Champlain thrust fault

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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:

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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:

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

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

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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…

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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:

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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:

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

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And another nice fold:

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This fold is transitioning into a shear band:

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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:

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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:

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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:

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

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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:

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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:

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

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

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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):

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

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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):

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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:

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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:

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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:

morancalc

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

Harpers Foldry

Cleaning out the backlog of photos I haven’t popped up here yet… Here’s three shots from last weekend, of folds (some kinky) which deform Harpers Formation foliation, just south of Harpers Ferry, West Virginia:

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The Harpers is a Cambrian-aged lagoonal mudrock, dated via Olenellus trilobites in Pennsylvania. It is part of a transgressive sequence that followed Iapetan rifting of the mid-Atlantic, and was later deformed during Alleghanian mountain-building. That’s when the pronounced foliation was imparted, and when that foliation was folded (also overturned). There are plenty of nice exposures of kink folds in this charismatic rock throughout historic Harpers Ferry. Check it out if you’re ever there on a history field trip.

“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:

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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:

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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:

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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:

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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:

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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)…

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…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:

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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:

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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):

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

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…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:

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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.

Sugarloaf

Sunday morning, NOVA adjunct geology instructor Chris Khourey and I went out to Sugarloaf Mountain, near Comus, Maryland, to poke around and assess the geology. Sugarloaf is so named because it’s “held up” by erosion-resistant quartzite. It’s often dubbed “the only mountain in the Piedmont,” which refers to the Piedmont physiographic province. Here’s a map, made with GeoMapApp and annotated by me, showing the general area:

A larger version of the map can be viewed by clicking here.

On the far west, you can see the Valley & Ridge province, which ends at the Blue Ridge Thrust Fault. Then the Blue Ridge province runs east from the Blue Ridge itself to Catoctin Mountain. From there, you enter the Piedmont, including both the “crystalline” Piedmont (Paleozoic metamorphism of various ocean basin protoliths, plus infusions of granite) and the Culpeper Basin, a Triassic/Jurassic rift valley. The Potomac River cuts a series of three spectacular water gaps across the Blue Ridge province just west of Sugarloaf. Harpers Ferry, West Virginia, is located at the confluence of the Potomac and the Shenandoah Rivers by the westernmost of these water gaps, and the name for the easternmost one is “Point of Rocks.”

Here’s a look at a detail from the southeastern corner of the geologic map of the Buckeystown, MD quadrangle, by Scott Southworth and David Brezinski:
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The map pattern shows a that the area around Sugarloaf Mountain is a doubly-plunging anticlinorium of Sugarloaf Mountain Quartzite [SMQ] and overlying (younger) Urbana Formation. Overall, it’s got that typical “Appalachian” northeast-southwest trend. Notice the thrust fault on the west side: a typical hanging wall anticline? The ridges, including the summit of Sugarloaf Mountain itself, are held up by the toughest quartzite. This overall “squashed donut” shape shows up pretty well in the physiographic map up at the top of this post.

Sugarloaf is quartzite (metamorphic), but you can clearly see the sand grains that composed its protolith (sedimentary). There’s also reports of cross-bedding, and so Chris asked me to take a look at a few structures to assess them with my point of view. I found a pervasive cleavage in the rock, far more than I would have suspected would be there. We did find bedding exposed as compositional/grain size layers in several locations, including on the summit. I also paid a lot of attention to the many quartz veins which cut the metasedimentary quartzite. These veins of “milky quartz” are often arranged in lovely en echelon series, like these tension gashes:

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I took the above photo several years ago on a visit there, but it’s typical of the sorts of stuff we saw Sunday. The kinematic sense of this outcrop would be “top to the right.” Interestingly, none of the Sugarloaf outcrops show really deformed tension gashes (i.e., they’re not folded into Z or S shapes like those I showed you a few days ago).

What we really wanted to get a sense of, though, was which way was up in these rocks. We were in search of geopetal structures: primary sedimentary structures that indicate the “younging direction” of the beds. Graded beds can do this, though I didn’t see any unambiguous graded beds in the SMQ on Sunday’s trip. We wanted some cross-beds. We found some hummocky / swaley examples, looking approximately like this USGS photograph (black & white; hammer for scale) of an outcrop somewhere “north of the summit”:

crossbedding_USGS_sugarloafImage source: USGS

Ours wasn’t as beautiful as the one pictured above, but it was clearly hummocky cross-bedding, and it was right-side-up (in beds tilted at ~30°). Interestingly, the SMQ has been correlated by Southworth and Brezinski (2003) with the Weverton Formation of the Chilhowee Group, a rock unit exposed in the Blue Ridge. Just as the Weverton is overlain by the finer-grained Harpers Formation, so too is the SMQ overlain by a finer-grained unit, the Urbana Formation. Both are interpreted as metamorphosed continental margin deposits. The Urbana is mostly phyllite in the areas I’ve seen it (including phyllite that’s full of quartz grains, a first for me). The Urbana is well exposed in a creek-side outcrop north of Sugarloaf Mountain, and I took Chris there to show him the lovely intersection of bedding and cleavage.

Here is a weathered piece of the Urbana Formation that Chris collected there, looking at the plane of cleavage (ruler in background for scale):

urbana Image source: Christopher Khourey

You can see the bedding running ~horizontally across it, though the photo cannot convey the lovely phyllitic sheen that results from waggling these samples back and forth in good light. It’s pretty cool. In places, the transition from sandy to phyllitic is gradational, probably relict graded bedding.

So, what does it mean if Southworth and Brezinski (2003) are correct in their correlation, and the Weverton and the SMQ are really the same rock layer, but in different provinces and at different metamorphic grades? Recall that the Blue Ridge province to the west is also a thrust-faulted anticlinorium, launched up and to the west by the Alleghanian Orogeny from an original position deeper in the crust and further towards the east. It’s a shard of the craton, snapped off and shoved bodily up and to the northwest. (In class, I often liken it to Joe Theismann’s leg: a compound fracture of the continental crust.) Might the Sugarloaf Mountain Anticlinorium [SMA] be a smaller version of the Blue Ridge pulling the same trick? It too is arched up and snapped off …but it would be a “Mini-Me” that’s only just surfacing, like a baby whale swimming above momma whale’s back…

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We know that deeper down in the Blue Ridge stratigraphy, we find the Catoctin Formation, the Swift Run Formation, and the basement complex. If we drilled down through the crest of the SMA, would we find the same units (or more metamorphosed equivalents thereof)? It’s an intriguing thought…

Transect debrief 7: Brittle-ductile deformation

On the transect trip, I also saw some nice meso-scale “minor” structures that probably formed during Alleghanian deformation. Prominent among the ones that really impressed me were these en echelon tension gash arrays, deforming the Antietam Formation quartz sandstone and well exposed in blocks used to construct the wall along Skyline Drive and the Sandy Bottom Overlook in Shenandoah National Park:

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Good Lord! Ain’t those things beautiful? They also give us a lovely sense of the kinematics (relative motions) of the blocks of Antietam sandstone on either side of this sheared zone. In the case of the image above, the left side of the photo has moved “down” relative to the right side. The rock in between has torn and stretched, with the gashes opening up at right angles to the maximum stretching direction. As deformation proceeds, of course, the gashes rotate and deform, folding into “S” shapes.

Here’s one that’s more subtle:

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What you’re looking at in the image immediately above is a tension gash array that was a zone of weakness, exploited by later brittle deformation. The fracture which defines the edge of this block cracked through those old brittle-ductile tension gashes and split them clean in half.

Neat, eh? …Now check this out:

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Remember the Skolithos trace fossils? Here, you’re looking at a sideways cross section through some cylindrical Skolithos as they are disrupted by this zone of shearing. Note that the burrows tend to be highlighted by rust (hematite) staining: the brown lines that run roughly from the top left of the photo towards the bottom right. But look what happens to the orientation of those tubes where they are cut by the tension gash arrays: they are deflected into a new orientation, rotated from their original orientation!

If that’s a bunch of gobbledygook to you, consider this annotation:

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I’ve drawn white lines to show the orientation of the Skolithos tubes in their undeformed and deformed states, colored the tension gashes yellow, and drawn on a set of blue arrows to show my kinematic interpretation (top to the left).

Here’s another block, showing the same phenomenon:

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Go ahead. Tell me you’re not impressed with that. I dare you. That is frakking AWESOME.

You are now dismissed.

Transect debrief 5: sedimentation continues

We just looked at the Chilhowee Group, a package of sediments that records the transition for the North American mid-Atlantic from Iapetan rifting through to passive margin sedimentation associated with the Sauk Sea transgression. Well, if we journey a bit further west, we see the sedimentary stack isn’t done telling its story. The saga continues through another two pulses of mountain building. Consider this “unfolded, unfaulted” east-west cross-section cartoon:

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Part A of the image above shows the overall stratigraphic sequence for the Blue Ridge and the Valley & Ridge provinces in Virginia and West Virginia. You’ll notice that the small, detailed stratigraphic column I used to start the last two posts covers just the bottom 6 layers in this stack. Zoomed out to the bigger picture, we see ~40 layers overall. Lynn Fichter of James Madison University, one of the leaders of the Transect Trip, has published an excellent information-dense guide to the mid-Atlantic column. It’s a terrific reference for anyone looking to learn more about these rocks and the story they tell.

Part B of the image above shows the tectonic interpretation of these different packages of rock — some represent rifting, some represent passive margin sedimentation, some represent clastic influence from various orogenies occurring to the east (Taconian and Acadian).

The cartoon cross-section below, modified from an original by Steve Marshak in his excellent introductory textbook Earth: Portrait of a Planet, shows the tectonic evolution of the east coast over the past ~1 billion years of geologic time. It is reprinted here with Steve’s permission.

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The story begins with the Grenville Orogeny, an episode of mountain building that completes the assembly of the Rodinian supercontinent. This is followed by Iapetan rifting, followed by three pulses of Appalachian mountain-building: the Taconian (“Taconic“) Orogeny, the Acadian Orogeny, and the culminating event of Pangean supercontinental assembly, the Alleghanian (“Alleghenian”) Orogeny. Finally, Pangea breaks up in the Mesozoic, an event also known as Atlantic rifting. Two complete Wilson Cycles are preserved by the Appalachian mountain belt!

The Valley & Ridge province received sediment courtesy of the Taconian and Acadian Orogenies, but wasn’t directly involved with the tectonic collision in any deformational way. Notice how west of both those orogenies in the Marshak diagram you see a fresh layer of sediment being deposited atop the North American craton.

During the field trip, I posted some iPhone photos of the sedimentary strata that accumulated in the Valley & Ridge during the mid-Paleozoic, shed off from the orogenic activity to the east. For example, the Brallier Formation’s turbidites record a time when sea was west and mountains were east. Or the Juniata Formation’s red beds speak of a time in the late Ordovician when an advancing clastic wedge had piled sediment up above sea level. This shot of some of those red beds preserves some beautiful depositional relationships from ~440 million year old river systems.

Let’s annotate that, shall we?

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Even in the Ordovician, rivers did what they do today, spilling over their bansk and building up natural levees. Same as it ever was, people.

That “sediment only; no deformation” regime for the Valley & Ridge changed with the Alleghanian Orogeny. That’s when deformation propagated to the west, encompassing the flat-lying Valley & Ridge strata into a proper fold-&-thrust belt. Later, differential erosion of these folded and faulted layers would etch the landscape into a series of valleys and ridges… hence the province name. More on that deformation in the next post.