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.

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