Part 6

The Phosphoria Formation and its equivalents of Permian age are unlike any other Paleozoic rocks because of their extraordinary content of uncommon elements. The formation consists of sandy dolomite, widespread black phosphate beds and black shale that is unusually rich not only in phosphorus, but also in vanadium, uranium, chromium, zinc, selenium, molybdenum, cobalt, and silver. The formation is mined extensively in nearby parts of Idaho and in Wyoming for phosphatic fertilizer, for the chemical element phosphorus, and for some of the metals that can be derived from the rocks as byproducts. These elements and compounds are not everywhere concentrated enough to be of economic interest, but their dollar value is, in a regional sense, comparable to that of some of the world’s greatest mineral deposits.

Figure 39. _A glimpse of the sea floor during deposition of the Madison Limestone 330 million years ago, showing the remains of brachiopods, corals, and other forms of life that inhabited the shallow warm water._

THE MESOZOIC—ERA OF TRANSITION

The Mesozoic Era in the Teton region was a time of alternating marine, transitional, and continental environments. Moreover, the highly diversified forms of life, ranging from marine mollusks to tremendous, land-living dinosaurs, confirm and reinforce the story of the rocks. Living things, too, were in transition, for as environment changed, many forms moved from the sea to land in order to survive. It was the time when some of the most spectacularly colored rock strata of the region were deposited.

Colorful first Mesozoic strata

Bright-red soft Triassic rocks more than 1,000 feet thick, known as the Chugwater Formation, comprise most of the basal part of the Mesozoic sequence (table 4). They form colorful hills east and south of the park. The red color is caused by a minor amount of iron oxide. Mud cracks and the presence of fossil reptiles and amphibians indicate deposition in a tidal flat environment, with the sea lying several miles southwest of Jackson Hole. A few beds of white _gypsum_ (calcium sulfate) are present; they were apparently deposited during evaporation of shallow bodies of salt water cut off from the open sea.

As the Triassic Period gave way to the Jurassic, salmon-red windblown sand (Nugget Sandstone) spread across the older red beds and in turn was buried by thin red shale and thick gypsum deposits of the Gypsum Spring Formation. Then down from Alaska and spreading across most of Wyoming came the _Sundance Sea_, a warm, muddy, shallow body of water that teemed with marine mollusks. In it more than 500 feet of highly fossiliferous soft gray shale and thin limestones and sandstones were deposited. The sea withdrew and the Morrison and Cloverly Formations (Jurassic and Lower Cretaceous) were deposited on low-lying tropical humid flood plains. These rocks are colorful, consisting of red, pink, purple, and green badland-forming claystones and mudstones, and yellow to buff sandstones. Vegetation was abundant and large and small dinosaurs roamed the countryside or inhabited the swamps.

Age Formation Thickness Description Where exposed (feet)

CRETACEOUS Harebell 0-5,000 Sandstone, olive Eastern and Formation drab, silty, drab northeastern parts siltstone, and of Jackson Hole. dark-gray shale; thick beds of quartzite pebble conglomerate in upper part. Meeteetse 0-700 Sandstone, gray to Spread Creek area. Formation chalky white, blue-green to gray siltstone, thin coal, and green to yellow bentonite. Mesaverde 0-1,000 Sandstone, white, Eastern Jackson Formation massive, soft, thin Hole. gray shale, sparse coal. Unnamed 3,500± Sandstone and Eastern Jackson sequence of shale, gray to Hole and eastern lenticular brown; abundant margin of the park. sandstone, coal in lower 2,000 shale, and feet. coal. Bacon Ridge 900-1,200 Sandstone, light Eastern Jackson Sandstone gray, massive, Hole and eastern marine, gray shale, margin of the park. many coal beds. Cody Shale 1,300-2,200 Shale, gray, soft; Eastern and thin green northern parts of sandstone, some Jackson Hole. bentonite; marine. Frontier 1,000 Sandstone, gray, Eastern and Formation and black to gray northern parts and shale, marine; many south-western persistent white margin of Jackson bentonite beds in Hole. lower part. Mowry Shale 700 Shale, Gros Ventre River silvery-gray, hard, Valley, northern siliceous, with margin of the park, many fish scales; and southern part thin bentonite of Jackson Hole. beds; marine. Thermopolis 150-200 Shale, black, soft, Gros Ventre River Shale fissile, with Valley, northern persistent margin of the park, sandstone at top; and southern part marine. of Jackson Hole. Cloverly and 650 Sandstone, light North end of Teton Morrison(?) gray, sparkly, Range and Gros Formations rusty near top, Ventre River Valley. underlain by variegated soft claystone; basal part is silty dully-variegated sandstone and claystone. JURASSIC Sundance 500-700 Sandstone, green, North end of Teton Formation underlain by soft Range, Blacktail gray shale and thin Butte, Gros Ventre highly River Valley. fossiliferous limestones; marine. Gypsum Spring 75-100 Gypsum, white, North end of Teton Formation interbedded with Range, Blacktail red shale and gray Butte, Gros Ventre dolomite; partly River Valley. marine. Nugget 0-350 Sandstone, North flank of Gros Sandstone salmon-red, hard. Ventre Mountains, southern Jackson Hole. TRIASSIC Chugwater 1,000-1,500 Siltstone and North flank of Gros Formation shale, red, Ventre Mountains, thin-bedded; one north end of Teton thin marine Range, southernmost limestone in upper Jackson Hole. third. Dinwoody 200-400 Siltstone, brown, North flank of Gros Formation hard, thin-bedded; Ventre Mountains, marine. north end of Teton Range, southernmost Jackson Hole.

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