Distribution and Origin of Iridium in Upper Triassic-Lower Jurassic Continental Strata of the Fundy, Deerfield and Hartford Basins, Newark Supergroup

Abstract:

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 extrusive units of the Central Atlantic Magmatic Province. We report here the first occurrence of elevated Ir above the oldest volcanic units, as well as additional horizons of Ir enrichment from other basins of the Newark Supergroup. 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. Similarly, sedimentary strata from between flows of North Mount Basalt show no Ir appreciable enrichment. The Deerfield Basin (Massachusetts) extension of the Hartford Basin contains only one CAMP extrusive unit, the Lower Jurassic Deerfield Basalt. Very 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. 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 (Holyoke, 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 REEs relative to other Hartford Basin samples, although there is no evidence of HREE enrichment. We consider the basalts of the Central Atlantic Magmatic Province, widely accepted as the driver of Late Triassic extinctions, as the origin of the elevated Ir levels in the Newark Supergroup.