In the geological timescale, the Berriasian is an age or stage of the Early or Lower Creteceous. It is the oldest or lowest subdivision in the entire Cretaceous. It spanned between 145.5 ± 4.0 Ma and 140.2 ± 3.0 Ma (million years ago). The Berriasian succeeds the Tithonian (part of the Jurassic) and precedes the Valanginian.
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 in part of Berriasian age. In fact, the first rocks to be described of this age were the beds of the English Purbeck Formation, named as the Purbeckian by Brongniart in 1829 following description by De la Beche, Buckland, Webster and Fitton.
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 International Subcommission on Cretaceous Stratigraphy (ISCS) of IUGS. A range of contender GSSP localities are currently being studied in detail by the ISCS's Berriasian Working Group. These include localities as far apart as Mexico, Ukraine, Tunisia , Iraq and the Russian Far East. Several markers are being employed to refine correlations and to work towards definition of a base for the Berriasian Stage. These include calcareous nannofossils, such as Nannoconus, calpionellids, ammonites, palynological datums and magnetostratigraphy, notably the base of chron M18r. The calibration of these markers, especially Nannoconus steinmannii minor, N. kamptneri minor, and Calpionella alpina, with precisely fixed magnetozones gives greater precision in trying to identify the best position for a boundary.
In the Tethyan Realm, the Berriasian consists four ammonite biozones, from top to bottom:
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.
- ^ See Gradstein et al. (2004) for a detailed geological timescale
- ^ a b Only known from this stage.
- ^ a b c d e f g h i Dating uncertain.