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.
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.
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.
Table 2.—Paleozoic sedimentary rocks exposed in the Teton region.
| Age | Formation | Thickness (feet) | Description | Where exposed |
|---|---|---|---|---|
| Permian | Phosphoria Formation | 150-250 | Dolomite, gray, cherty, sandy, black shale and phosphate beds; marine. | North and west flanks of Teton Range, north flank of Gros Ventre Mountains, southern Jackson Hole. |
| Pennsylvanian | Tensleep and Amsden Formations | 600-1,500 | Tensleep Sandstone, light-gray, hard, underlain by Amsden Formation, a domolite and red shale with a basal red sandstone; marine. | North and west flanks of Teton Range, north flank of Gros Ventre Mountains, southern Jackson Hole. |
| Mississippian | Madison Limestone | 1,000-1,200 | Limestone, blue-gray, hard, fossiliferous; thin red shale in places near top; marine. | North and west flanks of Teton Range, north flank of Gros Ventre Mountains, southern Jackson Hole. |
| Devonian | Darby Formation | 200-500 | Dolomite, dark-gray to brown, fetid, hard, and brown, black, and yellow shale; marine. | North and west flanks of Teton Range, north flank of Gros Ventre Mountains, southern Jackson Hole. |
| Ordovician | Bighorn Dolomite | 300-500 | Dolomite, light-gray, siliceous, very hard; white dense very fine-grained dolomite at top; marine. | North and west flanks of Teton Range, north and west flanks of Gros Ventre Mountains, southern Jackson Hole. |
| Cambrian | Gallatin Limestone | 180-300 | Limestone, blue gray, hard, thin-bedded; marine. | North and west flanks of Teton Range and Gros Ventre Mountains. |
| Gros Ventre Formation | 600-800 | Shale, green, flaky, with Death Canyon Limestone Member composed of about 300 feet of hard cliff-forming limestone in middle; marine. | North and west flanks of Teton Range and Gros Ventre Mountains. | |
| Flathead Sandstone | 175-200 | Sandstone, reddish-brown, very hard, brittle; partly marine. | North and west flanks of Teton Range and Gros Ventre Mountains. |
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 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