LOW LATITUDE ISOTOPIC RESPONSES TO THE LATE PALEOZOIC ICE AGE: TRACKING ICEHOUSE CARBONATE FACIES SHIFTS WITH WHOLE-ROCK CARBONATE CARBON AND OXYGEN ISOTOPES, THE ELY-BIRD SPRING BASIN, NEVADA AND UTAH

The Ely-Bird Spring basin (EBSB) in Nevada comprises Pennsylvanian cyclothemic carbonates deposited on a shallow equatorial shelf. Cyclothems are fairly ubiquitous throughout the low paleolatitudes during the Pennsylvanian, with cyclicity interpreted as being driven by glacioeustasy during the late Paleozoic ice age. Fine-scale correlation of these strata is difficult due to both syn-depositional variation in section preservation (i.e., local missed cycle beats) and post-depositional tectonic displacement. High resolution sampling at Arrow Canyon in the EBSB has previously shown that carbon isotope variations correlate with lithologic changes, with δ¹³C shifted to heavier values toward the tops of high-frequency meter-scale lithologic cycles, superimposed upon lower-frequency decameter-scale lithologic cycles (Tierney, 2005). We tested the utility of carbon isotope cycles as a proxy for glacioeustatic sea-level cycles, which could provide an order of magnitude better correlation of cyclothems when combined with biostratigraphy.

To test the hypothesis that carbon isotope shifts mirror lithologic shifts and that these systematic shifts can be used as a correlation tool, we densely sampled six sections throughout the EBSB. Carbon isotopic data from these sections show patterns similar to the existing Arrow Canyon data set, though with variation across different isotopic baselines and with shifts of different magnitude. Differences in isotopic baselines appear to be due to basin position and other local factors affecting average δ¹³C values at the time of deposition. Though the absolute carbon isotope values vary, the isotopic shift patterns are similar. Oxygen isotope data are more susceptible to diagenetic alteration, especially near cycle boundaries, but the results also demonstrate several orders of cyclicity similar to the δ¹³C data.

Pattern matching of whole-rock isotope shifts, tied to biostratigraphy, is a feasible method for high-resolution correlation within the Pennsylvanian Ely-Bird Spring basin and has potential in other basins with glacioeustasy-driven carbonate cyclothems. Carbon is the more robust isotopic system to use for correlation, but oxygen retained enough of the original signal to use for pattern matching.