Southern Province Chalk nomenclature - White Chalk Subgroup: Portsdown Chalk Formation
Name
First proposed in Rawson, Allen and Gale (2001) as part of the agreed standard for the Chalk Group of England. The term Portsdown Member was first used in Mortimore (1986) as part of his Sussex White Chalk Formation and adapted to the Portsdown Chalk Member of the Upper Chalk Formation by Bristow, Mortimore and Wood (1997).
Type section
Whitecliff Bay [SZ 638 854] on the Isle of Wight.
Primary Reference Section
Scratchell’s Bay [SZ 296 847] on the Isle of Wight.
Formal subdivision
Includes the Studland Chalk Member. Informally includes a number of laterally persistent flint and marl beds named in Mortimore (1986) that can be traced outside Sussex in the Southern and Transitional provinces.
Lithology
Composed of white chalk with marl seams (particularly in the lower part) and flint bands, although less flinty than the Culver Chalk Formation. The lowest part (up to and including the Farlington Marls at the base of the mucronata Zone) contains several belts rich in inoceramid shell debris, as well as isolated marl seams and pairs of marl seams that likewise contain much inoceramid shell debris. The highest part is essentially marl-free and termed the Studland Chalk ‘Member’ of Gale, Wood and Bromley (1987).
Definition of upper boundary
As defined the upper boundary of the Formation is the sub-Palaeogene erosion surface. The boundary with the Studland Chalk Member (sensu Gale et al., 1987) is conformable at a marly bed with included marl above which the chalk is marl-free in the Alum Bay/Scratchell’s Bay section on the Isle of Wight and at “the lower horizon of yellow-stained chalk on Old Harry Rock” in Studland Bay, Dorset (Gale et al., 1987). In the ‘Transitional’ Province the upper boundary is undefined but it is noted (Mortimore, Wood and Gallois, 2001) that the “highest chalk of the Isle of Wight and Dorset” is equivalent to beds within the Weybourne Chalk Member as formalised by Johansen and Surlyk (1990). These higher beds are informally termed the ‘Norwich Chalk’ and ‘Trimmingham Chalk’.
Definition of lower boundary
Conformable at the Portsdown Marl 1in the Farlington Redoubt on Portsdown and at Whitecliff Bay, Isle of Wight.
Thickness
62m at Whitecliff Bay but elsewhere thought to be thicker but limited by the sub-Palaeogene erosion surface.
Distribution
Limited to Sussex, Hampshire, Wiltshire and Dorset close to the Palaeogene cover in the Southern Province. Its possible occurrence and distribution beneath the Quaternary deposits of East Anglia in the Transitional Province is not well known.
Previous names
Portsdown Chalk Member of Mortimore (1986) and as adopted in Bristow et al. (1997).
Parent
White Chalk Subgroup.
Age and biostratigraphy
Upper Cretaceous, Campanian. Topmost Gonioteuthis quadrata and Belemnitella mucronata Zones.
References
Rawson, Allen and Gale (2001); Bristow et al. (1999); Bristow, Mortimore, and Wood (1997).
Studland Chalk Member
Name
The term Studland Chalk was used by Gale, Wood and Bromley (1987) to describe their highest Campanian chalks at Handfast Point in Studland Bay, it is considered as a member herein.
Type section
Studland Bay between Old Harry Rocks [SZ 0055 825] and the Palaoegene contact [SZ 045 824].
Primary Reference Section
Alum and Scratchell’s Bay [SZ 296 847] Isle of Wight.
Dorset coast at Studland.
Formal subdivision
None herein. Informally includes a number of laterally persistent flint and marl beds named in Mortimore (1986) that can be traced outside Sussex in the Southern and Transitional provinces.
Lithology
Marl-free, soft chalk with large irregular flints.
Definition of upper boundary
The sub-Palaeogene unconformity in the Southern Province. Relationship to the Upper Campanian and Maastrichtian chalks of the northeast ‘Transitional’ Province poorly understood due to poor exposure.
Definition of lower boundary
Conformable at a marly bed with included marl, above which the chalk is marl-free in the Alum Bay/Scratchell’s Bay section on the Isle of Wight and at “the lower horizon of yellow-stained chalk on Old Harry Rock” in Studland Bay, Dorset (Gale et al., 1987). It is not discernible from the published literature whether these two beds are in the same stratigraphic position. This doubt together with the lack of exposure and geomorphological expression of the member has led to the BGS reverting to the original concept of the Portsdown Chalk Member (Bristow, Mortimore and Wood, 1997). This is also the position stated in Rawson, Allen and Gale (2001) and thus the Portsdown Chalk Formation covers all of the chalk up to the sub-Palaeogene erosion surface.
Thickness
Up to 23m preserved beneath the sub-Palaeogene unconformity in Alum Bay on the Isle of Wight.
Distribution
Limited to the Isle of Wight and the Swanage area of Dorset in the Southern Province..
Previous names
Studland Chalk of Gale et al. (1987) as adopted informally in Bristow et al. (1997). Perhaps partly equivalent to the informal ‘Norwich Chalk’ as applied in north Norfolk. Perhaps now considered as a Member within the Portsdown Chalk Formation.
Parent
Portsdown Chalk Formation or White Chalk Subgroup.
Age and biostratigraphy
Upper Cretaceous, Campanian. Belemnitella mucronata s.l. Zone.
References
Bristow, Mortimore and Wood (1997); Rawson, Allen and Gale (2001); Bristow et al. (1999).