Vegetation distribution across the early Eocene depositional landscape from palynological analysis

Abstract

The usefulness of palynomorphs in reconstruction of vegetation in the pre-Quaternary has been hampered by uncertainty in distinguishing local from transported palynomorphs. Analysis of palynomorph transport in modern settings suggests that the importance of transported palynomorphs in a nonmarine depositional environment depends on its geomorphologic characteristics. Aerial transport will be a significant source of palynomorphs only if the environment is open. Waterborne transport depends on the access of water to the depositional environment. The importance of transported palynomorphs relative to the local palynomorphs varies among depositional environments, and can be assessed in each case. In the early Eocene of the Bighorn Basin (Wyoming), five depositional environments (pond, levee/crevasse splay, swamp margin, swamp, reduced floodplain) contain distinct palynomorph assemblages as measured by canonical variates analysis. The relative importance of transported palynomorphs is greatest in the pond, and progressively decreases in the levee, reduced floodplain, swamp margin, and swamp. In the pond, reworked marine dinoflagellates and bisaccate conifer pollen are high in abundance, reflecting transport. Taxodiaceae and Alnus are rare, and did not inhabit the vegetation surrounding the pond. Corylus, ulmaceous, and fagaceous (?) taxa lived surrounding the pond. The levee, like the pond, contains marine dinoflagellates and bisaccate pollen, but in lower numbers than the pond. Pollen of Fagaceae, Alnus (locally), and pteridophyte spores reflect the levee vegetation. The abundance of pteridophyte spores results from their ability to respond quickly to substrate disturbance. Reduced floodplains contain pteridophytes, Taxodiaceae, Platycarya, and Plicatopollis. Swamp margin and swamp palynofloras are similar and are dominated by local plants: Taxodiaceae, Alnus, Platycarya, Sparganiaceaepollenites, and locally Pistillipollenites. The palynofloral data is consistent with the megafloral record in the same environments; this supports the vegetation reconstruction from palynology.