The following is a field trip guide for a Structural Geology class’ trip to Thoroughfare Gap in northern Virginia.
Thoroughfare Gap (TFG) is the break in Bull Run Mountain where Broad Run, I-66 and some train tracks pass through the range. TFG is located along the western boundary of the Culpeper Basin and represents the end of the Piedmont and the beginning of the Blue Ridge provinces of Virginia.
As geologists, there are multiple reasons to travel to this particular area; the transgressive sequence of Rodinian rifting, well displayed joint sets and deformation from Appalachian mountain building, and conglomerates collected within the basin from the rifting of Pangaea.
Bull Run Mountain is composed primarily of the Weverton quartzite, a meta-quartz arenite with clasts ranging in size from fine sand to pebbles; including conglomeritic sections. The resistant Weverton is the lowest formation of the Chilhowee Group which is overlain by the Harpers and Antietam Formations . These sediments were deposited during the break up the supercontinent, Rodinia, at the end of the Ediacaran and beginning of the Cambrian periods. Underlain by the Catoctin, a metabasalt characterized by a green color from both epidote and chlorite minerals, the trangressive Chilhowee represents sea level rise as the depositional environment progressed from a beach, to a lagoonal setting, followed by a point bar; though the Antietam Formation is not found exposed at TFG. Eventually the Chilhowee was overlain by limestone deposits which can be seen west of TFG in the Valley and Ridge province of Virginia and West Virginia.
A major part of the assignment associated with this trip will be to use measurements to determine the orientation of the stress that was applied.
The next large scale tectonic activity to occur in the area of TFG was the Appalachian Orogenies that created the Blue Ridge and Bull Run Mountains. We will start out the field trip by hiking the trail to the western overlook atop Bull Run Mountain, making sure to take notice of a few easterly dipping outcrops of Weverton along the way. Once at the overlook we can correlate those outcrops with the Blue Ridge Mountains seen miles away. Here you will be standing on the eastern limb and looking at the western limb of the Blue Ridge Anticlinorium.
The next chapter of this tectonic trilogy involves the rifting of another supercontinent. Once the final phase of Appalachian mountain building was complete, Pangaea had been formed. Just like Rodinia before it, excess heat beneath the continent built up eventually leading to rifting and the subsequent creation of the Atlantic Ocean. While multiple faults were created only one would become the Atlantic, while the others, like the Culpeper Basin, would be left high and dry to collect the freshly weathered sediments coming off the Appalachian Mountains. East of Bull Run Mountain is the western boundary of the C. Basin where an outcrop of the Waterfall Conglomerate is located. Contained within the muddy matrix of the conglomerate are clasts of recognizable members of the Chilhowee Group among others.
Back down at the beginning of the trail we will walk west along the train track being mindful of any trains heading our way. From the parking lot heading west we will pass outcrops of the phyllitic Harpers Formation with good exposures of fissile cleavage. Eventually we will come to a large outcrop of Weverton where most of the measurements will be taken. At this location, a few joint sets are readily visible, and are perfect for measuring and plotting on stereonets. Some questions to consider would be: what direction did the continental collision come from? What happens to rocks as they are folded? How come the Broad Run water gap is located where it is?
This will be our first time running this field trip so some fine tuning will be expected while out on the trail. Remember to be observant of all structurally significant features as small features can relate to the regional picture.