Last week, I
mentioned some cool conglomerates I saw when
NOVA adjunct instructor Chris Khourey and I did some field scouting. The main purpose of that trip was not to focus on the Culpeper Basin's boundary conglomerates, however, but the "Great Valley" of Virginia's Valley and Ridge province. The "Great Valley" is usually called the Shenandoah Valley in Virginia, because the Shenandoah River flows north through it. (
Topographically, it continues north into Maryland, but the Shenandoah River isn't found there.) Sitting in the middle of the valley is a mountain range, Massanutten Mountain. And in the middle of Massanutten, there is another valley, the Fort Valley. As you can see below, Massanutten is a fence-like ridge separating the higher Fort Valley from the lower Shenandoah Valley:
In fact,
rumor has it that the name "Massanutten" is a native American term for "basket." This describes the overall shape of the mountain/valley quite well. It probably won't surprise you to learn that this valley-in-a-mountain-in-a-valley pattern is due to differential weathering of folded sedimentary layers. In fact, the entire Great Valley is one big downturned fold, a
syncline. Actually, it's not a perfectly smooth fold -- there are some wrinkles and minor folds within the overall down-turned structure, so we call it a
synclinorium. The oldest rocks are therefore at the eastern and western edges of the Great Valley, and the youngest rocks are at the center of the Massanutten Synclinorium, up in the Fort Valley. It turns out that some of these rock layers are easily eroded, and some are tough. Of particular note is the Massanutten Sandstone, a quartz-rich,
well-indurated rock that is responsible for the ridges of Massanutten Mountain. It weathers away more slowly than the shales and carbonates (limestones) above and below it. Here's a cross-section view to show how the subterranean structure influences the surface topography:
The map view up above (using Google Maps' super-cool new terrain feature) and this cross-section also show the difference in landscape texture (and geologic cause) of the Blue Ridge province in the SE corner of the images.
In discussing the geology of the area, I'm going to mix my pictures from
Thursday's scouting expedition with photos from Saturday's actual field trip with my Audubon class.
Let's start at the beginning. The first stop was in the Conococheague Formation, a late Cambrian limestone. Our field trip stopped at a nice exposure near Mulberry Run, west of Strasburg, VA. Here's the crew looking close at the outcrop, and trying out their geo-interpretive field skills for the first time:
Albert tests the outcrop with some dilute hydrochloric acid. It fizzes!
Soon, we spot the first of several
stromatolites:
There are also some nice spherical grains of calcite called
ooids (or ooliths). These form in wave-influenced carbonate banks today, like the Bahamas.
Interpretation of this environment then? Looks like a nice passive margin, far from any major terrigenous inputs (
i.e. mud or sand). Warm tropical temperatures leading to the chemical precipitation of lime mud from seawater.
What comes next? On to stop #2, the Tumbling Run section* south of Strasburg, we see a nice long exposure of the New Market, Lincolnshire, and Edinburg Formations, a series of Ordovician limestones, all dipping nicely towards the axis of the synclinorium. (
Last semester, one of my Honors students looked at silicified trilobites in the Edinburg Formation.) As you walk downhill (and up-section), you see a change in the limestones. They get darker in color, and they start splitting into thin sheets along clay-rich layers. Uh-oh, we're getting an increasing clastic influence on these sedimentary rocks. They no longer record pristine, Bahamas-type environments. Now the limestone is mixing with shale. Where is all that mud coming from? A hint may be found in several
bentonite layers, weathered volcanic ash deposits. There's some volcanoes getting closer to the area, it looks like.
In the late Ordovician, the east coast of North America experienced the first of three episodes of Appalchian mountain-building. Geologists infer that the Taconian Orogeny was caused by the collision of a volcanic island arc (like modern day Indonesia) with the east coast. The Tumbling Run section shows well the increasing clastic influence of the growing Taconian Mountains to the east.
It's also good for some small but interesting tectonic structures. Check out this conjugate pair of
en echelon tension gash arrays:
The black nodules you see along bedding in the above image are flint nodules, very characteristic of the Lincolnshire Formation. If you get close to them, you'll find that they exhibit different mechanical properties than the limestone that surrounds them. They are more likely to break (brittle behavior) than flow (ductile behavior):
But let's get back to the stratigraphy, shall we? (It just doesn't do to get distracted by these minor structures!) Our next stop was to look at the Oranda Formation (calcareous shale), indicating heavy clastic influence (but still a bit of carbonate). Then, after a lovely lunch at the
Strasburg Emporium, we headed off to the Buzzard Rock Trail, to look at the Martinsburg Formation. The Martinsburg is a nice thick batch of fine sand and mud interpreted as turbidite deposits. Various pieces of the
Bouma sequence can be seen throughout the formation, including graded beds, ripple marks, and cross-bedding. This picture conveys these alternating lithologies, representing fluctuating current strength as turbidity currents periodically brought coarser sediment into the deep (low-oxygen, as indicated by the dark color) basin.
Now, keep in mind that all these sedimentary layers later got folded during the final phase of Appalachian mountain-building, the Alleghenian ("Alleghany") Orogeny. At that same time of intense deformation, some of these mud layers began to convert to slate. The outcrop on the Buzzard Rock Trail shows this pretty well, in spite of being covered by lichen, algae, moss, and other horrible rock-obscuring growths:
The sandy layers outcrop as stiff, blocky strata. But look to the right of the quarter: in the muddy layers, a penetrative cleavage has developed, subperpendicular to the compressive stress. Here, let me draw for you what I saw at this outcrop:
The clay minerals in the mud are more susceptible to being alligned by tectonic forces than the grains of sand in the coarser layers. So the shaley intervals exhibit a more pronounced cleavage than do the sandy intervals.
But again, I'm getting distracted by the tectonic overprinting! This trip is supposed to be about stratigraphy, pure and simple. Doggone it! Okay, moral of the Martinsburg: no more carbonate by the late Ordovician. Instead, this sedimentary basin is getting filled with clastic debris shed off the Taconian Mountains** to the east.
Next layer up is the Massanutten Formation: mainly quartz sandstone, but also some quartz pebble conglomerate. We see it by entering the "basket" via a water gap near Waterlick, VA. Driving south (uphill) along Passage Creek, we were soon surrounded by looming cliffs of quartzite. It represents fluvial and beach facies as the depositional basin was filled to the brim. Here's a boulder of the conglomeratic portion:
Here's some nice cross-beds in the sandy portion exposed near Blue Hole, about 4 miles south of Waterlick, VA:
Other Massanutten Formation features include some fossils. Here's some poorly-preserved brachiopod external molds:
And here's some
Arthophycus horizontal trace fossils, probably made by polycheate worms:
Okay, I can't resist this tectonic structure: an awesome anticline exposed along the Veatch Gap Trail (eastern part of the synclinorium, where a small anticline in the Massanutten Formation is superimposed on the larger synclinal pattern):
Beyond the Massanutten Formation, we are in the Fort Valley proper, inside the "canoe" shape of the Massanutten Mountain ridge system. Next layer up is some upper Silurian / lower Devonian carbonates, representing a return to passive margin sedimentation after the end of the Taconian Orogeny and the erosional beveling of those ancient mountains. Unfortunately, there are no good places to stop on the narrow Fort Valley Road, so I don't have a picture of them to share. Trust me, though: they're there.
The next good stops are of Devonian shales. There's some nice ones exposed across the road from Elizabeth Furnace. More mud? From whence does it come? We interpret this again as the onset of an orogeny, in this case the Devonian-aged Acadian Orogeny, which dumped a big thick wedge of sediment into the Appalachian Basin. Here's a shot of the Needmore Formation, one of these shales with distinctive trace fossils highlighted by iron oxide:
The overlying Mahantango Formation (Devonian) is a siltstone that bears a decent number of body fossils, like these brachiopods:
Here's something that may be the back of a trilobite (if I'm not imagining the lobe to the left of the central line of knobs), or maybe a crinoid (if the "central" line is all there is):
Here's what appears to be the (vertically-oriented) trace fossil
Daedalus, which I learned for the first time this spring
in Silurian rocks near Buffalo, New York:
Finally, at the top of the stack, near Seven Fountains, there are exposures of more bentonite, in this case the Tioga Bentontite, a major stratigraphic marker bed throughout the Appalachians. Here's a shot of the bentonite exposure on the Fort Valley Road near Seven Fountains:
Here's Chris looking at the outcrop:
To summarize the Fort Valley portion of the story: after the Taconian Orogeny ends, we get a brief period of tectonic calm and passive margin sedimentation (carbonate), and then a return to orogenically-induced clastic sedimentation (augmented with volcanic eruptions). In the sedimentary sequence of the Massanutten Synclinorium, this records the onset of the Acadian Orogeny. The actual deformation of all these sedimentary horizons into a synclinorium shape was accomplished by the Alleghenian Orogeny: the much bigger mountian-building episode triggered with Africa and North America collided in the latest Paleozoic.
Hope you enjoyed joining us on this trip. Virginia's got some great geology, eh?
* For the Tumbling Run section, I highly recommend this excellent field guide:
Fichter, Lynn S., and Diecchio, Richard J., 1986, "The Taconic sequence in the northern Shenandoah Valley, Virginia." In: Geological Society of American Centennial Field Guide - Southeastern Section, p.73-78.
** Note I don't say "Taconic." The Taconic Mountains are a modern topographic feature in New York. They exhibit Taconian rocks well, and the orogeny is named for them, but the Ordovician Taconian Mountains would have been much bigger and more areally extensive.
Labels: fossils, primary structures, sediment, stratigraphy, structure,
