STRATIGRAPHY AND AMMONITE ZONES OF THE LOWER TRIASSIC THAYNES GROUP, SOUTHERN ELKO COUNTY, NEVADA

In southern Elko County, Nevada (T29-31N, R64-65E), an ~ 470-m-thick section of the Lower Triassic Triassic Thaynes Group crops out as an essentially homoclinal, northeasterly-dipping section just north of Currie, and in a SW-NE-trending syncline in the Southern Pequop Mountains. Most of the Thaynes section is shale with sparse, thin beds and nodules of limestone (~ 84% of the section), and a minority (16%) of the section is limestone, most of which is wackestone. Calcareous shale at the base of the Thaynes sharply overlies cherty, brachipod- and bryozoan-rich wackestones of the Lower Permian Phosphoria Formation; and fluvial, crossbedded conglomerate and sandstone of the Upper Triassic Shinarump Formation of the Chinle Group overlie calcareous siltstones and algal wackestones at the top of the Thaynes. In southern Elko County, Early Triassic ammonites occur at three intervals in the Thaynes: (1) Smithian Meekoceras assemblage, a 10-m-thick interval of limestone that begins only 11 m above the Phosphoria-Thaynes contact; (2) Spathian Tirolites assemblage, a 6-m-thick interval of black shale located about 230 m above the Thaynes base; and (3) late Spathian “Keyserlingites” assemblage, a 14-m-thick interval of limestone located about 370 m above the Thaynes base.The Phosphoria-Thaynes unconformity thus encompasses all of Middle and Late Permian time and at least half of the Early Triassic, contrary to recent suggestions that there is a more continuous Permian-Triassic section preserved in the Basin and Range. No Middle Triassic rocks are preserved in southern Elko County, and the Thaynes-Shinarump unconformity, which is the base of the Chinle tectonosequence, represents all of Middle Triassic and the earliest part of Late Triassic time.