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.
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.
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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.
