Palynomorph biozones for the fort Union and Wasatch formations (upper Paleocene‐lower Eocene), powder river basin, Wyoming and Montana, U.S.A.

Abstract

Three biozones are proposed for the Tongue River Member of the Fort Union Formation and the overlying Wasatch Formation in the northern Powder River Basin, Wyoming and Montana. The biozones are, in ascending order, the Aquilapollenites spinulosus Zone, Caryapollenites Assemblage Zone (incorporating the Pistillipollenites mcgregorii Subzone and Insulapollenites rugulatus Subzone), and Platycarya platycaryoides Zone. The zones are based on the stratigraphic ranges of several palynomorphs and changes in abundance of certain plant groups. The apparent evolution within the Momipites‐Caryapollenites lineage, previously defined and employed stratigraphically in the Wind River Basin, Wyoming, can also be applied in the Powder River Basin. The change in dominance upsection from Momipites to Caryapollenites essentially coincides with a major sedimentological change from a fluvial channel‐dominated environment to a fluvial‐lacustrine environment. The Paleocene‐Eocene boundary in the Powder River Basin lies in the lower part of the Wasatch Formation and is characterized by an abrupt floral change. This is in accord with previous observations in the Powder River Basin. There is good correlation of the boundary with other Western Interior basins, which is made possible by the simultaneous appearance of Platycarya pollen.

Notes

Work completed while the author was on leave at the U.S. Geological Survey, Denver, Colorado