The Late Triassic timescale: Age and correlation of the Carnian-Norian boundary

The Late Triassic timescale is poorly constrained due largely to the dearth of reliable radioisotopic ages that can be related precisely to biostratigraphy combined with evident contradictions between biostratigraphic and magnetostratigraphic correlations. These problems are most apparent with regard to the age and correlation of the Carnian-Norian boundary (base of the Norian Stage). We review the available age data pertaining to the Carnian-Norian boundary and conclude that the “long Norian” in current use by many workers, which places the Carnian-Norian boundary at ~ 228 Ma, is incorrect. The evidence supports a Norian stage that is much shorter than proposed by these workers, so the Carnian-Norian boundary is considerably younger than this, close to 220 Ma in age. Critical to this conclusion is the correlation of the Carnian-Norian boundary in nonmarine strata of Europe and North America, and its integration with existing radioisotopic ages and magnetostratigraphy. Three biostratigraphic datasets (palynomorphs, conchostracans and tetrapods) reliably identify the same position for the Carnian-Norian boundary (within normal limits of biostratigraphic resolution) in nonmarine strata of the Chinle Group (American Southwest), Newark Supergroup (eastern USA-Canada) and the German Keuper. These biostratigraphic datasets place the Carnian-Norian boundary at the base of the Warford Member of the lower Passaic Formation in the Newark Basin, and, as was widely accepted prior to 2002, this correlates the base of the Norian to a horizon within Newark magnetozone E13n. In recent years a correlation based solely on magnetostratigraphy has been proposed between the Pizzo Mondello section in Sicily and the Newark section. This correlation, which ignores robust biostratigraphic data, places the Norian base much too low in the Newark Basin section (~ at the base of the Lockatong Formation), correlative to a horizon near the base of Newark magnetozone E8. Despite the fact that this correlation is falsifiable on the basis of the biostratigraphic data, it still became the primary justification for placing the Carnian-Norian boundary at ~ 228 Ma (based on Newark cyclostratigraphy). The “long Norian” created thereby is unsupported by either biostratigraphic or reliable radioisotopic data and therefore must be abandoned. While few data can be presented to support a Carnian-Norian boundary as old as 228 Ma, existing radioisotopic age data are consistent with a Norian base at ~ 220 Ma. Although this date is approximately correct, more reliable and precise radioisotopic ages still are needed to firmly assign a precise age to the Carnian-Norian boundary.