Regional correlations, from the southern to northern Black Hills and across the central Powder River Basin to the Bighorn Mountains, serve as the framework for a depositional model of Middle Minnelusa sediments. In the eastern part of the study area, deposition took place in a carbonate sabkha environment. During transgressive periods most of this region was covered by a restricted, shallow sea. In the northern Black Hills, close to the limit of the transgression, deposition occurred in a coastal dune setting. During regressions, the sabkha prograded westward toward the Lusk Embayment. Coastal dune-fields to the north and isolated dune complexes to the south, migrated southwestward across this prograding sabkha. West of the Lusk Embayment deposition occurred in a sand-dominated tidal flat environment during transgressions and in the coastal edge of an eolian sand sea during regression.