6 June 2016

Cleavage refraction in Konnarock Fm. rhythmites – a virtual sample

Here’s a visualization combination that leverages the advantages of the GIGAmacro system with the 3D ‘virtual sample’ perspective of the Sketchfab-hosted model: the same sample presented in both formats.

In this case, it’s a lovely example of cleavage refraction going from meta-clay-shale (now ‘slate’) through a graded bed of fine sand and silt.

Link GIGAmacro by Callan Bentley

Photoscan model by Marissa Dudek

The sample is of the Neoproterozoic Konnarock Formation‘s lower “rhythmite” unit (maybe varves??) near Mount Rogers, in Virginia’s Blue Ridge. The deformation overprints the primary sedimentary features, and is certainly related to Appalachian mountain building – probably the terminal Alleghanian phase.

Students: can you tell which of these renditions is right side up? Explain. How does the cleavage angle reinforce this deduction?

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3 June 2016

Friday fold: Riggins, Idaho

There are some folds in this stunning west Idaho landscape. Perspective is looking toward the north, more or less. See if you can find them:
Link GigaPan by Callan Bentley

One example that will probably be obvious at first glance is this syncline/anticline pair, differentially weathered, with subvertical axial traces, and an apparently shallow plunge of the axes to the south:

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A more subtly expressed example is on the hill to the east (left), where differential weathering reveals a “Z” fold outcrop pattern:

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Here are two annotated versions of the initial GigaPan image, with the trace of the foliation of the Squaw Creek Schist traced out in yellow (and projected into the sky in black) and the axial surfaces of the folds sketched in using red:

Link Annotated GigaPan by Callan Bentley

Link Annotated GigaPan by Callan Bentley

The region (Riggins, Idaho) has smaller scale folding too: here are some cm-scale folds from just south of the Time Zone Bridge, north (downstream) of Riggins:

Example 1 (two perspectives):

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Example 2 (two perspectives):

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I visited Riggins the week before last on a GSA field trip (“Accretionary Tectonics of Western Idaho”) led by Keegan Schmidt, Reed Lewis, and Keith Gray. I’ve got plenty more photos to share from that trip… stay tuned!

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2 June 2016

Flood deposits of Glacial Lake Missoula outburst floods

Two weeks ago, I went on an awesome, informal field trip to eastern Washington State to visit the Channeled Scablands for the first time. My collegue Bill Richards of North Idaho College picked me up in Spokane and drove me halfway across Washington and back to Moscow, Idaho, over the course of a day. This is a region of the country where a glacier-dammed valley filled up with water (Glacial Lake Missoula), then floated its icy obstruction out of the way, draining out to the west in a series of catastrophic floods. The scale of the floodwaters boggles the imagination. They swept away the fertile loess that makes the Palouse so agriculturally productive, and incised downward into the underlying Columbia River flood basalts, etching out a series of deep channels called coulees. Where cataracts formed, escarpments rapidly retreated, deep plungepools were scoured, and giant potholes were drilled. All this eroded material tumbled downstream, formed enormous gravel bars topped with giant current ripples (gravel dunes).

Erosional buttes of Columbia River basalt (left) and giant gravel bars (right) in Crab Creek Coulee, near Wilson Creek, Washington:
Link

The guts of these enormous features are made of poorly sorted, rounded gravels (cobbles and boulders):

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You can explore a cross section of one Wilson Creek gravel bar in this GigaPan:
Link

We took a detour to drive through the Crab Creek Coulee north of Pinto Ridge, and saw both erosional and depositional landforms there:

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Pinto Ridge Road erosional butte and 2 giant gravel bars:
Link

South of Soap Lake, where the Grand Coulee opens up into wider terrane, there’s a giant subaqueous expansion bar called the Ephrata fan. It’s mostly made of cobbles of Columbia River basalt, but there are also examples of basement granites, like this boulder:

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Here is an enormous boulder of columnar-jointed basalt, with me for scale. This thing was transported by the floods!!

SONY DSCClick to enlarge to an ~8000 pixel wide version

Actually, it used to be even bigger, as it’s weathering in place and sloughing off columns left and right. There’s a nice raven’s nest on it too – just to the left of where I’m standing. Click to make it bigger.

Nearby, we saw a primary sedimentary signature of the prograding “fan”:

Foreset gravel beds, Ephrata Fan:
Link

Last, Bill and I journeyed to Crescent Bar, on the Columbia River, where we took many photos of the outstanding giant current ripple complex there:

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We shot multiple handheld GigaPans here:

Crescent Bar from two different perspective points. Note the erosional cliffs of basalt on the left and the enormous point bar covered in giant current ripples that are probably 30 meters tall:
Link

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Close-up on the giant current ripples covering Crescent Bar:
Link

This field trip really opened my eyes (or my mind) to the scale of the Missoula floods. It is astonishing. I was highly motivated to participate in a session at the Rocky Mountain GSA meeting that followed, focusing on various scientific aspects of giant flood events, both in the Channeled Scablands and elsewhere. I will certainly be going back someday to visit some of the other sites that record this enormous event.

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1 June 2016

Varves along Hawk Creek, Washington

The first stop on my Columbia Plateau / Channeled Scablands trip was to a place called Hawk Creek, that I found out about from the just-published (by Mountain Press) Washington Rocks! book, by Eugene Kiver, Chad Pritchard, and Richard L. Orndorff. I received a review copy of this slim, trim volume the week before I left for Spokane, so I was delighted to be able to put it to immediate good use. My colleague Bill Richards and I drove there, and found a it at tidy little valley on a tributary to Lake Roosevelt.

The lake level was low, but that was a blessing in disguise. It allowed us to see deposits from a previous lake, Glacial Lake Columbia, which existed here at the time of the Glacial Lake Missoula outwash floods (late Pleistocene). Rumor had it that there were varves hereabouts!

Varves are seasonal depositional couplets common in lacustrine settings that are situated in strongly seasonal climates – dark, fine grained clay-rich sediment during the winter (when the lake was frozen over during the winter, and therefore calm), and light-colored and coarser (silty) during the summer, when runoff brings bigger bits into the lake, tumbling along and mixing in and jacking up the oxygen levels en route.

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We spied a promising outcrop on the other side of the creek, with finely layered sediments peppered with swallow burrows:

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But the true varves were found as blocks in Hawk Creek itself, where we fished them out from amid their neighbors (SO much Columbia River Basalt!) and broke them open to spy their internal laminae:

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Here’s a thin piece, showing the very fine grain size and very thin laminations:

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The best ones I saw are shown here: that’s about a decade of sedimentation, with some years seeing more robust sedimentation than others.

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These structures speak of very, very, very calm water conditions. But our real quarry for the day was the opposite: raging, almost unimaginably powerful floods.

…So on we drove…

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31 May 2016

A baked horizon in the Columbia River flood basalts

Near Wilson Creek, Washington, we beheld a bright orange stripe on the cliff:

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It’s a bold color, and it begs the question: What’s going on here?

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Laterite, a tropical soil, has a similar color scheme, and I’ve seen laterite horizons sandwiched between lava flows before (e.g., at the Giant’s Causeway in northern Ireland). But when I zoomed in on this orange layer, it appeared to be vesicular basalt:

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Can a basalt flow contact metamorphose an older basalt flow?

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Maybe weathering is the key: So the story recorded here could therefore be: lava erupts, lava cools to basalt, basalt weathers from the top down, producing clay and rust-rich saprolite, new lava erupts, cooks weathered basalt beneath, baking it orange.

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Other thoughts? Chime in.

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30 May 2016

Scenes from a cut bank

My family and I went canoeing this weekend, and one of the more photogenic things we saw on the river was this fine cut bank:

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The bank is being actively undercut by the river, as evidenced by the overhanging soil + grass carpet, the slump scarps at the bottom (showing fresh, wet soil), and the bare tree roots that hang out like orange cables:

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I also shot a little video as we drifted by, if you want the full experience:

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27 May 2016

Friday fold: Castile Formation GIGAmacro

I’ve posted here before about the extraordinary intra-layer folds in the varved evaporite deposits of West Texas’ Permian Basin, but today I can go one better and offer a GIGAmacro look at these lovely folds:

Link GIGAmacro by Callan Bentley

Enjoy checking these amazing small-scale folds out. They will boggle your mind.

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26 May 2016

Glaciotectonic thrust at Waubonsee Community College

This is fun! Check out this photo of a trench wall during a sewer line installation through the Waubonsee Community College campus, southeast of Chicago, Illinois, in 2006:

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The photo is from my colleague Dave Voorhees, a professor at Waubonsee Community College. Dave says:

The handle of a trowel stuck into the trench wall is visible in the middle left side of the image. The top third of the image shows evidence of cryoturbation seen in many gelisols, that is here formed in an organic-rich (O horizon) silt over diamicton (till) of the Batestown Member of the Lemont Formation formed during the Quaternary invasion of northeast Illinois by the Lake Michigan Lobe of the Laurentide Ice Sheet. The lower 2/3 of the image are diamictons, sands, silts and clays that have been deformed by an over-riding glacier during the Putnam Phase of the Michigan Subepisode (~18,0000 to 17,500 yr B.P.). Of note is the glaciotectonic thrust fault from bottom left to top right.

Thanks for sharing, Dave – this is so cool. I’ve previously featured other glaciotectonic features from Canada and Montana.

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24 May 2016

Nine new GigaPans from Team M.A.G.I.C.

Alethopteris fern fossil:
Link GIGAmacro by Robin Rohrback

Rapid River Canyon, Idaho:
Link GigaPan by Callan Bentley

River cobble of brecciated Columbia River Basalt, Hammer Creek (Salmon River), Idaho:
Link GIGAmacro by Callan Bentley

Petersburg Granite exposed at Belle Isle, Richmond, Virginia:
Link GigaPan by Jeffrey Rollins

Ammonite:
Link GIGAmacro by Callan Bentley

Slickensides in ultramafic rocks of the Wallowa Terrane, just outboard of the paleo-Laurentian tectonic margin, Salmon River, Idaho:
Link GigaPan by Callan Bentley

Fossil cone:
Link GIGAmacro by Robin Rohrback

Boulder of brecciated Martin Bridge Formation marble, Rapid River Canyon, Idaho:
Link GigaPan by Callan Bentley

Graded bedding in Neoproterozoic Mechum River Formation, Sharp Top, Virginia:
Link GIGAmacro by Callan Bentley

Happy exploring!

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20 May 2016

Friday fold: core

At the Rocky Mountain Section meeting of the Geological Society of America this week, there were several displays of interesting cores. I’m not sure where this one came from, but it had a fold in it, and since no one else had volunteered a Friday fold for this week, I took a photo:

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It’s standard core diameter; I’d guess that’s about 2 inches.

Given that I’m headed out on an accretionary tectonics field trip today; I hope to have more impressive folds for you next Friday. Or: you could always send me a photo or two… hint, hint.

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