4 July 2014
A final guest Friday fold from reader Howard Allen, who I’m pleased to be meeting up with in Banff late next week…
Howard writes the following in describing this lovely scene:
Warspite Anticline, Peter Lougheed Provincial Park, Alberta. Photo is a telephoto shot (hence the strong blue alpine haze–the colour cast is an accurate rendition of the original daylight Kodachrome slide), looking southwest at an angle to regional strike. The peak at top centre is Mount Joffre, the highest peak in the area. The dark peak in centre, on the left limb of the anticline, is Mount Warspite. Photo was taken in 1998 from the summit of the north end of “Kent Ridge” (= the western ridge of Mount Inflexible) at 50.7706, -115.2210. The peaks in the middle ground, with nearly vertical bedding, between Mt. Warspite and Mt. Joffre, belong to Mount Lyautey. This view shows the opposite (north) side of Lyautey, and the Lyautey Syncline. The syncline axis is hidden in this view by the peak of Mount Warspite, but you can see it on the next range south, forming a snow-covered col on the left side of the photo, about 1/3 the way down from the top.
Thanks, Howard! See you next week!
To everyone else:
- Happy Friday!
- Happy American Independence Day!
- Happy Final Friday Fold For Now… I’ll resume the blogging in August after I get back from travels out in these same mountains.
3 July 2014
What are these Border to Beltway students up to?…
Clearly, they are all immersed in their field notebooks, sketching away. This was in March, in west Texas.
There must be something worth drawing at this road cut…
A clue can be seen on the wall of rock behind them. There, you can find features such as this:
Those are outsized clasts of gray limestone in fine-grained tan limestone. …An interesting mixture! How could such a mixing occur? Perhaps the outcrop has some more clues to share, something to inform our thinking before we rendered an interpretation…
So we asked the students to sketch out what they observed, at several scales, before we began to discuss it.
The clast-rich unit was overlain by a graded bed of limestone “breccia”…
Which fined upward…
And got finer and finer, but still part of one massive bed…
And then that massive bed was overlain by alternating layers of shaly limestone and ~2″ thick limestone beds:
We interpreted this ensemble, which was in the deep water basin seaward of the forereef of the Permian reef complex which sees expression in the Guadalupe Mountains a few miles to the north, as a massive submarine landslide, bringing big blocks of reef material avalanching down into the deep, where the big chunks were included in unlithified carbonate mud.
Thus, something as innocent looking as this…
…can imply a massive, chaotic, underwater catastrophe in the ancient past.
This is why I love geology – acquiring the ability to read the clues allows one to deduce fascinating stories about the otherwise inaccessible past.
Spotted in the yard this morning:
One fungus (yellow, cracked like a breadcrust) being consumed by another (whitish, hairy wisps).
2 July 2014
Here’s a breccia that Dan Doctor and I found in a tabular zone within the Helderberg Group (Devonian limestones) in one of the massive new roadcuts along Corridor H.
Is it a fault breccia or a sedimentary breccia? The breccia was bedding parallel, which suggests it could be just another bed, but it’s so darn coarse and angular (unlike the rest of the Helderberg) that we were skeptical. Indeed, pulling out a chunk (the sample you see above) and examining the bottom (the view you see below), brought additional evidence into consideration:
Those are slickensides! They are most prominent in the vein calcite patch on the left side. Zoom into this macro GigaPan by my student Robin Rohrback and check it out.
It’s a fault breccia. The fault happens to be parallel to bedding (because bedding planes are planes of weakness in sedimentary rocks), but it’s not actually a bed itself.
27 June 2014
Some folds this week from coastal exposures in western Boothbay Harbor, Maine, where I’m on vacation for one more day…
Acadian metamorphics (schist, gneiss), with injected granite pegmatite that has also been folded (and boudinaged):
26 June 2014
Okay, I photoshopped that one up. This one too…
Here are the originals…
And, if you’re a scale-off-to-the-side-of-your-main-subject purist, here’s a different shot of this quintessential boudin:
And, while we’re at it, here are some other fine boudins (of granite pegmatite) exposed along the coast of West Boothbay Harbor, Maine:
Some nice coastal ecology to be seen in that last shot, eh?
25 June 2014
Pemaquid Point, Maine, is a locally-owned and -managed park near an old lighthouse.
I went there yesterday with my family. We’re on vacation in coastal Maine for a week. At Pemaquid Point, the action of waves have cleaned the rocks, and they offer a delightful three-dimensional look at Acadian-aged metamorphics and granite pegmatite dikes, with a fair amount of structural geology superimposed on the whole lot.
Learn more about the site from this “Site of the Month” (July 2002) from the Maine Geological Survey. According to it, the sedimentary protolith of these rocks was Silurian in age, with Devonian metamorphism and igneous intrusion on top of that. (The Maine Geological Survey has a great collection of site reports like this, searchable in map format, here.)
The folds range from open…
Here are a few more:
There is also a lovely suite of examples of boudinage, on many scales:
As you can see, I brought the GigaPan with me, and took a couple of new images with it:
And here are two that are not mine (These next two from GigaPanner “eos101” at Bowdoin College):
20 June 2014
One of the real treats of the spring break field course in west Texas was visiting a road cut of the Castile Formation, a Permian evaporite formation south of the Guadalupe Mountains. It’s on the border between Texas and New Mexico, and I’d been wanting to see it for years after seeing a photo of the folds there in a structural geology paper about folding.
The strata of the Castile Formation are gypsum/anhydrite (the white layers) and organic-rich limestone (the dark layers). They are thought to represent changes in the salinity of the basin in which the strata were deposited (the Delaware Basin). The regular periodicity of the alternation of sediment type suggests some sort of rhythmic influence on the salinity of the basin, and many workers interpreted theses black and white layers as varves – seasonal deposits.
But it’s probably a more complicated story than just “seasons only” – as there are anomalously thick layers interspersed throughout:
But we didn’t come here for the layers themselves. We came to see their high-contrast outcrop pattern when folded. Though this post runs the risk of overwhelming you with gorgeous fold imagery, I’m going to just let loose the avalanche:
And, why not, let’s finish this feast with a single fault example:
19 June 2014
That’s the State Line outcrop south of the Guadalupe Mountains, along the Texas / New Mexico border. Know what you can find there? Tune in tomorrow to find out…
17 June 2014
Yesterday, I pointed out an example of differential weathering on Old Rag Mountain, in Shenandoah National Park, in Virginia. Today, I’d like to shine the spotlight on another example of weathering to be seen along the trail there: little weathering pits that occur on the top of the granite outcrops. These are opferkessel.
Some people call these “potholes,” a term I do not approve of in this context. To me, “pothole” implies focused abrasion (in a fluvial setting) – which is not how opferkessel form. Others opt to dub them “solution pans,” but I don’t love that one either, since the granite isn’t really “going into solution” at these sites so much as it is being subjected to increased levels of chemical (and, to a lesser extent, physical) weathering. The actual genesis of opferkessel is more prosaic and basic: the chaotic business of weathering a rock means some areas will be higher than others, and some will be lower. The lower spots tend to hang on to rainwater for longer, which means the minerals lining those low spots will be subjected to more chemical weathering (oxidation and hydrolysis mainly, in the context of granite). Consequently, those minerals “rot” faster than their neighbors in higher, drier positions. When the wet minerals weather away, the little low spot becomes bigger, and so can hold more water, and that can encourage more chemical weathering, and so on. It’s a self-amplifying process. Physical weathering may play a role, too – I can envision additional breakup of the granite due to the splashing action of impacting raindrops, the jostling of small waves, and even the grinding action of ice that forms in the opferkessel during winter months. Many of the opferkessel have living things in them, too, and the humic acids, hyphae, and even scrabbling of little legs may focus additional weathering power on the walls and floor of the opferkessel.
The actual summit (highest point) of the mountain (3,291 feet above sea level) has a very well-developed opferkessel, too:
(Note the quarter for scale on the “peninsula” at bottom center)
If you’re into weathering, Old Rag’s an excellent destination.
If you can’t get to Old Rag in person, here’s a GigaPan of an opferkessel there for you to explore: