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:

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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.

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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.

Scenes from a drill campaign

The past couple of days, I’ve been in the field, collecting samples with Dr. Fatim Hankard, a post-doctoral researcher from the University of Michigan, and Matt Domeier, a PhD candidate from that same fine school. We’re interested in using Virginia’s wealth of Catoctin formation feeder dikes to do paleomagnetism measurements that might help us constrain the latitude of Virginia during the emplacement of these dikes during the Neoproterozoic.

More later on the drilling technique and goals, but here’s a small batch of funny photos from Robin R., one of three Honors students who joined the researchers yesterday for drilling of Catoctin dikes along Skyline Drive in Shenandoah National Park*. The other two students were Elysia H. and Aaron Barth, former NOVA Honors student and now a George Mason University geology major. Thanks for the photos, Robin!

satansdriller

So here I am as a bad-ass driller. The reason I was feeling so aggressive was I was drilling out a beautiful core, when suddenly the rock face I was drilling in detached and the chunk of rock stuck to the drill, spinning around in the air. We all had a good laugh at that. It’s testament to what a nice core this would have been that you can see water burbling through the sample and dribbling down into the air behind it. Here, I’ll outline the sample (hard to see the dark rock against the dark background) and the water for you:

satansdriller_anno

Another funny moment occurred when we fired up the drill while the bit was still lying in the tall grass. Instantly, it would up a nice mantle of grass into a tube, like a fork twirled in spaghetti:

spaghetti

Lastly, I’d like to demonstrate how far I have advanced in my own arachnophobia by showing how close I got my finger to this fat orb weaver spider that was crawling over the basement complex adjacent to one of the dikes:

spider

…Okay, I’ll admit it: at one point, the spider changed direction, and brushed up against my finger, and I shrieked like a little girl. This prompted another round of laughs at my expense.

Great times, hopefully to yield great data… Stay tuned.

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* Yes, we had a permit to collect in the park. It is illegal to remove rocks or other natural resources from national parks without explicit written permission from the National Park Service.

“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:

not_pillow_01

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!):

not_pillow_02

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:

not_pillow_07

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

not_pillow_08

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:

not_pillow_09

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…)

not_pillow_09_anno

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

not_pillow_11

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

not_pillow_10

…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:

not_pillow_10_anno

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

boxwork2

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…

not_pillow_11
not_pillow_11_anno

not_pillow_01
not_pillow_01_anno

not_pillow_03
not_pillow_03_anno

not_pillow_05
not_pillow_05_anno

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

not_pillow_06
not_pillow_06_anno

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?

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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.

3,2,1, Contact!

On my structure field trip just over a week ago, we found the contact between the Mesoproterozoic-aged Blue Ridge basement complex and the overlying Neoproterozoic Catoctin flood basalts (now metamorphosed to greenstone). This nonconformity can be found just west of the Appalachian Trail at the Little Stony Man parking area in Shenandoah National Park. Here’s four photos, with my left index finger for scale, in raw and annotated versions:

unconf_01

unconf_01_anno

It’s not as glaringly obvious as some other unconformities profiled here, but it’s an important horizon in understanding the geologic history of the mid-Atlantic region.

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In places, small inclusions of the basement complex may be found inside the base of the Catoctin Formation, a nice example of the principle of relative dating by inclusions. The basement rock must be older than the Catoctin if pieces of the basement have been broken off and enveloped in the Catoctin:

unconf_03

unconf_03_anno

You’ll notice that the Swift Run Formation isn’t present at this location, though stratigraphically, it belongs between the basement and the Catoctin. The Swift Run is patchy and discontinuous, probably reflecting low-lying areas on the paleo-landscape, which paleo-hills poked up above the sediment-laden paleo-valleys, and were last to be smothered beneath the advancing flood basalts.

unconf_04

unconf_04_anno

It’s a great pleasure to be able to find and “put your finger on” such a significant surface, such a gap in the geologic record. Given that the basement complex formed during the Grenvillian Orogeny (1.1-1.0 Ga), and the Catoctin erupted sometime before 565 Ma, there’s probably more than 400 million years of time that passed between the formation of the rock below my finger and the rock above it. Unconformity surfaces like this are geologic contacts which are emblematic of time passing, but going unrecorded in the geologic record. They are high-contrast reminders of how incomplete the geologic record is at any single location on the planet. They remind us to be humble in our interpretations. They remind us to strive for a multi-referenced correlation between different locations’ outcrops in order to get closer to the full story of our planet’s checkered past.

Transect debrief 3: Rodinian rifting

The Grenville Orogeny, responsible for Virginia’s basement complex, was one mountain-building event among many that helped put together a Mesoproterozoic supercontinent called Rodinia. But Rodinia didn’t last: it broke apart during the Neoproterozoic to form the Iapetus Ocean basin. This rifting event is recorded in Virginia’s Blue Ridge province in the Swift Run Formation and the Catoctin lava flows.

It’s probably about time to start putting some of these rock units in stratigraphic context. Here’s my redrawing (and updating) of a cartoon Shenandoah National Park stratigraphic column based on an original by Tom Gathright (1976):
shenandoah_column

You’ll notice here that the Swift Run Formation is interbedded with the Catoctin Formation, a Neoproterozoic series of lava flows fed by fissure eruptions (kind of like what’s happening this week in Iceland).

Trickling downhill away from these fissure eruptions would have been flows of basaltic lava (tholeitic, indicating a mantle source chemistry). If you want a warmer modern analogue than Iceland, look to the Afar Triangle region of Ethiopia:

As with Neoproterozoic Virginia, the continental crust of modern Ethiopia is stretching, opening up topographic grabens which are being filled with clastic influx from the surrounding highlands and mafic lava which is formed from decompression melting in the underlying mantle, and funneled to the surface via feeder dikes. In places you will see streambed conglomerates interlayered with the mafic lava flows, and in places there are tuffs and rhyolites that are a (volumetrically-small) part of the package. Elsewhere there are lake sediments. The two bear a common geologic signature, despite being separated by thousands of miles and half a billion years. There’s that refrain again: Same as it ever was, same as it ever was.

Once on the surface, the lava cooled, and in some places, columnar jointing formed:

The cooling age on some of the rhyolitic upper units in the Catoctin Formation is 570-565 Ma (Rb/Sr on pyroxene by Badger and Sinha, 1988). Some mafic and felsic dikes (could be feeders) associated with the unit yield the same age via U/Pb (Aleinikoff and others, 1995).

At some point, ancestral North America (“Laurentia”) drifted away from the spreading center, and volcanism ceased. The crust cooled, subsided, and then a sequence of sedimentary rocks began to accumulate atop the cooled lava flows. This transgressive sequence of sediments (the Chilhowee Group) is the next thing up in the stack. More on that later.

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:
tt_3_anno

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:
tt_4_anno

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:
tt_2_anno

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.

When the Sturtian happened

ResearchBlogging.orgLast Friday, I spent the evening riding up to New York on a bus. To pass the time, I had my iPod and a new paper by Francis Macdonald and colleagues in Science. The paper examines the timing of one of the episodes of “Snowball Earth” glaciation. There’s some important new data in this paper, and it helps constrain the “Sturtian” glaciation in time.

So here’s the deal with Precambrian glaciations: there have been several. Generally speaking, there was a big episode of glaciation around 2.5 Ga (“Ga” = billion years ago, for those new to geo-temporal argot, and “Ma” = million years ago). There were also a series of at least two, and maybe upwards of four episodes during the Neoproterozoic era (~700 Ma). These latter glaciations have been collectively dubbed the Snowball Earth glaciations for evidence which suggests that they were global in extent. The evidence was high-precision paleomagnetic signatures which suggest some of the glacial sediments were deposited within a few degrees of the equator. If the equator was frozen over, it follows that the rest of the planet was too, due to ice-albedo feedback. That’s kind of a big deal, and the Snowball Earth hypothesis has been a rich source of research inspiration over the past decade and a half.

Now, figuring out just when the Snowball Earth glaciers flowed is a bit tricky. You can’t directly date glacial sediments using radiogenic isotopes, as they will be composed of the pulverized remains of pre-existing rock bodies, and will yield older-than-actual ages. It would be cool to find volcanic layers within the sedimentary package, because we can date those, or to find igneous intrusives (like dikes) which cut across the glaciogenic sediments, because those too are worthy of dating. The younger of the two “main” Neoproterozoic glaciations is called the Marinoan glaciation, and it has been dated using methods like these in Namibia (635.5 ± 0.6 Ma) and China (between 636 ±4.9 Ma and 635.2 ± 0.2 Ma). Locations as farflung as China and Namibia and other Canada can be correlated with one another on the basis of stable isotope chemostratigraphy. Basically, the idea is that there are global fluctuations in the carbon (or sulfur, or oxygen, or whatever) isotope “signature” that gets locked in the sediments, due to whatever was happening in the world at that time (e.g., life gobbling up certain isotopes, or climatic shifts, or other “big picture” events). So the chemostratigraphy allows us to match up rock units of the same age, and the few places where we are lucky enough to get igneous units interacting with the sedimentary package allow us to pin the whole lot to a specific date.

Great… for the Marinoan.

But an earlier “Snowball” episode, the Sturtian glaciation, has not been as precisely dated. Enter the Macdonald, et al. (2010) study. They report four new high-precision U/Pb dates from igneous rocks in the Ogilvie Mountains of northwestern Canada. Three of these are part of the Sturtian stratigraphic package, following the paradigm I outlined above. One, from a tuff unit, yielded a date of 717.43 ± 0.14 Ma, and another yielded a date of 716.47 ± 0.24 Ma: both of these were essentially right at the bottom of the Upper Mount Harper Group, which includes strata that are interpreted as belonging to the Sturtian glaciation on the basis of dropstones (A) and striated clasts (C) like these (from the supporting figure S2 for the paper):
fieldphotos

They also found evidence of “grounded ice”: soft-sediment folds that resulted when (they interpret) the nose of the glacier shoved its way forward. So this wasn’t just a floating glacier above: the glacier was in the muck, suggesting it was right there at sea level.

This is a lucky find: a datable volcanic ash layer right at the base of a big stack of glacial sediments. It’s a major advance for understanding the Sturtian in its own right.

They also report a date of 811.51 ± 0.25 Ma for strata deeper down in the stack, right before a global isotopic ‘excursion’ (a big, distinctive leftward squiggle on the carbon chemostratigraphy plot) called the Bitter Springs isotopic stage. Here’s a detail from the paper’s Figure 2, showing how this new date integrates absolute time with the relative time illustrated by the isotopic curve:
curve

That’s δ13C data plotted from three Neoproterozoic sections (in Namibia, Svalbard, and the Yukon). The thick central vertical black line is 0‰, with the left bound being -8‰ and the right bound being +8‰. The horizontal green lines show the new dates from this paper.

So all that is good, and a significant new batch of data for helping pin down the timing of these ancient glacial episodes. We’ve been able to date some Sturtian glacial units and a pre-Sturtian isotopic excursion.

The paper presents a fourth date, too: this is from a diabase sill that is part of the Franklin Large Igneous Province (LIP) exposed on Victoria Island, over 1000 km to the northeast of the Ogilvie Mountains (where the other three dates come from). The Franklin diabase gives a U/Pb age just like those from the Sturtian glacial sediments: 716.33 ± 0.54 Ma. But is this relevant, considering how different the rocks are, and how very far apart they are? Check out this map to see their lack of proximity, from the paper’s supporting figure S1:
map_ogilivie

Why would the paper’s authors bother with a rock unit so far away from the Ogilivie section? Well, the Franklin LIP is integral to the Snowball story on at least three fronts that I can think of. It ties this story together quite nicely, and I think that it is just as important as the Ogilvie data.

First, on a tectonic note, it’s a mafic unit that is associated with the breakup of Rodinia, a Proterozoic supercontinent. (Rodinia’s position on the paleo-equator is supposed to have sped up weathering of the continental crust and resulting CO2 drawdown, cooling the planet.) Second, it has paleomagnetic orientations which suggest it was emplaced within 10° of the magnetic equator. (This is important because it demonstrates that grounded ice was present within 10° of the equator at the time the Franklin LIP erupted… and due to ice-albedo feedback, it implies higher latitudes were frozen-over at that time, too.) Third, the Franklin LIP has been fingered as a possible culprit in causing Snowball Earth. This is because mafic igneous rocks suck CO2 out of the atmosphere when they are chemically weathered, producing carbonate rocks. The Franklin LIP has the potential to be a major driving force for the CO2 drawdown which initiated the Sturtian Snowball via global cooling. A big package of mafic rock delivered raw right to the tropical weathering belt could be sufficient to trigger an ice age, some workers have suggested. The Franklin LIP was in the right place at the right time: was it the culprit, or only an accomplice? Witness the way that the authors (properly) hedge their bet in their conclusion’s penultimate sentence:

…the synchrony among continental extension, the Franklin LIP, and the Sturtian glaciation is consistent with the hypothesis that the drawdown of CO2 via rifting and weathering of the low-latitude Franklin basalts could have produced a climate state that was more susceptible to glaciation.

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Macdonald, F., Schmitz, M., Crowley, J., Roots, C., Jones, D., Maloof, A., Strauss, J., Cohen, P., Johnston, D., & Schrag, D. (2010). Calibrating the Cryogenian Science, 327 (5970), 1241-1243 DOI: 10.1126/science.1183325