Pennine Lower Coal Measures Formation
- 1 Pennine Lower Coal Measures Formation (PLCM), Carboniferous, Northern England Province
- 2 References
Pennine Lower Coal Measures Formation is part of the Pennine Coal Measures Group
The formation has historically been referred to as the Lower Coal Measures, as defined by Stubblefield and Trotter (1957). The name was applied across Britain, despite different boundary definitions existing between England and Scotland (Browne et al., 1999). To distinguish the succession present within the Pennine Basin from that present within the Midland Valley of Scotland and the South Wales Basin, the formation has been renamed the Pennine Lower Coal Measures Formation.
Interbedded grey mudstone, siltstone and pale grey sandstone, commonly with mudstones containing marine fossils in the lower part, and more numerous and thicker coal seams in the upper part. In the Lancashire and East Pennines coalfields the formation can be broadly divided into three unnamed members (Chisholm, 1990; Aitkenhead et al., 2002). From the base of the Subcrenatum Marine Band to the 80 Yard Coal (Waters et al., 2009, fig. 13) or Pasture Coal (Waters et al., 2009, fig. 14) the cyclothems usually have a marine band at the base and a palaeosol at the top, and include micaceous sandstones. Between the 80.Yard or Pasture coals and the basin-wide Arley Coal (Waters et al., 2009, fig. 13) or Kilburn (Better Bed) Coal (Waters et al., 2009, fig. 14), coal seams are thin and rare, only the palaeosol beneath the Kilburn–Arley coals is notably leached and marine band faunas are very restricted. The sandstones include both micaceous and green, weakly micaceous sandstones. Between the Kilburn–Arley coals and the base of the Vanderbeckei Marine Band (Waters et al., 2009, figs. 13, 14) there is a thick succession of laterally impersistent cyclothems that lack true marine bands and have thick coals. In the subsurface of the East Midlands, coal and oil exploration has revealed an abundance of alkali basalt lavas and tuffs, limited to the Pennine Lower Coal Measures Formation. In the Northumberland and Durham Coalfield, sheet-like, coarse-grained sandstones, common marine bands and thin coals, are present in the Pennine Lower Coal Measures.
The deposits accumulated in a delta-top environment with large distributary channels. The main channels, up to 20 m thick and 20 km wide, were filled by relatively thick, sharp-based sandstones (Guion et al., 1995). Between the channels were freshwater lakes and lagoons associated with deposition of mudstones. The lakes and lagoons were filled by small deltas and crevasse splays producing the upward-coarsening siltstones and sandstones. Near-emergent surfaces became swamps or raised bogs colonised by plants which following burial formed coals. Subsidence rates were low along the southern margin of the Pennine Basin resulting in relatively few thick seams (Waters et al., 1994). Northwards, towards the basin depocentre subsidence rates are greater and seams split. The cyclothems are typically upward-coarsening lake-fills with the principal driving force being continuous subsidence.
Some of the cycles, particularly in the early Langsettian, commence with laterally widespread marine bands that probably result from sea level rises (Aitkenhead et al., 2002). The sandstones present within the lower part of the formation are considered to have the same source from the north or north-east as the underlying Millstone Grit (Chisholm et al., 1996). The middle part of the formation includes micaceous sandstones sourced from the north, and weakly micaceous sandstones sourced from the west (Chisholm, 1990; Chisholm et al., 1996; Hallsworth and Chisholm, 2000). The latter have the same source as the Haslingden Flags of the Millstone Grit of Yeadonian age. The upper part of the formation includes sandstones, which appear to be sourced from the west (Hallsworth and Chisholm, 2000; Chisholm and Hallsworth, 2005).
The type area is the North Staffordshire (Potteries) Coalfield, Stoke-on-Trent (SJ 50 90), where there are numerous borehole and shaft sections, but few exposures (see Waters et al., 2009, fig..13). Reference sections include, for the top boundary: mudstone in the Vanderbeckei Marine Band at Miry Wood, Apedale (SJ 8118 4940), which is to be designated a RIGS site (Rees and Wilson, 1998). For the top part: from 640 to 318.5 m in the constructed stratigraphical section (V5 British Coal, G7 British Geological Survey), Hesketh Back Cut (Crosscut), at Chatterley Whitfield Colliery, Stoke-on-Trent (SJ 884 533) (Rees and Wilson, 1998). For the lower part and base: from surface to 550.7 m depth in the Ridgeway Borehole (BGS Registration Number SJ85SE/14) (SJ 8923 5381) (Rees and Wilson, 1998). For the base: the River Little Don, Langsett (SE 2215 0041), which is the basal Stratotype for the Langsettian Stage with a section up to 2 m thick including the Subcrenatum Marine Band (Owens et al., 1985); and the Ballavaarkish (Shellag North) Borehole (NX 4625 0070), north Isle of Man, which includes the Pennine Lower Coal Measures Formation from 120.75 m (the top of the cored section) to the base of the Subcrenatum Marine Band at about 136.8 m depth (see Chadwick et al., 2001).
Lower and upper boundaries
The base of the formation, as defined by Stubblefield and Trotter (1957), is taken at the base of the dark grey fissile mudstone of the Subcrenatum Marine Band (SBMB) with eponymous fossils, or at the base of the lowest coal of the coal-bearing sequence if this marker cannot be recognised. Typically, the formation rests conformably upon the Rossendale Formation (Millstone Grit Group) (Figure.8, Column.9; Figure.9, Column 17). However, in north, west and east Cumbria (Figure 9, Column 13; Figure 14, Column 1) the Solway Basin (Figure 6, Column 7) (except the Canonbie area), the Stainmore (Figure 9, Column.16) and Northumberland troughs (Figure 8, Column 11; Figure 13, Columns 3, 4), on the Alston Block (Figure 9, Column 15) and in north-east Northumberland (Figure 8, Column 12), the formation overlies conformably the repetitive mudstones, siltstones, sandstones, thin limestones and thin coals of the Stainmore Formation (Yoredale Group). In the Canonbie Coalfield (Figure 8, Column 10), the formation is underlain by the Stainmore Formation (Yoredale Group) though the stratigraphical position and nature of the boundary is uncertain since the Subcrenatum Marine Band has not been recognised there (see Jones and Holliday, 2006).
The top of the formation is taken at the base of the mudstone of the Vanderbeckei Marine Band (VDMB) with eponymous fossils (Figure 8, Columns 10, 12; Figure.9, Columns 13, 16; Figure 10, Column 3; Figure 11, Column.1; Figure 13, Columns 3, 4; Figure 14, Columns 1, 2).
Up to 650.m thick in the North Staffordshire (Potteries) Coalfield, and 720.m thick in Lancashire. In the Canonbie Coalfield the Pennine Lower Coal Measures Formation is about 100.m thick (Picken, 1988; Jones and Holliday, 2006).
Distribution and regional correlation
Central and northern England, North Wales and the north Isle of Man. The formation broadly equates with the Scottish Lower Coal Measures and South Wales Lower Coal Measures formations, although in the Midland Valley of Scotland the definition of the base differs from that of the Pennine Basin. This reflects the limited recognition of the Subcrenatum Marine Band within the Midland Valley of Scotland.
Age and biostratigraphical characterisation
Langsettian. The base of the formation is taken at the base of the Subcrenatum Marine Band (SBMB), which marks the base of the Langsettian Stage. The top is taken at the base of the Vanderbeckei Marine Band (VDMB), which marks the base of the Duckmantian Stage.
The Subcrenatum Marine Band, is absent over much of north Cumbria and has a restricted occurrence in west Cumbria, but has an equivalent in north-east England known as the Quarterburn Marine Band that contains a shallow marine benthic productoid fauna (Calver, 1969). In the north Isle of Man, in the Ballavaarkish (Shellag North) Borehole (see above), the Subcrenatum Marine Band contains Lingula mytilloides and Gastrioceras subcrenatum.
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