A POSSIBLE RECORD OF THE LATE ORDOVICIAN POSITIVE CARBON ISOTOPE EVENT IN THE ELY SPRINGS DOLOMITE, NOPAH RANGE, CALIFORNIA

A detailed stratigraphic profile of carbonate d¹³C was obtained from a >200m section of the Ely Springs Dolomite (ESD) in the northern Nopah Range, California. d¹³C values in the lower half of the section are generally positive with limited variation. d¹³C values in the upper part of the section show a coherent shift towards negative values beginning at 142m, then trend towards positive values and reach a near-maximum of ~+2.0 ‰ (vs. PDB) at 174.5m. Higher units show rapid oscillation in d¹³C between 0 and +2 ‰. Although more definitive tests are pending, limited covariance of d¹³C and d¹⁸O values support an inference that d¹³C values may represent Late Ordovician secular variation.

The d¹³C profile suggests a possible correlation between the Nopah ESD and the Hanson Creek Formation in the Monitor Range (Nevada). The uppermost Hanson Creek preserves d¹³C values >+6 ‰ during the latest Ordovician glacioeustatic lowstand. Immediately before the rise in d¹³C values (and sea level fall) in the Hanson Creek, d¹³C values fall to –1.6 ‰ within an interval of sharp lithologic change. A similar chronology is seen in the Nopah Range ESD section. d¹³C values fall immediately above a (more subtle) variation in ESD lithology, then rise to a local maxima in conjunction with the occurrence of cold-water conodont faunas at 175m. The absence of very high d¹³C values in the Nopah Range ESD succession suggest a cryptic disconformity immediately above 175m.

The correlation is currently speculative. The ESD d¹³C profile is not fully diagnostic due to the absence of very positive values found in some latest Ordovician successions. However, other contemporaneous successions show truncation of rising d¹³C values, followed by earliest Silurian faunas and lower d¹³C values. If further work can substantiate the correlation, the Nopah Range ESD succession may preserve a nearly complete record of d¹³C values in the immediately pre-glacial interval, which is crucial to constraining paleoclimatic models of the Late Ordovician glacial epoch.