ISOTOPE AND PETROGRAPHIC ANALYSIS OF THE ONONDAGA FORMATION, CENTRAL NEW YORK

Stable isotope measurements are a powerful tool for understanding oceanic water chemistry. While δ¹⁸O values from Paleozoic carbonates may be overprinted during diagenesis, δ¹³C values can still record changes in the carbon cycle during the time of sediment deposition. Here we sampled Paleozoic carbonates from the Onondaga Formation and the Rondout, Manlius, and Kalkberg Formations within the Helderberg Group from exposures across Eastern and Central New York. Petrographic slides prepared for the Onondaga Formation samples reveal bioclastic grainstones to packstones, containing abundant micrite and fossil fragments. Most of the fossils are mollusc fragments but some samples also have crinoid columnals, suggesting that these samples are from the Edgecliff Member of the Onondaga Formation. The slides also reveal minimal sparry-calcite in the samples, suggesting minimal diagenesis, despite the presence of cracks in hand sample. Carbon and oxygen isotope analyses of the Onondaga Formation were compared to measurements from the Helderberg Group and to previous work from the Keyser, New Creek, and Corriganville Formations in West Virginia (Saltzman, 2002). Eighteen Onondaga Formation samples collected in Cherry Valley and Cazenovia, New York, have an average δ¹³C of 1.6 ‰, and average δ¹⁸O of -6.5‰ (VPDB). These values differ from those of the Kalkberg and Keyser Formations, suggesting changes in the rate of organic carbon burial in the Devonian oceans, probably due to the dramatic expansion and diversification of terrestrial plants.