Over my spring break from school last month I spent a few days as we say “doing geology” close to home with my good friend and geology accomplice, Aaron. With our time away from the University we decided to tackle one of our longest standing goals in local geology, finding evidence of the Blue Ridge Thrust Fault. Namely, a good fault breccia.
How about a little background info:
Here is diagram dividing the great state of Virginia into 5 geophysical provinces, from William and Mary’s website The Geology of Virginia
The igneous and metamorphic rocks at the core of the Blue Ridge are the oldest in the state and are associated with Grenville Orogeny marking the formation of Rodinia. On the flanks of the Blue Ridge are late Proterozoic volcanic rocks and early Paleozoic sediments which mark the break up of Rodinia. A Unakite sample that I wrote about last year is from the Blue Ridge as well as a post from my Structural Geology class trip last April to the Garth Run shear zone.
The Valley and Ridge make up most of the western Virginia boundary and into eastern West Virginia. The rocks of the Valley and Ridge are primarily carbonates and siliclastics from the lower Paleozoic. I also wrote a little about the Valley and Ridge in a post last year.
Here is a general cross section of the Blue Ridge province of Virginia, from James Madison University’s website The Geological Evolution of Virginia and the Mid-Atlantic Region
The Blue Ridge is structurally an eroded anticline, which has been overturned to the west(left in above diagram). This diagram illustrates how the Blue Ridge is an up-folded structure with older rocks in the center and younging outward. The western limb of the anticline has been overturned beyond vertical to the point where younger rocks of the Chilhowee group are below older rocks of the Catoctin and Swift run formations.
At the meeting between the Blue Ridge and Valley and Ridge is a thrust fault and major division between two of Virginia’s major provinces. The fault marks where the Blue Ridge is thrust up and over the younger Valley and Ridge. All this pushing and shoving is from crustal deformation due to the Alleghenian Orgogeny, the most recent mountain building event when Africa collided with North America for a final time forming the super continent, Pangea about 300 million years ago.
Here is another look of the Blue Ridge Province but this time focused on the region surrounding Rt 66 in Northern Virginia along with all the geologic units shown. With a little help from my friend Sasha with the annotations, I created this image using the Google Earth USGS map overlay for the state of Virginia, which is an amazing resource, the overlay that is…..and Sasha
This map shows the bedrock geology of the Blue Ridge in Fauquier and Warren Counties Virginia, and the eastern most unit of the Valley and Ridge. The Purple unit visible on both flanks is the Catoctin formation, a neoproterozoic flood basalt which I also wrote about last april while on the Limberlost Trail. In searching for the fault we used the Catoctin as a guide unit in the field while driving back and forth trying to figure out where we were in the stratigraphic column. As you can see on the map, the Catoctin is a relatively thick unit which is also easy to recognize due to its characteristic “pistachio green” color. On the outer-most of each of the flanks is the Chilhowee group in pale yellow, a sedimentary sequence associated with the weathering of the Grenville mountains.
Where the North West flank of the Blue Ridge meets the light blue area is where we hoped to find our fault breccia. Armed with the above map and a few good publications, we began our search east of Front Royal near the town of Linden, exploring roads which crossed the boundary between Blue Ridge and Valley and Ridge.
At our first stop, which we found while curving through some rural neighborhoods, we walked into the edge of the forest to look at a few suspect boulders. These were likely pulled up during the construction of new homes.
Although we did not find the fault breccia, we did find these skolithos worm tubes.
These skolithos are the preserved burrows of worm-like organisms that lived during the lower Cambrian period. Aside from being cool trace fossils they also are a geo-pedal structure, or a “up” structure, providing an indication of which direction was upward during the time of deposition. It also indicates the depth of water during deposition, as skolithos live in beach or near shore environments. Also, dating back to around 500 million years old, these are some of the earliest forms of life found in Virginia.
We were able to find a fresh un-weatherd surface on one of these boulders to gauge the mineralogy
Pretty much all quartz with little or no clay minerals.
Based on the mineralogy and abundance of skolithos, we assumed this to be Antietam quartzite, which is the the top of the Chilhowee group and the “roof” of the Blue Ridge stratigraphic column. The Antietam is interpreted as beach sand/ barrier island deposits, laid down during a transgressive stage when sea levels were much higher.
The fault breccia is actually a mapped unit with in the Antietam, so we knew we had found the right area generally, now just to find where the Antietam meets the Valley and Ridge
Continuing west, we knew we had to be getting really close as we saw billboards advertising Front Royal’s limestone caverns just 5 miles away. After circling around several times to look for an adequate place to put the vehicle, we ventured off the road and down a slope to inspect a suspiciously jagged looking outcrop.
Here it is ! A beauty of a fault brecchia If you ask me. Made of poorly sorted angular pieces of quartzite which were crushed and in some cases pulverized along the fault zone. The broken bits were then cemented in place by a Iron oxide cement which is caused by the infiltration of mineral rich fluids or ground water into the pore spaces in the rock.
This area is mapped as the Happy Creek Fault, and is a smaller fault on in the Blue Ridge Thrust Fault system, a series of faults dividing the Blue Ridge from Valley and Ridge which share similar kinematic orientation, East over West
Here is another which seems to have a different cement type?
We found this in the same area, but it seems to display a lighter colored matrix material in the inter-grain zones, and also has a weird gray patch in the center. Perhaps this is just a weathering phenomenon, or another idea is that maybe this piece found itself close enough to the fault zone to be crushed and shattered, but just far enough away to avoid chemical interaction with groundwater?
Here is another with the darker Iron oxide cement, and some very large grains, as scaled by a size 13 boot.
I like to think of these shattered quartzite pieces as brave little foot soldiers; imagining they were the first wave of infantry in the advancing front of a continental-scale compressive regime, whose selfless efforts were forever memorialized in an Iron oxide cement. But then again I always try to turn geology into an action movie.
Some other references I used:
- Virginia Department of Mines and Minerals, Geology of Front Royal Quadrangle, which can be found here.
- USGS 2010 groundwater study in Warren county, which can be found here.