AIRBORNE GEOPHYSICAL SURVEY CONFIRMS DECORAH IMPACT CRATER

Decorah_perspective_view-3Site of impact crater under Decorah in northeastern Iowa  Photo credit Adam Kiel, US Geological Survey

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.

Advertisements

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s