Category:Southern Province Chalk nomenclature - Chalk group

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Name

The term Chalk Group was introduced by Wood and Smith (1978) to cover all of the chalk in the Northern Province (as such it included the Hunstanton Formation). It had previously used offshore by Rhys (1974) and by Robinson (1986) to cover the entirely Upper Cretaceous chalks in Kent. Bristow, Mortimore and Wood (1997) used the term to cover the chalks of the Southern Province as part of their new chalk nomenclature and this usage was agreed and published by Rawson, Allen and Gale 2001 following the stratigraphical meeting held at BGS in 1999.

Type section

With the exception of well sections in the North Sea there is no single expanded and entire sequence known for the whole group. The most complete onshore exposure is in the near vertically inclined cliff sections on the Isle of Wight where, under favourable conditions, the greater part of the known Southern Province sequence can be seen.

The sea-cliffs of Sussex and Kent afford many of the individual formation stratotype sections for the Southern Province, whilst the sea-cliffs and extensive inland exposures in Yorkshire and Lincolnshire provide the stratotypes for the constituent Northern Province formations.

Primary reference section

See notes above.

Formal subdivisions

The group is formally divided into two subgroups, the Grey Chalk and White Chalk subgroups, and numerous formations throughout England with schemes developed for the Northern and Southern provinces. Well-known units used to divide the succession further are given member status. These units are defined herein.

Lithology

Chalk, with or without flint and discrete marl, sponge, calcarenite, phosphatic, hardground and fossil-rich beds.

Definition of upper boundary

Unconformable beneath the Palaeogene or Quaternary basal unconformity onshore in the UK. Conformable beneath Danian age Maureen Formation in parts of the North Sea. Elsewhere unconformable.

Definition of lower boundary

Generally unconformable (at a burrowed surface) on the underlying Lower Cretaceous strata (Upper Greensand and Gault Formations in the Southern Province and on the Hunstanton Formation in the Northern Province). Oversteps onto older strata in limited basin marginal situations.

Thickness

The thickness of the whole group is variable depending on the degree of post-Cretaceous erosion and the relative development of its constituent formations. Onshore the thickest development is within the Hampshire/Sussex area of the Southern Province, where up to about 560 m of strata are preserved; the most chronostratigraphically complete succession is in Norfolk but is thought there to be only some 400 m thick; within the Northern Province up to 530 m are preserved but the thickest succession is within the North Sea area where about 1000 to 1300 m are preserved.

Distribution

The group is known throughout the onshore outcrops in England and offshore in the Southern Central and Northern North Sea areas.

Previous names

Known simply as the Chalk, or the Chalk Formation since the earliest writings it is perhaps the most readily identifiable geological unit and rock type in the world. The word chalk is derived from the Saxon cealc or German kalk and has been in use since the Middle Ages with Martin Lister (1684) frequently being credited with the first publication of the name. The use of the term Chalk Group was formally proposed by Wood and Smith (1978) in their classification of the ‘Northern Province’; it was introduced to cover the Chalk of the North Sea by Rhys (1974) and is used by Robinson (1986) specifically to describe the whole of the Chalk of the North Downs; group is implied by the use of the term formation to describe its constituent parts by Mortimore (1986). The Correlation of Cretaceous Rocks of the British Isles (Rawson et al., 1978) uses the term Chalk Formation and utilises the traditional terms Lower, Middle and Upper Chalk without recourse to a formal status suffix. Bristow, Mortimore and Wood (1997) used the term Chalk Group to encompass their Lower, Middle and Upper Chalk Formations. Equivalent to the Inner Hebrides Group in Scotland (see Section 5), the Hibernian Greensand and Ulster White Limestone groups taken together in Northern Ireland (see Section 6) and the Shetland Group in the Northern North Sea.

Parent

None

Age and biostratigraphy

Upper Cretaceous, Cenomanian to Maastrichtian.

References

Rawson, Allen and Gale (2001); Wood and Smith (1978); Rhys (1974).

Subcategories

This category has the following 2 subcategories, out of 2 total.