Newly compiled and merged Canadian and United States potential field data for the area of southern Lake Huron reveal a large circular structure that has a diameter that may extend to 100 km. The aeromagnetic image shows a circular central high with a diameter of 6 km located at fat 43°14.21′N, long 82°19.88′W, surrounded by a principal ring with a diameter of 50 km and indications of conformable, arcuate structure extending to a diameter of about 100 km. The prevailing, northeast-striking aeromagnetic signature of the Grenville front and adjacent Grenville front tectonic zone is interrupted and may be overprinted by the circular feature. The Bouguer gravity field shows a strong positive anomaly that extends southeast from the Grenville front and terminates in an arcuate complex of positive anomalies that correlate with the aeromagnetic field. The gravity field illustrates the circular structure with an annular low and central high, as well as positive extensions that follow the foliate pattern of magnetic anomalies marking the outer area of the feature.
There is no strong evidence of the feature in the bathymetry data of southern Lake Huron, although the general coastline of the southern end of Lake Huron closely follows the outline of the main circular feature. The interruption of the Grenville trends, the clear, regular, circular signature of the feature, and the interpreted magnetic source depths at the Precambrian surface, suggest an event possibly as young as early Paleozoic. In the absence of basement samples, the origin of the feature is unconfirmed. However, the scale and potential field anomaly pattern of the feature are comparable to several of the larger impact structures exposed on the Canadian Shield.