Bright-red soft Triassic rocks more than 1,000 feet thick, known as the
Chugwater Formation, comprise most of the basal part of the Mesozoic
sequence (table 4). They form colorful hills east and south of the park.
The red color is caused by a minor amount of iron oxide. Mud cracks and
the presence of fossil reptiles and amphibians indicate deposition in a
tidal flat environment, with the sea lying several miles southwest of
Jackson Hole. A few beds of white _gypsum_ (calcium sulfate) are
present; they were apparently deposited during evaporation of shallow
bodies of salt water cut off from the open sea.
As the Triassic Period gave way to the Jurassic, salmon-red windblown
sand (Nugget Sandstone) spread across the older red beds and in turn was
buried by thin red shale and thick gypsum deposits of the Gypsum Spring
Formation. Then down from Alaska and spreading across most of Wyoming
came the _Sundance Sea_, a warm, muddy, shallow body of water that
teemed with marine mollusks. In it more than 500 feet of highly
fossiliferous soft gray shale and thin limestones and sandstones were
deposited. The sea withdrew and the Morrison and Cloverly Formations
(Jurassic and Lower Cretaceous) were deposited on low-lying tropical
humid flood plains. These rocks are colorful, consisting of red, pink,
purple, and green badland-forming claystones and mudstones, and yellow
to buff sandstones. Vegetation was abundant and large and small
dinosaurs roamed the countryside or inhabited the swamps.
[Illustration: Table 4.—Mesozoic sedimentary rocks exposed in the
Teton region.]
Age Formation Thickness Description Where exposed
(feet)
CRETACEOUS
Harebell 0-5,000 Sandstone, olive Eastern and
Formation drab, silty, drab northeastern parts
siltstone, and of Jackson Hole.
dark-gray shale;
thick beds of
quartzite pebble
conglomerate in
upper part.
Meeteetse 0-700 Sandstone, gray to Spread Creek area.
Formation chalky white,
blue-green to gray
siltstone, thin
coal, and green to
yellow bentonite.
Mesaverde 0-1,000 Sandstone, white, Eastern Jackson
Formation massive, soft, thin Hole.
gray shale, sparse
coal.
Unnamed 3,500± Sandstone and Eastern Jackson
sequence of shale, gray to Hole and eastern
lenticular brown; abundant margin of the park.
sandstone, coal in lower 2,000
shale, and feet.
coal.
Bacon Ridge 900-1,200 Sandstone, light Eastern Jackson
Sandstone gray, massive, Hole and eastern
marine, gray shale, margin of the park.
many coal beds.
Cody Shale 1,300-2,200 Shale, gray, soft; Eastern and
thin green northern parts of
sandstone, some Jackson Hole.
bentonite; marine.
Frontier 1,000 Sandstone, gray, Eastern and
Formation and black to gray northern parts and
shale, marine; many south-western
persistent white margin of Jackson
bentonite beds in Hole.
lower part.
Mowry Shale 700 Shale, Gros Ventre River
silvery-gray, hard, Valley, northern
siliceous, with margin of the park,
many fish scales; and southern part
thin bentonite of Jackson Hole.
beds; marine.
Thermopolis 150-200 Shale, black, soft, Gros Ventre River
Shale fissile, with Valley, northern
persistent margin of the park,
sandstone at top; and southern part
marine. of Jackson Hole.
Cloverly and 650 Sandstone, light North end of Teton
Morrison(?) gray, sparkly, Range and Gros
Formations rusty near top, Ventre River Valley.
underlain by
variegated soft
claystone; basal
part is silty
dully-variegated
sandstone and
claystone.
JURASSIC
Sundance 500-700 Sandstone, green, North end of Teton
Formation underlain by soft Range, Blacktail
gray shale and thin Butte, Gros Ventre
highly River Valley.
fossiliferous
limestones; marine.
Gypsum Spring 75-100 Gypsum, white, North end of Teton
Formation interbedded with Range, Blacktail
red shale and gray Butte, Gros Ventre
dolomite; partly River Valley.
marine.
Nugget 0-350 Sandstone, North flank of Gros
Sandstone salmon-red, hard. Ventre Mountains,
southern Jackson
Hole.
TRIASSIC
Chugwater 1,000-1,500 Siltstone and North flank of Gros
Formation shale, red, Ventre Mountains,
thin-bedded; one north end of Teton
thin marine Range, southernmost
limestone in upper Jackson Hole.
third.
Dinwoody 200-400 Siltstone, brown, North flank of Gros
Formation hard, thin-bedded; Ventre Mountains,
marine. north end of Teton
Range, southernmost
Jackson Hole.