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A revision of the 1954 publication of the "Stratigraphy of the Williston Basin" is nearing completion and will be published early in 1960. The revision is by six stratigraphic committees of the North Dakota Geological Society. The six committees also coordinate their nomenclature selections with the North Dakota Geological Survey. The Saskatchewan Canadian Government and the Saskatchewan Geological Society have given their approval to the nomenclature revision of the Mississippian group.
The committees have divided their work as follows: 1--Cambrian-Ordovician-Silurian; 2--Devonian; 3--Mississippian Madison group; 4--Mississippian Big Snowy group-Pennsylvanian; 5--Permian-Triassic-Jurassic 6--Cretaceous-Tertiary. Close coordination among the committees is being carried on at all times. In addition to the preparation of regional cross sections, each committee is preparing branch cross sections to the regional cross sections, maps, nomenclature charts and electric survey type-section logs pertinent to its particular problem.
Although some nomenclature revisions may be made before publication, the following are group and formation divisions by four of the committees.
I. The Cambrian-Ordovician-Silurian have been divided as follows. The Cambrian has been classified as the Cambrian-Deadwood formation. The Ordovician has been divided into the Winnipeg, Red River, Stony Mountain, and Stonewall formations. The Winnipeg formation contains a lower, middle and upper member. The Stony Mountain formation has been subdivided into Stoughton and Gunton members. The Silurian has been classified as the Interlake formation. In the latter formation, two electric survey markers have been selected and named the Tioga and the Croff.
II. The Devonian sediments have been divided into the Elk Point, the Manitoba, and the Jefferson groups overlain by the Three Forks formation. The Elk Point group has been subdivided into the Ashern, Winnipegosis, and Prairie formations. The Winnipegosis formation contains a lower and upper member. The Manitoba group has been subdivided into the Dawson Bay and Souris River formations. The Jefferson group has been subdivided into the Duperow and Bird Bear formations.
III. Overlying the Devonian is the Mississippian Bakken formation. Above the latter sediments is the Mississippian Madison group. Previous designation of the Madison group consists of the Lodgepole, Mission Canyon, and Charles formations. These formations have now been classified into the Lodgepole, Mission Canyon and Charles facies in the light of more intimate knowledge of the stratigraphy. In addition, five marker-determined intervals and two sub-intervals are defined wherever they are recognizable for correlation purposes. The Madison group facies are commonly crossed by several of the intervals which are defined by log deflections. A marker-determined interval may, and frequently does, occur in more than one facies. The three facies are not generally traceable throughout the Will ston Basin, but are identified and present in the correlation chart. For each of the six selected areas in the Williston Basin, a type log is presented. Each of the six type logs indicate porosity sections which are identified with the name or names they have come to be known by in the past. The Mississippian Madison group committee wishes to emphasize that the criteria for identification of the above mentioned intervals are based on log deflections or "markers," not on lithology.
IV. The Mississippian Big Snowy group-Pennsylvanian have been divided as follows. The Big Snowy group has been subdivided into the Kibbey, Otter, and Heath formations. The Kibbey formation contains a lower, middle and upper member. The Pennsylvanian
sediments comprise the Tyler and Minnelusa-Amsden formations.
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