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Friday Field Foto #41: Armored mudball (eroded out)

February 8, 2008

Realizing how fun posting a geopuzzle can be, this week I asked the readers to tell me what they think the photo is all about. I got some great responses fairly quickly. See answer and analysis below.

Click on photo for a bigger version.


It is from the Eocene Grès d’Annot Formation of southeastern France.

Unfortunately, I don’t have any fun prizes to give away…maybe I should get some t-shirts made. For now, if you win you get to brag to your friends!

Happy Friday!



Essentially, Gary gets it right with the first comment.

First things first … this is a cross-sectional view of part of thick (~1.5 m) turbidite bed. The thing that used to be there was an armored mudball (is that one of the best terms or what?).

Here’s the deal: mud/clay “clumps” are commonly ripped up from the sea floor as sand-laden turbidity currents come by. Since the mud is more cohesive, they can remain in clumps as they are being transported. Think about trying to make a mudball with your hands compared to making a sandball … the sandball will fall apart. Then, the mudball probably saltated on the sandy substrate (or grains in suspension just stuck to it?) picking up the grains as it went. Now, think about rolling a log of cheese in some chopped nuts … they’re gonna stick to the cheese (mmm…cheese log). The mudball becomes “armored” with very coarse to granule-sized grains just as the cheese log becomes armored with pecans. The armored mudball is deposited when the high-density turbidity current transporting it loses its momentum for one reason or another and drops its sediment load. The sediment becomes sedimentary rock, the area is uplifted and exposed to the surface and, finally, the mudstone preferentially erodes out of the sandstone leaving only the armor behind.

Callan gets the process kind of right … replace the jellyfish with mud, and replace the windstorm with a submarine turbidity current. :)

Andrew…had this been a bedding plane as you postulated, and not a cross section…I would’ve guessed exactly what you did as well. You can imagine the coarser material collecting in the pit similar to what’s seen in modern stream lag deposits.

Mathias brings up some interesting details regarding the composition of the material. Honestly, when I was there taking this shot I didn’t look at the grains very closely. So, unfortunately, I can’t answer you precisely. But, it does look like a bunch of quartz and feldspathic grains with perhaps some meta-sedimentary grains (dark ones near top?). As for slightly larger grains in the bottom…my guess is they simply fell from the top and have collected in there as the pit eroded bit by bit (remember, this is a vertical cross-sectional view).

Yet another interesting detail alluded to by both Mathias and Andrew is why such a large clast (pebble to cobble) is “floating” in a sand matrix. There are a couple of ways to do this. First, depending on the state of lithification of the mudball when it was ripped up and suspended in the turbidity current, it is could’ve been quite a bit lighter. That is, hydrodynamically it behaved similar to the medium-grained sand. We see this all over the place with unarmored mud clasts (or, mudstone rip-up clasts)…they’ll be “floating” within a matrix of sand. In this case, however, the armor may have precluded such a process. Another option is that the armored mudball simply rolled along the sandy substrate. Think of large pebbles/small cobbles on a beach…they are large enough that they can be rolled along the sand surface by the waves. This bimodality in grain size is seen quite often (e.g., pebbly sandstones). One thing not evident in the photo is that the grain size of the armor is present in the very lower part of this turbidite bed (~40 cm below bottom edge of photo) and in other beds stratigraphically below and above.

I’m not sure of the exact answer for this particular example … we could go to the French Alps and check it out? Any takers?

If you want to read a bit more about armored mudballs, this page talks about them a bit more (and I found this very short reference to the largest known armored mudball from 1982 Mt. St. Helens mudflows).


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13 Comments leave one →
  1. February 8, 2008 10:30 am

    Out of my usual geo-territory…was there a mudball in that space, maybe?

  2. February 8, 2008 11:22 am

    Well, I am seeing sand, gravel and that big piece right of the centre is that a pyrite crystal??? Anyways most gravel appears to be quartz. Some yellowish-brown pieces could be feldspar or maybe limonite (due to weathering?) or so. So this cannot have travelled so far. The quartz seems quite angular with little rounding, too.

    Maybe my imagination is seeing something that does not exist but do the others also see some very, very bad kind of grading? There are less gravel size grains in the upper section of the “pit” than in the lower section but perhaps that just coincidence.

    If that pyrite I would put this into a marine environment or at least one with plenty of water we need to make that pyrite somehow.

    An alluvial fan close to a coastal environment with a tiny shelf area? The hole looks like u made it yourself though…

  3. February 8, 2008 2:24 pm

    I assume the rock itself is a flysch and the whole surface is a bedding plane. I take it to be a weathering feature, not some kind of sole mark. The rugged inner surface shows me it’s not a space left by a large clast, and such a clast wouldn’t be here in a coarse turbidite of well-rounded grains. I will call it tafoni–salt weathering–although the sharp rim is unusual.

  4. February 8, 2008 2:38 pm

    Anybody else? One of the answers above is on the right track…but I’m not gonna say which just yet.

  5. February 8, 2008 2:57 pm

    Andrew … nice, pulling out the term “flysch” … old school. I like that.

  6. February 8, 2008 3:13 pm

    …probably from a eumiogeosyncline near a peneplain.

  7. February 8, 2008 3:14 pm

    How about this: One day, a poor little slug (or jellyfish, or other mucousy creature) gets tumbled down a slope covered in coarse sand. Before it has a chance to recover, a windstorm comes along and buries it in finer-grained, well-sorted sand. As lithification proceeds, the nudibranch (or tadpole, or whatever) decays away, but the coarse sand remains in place.

  8. February 8, 2008 4:59 pm

    i’ve updated the post with the answer … see above

  9. February 8, 2008 7:54 pm

    “Armored Mudball” would be a fantastic name for a rock band.
    Dave Barry reference and geology pun both equally intended.

  10. February 8, 2008 10:47 pm

    Cool cool cool! I love being found wrong. So the whole sequence in this view is pure sand except for the thin, thin shell of pebbles. And I thought there was a whole layer of pebbles behind the front surface.

    OK, but what kind of mud would erode out while sand around it remained cemented?

  11. February 9, 2008 9:19 am

    “OK, but what kind of mud would erode out while sand around it remained cemented?”

    For this particular feature we obviously don’t have the mud left to look at it. But, there are some preserved armored mudballs in this formation in other spots. It’s just regular ol’ siltstone/shale…ends up eroding a bit faster. In this picture, for example, the finer-grained layers are more recessive whereas the sandstone beds are sticking out.

  12. February 9, 2008 9:20 am

    Forgot to mention…the rocks I link to in comment above are not the same as the one in this post…just an example.


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