Friday, October 28, 2011

Armored Mudballs!

"Quiet water conditions" is the depositional mechanism most often evoked to explain the presence of fine-grained mud in the rock record.  Mud, generally made up of clay minerals, is defined on the Udden-Wentworth grain size scale as particles smaller than ~0.00015 inches, or around 0.003 mm.  That's pretty small, and any amount of turbulence or motion in a water column will keep particles of that size suspended for quite a while.  However, as I've discussed before, there are some hints that a fair portion of mudrock in the stratigraphic record records higher energy conditions; one of the nifty qualities of clay minerals is that they're weakly charged, and can aggregate together into larger "chunks" that behave as hydrodynamically heavier clasts.  Sometimes, the features that would record this sort of aggregate/flocculate behavior of mudclasts is subtle or cryptic, especially once muddy sediment gets compacted and flattened out.  Sometimes, however, the evidence for muddy clasts is obvious!

The picture below is from the Eocene Cathedral Bluffs Member in SW Wyoming/NW Colorado, a pretty thick succession of fluvial/alluvial sediments that form some fairly picturesque vistas in that area.  The channelform sandstones around here are pretty coarse - usually upper Medium at the small end, and sometimes even getting into coarse and very coarse sands (we're pretty near their source area!).  Anyway, in among the fluvial sandstones, are things like this:

That's a gravel-sized clast, maybe 50 mm across or so, made up entirely of clay, and coated on the outside by a nice armor of sandy grains glommed onto the outside!  In a moment of refreshing clarity, the official science word for these things is the surprisingly restrained term "armored mudball".  Nifty, huh?  Here's another shot:

You can see that these muddy clasts are floating in a coarse sandy matrix.  Because these mudballs are so large, they form a nice erosional lag at the base of the channels.

Here's a close up of some of the gravel-sized mudballs:

The shot below shows the muddy interior of these little fellows:

Neat, huh?  Like almost all things in sedimentary geology, mud is a lot more fun once it starts gettin' pushed around by turbulence!

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