Fusulinid and conodont biostratigraphy of the Upper Pennsylvanian–Cisuralian Riepe Spring Limestone, Spruce Mountain Ridge, Elko County, Nevada, U.S.A.

Abstract

The Upper Pennsylvanian-Cisuralian (lower Permian) mixed carbonate-siliciclastic strata exposed at Spruce Mountain Ridge, Elko County, Nevada, record the shallow-marine microfauna and oscillating depositional history of the Ferguson Trough during the final glacial interval of the late Paleozoic ice age. The 420 m thick North Spruce Mountain Ridge (NSMR) section includes the uppermost part of the Ely Limestone, the Riepe Spring Limestone, and the base of the Rib Hill Formation. A discontinuous Pennsylvanian-Permian transition is preserved within the approximately 390 m thick Riepe Spring Limestone along the west side of Spruce Mountain Ridge, where an erosional disconformity separates upper Gzhelian (Upper Pennsylvanian) strata of the middle informal member of the Riepe Spring Limestone from Asselian(?)-Sakmarian (lower Cisuralian) deposits of the upper informal member of the Riepe Spring Limestone. Age control of the system-bounding erosional vacuity and/or hiatus is determined from the integration of co-occurring fusulinid and conodont faunas. Six fusulinid assemblage zones are recognized in the uppermost part of the Ely Limestone and the overlying Riepe Spring Limestone at Spruce Mountain Ridge. Upper Pennsylvanian zones include the Eowaeringella-Triticites Group I assemblage zone, Triticites Group II-Dunbarinella assemblage zone, and the Triticites Group IV assemblage zone. Cisuralian zones include the Schwagerina wellsensis assemblage zone, Eoparafusulina linearis assemblage zone, and the “advanced” Schwagerina assemblage zone. Descriptions of 38 fusulinid species identified from loaned Smithsonian thin sections and recently collected material are provided. Additionally, eight species of Late Pennsylvanian conodonts such as Idiognathodus and Streptognathodus (i.e., idiognathodids) and 12 species of Cisuralian Sweetognathus and Neostreptognathodus (i.e., sweetognathids) are described from the sparse conodont faunas recovered from the Riepe Spring Limestone.