Part 5

Permian Phosphoria 150-250 Dolomite, gray, North and west Formation cherty, sandy, flanks of Teton black shale and Range, north phosphate beds; flank of Gros marine. Ventre Mountains, southern Jackson Hole. Pennsylvanian Tensleep 600-1,500 Tensleep Sandstone, North and west and Amsden light-gray, hard, flanks of Teton Formations underlain by Range, north Amsden Formation, flank of Gros a domolite and red Ventre Mountains, shale with a basal southern Jackson red sandstone; Hole. marine. Mississippian Madison 1,000-1,200 Limestone, North and west Limestone blue-gray, hard, flanks of Teton fossiliferous; Range, north thin red shale in flank of Gros places near top; Ventre Mountains, marine. southern Jackson Hole. Devonian Darby 200-500 Dolomite, dark-gray North and west Formation to brown, fetid, flanks of Teton hard, and brown, Range, north black, and yellow flank of Gros shale; marine. Ventre Mountains, southern Jackson Hole. Ordovician Bighorn 300-500 Dolomite, North and west Dolomite light-gray, flanks of Teton siliceous, very Range, north and hard; white dense west flanks of very fine-grained Gros Ventre dolomite at top; Mountains, marine. southern Jackson Hole. Cambrian Gallatin 180-300 Limestone, blue North and west Limestone gray, hard, flanks of Teton thin-bedded; Range and Gros marine. Ventre Mountains. Gros Ventre 600-800 Shale, green, North and west Formation flaky, with Death flanks of Teton Canyon Limestone Range and Gros Member composed of Ventre Mountains. about 300 feet of hard cliff-forming limestone in middle; marine. Flathead 175-200 Sandstone, North and west Sandstone reddish-brown, flanks of Teton very hard, Range and Gros brittle; partly Ventre Mountains. marine.

The regularity and parallel relations of the layers in well-exposed sections such as the one in Alaska Basin suggest that all these rocks were deposited in a single uninterrupted sequence. However, the fossils and regional distribution of the rock units show that this is not really the case. The incomplete nature of this record becomes apparent if we plot the ages of the various formations on the absolute geologic time scale (fig. 34). The length of time from the beginning of the Cambrian Period to the end of the Mississippian Period is about 285 million years. The strata in Alaska Basin are a record of approximately 120 million years. More than half of the pages in the geologic story are missing even though, compared with most other areas, the book as a whole is remarkably complete! During these unrecorded intervals of time either no sediments were deposited in the area of the Teton Range or, if deposited, they were removed by erosion.

[Illustration: Figure 32. _Paleozoic marine sedimentary rocks near south boundary of Grand Teton National Park. View is south from top of Teton Village tram. National Park Service photo by W. E. Dilley and R. A. Mebane._]

Madison Limestone Darby Formation Bighorn Dolomite Gallatin Limestone

Advance and retreat of Cambrian seas: an example

The first invasion and retreat of the Paleozoic sea are sketched on figure 35. Early in Cambrian time a shallow seaway, called the _Cordilleran trough_, extended from southern California northeastward across Nevada into Utah and Idaho (fig. 35A). The vast gently rolling plain on Precambrian rocks to the east was drained by sluggish westward-flowing rivers that carried sand and mud into the sea. Slow subsidence of the land caused the sea to spread gradually eastward. Sand accumulated along the beaches just as it does today. As the sea moved still farther east, mud was deposited on the now-submerged beach sand. In the Teton area, the oldest sand deposit is called the Flathead Sandstone (fig. 36).

The mud laid down on top of the Flathead Sandstone as the shoreline advanced eastward across the Teton area is now called the Wolsey Shale Member of the Gros Ventre Formation. Some shale shows patterns of cracks that formed when the accumulating mud was briefly exposed to the air along tidal flats. Small phosphatic-shelled animals called _brachiopods_ inhabited these lonely tidal flats (fig. 37A and 37B) but as far as is known, nothing lived on land. Many shale beds are marked with faint trails and borings of wormlike creatures, and a few contain the remains of tiny very intricately developed creatures with head, eyes, segmented body, and tail. These are known as trilobites (fig. 37C and 37D). Descendants of these lived in various seas that crossed the site of the dormant Teton Range for the next 250 million years.

[Illustration: Figure 33. _View southwest across Alaska Basin, showing tilted layers of Paleozoic sedimentary rocks on the west flank of the Teton Range. National Park Service photo._]

Mount Meek Madison Limestone Bighorn Dolomite Death Canyon Limestone Member Flathead Sandstone Precambrian Rock

As the shoreline moved eastward, the Death Canyon Limestone Member of the Gros Ventre Formation (fig. 33) was deposited in clear water farther from shore. Following this the sea retreated to the west for a short time. In the shallow muddy water resulting from this retreat the Park Shale Member of the Gros Ventre Formation was deposited. In places underwater “meadows” of algae flourished on the sea bottom and built extensive reefs (fig. 38A). From time to time shoal areas were hit by violent storm waves that tore loose platy fragments of recently solidified limestone and swept them into nearby channels where they were buried and cemented into thin beds of jumbled fragments (fig. 38B) called _“edgewise” conglomerate_. These are widespread in the shale and in overlying and underlying limestones.

[Illustration: Table 3. _Formations exposed in Alaska Basin._]

AGE (Numbers FORMATION (Thickness) ROCKS AND FOSSILS show age in millions of years)

(310) MISSISSIPPIAN MADISON LIMESTONE Uniform thin beds of (Total about 1,100 blue-gray limestone and feet, but only lower sparse very thin layers of 300 feet preserved in shale. Brachiopods, corals, this section) and other fossils abundant. (345) LATE AND DARBY FORMATION (About Thin beds of gray and buff MIDDLE DEVONIAN 350 feet) dolomite interbedded with layers of gray, yellow, and black shale. A few fossil brachiopods, corals, and bryozoans. (390) (425) LATE AND BIGHORN DOLOMITE (About Thick to very thin beds of MIDDLE 450 feet; Leigh blue-gray or brown dolomite, ORDOVICIAN Dolomite Member about white on weathered surfaces. 40 feet thick at top) A few broken fossil brachiopods, bryozoans, and horn corals. Thin beds of white fine-grained dolomite at top are the Leigh Member. (440) (500) LATE CAMBRIAN GALLATIN LIMESTONE (180 Blue-gray limestone mottled feet) with irregular rusty or yellow patches. Trilobites and brachiopods. (530) MIDDLE CAMBRIAN GROS VENTRE FORMATION PARK SHALE MEMBER Gray-green shale containing (220 feet) beds of platy limestone conglomerate. Trilobites, brachiopods, and fossil algal heads. DEATH CANYON Two thick beds of LIMESTONE MEMBER dark-blue-gray limestone (285 feet) separated by 15 to 20 feet of shale that locally contains abundant fossil brachiopods and trilobites. WOLSEY SHALE MEMBER Soft greenish-gray shale (100 feet) containing beds of purple and green sandstone near base. A few fossil brachiopods. FLATHEAD LIMESTONE (175 Brown, maroon, and white feet) sandstone, locally containing many rounded pebbles of quartz and feldspar. Some beds of green shale at top. (570) PRECAMBRIAN Granite, gneiss, and pegmatite.

[Illustration: Figure 34. _Absolute ages of the formations in Alaska Basin. Shaded parts of the scale show intervals for which there is no record._]

STRATIGRAPHIC SCALE ABSOLUTE ENLARGED PIECE OF TIME (Years YARDSTICK SHOWN ON ago) FIGURE 19

2 PALEZOIC PENNSYLVANIAN ? 300 million MISSISSIPPIAN MADISON DEVONIAN DARBY 3 400 million SILURIAN ORDOVICIAN BIGHORN 500 million 4 CAMBRIAN GALLATIN GROS VENTRE FLATHEAD 600 million PRECAMBRIAN 5

Figure 35. _The first invasions of the Paleozoic sea._

[Illustration: A. _In Early Cambrian time an arm of the Pacific Ocean occupied a deep trough in Idaho, Nevada, and part of Utah. The land to the east was a broad gently rolling plain of Precambrian rocks drained by sluggish westward-flowing streams. The site of the Teton Range was part of this plain. Slow subsidence of the land caused the sea to move eastward during Middle Cambrian time flooding the Precambrian plain._]

[Illustration: B. _By Late Cambrian time the sea had drowned all of Montana and most of Wyoming. The Flathead Sandstone and Gros Ventre Formation were deposited as the sea advanced. The Gallatin Limestone was being deposited when the shoreline was in about the position shown in this drawing._]

[Illustration: C. _In Early Ordovician time uplift of the land caused the sea to retreat back into the trough, exposing the Cambrian deposits to erosion. Cambrian deposits were partly stripped off of some areas. The Bighorn Dolomite was deposited during the next advance of the sea in Middle and Late Ordovician time._]

[Illustration: Figure 36. _Conglomeratic basal bed of Flathead Sandstone and underlying Precambrian granite gneiss; contact is indicated by a dark horizontal line about 1 foot below hammer. This contact is all that is left to mark a 2-billion year gap in the rock record of earth history. The locality is on the crest of the Teton Range 1 mile northwest of Lake Solitude._]

Once again the shoreline crept eastward, the seas cleared, and the Gallatin Limestone was deposited. The Gallatin, like the Death Canyon Limestone Member, was laid down for the most part in quiet, clear water, probably at depths of 100 to 200 feet. However, a few beds of “edgewise” conglomerate indicate the occurrence of sporadic storms. At this time, the sea covered all of Idaho and Montana and most of Wyoming (fig. 35B) and extended eastward across the Dakotas to connect with shallow seas that covered the eastern United States. Soon after this maximum stage was reached slow uplift caused the sea to retreat gradually westward. The site of the Teton Range emerged above the waves, where, as far as is now known, it may have been exposed to erosion for nearly 70 million years (fig. 35C).

The above historical summary of geologic events in Cambrian time is recorded in the Cambrian formations. This is an example of the reconstructions, based on the sedimentary rock record, that have been made of the Paleozoic systems in this area.

Figure 37. _Cambrian fossils in Grand Teton National Park._

A-B. _Phosphatic-shelled brachiopods, the oldest fossils found in the park. Actual width of specimens is about ¼ inch._

C-D. _Trilobites. Width of C is ¼ inch, D is ½ inch. National Park Service photos by W. E. Dilley and R. A. Mebane._

[Illustration: A.]

[Illustration: B.]

[Illustration: C.]

[Illustration: D.]

Younger Paleozoic formations

Formations of the remaining Paleozoic systems are likewise of interest because of the ways in which they differ from those already described.

Figure 38. _Distinctive features of Cambrian rocks._

[Illustration: A. _Algal heads in the Park Shale Member of the Gros Ventre Formation. These calcareous mounds were built by algae growing in a shallow sea in Cambrian time. They are now exposed on the divide between North and South Leigh Creeks, nearly 2 miles above sea level!_]

[Illustration: B. _Bed of “edgewise” conglomerate in the Gallatin Limestone. Angular plates of solidified lime-ooze were torn from the sea bottom by storm waves, swept into depressions, and then buried in lime mud. These fragments, seen in cross section, make the strange design on the rock. Thin limestone beds below are undisturbed. National Park Service photo by W. E. Dilley._]

The Bighorn Dolomite of Ordovician age forms ragged hard massive light-gray to white cliffs 100 to 200 feet high (figs. 32 and 33). _Dolomite_ is a calcium-magnesium carbonate, but the original sediment probably was a calcium carbonate mud that was altered by magnesium-rich sea water shortly after deposition. Corals and other marine animals were abundant in the clear warm seas at this time.

Dolomite in the Darby Formation of Devonian age differs greatly from the Bighorn Dolomite; that in the Darby is dark-brown to almost black, has an oily smell, and contains layers of black, pink, and yellow mudstone and thin sandstone. The sea bottom during deposition of these rocks was foul and frequently the water was turbid. Abundant fossil fragments indicate fishes were common for the first time. Exposures of the Darby Formation are recognizable by their distinctive dull-yellow thin-layered slopes between the prominent gray massive cliffs of formations below and above.

The Madison Limestone of Mississippian age is 1,000 feet thick and is exposed in spectacular vertical cliffs along canyons in the north, west, and south parts of the Tetons. It is noted for the abundant remains of beautifully preserved marine organisms (fig. 39). The fossils and the relatively pure blue-gray limestone in which they are embedded indicate deposition in warm tranquil seas. The beautiful Ice Cave on the west side of the Tetons and all other major caves in the region were dissolved out of this rock by underground water.

The Pennsylvanian System is represented by the Amsden Formation and the Tensleep Sandstone. Cliffs of the Tensleep Sandstone can be seen along the Gros Ventre River at the east edge of the park. The Amsden, below the Tensleep, consists of red and green shale, sandstone, and thin limestone. The shale is especially weak and slippery when exposed to weathering and saturated with water. These are the strata that make up the glide plane of the Lower Gros Ventre Slide (fig. 5) east of the park.

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._

[Illustration: A. _Slab in which fossils are somewhat broken and scattered. Scale slightly reduced. National Park Service photo by W. E. Dilley and R. A. Mebane._]

[Illustration: B. _Slab in which fossils are remarkably complete. Silver dollar gives scale. Specimen is in University of Wyoming Geological Museum._]

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.

[Illustration: Table 4.—Mesozoic sedimentary rocks exposed in the Teton region.]

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.

[Illustration: Figure 40. _The yardstick and the sea. The shaded part of the yardstick shows the 500-million-year interval during which Paleozoic and Mesozoic seas swept intermittently across the future site of the Tetons. When they finally withdrew about 85 million years ago, a little more than 5/8 of an inch of the yardstick remained to be accounted for._]

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.

[Illustration: Figure 41. _Grand Teton National Park region slightly more than 80 million years ago, just before onset of Laramide Revolution. The last Cretaceous sea still lingered in central Wyoming._]

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.