The Age of the Alaunian–Sevatian Substage Boundary in the Norian: Chronostratigraphic Implications for Marine to Nonmarine Correlation

The Alaunian–Sevatian (Norian) substage boundary, originally defined in the marine realm by ammonoid biostratigraphy, is now widely defined by conodont biostratigraphy (first occurrence of Mockina bidentata), particularly in the Tethyan realm. Although the taxon used is cosmopolitan, the zone base appears to be diachronous between Tethys and Panthalassa. The biostratigraphically anchored composite magnetostratigraphy for the marine Triassic places the base of the Mo. bidentata Zone at the base of the UT21n or high in the UT20r magnetochron as defined in the Geomagnetic Polarity Time Scale. When correlated to the terrestrial Newark Basin magnetostratigraphy, this zone appears equivalent to the base of the E15n or upper E14r magnetochron. The current version of the Newark Astronomic Polarity Time Scale, based on fitting the cyclicity of Newark Basin strata to established orbital cycle frequencies, places the E15n magnetochron base at an age of ca. 213.4 Ma. The Manicouagan impact event in eastern Canada is well-dated to 215.5 Ma, and an ejecta layer in Japan linked to the Manicouagan structure occurs very near the base of the Mo. bidentata conodont zone. If the age of the Manicouagan impact is accurately dated, the Newark time scale is inaccurate by as much as ca. 2.1 Myr.