LACUSTRINE DEPOSITION IN THE BRIDGER FORMATION: LAKE GOSIUTE EXTENDED!

The Green River Formation was deposited in Lake Gosiute, until the lacustrine system shifted to the more fluvial environment of the Bridger Formation. In the southern part of Bridger A exposures, Bridger A becomes increasingly lacustrine and interfingers with the main body of the Laney Shale member of the Green River Formation. Higher in Bridger A several widespread limestone marker beds are separated by mudstones. Bridger B also consists of mudstones alternating with limestones. A number of these limestones have now been mapped, and they extend across the entire existing Bridger B exposures, and represent basin-wide shallow lakes. These lakes were filled by volcaniclastic input from episodes of volcanism to the north. The lacustrine deposits of the Green River Formation consist largely of laminated kerogen-rich micrites (oil shales) that grade laterally into massive limestones or siliciclastic mudstones, whereas the lacustrine deposits of the Bridger Formation are widespread, massive limestones deposited in shallow, but very large lakes. Thus the large-scale lake that formed the Green River Formation did not really disappear. It became a shallow lake that periodically was filled by an episode of volcaniclastic deposition in a fluvial-lacustrine system, only to reappear when basin subsidence exceeded volcaniclastic input.