Chronostratigraphic units in the Jurassic and their boundaries: Definition, recognition and correlation, casual mechanisms

The historical development of the chronostratigraphic classification of the Jurassic into series, stages and zones is summarised. Traditionally, the series and stages are defined hierarchically in terms of the standard (ammonite) zones they contained but since Hedberg (1976)([1]) they have been defined by a basal boundary stratotype (GSSP). To date four Jurassic Stages have ratified GSSPs; progress with the others is outlined. The primary marker for the definition of the Stage boundaries is biochronology based on evolution of ammonites, with secondary markers based on biostratigraphy of other fossil groups, chemostratigraphy and magnetostratigraphy. Causal mechanisms for the environmental changes that caused ammonite evolution are reviewed and possible methods of correlation with non-marine successions are briefly discussed.