The BGS Lexicon of Named Rock Units — Result Details
Arden Sandstone Formation
| Computer Code: | AS | Preferred Map Code: | AS |
|---|---|---|---|
| Status Code: | Full | ||
| Age range: | Carnian Age (TC) — Carnian Age (TC) | ||
| Lithological Description: | Consists of grey, green and purple mudstone interbedded with paler grey-green to buff-coloured siltstone and fine to medium-grained, varicoloured (green, brown, buff, mauve) sandstone; thin pebble beds occur locally. Breccias occur at the base and top of the formation on the south Devon coast (Jeans, 1978), and may have been formed by penecontemporaneous solution of halite. Laminated and thinly interbedded sediments are commonly extensively bioturbated and show structures indicative of thixotropic deformation. Invertebrate and vertebrate macrofossils are present, locally in abundance, and miospores and burrows are common. The siltstones and finer sandstones show small-scale ripple drift cross-bedding; thicker sandstone beds show trough and planar cross-bedding. The proportion of fine to coarse clastics varies laterally within the formation. The thicker sandstone units, composed of several individual beds, have a lenticular geometry and occupy the inferred former courses of distributary channels in a deltaic or estuarine environment. Less arenaceous, mudstone- and siltstone-dominated successions represent intervening interdistributary areas. The formation is differentiated from the reddish brown, blocky weathering mudstones of adjacent formations by its predominantly greenish grey colour, the presence of a significant (though commonly subordinate) proportion of sandstone, the predominance of finely laminated lithologies throughout, and its comparatively fossiliferous nature. The formation is characterised by an exotic clay mineral assemblage, which is rich in mixed-layer clays and distinct from the illite-dominated units below and above (Jeans 1978, Bloodworth and Prior, 1993; Carney et al., 2004; Jeans et al., 2005). Late Triassic, late Carnian (Tuvalian), on palynological evidence. | ||
| Definition of Lower Boundary: | Placed at an abrupt upward change from the red mudstones or siltstones of the Sidmouth Mudstone Formation to the dominantly grey or green, partly or largely arenaceous beds of the Arden Sandstone Formation. On the south Devon coast, this boundary coincides with the base of a prominent breccia bed (see sections in Jeans, 1978), and is conformable in all areas. Where not exposed, the boundary is typically marked by a change from the reddish brown clayey soils of the Sidmouth Mudstone Formation to the grey, slightly sandy clay soils of the overlying Arden Sandstone Formation. Where the latter forms a marked cuesta feature, the boundary typically lies at a slight concave break in the scarp slope. | ||
| Definition of Upper Boundary: | Placed at the base of the predominantly red, structureless mudstones of the Branscombe Formation where they rest on the interbedded dark grey-green siltstone and pale grey sandstone of the underlying Arden Sandstone Formation. The boundary is abrupt or is a rapid interbedded transition. On the south Devon coast, the upper boundary coincides with the top of a prominent breccia bed (see sections in Jeans, 1978). This boundary is conformable in all areas. Where not exposed, the boundary is typically marked by a change from the grey, slightly sandy clay soils of the Arden Sandstone Formation to the reddish brown clayey soils of the Branscombe Mudstone Formation. Where the Arden Sandstone forms a marked cuesta feature, the boundary lies at the down-dip limit of the dip slope. | ||
| Thickness: | Typically 7 to 8 m, rising to 20 m in the Stowell Park Borehole (SP01SE/1, [SP 0835 1176] (Green and Melville, 1956) towards the south of the Worcester Basin, and 24 m on the south Devon coast (cf. Jeans, 1978). Locally, it thins to 2 to 3 m in parts of the Worcester Basin, Knowle Basin and East Midlands areas. | ||
| Geographical Limits: | Traceable nearly continuously from Gloucestershire northward through Worcestershire and eastwards though Warwickshire into Leicestershire and Nottinghamshire. Less continuous outcrop is mapped in Somerset and east Devon. In the subcrop, the formation is proved in several cored and geophysically logged boreholes in the subsurface in the central Midlands (Worcester and Knowle basins), East Midlands, and in southern England (Wessex Basin). It is not recognised in the Cheshire Basin and other basins farther north-west, though thin sandstones in the upper part of the Wilkesley Halite (Poole and Whiteman, 1966), might represent the formation in the Cheshire Basin. At depth in Dorset and Somerset, the partly equivalent Dunscombe Mudstone Formation has been interpreted to pass laterally into halite (Gallois, 2001, 2003). The type area is the Knowle Basin, Warwickshire. | ||
| Parent Unit: | Mercia Mudstone Group (MMG) | ||
| Previous Name(s): | Dane Hills Sandstone Group [Obsolete Name and Code: Use AS]
(-3804)
Hollygate Skerry [Obsolete Name and Code: Use AS, HLY] (-3805) Arden Sandstone Member [Obsolete Name and Code: Use AS] (-3177) Keuper Sandstone [Obsolete Name and Code: Use AS] (KS) Weston Mouth Sandstone Member [Obsolete Name and Code: Use AS] (WMSF) Dunscombe Mudstone Formation [Obsolete Name and Code: Use AS, BCMU] (DUM) Arden Sandstone Horizon [Obsolete Name and Code: Use AS] (-1314) North Curry Sandstone Member [Obsolete Name and Code: Use AS] (NCMB) Upper Keuper Sandstone [Obsolete Name and Code: Use MASS, AS] (-1683) Arden Sandstone Group [Obsolete Name and Code: Use AS] (-1956) Dane Hills Sandstone Member [Obsolete Name and Code: Use AS] (DHSM) Butcombe Sandstone Member [Obsolete Name and Code: Use AS] (BUS) Hollygate Sandstone Member [Obsolete Name and Code: Use AS] (HLY) Arden Sandstone [Obsolete Name and Code: Use AS] (*64) |
||
| Alternative Name(s): | none recorded or not applicable | ||
| Stratotypes: | |||
| Type Section | Canal cutting [SP 2118 6744], Shrewley, Warwickshire, showing the full thickness (about 9 m) of the formation (Old et al., 1991, plate 10; Benton et al., 2002). | ||
| Reference Section | Twyning Borehole (SO83NE/5) [SO 8943 3664], Twyning, near Tewkesbury, Gloucestershire: from 310.51 to 315.00 m depth (Barclay et al., 1997). Curated core held at the National Geosciences Records Centre, BGS, Keyworth. | ||
| Reference Section | Canal cutting [SP 2010 6905], Rowington, Warwickshire (Old et al., 1991) | ||
| Reference Section | Cropwell Bridge Borehole (SK63NE/28) [SK 6773 3547] Cropwell Bishop, Nottinghamshire: from 37.50 to 42.03 m depth (Howard et al., 2009). Curated core held at the National Geosciences Records Centre, BGS, Keyworth. | ||
| Reference Section | Higher Dunscombe and Weston Cliffs [SY 152 877 to 171 879], south Devon coast (Jeans, 1978; Warrington and Scrivener, 1980; Gallois, 2001). | ||
| Reference Section | Newnham river cliff, River Severn, Gloucestershire [SO 6923 1168]. | ||
| Reference(s): | |||
| Matley, C A. 1912. The Upper Keuper (or Arden) Sandstone Group and associated rocks of Warwickshire. Quarterly Journal of the Geological Society, London, Vol.68, 252-280. | |||
| Gallois, R W. 2001. The lithostratigraphy of the Mercia Mudstone Group (mid to late Triassic) of the south Devon coast. Geoscience in south-west England, Vol.10, 195-204 | |||
| Horwood, A R. 1913. The upper Trias of Leicestershire, Part 1. Geological Magazine, Vol.50, 21-32, 73-86, 109-121, 205-215. | |||
| Plant, J. 1856. On the upper Keuper Sandstone (included in the New Red Marls) and its fossils at Leicester. Quarterly Journal of the Geological Society of London, Vol.12, 369-373. | |||
| Symonds, W S. 1855. Notice of fossils from the Keuper Sandstones of Pendock, Worcestershire. Quarterly Journal of the Geological Society of London, Vol.11, 450-451. | |||
| Wills, L J and Campbell Smith, W. 1913. Notes on the flora and fauna of the Upper Keuper Sandstones of Warwickshire and Worcestershire. Geological Magazine, Vol.50, 461-462. | |||
| Howard, A S, Warrington, G, Ambrose, K, and Rees, J G. 2008. A formational framework for the Mercia Mudstone Group (Triassic) of England and Wales. British Geological Survey Research Report, RR/08/04. | |||
| Brodie, P B. 1870. On the geology of Warwickshire. 34th Annual Report of the Warwickshire Natural History and Archaeological Society, 10-34. | |||
| Charsley, T J, Rathbone, P A and Lowe, D J, 1990. Nottingham: A geological background for planning and development. British Geological Survey Technical Report WA/90/1. | |||
| Elliott, R E 1961. The stratigraphy of the Keuper Series in southern Notinghamshire. Proceedings of the Yorkshire Geological Society, Vol.33, 197-231. | |||
| Green, G W and Welch F B A, 1965. The geology of the country around Wells and Chedder. Memoir of the Geological Survey of England and Wales, Sheet 280. | |||
| Howard, A S, Warrington, G, Carney, J N, Ambrose, K, Young, S R and Pharaoh, T C. 2009. Geology of the country around Nottingham. Memoir of the British Geological Survey, Sheet 126 (England and Wales). | |||
| Kellaway, G A and Welch, F B A. 1993. Geology of the Bristol district. Memoir of the British Geological Survey Special Sheet [England and Wales]. | |||
| Poole, E G, and Whiteman, A J. 1966. Geology of the country around Nantwich and Whitchurch. Memoir of the Geological Survey of Great Britain, Sheet 122 (England and Wales). | |||
| Ussher, W A E. 1908. The geology of the Quantock Hills and of Taunton and Bridgewater. Memoir of the Geological Survey of Great Britain. | |||
| Wills, L J. 1970. The Triassic succession in the central Midlands in its regional setting. Quarterly Journal of the Geological Society of London, Vol.126, 225-285. | |||
| Phillips, J. 1848. The Malvern Hills compared with the Palaeozoic districts of Abberley, Woolhope, May Hill, Tortworth and Usk. Memoir of the Geological Survey of Great Britain, Vol. 2, Part 1. | |||
| Warrington, G, Audley-Charles, M G, Elliott, R E, Evans, W B, Ivimey-Cook, H C, Kent, P E, Robinson, P L, Shotton, F W and Taylor, F M. 1980. A correlation of the Triassic rocks in the British Isles. Special Report of the Geological Society of London, No.13. | |||
| Green, G W and Melville, R V. 1956. The stratigraphy of the Stowell Park Borehole (1949-1951). Bulletin of the Geological Survey of Great Britain, No.11, p.1-66. | |||
| 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. | |||
| Barclay, W J, Ambrose K, Chadwick, R A, and Pharaoh, T C. 1997. Geology of the country around Worcester. Memoir of the British Geological Survey, Sheet 199 (England and Wales). | |||
| Benton, M J, Cook, E, and Turner, P. 2002. Permian and Triassic Red Beds and the Penarth Group of Great Britain. Geological Conservation Review Series, No. 24. (Peterborough: Joint Nature Conservation Committee.) | |||
| Carney, J N, Ambrose, K, Brandon, A, Royles, C P, Lewis, M A, and Sheppard, H. 2004. Geology of the country around Melton Mowbray. Sheet Description of the British Geological Survey, 1:50 000 Series Sheet 142 Melton Mowbray (England and Wales). | |||
| Jeans, C V. 1978. The origin of the Triassic clay assemblages of Europe with special reference to the Keuper Marl and Rhaetic of parts of England. Philosophical Transactions of the Royal Society of London, Series A, Vol.289, 549-639. | |||
| Wills, L J. 1976. The Trias of Worcestershire and Warwickshire. Report of the Institute of Geological Sciences, No.76/2. | |||
| Bloodworth, A J, and Prior, S V. 1993. Clay mineral stratigraphy of the Mercia Mudstone Group in the Nottingham area. British Geological Survey Technical Report WG/93/29. | |||
| Gallois, R W. 2003. The distribution of halite (rock-salt) in the Mercia Mudstone Group (mid to late Triassic) in south-west England. Geoscience in south-west England, Vol. 10, 383-389. | |||
| Jeans, C V, Fisher, M J, and Merriman, R J. 2005. Origin of the clay mineral assemblages in the Germanic facies of the English Trias: application of the spore colour index method. Clay Minerals, Vol. 40, 115-129. | |||
| Warrington, G, and Scrivener, R C. 1980. The Lyme Regis (1901) Borehole succession and its relationship to the Triassic sequence of the east Devon coast. Proceedings of the Ussher Society, Vol. 5, 24-32. | |||
| Old R A, Hamblin, R J O, Ambrose, K, and Warrington G. 1991. Geology of the country around Redditch. Memoir of the British Geological Survey, Sheet 183 (England and Wales). | |||
| Murchison, R I and Strickland, H E. 1840. On the upper formations of the New Red Sandstone system in Gloucestershire, Worcestershire and Warwickshire. Transactions of the Geological Society of London, Series 2, Vol.5, 331-348. | |||
| 1:50K maps on which the lithostratigraphical unit is found, and map code used: | |||
| E216 E183 E184 E199 E326 E155 E156 E168 E182 E200 E234 E264 E280 E340 | |||