Cordilleran Section - 97th Annual Meeting, and Pacific Section, American Association of Petroleum Geologists (April 9-11, 2001)

Paper No. 0
Presentation Time: 2:50 PM


RIPPERDAN, Robert L., Univ Puerto Rico, PO Box 9017, Mayaguez, PR 00681-9017, BUGGISCH, Werner, Roethanger 35, Uttenreuth, 91080, Germany, COOPER, John D., Department of Geological Sciences, California State Univ Fullerton, PO Box 6850, Fullerton, CA 92834-6850, LEHNERT, Oliver, Institute for Geology and Mineralogy, Univ of Erlangen, Schlossgarten 5, D-91054, Erlangen, Germany and LEATHAM, W. Britt, Geological Sciences, California State Univ San Bernardino, San Bernardino, CA 92407-2397,

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.