ABSTRACT

In the Williston Basin, the Winnipegosis Formation is the major carbonate unit of the initial transgressive-regressive pulse of the Kaskaskia sequence. Following a brief hiatus separating the Ashern Formation from the Winnipegosis Formation, a clear, quiet shallow marine environment became widely established in the North Dakota portion of the Elk Point Basin. Preferential dolomitization of bioturbated areas was the major diagenetic alteration of the rocks.

During the transgressive phase, the basin differentiated into two distinct regions due to varying amounts of carbonate production: 1) a deeper basin with scattered pinnacle reefs, and 2) an encompassing carbonate shelf. In the basin region two environments became established: a) the pinnacle-reef environment where several lithofacies developed due mainly to exposure of the upper portion of the reefs; diagenesis was extensive and included pervasive dolomitization, cementation, stylolitization and vadose alteration; b) the deeper basin environment surrounding the reefs where slow deposition gave rise to laminated limestone. In the shelf region, proceeding landward, there developed shallow marine, patch reef, lagoon and tidal flat environments, each of which produced a variety of lithofacies. The dominant diagenetic alteration was preferential dolomitization of bioturbated areas, but stylolitization, anhydrite replacement and displacement, and pyrite formation also occurred.

The regressive phase is seen in the shelf region as a series of rocks with progressively shallower subtidal faunas and growth forms, capped by an intertidal to supratidal series of dolomites and anhydrites. Diagenesis of the subtidal rocks is similar to those of the transgressive phase for that environment; however, deposition and diagenesis of the rocks of the intertidal and supratidal regressive phase were penecontemporaneous. Dolomitization, anhydrite displacement and replacement and pyrite formation were the major diagenetic changes in these rocks. In the basin region, as sea level fluctuated, stromatolites and supratidal dolomites were deposited upon the pinnacle reefs. Eventually, when sea level fell substantially, redbeds formed on the exposed reefs. In the shallow inter-reef environment surrounding the emergent reefs, restricted shallow marine to supratidal dolomites formed and subaqueous algal mats developed. Rocks of the basin region were affected primarily by dolomitization and anhydrite displacement. Deposition of additional dolomite followed, then anhydrite and finally halite of the Prairie Formation.