LATE EOCENE PALEOVALLEYS OF THE NORTHERN LARAMIE MOUNTAINS AND EASTERN GRANITE MOUNTAINS, WYOMING

Eighteen upper Paleogene paleovalley fills occur in the northern Laramie Mountains, and three paleovalley fills occur in the eastern end of the Granite Mountains of central Wyoming. The valleys were cut into Archean crystalline rocks forming a surface of high relief. The basal rocks in the paleovalley fills are typically conglomerates with huge rounded blocks of Archean granite. Above these conglomerates are tuffaceous mudstone and siltstone beds of the White River Formation. The maximum preserved thickness of a White River paleovalley fill is 245 m. Capping the White River Formation is a series of conglomerate sheets that are thickest within the paleovalley fills, but thin laterally into conglomerate beds that cap pediments cut into the surrounding crystalline rocks.

The White River paleovalley fills have scattered fossil mammal remains and all are of the Chadronian land mammal age of late Eocene age. Radiometric ages of tuffs at Flagstaff Rim on the eastern edge of the Granite Mountains are all of late Eocene age. The age of the basal very coarse grained conglomerate has been suggested as old as middle Eocene, but many of these conglomerates are interbedded with tuffaceous mudrocks of the White River Formation suggesting they are of late Eocene age. These conglomerates represent the basal deposits of the paleovalleys as they were being cut. The upper conglomerate sheets grade laterally into sheet sandstones in the upper White River Formation, widespread blankets of coarse conglomerates in the upper Oligocene Arikaree Group, and conglomerate beds of the Miocene Ogallala Group. The paleovalley fills indicate a topography of high relief at the start of the late Eocene that were filled first by fine tuffaceous sediments of the White River Formation. These paleovalley fills were capped by gravels deposited during a long period of tectonic stability that resulted in pedimentation of the crystalline rocks. Post-Ogallala exhumation of these paleovalley fills is exposing late Eocene topography.