The only coral that you would ever find in the Great Lakes area would be incased in rock and broken into stones as a result of the movements of the glaciers which formed the Lakes.
The basement rocks under Michigan are sedimentary rocks that were laid down by a saltwater lagoon a half billion years ago. I’ve found shark’s teeth on the Canadian shore of Lake Huron.
But if the Mormon Tabernacle Choir was on it, it would be a choral reef.