Paper No. 14
Presentation Time: 12:15 PM


NESER, Laura, Department of Geological Sciences, University of North Carolina, 104 South Road, Mitchell Hall, Campus Box #3315, Chapel Hill, NC 27599 and STEWART, Kevin G., Department of Geological Sciences, Univ of North Carolina, 122 Mitchell Hall, CB 3315, Chapel Hill, NC 27599-3315,

In northwestern Wyoming, we recently mapped areas along the western edge of the Bighorn Basin with a focus on the Kimball Bench (~17 km2), Heart Mountain (~30 km2), Clarks Fork Canyon (~5 km2) and Line Creek (~18 km2) areas. Pierce (1937, 1938, 1955) last mapped these areas in detail, and Jason Lillegraven (2009) updated a few small areas with a focus on the contact between the upper Cretaceous and lower Paleogene strata.

The purpose of this mapping was to provide constraints on the age and nature of deformation along the western edge of the Bighorn Basin, as well as the nature of geologic contacts among upper Cretaceous and Paleogene layers. We were also able to map upper Cretaceous, Paleogene, and Quaternary formations in greater detail than Pierce. In the Kimball Bench area, our mapping shows a conformable contact between the tilted Late Paleocene-Eocene Willwood Formation and underlying Cretaceous strata. This indicates that deformation here postdates early Eocene deposition of the Willwood. In the Heart Mountain area, our map of Cretaceous and Paleogene strata shows that this region experienced two separate episodes of Laramide deformation during the Paleocene-Eocene. In this map, pronounced angular unconformities at the base of Paleogene sedimentary rocks are two different ages and overlie folded strata with two different orientations. In the Clarks Fork Canyon area, our mapping shows Paleogene and Cretaceous rocks conformably tilted together, indicating post-Paleocene deformation. In the Line Creek area, the Willwood is tilted to nearly vertical where it is in conformable contact with tilted Cretaceous rocks. Many of the folds and faults revealed by our maps were not previously mapped, including intraformational deformation as well as the nature of some unconformable contacts. Generally, these maps imply a deformation age postdating the early Eocene and therefore also postdating the previously accepted Paleocene uplift of the Beartooth Block.

This poster session will include cross sections from each of the mapping areas to illustrate the nature of deformation in this region.