Macrofossil palaeontology and stratigraphical interpretation of the Chalk Group in the Bradfield Southend Borehole (SU57SE229)

Macrofossils from the Chalk Group of the Bradfield Southend Borehole indicate assignment of the topmost interval (58.83 – 67.83 m depth), below the contact with Paleogene strata, to the Uintacrinus socialis and uppermost Micraster cornguinum zones; the boundary between these zones is tentatively placed at a depth of 67.6 m. The lithology of the chalk assigned to the U. socialis Zone corresponds with the typically marl-rich Newhaven Chalk Formation. The remainder of the chalk in the borehole, below 67.6 m to the base of the borehole at 89.6 m, is assigned to the upper (Santonian) part of the M. coranguinum Zone. This interval contains tentative evidence for Rowe’s Echinoid Band and the Barrois Sponge Bed, both indicative of levels near the top of the M. coranguinum Zone in the North Downs. There is no macrofossil evidence for the succession ranging below the base of the Santonian into the lower (Coniacian) part of the M. coranguinum Zone. The lithology of the chalk interval assigned to the M. coranguinum Zone in the Bradfield Southend Borehole is marl-rich, and unlike the typically marl-free upper Seaford Chalk that elsewhere in southern England generally corresponds with the upper M. coranguinum Zone. Similar, but less prolifically marly chalk occurs in the upper M. coranguinum Zone of the Faircross Borehole south of Reading. Provisionally, this atypical marly facies is designated the Bradfield Southend Member of the Seaford Chalk Formation. Regional gravity data suggest that local basin architecture may have influenced the successions found in the Bradfield Southend and Faircross boreholes.