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The Supai Formation in east-central Arizona, strategic for an understanding of the Permian of southwestern United States because it lies in a little-known area between the better-known regions of Grand Canyon and west Texas, is readily divisible into four members (from base to top): the Amos Wash (new), Big A Butte (new), Fort Apache Limestone, and Corduroy (new) members.

Sedimentary and stratigraphic data suggest that the Supai Formation was deposited marginal to and in a shallow epeiric sea under warm and dry conditions. Regional relationship and the cyclical nature of the deposits indicate eustatic fluctuations and periodic encroachment of a Permian sea from the south.

A dominantly molluscan fauna collected from the Fort Apache Limestone consists of silicified specimens obtained by dissolving approximately a ton of limestone in acid. These forms are described for the first time. Forms from limestone blocks collected at close vertical intervals, at two of the four collecting localities where complete thicknesses were preserved show little variation. Conditions were probably uniform throughout the area during deposition of the limestone.

Stratigraphic and paleontologic comparisons with other Permian sequences in southwestern United States indicate that the Supai Formation is Leonardian and Wolcampian, and that the Fort Apache fauna is Leonardian.

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