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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16 August 2016

Northwest Highlands unconformities (1 of 3): Sub-Stoer Group

There are three major unconformities in the North-West Highlands of Scotland. I’ll be profiling each over the coming weeks.

Schematically, this is their relationship in cross-section:

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The oldest of them is a nonconformity between the ~3.0 Ga Lewisian gneiss basement complex and the ~1.2 Ga Stoer Group sedimentary package (the number “1” on the diagram above). On the coast at Clachtoll, I made two GigaPans of this “gap in the geologic record.” Here’s the first of them:

Link GigaPan by Callan Bentley

That shows a paleo-valley, cut more than 1.2 billion years ago into the Lewisian gneiss, then filled with a jumble of boulders and cobbles. These, later, were cemented into a breccia.

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Here’s a view from the valley across the cove to the spot where the GigaPan was shot from:

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Some clean looks at the breccia filling this paleo-valley:

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Here’s another exposure of the unconformity surface, which dips at a steep angle, almost parallel to the outcrop surface:

Link GigaPan by Callan Bentley

See if you can find pre-erosional structures in the gneiss, as well as recent paleomagnetic research drill holes in the breccia!

It’s interesting to visit because the unconformity surface shows significant relief – in other words, it preserves the three-dimensional nature of the 1.2 Ga land surface, with hills and valleys all smothered by the Stoer Group red beds.

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15 August 2016

The Paps of Jura, with a nice example of orographic effect (plus a seal)

The isle of Jura in Scotland is where George Orwell wrote 1984. It’s just across a narrow channel from the eastern side of Islay, where I spent four lovely days geologizing this summer. Looking across the gap, you can see a cluster of prominent mountains on Jura. These are the “Paps” of Jura, and they are held up by quartzite. I took these photos when driving home after an afternoon of examining stromatolite-bearing Neoproterozoic rocks south of Bunnahabhain.

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I was struck by the clouds sitting over these landforms:

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Though I’m no meteorologist, that appears to me to be a nice example of orographic lifting – where air is forced up and over an obstacle, such as this Pap, and as it gets to higher altitude, the moisture in it condenses to form a cloud. As the air sinks again on the opposite side, the little droplets re-evaporate, and the cloud disappears…

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The more I learn about meteorology, the more I want to learn.

And I promised a seal, didn’t it? Here’s one:

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But they’re cuter closer up, though that means losing the mountains:

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One more shot of the Paps of Jura here:

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And, while we’re at it, here’s a handheld panorama of the scene, seals not included!

Link GigaPan by Callan Bentley

Sorry for the dearth of posting here lately – it’s been a deeply busy time. I hope to get back in the groove again once the semester begins.

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5 August 2016

Friday fold: roadside in Ladismith/Kleinkaroo

Martin Schmidt again contributes a fold – this time from his summer trip to South Africa:
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Pretty great! This is part of the Cape Fold Belt. Perhaps some of you will drive by it when you go to the International Geological Congress in South Africa later this month!

You can view the site in the context of Google Street view here.

Thanks for sharing, Martin.

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2 August 2016

Oldest fossils in the UK: M.I.S.S. in Stoer Group, Scotland

This is the Split Rock at Clachtoll, on the shore of the North-West Highlands of Scotland. You’re looking out to sea, over the Minch. It’s the site that graces the cover of the excellent book A Geological Excursion Guide to the North-West Highlands of Scotland, by Kathryn Goodenough and Marten Krabbendam.

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“Clach toll” apparently means “Split rock” — Go figure.

The Split Rock is an easy landmark to steer toward if you’re in the neighborhood. And steer that way you should, for there are MISS in them thar rocks.

Microbially-induced sedimentary structures (MISS) are essentially stromatolites preserved in siliciclastic sedimentary rocks. Like stromatolites, they are evidence of ancient microbial biofilms which trapped, baffled, and bound sediment. Unlike stromatolites, they do not typically display doming-upward morphologies, and instead look like finely laminated “crinkles” in mud and sand layers. They may be disrupted by erosion, generating flakes or “roll-ups”.

At Clachtoll in Scotland’s North-West Highlands, there are excellent exposures of the Clachtoll Formation of the Stoer Group, a package of ~1.2 Ga sedimentary rock that unconformably overlies the (Archean, ~3.0 Ga) Lewisian basement. Near Split Rock, there are some layers showing MISS in the Stoer Group. I took some photos at the site, as well as a GigaPan, and made a GIGAmacro and a 3D model when I got home, too. Feast your eyes:

Here is the GigaPan of the outcrop:
Link GigaPan by Callan Bentley

Within that panorama, see if you can find things that look like this:

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Here, just about the lens cap, erosion has cut into the microbial laminations and filled in the scour with coarser sand:

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Many of the most charismatic examples show crinkly crenulations:

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…And here is a GIGAmacro of a beach cobble I picked up nearby:
Link GigaPan by Callan Bentley

I made a 3D model of this sample, too:

 

Fossil slime never looked so good!

These subtle little features are the oldest evidence of life in the United Kingdom.

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29 July 2016

Friday fold: Lewisian gneiss near Tarbet, North-West Highlands, Scotland

A quick Friday fold here from the North-West Highlands of Scotland:

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These are Lewisian gneisses exposed on a headland northwest of the little outpost of Tarbet (where the Handa Island ferry departs from). I was out there in search of shear zones, but I found plenty of nice folds, too.

There are two main folds in the image – a synform in the foreground (yellow), and an antiform in the distance (pink):

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Happy Friday to you!

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28 July 2016

Currents of air and water, deposition and erosion

Yesterday I showed you two scenes, depicted in two photos each, that I saw on the beach at Machir Bay, Islay, last week. I suggested that it might be fun to compare and contrast them.

Scene #1 was this:

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Scene #2 was this:

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Scene #1 is a place where aeolian (wind) currents were at work. They appear to have stripped away some of the sand protecting these pebbles, and then deposited that sand in the lee of each pebble, building out little pointy ridges of sand to the left. These are additive features, standing up in positive relief. A smattering of rain bombarded the sand, roughening up the surface thereafter, helping hold everything in place for me to photograph it. In contrast, Scene #2 was further down the beach, where the tide had receded post-rainstorm, and so the sand appears much smoother in these photos. But as the water drained down the beach, it had to divert around the pebbles, focusing its erosive power immediately on each side of the pebble. These created little pebble-induced flutes, which flare out and disappear downstream (to the right). These are subtractive features, etched into the sand in negative relief. So overall, a few meters apart, we have a story of two currents, one wind and one water, and examples of what changes currents can impart: they can erode or they can deposit.

Did you notice anything else worth talking about here?

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27 July 2016

Compare and contrast: pebbles on a beach

Here are two scenes, depicted in two photos each, that I saw on the beach at Machir Bay, Islay, last week. Compare and contrast them.

Scene #1:

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Scene #2:

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What stories do these pebbles tell? Answers tomorrow…

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22 July 2016

Friday fold: The walls of Scalloway Castle

When in Shetland, one of my first stops was the museum in Scalloway, and one of the ancillary benefits of visiting there is the castle next door:

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Scalloway Castle includes building stones derived from the local limestone – a Neoproterozoic unit that has recently been chemostratigraphically correlated with Snowball Earth cap carbonates elsewhere in the world. But that need not concern us today. Today we are here for the folds!

Check out these fine examples:

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There are also some adjoining examples of boudinage:

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(Zooming  in on a boudin neck from the previous example…)

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Hope you’re having a happy Friday!

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