Wrangellian Posted October 14, 2015 Share Posted October 14, 2015 (edited) What are the chances this thin layer at Mt Tzuhalem is a volcanic ash layer? It is one of very few with a gap of at least 1m between it and the next (though the other seems to be less consolidated than this one). There are no more than 2 or 3 of these visible in this quarry and they appear to stretch right across with little or no variation in thickness (less than 1 cm), etc. The layer is above my wallet which is the only thing I had handy for scale: Edited October 14, 2015 by Wrangellian Link to comment Share on other sites More sharing options...
Wrangellian Posted October 14, 2015 Author Share Posted October 14, 2015 They seem so out of place within the otherwise uniform black shale (the nodules and such notwithstanding) As you can see the material is lighter in color and grainy. I brought home a piece and if I can get a decent closeup pic of it I will post it. Closeup in situ: Link to comment Share on other sites More sharing options...
Wrangellian Posted October 14, 2015 Author Share Posted October 14, 2015 The ferns like it: Link to comment Share on other sites More sharing options...
Wrangellian Posted October 14, 2015 Author Share Posted October 14, 2015 This is the layer that I photo'd closeup and collected a piece of: Unfortunately this quarry is north-facing so I'm having to deal with shadow when photo'ing. Link to comment Share on other sites More sharing options...
Wrangellian Posted October 14, 2015 Author Share Posted October 14, 2015 Link to comment Share on other sites More sharing options...
Wrangellian Posted October 14, 2015 Author Share Posted October 14, 2015 I think these are both separate layers from the one I opened with - below it, lower in the quarry. I didn't get a good look at the upper of the two but the lower seems to be more crumbly than the first layer pictured above. I should get back up there again and study them more carefully. Link to comment Share on other sites More sharing options...
Mediospirifer Posted October 14, 2015 Share Posted October 14, 2015 I don't know how likely an ash layer is, but it seems possible. I know that there's plenty of vulcanism in the present-day Rocky Mountains, so it's worth investigating. One thing I'd try is an acid test: drop a piece in vinegar and see if it effervesces. If it does, you have a carbonate-bearing layer, which is more likely limestone than ash. If it doesn't, the case for ash is stronger. Also, the grain size for ash will be larger than for mud. I'm not sure what the actual grain sizes are, or whether you can see them easily (in the case of ash). I'll try putting my ash rock under the microscope to see if the grains are visible, and we can compare. I'll post a photo. It won't be tonight, though; it's 2:00AM for me and I'm about to go to bed! Link to comment Share on other sites More sharing options...
Doctor Mud Posted October 14, 2015 Share Posted October 14, 2015 Could well be an ash layer or what is call a pyroclastic or tephra deposit. In Quaternary sediments tephra (ash) layers are fingerprinted using chemistry and catalogued as they can be used to link records from different sites. So quite a lot is written about tephra layers. As has already been pointed out the composition and structure would be useful. If you had a higher power transmission scope (that shines light through the sample from below) a thin section would be the way to go or even if you could break it up without destroying the original size and shape of the particles. Deflocculate in hot water if it is soft. A detergent can help to break up clays. Mix some sediment with water and put under a coverslip. You might see tiny glass shards in a tephra as opposed to mineral grains and biologicals in mudstone. Things to think about. Would the shale be a deep water deposit? I wonder if you would get such a pure tephra layer in this environment. Tephra can stay suspended for long periods of time. I wonder if the acid test would work as you may get fizzing from calc/alkaline tephras or if the tephra is mixed with background sedimentation. An alternative could be a turbidite or underwater landslide deposit but these tend to have characteristic "fining up" grain size with heavier (larger) particles at the base and finer material at the top. I look forward to seeing what you find. Link to comment Share on other sites More sharing options...
Wrangellian Posted October 14, 2015 Author Share Posted October 14, 2015 Thanks both. I agree that vulcanism must be possible - the Island (Wrangellia Terrane) was on the 'active margin' of the continent as it is now, and there must have been volcanoes along the coast, but I don't know. I am only hoping because I know the value of having a volcanic layer for dating. I can try the acid test for what it will be worth, but I don't have a microscope or anything for the more sophisticated tests. It would be nice if I knew of someone I could send a piece to that could do these things and tell me definitively. I am still trying to understand the sedimentology of my site but I believe it is deep water (dark shale usually is, no?) but the thin layer doesn't seem to have any fining up in particle size - I can just barely see the particles with my glasses off and they appear to be uniformly distributed. Now that you mention it, wouldn't a brief sediment dump from above be graded coarse to fine as well? Maybe a turbidite that flowed along the bottom would be more mixed, I don't know. That's why I'd like to send a piece to someone who could examine it with more expertise. Maybe there is someone at the museum, I will have to get in touch with them. Link to comment Share on other sites More sharing options...
Doctor Mud Posted October 14, 2015 Share Posted October 14, 2015 If you can't get someone locally you are welcome to send a sample to me. I know a few tephra specialists - yep they exist :-) tephrochronologists.... I can do some of the basic stuff in our lab and could organize for a thin section too. Just pm me if you are keen. Link to comment Share on other sites More sharing options...
Auspex Posted October 14, 2015 Share Posted October 14, 2015 At what depth were the sediments in this formation deposited? "There has been an alarming increase in the number of things I know nothing about." - Ashleigh Ellwood Brilliant “Try to learn something about everything and everything about something.” - Thomas Henry Huxley >Paleontology is an evolving science. >May your wonders never cease! Link to comment Share on other sites More sharing options...
erose Posted October 14, 2015 Share Posted October 14, 2015 Get out the microscope and take a look. Link to comment Share on other sites More sharing options...
Wrangellian Posted October 15, 2015 Author Share Posted October 15, 2015 At what depth were the sediments in this formation deposited? I don't know for sure! It was certainly not too near shore, it is fine-grained black shale, but there is some coarser stuff very nearby and there are fossils as you know, including conifer fronds and sticks. Link to comment Share on other sites More sharing options...
Wrangellian Posted October 15, 2015 Author Share Posted October 15, 2015 Get out the microscope and take a look. What microscope? Link to comment Share on other sites More sharing options...
Mediospirifer Posted October 15, 2015 Share Posted October 15, 2015 Here's a closeup of my fossil-bearing ash layer. The marks at the top are mm. I hope this is helpful! Unfortunately, it's not as clear as I'd like. Good luck with figuring out the layer. It looks interesting! Link to comment Share on other sites More sharing options...
Auspex Posted October 15, 2015 Share Posted October 15, 2015 I don't know for sure! It was certainly not too near shore, it is fine-grained black shale, but there is some coarser stuff very nearby and there are fossils as you know, including conifer fronds and sticks. I was wondering about how far from land it might have been. If it is ash, a layer that thick would most likely come from terrestrial erosion of an ashfall. Look for graded bedding! "There has been an alarming increase in the number of things I know nothing about." - Ashleigh Ellwood Brilliant “Try to learn something about everything and everything about something.” - Thomas Henry Huxley >Paleontology is an evolving science. >May your wonders never cease! Link to comment Share on other sites More sharing options...
ZiggieCie Posted October 15, 2015 Share Posted October 15, 2015 Since you are on Vancouver Island I would think that an anomly like that could very likely be an ash layer. A large volcano hundreds of miles away could deposit an ash layer with the right wind conditions and would be isolated and very different from the local deposits. Link to comment Share on other sites More sharing options...
Wrangellian Posted October 16, 2015 Author Share Posted October 16, 2015 I don't see any obvious grading in the layer in question, or in the shale for that matter. Does that make it less likely this is an ash layer? I am not sure how to tell how far from land a given layer of shale was deposited. I know that, in general, coarser grains sunk soon after being washed into the sea and finer particles could reach further out to sea, but then how do you judge the depth? It seems to me the calculated distance could vary depending on whether it was a gradual slope or a steep slope, and therefore your estimate of the depth as well. Link to comment Share on other sites More sharing options...
Wrangellian Posted October 16, 2015 Author Share Posted October 16, 2015 If you can't get someone locally you are welcome to send a sample to me. I know a few tephra specialists - yep they exist :-) tephrochronologists.... I can do some of the basic stuff in our lab and could organize for a thin section too. Just pm me if you are keen. It could be pretty expensive to ship that far, though a small piece maybe not so much. I'll have to look into it. If I can't find someone closer I'll let you know. Here's a closeup of my fossil-bearing ash layer. The marks at the top are mm. Ash close-up.jpg I hope this is helpful! Unfortunately, it's not as clear as I'd like. Good luck with figuring out the layer. It looks interesting! Thanks, How did you get this pic? If I had the means to do that I would try it with my piece and see how it compares, but... Link to comment Share on other sites More sharing options...
Mediospirifer Posted October 16, 2015 Share Posted October 16, 2015 I have an older stereo microscope (Stereozoom 4) with an inexpensive digital camera that fits into the eyepiece. It's a Celestron camera that mikecable reviewed in the Micro-Paleontology thread here: LINK. I collect enough micros that it was worth the price. Do you have a loupe? That plus digital camera might get you a decent picture. Good luck! Link to comment Share on other sites More sharing options...
Wrangellian Posted October 16, 2015 Author Share Posted October 16, 2015 I wish I had a stereo microscope (we used to call them binocular mic's)... I don't know offhand how I will line up a loupe with a camera, but will think about it. Something tells me I wont be able to get close enough in with this method to get a clear answer. Link to comment Share on other sites More sharing options...
Auspex Posted October 16, 2015 Share Posted October 16, 2015 I don't see any obvious grading in the layer in question, or in the shale for that matter. Does that make it less likely this is an ash layer?... Not at all; I'm just looking for clues to piece together the story on this anomalous layer. Graded bedding can indicate a down-slope turbidity flow. "There has been an alarming increase in the number of things I know nothing about." - Ashleigh Ellwood Brilliant “Try to learn something about everything and everything about something.” - Thomas Henry Huxley >Paleontology is an evolving science. >May your wonders never cease! Link to comment Share on other sites More sharing options...
Mediospirifer Posted October 16, 2015 Share Posted October 16, 2015 I wish I had a stereo microscope (we used to call them binocular mic's)... I don't know offhand how I will line up a loupe with a camera, but will think about it. Something tells me I wont be able to get close enough in with this method to get a clear answer. Try holding the loupe right up to the camera lens and taking a picture. Play with it and take a lot of pictures; before I had my scope camera, I used to aim my digital camera down the microscope eyepiece to get photos. If I took 12 pictures, I might have 1 that was acceptable! I'd imagine a similar ratio for using a loupe. It's worth a try, at least with a digital camera. The only thing you'll waste if it doesn't work is time! Link to comment Share on other sites More sharing options...
Wrangellian Posted October 16, 2015 Author Share Posted October 16, 2015 My digital camera is the classic type with a large lens, not the small cellphone type lens, does this make a difference? My loupe is no more than about 1" diam. and my other one is half that. Link to comment Share on other sites More sharing options...
Mediospirifer Posted October 18, 2015 Share Posted October 18, 2015 It might not work too well, given a large camera lens with loupe, but there are large hand lenses. I got a pack of 7 from a dollar store, then had fun watching kids using them to look at some rock displays we we showing. One kid grabbed two lenses and looked through both, saying (rather excitedly) that he could see so much more detail with two. You might be able to stack a pair of lenses for cheap magnification. It's worth experimenting. Link to comment Share on other sites More sharing options...
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