shale - hard tan shale from the Green River Formation, an Eocene lacustrine petroleum source rock - teaching hand specimen
The Green River Formation was deposited in a series of lake basins formed as a result of mountain building during the Laramide orogeny, with deposition largely occurring during the Eocene. The lakes alternated from fresh water to saline-alkaline phases and covered a large area in northeastern Utah, southwestern Wyoming and northwestern Colorado.
The lithology of the Green River Formation varies widely, in places 33% organic carbon by weight. Typical of the organic-rich shale, the “mahogany layer” of the Parachute Member is technically a kerogen saturated lime mud, though it is referred to as oil shale, and was experimentally mined, primarily in the 1970s, in Parachute Canyon, Colorado, with an eye on its petroleum potential. Costs were high and the potential was unrealized, though Unocal produced five million barrels of oil in their Parachute Canyon plant in the 1950s ‘60s and ‘70s.
Near Bonanza, Utah and at Douglas Pass, Colorado, the shale contains prolific fossil plant and insect sites, recording Rocky Mountain vegetation during the early middle Eocene, and providing information about the nature of the ecosystems that surrounded the Eocene lake systems. This shale is durable because of its laminated hydrocarbon-rich nature, allowing preservation of entire branches instead of the single leaves typical of most other fossil leaf localities.
This shale was collected near the FAA radar dome at Douglas Pass, in the divide between the Piceance Creek and Uinta basins. It has a leaner organic content than the hydrogen-enriched shale of the Piceance Creek basin to the east.
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