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Collection and analysis of fossil plants in several stratigraphic sections have produced a chronologic framework for plant-bearing rocks of the northwestern North American province. The geographic extent of this province varied during the Cenozoic, but the stage framework can generally be applied to the region west of the Rocky Mountains (including Alaska) during the Paleogene and Miocene. The occurrence of some plant megafossils in dominantly marine sections of Paleogene age provides only a general concept of relation to the marine geochronology. Numerous Neogene floras have been radiometrically dated and can thus be more accurately related to the marine geochronology. The changes in the floral assemblages are largely the result of major climatic changes. If these changes are assumed to represent secular phenomena, the provincial chronology can also be related to the Cenozoic terminology in those Northern Hemisphere regions where paleobotanical data indicate that a similar series of climatic changes occurred. Inferences based on a combination of radiometric ages, correlations of West Coast benthic foraminiferal stages to the planktonic chronologies, and marine-nonmarine correlations of West Coast units indicate that the Eocene/Oligocene boundary may be as young as 32 m.y.

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