Southern Province Chalk nomenclature - White Chalk Subgroup: Holywell Nodular Chalk Formation
Name
First proposed in Rawson, Allen and Gale (2001) as part of the agreed standard for the Chalk Group of England. Replaces the mapping member of the same name proposed in Bristow, et al. (1997).
Type section
Beachy Head [TV 59 95] and Holywell [TV 603 973] section west of Eastbourne, Sussex.
Primary Reference Section
Plenus Marls Member type-site at Merstham [TQ 295 542] of Jefferies (1963) is now obscured. Robinson (1986) used Shakespeare Cliff [TR 307 398] in Kent as a reference section. The thickest onshore sequence is seen at Eastbourne adjacent to the Holywell Cafe [TV 602 967].
Melbourn Rock originally defined at Melbourn in Cambridgeshire where the type-site is now poorly exposed in a car park beneath a building. A replacement type section is taken at Ashwell Quarry [TL 2687 3945] on the Biggleswade sheet (see Hopson et al., 1996). Melbourn Rock Bed or Beds defined by Mortimore (1986) and Robinson (1986) at Gun Gardens [TV 588 964] and Akers Steps [TR 297 394] respectively.
Chinor [SU 754 994] in Oxfordshire.
Formal subdivision
Includes the Plenus Marls Member and the Melbourn Rock Member. Comprises a number of laterally persistent and named marl and flint beds in Mortimore (1986) that can be recognised outside the Sussex area over much of southern England.
Lithology
Generally hard nodular chalks with thin flaser marls and significant proportions of shell debris in part. Base marked by the interbedded coloured marl and chalk succession characteristic of the Plenus Marls Member (a term applicable in both the Southern and Northern Provinces). The Melbourn Rock Member above the base can be distinguished by its lack of shell material.
Definition of upper boundary
Conformable. Taken at the Gun Gardens Main Marl in the expanded sequences of the Southern Province and at the highest shell detrital bed in mapping terms.
Definition of lower boundary
Conformable. Base of the formation taken at the bedding plane immediately below the lower marl bed in the Plenus Marls Member. (Note that the Plenus Marls Member is now considered as part of this formation thus providing a consistent datum throughout the Chalk Group of England and the North Sea).
Thickness
In thicker sequences this formation is generally 25 to 35m thick. It thins rapidly into the ‘Transitional’ Province of the North Downs, Berkshire Downs and the Chilterns (10 to 15m) and is almost absent in the southwest of Dorset and in Devon.
Distribution
Known throughout the Southern Province, within the Chilterns and northward into East Anglia in the Transitional Province.
Previous names
Previously known as the Holywell Nodular Chalk Member of Bristow, Mortimore and Wood (1997), under which name it did not include the Plenus Marls.
Equivalent to the Plenus Marl Beds, Lower and Middle Holywell Beds of Mortimore (1986); and the Plenus Marls Formation and lower part of the Shakespeare Cliff Member of the Dover Chalk Formation (Robinson, 1996).
Includes the Melbourn Rock (Ballard Head Member of Gale, 1996). The name, as applied erroneously by Shephard-Thorn, 1988, referred to the whole of the condensed sequence representing the Holywell Nodular Chalk Formation in East Kent.
Parent
White Chalk Subgroup.
Age and biostratigraphy
Upper Cretaceous, Cenomanian (uppermost) to Turonian. Metoicoceras geslinianum to Mytiloides spp. Zones. (the inferred top of the Mammites nodosiodes Zone).
References
Mortimore (1986); Rawson, Allen and Gale (2001); Bristow et al. (1999); Bristow, Mortimore and Wood (1997).
Plenus Marls Member
Name
Traditionally known as the Plenus Marls and defined by Jefferies (1963). Defined herein at member level marking the basal unit of the Holywell Nodular Chalk Formation.
Type section
Merstham Greystone Limeworks in Surrey [SU 295 542].
Primary Reference Section
Numerous throughout Southern and Northern Province see Jefferies (1963).
Formal subdivision
None herein but finely divided into eight beds of widespread geographical distribution be Jefferies (1963).
Lithology
Interbedded coloured marl and chalk.
Definition of upper boundary
Conformable at the bedding plane above the highest marl below the hard nodular limestone of the Melbourn Rock Member.
Definition of lower boundary
Conformable beneath the lowest marl on the underlying Zig Zag Chalk Formation. Sequence constitutes the lowest member of the Holywell Nodular Chalk Formation and the White Chalk Subgroup (see also definition for Northern Province).
Thickness
Usually in the range 1 to 1.5m but known to be up to 3.0 m at the Holywell Cafe cliff section in Eastbourne.
Distribution
Widespread within the Southern, Transitional and Northern provinces (see additional entry for Northern Province) and offshore in the North Sea area. Forms the most characteristic downhole geophysical signature in the whole of the Chalk Group.
Previous names
Plenus Subzone in various Survey publications; Belemnite Marls.
Parent
Holywell Nodular Chalk Formation.
Age and biostratigraphy
Upper Cretaceous, Cenomanian. Metoicoceras geslinianum Zone.
References
Jefferies (1963).
Melbourn Rock Member
Name
Recognition of the Melbourn Rock in the Cambridgeshire and Chiltern areas formed the basis on which the traditional tripartite scheme for the Chalk of southern England was erected by the Geological Survey (Jukes-Browne and Hill, 1903). It is herein regarded as of member status within the lower part of the Holywell Nodular Chalk Formation.
Type section
Named after Melbourn, in Cambridgeshire where two pits are described. The Melbourn Lime-kiln Pit and a second ‘a quarter of a mile distant’ both at the southern end of Melbourn. The type-site is in a car park beneath a modern industrial building developed in the old (?) lime-kiln pit and is now in poor condition and the full sequence cannot be seen. A substitute type section and essentially the new stratotype is described in the Hitchin memoir (p. 47) at Ashwell [TL 2697 3945] where the full sequence from the Plenus Marls to well above the Melbourn Rock can be seen in a clean section.
Primary Reference Section
Numerous throughout the Southern and ‘Transitional’ Province. Chinor in Oxfordshire, Folkestone-Dover in Kent, Eastbourne –Beachy Head in Sussex, Compton Bay on the Isle of Wight.
Formal subdivision
None
Lithology
Hard to very hard off-white, blocky fractured chalk with numerous nodular chalk beds and thin anastomosing marls.
Definition of upper boundary
Conformable but difficult to place in the ‘Transitional’ Province where it is usually considered to be at a horizon of less hard flaggy chalks with significant anastomosing marls. This is probably equivalent to the Meads Marls sequence of the Southern Province and Mortimore (1986) places the boundary at the Meads Marl 1 in Sussex.
Definition of lower boundary
Conformable on the highest marl seam of the Plenus Marls Member. This is the Foyle Marl of Mortimore (1986).
Thickness
About 3m in the classic Chiltern area but noted to be between 2 and 7 m in the Hitchin Memoir (Hopson et al., 1996) and a range of 2.7 to 4 m given in Mortimore (1986).
Distribution
Known throughout the Southern and Transitional provinces and forms the most readily identifiable geomorphological features in unexposed ground.
Previous names
Melbourn Rock as used in many BGS publications and external papers.
Equivalent to part of the Grit Bed of Price (1877).
Parent
Holywell Nodular Chalk Formation.
Age and biostratigraphy
Upper Cretaceous, Cenomanian to Turonian. Metoicoceras geslinianum, Neocardioceras juddi and basal Mytiloides spp. Zones.
References
Hill and Jukes-Browne (1886); Hill (1886).