"With the help of magnetic sensors, scientists have detected a rimmed circular structure, 30 miles in diameter, more than a mile beneath the floor of Lake Huron. They believe the magnetic ring marks a buried crater -- blasted by a meteorite at least 500 million years ago."
Editor's Note: Not that long ago -- just DEEP.
(Stolzenburg, W.; "Impact Crater May Lie beneath Lake Huron," Science News, 138:133, 1990.)
The story that you refer to in your query is an article published in
Geology. The full bibliographic reference is as follows:
Forsyth, D.A., Pilkington, M., Grieve, R.A.F. and Abbinett, D.,
1990. Major Circular Structure beneath Southern Lake Huron defined from
Potential Field Data. Geology, v. 18 (August), pp 773 - 777.
In summary, a major circular structure beneath Lake Huron defined by
magnetic and gravity data, centered at lat 43 deg, 14.21' N and long 82 deg, 19.88' W, is characterized by a magnetic anomaly pattern with a central peak, a 50 km diameter principal ring and conformable anomalies extending to a diameter of about 100 km. Depth estimates place the source of the magnetic anomalies at the top of the Precambrian basement. The structure is visible through 2 km of sediments, indicating a well preserved structure.
There is no strong evidence of the feature in the bathymetry data of
southern Lake Huron, though the general coastline of southern Lake Huron
closely follows the outline of the main circular feature.
(Saved from http://members.theglobe.com/algis/states/michigan.html
Author: Algirdas Ratnikas
"500 Million A -- 30-mile size crater, a mile underneath the bed of Lake Huron, just north of Port Huron, Michigan, marks the impact of a meteor. It was discovered in 1990 by scientists from the Geological Survey of Canada. (LSA, Spring 1995, p.31)
"440 Million A -- five-mile size crater in Michigan in Cass County by the village Calvin Center marks the impact of a meteor the size of a football field. It was discovered in 1987. (LSA, Spring 1995, p.31)
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