24 June 2016

Friday fold: Dalradian schist at Cushendun, Northern Ireland

Same beach as the Cushedun conglomerate post earlier in the week – but here we see the schist into which the rhyolite dikes intruded:

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It’s been folded!

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Happy Friday. Hope your weekend is rejuvenative and fun.

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23 June 2016

Mega-trace fossils in the floor of the Old Bushmills Distillery, Northern Ireland

We arrived at Old Bushmills at 4:06pm, and the last tour of the distillery for the day had left at 4:00. But all was not lost – We were delighted to see that the visitor center area was paved in slabs of shale with tremendously large, well-preserved trace fossils – sinuous burrows parallel to the bedding plane, in some cases cross-cutting or looping back over themselves!

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Great stuff – balm for the disappointed whiskey tourist soul.

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22 June 2016

Shattered chert breccia cobbles, Church Bay, Rathlin Island

My GigaPan expedition has landed at Rathlin Island, north of Northern Ireland, within view of Scotland, for a few days. The beach on Church Bay is cobble-covered and steep, and the cobbles reflect the island’s geology, with some anthropogenic components thrown in for flavor:

Link GigaPan by Callan Bentley

But I was struck by these two cobbles, each showing a pervasively shattered breccia of chert:

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To me, that is not only as lovely as a mosaic, but it’s indicative of an intriguing tale. The rocks in this part of the world are a layer cake of sediments and lavas, and so I wonder what processes were responsible for generating these shattered cobbles. The chert is a diagenetic feature within the Ulster White Limestone (“the Chalk” – namesake of the Cretaceous), but something violent must have happened to them after the chert precipitated in order to make the breccia. What was that? Some sort of faulting? There are several faults mapped on the island, as well as several intrusions that punch through. Later, of course, uplift brought the breccia-bearing rock to the surface, where it birthed a cobble or two, and these were subsequently tumbled in the surf to attain this fine, rounded morphology.

The presence of these breccias as a minor component of the beach sediment will make me take a closer look at the cliffside chalk exposures when I visit them tomorrow.

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21 June 2016

Porphyritic rhyolite dike seen on the beach at Cushendun

At the opposite end of the beach at Cushendun, Northern Ireland, we found some outcrops of schist – I’ll be featuring some of them as Friday folds later this week. But cutting across the schist was a pink porphyry, with big well-formed potassium feldspars. I splashed some water from the Irish Sea onto it to increase the contrast:

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Here’s a handheld GigaPan image, so you can explore it for yourself. Find a euhedral feldspar! Find a zoned feldspar! Find a beach fly!

Link GigaPan by Callan Bentley

Here is a shot showing the contact between the rhyolite (bottom) and the schist (top):

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You can probably tell that schist has a strong potential for featuring Friday folds. Indeed it does. Stay tuned!

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20 June 2016

Cushendun Conglomerate of the Cross Slieve Group, Northern Ireland

Want a geological irony? Here’s one!

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You’re looking at a rounded boulder of Cushendun Conglomerate, a Devonian “Old Red Sandstone” unit (Cross Slieve Group) exposed at Cushendun Caves, Northern Ireland, U.K. The irony lies in the repetition of history – a tumbling environment of high water energy, rounding cobbles and boulders and depositing them, in order to make the conglomerate. And now, ~400 million years later, history repeats itself, with the same rock! Note the surrounding cobbles, being tumbled and rounded in a very similar way. Compare and contrast:

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I found this boulder on the beach at Cushendun, a charming little town on the eastern coast of Northern Ireland. Just south of town are a series of rocky islets and caves cut into a headland of this conglomerate.

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Here’s my field assistant in the cave:

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The view out among the sea stacks to the countryside beyond:

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If you watch the HBO series Game of Thrones, you may recognize Cushendun Caves as the place where [spoiler alert] the red priestess Melisandre gave birth to a shadow demon thing. Check out the conglomerate on the wall behind her!

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That scene was not being reenacted when we visited, to mixed emotions.

Anyhow, we weren’t there to be GoT tourists (though there were plenty of other folks doing exactly that!); we were there for the rocks. During the Devonian, mountain-building shuddered to life in the British Isles. As the mountains rose, they shed sediment in vast quantities, and much of it was deposited along the flanks of that ancient range. There were arkoses aplenty to be seen in the building stones along our walk in from the parking area:

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But at the caves themselves, the arkose is joined by a bunch of big boulders of quartzite and related rocks, all very well rounded:

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In many places, as above, the clasts don’t touch much – the conglomerate is “matrix supported.” The locals call it puddingstone.

Elsewhere, the big grains touch each other, including along grain boundaries that are flush (apparently planar) or impinging on their neighbors in a concave/convex relationship. This is evidence of pressure solution; the dissolving of portions of these cobbles at the highest-stress areas where they touched. Check out the lower/middle right of this outcrop photo, for instance:

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Here are four GigaPans so you can check for these features yourself:

Link GigaPan by Callan Bentley

Link GigaPan by Callan Bentley

Link GigaPan by Callan Bentley

Link GigaPan by Callan Bentley

Post-depositional stresses fractured these conglomerates with a set of distinct co-parallel fractures, some of which acted as faults. Can you spot the fault in this next image?

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Here’s my favorite outcrop at the Cushendun Caves:

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Before I explain, see if you can suss it all out.

Okay, I’m not waiting any longer!

In this photo, there are many examples of flush/impinging grain boundaries (circled in white in the annotated version below), plus some of those flush grain boundaries (black) have been offset along a set of small faults (traced out in yellow):

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It’s a neat place to visit. If you find yourself in Cushendun, I can’t think of any reason why you wouldn’t make a point of visiting.

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19 June 2016

3D model of onion-skin weathering

I collected a photo set at the Giant’s Causeway to show the “textbook” examples of spheroidal (“onion skin”) weathering exposed on the road down to the causeway. My student Marissa Dudek used the photo set and Agisoft Photoscan to make a great 3D model of the site. She posted it on Sketchfab yesterday evening. Check it out!

Photoscan model by Marissa Dudek

Great work, Marissa!

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18 June 2016

Two virtual weathered-out dikes in a fjord in eastern Iceland

Two 3D models for you today, both produced by my student Marissa Dudek, using photo sets I gathered in Iceland:

Photoscan model by Marissa Dudek
(That one has paleomag holes drilled into it!)

Photoscan model by Marissa Dudek
(That one I’m particularly pleased with. Given the circumstances of image acquisition, this is a very good result!)

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17 June 2016

Friday fold: isoclinal flow structures in Icelandic lava

While soaking at some fine outdoor hot springs in southern Iceland (near Höfn) last week, I spied a Friday fold on the rock wall above the hot pots:

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Iceland is not a place where we would expect to find ductile folds in already-lithified rocks, so I’m guessing that these are folds related to flow in the lavas as they erupted at the surface.

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Happy Friday!

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16 June 2016

Dimmuborgir, Lake Mývatn, Iceland

Last week, I was in Iceland, driving around the country’s Ring Road and checking out its amazing geology with my family. We had a great time in particular exploring in the Lake Mývatn region, on the country’s subaerial expression of the Mid-Atlantic Ridge. Lake Mývatn itself is in a rift basin, peppered with a great variety of volcanic and hydrothermal features. One spot we enjoyed a short stroll was in Dimmuborgir, a phantasmagoric landscape of pinnacles and arches where a basalt flow collapsed into all sorts of odd shapes. Here are three handheld GigaPans (stitched with Microsoft ICE) that convey the strange shapes to be found there:

Link GigaPan by Callan Bentley

Link GigaPan by Callan Bentley

Link GigaPan by Callan Bentley

In addition, I brought my Theta 360 spherical photo camera, for it seemed like an ideal setting for that particular flavor of immersive medium. Here are a few of those shots:

Spherical Image – RICOH THETA

Spherical Image – RICOH THETA

Spherical Image – RICOH THETA

Spherical Image – RICOH THETA

Spherical Image – RICOH THETA

Spherical Image – RICOH THETA

Lastly, you can check out a Google Map of the area:

That should give a sense of the extent of this odd troll-tower landscape, as well as the larger regional context.

The mission of my trip is to generate lots and lots of images like these that can be used to convey geologic understanding and facilitate access to far-flung and exemplary locations. I’ve got support for this venture mainly from the Chancellor’s Commonwealth Professorship through the Virginia Community College System, and also the GEODE project, which is funded by the National Science Foundation.

I’ve been making a lot of pictures of cool stuff, and I can’t wait to share it with you. Stay tuned!

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10 June 2016

Friday fold: Mesoscopic structures in the Lightning Creek Schist

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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:

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

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Folded vein at center:

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This next one has a lot of structure going on…

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

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

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

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Two views of a fold expressed partly in 3D:

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A small squidge’um nearby:

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

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And let’s close with this lovely isoclinal fold…

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

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