REFINING THE PALEOCENE - EOCENE BOUNDARY AND UNDERSTANDING BASIN FILL OF THE HANNA FORMATION IN THE HANNA BASIN, WYOMING

During the Laramide orogeny (Late Cretaceous to Eocene), the Hanna Basin rapidly subsided and accommodated a thick succession of fluvial, paludal and lacustrine sediments. Greater continuity in sedimentation of the Hanna Basin compared to other surrounding Laramide basins provides a more continuous record of basin fill history and regional paleogeography during this time. New stratigraphic observations from detailed measured sections were integrated with new leaf macrofossil localities, palynology, δ¹³C isotope, mollusk and zircon analysis to constrain the stratigraphic location of the Paleocene- Eocene Thermal Maximum (PETM).

Approximately 1,250 m of upper Hanna Formation strata were measured and correlated across the northeast-trending syncline in the Hanna Basin, near Hanna, Wyoming and a detailed stratigraphic framework was developed. New paleobotanical observations and δ¹³C isotope analysis identify the location of the carbon isotope excursion (CIE) and the PETM within the section. Zircon crystals from silt- and claystones within the carbonaceous shale and coalbeds were analyzed by both laser ablation and TIMS techniques to provide age control for the stratigraphic framework.

Within the interval containing the PETM, laterally extensive fluvial sandstone bodies occur and orange- to red-mottled paleosols indicate dryer conditions than directly before or after, when carbonaceous shales with immature paleosols are more prevalent. Large fluvial sandstone bodies deposited during the PETM also occur in the nearby Bighorn and Piceance Basins. However, throughout the entire Hanna Formation, a repetitive pattern of similar, amalgamated fluvial sandstones occurs. This pattern could be caused by 1) autocyclic processes internal to the depositional system, like channel avulsion, 2) drainage reorganizations or mountain uplift, 3) orbital forcing as found in other nearby basins, or 4) the climatic hyperthermals of the Eocene. More precise age data is necessary to test these hypotheses.