Southern Province Chalk nomenclature - Grey Chalk Subgroup:Zig Zag Chalk Formation

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Name

First proposed in Rawson et al. (2001) as part of the agreed standard for the Chalk Group of England. This term replaces that defined by Bristow et al. (1997) [Fig.P895005/10] and specifically excludes the Plenus Marls Member

Type section

Bristow et al. (1997) did not give a type section for the whole of the formation in Dorset but named it from the lower boundary reference section at the foot of Zig Zag Hill [ST 891 207] near Shaftsbury in Dorset. They referred to the sections in Kent and Sussex that can be considered as primary reference sections.

Primary reference section

The three most significant reference sections are to be found in the cliff sections of the Kent and Sussex coasts and on the Isle of Wight.

Copt Point [TR 242 365] to Hay Cliff [TR 301 394] (including Abbots Cliff and path [TR 268 385]), Folkestone To Dover, Kent.

The Southerham Grey Pit and the adjacent Southerham Machine Bottom Pit, Sussex [TQ 427 090].

Compton Bay, Isle of Wight [SZ 350 855].

Chinor [SU 754 994] in Oxfordshire.

Formal subdivision

Includes the Totternhoe Stone Member and the laterally equivalent but informally named Cast Bed at its base.

Lithology

Mostly firm, pale grey to off-white blocky chalk with a lower part characterised by rhythmic alternations of marls and marly chalks with firm white chalk. Thin gritty, silty chalk beds act as markers in the sequence.

Definition of upper boundary

Conformable. The upper surface is redefined as the bedding plane beneath the lowest of the marls in the Plenus Marls Member in the overlying Holywell Nodular Chalk Formation. (Note that the Plenus Marls Member is now considered as part of the overlying formation thus providing a consistent datum throughout the Chalk Group of England and the North Sea).

Definition of lower boundary

Conformable in full sequences. The lower boundary is placed at the erosional contact at the base of the ‘Cast Bed’ (Southern Province) or the Totternhoe Stone (Transitional Province) in southern England. The former is the direct equivalent of bed C1 of Gale (1995). The formation oversteps onto older formations over the Mid-Dorset Swell where the preceding West Melbury Marly Chalk Formation is absent. The basal unit here is termed the Cenomanian Basement Bed (Drummond, 1970.

Thickness

Generally in the range 35 to 50 m in the Southern Province with the most expanded sequences (up to 75 m exceptionally) in the area of West Sussex, East Hampshire and the Isle of Wight. Thins markedly to the west and is at its least over the Mid-Dorset Swell where as little as 10 m may be preserved.

Distribution

Throughout the Southern Province and the Transitional Province of England.

Previous names

Broadly equivalent to the ‘Grey Chalk’ of the traditional scheme. Most of the Zig Zag Chalk Member, (But without the Plenus Marls) of Bristow, Mortimore and Wood (1997). Equivalent to the upper part of the East Wear Bay Chalk Formation and the Abbots Cliff Chalk Formation of Robinson (1986).

Parent

Grey Chalk Subgroup.

Age and biostratigraphy

Upper Cretaceous, Cenomanian. Acanthoceras rhotomagense to Calycoceras guerangeri Zones.

References

Rawson et al., 2001; Bristow et al., 1999; Bristow, et al., 1997.

Totternhoe Stone Member

Name

The Totternhoe Stone of the traditional scheme and implicitly of member status within the Zig Zag Chalk Formation herein.

Type section

Totternhoe Stone Quarry [SP 980 222] west of Dunstable. Now forms the lowest part of the much larger encircling lime workings. Regarded as an expanded atypical sequence, almost 5 m thick (Aldiss, 1990).

Primary reference section

In the Hitchin district the Green Lagoon [TL 1978 3486], near Arlesey is in thin ‘shelf’ facies. The western face of the disused Chalk Quarry near the redeveloped Fairfield Hospital site shows an atypical ‘channel’ development (Hopson, 1992).

Blue Lagoon [TL 1972 3444], Arlesey c. 1.0 m in typical shelf facies.

Formal subdivision

None. Informal quarrying terms used at the type site.

Lithology

A distinctly harder unit in the Grey Chalk Subgroup (Lower Chalk). Typically brownish-grey, fine-grained calcarenite. Has been described as 'sandy' because of coarse fossil fragments - not because of quartz sand grains. Thin to thickly bedded. Phosphatic in part with dark brown pellets a few mm across, up to nodules several cm across. Fossiliferous. Locally used as building stone.

Definition of upper boundary

Commonly indistinct conformable boundary with the overlying strata of the Zig Zag Chalk Formation (Lower Chalk, Grey Chalk unit). Upward reversion to softer, finer grained more typical chalk or marly chalk. May be difficult to locate, even in sections.

Definition of lower boundary

Commonly indistinct conformable boundary with the underlying West Melbury Marly Chalk Formation (Lower Chalk, Chalk Marl unit) but there may be an erosion surface with concentration of phosphatic pebbles in base of Totternhoe Stone above. May be difficult to locate, even in sections.

Thickness

Between 0.3 and 6.0 m in Hitchin district. Typically 1-2 m thick but expanded sequences in synsedimentary channels may be up to 6 m thick.

Distribution

Known from Berkshire, Hertfordshire and Cambridgeshire where the Southern Province and Transitional Province overlap.

Previous names

Equivalent to the Cast Bed of the basinal successions in the Southern Province. Called the Chilton Stone on the Abingdon Sheet (253) (see entry under that title).

Parent

Zig Zag Chalk Formation (unit is the basal member of formation).

Age and biostratigraphy

Upper Cretaceous, Cenomanian. Acanthoceras rhotomagense Zone, Turrilites costatus Subzone.

References

Whitaker, 1865b; Aldiss, 1990; Hancock, 1972; Hopson, 1992; Hopson et al., 1996; Jukes-Browne and Hill, 1887; Penning and Jukes-Browne, 1881; Rawson,et al., 1978.