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Geopuzzle (updated with answer)

May 19, 2009

Haven’t done one of these in a while.

This could be easy, might be difficult … I’m not sure — simply comment below explaining this photograph.

(© 2009 clasticdetritus.com)

(© 2009 clasticdetritus.com)

If you happened to be there with me you are not allowed to answer :)

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UPDATE: This was a tough one. I was on a field trip last weekend and as soon as I saw this feature I instantly knew I’d make it a great geopuzzle.

A few of the commenters speculated about this being some kind of collapse feature. That is exactly what it is — but the part of the story that you might not have guessed is that the collapse happened since the 1860s. This is from the Black Diamond coal mine (inactive since the early 1900s) near Antioch, California. The coal seam was removed and the overlying sedimentary rocks collapsed into the space. The dark material is bits of pieces of the lignite coal left.

Even though I knew what this was when we came across it (as our guide pointed it out), I was amazed how similar it looked to other soft-sediment or local-collapse deformation features I’ve seen from the geologic record.

To see more photos of this mine, check out my Flickr set here.

Thanks for playing!

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20 Comments leave one →
  1. bobreturns permalink
    May 19, 2009 7:49 am

    Soft sediment deformation due to dewatering?

  2. Sue Hutton permalink
    May 19, 2009 8:24 am

    At the risk of being laughed at, I can only say that the finely layered sediments seem to have been disrupted by a roundish, carbonised object.

    Would the fine layers be lake sediments and would the carbonised object be a tree bole.

    I’m not good at puzzles :(

  3. May 19, 2009 8:34 am

    No scale? For shame : )

  4. May 19, 2009 9:07 am

    Is this picture upside down? Otherwise, I’d say these must be disrupted turbidites, and wonder if there isn’t some kind of diapirism going on with the lower seds. What are those lower, dark clasts – and is there oil in the matrix of the lower breccia or just in the clasts?

  5. May 19, 2009 9:11 am

    Hey! I saw that picture on the daily “geology photos on Flickr” post (yesterday?).

  6. May 19, 2009 9:31 am

    Chris … no scale is part of the puzzle!

    Kim – yes, that is my photo on Flickr, but if you go there it’ll give the answer (I should’ve waited to upload it there).

    For those that still want to play, firstly, don’t go to Flickr :) … secondly, most have noticied the seds are disrupted, but no one has correctly stated the cause of that disruption.

  7. May 19, 2009 9:55 am

    I’m going to go with a paleokarst breccia interpretation.

  8. May 19, 2009 12:04 pm

    Ron Schott made the guess I was going to make… but I’ve got some problems with it. First, the sediments do appear to have been somewhat soft when disrupted, and second, the two sides seem to have converged. That is, there appears to be shortening accross the disrupted zone. That could happen in a cavern collapse, but it doesn’t look right.

  9. May 19, 2009 12:18 pm

    Is it a weakly explosive pepperite? It looks like it might be basalt injected into soft sediment. There’s a number of these on the central-northern Oregon coast from the CRB’s, but I’ve never seen one like this- the ones I’ve seen are much more chaotic, or much more organized. God, I’ve got to get out again…

  10. May 19, 2009 12:28 pm

    I’ll give it at least a few more hours … this is a good one!

  11. David D permalink
    May 19, 2009 3:00 pm

    Glad I wasn’t standing there, whatever happened!

  12. peccavi808 permalink
    May 19, 2009 5:56 pm

    Is it a result of soft-sediment (partially lithified) deformation from an underwater slump? The black layer looks like petroliferous shale (this is arm chair geologizing taken to the next level).

  13. May 19, 2009 7:59 pm

    I agree with Lockwood. The manner in which the laminated crust (caliche /supratidal?) has been broken up seem to suggest shortening.

  14. May 20, 2009 7:53 am

    see answer in updated post above

  15. Mark permalink
    May 20, 2009 1:28 pm

    I call shenanigans! A scale would have greatly helped, it is still unclear how large the feature is.

  16. May 20, 2009 3:31 pm

    Shoulda figured the black stuff would be coal, since you were twittering about your visit to Black Diamond.

  17. May 20, 2009 7:20 pm

    Mark … the biggest coal pieces are about fist-sized … and leaving off a scale is part of the fun, at least for me :)

  18. May 26, 2009 9:59 pm

    Ron Schott made the guess I was going to make… but I’ve got some problems with it. First, the sediments do appear to have been somewhat soft when disrupted, and second, the two sides seem to have converged. That is, there appears to be shortening accross the disrupted zone. That <i>could</i> happen in a cavern collapse, but it doesn’t look right.

  19. Pete Fred permalink
    June 17, 2009 9:05 am

    Just broke into geology as a hobby! Definately water related erosion however the blck coal like fragments tell me that this erosion is over along period of time aka in the millions of years.

  20. June 18, 2009 7:02 am

    Pete … look at the update, this is collapse due to mining.

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