Southern Province Chalk nomenclature - White Chalk Subgroup: New Pit 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

Gun Gardens [TV 588 964] and Beachy Head [TV 576 953] sections in Sussex.

Primary Reference Section

Akers Steps [TR 297 394], Kent; Compton Bay [SZ 350 855].

Formal subdivisions

None herein but contains a number of laterally persistent and named marl and flint beds in Mortimore (1986) which can be recognised outside the Sussex area over much of southern England.

Lithology

Principally blocky, white firm to moderately hard chalk with numerous marls or paired marl seams. Flint occurs sporadically in the upper part in the deeper basin areas of the Southern Province. In some localities flint, in seams, occurs to the base of the formation most notable over structural highs, towards the margins of the outcrop and within the ‘Transitional’ Province.

Definition of upper boundary

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 expands in response to the later inception of nodularity and hardground development (characteristic of the Lewes Nodular Chalk Formation) such that the upper 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).

Definition of lower boundary

Conformable at the bedding plane immediately below the Gun Gardens Main Marl (the Lulworth Marl of Gale, 1996) in the standard Sussex succession. The mapping boundary is placed at the highest shell-detrital chalk and at the incoming of sporadic flints. In the ‘Transitional’ Province the top of these shell-detrital chalks is at the Odsey Marl (See Hopson et al., 1996).

Thickness

Typically between 35 and 50 m but is as little as 10 to 25m metres thick in Wiltshire, north Hampshire and Berkshire over synsedimentary structural. The formation expands in the ‘Transitional’ Province where it is thought to be up to 75 to 80m thick.

Distribution

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

Previous names

Known as the New Pit Chalk Member prior to Rawson et al. (2001). Also the upper and thicker part of the traditional Middle Chalk. Equivalent to the Upper Holywell Beds and New Pit Beds of the Ranscombe Chalk Member (Mortimore and Pomerol, 1996); and the upper part of the Shakespeare Cliff Member and Aycliff Member of the Dover Chalk Formation (Robinson, 1986).

Parent

White Chalk Subgroup.

Age and biostratigraphy

Upper Cretaceous, Turonian. The greater part of the Terebratulina lata Zone.

References

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