Scientists confirm Iowa city sits on impact crater

DES MOINES – Federal researchers confirmed Tuesday that the city Decorah sits atop a 3 mile wide hole that formed 470 million years ago when a meteorite hit the area, one of just 180 or so known impact craters on earth.

Scientists with the U.S. Geological Survey said aerial gravity and electrical magnetic surveys of the area confirm the work of geologists that shows a buried crater concealed beneath bedrock and sediment.

Decorah, known for its annual Nordic Fest celebrating its Norwegian heritage and a family of bald eagles watched by millions of Internet nature enthusiasts, now has a new claim to fame.

Iowa Department of Natural Resources workers and Iowa Geological Survey Geologist Robert McKay first made the discovery in 2008 after examining well drilling samples. The samples uncovered an unusual shale formation. Scientists believe the shale was created when water filled the crater and over time sediment and rock filled it in.

Underlying the shale is a layer of broken rock fragments cemented together by a fine-grained matrix, which contained small grains of quartz that have been shattered – or as geologists say – shocked.

“There are only two known ways to create shocked quartz,” McKay said. “You find it in known meteorite impacts, and the second place you find it is in underground nuclear test sites.”

The quartz was studied by Bevan French, a scientist at the Smithsonian Institution’s National Museum of Natural History. He concluded it contained the characteristics found in meteorite craters.

He said the Decorah crater is intriguing because of its age and well-preserved rock structures.

The Decorah crater preserves very well the layer of rock broken up by the meteorite impact and the shale above it “that contains a very fascinating biological assemblage,” French said.

“We may have a crater producing a snapshot in the biology of this particular area which I think is the thing that interests a lot of paleontologists,” he said.

The confirmation that Decorah was built on a crater came recently when the U.S. Geological Survey was conducting aerial surveys using electromagnetic equipment that measures how well rocks conduct electricity and a gravity device measuring subtle changes in rock density.

The shale has a significant electrical contrast to surrounding rock and when measured in the recent survey, the shape of a nearly circular crater was revealed, said Paul Bedrosian, a USGS geophysicist in Denver.

“The shale is an ideal target and provides the electrical contrast that allows us to clearly image the geometry and internal structure of the crater,” Bedrosian said.

The crater appears to be about 3 miles wide and more than 700 feet deep. Determining the exact depth with take further study.