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ABSTRACT: A Major Circular Basement Structure beneath the Eastern Michigan Basin

D. A. Forsyth, M. Pilkington, B. Milkereit, R.A.F. Grieve

Newly compiled and merged Canadian and U.S. potential field data for the area of southern Lake Huron reveal a large circular structure with a diameter that may extend to 100 km.

The aeromagnetic image shows a circular central high with a diameter of 6 km located at 43°14.21^prime N, 82°19.88^prime 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 generally continuous, northeast-striking aeromagnetic signature of the Grenville Front and adjacent Grenville Front Tectonic Zone appears overprinted by the circular feature.

Estimated depths to magnetic sources for the anomaly pattern indicates sources near the top of Precambrian basement. Reflection data from the area suggest structural dips that vary from the regional southeast dips characteristic of the Grenville terrain.

The Bouguer gravity image also shows an arcuate complex of positive anomalies that terminate a high extending southeast from the positive anomaly over the Grenville Front. Similar highs extending southeast from the Grenville front are absent in the Great Lakes area. In detail, the gravity anomalies show symmetry with an annular low and central high that correlate with the principal circular magnetic anomalies, as well as the lobate pattern of magnetic anomalies marking the outer area of the feature.

There is no apparent 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 interpreted magnetic source depths near the Precambrian surface suggest a post-Grenville to possibly early Paleozoic event.

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.

AAPG Search and Discovery Article #90998 AAPG Eastern Section Meeting, London, Ontario, Canada, September 10-12, 1990