Great Limestone Member
Great Limestone Member is part of the Alston Formation
No longer bed status, now a member of the Alston Formation (Yoredale Group). The member includes the former Main Limestone of the Askrigg Block, the Catsbit Limestone of Langholm, and the Dryburn Limestone of Northumberland. See Cossey et al. (2004), and Burgess and Holliday (1979); Chadwick et al. (1995); Day (1970); Dunham (1990); Dunham and Wilson (1985); Eastwood et al. (1968); Fairbairn (1978, 1980, 2001); Johnson (1958); Johnson and Nudds (1996); Lumsden and Wilson (1961); Young and Boland (1992).
Limestone (bioclastic packstone), mid to dark blue-grey; thickly bedded with thin shaly mudstone partings along uneven or wavy bedding planes; there is much crinoid debris throughout and there are many other beds rich in brachiopods and/or corals. The internal stratigraphy of the member is in parts remarkably consistent throughout the region and four informal divisions are recognised (Fairbairn, 1980) and referred to, from the base upwards as the Bench, Main and Transitional posts and the Tumbler Beds. Beds rich in fossils are widespread. Most notable and widespread is the Chaetetes Band (Johnson, 1958) which occurs near the base; this comprises a sponge biostrome along with locally abundant brachiopods, corals and ostracods. Within the Main Posts are the Brunton Band comprising the rare alga Calcifolium (Johnson, 1958; Cossey et al., 2004), and the Frosterly Marble (Johnson, 1958). At the top, the Tumbler Beds, typically 2–5 m thick, comprise thick beds of limestone interbedded with persistent mudstone units.
The type section is Greenleighton Quarry, about 10 km south of Rothbury, Northumberland (NZ 034 917) (Figure.13, Column 3), which includes 12 m of limestone divisible into the Bench, Main and Transitional posts and Tumbler Beds of Fairbairn (1980) (see Cossey et al., 2004, pp. 144–147). Reference sections occur at Brunton Bank Quarry, about 1.km south-east of Chollerford, Northumberland (NY 928 699) where the Great Limestone is about 15 m thick and contains the type sections of the Brunton Band (with the rare alga Calcifolium), and the Chaetetes Band (a spectacular sponge biostrome) (see Cossey et al., 2004, pp. 141–144); Eastgate Quarry, Weardale, Co. Durham (NY 940 370) where a complete section, including the Frosterly Marble, can be seen; Tendley Hill Quarry, Cumbria (NY 088 289) where a full thickness of 15.8 m of mid grey, thickly bedded, prominently jointed packstone, typically with uneven or wavy bedding planes, and a development of the Chaetetes Band can be seen (Young and Boland, 1992, p..21); the Archerbeck Borehole (BGS Registration Number NY47NW/1) (NY 4157 7815), 2 km north-east of Canonbie, Dumfriesshire from 215.8 to 238.3 m depth with massive limestone except for a few calcareous mudstone beds (see Lumsden and Wilson, 1961); and the Rookhope Borehole (BGS Registration Number NY94SW/1) (NY 9375 4278), Rookhope, Weardale, Co. Durham from rockhead, apparently near the top of the limestone at 6.86 to 25.07 m depth (see Johnson and Nudds, 1996, p. 185).
Lower and upper boundaries
The lower boundary is taken at the generally conformable, sharp base of the first bed of limestone that overlies strata of the Alston Formation (which is characterised by cyclical sedimentary rocks that include thick-bedded, commonly bioclastic limestones). In most localities the limestone directly overlies the Tuft Sandstone, a brown, micaceous fine-grained sandstone that has a rooty upper part and which locally may be overlain by a thin carbonaceous smear or a coal.
The upper boundary of the member is taken at the top of the uppermost limestone bed that is overlain by a sequence of dark grey siltstones and mudstones at the base of the Stainmore Formation (SMGP).
About 12 m in the Vale of Eden; 16–22 m on the Alston Block; 22–24 m with a maximum of 40 m on the Askrigg Block and 8–15 m in the Northumberland Trough; about 22.m in the Langholm area and Archerbeck Borehole.
Distribution and regional correlation
Occurs throughout most of northern England at the top of the Alston Formation. This definition incorporates the Main Limestone of the Askrigg Block, and the Catsbit Limestone (Day, 1970) of the Scottish Borders near Langholm. Geographical distribution therefore includes, Northumberland, Durham and North Yorkshire. The Great Limestone has also been correlated with the the First Limestone (LM1) (Eskett Limestone Formation, Great Scar Limestone Group) of Cumbria (Figure 14, Column 1), and the Top Hosie Limestone (TOHO) (Lower Limestone Formation, Clackmannan Group) of the Midland Valley of Scotland (Figure 6, Column 4).
The member has been seen in numerous deep boreholes throughout the region including the Archerbeck and Rookhope boreholes (see above), the Barrock Park Borehole (BGS Registration Number NY44NE/28) (NY 4613.4660), the Throckley Borehole (BGS Registration Number NZ16NW/45 (NZ.1456.6762), the Harton Borehole (BGS Registration Number NZ36NE/80) (NZ 3966 6563), the Woodland Borehole (BGS Registration Number NZ02NE/4) (NZ 0910 2769) and the Seal Sands Borehole (BGS Registration Number NZ52SW/308) (NZ 538 239) (Chadwick et al., 1995).
Pendleian. At Fountains Fell on the Askrigg Block the Great Limestone (formerly the Main Limestone) overlies mudstone with the E1a Subzone ammonoid Cravenoceras (=Emstites) leion (Arthurton et al., 1988). A change in foraminiferal assemblage at the base of the Catsbit Limestone (the local equivalent of the Great Limestone Member) in the Archerbeck Borehole (see above) suggests that the base of the Namurian occurs at this level (Cummings, 1961).
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