MAGNETOSTRATIGRAPHY AND MEGAFLORA OF THE LOWER PALEOCENE FORT UNION FORMATION ALONG THE SOUTHWESTERN MARGIN OF THE WILLISTON BASIN, NORTH DAKOTA

A continuous series of Upper Cretaceous through Eocene age deposits occurs along the southwestern margin of the Williston Basin, in the Little Missouri River valley of North Dakota. Using the well constrained K-T boundary as the basal datum, a succession of stratigraphic sections have been measured allowing for the construction of an approximately 180 meter composite section of the lower Paleocene Ludlow Member of the Fort Union Formation. Paleomagnetic samples and fossil plants have been collected within this stratigraphic interval. Paleomagnetic samples analyzed from the section indicate that that this sequence of paleomagnetic reversals can be correlated to the part of the Geomagnetic Polarity Time Scale that extends from C29r to C28n. Based on these paleomagnetic data, the mean sedimentation rate on the southwestern margin of the Williston Basin from the K-T boundary to the top of 28r is estimated to be 101 m/Myr. Preliminary results indicate that the rate of deposition increased slightly from about 111 m/Myr in C29r to approximately 123m/myr in C29n and then decreased by more than half to about 44 m/Myr during C28r. The base of C28r is coincident with the Three V tongue of the marine Cannonball Member of the Fort Union Formation and suggests that rates of deposition declined during the retreat of the Cannonball Sea. Census collections from megafloral localities through this interval document low- diversity floral assemblages composed up of species that are common to the basal Paleocene across the Western Interior of the United States. The composition of this flora places the Ludlow Member within the early Paleocene FU1 floral zone and demonstrates that the FU1 megafloral assemblage existed within the Williston Basin for at least the first 1.4 Myr of the Paleocene.