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Thread: Geology Discussion

  1. #991
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    Are you trying to make a thin section?

  2. #992
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    Quote Originally Posted by dojmo View Post
    John I enjoyed seeing all 250+ pictures. I sliced the rock you gave me and on my saw it came out to thick for me, I will try at work when a saw comes available.
    Thanks Don, the image of the channel fill we studied I think is the biggest clue as to the formations origin.

  3. #993
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    Quote Originally Posted by geonuc View Post
    Are you trying to make a thin section?
    I don't think we can get that thin, but a flat surface will do better under my microscope. As it is we have to follow the topography with the focus knob and we think that flat would be better; light source from above.

  4. #994
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    Quote Originally Posted by jlhredshift View Post
    I don't think we can get that thin, but a flat surface will do better under my microscope. As it is we have to follow the topography with the focus knob and we think that flat would be better; light source from above.
    Ah. I recall trying to make decent thin sections in school and failing miserably. Fortunately, after demonstrating we knew how, my school allowed us to pay someone to make the sections for use in our projects.

    But those are for use with optical mineralogy.

    Never mind.

  5. #995
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    Quote Originally Posted by jlhredshift View Post
    I emailed Dr Szabo the pix link and he forwarded it on to people he knows at the Ohio Geological Survey and they are possibly going to look at the exposure in July.
    Update: well, unfortunately the owners of the Northridge exposure have now buried it.

  6. #996
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    Quote Originally Posted by jlhredshift View Post
    Thanks Don, the image of the channel fill we studied I think is the biggest clue as to the formations origin.
    I contacted Ron Blakey and with his permission here is his description of Papago Buttes:

    The Papago Buttes and Camelhead outcrops do indeed, line up. At Camelhead, the formation clearly overlies unconformably, the Precambrian basement. The Camelhead Formation (not sure if name has been formalized or not) is a Late Eocene to Early Oligocene "fanglomerate" (fan-deposited conglomerate). The unit formed in basins adjacent to Eocene uplifts in central Arizona similar to the Rocky Mountains. The conglomerate consists of basement (Precambrian)-derived clasts -- granites, rhyolites, schists, etc. Many of the clasts are angular because they had very local sources. I believe there have been one or more ASU graduate theses on these rocks but I am not aware of formal publication. At South Mountain and most of the Phoenix Mtns, this unit has been stripped off by erosion.

    One more thing -- the conglomerate may never have been deposited in areas where it is now absent. In other words, there are two possibilities 1- never deposited, 2- deposited an subsequently eroded away.
    (R. Blakey per com 6/29/2010)

    When I took the picture of the channel fill feature I failed to document direction and exact location; i.e. nothing in my notebook. If someone happens to live in the Phoenix area and could have a looksee to make up for my failure, I would be deeply appreciative.

  7. #997
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    Quote Originally Posted by jlhredshift View Post
    ...The red matrix, no idea, did not pack and acid, but throwing one at another barely makes even a small mark, they almost make a ringing sound when struck.
    check with Jon on this, but it sounds like an ironstone - a lithified iron-rich sediment (sand, silt or clay) like the Old Red continental stuff, or similar; if the matrix and the granite intrusions were proximal, then local metamorphism might have led to an increase in the hardness and the bell-tone.

  8. #998
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    Quote Originally Posted by cran View Post
    check with Jon on this, but it sounds like an ironstone - a lithified iron-rich sediment (sand, silt or clay) like the Old Red continental stuff, or similar; if the matrix and the granite intrusions were proximal, then local metamorphism might have led to an increase in the hardness and the bell-tone.
    You rang Sir?

    Priobably more like a siliceous cement to me, they ring nicely when it. The grey appearance with a red weathering coat is consistent with this.

    The extreme grain-size range is consistent with debris flows, these move as slurries, normal rules about sorting don't apply. Debris flows can be both submarine and subaqueous, I have seen small truck sized boulders in subaerial debris flows and 10 story building sized ones in submarine examples.

    I would assume that Terrtiary conglomerates in Arizona would be terrestrial rather than marine.

  9. #999
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    Quote Originally Posted by JonClarke View Post
    You rang Sir?

    Priobably more like a siliceous cement to me, they ring nicely when it. The grey appearance with a red weathering coat is consistent with this.

    The extreme grain-size range is consistent with debris flows, these move as slurries, normal rules about sorting don't apply. Debris flows can be both submarine and subaqueous, I have seen small truck sized boulders in subaerial debris flows and 10 story building sized ones in submarine examples.

    I would assume that Terrtiary conglomerates in Arizona would be terrestrial rather than marine.
    My bold

    Blakey agrees, and on the ground the physical altitude places it in the Tertiary in relation to the strata. Also, when the HCl did not attack the "red" coating it really susprised me, but I like the "silicified" (sp?) idea, how do I prove it?

    The last two "nose to rock" events of mine have pointed out that though I have access to a vast amount of geologic literature, when it gets down to "an" exposure, one may not find a previous publication describing it. They say that we havn't touched the oceans of this planet as far as exploration goes, I'm not so sure that it doesn't also apply to ground truth as well.

  10. #1000
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    Silca cements are hard to prove, but hardness of rock, the fact it rings when struck and lack of reactity to HCl are all good good pointers. A thin section would be the clincher. You should try stratching it with a piece of sharp steel as well, if it does not fragment into grains and there are metal streaks on the rock then it will almost certainly be silicified.

    You are certainly right about how poorly studied the Earth is. Some units have been studied to death, other untouched, even in well studied regions. Where I live, despite more than 150 years of investigation and four universities and the national geoscience agency in town and at least two geological monuments (one of global significance) there is not a single good overview of the geomorphology or the regolith, or detailed sedimentology of the major units.

    edit I should say hard to prove without a thin section.
    Last edited by JonClarke; 2010-Jul-05 at 10:01 PM.

  11. #1001
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    Well, when I try to scratch it a few little, vey tiny, dust almost, particles fly off but it doesn't really leave a mark. But, when Don tries to cut anything under an 1/8th inch it becomes easy to crumble.

  12. #1002
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    Quote Originally Posted by jlhredshift View Post
    Well, when I try to scratch it a few little, vey tiny, dust almost, particles fly off but it doesn't really leave a mark. But, when Don tries to cut anything under an 1/8th inch it becomes easy to crumble.
    That is consistent with moderate silica cement. I would not be surprised if 3 mm (1/8+) is the grainzise of the matrix. If you do want to cut it finer than this, you can trying soaking the first cut surface in a runny resin glue. This will fill in the pores and help hold it together.

  13. #1003
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    Apologies for the grave-digging, guys - I have an update on this:

    Quote Originally Posted by cran View Post
    going back to Big Don's question, briefly:

    I only have to go outside to see another example in the making -
    in front of my house is an electric power pole set in a bare and sloping nature strip
    (why bare? Shire policy, according to the environmental health officer,
    so it's poisoned twice a year ... but I digress) ...

    and over the past two years, the runoff from rainfall
    (which is not great in this part of Oz)
    has been carving a nice round depression in ground around the pole ...

    an' that pole ain't spinnin' nohow! ...

    leaning, sure ... but not spinning ...
    Well, it's not leaning anymore - it's lying across my front garden! In one of those freaks of luck, the factors which caused it lean and finally fall also saved the house from wearing it ... the concrete footpath and wet sand helped the pole base slip roadward (downslope) as it fell ... which it did in the middle of a rainy night - it's still raining this morning, and the workers are now outside trying to remove the evidence ...

  14. #1004
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    Quote Originally Posted by JonClarke View Post
    That is consistent with moderate silica cement. I would not be surprised if 3 mm (1/8+) is the grainzise of the matrix. If you do want to cut it finer than this, you can trying soaking the first cut surface in a runny resin glue. This will fill in the pores and help hold it together.
    We are going to survey the samples for grain size as time permits.

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