The BGS Lexicon of Named Rock Units — Result Details

Millstone Grit Group [See Also MIGR]

Computer Code: MG Preferred Map Code: MG
Status Code: Full
Age range: Namurian Stage (CN) — Namurian Stage (CN)
Lithological Description: The Millstone Grit Group (Millstone Grit facies) is of Namurian age. In Northern England, each of the seven Namurian Stage successions has been assigned a distinct formation name, with the exception of the thin, and usually mudstone-dominated successions of the Chokierian and Alportian, which have been joined to form a single formation. The component formations, in ascending order, are the Pendleton, Silsden, Samlesbury, Hebden, Marsden and Rossendale formations. Where marine bands cannot be recognised, or other biostratigraphical data is absent, it is necessary to leave the group undivided. The Cefn-y-fedw Sandstone Formation of north Wales and the Morridge Formation of Staffordshire and the East Midlands, both of Millstone Grit facies, were deposited along the northern margin of the Wales-Brabant High. Lateral continuity between these two broadly similar formations has not been established. Therefore, distinct formation names are maintained. The Millstone Grit Group comprises broadly upward-coarsening cyclic sequences of sandstone, siltstone and grey mudstone, with subordinate and typically thin coal seams and seatearth (palaeosol) horizons. The base of the cycles are typically marked by thin marine bands of dark grey to black, calcareous, shaly mudstone, with distinct ammonoid faunas. The widespread occurrence of the marine bands makes them of primary stratigraphical importance for correlating the successions.
Definition of Lower Boundary: In the northern part of the Central Pennine Sub-basin, the base of the group is taken at the base of the first thick quartz-feldspathic sandstone, of Namurian age, typically present above the Bowland Shale Formation. In the southern part of the Central Pennine Sub-basin, the Cefn-y-fedw Sandstone Formation conformably overlies the Dinantian Minera Formation to the south and locally rests on the Cefn Mawr Limestone in the north. It intertongues with and onlaps the Pentre Chert Formation, and both intertongue with, and are conformably succeeded by, the Bowland Shale Formation. The Morridge Formation conformably overlies, and intertongues with, the Bowland Shale Formation.
Definition of Upper Boundary: A conformable top, located at the incoming of thick coal-bearing Coal Measures facies, at the top of the Rough Rock, or more precisely at the base of dark grey shaly mudstone of the Subcrenatum Marine Band, with eponymous fauna where present (Earp and others, 1961, p.104).
Thickness: The thickest development is in the northern part of the Central Pennine Sub-basin, where 1225 m is recorded in Wharfedale (Ramsbottom et al., 1978). On the Askrigg Block the group is about 500 m thick, significantly thinner than the equivalent strata present within the Central Pennine Basin to the south.
Geographical Limits: The group extends across most of the Central England and North Wales Province, and occurs on the flanks of the Wales-Brabant High. It is limited within the Northern England Province to the Askrigg Block.
Parent Unit: Not Applicable (-)
Previous Name(s): Shale Group [Obsolete Name and Code: Use MG] (SH)
Millstone Grit [Obsolete Name and Code: Use COS, MG, CFS] (-2071)
Alternative Name(s): Millstone Grit Formation (Offshore) [See Also MG]
Stratotypes:
Reference Section  Base: Tansley Borehole (SK35NW/13) at 462 ft 7 ins depth, which shows the base of the siltstone and sandstone of the Ashover Grit, resting on Edale Shales. Ramsbottom et al., 1962. 
Type Area  Numerous natural sections, central and south Pennines, in SD, SE, SJ and SK National Grid map areas. 
Reference Section  Base: Little Mearley Clough, where a waterfall displays the base of the Pendle Grit, resting on dark grey shaly mudstone. Earp et al., 1961, fig. 8 and p.118. 
Reference Section  Base: Stream sections at Edale; e.g. Crowden Brook [SK 1021 8578], Grinds Brook [SK 1208 8649] and the River Noe [SK 1665 8586]. Stevenson and Gaunt, 1971, p.210. 
Reference Section  Base: Blake Brook, which displays the base of the Longnor Sandstones, resting upon dark grey mudstones with common marine shaly mudstones, and beds of turbiditic proto-quartzitic sandstone of the Bowland Shale Formation. Aitkenhead et al., 1985, p.84, and also fig. 28 which displays measured section SK06SE/17. 
Reference(s):
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. 
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. 
Collinson, J D. 1988. Controls on Namurian sedimentation in the Central Province basins of northern England. In: Besly, B M and Kelling, G (eds.) Sedimentation in a synorogenic basin complex: the Upper Carboniferous of Northwest Europe, 86-101. Blackie & Son, London. 
Ramsbottom, W H C, Calver, M A, Eagar, R M C, Hodson, F, Holliday, D W, Stubblefield, C J and Wilson, R B. 1978. A correlation of the Silesian rocks in the British Isles. Geological Society of London, Special Report No.10. 
Stephens, J V, Mitchell, G H and Edwards, W N. 1953. Geology of the country between Bradford and Skipton. Memoir of the Geological Survey of Great Britain, Sheet 69 (England and Wales). 
Collinson, J D. 1969. The sedimentology of the Grindslow Shales and the Kinderscout Grit: a delta complex in the Namurian of Northern England. Journal of Sedimentary Petrology, Vol. 39, 194-221. 
Collinson, J D, and Banks, N L. 1975. The Haslingden Flags (Namurian G1) of south east Lancashire: Barfinger sands in the Pennine Basin. Proceedings of the Yorkshire Geological Society, Vol. 40, 431-458. 
Jones, C M, and Chisholm, J I. 1997. The Roaches and Ashover Grits: sequence stratigraphic interpretation of a 'turbidite fronted delta' system. Geological Journal, Vol. 32, 45-68. 
McCabe, P J. 1978. The Kinderscoutian delta (Carboniferous) of northern England: a slope influenced by density currents. 116-126 in Sedimentation in submarine canyons, fans and trenches. Stanley, D J, and Kelling, G (editors). (Stroudsburg: Dowden, Hutchinson and Ross.) 
McLean, D, and Chisholm, J I. 1996. Reworked palynomorphs as provenance indicators in the Yeadonian of the Pennine Basin. Proceedings of the Yorkshire Geological Society, Vol. 51, 141-151. 
Walker, R G. 1966. Deep channels in turbidite-bearing formations. Bulletin American Association of Petroleum Geologists, Vol. 50, 1899–1917. 
Ramsbottom, W H C, Rhys, G H and Smith, E G, 1962. Boreholes in the Carboniferous Rocks of the Ashover district, Derbyshire. Bulletin of the Geological Survey of Great Britian, No.19, p.75-142. 
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. 
Stevenson, I P and Gaunt, G D, 1971. Geology of the country around Chapel-en-le-Frith. Memoir of the Geological Survey of Great Britain, England and Wales, Sheet 99. 
Earp, J R, Magraw, D, Poole, E G, Land, D H and Whiteman, A J, 1961. Geology of the country around Clitheroe and Nelson. Memoir of the Geological Survey of Great Britain, Sheet.68, England and Wales, pp.346. 
Phillips, J. 1836. Illustrations of the Geology of Yorkshire, Part II. The Mountain Limestone District. (London:John Murray). 
1:50K maps on which the lithostratigraphical unit is found, and map code used:
E024 E020 E059 E067 E077 E070 E141 E041 E040 E051 E069 E099 E111 E112 E124 E125 E050 E060 E061 E062 E075 E076 E084 E085 E110 E123