Southern Province Chalk nomenclature - White Chalk Subgroup: Seaford 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 Seaford Chalk Member was used by Mortimore (1987) as part of his Sussex White Chalk Formation and adopted as Seaford Chalk Member within the Upper Chalk Formation by Bristow, Mortimore and Wood (1997).
Type section
Seaford Head [TV 496 976] in Sussex.
Primary Reference Section
Light Point, through Birling Gap [TV 553 959] to the Seven Sisters and numerous other sites in Sussex; Whitecliff Bay [SZ 638 854] on the Isle of Wight; in a number of discontinuous sections between Hope Point [TR 379 463] and White Ness [TR 397 710] in Kent; Ballard Point [SZ 048 813] and White Nothe [SY 764 813] in Dorset.
Formal subdivision
Includes the Stockbridge Rock Member in the Hampshire Downs area. 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
Firm white chalk with conspicuous semi-continuous nodular and tabular flint seams. Hardgrounds and thin marls known from the lowest beds. Definition of upper boundary
Conformable at the Buckle Marl 1 in the Sussex succession. The incoming of common Zoophycos flints and the presence of the zonal Uintacrinus socialis crinoid at the base of the Newhaven Chalk Formation are useful indicators in the field.
Definition of lower boundary
Conformable at the base of Shoreham Marl 2 in Sussex that marks the change from regularly spaced nodular and grainy chalk beds of the upper Lewes Nodular Chalk Formation to smooth white chalks. This marl is equivalent to the East Cliff Marl 2 in Kent and which is also identified at the Anstey Quarry [TL 395 329] south of Royston, in the ‘Transitional’ Province.
In the field, this is the most difficult boundary to place precisely as it falls in the sequence where the predominance of interbeds of hard chalk reduces in favour of soft chalk. The most positive criterion is the incoming of abundant thick-shelled inoceramid debris (Platyceramus) in soft chalks, although this may not be applicable everywhere. The presence of a sequence containing carious flints within the low Seaford Chalk and high Lewes Chalk is a helpful field indicator of the boundary. Geomorphologically the formation characteristically forms long even dipslope crests.
Thickness
Generally in the range 50 to 80m in the basinal successions of Sussex and Hampshire within the Southern Province. Equivalent beds in Kent are in the range 55 to 60m. Over considerable areas of Southern England the thickness of this unit is limited by erosion beneath the sub-Palaeogene unconformity.
Distribution
Known throughout the Southern Province, within the Chilterns and northward into East Anglia in the Transitional Province.
Previous names
Seaford Member of Mortimore (1986) and as adapted in Bristow et al. (1997). Broadly equivalent to the Broadstairs Member of the Ramsgate Chalk Formation of Robinson (1986) in Kent.
Parent
White Chalk Subgroup.
Age and biostratigraphy
Upper Cretaceous, Coniacian to Santonian. Micraster coranguinum Zone.
References
Mortimore (1986); Rawson, Allen and Gale (2001); Bristow et al. (1999); Bristow, Mortimore and Wood (1997).
Stockbridge Rock Member
Name
Described in the Winchester Sheet Explanation (Booth, 2002) and Farrant (2001).
Type section
Type area only in the Stockbridge district. Ongoing research in progress but the unit is identified from its characteristics porcellanous limestone brash in the field.
Primary Reference Section
Poor exposure in a silage pit at Beech Farm, Nether Wallop [SU 2845 3537].
Formal subdivision
None
Lithology
Very hard, locally porcellanous, creamy white chalk, grainy in part with abundant sponge spicules.
Definition of upper boundary
Conformable at the top of the uppermost bed of very hard chalk.
Definition of lower boundary
Conformable at the change from soft white smooth chalk to very hard chalk. Placed within the higher part of the Seaford Chalk Formation.
Thickness
Not known in detail estimated to be 2 to 3m. Member may be a number of such beds of hard chalk between 5 and 10m below the mapped base of the Newhaven Chalk Formation.
Distribution
Occurs in the high part of the Seaford Chalk Formation in the Winchester, Stockbridge area of Hampshire and westwards within the Bourne and Avon valleys near Salisbury. Central Hampshire and Wiltshire in the Southern Province. May extend northwards into Berkshire.
Previous names
Informally described as the Winchester Hardground. May be the lateral equivalent of the Whitway Rock. The member occurs at about the level of the Barrois Sponge Bed and the Clandon Hardground of the North Kent succession.
Parent
Seaford Chalk Formation.
Age and biostratigraphy
Upper Cretaceous, Santonian (?). Micraster coranguinum Zone.
References
Farrant (1999, 2000).