HIGH RESOLUTION MAGNETOSTRATIGRAPHIC CORRELATION ACROSS THE PALEOCENE/EOCENE BOUNDARY IN THE BIGHORN BASIN, NORTHWESTERN WYOMING.

by

Walid M. Hamzi

University of New Hampshire, December, 2003

New paleomagnetic data resolve previous stratigraphic discrepancies from Wyoming’s Bighorn Basin and provide a chronostratigraphic framework with which to correlate the biotic and climatic events evidenced in the Paleocene and Eocene rocks comprising this region. This study reports on magnetostratigraphic correlations to the Geomagnetic Polarity Time Scale (GPTS) from the paleomagnetic analysis of 110 sites collected from 5 centrally located stratigraphic sections in the Bighorn Basin. Three components (a viscous remanent component, a present-day field component, and a characteristic remanent component) generally characterize paleomagnetic results for both the Paleocene Fort Union Fm. and Eocene Willwood Fm. Characteristic magnetizations have either N-NW and steeply down directions or antipodal S-SE and up directions. The characteristic component is interpreted as primary based on antipodal polarity directions and consistency with the expected Eocene direction for western Wyoming. The polarity sequence seen from the base of Sand Creek Divide to the top of Elk Creek correlates with the GPTS from Chron C26n to Chron C24n and encompasses approximately 4.5my.

Magnetostratigraphic correlation of the 5 study sections and 2 northern sections, Clarks Fork Basin and McCullough Peaks, show a significant thinning of Paleocene and Eocene beds from north to south. This indicates sedimentation flux and/or basin subsidence was greater in the northern basin. Previous work in the basin resulted in discrepancies in placement of reversal C24r/C24n relative to the First Appearance Datum (FAD) of Bunophorus. Results of this study show this reversal to occur slightly above the FAD of Bunophorus. This finding is consistent with that of Butler et al. (1981) and Clyde et al. (1994).