Champlain thrust fault

Over the summer, I went up to Vermont to visit my friends the Clearys. Joe Cleary is a college friend and a talented luthier. He and his wife Tree and their children Jasper and Juniper have settled in Burlington, a lively town with a lot of cool stuff going on. Joe took time out one morning to show us a superb example of a thrust fault on the shore of Lake Champlain. It is on private property, but Joe got permission for us to hike there first. Our group that day consisted of Joe, Lily, and me, plus by a stroke of good luck, my pal Pete Berquist was in Burlington at the same time, with his friend Amy. The five us were Team Burlington for the day.

There are two rock units involved in the faulting at this location. Consider the first:

This is the Dunham Dolostone. It’s early Cambrian in age. It’s resistant to erosion, and stands up in cliffs above Lake Champlain. The distance from my ten little piggies down to the water is probably fifty feet. Below the Dunham Dolostone, you can find the Iberville shale. It is actually younger than the overlying dolostone. (We know this from unfaulted stratigraphy elsewhere in the region.) The Iberville shales are Middle Ordovician in age. They are relatively weak (‘incompetent’) rocks, and have been sheared out by the faulting. Here, Team Burlington demonstrates the sense of shear, by leaning over in the direction that foliation has rotated towards:

Looking in one direction along the base of the fault to show the differential weathering of the two units:

Flip it around 180°, and you see the same thing in the other direction:

Pete, Joe, and I crawled underneath the ominously overhanging dolostone to check out the detailed structure of the fault. Here’s Pete tickling the sheared out shales, looking for little sigmas…

The shales had nice veins of calcite running through them, and the high contrast of light and dark reveals some lovely folds, like this one:

Pete goes into professor mode, gesticulating and using the verb “shmoo” to describe the reaction of the shale to a gazillion tons of dolostone sliding over top of it:

Another nice fold (little tiny blue Swiss Army knife, 5.7 cm in length, for scale):

And another nice fold:

This fold is transitioning into a shear band:

Here’s my favorite part of the outcrop, a big fold with little parasitic folds all over it, showing opposite senses of shear on the opposite limbs of the big fold:

S-folds on the upper limb, Z-folds on the lower limb. Sweet, eh?

Here, a sort of S-C fabric has developed, with foliation tipped over the the left, and then near-horizontal shear bands running along through it:

Here’s something weird. Perhaps a reader can explain it. Here’s a shot of some of the veins, with the same 5.7 cm knife for scale:

Now we’ve zoomed in, and you can see some detail in the vein:

What are those lines? Is that more “S-C” fabric? I mean, it can’t be cross-bedding in a vein… but I’m having trouble visualizing what process of shearing the vein could yield such a delicate, even distribution of dark material amid the vein fill. What the heck is going on here?

Okay, now that you’ve twisted your brain up thinking about that, you can relax with a structure whose meaning is obvious. Some artistic and romantic previous visitor (not a member of Team Burlington) had arranged pebbles weathered from the two rock units into a bimodal icon of love:

Displacement along the Champlain Thrust is estimated at 30–50 miles (48–80 km). These dolostones started off near the New Hampshire border, then crossed Vermont, almost but not quite making it into the Empire State! The Champlain Thrust is the westernmost thrust fault that has been associated with the Taconian Orogeny, a late Ordovician episode of mountain building associated with the docking of an island arc with ancestral North America. Looking up at the fault trace:

A final glance at the thrust outcrop, looking north and showing the fault’s gently-inclined easterly dip:

Joe, thanks for taking the time to bring us out there!

Fine faulting

Check it out: In the canyon of the Jefferson River, Montana, you can find yourself some limestone (Mississippian Madison Group, I think of the Lodgepole Formation) that has seen a wee bit of faulting:

And here’s an annotated copy… Both of these images are enlargeable by clicking through (twice):

Note the quarter for scale: this is very fine faulting (very small offsets). The thing that struck me as cool (and thus photo-worthy) about this outcrop is the sense of offset on the main “master” fault, which runs from upper left to lower right, branching into two strands as it goes. Compare this to the smaller faults which cut through the block between the two strands of the master fold. They show the opposite sense of offset! (Embiggen it if you don’t believe me.)

While the two strands of the master fault show dextral/clockwise kinematics (a “normal” sense of offset with the hanging wall moving down with respect to the foot wall), the smaller faults show sinistral/counterclockwise kinematics: here the right side is climbing up relative to the left side. It looks like what’s happening here is that there is a significant compactional element to the stresses these limestones suffered enjoyed, with σ₁ oriented from the upper right towards the lower left. As they were compressed, the broken slivers of the central pod of limestone (bounded by the two strands of the master fault) “bookshelfed” relative to their neighbors: think of encyclopedia volumes slumping down relative to the volume next door. If this is the right interpretation, it would have resulted in shortening of the rock from lower-left to upper-right. At least that’s the best explanation I can come up with for this anomaly. Anyone else want to chime in with an interpretation?

Mount Moran

The other day, Chris Rowan of Highly Allochthonous posted some pictures (and video!) of the Teton Range in Wyoming, a normal fault-bounded block of rock that has rotated along a north-south axis, with the west side dropping down and the east side rising up relative to the floor of Jackson Hole. This is classic “Basin and Range” extension, but the great thing about the Tetons is that it is so fresh and raw. Standing in Jackson Hole, you can look up at one particular peak which allows you to calculate how much offset has occurred along the Teton fault.

This peak is Mount Moran (slightly Photoshopified):

Here’s how the National Park Service would annotate that view, from here:

I’m interested in other details, though (like dates and elevations), so here’s a quick sketch I worked up on my new pad of NOVA sticky notes:

Most of the mountains are Arhcean gneisses of the “basement complex.” Cross-cutting these are a series of mafic dikes, including the prominent one that pokes out of the face of Mount Moran. The diabase dike, sometimes called “The Black Dike” is a prominent feature, but to me, the really interesting tidbit is that thin little scrap on top: a bit of the Cambrian-aged Flathead Sandstone. This sedimentary stratum overlies a profound nonconformity, and that same layer is found way down beneath Jackson Hole, at a depth of about 20,000 feet (20,000′) below the surface. (As Mount Moran is 12,605′ tall, that means that at its lowest point, the nonconformity is actually close to 14,000′ below sea level!)

Well, that sandstone layer can serve as a marker bed, seeing as how it’s been broken and offset along the Teton fault. Consider the following sketch to get a sense of how the Flathead Sandstone is 6000′ above the Jackson Hole valley floor on the west (right) and 20,000′ below on the east (left):

The Teton fault is 55 km long, and it dips to the east at 45°–75°. For the sake of simplicity, I’ll use a value of 60° to make my estimate of displacement. This is in accordance with the generally high-angle nature of normal faults, in accordance with Andersonian predictions (a topic which deserves a post of its own!). Given the vertical offset along with this angle, with can figure out how much offset has taken place. I’ve pulled out a highlighter now to color in the Flathead Sandstone:

Of course, this requires us to employ some trigonometry. We can do this with two separate triangles, as with the example above, or we can slide that vertical bar over to the right (west), and make it into one big triangle, where we add our vertical distances above and below the valley together:

The vertical part of this triangle, 26,000′ feet tall, is the “throw” of the fault, the vertical component of the displacement vector. We can use it, plus the dip angle, to figure out what the displacement is.

The way I was taught trigonometry in school, we memorized a pseudoIndian word, “SOHCAHTOA,” as a mnemonic device. For right triangles, this meant that: this relates the angle we’re interested in (let’s call it ψ) to the lengths of the sides of the right triangle, where S refers to sin(ψ), C refers to cos(ψ), and T refers to tan(ψ). That’s sine, cosine, and tangent, respectively. “O” is the length of the side opposite the corner of the triangle with the ψ angle. “A” is the length of the non-hypotenuse side adjacent to the ψ-angled corner. “H” is the length of the hypotenuse itself.

So with our Mount Moran calculation, we’re interested in the length of the hypotenuse, which is the same as the offset of the Flathead Sandstone. We use the “SOH” part of “SOHCAHTOA”:

sin(60°) = O/H

sin(60°) = 26,000’/H

H*[sin(60°)] = H*(26,000’/H)

H*[sin(60°)] = 26,000′

H = (26,000′)/[sin(60°)]

Let’s pull out the old TI-83:

So the length of the hypotenuse is 30,022′ — and assuming that all the slip along the fault has been dip-slip (no strike-slip or “transform” motion), then we’ve got our answer: the Flathead Sandstone marker bed has been offset by around 30,000′ feet. Nice!

This calculation has got me in a mathy mood. Let’s check out the rate of displacement, while we’re at it. It is estimated that extension began on the Teton fault around 13 million years ago (13 Ma). If we have seen 30,000′ (9,144 m) of displacement in that time, what is the average rate of displacement?

30,000′ / 13,000,000 years

3’/1,300 years (just lopping off four zeros from each side)

(12 inches/foot)*3′ = 36 inches/1,300 years

0.028 inches/year

or: 1 inch every ~36 years.

But of course fault motion usually doesn’t proceed at a slow and steady rate; it sticks and then slips infrequently in sudden jumps that we call earthquakes. The last major earthquakes on the Teton fault were 8,000 and 4,800 years ago. Both of these saw between 4 and 10 feet of offset. Check out the map of historical seismicity in the area, from the USGS:

Notice the intense cluster of quake epicenters associated with Yellowstone National Park, and the cluster in the Gros Ventre range, active this summer. Notice also the big blue smudge of Jackson Lake, a 25,540 acre lake where the Snake River is dammed up by first a glacial moraine, then augmented by humans via a dam.

Now notice the big gaping hole in seismic data in Jackson Hole… There has been no historical seismicity on the Teton fault. Jackson Lake is held up by an earthen dam, and earthen dams do poorly when shaken. The town of Jackson (8,000 residents, plus tourists) is downstream of Jackson Lake.

This strikes me as worrisome.

45°–75°E

Tipping your tension gash

Tension gashes are small veins that open up when rocks get stretched. Often, they are arrayed en echelon with respect to other tension gashes, all oriented in the same direction. Here is a sample of tension gashes I found this summer in rip-rap (i.e., not in situ) at some building site in New England. (I forget where, but it doesn’t matter, since it’s rip-rap. Could have come from anywhere!) Check out the lovely veins of milky quartz:

We’ve seen this sort of thing before. So how does this form? It takes a series of steps. First, the rock gets sheared along some zone. Tension fractures open up oblique to that zone (as shown by the arrows here) and get filled it with mineral precipitations:

As shearing continues (with the same kinematics), these short mineral veins experience rotation (dextral, in this case) and perhaps some folding:

The more shearing you get, the more rotation and folding of the gashes:

You get the idea, right?

Here it is in summary:

I’m loving animated GIFs these days. So flippin’ cool, right?

Here’s the back side of the same sample, where you can see that a central fault has ruptured through the lovely tension gashes. It’s not as well-developed on the front side:

Poor things. It’s such a shame when ductile structures go brittle.

Rocks of Glacier National Park

This is the second of my Rockies course student projects that I wanted to share here on the blog: it is a guest post by Filip Goc. Enjoy! -CB

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The Rocks around Glacier National Park, Montana: Introduction to the formations

The geology around Glacier National Park is great for beginners because the area is structurally straightforward and formations are generally easy to distinguish. Still, there is a lot to be excited about.

The rocks exposed firstly from the top down are old sedimentary rocks of the Belt Supergroup. It is called “Belt” after Belt, Montana, and “supergroup” because it is immense. These rocks were deposited in a Mesoproteozoic (1.6-1.2 Ga) sea basin, and show little to no metamorphism despite their age. Below belt rocks that make up the peaks of Glacier NP, there lay Cretaceous (~100Ma) shales; which brings us to the structure. How can these young shales get underneath MUCH older Belt rocks? Yes, there is a major thrust fault, and it is called Lewis Overthrust.

Simply put, the Belt rocks were first deposited in the sea basin. Then, Paleozoic rocks were deposited, but they are not exposed in Glacier NP, as they have eroded away. Then, Mesozoic rocks, including the Cretaceous shales and sandstones of the Western Interior Seaway, were deposited. Around 150-80Ma, as a result of the Sevier Orogeny, a HUGE slab of Belt rocks hundreds of miles wide and 15-18 miles thick slid over the Cretaceous formations more than 50 miles east! Slabs just love to glide on shales with their weak planes. Mr. Maitland from our group would call it the Banana Peel Principle, although most geologists who love French as much as I do prefer a much more refined term décollement horizon (yes, it is essentially a banana peel).

Check it out in this photo. The Belt rocks of the Altyn and Appekunny formations comprise the cliffs. They are much more resistant to erosion than the weak Cretaceous strata underneath them (low hills covered in trees). The striking white layer in the Appekunny formation is a quartzite bed.

Then erosion took the stage with its rivers and mass wasting. Finally, around 2 Ma the Ice Age came, and we derive the name of the park from the enormous glaciers that carved the peaks into their current shapes. The last of these huge glaciers melted ~12,000 years ago, and some people think the park should therefore be named Glaciated Park, ExGlacier Park, or just Glacier-No-More. The glaciers to be seen there today are young and tiny.

Glacier National Park is a great place to educate kids about geology because many formations can be identified by their colors. From old to young, there are Altyn (and Prichard), Appekunny, Grinnell, Empire, Helena (or Siyeh), Snowslip, and Shepard formations. Let’s get color-wise. First above the Lewis Overthrust are the light gray to white (or weathered into light tan) layers of limestone and dolostone of the Altyn formation. The Prichard formation exposed on the west side of the park is essentially of the same age as Altyn, but was deposited deeper, and is therefore darker as there wasn’t as much of oxygen in the depositional waters. It consists of dark gray to black argillite with slate-like appearance. (Argillite is slightly metamorphosed mudstone.) Then there is light green or burgundy argillite of Appekunny formation. Both versions have the same iron content, but the purple version is more oxidized. It was deposited closer to shore than Altyn. The Grinnell formation is the one dominated by burgundy argillite. The Empire formation is a transitional formation between Grinnell and Helena. Helena consists of medium to dark gray dolostone and limestone, often covered with honey-colored weathering rind. The Snowslip formation features red or green argillites, shades of orange or yellow, rusty colors, purple tones, white quartzite… pretty much “rainbow rock.” The Shepard formation is similar to Helena, gray limy rocks with orange buff.

Now for the fun stuff:

The Prichard formation is exposed on the west side of the park; is essentially of the same age as Altyn, but was deposited deeper, and is therefore darker as the increased pressure produced biotite. It consists of dark grey to black argillite with slate-like appearance. Aren’t these potholes beautiful? Also notice the joint sets. The diameter of the larger pothole is ~55cm.

There are great features to be seen in the Grinnell formation.
In this picture from McDonald Creek, there is cross-bedding in white quartzite, and a cross-section of ripple marks! A stream dumped sand onto muddy shore, ripples were created, and then mud leveled them out! Quarter for scale.

Annotated:

Close-up:

In this outcrop there are some mud cracks filled in with sand, exposed in cross section view:

There’s also cross bedding, and mud chip rip-up clasts. When the muddy shore gets exposed to the sun (low sea level), the mud dries up, loses volume, contracts, and cracks. That’s when a shot of sand came, probably with some rainstorm. Quarter for scale. As you can see in this picture of present-day drying mud, in the next stage mud cracks curl up:

Streams or waves can easily carry those “chips” away, and deposit them with sand. That’s the mudchip rip-up clasts. Very cool outcrop!

These are just another batch of nice mud cracks in Grinnell formation. Boot for scale.

This boulder has mud cracks overprinting ripple marks. Two in one! Swiss Army knife (11 cm long) for scale.

This one has it all. Cross bedding, mud chip clasts, ripples, and mudballs. Field notebook for scale.

These strange ripples in the Shepard formation are called interference ripple marks. They form when two currents go against each other at ~90°. Field notebook for scale.

Folded argillite and quartzite of the Grinnell formation with preserved ripple marks. Car keys for scale.

A bit more of the structure within the Grinnell Formation. This beautiful faulted fold lies on the way to Grinnell Glacier. Field notebook for scale.

Here’s a fold that hasn’t yet been breached by a through-going fault. Width of field of view is about 30 cm.

Note on prominent RED color in Glacier National Park: These red beds above St. Mary Lake are Grinnell formation.

Within the Helena formation, there is a conspicuous layer of diorite with contact metamorphosed rock above and below it – the Purcell Sill.

Part of this piece of Purcell Sill diorite has been altered to make the green mineral epidote. The horizontal field of view is ~80cm.

So when one hikes in the area at or above Purcell Sill, all the Grinnell is way down in the stratigraphic column. The red mudstones exposed around the center of the park ( like around Logan Pass) are mostly of Shepard formation. They look a lot like Grinnell, but they are younger. The width of the rock in the foreground is ~ 1m.

Mudcracks in Shepard formation near Hidden Lake. The trail to the Hidden Lake has one of the thickest Shepard exposures in the park.

There is one more red formation, even younger than the Shepard. This is the Kintla formation. Most visitors don’t encounter the Kintla. It can be seen as a red cap on the tops around Logan Pass (even above Shepard.) There are also exposures around Waterton Lakes on the north side of the Canadian border, and, of course, around Kintla Lakes and Hole-in-the-Wall on the northwest side.

This cross-section of mudcracks at the Boulder Pass are possibly within Kintla formation or Shepard formation. It is really hard to tell without a precise geologic map. At any rate, it is NOT Grinnell. The width of the rock in the foreground is ~ 70cm.

Since we are at Boulder Pass, there’s also plentiful of typical Snowslip at the trail to Hole-in-the-Wall. This sample shows why the formation is called Snowslip (at least as far as I figure it). The glacial striations show what direction the “snow slipped.”

Sometimes there are areas of low oxidation called reduction spots.

As the red color of Grinnell formation is caused by oxides of iron, non-oxidized Grinnell has a different color: the greenish Appekunny tone or shades of orange. There are whole greenish beds of reduction within Grinnell. The cool thing is that iron content is roughly the same throughout the rock! GAME for you: What do you see when you look at this reduction zone?

I see a very specific animal, and I know exactly what it is doing in there:

It is a bunny rabbit, and he is looking for stromatolites, or so-called “cabbage heads”!

So what is a stromatolite? Did the bunny choose the right formation to dig in? If not, what formation would be better?

Read on.

(Stromatolite layer along Going-to-the-Sun Road. The stromatolite layer is ~60cm high.)

Stromatolites are blue green algae or cyanobacteria that thrived on Earth in the Precambrian. The oldest stromatolite fossils on Earth are around 3.5 Ga. Stromatolites persist in the modern world in places where they are protected from grazing predators like snails. They were one of the first abundant photosynthetic organisms. They essentially remove CO2 from ocean; use the carbon for themselves while causing precipitation of calcium carbonate, and release the oxygen. They cover their cells with protective slime. When the slime gets too covered in sediment, they just grow a new layer, which results in dome-shaped layered “cabbage heads.” Stromatolites used to be so abundant that the sheer volume of oxygen they produced significantly changed the composition of our atmosphere. Stromatolites made our planet suitable for organisms like us!

Stromatolite beds are within many of Belt formations. The major stromatoliferous bed in Glacier National Park is the Helena formation. Some beds are in the Altyn and Snowslip formations also host stromatolites.

One of the best exposures of enormous stromatolites is at the Grinnell Glacier. Those honey-colored guys in Helena formation were ground down by the glacier so we can admire their cross-sections of their colony from the top. But first, a side view:

Here is our little group hanging out with the Helena stromatolites.

Notice the easily visible glacial striations!

This awesome 1.8m diameter stromatolite cracked in half!
In fact, this one was quite AGGRESSIVE, and had a DEADLY appetite.

Exposed at the Boulder Pass. Stromatolite in Shepard formation, as viewed from underneath. Stromatolites grow dome-shaped, but this is bowl-shaped. Therefore it is upside down. Field notebook for scale.

This exposure near Hole-In-The-Wall cirque I named “Stromatolite Wall.” All those columns you see are stromatolites. The wall is some 8m high (exposed).

Looking up the wall:

This stromatolite weathered into a three-dimensional column! One can easily see the separate slime layers.

Another absolutely stunning stromatolite bed is in the Snowslip formation. It is not a thick one, but special. The Snowslip formation was deposited closer to shore than the Helena, and the algae had trouble living there. There was quite some amount of organic material and mud periodically dumped in. Stromatolites caught the mud with its load of minerals into the slime layers, and those minerals later stained the fossils. The result: RAINBOW STROMATOLITES (my term). Next time somebody whines about how stromatolites are boring blue green grandpas, sitting around for billions of years doing nothing, just show them these playful buddies.

Oh yeah! A close-up:

One more (for good luck):

Elsewhere in the Helena Formation, you can see halite casts:

These are features that form when evaporation concentrates the dissolved Na & Cl ions so that they begin to bond together and crystallize salt. Later, when the water level rises again, the halite dissolves away and mud can fill in the empty cubic mold:

There’s one more interesting feature in the Helena formation. OOLITIC LIMESTONE. I made it uppercase because most people don’t see this in the park. It is exposed just next to the Going-to-the-Sun Road in the western part of the park.

In the photo, the gray beds are limestone, the brown ones are sandstone.

Oolites (also called ooids or ooliths) are little (0.25 to 2mm) round balls of limestone that for in warm shallow marine environments. The grain of limestone is gently rotated around by waves, and so the limestone precipitates in layers around the center…

If you look closely, you should see the oolites. They are ~0.5mm in diameter. Quarter for scale.

Although Glacier National Park is primarily famous for its jagged glacial landscape (and for a good reason!), the rocks that make up its horns and arêtes are remarkable as well. Despite having been displaced ~50miles east, they retained many of their primary sedimentary features. It’s common to spot beautifully preserved ripple marks or mudcracks. The abundance of fossil algae – stromatolites – is striking as well. Glacier National Park offers arguably one of the best Belt rock exposures in the US, which also makes it extraordinarily colorful. The deadly combination of colorful strata, white snow, and jagged peaks ensures the park is the one of the most scenic places around. It is just gorgeous up there.

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Filip now leaves NOVA; my Rockies course this summer was his final NOVA class. Now he’s off to the University of Virginia. Good luck, Filip! —CB

New “secondary structures” display at NOVA

…. And on the other side, we have secondary (tectonic) structures, focused on folds and faults:

Falls of the James II: fractures

In my previous post, I introduced you to the Petersburg Granite, as it is exposed south of Belle Isle, at the falls of the James River in Richmond, Virginia. I mentioned that it was fractured, and I’d like to take a closer look at those fractures today.

The geologically-imparted fractures were exploited by human granite quarriers, and in some parts of the river bed, you can see the holes they drilled to break out big slabs of the rock. Some of these block-defining perforations exploited pre-existing fractures.

This is also evident on the north side of Belle Isle itself, where there are several large abandoned quarries now mainly utilized as a rockclimbing locale. There are two dominant fracture sets in the area: one which parallels the schlieren (magmatic fabric), striking NNE; and a second which strikes ENE.

The meaning of these fractures are one of the problems Chuck Bailey (my host at Belle Isle) and his students have been considering. Under Chuck’s tutelage, James McCulla examined these fractures and reported his findings at the NE/SE GSA section meeting in Baltimore last March.

One of the first things Chuck showed me when we got to Belle Isle is some offset schlieren, like these:

Let’s annotate those up, so you can orient yourself:

So clearly, that looks like a right-lateral offset, right? Of course, it could just be an apparent right-lateral offset, as perhaps the inclined schlieren have been offset in a vertical sense, then exposed by erosion on the same horizontal section. We need to determine the true offset direction. If we look at a vertical exposure of the fracture surface itself, will slickensides back that up? Here’s one…

Yep, the slicks are very gently plunging (close to horizontal) and agree with the right-lateral offset we thought we saw in the horizontal exposures in the earlier photos. These are in fact true right-lateral offsets. Chuck is currently dating some muscovite that appears on these surfaces as a method of constraining the timing of deformation.

The other fracture set (NNE-trending, parallel to the schlieren) shows very little in the way of telling fracture-surface anatomy. There may be some weakly-developed steps facing to the upper left, but these surfaces are neither gouged nor mineralized:

Chuck and James therefore interpret the NNE-trending fractures as extensional fractures and the ENE-trending fractures as faults with small offsets. It is worth noting that the NNE-trending extensional joint set is parallel to extensional faults in the Richmond Basin, a Triassic rift valley 15 km upstream.

So which came first? Here’s a confounding exposure:

Allow me to lighten that up and annotate it for you:

We have two different relationships exposed here, less than a foot apart. At left, we see the NNE-trending joints truncating against the ENE-trending “fault.” At right, we see that the NNE-trending fracture steps to the right as it crosses the ENE-trending fracture. The left example suggests that the ENE “fault” is older, and the NNE joint came later, propagating to the pre-existing discontinuity but no further. The right example suggests that the NNE-trending joint was there first, but was then broken and offset (ever so slightly) in a right-lateral fashion, like the offset schlieren in the photos earlier in this post. In other words, the ENE “fault” is younger.

“Geology isn’t rocket science.” We know what’s going on with rockets — we built those suckers! This, on the other hand, is a bit more complicated!

Anyhow, Chuck and James have been over these rocks like gravy on rice, and they have documented many other instances of cross-cutting relationships. As James’ GSA abstract notes, they found enough exposures to feel confident interpreting the ENE-oriented set to be the older set to have formed as a result of WNW-directed contraction during the Alleghanian Orogeny. The NNE-oriented extensional fractures are the younger set, and are interpreted to have formed during Mesozoic extension accompanying the breakup of Pangea.

Next up, we should take a quick look at the James River itself, and the imprint it has left on this stupendous field site… Stay tuned…

“Geology of Skyline Drive” w/JMU

I mentioned going out in the field last Thursday with Liz Johnson‘s “Geology of Skyline Drive” lab course at James Madison University.

We started the trip south of Elkton, Virginia, at an exposure where Liz had the students collect hand samples and sketch their key features. Here’s one that I picked up:

Regular readers will recognize those little circular thingies as Skolithos trace fossils, which are soda-straw-like in the third dimension. Rotate the sample by 90°, and you can see the tubes descending through the quartz sandstone:

This is the Antietam Formation, a distinctive quartz sandstone / quartzite in the Blue Ridge geologic province. But at this location, on the floor of the Page Valley and butted up against the Blue Ridge itself, we see something else in the Antietam:

Parts of this outcrop are pervasively shattered: a variety of sized clasts of Antietam quartzite are loosely held together in porcupine-like arrays of fault breccia. Turns out that this is the structural signature of a major discontinuity in the Earth’s crust: the Blue Ridge Thrust Fault. This is the fault that divides the Valley & Ridge province on the west from the Blue Ridge province on the east. And here, thanks to a roadcut on Route 340, we can put our hand on the trace of that major fault. Here’s another piece of the fault breccia:

After grokking on the tectonic significance of this fault surface, we drove up into Shenandoah National Park, to check out some outcrops along Skyline Drive itself, but it was really foggy. Here’s a typical look at the team in the intra-cloud conditions atop the Blue Ridge:

We checked out primary sedimentary structures in the Weverton Formation at Doyles River Overlook (milepost 81.9), like these graded beds (paleo-up towards the bottom of the photo)…

…and these cross-beds. You can see that it was raining on us at this point: hence the partly-wet outcrop and glossy reflection at right:

Cutting through this outcrop was a neat little shear zone where a muddy layer had been sheared out into a wavy/lenticular phyllonite, with a distinctive S-C fabric visible in three dimensions:

Finally, we went to the Blackrock Trail, which leads up to a big boulder field of quartzite described as Hampton (Harpers) Formation. In some places, exquisite cross-bedding was visible, as here (pen for scale):

Here’s a neat outcrop, where you can see the tangential cross beds’ relationship to the main bed boundary below them:

…And then if you spin around to the right, you can see this bedform (with internal cross-bedding) in the third dimension. I’ve laid the pen down parallel to the advancing front of this big ripple:

That last photo also shows the continuing influence of the fog.

Thanks much to Liz for letting me tag along on this outing! It was a great opportunity for me to observe another instructor leading a field trip, and also to discover some new outcrops in the southernmost third of the park.

Easter egg

Searching through my photo archives this morning for something suitably “Eastery”… something in pastel colors, perhaps? … a petrified lagomorph? … how about an egg, or something egg-shaped?

This is as close as I got:

This is in the Owens Valley of eastern California, showing a boulder of the Mesozoic Sierra Nevada Batholith bearing a faulted xenolith. I love outcrops like this, with a combination of primary structures (like the xenolith) and secondary structures (like the fault). And the fault surface appeared to have hosted some fluid flow, encouraging epidotization (hydrous metamorphism) along its surface. How appropriate, considering both the “cracked egg” implication of the round xenolith and the pastel tones of the green epidote.

I’ll annotate it up for you, because I know you love it when I do that:

Happy Easter, folks. Focus on the bunnies and candy, and not the zombies.

Transect debrief 6: folding and faulting

Okay; we are nearing the end of our Transect saga. During the late Paleozoic, mountain building began anew, and deformed all the rocks we’ve mentioned so far. This final phase of Appalachian mountain-building is the Alleghanian Orogeny. It was caused by the collision of ancestral North America with the leading edge of Gondwana. At the latitude of Virginia, that means northwestern Africa (Morocco and/or Mauritania).

Whereas the first two pulses of Appalachian mountain building were relatively provincial affairs, this Alleghanian phase was a full-on continent-on-continent smackdown. The Himalaya (India colliding with Eurasia) would be a good modern analogue for the Pennsylvanian and Mississippian Appalachians.

When I was live-blogging the trip, I posted this photo of Judy Gap:

It was a bit hard to get it all into one measly iPhone frame (hence the tilted angle: those trees are in fact vertical!), but what you’re looking at here is the erosion-resistant Tuscarora Sandstone (Silurian in age; quartz-rich beach deposits) that outcrop as a ridge. However, here at Judy Gap, there are two ridges. What gives? This is where I was introduced to a new term that is apparently becoming a common phrase in the structural geology literature: contraction fault.

The story most Physical Geology students get about fault types is that tectonic extension causes normal faults, while tectonic compression causes reverse faults. Contraction faults are faults that display an apparent “normal” sense of motion, but were caused by a compressional tectonic regime. How the heck does that work, you may ask? Consider the following diagram:

So the deal with contraction folds is that they might start out “reverse” but are then rotated and tipped over as deformation proceeds. The former footwall becomes the new “hanging wall,” and the sense of motion is obscured by this new orientation. This means that they do represent contractional strain, but a freshman geology student is unlikely to spot it at first glance.

The Germany Valley to the east of Judy Gap is a big breached plunging anticline, as I attempted to show with this iPhone photo from the Germany Valley Overlook along Route 33:

It’s a bit easier to see if you jump up in the air 10 kilometers or so. Fortunately, that’s precisely why God created Google Earth:

The valley is hemmed in by a big V-shaped fence of mountains, all held up by the Tuscarora. It’s tough stuff. During Alleghanian folding, the crest of the anticline was breached, and water was able to get inside and gut the weaker rocks. The quarry annotated in the photo is mining the same Cambrian and Ordovician carbonates seen in the Shenandoah Valley back in Virginia (Lincolnshire and Edinburg Formation equivalents). A pattern geologists have noted with eroded anticlines is that older rocks are exposed in the middle of the structure, with younger rocks flanking them along the sides.

So that’s a glimpse of the big picture of deformation in the Valley & Ridge, but we can also see cool deformation at smaller scales… Stay tuned…