3 November 2016

A “triple tombolo” in northern Shetland

We’ve taken a look already at an exemplary tombolo from Shetland. Today, I’m dialing up the tombolosity of the blog with a Triple Feature:

triple-tombolo-sm Click to make much larger (8000 pixels wide)

If you look closely here, you’ll see that only the rightmost bar fully connects the two islands. It’s the only true tombolo, sensu stricto, at this site. The other two perhaps deserve another name, though I’m not sure what that would be… bayhead and baymouth bars, perhaps? (Though the “bay” is in fact defined by the tombolo, so that seems problematic.)

Here’s the place in Google Maps. It’s in the north of Mainland Shetland, just shy of the ferry to Yell:

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29 October 2016

New digital media of Shenandoah National Park feeder dikes

In Shenandoah National Park, astride Virginia’s Blue Ridge, feeder dikes of Catoctin Formation (meta-)basalt cut across the Grenvillian-aged granitoid basement. Due to their mafic composition and columnar jointing, these feeder dikes generally weather more rapidly than their host rocks. I led a field trip in the park on Thursday for my son’s school, and my student Marissa was there the weekend prior, checking out the autumn leaves and geology with her family. She and I have two new interactive media to share highlighting these dikes. I made a Theta 360° spherical photo of the feeder dike at Mary’s Rock Tunnel (south of Thornton Gap), and Marissa made a 3D model of the most prominent dike north of the Little Devil’s Stairs Overlook in the northern third of the park. Check them out:

~9 foot thick feeder dike of Catoctin Fm. meta-basalt cutting through Grenvillian basement in @shenandoahNPS Theta 360° Spherical Image by Callan Bentley

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28 October 2016

Friday fold: “the eye of Sauron” at Sibubule

Here’s a scene from the pre-IGC field trip I participated in, a week-long examination of the Barberton Greenstone Belt:

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Look at the mountain over to the right. It’s “looking” back at you!

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This is Sibubule, 1765m in elevation, as viewed from the R40 in Barberton Mountain Land, not far from the border with Swaziland.

It shows a more or less recumbent fold, like a taco lying on its side. The perspective of the photo is looking ~south at the ~eastern face of the mountain.

Here it is in Google Maps view:

Our field trip leader, Christoph Heubeck, furnished me with this information:

The mountain is incompletely mapped (that is, we only walked up to the base of the northern cliff and lateral equivalents) but there is a hinge at  25°57’9.10″S  31° 5’46.14″E well visible on Google Earth (best when viewed towards the south), and we have been there. I think the other hinge of the “eye” is at  25°57’35.84″S  31° 5’23.85″E but we have never climbed up there, either. Mapping the mountain would be beautiful and not difficult – alas, so little time…

The fold axial plane is moderately south-dipping. The big south-dipping slope of Sibubule appears [to be] a dip slope in overturned cherts of the uppermost Onverwacht (Mendon Formation).

These cherts bound a thick package of banded iron formation (BIF) of the Mapepe Formation (lower Fig Tree Group) in the middle. As a result, Chrisoph says,

“It’s a blood-shot eye!”

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26 October 2016

Archean meteorite impact evidence from the Fig Tree Group in Barite Valley, Barberton Greenstone Belt, South Africa

During the Archean eon of geologic time, the Earth was young, and had characteristics that were both similar to what we find in the modern planet, and also seemed to have some striking differences. Examples of the former: gravity, oceans, and metamorphism. Examples of the latter: a different atmospheric mix of gases, an absence of plate tectonics, and a much hotter internal temperature (of which the komatiites I shared yesterday are one signature).

Something else that was apparently quite different was regular bombardment by large extraterrrestrial impactors – the early, messy solar system was still in the process of cleaning itself up by gravitationally merging previously isolated bodies. Large meteorite impacts would of course create a crater at the site of the impact, but continual resurfacing on the planet due to the rock cycle makes it less likely to preserve their large region-scale forms. We’ll have better luck if we consider the sedimentary record of those impacts. What sedimentary signatures would a really big impact have on its neighboring or more distant regions?

Answers can be found in the Fig Tree Group of strata, as accumulated in what today we call the Barberton Greenstone Belt. In particular, a spot called Barite Valley in Barberton Mountain Land yields key insights into two separate impact events, dubbed S2 and S3 by Don Lowe, who first described and published what I’ll show you today. We were fortunate to have Don lead us on this excellent field day, along with collaborators Gary Byerly and Christoph Heubeck, and Don’s graduate student Nadja Drabon.

A GigaPan view towards the Barite Valley:
Link GigaPan by Callan Bentley

Two views across the Barite Valley… All the rock sample and outcrop photos you’ll see in the rest of the post are encompassed in these two views:
Link GigaPan by Callan Bentley
Link GigaPan by Callan Bentley

The Archean crust subject to the impact (and perhaps the mantle directly underneath) was ultramafic in composition. Komatiitic ash deposits look like this – like a lightweight dark green chert:

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In it, primary sedimentary structures such as cross-bedding may be observed:

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As you might expect, this stuff apparently has an elevated iridium anomaly, suggesting extraterrestrial elemental input:

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GIGAmacro images to show komatiitic composition ash deposits include primary sedimentary structures like cross-bedding:
Link GIGAmacro by Callan Bentley
Link GIGAmacro by Callan Bentley

These sedimentary breccias are interpreted as tsunami deposits, with the violent wave being the result of water displaced by the impact:

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…Zooming in to the lower center of the previous image, for a closer look at the details of the texture:

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Komatiitic ash with chunks of pre-impact chert in it:

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The impacts could vaporize rock at the site of impact, and this rock vapor condensed in the atmosphere soon thereafter, raining down as small lithologic hailstones called “spherules”. They are layered, like jawbreaker candy, or ooids:

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Each of those little spheres is an atmospheric condensate of rock vapor. They might not look like much, but realizing the utter, unimaginable violence of their formation is profound. That sort of thing is one of my favorite things about geology – being able to notice and identify something like a spherule horizon, and then to realize the enormity of what it implies.

The mind grows giddy staring so deep into the gory violence of geologic history…

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Here is an impactite breccia from one impact, the S2 impact, at Barite Valley. Note the angular chert fragments as well as the spherules, all mixed in together:

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Another sample, showing the Barite Valley’s gorgeous scenery in the background:

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GIGAmacro view of this same sample of the “S2” Impact-generated tsunamite deposits. Explore it until you find a concentrically-layered spherule amid all the other debris…
Link GIGAmacro by Callan Bentley

Three views of a sample of the “S3” impact spherulite layer, featuring both fresh and weathered spherules, as well as a mud drape:
Link GIGAmacro by Callan Bentley
Link GIGAmacro by Callan Bentley
Link GIGAmacro by Callan Bentley

The Barite Valley gets its name from barite horizons within the sedimentary succession, with ~20 cm thick layers of chemically-precipitated barium sulfate, crystals growing up off the seafloor and fanning out. It’s distinctive stuff not only due to its coarsely crystalline texture, but also due to its density:

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These unusual deposits speak of a radical change in ocean chemistry in the aftermath of these large meteorite impacts. Where did all the extra barium come from all of a sudden? Perhaps the impact punctured the crust, allowing some mantle fluid content to mix into the world ocean.

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I was particularly keen to see these barite layers because similar horizons are cited as evidence for fundamental marine chemical weirdness in the aftermath of the Neoproterozoic Snowball Earth glaciations.

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Outcrop of the barite horizon associated with the S2 impactite layer, suggesting a fundamental change in ocean chemistry as a result of the big event:
Link GigaPan by Callan Bentley

GIGAmacro view of two samples of the barite:
Link GIGAmacro by Callan Bentley

So there you have it: that’s what the sedimentary signature of an Archean meteorite impact looks like: It is full of tsunami breccia, fine komatiitic ash, hail-like spherules, and a barium-rich ocean’s crystal precipitation.

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24 October 2016

GIGAmacro views of komatiite

Erik Klemetti posted today at Eruptions about komatiite, which is apropos, considering I just finished imaging some samples of that ultramafic volcanic rock. Have a look at three samples from Barberton Greenstone Belt here, each from the 3.27 Ga Weltevreden Formation:

Link GigaPan by Callan Bentley

Link GigaPan by Callan Bentley

Link GigaPan by Callan Bentley

And, while we’re at it, here’s one from the Red Lake Greenstone Belt (~3.0 Ga) in northwestern Ontario (sample courtesy of McGill’s Christie Rowe):
Link GigaPan by Callan Bentley

Explore that spinifex texture to your heart’s content – jagged olivine ‘chandeliers’ hanging down off the roof of the flow into its interior.

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21 October 2016

Friday fold: a miniature ramp and flat

This is kind of fun – a folded and faulted block of chert (lens cap for scale) as seen along R40 in Barberton Mountain Land, South Africa. This is likely the Archean-aged Msauli Chert (~3.3 Ga). This block shows a nice “ramp and flat” fold/fault structure, a common “structural ingredient” in many fold and thrust belts, including the Appalachian Valley & Ridge province.

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The harsh sunlight made me take a second photo, where the entire sample was in even shadow. I’m not sure, however, which image I prefer.

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Here’s an annotated copy, with bedding highlighted in yellow, and faults in black. White arrows show the kinematics on the main fault. Minor back-thrusts on the trailing edge of this “proto-nappe” may

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A miniature ramp and thrust… Too bad it wasn’t a wee bit smaller – I could have brought it home with me!

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19 October 2016

Oddball Icelandic rocks, part III: conglomerate

At the Volcano Museum in Stykkishólmur, I learned that Iceland has fossils. Specifically, they have a display of bivalve (clam) fossils there, and when I asked where to find them, I was directed to a point further east on the Snæfellsnes Peninsula. The next day, I set off to find them.

Here was what I saw on a cliff high above me, at the spot nearest to where I thought I was supposed to be looking:

 

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There were some suspicious looking outcrops there…

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I climbed up. Here’s the view from on high…

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Rotating, I faced the cliff.

In detail, the outcrop object of my quest showed inclined packages of conglomerate:

iceland-seds-foresetssmClick to make much, much larger

A few adjacent details:

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This was the only outcrop of sedimentary rock I saw anywhere in Iceland. It was too coarse for delicate external molds of clams, but it was cool in its own right on account of its unique status in my experience.

There were also a few boulders showing rotated blocks of the stuff:

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And here’s a cobble hosting a decent graded bed:

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And there you have it. With the green ignimbrite and the granite intrusion, those are the only three rocks I saw in a week in Iceland that weren’t mafic volcanics.

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11 October 2016

A virtual field trip to Portrush, Northern Ireland

One of my favorite places in Northern Ireland is the east side of the peninsula that hosts the tourist town of Portrush. There, two early schools of geological thought engaged in a battle. The opposing sides were:

  • the Neptunists, who thought all stratified rocks, and in particular basalt, must form from precipitation from the sea, and
  • the Plutonists, who thought some rocks, including basalt, formed through intrusion of molten rock from deep within the Earth (potentially followed by its eruption from a volcano).

The Neptunists are exemplified by Abraham Gottlob Werner, and the Plutonists were represented by James Hutton. So why did these titans clash in Portrush, of all places?

This is a spot where ammonite fossils are found in dark-colored, hard, stratified rocks. The Neptunists said, “Aha! Here are sea creatures fossilized in basalt! Therefore basalt must be a rock precipitated by the oceans!” But then Hutton showed up and demonstrated that in fact the rock had been misidentified. The “basalt” was actually hornfels, a contact-metamorphosed mudrock. The strata in question were of the Lias, a latest Triassic / earliest Jurassic unit in British stratigraphy. The Liassic shale became Liassic hornfels when it was baked by the heat of a nearby gabbroic intrusion, the ~60 Ma Portrush Sill. So there is mafic igneous rock here, but it’s gabbro, not basalt. And Portrush therefore proves the Plutonists’ case, not the Neptunists’.

A diagram from an interpretive sign at the site:

img_7993(Note the error on the sign where they substitute “dolomite” for what surely was intended to be “dolerite,” another name common in the U.K., for what U.S. geologists might call “diabase.” In my opinion, the sill is coarse enough to qualify as gabbro. So I’m sticking with that.)

Let’s dig in. Here’s a Google Map of the place, showing that something odd must be up with the geology in order to have such a profound promontory on what’s otherwise a relatively smooth coast.

The Portrush outcrops look like this:

portrush-sm Click to embiggen

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Interesting intrusive structures along the contact zone between the Lias and the gabbro:

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My field assistant examples samples of the different rock types on the rocky beach…

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…the sandy beach…

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…and on the outcrop:

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Here’s what we find in terms of the two main lithologies:

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Hornfels:
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Gabbro:
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You kind of can’t blame the pre-petrographic-microscope Neptunists for mistaking the hornfels for basalt. It’s fine-grained and black and there’s a lot of basalt in this corner of Ireland. It’s reasonable for that to be their initial take.

Hornfels sample on gabbro background:

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Here’s another sample that I felt looked a lot like a basalt:

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Now for the ammonites:

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Here is a gallery of 16 more specimens of ammonites from the outcrops of Portrush. Click through to see them all:

This slideshow requires JavaScript.

My favorite ammonite fossil is this big one, decorated as it is with modern barnacles:

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Portrush is proud of its geologic heritage, and the lovely spiral forms of ammonites decorate many municipal structures:

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This isn’t an ammonite, but it might be Aviculopecten or another bivalve:

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Here are the GigaPans that I made of the site.

Overviews of the baked Liassic outcrops:
Link GigaPan by Callan Bentley

Link GigaPan by Callan Bentley

Link GigaPan by Callan Bentley

Link GigaPan by Callan Bentley

Link GigaPan by Callan Bentley

This one looks particularly basaltic:
Link GigaPan by Callan Bentley

The gabbroic intrusion:
Link GigaPan by Callan Bentley

Link GigaPan by Callan Bentley

Link GigaPan by Callan Bentley

Link GIGAmacro by Robin Rohrback & Callan Bentley

The fossils:
Link GigaPan by Callan Bentley

Link GigaPan by Callan Bentley

Link GigaPan by Callan Bentley

Here’s a close look at a similar ammonite fossil in another shale, close-up.
Link GIGAmacro by Robin Rohrback

Lastly, here’s some sand from the beach nearby. It’s dominated by shells, not by either of the rock types discussed above:
Link GIGAmacro by Robin Rohrback

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10 October 2016

Oddball Icelandic rocks, part II: granite!?!?

Around the corner from the Hvalnes Lighthouse in eastern Iceland is the second non-basaltic igneous rock I saw in Iceland.

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I couldn’t drive past something like that and not stop. Seriously – this was the only high-albedo rock I saw on the entire island!

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(Note the lens cap for scale, as in all the photos in this post.)

Yes, that’s what it appears to be – here on the flanks of the Mid-Atlantic rift, partial melting of the mantle yields basalt and partial melting of the basalt has apparently generated a granite.

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This was unexpected (to me), as I associate granites with more evolved crust – like that of the continents, where the material has been cycled through multiple rounds of “distillation” to pull out the most easily-melted components. (Felsic minerals melt at lower temperatures than mafic ones, so if you partially melt a given rock, it’ll segregate into a more-felsic liquid and a more-mafic solid residue.) Iceland’s apparently got a sort of “hot spot” underneath it (like Hawaii) as well as being situated on a divergent boundary (the Mid-Atlantic Ridge), so I would have expected nothing but basalt. (And indeed, that’s mostly what Iceland is made of!)

I wouldn’t have guessed any of the rocks here would have had the opportunity to get the granite sweated out of them. But: apparently it has happened, at least once.

The granite contains xenoliths of more mafic compositions:

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And now for a few GigaPan views of this place:

Link GigaPan by Callan Bentley

Link GigaPan by Callan Bentley

Link GigaPan by Callan Bentley

Link GigaPan by Callan Bentley

Finally, a GIGAmacro view of a sample I collected at the site. You can explore its phaneritic (coarse-grained) texture and see its quartz content readily with this image:
Link GIGAmacro by Robin Rohrback

A granite in Iceland! Who’d have thunk it?

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7 October 2016

Friday folds: Dalradian schists at St. Ninian’s Isle

Remember St. Ninian’s Isle?

It is connected to Mainland Shetland by a tombolo. But it has rocks there, too. Here are some outcrops on the beach:

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If you visit these schisty fins, you’ll find they are populated by a cavalcade of small folds.

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Some of the folds are crisp things known as kink bands:

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Annotated version:
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And finally, as a lagniappe, here’s a bit of boudinage. This quartz vein has broken into segments, and the segments have separated from one another along the plane of foliation. Note how the foliation deflects into the boudin neck from each side:

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Annotated version:
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