A high resolution chronostratigraphic study of the early Paleocene floral record in the northern Great Plains

To estimate the duration and age of the early and middle Paleocene members of the Fort Union Formation, I analyzed paleomagnetic samples from nine lithostratigraphic sections in the Williston Basin and from two sections in the Powder River Basin. The Williston Basin paleomagnetic samples document a series of reversal that can be correlated to C30n -- C27r of the geomagnetic polarity time scale (GPTS). U/Pb isotopic age determinations and sedimentation rate age models of the sequence suggest that the Ludlow-Tongue River formational contact represents a &sim2 myr long unconformity. These data indicate that in the Williston Basin the Ludlow and Tongue River Members of the Fort Union Formation were deposited from &sim65.5 -- 58.5 Ma. The Powder River Basin polarity sequence can be correlated to C29n -- C26r of the GPTS. Sedimentation rate age models of the sequence suggest that the Lebo-Tongue River contact represents a &sim500 kyr unconformity and that the Lebo and Tongue River Members of the Fort Union Formation were deposited from &sim64.5 -- &sim61 Ma. The age models also suggest that the onset of the unconformities in both basins was contemporaneous at &sim63 Ma.I describe 117 fossil plant morphotypes from the Williston and Powder River Basins. This impression flora includes one alga, one equisetum, five ferns, four conifers, four monocotyledonous angiosperms, and 102 dicotyledonous (dicot) angiosperms. The floral composition and stratigraphic ranges of taxa indicates that the Williston Basin floral record can be subdivided into three megafloral zones each representing &ge1 myr. Although floras from the contemporaneous Powder River Basin suggest that this floral biostratigraphy is restricted to the Williston Basin, the floral records of both basins imply that local and regional paleoenvironmental and climatic changes contributed to transitions in the early and middle Paleocene plant communities. Diversity analyses suggest that Paleocene plant communities remained significantly less diverse than the latest Cretaceous communities for at least 7 myr after the Cretaceous-Paleogene extinctions. The floral records in both basins also document a significant decrease in floral diversity that mirrors a general decrease in mean annual temperatures from the latest Cretaceous to middle Paleocene. These data suggest that the cooling climate played a major role in the floral composition and diversity of early and middle Paleocene floras.