AGE AND SIGNIFICANCE OF THE SEQUENCE BOUNDARY AT THE BASE OF THE EUREKA QUARTZITE IN CENTRAL NEVADA

Middle Ordovician quartzites have long been recognized in the Great Basin region as a prominent interruption of the thick and otherwise carbonate-dominated Paleozoic sequence. Zimmerman and Cooper (1999) placed the Eureka Quartzite of southern Nevada and southeastern California in a sequence stratigraphic framework and suggested that the sand was derived from the western part of the transcontinental arch during a major regression. However, the relative roles of eustasy versus local tectonics in exposing siliciclastic source areas has not been addressed, due in large part to the imprecise biostratigraphic control on the basal Eureka sequence boundary. Study of a thick and relatively conformable sequence through the top of the underlying Copenhagen Formation in the Antelope Range indicates that the base of the Eureka is no older than middle Mohawkian (early Chatfieldian) based on carbon isotope stratigraphy. This is in good agreement with the conodont biostratigraphy that suggests that the topmost Copenhagen represents the undatus-tenuis conodont zones. Preliminary carbon isotope data record values as high as +3.8 per mil in the upper Copenhagen Formation and reveal the presence of the well-established excursion described from the Guttenberg Limestone of the U.S. midcontinent, Lexington Limestone of Kentucky and the Salona and Coburn formations of the central Appalachians. This is consistent with correlation of the Eureka Quartzite with units assigned to the Taconic clastic wedge that drowned the carbonate factory in parts of the Appalachians (e.g., Antes and Reedsville Shales). If further study indicates that these sequence boundaries on opposite sides of Laurentia are the same age, a role for eustasy (possibly driven by ice buildup or plate interactions) seems likely.