B. Slab in which fossils are remarkably complete. Silver dollar gives scale. Specimen is in University of Wyoming Geological Museum.
THE MESOZOIC—ERA OF TRANSITION
The Mesozoic Era in the Teton region was a time of alternating marine, transitional, and continental environments. Moreover, the highly diversified forms of life, ranging from marine mollusks to tremendous, land-living dinosaurs, confirm and reinforce the story of the rocks. Living things, too, were in transition, for as environment changed, many forms moved from the sea to land in order to survive. It was the time when some of the most spectacularly colored rock strata of the region were deposited.
Colorful first Mesozoic strata
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.
Table 4.—Mesozoic sedimentary rocks exposed in the Teton region.
| Age | Formation | Thickness (feet) | Description | Where exposed |
|---|---|---|---|---|
| CRETACEOUS | ||||
| Harebell Formation | 0-5,000 | Sandstone, olive drab, silty, drab siltstone, and dark-gray shale; thick beds of quartzite pebble conglomerate in upper part. | Eastern and northeastern parts of Jackson Hole. | |
| Meeteetse Formation | 0-700 | Sandstone, gray to chalky white, blue-green to gray siltstone, thin coal, and green to yellow bentonite. | Spread Creek area. | |
| Mesaverde Formation | 0-1,000 | Sandstone, white, massive, soft, thin gray shale, sparse coal. | Eastern Jackson Hole. | |
| Unnamed sequence of lenticular sandstone, shale, and coal. | 3,500± | Sandstone and shale, gray to brown; abundant coal in lower 2,000 feet. | Eastern Jackson Hole and eastern margin of the park. | |
| Bacon Ridge Sandstone | 900-1,200 | Sandstone, light gray, massive, marine, gray shale, many coal beds. | Eastern Jackson Hole and eastern margin of the park. | |
| Cody Shale | 1,300-2,200 | Shale, gray, soft; thin green sandstone, some bentonite; marine. | Eastern and northern parts of Jackson Hole. | |
| Frontier Formation | 1,000 | Sandstone, gray, and black to gray shale, marine; many persistent white bentonite beds in lower part. | Eastern and northern parts and south-western margin of Jackson Hole. | |
| Mowry Shale | 700 | Shale, silvery-gray, hard, siliceous, with many fish scales; thin bentonite beds; marine. | Gros Ventre River Valley, northern margin of the park, and southern part of Jackson Hole. | |
| Thermopolis Shale | 150-200 | Shale, black, soft, fissile, with persistent sandstone at top; marine. | Gros Ventre River Valley, northern margin of the park, and southern part of Jackson Hole. | |
| Cloverly and Morrison(?) Formations | 650 | Sandstone, light gray, sparkly, rusty near top, underlain by variegated soft claystone; basal part is silty dully-variegated sandstone and claystone. | North end of Teton Range and Gros Ventre River Valley. | |
| JURASSIC | ||||
| Sundance Formation | 500-700 | Sandstone, green, underlain by soft gray shale and thin highly fossiliferous limestones; marine. | North end of Teton Range, Blacktail Butte, Gros Ventre River Valley. | |
| Gypsum Spring Formation | 75-100 | Gypsum, white, interbedded with red shale and gray dolomite; partly marine. | North end of Teton Range, Blacktail Butte, Gros Ventre River Valley. | |
| Nugget Sandstone | 0-350 | Sandstone, salmon-red, hard. | North flank of Gros Ventre Mountains, southern Jackson Hole. | |
| TRIASSIC | ||||
| Chugwater Formation | 1,000-1,500 | Siltstone and shale, red, thin-bedded; one thin marine limestone in upper third. | North flank of Gros Ventre Mountains, north end of Teton Range, southernmost Jackson Hole. | |
| Dinwoody Formation | 200-400 | Siltstone, brown, hard, thin-bedded; marine. | North flank of Gros Ventre Mountains, north end of Teton Range, southernmost Jackson Hole. | |