UPDATED RESULTS OF A MAGNETOMETER SURVEY AT THE DECORAH IMPACT STRUCTURE

The recently confirmed Decorah Impact Structure, located in Decorah, IA, is of Early to Middle Ordovician age (470 m.y.). Regional lithology of the area generally consists of sandstones, limestones, and dolomites. In addition, the Winneshiek Shale Formation is newly recognized and only found in the Decorah structure. Speculation of an unidentified impact structure began in the early 2000’s with the discovery of an unidentified, local breccia from drill cores. During mineralogical examination of retrieved core, shocked quartz was identified. However, not until 2013 when the US Geological Survey (USGS) completed an aerial geophysical survey of the region, incidental to the impact structure, was the structure sufficiently confirmed to be of impact origin.

In 2011, boring data were used by the Iowa Geological and Water Survey (IGWS) to construct a regional geological cross-section for water quality and resource usage. The IGWS cross-section also suggests a structure approximately 5 km in diameter.

During the summer and fall of 2015 I conducted a terrestrial magnetic survey of the impact structure consisting of 47 data points. Using these points, a magnetic anomaly map and profile was constructed and depicts a bowl shaped structure approximately 5 km in diameter supporting both the USGS and IGWS results.