The BGS Lexicon of Named Rock Units — Result Details
Brora Coal Formation
| Computer Code: | BOCO | Preferred Map Code: | notEntered |
|---|---|---|---|
| Status Code: | Full | ||
| Age range: | Bathonian Age (JN) — Callovian Age (JC) | ||
| Lithological Description: | Sandstone, medium-grained, white, soft, pure quartz, massive and cross-bedded, cross-laminated and parallel laminated, up to 20 m thick (Doll Sandstone Unit (=Bed) of Hurst, 1981), overlain by over 30 m of grey shaly mudstone with thin interbedded sideritic mudstone beds (cementstones of early usage) that are laterally extensive; together these two units constitute the Doll Member. Its top is marked by a prominent brecciated siderite-cemented mudstone horizon (Bed 1 of Hurst, 1981). Overlying Bed 1 are about 15 m of grey and black, laminated, bituminous fissile mudstones, some of which approach oil shale composition. Two coals are developed in this succession (the Inverbrora Member), including a thin coal developed about 8 m below the top of the formation and a prominent seam, 1 m thick, forming the uppermost bed and termed the Brora Coal (Bed). Biota is relatively sparse and comprises bivalves, plant debris and fish scales. Two bivalve-rich beds, largely comprising Isognomon and Neomiodon are present between 3 and 4 m below the top of the formation and these beds were interpreted as a maximum flooding surface by Stephen and Davies (1998). Offshore, in the Inner Morray Firth, the formation comprises interbedded, dark grey, carbonaceous, waxy mudstones and greenish grey sandy mudstones, sandstones and coals. Mudstones, which contain sideritic spheres, show convolute bedding. Sandstones are white to pale grey, well sorted and fine- to medium-grained, with cross-bedding and ripple lamination. The sandstones have a sharp base and grade up into mottled, bioturbated and veined mudstone with rootlets and coaly zones. Cyclicity can be recognised in the sandstones (Linsley et al., 1980; Stevens, 1991). Coals occur at various levels in some localities (e.g. Well 11/25-1). At the top of the formation, a thin coaly section, which is probably equivalent to the 'Brora Coal Bed' onshore, varies from about 1m thick (e.g. Well 11/30-1) to thin seams within mudstones. | ||
| Definition of Lower Boundary: | The base of the Brora Coal Formation is not exposed onshore. The putative contact is thought to be a sharp unconformable contact between the grey mudstones of the Lady's Walk Shale Member (Dunrobin Bay Formation; Lower Jurassic, Hettangian to Pliensbachian) and the white medium sandstone of the Doll Sandstone Unit of the Doll Member. Offshore in the Great Glen Sub-Basin in the western part of the Inner Moray Firth, the Brora Coal Formation overlies the Orrin Formation. Richards et al. (1993, p.79) illustrated the boundary between the fine to medium-grained sandstones of the Orrin Formation and the interbedded sandstones and mudstones of the Brora Coal Formation as unconformable. However in well 11/25-1, this boundary is gradational and is marked by a transitional passage from the massive sandstones of the Orrin Formation to the interbedded mudstones and sandstones with sporadic coaly horizons of the Brora Coal Formation (Richards et al., 1993, p.78). Further offshore the Brora Coal Formation is partially or entirely absent due to erosion (Andrews et al., 1990, figs.27, 28), and in some wells the Brora Coal (Bed) directly overlies the Orrin Formation (Richards et al., 1993, panel 8). The two members recognised onshore, comprising siderite-cemented mudstones and quartzose sandstone (Doll Member) and black, carbonaceous shales (Inverbrora Member), have not been recognised offshore. | ||
| Definition of Upper Boundary: | Onshore, the boundary with the overlying unit is sharp and planar and although apparently conformable is probably erosional. The Brora Coal is overlain by the transgressive, intensely bioturbated medium-grained sandstone of the Brora Roof Bed, which is the lowermost bed of the Brora Shale Member (Strathsteven Mudstone Formation) at Brora, or of the Cadh'-an-Righ Shale Member (Balintore Formation) at Balintore. Offshore, the boundary with the overlying Beatrice Formation is a sharp, planar junction between a coal-bearing mudstone or coal, which is equivalent to the Brora Coal (Bed) and the fine to medium-grained sandstones of the Beatrice Formation (Richards et al., 1993, p.78). On wireline logs the boundary is marked by two low velocity spikes. | ||
| Thickness: | Onshore, at the type section, Hurst (1981) reported a thickness of at least 65 m. This contrasts with at least 31.5 m reported by Neves and Selley (1975). A significant part of the Doll Member forms approximately half the thickness of the section at a foreshore outcrop [NC 893 025] 400 m north-east of Sputie Burn (Hurst, 1981, p.174). South of Brora at Balintore [NH 851 728], the Brora Coal Bed is significantly thinner than at Brora, where it is only 20 cm thick. The total thickness of the underlying Brora Coal Formation at Balintore is unknown because the base is not exposed. Approximately 3 m of the Inverbrora Member have been observed and Judd (1873) reported at least 8 m. Offshore in the western part of the Inner Moray Firth Basin, the Brora Coal Formation is significantly thicker than observed onshore; for example, in offshore wells 11/25-1, 11/30a-9 and 12/21-3, it is 116, 183 and 113 m thick respectively (Richards et al., 1993, p.79). | ||
| Geographical Limits: | The Brora Coal Formation is confined to the Moray Firth area, north-east Scotland. Onshore, it forms an outcrop, bounded to the north-west by a fault, along a coastal strip from the type section [NC 900 030], south of the River Brora estuary, and further south-west along the foreshore, to a point [NC 893 025] 400 m north of Sputie Burn (Hurst, 1981, fig.1), and is thought to continue at depth through to Strathsteven [NC 88 01]. It is also at depth north-west of the fault in the Brora area. It crops out on the foreshore south of Balintore [NH 849 723 to 857 739]. This is a narrow sliver, between the Great Glen Fault Zone offshore to the east and a parallel fault in the cliffs to the west (Riding, 2005, fig.1). Offshore it is present in the western part of the Inner Moray Firth Basin, where it thins rapidly toward the east (Andrews et al., 1990, figs.27, 28) (North Sea quadrants 11, 12, 17, and 18). | ||
| Parent Unit: | Fladen Group (FLDN) | ||
| Previous Name(s): | Estuarine Series [Obsolete Name and Code: Use BOCO]
(-4999)
Brora Lower Oolite (-5218) Brora Coal (-5216) Brora Coal Member (-5217) |
||
| Alternative Name(s): | none recorded or not applicable | ||
| Stratotypes: | |||
| Type Section | Foreshore south of the River Brora estuary, Sutherland [NC 900 030], between the Brora Fault and the railway, and the sea. | ||
| Reference Section | North Sea well 11/25- 1 between 2975 m and 3141 m (Richards et al., 1993). | ||
| Reference Section | North Sea well 11/30a- 9 between 2850 m and 3033 m (Richards et al., 1993). | ||
| Reference Section | North Sea well 12/21- 3 between 2805 m and 2918 m (Richards et al., 1993). | ||
| Type Area | Brora area, Sutherland, Scotland. | ||
| Reference(s): | |||
| Barron, A J M, Lott, G K, and Riding, J B. 2012. Stratigraphical framework for the Middle Jurassic strata of Great Britain and the adjoining continental shelf. British Geological Survey Research Report RR/11/06. | |||
| Hurst, A R. 1981. Mid Jurassic stratigraphy and facies at Brora, Sutherland. Scottish Journal of Geology, Vol. 17, p. 169 - 177. | |||
| Hudson, J D. 1962. The Great Estuarine Series (Middle Jurassic) of the Inner Hebrides. Unpublished PhD thesis, University of Cambridge, Cambridge, UK, 350pp. | |||
| Judd, J W. 1873. The secondary rocks of Scotland. Quarterly Journal of the Geological Society, London, Vol.29, 97-195. | |||
| Stephen, K J, and Davies, R J. 1998. Documentation of Jurassic sedimentary cycles from the Moray Firth Basin, United Kingdom North Sea. 481-506 in Mesozoic and Cenozoic Sequence Stratigraphy of European Basins. SEPM Special Publication 60. Graciansky, P-C, Hardenbol, J, Jacquin, T, and Vail, P R (editors). | |||
| Maclennan, A M, and Trewin, N H. 1989. Palaeoenvironments of the late Bathonian-mid Callovian in the Inner Moray Firth. Northwest European Micropalaeontology and Palynology. British Micropalaeontological Society Series. Batten, D J, and Keen, M C (editors). (Chichester: Ellis Horwood Limited.) | |||
| Riding, J B. 2005. Middle and Upper Jurassic (Callovian to Kimmeridgian) palynology of the onshore Moray Firth Basin, northeast Scotland. Palynology, Vol. 29, 87-142. | |||
| 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. British Geological Survey United Kingdom offshore regional report (London). | |||
| Read, H H and Phemister, J. The Geology of the country around Golspie, Sutherlandshire (Strath Fleet, Strath Brora and Glen Loth). Memoirs of the Geological Survey of Scotland, 143pp. | |||
| Stevens, V. 1991. The Beatrice Field, Block 11/30a, 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, 245-252. | |||
| Linsley, P N, Potter, H C, McNab, G and Racher, D. 1980. The Beatrice Oil Field, Inner Moray Firth, UK North Sea. In: Halbouty, M T (ed.) Giant oil and gas fields of the decade, 1968-1978. American Association of Petroleum Geologists, Memoir, 30, 117-129. | |||
| Lee, G.W., 1925. Mesozoic rocks of East Sutherland and Ross, In: Read, H.H., Ross, G., Phemister, J. (Eds.), The geology of the country around Golspie, Sutherlandshire. Memoir of the Geological Survey, pp. 65-114. | |||
| Neves, R and Selley, R C. 1975. A review of the Jurassic rocks in north-east Scotland. In: Finstead, K.G. & Selley, R.C. (eds) Proceedings of the Jurassic Northern North Sea Symposium, Stavanger 28-30 September 1975, 5/1-5/21. | |||
| Richards, P C, Lott, G K, Johnson, H, Knox, R W O'B. and Riding, J B. 1993. 3. Jurassic 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. | |||
| 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 | |||