29 June 2016

Trace fossils in sandstone from Barns Ness

Check out this sandstone cobble I saw at Barns Ness – it comes bearing gorgeous trace fossils. Can you spot them?

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Lens cap for scale in all these photos. The next three are close ups of the burrows from the previous image:

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Plus two more, from other cobbles I encountered::

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28 June 2016

“Dunbar marble” at Barns Ness, Scotland

Thanks to the website ScottishGeology.com, run by Angus Miller, I learned of Barns Ness, a Mississippian-aged limestone fossil site on the shore not far from where we are staying at Dunbar. We ventured out there on Saturday afternoon, in search of fossils.

The presence of the Dunbar Cemenet Works nearby is an indication that this is the most extensive limestone outcrop in central Scotland.

I set my field assistant loose on the beachy outcrops to find fossils:

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The first one we spotted was a familiar sight from the shales back homeZoophycos, a feeding trace fossil:

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Then, as we worked our way down-section, the crinoids started showing up:

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Other, meatier traces also appeared:

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We spied a few large cross-sectioned shells – though I’m not sure quite what they would be. Maybe massive brachiopods?

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Also, there were corals. Very nice corals.

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A few more examples:

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Finally, we found what, for me, was the object of the trip – the “Dunbar Marble,” a limestone (not a marble) that was quarried as a building stone (in particular, for fireplaces) because of its lovely super-rich content of rugose corals, which are the white bits shaped like little cornucopias:

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Here is the cleanest, clearest face I could find showing a cross-section of this extraordinary bed:

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All of the preceding examples of the “Dunbar Marble” have been battered a bit – exposed more through the action of physical weathering (think regular poundings by pebbles and beach sand) than chemical weathering (slow dissolution of the carbonate, a bit faster in some spots and a bit slower in other spots). But this example that my wife found bucks that trend. Click on it to see the fine details of the segmentation and septae in the corals. Bonus: there’s a modern limpet shell to examine, too:

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Here are three GigaPans of the outcrop, which as you can see is half buried in beach sediment, if you want to explore them a bit:

Link GigaPan by Callan Bentley

Link GigaPan by Callan Bentley

Link GigaPan by Callan Bentley

While the GigaPans were shooting, I went for a stroll across the tidal flats. Heading for a prominent outcrop in the middle distance, I hopscotched my way accross a lumpy surface that looked stromatolitic:

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I couldn’t detect any convex laminations on that surface, but since the tide was coming in, I also didn’t spend much time on the task. Another possibility is that these lumps and hollows represent original features in the bedding – perhaps they are associated with tree roots, according to the pamphlet linked to above. However, I did spy a boulder that had some dome-ish looking internal layering:

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If I’m right that these are stromatolites, then the sample is “upside down” in these photos – the dome would bulge upward toward the sun in life position. Stromatolites are thus geopetal (“way-up”) indicators.

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Elsewhere, I found very crinkly little CaCO3 features, which didn’t quite appear regular enough to qualify as stromatolites, but I wasn’t really sure what else to classify them as. They didn’t appear to have any kind of regular anatomy like I would expect from an invertebrate animal:

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On a few of those, I see a shape and internal anatomy that suggests articulate brachiopods, as well as slightly-lighter colored internal sediment, suggesting a hollow chamber. But elsewhere (like the lowermost left), it looks like a layered repetitive crinkly mass.

I guess it could be more than one type of fossil. Anyhow: the tide’s coming in! Keep moving!

I arrived at my target outcrop, expecting it to be the weathering face of a tough bed, but instead I found a wall-like tabular projection. It’s a fault – weathering out in positive relief!

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The fact that the fault weathers out in as a “wall” is plain if you look at Barns Ness from the perspective of Google Maps. It’s the feature that strikes northeast-southwest across the middle part of this view. Zoom in and you can see it casting its shadow:

Close-up, it shows lots of brecciation:

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Fluids percolating along the fault cemented these fragments back together, and indurated the fault trace, making it more resistant to weathering than the neighboring host rock.

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The rising tide cut off my retreat along the path I had just trod, so I headed inland, and found that beneath the fossiliferous strata was a recessively-weathering shale (aren’t they all?) and at its base a thin coal seam:

 

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This site, then, is a nice exemplar of the many small transgressions of sea level during the Carboniferous that buried coastal swamps (bayous) under offshore marine sediments, condemning the plants to become coal. Groundwater flow seems to percolate through the limestone and sandstone well enough, but finds the shale an aquitard, resulting in some small effusive springs above the beach:

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All told, Barns Ness had much to hold the geologist’s interest – and I’d spend more time here if I had it.

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27 June 2016

Small faults in upper Old Red Sandstone, Dunbar, Scotland

Dunbar, Scotland, is a nice little seaside town that also happens to be the birthplace of the conservationist John Muir. My family and I have been based out of here this week on our European geological GigaPan expedition. But on our first morning, upon visiting Siccar Point (which is nearby), I threw out my back, and spent most of the next two days recuperating. I did manage a short walk to the beach in a cove on the northwest side of town.

My son instantly keyed in on the puddles, and took his pet centipede there to play in the water:

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The clean exposures of the bright red sandstone there caught my attention, and I then did my own ‘keying in’ – this time to the many small offsets in the primary sedimentary layering. They are exquisite!

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According to the local geological map, these rocks are the Devonian- Mississippian upper portion of the Old Red Sandstone – the same stuff that caps the unconformity at Siccar Point.

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It was apparently quite disturbed by the intrusion of a collection of igneous masses at ~345 Ma. Many of these small faults looked to me like soft-sediment deformation, so I don’t know that I would have invoked volcanic molestation if I hadn’t read about it, but regardless, they are a delight to behold. I think the primary reason I fell in love with structural geology is that structures like these are just plain beautiful.

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25 June 2016

A virtual field trip to the Giant’s Causeway, Northern Ireland

The Giant’s Causeway, on the coast of Northern Ireland, is a classic site for beautifully-developed cooling columns in basalt. It should be a stop on any geologist’s Grand Tour of the world’s most important geological locales. Battered by the waves of the Irish Sea, the columns are exposed in three dimensions, and visitors can wander all over them. It’s an excellent way to gain an appreciation for the shape of the columns, and their detailed features – vesicles, ball and socket joints subdividing columns, laterite horizons between flows, spheroidal weathering, etc.

I visited the Causeway last week with the intent of collecting imagery for a virtual field experience. Let’s begin our exploration with some traditional photos:

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For those who are unfamiliar with cooling lava, this place looked so geometrically regular as to suggest intelligent design – it must be man-made! Or perhaps at least giant-made. The locals attributed the site to a giant, Finn MacCool, who tried to build a causeway to Scotland to aid in an altercation with another giant. The columns were thought to be hexagonal paving stones.

But geologists who are familiar with cooling columns from elsewhere in the world have another explanation.

To suss it out, I brought my field assistant with me:

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It didn’t take him long to find some mud – and in this case, the mud had dried out, producing a set of intersecting desiccation cracks. The mud loses volume when the water in it evaporates – so it contracts. The stresses generated by this contraction are greater than the strength holding together the sedimentary particles in the mud. So they separate. These cracks met one another at a more or less 120° angle. They subdivided the previously-continuous sheet of mud into a polygonal set of chunks.

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… This makes them easier to pull up and break out:

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The same process is true of cooling basalt. Here, the volume loss is not due to water evaporating, but simply due to the contraction of warm rock into cool rock. Cold rock is more dense than warm rock. Warm rock is more “poofed out” than cold rock. Once again, stresses are induced, and once again, the stresses are greater than the strength of the crystallized basalt. The 120° angle is again prevalent – as three directions of tension are the minimum number (simplest solution) needed to break up a contracting sheet of basalt with ~equally spaced centers of contraction. The result? Three main orientations to the fractures, and as they intersect they make hexagonal blocks.

Not every column is hexagonal in map view; there are also 5 (6-1) and 7 (6+1) sided columns reasonably commonly represented, as with this “daisy” like arrangement. But 6 is most common.

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Though the solid blocks created by these fractures appear polygonal on the surface of the flow, as they propagate downward (or upward), they divide the solid basalt into prismatic chunks, the ‘columns.’ These may fracture further as cooling progresses, and the shortest distance from one side to the other is orthogonal to the column’s long (~vertical, in this case) axis:

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Here is some fresh basalt – fine-grained (aphanitic), dark (mafic). It’s not actually a very common sight at the Causeway, due to the weathering that takes place more or less constantly in the damp Irish climate:

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Here are a few shots of the basalt that’s rotting in place to form semispherioidal “kernstones”:

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Okay, now for the interactive imagery. Henceforth, all the imagery in this post is “interactive” in the sense that you can manipulate it – choose the level of zoom for the Google Map and the GigaPans, control the level of zoom and the perspective for the 3D models and the 360° spherical photos.

If you need context, here’s a Google Map of the site, though the resolution isn’t especially impressive:

Here are seven GigaPans, showing various aspects of this amazing place:

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

Link GigaPan by Callan Bentley

Link GigaPan by Callan Bentley

Here are two 360° spherical photos made with a Ricoh Theta S:

Spherical Image of the Giant’s Causeway, Northern Ireland – RICOH THETA

Spherical Image of the Giant’s Causeway, Northern Ireland – RICOH THETA

And finally, here are three 3D models of the basalt columns – photo sets by me, with Photoscan model construction by my student Marissa:

Photoscan model by Marissa Dudek

Photoscan model by Marissa Dudek

Photoscan model by Marissa Dudek

We already featured one of Marissa’s 3D models of the “onion skin” spheroidal weathering, but here it is again, along with a second example of that phenomenon:

Photoscan model by Marissa Dudek

Photoscan model by Marissa Dudek

While these media convey a sense of what it’s like to visit the causeway’s geology, if you ever get the chance to visit in person, you simply must. In the meantime, enjoy exploring these images as a ‘virtual field trip.’

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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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