Road Log and Stop Descriptions

Day 1 (Sunday, March 28, 2004)

The field trip begins at the I-64/I-295 interchange west of Richmond.

Stop 1. Neoproterozoic granitoid along unnamed, east-flowing tributary of Dover Creek.

Outcrop in creek upstream (west) of driveway.

The slopes and creek bed here contain excellent exposures of a distinctive type of Neoproterozoic granitoid (Zgr) within the State Farm Gneiss (fig. 3). The rock here is characterized by a somewhat spotted appearance, reflecting oriented clusters of biotite+amphibole. This same rock type also occurs as a small body some 10 km (6 mi) to the north (not shown on figure 3 because of scale). Although the full dimensions of this pluton have not been completely mapped out, it is exposed in several drainages to the west of Dover Creek covering an area of at least 2 × 0.5 km (1–0.3 mi) (A.K. Teepe, 2001, undergraduate geology thesis, College of William and Mary). Thin sections show that the rock is dominated by microcline, plagioclase, and quartz; dark-green amphibole, yellow-brown biotite, allanite, and sparse garnet are accessory minerals. Samples from this pluton are typically quartz syenite, with SiO₂ ranging from 60 to 69 weight percent in four samples (Owens and Tucker, 2003). One sample (south of this stop) yielded a U-Pb zircon age of 588+9/-12 Ma (Owens and Tucker, 2003).

Stop 2. State Farm Gneiss and Neoproterozoic granitoid in roadcut on north side of Va. 6.

Outcrop is in roadcut on north side of road.

This location represents one of the few actual roadcuts in this part of the Goochland terrane. Although highly iron-stained, this exposure shows a key intrusive relation between a mafic variety of the State Farm Gneiss (~57 weight percent SiO₂) and a leucocratic variety of Neoproterozoic granite (~70 weight percent SiO₂). Both rock types are exposed at various places in the roadcut, but the west end shows that a number of small (centimeter-scale) dikes of granite have intruded the gneiss. These dikes also are folded, indicating deformation following emplacement. A larger mass of the granite is exposed near the east end of the main roadcut, and here shows only a faint foliation. The granite here is more leucocratic than it is at Stop 1, but is mineralogically similar. Some differences include more abundant garnet, local occurrences of distinctive euhedral magnetite, and the presence of sphene. Samples from this roadcut give U-Pb zircon ages of 1039+18/-11 Ma for the State Farm Gneiss and ~654 Ma for the Neoproterozoic granite. The poorly constrained age for the granite is based on a single concordant zircon analysis, with five additional analyses showing considerable scatter (see Owens and Tucker, 2003, for additional discussion). Pegmatites of Paleozoic age, common throughout the Goochland terrane, also are present in this roadcut.

Stop 3. Maidens Gneiss at Hidden Rock Park.

Outcrops in whaleback exposures near bathrooms.

This locality contains some of the best exposures (outcrops and large blocks) of the Maidens Gneiss in this area. The gneiss here is a medium-grained, biotite plagioclase+quartz gneiss, with additional amphibole and (or) clinopyroxene present in some samples. All samples also contain sphene. The gneiss has obviously been highly injected by pegmatites for pegmatite is present in most exposures of the Maidens in this area. The protolith of the Maidens Gneiss is uncertain, but work is in progress to help answer this question on the basis of chemical compositions. Analyses of rocks from this park and several other locations (six samples, including the type locality at Maidens Cave, 4 km (2 mi) to the south) show a narrow range of bulk compositions: SiO₂=~55–62 weight percent; CaO=~4.2–6.8 weight percent; and K₂O=~2.7–4.0 weight percent (B.E. Owens, unpub. data). In addition, one sample from this locality has yielded a well-constrained U-Pb zircon age of 407±2 Ma (B.E. Owens and R.D. Tucker, unpub. data). The nearly concordant nature of the zircon analyses, in conjunction with the chemical compositions, suggests that this variety of Maidens Gneiss may have originated from an igneous protolith of intermediate composition (quartz monzodiorite to quartz monzonite or volcanic equivalents). The significance of the Paleozoic age is unclear at the present time, but work is in progress on additional samples to evaluate this surprising result.

The penetrative foliation in the gneiss, defined by amphibolite-facies minerals and microstructures, is folded into a series of northwest-verging, gently plunging, overturned antiforms and synforms (fig. 5). Burton and Armstrong (1997), in a study approximately on strike about 50 km (30 mi) southwest of this locality, interpret this fabric to be of Alleghanian age. Some pegmatitic dikes are discordant to the foliation whereas other pegmatitic dikes are folded and boudinaged in dramatic fashion. We interpret these dikes to have intruded throughout the deformation. These dikes serve as minimum strain markers and record sectional strains of ~15:1 to 20:1. Elongation lineations, where discernible, plunge gently to both the northeast and southwest parallel to fold axes. Kinematic indicators are compatible with dextral transpressive shear. These exposures are ~10 km (~6 mi) southeast of the Spotsylvania geophysical lineament at the western edge of the Goochland terrane, but as evidenced by the pegmatite markers, the rocks are very strongly deformed. It is difficult to place a southeastern boundary on the Spotsylvania zone, as rocks throughout the Goochland terrane are deformed in this fashion.

Stop 4. Tectonically disrupted Elk Hill Complex along western margin of Spotsylvania high-strain zone, Gum Spring Quarry.

Blocks and small outcrops in creek.

This abandoned quarry was operated by the Salem Stone Corporation during the construction of I-64 in the 1960s. We cannot safely visit the pit itself, which is filled with water and surrounded by steep highwalls, but the large quarried blocks dumped here provide an excellent view of the material blasted out of the pit. The blocks are predominantly composed of protomylonitic amphibolite and porphyroclastic mylonite. Plagioclase porphyroclasts range up to 5 cm (centimeters) (2 in (inches)) in diameter, apparently remnants of tectonized pegmatite. Blocks of dark-green to black amphibolite form tectonic inclusions in the mylonite. These range in shape from blocky to lens-shaped to extremely flattened, suggesting a wide range of competency contrasts in relation to the enclosing mylonitic gneiss. In the quarry walls, average orientation of the mylonitic foliation is 035° 42° SE.

This locality is on strike with and lithologically similar to the Ca Ira mélange of Marr (1991). While these rocks are certainly highly deformed, we note the lack of truly exotic blocks; virtually all observable tectonic inclusions are varieties of amphibolite, the dominant rock type in the nearby Elk Hill Complex. Therefore, we interpret this locality not as true mélange, but as highly deformed Elk Hill lithologies in the footwall of the Spotsylvania high-strain zone (fig. 6). Just east of here, the presence of strongly pelitic rocks (muscovite-sillimanite-garnet schist) marks the transition into the Maidens Gneiss protolith of the Goochland terrane.

A narrow zone of poorly exposed siliceous breccia trends north-northwest across the south slope of this small valley. Lithologically, the rock is similar to the breccia along the Lakeside fault: nearly white, massive cryptocrystalline silica with both quartz-filled and open fractures enclosing angular clasts of the same lithology. The siliceous breccia can be traced from the crest of a low hill on the quarry entrance road in a north-northwesterly direction for at least 500 meters (m) (1,600 feet (ft)) (fig. 6). The re-brecciated breccia indicates multiple episodes of Mesozoic brittle reactivation along the western margin of the Spotsylvania high-strain zone, although at an angle oblique to the original mylonitic foliation.

Stop 5. Elk Hill Complex on U.S. 250 near East Leake.

Small outcrops in roadcut on north side of U.S. 250.

Small outcrops of the Elk Hill Complex here were fresh and more extensive when photographed by C.B. Brown in 1937 (his pl. 7). We can still observe the general nature of the complex, compositionally layered amphibolite interlayered with biotite gneiss, as it is expressed north of the James River. The compositional layering here is parallel to a moderately developed foliation that strikes 040° and dips 70° SE. A weak mineral lineation (mostly hornblende) plunges gently northeast (20°–48°). On the north side of the road, outcrops display northeast-plunging Z-shaped refolded folds.

The main purpose of this stop is to demonstrate that the Elk Hill Complex is a coherent body of relatively undeformed amphibolitic rocks between two parallel high-strain zones (fig. 6). It is bounded on the east by the Spotsylvania high-strain zone (Stop 4) and on the west by mylonitic rocks in the hanging wall of the Lakeside fault (Stop 6). Our mapping indicates that the Elk Hill extends from here at least as far as the Farmville basin, 40 km (25 mi) to the southwest. Its northeastward extent has not been determined. While these rocks superficially resemble amphibolites in the Chopawamsic Formation, they are structurally separated from the main body of the Chopawamsic Formation to the west.

Stop 6. Mylonitic rocks in the hanging wall of the Lakeside fault in unnamed creek north of U.S. 250.

Creek pavement outcrops near I-64 culvert.

Exposures in this streambed give us a view of mylonitic rocks in the hanging wall of the Lakeside fault (fig. 6). A fine-grained matrix of biotite, muscovite, and quartz encloses abundant porphyroclasts of plagioclase feldspar. Plagioclase-quartz pegmatite layers have been deformed into lens-shaped, generally foliation-parallel domains of various sizes. Foliation trends 042° with an average dip of 50° SE. Preliminary analysis of porphyroclast asymmetry indicates dextral oblique motion on this mylonitic zone, similar to that in the Spotsylvania high-strain zone. Creeks crossing this zone at a high angle yield outcrops that indicate a width of nearly 900 m (3,000 ft). Elsewhere, the trace of the Lakeside fault is marked by extensive development of siliceous breccia (for example, Stop 10A), an indication of Mesozoic brittle reactivation of this Paleozoic ductile fault zone. The outcrops here represent part of the case for extending the Lakeside fault northeast of the terminus shown in northern Cumberland County on the geologic map of Virginia (Virginia Division of Mineral Resources, 1993). Its northeastern termination has not yet been determined.

Stop 7. Amphibole-rich metapyroxenite at Hadensville, U.S. 250.

Roadcut at crest of hill on south side of U.S. 250.

This roadcut exposes one of the most prominent mafic to ultramafic dike-like bodies in the Piedmont province of Virginia. It was originally mapped as pyroxenite by Brown (1937), but labeled as hornblende gabbro on the 1963 geologic map of Virginia. Curiously, the body was omitted from the 1993 geologic map of Virginia (Virginia Division of Mineral Resources, 1993). The body is well exposed in outcrop or float for about 5.5 km (3 mi), ranges up to ~200 m (~660 ft) wide, and is oriented ~040° (Murray and Owens, 2002). Most exposures are dominated by green, medium- to coarse-grained (≤1 cm), blocky calcic amphibole, and textures range from massive to slightly foliated. The blocky shapes may reflect pseudomorphic replacement of pyroxene by amphibole. In thin section, the rocks typically consist of ~85 to 90 percent calcic amphibole (actinolite to actinolitic hornblende to magnesio-hornblende), minor epidote and quartz, and rare talc (and even rarer cummingtonite). Clinopyroxene (relict igneous?) occurs in small amounts in a few samples as ragged grains within amphibole. Eight samples from along the length of the body show similar whole-rock SiO₂ (50–55 weight percent), Al₂O₃ (3–7 weight percent), and Mg# (73–82), but a range in MgO (14–23 weight percent) and CaO (5–14 weight percent). Normative mineralogy of typical samples is dominated by clinopyroxene and orthopyroxene; all are quartz normative. These observations and data suggest that the body represents a pyroxenite metamorphosed at middle- to upper-amphibolite-facies conditions, and that the protolith was a cumulate consisting largely of two pyroxenes and minor plagioclase. The mode of emplacement is an unresolved issue. The body clearly has a dike-like form, but its original cumulate character suggests that it could not have been emplaced as a liquid. In addition, no other indicators of intrusive emplacement (chilled margins, contact metamorphism, xenoliths) have been observed. Alternatively, the body was emplaced tectonically. Its proximity to the Little Fork Church fault (fig. 6) may be significant in this regard, but no clear field evidence linking these features has thus far been observed. [Much of this information is from the 2002 undergraduate geology thesis by J.D. Murray, College of William and Mary; see also Murray and Owens (2002)].

Stop 8. Granitic gneiss, mylonitized pegmatite, and breccia in the pegmatite belt near Mill Creek.

Outcrops on hillside north of road.

This leucocratic, fine-grained, weakly layered granitic gneiss with thin pegmatites is typical of the structurally lowest unit of the pegmatite belt (fig. 3). The pegmatites are composed mostly of pinkish-white microcline with lesser quartz and muscovite, and are typically deformed into lens-shaped bodies. Pegmatite bodies are parallel to the foliation, which strikes 052° and dips 68° NW. “Floating” porphyroclasts surrounded by fine-grained gneiss suggest that some of the granitic gneiss may be derived from extreme deformation of pegmatite. Pegmatites in this unit are the coarsest grained rocks seen in this part of the Piedmont, with individual microcline crystals up to 15 cm (6 in) long. Feldspars in the pegmatite are strongly fractured, with development of subgrains having nearly parallel crystallographic axes. Float blocks on the slope to the northeast include re-brecciated breccia similar to that found along the Lakeside fault and the western margin of the Spotsylvania high-strain zone. These blocks suggest the presence of an unmapped breccia zone, perhaps related to the Little Fork Church fault.

This locality is about 700 m (2,300 ft) southeast of the mapped trace of the Little Fork Church fault (fig. 6). Considering the high degree of deformation apparent in these rocks, and the presence of possibly tectonically separated, enigmatic, mafic to ultramafic rocks nearby (Stop 7), we must consider the possibility that the Little Fork Church fault is a major structural discontinuity.

End of Day 1.

Day 2 (Monday, March 29, 2004)

Stop 9. L-tectonites in Columbia pluton at Cowherd quarry, Va. 6, Columbia.

Old dimension stone quarry on north side of Va. 6.

Granodioritic gneiss of the Columbia pluton is exposed in this old roadside quarry (fig. 3). Wilson (2001) obtained a U-Pb zircon SIMS age of 457±7 Ma on rock from this locality. The Columbia pluton ranges from granite to quartz diorite in composition and intrudes metavolcanic rocks of the Chopawamsic Formation (Smith and others, 1964; Goodman and others, 2001) (fig. 3). The Columbia pluton is part of a suite of Ordovician plutons that record magmatic activity associated with the Taconic orogeny. These plutons may have originated above a subduction zone associated with the accretion of the Carolina zone to Laurentia (Hibbard, 2000).

At the Cowherd quarry the granodioritic gneiss contains plagioclase, quartz, biotite, potassium feldspar, and epidote with minor amounts of garnet, muscovite, and opaque minerals. Feldspar and quartz microstructures are consistent with recrystallization under amphibolite-facies conditions. This rock forms a distinctive L-tectonite (fig. 7). The penetrative fabric is defined by aligned biotite and quartz aggregates. The lineation plunges ~25° towards ~050° and a very weak foliation strikes ~055° and dips ~80° NW (fig. 8A–C). Poles to biotite cleavage form a strong great circle girdle normal to the lineation (fig. 8B, C). Three-dimensional quartz fabrics in the XZ section range from 2.7 to 3.4 with K-values of 4 to 12 (strongly constrictional) (fig. 8D). Locally, L-tectonites are restricted to the nose of a map-scale northeast-plunging synform (fig. 3). Three-dimensional quartz fabrics in the Columbia pluton exhibit a complete range from L- to L/S- to S-tectonites.

We view the L-tectonite as the product of cumulative deformation. At the map scale, the foliation is folded into a series of asymmetric northeast-plunging folds with axes that parallel the elongation lineation, suggesting a post-foliation deformation event. However, there is no microstructural evidence for two deformational events in these rocks. Although the quartz fabrics do not, in a strict sense, record the finite strain, they are a measure of the approximate overall strain. Here at the Cowherd quarry and at many locations in the central Piedmont, crustal rocks were apparently elongated in an orogen-parallel direction late in the Paleozoic.

Stop 10. Fault breccia, pegmatite belt, and diabase at “Bodatious” race track.

A. Breccia associated with the Lakeside fault.

Outcrops on hillside northeast of road.

This prominent northeast-trending ridge is underlain by erosionally resistant siliceous fault breccia, which marks the trace of the Lakeside fault (fig. 4). This fault forms the western bounding normal fault of the Farmville basin, 20 km (12 mi) to the southwest. From here, the fault continues northeastward, crossing the James River between Columbia and Cartersville. Our mapping extends the fault at least as far north as I-64 in Goochland County (fig. 6, Stop 6).

Breccia occurs discontinuously along the fault trace. This, the largest mapped breccia body, is about 8 km (5 mi) long. The breccia is composed of cherty, cryptocrystalline silica with multiple generations of partly to fully filled fractures. Some fractures are filled with microbreccia, but more commonly they are fully or partly healed with crystalline quartz; partly open fractures display terminated quartz crystals. Angular clasts displaying truncated internal fractures are common; they are either supported by microbreccia, by other clasts, or completely surrounded by cherty silica. Bourland (1976) reports zeolite minerals and prehnite from fractures associated with this fault. The breccia apparently formed from silica-rich hydrothermal fluids migrating along the fault in the early Mesozoic; the still-active fault fractured the hydrothermal deposits repeatedly during their formation.

Cross road and enter “Bodatious” race track complex. Walk 250 yd to the northwest to dirt drag strip.

B. Structural complex in upper unit of pegmatite belt.

Large saprolite cut along north side of drag strip.

This large saprolite cut provides a rare view of the structure of the eastern part of the pegmatite belt in the footwall of the Lakeside fault (fig. 4). The strongly compositionally layered gneiss consists of amphibolite, quartzofeldspathic biotite gneiss, and thin concordant pegmatites. These lithologies are typical of the structurally highest lithologic unit in the pegmatite belt (pgl, fig. 4). Layers have been deformed into a series of upright outcrop-scale folds that plunge steeply to the northeast. The folds are cut by two families of younger normal faults, one dipping steeply to the southeast and one dipping gently to the northwest. The faults commonly contain thin discontinuous fills of vein quartz. Crosscutting relations at most fault intersections indicate that the northwest-dipping set is younger, but a few examples can be found in which the southeast-dipping set displays more recent movement.

Follow creek upstream (west) for 100 yd.

C. Granitoid gneiss and amphibolite in middle unit of pegmatite belt.

Creek pavement outcrops west of race track.

In this outcrop, light-gray, fine-grained granitoid gneiss is intruded by a 1.5-m (5-ft)-thick amphibolite dike. The irregular boundaries of the dike cut and trend more easterly than weak compositional layering in the granitoid gneiss. Foliation dips steeply to the southeast (038° 80° SE) and cuts both the dike and the country rock. The amphibolite has been deformed into Z-shaped folds and is locally stretched into boudins of various size. The presence of amphibolite boudins within granitoid gneiss is a characteristic feature of the middle unit of the pegmatite belt (pga, fig. 4). Fine brittle fractures in both lithologies, apparently related to Mesozoic deformation, are filled with epidote and zeolite minerals.

Continue upstream (west) on path along north bank for 50 yd.

D. Large diabase dike.

Stream pavement, boulders, and blocks in ruins of mill dam.

This old mill dam, now in ruins, was built of local boulders placed on top of stream pavement composed of diabase. The boulders themselves are primarily diabase, but light-gray granitic gneiss and white siliceous breccia also are evident. This diabase, one of several large, north-trending Jurassic dikes in the Virginia Piedmont, can be traced on aeromagnetic maps for at least 80 km (50 mi) to the south. To the north, this dike crosses the James River 2 km (1 mi) east of Columbia, where Brown (1937) measured its width at 75 m (250 ft).

South of this outcrop (1.2 km; 0.75 mi), the dike intersects the trace of the Lakeside fault near the Willis River (fig. 4). Although the intersection of the dike with the fault is beneath the flood plain of the river, mapping indicates that where the dike emerges from the flood plain in the hanging wall of the fault, it displays an apparent right-lateral offset of 600 m (2,000 ft) (fig. 4). This evidence suggests a degree of dextral offset on the Lakeside fault after intrusion of the diabase in Early Jurassic time.

Stop 11. Gneissic diorite near Whiteville.

Outcrops and blocks in creek.

This coarse-grained melanocratic gneiss (dg), described here for the first time, occupies an area of 6 km² (2 mi²) in northern Cumberland County (fig. 3). It forms an elongate fault block bounded on the west by mylonitic rocks in the hanging wall of the Lakeside fault, and on the east by a narrow band of mylonitic biotite schist that separates it from metavolcanic rocks in the Elk Hill Complex (fig. 3). Composition is dominantly hornblende+plagioclase with minor clinopyroxene, quartz, titanite, and epidote. Foliation strikes 042° and is nearly vertical; hornblende defines a mineral lineation plunging 12° and bearing 042°. The weak to moderate gneissic layering seen here (fig. 9) is quite variable, with some outcrops showing no layering or foliation. Superficially, this unit resembles dioritic rocks identified as high-grade equivalents of the Carolina slate belt generally on strike to the south (W.C. Burton, written commun., 2003). When considered with the metapyroxenite in western Goochland County (Stop 7) and poorly exposed talc-chlorite soapstone about 3 km (2 mi) to the northeast (D.B. Spears, unpub. field data), all spatially associated with mapped faults, we can speculate that we are seeing the remains of a tectonically dismembered mafic-ultramafic complex. More work is needed to establish the affinity of these units and their significance with respect to the terrane boundary.

Stop 12. Mylonitic rocks along west edge of Spotsylvania high-strain zone at Howard’s Neck Plantation.

Large natural pavement exposure on south side of small valley east of railroad.

This large natural outcrop exposes rocks and structures typical of the Spotsylvania high-strain zone. The country rock is a mica-rich mylonitic gneiss that has been intruded by dikes of medium-grained granitic gneiss and pegmatite. These dikes are variably dismembered; some retain a tabular shape and others form lozenge-shaped asymmetric boudins. It is possible that the granitic intrusions occurred prior to and during ductile deformation. Foliation strikes to the northeast and dips moderately to gently southeast; elongation lineations plunge subhorizontally in the plane of foliation. Asymmetric structures consistently display a dextral sense of shear. Minimum strain estimates based on boudinaged and folded pegmatites are in excess of 10:1 on lineation parallel faces. The Spotsylvania lineament is located within a few hundred meters to the west and is defined on the ground by a transition to amphibole-rich gneisses of the Elk Hill Complex that do not display a mylonitic texture.

Stop 13. Maidens Gneiss at Tamworth.

Roadcuts along west side of gravel road and outcrop on east bank of Muddy Creek at mill dam.

This small roadcut contains fine-grained, weakly layered, moderately foliated quartzofeldspathic gneiss with finely disseminated biotite. A few thin, coarse-grained quartz-feldspar-muscovite-garnet pegmatites are present, concordant with the compositional layering. This locality falls into an area defined as the “Central Piedmont” terrane on the most recent geologic map of Virginia (Virginia Division of Mineral Resources, 1993), but no other reference to this terrane appears in the literature. Farrar (1984) included this area in his mapping of the Maidens Gneiss and reported high-grade pelitic assemblages (sillimanite+potassium feldspar±muscovite; sillimanite+staurolite+muscovite) from outcrops nearby. Outcrops adjacent to the mill dam are more typical Maidens biotite-hornblende gneiss with feldspar porphyroclasts showing strong dextral asymmetry.

Stop 14. Fine Creek Mills Granite.

Stream pavement exposures along Fine Creek.

Although this stop has been included on at least one other field trip (Farrar and Owens, 2001), it is one of the more spectacular outcrops in this part of the Piedmont province (and therefore worth re-visiting!). The Neoproterozoic Fine Creek Mills Granite (629+4/-5 Ma; Owens and Tucker, 2003) intrudes the Middle Proterozoic State Farm Gneiss. It is well exposed here along both sides of Fine Creek. This medium- to coarse-grained granite is mineralogically similar to the other Neoproterozoic granitoids previously visited. The dark minerals include both biotite and amphibole. Discrete, narrow shear zones cut a moderately developed foliation in the granite, but these have not been systematically examined.

End of field trip.

End of Day 2.

U.S. Department of the Interior, U.S. Geological Survey
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