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==Great Limestone Member [http://www.bgs.ac.uk/lexicon/lexicon.cfm?pub=Gl (GL)], Carboniferous, Northern England Province ==
Great Limestone Member is part of the [[Alston Formation]]
=== Name ===
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)<ref name="Cossey">Cossey, P J, Adams, A E, Purnell, M A, Whiteley, M J, Whyte, M A, and Wright, V P.2004.British Lower Carboniferous stratigraphy.''Geological Conservation Review Series'', No 29. (Peterborough: Joint Nature Conservation Committee.)</ref>, and Burgess and Holliday (1979)<ref name="BandH">Burgess, I C, and Holliday, D W.1979.Geology of the country around Brough-under-Stainmore.''Memoir of the Geological Survey of Great Britain'', Sheet 31, parts 25 and 30 (England and Wales)</ref>; Chadwick et al. (1995)<ref name="Chadwick">Chadwick, R A, Holliday, D W, Holloway, S, and Hulbert, A G.1995.The structure and evolution of the Northumberland–Solway Basin and adjacent areas.''Subsurface Memoir of the British Geological Survey''. (Keyworth, Nottingham: British Geological Survey.)</ref>; Day (1970)<ref name="Day">Day, J B W.1970.Geology of the country around Bewcastle.''Memoir of the Geological Survey of Great Britain'', Sheet 12 (England and Wales)</ref>; Dunham (1990)<ref name="Dunham">Dunham, K C.1990.Geology of The Northern Pennine Orefield: Volume 1, Tyne to Stainmore (2nd edition).''Economic Memoir of the British Geological Survey'', Sheets 19 and 25, parts 13, 24, 26, 31 and 32 (England and Wales)</ref>; Dunham and Wilson (1985)<ref name="DandW">Dunham, K C, and Wilson, A A.1985.Geology of the Northern Pennine Orefield: Volume 2, Stainmore to Craven.''Economic Memoir of the British Geological Survey'', Sheets 40, 41 and 50, parts 31, 32, 51, 60 and 61(England and Wales)</ref>; Eastwood et al. (1968)<ref name="Eastwood">Eastwood, T, Hollingworth, S E, Rose, W C C, and Trotter, F M.1968.Geology of the country around Cockermouth and Caldbeck.''Memoir of the Geological Survey of Great Britain'', Sheet 23 (England and Wales)</ref>; Fairbairn (1978<ref name="Fairbairn78">Fairbairn, R A.1978.Lateral persistence of beds within the Great Limestone (Namurian E1) of Weardale.''Proceedings of the Yorkshire Geological Society'', Vol. 41, 533–544</ref>, 1980<ref name="Fairbairn80">Fairbairn, R A.1980.The Great Limestone (Namurian) of south Northumberland.''Proceedings of the Yorkshire Geological Society'', Vol. 43, 159–167.</ref>, 2001<ref name="Fairbairn01">Fairbairn, R A.2001.The stratigraphy of the Namurian Great/Main Limestone on the Alston Block, Stainmore Trough and Askrigg Block of northern England.''Proceedings of the Yorkshire Geological Society'', Vol. 53, 265–274</ref>); Johnson (1958)<ref name="Johnson58">Johnson, G A L.1958.Biostromes in the Great Limestone of northern England.''Palaeontology'', Vol. 1, 147–157</ref>; Johnson and Nudds (1996)<ref name="JandN">Johnson, G A L, and Nudds, J R. 1996.Carboniferous biostratigraphy of the Rookhope Borehole, County Durham.''Transactions of the Royal Society of Edinburgh: Earth Sciences'', Vol. 86, 181–226</ref>; Lumsden and Wilson (1961<ref name="LandW">Lumsden, G I, and Wilson, R B.1961.The stratigraphy of the Archerbeck Borehole, Canonbie, Dumfriesshire.''Bulletin of the Geological Survey of Great Britain'', No. 18, 1–89</ref>); Young and Boland (1992)<ref name="YandB">Young, B, and Boland, M P.1992.Geology and land-use planning: Great Broughton–Lamplugh area, Cumbria.</ref>.

=== Lithology ===
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<ref name="Johnson58"></ref>; Cossey et al., 2004<ref name="Cossey"></ref>), and the Frosterly Marble (Johnson, 1958<ref name="Johnson58"></ref>). At the top, the Tumbler Beds, typically 2–5 m thick, comprise thick beds of limestone interbedded with persistent mudstone units.

=== Stratotype ===
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)<ref name="Fairbairn80"></ref> (see Cossey et al., 2004, pp. 144–147<ref name="Cossey"></ref>). 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<ref name="Cossey"></ref>); 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<ref name="YandB"></ref>); 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<ref name="LandW"></ref>); 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<ref name="JandN"></ref>).

=== 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).

=== Thickness ===
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<ref name="Day"></ref>) 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<ref name="Chadwick"></ref>).

=== Age ===
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<ref>Arthurton, R S, Johnson, E W, and Mundy, D J C.1988.Geology of the Country around Settle.''Memoir of the British Geological Survey'', Sheet 60 (England and Wales)</ref>). 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<ref>Cummings, R H.1961.The foraminiferal zones of the Carboniferous sequence of the Archerbeck Borehole, Canonbie, Dumfriesshire.''Bulletin of the Geological Survey of Great Britain'', No. 18, 107–128.</ref>).

== References ==
<references/>
[[Category:Carboniferous lithostratigraphical nomenclature]]

Great Limestone Member - Revision history

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Geosource>Volunteer 01: Created page with "==Great Limestone Member [http://www.bgs.ac.uk/lexicon/lexicon.cfm?pub=Gl (GL)], Carboniferous, Northern England Province == Great Limestone Member is part of the Alston For..."