| Computer Code: |
BRIG |
Preferred Map Code: |
notEntered |
| Status Code: |
Full |
| Age range: |
Bolsovian Substage (CC)
— Asturian Substage (CAS) |
| Lithological Description: |
The Brig Formation is largely composed of primary red beds, but also contains a thin basal unit of coal measures in many sections (e.g. 53/12-3).
The red-bed unit consists of alternating grey, reddish-brown, or white sandstones and reddish brown, purple, or occasionally waxy, greenish grey mudstones and silty mudstones. Well 53/19a-1 also encountered minor partings of shelly, argillaceous limestone and pale grey, microcrystalline dolomite. The sandstone beds constitute between 15 and 50 per cent of the formation (Tubb et al. 1986), and are up to 35 m thick. Most sandstones are argillaceous and are associated with a blocky log response; some of the thicker beds are composite and include mudstone partings. Grain size of the sandstones is mainly very fine or fine, but coarser beds have also been encountered in Block 53/12.
The coal-measures unit is largely composed of grey, purple and reddish brown mudstones and siltstones, but commonly includes two or three thin coal seams, and locally also includes thin, very fine grained sandstone beds. Sonic-log response for this unit is characteristically spiky, suggesting rapid alternation of these lithologies. |
| Definition of Lower Boundary: |
The base of the Brig Formation is an intra-Westphalian unconformity, generally with late Westphalian C or Westphalian D coal measures resting on Westphalian B coal measures (Westoe Coal Formation), but early Westphalian C Coal Measures are also preserved beneath the unconformity locally. The unconformity in this case is marked by a downward change from rapidly alternating mudstones, siltstones and thin coal seams, associated with a spiky sonic-log response, to a relatively uniform succession of argillaceous coal measures. On wireline logs, this boundary is marked by a sharp downhole increase in velocity (e.g. 53/12-2), and sometimes also by a downhole decrease in gamma-ray response of the mudstones (e.g. 53/19a-1). In a few areas the late Westphalian coal measures are absent and the unconformity separates primary red beds from early Westphalian coal measures (e.g. 53/10-1). In such areas the boundary is marked by the lowest downhole occurrence of primary red beds. |
| Definition of Upper Boundary: |
The Brig Formation is unconformably overlain by Lower Permian aeolian and fluvial desert sandstones (Leman Sandstone Formation) in all sections, and the boundary is marked by a sharp downhole increase in gamma-ray response. |
| Thickness: |
Upper Brig maximum thickness of 277 m ( 53/19a-1); Lower Brig 30 m. |
| Geographical Limits: |
The Brig Formation occurs along the northern flank of the London-Brabant Massif, where it is restricted to the axial regions of NW-SE trending Variscan synclines. |
| Parent Unit: |
Conybeare Group (CNYB)
|
| Previous Name(s): |
none recorded or not applicable
|
| Alternative Name(s): |
none recorded or not applicable
|
| Stratotypes: |
| Type Section |
North Sea well 53/12- 2: 1491.5-1628 m (4893-5342 ft) below KB (Cameron, 1993). |
| Reference Section |
North Sea well 52/05- 11X: 1683-1758.5 m (5522-5770 ft) (Cameron, 1993). |
| Reference Section |
North Sea well 53/12- 3: 1265.5-1423 m (4152-4668 ft) (Cameron, 1993). |
| Reference Section |
North Sea well 53/19a- 1: 1457.5-1753.5 m (4782-5753 ft) (Cameron, 1993). |
| Reference(s): |
| Rhys, G H. 1974. A proposed standard lithostratigraphic nomenclature for the southern North Sea and an outline structural nomenclature for the whole of the (UK) North Sea. Report of the Institute of Geological Sciences, 74/8. |
| 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. |
| Besly, B M. 1990. Carboniferous. 90-119 in Introduction to the Petroleum Geology of the North Sea. Glennie, K W (editor). (Blackwell Scientific Publications, Oxford.) |
| Mykura, W. 1960. The replacement of coal by limestone and the reddening of Coal Measures in the Ayrshire Coalfield. Bulletin of the Geological Survey of Great Britain 16, 69-109. |
| Trotter, P M. 1953. Reddened beds of Carboniferous age in north-west England and their origin. Proceedings of the Yorkshire Geological Society 29, 1-20. |
| Tubb, S R, Soulsby, A and Lawrence, S R. 1986. Palaeozoic prospects on the northern flanks of the London-Brabant Massif. In: Brooks, J, Goff, J C and Van Hoorn, B (eds.) Habitat of Palaeozoic gas in NW Europe. Geological Society, London, Special Publication No.23, 55-72. |
| 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. |
| 1:50K maps on which the lithostratigraphical unit is found, and map code used: |
| none recorded or not applicable |
