Abstract

Paleosols at Polecat Bench, northern Bighorn Basin, Wyoming, show prismatic structures not previously described in the Paleogene Willwood Formation. Prisms are densely spaced 15–50 mm diameter cylinders with vertical to slightly sinuous paths up to 400 mm long, subangular to rounded vertical faces, and slightly concave to convex tops. Prism exteriors are coated commonly by < 1-mm-thick clay films; exteriors also commonly show knobby and striated surficial morphologies. Prismatic structures are nearly exclusive to thin-bedded (< 1 m thick) compound paleosols composed of red to red-purple sandy mudrock overlying gray to green-gray heterolithic units composed chiefly of fine-grained sandstone. In addition to prismatic structures, these paleosols are characterized by large gray mottles, slickensides, and calcareous rhizocretions which crosscut or more typically follow prism exteriors. The architectural and surficial morphology of the soil prisms suggest strongly that these are pedogenically modified freshwater crayfish burrows of the ichnospecies Camborygma litonomos.

At Polecat Bench, an ~ 40 m interval of the Willwood Formation records a transient episode of global warming known as the Paleocene–Eocene Thermal Maximum (PETM). The PETM interval is characterized by a series of predominantly red, cumulative paleosol profiles and increased soil carbonates. Prismatic soil structures and crayfish burrows are rare or absent in cumulic paleosols typical of the PETM, whereas they are locally abundant in red compound paleosols outside the PETM interval. The changes in paleosol morphology and a sharp decrease in crayfish burrows within the PETM interval suggest significantly improved soil-drainage conditions and lower water tables on the Willwood floodplain during the global warming event.