GeologyRocksThings that make you go hmmm
Feb
23
00:35
From a meme running elsewhere in the geoblogosphere - things that make you go hmmm...
I never have quite sussed this outcrop. What happens to the orange sandstone and the faults?
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my guess would be that the
Sat, 02/23/2008 - 22:13my guess would be that the middle rock has slid either in or out of the picture but at an angle. This would cut the orange sandstone off and either move it up and behind this outcrop or down and in front to be eroded off (or vice versa). And faults do have to end somewhere, it's not terribly uncommon to see the end of one as not all faults are miles deep. They can be only a few meters long.
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Out of plane of section
Sat, 02/23/2008 - 22:55I'm sure that some movement out of the plane of section has something to do with it but ...
All of the other structures here are thrust related (it is near the famous Lady Cave anticline) or early extensional dip slip faulting.
The geometry of the central portion is really weird terminating downwards in a vee shape. If it is strike-slip then its the strangest fault geometry I've seen
The faults on the right just just tip out, they stop and start suddenly.
I could find no good evidence of oblique slickenfibres.
Geologists like a nappe between thrusts
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Yeah I would guess thrust
Sun, 02/24/2008 - 15:53Yeah I would guess thrust related. If the stress wasn't even laterally across the thrust, you could have areas that want to pinch together harder than the block of rock on either side, and this I think would push the center rocks outward, causing something like this. I would guess it's in a V shape due to the folding action that causes the waves in thrust faulting areas. But I only had one dedicated class on structural geology so I'm no expert.
Is there a way to tell who wrote this blog entry (besides the person blatantly telling us)? I'm always confused by who writes them because there aren't names in the normal place.
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I'll look in to that
Sun, 02/24/2008 - 17:46Is there a way to tell who wrote this blog entry (besides the person blatantly telling us)? I'm always confused by who writes them because there aren't names in the normal place.
I'll look into rejigging the blog look on the next maintenance session - I agree it is a bit confusing.
Geologists are gneiss!!
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Blog Author
Sun, 02/24/2008 - 19:58I do try and include a hypo-theses tag graphic at the top of all my blog posts - but it is a bit confusing
Geologists like a nappe between thrusts
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I saw that and I wondered if
Sun, 02/24/2008 - 20:39I saw that and I wondered if it was your sign but wasn't sure. I mean there are quite a few ways to figure it out, one being it has the author on the recent posts page (but I usually forget! ha) and the other that it says "hypocentre's blog" in small writing under the post along with other links but I usually skip over that part in my haste to read the material and next posts. So it's not a huge issue, I just have to readjust my thinking to get used to it.
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Ok, I might throw a spanner
Mon, 02/25/2008 - 12:56Ok, I might throw a spanner in the works with this but also I might be completely wrong!!
Having never been to this locality and knowing nothing of the area and it's geological history beyond what is shown in the photograph I have one simple question: Are we sure the sandstone bed to the left of the fault is the same as the sandstone bed to the right?
Beneath the 'V' join of the main two faults there appears to be further faulting, which looks minor at first glance but may not be. The sequence appears to be mainly mudstones with some more distinctive sandstone beds and it may not be unreasonable to suppose that two different sandstone bands have be juxtaposed by the faulting to fool the eye into thinking that they are the same one. (The bed to the right looks thicker as well to me.).
Remember, I'm basing this off a photo. In real life it may be easy to match beds across the faults below the sandstone, confirming that the throw is relatively small, and therefore debunking this theory so please let me know if I'm wrong (or even if the idea has merit!).
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My preferred solution
Mon, 02/25/2008 - 20:54al,
This is my preferred solution too. I don't think that the sandstone beds are the same. My interpretation is that since the faults below the right-hand sandstone appear to be truncated I think the master normal fault comes down on the left-hand side of the vee and becomes listric, detaching at the base of the right-hand sandstone. The right side of the vee is an antithetic to this fault.
Along this coast near Amroth and Saundersfoot there are a number of early extensional faults that do have this kind of geometry. Some of these are modified by the later Variscan compression.
Geologists like a nappe between thrusts
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Cracked It!
Sun, 04/13/2008 - 08:10I've just been back to the section (hence the absence of a geology hunt this past week) and I've cracked it.
The upper faults are pure dip-slip (you can see the slicken-fibres on them) and the sandstone bed IS the same but ...
... what you can't see is that there is a thrust running along the base of the sandstone!
So the answer to where has the sandstone has gone is that it is under the beach to the left of the image.
Geologists like a nappe between thrusts