Abstract

The Muddy sandstones in the western Wind River Basin were deposited in a thin, but well-developed delta system during early Cretaceous Albian time. Open marine Skull Creek shales grade upward into the lower marine Muddy sandstones. The submarine depositional surface was raised to sea level by deposition of 65 feet (1.8 m) of prodelta sediments and delta-front sandstones at the northwest edge of the delta. Progradation of delta-plain deposits then covered these delta-front sandstones. Rapid subsidence allowed preservation of most Muddy sedimentary rocks northwest of Lander, but a lesser rate of subsidence or slight uplift to the southeast toward the Laramide Sweetwater Uplift allowed progressively deeper erosion, resulting in two intraformational unconformities. The lower-marine deposits were deeply eroded by rivers of the delta plain, and most of these lower-marine sedimentary rocks southeast of Lander were removed. The delta plain was successively covered by two main facies: 1) fluvial channel sandstones and 2) flood plain mudstones. The fluvial sandstones, deposited in a northwest-flowing river system, terminate on outcrops to the northwest of Lander. The overlying floodplain deposits are truncated southeastward from Lander by erosion at the base of the overlying transgressive beach deposits. Progressive onlapping by beach and nearshore deposits of the middle-marine Muddy sequence subsequently buried the older delta system. The unconformity at the base of the beach deposits cuts progressively deeper into underlying delta-plain deposits to the southeast, where reworked beach sandstones were deposited directly on the fluvial sandstones of the delta plain. A second, or upper-marine, sandstone and shale sequence buried the delta to the southeast. Northwestward these upper-marine sandstones grade laterally into marine shales.