A comprehensive surface and subsurface study of the Upper Cretaceous Lazeart Sandstone Member of the Adaville Formation and the lower coal-bearing part of the Adaville Formation in the southwestern Wyoming thrust belt reveals a complex inter-tonguing of marine and nonmarine strata. During late Santonian and early Campanian time, the Lazeart wave-dominated deltaic system prograded southeastward onto a storm-influenced microtidal shelf. Sediments of the Lazeart Sandstone Member accumulated within storm-dominated lower shoreface, barred fair-weather upper shoreface, foreshore, washover, mouth bar, flood tidal delta, and tidal channel subenvironments. Landward of the strand line, sediments of the Adaville Formation were deposited within active...