13 September 2019
AGU’s Chief Digital Officer Jay Brodsky offers up a fresh European fold for you today — and this one is on rather a smaller scale than Jay’s last Friday fold contribution…
These are lovely crinkly folds in highly foliated rocks. I love boxy little crenulations like these.
Jay tells me that this is from
Graines, Italy, in one of the valleys of the Val D’Aosta right by this castle.
Jay asked “What is it?” and I replied that “it looks metamorphic.” But I also made an attempt at digging deeper:
I brought up a geologic map of Italy, and draped it onto Google Earth, lining up major cities and Milan to get it positioned correctly, and then put the Castle of Graines into the search button:
… So that is this spot, and the rock type there appears to be rock type 99, “medium grade metamorphic rocks:”
Well, that doesn’t tell us too much, but these appear to be metamorphic rocks (‘scisti!’) that enjoyed deformation during Alpine mountain-building.
As always – if anyone else recognizes these lovely folds, or knows any greater detail about the geology of this region, chime in via the comments below.
Thanks — and Happy Friday!
6 September 2019
Science writer Gabe Popkin shared two fold photos with me this week – both from near Sargans, Switzerland, adjacent to the Rhine River Valley and the border with Lichtenstein. The photos shows the mountain called Gonzen. There, Jurassic limestones crop out in a very wavy pattern:
I don’t know the geology of this area in any kind of detail, but I decided to trace out a distinctive upper surface of these limestone beds to get a better sense of what’s going on.
…The big central antiform appears to be overturned!
It occurred to me that this potentially implies some faulting, like this:
These are merely armchair speculations as I try to make sense of the pixels in the photo – but it makes me eager to go and visit the Alps to see these rocks in person! In the interest of “doing my homework” on the site in the limited amount of time I have this week, I found this paper, which seems to show at least one thrust fault on the mountain in one figure, and suggests these Jurassic limestones are part of Glarus Nappe Complex just to the southwest, which would be awesome.
If anyone reading the blog knows more about this area, please chime in below…
Thanks for sharing the images, Gabe!
Happy Friday, all.
3 September 2019
The last outcrop I visited in Newfoundland this summer was an angular unconformity, exposed along the shore at a place called Bacon Cove, near the head of Conception Bay:
Here, I’ve traced out the unconformity surface in yellow:
Unconformities are gaps in the geologic record – structures which juxtapose two different geologic units along a surface formed through an extended period with no rock-forming activities at that site or (more commonly) erosion, which destroys rocks at the site. At Bacon Cove, erosion was the culprit. First a green shale formed, then it was tilted to a steep angle, then it was eroded, probably along a rocky coastline much like Bacon Cove appears today, and then it was buried in dark limy sediment, sand and pebbles and mud.
Here is another view of the unconformity where the different orientations of the strata are plain:
There are additional aspects of this site that are intriguing: note the differential weathering of the one layer in the upper unit, weathering out to form a series of little hollows. Also note the prominent joint set that transects both units, indicating it was imposed after both units existed and were lithified.
One nice thing about Bacon Cove is how high-relief the unconformity surface is: there are lots of swales and bumps on it.
It’s also kind of patchy: there were spots where ancient “potholes” had been filled in with the upper limestone, and were preserved today as elliptical patches of the younger unit. Here is a spot where erosion making the modern rock surface has penetrated through to the unconformity and into the lower unit, breaking the upper unit into several patches:
Here is a place where the upper unit is particularly sandy:
There were also boulders lying around that crossed the unconformity surface – what an amazing sample this would be to have in one’s rock garden! Earlier in the trip, I was impressed by several of these on display at the Johnson Geo Centre in St. John’s.
Here are a few more shots of in situ unconformity exposures, where I’ve cleaned up the unsightly coring holes using Photoshop:
I like these because they show the presence of substantial pebbles/cobbles as inclusions in the overlying unit. In this next shot, those all appear to be locally derived examples of the older greenish shale:
The lower (older) unit here is Ediacaran in age, and is correlated with the fossil-bearing rocks at Mistaken Point. The upper (younger) unit here is Cambrian. So in the grand sweep of geologic time, there’s not a huge amount of time missing at Bacon Cove. But it was enough to take horizontally-deposited mud, turn it to rock (lithify it) , and then rotate it and lift it up to where it could be partially eroded, before it was re-submerged and buried anew.
30 August 2019
Happy Friday, good people!
We close out the workweek with a fold, and today it comes to us from Larry O’Hanlon, who manages the AGU Blogosphere.
Larry writes that he was recently in California, and…
I noticed a little fold in a sea cliff at Calafia State Beach in San Clemente. I barely managed to snap a few pictures before being pulled away by the kids. Looks like soft sediment deformation of beach sands? As it is about 100 m from the surf and maybe less than 10 m above sea level, I’m guessing this is a fairly young deposit exposed by uplift.
Here it is:
I’ve attempted to trace out bedding here, but it’s hard to do in the most tightly folded bit:
I agree that this looks like a soft sediment slump fold.
Thanks for thinking of the Friday fold when you were traveling, Larry! I hope all readers of the blog will consider snapping some fold photos when they see them during their global peregrinations…
23 August 2019
Eight years ago, I featured a spectacular Friday fold here, showing an antiform exposed dramatically on the coast of Greenland‘s King Oscar Fjord. The photo was by Alistair Knock. Alistair and his wife Marie are astoundingly good photographers, and their website, Taraji Blue, showcases other visually astonishing work that has come out of their cameras.
Eight years later, I’m in the business of planning an “art” exhibition of superb works of geo-visualization as part of a Pardee Symposium at the annual meeting of the Geological Society of America. Among the ~35 works I’ve solicited for inclusion in the show, I had Alistair’s Greenland fold photo in mind as an excellent example of a visually compelling image that was packed full of geoscience content – atmosphere, hydrosphere, lithosphere: it’s all there! I am pleased that Alistair’s original photo of the fold is joining the show. The rest of the exhibition will be diverse and varied, in both medium and subject matter. It’s a really powerful collection, and I am very pleased at all the astounding contributions. If you are going to be at GSA, don’t miss it!
Anyhow, back to Alistair’s photo…
Greenland’s been in the news a bit lately, you may have noticed – not only has it seen record levels of melting of its central ice sheet, but the addled President of the United States apparently has gotten snippy about not being allowed to make an offer on the purchase of the huge island, resulting in a brand new oddity of a diplomatic row with Denmark. (I know we’re probably all desensitized to such things at this point, but It. Is. So. Strange.)
Anyhow, let’s set aside the ice and the politicians, and let’s return to this outcrop for today’s Friday fold, with several new views:
And here’s a GIF, from video, zooming out to show the wider context:
Who wouldn’t want to buy that fold? I know I would!
Happy Friday to you and yours…
16 August 2019
Quick, awesome Friday fold here from the Canadian Rockies and Maggie Romuld:
Maggie also posted another intriguing image of her hiking in the Canadian Rockies – and set geoTwitter abuzz with a discussion of whether she had captured load casts bulging out of the bottom side of a bed or stromatolites projecting upward from the top of a bed. Have a look & read the ensuing discussion here.
9 August 2019
It’s Friday, so it’s time to turn our alliterative attention to folds.
Here is a sweet specimen I collected this summer in St. Anthony, northern Newfoundland:
Link 1.39 Gpx GIGAmacro by Callan Bentley
I’m still trying to wrap my mind around just how this rock formed. Here’s what I’ve got so far, and I’d appreciate any insights you have to share. This appears to be a porphyritic volcanic rock that has been metamorphosed (green color), but its most distinctive feature is the very thin layers which record the deformation so well. The white “phenocrysts” are not individual crystals however, but lumpy aggregates that appear to include both opaque feldspar and transparent/translucent quartz. Having quartz phenocrysts would imply a felsic original composition to the volcanics, but I’m not sure how that gets us to what I would guess (again based solely on color) to be chlorite, which needs a healthy dose of iron and/or magnesium.
Note also the differential weathering that is apparent between some of these layers. Some bright green lichens have roosted in the interstices provided by the recessive layers.
At the same site, there were more massive outcrops that showed a well-developed visible alignment of mineral grains. I spotted this distinctive rock halfway up the cliffs, long before I found the thinly layered, folded lithology up top.
Two views of one sample of the more massive lithology here:
Link 1.24 Gpx GIGAmacro by Callan Bentley
Link 1.05 Gpx GIGAmacro by Callan Bentley
(The first one of these is untreated; the second is polished.)
My first instinct was to lock on to their green and white pattern as a mylonitic fabric. But upon closer inspection and pondering, I have shifted to interpreting the alignment as a primary volcanic flow banding, on the basis of lack of ribboning of quartz and the presence of small void spaces in some locations (empty spaces wouldn’t persist under the high pressures needed to make a mylonite).
What do you think of these rocks? I tried to find out more about them, but unfortunately the site was not written up in any substantial detail in either of the two field guides I used in Newfoundland this summer. (One said, in essence, “There are a bunch of volcanic rocks near St. Anthony.”) And I was not able to access a detailed geologic map of this part of the island. So I’m left with a vague sense that I’m missing something, and I suppose the next step would be to make some thin sections to get a better handle on the petrology and mineral assemblage.
Regardless, have a happy Friday!
2 August 2019
So there I was, driving west on the southern Burin Peninsula, when we passed a roadside quarry, and a shimmery apparition caught my eye – low angle “X” shapes, gleaming in diamonds and criss-crosses in silver and dark gray, the colors shifting as the car carried me past.
Hit the brakes!
Back up, park, jump out with the camera.
The rocks were gleaming with the sheen of muscovite, but they also had gritty pebble-sized bits in them – a phyllitic diamictite? A meta-tuff? (The latter’s more likely on the Burin Peninsula, which is dominated by volcanic strata.)
Regardless, these suckers got kinked!
Kink bands are crisp folds that occur in highly mechanically layered rocks when they are compressed at some angle to that layering.
These boulders show nice conjugate pairs of kink band orientations. If they were in place, rather than piled up higgeldly piggledly, then a structural geologist would be able to work out the direction of maximum compressional stresses that acted on these rocks.
27 July 2019
Yesterday, I featured some folds from Broom Point, but there are also faults there. With the intriguing local limestone conglomerates providing easily-discernible marker beds, these apparently vertical faults are easy to spot. Here are three examples:
26 July 2019
Here’s a look at some of the outcrops at Broom Point, within sight of the famous uplifted fjord called Western Brook Pond:
The limestone beds here are Ordovician in age, and they dip to the east:
In places through there are folds to be spotted in the beds: