Revised Stratigraphy and Structural Relationships in the Roberts Mountains Allochthon of Nevada (United States) Based on Radiolarian Cherts

ABSTRACT
Seven radiolarian assemblages are currently recognized from
the Roberts Mountains allochthon, spanning from Late Ordovician
through Late Devonian. These assemblages fill in existing stratigraphic
gaps for which fossil data are sparse, reveal local structural
complexities, and clarify stratigraphic relations within a complexly
deformed and poorly exposed orogenic belt. The oldest assemblage
is the Late Ordovician (late Caradocian) Haplotaeniatum spinatum
assemblage, which occurs in the upper Vinini Formation in the PK
chert in the Tuscarora Mountains and a white porcellanite in the
Roberts Mountains. A 1- 2 m lenticular chert bed is found stratigraphically
higher in the upper Vinini formation and contains the
Kalimnasphaera assemblage, a younger Late Ordovician assemblage.
The Kalimnasphaera assemblage also occurs in shelfal rocks
of the lower Hanson Creek Formation at Copenhagen Canyon. The
Cherry Spring chert is a varicolored chert that is commonly found
above the Vinini Formation and contains a distinct early Silurian
(Llandoverian) fauna, the Haplotaeniatum Ð Orbiculopylorum
assemblage. The Cherry Spring chert is now recognized throughout
the Northern Adobe, Independence, and Sulfur Springs Ranges, in
the Tuscarora Mountains, and tentatively in the Roberts Mountains.
The Elder Sandstone and equivalents presently yield one distinct
assemblage, the Late Silurian Devoniglansus assemblage that occurs
in porcellanite beds in the Independence and Shoshone Ranges.
Three Devonian assemblages are currently recognized and within
the Slaven Chert and equivalent units: the Middle Devonian
(Eifelian) Ceratiokiscum lyratum assemblage, the Late Devonian
Durahelenifore (late Frasnian) assemblage, and Late Devonian (late
Famennian) Holoeciscus assemblage. At present, the Durahelenifore
assemblage is recognized only in the northern Shoshone
range, and the Ceratiokiscum lyratum and Holoeciscus assemblages
are recognized within the northern Adobe and Independence ranges,
and the Roberts Mountains. Future biostratigraphic work will focus
on refining and correlating the Devonian stratigraphy within the
allochthon and on attempting to extract identifiable radiolarians
from the older cherts in the Vinini and Valmy Formations.