Green River Formation

The Green River Formation is an Eocene geologic unit recording sedimentation in a group of intermountain lakes across Colorado, Wyoming and Utah, dating to roughly 53.5 to 48.5 million years ago. It extends across three basins surrounding the Uinta Mountains, in northwestern Colorado, southwestern Wyoming (ancient Lake Gosiute) and northeastern Utah and western Colorado (ancient Lake Uinta). The rock is finely layered into varves, alternating dark organic-rich layers laid down during growing seasons and lighter inorganic layers from dry periods, each pair representing one year; the sequence documents roughly six million years of continuous sedimentation with individual varves averaging about 0.18 millimeters thick. The formation preserves a wide variety of complete and detailed fossils, including fish, insects, plants, reptiles and early mammals and bats, with some 35,000 fossiliferous specimens held at the Smithsonian Institution and Wyoming's Fossil Butte National Monument occupying a particularly fossil-rich section. It also contains the largest oil shale deposit on Earth, with reserves estimated at up to 3 trillion barrels of recoverable shale oil. This entity documents the Green River Formation specifically as a geological formation, an Eocene lacustrine sedimentary deposit.