THE FIRST HALF MILLION YEARS OF THE JURASSIC AS SEEN IN THE STRATIGRAPHY AND PALEOECOLOGY OF THE NEWARK SUPERGROUP OF EASTERN NORTH AMERICA

In the rift basins of eastern North America, the continental Triassic-Jurassic boundary is recognized by a level of concentrated extinction in palynomorph and vertebrate taxa, but without appreciable facies change. The boundary and associated "fern spike," negative d¹³C excursion, and Ir anomaly occur in cyclical lacustrine sequences best documented in the Newark and Fundy basins. The boundary level is succeeded by a relatively thin interval of Early Jurassic strata containing floral and faunal assemblages that are of remarkably low diversity, but contain the first truly large theropod dinosaur traces. This is followed by the oldest documented flood basalts of the Central Atlantic Magmatic Province. These in turn are overlain by cyclical lacustrine strata, most completely known in the Newark, Hartford, and Fundy basins, the lower 300 ky of which (based on Milankovitch cyclostratigraphy) are especially distinctive and well displayed in cores from the Newark and Hartford basins. All these basins contain 30 ky of well-developed limestone-bearing lacustrine cycles that are richly fossiliferous. While the assemblages are again of low diversity at the family and perhaps genus level, the fish assemblages are characterized by species flocks, and the same may be true of the theropod dinosaurs. Unlike younger Newark Jurassic age strata, those deposited in the first half million years of the Jurassic have abundant remains of the dipteraceous fern Clathropteris, many preserved in growth position, and also show high concentrations of small-leaved cheirolepidaceous conifers. Our interpretation of the paleoecology and environmental milieu of these strata is that they are representative of a post-catastrophe environment, possible during a super-greenhouse time.