This all lead to a more recent examination of the area, and the crater possibility, by scientists from the USGS and the Iowa and Minnesota Geological Surveys. They did a high-resolution geophysical survey and additional borehole investigations of the area specifically for the groundwater and mineral resources that might be available. Their results showed a definite difference in rock density and more clearly revealed the circular nature of the geologic anomaly.
Well, long story short, turns out there is a 470 million year old impact crater lying under Decorah, Iowa - confirmed now by three data sets that correlate extremely well. All because someone wanted to assess the groundwater availability of the area by looking at drill cuttings from their water well.
And the shale layer that isn't supposed to exist but which started the whole inquiry? Turns out the entire region had been covered by shale deposited by the intrusion of an inland sea after the impact event but has since been eroded away. This shale just happened to get preserved inside the crater.
Next step is to assess if this crater is a unique event or might be related to three other known midwestern impact craters - in Ames, Oklahoma., Rock Elm, Wisconsin and the Slate Islands of Lake Superior in Ontario, Canada. All four have been dated to roughly the same time period.
Just goes to show you - you never know what's beneath your feet until you look closely.