Ashfell Limestone Formation

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Ashfell Limestone Formation (AFL), Carboniferous, Midland Valley of Scotland[edit]

Ashfell Limestone Formation is part of the Great Scar Limestone Group

Name[edit]

The name is derived from Ashfell in Ravenstonedale, Cumbria; see Ramsbottom (1973)[1]; George et al. (1976)[2]; Mitchell (1978)[3]; Dunham and Wilson (1985)[4]; Pattison (1990)[5].

Lithology[edit]

The distinctive Ashfell Limestone Formation is characteristically dark grey to dark blue-grey and mainly a packstone, shelly grainstone or lime mudstone. Strongly cross-bedded units, with thin interlaminae of siltstone and fine-grained sandstone occur at the base of the formation. Convolute beds and nodules of dark grey chert occur locally, and partial dolomitisation is common. The top of the formation is marked by the presence of dark grey, cross-bedded, coarse-grained crinoidal grainstone, grey mudstone with bivalves, and vuggy porcellanous limestone (the ‘Bryozoa Band’ of Garwood, 1913).

Genetic interpretation[edit]

Shallow marine, perhaps slightly restricted carbonate environment.

Stratotype[edit]

Partial type sections occur in Potts Valley (NY 6988 0827), (NY 6981 0820) and (NY 6972 0805 to 6969 0813) where more than 48 m of limestones in the upper and middle parts of the formation are exposed, and at Ewe Fell (NY 6971 0702 to 6999 0711) where 60 m of the lowest part of the formation, comprising limestone and 7 m of sandstone, are exposed (see Pattison, 1990[5]).

Lower and upper boundaries[edit]

The mainly packstone, shelly grainstone or lime mudstone of the Ashfell Limestone Formation conformably overlies the sandstones, mudstones and thin limestones of the Ashfell Sandstone Formation.

On the northern margin of the Stainmore Trough, the ‘Bryozoa Band’ (BZB) at the top of the Ashfell Limestone Formation is overlain either conformably by the mainly sparry packstone of the Potts Beck Limestone Formation (Figure 9, Column 16), or disconformably by the rhythmically bedded limestone (mainly wackestone and packstone) of the Knipe Scar Limestone Formation (Figure 14, Column 3).

Thickness[edit]

The formation is 40–50 m thick at Penrith and Brough, but thickens to 100 m in the Stainmore Trough.

Distribution and regional correlation[edit]

Penrith, Brough and the Stainmore Trough.

Age and biostratigraphical characterisation[edit]

Holkerian. Shelly beds contain stromatolite colonies, coral fragments (including Lithostrotion minus), bryozoa, brachiopods (including rhynchonellids, spiriferoids and Davidsonina carbonaria), gastropods, crinoid ossicles, fish scales and burrows. The characteristic Holkerian brachiopod Davidsonina carbonaria is found in the Orton and Ravenstonedale area (Pattison, 1990[5]). The ‘Bryozoa Band’ of Garwood (1913)[6] occurs at the top of the formation.

References[edit]

  1. Ramsbottom, W H C.1973.Transgressions and regressions in the Dinantian: a new synthesis of British Dinantian stratigraphy.Proceedings of the Yorkshire Geological Society, Vol. 39, 567–607
  2. 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
  3. 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
  4. 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)
  5. 5.0 5.1 5.2 Pattison, J.1990.Geology of the Orton and Sunbiggin Tarn districts: 1:10 000 sheets NY60NW and NE.British Geological Survey Technical Report, WA/90/12
  6. Garwood, E J.1913.The Lower Carboniferous succession in the north-west of England.Quarterly Journal of the Geological Society, Vol. 68, 449–596