Correlation of the lower and middle Turonian chalks of south-west England and north-west France — condensation towards the Cornubian and Armorican massifs

The condensation of lower and middle Turonian chalks towards the western margins of the Anglo-Paris Basin in SW England (UK) and Haute Normandie (northwest France) is investigated using basin-centre to basin-margin transects. Correlations are based on the identification of key marker beds and high-resolution microcrinoid biostratigraphy which enable detailed analysis of the patterns of condensation. Hardgrounds develop and become more intensely lithified and mineralised towards the basin margins. The formation of hardgrounds at the bases of the lower and the middle Turonian can be related to widely recognised eustatic sea-level events. The distribution of a hardground which underlies a major hiatus higher in the middle Turonian (Ogbourne–Tilleul) is related to reactivated thrusts in the Variscan Basement and its formation was caused by early (92.4 Ma) inversion in the Subhercynian Phase, generated by the collision of North Africa and Europe.