The BGS Lexicon of Named Rock Units — Result Details

Westoe Coal Formation

Computer Code: WSCO Preferred Map Code: notEntered
Status Code: Full, pending ratification
Age range: Langsettian Substage (CA) — Bolsovian Substage (CC)
Lithological Description: The Westoe Coal Formation is dominated by grey mudstones and silty mudstones, but contains varying proportions of seat earths, coal seams, and sandstone beds. Beds of dark grey or black marine mudstone form a minor component of most sections, but form marker horizons on spectral gamma-ray logs. White and grey, fine to very fine grained sandstone beds occur but constitute less than 10 per cent of the formation and are mostly less than 10 m thick. Coal seams up to 5 m thick are conspicuous in all sections, and constitute up to 5 per cent of the formation. Some of these may be very widespread. All sections show wireline-log responses indicative of small-scale, upward-coarsening sedimentary cycles, i.e. a gradual upward increase in velocity and resistivity, followed by a distinct trough. They characteristically display an upward gradation from mudstone to siltstone, with or without a sandstone bed, and are capped by a coal seam or carbonaceous mudstone. The cycles are mostly less than 20 m thick, but are exceptionally up to 50 m thick.
Definition of Lower Boundary: The Westoe Coal Formation generally rests on sandy coal measures (Caister Coal Formation) north of 53°30'N. The lithofacies transition is typically sharp (e.g. 44/27-1; 44/23-4), with bed thickness and abundance of the sandstones significantly reduced above the boundary. The top of the highest sandstone bed of the sandy coal measures defines the formational boundary. Because sandy coal measures are largely absent south of 53°30'N, the Westoe Coal Formation has a wider stratigraphic range and commonly rests on the Millstone Grit Formation. The lithofacies transition is marked by a downward change from coal-bearing to coal-free strata. The formation boundary is defined by the base of the lowest coal seam; it is easily identified in most long sections (e.g. 52/5-11X), but is less clear in sections where the Westphalian A strata include relatively few coal seams. Along the northern flank of the London-Brabant Massif, the Westoe Coal Formation oversteps the Millstone Grit Formation on to Dinantian platform carbonates or Lower Paleozoic basement.
Definition of Upper Boundary: Over large areas the Westoe Coal Formation is unconformably overlain by Lower Permian desert sediments, with secondary reddening of the uppermost coal measures penetrating up to 30 m below the unconformity in some sections. In such areas south of 54°N, the formation is normally overlain by the mainly uniform, red or grey sandstones of the Leman Sandstone Formation, whereas to the north the overlying Lower Permian is largely composed of lacustrine mudstones and evaporites. The downward facies transition to coal measures is generally sharp. Even where this transition is not clear from wireline logs, the boundary may be recognized by a downward change from the brighter hues of the Permian red-beds to the duller hues of the secondarily reddened sediments of the Westoe Coal Formation. Along the northern flank of the London-Brabant Massif, younger Westphalian C-D coal measures and primary red beds (Brig Formation) are preserved above the Westoe Coal Formation in the axes of Variscan synclines. The boundary is commonly defined by a downward change from thinly bedded mudstones, siltstones and coal seams (Lower Brig unit), associated with a spiky sonic-log response, to a cyclic coal-measures facies. On wireline logs, the boundary is marked by a sharp downhole increase in velocity (e.g. 53/12-2), that is sometimes accompanied by a downhole decrease in gamma-ray response of the mudstones (e.g. 53/12-3, 53/19a-1). Primary red beds (Upper Brig unit) rest unconformably on the Westoe Coal Formation locally (e.g. 53/10-1), the boundary being marked by the lowest occurrence of red mudstones. Farther north, in the Silver Pit area and in the axes of other deep Variscan synclines, the basal sandy coal-measures unit of the Schooner Formation rests conformably on the Westoe Coal Formation. The downward transition to argillaceous coal measures is marked by a sharp decrease in abundanceand bed thickness of the sandstones.
Thickness: 450 m +.
Geographical Limits: In the north, the Westoe Coal Formation is largely confined to the axes of regional Variscan synclines, and ranges up to 450 m in thickness. The formation is more widespread south of 53°30'N, and is commonly thicker, reflecting its wider stratigraphic range in this area. The southern limit of the formation is determined by pinch out between base-Silesian and base-Permian unconformities.
Parent Unit: Conybeare Group (CNYB)
Previous Name(s): none recorded or not applicable
Alternative Name(s): none recorded or not applicable
Type Section  North Sea well 44/27-1: 4438-4891 m (14561-16047 ft) below KB. 
Reference Section  North Sea well 41/20-1: 2050.5-2190.5 m (6727-7187 ft). 
Reference Section  North Sea well 44/21-2: 4203-4411.5 m (13789-14473 ft). 
Reference Section  North Sea well 52/5-11X: 1758.5-2215 m (5770-7266 ft). 
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. 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
none recorded or not applicable