|Computer Code:||BSG||Preferred Map Code:||BSh|
|Status Code:||Full, pending ratification|
|Age range:||Asbian Age (CR) — Yeadonian Sub-age (CY)|
|Lithological Description:|| Mainly dark grey fissile and blocky mudstone, weakly calcareous, with subordinate sequences of interbedded limestone and sandstone, fossiliferous in more-or-less discrete bands.
Furness and Settle areas: thick-bedded, blocky to sub-fissile, dark grey and black, organic-rich mudstone, with subordinate beds of dark grey siltstone, sandstone and pale brown dolomitic limestone. Marine bands are also present. Shows an upwards decrease in carbonate turbidites and a concomitant increase in siliciclastic sandstone turbidites.
South Isle of Man: Black claystone with localized deposition of carbonate turbidites, debris flows, olistoliths, volcaniclastic deposits and lavas. At the base of the formation, the Scarlett Point Member comprises cherty and pyritous tabular beds of pale wackestone and lime-mudstone (dolomitized in places), which display gradational boundaries with interbedded black, fissile, blocky claystone. The limestone is burrowed and has inadunate crinoidal lags and scattered ammonoids. At the top of the formation, the Scarlett Volcanic Member is dominated by a series of volcaniclastic debris flows and gravity slides. Claystone rafts and megaclasts are entrained within the volcaniclastic rocks, and carbonate olistoliths and pillow lavas also occur. Between the members, where the formation oversteps the Balladoole Formation (Great Scar Limestone Group), coarse-grained detrital carbonates and debris beds are common. These include erosively based, graded packstone beds, conglomerate, megaclasts and large olistoliths (with reef limestone and ammonoid faunas) derived from the Balladoole Formation. The middle part of the exposed formation comprises black, calcareous, platy claystone with subordinate beds (up to 2 m thick) of dark wackestone, and dark detrital packstone debris.
North Isle of Man: Siltstone, claystone and ironstone dominate, but includes two marine bands and five palaeosols.
The bulk of the formation displays uniformly high gamma-ray values, typically exceeding 100 API units. However, superimposed on this are sporadic intervals with very high gamma-ray values which, in well 48/3-3, were shown by Leeder et al. (1990) to represent uranium-rich marine bands. The formation displays occasional high-velocity spikes that may correspond to beds of siltstone or of highly calcareous mudstone.
|Definition of Lower Boundary:|| The conformable base upon the Pendleside Limestone Formation in the Craven Basin, on the Widmerpool Formation in the East Midlands, and the Pentre Chert and Cefn-y-Fedw Sandstone Formation in North Wales, on the Hodderense Limestone Formation in the south of the Isle of Man, is taken at the first appearance of black mudstone above variegated mudstones or fine-grained limestones.
In south Cumbria the lower boundary of the formation is taken at the base of the Cravenoceras leion Marine Band, where the dark grey to black marine mudstone rests upon thinly interbedded limestones, mudstones and subordinate sandstones of the Alston Formation.
The Bowland Shale Formation onlaps onto and eventually, by Pendleian times, extends over the carbonates of the Central Lancashire High (Trawden Limestone Group).
In the Craven Reef Belt, in the Malham Cove/Gordale Scar area, the fissile mudstones with nodules and thin beds of ironstone and limestone of the Bowland Shale Formation, rest unconformably and diachronously on the Malham Formation (Great Scar Limestone Group), or Yoredale Group.
In well 41/24a-2, the base of the Bowland Shale Formation is marked by a sharp downhole increase in velocity and decrease in gamma-ray response, corresponding to the top of the highest limestone bed of the Yoredale Formation. The base of the formation has not been penetrated in other North Sea wells.
|Definition of Upper Boundary:|| The top of the formation is taken at the base of the Millstone Grit Group over most of the Pennine Basin and at the base of the Morridge Formation in Staffordshire and the East Midlands. The formation shows complex intertongueing with the Morridge and Cefn-y-Fedw Sandstone formations. It is seen as a conformable boundary defined by the base of the lowermost thick feldspathic sandstone of the Millstone Grit Group, or quartzitic sandstone of the Morridge and Cefn-y-Fedw formations, above thick dark grey mudstone of the Bowland Shale Formation.
The transition is abrupt in wells 48/3-3 and 48/23-3, although occasional sandstone stringers do occur below the boundary. In well 41/24a-2, the lithofacies transition is gradational, but the boundary is marked by a sharp downhole decrease in velocity.
|Thickness:|| Generally between 120 m and 620 m. The formation thickens northeastwards along the axis of the Central Lancashire High, from about 22 m in the Roddlesworth Borehole (SD62SE/6), 68 m thick in the Holme Chapel Borehole (SD82NE/68), and 102 m in the Boulsworth Borehole (SD93SW/14). The underlying Trawden Limestone Group shows a thinning in the same direction (Evans and Kirby, 1999), suggesting the thickening of the Bowland Shale Formation reflects available accommodation space.
In the Craven Reef Belt the Bowland Shale Formation is perhaps 30 m to 200 m thick (see Arthurton et al., 1988, figure 22).
In south Cumbria, the Roosecote Borehole proved the formation to be 130 m thick (see Johnson et al., 2001; Rose and Dunham, 1977).
On the Isle of Man, the Bowland Shale Formation might be at least 186 m thick. In the north Isle of Man, the Shellag Point Borehole [NX 4565 9965] proved only a part of the formation, reaching 27.55 m in thickness.
|Geographical Limits:|| Widespread in the Craven Basin, including the Lancaster, Garstang, Settle, Clitheroe and Harrogate districts, south Cumbria and the Isle of Man; also in North Wales, Staffordshire and the East Midlands.
As few offshore wells have been drilled to sufficient depths to penetrate early Namurian sediments, the distribution of the Bowland Shale Formation is poorly constrained. Basinal mudstones are only 47 m thick in well 41/24a-2. Wells 48/3-3 and 48/23-3 terminated after drilling 217 m and 277 m of basinal mudstones respectively, but the Bowland Shale Formation is likely to be appreciably thicker in areas adjacent to Dinantian growth faults.
|Parent Unit:||Craven Group (CRAV)|
|Previous Name(s):|| BOWLAND SHALE GROUP
BOWLAND SHALES (BSG)
EDALE SHALE GROUP (ESH)
EDALE SHALES (ESH)
HALKYN FORMATION (HAF)
HOLYWELL SHALE FORMATION (HSH)
HOLYWELL SHALES (HSH)
|Alternative Name(s):||none recorded or not applicable|
|Reference Section||Shellag Point Borehole, north Isle of Man cored from 100.60m to 128.15m, with neither the base nor top proved.|
|Reference Section||Roosecote Borehole, Barrow-in-Furness (SD26NW/19), includes an entire thickness of the formation from 158.13 to 613.31m depth.|
|Reference Section||River Ribble at Dinckley, on the south bank of the River Ribble between Dinckley Hall and the suspension footbridge, about 300m to the west downstream.|
|Reference Section||Little Mearley Clough, 400m to 1100m southeast and upstream from Little Mearley Hall.|
|Reference Section||Coed Pen-y-Maes stream section at Holywell, Clwyd, North Wales. Intermittent exposures of the Formation showing mudstone lithologies typical for the North Wales region; base and top not seen.|
|Reference Section||Duffield Borehole, near Derby (SK34SW/5), from 0.86m to 405.78m depth, from the H1a marine band to the base od the Formation.|
|Whittard, W F and Simpson, S. 1960. Lower Carboniferous. 54-55 in Whittard, W F and Simpson, S (editors), Lexique Stratigraphique International, Part 3a VII, Vol.1.|
|Earp, J R and others, 1961. Geology of the country around Clitheroe and Nelson. Geological Survey of Great Britain Memoir, Sheet 68.|
|Aitkenhead, N, Chisholm, J I and Stevenson, I P. 1985. Geology of the country around Buxton, Leek and Bakewell. Memoir of the British Geological Survey, England and Wales Sheet 111.|
|Aitkenhead, N, 1977. The Institute of Geological Sciences borehole at Duffield, Derbyshire. Bulletin of the Geological Survey of Great Britain, No.59, 1-38.|
|Davies, J R, Wilson, D and Williamson, I T, The geology of the country around Flint. Memoir of the British Geological Survey, Sheet 108.|
|Fewtrell, M D and Smith, D G, 1980. Revision of the Dinantian stratigraphy of the Craven Basin, N. England. Geological Magazine, Vol.117, p.37-49.|
|Chadwick, R A, Jackson, D I, Barnes, R P, Kimbell, G S, Johnson, H, Chiverell, R C, Thomas, G S P, Jones, N S, Riley, N J, Pickett, E A, Young, B, Holliday, D W, Ball, D F, Molyneux, S G, Long, D, Power, G M and Roberts, D H. 2001. Geology of the Isle of Man and its offshore area. British Geological Survey Research Report, RR/01/06.|
|1:50K maps on which the lithostratigraphical unit is found, and map code used:|
|E108 E123 E059 E060 E067 E069 E096|