GOL 135: The Geology of Prince William Forest Park, Virginia

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GOL 135: Geology of Prince William Forest Park, Virginia

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Geologic map of the park:
from Southworth and Denenny (2005)

Part 1: Deposition of clastic sediments in a deep ocean basin

(Above) Cross section through the Iapetus Ocean, predecessor to the Atlantic Ocean. (And as the Atlantic is named for the Greek titan Atlas, so is the Iapteus Ocean named for Atlas's father, Iapetus.) In the vicinity of Prince William Forest Park, "dirty" (impure) sandstones called greywackes (see below) were accumulating on the bottom of the Iapetus Ocean. The ocean basin immediately adjacent to the ancestral North American margin was closing, as a subduction zone consumed the oceanic crust and brought an offshore chain of volcanic islands (a "volcanic island arc," labelled as the Chopawamsic Terrane in the above image) closer and closer to North America. Eventually, the Chopawamsic Terrane would collide with North America in a mountain-building event known as the Taconian Orogeny.

Map view of the tectonic situation about 510 million years ago, in the late Cambrian period of geologic time. "PWFP" locates Prince William Forest Park's location on the map. Note the volcanic island arc located offshore, in the Iapetus Ocean, moving closer to North America as the intervening oceanic crust is subducted. Map by Ron Blakey, Northern Arizona University.

Many more of Blakey's highly detailed maps are available for different geologic time periods here.

(Above) Three different kinds of sandstone, each indicative of a different sedimentary environment. (A) Quartz sandstone is a mature sandstone, consisting (as the name implies) of mostly quartz. It is deposited in beach and dune settings. (B) Arkose is an immature sandstone with large angular feldspar grains. It is deposited in continental basins (rift valleys). (C) Greywacke is an immature sandstone, composed of sand mixed with mud. It has a dark color, indicating its deposition in the low-oxygen environment of the deep sea. Typically, greywackes are associated with graded bedding (see below) , indicating that the sediments were deposited by a turbidity current -- again, indicating the deep ocean as the site of deposition.

Graded bedding is a characteristic sedimentary structure associated with deposition of sediments in a turbidity current. Turbidity currents are dense, sediment-laden flows which flow along the bottom of the ocean, starting from a source area adjacent to a land mass. As they slow down, they deposit their largest grains first, and their finest grains last. Turbidity currents are found today in deep ocean basins. So, when we find graded bedding in greywacke rocks, we assume it formed in the past the same way similar structures form today: in the deep ocean.

(A) schematic of the turbidity current / graded bedding depositional system.

(B) model turbidity current in a test tank, NVCC-Annandale geology lab.

Part 2: Eruption of lava in a volcanic island arc

This is an example of greenstone, a common rock in the eastern portion of Prince William Forest Park. Part of the Chopawamsic Formation, this greenstone was originally deposited as a basaltic lava flow. (Basaltic lava is dense, runny, dark-colored lava, low in silica and with only 2% gas content -- as opposed to more "felsic" lavas, which contain up to 6% dissolved gases, and are lighter in color and more viscous as well.) The gas content of the basalt's lava (mostly water vapor and carbon dioxide) bubbled out of solution when the lava was erupted in the Chopawamsic volcanic island arc. the bubbles that formed in the magma were preserved as small Swiss-cheese-esque bubbles called vesicles. Later, minerals filled the vesicles with light-colored deposits called amygdules, probably at the same time that the basalt was being metamorphosed to become greenstone. (Typically, amygdules are filled with the minerals quartz, zeolites, epidote, or jasper. These ones appear to be mostly quartz, judging by the way they weather out in high relief.) The amygdules are readily visible in this photograph, taken near Quantico Creek. Key for scale.

More- and less-gas-rich layers are visible in this photograph of the Chopawamsic greenstone, as exposed in Quantico Creek. The different-composition lavas are preserved as layers that are relatively rich in amygdules (white dots), alternating with layers that are relatively poor in amydules. Later metamorphism and deformation of these rocks during the Taconian Orogeny has squashed the amygdules top-to-bottom. They appear flattened as a result of these tectonic stresses. Keys for scale.

During eruption, or possibly during the Taconian Orogeny's deformation and metamorphism, clasts (chunks) of amygdular basalt/greenstone were broken off and included in less-amygdular areas of the rock. In the center of this photograph is an amygdular clast (with tectonically-flattened amygdules) surrounded by a less-amygdular "matrix". Keys for scale.

Here is a very large clast surrounded by a greenstone matrix. The clast is approximately one meter long by half a meter in width. It is lying on its side to the left of the keychain. This rock, which is certainly depositional (in other words, the clast was deposited there at the same time as the lava; it was not tectonically reshuffled later on) is exposed in the creekbed of Quantico Creek, south of the confluence with the South Fork of Quantico Creek. Rocks such as these are named "volcanic breccia" in acknowledgement of their mix of liquid lava and solid clasts. Keychain for scale.

Part 3: The Taconian Orogeny: collision between the island arc and ancestral North America

As the island arc approached the North American margin from the east, it scraped sediments off the top of subducted oceanic crust. Islands, seamounts, and other protuberances were also scraped off and added to a large, jumbled pile of ocean-bottom debris called an accretionary wedge.

The Taconian Orogeny, as viewed from above, about 450 million years ago. The Chopawamsic Terrane has begun to collide with ancestral North America, adding the volcanic rocks (basalt) and sedimentary rocks (greywacke) to the eastern margin of the continent. Map by Ron Blakey, Northern Arizona University.

Many more of Blakey's highly detailed maps are available for different geologic time periods here.

As the two pieces of crust collided, temperatures rose. As things got warmer, partial melting occured. Minerals present in the greywacke that had low melting temperatures, like quartz, potassium feldspar, and muscovite mica ("felsic minerals") all melted, and trickled out of the area. Minerals with higher melting temperatures, like plagioclase feldspar, augite, hornblende, and olivine, were left behind as solids. These minerals (the "mafic" ones) didn't melt. The magma, being less dense than the solid residue, rose to higher levels in the crust.

At Prince William Forest Park, that magma (made of low-melting temperature minerals) lodged in the crust and cooled to form granite. Because it cooled slowly (deep underground), there was plenty of time for the growth of large (readily visible) crystals. Pen for scale.

As the magma worked its way upward through the overlying solid crust, bits of older rocks broke off and dropped into the lliquid magma chamber. These fragments are called xenoliths, from the Greek for "alien rock." The round shape of these particular xenoliths show that they had plenty of time to equilibrate with the magma, melting off their angular corners in much the same way that an ice cube, initially rectangular, will melt into a ellipsoidal form after it has sat in a glass of lemonade for a little while. Pen for scale.

As the magma blobs ascended to shallower levels in the crust, they adjusted to the lower pressures they found there. One result of this was that certain compounds, previously dissolved in the magma, now "sweated" out as mineral-rich hot solutions. These hydrothermal fluids percolated into surrounding rocks, including fractures. As the hydrothermal solutions encountered cooler temperatures, they precipitated out their mineral load, resulting in "aura" deposits (purple in image) and veins (red in image) of mineral ore.

At Prince William Forest Park, the hydrothermal solutions were rich in iron and sulfur. When the solutions cooled down, these elements combine to make deposits of iron sulfide, known by the mineral name pyrite. Pyrite has a glittery golden luster, superficial similar to (more valuable) gold. As a result, it is often informally referred to as "fool's gold." Much later, the Cabin Branch Mine would be founded to harvest this concentration of pyrite.

As the magma cooled to form granite, it contracted. This opened up some fractures in the solid granite. Fresh magma, usually of a lighter color (i.e. more felsic composition) was injected into these fractures, and cooled. The broad white stripe in this photograph is just such a granite dike, cutting across the older (darker) granite. Pen for scale.

In places, the granites preserve evidence of ductile deformation (smearing, as opposed to breaking). Here, a shear band running through the granite shows smeared-out ribbons of quartz and a strong allignment of other minerals in a small band a few inches wide. This would have been the spot where movement had taken place in the magma chamber before it completely cooled into solid granite. Pen for scale.

Here is evidence of brittle behavior (breaking as opposed to smearing) in the granite. Two sets of joints (fractures) in the granite intersect at approximately right angles in this lichen-covered outcrop. Pen for scale.

Some of the dikes serve as excellent marker units, showing us places where the granite has been deformed. Here, a dike is offset (top to the left) along a series of small faults. The dike is broken up into sections by the faults, and then the sections are slid laterally past one another. These offset dikes show brittle behavior that could only have occured once the granite had solidified. Pen for scale.

Another thing that happened in the Taconian Orogeny was the metamorphosis of the sedimentary rocks that were originally deposited in the Iapetus Ocean basin. Elevated temperatures helped fuse the sedimentary grains together into solid rock, and tectonic pressures re-alligned the minerals into parallel orientations. Greywacke was transformed to metagreywacke, and oceanic mud was transformed in mica-rich schist.

Certain minerals will grow under metamorphic conditions that weren't there before metamorphosis. Growth of micas, for instance, provide the foliation plane of a schist (see above). Here, amphibole crystals grow as long, bladed black forms lying within the plane of foliation. Geologists call these big, eyecatching "metamorphic-only" crystals porphyroblasts. Quarter for scale.

Our penultimate stop of the day will be to examine roadside outcrops of phyllite of the Quantico Formation in downtown Dumfries. Originally described as the "Quantico Slate," we will deduce that this rock is actually a phyllite, based on its shiny foliation surfaces. It is much finer-grained than the metamorphosed greywacke sediments we saw earlier in the park. This indicates it was likely originally deposited as mud in the Iapetus Ocean. Later, of course, it too was caught up in the tectonic squeeze of the Taconian Orogeny, and developed a strong foliation at that time. The foliation wraps around large golden cubes: porphyroblasts of metamorphic pyrite. Scale bar is in centimeters.

From this angle, you can see that the phyllite of the Quantico Formation has been folded by later deformation. This outcrop is in downtown Dumfries. Keychain for scale.

Part 4: Erosion of the Appalachian Mountain range

The Appalachian Mountains were quite tall (1) at their height -- much taller than they appear today. After the collision that built them ceased, the mountains were subject to the down-grinding forces of erosion. The mountains served as a source of sediment, and that sediment filled in the lowest-lying areas of the landscape first (2). Eventually, the mountains had been worn down to their nubs, and sediment could now be deposited across the landscape (brown layer at the top of 3).

Part 5: Deposition of younger layers of sediments on top of the erosional surface

Cross section of the geology of Prince William Forest Park, from Southworth and Denenny (2005). Note the eroded remnants of the Appalachian mountain belt (all the "O" prefix rocks at the bottom of the diagram, indicating their Ordovician ages) are capped with younger sedimentary layers (Ttu and Kp). These sedimentary deposits are loose (unlithified) sediments, and the ancient erosional surface that separates them from the older rocks below is called an unconformity. Unconformities are gaps in the geologic record: they represent "missing time," in this case the absence of any geologic record between the late Ordovician (~450 million years ago) and the Cretaceous (~100 million years ago). About 360 million years of geologic time are missing from this geologic cross section, and we infer that erosion was operational in this area for most of that time.

Cobbles and pebbles on a hilltop in Prince William Forest Park. The highly-rounded shapes of these cobbles indicate that they have travelled a long distance. Their large size indicates that they were transported by fairly energetic (fast-flowing) waters. These deposits are part of the Potomac Formation (Kp on the above cross section), a series of Cretaceous-aged (~100 Ma) river gravels deposited atop the eroded roots of the Appalachian mountain belt. Keychain for scale.

Close-up of a single cobble from the Potomac Formation, bearing linear tube-shapes. These tubes are a trace fossil called Skolithos, interpreted as the burrows of a worm from the Cambrian period of geologic time. The worm burrowed into clean quartz sands on a beach, and later its burrow was filled in with more sand. In Virginia, Skolithos-bearing quartz sandstones are exposed in the Blue Ridge province's Antietam Formation. These trace fossils therefore help us pinpoint the source area (provenance) of the sediments included in the Potomac Formation. They also let us know that rivers were draining east from the Appalachians by about 100 million years ago. Length of cobble is about seven inches.

At the same time the gravels were being deposited by rivers, bald cypress trees (Taxodium sp.) were growing in slower-moving portions of the waterway, perhaps in a swampy environment. Some fossilized cypress trees were uncovered in the Potomac Formation during construction work just north of the park in the 1950s. Classic 1950s man for scale.

One of these fossil bald cypress trees, flared at one end indicating it was the trunk of the tree, is stored in the Prince William Forest Park maintenance yard. The tree trunk is silicified, meaning that the wood has been replaced with deposits of silica (quartz). Length of the tree trunk is about 1.5 meters.

Bottom-end view of the tree trunk, showing an area that was obscured by sediment in the original 1950s photograph above. Note the concentric layers of "tree rings" indicating growth by the bald cypress during its life. Keychain for scale (center, top).

Tension gashes in the fossilized tree trunk, filled in with quartz. Either before the tree was buried, or during the petrification process, the tree's wood split and opened up small gaps. Later deposits of quartz filled in these gaps. Hydrochloric acid bottle for scale, length about two inches.

Another bald cypress fossil, also stored in the Prince William Forest Park maintenance yard. This one does not flare at the base, and is likely a higher-level portion of the trunk of the tree. Keychain for scale.

Part 6: Erosion of all rock units when local rivers incise due to sea level drop

Potholes are one way that water effectively scours into solid rock. Swirling vortexes of water are full of suspended grit (sand and silt) which can act like a "sandpaper tornado" boring into the rock face. Here, greenstone of the Chopawamsic Formation hosts a pothole which has been "shut down" due to the dumping of very large particles (cobbles) into the pothole. One-liter water bottle for scale.

This pothole preserves evidence showing the direction of spin in the vortex of water which carved it. The clockwise spin in past water currents is preserved today in this elegant spiral of rock. Keychain for scale.

More resistant rocks (usually with higher quartz content, or lower pore space volumes) crop out as ledges which traverse the stream. These ledges provide the many beautiful cataracts along Quantico Creek which will showcase the trip's earliest rocks. Backpack (black; upper left) for scale.

Waterfall along the South Fork of Quantico Creek, cutting into felsic intrusive rocks (granites, essentially). Because of their high quartz content, granites are more stable at Earth-surface conditions of temperature and pressure, and so they are more resistant to erosion than rocks like greenstone, metagreywacke, or schist. Often, such waterfalls develop at the contact between less stable rocks (downstream) and resistant quartz-rich rocks like granite (upstream). At its narrowest point, the creek is about a meter wide.

Also easily eroded are the unlithified sedimentary deposits of the Coastal Plain. ("Unlithified" means that the sediments are still loose -- they have not been compacted or cemented into sedimentary rock yet.) The boundary between the easily-eroded Coastal Plain and the more-resistant Piedmont is known as the Fall Zone. As the name suggests, this physiographic boundary is marked by waterfalls on all the rivers which drain across the Zone. As this cross-section shows, in Washington, DC, the Fall Zone extends from the city upstream to Great Falls. At Prince William Forest Park, the Fall Zone is concentrated in the southeastern corner of the park.

Here is a map view look at the Fall Zone, as it stretches parallel to the east coast dividing Piedmont from Coastal Plain. Notice that the Fall Zone runs right through the major cities of Atlanta, Richmond, Fredericksburg, Washington, Baltimore, Philadelphia, and New York. Why? Because these cities were all settled at the furthest-upstream navigable point on their respective river systems. Beyond the Fall Zone, boats start running into hard Piedmont rocks sticking up from the bottom of the river.

After our time in the park, we will drive through neighboring Dumfries, Virginia, where we will see the Fall Zone first hand. Even from this aerial photograph of Dumfries, it's obvious which portions of Quantico Creek are in the Piedmont and which are in the Coastal Plain. Rising sea level since the last Ice Age has flooded the lower reaches of Quantico Creek. Modified from a Google Earth image.

Part 7: Mining, and mine reclamation

The Cabin Branch Pyrite Mine was a organized in 1889 to extract pyrite from the hydrothermal deposits in the Prince William Forest Park area (before it was a park). Until 1920, the mine's pyrite was processed for its sulfur content, which was used (as sulfuric acid) in gunpowder, soap, and fertilizer. Unfortunately, much of that sulfuric acid ended up in the soil immediately around the mine, and Quantico Creek, which drains the mine area. Historical photo courtesy of NPS.

A topographic map by the United States Geological Survey (USGS) showing reclamation efforts at the Cabin Branch Pyrite Mine site. Diversion channels direct water around the outside of the major acid-contaminated areas, so that the water is as uncontaminated as possible when it runs into Quantico Creek. Artificial ponds catch runoff and keep it from flowing directly into Quantico Creek. Image by USGS.

The mine site today looks much better thanks to reclamation efforts, but there are still large areas where trees will not grow, and some areas where no plant life grows, due to the acidification of the soil. Photo courtesy of NPS.

Summary

A summary of major events in the area we now call Prince William Forest Park, presented adjacent to the geologic timescale.

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