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.
[Illustration: 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.
[Illustration: Table 3. _Formations exposed in Alaska Basin._]
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.
[Illustration: 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 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