Wrangellian Posted October 18, 2015 Author Share Posted October 18, 2015 my larger loupe has 2 lenses already and it still doesn't magnify much! I have a feeling I don't have the equipment to get a sufficiently close look at the grains, and I'll need to send it to somebody who does. Link to comment Share on other sites More sharing options...
Wrangellian Posted October 19, 2015 Author Share Posted October 19, 2015 (edited) I just got back from the site and I counted 4 layers. The top one that I opened with (that I will call #4) is the most interesting, but it is also interesting that they are all so different - the only similarity between them is that they are thin and weather more readily than the surrounding shale and therefore stick out like a sort thumb (Layer 4's color makes it stick out more than its degree of weathering): Layers 1 and 3 are crumbly and an inch or two thick. Layers 2 and 4 are thinner, but 4's light color and grain size are so different from 2 which resembles most closely the surrounding shale, but it makes a distinctive 'incised' line across the rockface. I suppose the number and variety of these layers favors the turbidite theory? Anyway I brought home samples of each in case anyone is able and willing to examine them for me. Edited October 19, 2015 by Wrangellian Link to comment Share on other sites More sharing options...
Doctor Mud Posted October 19, 2015 Share Posted October 19, 2015 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. 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... Shouldn't be too much. I'd only need small bits, say 1 cm cubed (a cc). Take lots of photos and field notes. Very interesting stuff. It might be in Bobbys how to guide, but get a hold of field cards for grain size, sorting, angularity etc.. There might be copies on the USGS website. Photographing and sketching the outcrop would help. A hand lens is always useful for sedimentology. You can use the card to determine if you have sand, silt, clay etc. particle size. Clay and silt often look the same but silt will feel slightly gritty when a sample is rubbed between your fingers. Look also for cross cutting relationships in the layers. If it was ash fall it would drape existing layers. A turbidite might scour existing layers. As Auspex said, it's possible to be both, a reworked tephra that slumped in a landslide, although I doubt it would be so pure. I would expect a turbine to take other sediment with it and not just a tephra layer. Link to comment Share on other sites More sharing options...
Doctor Mud Posted October 19, 2015 Share Posted October 19, 2015 This is the sort of thing I was talking about. It's always good to carry this in your wallet or on a cord with your hand lens when in the field. They come laminated or you could print one out (being careful of scale) and laminate it. Useful for scale in closeups too. Link to comment Share on other sites More sharing options...
Doctor Mud Posted October 19, 2015 Share Posted October 19, 2015 Just noticed there's a grain size on this one for tephra :-) Link to comment Share on other sites More sharing options...
Wrangellian Posted October 19, 2015 Author Share Posted October 19, 2015 I think I saw cross-bedding in the other layers, at least Layer 2, but Layer 4 is so out of place, and fairly uniform in thickness and very distinct in color and grain size, and in the samples I brought home I don't see any obvious cross-bedding. Iw I am still leaning heavily toward turbidite, but I'll send you some samples if you give me your address, then we'll put this thing to rest. It's really hard to me to judge based on things like "Am I seeing cross bedding or am I not?" The grain size in Layer 4 is definitely a fine sand, that much I can say. The grains are just visible to the naked eye. I'll send you samples from each of the 4 layers if you can handle them all. Maybe you'll be able to tell just by looking at them in hand, unlike myself. I do have a loupe but I find that, for most things, my nearsightedness allows me almost the same capability just by taking my glasses off and holding the item 3" away from my eye (The only good thing about nearsightedness)... Link to comment Share on other sites More sharing options...
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