THE UNCONFORMITY AT THE BASAL EUREKA QUARTZITE IN NEVADA AND CALIFORNIA: IMPLICATIONS FOR SEA-LEVEL CHANGE AND THE INITIATION OF LATE ORDOVICIAN GLACIATION

The positive carbon isotope excursion from the late Middle Ordovician Copenhagen Formation that underlies the Eureka Quartzite has been hypothesized to record the initiation of the Late Ordovician climatic transition ~10 Myr prior to the well-recognized Hirnantian glaciation (~443 Ma). However, direct evidence for pre-Hirnantian continental glaciation remains scarce, raising uncertainties on the relationship between sea-level change, glaciation, and the origin of the Chatfieldian positive carbon isotope excursion.

In southern Nevada and eastern California, the Copenhagen Formation and d¹³C excursion are partially or completely missing. In addition, physical stratigraphic studies in this region have documented sand-filled karstic breccias and grikes locally associated with the basal Eureka Quartzite, extending downward ~120 m into the underlying Pogonip Group. If the absence of the Copenhagen Formation and positive d¹³C excursion resulted from erosion, and the karstic features record an additional amount of sea-level fall, then a total of >180 m base-level fall is implied. Considering the lack of an extensive glacial record during Chatfieldian time, the magnitude of base-level fall necessary to form these karstic features is inconsistent with a glacio-eustatic origin, and requires a tectonic influence as well. An alternative explanation for the absence of the Copenhagen Formation in southern Nevada and eastern California could be related to the presence of ancient regional highlands or formation of an embayment. If this proves to be correct the Copenhagen Formation of central Nevada would have been deposited in a localized or restricted basin, and the positive d¹³C excursion may not have global significance. An integrated physical and chemostratigraphic study across the late Middle to Late Ordovician strata of central Nevada and California is needed to further clarify the Middle-Late Ordovician greenhouse to icehouse transition.