TOPOGRAPHY OF THE LATE EOCENE EROSION SURFACE IN THE MIDDLE ROCKY MOUNTAINS

The topography on the widespread late Eocene unconformity in the Middle Rocky Mountains was buried by upper Eocene and lower Oligocene volcaniclastic sedimentary rocks. These sediments occur in well-preserved paleovalleys within all the major Laramide uplifts in the Middle Rockies, but are best preserved in the northern Laramie and the eastern Uinta mountains. In both the Laramie and Uinta mountains, major changes in drainage patterns between the late Eocene and the modern streams indicate local tectonic tilting during the late Cenozoic.

In the northern Laramie Mountains, 16 paleovalleys are filled with the upper Eocene (Chadronian) White River Formation that is composed primarily of tuffaceous mudstone and siltstone beds. The paleovalleys occur on both sides of the range, and indicate symmetric drainage basins on both sides of the mountains. The local relief of the Proterozoic crystalline ridges on the sides of these paleovalleys range between 450 to 815 m. Laramie Peak currently rises 1200 m above the nearest paleovalley fill.

The eastern Unita Mountains contain 15 paleovalleys filled by the upper Eocene to lower Oligocene Bishop Conglomerate. The Bishop Conglomerate is a typically coarse grained conglomerate that contains detrital zircons of latest Eocene and early Oligocene age and interbedded volvaniclastic mudstones containing tuffs dated to the early Oligocene. These paleovalleys head along the high crest along the northern part of the range and merged into a broad plain to the south of the crest. The local relief of the headwaters of these paleovalleys is between 300 and 600 m between the paleovalley fill and the surrounding ridges supported by the Proterozoic Uinta Mountain Group. The Bishop Conglomerate extends well into the Green River Basin , and its position indicates at least 1600 m of relief between the basin and the mountain crests in latest Eocene time.