The base of the Portsdown Chalk Formation (Chalk Group, White Chalk Subgroup) in the BGS Big Almer Wood Borehole, Dorset, southern England

Abstract

The mapped boundary between the Spetisbury Chalk Member of the Culver Chalk Formation and the overlying Portsdown Chalk Formation of the White Chalk Subgroup is taken in central Dorset at a pronounced negative feature break. In the absence of any section across the junction, the feature was thought to mark the upward change from marl-free chalk into chalk with numerous marl seams, as in the Isle of Wight. However, in the BGS Big Aimer Wood Borehole, drilled to investigate this boundary, marl seams are largely absent in the lower part of the Portsdown Chalk, and it is the upward change from flinty to flint-free chalk which marks the Spetisbury/Portsdown Chalk boundary. The basal beds are rich in comminuted inoceramid debris. A new member, the Big Aimer Wood Member, is introduced for this basal marl- and flint-free, ‘gritty’ chalk. This paper includes a revised foraminiferal zonal scheme for that part of the Lower Campanian corresponding to the G quadrata macrofossil Zone. In this scheme, the Big Aimer Wood Member is more-or-less co-extensive with the BGS20iv Subzone. New foraminiferal evidence shows that the basal Portsdown Chalk boundary in this part of Dorset lies at about the level of the Scratchell's Marls, i.e. a short distance above the basal boundary marl seam (Portsdown Marl) of the Portsdown Chalk of the Isle of Wight standard successions; i.e. the base of the Portsdown Chalk is slightly diachronous.