ABSTRACT

The Madison Limestone of west-central Wyoming forms a wedge-shaped body of carbonate strata that tapers unevenly from a thickness of approximately 900 feet at the northern end of the Wind River Mountains to about 500 feet at Wind River Canyon and less than 200 feet in the Rawlins Hills. This thinning is mainly the result of a progressive decrease in the sedimentation rate but also has been affected by removal of at least 250 feet of rock from the top of the formation at some localities by post-Madison, pre-Amsden erosion. The formation becomes progressively more dolomitic southeastward along the Wind River Mountains and eastward along the Owl Creek Mountains, but this facies trend is reversed in the Rawlins Hills.

A dolomite member is recognized at the base of the formation. Two distinctive lithic units in the upper half of the formation are interpreted as solution zones that were affected by the leaching of evaporite beds during post-Madison, pre-Amsden karst development. These solution zones are correlated with similar solution zones in the Mission Canyon Limestone in the type section of the Madison Group at Logan, Montana, and with evaporite intervals in the Mission Canyon at Hoback Canyon, Wyoming. Two faunal zones, the Spirifer cf. S. madisonensis Zone and the Diphyphyllum Zone, in the upper part of the formation are of great value for local and regional correlation.

The Sacajawea is here used in the sense of Strickland (1957) as a member of the Madison Limestone and includes the uppermost beds of the Madison. A revised interpretation of the age, correlation, and geologic significance of this unit is supported by a thorough study of the Sacajawea problem.

Corals and brachiopods collected from the Madison of west-central Wyoming suggest that the upper part of the formation is correlative with the Mission Canyon Limestone of Montana. However, the lowermost part of the formation contains Kinderhook conodonts at some localities and may be the same age as a part of the Lodgepole Limestone of Montana. The position of the boundary between Mission Canyon and Lodgepole age equivalents in west-central Wyoming is not indicated by available data.

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