IMPLICATIONS OF HIGH-RESOLUTION BIOSTRATIGRAPHY, AND MORPHOGUILD ANALYSIS OF UPPER ORDOVICIAN-LOWER SILURIAN CONODONT DISTRIBUTION IN CARBONATES FROM CALIFORNIA, NEVADA, AND UTAH, USA: PALEOBIOGEOGRAPHIC, LITHOSTRATIGRAPHIC, AND STABLE ISOTOPE CHEMOSTRATIGRAPHIC ASSOCIATIONS

Over three decades of collection, Katian through Llandoverian conodont faunas have been recovered from lowermost to middle Tippecanoe miogeoclinal carbonates of California, Nevada, and Utah. The faunas are diverse and elemental abundance is amazing, when compared with earlier reports that they were practically nonexistent in the western US. Species represented in those collections provide a high-resolution biostratigraphic framework for both global and regional correlation. Paleogeographic variance of those collections indicates that position in miogeoclinal transects from epeiric margins to the upper slope/distal shelf has a great influence on faunal composition, only secondary to stratigraphic position. Stratigraphic variance appears to have two major controls, first evolution, followed by a close second, paleoecologic migration.

Conodont elements recovered from shallow shelf dolomites at those localities generally typify the Maysville, Eden, and Richmond stages of the classic western Midcontinent faunas of the North American Cincinnatian Series. Conodont faunas identified from locations of the outer shelf/upper slope are likely to contain species more common with the Ohio/Kentucky Cinncinnatian Stratotype, as well as species of the North Atlantic/Baltic Provinces.

Identification of terminal Ordovician faunas at several locations indicates an incursion of more offshore forms into more onboard localities. Those incursions include laterally compressed M element-bearing species, and sharply costate coniform-bearing species. The presence of Gamachignathus sp,, which forms an acme zone (i.e. Fauna 13) of the Gamachian (=Hirnantian), with species typifying the latest Richmondian faunas of the Cincinnatian occur at several outboard localities. This acme of laterally compressed M element-bearing species seems to co-occur with the HICE (i.e. the Hirnantian Isotopic Carbon Excursion) at several localities where isotopic variance of that interval has been tested, e.g. the Nopah Range in eastern California, and the South Egan Range (83LE) of central Nevada. The highest resolution correlation of 83LE with the Ordovician Composite Standard Section (CSS) suggests that HICE might actually extend into the Richmondian, making the Gamachian (=Hirnantian) faunas coeval to the more epeiric/shelfal faunas of the North American Richmond.