The Manson Impact Structure: 40Ar/39Ar age and its distal impact ejecta in the Pierre Shale in southeastern South Dakota

The ⁴⁰Ar/³⁹Ar 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 (64.43 ± 0.05 Ma). The MIS sanidine is 9 million years older than ⁴⁰Ar/³⁹Ar age spectra of MIS shock-metamorphosed microcline and melt-matrix breccia interpreted earlier to be 64 to 65 Ma. Grains of shock-metamorphosed quartz, feldspar, and zircon were found in the Crow Creek Member (upper Campanian) at a biostratigraphic level constrained by radiometric ages in the Pierre Shale of South Dakota that are consistent with the ⁴⁰Ar/³⁹Ar age of 73.8 ± 0.3 Ma for MIS reported herein.