U-PB ISOTOPIC AGE DATING OF DEVONIAN CONODONTS: A NEW METHOD FOR DATING PALEOZOIC MARINE SEDIMENTARY DEPOSITS?

Conodonts are phosphatic microfossils common in many marine deposits of Cambrian through Permian age and they provide some of the highest resolution biostratigraphic age control available for rocks of this age. U-Pb isotopic age dates from conodonts would provide much needed absolute age control for many stratigraphic successions which contain conodonts. We tested the utility of conodonts for U-Pb isotopic age dating using limestone samples from the Middle Devonian Onondaga Formation (Cherry Valley, New York) and compared the U-Pb age to that of the well dated Tioga "B" bentonite which immediately overlies the sampled limestone layer. Zircons and monazites from this bentonite have been dated by Tucker et al. (1998) and Roden et al. (1990) as 391 ±1.8 Ma and 390 ± 0.5 Ma, respectively.

Approximately 6 kg of skeletal limestone was collected and 70-120 individual conodont elements (including Polygnathus costatus costatus, Polygnathus sp. aff. P. trigonicus and Icriodus latericrescens robustus) were analyzed for each data point using standard isotopic dilution techniques. Preliminary results indicate that the conodont ages overlap with the bentonite age; the conodonts are dated as 406 ± 18 Ma. These promising results from the Middle Devonian conodonts may partially be a function of the fact that the conodonts were partially silicified soon after deposition which limited the affects of burial diagenesis.