Rise and burial of mountains
The enormous section of Tertiary sedimentary rocks in the Jackson Hole area ([table 5]) is one of the most impressive in North America. If the maximum thicknesses of all formations were added, they would total more than 6 miles, but nowhere did this amount of rock accumulate in a single unbroken sequence. No other region in the United States contains a thicker or more complete nonmarine Tertiary record; many areas have little or none. The accumulation in Jackson Hole reflects active uplifts of nearby mountains that supplied abundant rock debris, concurrent sinking of nearby basins in which the sediments could be preserved, and proximity to the great Yellowstone-Absaroka volcanic area, one of the most active continental volcanic fields in the United States. The volume and composition of the Tertiary strata are, therefore, clear evidence of crustal and subcrustal instability.
Figure 46. Teton region near end of deposition of Paleocene rocks, slightly less than 60 million years ago. The ancestral Teton-Gros Ventre uplift formed a partial barrier between the Jackson Hole and Green River depositional basins; major drainages from the Targhee uplift spread an enormous sheet of gravel for 100 miles to the east. See [figure 41] for State lines and location map.
The many thick layers of conglomerate are evidence of rapid erosion of nearby highlands. The Pinyon Conglomerate ([fig. 45]), for example, contains zones as much as 2,500 feet thick of remarkably well-rounded pebbles, cobbles, and boulders, chiefly of quartzite identical with that in the underlying Harebell Formation and derived from the same source, the Targhee uplift. Like the Harebell the matrix contains small amounts of gold and mercury. Rock fragments increase in size northwestward toward the source area ([fig. 46]) and most show percussion scars, evidence of ferocious pounding that occurred during transport by powerful, swift rivers and steep gradients.
Figure 47. Teton region at climax of Laramide Revolution, between 50 and 55 million years ago. See [figure 41] for State lines and location map.
Conglomerates such as the Pinyon are not the only clue to the time of mountain building. Another type of evidence—faults—is demonstrated in [figure 16]. The youngest rocks cut by a fault are always older than the fault. Many faults and the rocks on each side are covered by still younger unbroken sediments. These must, therefore, have been deposited after fault movement ceased. By dating both the faulted and the overlying unbroken sediments, the time of fault movement can be bracketed.
Observations of this type in western Wyoming indicate that the Laramide Revolution reached a climax during earliest Eocene time, 50 to 55 million years ago. Mountain-producing upwarps formed during this episode were commonly bounded on one side by either reverse or thrust faults (fig. [16B] and [16C]) and intervening blocks were downfolded into large, very deep basins. The amount of movement of the mountain blocks over the basins ranged from tens of miles in the Snake River, Salt River, Wyoming, and Hoback Ranges directly south of the Tetons to less than 5 miles on the east margin of Jackson Hole (the west flank of the Washakie Range shown in [figure 1]). The ancestral Teton-Gros Ventre uplift continued to rise but remained one of the less conspicuous mountain ranges in the region ([fig. 47]).
The Buck Mountain fault, the great reverse fault which lies just west of the highest Teton peaks (see [geologic map] and [cross section]), was formed either at this time or during a later episode of movement that also involved the southwest margin of the Gros Ventre Mountains. The Buck Mountain fault is of special importance because it raised a segment of Precambrian rocks several thousand feet. Later, when the entire range as we now know it was uplifted by movement along the Teton fault, the hard basement rocks in this previously upfaulted segment continued to stand much higher than those in adjacent parts of the range. All of the major peaks in the Tetons are carved from this doubly uplifted block.
The brightly colored sandstone, mudstone, and claystone in the Indian Meadows and Wind River Formations (lower Eocene) in the eastern part of Jackson Hole were derived from variegated Triassic, Jurassic, and Lower Cretaceous rocks exposed on the adjacent mountain flanks. Fossils in these Eocene Formations show that it took less than 10 million years for the uplifts to be deeply eroded and partially buried in their own debris.
The Laramide Revolution in the area of Grand Teton National Park ended during Eocene time between 45 and 50 million years ago, and as the mountains and basins became stabilized a new element was added. Volcanoes broke through to the surface in many parts of the Yellowstone-Absaroka area and the constantly increasing volume of their eruptive debris was a major factor in the speed of filling of basins and burial of mountains throughout Wyoming. This entire process only took about 20 million years, and along the east margin of Jackson Hole it was largely completed during Oligocene time ([fig. 48]). However, east and northeast of Jackson Lake a Miocene downwarp subsequently formed and in it accumulated at least 7,000 feet of locally derived sediments of volcanic origin.
Figure 48. Teton region near the close of Oligocene deposition, between 25 and 30 million years ago, showing areas of major volcanoes and lava flows. See [figure 41] for State lines and location map.