The Late Paleozoic Alleghanian Orogeny
Mississippian, Pennsylvanian, and Permian; 320 my to 250 my
~A Synopsis of the Alleghanian Orogeny~
The Alleghanian orogeny was the final major orogenic event to affect the Mid-Atlantic region. The present Appalachian mountains are the result of gentle uplift not involving plate boundary processes, and are thus not true mountains in a geological sense. The orogeny occurred when northwest Africa, a part of the supercontinent Gondwana collided with the eastern seaboard of North America closing the Proto-Atlantic (and Rheic) oceans. The collision resulted in several associated orogenies; the Mauritanide (the complementary orogeny on Africa opposite the Alleghanian), the Ouachita (South America striking the Gulf Coast region), and the Hercynian (Gondwana colliding with southern Europe). All of these orogenies resulted in the formation of the supercontinent called Pangaea.
By the time of the Alleghanian orogeny, Virginia had already been through two collision orogenies (Taconic and Acadian), but the Alleghanian differed significantly from the previous two. With the Taconic and Acadian orogenies, Virginia not only had mountain ranges in the east, but also large foreland basins in the west in which excellent sedimentary records were deposited. Furthermore, although the western part of the state was downwarped into a foreland basin, the pre-existing underlying rocks were relatively unscathed structurally (almost no recognizable internal folding or faulting).
With the Alleghanian orogeny, however, virtually the entire state became a mountain range, probably close to the size of the present day Himalayan Mountains (click photo to the right for more details). Imagine Richmond, Va sitting at the bottom of a Mt. Everest sized mountain. All of Virginia's rocks were structurally deformed, folded, thrust faulted, and moved significant distances from their original locations as a result of the Alleghanian orgogeny. The only exceptions are the Triassic Basin and the Coastal Plain, which did not exist until after the Alleghanian orogeny.
During the Alleghanian orogeny, Africa (Gondwana) became a hinterland, meaning it was thrust up and over the North American edge. We do not know how far Africa slid over Virginia, but it may have traveled as far as the Allegheny Front
, indicating that the North American rocks were shoved down deep in the earth by the weight of Africa, and most of Virginia was underneath Africa. All traces of Africa were later removed by erosion and transported westward as sediments into the foreland basins.
The Alleghanian foreland basin then developed farther west through Kentucky, West Virginia, western Pennsylvania, Ohio, and beyond. Virginia has only a small part of this foreland record in the southwest part of the state, but it is typical and representative; deltas, coastal margins, and coal swamps teeming with evidence of life.
~Alleghenian Orogeny and the End of a Wilson Cycle~
The Alleghanian orogeny also represents the completion of a nearly 300 million year cycle. The cycle began with one late Precambrian supercontinent, Rodinia, and one super-ocean. All the individual continents were packed together, with North America in the center, and Virginia somewhere in the middle, protected and geologically stable. The cycle continued into the latest Precambrian and earliest Cambrian when the supercontinent rifted apart creating the Proto-Atlantic ocean (Stages B and C). The new Virginia coast was left exposed and vulnerable to future plate tectonic events. The rifting event is reflected in the outpouring of basalt lava flows (i.e. the Catoctin Formation) now exposed in the present Blue Ridge.
After the continents scattered across the globe, the rifting reversed, convergent boundaries and subduction zones became active, and the Protoatlantic/Rheic oceans closed. The oceans closed in stages causing the Taconic, Acadian, and the Alleghanian orogenies. The Alleghanian orogeny assembled all the scattered continents of the late Precambrian supercontinent back into another supercontinent, Pangaea, closing the Protoatlantic/Rheic oceans, which enclosed Virginia in the middle again, and completed the Cycle. With the creation of Pangaea the earth now contained one supercontinent again, and one large ocean, Panthalassa, although the Tethys Sea lies between northern Africa and southern Europe. One could walk from Virginia to Africa, although for a time it required climbing a Himalayan-sized mountain range.
The Alleghanian orogenic record exists in two forms: structural deformation, preserved throughout most of Virginia, and a sedimentary record, preserved west of the Allegheny Front. Each of these is distinct. The structure is important, in part, because it creates the basis of our modern landscape, and also because it is an excellent record of this kind of mountain building; a classic case of great scientific significance. The sedimentary record is important, in part, because some of the largest coal reserves in the world are contained within them. These reserves fueled the industrial revolution, and helped build the United States into the superpower it is. The structure of the Mid-Atlantic region and the Sedimentary Record are discussed below.
~Tectonic Effects of the Alleghanian Orogeny~
Virginia; Pre-Alleghanian Orogeny (See Detailed Analysis for a more thorough explanation.)
The Alleghanian orogeny not only faulted and folded the rocks of Virginia, but it also displaced most of them westward. To reconstruct the pre-Alleghanian world, the cross section (below) is presented. This is the Mississippian Cross Section, but approximate modern geographic positions are indicated on it.
At the bottom of the rock pile are the Grenville basement rocks, stretched and thinned during the Proto-Atlantic rifting. These are not just the rocks now exposed in the Blue Ridge, but all the related basement rocks underlying the east coast.
Superimposed on the Grenville rocks are the Proto-Atlantic rift graben, and all the other deposits associated with the rifting event. Since rift grabens normally form and exist on the edge of continents, the pre-Alleghanian location of this rift zone would represent the edge of the ancient continent. Upon unfaulting and unfolding the rocks, we find that boundary close to the eastern edge of the present day Piedmont
. Sedimentary basins in the Valley and Ridge (DCM, foreland basins, etc) covered most of what is present day Virginia, including areas now occupied by the Blue Ridge and Piedmont.
To appreciate what all this means for ancient and modern geography, observe that the reconstructed location of the Taconic and Acadian terranes in the drawing above is east of Richmond, while their present locations in the Piedmont are west of Richmond. Similarly the Proto-Atlantic rift is now exposed in the Blue Ridge province, not buried deep in the earth under Richmond as it was before the Alleghanian orogeny.
By the time the Alleghanian orogeny commenced, the Taconic and Acadian terranes had both been peneplained; eroded down to virtually flat features. Beginning at Richmond, if you walked east across them you would have seen first the exposed batholiths of the Taconic volcanic arcs/microcontinents, and east of that the suture zone where the Taconic and Acadian terranes joined together, and east of that the exposed basement batholiths of the Avalon terrane.
From Richmond westward, to the inland coast of the Absaroka sea, along the edge were the Carbonate tidal flats of the Greenbrier formation, and west of that the oolitic shoals and crinoid meadows extending across the continent. It is not known how far east this coastline extended. On the very eastern coast of the eroded Avalon terrane, looking out into the Rheic ocean, the tops of volcanoes building along the edge of northern Africa (Gondwana) by a subduction zone may have been visible. Soon Gondwana will slam into this older Mid-Atlantic coast line, slide up over the eastermmost edge, and move tens of miles inland crumpling and displacing all the North American rocks underneath and in front of it.
~Large Scale Structural Relationships and Displacements~
When Gondwana collided with the older Mid-Atlantic, it collided first with the Avalon terrane and as it hit, the east-dipping subduction zone acted as a ramp, allowing Gondwana to slide up over the edge of Proto-North America (cross section at top). Just the weight of Gondwana, and the horizontal stresses of its movement would be enough to deform the underlying rocks, but additional zones of weakness existed. Old sutures associated with extinct subduction zones and the faulted Proto-North American continental edge detached and began to move as well.
The most characteristic feature of Alleghanian structural deformation is that large sheets of rock slid westward along great, nearly horizontal thrust faults. The scale and scope of this is captured diagrammatically in the cross- section at the top of the page (section K). As Gondwana slid up over the edge of Proto-North America, both the Taconic and Acadian terranes detached and moved inland. The detachments penetrated deeper still, all the way down to the once thinned and stretched Grenville basement rocks at the old continental edge. Great slabs of this break free, and moved inland, taking the Proto-Atlantic rift graben with them; great blocks of the earth, breaking free and moving along slippery zones of weakness.
The Taconic and Acadian terranes were originally east of Richmond, while today they are west of Richmond. Similarly, Richmond today lies above the original location of the Proto-Atlantic continental edge but the Proto-Atlantic axial rift, originally buried at the bottom of the rock pile, is now preserved further west in the Blue Ridge. That is to say, most of the rocks originally east or below our Richmond reference point moved west and up during the Alleghanian orogeny to become the modern day Piedmont and Blue Ridge provinces (Details). All along the easternmost margin, this activity took place underneath the overriding African continent. At depth, the rocks were hot, and the pressure was very high, and the normally brittle rocks were soft and plastic, moving and folding easily.
The stacks of DCM and foreland basin sedimentary rocks that originally occupied those regions west of the original locations of the Taconic and Acadian terranes began to move too but because they are sedimentary rocks, layered and flat lying, with slippery zones scattered among them, they behaved differently. Great thick slabs of rock, greased by the slippery zones, begin to break and slide, stacking up in multiple layers tens of thousands of feet thick. Underneath the moving Blue Ridge and Piedmont rocks the weight caused the sedimentary layers to slide over each other largely horizontally, but out in front, west of the Blue Ridge front, the sedimentary layers are bull dozed (crimped and folded).
While it is not known how far Africa slid over Proto-North America, one can see how far the stress affected the rocks. Rocks throughout the Valley and Ridge are thrust faulted and folded, but at the Allegheny Front, folding of present day rocks is absent. Just to the east of the front the rocks are severely deformed but then just a few miles west, out in the high Allegheny plateau, the rocks are nearly flat lying or mildly folded. We can see this well in the Cumberland Satellite Photo and description. We do not know why deformation stopped at the Allegheny Front, but this front begins in Pennsylvania and runs continuously all the way down to Tennessee and it may represent the farthest inland that the African terrane overrode North America.
~Alleghanian Structural Deformation~
Structural geology is Fractal. The large scale structures can be inferred by the smallest structures, and from the largest structures we can predict the nature of microscopic structures. We want to illustrate this structural diversity, looking at both smaller structures (macroscale), the kind you can see easily in road cuts, as well as structures so large (megascale) they dwarf us, and can be recognized only if you know what you are looking for. None of these will truly capture the regional picture of Alleghanian deformation, which exists at an even larger, continental, scale. Structural geology is a discipline all its own, and requires a little theory before we systematically explore Appalachian geology.
Some Expressions of Alleghanian Structural Deformation
In the eastern U.S. most rocks are lost in the vegetation, which makes studying eastern geology far more challenging than western U.S. geology. In the past few decades this situation has significantly changed. Construction of the interstate highway system has torn great gashes in the sides of mountains, and even the widening of secondary roads has produced a wealth of outcrops.
These days it is hard to take a trip very far without seeing an outcrop of rock, such as the folds in the picture to the right along U.S. Rt. 50 west of Winchester. The rocks are Devonian, deposited in a submarine fan during the Acadian orogeny in the Catskill foreland basin. But, of course, rocks are not deposited in this folded, twisted pattern. Sedimentary rocks are deposited horizontally, so when we see them as folded exposures, we know it is the result of a later event and in the Appalachians, that deformation is generally (with some exceptions) Alleghanian in origin. Often it is possible in only a few miles to pass from rocks deposited under one set of geologic circumstances to those deposited in an entirely different set of circumstances.
Blue Ridge Thrust Fault:
We can easily drive over the most interesting geology, and not know it. For example, the picture to the right was taken from New Market Gap on Massanutten Mountain, a few miles west of Luray, Va., looking eastward toward the Blue Ridge. Page Valley is in the foreground.
The Blue Ridge contains the Grenville basement rocks, and the Catoctin lava flows that accompanied the opening of the Proto-Atlantic (Cross Section). Furthermore, from the explanation above we also know the Blue Ridge rocks have been moved westward along the Blue Ridge Thrust Fault (Cross Section). So, as we approach the base of the Blue Ridge mountains along Rt 211 we cross a great fault. You will not see it as you drive, and even a geologist new to the area would probably miss it. The rocks are so different across the fault the geologist may, however, suspect some geologic event change.
As soon as we cross the fault, traveling west, we leave the Blue Ridge Province and the Grenville suite of metamorphic and crystalline rocks. Now within the Valley and Ridge, the first rocks we encounter, although it is hard to find exposures, are Carbonate tidal deposits of the Proto-Atlantic divergent continental margin. All these Carbonate rocks have been folded and faulted during the Alleghanian orogeny, and thereby Massanutten Mountain represents the axis of a huge syncline (Cross Section).
Allegheny Front Transition:
The picture below was taken at Germany Valley, Pendleton Co. W.Va. Looking just to the right of center is looking northeast, along the trend of the mountains. That is the direction all the mountain ridges in the Valley and Ridge trend, as can be seen in the Cumberland Satellite Image, or at the Smoke Hole Link. The highest, long ridge on the left side of the picture is the Allegheny Front. It is the boundary between the Valley and Ridge and the High Allegheny Mountain Plateau. The valley in the foreground is Germany Valley.
Structurally it lies within an overturned breeched anticline. That is, before the valley eroded out it was roofed by a great dome of rock, arching high above the picture. The cross section to the right shows this arch diagramatically; the far right ridge in the photo is the eroded right side of the drawing; the left side of the drawing is the closest low ridge in the photograph. The jagged edge of the ridge on the right is comprised of the Tuscarora sandstone; a tough, resistant unit. The Tuscarora originally arched out over the center of the photo connecting with the nearest long sharp ridge on the left side (just a few miles NE it forms Seneca Rocks). Below ground is a thrust fault along which all these rocks moved during the Alleghanian orogeny (it is similar to the Blue Ridge Thrust Fault And Overturned Anticline
Major structural deformation of the Alleghanian orogeny seemingly came to a halt as represented by this photograph. East of the Allegheny Front, the rocks are severely deformed. West of the Front, expressed at the surface, they are mostly flat lying with only minor undulations. The analogy would be the front of the Siwalik Hills in the Himalayan Mountains; the front where structural deformation stopped.
~The Alleghanian Foreland Basin and the Coal Swamps~
Mountains the size of the Alleghanians, 20-30,000 feet, covering most of the eastern United States out to central Pennsylvania, Eastern West Virginia, and south, produced an enormous amount of sediment. Rivers from these great mountains drained westward into the Absaroka epicontinental sea along a broad front spreading from West Virginia, to Ohio, to Indiana and Illinois, across the Mississippi river (which did not exist at this time; all drainage was from east to west) to Iowa and Missouri and out eventually into Kansas and Nebraska (Pangaea Map, red arrows).
As this sediment traveled it mixed with sediment traveling north from the Ouachita mountains along the Gulf Coast (which also did not exist, instead being replaced by South America) and together they spread through Oklahoma and Texas (Pangaea Map, blue arrows). A large chunk of Africa, and South America, eroded into uncountable grains of sand and spread across two-thirds of the North American continent. By the time this was finished, so much sediment had been deposited that the Absaroka sea was pushed right off the continent through a small corridor in southwest Texas. North America would never again experience the great inundations of the sea that so dominated the previous 300 million years. As you drive across much of the present-day eastern United States what you drive across are these sedimentary layers from the Alleghanian orogeny.
Unfortunately the Mid-Atlantic region has virtually no record of all this sediment; instead this region was the sourceland, destined to be eroded down flat to a peneplain. Only Virginia has a record, and it is confined to a small outcrop belt in the southwestern corner of the state (including Scott, Russell, Tazewell, Lee, Wise, Dickenson, and Buchanan counties; that is the Appalachian Plateau portion of the state). But it is the beginning of the huge clastic wedge of sediments that spreads westward. Southwestern Virginia, being closest to the mountains, of course receives some of the thickest Accumulations; nearly three miles thick. From there it thins westward.
When mountain building was just beginning, marine shelf and Shoreline (beach, lagoon, swamp) environments dominate the record. These Shoreline environments were similar to the east coast of Virginia and North Carolina today with their barrier islands, lagoons, and tidal marshes, although the tectonic conditions are very different. Additionally, the geography was reversed; the mountains were in the east and the Shoreline in the west. As sediment accumulated and spread westward, deltaic Coal swamps and terrestrial environments take over. It is not clear why, but deltaic depositional environments did not exist before the late Paleozoic.
By the Late Paleozoic most of eastern North America (west of the Alleghanian mountains) looked something like the Everglades of Florida, or the bayou country of southern Louisiana. North America lay in a subtropical region and conditions were very humid most of the year with abundant rain. Vegetation was heavy; seed ferns, scale trees, and sphenopsids growing dense and thick, and large trees towered over head. The ground was a thick undergrowth littered with the tangled remains of decaying plants, accumulating layer after layer of Peat, eventually to become Coal.
Arthropods were abundant, crawling in the dank undergrowth, and buzzing through the air; centipedes, and millipedes, and insects, (some as much as a foot long), and dragon flies with wing spans of 29 inches along with labrynthodont amphibians, now extinct, but some up to six feet long and behaving something like crocodiles (which have not yet evolved) were dominant. Llepospondyl amphibians, some snake-like, some salamander-like, existed alongside the earliest reptiles just coming out of the water.
Contributed by Lynn Fichter