Massanutten Special

Massanutten is a synclinorium located within the Valley and Ridge province and is the dominant topographic and geologic structure of the Shenandoah Valley of Virginia, extending for approximately 160 km along a northeast strike.  It is in this region where we start our geology field trip for Structure.

The range bisects the Shenandoah Valley and is divided into northern and southern sections. 

The northern section consists of 3 roughly parallel ridges, forming 2 valleys. The wider, main valley, is called Fort Valley, while the smaller one is known as Little Fort Valley. The ridges of the northern section converge at New Market Gap. The southern section consists of a series of closely gathered ridges, separated by precipitous creek gorges.

The geology of the Massanutten Mountains is dominated by sandstone and supported by shale. Erosion of the underlying shale in some areas of the mountain caused the sandstone to break and slide to form talus slopes. Generally the Massanutten Sandstone is folded into a syncline, and it outcrops at the ridge tops. The synclinal structure of the mountain gives it its characteristic shape.

A syncline is a downward-curving fold, with layers that dip toward the center of the structure. On a geologic map, synclines are recognized by a sequence of rock layers that grow progressively younger, with the youngest layers at the fold's center or hinge followed by progressively older layers outward. Therefore, a reverse sequence of the same rock layers occurs on the opposite sides of the hinge. 

On the first day of our fieldtrip we headed to a location that I had never been to or heard of, for that matter—the Garth Run outcrop.  It is located four miles Northwest of Wolftown, Virginia, off of State Route 665—Garth Run Road within the Blue Ridge Province of Virginia.  When I first got to this spot, it just looked like a bunch of granitic rocks.  But, upon closer inspection I immediately recognized the Grenville Basement Complex by the characteristic anorthosite and blue quartz rock depicted below.

This gave me an indication of where it was in the stratigraphic time record.  Upon closer inspection, there were several other things going on here that would give me clues as to what events took place that aided in the deformation of these rocks.  The rocks at Garth Run consist of Granite, Meta-Granite, and gneiss and zones of ductile shear that cut through Garth Run indicating a high strain zone.  My mission here was to find evidence to support the hypothesis that this area was affected by two different orogenies—the Grenville orogeny, approximately one billion years ago and the Alleghenian orogeny approximately 300 million years ago.  Furthermore, to support the idea that this is a thrust dominated shear zone.  Below are some of my findings.  

Here, we found S and C fabrics as annotated in the picture above, which indicate as sinistral sense of shear.  Although not visible in this picture, lineations were present in this outcrop and I found two different sets of foliation.  

One set of foliation that was easy to see was one the rock by the creek.  Here, the water scours out the sides of these boulders leaving the surface fresh and easy to view.   Although hard to see due to the extreme amount of poison ivy,  there are rocks there that show progressive deformation into ultra-mylonite.  These classmates trudge through regardless of the poison ivy.

  

Many of the boulders were strongly foliated and augens gneiss with progressive deformation was present.  These Augens were primarily feldspathic and by the degree of deformation I classified the majority of them as Mylonite.  

We piled back into the vans after collecting all the great information that this outcrop could tell us and headed off to the Swift Run stop.

We began our journey to the next outcrop by bushwacking a little over 1000ft from the road.  I had no idea where we were just that my trust was in my professor.  As we walked, several rocks stood out to me and upon closer inspection they turned out to be amigdules.  Amigdules are commonly found in basalt where vesicles form.  These vesicles fill with solution and eventually crystallize.  Below is an excellent example of one that we found on the trail.  The presence of this suggests there was volcanic activity, possibly from the Catoctin during the rifting event of the opening of the Protoatlantic. 

The Swift Run Formation rests disconformably above the Greenville basement complex.  The rocks here are primarily sedimentary.  Blue quartz sediment is abundant in the sedimentary rocks here and this most likely came from the Grenville Basement as it was being eroded.  These sediments are arkosic sandstone conglomerates and appear to be deposited in a basin environment.  I also observed primary features such as these large ripped up clasts throughout the outcrop and cross bedding with coarse to fine layering.

Here are some nice examples of ripped up clasts embedded in the Swift Run Formation

Swift Run Formation- bedding showing coarse to fining upward.

Foliation is shown in the picture below.  Folding within these beds were a secondary feature most likely caused from the Alleghenian orogeny.  These tectonic events took place approximately  325 – 260 Mya when Africa slammed into North America.  The collision formed a huge mountain chain that rivaled the Himalayan Mountain Range exerting massive stress that caused tilting and fracturing and folding.  What remains of these mountains is explained in my Thoroughfare Gap blog here http://whattherockstellus.blogspot.com/2012/04/thoroughfare-gap-va.html

Pictured below is and example of sinistral parasitic folding from tectonic forces compressing from the west.  This is characterized by ductile shear rather than brittle shear.

Bedding here (below) measured approximately N10E, 75° and  the cleavage measured N75E, 80°

Chilhowee Group Quick Stop

At our next stop, we me up with Dr. Rick Diecchio to explore an outcrop of the Chilhowee Group primarily the Weverton and the Harpers.  For me, these stick out like a sore thumb.   The Weverton is a meta-quartzite that is typically grey to tan in color with some areas of conglomerate facies.  These sediments were laid down in succession during the breakup of Rodinia at the end of the Ediacaran and up into the Early Cambrian.  The Harpers is characteristic of a dark phyllite meta-siltstone.  The original sediment would most likely have come from a lagoon setting where mud predominated as deposition of sediments were laid down while the Iapetus Ocean sea level rose. Siltstone implies that these deposits came from a quieter environment.  In time, metamorphism of this sediment took place forming shale layers.  Below we can see the fissile nature of this formation.

Annotated below are bedding layers that are possibly part of a bouma sequence.  This package is grading from coarse to fine.

  

Here, we also took measurements of foliation and bedding and came up with this stereonet.  This is a classic example that we see in the Blue Ridge where thrusting develops this type of axial plane cleavage.

Skyline Drive

Antietam Sandstone is quite abundant in certain parts of skyline drive and even used as decorative borders along lookout points.  It was here that the following photos were taken. I have already discussed the Antietam Formation in detail in my Thoroughfare Gap blog page but I will recap here.   This formation lies atop the Harpers Formation and is very easy to spot due to the lovely skolithos tube fossils that are abundant within the meta-sandstone matrix.  The Antietam Formation is part of the Chilhowie group (discussed above) which represent a passive margin while sea level rose during the Early to Middle Cambrian.  The pictures below tell us that a tectonic event must have taken place after this formation had been deposited due to the tension gashes of the quartzite veins.  I have annotated the shear direction in the pictures below.  These tension gashes form at right angles (perpendicular) to the direction of maximum stretching.

These are not found in the original location and are thus considered float so a direction at which the forces came to cause this deformation is unknown.  However, we can see that relative to the skolithos, this deformation affected all of the grains including the skolithos.  Let’s take a look.  The first picture just shows the skolithos from the top view looking down as though you were looking at the sea floor.

And here below, the skolithos are deformed.

...and more deformation in tension gashes on Antietam Sandstone

Franklin Overlook was our next quick stop but one of my favorites.  This is because we were able to learn a little more about the Catoctin Formation.  Throughout my college geology courses, the Catoctin often gets mentioned but never more than that.  The Catoctin Formation was so named for the Catoctin Mountain.  This formation comprises lava flows that happened during the breakup of Rodinia. 

 Here, while the rifting was occurring, feeder dikes cut up through the crust filling some areas with meta-basalt aka “Greenstone”.  The swift run also continued to fill in low areas and consequently some of these basalts are rich in felsics, rhyolite and meta-tuff.  Evidence found in the Catoctin Formation show meta-sedimentary rocks primarily of interest is the black phyllite which we interpret as being from an old lake.  Here at this outcrop, we find a strange conglomerate rock from the Catoctin and seek to understand its origins.  The green matrix is primarily epidote with large clasts of quartz, basalt, and amygdule abundant basalt.  The clasts are quite angular which must have been immature when deposited.  We decided this must be epidotized volcanic breccia from a pyroclastic flow that later became metamorphosed.

We headed along Skyline Drive until we pulled over for a quick little hike along the Limberlost Trail.  Here we found another opportunity to see some of the lovely Catoctin…this time in the form of Columnar Basalt.  So as we step back in time, Rodinia was breaking up and all around the land would have been a naked landscape which was smothered in lava flows.  As these flows began to cool, hexagonal jointing began to take place.  Here is a picture of the columnar basalts of the Limberlost Trail. 

 Now most of the time, columnar jointing fractures in nice 120° pieces.  But here something must have happened to offset this jointing.  My suspicion was that the Alleghenian orogeny had a hand in deformation of these basalt columns.  Further measurements concluded that indeed there was a force coming from the east and that force deformed not only the angles between the joints as much as 25°+/-, but that the arrest lines were actually tilted offset from perpendicular to their normal position (annotated in picture above.)

In addition, there were three sets of columnar basalt units at this outcrop.  Being so close together, I felt they should all have received close to the same amount of movement and deformation.  But it was not so.  I labeled them as A, B, and C units and discovered the following:

Unit A was tilted at N10E, 64°

Unit B was tilted at N2E, 76°

Unit C was tilted at N2W, 60°

On a steronet they look like this.  Why they are not similar is beyond me for now.  I will make another visit to this location soon and see what I can piece together.

We all headed back down the trail with Callan testing us all along the way with rocks from the ground. Back in the Vans, we headed off to see some really nice examples of feeder dikes along skyline drive.  The contact between the outcrop and the dike was contrasted well as depicted in the picture below.  Notice the dark mafic microcrystalline characteristic of the basalt.  These dikes were striking at N25E and dipping at roughly 65° which gives insight to the orientation of breakage in this area where they opened up.  Scientists study feeder dikes such as these to try and understand the relationship of age and cross cutting relationships in terms of paleomagnatism.  The samples taken here will help to establish the position of Virginia on Earth at the time of Rodinia. 

More basalt columns were spotted along the trail up to Comptons Peak.  Growing up in the Columbia Basin area in Washington State, columnar basalt is quite abundant and I have to say I wasn’t too impressed.  But, given the fact that there are not too many areas in the east with this type of structure, I am rethinking my opinion.  One of the neat things at this outcrop that I noticed for the first time with basalt is the convex and concave nature on the faces of the columns where they have fractured and broke off.  More than likely, these happened along the arrest lines and I am left to ponder as to why this happens and is there a pattern.  

As we headed down the mountain, we began to get the feeling that it was going to rain.  Weather reports stated there was a chance of snow coming our way.  By the time we hit the next stop, wet weather was upon us.  We scrambled to Veach Gap where we saw many anticlines poking out from the rock outcrops.  All were facing in the same general direction and all were symettrical based on our measurments.  Slickenlines were found on the site indicating there was a fault nearby.  The rain made the rocks very wet and we soon found ourselves in a dangerous situation.  We quickly scrambled back to the vans where we made the decision to cut the trip short.  Here is a picture of one of these amazing anticlines in the middle of nowhere.