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Science 29 October 1993:
Vol. 262. no. 5134, pp. 729 - 732
DOI: 10.1126/science.262.5134.729
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Articles

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

G. A. Izett 1, W. A. Cobban 1, J. D. Obradovich 1, and M. J. Kunk 2

1 U.S. Geological Survey, Denver, CO 80225
2 U.S. Geological Survey, Reston, VA 22092

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.

Submitted on July 9, 1993
Accepted on August 31, 1993


THIS ARTICLE HAS BEEN CITED BY OTHER ARTICLES:
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