HOW CONODONTS, BRACHIOPODS, CARBON ISOTOPES, AND SEQUENCE STRATIGRAPHY MOVED THE CAMBRIAN-ORDOVICIAN BOUNDARY IN SOUTHERN NEVADA

Upper Cambrian–Lower Ordovician strata in southern Nevada north and east of the Las Vegas Shear Zone in the Sheep Range, Arrow Canyon Range, Meadow Valley Mountains., and Delamar Mountains. have had little attention since the 1950’s and 1960’s. In these early map-scale studies, the Cambrian–Ordovician boundary was assumed to be at the top of dark gray dolomites, which was taken to be the Nopah Formation-Pogonip Group contact.

We have detailed biostratigraphic (conodonts, phosphatic brachiopods) and carbon-isotope data from these strata primarily from the Delamar Mountains. These data reveal that the top of the Nopah Formation in eastern Nevada is the top of the Proconodontus posterocostatus Zone, nearly 90 m below the base of the Laurentian Ibexian Series (base of Cordylodus proavus Zone). The base of the Ordovician System (base of the Iapetognathus fluctivagus Zone) is five conodont zones above the top of the Nopah Formation. This indicates that the lower 160 m of the Ordovician Pogonip Group are, in fact, Late Cambrian in age.

Our isotopic and biostratigraphic correlations allow precise identification of sequence stratigraphic surfaces originally recognized in the Utah succession, resulting in the correlation of thin units ranging from 5 to 20 m thick. These Nevada strata correlate with the six members of the Notch Peak and House formations in western Utah, and major lithostratigraphic reassignment is indicated.

Such high-resolution correlations reveal remarkable similarities in the details of lithologic succession between Nevada and Utah, despite facies differences that confounded earlier attempts at correlation. For example, certain types and forms of microbialite buildups are found at predictable horizons in both areas. These include one horizon that locally contains one of the five oldest known Anthaspidellid sponge reef mounds in the world. This supports the hypothesis that sea-level fluctuations strongly influenced the evolution of shallow marine ecological communities.

The recognition of thin, distinctive cherty horizons over distance and across facies changes suggests that cherty zones result from a basin wide or extrabasinal influence. Some chert has gross characteristics suggestive of derivation from eolian sand, possibly linked with drier climate or sea level low stand.