Rumeli Hisarı

Right after I got to Istanbul on this most recent trip, I took a taxi from my hotel down to the Bosphorus, to check out the Rumeli Hisarı, a fort complex built in 1452 by Sultan Mehmet the II in anticipation of the following year’s siege of Constantinople. It’s constructed at the narrowest point on the Bosphorus (660 m wide), with the aim of controlling boat traffic coming from the Black Sea. This narrow spot is today where they have the second of two bridges spanning the Bosphorus. It looks like this:

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It’s in Europe; that’s Asia on the far right of the photo. A few more shots of the fortress’s pattern of towers and interconnecting walls:

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Inside, I was pleased to note the variety of building stones. Here’s a nice porphyritic andesite which was a common constituent of the walls:
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And a folded limestone:

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Here are some yellowish blocks that are weathering away faster than the mortar which holds them in place. There is a Turkish 1-lira coin in front of the dark block near the center, to provide a sense of scale:

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Here’s a similar phenomenon playing out with some bricks used to make an archway, except here the mortar is the more rapidly weathering component:

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Check out this slab of brick… it’s got a curious adornment:

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Zoomed in to show this detail:

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Dog prints! Sometime a long time ago, maybe more than 500 years ago, a brick maker put out slabs of clay to dry, and some long-dead dog walked across it. The dog’s footprints are a kind of “historical trace fossil” that was then incorporated into this ancient structure.

Visiting the Rumeli Hisarı was a pleasant experience. I walked down along the Bosphorus next, peering into its surprisingly clear waters and counting jellyfish, then got a pide at a cafe. I caught another cab back to the hotel, and eventually fell asleep, a victim of jet lag…

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Deducing my first anticline

When I was done with my sophomore year at William & Mary, I embarked on a time-honored tradition among W&M geology majors: the Geology 310 Colorado Plateau field course. Jess alluded to this same course in her Magma Cum Laude contribution to this month’s Accretionary Wedge geology blog “carnival,” too.

My version of Geology 310 was led by the legendary Gerald Johnson (a.k.a. “Dr J”), a dynamic and enthusiastic educator who seemed particularly at home in the field. One day, he had us out in Utah (I think) somewhere, and pulled over to the side of the road so we could examine some tilted sandstone layers. We took a strike and dip reading, and plotted it on a map.

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Then we descended into a narrow valley, where Dr. J did some “geology at 60 miles per hour,” pointing out shale outcrops in a few places in the valley. Then we drove up the opposite side. We pulled over again. Same sandstone strata: we again took a strike and a dip on the beds. The data was then recorded on our maps with a strike and dip symbol, a broad, squat “T” shape, where the upper bar of the “T” is parallel to the strike of the bedding, and the vertical prong of the “T” is pointing in the dip direction.

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“Well,” Dr. J asked us, “What’s going on here?”

We were all silent, trying to puzzle it out. What’s the deal? What is he fishing for? Seconds ticked by, and no one had the right answer. We started to sweat… “Um, the sandstone beds are dipping to the west on the ridge west of the valley,” someone ventured, “and they are dipping to the east on the ridge east of the valley?”

“Yes, but what does that mean?” he replied. Silence…

Eventually, he relented, and spelled it out for us. Imagine this situation from the sides, he suggested, gesticulating the layers dipping off in opposite directions. “These are the same layers, so they were once laterally continuous…” He mimed a cross-sectional perspective:

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How could we connect these disparately oriented strata together?

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Bam! It hit me: I got the idea of an anticline at that point — the idea that a structure like an anticline could be so large that I couldn’t actually see it from my earthbound human-sized perspective, and I could only infer it from detailed measurements of the rock structures. It was a revelation to me: this valley and its surrounding ridges were part of a massive fold. The anticline must have breached in the middle, with the shale eroding away faster than the sandstone, producing a valley flanked by two ridges.

I’m grateful to Dr. J for putting us through all stages of this exercise: collecting the incremental pieces of data, being forced to think about it in an attempt to come up with an interpretation, and then finally giving us the proper interpretation, once it had become obvious we weren’t going to get it on our own. This last bit is particularly important to me as an educator: sometimes it’s okay to spell it out for students, particularly if it’s their first time walking down a particular path. By revealing the “answer,” Dr. J guided my thinking from data to big picture structure to geomorphological interpretation in a way that I can only describe as “opening up a new pathway” in my mind. Once he showed the way to think about this sort of thing, it was suddenly very easy for me to visualize this sort of complicated four-dimensional story. Once the pathway was there, it was almost effortless to let my thoughts flow along that pathway. Weird how one’s perspective can change in a moment, and how that influences everything that comes after.

For me, this exercise and ensuing discussion constituted an important moment in developing my ability to think like a geologist. I don’t think my brain will ever be the same.

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!

“Those aren’t pillows!”

In the 1987 comedy Planes, Trains, and Automobiles, John Candy and Steve Martin have a funny experience. It involves a cozy hotel room (one bed only) and the two travelers are huddled up for warmth. As he wakes up, John Candy thinks he is warming his hand “between two pillows.” At hearing this, Steve Martin’s eyes pop wide open, and he yells, “Those aren’t pillows!”

They jump up, totally discombobulated. An awkward moment follows.

Well, it’s not quite as awkward, but I had a similar “those aren’t pillows” moment recently. I was out in Shenandoah National Park with my GMU structural geology students, and we stopped off at the Little Stony Man parking area (milepost 39.1 on Skyline Drive). Here’s a figure showing the area in question, from Lukert & Mitra (1986):

You’ll note in the detail map at the right that it shows the nonconformable contact that separates the basement complex (here, the “Pedlar” Formation) from the overlying metabasalts of the Catoctin Formation.You’ll also note that it says “PILLOWS” with an arrow pointing at a specific spot on the trail. The word refers to basaltic pillows, which are breadloaf-shaped primary volcanic structures that form when lava erupts underwater. They are typically the size of a bedroom pillow (especially overstuffed pillows). Here’s some video of pillows erupting.

Pillows have been reported elsewhere in the Catoctin (e.g., near Lynchburg, according to Spencer, Bowring, and Bell, 1989), but this is the only location that I’m aware of where they have been reported in northern Virginia. The implications are not all that tremendous: just that a portion of the Catoctin erupted subaqueously, but it would be a neat thing to show students, especially seeing how close the outcrop is to safe parking.

Well, I’ve been to this area a half-dozen times, and I’ve never been able to find those damn pillows. It’s frustrated me, but I had an additional impetus this time around: I ran into Jodie Hayob, the petrology professor from Mary Washington University, who was out there with her students for the day. First thing we said to one another? You guessed it: “Did you find the pillows?”

While the students ate their lunches, I went off downhill (to the west), exploring and looking for these confounded pillows. Pretty soon, I found something that looked vaguely pillowy, at least in terms of have a well-defined “crust” with a dark interior (click through that link for a fine Canadian pillow, courtesy of Ron Schott). Prepare yourself for a lot of photos today… Here’s what I saw:

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A few meters further downhill, I found another outcrop of the same stuff, this one veiled in a thin layer of algae (ahh, the joys of east coast geology!):

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Little double-ridges which varied in parallel, defining small chunks of rock. Could these be the fabled pillows? But they’re …so small! They’re almost pincushions! I know they say size doesn’t matter, but it’s hard for me to picture a volume of lava this small hitting water and “inflating” to such a puny volume with a nice quenched glassy rind, but then having the interior to stay hot enough to crystallize into basalt. Hmmm. Starting to think something’s fishy with this subaqueous tale…

I then found a nice big cliff, 10 meters high and 20 meters wide, which was made of almost nothing but these structures. Here’s some of them highlighted by the sun (the boundary ridges weather out in high relief), despite being obscured beneath several layers of lichen:

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A relatively clean, but relatively unweathered sample:

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Aha, now that’s better:

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The next two show more of a “classic” Catoctin coloring: chlorite green when fresh, with buff weathered surfaces on the outside:

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Zooming in on one small, skinny purported “pillow”:

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I climbed back up and coerced some students into joining me to check these weird things out, and they clambered down. Danny W. found a nice chunk of float which showed one of the “pillows” in three dimensions. Check it out at the top of this sample:

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Three-dimensional extension courtesy of Photoshop; red line shows the long axis of this oblate ~ellipsoid plunging towards the camera. (Lara laughs in the background…)

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Okay; two more… Check out how angular the boundaries of these “pillows” are:

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Seeing this one really made me think: No way; “those aren’t pillows!“…

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…Seeing that angular “break” on the left led me to realize that not only are these things too small* to be pillows, they also don’t have the right shape. Instead of being “pillowy,” (i.e., round) they are very angular, defined by edges that are aligned in a common direction and continue from one to the next.

* Where “too small” is defined as “smaller than anything Callan has seen before.”

I sketched in some of these planar edges:

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To me, it looks like what’s happening here is that original homogeneous rock of the Catoctin Formation fractured, and then fluids flowed along those fractures, altering the rock that the fluids came into direct contact with. This produced the “double ridge” of buff-colored rock (on either side of the fracture), with the less-altered greenstone interiors being beyond the reach of these altering fluids. The intersection of the various joints and their subsequent boundary-defining alteration would look something like this example (from the online structure photo collection of Ben van der Pluijm): definitely click through to check it out.

In other words, I interpret these structures to be secondary, not primary. The end result is something that looks a lot like “boxwork” (again, please click through to get a sense of what I’m suggesting here): a phenomenon that occurs when limestone fractures, more resistant mineral deposits are precipitated in those fractures, and then the limestone blocks are dissolved away, leaving behind the “fractures” as planar ridges separating little “boxes” from one another.

Here’s two photos of boxwork, one whole-sample, one zoomed-in. This sample is in the USGS library in Reston, Virginia, and both photos were taken at my request by Bill Burton of the Survey. (Thanks Bill!)
boxwork1

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At Little Stony Man, of course, the greenstone hasn’t “dissolved” away, but it does appear to be weathering more rapidly than the resistant buff-colored edges to these blocks, producing a distinctly boxwork-like effect.

Let’s look back at some of my field photos again, this time with the pillow boundaries highlighted in red…

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(…I definitely could have hit a few more boundaries on that last one; forgive me for being haphazard and slapdash…)

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This exercise convinced me that these things are not pillows, but some sort of fluid-rock interaction effect that took place on a complex fracture network. There’s no reason for the sharp edges of two adjacent pillows to be perfectly parallel and aligned.And it strains credulity to imagine ultra-tiny pillows in the first place (the size of my fingernail? Come on!).

I’ve e-mailed one of the authors of the original paper claiming pillows in this area with a link to my photos asking if these things are what he and his co-author were referring to, but I haven’t heard back anything. (I’ll update this post if he responds.) I might be totally off base here, but I can see how someone could make the claim that these were pillows. It’s just not a claim that convinces me, based on these outcrops.

What do you think? Do these look like any pillows you’ve ever seen?

__________________________________________

References:

M.L. Lukert and G. Mitra (1986). “Extrusional environments of part of the Catoctin Formation.” Trip #45 in Geological Society of America Centennial Field Guide – Southeastern Section, pp.207-208.

E.W. Spencer, C. Bowring, and J.D. Bell (1989). “Pillow lavas in the Catoctin Formation of Central Virginia.” in Contributions to Virginia geology, volume VI. Virginia Division of Mineral Resources publication 88, pp. 83-91.

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:

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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:

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Oblique view of the same outcrop:

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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:

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…and here’s a bonus tiger beetle:
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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 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:tt_3

Annotated:
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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:tt_4

Annotated:
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In spite of small folds and well-developed cleavage, I was shocked when someone on the field trip noticed this:tt_2

That’s two recumbent isoclinal folds! Annotated:
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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.

Transect debrief 1: starting in the basement

It is time to debrief the post-NE/SE-GSA field trip that I went on, affectionately dubbed the “Transect Trip” for the past 27 iPhone-uploaded “live”-geoblogged posts.

First off, I’d have to say that I enjoyed the live-field-blogging experiment overall, though I’ve got some critiques of the process and products. I think it’s amazing that I can upload photos and short blog posts from my iPhone to this site with a minimum of hassle. However, I can’t do much more than that. It’s not as easy to tag the posts or geotag the photos. I can’t compose annotations. In fact, I can’t even be sure the photos will be in focus, since the iPhone camera is a static lens. And there’s no macro feature on the iPhone camera, a source of some frustration for a guy like me that likes to photograph small things. Further, typing with my thumbs is laborious, keeping the live-geoblogged posts on the terse side.

So, when I asked what readers thought of the whole enterprise, I wasn’t surprised to get feedback that it would be nice to put things in a bit more context. I aim to start that process today, with the first rock we encountered, a charnockite (orthopyroxene-bearing granitoid). The rock type is named for Job Charnock, founder of Calcutta, India, whose tombstone is made of charnockite:

Charnockites are common rocks in the core of Virginia’s Blue Ridge “anticlinorium.” Here’s a nice photo of a fresh sample, showing the rusty/clayey weathering “rind” on the sample:

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Compare that image with this version, the original that I uploaded from the field trip via my iPhone:

Pretty profound difference in quality, eh?

So, here’s the deal with these charnockites. Volumetrically, they are a big part of the “basement complex” that cores the Blue Ridge. There are also a bunch of other flavors of granitoid down there; about fifteen discernible rock units in all. Our understanding of the basement complex has gotten a thorough re-working in recent years thanks to the coordinated efforts of many geologists who have focused on reexamining the Blue Ridge. Chief among these scientists in Scott Southworth of the USGS in Reston, who led an effort to remap the area in and around Shenandoah National Park. Dick Tollo (GWU), Bill Burton (USGS), Joe Smoot (USGS), Chuck Bailey (W&M), and John Aleinikoff (USGS) were part of the effort, too. The rocks were found to be more diverse than previously thought, and thus “complex.” Aleinikoff was responsible for a suite of new dates on the granitoids and their metamorphic successors in the basement complex. They have crystallization ages ranging from 1,183 Ma (±11 Ma) to 1,028 Ma (± 9 Ma): all Mesoproterozoic in age, and thought to be related to the Grenville Orogeny.

Some of these granitoids were deformed during Grenvillian mountain-building and attained a foliation which strikes northwest, in contrast to the later (Paleozoic) Appalachian foliation, which strikes northeast.

The plutonic rocks of the Blue Ridge province’s basement complex are the oldest rocks in Virginia, and they were the first ones we encountered on this field trip. All through that first day, we climbed upward through the stratigraphic column, meeting younger and younger rocks.

Rusty weathering rind

On a granite block

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