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 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.