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Biostratigraphy Research Group, British Geological Survey, Keyworth, Nottingham, NG12 5GG, UK
The basis of the chronostratigraphic subdivision of the Jurassic System is the sequence of ammonite faunas. At present, the 11 Jurassic stages (representing approximately 70 million years) can be divided into 145 ammonite-based zones or subzones. Methods of subdivision and correlation by various microfossil groups have also been developed for practical and economic reasons. Dating by fossils (biostratigraphy) underpins the broader-based approachs of event and sequence stratigraphy. It also supplies the primary stratigraphic control in the development of a standard magnetic polarity time-scale (magnetostratigraphy) of global applicability. At present, the Jurassic System of the UK area is divided into 38 units (biozones and subzones) on the basis of dinoflagellate cysts and 23 on the basis of calcareous nannofossils. Coverage is less comprehensive using the benthonic groups (foraminifera and ostracoda) which are more affected by environmental controls. The Lower Jurassic is divided into 16 units on the basis of foraminifera, but higher in the System only local divisions can be recognised in the Bathonian and informal divisions in the Callovian and oldest Oxfordian. There is fuller but still incomplete coverage using ostracoda which have proved particularly useful in the non-marine, brackish and marginal marine sequences of the Bathonian and Portlandian. The recovery of radiolaria from Jurassic sediments in the UK area is a new and exciting development which allows division of Kimmeridgian and Portlandian strata in graben areas in the North Sea Basin.
