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 (USGS) 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 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 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 Andy Kass, a USGS geophysicist working on the effort.