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:
19 July 2019
A final Friday fold from Madison, Wisconsin: this one a slab of cut and polished banded iron formation from Australia:
What exquisitely beautiful rock!
12 July 2019
Here is an outcrop of folded limestone along route 430 in Newfoundland, inside Gros Morne National Park, just west of the crossroads called Wiltondale:
A detailed look at the left antiformal portion of the outcrop:
A zoomed-in examination of the rightmost part, where a goopy looking synform resides:
Just down the way, a second outcrop shows another fold with the same sense of asymmetry, on a smaller scale:
Happy Friday to you. Enjoy your weekends.
11 July 2019
I’ve been traveling on the island of Newfoundland for the past few weeks, seeing world-class geology, but not having a lot of time to delve into blogging about it. I have been posting a few pictures each day on Twitter, though, including this one:
OK, geoTwitter super sleuths, what do you see here? pic.twitter.com/SSIqljzj99
— Callan Bentley (@callanbentley) July 10, 2019
There’s a lot to see there. It’s from a place called Sandy Cove, specifically a headland called “Greenings Point:”
This is a site (#63) from an excellent map + guide for the geology of Newfoundland, called Newfoundland & Labrador Traveller’s Guide to the Geology: Guidebook to Stops of Interest, by Stephen Colman-Sadd and Susan Scott (first published in 1994, reprinted in 2003). I was sent a copy of this invaluable guide by Tim Sherry, and I’m very grateful for its guidance, for I doubt I would have dropped $100 for it on Amazon on my own. Thanks Tim!
Okay, back to the rocks! Here’s a blog-hosted version of that same image:
To me, the key elements of the outcrop are bedding and cleavage. There is a variable amount of clay in the beds, resulting in variable development of cleavage. There is a prominent contorted bed in the middle of the outcrop. And both cleavage and bedding are highlighted by differntial weathering of the outcrop in its sea-spray-blasted coastal location.
Overall, contorted bedding is what drew me to this site: the Proterozoic sedimentary rocks here are interpreted as a massive submarine slump deposit, mixing mud and sand and gravel and beds of each of those sediments into a big chaotic jumble. Later, Appalachian mountain-building overprinted the whole mess with a penetrative cleavage.
Let’s start our examination with the slumpy aspects of the site…
There is a diamictite of big bits surrounded by finer stuff:
A boulder of “mixite”
Mudchip-rip up clasts surrounded by coarse sand and grit:
A seam of conglomerate stretches down through this next photo, surrounded on either side by sandstone, with a huge block of (white-weathering) mudrock :
Note that there are no clasts of igneous or metamorphic rock here. This is not a glacial tillite, bringing in material from all over. Instead, it’s just different varieties of clastic sediment. That, and the folded bedding suggest soft sediment deformation and an overall submarine slumping interpretation for the genesis of these rocks:
One curious aspect of the site is that the mudrock weathers white, though on a fresh surface it’s almost jet black. You can see that in this next photo (in the lower right), which shows coherent beds of mudrock on the flanks of a central blob of diamictite (“mixtite”) which contains a few folded slabs of mudrock (presumably ripped off one side or the other – I’m not at all confident which way is “up” in this mess):
On the scale of beds, consider this outcrop:
Relatively undeformed beds there overlie relatively deformed ones. This deformation is not tectonic in nature — it’s ‘soft sediment deformation,’ which is to say the folding and sqloorping* was post-deposition, but pre-lithification.
* not a real word, but it’s got onomatopoeia going for it.
Another fine example of folding:
A thinner example of a convoluted bed, so thin that the convolutions (± subsequent erosion) have separated the bed into distinct segments:
So we’ve got mud and silt and sand and gravel all being deposited somewhere in proximity to one another, and later slumping of these sediments underwater shuffles them up on both the grain-by-grain and the bedding scale, depending on where you look.
Here’s a coherent bed, folded up and surrounded by chaoticly sorted diamictite:
Here’s a similar spot where you can see big slabs of sandstone folded up and mixed in with grit and gravel:
The most spectacular example of contorted bedding I saw at the site was this one:
Note the flat-lying bedding at the bottom (easy to miss it, given the glory of the jellyroll above!).
Here are a set of coherent beds shuffled into a few different orientations, with an upper curved package capping a more steeply dipping thick lower package:
A big overturned antiform:
Here’s a nice view showing three packages of sediment: a lower, darker one that’s more or less horizontal and in its original form, a ~20cm (~1 foot) thick contorted bed that weathers white and crops out in a blocky pattern, and a thick brownish upper package showing serious internal deformation:
Okay, so that’s the sedimentary scene. But another prominent aspect of the site was the cleavage, a tectonic structure superimposed upon pre-existing sedimentary features during later Appalachian mountain-building.
Here’s a Z-fold with cleavage running cleanly through the whole thing:
The trace of the cleavage and the bedding could both be really enhanced by a bit of seaside weathering:
Here’s a neat spot that shows clasts of mudrock surrounded by sandstone, but only the mudrock is discernibly cleaved:
I should also mention: there was also a substantial igneous dike at the site:
Another perspective (from the opposite side), though lacking the same lovely sense of scale:
The rock making up this dike was a greenish sort of basalt, hosting a few amygdules:
Finally, there was a chasm that kept us from exploring further along the outcrop, but its deep declivity hosted waves that impacted at the back end and echoed/reverberated in the deepest tones through the rock and air – a sound felt as much as heard. Here is a look into the abyss —
…Stare not too long into it, lest it stare into thee…
I think that about covers it! Thanks for joining me in exploring this super cool outcrop.
5 July 2019
Today’s Friday fold is rendered in stained glass, along with a bunch of other geological details, as seen at the entrance to the geology museum at the University of Wisconsin (site of our previous Friday fold):
21 June 2019
I’m at a workshop in Madison, Wisconsin, this week.
I took the lunch hour today and walked over to the geology department to check out their rock garden and geology museum.
I was pleased to find a Friday fold in the rock garden: a limestone with cherty nodules/layering that has been folded….
Bonus: some nice bookshelfing/boudinage of the chert:
…And here’s another boulder of the same lovely stuff.
And here is another little train of cherty parallelograms:
There was also a chunk of what appears to be the same thing in the museum inside…
14 June 2019
Last weekend was the annual meeting of the eastern section of the National Association of Geoscience Teachers. On Friday afternoon, we visited Harpers Ferry, West Virginia, and my colleague Beth Doyle led a great field trip to examine the rocks exposed there. This was my favorite outcrop we saw:
Here is a close up of this outcrop, which is framed by (anthropogenic) rock wall:
Dipping shallowly from upper left to lower right is a tectonic foliation: that is to say, slaty cleavage. This foliation has been folded into a well-defined kink band by late Paleozoic Alleghanian deformation. The rock itself was a mudrock (~shale) when it was first deposited: the Cambrian-aged Harpers Formation. Bedding is also visible in places at Harpers Ferry, but I can’t see any in this outcrop.
Here is my sketch, from my field notebook:
…And lastly, here’s an annotated copy of the second photo:
…Happy Friday to you!