Sedimentation patterns around the Paleocene–Eocene Boundary in the Hanna Basin, Wyoming

The Hanna Basin in south central Wyoming was one of the most rapidly subsiding basins in the foreland of the Laramide orogeny. The Cretaceous to Eocene basin fill is divided into the Medicine Bow, Ferris, and Hanna Formations characterized by an upward succession from marine to fluvial to paludal and lacustrine deposits. The Paleocene to Eocene Hanna Formation (approximately 58–54 Ma) contains abundant, thick coalbeds preserved from extensive coal swamps interbedded with fluvial and lacustrine sandstones and shales. The Paleocene–Eocene Boundary (~56 Ma) is associated with a rapid global release of carbon into the atmosphere that led to a thermal event lasting about 170 k.y. Our new detailed stratigraphic studies in the Hanna Basin demonstrate that the Paleocene–Eocene Boundary is recorded by a remarkable upward shift from coalbeds and carbonaceous shales to an expanded fluvial system with coarser-grained channels and abundant floodplain deposits containing red-mottled paleosols that are otherwise rare in this basin-fill sequence. Sudden expansion of fluvial systems like this are also observed in the nearby Bighorn Basin and elsewhere in the world during the same stratigraphic interval. Here we present new data on timing of sedimentation and vegetation changes and discuss basin-fill processes in the Hanna Formation before, during, and after the Paleocene–Eocene Thermal Maximum.