Drawing the line: the history of the Jurassic—Cretaceous boundary

The first System boundaries were based on lithostratigraphical criteria — the junction between two formations. As biostratigraphy and chronostratigraphy developed, ammonite faunas were used in the Mesozoic to define stages and form the basis of most zonal schemes. In the uppermost Jurassic and lowermost Cretaceous, this process is complicated by extreme ammonite faunal provincialism; from this developed separate ammonite zonal schemes, separate stages and the recognition of different Jurassic-Cretaceous boundaries, dependent upon the ammonite faunal province. The acceptance of the Tithonian Stage as the Primary Standard for the terminal Jurassic means that the basal Cretaceous boundary will be drawn at the base of the Berriasian Stage. Selection of appropriate boundary stratotype sections is under consideration. Correlations of the southern English non-marine sections with the Tethyan marine limestones have been made using magnetostratigraphy and various microfossil groups; although the conclusions vary somewhat, it is apparent that the majority of the Purbeck Limestone Group of southern England is of Cretaceous age.