Stratigraphic Definitions
The Berriasian stage was introduced in scientific literature by Henri Coquand in 1869. It is named after the village of Berrias in the Ardèche region of France. The largely non-marine English Purbeck Formation is of Berriasian age and in the past the names Purbeck and Wealden were also used to address rocks of the lowest Cretaceous.
The base of the Berriasian (also the base of the Cretaceous system) has traditionally been placed at the first appearance of fossils of the ammonite species Berriasella jacobi. A global reference profile (a GSSP) for the Berriasian is under active consideration by the Cretaceous International Subcommission (ISCS) of IUGS. A range of contender GSSP localities are currently being studied by the ISCS's Berriasian Working Group. Several markers are being employed to refine correlations and to work towards definition of a stage base. These include calcareous nannofossils, such as Nannoconus steinmannii, calpionellids, ammonites, palynological datums and magnetostratigraphy, notably the base of chron m18r.
In the Tethyan Realm, the Berriasian consists four ammonite biozones, from top to bottom:
- Thurmanniceras otopeta
- Subthurmannia boissieri
- Subthurmannia occitanica
- Berriasella jacobi
The top of the Berriasian stage (the base of the Valanginian) is at the first appearance of calpionellid species Calpionellites darderi in the stratigraphic column. This is just a little below the first appearance of ammonite species Thurmanniceras pertransiens.
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