The Bighorn Basin of Wyoming preserves a remarkably continuous continental record of late Paleocene to early Eocene biotic and climatic change, but incomplete paleomag-

New paleomagnetic data from six different sections in the Bighorn Basin are combined with previously published results to construct a basin-wide magnetostratigraphic framework. Geomagnetic polarity reversals between chrons C26r, C26n, C25r, C25n, C24r, and C24n have been identifi ed among multiple stratigraphic sections in different parts of the basin. Using the new magnetostratigraphic framework, paleontological, paleobotanical, and isotopic information from these varied locations in the basin can now be correlated and compared to similar records from elsewhere in the world. These new data resolve previous uncertainty concerning the timing of an important episode of faunal turnover known as Biohorizon B, which is slightly below the chron C24r-C24n boundary, close to the position of the Elmo isotope excursion in marine records. Backstripping analysis using these new magnetostratigraphic data helps defi ne the time-transgressive onset of basin formation and shows the different subsidence histories of the northern and southern parts of the basin.