GRAND TETON Creation of the Teton Landscape: The Geologic Story of Grand Teton National Park

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.

Figure 31. Paleozoic rocks on the west flank of the Teton Range, air oblique view west. Ragged peaks in the foreground (Buck Mountain on the left center, Mt. Wister, with top outlined by snow patch on the extreme right), are carved in Precambrian rocks. Banded cliffs in the background are sedimentary rocks. Alaska Basin is at upper right. Teton Basin, a broad, extensively farmed valley in eastern Idaho, is at top. Photo by A. S. Post, University of Washington, 1963.

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).

Figure 32. Paleozoic marine sedimentary rocks near south boundary of Grand Teton National Park. View is sooth from top of Teton Village tram. National Park Service photo by W. E. Dilley and R. A. Mebane.

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.

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.

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.

Figure 34. Absolute ages of the formations in Alaska Basin. Shaded parts of the scale show intervals for which there is no record.

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).

Figure 35A. The first invasions of the Paleozoic sea. 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. Figure 35B. 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. Figure 35C. In Early Ordivician 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. (click on image for an enlargement in a new window)

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.

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 B) 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 D). Descendants of these lived in various seas that crossed the site of the dormant Teton Range for the next 250 million years.

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 1/4 inch. C-D. Trilobites. Width of C is 1/4 inch, D is 1/2 inch. National Park Service photos by W. E. Dilley and R. A. Mebane.

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 wide spread in the shale and in overlying and underlying limestones.

Figure 38A. Distinctive features of Cambrian rocks. Algal heads in the Park Shale Member of the Gros Ventre Formation. These calcareous mounds were built by algae growing in shallow sea in Cambrian time. They are now exposed on the divide between North and South Leigh Creeks, nearly 2 miles above sea level!

Figure 38B. Distinctive features of Cambrian rocks. 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.

Table 3. Formations exposed in Alaska Basin.

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.

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.

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.

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. 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. B. Slab in which fossils are remarkably complete. Silver dollar gives scale. Specimen is in University of Wyoming Geological Museum.

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, comparaible to that of some of the world's greatest mineral deposits.