10 June 2016

Friday fold: Mesoscopic structures in the Lightning Creek Schist

IMG_2488

There are some structural goodies here at the confluence of the Rapid River and the Salmon River in west-central Idaho. I visited these outcrops three weeks ago on a field trip after the Rocky Mountain section meeting of GSA. The rocks are the Lightning Creek Schist, a schist that’s part of the Wallowa Terrane, an accreted chunk of crust that docked with western North America during the Mesozoic.

Here is the trace of foliation:

IMG_2485

And here it is folded, with a few veins thrown in for good measure:

IMG_2487

Folded vein at center:

IMG_2489

This next one has a lot of structure going on…

IMG_2493

Now let’s zoom in to three areas of that image for some close-ups:

IMG_2494

IMG_2495

IMG_2496

Nearby, there was some sweet boudinage to be seen with a vein segment:

IMG_2491

Note the blocky bottom margin of the vein and the way the foliation warps upward into the boudin neck:

IMG_2492

Two views of a fold expressed partly in 3D:

IMG_2497

IMG_2498

A small squidge’um nearby:

IMG_2508

Next up, a big vein catches the attention, but then let’s zoom in to the recess just above it…

IMG_2500

IMG_2501

IMG_2502

And let’s close with this lovely isoclinal fold…

IMG_2504

That should satisfy your appetite for deformation for at least an hour or so. Happy Friday!

Comments/Trackbacks (0)>>



Green River Formation stromatolite – a virtual sample

Today concludes a weeklong run of  virtual samples. For the past five days, I’ve been presenting examples of 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.

Today, we finish up with a stromatolite from the hypersaline Wilkins Creek member of the Eocene-aged Green River Formation of southwestern Wyoming:

Link GIGAmacro by Callan Bentley

Photoscan model by Marissa Dudek

This is such a neat little piece of Earth history – I’m a fan of complete stromatolites so tidy and compact that they can be tucked into a pocket. This one has little papilliform bumps on it – like the taste buds on your tongue. In three dimensions, its overall domelike shape (convex up) is readily apparent.

Enjoy your exploration of this virtual sample.

Comments/Trackbacks (0)>>



9 June 2016

Storm deposits in Tonoloway Fm. limestone – a virtual sample

Our series of  virtual samples continues! This week, I’m presenting examples of 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.

Today, we feature a lovely slab of limestone, chock full of fossil material, and showing both primary sedimentary structures like graded bedding and cavity fills (both geopetal indicators), but also stylolites and small mineralized joints and faults:

Link GIGAmacro by Robin Rohrback

Photoscan model by Marissa Dudek

With this sample, I’m not sure that the 3D version adds too much to the understanding of the processes at work, since it’s been trimmed with a saw on both of the main two sides. But it does at least afford a glimpse of the “dark side of the sample,” which the GIGAmacro version doesn’t include.

Comments/Trackbacks (0)>>



8 June 2016

Differential weathering in dolostone – a virtual sample

Our series of  virtual samples continues! This week, I’m presenting examples of 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.

Today, we feature a differentially weathered dolostone.

Link GIGAmacro by Callan Bentley

Photoscan model by Marissa Dudek

Differential weathering (i.e., different rates of weathering for different substances subject to the same conditions) is an important control on geomorphic features. The mountain range where I live, for instance, only exists because of differential weathering of quartz sandstone (weathers slowly) relative to carbonate and shale (which weather relatively rapidly). Here’s that same process in microcosm.

Comments/Trackbacks (0)>>



7 June 2016

Variably-cemented conglomerate – a virtual sample

Here’s another example of 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 a conglomerate that I found in our lab at NOVA, showing variations in the level of cementation around a central ‘nucleus’.

Link GIGAmacro by Callan Bentley

Photoscan model by Marissa Dudek

It’s interesting to me that the center of the specimen is less well-cemented than the intermediate depth. I guess there must have been some sort of piping process to produce that “doughnut” shaped pattern in cross-section, or else a two-stage process of cementation and then de-cementation over a lesser area.

At any rate, have fun examining it!

Comments/Trackbacks (0)>>



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?

Comments/Trackbacks (1)>>



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:

IMG_1988

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

IMG_1965

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):

IMG_1958

IMG_1959

Example 2 (two perspectives):

IMG_1960

IMG_1962

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!

Comments/Trackbacks (0)>>



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):

IMG_0840

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:

IMG_0860

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:

IMG_0982

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:

IMG_1116

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

Link

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.

Comments/Trackbacks (0)>>



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.

IMG_0739

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

IMG_0740

IMG_0746

IMG_0747

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:

IMG_0749

IMG_0751

IMG_0752

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

IMG_0753

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

IMG_0760

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…

Comments/Trackbacks (2) >>



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:

IMG_0828

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

IMG_0829

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:

IMG_0830

Can a basalt flow contact metamorphose an older basalt flow?

IMG_0831

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.

IMG_0832

Other thoughts? Chime in.

Comments/Trackbacks (1)>>