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Pennsylvanian Stratigraphy of Powder River Basin and Adjoining Areas1

By
Philo C. Wilson
Philo C. Wilson
Calgary, Alberta
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Published:
January 01, 1962

Abstract

Currently accepted terminology for Pennsylvanian units in the area includes the Tensleep, Amsden, Quadrant, Minnelusa, Hartville, Casper, Ingleside, and Fountain formations. A review of nomenclatural history, lithologies, and lateral continuity of these units results in elimination of the terms Quadrant, Hartville, Casper, and Ingleside. The Minnelusa is raised to group status and is extended to include the strata formerly designated as Hartville. The Amsden is restricted vertically to the carbonate upper portion of the original formation, and Branson’s term Sacajawea is applied to the clastic formation below. The Ten-sleep, Amsden, and Sacajawea formations are extended laterally to include lithogenetically equivalent strata.

Age variations within the units suggest a southern Wyoming land mass connecting with the Black Hills until Morrowan time. Amsden-Sacajawea seas, initiated in Chesterian time to the north and southeast of this land bridge, were later joined. Tensleep sands advanced southward from Montana forcing the Amsden seas to retreat southeastward until overwhelmed by the youngest Tensleep deposits in the Hartville uplift area during Wolfcampian or Leonardian time.

Isopach variations are relatively constant from one unit to another, and show tectonically negative areas in the Lusk embayment and southwestern corner of the map area. The northern zero isopach is interpreted as an erosion edge, but areas of thinning on the Montana-Wyoming border, in the Black Hills, and in southern Wyoming are thought to be related to tectonic instability or position of former land masses. A superjacent strata map illustrates post-depositional erosion intervals by showing the age of the overlying formations.

Lithofacies analyses reveal a southern and southwestern source for the Tensleep, Amsden, and Sacajawea sediments. Reef-structures are postulated in the Amsden on the basis of lithofacies patterns, and coincide with the margins of evaporite distribution. A lithofacies map of the entire sequence emphasizes the masking effect introduced when this technique is used with large units involving long depositional intervals.

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AAPG Special Publication

Pennsylvanian System in the United States

Carl C. Branson
Carl C. Branson
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American Association of Petroleum Geologists
ISBN electronic:
9781629812373
Publication date:
January 01, 1962

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