IMPLICATIONS OF RADIOMETRIC AGES FROM STROMATOLITES, COPROLITES, AND CALICHES FROM THE NEWARK AND HARTFORD TRIASSIC-JURASSIC RIFT BASINS

One of the striking aspects of early Mesozoic rift basin strata of eastern North America is the prevalence of cyclical lacustrine strata interpreted as the result of Milankovitch climate forcing. In the Newark and Hartford basins, this cyclicity has been used as the basis for an astronomically calibrated geomagnetic polarity time scale. While the Milankovitch character of this cyclicity is corroborated by multiple workers, the interpretations have always suffered from a lack of a time scale independent of the cyclicity itself. The only radiometric dates from the sequences have been from earliest Jurassic basalt flows and associated intrusions that come from a very brief (<1 m.y., centered at about 201 Ma) interval in the 32 m.y. long sequence. Recently a series of U-Pb radiometric ages have been obtained from three sedimentary and fossil material that provide independent age constraints on the cyclical sequences. First, a tree-encrusting stromatolite (Whiteside, 2004, in Lowman, and Rinker, Forest Canopies, 2nd edition. Academic Press, 147-149) from the Metlars Member of the Passaic Fm. of the Newark basin provides an age of 208.5 ± 2.1 (Rasbury et al, 2003, Geol. Soc. Amer., Abst. Progr., 35(6):508); second, calcite from a caliche in the lower New Haven Fm. of the Hartford basin has yielded an age of 211.9 ± 2.1 Ma (Wang et al., 1998, Geochim. Cosmochim. Acta 62:2823-2835); and third, a coprolite from the Early Jurassic age Shuttle Meadow Fm. has an age consistent with the basalt flows that surround the formation. These ages, plus a U-Pb tuff age of 227.8 ± 0.3 Ma (Rodgers et al., 1993, Science 260:794-797) from the Ischigualasto Fm. correlated biostratigraphically to the Newark basin, are consistent within error with and provide the first independent corroboration of the Milankovitch cyclostratigraphic ages. Radiometric ages from such unconventional materials have great promise to calibrate the cyclostratigraphy of the Newark Supergroup and by extension, provide a time scale for the global Late Triassic and Early Jurassic.