Güvem geoheritage site, Turkey

Looks like I’m late to the party…

While I was away, apparently the geoblogosphere went on a rampage of cooling columns. Everyone was posting images of their favorite columnar joints, and I was left out in the cold. Let me remedy that now. As it turns out, I was visiting some columns while everyone else was writing about them. Here are some images from the Güvem area of Turkey, north of Ankara, where there are a mix of late Miocene lake sediments and intercalated volcanic rocks, including these basalt flows. We stopped to visit them last Wednesday on our way to the North Anatolian Fault:

The dark entablature looms above:

A nice central panel with a good cross-section of the flow:

Around the corner, some more:

I ran across the street (and a stream) to check out a similar exposure there:

Zooming in:

Close-up of a few columns (with my hand for scale):

Looking up along the columns:

And a few more shots of the scene:

A full list of Turkish geoheritage sites may be found at the end of this document. Lockwood maintained a list of the other blog posts in this meme here, which I’ll quote below since it’s so nicely laid out already:

Geotripper, here, here and here,
Sam at Geology Blues
Phillip, also at Geology Blues
Silver Fox, and another columnar post here.
Glacial Till and another!
Life in Plane Light: Squashed columns!
Aaron at Got The Time
Geology Rocks
Dana at En Tequila Es Verdad
Cujo 359 (see comment on Dana’s post for description)
Wayne at Earthly Musings has a gorgeous photo of columns below the rapids at Lava Falls in Grand Canyon.
MB Griggs at The Rocks Know has photos of what may well be the most perfect columns in the world.
Jessica, AKA Tuff Cookie, showcases a variety in different rock types.
Hypocentre finds columns in a very unlikely place, as well as a spectacular photo of radiating columns.
Dave Tucker at Northwest Geology Field Trips displays precisely one slew of columnar displays in Washington State.
Dave Bressan at History of Geology shares the first printed image of columnar basalts, from 1565.
A couple more variations from Dana’s and my driving about W. Oregon.
Dr. Jerque has some spectacular examples from the bottom of the Grand Canyon.
Silver Fox Has another (better than mine) photo of horizontal columns in a set of dikes, and points out a couple more links to columny goodness (not to be confused with calumny, which is not good)
Dan McShane offers some more Washington State columns.
Garry Hayes, who deserves credit for starting this meme (see first links in the list, above), adds yet another set of photos from the opening of the Atlantic Ocean, and a lovely guest photo by Ivan Ivanyvienen, of columnar jointing in rhyolite at the San Juan Precordillera.
Update, October 4: Eric Klemetti- who did his doctoral work just down the street from where I’m sitting- has joined the fray. (Also, check out the links readers have left in the comments)
Helena Heliotrope at Liberty, Equality and Geology shows off some more Washington columns.
Chris and Anne at Highly Allochthonous each toss in a photo- Tokatee Falls looks awesome!
Some more Cape Perpetua jointed dike photos from Cujo359, and Devil’s Churn– again, numerous dikes with horizontal columns.

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:

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:

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:

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:

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:

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:

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).

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:

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?

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:

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:

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:

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:

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:

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:

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:

Annotated:

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:

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:

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

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:

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:

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:

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:

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.

Lessons from a broken bottle

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

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

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

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

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

Annotated copy:

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

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

Jointed Virgelle

One of the stops my Rockies students and I made this summer was a dinosaur paleontology tour through the Two Medicine Dinosaur Center in Bynum, Montana. The folks there are very accommodating, and at my request gave the class a bit of stratigraphic context for the dinosaur fossils. For instance, we visited the geologic formation which underlies the dinosaur-bearing Two Medicine Formation: it’s a beach sandstone called the Virgelle Formation. The Virgelle was deposited along the shore of the Western Interior Seaway, a Cretaceous-aged transgression of seawater onto the North American continent.

While our guide Corey discussed the primary structures that showed the unit to be “beachy” to my students, I got distracted by this outcrop:

Field notebook for scale (long side 18cm).

So what’s so great about this? It struck me as a nice little demonstration of the relationship between stress directions and joint orientations. σ₁ is our maximum principal stress direction (i.e., the direction of greatest stress), in this case caused by acceleration due to the force of gravity. σ₂ is perpendicular to the screen of your computer (and the plane of the photograph): that is the intermediate principal stress direction. σ₃ is our minimum principal stress direction (weakest stress), in this case pushing in from the sides (atmospheric pressure only, no overlying rock weight):

By definition, σ₁ is greater than σ₃.

So we have a low-level confining stress paired up with the differential stress imparted by the heavy rock pushing down on the slab of sandstone beneath it. As long as that difference in stresses is greater than the strength of this weakly lithified Virgelle sandstone, then the rock will break, and the orientation of those breaks will be ~parallel to σ₁, and ~perpendicular to the extension direction, σ₃:

You’ll also note that the bedding planes in the Virgelle sandstone are planes of weakness, accommodating the extension by allowing blocks of sandstone to slip sideways over what amount to small-scale “detachment faults” (low-angle, upper block sliding downward relative to lower block).

So does an understanding of these stress directions and the resulting structures’ orientation do us any good beyond this one lone slab of fractured sandstone?

Indeed it does. Keeping in mind that we are rotating our perspective from horizontal (“side view”) to vertical (“bird’s eye view”), consider the following map of central Asia:

As the Indian subcontinent impacts the Eurasian continent, it moves towards the northeast. This results not only in the northwest-southeast-trending Himalayan mountain front at the site of impact, but also in extensional faulting further into the heart of the continent. Down-dropped blocks of crust in desert areas show up as northeast-southwest-striking rift valleys, but in wetter areas, those low-lying cracks fill with water, and show up to us as linear lakes.

Lake Baikal in Russia is a famous example of this, but Mongolia’s Lake Hovsgol is a smaller version of the same thing. The lakes are oriented with their long axis ~parallel to the σ₁ direction, as they have been opened up due to stretching in the σ₃ direction.

Caveat blog-reader: The kinematics and dynamics of central Asia are actually a lot more complicated than this simplistic picture I’ve painted. My main point in drawing the parallel between the two examples is that outcrop-scale structures can serve as analogues that can help us understand regional-scale processes.

Tipping your tension gash

Tension gashes are small veins that open up when rocks get stretched. Often, they are arrayed en echelon with respect to other tension gashes, all oriented in the same direction. Here is a sample of tension gashes I found this summer in rip-rap (i.e., not in situ) at some building site in New England. (I forget where, but it doesn’t matter, since it’s rip-rap. Could have come from anywhere!) Check out the lovely veins of milky quartz:

We’ve seen this sort of thing before. So how does this form? It takes a series of steps. First, the rock gets sheared along some zone. Tension fractures open up oblique to that zone (as shown by the arrows here) and get filled it with mineral precipitations:

As shearing continues (with the same kinematics), these short mineral veins experience rotation (dextral, in this case) and perhaps some folding:

The more shearing you get, the more rotation and folding of the gashes:

You get the idea, right?

Here it is in summary:

I’m loving animated GIFs these days. So flippin’ cool, right?

Here’s the back side of the same sample, where you can see that a central fault has ruptured through the lovely tension gashes. It’s not as well-developed on the front side:

Poor things. It’s such a shame when ductile structures go brittle.

The Purgatory Conglomerate

After my thesis defense at the University of Maryland, my mentor and friend E-an Zen asked me if I had ever heard of the Purgatory Conglomerate. I had not. Over the years, E-an has been a great source of new ideas and information to me, and so when he raises a notion, I pay attention.

In my thesis, I had done some strain analysis on volcanic clasts in a meta-ignimbrite that had developed foliation and lineation in Mesozoic shear zone in California’s high Sierra, and that reminded E-an of a rock he had once seen which was screaming for similar treatment: the Purgatory Conglomerate.

On my summer travels this year, I finally had the opportunity to swing through Newport, Rhode Island, and check it out in person. To me as a structural geologist and Zen devotee, this was like nirvana. Check it out:

I was very excited to be there. Here’s me enthusiastically embracing a watermelon-sized clast:

Looking along the trend of the stretching lineation (which is pretty much non-plunging):

Most of the clasts are quartzites of various flavors… Depositionally, it’s a relatively mature conglomerate.

Here’s looking “down the barrel” of the stretched clasts in a big boulder sitting atop the outcrop:

Here’s a really big clast, with local beer cans for scale (not mine, I swear):

Here’s a really long clast:

Recall that my Swiss Army knife is 11 cm long, but even without the specific unit, you can see that this clast has an axial ratio (on the plane of the outcrop) of roughly 7:1.

Here’s another long one with an axial ratio of 7:1, with a bonus feature. It displays internal bedding (of the sandstone it was originally derived from):

This is totally awesome. These cobbles, boulders, and pebbles have flowed into elongated shapes! We can use the geometric term “prolate” to describe their cigar-like or hot-dog-like forms.

It’s not all conglomerate there. There are some meta-sandstone bodies too. Here’s an example of sandstone meeting conglomerate:

Even cooler: here’s bedding: lenses of conglomerate within sandstone:

Annotated copy of that same photo:

(I once showed you something similar from the Sierra Crest Shear Zone: check photo C of this archived post.)

So how did the Purgatory Conglomerate get so distinctively deformed? Close examination of the rock suggests the main mechanism was pressure solution:

In the photo above, look below the Swiss Army knife for a triangular clast, and trace out its boundaries. You will see that it impinges on the hot-dog-shaped clasts immediately next to it. This triangular grain is encroaching on its neighbors’ territory! Now, one way to interpret this is that the original clasts had shapes which, jigsaw-puzzle-like, were perfectly formed to accommodate their neighbors’ shapes. But that seems rather unlikely, especially when you consider the ten gazillion clasts in this outcrop, all perfectly locked together.

Instead, the idea is that high pressure points (the edge of one round cobble touching an adjacent round cobble, for instance) are sites where certain minerals will go into solution. Quartz is both a common mineral and a mineral which will dissolve under high pressure and re-precipitate under lower pressure. Calcite pulls the same trick — that’s where stylolites come from. [Many nice examples of stylolites and other pressure-solution features here.]

Here’s another nice example showing how the individual clasts lock together with one another, suggesting part of their outer edge has dissolved away:

Here, too. See if you can pick out a few examples of where one clasts impinges on its neighbor. A refresher course may be found here.

Time for a different perspective. Unlike most of the previous pictures, this one is taken looking along the long axis of the clasts.

Zooming in for a closer look at that same photograph, the yellow areas highlight areas where one grain impinges on a neighboring grain:

If there are particularly large clasts, they may shelter smaller neighbors in their “pressure shadow,” immediately adjacent to them. Think of a building collapsing during an earthquake, with a strong central pillar. If you stand next to that pillar, you’re less likely to have the ceiling collapse on your head, since the pillar is protecting you. With that in mind, examine this part of the outcrop:

Since the long axes of the clasts runs left-to-right, that suggests that they were squeezed top-to-bottom. Therefore, the area immediately to the left of the giant clast would be “protected” from the highest pressures by the bulk of its large neighbor. If we zoom in there…

…we see clasts whose long axes are not aligned with the rest of the outcrop. They are pointed in several other directions, and/or are not as prolate as the rest of the clast population.

Here’s an annotation of the zoomed-in view, showing the orientation of the long axis of the grains in this outcrop plane:

The implication is that these “protected grains” were less subject to pressure solution than the grains which weren’t lucky enough to have a giant neighbor immediately “next door” (along strike).

In addition, it seems that the strain (deformation/stretching) of the clasts was more severe in some locations, and less severe in other locations. Here, my hands bracket a zone of less deformed (more spheroidal, less prolate) clasts within the overall outcrop of strongly deformed clasts:

The Purgatory Conglomerate is preserved at a spot called Purgatory Chasm. Here’s a shot of the chasm itself, cutting through the conglomerate outcrop down to the Atlantic Ocean. I’d guesstimate that it’s 10 m deep or so:

Here’s the Chasm, further out where the rock is clean of vegetation:

For a nice perspective on the whole area, check out this Quicktime 360° view.

So why is the Chasm there? We may get some insights by taking a look at another feature seen at the outcrop: a joint set which runs ~perpendicular to the long axis of the clasts:

A wider view, showing the same orientation of joints cutting across the conglomerate, ~perpendicular to the stretching lineation. Bikini babes at upper left for scale:

Now let’s go back to the Chasm, and take a look into it. Windsurfing board (washed out in daylight beyond the cleft) for scale:

There are a whole lot of joint faces there, all (a) perpendicular to lineation, and (b) parallel to the Chasm. You can see them all as parallel lines to the left of the Chasm. The large concentration of fractures in the area of the Chasm suggests that the Chasm was eroded out along a zone of more pervasively fractured rock. As you stand there and peer in, waves will come in and slosh towards the back end of the Chasm. But why is it so fractured here? I’m not sure.

In other places, you can see fractures that have “healed” into quartz veins:

This is where some of that dissolved quartz ends up, sealing shut these cracks. But not all the quartz veins I saw were perpendicular to lineation; there were some that were ~parallel to it, as in this photograph:

As a totally gratuitous bonus, the Purgatory outcrop also features glacial striations. In both of the following two photographs, the striae run from upper right to lower left:

Another spot, showing the same thing:

Annotation showing which lines are bedding traces internal to the quartzite cobbles, and which are glacial striations:

I’ve  got a few more photos of my visit to Purgatory in this Flickr photo set.

So: Thanks, E-an, for another great idea! What a cool place; I can’t wait to bring students back here…

Falls of the James II: fractures

In my previous post, I introduced you to the Petersburg Granite, as it is exposed south of Belle Isle, at the falls of the James River in Richmond, Virginia. I mentioned that it was fractured, and I’d like to take a closer look at those fractures today.

The geologically-imparted fractures were exploited by human granite quarriers, and in some parts of the river bed, you can see the holes they drilled to break out big slabs of the rock. Some of these block-defining perforations exploited pre-existing fractures.

This is also evident on the north side of Belle Isle itself, where there are several large abandoned quarries now mainly utilized as a rockclimbing locale. There are two dominant fracture sets in the area: one which parallels the schlieren (magmatic fabric), striking NNE; and a second which strikes ENE.

The meaning of these fractures are one of the problems Chuck Bailey (my host at Belle Isle) and his students have been considering. Under Chuck’s tutelage, James McCulla examined these fractures and reported his findings at the NE/SE GSA section meeting in Baltimore last March.

One of the first things Chuck showed me when we got to Belle Isle is some offset schlieren, like these:

Let’s annotate those up, so you can orient yourself:

So clearly, that looks like a right-lateral offset, right? Of course, it could just be an apparent right-lateral offset, as perhaps the inclined schlieren have been offset in a vertical sense, then exposed by erosion on the same horizontal section. We need to determine the true offset direction. If we look at a vertical exposure of the fracture surface itself, will slickensides back that up? Here’s one…

Yep, the slicks are very gently plunging (close to horizontal) and agree with the right-lateral offset we thought we saw in the horizontal exposures in the earlier photos. These are in fact true right-lateral offsets. Chuck is currently dating some muscovite that appears on these surfaces as a method of constraining the timing of deformation.

The other fracture set (NNE-trending, parallel to the schlieren) shows very little in the way of telling fracture-surface anatomy. There may be some weakly-developed steps facing to the upper left, but these surfaces are neither gouged nor mineralized:

Chuck and James therefore interpret the NNE-trending fractures as extensional fractures and the ENE-trending fractures as faults with small offsets. It is worth noting that the NNE-trending extensional joint set is parallel to extensional faults in the Richmond Basin, a Triassic rift valley 15 km upstream.

So which came first? Here’s a confounding exposure:

Allow me to lighten that up and annotate it for you:

We have two different relationships exposed here, less than a foot apart. At left, we see the NNE-trending joints truncating against the ENE-trending “fault.” At right, we see that the NNE-trending fracture steps to the right as it crosses the ENE-trending fracture. The left example suggests that the ENE “fault” is older, and the NNE joint came later, propagating to the pre-existing discontinuity but no further. The right example suggests that the NNE-trending joint was there first, but was then broken and offset (ever so slightly) in a right-lateral fashion, like the offset schlieren in the photos earlier in this post. In other words, the ENE “fault” is younger.

“Geology isn’t rocket science.” We know what’s going on with rockets — we built those suckers! This, on the other hand, is a bit more complicated!

Anyhow, Chuck and James have been over these rocks like gravy on rice, and they have documented many other instances of cross-cutting relationships. As James’ GSA abstract notes, they found enough exposures to feel confident interpreting the ENE-oriented set to be the older set to have formed as a result of WNW-directed contraction during the Alleghanian Orogeny. The NNE-oriented extensional fractures are the younger set, and are interpreted to have formed during Mesozoic extension accompanying the breakup of Pangea.

Next up, we should take a quick look at the James River itself, and the imprint it has left on this stupendous field site… Stay tuned…

Falls of the James I: pluton emplacement

Last Friday, NOVA colleague Victor Zabielski and I traveled down to Richmond, Virginia, to meet up with Chuck Bailey of the College of William & Mary, and do a little field work on the rocks exposed by the James River.

Our destination was Belle Isle, a whaleback-shaped island where granite has been quarried for dimension stone for many years. The island has also served as a Confederate prison for captured Union soldiers during the U.S. Civil War, and later for various industries. Today, it is preserved as park land, utilized by a wide swath of Richmond’s populace for recreational activities, both licit and non.

Fortunately, a large area of the James’ river bed south of Belle Isle is kept relatively dry by a long low diversion dam upstream. As a result, there are some mighty fine horizontal outcrops of rock:

The dam fed water into a hydroelectric power generation station, but that station has been abandoned for some time now:

The power plant dam has yielded enough exposure that some bedrock mapping is possible for those with the curiosity and fortitude to attempt it. Here’s a simplified geologic map of the area, authored by Chuck and his student James McCulla:

So you can see that most of the area is covered by sedimentary deposits of both modern and early Cenozoic vintage. Our goal, however, was the more interesting stuff beneath that. (All due respect to my sedimentological colleagues; the Coastal Plain just doesn’t get my juices flowing like ‘crystalline’ rocks do!)

So here’s what we came to see, the Petersburg Granite:

This is an Alleghanian pluton, ~320 Ma, and quite large: it extends for tens of kilometers north and south (Petersburg, the namesake locality, is to the south). It disappears beneath the Coastal Plain to the east, and beneath the Richmond Basin (a Triassic rift valley) to the west.

You can see from the photo above that in some places the Petersburg Granite is massive and equigranular, and in other places it’s “foliated,” with long dark lines running through it. These lines are schlieren, curtainlike zones of differing mineral ratios: more mafics than felsics, for instance. The schlieren (German for “lines”) are usually interpreted as magmatic flow structures as higher-temperature-crystallizing mafic crystals raft together in a more felsic flow. At Belle Isle, the schlieren are steeply dipping and trend NNE.

In places, there were also pegmatite bodies that were concordant (~parallel) with this overall magmatic fabric. Here’s an example of that texture:

And here’s a really big crystal of K-feldspar set amid finer-grained granitic groundmass. I guess you could call this a “megacryst”:

Another thing we saw a lot of were dark-colored inclusions in the granite. These were dark due to lots and lots of biotite mica in them. Here’s an example; notice how the schlieren wrap around it:

And another, with its long axis oriented parallel to the strike of the schlieren, suggesting alignment in the magma chamber before the granite set up:

How should we interpret these mafic inclusions? Are they xenoliths; fragments of country rock that were broken off and included in the intruding granitic magma? Or do they represent a plutonic emplacement process — perhaps an earlier stage of crystallization, or an immiscible bolus of mafic magma floating like a lava lamp blob in the surrounding felsic melt? When they’re fine grained and lacking internal structures, as with the above examples, it’s really hard to make that call.

On the other hand, this one clearly shows fragmentation along the right edge, suggesting to me that it was a coherent xenolith at the time the enveloping granite set up into solid rock:

That rules out the fluid-blob-within-another-fluid hypothesis, but is it country rock?

This one suggests that it is indeed country rock, as it is both foliated and kinked internally:

Here’s a heart-shaped inclusion which also suggests that it is a genuine xenolith. As with the previous example, it displays internal foliation that has been folded:

Victor ponders these xenoliths, as well as a dense clot of biotite (dark steak next to the yellow field notebook – not Chuck’s shadow, but parallel to it and closer to the photographer’s vantage point):

The photo above also shows how the schlieren wrap around these xenoliths. Here’s an example where the schlieren “tails” leave the xenolith “higher up” on the left side than the right side, suggesting a sinistral (counterclockwise) sense of magma-flow kinematics:

This one is a beauty. It’s almost perfectly circular in cross-section, though with little flanges coming off the upper left and lower right. However, the “tails” are both on the same side of the xenolith, so I don’t really feel like I’ve got a good bead on its kinematics:

A few more shots of these xenoliths:

This one is a cool one…

… because when you zoom in on the edge, you can see it has some ptygmatic folding inside it. Like the foliation and the broader folding we observed earlier, this internal structure suggests that these are genuine xenoliths; fragments of pre-deformed country rock.

Another xenolith, also showing this internal deformation of ptygmatically-folded granite dikes:

…And this one shows internal boudinage:

Chuck examines a small vertical surface to get a sense of what these xenoliths are doing in the third dimension:

This next bit was a real treat for me. It’s no secret that I’m a huge fan of boudinage, that brittle-ductile phenomenon that separates a more competent rock type into sausage-like chunks while a less competent rock type flows into the void between those chunks. Here’s some schlieren that evidently became thick enough slabs of biotite that they were able to behave as semi-coherent sheets, subject to boudinage:

…Not only that, but if you back out and follow these boudinaged schlieren along strike, you can see that they are folded, too! Check out these sweet isoclinally folded, boudinaged schlieren:

Biotite-rich inclusions which I interpret as similar “scraps of schlieren” which became entrained in later magmatic flows:

While everything I’ve talked about so far has been concordant with the dominant schlieren orientation (and thus reflective of main-stage magmatic flow in the Petersburg Granite), there are also some discordant features, like dikes, which cut across the regional fabric.

Here, for example, is an aplite dike:

Aplite is very felsic and displays a “sugary” fine-grained texture. This aplite dike is quite a nice feature, traceable over a long distance across the outcrop. We followed it a ways to a spot that Chuck was particularly eager to show us: a spot where the aplite dike crosses an earlier pegmatite dike, and then both dikes are cut by a right-lateral fault and a fracture set which parallels the schlieren. Check it out in outcrop (note the positive relief on the aplite dike):

And here’s a sketch of this outcrop (above photograph from the perspective of the lower right corner):

What a fine spot to bring students and have them suss out the order of events! First came the massive granite, then the pegmatite dike, then the aplite dike, then sometime later under very different P/T conditions, the rock was fractured and we get fractures: some of which show an apparent right-lateral offset (faults; oriented ENE), and others where no offset is apparent (joints). This second set appears to be utilizing the schlieren as zones of weakness, as it is parallel to the schlieren (NNE) and often occurs along their biotite-rich traces.

Whether the faulting or the jointing came first is a question we’ll examine in the next episode…

“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:

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!):

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:

A relatively clean, but relatively unweathered sample:

Aha, now that’s better:

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

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

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:

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…)

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

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

…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:

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!)

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…

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

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.

Hackles, ribs, plumes

Today, you get a photo from GMU structure student Nik D. This is a small exposure in the Hampshire Formation (Devonian) on New Route 55 in West Virginia. It shows a fine example of plumose structure with the not-often-seen concentric ribs running perpendicular to the ‘plumes.’ At the edge of the joint, you can see the flaring fringe of hackles. Top edge of a Rite-In-The-Rain field notebook for scale:

Where shall we zoom in? How about these two boxes?

Close-up of the concentric ribs:

Close-up of the hackle fringe on the edge of the joint:

…Even the hackles have hackles!

Good stuff: I love me some fine plumose structure!