Coal-bearing strata of Late Cretaceous age in the western part of the Wind River Basin show transitions in depositional environment from coastal marine deltaic (neritic and paralic) to nonmarine intermontane (limnic). Earliest peat accumulation in the Frontier Formation coincided with several extensive marine regressive cycles. Deposition of terrestrial strata in this deltaic environment ended with encroachment by the sea from the east, which resulted in an extensive period of marine-dominated deposition of the thick Cody Shale.

The overlying Mesaverde Formation was initially deposited as a prograding delta shoreline complex of sand bodies on which coastal swamps were established. Peat accumulation...