[meteorite-list] Iowa Meteorite Crater Confirmed

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Tue, 5 Mar 2013 11:50:32 -0800 (PST)
Message-ID: <201303051950.r25JoW2i026472_at_zagami.jpl.nasa.gov>

http://www.usgs.gov/newsroom/article.asp?ID=3521

Iowa Meteorite Crater Confirmed
USGS Airborne Surveys Back Up Previous Decorah Research
US Geological Survey
March 5, 2013

Recent airborne geophysical surveys near Decorah, Iowa are providing an
unprecedented look at a 470- million-year-old meteorite crater concealed
beneath bedrock and sediments.

The aerial surveys, a collaboration of the U.S. Geological Survey with
the Iowa and Minnesota Geological Surveys, were conducted in the last 60
days to map geologic structures and assess the mineral and water
resources of the region.

"Capturing images of an ancient meteorite impact was a huge bonus," said
Dr. Paul Bedrosian, a USGS geophysicist in Denver who is leading the
effort to model the recently acquired geophysical data. "These findings
highlight the range of applications that these geophysical methods can
address."

In 2008-09, geologists from the Iowa Department of Natural
Resources' (Iowa DNR) Iowa Geological and Water Survey hypothesized what
has become known as the Decorah Impact Structure. The scientists
examined water well drill-cuttings and recognized a unique shale unit
preserved only beneath and near the city of Decorah. The extent of the
shale, which was deposited after the impact by an ancient seaway,
defines a "nice circular basin" of 5.5 km width, according
to Robert McKay, a geologist at the Iowa Geological Survey.

Bevan French, a scientist the Smithsonian's National Museum of Natural
History, subsequently identified shocked quartz - considered strong
evidence of an extra-terrestrial impact - in samples of sub-shale
breccia from within the crater.

"The recognition of this buried geological structure was possible
because of the collaboration of a local geologist, water well drillers,
the USGS STATEMAP program, and the support of the Iowa DNR concerning
research on fundamental aspects of Iowa geology," said McKay.

The recent geophysical surveys include an airborne electromagnetic
system, which is sensitive to how well rocks conduct electricity, and
airborne gravity gradiometry, which measures subtle changes in rock
density. The surveys both confirm the earlier work and provide a new
view of the Decorah Impact Structure. Models of the electromagnetic data
show a crater filled with electrically conductive shale and the
underlying breccia, which is rock composed of broken fragments of rock
cemented together by a fine-grained matrix.

 "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.

More analysis of the data will provide additional detail. These data
show the impact as a nearly circular region distinct from the
surrounding area to a depth of several hundred meters.

"These data, when coupled with physical property measurements on drill
core samples, will form the basis for modeling efforts to constrain the
impact geometry and energy of the meteorite," said Dr. Andy Kass, a USGS
geophysicist working on the effort.

The Iowa and Minnesota airborne geophysical surveys are targeting an
igneous intrusion, known as the Northeast Iowa Igneous Intrusive
complex, that may be similar to the Duluth layered igneous complex
exposed in the Lake Superior region of northern Minnesota. Known copper,
nickel, and platinum group metal resources were deposited during the
formation of the Duluth complex. Both of these complexes are associated
with a large structural feature known as the Midcontinent Rift, which is
exposed in the Lake Superior Region but is covered by younger rocks as
it extends to the south through Iowa, Nebraska, Kansas, and Missouri.

This geophysical 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.

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Received on Tue 05 Mar 2013 02:50:32 PM PST