Sequence and chemostratigraphic study across the basal Eureka Quartzite unconformity in the Great Basin, Nevada: Implications for the origin of the Late Ordovician carbon isotope excursion

摘要

The positive carbon isotope (δ 13 C) excursion from the late Middle Ordovician Copenhagen Formation that underlies the Eureka Quartzite in central Nevada has been hypothesized to record a global cooling event ∼10 Myr prior to the well-recognized Late Ordovician Hirnantian glaciation (∼443 Ma). However, direct evidence for pre-Hirnantian continental glaciation remains scarce, raising uncertainties in the relationship between the inferred eustatic sea-level fall and the origin of the positive δ 13 C excursion. Additionally, previous stratigraphic studies from southern Nevada and California suggest relative sea-level fall in excess of ∼ 150 meters, which exceeds estimates for younger glacial periods (e.g., Hirnantian: ∼45-60 m, Late Paleozoic: ∼ 60-75 m, and Last Glacial Maximum: ∼125 m of sea-level fall) despite the paucity of glaciogenic deposits at this time.An integrated sequence and chemostratigraphic study across the basal Eureka Quartzite unconformity and underlying carbonate strata (Antelope Valley Limestone) indicates that the Copenhagen Formation in central Nevada was most likely deposited in a fault-controlled sedimentary basin. The absence of a comparable positive δ 13 C excursion in sections away from the Monitor and Copenhagen valleys suggests that the δ 13 C excursion reported from the Copenhagen Formation in central Nevada was likely a localized signal formed through carbon cycling (e.g., high biologic production and eutrophication) in a restricted, geographically limited formation that was deposited during the period of exposure and karstification of the paleo-shelf in southern Great Basin locations.