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Facies patterns and associated vertical sequences of kerogenous carbonates (oil shales) of the Green River Formation in Fossil basin, Wyoming, provide new insights into the deposition of oil shale. Unique to Fossil basin is a facies pattern consisting of kerogen-rich calcimicrite at the basin's depocenter succeeded laterally by laminated calcimicrite, bioturbated calcimicrite, and
finally calcareous siliciclastics. This same pattern occurs cyclically as a vertical sequence with oil shale at the base.
The calcimicrites at the depocenter increase in dolomite content dramatically in the upper third of the section, and oil shale units become more widespread. There is an accompanying increase in saline minerals as well as zeolites indicating hypersalinity.
Bioturbation is absent from both oil shale and dolomicrites. The marginal bioturbated facies are always dominated by calcimicrite, even in the upper third of the section. Well-preserved fossil fish occur in kerogen-rich calcimicrites, but never in dolomicrites.
The facies patterns and vertical sequences in Fossil basin are interpreted as being deposited in a closed-basin occupied by a lake with an ephemeral and hypersaline hypolimnion that underwent frequent vertical fluctuations that affected large areas of the lake bottom. The "transgression-regression" of the hypolimnion over large areas of lake bottom was possible because of a low topographic gradient on the lake bottom. Deposition of oil shale occurred within the denser, cooler, and hypersaline hypolimnion waters--laminated calcimicrite in a zone of chemocline fluctuation--and bioturbated calcimicrite was deposited in fresher nearshore zones. Kerogen-rich carbonates became more widespread in the later stages of the lake when it evolved into a shallow hypersaline lake. This suggests that kerogen deposition was controlled by high salinity rather than anoxic conditions.
It is concluded that the carbonate facies patterns, vertical sequences, and oil shale genesis required a dynamic and fluctuating lake as well as a fluctuating chemocline level in a closed basin with a low topographic gradient. This differs from the static, anoxic, and deep basin model frequently postulated for Green River Formation oil shale deposition.
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