Friday fold: twice-folded turbidites at Black Pond

Today’s Friday fold comes to us courtesy of Gary Fleming, botanist extraordinaire and brother of Tony Fleming, geological Jack of All Trades. Together, the Fleming brothers led a field trip for the Geological Society of Washington. While I was on that field trip, the topic of polyphase deformation came up, which led a couple of weeks later to Gary sending me this photograph. He took this photo in the Black Pond area, on the Virginia side of the Potomac River near the property of Madeira School:

mathergorgefm_blackpond

That’s a set of twice-folded folds. The earlier generation of folds are quite tight enough that their limbs are parallel; we call this “isoclinal.” They display axial planes that run left-to-right across the photo. They are overprinted by a second generation of folds which are more open and broad. The second generation folds have axial planes which run top-to-bottom across the photo. Here’s an annotated copy showing the undulating form of the folds:

mathergorgefm_blackpond_anno

And here I’ve tacked on some color-coded axial plane traces: the first generation of folding (F1) is in yellow; the second generation (F2) is in blue:

mathergorgefm_blackpond_axes

The rocks in question are turbidites of the Mather Gorge Formation, folded up during the late-Ordovician episode of mountain building called the Taconian Orogeny. Relative to the orientation of this photograph, the F1 folds would have resulted from top-to-bottom compression, while the F2 folds would have resulted from a later episode of side-to-side compression.

It’s also worth noting the collection of small parasitic F2 folds in the schisty section at the top of the photo (greenish-gray, and partially obscured by mud).

Happy Friday! If your week has left you as contorted as these rocks, I hope you have a relaxing weekend…

Thanks to Gary Fleming for sharing this image and letting me publish it here.

Photos from Virginia Geological Field Conference

For the second year in a row, more exotic travel plans meant that I wasn’t able to attend the superb Virginia Geological Field Conference. I see that they have now posted some photos on the group’s Facebook page, so go check them out to see what we both missed last weekend. Here’s a taste:

Sheared meta-conglomerate:

Metamorphosed mantle (?) xenoliths:

Drilling: what, why, and how

As mentioned, I spent a significant part of last weekend was spent on a paleomagnetic sampling project with collaborators from the University of Michigan. On Friday, this was our field area:

drilling06

That’s the south slopes of Old Rag Mountain, a popular Blue Ridge hiking destination because unlike many Virginia peaks, when you get to the top, you see some rocks instead of 100% trees:

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But we didn’t come here for the view. We came here for the dikes. Here’s the edge of one, with a pen for scale:

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These are dikes of basalt and meta-basalt of the Catoctin Formation which are presumed to be feeder dikes pumping mafic lava to the surface of Virginia around 570 million years ago, during the breakup of the supercontinent Rodinia and the opening of the Iapetan Ocean basin. The dikes cut across the Grenville-aged basement rocks, in this case the Old Rag Granite of about 1000 million years age. The Old Rag area is especially great because the dikes are less metamorphosed than they are in other parts of the Blue Ridge province, where the Catoctin has been cooked into greenstone. Here’s an annotated view of the previous photograph:

drilling04anno

As far as this project goes, we are interested in these dikes for the information that they (potentially) contain about the orientation of the Earth’s magnetic field in Virginia at the time of the supercontinent Rodinia’s breakup. By sampling these dikes and then analyzing the samples at their paleomagnetism lab back in Ann Arbor, Fatim and Matt hope to put some constraints on the question of paleo-Virginia’s latitude when these dikes cooled into solid rock.

As a reminder, you are not allowed to sample any rocks in any national park unless you have first applied for and been granted a research sampling permit by the National Park Service.

Close to the planet’s surface, the Earth’s magnetic field is shaped like a torus (or, in less technical terms, a doughnut, but one of those donuts with a pinched up midsection, and more of a dimple than a hole). It exits at the south magnetic pole, wraps north around the Earth, and plunges back into the inner core at the north magnetic pole:

magfield_normal

A magnetically-sensitive mineral forming in a modern rock would have an upward-oriented high-angle magnetism if it formed at high southerly latitudes, a moderate-angled upward orientation at moderate southerly latitudes, a horizontal, northward-pointing orientation at the magnetic equator, and then the reverse as you head towards the north pole: a moderate-angled downward orientation at moderate northerly latitudes, and a downward-oriented high-angle orientation if it formed at high northerly latitudes, just like the red arrows show in the above image.

Of course, the flow of the magnetic field occasionally reverses direction (emerging at the north magnetic pole instead, and flowing south), but the shape of the field doesn’t change:

magfield_reversed

So the angle of inclination of a fossil magnet should be the same regardless of whether it’s poking up or plunging down, relative to the surface of the Earth. In this way, paleomagnetism can reveal the approximate latitude (but not longitude) at which a rock formed.

But wait, is it really so simple? No, of course not. Check out the map below, showing the positions of the north geomagnetic pole over the past 2000 years, with numbers showing the position of the pole in a specific year CE. It moves! The circles around geomagnetic poles at 900, 1300, and 1700 CE are 95% confidence limits on those geomagnetic poles; the mean geomagnetic pole position over the past 2000 yr is shown by the square with stippled region of 95% confidence. These data were compiled by Merrill and McElhinny (1983) and plotted by Butler (1982).

secvar

So this map shows us that even though the magnetic pole does wander about a bit, 2000 years of data is enough to generate an average which is more or less coincident with the geographic pole. And therefore a statistically significant batch of data (spread over a 2000-year-or-greater spread of time) will also reflect that average pole position.

Meert, Van der Voo, and Payne (1994) made a first attempt at constraining the paleomagnetics of the Catoctin Formation. Four of their 32 sites were feeder dikes, sills, and host rock (Grenvillian basement complex). One of the things these authors did was that they performed a “contact test” on two of their dikes. A contact test is a way of using an igneous contact (as with a dike) to determine whether the whole region has been magnetically reset, perhaps by thermal activity accompanying contact metamorphism. Consider this situation:

contacttest1

You sample a dike and its surrounding host rock, at several distances away from the dike. You find that they all give you the same magnetic orientation. This suggests you have the magnetic signature of a later overprinting, not the original orientations of dike and host rock.

Now what if you found this, instead?

contacttest2

Here, your dike shows a distinct signature that is different from the host rock, and the host rock shows a uniform orientation except right next to the dike, where the heat of the intrusion has partially reset the (older) host rock’s magnetism. If I were to annotate this up (with color coding!), it would look something like this:

contacttest3

Passing the contact test is critical to tying the two rocks’ magnetic data to their age data. It’s only with a positive contact test that you can use this data to say anything about where Virginia (and thus ancestral North America, often dubbed “Laurentia”) was when the Catoctin dikes were intruded.

The contact test is something that our team wanted to repeat, with more dikes than just the two that were featured in the Meert, et al. (1994) paper. We also wanted to double-check their results, and verify, reject, or modify them as our data warranted.

The key to constraining the magnetic orientation of these rocks as precisely as possible is to constrain the current orientation of the samples as precisely as possible. We measured the strike and dip of the surface of each sample very carefully, before we extracted it from the bedrock. At Old Rag Mountain, we were not allowed to drill (Old Rag is a wilderness area with no motorized equipment allowed), so we were collecting oriented hand samples.

Here’s Fatim Hankard writing orientation data in her field notebook while Matt Domeier takes a strike and dip reading in the background, using his Brunton compass:

drilling02

Because these rocks are inherently magnetic (that’s why we’re sampling them, after all!), we have to control for the possibility that the rocks themselves might be throwing off our Brunton compass needles. A second compass is employed to control for any magnetic field coming off the rocks themselves. This is a solar compass. If you know exactly where you are (note Fatim’s GPS unit in the above photo), and when you are taking the measurement, you can use this solar compass to double-check the orientation you get from the Brunton compass.

Here’s Matt’s solar compass butted up against one of our Old Rag samples. Note the shadow being cast by the compass’s nomen, and also note the “arrow with a prong” strike and dip symbol that we wrote directly onto the face of the sample with a Sharpie:

drilling01

Next, take a look at a photo of a sample once extracted. We label it redundantly, not only in terms of the orientation lines, but also in terms of the sample’s identity. That way, we’re less like to find a bunch of scratched-up but un-identified and un-orientable rock samples once the van gets back to Michigan:

drilling03

While poking around, I found this interesting feature at the edge of one of the dikes. I’m hoping one of my more petrologically-inclined readers may be able to offer me some kind of interpretation of this pattern:

drilling05

What I noticed is that in the first few mm of the dike, right up against the contact with the host rock, there are no white lathes of plagioclase feldspar. These relatively large feldspar crystals are phenocrysts, big chunky crystals that grow in the magma when it’s cooling relatively slowly underground, but then entrained in the flow as it moves upwards into the dikes, whereupon the surrounding liquid chills rapidly to make fine-grained basalt. So there are no phenocrysts right at the edge of the dike, then there are a bunch, all aligned with one another (but with no preferred sense of imbrication, so far as I can tell), and then there are more phenocrysts in the bulk of the dike, but they are (a) less concentrated, and (b) lack any preferred orientation. Let me annotate it for you, then go back and take another look at the unannotated version, so you can see what I’m referring to:

drilling05anno

Okay, petrologists, I want to hear from you: How should I interpret this?

Back to the paleomag… On Saturday, we went to another location to sample. This one was much more convenient because (a) it was right on the side of the road, and (b) it wasn’t a wilderness area, so drilling was allowed. This was at the lovely selection of Catoctin dikes downhill (north) from the Little Devils Staircase overlook, on Skyline Drive in Shenandoah National Park. Here’s a charismatic dike with Matt acting as a sense of scale:

drilling08

Annotated:

drilling08ANNO

We unpacked the gasoline-powered diamond-grit-tipped drills and hooked them up to the water pump. We put on ear- and eye-protection, and got to work:

drilling09

One the sample has been drilled out, you’re left with an empty hole. The white liquid is the cooling water with suspended dust from the abraded rock. This hole is about 3 cm in diameter:

drilling11

The core (2.5 cm diameter) that came out of that hole:

drilling10

In our field area, a core this size of the dike rock takes about ten minutes to extract. Basement rock (host rock) takes longer, as it’s made of harder minerals.

One worry is that the core will snap loose while you are drilling it out. If this happens, it may start rotating in the hole, and you will lose all sense of how it was originally oriented, which means you’ve just wasted a lot of time for no gain in data. To protect against this possibility, we used a technique of scoring a second circle with the drill bit, partially overlapping our actual core like a Venn diagram:

drilling12

This way, if the core snaps off, you can line up its arc with the rest of the circle inscribed on the outcrop next to the hole. Whew! Core saved!

Fatim extracting another core:

drilling13

After the core is drilled out (but still in its hole), Fatim oriented it. Notice the new array here – it’s a stand with slots into the drill-hole, then has a Brunton compass atop it with a solar compass atop that:

drilling14

As you can see with this example, the solar compass is just about to become useless as the afternoon shadows advance! Next up, record all the orientation information (trend and plunge of the cylinder’s axis), and then score the core with a line:

drilling15

Fatim and Matt sampled for two more days after I had to leave them due to other obligations, like teaching. They are headed back to Michigan today. Soon, hopefully, we’ll see whether our sampling campaign yields any meaningful results… Stay tuned!

As a final note, I would like to point out that this collaboration was born when Fatim read my blog post on feeder dikes and then proposed that we combine her paleomag skillz with my dike-location knowledge. It’s not the first time that my blogging has yielded a great opportunity, but it seems to be a shining example of how virtual connections online can lead to tangible work in the real world. The blog-curious should take note.

_______________________________________

References cited:

R.F. Butler. PALEOMAGNETISM: Magnetic Domains to Geologic Terranes. Originally published by Blackwell in 1984, 248 pp. Updated online 2004. Retrieved September 15, 2010, from http://www.pmc.ucsc.edu/~njarboe/pmagresource/ButlerPaleomagnetismBook.pdf.

J. G. Meert, R. Van der Voo, and T.W. Payne. “Paleomagnetism of the Catoctin volcanic province: A new Vendian-Cambrian apparent polar wander path for North America,” March 10, 1994. Journal of Geophysical Research 99, No. B3, pp. 4625-4641.

R. T. Merrill and M. W. McElhinny, The Earth’s Magnetic Field, Academic Press, London, 401 pp., 1983.

Aeolian sand in Hampton, VA

This video was produced by my friend Pete Berquist. It shows rapidly moving “sheets” of sand saltating down Grandview Beach in Hampton, Virginia, during high winds associated with Hurricane Earl.

What do you notice here? A couple of things jump out at me, but I’d be curious to hear what this video makes Mountain Beltway readers think about…

Geology of Massanutten Mountain, Virginia

Here’s a new video from Greg Willis, the same guy who brought us a fine video on Piedmont geology. In this new opus (20 minutes), Greg details the geology of the Massanutten Synclinorium (Shenandoah Valley, Massanutten Mountain, and Fort Valley) in western Virginia. WordPress isn’t letting me embed it here, but you should go and check it out!

At the edge of the intrusion

Mountain Beltway reader Greg Willis attended my colleague Ken Rasmussen’s Triassic Rift Valley field course last weekend, and sent me this photo of the view inside the Luck Stone diabase quarry in Centreville, Virginia:

Here’s an annotated version:

Both photos are enlargeable by clicking on them (twice).

This quarry chews into rock right along the contact between a mafic igneous intrusion and lake sediments that formed when water pooled in a low-lying continental basin that formed during the breakup of Pangea. This rift valley, the Culpeper Basin, is just one prominent basin in a whole series of Triassic grabens and half-grabens that run through the Piedmont north and south of here, including all the way to the Bay of Fundy.

A similar environment can be seen today in east Africa, where a modern rift valley hosts similar lake deposits and mafic igneous rocks:

If you were to drop maybe half a kilometer below the surface of the Afar region, you’d see a similar situation to the one that produced Greg’s quarry photo ~200 million years ago.

Visiting the Centreville quarry is by permission of the Luck Stone corporation only; the best way to see it is by signing up for Ken’s course the next time it rolls around!

A day in the field

I spent last Thursday on a long field trip in the Valley and Ridge province of northernwestern Virginia. Leading the trip was Dan Doctor of the USGS-Reston. Accompanying Dan was a UVA environmental science student named Nathan. And the NOVA crew rounded it out: professor Ken Rasmussen from the Annandale campus, associate professor Victor Zabielski from the Alexandria campus, and me. We met at the Survey at 9am, and headed west towards Strasburg, site of my Massanutten field trip.

We started off by examining three Ordovician carbonate units (all above the Knox Unconformity) on the I-81 exit ramp at Route 11. This is the same sequence seen at the classic Tumbling Run outcrop: the New Market limestone, the Lincolnshire limestone, and the overlying Edinburg Formation. We looked at fossils, stratigraphy, some minor structures, and some interesting lithified gunk on the inside of some solution cavities (small caves). Dan interpreted it as collapse breccia: lithified sediment from inside the cave. The question was: when did it form? We wrestled with the best way to test its age, and didn’t come to any clear conclusions. I love moments like that one: out in the field, one geologist shows another something that’s caught his or her attention, and the other geologist reacts, and the two toy with the idea, batting it around like a cat with an unknown object. Like the cat, geologists will either then get really excited and attack the new idea, or get bored, shrug, and walk away.

Our next stop was Crystal Caverns, a commercial cave that is in ownership limbo. Our spirited guide Babs said that it was likely the last time she would lead a tour down in the cave. She was busy liquidating the artifacts of the adjacent Stonewall Jackson Museum, which had recently been shut down by its board of directors. The cave is accessed via a small building that has been built over its mouth. It was a cool cave with a significant 3D aspect: we descended in a corkscrew like fashion, then came back up via a different route. Very cool. A shame that it is being closed (at least temporarily) to the public.

We followed the cave with lunch at a local Mexican restaurant, and while we were there, a big thunderstorm rolled through. Victor, Dan, and I played dueling iPhones to get imagery of the weather front and plot out our plan for the rest of the afternoon.

The afternoon was spent visiting outcrops on the west side of the Great Valley, working our way up to Route 50, and then west to Gore, VA. I wasn’t especially fastidious about photographing everything we saw, but here’s a sample of where I opened the camera shutter…

Ooids in the Conococheague Formation:

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Same shot, zoomed in to the middle:

dd_3

Fossil (blastoid? crinoid?) stem, Needmore Formation:

dd_8

There were some lovely Opuntia cactus blooming among the vetch at this Needmore outcrop:

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dd_5

dd_4

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From there, we checked out the Chaneysville Member of the Mahantango Formation, where we saw some snail fossils…

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…and some spiriferid brachiopod fossils:

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Our last stop of the day was at the Clearville Member of the Mahantango Formation, which had lots of lovely coral fossils in it:

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dd_14

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dd_15

dd_1

Dan put together a Google Map of our 17 stops; if you’re interested in checking out some of these places yourself, then this is a great resource.

I’d like to publicly thank Dan for taking a work day to contribute to our understanding. It was a lot of fun!

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.

Straight nautiloid fossil

It seems I forgot to show this fossil when I found it in February of last year with my MSSE advisor John Graves. We were out in the Needmore Formation of the Fort Valley then. The Needmore is a formation I visited again yesterday with some colleagues in other outcrops further to the west.

In shade (penny for scale):

IMG_0164

In sideways-angled sunlight (thumb for scale):

IMG_0173

I’ll debrief yesterday’s field trip when I get some more time… for now, let me just toss this little fellow out there for your enjoyment.

If anyone can I.D. it based on these two images, please leave your assessment in the comments section.

Falls of the James III: river work

In today’s post, I’ll finish up with my geologic discussion of the falls of the James River in Richmond Virginia, south of Belle Isle. Previously, we’ve examined the bedrock at this location (the Petersburg Granite) and a series of fractures – some faults and some extensional joints – that deform that granite. Now we come to the final chapter in this story — the story of the river carving up these rocks as it incises downward along the Fall Zone.

Unlike my native Potomac River, there is no gorge carved along the James at the boundary between the metamorphic & igneous rocks of the Piedmont and the overlying Coastal Plain strata to the east.

But there is still some cool stuff to see. In my first post on Belle Isle, I mentioned the diversion dam that keeps the river-bottom bedrock (mostly) dry and available to geological scrutiny. That dam diverted some of the James River into a mill race which led to a hydroelectric power plant that was abandoned a half-century ago. Here’s a map showing the dam, mill race, and some other key features:

james_GE1

You’ll note that’s a Google Earth image that I’ve rotated 90° clockwise to fit it into this vertical-friendly blog space. North is to the right. I’ve highlighted the trends of the NNE- and ENE-oriented fracture sets that I discussed here yesterday, as well as the quarry pond on the north side of Belle Isle. You’ll also note a Δ-shaped logjam at the intake for the mill race. The mill race itself is choked with mud (the bigger debris is strained out at the inlet; but the mud makes it through). There are some mudcracks visible there:

fallsjames_river_01

Chuck is trying to talk a colleague into drilling a sediment core through this deposit — easily two meters thick. It might potentially provide an interesting sedimentological (and geochemical) account of the last 50 years.

Let’s zoom in further to the ~dry area of the river bed south of Belle Isle; the part the dam makes accessible to sunbathers, dope-smokers, fisherman, graffiti-artists, and geologists:

james_GE2

Again, north is to the right. You’ll note the obvious trend of the two dominant fracture sets, as well as a large number of elliptical ‘dots.’ These dots are potholes, semi-cylindrical holes that get drilled into the bedrock when water currents maintain vortices (plural of vortex: think a liquid tornado) in one place over an extended period of time. Some of these potholes line up in rows — like perforations at the top of a checkbook, these aligned potholes create planes of weakness that make it easier to pop out large slabs of rock in a quarrying process not unlike what humans do with their ‘plug & feather’ techniques.

In the close-up Google Earth image, at the lower left (southeast), you can see that I’ve placed a pair of black arrows pointing to a train of four potholes. One has a big boulder in it, and then there are three others with an elongated axis in the NNE-direction. We visited this particular chain of holes, and saw something interesting.

Here’s Chuck standing in the second pothole, with the boulder-containing pothole in the foreground:

fallsjames_river_02

You can just barely make out the two more northerly potholes in the far distance, but here’s a second shot showing them from the vantage of the northerly tip of the second pothole. The backpack and power plant should provide orienting landmarks:

fallsjames_river_03

Now take a look at that first photo again — take particular note of the magmatic schlieren in the bedrock. Recall that schlieren are curtain-like zones of more mafic minerals in the granite. You can see that the schlieren wrap around these potholes. Here, I’ll trace them out (crudely) for you:

fallsjames_river_02b

Potholes are recent geomorphological features imparted by the river. The schlieren formed ~320 million years ago — how could the older structure wrap around the younger carving? Chuck interprets this to mean that the potholes were etched out from some weaker/less stable rock type — perhaps some of the mafic-composition xenoliths that pepper the Petersburg Granite in this area. Certainly, we can see that the xenoliths often appear in long trains, strung out along the plane of magmatic foliation, and the schlieren wrap around those. If the river exploits the outcropping xenoliths as areas where it’s easier to drill, the ancient positioning of the xenoliths could lead to the modern positioning of the potholes. I’ve seen something very similar on the Billy Goat Trail (Potomac River, downstream of Great Falls), so it wasn’t too difficult for me to buy into this interpretation.

Have any of my readers seeing compositional variations (like xenoliths) controlling river geomorphology elsewhere? Do tell!

Finally, thanks again to Chuck for taking the time to show us around last Friday. Belle Isle is a cool place on many levels, and I’m glad I got the chance to check it out in person.

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