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The Williston basin is a structural and sedimentary basin which covers 51,600 square miles in North Dakota and contains sedimentary rocks of every geologic period from the Cambrian through the Tertiary. The maximum known thickness is 15,128 feet in a well in McKenzie County in Western North Dakota.
The Upper Cambrian to Lower Ordovician Epochs are represented by the Deadwood Formation which represent stable shelf deposits extending eastward from the Cordilleran geosyncline. The Williston structural basin began in Middle Ordovician time with a relatively thin clastic sequence (Winnipeg Group) followed by predominantly carbonate deposition (Red River, Stony Mountain and Stonewall Formations). Carbonate deposition continued through Lower and Middle Silurian (Interlake Formation) followed by a period of erosion marked by a major unconformity.
During the Middle and Upper Devonian Epochs the Williston basin was a part of the larger western Canada basin of deposition which was marked by predominantly carbonate deposition with a thick evaporite in the lower part (Prairie Formation) and cyclical carbonates with some thin clastic and evaporite beds in the upper part
(Duperow, Nisku, Three Forks). Deposition was continuous or nearly continuous into the Mississippian, but the center of the Madison depositional basin was nearly coincident with the present Williston basin. It began with predominantly carbonate deposition with increasing evaporites in the upper part. The evaporites are mostly halite in the central basin area with anhydrite toward the flanks of the basin. Predominantly clastic deposition (Big Snowy Group) followed the evaporites and this was followed by another unconformity.
The Pennsylvanian and Permian periods are represented by clastics with minor carbonates (Minnekahta Formation) and some evaporites. This was a time of slight subsidence with the Williston basin area being part of a larger depositional area extending to the south and west. Similar conditions continued through the Triassic with fine grained clastics and some evaporites being deposited, followed by some non-marine redbeds and another unconformity.
The Williston structural basin had little effect on Jurassic or Cretaceous sedimentation so these periods are represented by eastward extensions of the predominantly fine grained clastics from the Rocky Mountain area seas. The Tertiary Period is represented by a wedge of predominantly non-marine beds which thickens westward toward the Rocky Mountain area.
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