9 September 2016

Friday folds: A study in contrasts at the Walls Boundary Fault, Shetland

Today, in search of folds, we head for a fault!

This is the Walls Boundary Fault, looking north along its trace near Ollaberry in mainland Shetland, U.K.:

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Can’t see it? Here, let me help:

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Still not obvious? Okay, let’s try this, the view from the same spot but looking in the opposite direction (south):

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…Annotated with the trace of the fault:

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Still not clear? Okay, I guess I’ll have to bring out the big guns. Northward view again, but closer to the north side of the peninsula:

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Slightly oblique view, with local geologist and guide Allen Fraser standing on the opposite side for scale:

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Peering across the fault from Allen’s vantage, at the opposite side:

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…And the reciprocal view, from the beach below, looking back the way that I came:

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Here’s the Google Maps view of this neat place. The trace of the fault strikes about 010° here, and the Ollaberry Peninusla just barely includes the fault. The eastern tip of the peninsula is a hard pink granite. West of the Walls Boundary Fault are folded Dalradian metasedimentary rocks – and if we want to find a Friday fold today, that’s where we are going to have to turn our attention!

I spent about two hours here. I put the GigaPan to work, documenting the vertical fault plane:

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Here is the result:
Link 2.12 Gpx GigaPan by Callan Bentley

The Walls Boundary Fault is interpreted as the northernmost strand of the Great Glen Fault – the one that’s so famous in Scotland for being etched out into Moray Firth, Loch Linnhe, Loch Lochy, Loch Oich, and (of course) Loch Ness. It runs from Inverness down to Fort William, and beyond — in both directions!

Here, as noted, it has juxtaposed granite and metasedimentary rocks. But a big mass of granite doesn’t fold very well, and today’s Friday so if we want a fold, we’re going to have to look at the western side of the fault, where Dalradian quartzites (metaarenites) and phyllites (metapelites) may be found.

And:

They. Are. Folded.

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You may have noticed that I just showed you two very different styles of folding – tight kink folds in the strongly layered pelites, and broader, loopy tight folds in the thicker mechanical units of the quartzites.

Presumably, these two different lithologies were subjected, broadly speaking, to the same stresses as the rocks of the west side of the fault were dragged along the rocks on the east side of the fault. But they reacted differently. They strained differently.

Compare and contrast:

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Here is another GigaPan, of one of the folds in quartzite, that I shot at the site:

Link 1.16 Gpx GigaPan by Callan Bentley

Now try getting a 3D perspective on each kind of fold:


3D model by Marissa Dudek (using a photo set by Callan Bentley)


3D model by Callan Bentley

Pretty nice folds, amiright?

Here’s the view I had as I hiked out of this extraordinary place, back up the trace of the fault:

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8 September 2016

Tension gashes in Malmesbury metasediments near Sea Point Contact

While en route to the Sea Point migmatite contact last week, I saw some lovely quartz-filled tension gashes in the Malmesbury Group metasediments adjacent to the contact with the Cape Granite. These en echelon tension gash arrays speak of tremendous stresses ripping and stretching these rocks, often in complicated patterns where blocks moved relative to their neighbors, and the strain was taken up in these brittle/ductile arrays of ‘stretch marks’. Here are three examples, worthy of inclusion in a structural geology textbook:

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Annotated versions of the same three photos:

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Nothing like a little structural geology to get the blood flowing on a Thursday morning, amiright? Enjoy!

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4 September 2016

Lab Girl, by Hope Jahren

lab-girl-by-hope-jahren-275Lab Girl, by Hope Jahren, is a unique memoir. It describes Jahren’s journey through life from her childhood in the frigid northern Midwest to her eventual success as a celebrated scientist and an original thinker. The first thing you should know about it is that it is exceptionally well written (Hope Jahren Sure Can Write, after all) – evocative with joy and pathos and well articulated anxiety and the deep, mindless satisfaction of true friendship. The friendship, with her lab manager Bill, is the unexpected theme of the book. I had expected the childhood stories of becoming interested in science, and I had expected anecdotes of the challenges of making one’s way in academia as a woman. But I hadn’t expected the heartfelt, poignant revelation of her deep, fraternal connection to Bill. It shines through the book as the most profound relationship in her life, connected essentially with her very nature as a scientist. Her relationship with her husband in contrast, while described in clearly adoring terms, falls into the shadow relative to the book’s depiction of her relationship with Bill. Their dialogue is a delightful repartee – a treat to read or listen to (I listened to the audiobook, read by Jahren herself, which I recommend). Another unexpected surprise was the eloquence with which Jahren described her bipolar disorder, starting (uniquely in my experience) with an “insider’s view” of a manic episode as manifested in the scientific method. It’s insightful, and unsettling.

Jahren works on plants as aspects of the geobiosphere. The biographical parts of the book are interspersed with short chapters devoted to the extraordinary abilities of plants, as viewed from the plants’ perspective. These chapters are interesting in their own right, but I think they really work well in the context of Jahren’s driving passions as a geobotanist, soil scientist, and geochemist. They illustrate the unique way she thinks about botanical and pedological systems – a mind free to ask questions that haven’t even occurred to anyone else. Jahren’s work has been recognized with several prestigious awards, including the James B. Macelwane Medal from AGU and the Donath Medal from GSA. She’s a decorated investigator, and appears to have near boundless energy for innovating in the mentorship of her students. That alone would be reason enough to motivate you to read her book – but it’s a really intense personal tale too, one which will resonate with almost everyone, and after all it’s told very well.

Recommended.

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3 September 2016

Viewing the Sea Point migmatite through the lens of GigaPan

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It was five years ago when I first visited Sea Point, the outcrop on the coast of the Cape Peninsula where the Cape Granite (~540 Ma) intrudes the (meta-)sedimentary rocks of the Malmesbury Group. The outcrop is (a) beautiful and evocative, and (b) of historical importance, as Charles Darwin visited it while on the voyage of the Beagle, contemplating and confirming Lyell’s assertions of the validity of plutonism as he walked on these very rocks. This is a migmatite that formed due to the injection of porphyritic felsic magma between bedding planes of the sedimentary rock, cooking them thoroughly in the process. Migmatites can also form due to anatexis – the partial melting of a rock, but that’s not the case here. Rather than one rock splitting into two components (fractionating), this is two components merging to create one composite zone of swirling mixed-up rock.

I revisited the site this week during the IGC meeting in Cape Town, toting along two geologist friends who hadn’t seen it previously. I brought the GigaPan. Here are the images that resulted.

First, some still shots to give you the flavor of this classic granitic lit-par-lit injection:

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A close-up of the porphyritic texture defined by potassium feldspar megacrysts in the Cape Granite:

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Now for the GigaPans! Enjoy exploring the site from the comfort of your computer, tablet, or phone.

Link GigaPan by Callan Bentley

Link GigaPan by Callan Bentley

A relatively homogenous sill of Cape Granite located a few hundred meters away:
Link GigaPan by Callan Bentley

Here is a sample of Cape Granite (much fresher, and phaneritic rather than porphyritic)  I collected last time I was in South Africa, 5 years ago:
Link GigaPan by Callan Bentley

And, finally, one more shot of the lovely migmatitic injection textures:
Link GigaPan by Callan Bentley

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2 September 2016

Friday fold: isoclinal fold in ferruginous pelite and chert of the Fig Tree Group

A quick Friday fold – Ulundi Formation, basal Fig Tree Group of the Barberton Greenstone Belt, exposed in a creekbed etched into the trace of the Sheba Fault. This is one of the outcrops I visited one week ago today as part of the pre-IGC field trip to the Barberton. The rocks are iron-rich cherts and pelites that have enjoyed some serious strain, presumably due to movement along the Sheba Fault.

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Today’s the last day of the IGC meeting – Off I go to make the most of it! Enjoy your Friday, wherever you are.

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1 September 2016

Archean microbial mats in the news and in GigaPan

Yesterday, an article was published in Nature (Nutman, et al. 2016) announcing the discovery of what may be the oldest macroscopic fossils on Earth, some microbially-induced sedimentary structures (MISS) that show stromatolite-like internal layering. They were found in 3.7 Ga rocks in Isua, Greenland.

The photos I’ve seen of the newly described Greenland structures look pretty compelling. It’s a nice coincidence for me, since last week I saw some of the oldest macroscopic fossils in Africa – MISS in the Moodies Group clastics from the Barberton Greenstone Belt (not carbonates, which would be necessary for them to be stromatolites, sensu stricto). The MISS are preserved as kerogen – indicating that they used to be biological carbon. They don’t show domal structure. The geologists leading our field trip, Christoph Heubeck, Gary Byerly, and Don Lowe, refer to them as “crinklies” because this is what they look like in cross-section:

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The sedimentas are apparently fluvial sandstones and conglomerates, and there were a few cases where we could see the relationship between the mats and large clasts like this chert cobble:

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This one is fairly extraordinary – a ripple train draped in microbial mats; cut I think at an oblique angle (making the high apparent amplitude):

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I GigaPanned the Moodies MISS at six sites on a plateau where they are reasonably well exposed. While these are merely 3.22 Ga in age, I’ll venture to guess that the imagery below represents more pixels of MISS imagery than have been recorded anywhere else.

Enjoy checking them out. For me, visiting these rocks of the Moodies Group was inspirational – these slimy biofilms are the oldest evidence of life that I’ve ever seen, in the field or in the hand.

*If you’re having issues viewing the GigaPans here embedded in the blog post, then just click the “link” link after each one to visit its GigaPan page.

Link GigaPan by Callan Bentley

Link GigaPan by Callan Bentley

And now some variations on the theme…

Fluid escape structure disturbing the microbial mats:
Link GigaPan by Callan Bentley

Microbial mats draped over a rounded chert cobble:
Link GigaPan by Callan Bentley

Conglomeratic layer with microbial mats draped over cobbles and a fluid escape structure:
Link GigaPan by Callan Bentley

Channel cross-bed with microbial mats coating the foreset laminations:
Link GigaPan by Callan Bentley

Lastly — your lagniappe — here’s a baboon who watched me warily when I first climbed up onto the plateau where the MISS are exposed:

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…I guess the next stop for me is going to have to be Greenland! 🙂

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31 August 2016

Ball & pillow and other sedimentary structures in Graafwater Formation, Table Mountain

I hiked Table Mountain yesterday, south of Cape Town, via the notoriously steep Platteklip Gorge. I detoured a wee bit on the “contour trail” which heads toward Devil’s Peak to see some fine exposures of the Graafwater Formation. It’s mainly a red shale with some green shale, and some fine sandstone. This outcrop particularly caught my eye:

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Those are pretty sweet “ball and pillow” structures. Here’s the most photogenic example:

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“Ball & pillow” is a type of soft sediment deformation, when fresh sand is deposited atop soft, squishy mud. The density inversion triggers a load/sag phenomenon, and the sand pooches downward while the mud squooches up alongside, folding the sand into a shape much like an up-side-down mushroom cap.

Want to explore more? I shot a GigaPan there for your edification and amusement:
Link GigaPan by Callan Bentley

I also observed rippled sands (big wavy-topped pink unit in the photo below)…

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…and trace fossils (look for the little white / light-green burrows cutting across the red shale in the close-up photo below:

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And then there was this feature – at first I thought it was a mudcrack (which have been documented in the Graafwater), but then I noticed it had a spiral sort of shape in 3D, and there was a smaller 3D offshoot that pokes out to the right, about halfway up. So maybe it’s a trace fossil. Anyone want to chime in on their interpretation?

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30 August 2016

Northwest Highlands unconformities (3 of 3): Sub-Ardvreck

It’s time to cover the third and final unconformity I observed this summer in the North-West Highlands of Scotland: the unconformity between the Neoproterozoic Torridonian Group below and the Cambrian Ardvreck Group above.

Where I saw it, south of Loch Assynt on the mountain called Canisp, it actually is displayed alongside the sub-Torridonian unconformity. The mountain hosts a “double unconformity”! Here is a view, looking south:

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The bottom of the mountain – its “plinth” is on Lewisian gneiss. Atop that is horizontal Torridonian. The left “flank” of the mountain is Ardvreck that dips at about 15° to the east. So there’s a nonconformity and an angular unconformity in this view.

Here, I’ve highlighted the three units and two unconformities in this annotated version:

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The weird thing about thisangular unconformity is that the Torridonian (older) is horizontal, and the Ardvreck (younger) is tilted. What this implies is kind of extraordinary: The Torridonian was deposited horizontally, then tilted (at about 15° to the west) and then eroded. The Ardvreck was deposited atop this erosional surface (horizontally) and then lithified. Then the whole mess was tilted back to the east, again by about 15°!

Here is a GigaPan of Canisp, from a position slightly further to the west along Loch Assynt’s north shore:

Link GigaPan by Callan Bentley

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22 August 2016

Northwest Highlands unconformities (2 of 3): Sub-Torridonian Group

Let’s now profile the next unconformity in the sequence of pulses of erosion and deposition recorded in the North-West Highlands of Scotland. For a reminder, here’s a cartoon cross-section through the four relevant units:

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Today, we’ll look at the contact between the Lewisian Gneiss and the Torridonian Group (Diabeg Formation), marked with a “2” on the cartoon above, where it is exposed along the shore of Loch Assynt.

There are three outcrops where the contact is well exposed. This one is the most obvious:

Link GigaPan by Callan Bentley

Photo:

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…and with annotations added:

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Here’s a similar outcrop a few feet further west:

Link GigaPan by Callan Bentley

That’s a nice split – 50%/50%. But you can also see ~8 inches of Lewisian at the very bottom of this outcrop too. (Note also the ugly set of paleomag drill holes!):

Link GigaPan by Callan Bentley

Note how the unconformity surface here is more or less horizontal, in contrast to the high-relief sub-Stoer unconformity we saw last week.

Here is an exquisite outcrop of the Diabeg Formation (Torridonian Group) that I found down by the lochside, showing “Dreikanter” pebbles in cross-section (Brazil-nut-shaped clasts, thought to be wind-sculpted in a desert setting a billion years ago):

Link GigaPan by Callan Bentley

Finally, a small sample of the sandstone / conglomerate of the lower Torridonian (Diabeg Formation):

Link GIGAmacro by Callan Bentley

As far as unconformities in the North-West Highlands are concerned, we’re at: Two down, one to go … and the last one is an oddball.

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19 August 2016

Friday fold: marble, Kings Canyon, California

My friend Bill Burton (USGS, Reston) shared today’s Friday fold — Chevron folds in marble, Kings Canyon, California.

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Nice! Thanks for sharing, Bill!

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