Pebbles of pre-Paleozoic rocks in latest Cretaceous sediment of the Laramie Basin indicate that some of the ranges of the Rockies in Wyoming were elevated by the end of the Mesozoic Era to such an extent that streams had cut through the thick sedimentary cover to the basement rocks. The deformation in this segment of the Cordillera has resulted in vertical uplifts; local folds and reverse faults can be interpreted as secondary effects of the upward movement of the pre-Paleozoic cores of the ranges.
The first elevation of the Wyoming-Colorado sector of the Rockies took place in the Paleocene epoch, and by Eocene time the ranges may have had a relief comparable with that of the present day. The sediments from the erosion of these early Cenozoic mountains were deposited in adjacent basins as coarse conglomerates near the mountain fronts and as variegated sandstones and red banded shales farther out in the basins. The spectacular turreted scenery of the Bryce Canyon, Utah, is cut in the Eocene clastic sediments of the Wasatch Formation, eroded from the rising mountains.
During Eocene time large lakes filled the Green River Basin of southwestern Wyoming and the Uinta Basin of eastern Utah and northwestern Colorado. Although the deposits of these two basins are now separated, the two lakes probably joined around the eastern end of the Uinta Arch during some stages of their history. In the lakes a great variety of segments, including oil shale, algal limestone, phosphate rock, anhydrite, salt, and sodium carbonate (trona) were laid down in thick beds.
Oil shale is a light brown to black shale that contains hydrocarbons in the form of waxy spores and pollen grains. Under proper treatment these hydrocarbons can be distilled and used to prepare petroleum products. The petroleum that can be distilled readily from the minable beds of oil shale of the Green River Formation (that is, those yielding 30-35 gallons per ton, 125-150 liters per ton) have been estimated as 160 billion barrels.
The fine, regular laminations in these sediments could be annual deposits, or varves. Counts of the layers suggest that the 600 m of the Green River beds took 4 million years to be deposited. In order for evaporite minerals such as sodium carbonate, anhydrite, and dolomite to be associated with fossil-bearing, organic-rich shales, the lake in which the formation was deposited must have fluctuated greatly in level like the ephemeral lakes, called playa lakes, that occupy basins between the ranges in Nevada today. During arid times the lake water was concentrated to a brine by evaporation and confined to the center of the lake. Sodium carbonate was precipitated. During periods of greater rainfall the lake expanded greatly and algal blossoms formed a large quantity of organic ooze on the bottom of the lake from which the waxy hydrocarbons were incorporated in the oil shale. From the fossil plants found in the Green River Formation paleobotanists have learned that the climate was temperate to sub-tropical, with an annual temperature in the range of 15-20 degrees Celsius (60-70 F) and a seasonal rainfall of 60 to 76 cm (24 to 30 in). Some of the most perfectly preserved fossil fish in the world are found pressed flat on the bedding plains of the Green River mudstones.
Although Eocene sediments are thick and widespread in the basins between the mountains, they are missing from the wedge of sediments that spread eastward into the plains from the Rockies. Possibly, regional warping of the Rocky Mountains area during early Cenozoic time resulted in the drainage from the mountains being deflected from an eastward course, or else the successor basins may have acted as such efficient sediment traps that little of the products of erosion escaped the mountain terrane.