Correlation problems in the Kimmeridge Clay Formation (Upper Jurassic, UK): lithostratigraphy versus biostratigraphy and chronostratigraphy.

A scheme of grouped lithostratigraphical units ('beds') proposed for the English Upper Jurassic Kimmeridge Clay Formation has been claimed to be also chronostratigraphical, but some of the resulting time-correlations conflict with those of the standard chronozonation based on ammonite biostratigraphy. Review of some critical ammonite species reaffirms the validity of the ammonite zonal scheme and shows that mismatching of lithologies (facies-correlations) has led to incorrect time-correlations. Because the numbering scheme of 'beds' was based on correlations of attenuated successions, it is on too coarse a time-scale to identify many non-sequences, and its usefulness as a chronostratigraphical tool is questioned. Evidence suggests that at least some calcareous concretions in the Kimmeridge Clay formed at shallow depths, which is relevant to discussions of the succession in terms of basin analysis. [ABSTRACT FROM AUTHOR]