In northern Wyoming and southern Montana, the AalenianBajocian Gypsum Spring and Piper formations preserve the earliest deposition in the Sundance Seaway. The facies and sequence stratigraphy of these formations were examined along the eastern flank of the Bighorn Basin, where they record deposition on a marine evaporite-carbonate ramp. Shallow-subtidal facies on this ramp pass landward into tidal flats, and then to mosaic of supratidal flats and marine-fed salinas. Three surfaces are recognized as sequence boundaries, the J-1a, J-1b, and J-2. The J-1a separates the lower and upper portions of the Gypsum Spring, and the J-1b separates the Gypsum Spring from the Piper. The position of the J-2 is resolved, separating the Piper from the Sundance Formation. The gypsum deposits of the Gypsum Spring are interpreted as basin-marginal evaporites based on facies association, and placed in the transgressive systems tract of the J-1 sequence.