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.

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.

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

STRATIGRAPHIC SCALE			ABSOLUTE TIME (Years ago)	ENLARGED PIECE OF YARDSTICK SHOWN ON [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.

A. 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.

B. 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.

C. In Early Ordovician 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.

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.

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.