ABSTRACT

The depositional history of the Williston Basin was significantly influenced by the events that resulted in passive margin and later convergent margin foreland basin sedimentation in western North America. The basin was an actively subsiding structure during much of the Phanerozoic, but its geological history can be divided into five sequence packages, each separated by a major unconformity. The first three sequences represent the influence of the passive margin, and the later two, the effects of foreland basin sedimentation.

The earliest sequence, Sauk, marks the initial inundation of the craton and is represented by a coarse to fine clastic succession. The next two sequences, Tippecanoe and Kaskaskia, were dominated by carbonate-depositing seas. Tippecanoe rocks suggest that the sedimentation rate and rate of subsidence in the basin kept pace, but with indications of marked shallowing in the late middle Silurian. In the Kaskaskia, the basin was coupled to a much larger structural feature, the Elk Point Basin, in which, firstly, evaporites and reef growth dominated, followed by shelf carbonate and evaporite sedimentation. In late Kaskaskia (Mississippian), the basin was uncoupled and a distinct paleobathymetric zonation developed, from peritidal-shallow shelf conditions at the margins to deep water in the basin centre. The Zuni sequence marks initiation of the convergent margin. In the earliest phase (Jurassic), paleobathymetry continued to control sediment distribution but was dominated by fine and coarse siliciclastics. Late Zuni is marked by cycles of the same siliciclastic marine transgressions and continental progradations that are evident throughout western North America.

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