Tom Croft Limestone Formation

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Tom Croft Limestone Formation (TCL), Carboniferous, Northern England Province[edit]

Tom Croft Limestone Formation is part of the Great Scar Limestone Group

Name[edit]

The formation was first proposed by Burgess (1986[1]). The name is derived from Tom Croft Wheel (SD 6948 9147), Garsdale, Yorkshire. Similar in lithology and identical in age range to the Breakyneck Scar Limestone Formation (see above) of the Ravenstonedale area, it may be argued that either one or the other of these formation names be deemed redundant. However, whilst BGS is not actively working on these successions rationalisation must be deferred.

Lithology[edit]

The Tom Croft Limestone Formation comprises mid to dark grey, poorly bedded biosparites (grainstone containing peloids, shelly debris, crinoid fragments and variable quantities of dolomite grains). The formation includes a macrofauna comparable with that of the ‘Dalton Beds’ of south Cumbria (see Rose and Dunham 1977) and may include a thin basal unit of limestone breccia with siltstone and quartz pebbles.

Genetic interpretation[edit]

Shallow marine carbonate.

Stratotype[edit]

Reference sections in the formation include: the River Clough (SD 6954 9143 to 6947 9147), Garsdale, where the lowest beds are cut out by faults (Burgess, 1986, p. 7) but there is an 80 m-thick run (possibly including some repetition) of dominantly dark grey limestone virtually free of mudstone partings (Burgess, 1986, p. 8[1]; see Dunham and Wilson, 1985, p. 28[2]); the Beckermonds Scar Borehole (BGS Registration Number SD88SE/1) (SD 8636 8016) from 209.15 to 267.88 m depth including dominantly dark grey limestone with some mid grey runs (see Wilson and Cornwell, 1982); and the BGS Raydale Borehole (BGS Registration Number SD98SW/1) (SD 9626 8474) from about 273 to 350 m depth (see Dunham and Wilson, 1985, fig. 5, cols. 5 and 7).

Lower and upper boundaries[edit]

The base of the formation is disconformable on the limestone, dolostone, sandstone and siltstone of the Penny Farm Gill Formation, Ravenstonedale Group (Figure 9, Column 17).

The top of the formation underlies the diachronous Ashfell Sandstone Formation in the Stainmore Trough and the same formation and the Fawes Wood Limestone Formation on the Askrigg Block (Figure 15, Column 3).

Thickness[edit]

The formation is 60–80 m thick.

Distribution and regional correlation[edit]

Askrigg Block.

Age and biostratigraphical characterisation[edit]

Arundian. The macrofauna includes the corals Koninckophyllum sp. and Michelinia megastoma, and the brachiopod Delepinea carinata.

References[edit]

  1. 1.0 1.1 Burgess, I C.1986.Lower Carboniferous sections in the Sedbergh district, Cumbria.Transactions of the Leeds Geological Association, Vol. 11, 1–23.
  2. 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).