6 February 2012

Folded BIFs of Soudan, Minnesota

Posted by Callan Bentley

Why are these people smiling?

photo by Yvette Kuiper

Because they are structural geologists, and they are psyched to be at an extraordinary outcrop:

This is a famous pavement outcrop of polyphase-folded banded iron formation (BIF) near Soudan, Minnesota.

I went there last fall before the annual meeting of the Geological Society of America, on the Structural Geology of the Superior Craton’s sub-province boundaries field trip.


What you’re seeing here are layers of hematite (it’s listed as magnetite, but we found that it was not magnetic) and jasper. These are usually interpreted as chemical sediments whose formation would only be possible in a largely anoxic ocean. Later — perhaps while they were still precipitated goo on the seafloor, or perhaps post-lithification, they were folded up multiple times, resulting in distinctive fold interference patterns…

The Soudan BIFs are part of a larger stratigraphic sequence that includes lava flows and sedimentary deposits:

That last one (just above) is a classic fold interference pattern. See if you can produce it yourself with Visible Geology (hint: it will take two generations of folding at a specific angular relationship to one another)…

Got it yet? Okay, try these instead:

Here’s a big chunk that someone (not me) found off in the woods:

After my initial shock wore off, I was able to look more closely at the outcrop, and I saw some intriguing smaller-scale clues. For instance, consider this layer:

There are little grains in there! Sand-sized grains!

And not only that, but they’re elongated parallel to bedding… They appear to have been strained!

Another intriguing observation was that some of the big cherty layers had thin laminations within them. Check out the stripes inside this red jasper layer, for instance:

…or the ones inside this white cherty layer:

What do these laminations imply? Something was changing, semi-rythmically, with the deposition. And then there are the larger beds themselves, which exhibit variation over a longer tempo. What’s up with that? Are the big variations representative of daily tides and the smaller ones of fortnightly tidal cycles? Are we looking at “varve”-like seasonal variations with the small layers and perhaps some sort of orbital forcing with the larger ones? Or what? I wish I knew. These are intriguing data — I’m itching for a good story to tie them all together into something coherent.

And then there was the brittle deformation. Here are a suite of little fractures (full of white-colored milky quartz) as the stiff jasper layers broke during late, brittle deformation:

This area’s been shattered into shards of brittle BIF, and then glued back together with milky quartz:

Sans any quartz veins, here are some offset strata that serve as markers recording small-scale faulting:

A few more “classics” to go out on…

It’s been a good year for me and the world’s Archean banded iron formations. Between Soudan, Minnesota, and the Witwatersrand’s “Contorted Bed”, you might say that I’ve become BFFs with the BIFs.