A Late Frasnian (Late Devonian) Radiolarian, Sponge Spicule, and Conodont Fauna from the Slaven Chert, Northern Shoshone Range, Roberts Mountains Allochthon, Nevada

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Co-occuring conodonts, radiolarians, and sponge spicules from the type locality of the Slaven Chert, northern Shoshone Range, Nevada, indicate that the radiolarian and sponge spicule assemblage described herein correlates with the Late rhenana conodont Zone (late Frasnian). The moderately well preserved radiolarians are the first Frasnian-age fauna described from the Western Hemisphere. They include spumellarians, Ceratoikiscum, and Paleoscenidium, and a radiolarian which we have assigned to a new genus, Durahelenifore Boundy-Sanders and Murchey, with its type species, Durahelenifore robustum Boundy-Sanders and Murchey. Sponge spicules include umbellate microscleres of the Subclass Amphidiscophora, Order Hemidiscosa, previously documented only in Pennsylvanian and younger rocks. INTRODUCTION Most of the documentation of Devonian radiolarian faunas, and all of the documentation of Western Hemisphere faunas, has been of Famennian taxa (Foreman 1963; Coles and Snyder 1985; Cheng 1986; Aitchison 1990; Coles 1991; Noble 1992; Aitchison, in press). Previously, the only well-dated Frasnian faunas have been from the southern Urals (Nazarov 1975), the Gogo Formation of Western Australia (Nazarov et al. 1982; Nazarov and Ormiston 1983; Aitchison, 1993), from the Yarrimie Formation near Tamworth, New South Wales, eastern Australia (Aitchison 1990), and from the Liukiang Formation, eastern and southern Guangxi Province, China (Li and Wang 1991). The fauna described in this paper is the first Frasnian fauna described from the Western Hemisphere. We focus on samples from the type locality of the Slaven Chert in Nevada, but the most distinctive and previously undescribed species in the assemblage also occurs in the Barita de Sonora district near Hermosillo, Mexico, and in the Yugambal terrane of the eastern New England orogen, Western Australia (J.C. Aitchison, written commun. 1992). A similar radiolarian occurs in the Gamilaroi terrane of the New England orogen, Eastern Australia, in rocks thought to be Middle Devonian in age (J.C. Aitchison, written commun. 1993). The Nevada locality has yielded not only sponge spicules and the new radiolarian species, but also an abundance of conodonts which indicate that this assemblage correlates with the late Frasnian Late rhenana Zone (Ziegler and Sandberg 1990). This *Present address: 17859 149th Ave. NE, Woodinville WA 98072-6202 co-occurrence provides an important tie between radiolarian and conodont biochronologies. The new radiolarian, Durahelenifore robustum n. gen. and n. sp., shares many characteristics with the genus Helenifore. We suggest that the heleniforids may be diagnostic of the upper part of the Frasnian Stage, although they do not occur in every deep-water Frasnian assemblage (Li and Wang 1991). The occurrence of hemidiscid sponge spicules in the Slaven Chert samples extends the range of this spicule morphology downward from its previous lowest known occurrence in Pennsylvanian rocks (Finks 1983). Previously published radiolarian biostratigraphies cover Famennian and higher strata (Holdsworth and Jones 1980a, b; Miller et al. 1984; Cheng 1986 and Holdsworth therein; Schwartzapfel 1990). A radiolarian biostratigraphy for Frasnian and lower strata awaits the discovery and study of coherent, continuous sections. GEOLOGIC SETTING The Devonian Slaven Chert traditionally has been regarded as the youngest formation in the Roberts Mountains allochthon (Madrid 1987; Gilluly and Gates 1965). The Roberts Mountains sequence was deposited in a basin that may be described as episodically sediment-starved: cherts of Ordovician, Silurian, and Devonian age alternate with numerous and in places voluminous sequences of clastic rocks. Although the connection between the Roberts Mountains allochthon and other North American sequences is well accepted, several lithostratigraphic, sedimentological, and faunal differences warn against drawing an overly facile correlation between the Roberts Mountains allochthon and the distal miogeoclinal section of east-central Alaska (Turner, Madrid, and Miller micropaleontology, vol. 45, no. 1, pp. 62-68, text-figures 1-2, plates 1-2, table 1, 1999 62 This content downloaded from 207.46.13.120 on Wed, 14 Sep 2016 04:13:00 UTC All use subject to http://about.jstor.org/terms Micropaleontology, vol. 45, no. 1, 1999