RECONSTRUCTING THE TIMING AND STRUCTURAL EVOLUTION OF THE EASTERN BEARTOOTH UPLIFT USING PALEOSEISMITES AND SYNKINEMATIC ALLUVIAL FANS

The eastern Beartooth uplift of south-central Montana and northwest Wyoming is a Laramide basement-cored structure with a steeply dipping monocline of sedimentary cover flanking the edge of the uplift. Synkinematic alluvial fans of the Paleocene Fort Union Fm. have been used by previous workers to reconstruct the progressive uplift of the Beartooth block. In these models, the initial phase of deformation, prior to deposition of the synkinematic fan conglomerates, involved uplift of several thousand meters and rotation of the monocline limb to a dip of about 60°. Our recent field work shows that, contrary to the existing model, the initial phase of deformation involved minor uplift and rotation of the monoclinal limb to only 20°, or less. Near Clarks Fork canyon, along the eastern flank of the uplift, synkinematic Fort Union conglomerates are steep-to-overturned and are in nearly conformable contact with the underlying rocks. The conglomerates could not have been deposited against an already-steep limb. The main phase of uplift must have occurred after deposition of the lower conglomerate beds and likely corresponds to a major episode of thrusting that transported the Beartooth hangingwall to the north and northeast. The basement-rock core of the Beartooths appears to have been uplifted over a buried ramp located near Clarks Fork Canyon. The timing of the Beartooth uplift is recorded by both the presence of synkinematic conglomerates in the Paleocene Fort Union and abundant preserved paleoseismites in both the Fort Union and Eocene Willwood Formation. Conglomeratic Fort Union alluvial fan sediments adjacent to the mountain front contain rare slide blocks of Paleozoic carbonates. More distal, but coeval sandstones of the Tongue River Member of the Fort Union contain dozens of upwardly injected clastic dikes. Further away from the mountain front, fine sand and silt of the Tongue River Member contain dewatering structures and contorted lamina, but no dikes. Uplift of the Beartooths continued into the Eocene as shown by abundant sand lenses with liquefaction structures within the Willwood Formation. These new data show that the main phase of deformation of the Beartooth uplift did not predate the deposition of the Fort Union conglomerate, but instead may have occurred as late as the early Eocene.