is yellowish-orange to reddish-orange in color. Pyrite crystals are found indicating a partially anaerobic or reducing environment. Pollen, waxy spores, filaments of algae, and other plant parts are preserved along with insects and larvae. The preservation is akin to mummification. Crystals of calcite, dolomite, or authigenic feldspar are also found in the oil shale (Bradley 1966).

The exact mode of origin of oil shale is not positively known because of a lack of a modern analogue for comparison. Oil shale probably originated as an organic ooze on the bottom of the Fossil Lake. This ooze was composed of the remains of phytoplankton, blue-green algae, zooplankton, bacteria, and some pollen and spores. The ooze was dense and uncompacted. Little clastic debris is found, either because the ooze accumulated in deep water or plants near the shore filtered out the debris.

Decay was reduced effectively in the ooze because of either an antibiotic in the ooze which inhibited bacteria of decay or the ooze accumulated in waters where anaerobic conditions prevented decay.

With time and the weight of overlying sediments, the ooze was compacted and most of its water driven off. Continuing pressure from compaction and heat generated by burial and compaction caused a variety of complex chemical reactions which converted the ooze into a petroleum product called kerogen. Kerogen is distillable and is the important constituent of oil shale.

An alternate hypothesis (Eugster and Surdam 1973), would have some oil shale forming in a desert-playa environment. This is based on geochemical evidence found in Gosiute Lake sediments to the east of Fossil Lake. There, certain minerals are found in association with some oil shale that could only have been deposited during periods of extreme evaporation and in a shallow lake. Much study is now being directed toward a solution to these problems.

The combustible quality of oil shale has been known for a long time. Many of the pioneers used it as a fuel for cooking and heat. Hayden (1871:142) wrote of how workmen on the Union Pacific accidently ignited the oil shale in a cut they were excavating. The burning shale provided enough light for night work.

Many of the shales of the Green River Formation appear to be varved. A varve consists of two layers, one of calcium or magnesium carbonate and one of organic material. The limnological conditions that led to the formation of varves will be discussed in their proper place in the section on Paleoecology.

Fowkes Formation

This is the middle formation of Veatch’s (1907) tripartite division of the Wasatch Group. It is now found to be the youngest formation in Fossil Basin. It is not exposed in Fossil Butte National Monument.

Oriel and Tracey (1970) have formally divided and named three members of the Fowkes Formation: a lower Sillem Member, a middle Bulldog Hollow Member, and an upper Gooseberry Member.

SILLEM MEMBER.

Like most of the Fowkes Formation, this sequence of rocks has been eroded extensively and is preserved as erosional remnants, where protected by faulting, in the western part of Fossil Basin.

The Sillem Member consists of a lower conglomeratic sequence with some sandstone and mudstone. The conglomerate contains well-rounded clasts of gray quartzite, chert, and Paleozoic limestone. The sandstone is light gray, calcareous to muddy, and coarse to medium-grained. The mudstone is pink, gray, or tan in color.

The upper part of the Sillem Member is a mudstone and claystone unit. It ranges in color from pink and yellow to gray and green. Some volcanic debris is found. There are also interbedded layers of marlstone and limestone. Some sandstone is present.

The Sillem Member is between 100 and 400 ft thick and most probably rests unconformably on the Bullpen Member of the Wasatch.

BULLDOG HOLLOW MEMBER.

This middle member of the Fowkes Formation has the thickest and most extensive outcrops. The Bulldog Hollow Member is exposed along the west side of the basin.

Included rocks are green, white, and blue-green mudstone with ash beds, green and buff claystone, and tuffaceous, limy sandstone. A high percentage of the iron mineral, magnetite, occurs in the sandstone. Conglomerate occurs as lenses.

The Bulldog Hollow Member has a gradational contact with the underlying Sillem Member. The amount of volcanic material increases upward from the Sillem, indicating an increase in volcanic activity during the deposition of the Bulldog Hollow Member.

GOOSEBERRY MEMBER.

Oriel and Tracey (1970:55) place this uppermost member provisionally within the Fowkes Formation. Most of the Gooseberry Member is a puddingstone, a lithology with well-rounded, spherical pebbles in a marlstone, sandstone, or sandy limestone matrix. The pebbles are too rounded for the rock to be a diamictite, and too separated from each other to be called a conglomerate.

The nature of the Gooseberry-Bulldog Hollow contact is not completely known. It appears to be gradational in some areas and to be an angular unconformity in others.

AGE OF FOWKES FORMATION.

Fossils date the Sillem and Bulldog Hollow members as middle Eocene in age. These fossils consist of ostracodes, gastropods, leaves, and vertebrates from the Bulldog Hollow Member (Nelson 1973). The Gooseberry Member has yielded a few vertebrate remains and is late Miocene or early Pliocene in age (Oriel and Tracey 1970).

DEPOSITIONAL ENVIRONMENT.

The Fowkes Formation is an alluvial deposit, much like the Wasatch Formation. The chemical and climatic conditions of deposition were different from those of the Wasatch, and the extensive red-beds are not developed.

Small lakes were present in which limestone and marlstone accumulated. The puddingstone may be a mudflow. Volcanic activity left its record in the ash found in the Fowkes Formation.

In the past, the Fowkes Formation had a greater distribution. Postdepositional faulting down dropped parts of the Fowkes protecting them from subsequent erosion.

QUATERNARY

Rubey et al. (1968a, b) have mapped several forms of Quaternary deposits in Fossil Basin. These include stream alluvium, rock and landslide debris, river terraces, and gravels, all derived from local formations. These deposits are the work of water, wind, and ice acting in relatively Recent time.

THE GEOLOGIC STRUCTURE OF FOSSIL BASIN

The Fossil Basin is a small, linear and structurally controlled basin in the southeastern part of the Wyoming overthrust belt. This “overthrust belt” is represented by a number of small mountain ranges and high ridges formed by the “thrusting” of sedimentary rocks over other sedimentary rocks. Topographically, the Fossil Basin is bounded by the Crawford Mountains and Tunp Range on the west, by Oyster Ridge on the east, and by the Uinta Mountains on the south. The Crawford Mountains, Tunp Range, and Oyster Ridge (Fig. 2) are areas of high relief developed upon southerly extended salient ridges of deformed Paleozoic and Mesozoic strata. In the center of the Fossil Basin, these earlier rocks are covered by a veneer of early Tertiary sediments. Superficially, the Fossil Basin appears to be a broad syncline with tilted beds dipping sharply or gently basinward from the basin margins. The Tertiary sedimentary cover, however, partially obscures what is a more complex structural history.

Following deposition of the Late Cretaceous Adaville Formation, the Fossil Basin was included in a period of intense structural deformation. This deformation was the result of compressional forces acting along a more or less east-west alignment. The strain, or the resolution of these forces,