THE MANSON IMPACT STRUCTURE - AR-40 AR-39 AGE AND ITS DISTAL IMPACT EJECTA IN THE PIERRE SHALE IN SOUTHEASTERN SOUTH-DAKOTA/

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