Conversion of randomly ordered illite-smectite to ordered illite-smectite in the Upper Jurassic Kimmeridge Clay Formation from the North Sea has been recorded in the literature as occurring within the ‘oil window’ and has been suggested as an indicator of oil source rock maturity. Studies of authigenic clay minerals in the fine fraction (>0.5 µm) of the Kimmeridge Clay Formation mudstones from fourteen locations along the UK onshore outcrop between Dorset and North Yorkshire show that they comprise mainly ordered illite-smectites. The onshore Kimmeridge Clay section is organically immature, suggesting that the illite-smectite ordering reaction cannot be extrapolated between basins as an inorganic indicator of ‘oil window’ levels of maturity. These results also have important implications in source rock hydrocarbon expulsion and migration models which involve shale dewatering as a flushing agent. However, dewatering of shales may aid migration as it could cause fracturing of the shale bands separating the organic-rich layers within the source rock, prior to hydrocarbon generation.

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.