The BGS Lexicon of Named Rock Units — Result Details
Clifton Down Mudstone Formation
| Computer Code: | CDM | Preferred Map Code: | notEntered |
|---|---|---|---|
| Status Code: | Full | ||
| Age range: | Arundian Substage (CJ) — Arundian Substage (CJ) | ||
| Lithological Description: | Thin- to medium-bedded poorly fossiliferous, calcite mudstones, dolomite mudstones and mudstones. A unit of crinoidal and oolitic limestones occurs in the middle of the Formation, and a 15m-thick unit of oolitic and crinoidal limestone occurs in the upper part of Formation [said to correlate with the Goblin Combe Oolite Formation of Broadfield Down, but this is not proven]. Basalt, tuff and ashy limestone are present in the middle of the Formation on Broadfield Down. Carbonates of the Formation were deposited mainly in a back barrier setting. | ||
| Definition of Lower Boundary: | Taken at irregular erosive contact of dolomite and calcite mudstones and mudstones of the formation above the clay palaesol of the underlying Gully Oolite Formation. | ||
| Definition of Upper Boundary: | In the Bristol area the upper boundary is taken at the sharp contact between the interbedded dolomite and calcite mudstones and mudstones of the formation and the basal sandy limestone of the overlying Clifton Down Limestone Formation. The boundary needs further investigation. In Broadfield Down the upper boundary is taken at the incoming of ooidal limestones of the Goblin Combe Oolite Formation above the interbedded dolomite and calcite mudstones and mudstones of the Clifton Down Mudstone Formation. | ||
| Thickness: | 60 m at Bristol and northern Broadfield Down thinning to a feather edge on the southern side of Broadfield Down. | ||
| Geographical Limits: | Bristol [ST 60 74] area and Broadfield Down [ST 50 65]. Passes northwestwards into the Llanelly Formation of South Wales and southwards into the Burrington Oolite Subgroup of the Mendip Hills. | ||
| Parent Unit: | Pembroke Limestone Group (PEMB) | ||
| Previous Name(s): | Caninia Shales And Dolomite
(-760)
Lower Clifton Down Mudstone [Obsolete Name and Code: Use CDM] (-2066) Middle Limestone Shales [Obsolete Name and Code: Use CDM] (-2067) Upper Clifton Down Mudstone Formation [Obsolete Name And Code: Use CDM] (UCDM) Lower Clifton Down Mudstone Formation [Obsolete Name And Code: Use CDM] (LCDM) Upper Clifton Down Mudstone [Obsolete Name and Code: Use CDM] (-118) |
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| Alternative Name(s): | Whitehead Limestone [Obsolete: use LLY] |
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| Stratotypes: | |||
| Type Section | Portway road section, east bank of the Avon Gorge, Clifton, Bristol. Entire unit (59 m) exposed, comprising calcite and dolomite mudstones and mudstones with a 15m-thick unit of crinoidal and ooidal limestone in the upper part. Lower contact with Gully Oolite Formation seen, as is the upper contact with the Clifton Down Limestone Formation (Kellaway and Welch, 1993). | ||
| Reference(s): | |||
| Kellaway, G A and Welch, F B A. 1955. The Upper Old Red Sandstone and Lower Carboniferous rocks of the Bristol and the Mendips compared with those of Chepstow and the Forest of Dean. Bulletin of the Geological Survey of Great Britain, No.9, p.1-21. | |||
| Kellaway, G A and Welch, F B A. 1993. Geology of the Bristol district. Memoir of the British Geological Survey Special Sheet [England and Wales]. | |||
| Morgan, C Lloyd. 1885. Subaerial denudation of the Avon Gorge. Proceedings of the Bristol Naturalists' Society, New Series, Vol.4, 171-197. | |||
| Ramsbottom, W H C. 1973. Transgressions and regressions in the Dinantian: A new synthesis of British Dinantian stratigraphy. Proceedings of the Yorkshire Geological Society, Vol.41, 261-291. | |||
| Vaughan, A, 1905. The palaeontological sequence in the Carboniferous Limestone of the Bristol area. Quarterly Journal of the Geological Society of London, Vol.61, 181-307. | |||
| 1:50K maps on which the lithostratigraphical unit is found, and map code used: | |||
| E250 E251 E264 E265 | |||