H - Ordovician Taconic Orogeny


 
The Ordovician Taconic Orogeny and Martinsburg Flysch and Queenstone Clastic Wedges
 
Middle to Late Ordovician; 450 - 435 mya

 
~A Synopsis of the Taconic Orogeny~ 
 
The Taconic orogeny occurred when a terrane or series of terranes (volcanic arcs and microcontinents) collided with eastern North America along an east dipping subduction zone:Stage G to this stage. Because subduction was dipped to the east, it acted like a ramp, and the terrane was forced to slide up onto and over the continental edge (along the "suture zone" in the cross section at the top), forming the Taconic mountains. As the mountains rose up, the Craton to the west was pressed down into a deep-water foreland basin. The overriding terrane that formed the mountain is termed the hinterland
     
Eroded sediment from the mountains filled in the foreland basin and by the time the basin was filled, the mountain was completely eroded to sea level. What was a mountain became a flat, featureless, and stable (Stage I) region. The terrane became solidly welded to North America and the orogeny came to a close. The cross section at top illustrates the collision at full force.

~Complexities of the Taconic Orogeny~
 
The Taconic orogeny is more complex, however.  First, the eastern edge of the Cambo-Ordovician continent was rifted into a zig-zag pattern (Cambro-Ord. Map). Portions of land like the Southwest Virginia High and the southeastern Pennsylvania promontory jutted out and were hit hardest while areas of protected reentrants existed into between where collision was soft. Equal trajectories and strengths for collision to take place along a rugged margin is impossible. Second, the Taconic terrane approached North America at an angle from southeast (Pre-Taconic Map), further justifying the impossibility of a uniform collision. Third, the Taconic terrane collided first in southwest Virginia and then a few million years later with southeast Pennsylvania. In effect, the orogeny occurred in pulses and thus, shows different character in different places. It is also possible that the different belts that make up the Taconic terranes (Piedmont Belt Map) came in and collided at different times (Stage G). 
     
The points of major collision resulted in major thrust mountains, probably as high as the Swiss Alps, however,  the areas in between (northwest Virginia and Maryland for example; Taconic Map) were located in a protected reentrant, and were certainly squeezed, but not crushed. Deformation in this area was more of a gentle folding into an arch that may never have risen above sea level. 
 
The major processes of the Taconic Orogeny are sketched in the cross-section below.  A more thorough discussion can be accessed through the link at the bottom of the page. Please refer to the Pre-Taconic and Taconic maps - Pdf, and cross sections - Pdf for more information.

~The Setup~
 
The orogeny began when the Taconic terrane converged on the Southwest Virginia High from the southeast (Pre-Taconic Map). Along the western border of the terrane oceanic lithosphere is descending down a subduction zone, keeping the volcanoes in the terrane active (A-A'). Between the terrane and the continent edge is the Proto-Atlantic remnant ocean basin (ROB), quickly disappearing down a subduction zone. When it is gone, the continent and terrane collide. 

     
 
In the early Ordovician volcano tops over the edge of the horizon to the southeast may have been visible and later in the Ordovician rose even higher, but until the actual impact, North America experienced only minor effects from the terrane additions. Instead, carbonate rocks continued to flourish throughout the tectonically stable Mid-Atlantic region (Stage G). These carbonate rocks give no clue that an orogeny is forthcoming, but scattered in these rocks of the modern Valley and Ridge province are volcanic ash beds. In the early Ordovician eastern North America lay below the equator in the southern trade wind belt, and prevailing winds easily transferred volcanic ash from the terrane where it settled across the shallow carbonate seas, long before the collision. Perhaps long before the volcanoes could even be seen.

~The Collision~
 
As the last of the remnant ocean basin descended into the subduction zone, the terrane finally collided with the Southwest Virginia high. Upon collision, the terrane becomes a hinterland, overriding the Southwest Virginia High along two thrust faults, the Haysville-Fries-Rockfish Valley (HFRV) fault, and a Taconic overthrust (C-C'). The HFRV fault is not new; it was active in the Grenville (Stage A), and is reactivated by the Taconic collision. These mountains were large and the thrust faults introduced Cambrian/early Ordovician divergent continental margin rocks (Stage D), as well as terrane rocks up into the mountain to be later eroded (C-C'). Inland from the first impact, the first foreland basin develops, the Bount. Refer to cross-section C-C' to see this feature develop west of the HFRV thrust. 

A few million years after the first impact, the Taconic terrane wraps around and collides for a second time with southeastern Pennsylvania, building a second hinterland mountain. The Queenston foreland basin develops to the west in central Pennsylvania (Taconic Map).
     
Of the two impact sites, little is preserved of the one in southern Virginia; later events have masked or destroyed the evidence. The second impact is preserved, however, in Pennsylvania as a record of intense metamorphism and structural deformation. This is typical of what happens to foreland rocks that are shoved deep into the earth by an overriding hinterland.

~The Protected Reentrant~
 
 When the new Taconic terranes collided with the aforementioned promontories, movement was halted.  In between the two collision points lay a protected reentrant, which was set back too far from the terrane to undergo major deformation (Taconic Map).  This is known with certainty because the HFRV fault shows less and less reactivation farther to the north. Somewhere north of Charlottesville, presence of reactivation is absent.
     
The reentrant is not unscathed, however, and subduction zone dynamics were prominent in this region. Running in front of the subduction zone on the oceanic side of the trench was a peripheral bulge, or an arch in the oceanic floor. This bulge may be likened to a wave continuously running away from the front of a boat as it travels in the water. During the Ordovician, as the terrane approached, this bulge was the first thing to hit North America, and it slightly lifted the continent into an arch, moving like a wave inland. At the major impact sites we would see no evidence of the bulge, since it would have been destroyed by subsequent mountain building.

The bulge is currently preserved (stratigraphically; there is no actual bulge today) as the Little North Mountain arch. The arch divides the protected reentrant into two foreland basins, a Western Cratonic basin and an Eastern flysch basin (Taconic MapB-B'). On the east side of the flysch basin the terrane is hung up and acts as a dam sealing the flysch basin off from the ocean on the other side. The Western Cratonic Basin is continuous with the shallow Tippecanoe sea (Stage G) covering most of North America at the time, and it also merges north with the Queenston foreland basin.
 
Following the collision of the Taconic terranes, the Proto-Atlantic ocean was subducted underneath the land in the subduction zone. But there was still an ocean east of the terrane, known as the Rheic ocean. 

~The Foreland Basins~
 
With movement of the approaching terrane halted,  the Taconic orogeny was effectively over and no more mountain building occurred. The only thing that remains is for the mountains to erode, and the foreland basins to fill with sediment, however, in the Taconic event, two major source lands, one in the north and one in the south (not to mention the terrane blocking the opening of the reentrant), and several foreland basins exist. From the southern Virginia source land, sediment spreads out in two directions. One forms the Blount clastic wedge (foreland basin), spreading westward through southwest Virginia into Kentucky and Tennessee (Taconic Map). The second is the Martinsburg flysch, traveling northeast, down the deep axis of the flysch basin, trapped between the terrane on the east and the Little North Mountain arch on the west. The Martinsburg flysch is made of very immature sediments (rich in feldspar and rock fragments) flowing as deep water turbidity currents (underwater avalanches) down the axis of the basin. These currents traveled all the way to Pennsylvania where they stopped at the high walls of the transform faults, piling up at their base.
     
We might also expect a record of sediment flowing westward from the terrane into the flysch basin, but no such record exists. Inevitably some sediments must have come from there but the record is not preserved. From the Pennsylvania source land, sediment flowed westward into the Queenston foreland basin (clastic wedge) in central Pennsylvania and from there it traveled southwestward into the Western Cratonic basin of West Virginia. Some of it encroaches up onto the back side of the Little North Mountain Arch, but was not able to cross over into the flysch basin. Not until the very end, when the basins are full, and the mountains virtually all gone, was sediment spread from one basin into another.

~The End of the Taconic~
 
The end of orogenies in general, and the Taconic in particular, are easily recognized in the rock record. Orogenies produce very thick accumulations of immature sediments. An immature sediment is one filled with easily weathered particles, feldspars and rock fragments. Because of rapid erosion they are buried too rapidly to weather much. As the orogeny draws to a close, the supply of immature clastic sediment wanes, and sediments return to non-mountain building types; carbonates and quartz sandstones. 
     
Pure quartz sand is "mature" and indicative of tectonic stability because quartz is the only mineral that does not weather. For a sediment to be pure quartz, it must have the time for complete weathering of everything that is not quartz on a very low, stable source land. In the field we know mountain buildings events have ended and there tectonic stability established when quartz sand and carbonates appear (reappear) in the record.

The end of the Taconic orogeny is marked by the presence of the Clinch/Tuscsarora Formation (and its equivalents like the Massanutten sandstone), a pure quartz qandstone that blankets the entire Appalachian region from New York to Tennessee. The photograph to the right is the Tuscarora at Seneca Rocks, Pendleton Co, West Virginia. It is a resistant sandstone that holds up many ridges in the area. The maturity of the sandstone indicates the Taconic mountains have been eroded down to stability and also that all the various foreland basins had filled in to allow the Tuscarora sand to spread more or less uniformly throughout the region.

~One Final Complication~
 
There were two more orogenies that took place following the Taconic orogeny;  the Acadian (Stage J) and the Alleghenian (Stage K ). The Alleghenian in particular had a major impact on the Mid-Atlantic region. Virtually all the rocks in the region are faulted, folded and telescoped; shuffled like a deck of cards resulting in complete translocation of all rocks that were deposited. 
     
Most of the geology on the paleogeographic maps we used in the history above is based on the present location of the rocks, especially for the Western Cratonic and Eastern Flysch basins. It implies these basins are closer together than they were originally but we can get some idea of their true original locations from Interpretive Cross Sections. The first (Pre-Taconian) cross section shows the Virginia divergent continental margin stretched out with all the subsequent folding and faulting undone. The second cross section (Taconian) shows development of the Western Cratonic and Eastern Flysch basins in the Taconic. Note the distance between reference arrows A And B. The third cross section (Alleghenian) shows how the rocks have telescoped together by folding and faulting. Note especially how the reference arrows A and B have moved close together, and how the Western Cratonic basin rocks (yellow beds) extend far to the east below the surface.

The Taconic orogeny is a complicated one to unravel,  but it is a good example of what orogenies are like and how we interpret them. The Mid-Atlantic enters a period of stability following the Taconic orogeny, which leads into Stage I
 
Contributed by Lynn Fichter 
Friday, March 06, 2015
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