An airborne geophysical survey mapping mineral resources revealed the existence of a previously unknown 470,000,000 year-old impact crater below Decorah, Iowa, home of the famed and much-filmed Decorah Bald Eagle nest.
In 2008, geologists digging water wells first suggested the presence of the crater because they’d found evidence of a previously unknown shale deposit. Robert McKay from the Iowa Geological Survey found the deposit formed a perfect circle about 5.5 kilometers across—nearly five times the size of Barringer (Meteor) Crater in Arizona.
Further examination of sub-shale breccias (sharp fragments embedded in clay or sand) revealed shocked quartz—a telltale sign of an impact.
Andy Kass, a geophysicist at the US Geological Survey in Denver says, “We were really lucky in this case that the shale layer was preserved within the crater, as it was eroded away nearly everywhere else. If you travel to Decorah, you see a beautiful town, but certainly no impact structure.”
The diameter of the crater suggests that the impactor was a meteorite about 250 meters in diameter. As such, it becomes a part of a group of impact craters in the Midwest—craters in Ames OK, Rock Elm WI and the Slate Islands of Lake Superior in Ontario Canada. These craters may or may not have resulted from the same impactor.
Kass says, “It’s a tantalizing possibility. Unfortunately, it’s impossible to use dating techniques to see if all the impactors happened on a single day.” He also said that statistically, impactors of the size of those in the Midwest hit somewhere on Earth every 30,000 to 60,000 years.
SOURCE: Earth Magazine, July 6, 2013 Airborne geophysical survey funded by US Geological Survey Mineral Resources Program.