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The AAPG/Datapages Combined Publications Database

AAPG Bulletin

Abstract


Volume: 40 (1956)

Issue: 2. (February)

First Page: 421

Last Page: 422

Title: Stratigraphy of Madison Group of Montana and Wyoming: ABSTRACT

Author(s): John Andrichuk

Article Type: Meeting abstract

Abstract:

Lithologic sequences characterized by distinct evaporite cycles are recognized in the Madison group of Montana and Wyoming. Throughout the basinal area of Montana and North Dakota, the

End_Page 421------------------------------

cycles are represented by vertical variations from oolitic-fragmental and normal marine limestone (or dolomitized equivalents) to evaporitic rocks including anhydrite, locally halite, and associated cryptocrystalline or dense carbonates. Within the Wyoming shelf, the cycles exhibit only partial development in which the evaporitic interval is thinly represented by primary cryptocrystalline or dense dolomite. Dolomitized limestones (fine to coarse crystalline dolomites) generally form the marine intervals in the shelf areas.

By recognition of these lithologic cycles, the Lodgepole, Mission Canyon, and Charles formations may be correlated to their respective eroded edges in Wyoming, South Dakota, and north-central Montana.

Mississippian sedimentation commenced earliest in the Williston basin and central Montana trough with the deposition of black shales (Bakken formation) followed by dense limestones and shales (lower Lodgepole). Preceding and accompanying the initial marine transgression on the Wyoming shelf, minor terrestrial to near-shore marine clastic deposition took place in the Black Hills area (Englewood formation). Widespread marine deposition formed normal marine and fragmental-oolitic limestones which were subject to dolomitization in the shelf areas (upper Lodgepole, lower Pahasapa, and Guernsey formations).

Marine limestone and dolomite sedimentation, interrupted by two episodes of evaporite precipitation, resulted in the development of the Mission Canyon formation and the superjacent evaporites of the basal Charles, and their equivalents in the Pahasapa formation of the Black Hills area.

Restoration of normal marine conditions and development of a complex evaporite cycle followed (Charles formation, excluding basal evaporites and uppermost few beds of the Pahasapa).

Big Snowy clastics were deposited in Chester time in the basinal areas of Montana. In areas to the south, pre-Amsden, pre-Minnelusa, or pre-Pennsylvanian weathering and erosion affected the Mississippian beds. In north-central Montana, pre-Middle Jurassic erosion strongly truncated the upper Madison strata.

Oil occurrence in the Madison group is related to the two main provinces of deposition. In general, the oil from the basinal or limestone province is of relatively high gravity, whereas that from the dolomite or shelf province is of much lower gravity.

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