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.