ABSTRACT

A new Permian unit, the Blacktail Member of the Park City Formation, clarifies the upper boundary of the Permian in that it forms the top of the Permian sequence over a large area in western Wyoming and southwestern Montana The Blacktail is laterally equivalent to the Shedhorn Sandstone and the Ervay Member of the Park City Formation. The Blacktail Member is composed of brownish-gray, cherty dolomitic mudstone, siltstone and silty carbonate rocks. Thin beds of chert and sandstone are locally present. Fossiliferous strata are characterized as the echinauris n. sp. assemblage, which is dominated by several forms of articulate brachiopods. A shallow-water conodont fauna also characterizes the unit. The new member is Wordian to early Capitanian, (which are Late Permian according to some paleontologists or Early and Late Permian according to others) in age, similar to that of other units at the top of the Permian sequence in this area. Lithologic changes across the Permian-Tnassic boundary are not pronouced in sequences where the Blacktail Member forms the topmost Permian unit Mudstone and claystone of the Dinwoody Formation are differentiated from rocks of the Blacktail Member primarily by their noncherty and highly argillaceous character. The boundary is represented by a very sharp change in fossil assemblages. The contact with the overlying Dinwoody is represented by a disconformity. Evidence of the disconformity is subtle, but widespread. The disconformity represents a hiatus from early Capitanian (early Late Permian) to latest Griesbachian (Early Triassic).