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Madison group sediments accumulated in two depositional realms: a basinal, predominantly limestone province in Montana and adjoining Dakotas; and the Wyoming shelf, characterized by prominent dolomitization on the south. The belt of transition in southern Montana and adjoining limestone and shale deposition took place in the Idaho geosynclinal trend. Southeastward to the Cambridge arch, Madison group sediments were affected by increasing amounts of pre-Pennsylvanian erosion in Wyoming and western South Dakota. Mississippian sediments are absent from a part of southeastern Wyoming.
Early, medial, and late stages of sedimentation may be differentiated on the basis of lithologic cycles or sequences developed in the Madison group. Early sedimentation was characterized by deposition of the approximate Lodgepole formation and equivalents, Guernsey, Englewood, and lower Pahasapa formations. Deposition commenced earliest in the Williston basin and central Montana trough with the development of black shales, followed by dense limestone and shales; minor terrestrial to near-shore marine clastic sedimentation in the Black Hills area commenced somewhat later, preceding and accompanying the initial marine transgression on the Wyoming shelf. Widespread deposition of normal marine and fragmental oolitic limestones (mainly dolomitized equivalents in the shelf areas) occurred. hoaling was prominent, and as a result local minor restricted deposition took place.
Marine limestone and dolomite sedimentation, modified by two episodes of evaporite precipitation, occurred during medial Madison group time. Mission Canyon carbonates containing an intermediate evaporite zone and overlain by the prominent basal Charles evaporites constitute the resulting middle unit. Evaporitic deposition was characterized by the precipitation of anhydrite and halite in the basinal areas at the north. Southward, progressively less restriction is demonstrated by the disappearance of halite, thinning of anhydrite, and the development of primary dense carbonate zones.
Late sedimentation was characterized by restoration of normal marine conditions and development of a complex evaporite cycle (Charles formation, excluding basal evaporites).
Big Snowy clastics were deposited in Chester time in the northern basinal areas. In other areas, pre-Amsden or pre-Pennsylvanian to pre-Middle Jurassic erosion had varying effects on the Madison group surface.
Limestone reservoirs in the basinal areas of Montana and North Dakota, and dolomites in northwestern Wyoming yield group oil. Optimum facies changes and post-depositional tectonic effects make the madison group an excellent prospective unit in many areas.
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