Feb
11
2014

New evidence confirms Iowa impact crater

3D perspective of Decorah Impact Structure
3D perspective of Decorah Impact StructureCourtesy Public domain via USGS
By studying magnetic and electrical data, geologists have found further evidence that the town of Decorah, Iowa is built upon an ancient impact crater created around 460 million years ago.

Site map of Decorah crater
Site map of Decorah craterCourtesy Vkil via Wikipedia Creative Commons
Decorah is located in northeast Iowa near the Minnesota border about 150 miles south of Minneapolis. Scientists think the Decorah Impact Structure resulted from the same meteorite barrage - known as the Ordovician meteor event - that produced similarly-aged craters found in Oklahoma, Wisconsin, and the Slate Islands in Lake Superior. The Decorah crater lines up nicely with the others.

The first evidence that an ancient crater might exist under Decorah came in 2008 when well-drilling cores from the area collected and examined by Iowa's Department of Natural Resources and Geological and Water Survey indicated that a wide-ranging layer of an unusual type of shale set beneath the surface and encircled the town. Recent aerial geophysical measurements (both gravity and electrical magnetic) by the US Geological Survey and other agencies, including the Minnesota Geological Survey, affirmed the crater's existence.

The unusual shale layer is situated 50 feet under the bed of the Upper Iowa river and was probably deposited after the crater's creation when an ancient seaway invaded the area and filled in the basin with mud and sediment. Shocked quartz found in the rock layer directly beneath the shale adds further evidence that some sort of major impact took place. Shocked quartz is a highly stressed and shattered quartz produced only one of two ways: either by a bolide impact or from a nuclear blast. The impact that created the Decorah structure is estimated to have released the energy equal to the blast of 100 megatons of TNT. To put things in perspective, the nuclear bomb dropped on Hiroshima during World War II was equal to 15 kilotons of TNT. One megaton equals 1000 kilotons so the bolide blast in Decorah would have released the energy of more than 6500 Hiroshima bombs!

Paleobiologist Bevan French, an adjunct professor at the Smithsonian Institution's National Museum of Natural History studied quartz samples from the underlying layer of breccia and concluded they held characteristics indicative of an extra-terrestrial impact event.
When it slammed into Earth, the Decorah impactor created a 3.5 mile-in-diameter crater in the planet's surface and shattered existing layers of Early Ordovician and Cambrian rocks pushing them deeper underground. Several other meteor craters discovered on earth date back to around the same time period 450 to 470 million years ago, causing French to wonder if the Decorah crater should be included in that spike in impact frequency.

According to French the shale above the breccia layer is very well preserved and contains "a very fascinating biological assemblage,"which could also be of interest to paleontologists.

"Finding structures like these and being able to study them in the geological context," French said, "is going to yield a lot of very fascinating information about the relations between the terrestrial system and the extraterrestrial influences."

Sign of demise and new beginnings: A distinct layer of white clay in southern Colorado rock exposure (and found elsewhere around the world) marks the end of the dinosaurs (non-avian, anyway) and the beginnings of the reign of mammals.
Sign of demise and new beginnings: A distinct layer of white clay in southern Colorado rock exposure (and found elsewhere around the world) marks the end of the dinosaurs (non-avian, anyway) and the beginnings of the reign of mammals.Courtesy Mark Ryan
Has past life on Earth been influenced by these impacts? If you consider the Chicxulub impact crater in the Yucatán Peninsula of Mexico and its alleged effect on non-avian dinosaurs at the end of the Cretaceous Period, you have to wonder if similar impact events weren't responsible for other extinctions and biological radiations during Earth's long history.

SOURCE and LINKS

USGS story
Wunderground news story
Live Science report
Ordovician Meteor Event
Tiny Traces of Big Asteroid Breakup

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