W. A. Cobban J. Obradovich Michael J. Kunk G. A. Izett 1993 <p><span>The&nbsp;</span>⁴⁰<span>Ar/</span>³⁹<span>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 (64.43 ± 0.05 Ma). The MIS sanidine is 9 million years older than&nbsp;</span>⁴⁰<span>Ar/</span>³⁹<span>Ar 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&nbsp;</span>⁴⁰<span>Ar/</span>³⁹<span>Ar age of 73.8 ± 0.3 Ma for MIS reported herein.</span></p> application/pdf 10.1126/science.262.5134.729 en American Association for the Advancement of Science The Manson Impact Structure: 40Ar/39Ar age and its distal impact ejecta in the Pierre Shale in southeastern South Dakota article