Skip to main content Accessibility help

Clay mineralogy of the Old Red Sandstone and Devonian sedimentary rocks of Wales, Scotland and England

  • S. Hillier (a1), M. J. Wilson (a1) and R. J. Merriman (a2)


The Devonian sedimentary rocks of the UK are made up of a continental red bed facies, the Old Red Sandstone (ORS), and sediments of a marine origin. The latter are confined to southwest England whereas the ORS occurs much more extensively, particularly in South Wales, the West Midlands, Northern England, the Midland Valley of Scotland and the Orcadian basin. The ORS also occurs extensively offshore where it contains important hydrocarbon reservoirs. Highly variable suites of clay mineral assemblages are characteristic of the ORS. In the West Midlands and Monmouthshire, the Downton Group is characterized by illitic, smectitic, and mixed-layer illite-smectite minerals. A tuff bed (Townsend Tuff) also contains expansible minerals but when the bed is traced westwards it is found that the clay mineralogy changes progressively to an illite-chlorite assemblage, suggesting the influence of diagenetic or metamorphic change. It is not known, however, whether such a transformation is typical of the Downton Group as a whole. The overlying Ditton Group in its eastern outcrops contains a high-spacing mineral identified as tosudite, together with regularly interstratified illite-smectite and well crystallized kaolinite. Further west this assemblage gives way to illite and chlorite, with the latter being trioctahedral or dioctahedral, while in Dyfed the Ditton Group may contain smectite and poorly crystallized kaolinite in addition to illite and chlorite. The geographical distribution of clay minerals in the Ditton Group may also be accounted for by progressive diagenetic to low-grade metamorphic reactions, although it is necessary to postulate retrogressive diagenesis to account for the smectite and kaolinite that occur in the Dyfed samples. The clay mineralogy of the overlying Brecon Group and the Upper ORS also consists of mixtures of illite and chlorite in the west and central parts of the region. No data are available for the Brecon Group in the eastern parts of the outcrop but the Upper ORS from around Monmouth and Portishead contains assemblages rich in kaolinite and mixed-layer illite-smectite with only minor amounts of chlorite. The distribution of clay minerals in the Upper ORS is again suggestive of a progressive westerly increase in the influence of diagenetic alteration, although the influence of provenance cannot be discounted.

In Scotland the clay mineralogy of the Lower ORS in the Midland valley is characterized by a variety of interstratified minerals, including regularly interstratified trioctahedral chlorite-vermiculite, a tosudite mineral similar to that described from South Wales and illite-smectite, as well as occasional illite, chlorite and smectite. The oldest Stonehaven Group is kaolinitic but in the younger groups kaolinite is either completely absent or present in only minor amounts. It is clear that detrital inputs, particularly from associated volcanic rocks, have contributed to the clay minerals found in these rocks, although the contribution could be indirect with diagenetic clay minerals forming from volcanic detritus after deposition. Diagenetic alteration may also be important, particularly with respect to the origin of the tosudite mineral. The Middle ORS lacustrine sediments of the Orcadian Basin of Scotland are characterized by mixtures of trioctahedral chlorite and dioctahedral illite, with interstratified chlorite-smectite, illite-smectite, kaolinite and occasional reports of minor montmorillonite. The most recent interpretations of the origins of the clay minerals in these rocks emphasize the role of progressive diagenetic and low-grade metamorphic reactions, based on correlations of clay mineral assemblages and parameters such as illite crystallinity with organic maturation data. This interpretation argues that the illite-smectite in the shales is derived largely from a precursor detrital smectite. However, the finding of two populations of morphologically and structurally distinct illite-smectite particles in the Middle and Upper ORS sandstones suggests a more complex picture involving different diagenetic episodes. In addition, the likelihood of a smectitic-rich detrital input to the ORS may also be a point of debate. The clay mineralogy of the North Sea offshore is also described briefly, in addition to the marine Devonian in southwest England. The latter is characterized by chlorite and illite assemblages of low-grade metamorphic origin, although smectite and kaolinite are also found occasionally in these rocks. The offshore ORS, however, contains a variety of clay minerals, including an assemblage similar to that found in the Lower ORS south of the Highland Boundary Fault and notably contains a tosudite-like mineral.


Corresponding author


Hide All
Allen, J.R.L. (1985) Marine to fresh water: the sedimentology of the interrupted environmental transition (Ludlow-Siegenian) in the Anglo-Welsh area. Pp. 85–104 in: Evolution and environment in the late Silurian and early Devonian. (Chaloner, W.G. and Lawson, J.D., editors) Philosophical Transactions of The Royal Society, B309.
Allen, J.R.L. (1986) Pedogenic calcretes in the Old Red Sandstone facies (Late Silurian-early Carboniferous) of the Anglo-Welsh area, southern Britain. Pp. 58–86 in: Palaeosols: Their Recognition and Interpretation (Wright, V.P., editor). Blackwell Scientific, Oxford.
Allen, J.R.L. & Williams, B.P.J. (1981) Sedimentology and stratigraphy of the Townsend Tuff Bed (Lower Old Red Sandstone) in South Wales and the Welsh Borderland. Journal of the Geological Society London, 138, 15–29.
Allen, P.A. & Mange-Rajetzky, M.A. (1992) Devonian- Carboniferous sedimentary evolution of the Clair area, offshore north western UK; impact of changing provenance. Marine and Petroleum Geology, 9, 29–52.
Bevins, R.E., White, S.C. & Robinson, D. (1996) The South Wales Coalfield: low grade metamorphism in a foreland basin setting? Geological Magazine, 133, 739–749.
Bloxam, T.W. & Owen, T.R. (1985) Anthratization of coals in the South Wales coalfield. International Journal of Coal Geology, 4, 299–307.
Burolett, P.F., Byramjee, J. & Couppey, C. (1969) Contribution a l'étude sédimentologique des terrains Devonien du Nord-est de L'Ecosse. Notes et Memoires No.9. Companie Francaise des Pétrole, Paris, p. 1–85.
Brindley, G.W. (1961) Kaolinite, serpentine and kindred minerals. Pp. 51–131 in: The X-ray identification and crystal structures of clay minerals. (Brown, G., editor). Mineralogical Society London.
Cope, J.C.W., Ingham, J.K. & Rawson, P.F. (1992) Atlas of palaeogeography and lithofacies. Geological Society, London, 164 pp.
Dineley, D.L. (1992) Devonian. Pp 179–205 in: Geology of England and Wales (Duff, P.McL.D and Smith, A.J., editors). Geological Society, London.
Drits, V.A. (2003) Structural and chemical heterogeneity of layer silicates and clay minerals. Clay Minerals, 38, 403–432.
Drits, V.A., Weber, F., Salyn, A. & Tsipursky, S. (1993) X-ray identification of 1M illite varieties. Clays and Clay Minerals, 28, 185–207.
Drits, V.A., Besson, G. & Muller, F. (1995) An improved model for structural transformations of heat-treated aluminous dioctahedral 2:1 layer silicates. Clays and Clay Minerals, 43, 718–731.
Drits, V.A., Lindgreen, H., Salyn, A.L., Ylagan, R. & McCarty, D.K. (1998) Semiquantitative determination of trans vacant and cis vacant 2:1 layers in illite and illite-smectites by thermal analysis and X-ray diffraction. American Mineralogist, 83, 1188–1198.
Fairbridge, R.W. (1967) Phases of diagenesis and authigenesis. Pp 19–89 in: Diagenesis in sediments (Larsen, G. & Chiliingar, G.V., editors) Elsevier Amsterdam.
Garvie, L.A.J. (1992) Diagenetic tosudite from the lowermost St Maughan's Group, Lydney harbour, Forest of Dean, UK. Clay Minerals, 27, 507–513.
Gill, W.D., Khalaf, F.I. & Massoud, M.S. (1977) Clay minerals as an index of the degree of metamorphism of the carbonate and terrigenous rocks in the South Wales coalfield. Sedimentology, 24, 675–691.
Glennie, K. (1998) Introduction to the Petroleum Geology of the North Sea. 4th edition. Blackwell Science, Oxford, UK. 636 pp.
Grim, R.E. & Güven, N. (1978) Bentonites: Geology, Mineralogy, Properties and Uses. Elsevier, Amsterdam. Developments in Sedimentology, 24, 256 pp.
Hayashi, H. & Oinuma, K. (1964) Behaviours of clay minerals in treatment with hydrochloric acid, formamide and hydrogen peroxide. Clay Science, 2, 75–91.
Hillier, S. (1989) Clay mineral diagenesis and its relationship to organic maturity indicators in Devonian lacustrine mudrocks from the Orcadian basin, Scotland. PhD thesis, University of Southampton, UK. 193 pp.
Hillier, S. (1993) Origin, diagenesis and mineralogy of chlorite minerals in Devonian lacustrine mudrocks, Orcadian Basin, Scotland. Clays and Clay Minerals, 41, 240–259.
Hillier, S., Mátyá, s., Matter, A. & Vasseur, G. (1995) Illite/ smectite diagenesis and its variable correlation with vitrinite reflectance in the Pannonian Basin. Clays and Clay Minerals, 43, 174–183.
Hillier, S., Son, B.K. & Velde, B. (1996) Effects of hydrothermal activity on clay mineral diagenesis in Miocene shales and sandstones from the Ulleung (Tsushima) back-arc basin, East Sea (Sea of Japan), Korea. Clay Minerals, 31, 113–126.
Hillier, S. & Clayton, T. (1989) Illite/smectite diagenesis in Devonian lacustrine mudrocks from northern Scotland and its relationship to organic maturity indicators. Clay Minerals, 24, 181–196.
Hillier, S. & Marshall, J. (1992) Organic maturation, thermal history and hydrocarbon generation in the Orcadian Basin, northern Scotland. Journal of the Geological Society of London, 149, 491–502.
Hillier, S. & Ryan, P.C. (2002) Identification of halloysite (7Å) by ethylene glycol solvation: the MacEwan effect. Clay Minerals, 37, 487–496.
Hillier, S. (2003) Chlorite in sediments. Pp 123–127 in. Encyclopedia of Sediments and Sedimentary Rocks (Middleton, G.V., Church, M.J., Coniglio, M., Hardie, L.A. and Longstaffe, F.J., editors). Kluwer Academic Publishers, Dordrecht.
Hillier, S. (in press) Appendix A: Mineralogical and chemical data. in: Clay Materials Used in Construction (Reeves, G.M., Sims, I., and Cripps, J.C., editors). Geological Society of London, Engineering Geology Special Publication, 21.
Holder, M.T. & Leveridge, B.E. (1994) A framework for the European Variscides. Technical Report of the British Geological Survey WA/94/24/R.
House, M.R., Richardson, J.B., Chaloner, W.G., Allen, J.R.L., Holland, C.H. & Westoll, T.S. (1977) A correlation of Devonian rocks of the British Isles. Geological Society of London, Special Report, 7, 110 pp.
Jeans, C.V. (1995) Clay mineral stratigraphy in Palaeozoic and Mesozoic red bed facies onshore and offshore UK. Pp 31–55 in: Non-biostratigra-phical Methods of Dating and Correlation (Dunay, R.E. and Hailwood, E.A., editors). Geological Society of London, Special Publication 89.
Kelm, U. (1986) Mineralogy and illite crystallinity of the pelitic Devonian and Carboniferous strata of north Devon and western Somerset. Proceedings of the Ussher Society, 6, 338–343.
Kulke, H. (1969) Petrographie und Diagenese des Stuben-sandsteins (mittlerer Keuper) aus Tiefbohrungen im Raum. Memmingen (Bayern). Contributions to Mineralogy and Petrology, 20, 135–163.
Kübler, B. (1973) La corrensite, indicateur possible de milieux de sedimentation et du degré de transformation d'un sédiment. Bulletin Centre Recherche Pau-S.N.P.A. 7, 543–556.
Lee, M. (1996) 1M(cis) illite as an indicator of hydrothermal activities and its geological implication. 33rd Annual Meeting of the Clay Minerals Society, Program and Abstracts, p 106.
Leveridge, B.E. & Holder, M.T. (1985) Olistostromic breccias at the Mylor/Gramscatho boundary, south Cornwall. Proceedings of the Ussher Society, 6, 147–154.
Leveridge, B.E., Holder, M.T., Goode, A.J.J., Scrivener, R.C., Jones, N.S. & Merriman, R.J. (2002) Geology of the Plymouth and south-east Cornwall area. Memoir of the British Geological Survey, Sheet 348 (England and Wales).
Lindgreen, H.A., Drits, V.A., Sakharov, B.A., Salyn, A.L., Wrang, P. & Dainyak, L.G. (2000) Illite-smectite structural transformations during metamorphism in black Cambrian Alum shales from the Baltic area. American Mineralogist, 85, 1223–1238.
Mackenzie, R.C. (1957) The illite in some Old Red Sandstone soils and sediments. Mineralogical Magazine, 31, 681–689.
Marriott, S.B. & Wright, V.P. (2004) Mudrock deposition in an ancient dryland system: Moor Cliffs Formation, Lower Old Red Sandstone southwest Wales, UK. Geological Journal, 39, 277–298.
Marshall, J.E.A., Haughton, P.D. & Hillier, S. (1994) Vitrinite reflectivity and the structure and stratigraphy of the Old Red Sandstone of the Midland Valley of Scotland. Journal of the Geological Society of London, 151, 425–438.
Marshall, J.E.A. & Hewett, A.J. (2003) Devonian. Pp. 65–81 in: The Millennium Atlas: Petroleum Geology of the Central and Northern North Sea. (Evans, D., Graham, C., Armour, A. and Bathurst, P., editors). The Geological Society of London.
Maskall, R. (1985) Diagenesis of Air-Fall Tuffs. PhD thesis. University of Reading, UK.
Maxwell, D.T. & Hower, J. (1967) High grade diagenesis and low grade metamorphism of illite in the Pre-Cambrian Belt series. American Mineralogist, 52, 843–857.
Merriman, R.J. (2005) Clay minerals and sedimentary basin history. European Journal of Mineralogy, 17, 7–20.
Merriman, R.J., Kemp, S.J., Warr, L.N. & Prior, S.V. (1996) Regional low grade metamorphism in the Plymouth district, 1:50k geological sheet 348. British Geological Survey Technical Report WG/96/9.
Merriman, R.J. & Frey, M. (1999) Patterns of very low-grade metamorphism in metapelitic rocks Pp 161–107 in: Low-grade Metamorphism (Frey, M. and Robinson, D., editors). Blackwell Science, Oxford, UK.
Monnier, F. (1982) Thermal diagenesis in the Swiss Molasse basin: implications for oil generation. Canadian Journal of Earth Sciences, 19, 328–342.
Moore, D.M. & Reynolds, R.C. Jr. (1997) X-ray Diffraction and the Identification and Analysis of Clay Minerals. 2nd Edition, Oxford University Press, New York.
Morrison, S.J. & Parry, W.T. (1986) Dioctahedral corrensite from Permian red beds, Lisbon Valley, Utah. Clays and Clay Minerals, 34, 613–624.
Mykura, W. (1991) Old Red Sandstone. Pp. 297–346 in: The Geology of Scotland (Craig, G.Y., editor) 3rd Edition. Geological Society of London.
Nieto, F., Mata, P.M., Bauluz, B., Giorgetti, G., Árkai, P. & Peacor, D.R. (2005) Retrograde diagenesis, A a widespread process on a regional scale. Clay Minerals, 40, 93–104.
Pamplin, C.F. (1990) A model for the tectono-thermal evolution of north Cornwall. Proceedings of the Ussher Society, 7, 206–211.
Parker, A., Allen, J.R.L. & Williams, B.P.J. (1983) Clay mineral assemblages of the Townsend Tuff Bed (Lower Old Red Sandstone), South Wales and the Welsh Borderland. Journal of the Geological Society of London, 140, 769–779.
Pay, M.D., Astin, T.R. & Parker, A. (2000) Clay mineral distribution in the Devonian-Carboniferous sandstones of the Clair Field, west of Shetland, and its significance for reservoir quality. Clay Minerals, 35, 151–162.
Perrin, R.M.S (1971) The Clay Mineralogy of British Sediments. Mineralogical Society, London. 247 p.
Primmer, T.J. (1985) A transition from diagenesis to greenschist facies within a major Variscan fold/ thrust complex in south-west England. Mineralogical Magazine, 49, 365–374.
Robinson, D., Nicholls, R.A. & Thomas, L.J. (1980) Clay mineral evidence for low grade Caledonian and Variscan metamorphism in south-western Dyfed, south Wales. Mineralogical Magazine, 43, 857–863.
Rogers, D.A., Marshall, J.E.A. & Astin, T.R. (1989) Devonian and later movements on the Great Glen fault system. Journal of the Geological Society London, 146, 369–372.
Soper, N.J. & Woodcock, N.H. (2003) The lost Lower Old Red Sandstone of England and Wales: a record of post-Iapetan flexure or Early Devonian transten-sion? Geological Magazine, 140, 627–647.
Środoń, J. (1984) X-ray identification of illitic materials. Clays and Clay Minerals, 32, 337–349.
Środoń, J. (1999) Nature of mixed-layer clays and mechanisms of their formation and alteration. Annual Review of Earth and Planetary Science, 27, 19–53.
Sudo, T. & Kodama, H. (1957) An aluminium mixed-layer mineral of montmorillonite. Zeitschrift für Kristallographie, 109, 379–387.
Thirwall, M.F. (1988) Geochronology of Late-Caledonian magmatism in northern Britain. Journal of the Geological Society of London, 145, 951–961.
Warr, L.N. (1995) A reconnaissance study of very low-grade metamorphism in south Devon. Proceedings of the Ussher Society, 8, 405–410.
Warr, L.N., Primmer, T.J. & Robinson, D. (1991) Variscan very low-grade metamorphism in southwest England: a diastathermal and thrust-related origin. Journal of Metamorphic Geology, 9, 751–764.
Watts, N.L. (1977). comparative study of some Quaternary, Permo-Triassic and Siluro-Devonian Calcretes. PhD thesis. University of Reading, UK.
Wilson, M.J. (1971) Clay mineralogy of the Old Red Sandstone (Devonian) of Scotland. Journal of Sedimentary Petrology, 41, 995–1007.
Wilson, M.J. & Duthie, D.M.L. (1981) Some aspects of intrastratal alteration of biotite in the Old Red Sandstone. Scottish Journal of Geology, 17, 65–72.


Clay mineralogy of the Old Red Sandstone and Devonian sedimentary rocks of Wales, Scotland and England

  • S. Hillier (a1), M. J. Wilson (a1) and R. J. Merriman (a2)


Full text views

Total number of HTML views: 0
Total number of PDF views: 0 *
Loading metrics...

Abstract views

Total abstract views: 0 *
Loading metrics...

* Views captured on Cambridge Core between <date>. This data will be updated every 24 hours.

Usage data cannot currently be displayed