S-C fabric in meta-ignimbrite

Here’s a sample from my 2004 geology M.S. thesis work in the Sierra Crest Shear Zone of eastern California. The rock is a sheared ignimbrite (ashflow tuff) tuff bearing a porphyritic texture and a nicely-developed “S-C” fabric.

With annotations, showing the S- and C-surfaces, and my kinematic interpretation:

S-C fabrics develop in transpressional shear zones: ~tabular zones of rock that are subjected both to compression and lateral shear (“transform” motion). The S-surfaces (foliation) initially form at about 45° to the shear zone boundary, and then progressively tilt over in the direction of shearing as deformation proceeds. This gives this sample (when viewed from this angle) a dextral (top to the right) sense of shear. (previous examples on Mountain Beltway) The C-surfaces are shear bands, where a large amount of shear strain (parallel to the shear zone boundary) is accommodated.

You should be able to click through (twice) for big versions of these images.

I polished up this little slab and made a refrigerator magnet out of it. I think it’s a lovely rock.

Strath vs. terrace graphic

There is an old Chinese aphorism that “the beginning of wisdom is to call things by their proper names.” One of the naming conventions that tends to trip up NOVA students who hike the Billy Goat Trail with me is the difference between a “terrace” and a “strath.” This morning, I created a graphic that illustrates the difference between these two landforms as I understand it:

strath_vs_terrace

Both features are shown in cross-sectional cartoon view. Terraces are cut into alluvium, the unconsolidated sediment deposited by the same river which is now incising. Straths, on the other hand, have the same shape but are etched into bedrock. Another name for straths would be “bedrock terraces.” Straths will sometimes have a thin veneer of alluvium atop them: in my experience along the Billy Goat Trail, this consists of abandoned bedload from older, higher base levels, augmented by lighter-weight flood deposits.

Would anyone with more geomorphological knowledge than me care to qualify / critique / correct my understanding on this terminological issue? Thanks in advance!

UPDATE: Based on Anne’s comments below, I’ve tweaked it a bit:

strath_vs_terrace2

Vector maps by Eric Fischer


“The Geotaggers’ World Atlas #8: Washington, DC” by Eric Fischer

If you like maps, you should go check out these images by photographer Eric Fischer. (via here) The different colors represent different modes of transportation: Black is walking (less than 7mph), red is bicycling or equivalent speed (less than 19mph), blue is motor vehicles on normal roads (less than 43mph); green is freeways or rapid transit.

The coming flood

In January, a large landslide occurred in the Hunza Valley of Pakistan’s Karakoram Range, near the village of Attabad. Like the Madison River landslide in Montana (1959), or the Gros Ventre landslide in Wyoming (1925), a river was dammed by the slide debris, and the impounded waters began to rise.

At Gros Ventre, the landslide-dammed lake overtopped the debris and caused a catastrophic flood which killed 6 people in Kelly, Wyoming. At the Madison River, the U.S. Army Corps of Engineers feared another Kelly-style flood, with Ennis, Montana being the (larger) vulnerable town downstream. They carved a spillway through the debris which accommodated the flow the Madison River, though a “Quake Lake” still remains upstream of the dam.

Dave Petley has been covering this growing threat at Attabad since the initial landslide on his blog, Dave’s Landslide Blog. I think Dave’s coverage has been absolutely superb — it represents the best of what geoblogging can be. He has been soberly reporting the facts and offering his considered interpretations for more than four months. He has tracked the continuing mass wasting in the area, the Pakistani government’s attempts to dig a spillway, and the growing seepage through the dam (with attendant erosion). On an almost daily basis, he has been posting graphs showing the rising lake levels and decreasing “freeboard” (distance between the lake’s surface and the lowermost point on the dam — the spillway mouth).

Now, the day has arrived when the rising lake is projected to finally overtop the dam. Dave’s prognosis is not a positive one: the spillway appears to be inadequate in size to handle the flow of the river even at normal rates of discharge (and certainly not during floods). The material composing the dam appears to be easily erodible, which raises the likelihood that the overtopping waters will rapidly incise downward, widening the spillway gorge rapidly into a lake-draining chasm. A flood is not guaranteed, nor is it guaranteed that if there is a flood, that it will happen today — but the situation offers little hope for optimism. We might get lucky and avoid a catastrophe — but there seems to be ample reason for grave concern.

Dave Petley seems to have been a lone western voice raising awareness of this growing hazard, and I feel he should be strongly commended for it. Dave  is accompanied by coverage from the Pamir Times, and a daily lake level dataset being gathered by an on-the-ground volunteer team called “Focus.” One can only hope that their collective efforts have not been in vain. The people downriver of the slide will need to move to higher ground until the threat has abated. It seems unrealistic to expect Dave, the Focus team, and the Pamir Times don’t convince them via blogging. I would venture to say that the Pakistani government should have called a mandatory evacuation of the area several days ago. It is their responsibility to be sufficiently on top of things and protect their citizens.

Good luck and best wishes to the people of the Hunza Valley.

New “secondary structures” display at NOVA

…. And on the other side, we have secondary (tectonic) structures, focused on folds and faults:

New “primary structures” display at NOVA

One of the things I managed this week was to fill up a new display case in our Student Study area with a structural geology display. On one side is primary structures, both igneous and sedimentary….

Falls of the James III: river work

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

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

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

james_GE1

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

fallsjames_river_01

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

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

james_GE2

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

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

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

fallsjames_river_02

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

fallsjames_river_03

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

fallsjames_river_02b

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

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

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

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.

fallsjames_frac_07

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:

fallsjames_frac_01

fallsjames_frac_02

fallsjames_frac_03

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

fallsjames_frac_01b

fallsjames_frac_02b

fallsjames_frac_03b

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…

fallsjames_frac_04

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:

fallsjames_frac_06

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:

fallsjames_frac_05

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

fallsjames_frac_05b

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:

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

“Geology of Skyline Drive” w/JMU

I mentioned going out in the field last Thursday with Liz Johnson‘s “Geology of Skyline Drive” lab course at James Madison University.

We started the trip south of Elkton, Virginia, at an exposure where Liz had the students collect hand samples and sketch their key features. Here’s one that I picked up:

skyline01

Regular readers will recognize those little circular thingies as Skolithos trace fossils, which are soda-straw-like in the third dimension. Rotate the sample by 90°, and you can see the tubes descending through the quartz sandstone:

skyline03

This is the Antietam Formation, a distinctive quartz sandstone / quartzite in the Blue Ridge geologic province. But at this location, on the floor of the Page Valley and butted up against the Blue Ridge itself, we see something else in the Antietam:

skyline02

Parts of this outcrop are pervasively shattered: a variety of sized clasts of Antietam quartzite are loosely held together in porcupine-like arrays of fault breccia. Turns out that this is the structural signature of a major discontinuity in the Earth’s crust: the Blue Ridge Thrust Fault. This is the fault that divides the Valley & Ridge province on the west from the Blue Ridge province on the east. And here, thanks to a roadcut on Route 340, we can put our hand on the trace of that major fault. Here’s another piece of the fault breccia:

skyline04

After grokking on the tectonic significance of this fault surface, we drove up into Shenandoah National Park, to check out some outcrops along Skyline Drive itself, but it was really foggy. Here’s a typical look at the team in the intra-cloud conditions atop the Blue Ridge:

skyline05

We checked out primary sedimentary structures in the Weverton Formation at Doyles River Overlook (milepost 81.9), like these graded beds (paleo-up towards the bottom of the photo)…

skyline07

…and these cross-beds. You can see that it was raining on us at this point: hence the partly-wet outcrop and glossy reflection at right:

skyline09

Cutting through this outcrop was a neat little shear zone where a muddy layer had been sheared out into a wavy/lenticular phyllonite, with a distinctive S-C fabric visible in three dimensions:

skyline06

Finally, we went to the Blackrock Trail, which leads up to a big boulder field of quartzite described as Hampton (Harpers) Formation. In some places, exquisite cross-bedding was visible, as here (pen for scale):

skyline10

Here’s a neat outcrop, where you can see the tangential cross beds’ relationship to the main bed boundary below them:

skyline11

…And then if you spin around to the right, you can see this bedform (with internal cross-bedding) in the third dimension. I’ve laid the pen down parallel to the advancing front of this big ripple:

skyline08

That last photo also shows the continuing influence of the fog.

Thanks much to Liz for letting me tag along on this outing! It was a great opportunity for me to observe another instructor leading a field trip, and also to discover some new outcrops in the southernmost third of the park.

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