THE TRIASSIC-JURASSIC BOUNDARY IN THE NEWARK SUPERGROUP; QUO VADIS?

Recent emphasis of the possible roles of impacts and flood basalts of the Central Atlantic Magmatic Province (CAMP) on the Triassic-Jurassic (Tr-J) mass extinction has raised serious questions about both the existence of the mass extinction, and identification of the Tr-J boundary in the Newark. Adding to the uncertainty is the lack of a GSSP or definition for the boundary. For over 30 years, the Tr-J boundary in the Newark Supergroup has been identified by a palynological transition from relatively higher diversity assemblages with typical Triassic taxa to lower diversity assemblages lacking Triassic taxa and with very high proportions of Classopollis. So, we must ask ourselves: what do we really know about this transition in the Newark and what observable phenomena do we wish to explain?

First, where is there a concentration of last appearances (CLA) in the Newark, and what relationship does that have to other phenomena? Looking in the younger part of the Newark, there is only one CLA, and that is seen in palynofloral, vertebrate, and possibly crustacean assemblages within tens of meters of strata below the oldest CAMP flows. Specifically, there are last appearances of such long-ranging Triassic forms as the pollen taxa Patinasporites and Ovalipollis, the tetrapod groups Procolophonidae and Phytosauria, and four ichnogenera, Brachychirotherium, Apatopus, “new genus A” and Gwynnedichnium, with the former three being proxies for non-crocodylomorph cruotarsians, and the latter being tanystropheids. The oldest Eubrontes giganteus occur below the oldest basalts associated with a palynoflora lacking Triassic taxa. Does this transition constitute a mass extinction? That is unclear, but as a percentage of Newarkian taxa this is about a 50% extinction at the million-year-level of resolution. Thus, if there is a turnover phenomenon to explain, this is it. The most parsimonious hypothesis is that the marine invertebrate and palynomorph CLA in the candidate European GSSP sections is correlative with the Newark CLA, consistent with new published U-Pb dates. Thus, the known basalts of the Newark Supergroup simply could not have caused this CLA, because it occurs beneath the basalts. Whether this CLA ends up as the Tr-J boundary depends on its definition at its GSSP, but this is of little phenomenological or process interest.