Friday fold: Siccar Point, Scotland

As with last week, I’m going to show you someone else’s fold today. This one should have strong resonance with most geologists, because it’s a fold in the tilted (and contorted) older strata exposed below the famous unconformity at Siccar Point, Scotland:

I found this image on the British Geological Survey’s online repository of images, which are available for public use with attribution. I found out about the BGS photo repository via a post on StructuralGeology.org.

The photo was taken by T.S. Bain in 1979. Rock hammer (lower left) for scale.

The specific rock type here is shale, and their age is Silurian. Note the thinning of the limbs of the fold, and the relatively thick hinge area.

Happy Friday – may your workday rapidly thin (like the limbs of this “similar” fold), and your weekend be as thick as this fold hinge!

Building stones of the Haghia Sophia

The Haghia Sophia (or “Ayasophia”) is an astounding building in old town Istanbul. It is an ancient cathedral turned mosque turned museum. Through all these incarnations, the Hagia Sophia has retained some features and had other ones added on: it is a palimpsest of architecture, symbology, and history. Walking through its soaring main chamber, or side passages and alcoves, visitors like me stand with necks bent and mouths agape. It is an unparalleled location for peeling back the layers of time.

Built in 532 CE by the Emperor Justinian, the cathedral rose on the same spot where two earlier churches had stood, the first of which was built in 360 CE. The name “Haghia Sophia” comes from the Greek for “holy wisdom.” For more than a thousand years, it served as the principal church of the Byzantine Empire. It was the world’s largest cathedral for thousands of years. The minarets were tacked on in 1453, after Constantinople fell to the Ottoman Empire:

There’s a gazillion aspects of this building to discuss, but today I’d just like to share some images of the different building stones seen in and around the Haghia Sophia. To start with, here’s a “Verde Antique” (serpentenite breccia) sarcophagus outside the building:

The floor stones in an interior hallway, worn smooth and shiny by millennia of human shuffling:

And a bunch of shots of stones used in the interior walls …

Granite (verging on unakite?):

Conglomerate:

Rhyolite porphyry:

Rhyolite porphyry with xenoliths (also used to construct a sarcophagus outside):

Marble gneiss:

Darker granitoid:

There are also some structurally interesting rocks, like this red and white marble breccia that shows pressure solution. Notice the sutured boundaries of the white grains, and their pronounced long axes, 90° to that maximum pressure direction.

Kind of reminds you of the Purgatory Conglomerate, right? (Me too.)

My favorite rock there is this lurid, gory red/white/black marble gneiss, as it displays ptygmatic folding (elsewhere it is also boudinaged):

I wish I had more photos of this stuff. It’s great. It reminds me of guts!

Here it is in a typical display (pardon the blurriness of the photo): they “fillet” the rock and spread it open in the manner of a Rorschach blot. This produces an attractive symmetrical design, with minimal artistic effort:

Another nice “butterfly” spread, this one of folded marble gneiss:

Another one:

Look close at this one. Note the little gray crosses in there? Let’s zoom in…

Here’s one closer-up:

These are ancient Christian crosses, or rather, the holes where ancient Christian crosses were once mounted on the wall. When the Haghia Sophia was converted to a mosque in 1453, these Christian symbols were removed, and the holes cemented over to obliterate traces of the old religion. Here’s another one, where the cement has fallen away:

Along similar lines, here’s some Arabic script carved into the railing of the second floor, marring a lovely marble breccia:

Stuff like this just floors me. I mean, think about all the different people to lean on this railing over the past 1500 years. The Haghia Sophia’s history is so deep, with so many distinct overlapping layers. The mind reels…

A fantastic concentration of building stones may be found at the “Coronation” spot on the main floor of the building, where Byzantine kings were crowned:

After several pleasant hours touring the Haghia Sophia, we got lunch at a great cafe nearby. Lily got lentil soup:

…and I got an amazing pide, the Turkish style of “pizza”:

Delicious rocks followed by delicious repast! Can’t complain…

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!

Friday fold: granite dikes, Barberton greenstone belt

Folded & boudinaged granite dikes in tonalitic gneiss, Barberton granite-greenstone belt, South Africa. From Passchier, CW, Myers, JS, and Kroner, A., (1990). FIELD GEOLOGY OF HIGH GRADE GNEISS TERRANES.

Very crudely annotated:

This is a sweet example of how you can get different structures developing in different orientations relative to the principal stress directions. In this particular part of the Barberton Greenstone Belt, compression (orange arrows) operated from the top of the photo towards the bottom, and the rock stretched out from left to right (green arrows). Folds formed where granite dikes were compressed, but the same rock in a different orientation was boudinaged… Cool, eh?

So that’s your Friday fold! The boudinage is just a little bonus for you, because, hey, it’s Friday.

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?

Friday fold: Kinky metagraywacke from DC

Fourth edition of the “Friday fold;” second one via video. Happy Friday!

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

Lessons from a broken bottle

Whilst hiking at Dolly Sods over the weekend, I found this old artifact:

Upper 10 is apparently a “Sprite”-esque lemon-lime soda, discontinued in America but still being marketed abroad. But that wasn’t what got me jazzed, of course. Look more closely…

That is a lovely little conchoidal fracture, and it’s so exquisite because it preserves not only the concentric “ribs” that are typical of conchoidal fractures, but also delicate little traces of plumose structure. Note that the conchoidal “ribs” are parallel to the advancing joint front (leading edge of the fracture), and the plumes are perpendicular to the joint front.

Here’s an annotated copy to make this more explicit:

The same pattern can be observed in a second fracture, this one located within the glass (not on the surface):

Annotated copy:

Nice! This is the same pattern that we observe with the fine-scale topography of joint surfaces in rocks, as I have blogged on several occasions.

Thank you, Upper 10, and thank you, nameless Dolly Sods litterbug, for providing us with this fine lesson in fracture anatomy.

Dolly Sods

Over the long Labor Day weekend, my fiancée Lily and my friend Seth and I took a three-day backpacking trip in the Dolly Sods Wilderness area of West Virginia:

Dolly Sods is a unique place, a little patch of flora that is more typical of Canada. It sits atop the eastern Continental Divide, and most of the area drains to the Gulf of Mexico via the Ohio River. Parts of Dolly Sods are sparsely treed, and resemble Arctic tundra. It is the easternmost bit of the Appalachian Plateaus province. Many places reminded me of Alaska:

Rolling meadows and bogs occur in patches, interspersed with forest of spruce, hemlock, and aspen (yes, aspen!):

The area was used as a proving ground during World War II, and there are still some dangerous bits and pieces left over from that time:

Here’s our happy trio, ready to set off on Friday afternoon:

Very quickly, I clued into the wealth of small blueberries which were omnipresent in the “tundra” landscapes. I snacked on these continuously throughout the weekend:

A glimpse of two forms of power generation off to the north: Mount Storm on the left (a coal-fired electric generation plant) and a field of windmills on the right:

Here and there, outcrops of white rock rose up above the lichens and shrubs:

This is the Pennsylvanian-aged quartz sandstone of the Conemaugh Group. Occasionally, it outcrops as bedrock, and other times, you just get these clean boulder fields, surrounded by tundra vegetation:

So what do we see when we zoom in on these outcrops and boulder fields? Well, mostly, we see quartz sandstone:

…Although there is a regular smattering of quartz-pebble conglomerate, too:

Occasionally, primary structures jump out at the eye, like some graded bedding…

…or these cross-beds:

Annotated copy:

There were even some fossils, like these plant scraps:

Plant scraps compressed en masse make coal, and there are coal interbeds to be found in places in Dolly Sods, and bituminous coal can also be found as float, as with these chunks:

There was even some structure to observe!

Annotated version:

A bigger outcrop, right around the bend, showed even more pervasive distortion of the sedimentary layers:

Annotated version:

What’s going on with these folds? After all, the Allegheny Plateau isn’t known for pervasive structural shenanigans… I’m guessing this might be soft-sediment deformation: slumping and sliding of sedimentary layers before they got lithified… Any other thoughts? (chime in via the comments section below, if so).

Here is sand weathering out of the sandstone, the grains free and loose again for the first time in ~300 million years:

The plants were a joy:

Here’s the view at sunset from our third campsite:

Yesterday (Monday) morning, when we woke, we found that the temperature had dropped below freezing overnight, and a coarse layer of frost covered everything:

Detail of the frost crystals on my tent’s rain fly:

The sun rose, and starting melting off the frost and dissipating the fog:

Soon only the fog remained:

In the warmth of the new day, we hiked out, got apple dumplings at the Front Porch Restaurant across from Seneca Rocks, and drove back home along good old (new) Route 55. It was a great weekend away, just the right distance, in wild country, with great weather. I felt rejuvenated by the experience.

Friday fold: tight syncline in Montana

This fold is located on Highway 287, north of Wolf Creek, Montana.

Annotated version:

As with last week’s Friday Fold, this fold owes its existence to (a) deposition of sandstone and shale in the Western Interior Seaway, and (b) deformation under a giant thrust sheet during the thin-skinned compressional tectonics of the  Sevier Orogeny.

In this case, we’re south of the Lewis Thrust, and the local equivalent is called the Dorado Thrust. It’s basically the same exact thing you find up north in Glacier National Park: Mesoproterozoic Belt Supergroup metasedimentary rocks thrust as a relatively coherent sheet over weaker, younger Mesozoic sedimentary strata.

The fold is moderately plunging towards the road, which is how I was able to nestle in, tucked into the trough of the syncline, the length of my body parallel to the axis of the fold.

Happy Friday, everyone!