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Regional correlations of the middle member of the Minnelusa Formation (middle Upper Pennsylvanian) shed new light on the stratigraphic relationships of Pennsylvanian rocks in the central Powder River basin and serve as the basis for a regional depositional model.
The middle Minnelusa (Leo section) stratigraphy is best known in the southeastern part of the study area. To the north, the upper boundary (Red Marker shale) correlates with the base of the "B" dolomite interval, an upper Minnelusa marker. The underlying "C" sandstone of the upper Minnelusa is therefore Pennsylvanian in age. To the west the Red Marker becomes silty and disappears into the sand of the Tensleep Formation along the eastern Bighorn Mountains. There the middle Minnelusa is equivalent to the lower three-fourths of the Tensleep.
The two main factors influencing the depositional environments of the middle Minnelusa were paleogeography and sea level change. In the southeastern and south-central parts of the area, the proximity of the Lusk embayment and fluctuations in sea level caused frequent flooding of a broad, flat sabkha surface over which isolated dune complexes were migrating. The sediments deposited were subtidal carbonates and black shales interbedded with supratidal dolomites, evaporites, and eolian sandstones. Farther north, closer to the sand source, the dune complexes were larger, more continuous, and were affected only by major transgressive events. Laterally extensive eolian sands were also deposited to the west. The lack of black shales and presence of sandy dolomites with Skolithus burrows, how ver, suggests more normal marine conditions and deposition on a sand-dominated tidal flat.
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