The Geological History of Fossil Butte National Monument and Fossil Basin

by Paul O. McGrew; Michael Casilliano

In the Fossil Butte Member an interesting sequence of facies changes can be seen which reflect lateral changes in the environment of Fossil Lake. In those areas where the member represents a deep-water environment, organically formed limestones and shales are predominant. Shoreward, these rocks grade into ostracodal limestone, gastrapodal limestone, and, closest to shore, algal limestone. Then a muddy, sandy beach facies is encountered where the Green River and Wasatch formations intergrade.

The contact with the underlying Wasatch is conformable and sharp. It is often marked by a bench or by slump blocks of Green River Formation.

The Fossil Butte Member may in some cases overlap the Wasatch Formation and be deposited on Paleozoic and Mesozoic rocks. This is a reflection of topographic relief during the Eocene.

The thickness of the Fossil Butte Member, as reported by Oriel and Tracey (1970:37), is from 208 to 269 ft.

ANGELO MEMBER.

In the northern part of Fossil Basin, the Mudstone Tongue of the Wasatch Formation separates the Angelo Member of the Green River Formation from the Fossil Butte Member. Further south, the two members rest directly on one another. The southern extent of the Angelo Member has not been determined as yet, but Oriel and Tracey (1970:32) believe it extends up to, and intertongues with, Wasatch conglomerates near the Uinta Mountains. Toward other edges of Fossil Basin, the Angelo Member thins and pinches out into the Wasatch Formation. The Bullpen Member of the Wasatch rests on top of the Angelo Member.

The Angelo Member consists of white to blue-white weathering limestone, marlstone, and mudstone. Some sandstone lenses, claystone, oil shale, and siliceous limestone are present as well. In general, Oriel and Tracey (1970) have found that buff limestones prevail to the north and white, siliceous limestone, to the south. Like the Fossil Butte Member, the Angelo Member shows a facies change from deep-water to shore.

This member forms the very uppermost, rounded slopes of Fossil Butte in contrast to the more vertical cliffs formed by the Fossil Butte Member.

AGE OF THE GREEN RIVER FORMATION.

The age of the Green River Formation is dated on its intertonguing relationships with the Wasatch Formation because the latter has datable mammal fossils. The well-preserved fossils of fish, leaves, and insects from the Green River are, unfortunately, of little value in dating the Green River Formation.