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

G. A. Izett; W. A. Cobban; J. Obradovich; Michael J. Kunk

doi:10.1126/science.262.5134.729

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

G. A. Izett·W. A. Cobban·J. Obradovich·Michael J. Kunk

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Abstract

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 (64.43 ± 0.05 Ma). The MIS sanidine is 9 million years older than 40 Ar/ 39 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 40 Ar/ 39 Ar age of 73.8 ± 0.3 Ma for MIS reported herein.

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Source-reported bounding extent: 42.479635° to 45.94531° latitude; -104.057698° to -96.436589° longitude. This indicates report coverage, not an exact sampling location. View area on OpenStreetMap.

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G. A. Izett, W. A. Cobban, J. Obradovich, Michael J. Kunk. 1993. The Manson Impact Structure: 40Ar/39Ar age and its distal impact ejecta in the Pierre Shale in southeastern South Dakota. https://doi.org/10.1126/science.262.5134.729

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