Bowland Shale Formation

Bowland Shale Formation (BSG), Carboniferous, Midland Valley of Scotland

Bowland Shale Formation is part of the Yoredale Group

Since the Bowland Shale Formation extends into southern Britain the definition that follows has been unified with that of Waters et al. (2009). Information relevant solely to northern Britain is, however, provided under the subhead ‘Local notes’.

Name

The Bowland Shale Group of Earp et al. (1961) was subdivided into Lower and Upper Bowland Shale formations, the division between the two taken at the base of the Cravenoceras leion Marine Band. The equivalent succession was referred to as the Bowland Shale Formation by Fewtrell and Smith (1980). The formation is formally redefined, with the term extended to replace other dark grey euxinic shales in North Wales (Holywell Shales) (Davies et al., 2004) and the East Midlands (Edale Shales), which were in lateral continuity with the deposits of the Craven Basin at the time of deposition.

Lithology

Mainly dark grey fissile and blocky mudstone, weakly calcareous with subordinate sequences of interbedded limestone and sandstone, fossiliferous in more-or-less discrete bands. In the Furness and Settle areas the formation comprises thick-bedded, blocky to subfissile, 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. The formation shows an upwards decrease in carbonate turbidites and a concomitant increase in siliciclastic sandstone turbidites (see Rose and Dunham, 1977; Johnson et al., 2001; Arthurton et al., 1988). In the south Isle of Man, the Bowland Shale Formation includes black claystone with localised deposits of carbonate turbidites, debris flows, olistoliths, volcaniclastic deposits and lavas (see Chadwick et al., 2001; Dickson et al., 1987). At the base of the formation, the Scarlett Point Member comprises cherty and pyritous tabular beds of pale wackestone and lime-mudstone (dolomitised 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 Bowland Shale 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 spectacularly preserved ammonoid faunas) derived from the Balladoole Formation. The middle part of the exposed part of the formation comprises black, calcareous, platy claystone with subordinate beds (up to 2 m thick) of dark wackestone, and dark detrital packstone debris. In the north Isle of Man, the Shellag Point Borehole (see below) cored through 27.55 m of siltstone, claystone and ironstone. These included two marine bands.

Genetic interpretation

The mudstones accumulated as hemipelagic deposits, predominantly from suspension in moderately deep water, largely below the storm wave-base. For much of the time the water was brackish or fresh and occurred in the photic zone (Collinson, 1988). Marine bands developed during periods of higher salinity when connections with the open ocean were established. The thin limestones and sandstones were introduced by storms and/or as turbidites; the limestones sourced from active carbonate shelves, the siliciclastic sediments from active deltas accumulating on the margins of the Central Pennine Basin. In the south of the Isle of Man, lavas, volcaniclastic debris flows and gravity slides are apparently submarine. In the north Isle of Man, the siltstone, claystone and ironstone of the Shellag Point Borehole was mostly of marine origin.

Stratotype

Reference sections include: River Ribble at Dinckley (SD 689 366 to 686 366), on the south bank of the River Ribble between Dinckley Hall and the suspension bridge, about 400 m to the west downstream (Earp et al., 1961); Little Mearley Clough (SD 779 414 to 785 411), 400 to 1100 m south-east and upstream from Little Mearley Hall (Earp et al., 1961; Fewtrell and Smith, 1980); Coed Pen-y-Maes stream section at Holywell (SJ 1939 7650 to 1962.7687) (Davies et al., 2004); Duffield Borehole, near Derby (BGS Registration Number SK34SW/5) (SK 3428 4217), from 335.23 to 405.78 m depth (see Aitkenhead, 1977); Roosecote Borehole, Barrow-in-Furness (BGS Registration Number SD26NW/19) (SD 2304 6866), which includes an entire thickness of the formation from 491.68 to 613.31 m depth (see Rose and Dunham, 1977; Johnson et al., 2001); Shellag Point Borehole, north Isle of Man (NX 4565 9965) cored from 100.60 to 128.15 m, but proving neither the base nor the top of the formation (Chadwick et al., 2001).

Lower and upper boundaries

The conformable base of the formation upon the Pendleside Limestone Formation in the Craven Basin, on the Widmerpool Formation in the East Midlands, on the Pentre Chert and Cefn-y-Fedw Sandstone Formation in North Wales, and on the Hodderense Limestone Formation in the south of the Isle of Man (Figure 8, Column 8), 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 (E1A1), where the dark grey to black marine mudstone rests upon thinly interbedded limestones, mudstones and subordinate sandstones of the Alston Formation (Figure 9, Column 14).

The Bowland Shale Formation onlapped onto, and eventually, by the Pendleian, extended 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 (Figure 15, Column 6).

The top of the formation is taken at the base of the Millstone Grit Group over most of the Pennine Basin (Figure 9, Column 14; Figure 15, Column 6) 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.

Thickness

Generally 120–620 m thick. The formation thickens north-eastwards along the axis of the Central Lancashire High, from about 22 m in the Roddlesworth Borehole, 68 m thick in the Holme Chapel Borehole, and 102 m in the Boulsworth Borehole (Waters et al., 2009, fig. 11). 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 30–200 m thick (see Arthurton et al., 1988, fig. 22). In south Cumbria, the Roosecote Borehole (see above) proved the formation to be 121.63 m thick (see Johnson et al., 2001; Rose and Dunham, 1977). On the Isle of Man, the Bowland Shale Formation may be at least 186 m thick; in the north, the Shellag Point Borehole (see above) proved only a part of the formation 27.55 m thick.

Distribution and and regional correlation

Widespread in the Craven Basin (the type area), including Lancaster, Garstang, Settle, Clitheroe, Harrogate districts, south Cumbria and the Isle of Man, but also in North Wales, Staffordshire and the East Midlands. The upper part of the Bowland Shale Formation passes northward into the Millstone Grit Group and to the south into the Morridge Formation (Waters et al., 2009, fig. 10).

Age and biostratigraphical characterisation

Asbian to Yeadonian. In the type area of the Craven Basin the formation ranges from late Asbian (Cf6ð¨ Zone) to early Pendleian (E1c1 Zone). The top of the formation ranges to younger ages toward the south of the Pennine Basin, with Yeadonian strata present in North Wales. The Bowland Shale Formation of south Cumbria and the Craven Reef Belt is Pendleian. On the Isle of Man, the formation is Asbian to late Arnsbergian. In the south of the island, the Scarlett Point Member is of (possibly early) Asbian age and includes in the limestone the deep water trace fossil Helminthoides sp., and the ammonoid genera Beyrichoceras and Bollandoceras. In the north of the island, the Shellag Point Borehole (see above) includes the two latest Arnsbergian marine bands with the ammonoid Nuculoceras nuculum, an associated shelly fauna and the trilobite Paladin.

Local notes

In the south Isle of Man, the Bowland Shale Formation oversteps the Balladoole Formation along the foreshore north-west of Salt Spring Cottage (SC 244 679), and the exposed middle part of the formation is seen for example at Black Marble Quarry (SC 244 676). In northern England the Bowland Shale Formation is ?early Asbian to Namurian in age and includes the Emstites leion, Eumorphoceras pseudobilingue and C. malhamense Marine bands.