Eskett Limestone Formation
Eskett Limestone Formation is part of the Great Scar Limestone Group
The name is newly erected for individually named units, or sets of units in north and west Cumbria (see Akhurst et al., 1997), that represent minor transgression/regression carbonate cyclothems (see Stabbins, 1969) separated by emersion surfaces and palaeosols.
The Eskett Limestone Formation is dominated by pale to dark grey limestones with subordinate, commonly bentonitic, and pale grey to buff, fine- to medium-grained micaceous and cross-bedded sandstone. There is also interbedded limestone and mudstone, black shale, siltstone, and seatearth that can include crinoidal and shelly fossils or plant remains. Historically, the lithostratigraphical succession has been divided into individually named units, or sets of units (see Eastwood et al., 1931; Eastwood et al., 1968; Akhurst et al., 1997) that represent minor transgression–regression carbonate cyclothems (see Stabbins, 1969) separated by emersion surfaces and palaeosols. The names of these local units, in ascending stratigraphical order, are: ‘Sixth Limestone’ (including the ‘Sixth Shale’); ‘Fifth Shale’; ‘Fifth Limestone’; ‘Fourth Shale’; ‘Fourth Limestone’; ‘Third Shale’; ‘Third Limestone’; ‘Orebank Sandstone’ (or ‘Second Shale’); ‘Second Limestone’; ‘First Shale’; ‘First Limestone’. The middle part of the succession, the ‘Fourth Limestone’, includes limestones with prominent palaeokarst surfaces and distinctive fossils, which can characterise various fossil-rich beds (for example the Girvanella and Orionastrea beds and the Junceum Limestone).
A platform carbonate facies with minor marine transgression–regression and emergence.
The type section is Eskett Quarry (NY 0534 1673), Frizington, Cumbria which exposes from the Rough to the Junceum limestones and includes the Orionastrea Band, and the Ethrospongia Band at the base of the Junceum Limestone (Eastwood et al., 1931, pp. 88–89). Reference sections include: Stockhow Hall Quarry (NY 0665 1755) (Sixth Limestone and Fifth Shale); Clints Quarry (NY 0080 1240) (Fourth Shale and Fourth Limestone); and a quarry at Tendley Hill (NY 088 286) (First Limestone).
Lower and upper boundaries
The base of the formation is taken at the base of the Sixth Limestone unit (Figure 14, Columns 1, 2). This is at a level, which may only be defined in borehole logs, as there are no known surface exposures. Barclay et al. (1994) located the top of the underlying Frizington Limestone Formation in a borehole at Sellafield on the perceived late Holkerian/Asbian boundary. However, they were unable to draw a correlation with the west Cumbria outcrop.
The top of the formation, in the Egremont–Whitehaven–Maryport–Cockermouth area, is marked by the cessation of continuous limestone deposition at the top of the First Limestone, which is conformably overlain by the mostly clastic marine and deltaic facies of the Stainmore Formation (Yoredale Group). Here, at outcrop, this overlying formation is generally represented by the coarse-grained, fluvial Hensingham Grit, with a basal mudstone. Immediately west of the Bothel Fault, the upper boundary of the Eskett Limestone Formation occurs at the top of the limestones with palaeokarst surfaces and distinctive fossils, the Fourth Limestone, which is overlain by the Alston Formation (Yoredale Group) (Figure 9, Column 13) with cyclical sequences of coarsening upwards sedimentary rocks including relatively thick and common limestones. East of the Bothel Fault, however, the top of the Eskett Limestone Formation is close below the Asbian–Brigantian boundary, at the top of the White Limestone unit, which is included at the base of the Fourth Limestone. Here again, the Alston Formation, just described, overlies the boundary. It should be stressed that division east and west of the Bothel Fault is purely for ease of description. The Bothel Fault is not implied to have exerted any penecontemporaneous effects on deposition.
In west Cumbria the formation is about 85 m thick.
Distribution and regional correlation
North and west Cumbria. The uppermost unit, the First Limestone, is the lateral equivalent of the Great Limestone Member (Alston Formation, Yoredale Group) of the Alston Block, and the ‘Main’ (now Great) Limestone Member of the Askrigg Block.
Age and biostratigraphical characterisation
Holkerian to Pendleian. The presence of Productus garwoodi in the lower part of the Sixth Limestone at Rowrah (Ramsbottom, 1955) indicates that the Holkerian/Asbian boundary occurs within, and not at the base of the formation. In west Cumbria the platform carbonate facies continued to dominate until the early Pendleian. The distinctive fossils of the Fourth Limestone include Girvanella sp., Saccamminopsis sp., Orionastrea sp. Siphonodendron junceum and Spirorbis sp.
- Akhurst, M C, Chadwick, R A, Holliday, D W, McCormac, M, McMillan, A A, Millward, D, and Young, B.1997.Geology of the west Cumbria district.Memoir of the British Geological Survey, Sheets 28, 37 and 47 (England and Wales).
- Stabbins, R.1969.The depositional history of the Lower Carboniferous rocks of West Cumberland. Unpublished PhD thesis, University of London.
- Eastwood, T, Hollingworth, S E, Rose, W C C, and Trotter, F M.1968.Geology of the country around Cockermouth and Caldbeck.Memoir of the Geological Survey of Great Britain, Sheet 23 (England and Wales).
- Eastwood, T, Dixon, E E L, Hollingsworth, S, and Smith, B.1931.The geology of the Whitehaven and Workington District.Memoir of the Geological Survey, Sheet 28 (England and Wales).
- Barclay, W J, Riley, N J, and Strong, G E.1994.The Dinantian rocks of the Sellafield area, West Cumbria.Proceedings of the Yorkshire Geological Society, Vol. 50, 37–49.
- Ramsbottom, W H C.1955.Unpublished palaeontological report on boreholes NY01NE 8 and 9. British Geological Survey.