STRATIGRAPHY AND PALEONTOLOGY OF THE LOWER DINWOODY FORMATION, AND ITS RELATION TO THE PERMIAN-TRIASSIC BOUNDARY IN WESTERN WYOMING, SOUTHEASTERN IDAHO AND SOUTHWESTERN MONTANA.

摘要

Regional stratigraphic analysis of uppermost Permian and lowermost Triassic rocks has clarified the Permian-Triassic boundary in western Wyoming and adjacent states. Controversial Permian rocks, formerly assigned to the Dinwoody, are recognized as part of the newly-defined Blacktail Member of the Park City Formation, and the systemic boundary is maintained at the base of the Dinwoody Formation.; The Blacktail Member of the Park City Formation is composed of light-colored cherty mudstone and siltstone not previously recognized as typical of the topmost Permian sequence. The type sequence is 7m thick in the type area along the West Fork of Blacktail Deer Creek in the Snowcrest Range of southwestern Montana. Fossiliferous Blacktail rocks are characterized by productoid and rhynchonelloid brachiopods of the Echinauris sp. A assemblage which is late Wordian to early Capitanian in age.; The mudstone member of the Dinwoody Formation forms the base of the Triassic sequence over most of the study area. Basal Triassic strata are composed predominantly of yellowish-grey dolomitic mudstone, claystone and siltstone. Fossils of the mudstone member are categorized as the Lingula borealis assemblage which is dominated by linguloid brachiopods and pteriomorph pelecypods. Fossils of the lower Dinwoody define one of the oldest Triassic assemblages in North America, and are considered Late Griesbachian in age.; Lithologic changes across the systemic boundary are not abrupt where the Blacktail Member forms the topmost Permian unit. Dinwoody rocks are differentiated from those of the Blacktail Member primarily by their non-cherty and highly argillaceous character. Blacktail strata are readily distinguished from the Dinwoody on the basis of sharp differences in fossil assemblages. The stratigraphic segregation of Permian and Triassic taxa is well-defined, and all species and most genera are restricted to their respective assemblage.; The Permian-Triassic boundary is considered disconformable and represents a major gap in the rock record of the Wyoming-Idaho-Montana area