δ¹³C TRENDS FROM NEW YORK'S ONONDAGA FM.: GLOBAL OR LOCAL? IMPLICATIONS FOR STRATIGRAPHY

Carbon isotope (δ¹³C_carb) values were determined from carbonate deposits in the Onondaga Formation of the Appalachian foreland basin in order to help fit the Early-Middle Devonian (Emsian-Eifelian) strata of New York into a global stratigraphic framework. δ¹³C_carb values representing the lower part of the Onondaga (Edgecliff Member) were sampled at the Neid Road quarry near the town of LeRoy, eastern New York and values through the entire Onondaga (Edgecliff-Seneca Members) were obtained from a New York State Geological Survey drill core from the central Finger Lakes region of New York. New York values are compared to δ¹³C_carb from shelf deposits preserved in the Northern Antelope Range, Nevada, and from central and southern European sections of Buggisch & Mann (2004), and van Geldern, et al. (in press).

The δ¹³C_carb trend through the late Polygnathus costatus patulus-early Polygnathus costatus partitus zones in the Neid Road quarry section is steady, which agrees with European values of this time interval (Buggisch & Mann, 2004). Data from the drill core also show the steady values observed at the quarry, followed by a well defined 3 ‰ decrese in δ¹³C_carb through the Polygnathus costatus costatus zone. These trends are similar to those found in Northern Antelope Range, Nevada (Saltzman, 2000), as well as recently obtained data from Morocco, Spain and the Eifelian type section in Germany (van Geldern, et al, in press).

In contrast to the relatively pure limestones of the Onondaga, the underlying and overlying units Onondaga in the drill core (Tristates Group and Hamilton Group, respectively) consist of dark, organic rich, argillaceous limestones to calcareous mudstones, with dark shaly interbeds. The occurrence of organic rich strata coincident with a decrease in δ¹³C_carb values can be explained by the upwelling of nutrient rich deep water into shallower settings, depositing fine organic rich material while simultaneously inundating surface waters with ¹²C, thus, shifting the δ¹³C_carb baseline to lighter values. However, because these trends have been identified worldwide, a global cause cannot be ruled out.