Potts Beck Limestone Formation
Potts Beck Limestone Formation (PBK), Carboniferous, Midland Valley of Scotland
Potts Beck Limestone Formation is part of the Great Scar Limestone Group
The name is derived from Potts Beck near Ravenstonedale, Cumbria (see George et al., 1976).
See Mitchell (1978); Dunham and Wilson (1985); Millward et al., (2003). The Potts Beck Limestone Formation is composed largely of pale grey, thick-bedded, strongly jointed, sparry packstone. Rubbly weathered calcrete horizons are present, as are subordinate sandstones and mudstones.
The type section is at Great Kinmond (NY 6728 0892 to 6728 0927) where some 39 m of mostly fossiliferous limestone is exposed. The type area is at Little Asby Scar (NY 6988 0827) on the north side of Potts Valley, near Newbiggin-on-Lune, Cumbria where the base of the formation is exposed.
Lower and upper boundaries
The base of the formation is conformable, and taken at the change from dark grey, cross-bedded, coarse-grained, crinoidal grainstone with mudstone interbeds (the ‘Bryozoa Band’ of Garwood, 1913) at the top of the underlying Ashfell Limestone Formation (Figure 9, Column 16), to grey, coralline packstone.
The upper boundary of the formation is overlain by the grey, scar-forming, rhythmically bedded, limestone (mainly wackestone and packstone) of the Knipe Scar Limestone Formation.
The formation is up to 45 m thick in the Stainmore Trough, but thins rapidly towards the surrounding block areas. It is, for example, absent north of Shap village.
Distribution and regional correlation
Age and biostratigraphical characterisation
Latest Holkerian to early Asbian. Fossils, which can be abundant, include the corals Axophyllum sp., Dibunophyllum bourtonense, Palaeosmilia murchisoni, Siphonodendron junceum and S. martini, and the brachiopods Daviesiella llangollensis and Gigantoproductus sp. The Stainmore Trough includes the basal Asbian Stratotype at Little Asby Scar (NY 6988 0827) (George et al., 1976). Faunal analysis of this section places the Holkerian/Asbian boundary above the base of the formation (Ramsbottom, 1981).
- George, T N, Johnson, G A L, Mitchell, M, Prentice, J E, Ramsbottom, W H C, Sevastopulo, G D, and Wilson, R B.1976.A correlation of Dinantian rocks in the British Isles.Geological Society of London Special Report, No.7
- Mitchell, M.1978.Dinantian.168–177 in The geology of the Lake District. Moseley, F (editor).Occasional Publication of the Yorkshire Geological Society, No. 3
- 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)
- Millward, D, McCormac, M, Hughes, R A, Entwistle, D C, Butcher, A, and Raines, M G.2003.Geology of the Appleby district.Sheet explantion of the British Geological Survey,Sheet 30 (England and Wales)
- Ramsbottom, W H C.1981.Fieldguide to the boundary stratotypes of the Carboniferous stages in Britain. Biennial Meeting of the Subcommisson of Carboniferous Stratigraphy, Leeds