The Decorah impact crater, also called the Decorah impact structure, located on the east side of Decorah, Iowa, is where a ca. 200-meter-wide asteroid struck the Earth during the Middle Ordovician Period, 470 million years ago. The crater is estimated to be 3.5 miles (5.6 km) in diameter, covered by Winneshiek Shale. There is no surface evidence of the impact, as the Winneshiek Shale is more than 50 feet below the bottom of the Upper Iowa River. The impact, equivalent to 1,000 megatons of TNT, did not appear to penetrate the Earth's mantle, but it did push down the underlying Ordovician and Cambrian bedrock several hundred feet. It may be one of several Middle Ordovician meteors that fell roughly simultaneously 469 million years ago, part of a proposed Ordovician meteor event, including the Rock Elm crater in Wisconsin, the Slate Islands crater in Lake Superior, and the Ames crater in Oklahoma.
- Vastag, Brian (18 February 2013). "Crater found in Iowa points to asteroid break-up 470 million years ago". Washington Post. Retrieved 19 February 2013.
- "Geological survey: Ancient meteorite crater sits below Decorah". Cedar Rapids Gazette. 5 March 2013. Retrieved 6 March 2013.
- US Geological Survey. "Iowa Meteorite Crater Confirmed". Retrieved 7 March 2013.
- Iowa Department of Natural Resurces. "GEOLOGIC MAPPING FOR WATER QUALITY PROJECTS IN THE UPPER IOWA RIVER WATERSHED". Technical Information Series No. 54, 2011. Retrieved 19 February 2013.
- Heck, Philipp; Birger Schmitz, Heinrich Baur, Alex N. Halliday. Rainer Wieler (15). "Fast delivery of meteorites to Earth after a major asteroid collision". Nature 430: 323–325.