FLORAL CHANGE IN THE NORTH AMERICAN PALEOCENE

Using a nearly continuous sequence of Paleocene terrestrial sediments in the Little Missouri River Valley of North Dakota, USA, we aim to establish when and if there are changes in the Williston Basin megafloral record, and to examine overall floral change in the Western Interior Paleocene. We have constructed a ~325 meter composite section for the Cretaceous Hell Creek Fm. and the Paleocene Ludlow and Tongue River Mbrs. of the Fort Union Fm. that demonstrates a series of geomagnetic reversals that can be correlated to the part of the GPTS that extends from C30n to C27r. Census collections of megafloral localities through this interval document three distinct floral assemblages. The Fort Union I flora (FU I; the first floral assemblage) persisted until ~64 Ma. The subsequent change in the megaflora record corresponds to the transition from palynostratratigraphic zone P2 to P3 and is likely related to the coincident regression of the Cannonball Seaway. The second floral assemblage lasted ~1 – 1.5 myr and may relate to a local environmental changes accompanying the transition from Ludlow to Tongue River deposition and perhaps to the onset of cooler climates in the mid-Paleocene. The third floral assemblage is first seen at least 2.5 – 3 myr after the K-T boundary and suggests an even cooler climate than do the two preceding floral assemblages. Using published floral lists and our Williston Basin megafloral data, we recognize three major floral regimes in the Western Interior of North America. The earliest Paleocene floral regime, FU I, is similar across all Western Interior basins and persists for at least 3 myr. The second floral regime is lower in diversity and probably indicates a cooler climate for the Western Interior. The third floral regime shows an increase in diversity and the onset of significant differences in floral composition between basins. These new data indicate that 1) the FU I “recovery” flora persists for at least the first 1/3 of the Paleocene; 2) there are both large scale (Western Interior) and small scale (within basin) changes in the Paleocene megafloral record; and 3) megafloral changes in the Western Interior are likely related to both local environmental changes, such as sea level transgression and regression, and to global changes, such as long term cooling and warming trends in global temperatures.