Friday Field Foto #41: Armored mudball (eroded out)
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 ClasticDetritus.com t-shirts made. For now, if you win you get to brag to your friends!
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).