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

Abstract

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 ...