The lithostratigraphical context of the English Chalk Rock (Turonian)

Correlations of borehole geophysical logs in the middle and upper Turonian Chalk Group are used to consider recent proposals for a revision in understanding of a unit of hardgrounds (Chalk Rock) and associated stratigraphy developed across parts of southern England. Along the northern edge of the London and Wessex basins, geophysical logs reveal a laterally continuous framework of correlatable inflection patterns in the New Pit Chalk, with the package of sediment immediately below the Chalk Rock showing a trend of lateral thinning and increasingly condensed sedimentation westwards into areas where the oldest of the Chalk Rock Hardgrounds (Ogbourne Hardground) is present. However, apart from local absence of the Glynde Marls Complex near the top of the New Pit Chalk, there is no evidence for the presence of a major erosion event. This questions recent interpretations of microcrinoid data, reportedly showing that the Ogbourne Hardground lithifies a stratigraphical level in the lower part of the New Pit Chalk, with the middle and upper parts of this unit corresponding with a hiatus and related short-lived globally significant sea level fall. Macrofossil biostratigraphy supports the geophysical log interpretations, with evidence of both younger and older parts of the New Pit Chalk below the Ogbourne Hardground. The data are consistent with the Ogbourne Hardground in Wiltshire and Berkshire representing a highly condensed equivalent of thickened nodular chalk fabrics at the base of the Lewes Nodular Chalk in the eastern Chilterns.