NEW REPORT OF ELEVATED IR IN UPPER TRIASSIC-LOWER JURASSIC STRATA OF THE NEWARK SUPERGROUP BASINS

We report new data on the occurrence of Ir in strata of the Newark Supergroup basins. To date, elevated Ir levels in continental sediments proximal to the Triassic-Jurassic boundary (TJB) have been reported only from Upper Triassic strata of the Newark and Fundy basins, below the basal extrusions of the Central Atlantic Magmatic Province. Here we present the first report of the occurrence of elevated Ir above the oldest volcanic units, as well as additional horizons of Ir enrichment from other basins of the Newark Supergroup. The uppermost New Haven Formation (Upper Triassic) at the Silver Ridge locality (Guilford, CT) in the Hartford Basin contains abundant plant debris, but no evidence of elevated Ir. At the Clathopteris locality to the north (Southampton, MA), potentially correlative strata that are fine grained and rich in plant remains have Ir enriched to 542 pg/g, an order of magnitude higher than in the coarser-grained strata in direct stratigraphic contact. The high-Ir beds also have elevated Rb, Yb and Ta relative to other Hartford Basin samples. The Deerfield Basin (Massachusetts) is a structural extension of the Hartford Basin that contains only one CAMP extrusive unit, the Lower Jurassic Deerfield Basalt. Modest Ir enrichment, up to 90 pg/g, occurs in the Fall River Beds of the Sugarloaf Formation, several meters below the basalt, and up to 70 pg/g in the Turners Falls Formation less than 2 meters above the basalt. In the Fundy Basin (Nova Scotia, Canada), lacustrine sediments of the Scots Bay Member of the McCoy Brook Formation that directly overlie the North Mountain Basalt contain Ir up to 413 pg/g in fish-bearing strata very close to the palynological TJB. Higher in the formation the strata lack significant Ir enrichment. Sedimentary strata from between flows of North Mount Basalt show no Ir appreciable enrichment. We consider the basalts of the Central Atlantic Magmatic Province, widely accepted as the driver of Late Triassic extinctions, the origin of the elevated Ir levels in the Newark Supergroup, potentially through aerosol fallout.