Part 5

More than 700 million years elapsed between intrusion of the black dikes and deposition of the first Paleozic sedimentary rocks—a longer period of time than has elapsed since the beginning of the Paleozic Era. During this enormous interval the Precambrian rocks were uplifted, exposed to erosion, and gradually worn to a nearly featureless plain, perhaps somewhat resembling the vast flat areas in which similar Precambrian rocks are now exposed in central and eastern Canada. At the close of Precambrian time, about 600 million years ago, the plain slowly floundered and the site of the future Teton Range disappeared beneath shallow seas that were to wash across it intermittently for the next 500 million years. It is to the sediments deposited in these seas that we turn to read the next chapter in the geologic story of the Teton Range.

ABSOLUTE TIME (Years ago) INCHES

Beginning of the Paleozoic. First abundant fossils → 4 1 billion 8 Maximum age of black dikes → 10 Oldest known fossils 15 2 billion 16 Old granite and pegmatite 20 3 billion 24 Gneisses and schists formed sometime in this interval 20-27 Oldest dated rocks → 28 4 billion → 32 Minimum age of the earth 36

THE PALEOZOIC ERA—TIME OF LONG-VANISHED SEAS AND THE DEVELOPMENT OF LIFE

The Paleozoic sequence

North, west, and south of the highest Teton peaks the soaring spires and knife-edge ridges of Precambrian rock give way to rounded spurs and lower flat-topped summits, whose slopes are palisaded by continuous gray cliffs that resemble the battlements of some ancient and long-abandoned fortress (fig. 31). As mentioned previously, the cliffs are the projecting edges of layers of sedimentary rocks of Paleozoic age that accumulated in or along the margins of shallow seas. At one time the layers formed a thick unbroken, nearly horizontal blanket across the Precambrian basement rocks, but subsequent uplift of the eastern edge of the Teton fault block tilted them westward. They were then stripped from the highest peaks.

The Paleozoic and younger sedimentary rocks in the Teton region are subdivided into _formations_, each of which is named. A formation is composed of rock layers which, because of their similar physical characteristics, can be distinguished from overlying and underlying layers. They must be thick enough to be shown on a geologic map. Table 2 lists the various Paleozoic formations present in and adjacent to Grand Teton National Park and gives their thicknesses and characteristics. These sedimentary rocks are of special interest, for they not only record an important chapter of geologic history but elsewhere in the region they contain petroleum and other mineral deposits.

The Paleozoic rocks can be viewed close at hand from the top of the Teton Village tram (fig. 32) on the south boundary of the park. A less accessible but equally spectacular exposure of Paleozoic rocks is in Alaska Basin, along the west margin of the park, where they are stacked like even layers in a gigantic cake (fig. 33).

Alaska Basin—site of an outstanding rock and fossil record

Strata in Alaska Basin record with unusual clarity the opening chapters in the chronicle of seas that flowed and ebbed across the future site of the Teton Range during most of the Paleozoic Era. In the various rock layers are inscribed stories of the slow advance and retreat of ancient shorelines, of the storm waves breaking on long-vanished beaches, and of the slow and intricate evolution of the myriads of sea creatures that inhabited these restless waters.

Careful study of the fossils allows us to determine the age of each formation (table 3). Even more revealing, the fossils themselves are tangible evidence of the orderly parade of life that crossed the Teton landscape during more than 250 million years. Here is a record of Nature’s experiments with life, the triumphs, failures, the bizarre, the beautiful.

Age Formation Thickness Description Where exposed (feet)

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.

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.

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.