The nonmarine Triassic-Jurassic boundary in the newark supergroup of eastern North America

摘要

For the last three decades, the position of the Triassic-Jurassic boundary (TJB) in nonmarine strata has largely been based on its palynostratigraphic placement just below the oldest basalts of the Central Atlantic magmatic province (CAMP) in the Newark Supergroup of eastern North America. This boundary, however, is demonstrably older than the TJB as it is defined in marine strata. Thus, the palynostratigraphic change used to place this TJB in the Newark most resembles a late Norian change in the European section and does not correspond to any palynological event at the marine-defined TJB. Conchostracan biostratigraphy suggests that the TJB is in the Newark extrusive zone above the oldest CAMP basalt, and vertebrate biostratigraphy is consistent with this placement of the TJB. Radioisotopic ages indicate that the TJB defined in marine strata is no older than 200 MA, but the oldest CAMP basalts of the Newark Supergroup consistently yield ages averaging about 201 Ma. Magnetostratigraphic correlation of the Newark section to the TJB section at St. Audrie's Bay in England also indicates that the onset of CAMP volcanism ill the Newark Supergroup is older than the marine TJB. Correlations based on carbon and osmium isotopes are also consistent with this correlation. Thus, all data indicate that the TJB in the Newark Supergroup is above the lowest CAMP basalt in the Newark extrusive zone. Correct placement of the TJB in the Newark Supergroup allows key nonmarine and marine events to be sequenced across the TJB, indicating the following succession of events: crurotarsan ("thecodont") extinction, the palynofloral turnover in the Newark, the first CAMP eruptions, the extinctions of Rhaetian ammonites and conodonts, and the first appearance of Jurassic psiloceratid ammonites. (C) 2007 Elsevier B.V. All rights reserved.