Rochester, Minnesota — Team Minnesota traveled south today to visit exposures of our three favorite formations: the Platteville Limestone, Decorah Shale, and Cummingsville Limestone. Where best to see the Decorah Shale than in Decorah, Iowa? Above the crew is scattered in the abandoned Decorah Bruening Quarry. They are walkinng on top of the Carimona Member of the Decorah, with the shaley units above topped by the Cummingsville Limestone.
We began at the bottom with the Platteville and a bit of rare shade. Nikki Bell and Etienne Fang have their hands on the iconic Deicke Bentonite. A very handy time indicator, that volcanic ash deposit.
Our excellent guide Andrew Retzler of the Minnesota Geological Survey is examining the contact between the upper Decorah Shale and Lower Cummingsville Limestone. We found here several specimens of the “gumdrop” bryozoan Prasopora.
Where freshly exposed, the Cummingsville reveals itself to be a fascinating unit with alternating limestone lithologies. The darker layer here is a packstone with fine fossil debris. It is almost certainly a storm deposit.
This slab of Cummingsville is covered with beautiful Chondrites trace fossils.
In the afternoon we returned to Minnesota and explored a very overgrown exposure of the Decorah Shale at the Golden Hill abandoned quarry along US 52 near Rochester. The main attraction here for us is the abundance of “iron ooids”, small spheres of iron oxides. Etienne Fang is studying their composition and origin for her Independent Study thesis. It’s a steep and muddy slope after a journey through head-high brush, but the bags full of samples made it worthwhile.
It was a great day of exploration. Tomorrow we examine localities north of Rochester.