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Ejecta from Sudbury Impact found in Minnesota: Photo by Mark Jirsa, Minnesota Geological SurveyNew evidence of an ancient meteorite impact in Ontario, Canada, has been found nearly 500 miles away in Minnesota.

Mark Jirsa of the Minnesota Geological Survey said the recent Ham Lake forest fire that raged over 118 square miles near the Gunflint Trail in northern Minnesota had caused the annual meeting of the Institute of Lake Superior Geology (ILSG) Institute of Lake Superior Geology (ILSG) to cancel one of its planned field trip into the region last May.

But because he’s a geologist, Jirsa wasn’t going to let a little conflagration ruin his chances to examine outcrop, so he sojourned into the area on his own to look for possible alternate sites.

Map showing Sudbury Impact and Gunflint Trail locations: Diagram by Mark RyanWhile examining some of the Gunflint Iron Formation in an area he hadn’t visited before, he discovered an odd jumble of rounded black rock overlying the formation, which he now thinks may be partially composed of ejected material from the Sudbury Impact which took place 1,875 million years ago in Ontario.

“It’s fairly dark rock,” Jirsa explained. “They look like concrete, but in this concrete you would throw pieces of rock of all sizes and shapes and in all possible orientations.”

It’s thought that the Sudbury Impact site was created when an extraterrestrial object such as a comet or asteroid between 6 and 12 miles in diameter slammed into the Earth near the present-day town of Sudbury, Ontario, just north of Lake Huron. The effects of the impact were widespread, releasing as much energy as several billion atomic bombs, blasting out more than 6500 cubic miles of debris that blanketed nearly one million square miles with impact ejecta. The crater it left was over 160 miles in diameter, the second largest on Earth.

Some of this ejecta fell into the shallow ocean that covered northern Minnesota at the time. But soon after, shock wave-induced tsunamis would have tore across the area with such force the seabed would have been severely disrupted.

“When the meteorite hit, it’s very likely that the seas went out and then the seas came back in with a vengeance,” Jirsa said.

Consequently, the ejecta and the seabed material would become mixed into a jumbled mess of sediment that would later harden into rock. This is what Jirsa thinks he’s come across along the Gunflint Trail.

Studies of lunar impact craters show that an ejecta blanket five times larger than the crater is usually created from a typical impact. Products of impact range from angular fragments of preexisting rocks and partially melted, recrystallized, or glassy fragments. But unlike the airless Moon, Earth’s atmosphere would have additional effects on the ejecta, producing such things as spherules that condense from vapor in the ejecta cloud (like hail stones forming in rain clouds).

Jirsa showed his discovery to an ILSG colleague named William Addison, who with Gregory Brumpton had first identified Sudbury Impact ejecta 15 years ago near Thunder Bay, Ontario. Addison immediately recognized that the samples exhibited typical textures of material formed in an impact. Interestingly, Addison and Brumpton are both high school earth science teachers, and not professional geologists.

William Cannon of the United States Geological Survey joined the search and has located and documented exposures of ejecta blanket in or near five iron ranges in the Lake Superior region. And Jirsa’s Gunflint Trail discovery, 500 miles from the impact’s “ground zero”, adds another iron formation to the list.

Stromatolite fossils along Gunflint Trail: Photo by Mark Jirsa, Minnesota Geological SurveyIron deposits the same age as the Gunflint Iron Formation (about 2 billion years old) occur world-wide, and are thought to be result of high levels of oxygen present in the atmosphere, due to photosynthesis by cyanobacteria, Earth’s earliest preserved life form. Fossils of these ancient creatures called stromatolites are exposed in the upper layers of the Gunflint Iron Formation in the same area where Jirsa found the ejecta blanket evidence.

Interestingly, the deposition of iron-forming sediments declined right around the same time as the Sudbury Impact (1.875 billion years ago), and Cannon wonders if there may be a connection between the two events. Could the impact have caused the cyanobacteria to go extinct, thereby ending the favorable conditions for iron deposit formation?

“There’s a lot of work that needs to be done in the field to see what this deposit tells us that other sites don’t,” Jirsa said. “That’s the critical thing. This is a different geological setting; it’s a little farther away from the impact, the rocks are altered differently. It may reveal some secrets about the impact that other discoveries haven’t yet. That’s what we’re hoping.”

LINKS and MORE INFO

Story in Minneapolis Star Tribune
Another story source
Yet another story source
More on impact craters
More on cyanobacteria
Sudbury Impact layer in Michigan

Tags : geology, impact crater, Gunflint Trail, Minnesota geology, stromatolites, cyanobacteria, impact crater ejecta

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