Part 10

Drab Cretaceous strata

The youngest division of the Mesozoic Era is the Cretaceous Period. Near the beginning of this period, brightly colored rocks continued to be deposited. Then, the Teton region, as well as most of Wyoming, was partly, and at times completely, submerged by shallow muddy seas. As a result, the brightly variegated strata were covered by 10,000 feet of generally drab-colored sand, silt, and clay containing some coal beds, volcanic ash layers, and minor amounts of gravel.

The Cretaceous sea finally retreated eastward from the Teton region about 85 million years ago, following the deposition of the Bacon Ridge Sandstone (fig. 40). As it withdrew, extensive coal swamps developed along the sea coast. The record of these swamps is preserved in coal beds 5 to 10 feet thick in the Upper Cretaceous deposits. The coal beds are now visible in abandoned mines along the east margin of the park. Coal is formed from compacted plant debris; about 5 feet of this material is needed to form 1 inch of coal. Thus, lush vegetation must have flourished for long periods of time, probably in a hot wet climate similar to that now prevailing in the Florida Everglades.

Sporadically throughout Cretaceous time fine-grained ash was blown out of volcanoes to the west and northwest and deposited in quiet shallow water. Subsequently the ash was altered to a type of clay called bentonite that is used in the foundry industry and in oil well drilling muds. In Jackson Hole, the elk and deer lick bentonite exposures to get a bitter salt and, where the beds are water-saturated, enjoy “stomping” on them. Bentonite swells when wet and causes many landslides along access roads into Jackson Hole (fig. 17).

The Cretaceous rocks in the Teton region are part of an enormous east-thinning wedge that here is nearly 2 miles thick. Most of the debris was derived from slowly rising mountains to the west.

Cretaceous sedimentary rocks are much more than of just scientific interest; they contain mineral deposits important to the economy of Wyoming and of the nation. Wyoming leads the States in production of bentonite, all of it from Cretaceous rocks. These strata have yielded far more oil and gas than any other geologic system in the State and the production is geographically widespread. They also contain enormous coal reserves, some in beds between 50 and 100 feet thick. The energy resources alone of the Cretaceous System in Wyoming make it invaluable to our industrialized society.

ABSOLUTE TIME (Millions of years ago) INCHES

{submerged} 85-585 ⅝-4⅝ CENOZOIC 0-80 0-½ MESOZOIC 80-180 ½-⅞ PALEOZOIC 180-570 ⅞-4⅞ PRECAMBRIAN 570- 4⅞-

As the end of the Cretaceous Period approached, slightly more than 80 million years ago, the flat monotonous landscape (fig. 41) which had prevailed during most of Late Cretaceous time gave little hint that the stage was set for one of the most exciting and important chapters in the geologic history of North America.

Birth of the Rocky Mountains

The episode of mountain building that resulted in formation of the ancestral Rocky Mountains has long been known as the Laramide Revolution. West and southwest of Wyoming, mountains had already formed, the older ones as far away as Nevada and as far back in time as Jurassic, the younger ones rising progressively farther east, like giant waves moving toward a coast. The first crustal movement in the Teton area began in latest Cretaceous time when a broad low northwest-trending arch developed in the approximate area of the present Teton Range and Gros Ventre Mountains. However, this uplift bore no resemblance to the Tetons as we know them today for the present range formed 70 million years later.

One bit of evidence (there are others) of the first Laramide mountain building west of the Tetons is a tremendous deposit of quartzite boulder debris (several hundred cubic miles in volume) derived from the Targhee uplift (fig. 42). Nowhere is the uplift now exposed, but from the size, composition, and distribution of rock fragments that came from it, we know that it was north and west of the northern end of the present-day Teton Range. Powerful streams carried boulders, sand, and clay eastward and southeastward across the future site of Jackson Hole and deposited them in the Harebell Formation (table 4). Mingled with this sediment were tiny flakes of gold and a small amount of mercury. Fine-grained debris was carried still farther east and southeast into two enormous depositional troughs in central and southern Wyoming. Most of the large rock fragments were derived from Precambrian and possibly lower Paleozoic quartzites. This means that at least 15,000 feet of overlying Paleozoic and Mesozoic strata must first have been stripped away from the Targhee uplift before the quartzites were exposed to erosion.

Remains of four-legged horned ceratopsian dinosaurs, possibly Triceratops (fig. 43), reflecting the last population explosion of these reptiles, have been found in pebbly sandstone of the Harebell Formation in highway cuts on the Togwotee Pass road 8 miles east of the park.

Near the end of Cretaceous time, broad gentle uplifts also began to stir at the sites of future mountain ranges in many parts of Wyoming. The ancestral Teton-Gros Ventre arch continued to grow. Associated with and parallel to it was a series of sharp steepsided elongated northwest-trending upfolds (anticlines). One of these can be seen where it crosses the highway at the Lava Creek Campground near the eastern margin of Grand Teton National Park.

During these episodes of mountain building, erosion, and deposition, the dinosaurs became extinct all over the world. The “Age of Mammals” was about to begin.

TERTIARY—TIME OF MAMMALS, MOUNTAINS, LAKES, AND VOLCANOES

STRATIGRAPHIC SCALE THE LAST INCH OF ABSOLUTE TIME (million THE YARDSTICK years ago)

CENOZOIC QUATERNARY Recent and 0 0 Pleistocene TERTIARY Pliocene 0 0 Miocene ⅛ 12 Oligocene ¼ 25 Eocene ⅜ 40 Paleocene ⁷/₁₆ 55 MESOZOIC CRETACEOUS ½ 65

The Cenozoic (table 1), last and shortest of the geologic eras, comprises the Tertiary and Quaternary Periods. It began about 65 million years ago and is represented by only the final one-half inch of our imaginary yardstick of time (fig. 19). Nevertheless, it is the era during which the Tetons rose in their present form and the landscape was sculptured into the panorama of beauty that we now see. In order to show the many Tertiary and Quaternary events in the Teton region, it is necessary to enlarge greatly the last part of the yardstick (fig. 44). There are two reasons for the extraordinarily clear and complete record. First, the Teton region was a relatively active part of the earth’s crust, characterized by many downdropped blocks. The number of events is great and their records are preserved in sediments trapped in the subsiding basins. Second, the geologically recent past is much easier to see than the far dimmer, distant past; the rocks that record later events are fresher, less altered, more complete, and more easily interpreted than are those that tell us of older events.

Age Formation Thickness Description Where exposed (feet)

QUATERNARY Recent Modern 0-200± Sand, gravel, and Floor of Jackson stream, silt along present Hole and in canyons landslide, streams; jumbled and on glacial and broken rock in mountainsides talus deposits landslides and on throughout the talus slopes; region. debris around existing glaciers. Pleistocene Glacial 0-200± Gravel, sand, silt, Floor of Jackson deposits and and glacial debris. Hole. loess Unnamed upper 0-500 Shale, brown-gray, Gros Ventre River lake sequence sandstone, and Valley. conglomerate. Unnamed lower 0-200 Shale, siltstone, National Elk Refuge. lake sequence and sandstone, gray, green, and red. ? Pleistocene or Pliocene Bivouac 0-1,000 Conglomerate, with Signal Mountain and Formation purplish-gray West Gros Ventre welded tuff in Butte. upper part. TERTIARY Pliocene Teewinot 0-6,000 Limestone, tuff, National Elk Formation and claystone, Refuge, Blacktail white, soft. Butte, and eastern margin of Antelope Flat. Camp Davis 0-5,500 Conglomerate, red Southernmost tip of Formation and gray, with Jackson Hole. white tuff, diatomite, and red and white claystone. Miocene Colter 0-7,000 Volcanic Pilgrim and Ditch Formation conglomerate, tuff, Creeks, and north and sandstone, end of Teton Range. white to green-brown, with locally-derived basalt and andesite rock fragments. Oligocene Wiggins 0-3,000 Volcanic Eastern margin of Formation conglomerate, gray Jackson Hole. to brown, with white tuff layers. Eocene Unnamed upper 0-1,000 Tuff, conglomerate, Eastern margin of and middle sandstone, and Jackson Hole. Eocene claystone, green, sequence underlain by variegated claystone and quartzite pebble conglomerate. Wind River 2,000-3,000 Claystone and Eastern margin of and Indian sandstone, Jackson Hole. Meadows variegated, and Formations locally-derived conglomerate; persistent coal and gray shale zone in middle. Paleocene Unnamed 1,000-2,000 Sandstone and Eastern margin of greenish-gray claystone, Jackson Hole. and brown greenish-gray and sandstone and brown, claystone intertonguing at sequence base with quartzite pebble conglomerate. Pinyon 500-5,000 Conglomerate, Eastern part of Conglomerate brown, chiefly of Jackson Hole, Mt. rounded quartzite; Leidy and Pinyon coal and claystone Peak Highlands, and locally at base. north end of Teton Range.

During the early part of the Tertiary Period, mountain building and basin subsidence were the dominant types of crustal movement. Seas retreated southward down the Mississippi Valley and never again invaded the Teton area. Environments on the recently uplifted land were diverse and favorable for the development of new forms of plants and animals.

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.

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.

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.