Sunday, May 28, 2023

Uŋčí Makhá Park Revisited, Part 1: Freeze-Thaw

After I'd come across the new exposures at Uŋčí Makhá Park last fall, I was very curious about how a Minnesota winter and spring would treat them. After all, these were fresh, with no previous direct exposure to snow, ice, and freeze-thaw cycles. Would they rapidly degrade, or were they made of sterner material? Last week I had the opportunity to spend some quality time at the park, in preparation for and leading a training session for Mississippi National River & Recreation Area seasonals (and if any of the participants happen on this post, hello! I hope you had a good time!).

What were the results of this natural experiment? A few observations:

The Carimona Member of the Decorah (blue-gray upper interval), particularly the blocks used as landscaping, suffered appreciably more than the Magnolia Member of the Platteville (tan lower interval). I attribute this to the greater shale content of the Carimona.

This is a pretty illustrative comparison. The blue-gray block on the upper left is Carimona, and the tan block on the lower right is Magnolia. The Carimona block's upper surface is littered with small chips, while the only chips on the Magnolia block came from the Carimona block. (Note also the large burrow on the Magnolia block.)

More Carimona landscaping showing exfoliation.

This indicates that the Carimona blocks will weather faster than the Magnolia blocks; eventually, both lithologies will reach equilibrium with their new surroundings, but the "fucoidal" surfaces on the landscaping are going to go away faster than the shell beds.

Note the burrows popping off the surface in some places.

It wasn't all smooth sailing for the Magnolia, though. Although many blocks and beds seemed fine, others had definite signs of damage.

Here a thin bed is breaking up.

This isolated block appears to be shattered. (Colors are weird because when I took this photo, I'd forgotten to reset the lighting from tungsten bulbs.)

Unlike last fall, which was a time of drought, this spring we can also definitely see where the seeps are.

And they're concentrated at the bentonite layers in the Carimona.

Many fossils and features came through without particular damage, though. I included a photo of a bivalve in the fossil guide post. Here it is last week:

Dare I say that it's "happy as a clam"? (Ignore the color balance differences.)

With that out of the way, did we find other fossils I hadn't seen in the fall? Well, of course! Tune in next week for some less-typical fossils!

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