Road Log and Stop Descriptions

Road log begins at entrance station to Leesylvania State Park, Va.

Mixon and others (1972) refer to the strata exposed at Freestone Point (and elsewhere on the Quantico quadrangle) as the Potomac Group. Seiders and Mixon (1981) refer to the same strata exposed on the adjacent Occoquan quadrangle (see Stop 2) as the Potomac Formation. For consistency, we will use the more recent terminology and will refer to these strata as the Potomac Formation.

Part of the Lower Cretaceous Potomac Formation is exposed at Leesylvania Park in the bluffs along the shore, beyond the end of the road at Freestone Point, in the northeast end of the park. This is a good exposure of one of the sandstone units in the Coastal Plain. These strata were probably deposited in a beach environment, as indicated by the sorting and the bimodal crossbeds. Note the presence of blue quartz. Blue quartz is common in the Mesoproterozoic Grenville rocks of the Blue Ridge province, indicating the source area of this sediment.

These Cretaceous beach sands occur above present-day sea level. During the Cretaceous, the Earth was probably free of glaciers, climates were warmer, and sea level was higher. In the current global warming controversy, it is significant to note that these conditions existed long before humans could influence the climate system. In the regional historical picture, the sandstone here represents a coastline developed along the eastern shore of North America during the Cretaceous.

Stop 2. Neabsco Creek, Gar-Field High School.

This outcrop contains a nonconformity between the gneisses and schists of the Middle Ordovician Chopawamsic Formation (fig. 3) below and the sandstones of the Lower Cretaceous Potomac Formation above (Mixon and others, 1972). The hiatus represents over 300 million years. The surface expression of this nonconformity defines the boundary between the metamorphosed rocks of the Piedmont province to the west and the unconsolidated strata of the Coastal Plain province to the east.

The regional metamorphic event that formed the schist here, along with the other foliated rocks of the Piedmont, can be used to determine the relative ages of other exposures we will see on this trip. This metamorphic event will be the basis for our discussion of the older rocks that predate metamorphism, and the younger rocks that postdate metamorphism.

The northeast strike of the foliation in the schists is consistent with the regional Appalachian trends of metamorphic foliation, as well as the trends of fold axes and thrust faults in the Blue Ridge and Valley and Ridge provinces. In other words, the metamorphism was part of Appalachian mountain building during the Paleozoic.

At this stop the nonconformity and the overlying strata of the Coastal Plain dip gently seaward and comprise the subsurface of the eastern shore. Permeable units, such as these, are aquifers and are usually confined between impermeable clay-rich aquitards. Regional ground-water flow is downdip to the east.

The Occoquan Granite is exposed here and along the fall zone of the Occoquan River. The granite contains a foliation indicated by the orientation of biotite, and therefore the Occoquan is clearly premetamorphic. According to Seiders and Mixon (1981), the Occoquan Granite was emplaced during the Early Cambrian. A recent age determination (Aleinikoff and others, 2002) indicates the Occoquan was emplaced in the Ordovician during the Taconic orogeny. The Occoquan probably underwent metamorphism during a subsequent orogeny.

We will observe one of several mafic dikes that intrude the granite (fig. 4). These dikes are nonfoliated and therefore postdate the metamorphism. Their ages are assumed to be Mesozoic and, if so, provide additional field evidence that basaltic volcanism occurred after emplacement and metamorphism of the granite.

Historically, the Occoquan Granite represents a volcanic arc that existed during the early Paleozoic. The mafic dikes are probably associated with the rifting that occurred during the Mesozoic.

Metasedimentary rocks of the Piedmont are well exposed at this stop. We will look at the slates and metasandstones of the Upper Ordovician Quantico Formation, which are the primary rocks exposed in the park. Exposures of the Chopawamsic Formation and Occoquan Granite also are present in the park (Seiders and Mixon, 1981). All of these units are overlain nonconformably by the Lower Cretaceous Potomac Formation. We observed this nonconformity at Gar-Field High School and will not take the time to see it again here.

This locality is on the eastern edge of the Culpeper basin, a Mesozoic continental rift basin. Here is the nonconformable relation between the late Precambrian or Early Cambrian Piney Branch Complex and the Late Triassic Reston Member of the Manassas Sandstone (Drake and others, 1994).

Proceed on foot back to Clifton Road and turn right.
Walk to intersection of Moore Drive and observe small outcrop of the Piney Branch Complex on northeast corner of intersection.

The Piney Branch Complex clearly is pre-metamorphic, with a northeast-trending foliation.

Walk north on Clifton Road to a small outcrop on the east side of the road across from Regal Crest Drive.

The Reston Member here is a conglomerate (polymict diamictite, see figure 5). Various types of pebbles occur here and most contain a metamorphic foliation. The pebbles are contained in a nonfoliated hematitic mud matrix. The overall rock itself is therefore non-metamorphic or post-metamorphic. However, the foliated pebbles are clearly derived from an older, pre-metamorphic source. The contact between the Piney Branch Complex and the Manassas Sandstone is a nonconformity that is not exposed here and represents over 300 m.y. missing.

The historical significance of this stop is to illustrate the relative ages of the older premetamorphic Piedmont rocks and the younger post-metamorphic strata of the Culpeper basin.

The quarry is in a Lower Jurassic diabase sill (Drake and others, 1994) that was intruded into the sedimentary rocks of the Culpeper basin. The strata have a gentle westward dip due to the downdropping along the western border fault between the Culpeper basin and the Blue Ridge (fig. 2). Basalts are known to occur in the western part of the basin; hence there was also extrusive volcanic activity. Because the igneous rocks of this area are more resistant to erosion than the sedimentary rocks, the intrusive and volcanic rocks form topographic highs within the basin. Elsewhere in the basin, contact-metamorphosed sedimentary rocks, such as hornfels, also form topographic highs.

Proceed through the tunnel under U.S. 29 to the north end of the quarry.

An east-dipping normal fault separates the Mesozoic strata from the diabase (fig. 6). The diabase does not have any metamorphic texture. It is therefore post-metamorphic and represents mafic plutonism-volcanism associated with extensional tectonics after Appalachian mountain building. The historical significance of this stop is Mesozoic continental rifting that accompanied the initial opening of the Atlantic Ocean.

Stop 7. Stone Bridge over Bull Run.

Upper Triassic red clastic sediments of the Balls Bluff Siltstone (Lee and Froelich, 1989; Southworth and others, 2000) are exposed here along Bull Run. These strata also display the regional westward dip seen at Stop 6. Note that this is a finer grained facies than the red conglomerates of Stop 5 even though both had a common source. The conglomerates near both the east and west margins of the basin are closer to the source. The much finer sediments (now rocks) in the center of the basin were transported a greater distance from the source.

Route 15 runs along the western side of the Culpeper basin. The western border fault lies to the west of U.S. 15, and just west of the fault is the Blue Ridge, which is visible on the horizon to the left. As we drive north, U.S. 15 gets closer to the western edge of the basin.

Within a few hundred feet is the western border fault of the Culpeper basin.

Stop 8. Goose Creek near Steptoe Hill.

The Neoproterozoic Catoctin Formation is a foliated greenstone and greenschist interbedded with metasedimentary rocks. The Catoctin greenstone is metabasalt as indicated elsewhere by vesicles and amygdules, porphyritic texture, flow-top breccias, and pillows (Kline and others, 1991). The Catoctin is commonly referred to as greenstone (a field term) because of the abundance of green-colored chlorite and epidote.

The Catoctin Formation represents an episode of continental rifting. Its pre-metamorphic age (Southworth and others, 2000) indicates that it was emplaced before Appalachian mountain building in the Paleozoic, and certainly before the opening of the Atlantic Ocean in the Mesozoic. The Catoctin rifting occurred during the Neoproterozoic (latest Precambrian) and is good evidence of the opening of the Proto-Atlantic Ocean or Iapetus.

Stop 9. Goose Creek at Groveton Farm.

Here is the nonconformity between the gneiss (metagranite) of the Mesoproterozoic Marshall Metagranite and the arkosic metaconglomerate of the Neoproterozoic Fauquier Formation, as described by Kline and others (1991) and Southworth and others (2000). The Fauquier Formation is overlain by the Catoctin Formation.

Compare the relations here with the nonconformities observed earlier today at Gar-Field High School (Stop 2) and on Union Mill Road in Centreville (Stop 5). Similar to the earlier stops, this nonconformity represents younger sediments overlying older metamorphic rocks. However, one major difference exists here: the younger sediments (Fauquier Formation) are foliated (like the overlying Catoctin Formation) and therefore are pre-metamorphic. The older basement gneisses here are more severely metamorphosed than the overlying metasediments. This indicates that the older basement rocks had undergone metamorphism before deposition of the overlying sediments. Subsequently, these sediments and the underlying metamorphic basement rocks were metamorphosed. Two episodes of metamorphism are therefore evident, the older of which predates the metamorphism of the Appalachian events. This older metamorphism is associated with the Mesoproterozoic Grenville orogeny, which represents mountain building prior to the opening of the Proto-Atlantic Ocean.

Return to Leesylvania State Park.