U-PB AGE OF STROMATOLITE CALCITE FROM THE TRIASSIC PASSAIC FORMATION OF THE NEWARK BASIN

Stomatolites that encrust vegetation during lacustrine transgressions are common in the terrestrial record. We have studied one such occurrence from the Metlars Member of the Triassic Passaic Formation of the Newark Basin near Pottsdam, PA to test the potential for U-Pb dating of such materials. The sample has well developed concentric layers that show highly variable U enrichment based on phosphor imaging. Five aliquots give Pb concentrations that range from 15-681 ppm and U concentrations that range from 25-42 ppm. These five aliquots give a Total U-Pb isochron age of 208.5 ± 2.1 Ma (2 sigma uncertainty) with a MSWD of 13. The best age constraints for the Newark Supergroup are from sills and basalt flows that are constrained on the basis of biostratigraphy to the lower Jurassic. Based on Ar geochronology (Hames et al., 2000) and on U-Pb zircon and baddelyite analyses (Dunning and Hoydch, 1990) these flows and sills have been dated at about 201 ± 1 Ma. The age of the Triassic-Jurassic (T-J) boundary should be slightly older than this age. However, Palfy et al. (2000) estimates the T-J boundary at 199 ± 0.3 Ma on the basis of U-Pb zircon ages from ashes that occur in a marine section in British Columbia. The well-developed McLaughlin cycles in the Newark sequence are interpreted to represent the 404 ky Milankovitch cycle. The Metlars Member is 23 cycles below the T-J boundary. If we consider the ages of the basalts a reasonable estimate of the boundary age this would suggest the Metlars Member should be 211 ± 1 Ma. This result is within uncertainty of the U-Pb age of the stromatolite. If the Palfy et al. (2000) age of 199 for the boundary is used, the Metlars Member should be 209 Ma, precisely the age we obtain from the stromatolite. Because stromatolites are fairly common in the Newark sequence as well as in other lacustrine deposits, the possibility for obtaining U-Pb ages with a precision of 1% offers great potential for providing sorely needed time constraints for terrestrial strata. Importantly, our result is consistent with the interpretation of Milankovitch cyclicity in the Newark Supergroup.