Geological survey: Ancient meteorite crater sits below Decorah

City built atop just 180 known impact craters on Earth

Looking due north, this 3-D view of Decorah and the Upper Iowa River shows the impact area of a meteorite that crashed into the area mearly half a billion years ago. (Credit: Adam Kiel graphic/Northeast Iowa RC&D)
Looking due north, this 3-D view of Decorah and the Upper Iowa River shows the impact area of a meteorite that crashed into the area mearly half a billion years ago. (Credit: Adam Kiel graphic/Northeast Iowa RC&D)

Recent aerial surveys in the Decorah area have confirmed that a huge meteorite crashed into the earth there about 470 million years ago.

“Yeah, we have a very high confidence level” that a meteorite about 200 meters across struck there, said Robert McKay, a geologist with the Iowa Geological and Water Survey, which is partnering with the U.S. Geological Survey in the scientific study.

“I would say it is a pretty big scientific discovery,” said Paul Bedrosian, a USGS geophysicist in Denver, Colo., who is leading the effort to model the recently acquired geophysical data.

Only 183 meteor impact craters have been documented worldwide, according to an international database at the University of New Brunswick.

No evidence of the giant underground crater – more than 3 miles in diameter -- is visible to anyone studying the area through satellite images, aerial photos or even from a lofty vantage, said McKay, who pieced together several disparate clues to nail down the discovery.

The shale formation that led to the discovery pokes through the earth’s surface at only one known location, he said.

McKay credits the late Jean Young, a Decorah-area independent geologist who died in 2007, with finding the first clue.

“She called to my attention in 2000 an unusual cluster of well boring samples containing thick layers of shale, all from a confined area in and immediately around Decorah,” McKay said.

When McKay plotted them on a map, they described a circular basin 3.5 miles wide overlaying the city of Decorah.

Beneath the shale, formed when an ancient sea deposited sediment in the crater, was another layer of material unique to that particular spot, a substance McKay suspected was shocked quartz, shattered crystals often associated with meteorite impact structures.

McKay sent photos and samples to Bevan French, a scientist at the Smithsonian's National Museum of Natural History, who subsequently identified shocked quartz -- considered near definitive evidence of an extra-terrestrial impact - in the samples.

Finally, the recent aerial survey, which collected electromagnetic and rock density information, has yielded data consistent with the existence of the crater hidden beneath Decorah.

Bedrosian said “the one-to-one correspondence of data collected in completely independent studies provides pretty definitive evidence” of an extraterrestrial impact.

The energy released in that long-ago collision, he said, would have been equivalent to the explosion of a bomb rated at more than 1,000 megatons of TNT. That compares with the 57-megaton rating of the most powerful nuclear weapon ever detonated and the estimated 10 megatons of force generated by the meteor that exploded over Russia last month.

Bedrosian said the survey is part of a larger USGS effort to evaluate the concealed mineral resource potential of the greater Midcontinent Rift region that formed about 1.1 billion years ago.

The recent flights, he said, targeted the Northeast Iowa Igneous Intrusive complex, which geologists suspect may be similar to an area of northern Minnesota known for deposits of copper, nickel and platinum.

While more analysis will be required to determine the mineral potential, the study has unearthed a valuable finding about the area’s water resources, McKay said.

“We found that the presumed impact shattered the Jordan sandstone layer, disrupting the Jordan aquifer in that locale,” he said.

That means, for example, that if the city of Decorah wanted to supplement its shallow alluvial wells with a deeper source of water, drilling beyond the crater boundaries would be advised, he said.

McKay also said the shale in the crater is too young in the geological sense to be a source of oil or natural gas.

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