The stratigraphy of the Chalk Group of the Berkshire Downs

The Chalk Group in three cored boreholes in the Berkshire Downs has been lithostratigraphically classified using lithology, biostratigraphy and geophysical log characteristics. Six formations are recognized in these boreholes, and together with an overlying possible seventh formation, their presence in a further 60 boreholes has been determined from geophysical log interpretation and correlation with the cored successions. Highly detailed imagery from an optical borehole imaging system has been used to augment the interpretation of the cored succession at Banterwick Barn. It superbly illustrates the potential of this technique for stratigraphical evaluation in the many hydrogeological boreholes that penetrate the Chalk Group, especially for those where no core exists. This work shows that subdivision of the Grey Chalk Subgroup into West Melbury Marly Chalk overlain by Zig Zag Chalk is locally possible in some borehole successions in the Berkshire Downs. The White Chalk Subgroup can be divided into the Holywell Nodular Chalk, New Pit Chalk, Lewes Nodular Chalk and Seaford Chalk formations; the Newhaven Chalk Formation may be locally present. Geophysical log correlations also demonstrate that, locally, the New Pit Chalk Formation is significantly attenuated, as a result of non-deposition and/or erosion associated with the development of the lowest ‘Chalk Rock’ hardground present in the area, the Ogbourne Hardground, and suggest a more complex model for its geographical distribution than has hitherto been proposed. The top of the Lewes Nodular Chalk, as defined by lithology and the resistivity log signatures in the Berkshire Downs, is diachronous, occurring below the likely correlative of Shoreham Marl 2, the marker-bed for the Lewes Nodular-Seaford Chalk junction in Sussex. This relationship, which is contrary to what might be expected, may be a consequence of changes in coccolith productivity connected to an Early Coniacian marine transgression. Confirmatory evidence is given for the occlusion of the Fognam Marl (= Southerham Marl 1) by Mid-Turonian erosion close to the Goring Gap.