Four Fathom Limestone Member
Four Fathom Limestone Member is part of the Alston Formation
No longer bed status, now a member of the Alston Formation. Previously known as the Four Fathom Limestone, Sandbanks Limestone, Low Dean Limestone (Gunn, 1900), Eight Yard Limestone, Second Limestone, Buccleuch Limestone (Lumsden and Wilson, 1961), Undersett Limestone. See also Burgess and Holliday (1979); Chadwick et al. (1995); Day (1970); Dunham (1990); Dunham and Wilson (1985); Eastwood et al. (1968); Frost and Holliday (1980); Holliday et al. (1975); Johnson et al. (1962); Johnson and Nudds (1996); Young and Boland (1992).
Limestone, packstone, fine-grained, mid and dark grey, thick bedded and wavy-bedded, with few mudstone partings; somewhat argillaceous, particularly at the top. Typically contains layers of nodules and lenses of black chert. Crinoidal but not particularly fossiliferous on the Alston Block; contains conspicuous coral biostromes to the north and south of the Alston Block.
The type area is coastal exposures of Near Skerr, Middle Skerr and Far Skerr, about 4 km south-east of Berwick-upon-Tweed (NU 028 487 to 036 481). Here the limestone is earthy and bluish with many shale partings, about 8.5.m thick (Fowler, 1926, pp..26–27). The type section is the Middlehope Burn, Westgate, Upper Weardale (NY 905 396) where a nearly complete section can be seen. A reference section is the Rookhope Borehole (BGS Registration Number NY94SW/1) (NY 9375 4278) Rookhope, Weardale, Co. Durham from 57.99 to 63.70.m depth (see Johnson and Nudds, 1996, p..187).
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. On the Alston Block, it usually overlies less than 2 m of dark grey seatearth mudstone and mudstone with abundant plants that overlies a prominent sandstone referred to there as the Natrass Gill Hazle. In the Bellingham district a coal underlies the limestone.
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 mudstones, the lowermost of which are calcareous and fossiliferous. On the Askrigg Block the limestone member is overlain by a dark grey or almost black chert called the Underset Chert.
Between 7 and 10 m north of the Lake District and in Northumberland; 5–12 m on the Alston Block; about 9 m on the Askrigg Block, but locally up to 25 m.
Distribution and regional correlation
Occurs throughout northern England and the Scottish Borders, lying below the Great Limestone near the top of the Alston Formation (Figure 10, Column 3; Figure.11, Columns 1–3; Figure 12, Columns 1, 3, 4; Figure.13, Columns 3, 4; Figure 14, Columns 2, 3; Figure.15, Columns 1–4). This definition incorporates the various other local names of Sandbanks, Lowdean and Eight Yard in Northumberland, the Buccleuch Limestone (Lumsden and Wilson, 1961) of the Langholm and Bewcastle area; the Second Limestone of Cumbria (Eastwood et al., 1968; Young and Boland, 1992) and the Underset Limestone of the Askrigg Block (for example Dunham and Wilson, 1985). Geographical distribution therefore includes Cumbria, Northumberland, Durham and North Yorkshire. The member has been seen in numerous deep boreholes throughout the region including the Archerbeck Borehole (BGS Registration Number NY47NW/14) (NY 4160 7820) (Lumsden and Wilson, 1961), the Rookhope Borehole (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). The member has also been widely seen on the Alston Block in many mine shafts and workings.
Brigantian. Goniatites indicative of the Lyrogoniatites georgiensis P2c Subzone have been found in mudstones above the Four Fathom Limestone in the Mount Pleasant Borehole (NZ 034 152) near Barnard Castle (Johnson et al., 1962).
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