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.
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.
Table 3. Formations exposed in Alaska Basin.
| AGE (Numbers show age in millions of years) | FORMATION (Thickness) | ROCKS AND FOSSILS | |
|---|---|---|---|
| (310) | |||
| MISSISSIPPIAN | MADISON LIMESTONE (Total about 1,100 feet, but only lower 300 feet preserved in this section) | Uniform thin beds of blue-gray limestone and sparse very thin layers of shale. Brachiopods, corals, and other fossils abundant. | |
| (345) | |||
| LATE AND MIDDLE DEVONIAN | DARBY FORMATION (About 350 feet) | Thin beds of gray and buff dolomite interbedded with layers of gray, yellow, and black shale. A few fossil brachiopods, corals, and bryozoans. | |
| (390) | |||
| (425) | |||
| LATE AND MIDDLE ORDOVICIAN | BIGHORN DOLOMITE (About 450 feet; Leigh Dolomite Member about 40 feet thick at top) | Thick to very thin beds of blue-gray or brown dolomite, white on weathered surfaces. 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 feet) | Blue-gray limestone mottled with irregular rusty or yellow patches. Trilobites and brachiopods. | |
| (530) | |||
| MIDDLE CAMBRIAN | GROS VENTRE FORMATION | ||
| PARK SHALE MEMBER (220 feet) | Gray-green shale containing beds of platy limestone conglomerate. Trilobites, brachiopods, and fossil algal heads. | ||
| DEATH CANYON LIMESTONE MEMBER (285 feet) | Two thick beds of dark-blue-gray limestone separated by 15 to 20 feet of shale that locally contains abundant fossil brachiopods and trilobites. | ||
| WOLSEY SHALE MEMBER (100 feet) | Soft greenish-gray shale containing beds of purple and green sandstone near base. A few fossil brachiopods. | ||
| FLATHEAD LIMESTONE (175 feet) | Brown, maroon, and white sandstone, locally containing many rounded pebbles of quartz and feldspar. Some beds of green shale at top. | ||
| (570) | |||
| PRECAMBRIAN | Granite, gneiss, and pegmatite. | ||
