STRUCTURAL AND DEPOSITIONAL HISTORY OF PALEOGENE SEDIMENTARY ROCKS ALONG THE WESTERN EDGE OF THE BIGHORN BASIN, NORTHWESTERN WYOMING

Synkinematic Paleogene sediments reveal a complex depositional and deformation history along the western edge of the Bighorn Basin in northwestern Wyoming. The Late Paleocene-Eocene Willwood Formation rests conformably on top of tilted Cretaceous rocks along at least a 30 km-long segment of the deformed basin edge. The Cretaceous rocks in contact with the Willwood vary in age along the basin edge: the youngest Cretaceous rocks that are preserved are Meeteetse and these are present at the northern end of our study area (Line Creek) and the southern end (Kimball Bench). In between these areas (Clarks Fork Canyon), the Willwood rests on Mesaverde formation, which is the unit below the Meeteetse. Missing in all of these areas is the upper Cretaceous Lance Formation. This indicates that, before these layers were tilted and before the Willwood was deposited, there was differential erosion of the upper Cretaceous rocks. In the Clarks Fork Canyon area, at least 1,200 meters of Cretaceous sediments were eroded away before deformation began. In the Kimball Bench area to the south, about 1,000 meters of Cretaceous sediments were removed and in the Line Creek area about 600 meters were removed. The resulting topographic depression that formed in the Clarks Fork Canyon area and to a lesser extent in the Kimball Bench area began filling with sediments during the earliest erosion of the nascent Beartooth Mountains to the west. Deposition during early unroofing resulted in predominantly limestone-clast conglomerate being deposited in the basal Willwood of the Clarks Fork Canyon area. After the sediments filled the Clarks Fork Canyon depression, continued uplift of the Beartooths resulted in Willwood deposition in the Kimball Bench area. The basal Willwood here contains mixed-clast conglomerate, indicating that unroofing had progressed to the point where the crystalline core of the Beartooths was exposed. The basal Willwood in the Line Creek area contains crystalline-clast conglomerate, indicating deposition during the later stages of Beartooth unroofing. After deposition of basal Willwood in all three areas, they were tilted conformably with their respective underlying Cretaceous rocks.