The Orogeny Zones and Virginia Geology

USGS graphic showing Taconic Orogeny Along the shoreline of Virginia 500 million years ago, as the Iapetus Ocean widened and the continental plates drifted apart during the Cambrian period, Virginia sediments washed to the ocean. There they formed beaches on the shoreline. Offshore, layer after layer of sediments settled out from whatever rivers were eroding the North American craton, creating a continental shelf of mud and quartz (silica) sand.

Today in Shenandoah National Park, the Chilhowee Group of sediments documents the initial burial of the Catoctin basalts (and any basement rocks not covered with basalt). The Weverton Formation is composed of relatively large sand particles that eroded off the Catoctin basalts and basement rocks, and settled in the Iapetus Ocean.

As the mountains eroded to a lower level (or the ocean basin dropped to create a deeper water body), rivers eroding the land had less energy. Smaller particles of silt were deposited on top of the Weverton sandstone. The smaller mud and silt particles were compacted underneath other sediments, to form today's Harpers Formation. On top of the Harper's Formation are today's Antietam Formation of sand-sized particles, suggesting the formation of barrier islands comparable to those on the Eastern Shore or at Cape Hatteras today.

Then the old basement rocks were buried under more sediments that formed in the salt waters of that Iapetus Ocean or another sea. Zillions of tiny sea creatures with tiny shells of limestone lived and died in the waters on top of the buried stack of rocks, with the Antietam Formation at the top. After the sea creatures died, their tiny shells drifted to the ocean bottom and formed an ooze of limey mud. More and more sediments of shells accumulated, covered by chemically-precipitating lime and an occasional layer of mud and sand (after major storms onshore created spectacular erosion).

The sedimentary layers compacted and the tiny individual shells merged into thick beds of limestone (calcium carbonate) and dolomite (calcium magnesium carbonate). If you go to the Bahamas and the Carribean, you can the same process happening today, forming a "carbonate bank." (Another option: keep adding spoon after spoon of sugar to your Cheerios, and see the grains accumulate on the bottom of the bowl.)

As much as 25,000 feet of limestone sediments accumulated on top of the Virginia basement rocks, in the early days of the Paleozoic Era (the Cambrian and Ordovician periods). Virginia - and most of the United States west to California - was covered by a shallow ocean sea. The basalts and Grenville rocks were pressed down and metamorphosed, perhaps creating at this time the mineral "epidote" that gives the Catoctin basalts a particular greenish color.

Roughly 1 billion years ago, the igneous and metamorphic crystalline basement rocks of Virginia had been formed in a continental collision/collisions known as the Grenville Event. (The basement is what you find, if you keep digging down, down, down...) When the supercontinent of Rodinia continent rifted apart 600 million years ago, lava flows of volcanic basalt coated much of the surface that would ultimately become the Blue Ridge, and then everything was buried in lime-rich sediments underneath the Iapetus Ocean.

So far, so good? You've seen the Cheerios float together a billion years ago. You've seen an eruption of "milk" - basalt - coat the basement rocks 600 million years ago. You've seen the Cheerios drift apart, creating the Virginia shoreline on the Iapetus Ocean. Then the whole clump of Cheerios was submerged completely under the milk as layers of "sugar" (limestone) accumulated on the bottom of the bowl, on top of the limestone, basalt, and cystalline basement bedrock.

Now comes the good part. The Iapetus Ocean did not grow steadily; the accumulation of limestone sediments did not end the story.

For whatever reason, the continental plates reversed direction. Africa and North America started to move together again, and the Iapetus Ocean began to close. The bottom of the ocean buckled, and one continental plate slid underneath the other. (That's how the Cheerios actually merge together.)

Ultimately, the Iapetus Ocean closed completely, and Africa crunched into Virginia and the rest of eastern North America. That merger of continental plates occurred in three major collisions that formed mountains: the Taconic, Acadian, and Appalachian (Alleghanian) orogenies.

The Taconic Orgeny, roughly 460 million years ago during the Ordovician Period, involved volcanic island chains in the Iapetus Ocean. Chunks of land equivalent to Japan or the Aleutian Islands today off the coast of Alaska were "rooted" to the ocean floor - but that ocean floor moved, and was subducted underneath the North American continental plate.

When the Iapetus Ocean began to close and the ocean crust began to move towards North America, the volcanic islands were carried towards the continent too. One chunk, known today as the Choppowamsic Terrane, was shoved against continenal edge. As the plates drifted and the Iapetus Ocean closed, a smaller and smaller gap existed between the shoreline of North America and North Africa.

Several island arcs and chunks of crust (micro-continents) existed in the gap between the two continents. Geologists refer to the water separating North America and the closest island arc as the Iapetus Ocean. Behind that island arc, the Rheic Ocean stretched to the African plate.

The limestone that accumulated on the edge of the North American plate is exposed today throughout the Shenandoah Valley. Those formations developed in a quiet tectonic environment, with minimal sand or clay washing in from the cntinental crust, suggestin that the edge of the continent was flat.

However, the Iapetus Ocean was shrinking, and ultimately new sediments were added to the ocean floor. As the Iapetus Ocean floor was subducted undeneath the North American plate, volcanic ash was blown in from the approaching islands, and mud washed from the mountainous islands onto the ever-shrinking seafloor. As the ocean closed, the depth and chemistry of the water changed. Ultimately mud, sand, and volcanic debris from the islands covered the bottom; the remaining water was too shallow for creating more layers of lime-rich sediments.

geologic time scale Finally, the first part of the ocean seafloor closed completely when one island arc collided or "docked" with the continent. The scrunching together of the island arc with the continental margin formed mountains perhaps as high as the Alps in Europe, in a mountain-building event known as the Taconic orogeny. (Try squeezing a balloon between your hands, and you can see it rise up higher and higher as the distance shrinks between the hands. That's what happened to the earth's crust - it rose higher into mountains, as the width occupied by the crust got smaller and smaller.)

The Taconic-time mountains, located east of the current Blue Ridge, were roughly 13,000 feet high. In the vegetation-free Ordovician Period 450 million years ago, erosion of those mountains would have been immense and rapid. As the chunks of continenal crust collided, land was uplifted but rushing rivers carried sediments away.

Erosion finally won the battle. The Taconic-time mountains in what would become Virginia were flattened, and the rivers spread the eroded sediments horizontally in flat layers. As the Taconic-time mountains eroded to near sea level, a "clastic wedge" known as the Queenston Delta formed in what is now West Virginia and western Pennsylvania. In southwestern Virginia, erosion from another section of the Taconic-time mountains formed the Blount clastic wedge, extending into Kentucky.1

As the mountains washed away and became lower lower lower, the energy in the rivers to move the rock lessened. The clastic wedges, formed by the eroding Taconic-time mountains, were capped by a final layer of quartz sand particles. The final sedimentary layer marks the last stage of natural mountaintop removal, when most of the topographic relief was gone and just the hard-to-dissolve quartz minerals remained.

The sands were later compacted into sandstone, and the layer marking the end of the Taconic orogeny is known today (depending upon your location) as the Tuscarora, Clinch, or Massanutten sandstone. After being buried by other sediments, that sandstone is exposed on the surface again. It is visible today as the slow-to-erode bedrock that forms many of the Appalachians ridges, including the eastern and western ridges of Massanutten Mountain.

Virtually the last erosion from the island arc in Taconic-time was iron-rich sediments deposited in the Silurian period. These Clinton/Cayugan deposits did not form thick layers; they were created when there was little relief and the high-energy, powerful rivers of the Ordivician period had been replaced by mild, meandering streams. The Silurian sediments deposited in patches by these streams were the sources for much of the Virginia iron industry in the 1800's.2

At the same time the Silurian iron deposits were formed in sandstones, some areas were shallow seas where carbonate (limestone) sediments were deposited. There were a few zones where the water evaporated, and thick salt deposits developed.

In the colonial era, before transportation of bulk commodities became relatively easy, these salt deposits were highly valued by pioneers and settlers in the mountains - and by the Confederacy in the Civil War. Before the railroad came to town, Roanoke was known as "Big Lick." Saltville relied upon its salt and gypsum deposits as the primary source of employment for over a century, until Tennessee's complaint about salt pollution affecting the drinking water of the Holston River finally forced closure of the factory.

The Silurian period was the calm before the storm. The Taconic Orogeny eliminated the Iapetus Ocean, but the Rheic Ocean still separated the North American tectonic plate from the African Plate. Within the Rheic Ocean were other microcontinents and island arcs. As continental crust was subducted, the Rheic Ocean also got smaller in a pattern similar to the disappearance of the Iapetus Ocean.

In the Devonian Period, a second island arc known as the Armorica terrane docked against the North American Plate. That triggered the Acadian orogeny, and the rocks of Virginia record a second sequence of ocean sedimentation shifting to mountain-building and fnally erosion.

The oldest rocks in the sequence are low-energy carbonates deposited in shallow seas (or on a continental shelf) before the Rheic Ocean closed. On top of them, as the Armorica terrane approached, erosion and volcanic eruptions deposited black shales and volcanic material. A chunk of land, now called Avalonia, was pushed onto the edge of Virginia, expanding the width of the North American plate and creating a new mountain range.

Just like the Taconic-time mountains, the Acadian-time mountains washed away. Thick wedges of sediments (the Catskill delta) eroded from the Acadian-time mountains. Once again, a final layer of relatively-pure sandstone was deposited at the end of the cycle, after everything else has washed away and the mountain range was becoming a flatland. Just as the deposition of the Tuscarora/Clinch/Massanutten sandstone marks the end of the Taconic orogeny, the deposition of the Oriskany sandstone marks the end of the Acadian orogeny.

The Acadian orogeny, like the Taconic, widened the North American continent. The "terranes" or island arcs that docked onto the shores of Virginia extended North America east of the Grenville-age Blue Ridge, as well as increased the depth of the younger sediments that eroded to the west and the east. The arrival of Avalonia pushed the earlier-arriving Choppowamsic Terrane further west, adding more heat/pressure to alter the bedrock in the older terrane.

During these two orogenies, the modern-era Blue Ridge was buried deep beneath the surface. The rocks of the modern-era Blue Ridge were also 40 or so miles east of their current location. The peaks of the eroding Taconic-time mountains and the Acadian-time mountains were located roughly along the route of Interstate 95 today, while the buried Blue Ridge was just to the west... or underneath the Taconic-time and Acadian-time mountains.

The third and last collision was different from the Taconic and Acadian collisions, which involved just small "terranes" of land. When Africa finally slammed into North America, it created the supercontinent of Pangea - and squeezed Virginia from the side. It's possible the African plate slid inland and reached as far as the Allegheny Front - nearly the western boundary of Virginia. The pressure from Africa caused the rocks on the eastern side of Virginia to break free from their roots.

The eastern rocks pressured by Africa were folded and thrust, like a stack of slick Sunday newspaper inserts sliding off a table. Ocean sediments that were once east of Richmond ended up as metamorphosed Piedmont crystalline rocks at Charlottesville. The buried Blue Ridge was thrust 40-60 miles to the west. The sediments that had eroded off the Taconic-time and Acadian-time were folded and bent into anticlines and synclines, as the rocks layers reacted to the pressure of the collision.3

Picture a spectacular car crash in a Hollywood movie. One car bangs into another, and they both stop - and then a second car with a high bumper rams them, and slides up onto the trunk. Other cars and trucks come by, and ride over top of the smashed cars to create a tall stack. In the movie, as everyone scatters and the gas tanks begin exploding, a tractor trailer might roll in and climb halfway up the pile before pushing over the cars at the top. Forget the gasoline explosions, speed up the film so millions of years are compressed into a minute, and replace the layers of cars with layers of rocks.... and you have the Appalachian orogeny, when Africa came calling.

In addition to breaking and thrusting the rocks on the coastline to the west, the energy of the collision with Africa wrinkled the layers of sediment as well. Those limestones and shales and sandstones from the Taconic and Acadian erosion cycles were folded, crumpled, and (close to the shoreline) broken. In some cases the flat-lying layers of sediments cracked and lifted slightly, and then the eastern portion slid overtop the western part of the same layers. This doubled the thickness of the sediment layers and shrank the width of Virginia, while absorbing the energy of the collision. The width of Virginia may have been compressed as much as 200 miles.4

The African collision produced its own mountain range on top of all the Virginia rocks left behind earlier by the Grenville event, the Taconic orogeny, and the Acadian orogeny.. [Surely you have figured out by now that the solid earth is anything but static, and that "native Virginia soil" came from rocks that are constantly moving around the continent...] Once again, all the drainages were rearranged by a new series of mountains. Only one river, known ironically as the New River, seems to have had the ability to continue to continue in its old channel, cutting across the rising Appalachian Mountains as the African continent wrinkled the surface into new ridges and valleys.

folded rocks in the Valley and Ridge province
rocks layers in the Valley and Ridge province, folded by the collision with Africa

References

1. Lynn S. Fichter, Steve J. Baedke, "The Ordovician Taconic Orogeny And Martinsburg Flysch and Queenstone Clastic Wedges - Middle to Late Ordovician; 450-435 mya," The Geological Evolution of Virginia and the Mid-Atlantic Region, 1999, http://csmres.jmu.edu/geollab/vageol/vahist/H-MidO.html (last checked March 6, 2013)
2. Lynn S. Fichter, Steve J. Baedke, "Orogenic Calm in the Central Appalachian Basin - Silurian and Early Devonian; 435-370 mya," The Geological Evolution of Virginia and the Mid-Atlantic Region, 1999, http://csmres.jmu.edu/geollab/vageol/vahist/I-siledev.html (last checked March 6, 2013)
3. Evans, M.A., 1989, The structural geometry and evolution of foreland thrust systems, northern Virginia: Geological Society of America Bulletin, v. 101, p. 352354, http://bulletin.geoscienceworld.org/cgi/content/abstract/101/3/339 (last checked June 27, 2011)
4. U.S. Geological Survey, "Geology of NYC Region - Valley and Ridge Province," http://3dparks.wr.usgs.gov/nyc/valleyandridge/valleyandridge.htm (last checked June 23, 2011)

On to: The Rest of the Story About Virginia Geology
Back to: The Grenville Event and Virginia Geology


Using Cereal Bowls and Car Crashes to Understand Virginia Geology
Rocks and Ridges - The Geology of Virginia
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