The Manson Impact Structure: 40Ar/39Ar Age and Its Distal Impact Ejecta in the Pierre Shale in Southeastern South Dakota.

The (40)Ar/(39)Ar ages of a sanidine clast from a melt-matrix breccia of the Manson, Iowa, impact structure (MIS) indicate that the MIS formed 73.8 +/- 0.3 million years ago (Ma) and is not coincident with the Cretaceous-Tertiary boundary (64.