It seems to me that Michigan's geological substrate suggests that the area took a major direct hit in the pre-dinosaur era. See http://www1.snapfish.com/slideshow/AlbumID=43805012/PictureID=1291284304/a=53099727_53099727/t_=53099727
If so, the rock probably hit some time before 150 MYA, when the area became the center of a great inland sea.
I agree. It was probably a "doublet" impact, that is, multiple, nearly simultaneous impacts from a large body torn apart by an earlier encounter, or by reaching its Roche point during terminal descent, or even more likely, by never having been a single body in the first place, a la TVF. :')
An Asteroid, Cobbled Together
ScienceNOW Daily News | 2 June 2006 | Phil Berardelli
Posted on 06/03/2006 3:34:15 AM EDT by neverdem
http://www.freerepublic.com/focus/f-news/1642717/posts