Part 9

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.

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.

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.

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.

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.

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.

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.