The BGS Lexicon of Named Rock Units — Result Details
Craven Group
| Computer Code: | CRAV | Preferred Map Code: | notEntered |
|---|---|---|---|
| Status Code: | Full | ||
| Age range: | Chadian Substage (CI) — Yeadonian Substage (CY) | ||
| Lithological Description: | The group includes many formations, with separate nomenclature for geographically isolated areas: Craven Basin (the type area): Hodder Mudstone, Hodderense Limestone and Pendleside Limestone formations, which were all formally defined by Riley (1990) for the now redundant Worston Shale Group, and the redefined Bowland Shale Formation (formerly of group status); West of the Derbyshire High: Ecton Limestone, Widmerpool and Bowland Shale formations (Aitkenhead and Chisholm, 1982); Widmerpool half-graben: Long Eaton, Lockington Limestone and Widmerpool formations (Carney, 2001) and Bowland Shale Formation (formerly Edale Shales); North Wales: Prestatyn Limestone, Teilia, Pentre Chert and Bowland Shale formations (formerly Holywell Shales) (Davies et al., 2004); In south Cumbria and in the Askrigg Block-Craven Basin 'Transition Zone' it comprises the Bowland Shale Formation; On the Isle of Man it comprises the Hodderense Limestone and Bowland Shale formations; Knoll-reef and slope carbonate turbidite facies also occur on the southern margin of the Askrigg Block. The Craven Group typically comprises a Tournaisian-Visean succession of calcareous mudstone and siltstone interbedded with limestones and subordinate limestone breccias, conglomerates and sandstones. This is overlain by a late Brigantian and Namurian succession of the Bowland Shale Formation, which comprises dark grey and black, organic-rich mudstone, with subordinate beds of siltstone, sandstone and dolomitic limestone. Impersistent coal seams and seatearths are present in north Wales (Davies et al., 2004). The Tournaisian-Visean formations are broadly defined by the relative abundance of limestone and mudstone. The Hodder Mudstone, Long Eaton, Widmerpool and Pentre Chert formations are mudstone-dominated. The Hodderense Limestone, Pendleside Limestone, Lockington Limestone, Ecton Limestone, Prestatyn Limestone and Telia formations are limestone-dominated. The limestone beds are typically sharp-based, pale and coarsely bioclastic towards the base and more argillaceous towards the top. Locally the upper part of the Widmerpool Formation includes Brigantian subaqueous basaltic lavas, tuffs, agglomerates and hyaloclastites. Unconformities occur internally within the group at the base of the Pendleside Limestone and Hodder Mudstone formations. The absence of proven late Chadian and Arundian strata within the Long Eaton Formation of the Widmerpool half-graben, may also indicate the presence of an unconformity. The Craven Group ranges from Chadian to Yeadonian age. | ||
| Definition of Lower Boundary: | The base of the Craven Group within the Craven Basin rests unconformably upon the Clitheroe Limestone. In the Widmerpool half-graben the group rests unconformably upon Cambrian strata. West of the Derbyshire High the group may rest conformably upon upper Devonian strata. The base of the group in north Wales, taken at the base of the Pentre Chert Formation, is considered to be a disconformity above the Clwyd Limestone Group. In south Cumbria the upper boundary of the Yoredale Group occurs at the base of the Cravenoceras leion Marine Band, where the thinly interbedded limestones, mudstones and subordinate sandstones of the Alston Formation pass upward into the thick mudstones and fine-grained siltstones of the Bowland Shale Formation, Craven Group. On the south Isle of Man, the upper boundary of the Great Scar Limestone Group occurs at a change from the wackestones and packstones of the Knockrushen Formation to massive lime mudstones with micrite nodules of the Hodderense Limestone Formation (see Dickson et al., 1987) and at a change from the limestones with subordinate claystones and siltstones of the Balladoole Formation to the overstepping claystone with detrital carbonates of the Bowland Shale Formation. | ||
| Definition of Upper Boundary: | The conformable, though highly diachronous, base of the Millstone Grit Group defines the top of the group over most of the Central Pennine Sub-basin. However, at the northern margin of the Wales-Brabant High the top of the group occurs at the base of the Cefn-y-fedw Sandstone and Morridge formations. | ||
| Thickness: | Within the Craven Basin the group is in excess of 1450 m thick. In the Widmerpool half-graben the Craven Group may be in excess of 5000 m thick. The succession is dominated by the Long Eaton Formation, which seismic interpretations suggest is up to 4000 m thick. In north Wales, where the group is up to 400 m thick, both the Prestatyn Limestone and Telia formations pass southwards into Visean shelf carbonate facies of the Clwyd Limestone Group. In south Cumbria the group is about 120 m thick. In the Isle of Man it is about 264 m thick. | ||
| Geographical Limits: | The type area of the Craven Group is the Craven Basin of Lancashire (after which it is named) and its main extent is across central England and North Wales. | ||
| Parent Unit: | Not Applicable (-) | ||
| Previous Name(s): | Gronant Group [Obsolete Name and Code: Use CLWYD, CRAV]
(GRN)
Bowland Shale Group [Obsolete Name and Code: Use CRAV] (-3323) Dyserth Limestone Group [Obsolete Name and Code: Use CRAV] (-161) Gronant Group [Obsolete Name and Code: Use CRAV] (-162) Mallardale Shales [Obsolete Name and Code: Use CRAV] (MLDS) Worston Shale Group [Obsolete Name and Code: Use CRAV, CLL] (WSG) |
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| Alternative Name(s): | none recorded or not applicable | ||
| Stratotypes: | |||
| Type Area | Craven Basin, Lancashire. See component Hodder Mudstone, Hodderense Limestone, Pendleside Limestone and Bowland Shale formations for reference sections. Fewtrell and Smith, 1980. | ||
| Reference Section | Long Eaton No.1 Borehole (SK43SE/161) from 695 m to 2747 m depth (partially cored). | ||
| Reference(s): | |||
| Waters, C N, Gillespie, M R, Smith, K, Auton, C A, Floyd, J D, Leslie, A G, Millward, D, Mitchell, W I, McMillan, A A, Stone, P, Barron, A J M, Dean, M T, Hopson, P M, Krabbendam, M, Browne, M A E, Stephenson, D, Akhurst, M C, and Barnes, R P. 2007. Stratigraphical Chart of the United Kingdom: Northern Britain. (British Geological Survey.) | |||
| Waters, C N, Smith, K, Hopson, P M, Wilson, D, Bridge, D M, Carney, J N, Cooper, A H, Crofts, R G, Ellison, R A, Mathers, S J, Moorlock, B S P, Scrivener, R C, McMillan, A A, Ambrose, K, Barclay, W J, and Barron, A J M. 2007. Stratigraphical Chart of the United Kingdom: Southern Britain. British Geological Survey, 1 poster. | |||
| Dickson, J A D, Ford, T D and Swift, A. 1987. The stratigraphy of the Carboniferous rocks around Castletown, Isle of Man. Proceedings of the Yorkshire Geological Society, Vol.46, 203-229. | |||
| Riley N J. 1990. Stratigraphy of the Worston Shale Group (Dinantian) Craven Basin, north-west England. Proceedings of the Yorkshire Geological Society, Vol. 48, 163-187. | |||
| Aitkenhead, N, and Chisholm, J I. 1982. A standard nomenclature for the Dinantian formations of the Peak District of Derbshire and Staffordshire. Report of the Institute of Geological Sciences, No. 82/8. | |||
| Carney, J N, Ambrose, K and Brandon, A. 2001. Geology of the country between Burton, Loughborough and Derby. Description of 1:50k Sheet 141 (England and Wales). | |||
| Waters, C N, Waters, R A, Barclay, W J, and Davies, J R. 2009. Lithostratigraphical framework for Carboniferous successions of Southern Great Britain (Onshore). British Geological Survey Research Report, RR/09/01. 184pp. | |||
| 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. | |||
| Waters, C N, Browne, M A E, Dean, M T and Powell, J H. 2007. Lithostratigraphical framework for Carboniferous successions of Great Britain (Onshore). British Geological Survey Research Report, RR/07/01. | |||
| Davies, J R, Wilson, D and Williamson, I T. 2004. Geology of the country around Flint. Memoir for 1:50 000 Geological Sheet 108. | |||
| 1:50K maps on which the lithostratigraphical unit is found, and map code used: | |||
| E059 E060 E067 E068 E069 E095 E096 E108 E123 E124 E125 | |||