The BGS Lexicon of Named Rock Units — Result Details
Cementstone Formation
| Computer Code: | CMNT | Preferred Map Code: | notEntered |
|---|---|---|---|
| Status Code: | Full | ||
| Age range: | Courceyan Substage (CF) — Chadian Substage (CI) | ||
| Lithological Description: | The Cementstone Formation consists of sandstones and shaly mudstones with thin beds of dolomite and dolomitic limestone. The sandstones are grey or white, and in well 44/2-1 are thinly bedded and mainly fine grained. However, they are very fine to medium, or occasionally coarse grained, and moderately or poorly sorted in well 43/2-1, where they form beds up to 25 m thick that are commonly cemented by calcite. Most sandstone beds display blocky log motifs, but some form upward-fining units. The mudstones are silty, yellowish brown, reddish brown or grey, and non-calcareous. Dark grey, carbonaceous and pyritic mudstone beds have also been reported from well 43/2-1. The dolomites are microcrystalline or, locally, cryptocrystalline and are brown, brownish grey or grey, and commonly argillaceous. They are responsible for the spiky velocity log signature that characterizes the Cementstone Formation in well 44/2-1. In well 43/2-1, the spiky signature is muted because there are fewer dolomite beds. Cyclicity of facies, which characterizes the Cementstone Group on the Berwickshire coast (Smith 1967), is also apparent in well 43/2-1. | ||
| Definition of Lower Boundary: | Offshore, the base of the Cementstone Formation has been penetrated only in well 44/2-1, where it is defined by the velocity-log spike corresponding to the lowest Cementstone bed. In north-east England, the base of the Cementstone Group occurs within a gradual transition from terrestrial red-bed facies to mixed lacustrine/quasi-marine and fluvial sediments (George et al., 1976). | ||
| Definition of Upper Boundary: | The top of the Cementstone Formation in well 43/2-1 is defined by a downward change from massive sandstones of the Fell Sandstone Formation to more thinly bedded sandstones, dolomites and mudstones. It is marked by a sharp downward increase in gamma-ray response. The facies change is more gradational in well 44/2-1, where the boundary is placed along the top of the uppermost Cementstone bed and sonic-log spike. In onshore sections, the boundary with the Fell Sandstone is associated with a downward change from mica-free to micaceous sandstone (Smith 1967); this feature may help locate the Fell Sandstone / Cementstone formation boundary offshore in relatively sandy sections that lack significant cementstone development. | ||
| Thickness: | 183 m thick in well 44/2-1, and at least 390 m thick in well 43/2-1. | ||
| Geographical Limits: | The Cementstone Formation may be continuous from the Northumberland coast across western crestal areas of the Mid North Sea High, and along its southeastern flank to the median line. The southern limit of the formation cannot be defined offshore because there are no well penetrations of Dinantian strata in the central Southern North Sea area. The formation is 183 m thick in well 44/2-1, and at least 390 m thick in well 43/2-1. | ||
| Parent Unit: | Farne Group (FRNE) | ||
| Previous Name(s): | Cementstone Group (B) [Obsolete Name and Code: Use INV, BGN, CMNT]
(CMST)
Cementstone Group (A) [Obsolete Name and Code: Use INV, BGN, CMNT] (CST) |
||
| Alternative Name(s): | none recorded or not applicable | ||
| Stratotypes: | |||
| Reference Section | North Sea well 43/02- 1: 3465.5-3855 m TD (11370-12647 ft TD) below KB (Cameron, 1993). | ||
| Reference Section | North Sea well 44/02- 1: 3198-3381 m (10493-11092 ft) (Cameron, 1993). | ||
| Reference(s): | |||
| Waters, C N, Smith, K, Hopson, P M, Wilson, D, Bridge, D M, Carney, J N, Cooper, A H, Crofts, R G, Ellison, R A, Mathers, S J, Moorlock, B S P, Scrivener, R C, McMillan, A A, Ambrose, K, Barclay, W J, and Barron, A J M. 2007. Stratigraphical Chart of the United Kingdom: Southern Britain. British Geological Survey, 1 poster. | |||
| Cameron, T D J. 1993. 5. Carboniferous and Devonian of the Southern North Sea. In: Knox, R W O'B and Cordey, W G (eds.) Lithostratigraphic nomenclature of the UK North Sea. British Geological Survey, Nottingham. | |||
| George, T N, Johnson, G A L, Mitchell, M, Prentice, J E, Ramsbottam, W H C, Sevastopulo, G D and Wilson, R B. 1976. A correlation of the Dinantian rocks of the British Isles. Special Report of the Geological Society of London, No 7. | |||
| Smith, T E. 1967. A preliminary study of sandstone sedimentation in the Lower Carboniferous of the Tweed Basin. Scottish Journal of Geology 3, 282-305. | |||
| 1:50K maps on which the lithostratigraphical unit is found, and map code used: | |||
| E007 E001 E002 | |||