Abstract

A palynostratigraphic zonation has been developed for Paleocene and lower Eocene strata that has been successfully applied throughout the Western Interior of North America. The original zonation was developed in central Wyoming, but it has been extended by subsequent studies to other Cenozoic basins in Wyoming, where it has been modified and refined. A revised version of the zonation was recently applied in North Dakota. Correlative zonations based in part on the same palynomorph species were developed in Colorado and New Mexico, and in Alberta, Canada. With attention to the influence of paleolatitude, these local zonations can be integrated into a single, comprehensive, and utilitarian nonmarine biostratigraphy for the lower Cenozoic of the entire region. Toward this end, seven biozones are redefined and formally named here.

Regional effects of the terminal Cretaceous extinction event on plants provide a basal datum for the zonation. Recovery and proliferation of angiosperms in the Paleogene resulted in new species having biostratigraphic potential. Six biozones defined by ranges of species within the Momipites-Caryapollenites lineage and a seventh defined by the occurrence of a species of the closely related genus Platycarya are the basis of the palynostratigraphy. These are all fossil species of the modern family Juglandaceae. Additional, unrelated species of angiosperm pollen supplement the definition of the biozones. Local ranges of some of these species differ within the Raton, Denver, Hanna, Wind River, Bighorn, Powder River, and Williston Basins, and in the Alberta Syncline in Canada. These differences imply that the Juglandaceae spread from south to north within the Western Interior of North America. Understanding the similarities and differences in occurrences of key pollen taxa enhances the utility of palynostratigraphy in the lower Cenozoic rocks of the region.