REEVALUATION OF PALEOGENE DEFORMATION ALONG THE WESTERN EDGE OF THE BIGHORN BASIN, WYOMING AND MONTANA

Synkinematic sediments of the Paleocene Fort Union and late Paleocene-Eocene Willwood Formations in the Bighorn Basin of northwestern Wyoming and adjacent Montana have been used to constrain the timing of uplift of several major Laramide structures, including the Beartooth Mountains and Rattlesnake Mountain. Previous workers have shown that the Beartooths in Montana were rising during the deposition of the Fort Union Formation. A problem exists, however, at the state line between Wyoming and Montana; sedimentary rocks north of the line are mapped as Fort Union Formation, while south of the line contiguous strata are mapped as Willwood Formation. This would imply that the Beartooths in Wyoming were active more recently than the Beartooths in Montana. Additionally, Laramide anticlines further to the south have been interpreted to predate the uplift of the Beartooths. This is based on maps that show tilted Cretaceous rocks overlain by nearly horizontal Paleogene sediments with a strong angular unconformity.

Our recent mapping in this area has resolved the state-line problem, and the laterally equivalent strata in Wyoming are in fact part of the Paleocene Fort Union Formation. This now supports uniform uplift of the Beartooths during the deposition of the Fort Union Formation. Further to the south in the area near Kimball Bench in Wyoming, our mapping shows that the contact between the Paleogene rocks is conformable with the underlying Cretaceous Lance Formation. The conformable relationship implies that uplift postdates deposition of lower Eocene Willwood Formation and therefore also postdates the accepted Paleocene uplift of the Beartooth block. In the same area, Lillegraven suggests a large scale west-directed thrust fault along the base of the Willwood Formation, which caused several kilometers of shortening and placed Willwood sediments directly atop the Cretaceous Mesaverde Formation (2009). Although we have seen some evidence of shearing along the Willwood-Cretaceous contact, it is minor.