The BGS Lexicon of Named Rock Units — Result Details

Portsdown Chalk Formation

Computer Code: PCK Preferred Map Code: PCk
Status Code: Full
Age range: Campanian Age (KC) — Campanian Age (KC)
Lithological Description: 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 Lower Boundary: The lower boundary is conformable at the base of the Portsdown Marl 1 where it overlies white chalks of the Culver Chalk Formation (Spetisbury Chalk Member) in the Farlington Redoubt on Portsdown and at Whitecliff Bay, Isle of Wight (Mortimore, 1986).
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 (Norwich Chalk) as formalised by Johansen and Surlyk (1990). Higher beds in Norfolk are informally termed the "Norwich Chalk" and "Trimmingham Chalk".
Thickness: 62m at Whitecliff Bay. Thought to be thicker elsewhere, but limited by the sub-Palaeogene erosion surface.
Geographical Limits: Limited to Sussex, Hampshire, Isle of Wight, 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 (informal Norfolk NKCK and Trimmingham TRCK chalks) is not well known.
Parent Unit: White Chalk Subgroup (WHCK)
Previous Name(s): Portsdown Chalk Member (-4246)
Alternative Name(s): none recorded or not applicable
Stratotypes:
Reference Section  Cliffs between Scratchell's Bay and Alum Bay, western Isle of Wight. Access is by boat to Scratchell's Bay where the oldest part of the succession is visible. Access to the youngest part of the formation around the Needles Headland and into Alum Bay is only possible for a short time at extremely low tides. 
Partial Type Section  Basal boundary stratotype: disused quarry, Farlington Redoubt, Portsmouth. 
Type Section  Cliff section, Whitecliff, eastern Isle of Wight. The full succession up to the Palaeogene unconformity is visible but access is difficult at certain stages of the tide. 
Reference(s):
Mortimore, R N. 1986. Stratigraphy of the Upper Cretaceous White Chalk of Sussex. Proceedings of the Geologists' Association, Vol.97(2), 97-139. 
Bristow, C R, Mortimore, R N and Wood C J. 1997. Lithostratigraphy for mapping the Chalk of southern England. Proceedings of the Geologists' Association, Vol.108(4), 293-315. 
Gale, A S, Wood, C J and Bromley, R G. 1987. The lithostratigraphy and marker bed correlation in the White Chalk (Cenomanian-Campanian) of southern England. Mesozoic Research, Vol.1, 107-118. 
Hopson, P M. 2005. A stratigraphical framework for the Upper Cretaceous Chalk of England and Scotland, with statements on the Chalk of Northern Ireland and the UK Offshore Sector. British Geological Survey Research Report RR/05/01 102pp. ISBN 0 852725175 
Rawson, P F, Allen, P M and Gale, A. 2001. A revised lithostratigraphy for the Chalk Group. Geoscientist, Vol.11, p.21. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
E314 E317 E332 E341 E342 E343