Dual biozonation scheme (benthic foraminifera and “calcareous” green algae) over the Jurassic–Cretaceous transition. Another plea to revert the system boundary to its historical Orbigny's and Oppel's definition

Abstract

The Tithonian–Valanginian interval is subdivided into 5 biozones (zones and subzones) combining mostly large benthic foraminifera and “calcareous” green algae, which are typical of photozoan assemblages from the shallow-water carbonate facies of the Tethysian realm. This dual zonation is calibrated on a sequence stratigraphic framework for the Jura Mountains where earlier findings of ammonites and calpionellids are plotted. The microfossil diversity was low in the Tithonian–early Berriasian times; then it significantly increased over the middle–late Berriasian times before a first extinction event at the Berriasian/Valanginian boundary followed by a second extinction event in the early Valanginian times. The lower/middle Berriasian and the Berriasian/Valanginian boundaries can be traced consistently from West to East through the whole Tethysian realm. By contrast, the upper Tithonian and the lower Berriasian cannot be distinguished. Because the location of the Jurassic/Cretaceous system boundary has been pending a decision of the International Commission on Stratigraphy since 1975, this survey provides one more argument to abandon the early 20th century Kilian's view which placed it at the Tithonian/Berriasian stage boundary, and to return it to the original 19th century Orbigny's and Oppel's view, i.e., back at the Berriasian/Valanginian stage boundary.