ABSTRACT

Three formations, in upward sequence the Bakken, the Madison Limestone and the "Charles" Evaporites, comprise the Mississippian succession preserved in the subsurface of the northern slopes of the Williston Basin. They lie unconformably beneath Mesozoic sediments.

The Bakken, composed of shales and siltstones, rests with slight unconformity on Devonian continental beds along the northeastern margin of the basin; a strongly phosphatic pebble bed is present at the junction.

The carbonate and evaporite sequence above the. Bakken reveals the history of a transgression and regression which culminated in the deposition once more of basin-wide continental beds. Small marine advances which punctuated the regression caused a cyclic deposition of interbedded evaporites and carbonates. Lower evaporites fringing the northeastern margin of the basin die out basinward, but higher evaporites apparently thicken over the basin centre.

Limestone diagenesis included early dolomitization and anhydrite metasomatism, and was probably accelerated by the intermittent retreats of the sea. Later changes (some Jufa-Triassic) included dolomitization and sulphate impregnation (anhydrite and celestite) particularly near the eroded Mississippian surface beneath the pre-Mesozcic unconformity.

Numerous oil pools lie in stratigraphic-type traps formed either by evaporites interleaved with the carbonates or by an altered limestone layer at the unconformity.