Thoroughfare Gap

            A week or so ago my structural geology class took another field trip.  This time we went to Thoroughfare Gap located near Haymarket Virginia.  The location is a water gap in the Bull Run Mountain created by Broad Run, a tributary of the Occoquan River.  The gap has provided a vital means of transportation across Bull Run Mountain.  During the civil war it was a strategic position from which one could control the passage of armies and good s from the Shenandoah Valley into the east.  Today the gap is used by the Broad Run, a train track, and two highways (Interstate 66 and Route 55).

            Aside from its geographic value Thoroughfare Gap shows evidence of a diverse series of tectonic events.  Thoroughfare Gap is located on the border between the Blue Ridge and the Piedmont geological provinces of Virginia.  Bull Run Mountain shows geologic features from one and a half Wilson Cycles.  Wilson Cycles are the process by which the continents alternate between being arranged together as supercontinents and being spread out across the earth.  The cycle takes roughly 600 Ma between supercontinents.  This is thought to be caused by heat from within the crust, convection cells within the earth’s mantle move material and heat up from the core mantle boundary which then spreads as it hits the surface along spreading centers.  I will now attempt to explain the complicated history of Thoroughfare Gap.

            The first story told by Thoroughfare Gap is that of the breakup of Rodinia which occurred 650 Ma ago.  This was known as the Post-Grenville Rifting.  We shall begin our tour at the western edge of the gap where the evidence of the rifting is readily apparent.  During periods where supercontinents exist excess heat builds up underneath them.  This heat then causes rifting.  As Rodinia broke apart there was a decrease in pressure in the upper mantle which allowed for melt to form.  This melt then spilled onto the surface in the form of the Catoctin Flood Basalt.  The Catoctin can also be seen in Shenandoah National Park as igneous dikes in the Old Rag Granite.  This was the oldest formation we saw on our trip.  Since its deposition it has undergone metamorphism, today the basalt is known as the Catoctin Greenstone. Originally it was a microcrystalline, mafic igneous rock.  When Rodinia broke up the area that is now Thoroughfare Gap became the shoreline of the Proto-Atlantic or Iapatus Ocean.  Then, after the break-up of Rodinia sediment began to accumulate.  During the early Cambrian the Chilhowee Group was deposited.  The Chilhowee Group is made up of the Weverton, Harpers, and Antietam Formations.  Of these three the Weverton and the Harpers are exposed at Thoroughfare Gap.  The Weverton is located just above the Catoctin stratigraphically.  At Thoroughfare Gap it was a quartz arenite which has since been metamorphosed into a quartzite.  The nature of the sandstone shows that this was once an ancient beach.  Regions of the formation are made up of oligomict quartzose paraconglomerates.  Above the Weverton is the Harpers which is a thin bedded shale with lenses of sand.  This shows that there was a shift to a lower energy environment.  This was likely a lagoon area.  According to Hess’s Law which states that vertical changes in stratigraphy represent the horizontal changes, this represents a transgressive sequence where sea level rose.  In other localities the Antietam Formation overlies the Harpers, but at Thoroughfare gap this is an unconformity.  In the area that we are interested all of these layers dip to the west.  It is necessary to clarity’s sake that this is stated now, however, the evidence for this will be explained later.

            Now we get into the convergent part of the Wilson Cycle.  Eventually the Iapatus Ocean began to close back up resulting in a series of continental collisions which made up the Appalachian Orogeny.

Map by Ron Blakey

            First, around 460 Ma during the Mid-Ordovician to Mid-Silurian the Taconian Orogeny took place.  During this time the Chopawamsic Terrane collided with what is now the east coast of the North American Plate.  Then, at 360 Ma the microcontinent Avalonia impacted on the North American Plate.  Finally the Iapatus Ocean closed completely as the African Plate crashed into the North American Plate in what was known as the Allegenian Orogeny which formed Pangea roughly 275 ma.  Strucural features throughout the area are evidence of the Appalachian Orogeny.  We will begin our tour of these features in the oldest rocks and then move up column into the younger rocks.

            Following the railroad tracks provides a unique opportunity to observe an east-west cross section of the structural features of Bull Run Mountain.  In many places the railroad company had to remove portions of the rock exposing fresh faces which show the true, unweathered traits of the rocks and the structures they are a part of.

            In the Catoctin Flood Basalts there are four features which show the east-west compression in the region.  First of all, the basalt has been metamorphosed into the Catoctin Greenstone.  Below is an image of the greenstone which is prominent in much of the Blue Ridge Geological Providence of Virginia. 

            The feature I found farthest to the west was a small fold in the rocks on the north side of the railroad track.  It is a close fold meaning the interlimb angle is less than 90°.  The straight limbs and hinge make it a chevron fold.  It is overturned and harmonic.  The orientation of this fold indicates that the primary stress (d1) was compressing the area along an east-west axis. 

Next, moving upward in the stratigraphic column (Moving east along the train tracks in our case) I came across another fold.  This one was much more rounded than the previous fold.  It is also harmonic, this one is upright, and it is also a close fold. 

 

Below is the same image annotated.

The next feature is what may have been a boudinage or perhaps a “proto-boudinage” which I found in a wall on the south side of the tracks.  This is where the compressive forces forced a rigid material (in this case what appears to be feldspar) to smear and fracture.  The less rigid meta-basalt then fills in between the fragments of the feldspar.  In this case the direction of least stress (d3) is nearly vertical with the two other stresses being north-south and east-west.

Below is the same picture annotated.  d2 would be into the page.

Next we move into the Weverton Formation.  It is important to note here that while we are moving up-column and that this formation is younger than the Catoctin the structural features mentioned here are roughly the same age as those in the Catoctin below.  This formation includes some of the more interesting structural features of the area.  At the least, it was the area that we focused on the most during our field trip. 

            The Weverton Formation contains various sets of joints which are shown in the picture below.

Photograph courtesy of Laura S.

Along with these joints we found plume structures everywhere.  The plumes are interesting because they tell the story of how the joints formed.  When a joint forms the direction of propagation is in the direction in which the plume opens.

Again, below is the same image annotated.

Using stereonets it is possible to identify the major sets of joints present in the Weverton Formation.  They are indicated below.    It is important to notice that all of the stereonets included in this post are using poles to display data for planar features.  I find that poles make paterns more easily recognizable than great circles with large sets of data.

 

The orientation of the joints is valuable because it can give us an insight into the major stresses acting on the region.  Typically the plane of the joint is oriented parallel to the plane created by the primary and secondary stress directions. (dandd2).  The direction of least principle stress is generally perpendicular to the plane mentioned before.  This being said, this explains joint set 3 as the plain is parallel to the primary stress direction (east- west) during the Appalachian Orogeny.  However, joint sets 1 and 2 are more difficult to explain.  They each appear to be dipping at 50-60 degree angles in opposite directions (roughly east and west).  To me this suggests a conjugate pair of normal faults even though there is no evidence of displacement along the joint faces I examined.  As such, I propose the hypothesis that joint sets 1 and 2 formed during the rifting of pangea (discussed later).  The extensional forces exceeded the tensile strength of the quartzite forming the joints.  Perhaps, this is evidence of a failed right which failed even earlier in its existence than the Culpeper Basin to the east.  Possibly, the extensional stress was relieved by the formation of other basins before the joints underwent displacement.

            Another feature seen in the Weverton is large quartz veins which have proceeded to fill the gaps left by joints.  The stereonet below shows the vein information we have overlaid on the joints from above.  It is evident that the quartz veins followed the pre-existing joints.  This was caused by the high pressures and relatively high temperatures during the Alleghenian Orogeny.  These parameters metamorphosed the sandstone into a quartzite.  Also, some areas of the quartz experienced more extreme conditions and went into solution.  Another scenario is that some areas of the quartz were more pure than others meaning that they required a higher energy conditions in order to melt, which allowed some areas to go into solution and others to not.  The principle behind this hypothesis is that contaminants lower the melting point of a solution similar to putting salt on icy roads in the winter.  Regardless of the cause, the silica solution then moved to the pores (in this case the joints) and then recrystallized.  Interestingly the veins seem to have preferentially formed in what was earlier referred to as joint set 1.

            Another interesting feature in the Weverton Formation was the metamorphism of the conglomeratic sections.  The quartz clasts were made somewhat prolate and aligned.  Thus it has been given the term “stretch pebble conglomerate”.  Sadly, I never saw any in situ so analysis of the stress directions is somewhat pointless.  Since the clasts are prolate, d3 would be parallel to the lineation formed by the quartz clasts.  d2 and d1 would then form a plane perpendicular to d3.  The picture shown below is one of float used to line the path.

The keys are roughly 5 cm long for scale.

As we move farther up column we reach the boundary between the Harpers and the Weverton Formations.  Here we find several valleys which have formed along joint planes.  They are likely part of the overall imbricated thrust fault which runs under the entire Blue Ridge Providence.  This will be further explained later as a overall discussion of the features created by the Appalachian Orogeny.  Since its creation the fault has been weathered and is now only visible as a north-south oriented valley.

Later in the column we came across a kink fold in the Harpers formation.  This fold is one of the more interesting features that we saw today.  That is because it is a prime example of Pumpelly’s Rule which states that small scale structures can tell the story of the regional structure.  The shear of this fold shows a regional shearing “up and to the left”.  This picture was taken looking north which means that the “up and to the left” translates to a shear to the west.  In this case, it provides evidence that this is in fact the eastern limb of an anticline with its center to the west.

Photograph courtesy of Laura S.

The next part of our story deals with the next half of a Wilson Cycle, the breakup of a supercontinent.  The setting is 210 Ma, Pangea has been around for a while and heat is building up underneath it.  The convection causes the crust to begin to pull apart exceeded the tensile strength of the continental plate.  These extensional forces led to the rifting of Pangea.  The center of the rift was what is now the Mid-Atlantic Ridge.   The Atlantic Ocean began to open up between Africa and the Americas.  Below is an image of the paleogeography as Pangea broke apart.

Map by Ron Blakey. 

Another product of this rifting was the failed rifts.  The Newark Supergroup is a set of failed rifts which formed basins running from the post-Pangea rifting which runs nearly parallel to the east coast of North America from South Carolina to New Jersey.  The Culpeper Basin (part of the Newark Supergroup) is located just east of Thoroughfare Gap.  As Pangea split apart a rift began to form just east of the Blue Ridge.  The rift ultimately failed but it left behind a depositional basin which then filled with Mesozoic sediment.  Just east of the gap one can find exposures of the Waterfall Conglomerate which is a formation of the Culpeper Basin.  The contact between the Harpers Formation and the Waterfall Conglomerate marks the dividing line between the Blue Ridge and Piedmont Geological Providences.  Below is a picture of the conglomerate.  It formed as sediment from the Blue Ridge Mountains and points west filled in the basin left by the failed rift.

Just as a side note here, it is important to record the strike and dip of the beds.  The following shows the strike and dip of the Weverton and Waterfall Formations.  (Waterfall is in red)

Both appear to be dipping at ~45 degrees to the east. 

During and after the rifting of Pangea the Blue Ridge Mountains were slowly being eroded by time and weather.  The center of the mountain has since been eroded away; as a result there is a valley where the core of the mountain once was.  Today all that is left is the two flanks of the mountain, one being Bull Run Mountain and the other being the Blue Ridge Mountains to the west.  Also, the Bull Run River incised into the mountain creating what is now Thoroughfare Gap. 

Some other interesting erosional features we found were the opferkessels at the top of the mountain.  Water gets down in pre-existing pits and gets blown around or frozen which enlarges the pit eventually becoming the large bowl shape seen below.  This picture is particularly interesting because it shows the ice in action.

 

Now that we have covered all of the “small scale” structural features of the area we can begin to understand the forces acting on the area and piece together the mega-scale structure of the region.  All of the evidence points to compressional forces from east to west which is understandable since the three orogenies which make up the Appalachian Orogeny involved terranes moving in a westward direction and impacting the eastern front of the North American Plate.  The folds tell us a story of left lateral shear (when looking north along the Bull Run Mountain Ridge).  Additionally the dip of the beds within the Weverton and Harpers Formations all point east.  At the top of the mountain we looked west through the heart of the Blue Ridge to the current Blue Ridge Mountains.  We were also informed that the formations on that side of the valley dipped to the west.  This indicates that the entire area used to be a massive anticlinorium.  Also, we were informed that the western limb of the anticlinorium is overturned, evidence for which lies in the stratigraphic relationships of the formations found on the western side of the Blue Ridge.  Referring to the picture above, the ridge in the distance is the Blue Ridge Mountain and the area from there to the mountain I was standing on is the weathered out core of the mountain.  Earlier in the post I mentioned imbracated faulting which is associated with low angle reverse faults or “thrust faults”.  Such a thrust fault (with vergance to the west) would result in an overturned limb to the west.  Below is a cartoon west-east cross section of the region.  Its goal was to show Pumpelly’s Rule, however, I thought it gave a great overview of the regional scale structure.

Image courtesy of Callan B.

References

Bentley, Callan. May 2011.  Mountain Beltway: Friday Fold(s): The Outdoor Lab.  American Geophysical Union.  < http://blogs.agu.org/mountainbeltway/2011/05/27/friday-fold-outdoor-lab/>. (19 April, 2012) 

Blakey, Ron.  Paleogeography and Geologic Evolution of North America.  NAU Geology. < http://jan.ucc.nau.edu/rcb7/nam.html> (19 April, 2012)

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