Southern Province Chalk nomenclature - White Chalk Subgroup: Lewes Nodular Chalk Formation

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Name[edit]

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). The term Lewes Nodular and Flinty Chalk Member was used by Mortimore (1987) as part of his Sussex White Chalk Formation and adapted to Lewes Nodular Chalk Member within the Upper Chalk Formation by Bristow, Mortimore and Wood (1997).

Type section[edit]

The type-sites for the Lewes Chalk as defined by Mortimore (1986) are at the Caburn Pit [TQ 447 089], Navigation Pit [TQ 426 099] and on the coast between Beachy Head [TV 576 953] and Light Point [TV 566 954], all in Sussex.

Primary Reference Section[edit]

Compton Bay [SZ 350 855] and Whitecliff Bay [SZ 638 854] on the Isle of Wight; White Nothe [SY 764 813] in Dorset; Akers Steps [TR 298 394] and Langdon Stairs [TR 341 422] near Dover, Kent; the Aston Rowant (Stokenchurch) M40 cutting [SU728 965 to 740 965] Oxfordshire; Kensworth Chalk Pit [TL 015 197] in Bedfordshire.

Formal subdivision[edit]

Includes the Chalk Rock Member, the Kensworth Nodular Chalk Member and the Top Rock Bed. Includes a number of laterally persistent and named marl, hardground and flint beds in Mortimore (1986) that can be recognised outside the Sussex area over much of southern England.

Lithology[edit]

Composed of hard to very hard nodular chalks and hardgrounds (which resist scratching by finger-nail) with interbedded soft to medium hard chalks (some grainy) and marls; some griotte chalks. The softer chalks become more abundant towards the top. Nodular chalks are typically lumpy and iron-stained (usually marking sponges). Brash is rough and flaggy or rubbly, and tends to be dirty. First regular seams of nodular flint, some large, commence near the base and continue throughout.

Definition of upper boundary[edit]

Conformable at the Shoreham Marl 2 which is equivalent to the East Cliff Marl 2 in Kent and 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 because of the gradual upward transition in predominant lithology, with interbeds of hard chalk in soft chalk and vice versa. 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.

Definition of lower boundary[edit]

Conformable at the base of Glynde Marl 1 in Sussex, but one of the higher marls elsewhere although invariably in the interval Glynde Marls to Southerham Marls in the Southern Province. The mapping boundary is placed at the appearance of nodular chalks and significant flint development within that range of marls.

In the ‘Transitional’ Province the formation is condensed in response to the later inception of nodularity and hardground development such that the lower boundary is diachronous here and placed below the ‘Chalk Rock Member’ at the Reed Marl (the lateral equivalent of the Bridgewick Marls of the Southern Province).

Thickness[edit]

The formation is thickest in the basinal successions of the Southern Province in east Hampshire and Sussex where it attains c. 35 to 60 m in the east and up to 80 m in west. The formation thins towards the margins of the outcrop and over synsedimentary structural highs.

Distribution[edit]

Known throughout the Southern Province, within the Chilterns and northward into East Anglia in the Transitional Province.

Previous names[edit]

Lewes Nodular and Flinty Chalk Member of Mortimore (1986) in Sussex and the Lewes Nodular Chalk Member (Bristow et al., 1997). Equivalent to the Akers Steps Member of the Dover Chalk Formation and the St. Margarets Member of the Ramsgate Chalk Formation of Robinson (1986) in Kent. Forms the lowest part of the traditional Upper Chalk.

Parent[edit]

White Chalk Subgroup.

Age and biostratigraphy[edit]

Upper Cretaceous, Turonian to Coniacian. Top Terebratulina lata, Sternotaxis plana (now Plesiocorys plana)and Micraster cortestudinarium Zones.

References[edit]

Mortimore (1986); Rawson, Allen and Gale (2001); Bristow et al. (1999); Bristow, Mortimore and Wood (1997).

Chalk Rock Member[edit]

Name [edit]

Described as the Chalk Rock in the traditional scheme as exemplified in Jukes-Browne and Hill (1904). The term Chalk Rock Formation was adopted in Bromley and Gale (1982) and later as a member in Gale, Wood and Bromley (1987). Defined herein as a member within the Lewes Nodular Chalk Formation.

Type section[edit]

Ogbourne Maizey. Old farm west of Ogbourne Maizey, 3km north of Marlborough, Wiltshire.

Primary Reference Section[edit]

Noted at 55 sites throughout the ‘Transitional’ and Southern Provinces in Bromley and Gale (1982).

Formal subdivision[edit]

None herein but is divided into a number of named hardgrounds (of bed status) in Bromley and Gale (1982).

Lithology[edit]

Very hard chalk and chalkstone, some nodular, including mineralised hardground surfaces, and marl seams.

Definition of upper boundary[edit]

Where the Hitch Wood Hardground is developed the boundary is placed at the flint nodule horizon that marks the base of the overlying nodular chalk unit. In other areas, at the Pewsey Hill Hardground or the Ogbourne Hardground.

Definition of lower boundary [edit]

The diffuse lower limit of cementation of the lowest bed of chalkstone at this level.

Thickness[edit]

4 to 5m where fully developed.

Distribution[edit]

Known throughout the Southern Province and in the Chiltern area of the Transitional Province.

Previous names[edit]

Chalk Rock (CkR).

Chalk Rock Formation[edit]

NOTE 1: Although Bromley and Gale's (1982) definition of the Chalk Rock is accepted, it is BGS practice to include the unit in the Upper Chalk of the traditional scheme and as a constituent part of the Lewes Nodular Chalk Formation in the new scheme. Thus the Chalk Rock is reduced in rank from 'formation' (Bromley and Gale, 1982) to member.

NOTE 2. Gale, Wood and Bromley (1987) treat the Chalk Rock as a Member of the 'White Chalk Formation' (= Middle and Upper Chalk of BGS).

Equivalent to part of the Kingston Beds of Mortimore (1986) and part of the St Margarets Member of Robinson (1986).

Parent[edit]

Lewes Nodular Chalk Formation.

Age and biostratigraphy[edit]

Upper Cretaceous, Turonian. Sternotaxis plana (now Plesiocorys plana) Zone.

References[edit]

Whitaker (1861); Bromley and Gale (1982); Jukes-Browne and Hill (1904); Gale, Wood and Bromley (1987).

Kensworth Nodular Chalk Member[edit]

Name [edit]

First used and defined in the Hitchin Memoir (Hopson, Aldiss and Smith, 1986)

Type section[edit]

Kensworth Chalk Quarry [TL 017 197], near Dunstable, Bedfordshire.

Primary Reference Section[edit]

Reed Chalk Pit [TL 3595 3704], south of Royston, Hertfordshire.

Formal subdivision[edit]

Includes the Top Rock Bed

Lithology [edit]

Very hard nodular chalk and chalkstone, including mineralised hardground surfaces.

Definition of upper boundary[edit]

Conformable at the top of a succession of obviously indurated chalks within the Lewes Nodular Chalk Formation overlying the Chalk Rock Member. Note that this does not necessarily coincide with the top of the Top Rock Bed.

Definition of lower boundary [edit]

Conformable at the top of the Chalk Rock Member above the flint nodule horizon marking the top of the Hitchwood Hardground.

Thickness[edit]

About 2.5 to c.4m in Hitchin and Leighton Buzzard districts.

Distribution[edit]

Known within the Leighton Buzzard and Hitchin areas of the Chilterns.

Previous names[edit]

None

Parent[edit]

Lewes Nodular Chalk Formation.

Age and biostratigraphy [edit]

Upper Cretaceous Turonian and Coniacian. Sternotaxis plana (now Plesiocorys plana), Micraster cortestudinarium Zones.

References[edit]

Hopson, Aldiss and Smith (1996); Shephard-Thorn, Moorlock, Cox, Allsop and Wood (1994).

Top Rock Bed[edit]

Name [edit]

The name was first used by Penning and Jukes-Browne (1881) to describe a distinctive bed(s) of hardground and nodular chalk development at a distinctly higher stratigraphical level than the Chalk Rock.

Type section Reed Pit [TL 3595 3704] near Royston in Cambridgeshire.

Primary Reference Section[edit]

Kensworth [TL 017 197} (Leighton Buzzard); Hitch Wood Hill End [TL 1973 2398] (Hitchin).

Formal subdivision None

Lithology=[edit]

Hard cream limestone with scattered brownish phosphatic nodules commonly green-coated at the top. Mineralised hardground or chalkstone bed.

Definition of upper boundary[edit]

Conformable at the hardground surface marked by the change from mineralised hard chalkstone to soft white smooth chalk.

Definition of lower boundary [edit]

Conformable within the Kensworth Nodular Chalk Member at an indistinct boundary marking a significant increase in nodularity between 2 and 3 m above the uppermost hardground of the Chalk Rock Member.

Thickness[edit]

0.8 to 1.0m in Cambridgeshire. 0.3m to 1.0m in Chilterns to where it is essentially confined.

Distribution[edit]

The Chilterns within Hertfordshire and possibly Buckinghamshire.

Previous names[edit]

Forms part of the Kensworth Nodular Chalk Member.

Parent[edit]

Lewes Nodular Chalk Formation.

Age and biostratigraphy[edit]

Upper Cretaceous, Turonian to Coniacian. Micraster cortestudinarium Zone.

References[edit]

Penning and Jukes-Browne (1881); Brighton (1928); Worssam and Taylor (1969).