Friday fold: multilayer buckle folding demo

Check out this video I found online whilst uploading last week’s Friday fold:

This video was produced and published on YouTube by Markus Beckers, Michael Ketterman, Dennis Laux and Janos Urai.

It’s a nice demonstration of how multiple layers of material of different properties and different thicknesses can yield up different flavors of folds. In the movie, there are two materials present: white silicone and gray foam. The silicone layers are stronger (“more competent”) than the foam. But the two silicone layers are different thicknesses. It turns out that this ends up being a decisive factor in determining the way they fold.

We can explain this behavior using the Ramberg-Biot equation:

L = 2 π t (η / 6ηo)

where L is the wavelength of the fold (in other words, the distance from one antiform fold hinge to the next antiform fold hinge); t is the thickness of the folded layer; η is the viscosity (resistance to flow) of the silicone layer (or, in general, the more competent of the two layers); and ηo is the viscosity of the foam layers.

In other words, the (η / 6ηo) part of the equation reflects the viscosity contrast between the affected layers. In the video, this viscosity contrast is a constant, since we’re looking at two layers of the same stuff surrounded by the same matrix of other stuff. The only difference is the thickness of the two silicone layers.

So as far as our video up top is concerned, pay attention to the t value and the L value: the thicker the layer is, the larger the wavelength of the resulting fold. The thin layer has a lower t value, and so it ends up with a shorter wavelength: i.e., there are more folds packed into the same amount of vertical space as its stouter neighbor. The thick layer’s higher t value means it wıll have a proportıonately higher L value. It will have a longer wavelength, and fewer undulations will fit into the available vertical space.

Happy Friday, everyone! I’m heading back to DC tomorrow (from Turkey), so more regular posting wıll resume next week.

Lessons from a broken bottle

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

dollysods_20

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…

dollysods_21

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:
dollysods_21anno

The same pattern can be observed in a second fracture, this one located within the glass (not on the surface):
dollysods_22

Annotated copy:
dollysods_22anno

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.

Pristine stratigraphy vs. bioturbated

Beautiful fiancée for scale.

What does Callan see here?

leaves_puzzle

Tell me why I took this iPhone picture, and I’ll mail you a GEOLOGY ROCKS bumper sticker! Answer in the comments below…

Uniformitarian

polygonal_cracking_DC_map

Heat-stressed map of the Chesapeake Bay / Washington, DC region, as seen at Kenilworth Aquatic Gardens. Looks like mudcracks, eh?

Similar stresses; similar strains.

Butter Buster animation

A million years ago, I posted about my inaugural attempt to use the Butter Buster to illustrate shear zone deformation to my structural geology students.

Today, using the UnFREEz program to make an animated GIF (Thanks, Lockwood!), I give you the Butter Buster animation:

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:
sugarloaf_geol
sugarloaf_geol_key

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:

tension_gash_array_sugarloaf_web

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…

whales_analogy

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…

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