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!

Replacement of Fossils

You might think that getting a shell or bone or wood chunk safely buried is the tough part for fossilization, that once something's entombed in sediment it's all smooth sailing. Burial is certainly important, but it's not the end of the story. A lot of things can happen between deposition and exposure. Pore spaces are filled with new minerals. Existing minerals are replaced. Entire structures can be replaced, then lost. These changes all fall under diagenesis. What exactly happens depends on things like the physical and chemical structure of the object in question, temperature and pressure of burial, and the chemical composition of the fluids in the sediment. Denser fossils like teeth are less vulnerable to changes than more porous materials. The form of calcium carbonate known as aragonite is less stable than calcite. Many different minerals and mineraloids can get involved in the fun; for example, there are opalized fossils and pyritized fossils.

Bivalve mold and internal cast (steinkern). Not pictured: bivalve shell.

Because silica and carbonate minerals are so abundant at typical surface and near-surface temperatures and pressures, they are the minerals most frequently involved. In Minnesota, we generally get dolomitization. This is somewhat inconvenient, because dolomitization has a tendency to destroy fossils, and even when it doesn't, it usually leaves behind molds and casts that aren't as crisp as the original. It's a bit like replacing the Venus de Milo or Michelangelo's David with nothing but 2x4 Lego bricks; you'll notice a difference. Dolomitic replacement may give a fossil a quirky sparkly appearance thanks to the dolomite rhombs, but that's about the only plus. Pervasive dolomitization is why many fossils in the Platteville are gray with a sugary appearance: you're actually looking at a natural mold or cast of the original in dolomite.

Sometimes diagenesis gives you exotic, spectacular fossils, and sometimes it gives you dolomite.

Although once in a while you get something to write home about; this is a nautiloid in the Science Museum of Minnesota collections with its internal structures replaced.

I was inspired to write a note about this topic by a different kind of replacement. Someone reviewing one of my work projects commented on a type of replacement seen in some of the fossils, consisting of circular mineralizations. They informed me this was a kind of silicification known as beekite. This immediately twigged my memory banks, because I'd also seen it in photos of fossils from other work projects. Like dolomitization, it's not exactly faithful reproduction, although it can be aesthetically pleasing.

A beekitized (beekitified?) lower Permian brachiopod, central Kansas.