How to prep your pocket fold

“Pocket folds,” as my Rockies co-instructor Pete Berquist has defined them, are rock samples exhibiting folds that are small enough to stick in your pocket (and take back to your lab). Here’s a pocket fold that I found last week in the White Mountains of New Hampshire:

I brought it home, and today I unpacked it from the car, along with about 70 pounds of other samples. I turned on the hot water tap and took out a critical piece of sample prep equipment, the wire brush (suitable for cleaning a grill), and scrubbed off all the algae. You should only use wire brushes on relatively hard samples. Because the steel wires have a hardness of 5.5, they will scratch rocks like limestone, which is made of 3.0-hardness calcite. I also gave it a quick bath in 4.5 molar HCl, to kill off any remaining algae. A rinse, and then I set it in the sun to dry.

My White Mountain pocket fold is a quartzite, with some nice graded beds, which probably show up better when they are filled in with algae. Regardless, here’s the cleaned up sample, a gray ghost of a pocket fold:

If you collect field samples for teaching purposes or just because you find them beautiful, what steps do you take to prepare them?

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:

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.

New “secondary structures” display at NOVA

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

Crenulation lineation

Hiking last Sunday in Rock Creek Park, DC, I saw this boulder and my eye was immediately drawn to the linear pattern running from upper left towards lower right (Swiss Army knife at upper right for scale):

Because that photo is not especially large, let’s zoom in a bit to two sections… Here is Photo 1, annotated to show the areas we will look at next:

Here’s a cropped and higher-resolution look at the diagonal lineations that caught my eye:

There are lots of different linear elements that can show up in rock fabric (as distinguished from the many kinds of planar elements that could be found). Some lineations are primary, but the ones that interest me are secondary (i.e., tectonic in origin). Let’s rotate our perspective, moving to the left of the first photo, and turning our head ~70° to look towards the right. This closer look at the left face of the boulder reveals the origin of these particular linear elements:

…They are crenulation lineations, essentially very small folds that deform the cleavage of these highly-foliated rocks. The crenulations’ fold axes were popping out in very slight 3D relief on the face of the boulder that initially caught my eye, like tectonic “ripple marks.” On the right side of Photo 3, you can see the lineations (fold axes) stretching away into the blurry distance.

In addition, some of the convex-outward crenulations had been breached, which means that the trace of the foliation was outcropping along the same trend as the fold axis. This is a variety of intersection lineation: two planar elements intersecting in a line. In this case the planar elements are [a] the foliation and [b] the outcrop surface.

(The other, more “classic” variety of lineation is a mineral stretching lineation, like the lineated gneiss I showcased last November.)

So, how should we interpret these rocks? I’d say that an initial foliation was imparted to them due to shearing along the Rock Creek Shear Zone, a prominent north-south-trending zone of smeared rocks in northwest DC; about 1 km wide. The foliation formed perpendicular to an original σ₁ maximum principal stress direction. Later, the stress field changed, and deformed this pre-existing foliation. The new σ₁ was oriented (using Photo 1 as our reference) from the lower left towards the upper right. The new σ₂ was oriented parallel to the crenulation fold axes (upper left towards lower right). And the new σ₃ was oriented in the direction perpendicular to the main outcrop face — that’s why the folds pooched out in that direction. (It offered the least resistance to being pushed.)

Recall that we saw something similar in the snow back in February.

Anyhow, I had just gotten through discussing lineations with my GMU structure students, so I figured I should photograph this particular outcrop for their benefit…

…and, I suppose, for your benefit as well, dear blog reader.

Using bedding / cleavage to detect overturned beds

One of my students wrote to me this morning with a question about the relationship between bedding, cleavage, and folding. He asked:

I am not sure how we use the relationship between bedding and cleavage to interpret fold limbs.  It seems if bedding is near vertical and cleavage is closer to horizontal, this would be an upright fold limb.  To be overturned, wouldn’t the bedding need to be closer to horizontal?  I guess I don’t understand how does cleavage help dictate the bedding orientation.

So here’s the deal: when rock strata (layers) get compressed, they develop a couple types of structures: one is that they tend to fold, and the other is that they tend to cleave. Cleavage and folding have a distinctive relationship.  Say bedding starts off horizontally-oriented, and is subjected to a horizontal compressive stress. Cleavage will form that is vertically-oriented (perpendicular to σ₁). As deformation proceeds and the bed begins to shorten by buckling up and down, the cleavage “tips” over (rotates) as the top of the bed moves towards the fold crest. (I have previously discussed a similar aspect of vergence, using S and Z folds. The same thing that applies to the axial planes of parasitic folds also applies to cleavage.)

Assuming a simple single episode of deformation (no overprinting), the orientation of the cleavage plane will be approximately the same as the axial plane of the main fold (an imaginary geometrical plane that “divides a fold into left and right halves”).

Here’s a quick sketch I just drew of a folded bed (yellow) being cut by cleavage (parallel brown lines):

In the example on the right, the cleavage and folding agree with one another (that is, the folds’ axial planes and the cleavage planes are parallel): one episode of compression could produce these two structures in these orientations. In the example on the left, the cleavage cuts across the fold at an angle which is close to orthogonal (perpendicular) to the axial plane of the folds — this is an impossible situation to produce with a single episode of deformation.

If you were to find an outcrop of the yellow layer (circular zones of exposed rock in the diagram below) that showed the relationship between the cleavage and the bedding, you can interpret the overall structure:

Outcrop 1 shows bedding and cleavage dipping in opposite directions. Outcrop 2 shows bedding and cleavage dipping in the same direction, though bedding is dipping more steeply than cleavage.

Because of this relationship, Outcrop 2 is best interpreted as an overturned limb of a fold. But Outcrop 1 doesn’t make any sense as it is drawn above. The best way to interpret that circular exposure is shown here:

…That is: it is an upright limb of a fold, not an overturned limb.

So: if you have a steeply-dipping bed cut by more-shallowly-dipping cleavage, pay attention to the direction of the cleavage’s dip: (a) If it is dipping in the opposite direction as bedding, your fold is upright or asymmetric. (b) If your bedding and cleavage are dipping in the same direction, your fold is overturned. If the bedding and cleavage are both close to horizontal (and part of a larger fold), then you’ve likely got the limb of a recumbent fold. If bedding is vertical and cleavage is horizontal, you’re likely on the nose of a recumbent fold, where the axial plane cleavage is intersection bedding at a right angle. If bedding is horizontal but cleavage is vertical, then the deformation likely hasn’t proceeded very far. Obviously, checking for geopetal structures like cross-bedding or mudcracks can help you determine whether the beds are overturned or not from a purely sedimentological point of view.

Hopefully this post helps elucidate the structural relationship between bedding and cleavage a bit more. If not, read here about a classic example in Wisconsin (Van Hise Rock).

Quartz veins on Pimmit Run

Last Sunday, I took a solo hike along Pimmit Run in Virginia, accessing the valley via Fort Marcy, a Civil War fortification off of the George Washington Memorial Parkway. As always, I did a bit of geologizing along the route. One theme that emerged from the day’s photos was quartz veins. These veins form when the host rock (in this case, the Sykesville Formation) cracked open in a brittle fashion, then silicon- and oxygen-bearing hydrothermal fluids flowed into that fracture. As the fluids cooled, the silicon and oxygen bonded together and precipitated quartz, sealing shut the fracture like a seam of glue.

Here’s one that I liked because it outcropped both above and below stream level:

In several places along Pimmit Run, I saw small zones of saprolitic bedrock, which is basically “rotten rock,” where the Sykesville Formation outcrops have been more pervasively chemically weathered. This one was so soft that I was able to dramatically plunge the blade of my Swiss Army knife into the rotted rock adjacent to an unweathered quartz vein:

Oblique view of the same outcrop:

As a structural geologist, quartz veins are interesting because they are extensional features whose orientation relates to the stress field these rocks experienced in the distant past. Once formed, however, they can also act as strain markers to show how subsequent deformations have affected these rocks. Here, for instance, is a folded quartz vein:

…and here’s a bonus tiger beetle:

Here, ptyggie ptyggie ptyggie!

Yesterday, I took my GMU structural geology class to the Billy Goat Trail, my favorite local spot for intriguing geology. Unlike last year, we managed our time well enough that we got to clamber around on the rocks downstream of the amphibolite contact. Here’s Sarah, Lara, Kristen, and Alan, negotiating a steep section:

Justin, Joe, Nik, Aaron, Jeremy, and Danny find a chunky amphibolite boudin in metagraywacke. Notice how Jeremy is gesturing about the orientation of the metagraywacke foliation wrapping around the boudin.

The thing that we found that really made me happy were these ptygmatic folds. Most of my readers will doubtless already be familiar with ptygmatic folding, but in case you’re new to this, check out this photo (ballpoint pen for scale):

Ptygmatic folding is “intestine-like” in appearance. It results where there is a particularly high viscosity contrast (viscosity is resistance to flow) between the folded layer and the surrounding matrix. The higher viscosity material makes broad lobes, while the lower viscosity material may be found in the pointy cusps between those lobes. If ptygmatic folding is well developed, the limbs become parallel to one another (isoclinal), and the visual similarity to guts is disconcerting. Here’s a smaller version, a few feet away from the first one:

I’m headed back to the Billy Goat Trail today to discuss the trail’s geology with a crew from Sigma Xi‘s D.C. chapter. I wonder what we will discover today?

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:

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:

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”:

Image 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):

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…

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 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…

Transect debrief 2: weathering the Grenvillian landscape

From the basement complex, the next unit up in the Blue Ridge province’s stratigraphic sequence is the Swift Run Formation. It rests atop an erosional unconformity. After the Grenville Orogeny (~1.1 Ga) added a swath of new crust along the margin of the North American continent, the landscape began to weather and erode. Eventually, an episode of rifting broke open rift valleys and a new ocean basin, the Iapetus. The Neoproterozoic rift valleys filled with sloughed-off detritus from the exposed Grenvillian rocks (granitoids, mainly), resulting in arkosic sediment. This arkose is mixed in with muddy layers: it looks very much like the much-younger rift valley sediments in the Culpeper Basin (Triassic rifting for those, not Neoproterozoic). This is the principle of uniformity at work. The same tectonics yield the same signature, even though they happen at different times. Same as it ever was, same as it ever was.

Here’s a reposted iPhone photo of some of the Swift Run, showing rip-up clasts of mudstone in the arkose:

Some of it is conglomeratic, with rounded quartz pebbles surrounded by immature-composition sand (reposted iPhone photo):

Later, during Paleozoic mountain-building (Alleghanian Orogeny), the Swift Run developed a penetrative cleavage. Here’s a photo showing bedding and cleavage intersecting in the Swift Run:

Annotated:

This is a cool outcrop: In spite of being polka-dotted with lichens, it shows primary bedding truncations (a primary geopetal sedimentary structure that tells us that up is “up” in this photo) as well as a small S-fold (top to the left) that probably resulted from Paleozoic Alleghanian deformation:

Annotated:

In spite of small folds and well-developed cleavage, I was shocked when someone on the field trip noticed this:

That’s two recumbent isoclinal folds! Annotated:

These folds may be just a local phenomenon formed as one layer of the Swift Run slipped over its neighbor… but they also may hint that deformation is more pervasive in this unit than a cursory glance would indicate. Quite interesting, if you ask me.

Take home lessons: (1) The Swift Run Formation is a post-Grenville rift-related sedimentary deposit. It is compositionally and texturally immature. (2) The Swift Run, like everything else in the Blue Ridge province, got deformed millions of years later during the Alleghanian phase of Appalachian mountain-building.