Middle Miocene (ca. 10.5-16.0 Ma) deposits in the Greater Yellowstone region are confined to the newly defined Hepburn's Mesa Formation in the Yellowstone Valley, Park County, Montana, and the Colter Formation in Jackson Hole, Teton County, Wyoming. The Hepburn's Mesa Formation was deposited in and adjacent to a perennial saline lake, as shown by the mineral assemblage (clinoptilotite, smectite, calcite, halite, and gypsum) and sedimentary features (fine grain size, laminations, mud cracks, and mud flakes in pebble-sandstone lenses). Biostratigraphic dating indicates that the saline lake formed prior to 16.0 Ma and persisted until after 14.8 Ma. The presence of a saline lake and a fossil mammal fauna dominated by geomyoid rodents indicates an arid or semi-arid climate. Some of the abundant tuffaceous components of the Hepburn's Mesa Formation may have blown from coeval volcanic vents represented by the Colter Formation in Jackson Hole.
The Hepburn's Mesa and Colter Formations suggest that both the northern and southern ends of the Greater Yellowstone region were reacting similarly to tectonism during the middle Miocene. (1) Erosion dominated between ca. 18 to 17 Ma. (2) Subsidence of the basins accompanied by extensional block faulting took place from at least 16.0 to 14.8 Ma. (3) After 14.8 Ma but before 8.6 Ma, uplift occurred in northern Yellowstone Park; strata in the Yellowstone Valley and Jackson Hole tilted and eroded, then subsided faster than they had prior to 14.8 Ma. Inasmuch as the chronology of extensional deformation and volcanism in the Greater Yellowstone region is so similar to that in the Northern Rocky Mountains, the Columbia Plateau, and the central part of the Basin and Range, all of these areas appear to be related by a common tectonic process.