ABSTRACT

A magnetostratigraphic study in the northeast part of the Crazy Mountains Basin of Montana sampled strata that includes both the Cretaceous–Paleogene boundary sequence as well as numerous, well-documented fossil mammal and invertebrate localities. Ninety-five horizons were sampled from nine separate sections, with a composite stratigraphic range from the upper part of the Hell Creek Formation, through the Bear and Lebo Members of the Fort Union Formation, and into the lower part of the Melville Member of the Fort Union. Data show that the (1) Hell Creek Formation–Bear Member contact lies within a magnetozone of reverse polarity (chron C29r); (2) Simpson Quarry and the Bear Member–Lebo Member contact occurs within the overlying normal polarity magnetozone (chron C29n); (3) magnetozones defining chron C28 fall completely within the Lebo Member; and (4) Gidley and Silberling quarries and the Lebo Member–Melville Member contact occurs within a reverse polarity magnetozone (chron C27r). The resulting high-resolution chronostratigraphic framework—incorporating both biostratigraphic and magnetostratigraphic data—places the Crazy Mountains Basin strata and fossil localities into a more well-defined context with other depositional basins in the Western Interior of North America.

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