The Green River Formation of Wyoming, Colorado, and Utah covers more than 38,000 square miles (100,000 sq km) and has a maximum thickness of almost two miles (3 km) which accumulated over a period of 10 million years. This formation, which has the world's richest deposits of oil shale, was deposited in four separate basins , including the Piceance basin in Colorado, the Uinta Basin in Utah, and the Green River and Washakie basins in Wyoming. The oil shale contains kerogen layers interlaminated with the marlstone which is believed to have accumulated with algal blooms. Early workers such as Bradley (1929 and 1948) suggested that couplets (an organic or kerogen layer and a marlstone layer) of the laminae can represent true varves. If this is correct, then each one of the couplets would represent one year's deposition. Based on the measurement of the varves, Bradley estimated that the Green River Epoch lasted 5,000,000 years.
This environment of deposition would require tranquil water between 16 and 100 feet (5 and 30 m) in depth (Ryder et al., 1976). Eocene Lake Uintah filled the Wyoming basins and Lake Goshiute filled the other basins. The Green River Formation of Lake Uinta contains exceptionally well preserved fossil fish. The deeper portions of the lake were anoxic, allowing the deposition of oil shale and preventing scavengers from disturbing the carcasses on the bottom. Algal stromatolites flourished on the margin of Lake Goshiute. The highly restrictive environment necessary to deposit the Green River Formation include: (1) stagnant water necessary for producing oil shale; (2) warm clastic-free water needed to deposit the carbonate layers; (3) the algal blooms; (4) the shallow water to deposit the stromatolites; and (5) the deposition of finely laminated sediments up to 9843 feet (3 km) thick.... Evidence of a lacustrine environment include freshwater fossils, especially plant material in fine-grained laminated sediments, oscillation ripples instead of current ripples, and a lack of tidal structures.