EXTRA-BASINAL SOURCE FOR THE TR-J BOUNDARY CLAY LAYER IN THE NEWARK BASIN

A palynologically identified terrestrial Tr-J boundary section occurs within the Newark Basin in the Exeter Township member of the Passaic Formation. The boundary is marked by a clay layer containing a spore spike and Ir anomaly, and is overlain by a black carbonaceous shale or coal. These features are comparable to other spore spike, Ir anomaly, and coal layers found at many K-T boundary sites. These analogous features, as well as reports of shocked quartz at two other Tr-J boundary sections in Europe, led to suggestions that this clay layer may represent a meteorite impact ejecta layer coinciding with the end-Triassic mass extinction. Previous efforts to find shocked quartz were unsuccessful, in part due to coarse sampling of the layer. In order to concentrate shocked minerals that may be present in low abundance, mineral separates were recovered from several kg of the boundary clay. An extensive series of analyses were conducted on these mineral separates to identify shock metamorphosed material. A petrologic examination was conducted to search for microspherules/microkrystites. Silica grains were analyzed with 29Si MAS NMR and XRD to look for the high-pressure quartz polymorphs coesite and stishovite. Heavy liquid separations were conducted to recover zircons, and high-pressure quartz polymorphs. Zircons were etched in concentrated NaOH and then analyzed for planar deformation features using SEM. None of the analyses performed indicated the presence of shock metamorphosed material. KTbase, the comprehensive database of K-T boundary data, indicates the occurrence of several sites containing Ir anomalies without the recognition of shocked quartz. If the Exeter Township clay is an impact ejecta layer, or a volcaniclastic layer, it should have a geochemical signature distinct from the background of the Newark Basin. Nd analyses of the clay layer represent an excursion of approximately 3 epsilon Nd units from the immediately adjacent samples. These data represent a significant first step at establishing an event horizon that might be used to correlate other Tr-J boundary sections.