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Friday Field Foto #65: Geopuzzle!

September 26, 2008

I was reading Callan’s fantastic summary of his geoblogosphere survey and was reminded about how fun geopuzzles can be. So, this week’s Friday Field Foto is a geopuzzle.

Your goal is to be the first to describe what is going on with this rock … I’m gonna leave it at that, no clues.

To make it a bit more difficult there is no scale shown (on purpose).

Happy Friday!


UPDATE (9/30/2008): Answer!

Silver Fox basically got it right … and Andrew knows me too well – these are, of course, turbidites. This particular image is of mudstone-rich deposits with one silty bed in it. Stratigraphic up is to the left and the entire image is about one meter across.

The diagonal lineaments (from upper left to lower right) are fractures that, we were told, are related to the uplift of these ~600 million year-old rocks during the Mesozoic. The very obvious, nearly horizontal lineaments are very recent glacial striations.

If you want to learn more about this outcrop (Canadian Rockies), see this post for details and more photographs.


13 Comments leave one →
  1. September 26, 2008 7:49 am

    I’ll cast a vote for a glacially polished mylonite.

  2. September 26, 2008 9:33 am

    I like the glacial scour/polish idea – but if it’s a mylonite, then we aren’t looking perpendicular to the motion direction, because mylonite-like motion indicators seem inconsistent, mostly top-to-the-left.

    I vote for some kind of turbiditic fine-grained to very fine-grained, laminated, possibly tuffaceous sandstone to mudstone deposited in a somewhat chaotic environment, with some laminae showing minor offset on minor fractures.

  3. September 26, 2008 11:56 am

    anyone else out there?

  4. September 26, 2008 3:18 pm

    A bunch of dull weekenders! :-)

  5. September 26, 2008 8:01 pm

    original lamination? but also low grade metamorphism. and some iron hydroxide stains

  6. September 28, 2008 7:01 am

    A sandstone with transport direction to the left.

  7. September 28, 2008 8:00 am

    Silver Fox … you’re first answer (second comment above) is pretty close.

  8. September 28, 2008 2:02 pm

    Well, then I’ll delete my last comment (figuratively) – unless part of it’s correct, also! :)

  9. Dominion permalink
    September 28, 2008 7:17 pm

    a turbidite mudstone containing calcite pressure solution precipitant layers would be my (somewhat educated) guess. I just returned from a trip around the Mountain City window in east Tennessee and got to view something similar but with more shortening.

  10. September 29, 2008 6:59 am

    Hmmm. the laminae are visible only on the surface, not in the holes where chips are missing. So, either they are a S feature emphasized by weathering, or they are a L-feature.
    aeolian ashfall
    slickenlines on a fault surface in diatomaceous mudstone.

  11. September 29, 2008 3:03 pm

    Well, let’s see: I’m thinking the surface is horizontal from the pattern of the brown stains. I don’t think the striations are mechanical but stratigraphic, accentuated by weathering. The color points to limestone. But you know, Brian, you are a fiend.

  12. September 29, 2008 3:06 pm

    this is gettin’ fun … I like the creativity in the guesses … I’ll give it another day or so

  13. September 29, 2008 5:09 pm


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