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:

fallsjames_05

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

fallsjames_09

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:

richmond_map

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:

fallsjames_10

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:

fallsjames_01

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

fallsjames_04

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:

fallsjames_06

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:

fallsjames_07

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:
fallsjames_08

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:
fallsjames_11

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:

fallsjames_12

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):

fallsjames_13

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:

fallsjames_26

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:

fallsjames_19

A few more shots of these xenoliths:

fallsjames_22

fallsjames_20

This one is a cool one…

fallsjames_16

… 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.

fallsjames_17

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

fallsjames_21

…And this one shows internal boudinage:

fallsjames_14

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

fallsjames_23

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:

fallsjames_15

…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:

fallsjames_18

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

fallsjames_25

fallsjames_24

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:

fallsjames_02

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):

fallsjames_03

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

cross-cutting-belle_isle

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

Advertisements

“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:

not_pillow_03

not_pillow_04

A relatively clean, but relatively unweathered sample:

not_pillow_05

Aha, now that’s better:

not_pillow_06

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?

__________________________________________

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:ribs_plumose

Where shall we zoom in? How about these two boxes?ribs_plumose4

Close-up of the concentric ribs:ribs_plumose2

Close-up of the hackle fringe on the edge of the joint:ribs_plumose3

…Even the hackles have hackles!

Good stuff: I love me some fine plumose structure!

Hol(e)y basalt, Batman!

Today, our theme is vesicles. Here are some images of vesicles in basaltic lava flows in the Owens Valley of California, the same spot where we saw the baked fanglomerate that I showcased a few days back.

owens6_02

In this photo (and the zoomed-in detail shot below), you can see a couple of things. One is the size difference of the vesicles as you go up in the flow. Bigger bubbles represent larger loci of low density, and hence will be more likely to rise in a fluid batch of lava. This is the inverse of the phenomenon that causes graded bedding (heaviest grains sinking first). The result is a “graded vesicular lava flow.”

Also visible are several cooling joints that intersect to form columns. At the lower part of these columns, you can see arrest lines perpendicular to the column. Each of these subhorizontal lines represents a single instance of fracture propagation as the column separated from the rest of the flow. In composite, they form a “crack panel” like others showcased here in the past.

Let’s take a closer look at these distinctive features:

owens6_06

…And here’s some big vesicles, big enough to host a Swiss Army knife for scale:

owens6_03

They aren’t as big as some I’ve shown here in the past, but they were the largest vesicles I saw on the Owens Valley Field Forum last September. One thing I find interesting about this batch of vesicles is how they deform one another. The big one in the upper right has several smaller ones above it that “wrap around” its left edge. I envision this as the small bubbles hanging out with ~neutral buoyancy (ascendancy power), when up from below comes this massive bubble. As it pushes up (with its greater buoyancy), they smear out to the side, out of the way.

Likewise with the pair of large vesicles at lower right: it looks like the big flat one was there first, with the smaller “egg-shaped” one rising up from below and impinging on its larger upstairs neighbor. If the lava has been less viscous, the two may have merged into one, as blobs in lava lamps may be seen to do: a minimizing of surface tension, a lowering of the surface-area-to-volume ratio. Why would the smaller impinge on the larger? As I’m envisioning it, there would be a viscosity gradient in the cooling flow, with cooler temperatures towards the top (and hence higher resistance to flow). Deeper in the lava, temperatures would remain warmer, and hence the lava would be less viscous. I’m thinking that the big flat bubble had essentially risen as far as it could, but its top side was cooler than its more ductile bottom side, and so the bottom side was less resistant to the nosy intrusions of upstart bubbles from below.

Do you see anything else worth discussing in these photos?

Baked fanglomerate

A quick post to share a few images of an outcrop I visited last September out in California’s Owens Valley. This is a spot where alluvial fans coming off the eastern Sierra Nevada were overrun by a basaltic lava flow (Jeff, Kim, Fred, and Kurt for scale):

contact_meta_sub_lavaflow02

The unofficial term for these conglomerates deposited by alluvial fans is “fanglomerate,” and it’s pretty cool to see the contact metamorphism at the top of the fanglomerate. There’s also some weakly-developed columnar jointing in the basalt. Here’s an annotated version, in case the contact wasn’t quite obvious enough:

contact_meta_sub_lavaflow02_anno

Here’s a close up (Doug for scale), showing the orange zone of thermal metamorphism at the top of the fanglomerate as the lava flow above baked the hell out of it:

contact_meta_sub_lavaflow01

Groovy, eh? Where’s your favorite example of contact metamorphism?

Transect debrief 8: late brittle deformation

The final chapter in our Transect saga is now here. In some ways, it’s the least thrilling of the bunch. On the other hand, when I see a nice example of this structure, it makes me squeal like a little girl.

I refer, of course, to plumose structure, the small-scale architecture of a joint surface. We saw multiple great examples on the trip, but my favorites came with the first post-lunch stop on Transect Trip day #1, at an outcrop of the Weverton Formation showing a fine-grained deposit of siltstone.

I posted versions of both these photos previously via iPhone, but here I’ll give you the crisper Canon Elph version coupled with reposting of the iPhone shots for comparison purposes.

tt_11

tt_10

Plumose structure branches out in the joint propagation direction, the curvilinear “plumes” are thought to be perpendicular to the leading edge of the joint as it cracks through the rock. These late-stage brittle features may be related to the very latest part of Alleghanian deformation, or they may be related to recent uplift of these rocks.

All righty, then. I think that brings us up to the present day. Those of you who requested more details about the live-geoblogged photos, has this series answered your questions? If not, what do you need more details on?

Transect Trip 23: hackle fringe

Nice set of twist hackles on the fringe of this joint face: