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Figure 6.2 Stratigraphic nomenclature for the uppermost Cretaceous and lowermost Paleogene of the Williston Basin in eastern Montana and western and south-central North Dakota. The Hell Creek Formation and Tullock and Ludlow Members of the Fort Union Formation are nonmarine; the Bearpaw Shale and Pierre Shale and the Cannonball Member of the Fort Union Formation are marine (from Johnson et al. 2002). Reprinted by permission.

were rare, but tended to develop as the water table rose during minor incursions of the sea. A diverse forest dominated by angiosperms covered the area. The flora comprised hundreds of species and included both familiar groups such as palms and a variety of lobe-leafed species, many of which are unknown today in forests anywhere in the world. The Maastrichtian climate was warm, especially during the last million years. Toward the end of Maastrichtian time, the warmest temperatures promoted an influx of thermophyllic (warmth-loving) plant species into the basin; these additions to the flora served to bring species diversity to a peak. At 65.5 Ma, at the K-T boundary, the vegetation of the Williston Basin was abruptly, profoundly, and permanently changed. The destruction of the flora involved extinction of all of the dominant plants of the Maastrichtian.

The Paleocene climate did not change radically from what had prevailed during Maastrichtian time. Mean annual temperatures remained about the same as they had been, as indicated by the persistence of thermophyllic species such as palms. The terrain changed somewhat in early Paleocene time, largely in response to a major re-advance of the Western Interior seaway in the form of the Cannonball Sea. Rivers were effectively dammed, the water table rose across the region developing extensive mires in which peat accumulated. A different flora ell Creek m MT

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