Samlesbury Formation
Samlesbury Formation (SAML), Carboniferous, Northern England Province
Samlesbury Formation is part of the Millstone Grit Group
Name
The new name Samlesbury Formation is proposed to identify all Millstone Grit Group strata of Chokierian and Alportian age. The term Middleton Grit Group of Bradford (Stephens et al., 1953[1]) is unsuitable as there is a sandstone already called Middleton Grit, and it is Arnsbergian in age. The new name was chosen from Samlesbury Bottoms (SD 618 291) where a complete basinal succession in the Chokierian and Alportian is exposed.
Lithology
Dark grey mudstone, common thin shaly mudstones, subordinate sandstone and siltstone, and locally, in the Bradford area, a thick, coarse-grained, cross-bedded sandstone (the Brocka Bank Grit) (Waters, 1999[2]).
Genetic interpretation
During the Chokierian and Alportian there appears to be little clastic input into the Pennine Basin. The succession is condensed with some hemipelagic marine shales. A shallow-water lobate delta, the Brocka Bank Grit prograded about 15 km into the Harrogate sub-basin (Waters, 1999[2]).
Stratotype
The type section is a composite section in the River Darwin at Samlesbury Bottoms (SD 618 291). This shows dark grey mudstones and common marine shales, including the marine bands that define the base and top of the formation (Moore, 1930[3]; Price et al., 1963[4]). Stonehead Beck, Cowling, North Yorkshire (SD 9473 4330), shows a continuously exposed 40.m succession including the base Chokierian stratotype. This equates to the base of the Samlesbury Formation, and defines the position of the mid Carboniferous boundary (Riley et al., 1995[5]).
Lower and upper boundaries
The sharply conformable base of the formation is taken at the base of the dark grey shaly mudstone of the Isohomoceras subglobosum Marine Band with an eponymous fauna, where the formation overlies the interbedded sandstone, siltstone and mudstone succession of the Silsden Formation (Figure 9, Column 17; Figure 15, Column 5). Where the marine band is absent from the interval, the base of the formation is taken at the top of the Lower Follifoot Grit in the southern part of the Askrigg Block, or at the base of a mudstone succession with Lingula in the Stainmore Trough. In the southern part of the Central Pennine Basin, the boundary is taken at the base of the first thick quartz-feldspathic sandstone of Chokierian to Alportian age that is present above the Bowland Shale Formation.
The top is taken at the sharp conformable base of the dark grey shaly mudstone of the Hodsonites magistrorum Marine Band, including an eponymous fauna, overlain by the siltstone-dominated lower part of the Hebden Formation. The marine band is underlain by the mudstone-dominated succession of the Samlesbury Formation. Where the marine band is absent from the interval, the top of the formation equates with the base of the Cayton Gill Shale in the south of the Askrigg Block (Wilson, 1977[6]) and the base of the Mousegill Marine Beds in the Stainmore Trough (Owens and Burgess, 1965[7]).
Thickness
In Lancashire there are 58.m of Chokierian and 6 m of Alportian strata, entirely within shaly mudstones (Brandon et al., 1998[8]). In Bradford the succession including the Brocka Bank Grit is 75.m thick (Waters, 1999[2]). The formation is less than 30 m thick along the southern part of the Askrigg Block, and in the Stainmore Trough, the thin Chokierian succession is no more than 20 m thick.
Distribution and regional correlation
The Craven Basin of north Lancashire and north Yorkshire, between Lancaster (SD 47 61), Pendle Hill (SD 80 41), Skipton Moor (SE 00 50) and Harrogate (SE 30 55) and the southern part of the Askrigg Block and the Stainmore Trough. The formation passes southward into basinal mudstones of the Bowland Shale Formation (Craven Group).
Age and biostratigraphical characterisation
Chokierian to Alportian (H1–H2). Part or all of the Alportian succession may be missing regionally due to a mid Carboniferous unconformity (Riley et al., 1995[5]). The Isohomoceras subglobosum Marine Band occurs at the base of the formation, and the top is taken at the base of the Hodsonites magistrorum Marine Band (R1a).
Local notes
Along the southern part of the Askrigg Block, the formation comprises a lower mudstone-dominated Follifoot Shale, overlain by the Upper Follifoot Grit (Wilson, 1977[6]; Dunham and Wilson, 1985[9]). Unlike in the Craven Basin to the south, no strata of Alportian age have been proved in the Askrigg Block and Stainmore Trough (Ramsbottom et al., 1978[10]). In the Stainmore Trough, a thin Chokierian succession is postulated from miospore evidence (Owens and Burgess, 1965[7]). A non-sequence is considered to be present above the Upper Follifoot Grit.
References
- ↑ Stephens, J V, Mitchell, G H, and Edwards, W.1953.Geology of the country between Bradford and Skipton.Memoir of the Geological Survey of Great Britain, Sheet 69 (England and Wales).
- ↑ 2.0 2.1 2.2 Waters, C N.1999.Geology of the Bradford district.Sheet description of the British Geological Survey, Sheet 69 (England and Wales).
- ↑ Moore, E W J.1930.A section in the Sabden Shales on the River Darwen, near Blackburn.Journal of the Manchester Geological Association, Vol. 1, 103–108.
- ↑ Price, D, Wright, W B, Jones, R C B, and Whitehead, T H.1963.Geology of the country around Preston.Memoirs of the Geological Survey of Great Britain, Sheet 75 (England and Wales).
- ↑ 5.0 5.1 Riley, N J, Claoué-Long, J C, Higgins, A C, Owens, B, Spears, A, Taylor, L, and Varker, W J.1995.Geochronometry and geochemistry of the European mid-Carboniferous boundary global stratotype proposal, Stonehead Beck, North Yorkshire, UK.Annales de la Société géologique de Belgique, Vol. 116, 275–289.
- ↑ 6.0 6.1 Wilson, A A.1977.The Namurian Rocks of the Fewston Area.Transactions of the Leeds Geological Association, Vol. 9, 1–44.
- ↑ 7.0 7.1 Owens, B, and Burgess, I C.1965.The stratigraphy and palynology of the Upper Carboniferous outlier of Stainmore, Westmorland.Bulletin of the Geological Survey of Great Britain, No. 23, 17–44.
- ↑ Brandon, A, Aitkenhead, N, Crofts, R G, Ellison, R A, Evans, D J, and Riley, N J.1998.Geology of the country around Lancaster.Memoirs of the Geological Survey of Great Britain, Sheet 59 (England and Wales).
- ↑ 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).
- ↑ Ramsbottom, W H C. 1978.Namurian. 178–180 in The geology of the Lake District. Moseley, F (editor).Occasional Publication of the Yorkshire Geological Society, No. 3.