Southern Province Chalk nomenclature - Grey Chalk Subgroup:West Melbury Marly Chalk Formation

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Melbury Marl Chalk Formation

Name

First proposed in Rawson, Allen and Gale (2001) as part of the agreed standard for the Chalk Group of England. This term is essentially the same as the member defined in Bristow et al. (1997) [Fig.P895005/10]

Type section

Disused Melbury Quarry [ST 8753 2015] south of Shaftsbury as in Bristow, Mortimore and Wood, (1997) but better defined and seen at the references sections below.

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.

Southerham Grey Pit, Sussex [TQ 427 090].

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

Chinor [SU 754 994] in Oxfordshire.

Formal subdivision

Includes the equivalents of the Melbury Sandstone Member, Glauconitic Marl Member and the Cambridge Greensand Member at the base.

Lithology

Buff, grey and off-white, soft, marly chalk and hard grey limestone arranged in couplets.

Definition of upper boundary

Conformable in ‘full’ sequences at the erosional base of the overlying Cast Bed ‘Member’ (Bed C1 of Gale, 1995) of the Zig Zag Chalk Formation in the Southern Province and by the base of the Totternhoe Stone Member, its lateral equivalent, in the ‘Transitional’ Province north of the Thames. This mid Cenomanian erosional break marks the change from well-defined limestone marl couplets below to chalks with marls above and also the boundary between the ‘B’ and ‘C’ Chalk Marl couplets of Gale (1995). In full sequences, the ‘Tenuis Limestone’ is the bed immediately beneath the mid Cenomanian break.

Definition of lower boundary

Unconformable. Base marked by erosion surface, frequently bored, at the base of the Glauconitic Marl Member (Southern Province), Melbury Sandstone Member (western Southern Province) or Cambridge Greensand Member (‘Transitional’ Province).

Thickness

In general between 15 and 25 m of these beds are preserved but they are absent over the Mid-Dorset Swell (Drummond, 1970; Kennedy, 1970; Bristow et al., 1995) and thin over other syndepositional structural ‘highs’.

Distribution

Throughout the Southern Province and the Transitional Province of England. Is known to be absent or attenuated over structural highs. Most notably the Mid-Dorset Swell in Dorset.

Previous names

In part equivalent to the Chalk Marl of the traditional scheme. Formerly known as the West Melbury Marly Chalk Member as first proposed (Bristow, et al., 1997) and upgraded to formation status in Rawson et al. (2001). Equivalent to the lower part of the East Wear Bay Chalk Formation of Robinson (1986).

Parent

Grey Chalk Subgroup.

Age and biostratigraphy

Upper Cretaceous, Cenomanian. Mantelliceras mantelli to Cunningtoniceras inerme Zones.

References

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

Glauconitic Marl Member

Name

Traditionally termed the Glauconitic Marl for example in Jukes-Browne and Hill (1903). By implication a member in Robinson (1986), and in Survey memoirs in Sussex e.g. Young and Lake (1988) and as defined herein.

Type section

East Wear Bay [TR 261 383], Kent.

Primary reference section

Isle of Wight: coastal section, Compton Bay.

Formal subdivision

None

Lithology

Calcareous glauconitic sand and glauconitic sandy silty chalk with phosphatic nodules.

Definition of upper boundary

Upward cessation of sand and/or abundant glauconitic grains (and phosphatic pebbles)(can occur as a rapid transition over 10 cm or less) into grey, sparsely glauconitic chalky marl or marly chalk, comprising the rest of the West Melbury Marly Chalk Formation and part of the Zig Zag Chalk Formation (broadly equivalent to the traditional scheme 'Chalk Marl').

Definition of lower boundary

Base of glauconite-rich sand or clay resting disconformably, at a strongly burrowed surface, on pale grey siltstones of the Upper Greensand (or grey mudstones of the Gault), which contain only a trace of glauconite at most. [Note: bioturbation may carry glauconitic sediment down into underlying formation].

Thickness

Some 2-4 m throughout much of Southern Province; may be only a few tenths of a metre elsewhere over synsedimentary structural highs. Up to 5 m in Kent and at Eastbourne, East Sussex.

Distribution

Known throughout much of the Southern Province in Kent, Surrey, Sussex, Hampshire and Berkshire.

Previous names

Chloritic Marl in older publications due to the misidentification of the glauconite. Laterally equivalent to the Cambridge Greensand of the ‘Transitional’ Province; and the Melbury Sandstone Member in the western outcrops.

Over the Mid-Dorset Swell and other synsedimentary highs in the western area, Kennedy (1970) uses the term ‘Basement Bed’ to describe the diachronous phosphatic and glauconite-rich basal unit of the Chalk whose age ranges up to the Upper Cenomanian.

Parent

West Melbury Marly Chalk Formation.

Age and biostratigraphy

Upper Cretaceous, Cenomanian. Mantelliceras mantelli Zone, Neostlingoceras carcitanense Subzone. (Note: over synsedimentary highs such as the Mid-Dorset Swell the age range may well included zones up to Acanthoceras rhotomagense).

References

Jukes-Browne and Hill (1903); Kennedy (1969); Robinson (1986); Young and Lake (1988); Lake, et al., 1987).

Melbury Sandstone Member

Name

Defined in Bristow (1989) where it was considered to be the highest unit within the Upper Greensand Formation and formerly known as the Cenomanian Sands, Passage Beds or Warminster Greensand. Regarded in recent BGS publications and herein as the lateral equivalent of the Glauconitic Marl Member.

Type section

Old quarry (now backfilled), 700 m at 276 degrees from Melbury Abbas church. Section recorded by Jukes-Browne and Hill (1900, 1903).

Primary reference section

See above

Formal subdivision

None

Lithology

Richly fossiliferous, glauconitic, fine-grained sand and weakly cemented sandstone.

Definition of upper boundary

The incoming of marl is taken as the basal bed of the overlying remainder of the West Melbury Marly Chalk Formation. This can frequently be a minor erosion surface.

Definition of lower boundary

Above the highest bed of chert of the Boyne Hollow Chert.

Thickness

0-6m (1.22m at type locality).

Distribution

Known for Dorset and Wiltshire

Previous names

Cenomanian Sands, Passage Beds, Warminster Greensand. Laterally equivalent to the Glauconitic Marl Member and the Cambridge Greensand Member. Formerly considered to be the highest member of the Upper Greensand Formation.

Parent

West Melbury Marly Chalk Formation.

Age and biostratigraphy

Upper Cretaceous, Cenomanian. Mantelliceras mantelli Zone, Neostlingoceras carcitanense Subzone.

References

Bristow (1989); Drummond (1970); Kennedy (1970); Jukes-Browne and Hill (1900, 1903); White (1923).

Bookham Conglomerate Bed

Name

The Bookham Conglomerate is defined in Bristow et al. (1995) where it was considered to pass laterally into the basal part of the Melbury Sandstone Member (which itself was considered therein to be the highest unit within the Upper Greensand Formation at that time).

Type section

Bookham Farm [ST 7065 0414]

Primary reference section

See above

Formal subdivision

None

Lithology

Clasts of very glauconitic shelly sandstone, up to cobble size, commonly with a phosphatic rind, together with phosphatised shells (bivalves & ammonites) in a matrix of sandy glauconitic chalk. The derived fauna is Latest Albian; the indigenous fauna is Early Cenomanian.

Definition of upper boundary

At the disconformable contact with phosphatic pebble bed at the base of the Zig Zag Chalk Formation or the West Melbury Marly Chalk Formation (at the base of the Chalk Basement Bed of Kennedy, 1970)

Definition of lower boundary

Unconformable, locally strongly erosive boundary with underlying Shaftesbury Sandstone Member.

Thickness

0-1 m.

Distribution

Known for Dorset and Wiltshire over the Mid-Dorset Swell

Previous names

Passage Beds. Formerly considered to be the highest bed of the Upper Greensand Formation.

Parent

West Melbury Marly Chalk Formation.

Age and biostratigraphy

Upper Cretaceous, Lower and Upper Cenomanian. Up to Acanthoceras rhotomagense Zone,

References

Bristow et al. (1995); Drummond (1970); Kennedy (1970); Jukes-Browne and Hill (1900, 1903); White (1923); Smart (1955).

Cambridge Greensand Member

Name

Known by a number of different names (see below) and given member status in Hancock (1972). Called the Cambridge Greensand on the Hitchin Sheet and formally regarded as a member equivalent to the Glauconitic Marl Member herein.

Type section

Established in brick pits as exposures become available. Currently Arlesey Brickpit. Active clay pit to west of mainline railway, west of Arlesey. Disused parts of pit used for landfill. Varies according to working.

Primary reference section

Arlesey Brickpit [TL 1879 3476] and Borehole TL13SE 45 [TL 1887 3463].

Formal subdivision

None

Lithology

Glauconitic marl: Thin but distinctive condensed basement bed of pale greenish grey marl rich in phosphatic nodules (so called "coprolites") at base. Much dark green glauconite as sand-sized grains, disseminated or concentrated in pods and layers giving a sandy texture and hence name "greensand". Rare erratic pebbles at base (Hawkes, 1943).

Definition of upper boundary

Rapid upward transition to "Chalk Marl" or Lower Chalk lacking distinctive glauconite grains and nodule concentrations.

Definition of lower boundary

Disconformity. Sharp contact; commonly burrowed on underlying grey Gault clay; glauconitic burrow infill to 0.3 below junction.

Thickness

Typically 0.1 - 1.0m thick. Locally thicker developments infilling hollows on top of Gault; 0.5 - 0.7m at Arlesey Brickpit (TL 188 347) (Hopson, 1992).

Distribution

Known from the north Hertfordshire and Cambridgeshire area within the Transitional Province.

Previous names

Cambridge Bed, Chloritic Marl, Coprolite Bed, Glauconitic Marl, Nodule Bed, Phosphate Bed. Laterally equivalent to the Glauconitic Marl and Melbury sandstone members.

Parent

West Melbury Marly Chalk Formation.

Age and biostratigraphy

Upper Cretaceous, Cenomanian. Mantelliceras mantelli Zone, Neostlingoceras carcitanense Subzone.

References

Hancock (1972); Edmonds and Dinham (1965); Fitton (18360; Hart (1973); Hawkes (1943); Hopson (1992); Jukes-Browne (1875); Rawson et al. (1978); Seeley (1866).