Eve of the Alleghanian Orogeny

 
Virginia on the Eve of the Alleghanian Orogeny
 

The spatial relationships among rock formations prior to and following  the Alleghanian orogeny changed significantly. A reconstruction of Virginia on the eve of the Alleghanian orogeny is developed on this page.  When Gondwana collided with North America neither the Coastal Plain nor the Triassic Rift Basins existed. We ignore the Allegheny Plateau since it was far enough inland that it was simply accumulating layer after layer of sediment brought in from the east. What remains is the Valley and Ridge, Blue Ridge, and Piedmont (Physiographic Map). We  know that many of these rocks were formed from terranes, volcanic arcs and microcontinents brought in in stages during the Taconic and Acadian orogenies. These rocks underwent structural displacement during the Alleghanian orogeny. Below is a review of the rocks and their modern spatial relationships (from west to east):
  •  Allegheny Plateau: sedimentary rocks, virtually unmoved since their deposition.
  • Valley And Ridge: Proto-Atlantic divergent continental margin sedimentary wedge, foreland basins (Taconic Queenston, and Acadian catskill), and Cratonic episontinental seas (Central Appalachian Basin and Mississippian Kaskaskia). Note that these rocks also extend east under the Blue Ridge and Piedmont (Cross Section).
  • Blue Ridge Province: Grenville basement rocks, with the Proto-Atlantic rift graben faulted into them.
  • Piedmont: suspect terranes, brought in from elsewhere prior to the Alleghanian orogeny.
  • Coastal Plain: does not exist in the late Paleozoic

~Restoration Work: Getting Rocks Back to their Original Locations~
 
The Valley and Ridge and related sedimentary rocks are found in two locations. First are rocks in the modern Valley and Ridge (Cross Section). Although these are folded and thrust faulted, they are essentially in their Paleozoic position relative to the other provinces, i.e., east of the Allegheny plateau and west of the Blue Ridge/Piedmont. But, because they have been foreshortened by the Alleghanian structural deformation, the pre-Alleghanian Valley and Ridge province must have been wider than it is today, perhaps several times wider. We can get some idea of the degree of shortening by looking at the second group of Valley and Ridge-type rocks.
 
The second group of pre-Alleghanian sedimentary rocks extend east under the Blue Ridge and Piedmont all the way out to what is now Richmond (Cross Section). We presume these are Cambrian and Ordovician in age, making them part of the divergent continental margin wedge, but since they are buried we really don't know for sure. These subsurface sedimentary rocks are mostly horizontal and unfolded, and at least the bottom layer is sitting conformably on basement; that is, they are sitting as they were deposited on the underlying igneous and metamorphic rocks of the North American continent. These rocks have probably not moved since they were deposited, and we can use them as a reference for figuring out where all the other rocks went.
 
The result of these relationships indicate that the Valley and Ridge-type rocks were originally in a belt about two or three times as wide as the present Valley and Ridge (Cross Section), extending about out to the boundary of the current Piedmont and Coastal Plain Provinces. We have reasoned our way to the idea that most of Pre-Alleghanian Virginia, at least out to the eastern edge of the Piedmont, existed as Valley and Ridge-type rocks (sedimentary basins). There is also geophysical evidence that this is true; both seismic studies and gravitational studies indicate that deep in the subsurface under the eastern Piedmont are rocks and structures created during the rifting that Opened The Proto-Atlantic ocean. This, despite the fact that the axial graben is today located in the Blue Ridge. 

The Blue Ridge rocks are the Grenville basement and the Proto-Atlantic rift graben. Even though they are older than the Valley and Ridge rocks, and should be underneath them, they are actually sitting on top of Cambro-Ordovician divergent continental margin sediments, deposited after the rift graben had formed and filled (Cross Section). What is more, normally rift rocks lie at the edge of a continent, but if the DCM sediments actually extend to the eastern edge of the Piedmont province, then the Blue Ridge rocks are displaced west. So, these rocks are not only on top of rocks they are older than, but they are farther west than they should be. To reconstruct the pre-Alleghanian Mid-Atlantic we have to put these rocks back geographically where they belong, probably just about the piedmont/Coastal Plain boundary, and stratigraphically where they belong (deep in the earth).

The Piedmont rocks also shows complexities. The Piedmont has dozens of imbricated Thrust Faults, which means that many rocks now found side by side were separated in the past and the overall effect is that the Piedmont province is now narrower than it once was. Perhaps it was twice as wide as it is now. Also, the Piedmont rocks, like the Blue Ridge rocks, have been thrust over the underlying horizontal DCM rocks, implying they originally must have existed much farther east than the current location, and farther east than the Proto-Atlantic rift graben now in the Blue Ridge (Cross Section).

To reconstruct the pre-Alleghanian world we must restore all this displacement and dislocation in the Valley and Ridge, Blue Ridge, and Piedmont and the result is something like the Mississippian cross section below. At the bottom of the 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.


Faulted into the Grenville rocks are the Proto-Atlantic rift graben, and all the other rocks associated with the rifting event. Fortunately, most of these rocks are now exposed in the Blue Ridge province. Since these rocks are the oldest they are, appropriately, on the bottom of the cross section. Sitting on top of the Grenville/Rift rocks is the DCM sedimentary wedge. However, just before the Alleghanian orogeny, these rocks are buried under sedimentary deposits of the Taconian foreland basins, and Acadian foreland basin, as well as interorogenic sediments, a thick Accumulation of sediments; several tens of thousands of feet.
 
Just before the start of the Alleghanian orogeny, the Greenbriar Carbonates were being deposited in the tectonically stable Absaroka sea. Virtually none of these rocks have been displaced from their site of deposition at this Pre-Alleghanian time. At this point, the cross section above restores all the pieces spatially, but does not yet restore them geographically.

~Pre-Alleghanian Geographic Restoration~
 
 In the pre-Alleghanian world, virtually all of Virginia is sedimentary basin; that is from the far west all the way out to the Paleozoic eastern edge of the continent at the Piedmont/coastal plain Boundary. This sedimentary basin contains, from bottom to top, rift graben, buried by rocks of the DCM, buried by foreland basin rocks. If we go any farther east than about Richmond we move off the edge of the ancient continent and out into the Proto-Atlantic ocean basin. But out there are the eroded roots of the Taconic and Acadian terranes. Today these rocks are in the piedmont, but in the pre-Alleghanian they must have existed much farther east, certainly east of the original Proto-Atlantic edge of North America. In today's geography this would put the terranes out in the Coastal Plain region, probably out to the edge of the present continental shelf, and beyond. East of that then, in the Pennsylvanian, is the Rheic ocean, containing Gondwana, closing fast. 


(Click image to see full cross section)

In the cross section above we have indicated possible modern geographic reference points to give an idea of the Pre-Alleghanian locations of terranes. Observe that all these rocks today are found west of Richmond, but in the Pre-Alleghanian they were east of, or in the case of the rift graben, underneath Richmond.

Prior to collision, across the area of the present Coastal Plain, are the Taconic and Acadian terranes, peneplained, virtually flat, eroded down almost to sea level. These rocks would have been rich in complexity since they contain more than one volcanic arc, and the suture zones between them, and likely microcontinents as well (Terrane Map). To the east of the Taconic terrane is the suture zone where the Taconic and Acadian terranes join together, and east of that the exposed basement rocks of the Avalon terrane. And finally to the east of all this, off the eroded edge of the Avalon terrane, is the Rheic ocean.
 
To the west is the inland coast of the Absaroka sea. Along the edge are the Carbonate tidal flats of the Greenbriar Formation, and west of that the oolitic shoals and crinoid meadows. The presence of tidal flat deposits along the present Allegheny front indicates that at times it was that far west but with a higher sea level and a low sourceland it could have come farther east; in the cross section it is brought rather far east, but it is not known with certainty.




~Virginia After the Alleghanian Orogeny~

As Gondwana slid up and over the North American continent, it caused many of the underlying terranes and blocks of crust to detach and move westward. This movement displaced both the Taconic terranes (Arvonia arc, Carlonina slate belt, etc. (Terrane Map)) and the Acadian terrane (Avalon). It also reached down to the roots of the continent and detached the stretched and thinned edge of the continent containing the Proto-Atlantic rift systems and shoved them westward to their present location, where they stopped as an Overturned, Ramped Anticline.
 

Some idea of the extent of this displacement can be seen in the drawing above comparing pre-Alleghanian and post-Alleghanian locations. The location of modern Richmond is a reference point. Prior to the orogeny the Taconic and Acadian terranes lay east of Richmond, and the Proto-Atlantic rift grabens somewhere below in the subsurface. Directly to the west in what is now the piedmont was the outer edge of the Proto-Atlantic divergent continental margin wedge of sediments, and superposed on that the foreland basins of the Taconic and Acadian orogenies. 
 
After the orogeny the Taconic/Acadian terranes had been moved west of Richmond to form the modern Piedmont and west of that was the Proto-Atlantic rifts, brought up from below by the collision and shoved westward in front of the moving terranes. This now constitutes the Blue Ridge province with its Grenville basement. When we look at the structure of the Piedmont and Blue Ridge we see they are not that different (Cross Section). What distinguishes the Blue Ridge from the Piedmont is that the Blue Ridge exposes Grenville basement while the Piedmont contains the suspect terranes. Otherwise the whole complex is cut extensively by decollement imbricated thrust faults.
 
Lying under the Piedmont are horizontal layers of DCM and foreland basin rocks. These rocks probably acted as slippery glide planes along which the Blue Ridge and Piedmont rocks could slide. Further west, however, in front of the moving Blue Ridge front,  the Blue Ridge rocks bull dozed the sediments, faulting and folding them in complex patterns.
 
Contributed by Lynn Fichter 
 
Sunday, August 10, 2014
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