The BGS Lexicon of Named Rock Units — Result Details

Firth Coal Formation

Computer Code: FICO Preferred Map Code: notEntered
Status Code: Full
Age range: Arundian Substage (CJ) — Arnsbergian Substage (CG)
Lithological Description: The Firth Coal Formation is mainly composed of cyclic alternations of sandstones and mudstones, but it includes occasional coal seams and local limestone beds and volcanic horizons. The sandstones are mostly white or brownish grey in colour. They are rather variable in grain size, but sorting is moderate or good, and grainstone textures predominate. Ferroan calcite cement can comprise up to 40 per cent of the total rock (e.g. in the Claymore Field, Maher and Harker, 1987). The siltstones and mudstones are generally grey or dark grey, but are occasionally yellowish green, purple, or reddish brown. A few thin beds of white and brownish grey, microcrystalline limestone have been recorded from the Forth Approaches Basin. A thin bed of oolitic limestone was also encountered in well 26/7-1, but contemporary limestones are notably rare or absent in the Outer Moray Firth Basin (Leeder and Boldy, 1990). Sandstone constitutes between 10 and 50 per cent, siltstone and mudstone between 50 and 90 per cent, and coal up to 13 per cent of the Outer Moray Firth coal measures (Leeder and Boldy, 1990). Leeder and Boldy (1990) subdivided the Outer Moray Firth coal measures into five discrete lithofacies (Lithofacies 1 to 5). Comparable lithofacies can be recognized in the contemporary coal-measures sequences elsewhere in the Central North Sea.
Definition of Lower Boundary: The Firth Coal Formation is underlain by the Tayport Formation in all wells where the base has been penetrated. The boundary between the two formations is defined by the base of the lowest coal seam. A downward transition from grey to red-brown mudstones occurs close to the boundary, except in Block 21/2, where the colour change takes place lower in the Tayport Formation. In most sections the Tayport Formation is characterized by a higher proportion of sandstone (e.g. 26/7-1).
Definition of Upper Boundary: The Firth Coal Formation is overlain by Permian or post-Permian sediments in all sections, with a marked downward change to grey, carbonaceous or coalbearing sediments.
Thickness: The Firth Coal Formation occupies a broad, synclinal basin (Leeder and Boldy, 1990), and is more than 1500 m thick at the centre (Andrews et al., 1990). South of the Buchan Horst, equivalent coal measures may be widespread, in an offshore continuation of the Scottish Midland Valley (Gatliff, 1994). Well 26/7-1 penetrated 551 m of these coal measures; equivalent sequences are locally more than 3000 m thick in the hanging walls of growth faults onshore, in central Scotland (Cameron and Stephenson, 1985). As onshore, early Carboniferous coal measures may be thin or absent south of the Southern Uplands Fault and its offshore continuation (Gatliff, 1994). However, wells 31/27-1 and 39/7-1, on the northeastern flank of the Mid North Sea High, encountered poorly dated coal measures which are assigned to the Scremerston Coal Formation.
Geographical Limits: Eastern Quadrant 14 and central Quadrant 15.
Parent Unit: Not Applicable (-)
Previous Name(s): none recorded or not applicable
Alternative Name(s): none recorded or not applicable
Stratotypes:
Reference Section  North Sea well 15/19- 2: 2016-2613.5 m TD (6614-8575 ft TD) (Cameron, 1993). 
Reference Section  North Sea well 20/10a- 3: 3515-3922 m (11533-12867 ft) (Cameron, 1993). 
Type Section  North Sea well 26/07- 1: 1285-1836 m (4216-6024 ft) below KB (Cameron, 1993). 
Reference Section  North Sea well 14/19- 1: 2434-2920 m TD (7986-9580 ft TD) (Cameron, 1993). 
Reference(s):
Cameron, T D J. 1993. 4. Triassic, Permian and pre-Permian of the Central and Northern North Sea. In: Knox, R W O'B and Cordey, W G (eds.) Lithostratigraphic nomenclature of the UK North Sea. British Geological Survey, Nottingham. 
Andrews, I J, Long, D, Richards, P C, Thomson, A R, Brown, S, Chesher, J A, and McCormac, M. 1990. The geology of the Moray Firth. (London: British Geological Survey.) 
Armstrong, L A, Ten Have, A, and Johnson, H D. 1987. The geology of the Gannet Fields, Central North Sea. In Brooks, J and Glennie, K W (eds.). Petroleum geology of North West Europe, 533-548 [Graham and Trotman, London]. 
Cameron, I B and Stephenson, D, 1985. British Regional Geology: The Midland Valley of Scotland (3rd Edition). (London: HMSO for British Geological Survey). 
Forsyth, I H and Chisholm, J I, 1977. The geology of east Fife. Memoirs of the Geological Survey of Great Britain. 
Francis, E H. 1991. Carboniferous. 347-392 in Geology of Scotland (3rd edition). Craig, G Y (editor). (London: The Geological Society.) 
Gatliff, R W, Richards, P C, Smith, K, Graham, C C, McCormac, M, Smith, N J P, Long, D, Cameron, T D J, Evans, D, Stevenson, A G, Bulat, J, and Ritchie, J D. 1994. United Kingdom offshore regional report: the geology of the central North Sea. (London: HMSO for the British Geological Survey.) 
Harker, S D, Green, S C H and Romani, R S. 1991. The Claymore Field, Block 14/19, UK North Sea. In: Abbotts, I L (ed.) United Kingdom oil and gas fields, 25 years commemorative volume. Memoir of the Geological Society, London, 14, 269-278. 
Knox, R W O'B and Holloway, S. 1992. 1. Paleogene of the Central and Northern North Sea. In: Knox, R W O'B and Cordey, W G (eds.) Lithostratigraphic nomenclature of the UK North Sea. British Geological Survey, Nottingham. 
Maher, C E and Harker, S D. 1987. Claymore OIlfield. In: Brooks, J and Glennie, K W (eds.) Petroleum Geology of North West Europe, 835-845. [Graham & Trotman, London] 
Stoker, M S, Long, D, and Fyfe, J A. 1985. A revised Quaternary stratigraphy for the central North Sea. Report of the British Geological Survey, Vol. 17, No. 2. 
Leeder, M R, and Boldy, S R. 1990. The Carboniferous of the Outer Moray Firth Basin, quadrants 14 and 15, Central North Sea. Marine and Petroleum Geology 7, 29-37. 
Waters, C N, Gillespie, M R, Smith, K, Auton, C A, Floyd, J D, Leslie, A G, Millward, D, Mitchell, W I, McMillan, A A, Stone, P, Barron, A J M, Dean, M T, Hopson, P M, Krabbendam, M, Browne, M A E, Stephenson, D, Akhurst, M C, and Barnes, R P. 2007. Stratigraphical Chart of the United Kingdom: Northern Britain. (British Geological Survey.) 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
none recorded or not applicable