Crucifix Site 1: Sediments

On the September 2009 GSA field forum in the Owens Valley, the final stop of our first day was to check out the so-called “Crucifix Site,” along Chalk Bluff Road (north of Bishop, California, at the southern margin of the Volcanic Tableland). It’s called the “Crucifix Site” because there is a metal cross erected there:chalk_bluff_stuff_13

This is the site of some pre-Bishop-Tuff volcaniclastic sediments. The place is interesting on several levels, including the sediments themselves, and the subsequent deformation they have experienced. Here’s a look at the outcrop:chalk_bluff_stuff_03

Some annotations help to call one’s attention to primary sedimentary structures and interpretations:chalk_bluff_stuff_03anno

Lots of the sediment itself was made of little beads of obsidian, usually surrounded with a “chalky” weathering rind:chalk_bluff_stuff_07

A cool little channel cross-section was visible, too:chalk_bluff_stuff_04

For some reason, this is what pops into my mind when I run into a well-exposed Cheshire channel cross-section:
chalk_bluff_stuff_04anno

Birds and wasps had tunneled into the softer layers, resulting in horizontal rows of holes. I tried to ignore these modern bioturbations so I could focus on the ancient tale in the rocks themselves. Some cool soft sediment deformation was visible, like these flame structures (upper part of the central gray layer):
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Zoomed in on a pair of flame structures, and the down-sagged material between them:chalk_bluff_stuff_11

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…And another set:chalk_bluff_stuff_12

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This was odd: The lowermost layer (upon which my field notebook rests) is unperturbed, but the layers above it are all churned up in one small area (center), flanked by a couple of bird holes:
chalk_bluff_stuff_08

Zoomed-in on the area in question:chalk_bluff_stuff_09

Annotated, for your viewing pleasure; green is top of undisturbed layer; red shows boundaries of zone of disrupted sediment:chalk_bluff_stuff_09anno

I would be pleased to hear from anyone who knows more about sedimentology than me about the wavy bedding in the second (& third) photo, and this weird sediment disturbance in the lowermost photo. Also: with the flame structures, it looks like coarser material in the lower layer (gray) is the less viscous participant, while finer-grained (white) material is sinking downwards. Isn’t this the opposite of the way it usually works?

Virginia water well shows seismic waves

This site, from the USGS, shows depth to the water table for a well in Virginia:

water

You’ll notice the tidal influence on the water table (broad sine-curve-like up and down crests and troughs at ~12 hour intervals), and then a sudden perturbation which caused some wiggles almost two and a half feet of magnitude! This, presumably, is the seismic waves from the Chilean earthquake arriving — surface waves, I would guess, but I’ll happily be corrected if that’s not the case.

Major hat tip to Cian Dawson, who tweeted a link to the site around 6am D.C. time (which would be, what, 3am in California, Cian? Sheesh!). In the same tweet, a link to this hydrograph in Christiansburg, Virginia, showing its response to various historical earthquakes.

Frozen soil lifts off

When I was out poking around in the woods, confirming for local geophile Barbara that indeed her local geologic map wasn’t 100% accurate, I noticed this on the frozen ground:
barbara_03

We have seen this before, in a post back on NOVA Geoblog, almost exactly a year before I took this photo. Here’s another shot from the more recent excursion, taken a foot or so over from the first one:

barbara_04

What’s happening here is not that I am showing you particularly high-contrast photos of pebbles and cobbles in the mud. Instead, the reason for the dark line around the sedimentary clasts is that the mud is frozen. When water freezes into ice, it expands in volume by about 9%. This extra volume means that the ice can’t occupy the same space that the liquid water did. So it pooches upwards, as “up” is the direction in which it is least “hemmed in.” Down? No — the expanding ice is not capable of pushing the entire Earth out of its way. North/south? or East/west? Well, there’s already soil there, and it’s pushing back, so there’s no expanding out in those (horizontal) directions. So, “up” it is. That’s all we’re left with: “up” is σ3.

If I were to draw this as a cartoon, here’s the “before” picture:

mud1

As the sheet of frozen mud expands upwards, it detaches from the non-expanding (in fact, shrinking, but not by anywhere near 9%) cobbles and pebbles. As the mud ice lifts up higher and higher, the gap between it and the clasts gets more and more pronounced.

mud2

Shadows in those gaps make them appear dark to the camera lens.

Ice pulls all kinds of neat tricks like this in the winter. What’s a cool ice phenomenon you’ve observed lately?

Piedmont rocks exposed in a creek

One of the cool things about being the local geoblogger is that people get in touch with you about local geology. Sometimes this even leads to meeting up for field trips. Here’s two quick photos from a recent (January 2010) field trip to a creek near Springfield, Virginia.

My host was Barbara X, a local aficionada of Piedmont geology. She has lived in this particular neighborhood for many years, and is very familiar with the local woods and drainages through decades of dog-walking there.

Her main question for me was “Could the geologic map of this area be wrong?” She showed me the map, and then took me out to an outcrop which clearly was of a different rock type than the map indicated it “should” be.

The offending intruder, a meta-basalt with two prominent joint-sets:

barbara_01

A short distance downstream, a cut bank revealed some saprolitic rock that is more typical of the Piedmont province:

barbara_02

I think we’re seeing bodies of schist/ gneiss (highly foliated in cross-section), as well as coarse-grained, lighter-colored bodies of granite. All of them have been weathered to hell: you can scoop handfuls of this “rock” out of the outcrop if you want. If you’re a plant, you can plunge your apical meristem right into it, and let the roots follow.

This is typical “outcrop” around here: though the mid-Atlantic region has a fascinating story (including the Appalachian mountain belt, like these rocks), the wet climate has rotted most rock away. The only other thing that’s worth mentioning about this particular outcrop are the upper-left-to-lower-right brown lines: those are fracture traces decorated with rust. The fractures serve as plumbing to move fluids around in the subsurface, and their dissolved cargo of elements can then react with the rock on either side of the fracture.

Rockies course applications open

For those of you who are potential NOVA students (really, that’s pretty much anyone on the planet), I wanted to let you know that applications are now open for the July 2010 Regional Field Geology of the Northern Rockies course that I co-teach with Pete Berquist of Thomas Nelson Community College. A more detailed description is available on my website.

Contact me via e-mail if you want more information or download an application here.

To whet your appetite, here’s Rockies 2009 student Jason Von-Kundra mapping Mississippian-aged carbonates in the Bridger Range of Montana:

Jason_maps

Lola → ammonite

Seeing my cat in this posture:
lola_2_legs
…made me think of this:
lola_ammonite

Where did those hind legs go?

Snowy décollement

Earlier in the month, during the big snowstorms, my window got plastered with snow. This snow formed a vertical layer which then deformed under the influence of gravity. Looking at it through the glass, I was struck by how it could serve as a miniature analogue for the deformation typical of a mountain belt.

Let’s start our discussion by taking a look at an iPhone photograph of the snow:

So here’s what I notice about this (vertically-oriented) photo:

The big sheet of snow is sliding downward over the face of the glass. This surface of slip is thus analogous to a low-angle thrust fault. Here, the maximum principal stress (known as σ1 to structural geologists) is gravity. The minimum principal stress (σ3) is perpendicular to the window, and the intermediate principal stress (σ2) is horizontal, parallel to the bottom edge of the window (i.e., left-to-right). As deformation proceeds, the snow slab folds up on itself and pooches outward in the area of least stress (σ3); away from the surface of the window.

As the snow layer moves downward, it creates a major fold which thickens the snow in a big line perpendicular to gravity, parallel to σ2. Along the vertical part of the window frame, the snow sheet has detached in a vertically-oriented fracture (i.e., parallel to σ1). Oblique to both σ1 and σ2 is a series of smaller folds with diagonal axes.

We can see a similar pattern in this map of the Himalayan mountain belt:

Note that the map* is oriented with north at the bottom, and south at the top, so as to be able to better compared it to my window. Note the broad arc of the Himalayan mountain front (~parallel to the Nepali border) which is perpendicular to the motion of India relative to Eurasia. The minimum principal stress direction (σ3) is vertical, which is why the mountains grow upwards (and the crust thickens downwards into the mantle, too, making the Himalayan mountain belt the site of the thickest crust on the planet). Along the edge of the impactor (analogous to our snow sheet), for instance in northern Burma, we see the same “splay” of folds with axes perpendicular to the the India-Eurasia convergence vector. The crust there is not as thickened.

Though a gooey slab of snow on my window isn’t a perfect analogue for Himalayan mountain-building, we can see some similarities in gross morphology — structural similarities that are fundamentally tied to the orientation of the principal stress directions.

_______________________________________

* Modified by me from a Google Maps “terrain” view.

Plutonic contacts in eastern Sierras

Last September, at the location of the faulted moraine (eastern Sierra Nevada, California), I took some photos of some of the sexier plutonic contacts exposed in big boulders (erratics) of the glacial till composing the moraine. Check them out. What do you see here?

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eastern_sierra_05

eastern_sierra_03

Pacaya Volcano, Guatemala

Today, some photographs from Guatemala. I know one of my geoblogopeers is down in Guatemala doing research, so I’ll be interested to hear her take on these photos. These photos all come to us courtesy of my friend Courtney, who is a librarian at M.I.T., and a fellow M.S.-graduate of the University of Maryland geology department. She shared these images with me about a year ago, and I intended to post them on NOVA Geoblog, but never got around to it. Well, the wait is over… it is time to feast your eyes on some lava! The full set of images is here.

Pacaya Volcano is about 50 miles south of Antigua. It is one of about 35 volcanoes in the country — and one of four active volcanoes. Courtney and her friends four-wheeled it part way up to save a couple hours of hiking time. The trip door to door from Antigua was seven hours or so. They drove to about 1850′ elevation (564 m) and hiked the rest. Pacaya is 2,552′ high (778 m), but they didn’t go all the way to the top, just high enough to get up close and personal with some lava.

Here they are hiking up across relatively fresh lava flows:

“Lava stairs” that the group used to hike up. Courtney says, “This was live lava about two weeks prior to this picture. When the guide told me how recent it was, I started to get a little panicked. I put my hand down on the ‘stairs’ and it was very warm to the touch. Yikes.”

The group approaches the incandescent lava river (seen here as a faintly orange band running from upper left to lower right):

Approaching the lava river itself:

Wow. I’m struck by the ‘natural levees’ that form on either side of the liquid flow. The overall morphology calls to mind the neural tube of an embryo…

Here’s an unsettling sight to see on the “trail.” Courtney reports very hot feet on this hike, so I’m really not sure whether this is safe or foolhardy.

In these next two images, watch a big chunk of solidified basalt (shaped like an anvil, dark in the first picture, rolled over to appear orange in the second picture) ride the current downstream, like a log floating down a river:

The hikers, evidently happy with their experience. You can see the lava river in the distance as an orange stripe on the side of the volcano:

I’d like to thank Courtney for sharing these photos with us. What do you think? Was this safe? Was it awesome regardless?

Transtensional quartz vein

On last May’s GSW spring field trip to Chain Bridge Flats, I saw a quartz vein:

gsw_sp_FT_09_04

Surely, upon looking at this photograph, you will be struck by the way the vein is not the same thickness along its length, and parts of it appear to be a white line transitioning into a parallelogram, and back into a white line again. What, you make ask, gives?

I think what you’re looking at here is a transtensional quartz vein. Like all veins, this one formed when the host rock (in this case, metagraywacke of the “Sykesville Formation”) cracked open and hot fluids squirted into that fracture. Elements dissolved in the fluid organized themselves into mineral crystals, and precipitated in the void space of the crack, sealing it shut with quartz “glue.”

gsw_sp_FT_09_04_anno

“Transtension” is the word used to describe a kinematic regime which contains elements of transform “shear” (in this case, right-lateral) and tensional stress. Because of the jagged shape of the fracture here, some parts of the fracture are grinding past their neighbors, while other parts are dilating. The dilating parts are only dilating because of the shape of the fracture. The actual motion of the blocks of rock is uniform and non-rotational. We call these little pulling-apart areas “releasing bends.”

On a much larger scale (lithosphere-scale), releasing bends near the surface create pull-apart basins like the Dead Sea. Deeper in the crust, pull-aparts may serve to accommodate pluton emplacement, as has been suggested by Tikoff & Teyssier (1992) for the Tuolumne Intrusive Suite of the high Sierra in California.

This “part-sliding, part-extension” pattern is actually quite common. Here’s another example, this one in a brick sidewalk on Capitol Hill:
cap_03

The same pattern also shows up at the Mid-Atlantic Ridge, where the extensional segments (north-south-oriented) are sites of new oceanic crust being formed, and the fracture networks (east-west-oriented) are sides of transform faults, where the South American Plate slides laterally past the African Plate:
MOR

Where else have you seen this pattern? Use the comments section to share an example or two.

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