Fossil mollusks in the Bullpen Member are not completely diagnostic but suggest either a late early Eocene (Lostcabinian) (Fig. 6) or mid-Eocene age. The Tunp Member interfingering as it does with the entire Wasatch section represents all of early Eocene age.

DEPOSITIONAL ENVIRONMENT.

The sediments that make up the Wasatch Formation in the Fossil Basin were deposited mainly by streams flowing into the basin from the surrounding uplands. Rock types are variable and for the most part individual types cannot be traced over large areas, a condition typical of fluvial environments. The mudstones and siltstones probably were deposited along flood plains, while the lenticular sands and conglomerates were laid down in stream channels. Where streams entered the lake tongues of deltaic deposits, sands or mudstones wedge into the Green River Lake sediments.

The reason for the red color of the Wasatch Formation is not fully known. According to Van Houten (1948), the red is most likely the result of tropical, red lateritic soils forming in the uplands and being deposited on flood plains. The oxidation of organic material in a humid, subtropical environment causes the red color. The bands of purple, gray, orange, and other colors are due to various stages in the reduction of the iron oxides in the rocks.

The presence of palms in the lake sediments and of bones of primates and crocodiles in both the lacustrine and fluvial sediments indicate a heavily forested, tropical environment.

Green River Formation

The Green River Formation is easily recognized by its light color and continuous bedding, in strong contrast to the red, discontinuous, variegated Wasatch Formation below. The Green River Formation can be thought of as a gigantic lens of lacustrine sediments enclosed in the fluvial Wasatch Formation. The name Green River Formation is applied to all of the roughly contemporaneous deposits laid down in lakes of Utah, Colorado, and Wyoming. It is probable that at one time or another all of these lakes were connected.

Oriel and Tracey (1970) have divided the Green River Formation of the Fossil Basin into two members: the Fossil Butte Member and the Angelo Member.

FOSSIL BUTTE MEMBER.

The type section for this member is near the southeastern end of Fossil Butte within the monument boundaries. In the type area the Fossil Butte Member can be seen to consist of four lithologic units.

The lowermost unit is predominantly mudstone. It occurs in a sequence about 45 ft thick and contains light gray, fine-grained, calcareous mudstone and siltstone.

The next overlying unit is about 75 ft thick and is mainly a limestone unit. It consists of tan to gray limestone, shaly limestone, siltstone, and paper shale which weathers into thin, curled flakes. A yellow-brown mudstone tops this unit.

The third unit in the sequence is 45 ft thick and mainly composed of shales. These shales weather a buff color and are calcareous. Oil shale, organic rich paper shale, and marlstone comprise the actual layers in the unit. A few, thin ash beds are also present. This unit is the most significant one for Fossil Butte National Monument. About 10 ft below the top of this unit (about 155 ft above the Wasatch) is a bed of varved shales, one foot thick, that contains the fossil fish for which the monument was established.

The uppermost unit contains a number of beds of oil shale that are brown on a fresh surface but weather a grayish-white. An orange-yellow-weathering limestone caps the 40-ft thick upper unit. Ash beds are common and traceable over a wide area of Fossil Basin.

South of the monument the Sandstone Tongue of the Wasatch Formation wedges in between the lowermost unit and the overlying limestone unit of the Fossil Butte Member.

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.

Although no Lostcabinian mammals are known from Fossil Basin, the Green River Formation is believed to be of that age (Gazin 1959; Schaeffer and Mangus 1965).

DEPOSITIONAL ENVIRONMENT.

The Green River Formation, on the basis of its lithology and fossil content, is a fresh-water, lacustrine deposit. Clay and silt were dumped in Fossil Lake by streams. Most of this fine debris was deposited near the shore. Chemical and organic processes formed limestones and marlstones in the deeper central part of the lake.

Examination of the edge of the Green River Formation indicates that Fossil Lake expanded and contracted several times.

Fossil Lake was eventually filled in with chemical precipitates and deltaic deposits. The end of the lake was gradual as seen in the transitional and gradational Green River-Bullpen contact.

Although oil shale is not extensively developed in the Green River Formation of Fossil Basin, it does occur in small quantities.

Oil shale is a fine-grained sedimentary rock containing organic matter which was derived chiefly from aquatic organisms, waxy spores and pollen grains ... and of which a large portion is distillable into a liquid similar to petroleum. Despite the name, most rich beds of oil shale in the Green River Formation cannot be regarded strictly as shale. Instead, they are dolomitic marlstones rich in organic matter. Nevertheless, a few are shaly (Bradley 1964b:19).

Oil shale has a structureless ground mass that