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TitreTowards the definition of the Carnian/Norian boundary: new data on ammonoids and conodonts from central Nevada
AuteurBalini, M; Orchard, M J; Jenks, J A; Silberling, N J; Martin, R
SourceProceedings of the 21st Canadian Paleontology Conference; par Haggart, J W (éd.); Smith, P L (éd.); Canadian Paleontology Conference Proceedings no. 9, 2011 p. 9-10
Séries alt.Secteur des sciences de la Terre, Contribution externe 20120097
Réunion21st Canadian Paleontology Conference; Vancouver; CA; août 19-21, 2011
Documentpublication en série
Sujetsassemblages fossiles; distribution de fossiles; fossiles; Carnien; Norien; zones fossilifères; Trias supérieur; paléontologie; Mésozoïque; Trias
Programmebassins sédimentaires du Yukon, GEM : La géocartographie de l'énergie et des minéraux
Résumé(disponible en anglais seulement)
The Berlin-Ichthyosaur State Park in central Nevada is a very important locality for the understanding of the Carnian-Norian boundary (CNB) in North America. Rich ammonoid faunas from this site within the Luning Formation were studied by Silberling (1959) and provided support for the definition of the Schucherti and Macrolobatus zones of the latest Carnian, which are here overlain by well preserved faunas of the earliest Norian Kerri Zone. Despite its importance, no further investigations have been done at this site during the last 50 years. Here we present the preliminary data of a new bed-by-bed sampling for ammonoids and conodonts carried out in West Union Canyon during October 2010. The eastern side of the canyon provides the best record of the Macrolobatus Zone, which is represented by several beds yielding ammonoids of the Tropites group, together with Anatropites div. sp. Conodont faunas from both these and higher beds are dominated by ornate 'metapolygnthids' that would formerly have been collectively referred to Metapolygnathus primitius, a species long known to straddle the CNB. Within this lower part of the section, they resemble forms that have been separated as Metapolygnathus mersinensis. Slightly higher, forms close to 'Epigondolella' orchardi and a single 'Orchardella' n. sp. occur. This association can be correlated with the latest Carnian in British Columbia.
Higher in the section, the ammonoid fauna shows a sudden change and is dominated by Tropithisbites. Few tens of metres above, but slightly below the first occurrence of Norian ammonoids Guembelites jandianus and Stikinoceras, two new species of conodonts (Gen et sp. nov. A and B) appear that also occur close to the favoured Carnian/Norian boundary at Black Bear Ridge, British Columbia. Stratigraphically higher collections continue to be dominated by forms close to M. mersinensis and 'E.' orchardi.
The best exposure of the Kerri Zone is on the western side of the West Union Canyon. Ammonoids, dominated by Guembelites and Stikinoceras div. sp., have been collected from several fossil-bearing levels. Conodont faunas replicate those of the east section. The collected ammonoids fit perfectly well with the faunas described by Silberling in 1959, but they differ somewhat from coeval faunas of the Tethys and Canada. The genus Gonionotites, very common in the Tethys and British Columbia, is for the moment unknown in Nevada. More in general, the Upper Carnian faunas are dominated by Tropitidae, while Juvavitidae are lacking.