Have you seen this migmatite?

33 years ago on her first day of work at a hospital, my friend’s mother inherited, in her words, an “antique doorstop and/or paperweight …we think it is petrified wood”. It is fist-sized, shiny, and much heavier than it appears. It is stumpy and rhombohedral-ish, with many semi-parallel lines along the sides and curving bands along one face. It kind of looks like petrified wood…but it is not. Far from it.

The mystery migmatite. Dime for scale. Click for full resolution.

Petrified wood results from rapid burial and slow hydrous alteration into silicified casts (permineralization). Lying underground in a wet, mineral-rich environment, picking up hues of red and yellow and gray. Calmly, coolly, entirely without incident. A history about as far removed as possible from the sample that arrived in the mail over the weekend. Migmatites (from the Latin migma for mixture) are the product of intense heat and pressure that result both high-grade metamorphism and partial melting. Check out the Georneys post M is for Migmatite for fantastic coverage of all things migmatite.

“far side” of the migmatite from previous photo. Was there a vug/gap here that allowed free crystallization?

What’s missing from this story is the provenance (origin) of this fantastic rock. Not all migmatites look the same – some lack the leucosomes (light bands) seen here, and they are not all black-and-white – and my hope is that this sample is from the Pacific Northwest… maybe someone out there knows where. For the past three decades it was hanging out east of the Cascades in Central Washington, which is a good place to start. The crustal accumulation and volcanic history of the Pacific Northwest is a prime migmatite-forming environment. I’ve found references to the Okanogan dome/highlands and the Skagit migmatite as starting points, but detailed online photographic records are somewhat lacking. Now I reach out to the ether: Have you seen this migmatite?

So an igneous petrologist walks into Indiana…

…and says, “Hey, wait just a minute…”
It’s not a funny joke, or even a joke, really. It’s a thought that crosses my mind every now and again: What am I doing in the Midwest? I’m a hard rock geologist. Give me volcanoes and basalts, faults and structures…heck, I’d even settle for a roadcut. On a field trip to Michigan’s Upper Peninsula we didn’t see an outcrop until hour six. There is, however, plenty of this:

Make your own GD roadcut.

At first you might think that’s soil…and you’d be correct, but that’s not what we were digging for. Beneath that veneer of Quaternary flim-flam is the edge of a glacial lake. That was in New York, but northern Indiana is rife with unconsolidated glacial deposits. In some areas you’ll dig 500 feet and still be scraping glacial drift off of your shovel. But maybe you’re persistent. Surely, you think, it all pays off when you hit bedrock. Could be, if you’re a fan of gray rocks…

Maybe you like nice big gray chunks of steadily accumulated, neatly bedded shale? Or how about some nice gray fossil corals in gray dolostone? Under the gray permacloud, of course.

Whoop-dee-freaking-doo.

Or, at least, that was my initial self-important, half-serious reaction. I know of many important, interesting, varied and difficult facets of sedimentary geology. Those two gray photos are from a summer research project to constrain an extinction boundary (primarily in NY). To get that internship, I had to submit an application. Get letters of recommendation. Say to myself, “Yes, I want to spend my time looking at black microfossils in black shale collected from cold black streams and battered core tubes. Give me a summer of that.” And in the end, a summer was enough and a bag of chips. It ended up as a way to narrow potential fields of study. Igneous petrology was my primary interest before and after that project, but at least afterward I could say I had tried something different.

The problem with Indiana was that it was too similar to New York in all the wrong ways. The bedrock geology spans over 150 million years from the Mississippian back to Ordovician time, but look at the dominant lithology (youngest to oldest): limestone, shale, sandstone, siltstone, shale, shale, dolomite, limestone, limestone, dolomite, and finally shale and limestone. I had such a bias against Indiana that none of these pictures are actually from the Hoosier state. And maybe that’s the problem: I haven’t tried it. Until recently, I hadn’t even researched Indiana geology, and what I found shouldn’t have surprised me. Turns out Indiana actually has some interesting geology for work and play, and most of it’s not more than four hours away: sand dunes, geodes, caves, impact craters..wait, what?

Kentland structure. Click to visit interactive USGS Indiana geology map

Yeah, there’s this anomalous bulls-eye on a map of Indiana’s bedrock geology. A puncture wound in the Mississippian-age siltstone of west-central Indiana, the Kentland impact structure. It left an ~8 mile diameter wide dome-like structure, bringing Ordovician dolomite to the surface where it remains exposed today. That’s pretty cool.

So there are no ancient volcanoes here. No columnar basalts or peridotite. It’s not paradise, but you’re all right in my book, Indiana.

Additional Info:

Indiana Geological Survey; maps of bedrock and surficial geology, and more.

Roadside Geology of Indiana, Mountain Press Publishing; where I first read about the Kentland crater.

McRocks reports of geode/mineral/rock collecting expeditions

Indiana Dunes National Lakeshore, National Park Service