the Luray area. The first of these is a dark rudely layered granodiorite or quartz monzonite, shown as the Pedlar Formation on the geologic map of Virginia (Virginia Division of Mineral Resources, 1963) and by Allen (1963). It is exposed chiefly along the crest of the Blue Ridge and on the western flank of the mountains. The second, a coarse-grained light-colored granite designated the Old Rag Granite by Furcron (1934) for exposures on Old Rag Mountain, is exposed in the area east of the crest of the Blue Ridge.

The granodiorite is medium to coarse grained, light greenish gray where fresh and weathers to light brownish gray or white. It is composed chiefly of plagioclase, quartz, potassium feldspar, hypersthene, biotite, chlorite, magnetite, and garnet, and it contains minor amounts of epidote and albite. The chlorite forms dark-green clots, probably derived from alteration of ferromagnesian minerals. The plagioclase grains are generally greenish white, although where alteration has been minor they may be clear and show twin striations. In most places, shearing has broken the feldspars and mashed the chlorite blebs.

Most of the granodiorite exposed on the western slopes of the Blue Ridge is only faintly foliated. Along the Skyline Drive between Hughes River Gap and Thornton Gap the granodiorite is well foliated, the foliation being defined by parallel clots and layers of light and dark minerals that are 1 millimeter to 10 centimeters thick. In some outcrops the dark minerals are in parallel spindle-shaped clots which give a spotted appearance to the rock where it is broken across the lineation. A few pegmatitic stringers and pods lie parallel to the foliation. At several places along U.S. Highway 211 east of Thornton Gap, the gneissic granodiorite contains layers composed principally of quartz. Locally, hydrothermal alteration of the granodiorite gneiss has produced a rock called unakite, which is composed of epidote, pink feldspar, and blue quartz. Jonas and Stose (1939) have suggested that the granodiorite is at least in part derived from granitization of a sedimentary series. Radiometric age determinations on zircon from the granodiorite near the south end of the Marys Rock tunnel indicate that the rock was formed about 1,100 million years ago (Davis and others, 1958).

The Old Rag Granite of Furcron (1934) is considerably coarser grained than the granodiorite; individual feldspar crystals are as much as 3 centimeters long. The only abundant minerals are milky-white perthitic microcline and dark-gray or blue smoky quartz. A very small amount of chlorite is present, but no primary ferromagnesian minerals remain. The rock is commonly light gray or white; foliation is defined by parallel stringers or elongate patches of dark quartz. Along faults the Old Rag Granite is commonly intensely sheared and, in places, is finely ground to form white mylonite.

The contact between the granodiorite and the Old Rag Granite is gradational through a zone several miles wide. In the transition zone the granodiorite is more gneissic, finer grained, and much richer in biotite than it is elsewhere. Garnet seems to be confined exclusively to this zone. Dikelets and pods of blue-quartz granite similar to the Old Rag Granite occur in gneissic granodiorite at several places along Skyline Drive, on the flanks of Marys Rock, and along the fire road west of Skyland.

The foliation and layering in the granitic rocks are rudely parallel to the cleavage in the overlying Catoctin Formation over wide areas, but locally there is a distinct divergence between structures in the basement and those in the overlying rocks. A faint fracture cleavage in the basement rocks in many areas is believed to be related to the cleavage in the rocks above.

Pebbles of foliated granodiorite in the basal sedimentary member of the Catoctin Formation show that the foliation of the basement rocks antedated the deposition of the Catoctin sedimentary rocks. The fracture cleavage in the basement rocks was probably formed at the same time as the cleavage in the Catoctin and overlying rocks. The low-grade metamorphism of the basement rocks probably dates from this period.

Relation to the Catoctin Formation

The flows of the Catoctin Formation rest on an eroded surface of granodiorite and granite that has a relief of as much as 1,000 feet. Valleys in the erosion surface contain a layer of clastic sedimentary rocks as much as 150 feet thick beneath the greenstone. In other places the sedimentary rocks are only a few inches thick. Buried hills of granitic rocks beneath the Catoctin are exposed in cross section in the valleys of Whiteoak Run and Rose River. These exhumed hills superficially resemble intrusive masses of granite, but a widespread basal sedimentary layer surrounding them contains pebbles of gneiss and granite, which shows that the contact is an unconformity, not an intrusive contact.

Figure 4 shows the relationship of the basal sedimentary member of the Catoctin Formation to the pre-Catoctin erosion surface as it is exposed on the western slopes of the Blue Ridge between Hawksbill Mountain and Big Meadows. The diagram is based on a series of well-exposed sections from the base of the lowest porphyritic flow in the Catoctin to the base of the formation. The floodlike character of the lavas and the thickening of the basal sedimentary member in the lower areas in the pre-Catoctin surface are well illustrated in this area. These relationships are typical of those observed wherever the bottom contact of the Catoctin Formation was examined.

Figure 4. Stratigraphic relations between the Catoctin Formation and the granitic basement rocks along the western edge of the Catoctin outcrop area between Hawksbill Mountain and Big Meadows.

ROCKS OVERLYING THE CATOCTIN FORMATION

General character

The Chilhowee Group, which overlies the Catoctin Formation, has been well described in other areas in northern Virginia and Maryland (King, 1950; Nickelsen, 1956; Whitaker, 1955; Allen, 1963), and only a general description is necessary here. The Chilhowee Group is subdivided into three formations—the Weverton Formation (at the base), the Harpers Formation, and the Antietam Quartzite (at the top); however, these formations are not distinguished on the geologic map of the Luray area (in pocket). With the exception of the Shenandoah salient and a few small areas around Chapman Mountain on the south and Neighbor Mountain on the north, all the Chilhowee exposures in the area studied are confined to the nose and lower limb of the Blue Ridge anticline where shearing has been intense and outcrops are poor. The lower beds (equivalent to the Weverton Formation) are coarse-grained, thick-bedded, ferruginous quartzites and graywackes, commonly dark gray, brown, purple, or bluish gray with color banding parallel to bedding. Scattered quartz pebbles are common and crossbedding is present in many exposures. Dark slaty interbeds are common. These beds grade upward into a poorly exposed sequence of buff or light-gray sandy phyllite and argillite containing thin beds of quartzite; this sequence represents the Harpers Formation. Bedding in the Harpers Formation is obscured by cleavage except where competent quartzite beds are present. The upper part of the formation is characterized by jetblack, blue, or purple quartzite beds within the phyllite sequence. The combined thickness of the Weverton and Harpers Formations, measured along U.S. Highway 211, is about 2,300 feet; in the Shenandoah salient and in the Elkton area just to the south, King (1950) found the thickness to be 1,900 to 2,500 feet.

The Antietam Quartzite is a white, medium- to fine-grained, sugary to glassy rock that forms prominent cliffs and ledges in areas where it has not been completely shattered during deformation. Where shattering has been intense, outcrops of the Antietam are rare, and the ground is littered with quartzite fragments a few inches in diameter. The upper part of the formation contains abundant worm tubes (Scolithus) normal to bedding, but they are absent in the lower part. Megascopic cleavage is poorly developed, and bedding is obscure in many outcrops. According to King (1950), the Antietam is about 800 feet thick in the Elkton area; no reliable estimate was made in the Luray area. The Antietam passes upward through a few feet of shaly beds into the Tomstown Dolomite of well-established Early Cambrian age; the Tomstown marks the bottom of the great Cambrian and Ordovician carbonate sequence of the Appalachian Valley.

Relation to the Catoctin Formation

The contact between the Catoctin Formation and the overlying sedimentary beds is marked by a layer of purple or blue slate as much as 150 feet thick, which was mapped by King (1950) as the Loudoun Formation in the Elkton area. The volcanic nature of this rock is clear; it contains numerous amygdules (filled gas bubbles), and in thin section it shows a relic basaltic fabric. Furcron and Woodward (1936) have described this as an altered rhyolite flow, but the fabric and composition indicate that it is more probably derived from an andesite or basalt. Its petrographic character will be discussed in more detail, but its stratigraphic relations have an important bearing on the relation of the Catoctin Formation to the overlying rocks. In the western part of the Shenandoah salient (map in pocket) the greenstone of the Catoctin is absent, and the purple slate lies directly on granodiorite. Still farther west the slate is absent and the basal beds of the Chilhowee rest directly on faintly foliated coarse-grained granodiorite. The purple slate is thus present in several places where the greenstone is absent. Furcron and Woodward (1936) consider this to be evidence of an angular unconformity beneath the Chilhowee and the purple slate. As no discordance was noted between the Catoctin and Chilhowee, however, it seems more likely that the relationship is a result of overlap of the Catoctin Formation onto a topographic high of basement rocks; the purple slate can then be assigned to the Catoctin Formation, where its volcanic character and similarity to other rocks in the volcanic sequence seem to indicate that it belongs.

Along the Skyline Drive about 4 miles north of Thornton Gap, the slate is absent, and the basal quartzite of the Chilhowee contains rounded cobbles of fine-grained massive Catoctin lava; this indicates that erosion preceded deposition of the Chilhowee sedimentary rocks. Evidence of a major unconformity is lacking, however. Cloos (1951) reports that volcanic rocks are interlayered with the lower beds of the Chilhowee Group in some areas in Maryland, and Bloomer (1950) has pointed out that the similarity of the volcanic rocks of the Catoctin to the volcanic flows in the Chilhowee Group near Tye River Gap and the James River indicates that there was no major erosion interval between the Catoctin and the Chilhowee.

GENERAL CHARACTER OF THE CATOCTIN FORMATION

In the Luray area, most of the Catoctin Formation consists of lava flows altered to greenstone by low-grade regional metamorphism. Sedimentary members are common but not extensive, and none of these interbeds is more than 40 feet thick, although the basal sedimentary layer locally reaches 150 feet. These sedimentary members mostly are graywacke, arkose, and conglomerate derived from the underlying granitic rocks and micaceous (sericite) phyllite perhaps derived from volcanic ash. The greenstone is green, blue, purple, or gray and generally is so fine grained that only chlorite and epidote can be recognized with the hand lense. Porphyritic varieties containing conspicuous crystals of plagioclase (phenocrysts) as much as 1 cm long are present in several places in the greenstone sequence. In thin section the chief minerals of the greenstone can be identified as albite, chlorite, epidote, actinolite, and sphene, and minor amounts of pyroxene, magnetite, hematite, and ilmenite are also present. Most exposures show a well-developed cleavage, but cleavage is absent in some places. Primary structures such as bands of amygdules, columnar and platy jointing, and sedimentary dikes are preserved where deformation has not been intense. Flow breccias are common. Veinlets of epidote, milky quartz, and anthophyllite cut the greenstone in many places. Complete replacement of greenstone by irregular pods and veinlets of epidote and quartz has formed yellowish-green epidosite which is very common in some areas.

The present study of the Catoctin Formation was concentrated in the southern half of the area shown in the geologic map (in pocket) where excellent exposures of the volcanic sequence occur along the crest of the Blue Ridge between Stony Man and Big Meadows. North of Thornton Gap the Catoctin was not studied in detail.