Abstract

The Phosphoria Formation of Permian age consists of chert, carbonaceous mudstone, and phosphorite in its typical area in southeastern Idaho. These rocks intertongue with, and pass laterally into, sandstone in south-central Montana and northwestern Ywoming, carbonate rock in west-central Wyoming, and carbonate rock and subordinate sandstone in northwestern Wyoming, carbonate rock in west-central Wyoming, and carbonate rock and subordinate sandstone in northeastern Utah; thin tongues of phosphatic and cherty rocks, however, persist over all of these areas. The carbonate rocks in turn intertongue with, and pass laterally into, greenish gray and red beds in eastern Wyoming, southeastern Montana, eastern Utah, and northwestern Colorado. The plan of nomenclature developed to describe these rocks and their relationships has the following chief elements: (1) it retains the name Phosphoria Formation for the chert-mudstone-pbosphonte facies and identifies as tongues of the Phosphoria Formation rocks of these lithologic types that interfinger with sandstone and carbonate rock along the fringe of the phosphate field; (2) it retains the name Park City Formation (Permian) for the sequence of carbonate rock and sandstone in Utah, restores this name for similar carbonate rock in west-central Wyoming, and identifies as tongues of the Park City Formation beds of carbonate rock that interfinger with other formations in Idaho, western Wyoming, and Montana: and (3) the plan introduces a new name, the Shedhorn Sandstone, for sandstones of Phosphoria age in northwestern Wyoming and adjacent parts of Montana and identifies as tongues of the Shedhorn Sandstone the beds of sandstone that interfinger with the Phosphoria and Park City Formations in northwestern Wyoming and southwestern Montana.