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.
[Illustration: 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.
[Illustration: 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.
[Illustration: 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._]