- © The Mineralogical Society
The regional distribution, mineralogy, petrology and chemistry of the detrital and authigenic clay minerals associated with the Permo-Triassic strata (excluding the Rotliegend: see Ziegler, 2006; this volume), of the onshore and offshore regions of the British Isles are reviewed within their stratigraphical framework. The origin of these clay minerals is discussed in relation to current hypotheses on the developments of the Mg-rich clay mineral assemblages associated with the evaporitic red-bed Germanic facies of Europe and North Africa.
Composite clay mineral successions are described for seven regions of the British Isles – the Western Approaches Trough; SW England; South Midlands; Central Midlands; the Cheshire Basin; NE Yorkshire; and the Central North Sea. The detrital clay mineral assemblages of the Early Permian strata are variable, consisting of mica, smectite, smectite-mica, kaolin and chlorite, whereas those of the Late Permian and the Trias are dominated by mica, usually in association with minor Fe-rich chlorite. The detrital mica consists of a mixture of penecontemporaneous ferric mica, probably of pedogenic origin, and recycled Pre-Permian mica. In the youngest Triassic strata (Rhaetian), the detrital clay assemblages may contain appreciable amounts of poorly defined collapsible minerals (irregular mixed-layer smectite-mica-vermiculite) and kaolin, giving them a Jurassic aspect. There are two types of authigenic clay mineral assemblages. Kaolin may occur as a late-stage diagenetic mineral where the original Permo-Triassic porewaters of the sediment have been replaced by meteoritic waters. A suite of early-stage diagenetic clay minerals, many of them Mg-rich, are linked to the evaporitic red-bed facies – these include sepiolite, palygorskite, smectite, irregular mixed-layer smectite-mica and smectite-chlorite, corrensite, chlorite and glauconite (sensu lato). The sandstones and mudstones of the onshore regions of the British Isles display little or no difference in their detrital and authigenic clay mineral assemblages. In contrast, the sandstones of the offshore regions (North Sea) show major differences with the presence of extensive chloritic cements containing Mg-rich and Al-rich chlorite, irregular mixed-layer serpentine-chlorite, and mica.
The Permo-Triassic strata of the British Isles form part of the Germanic facies of the European Permo-Trias and consist predominantly of unfossiliferous hypersaline red-bed facies laid down under arid conditions. Their clay mineralogy is of exceptional interest and has benefited from a long history of research starting in the 1950s. These Permo-Triassic rocks contain an association of clay mineral assemblages in a geological setting that has so far proved unique. The best documented assemblage consists of Mg-rich clay minerals ranging from smectite, through irregular and regular mixed-layer smectite-chlorite, to chlorite. The origin of this assemblage has been a matter of controversy. Most of the researchers, who have examined this clay assemblage within its host strata, consider it to be an integral development of the particular palaeo-environments in which the Germanic facies was deposited. Until now no modern example of the Germanic facies with its peculiar clay mineralogy has been recorded (Calvo et al., 1999). This has led to the suggestion that the Mg-rich clay mineral assemblage has been formed by low-grade metamorphic reactions between smectite and Mg-rich pore solutions. Similar Mg-rich clay assemblages are known from situations where basic igneous rocks have been altered in low-grade regional metamorphic or hydrothermal environments. However, recent research indicates that a metamorphic origin is unlikely, at least for the Germanic facies of England. Possibly even more remarkable than the Mg-rich clay assemblage, but certainly less well known, is the mica assemblage. This consists of little more than an Fe3+-rich mica which K/Ar dating has demonstrated to be of penecontemporaneous origin.
In England, the distinctive clay mineral assemblages of the Germanic facies are first found in beds of Tatarian (Late Permian) age, and they continue throughout the Triassic. These assemblages are replaced in the latest Triassic (Rhaetian) by clay mineral assemblages of Jurassic aspect (Jeans, 2006, pp 196, 198). The first appearance of the exceptional clay mineral association coincides approximately with the Earth’s greatest mass extinction of organisms – as judged by the fossil record. Approximately 60% of all known fossil genera living in the Late Carboniferous and Early Permian became extinct in the Late Permian (Saunders, 2005) and this occurs close to the Permian-Triassic boundary at approximately the same time as the emplacement of the Siberian Trap Volcanics (250–248 Ma), a huge eruption (106–107 km3) of continental flood basalts. It has been suggested that the development of this large igneous province could have resulted from the extensive melting of the crust and mantle by the impact of a major meteorite (Jones, 2005). The degassing of these subaerial flood basalts may have released large amounts of SO2, CO2 and NOx (Self et al., 2005) causing worldwide changes in the atmosphere, surface layers of the continents and the oceans that led to widespread biological extinction (Kiehl & Shields, 2005; Huey & Ward, 2005; Kerr, 2005; Wignall, 2005). Whether these changes, or perhaps enhanced heat flow through the crust as the result of mantle perturbations, played a role in the development of the exceptional association of clay mineral assemblages in the late Permian-Triassic of Europe is a fascinating but open question. So far there is no evidence of meteoritic or volcanic material from strata in Europe close in age to the Permo-Triassic boundary, although in the late Triassic succession in England there is good evidence of microtektites (Fig. 1⇓), and some slight indication that volcanic ash may have been an important component of the Triassic sediment. The evidence for this is discussed below.
This review deals first with the hypotheses put forward to explain the clay mineralogy of the Permo-Triassic Germanic facies of western Europe and north Africa and then describes the clay mineral stratigraphy of the Permo-Triassic strata of the British Isles. It discusses geologically important features of the clay mineral assemblages and concludes with some suggestions to advance the understanding of these exceptional clay mineral assemblages.
It was Géologic des Argiles (Millot, 1964) that brought to the general earth science audience the extent and nature of the extraordinary Mg-rich clay mineral assemblages which Jacques Lucas (Fig. 2⇓) had described from the Permo-Triassic strata of France, Spain and North Africa (Lucas, 1962). These assemblages had already been described from localities in England, France, Germany and Spain (Stephen & MacEwan, 1950, 1951; Honeybourne, 1951; Lippman, 1954, 1956, 1959; Keeling, 1956; Martin-Vivaldi & MacEwan, 1957, 1960). The association of these Mg-rich mineral assemblages with a red-bed facies containing concentrations of evaporitic carbonates, sulphates and halides was the geological framework in which various researchers were to study their origin. By the end of the 1970s two contrasting hypotheses had developed that reflected different areas of study and styles of approach.
In France, based on the Jura region in particular, Lucas (1962) developed the transformational hypothesis. This was slightly modified by Lucas & Ataman (1968) and by Krumm (1969) and Dunoyer de Segonzac (1969). It suggests that continentally derived clay detritus, predominantly of poorly crystalline mica and minor chlorite, was washed into restricted basins containing hypersaline, Mg-rich brines. Reaction took place between the poorly crystalline mica and the Mg-rich brines both within the water body and in the porewaters of the sediments. This resulted in the development by transformational reactions of two mineral series which Lucas had traced from the edges of these restricted basins into their hypersaline centres. The extent to which the detrital mica was altered depending upon the Mg2+ concentration, the volume of detrital mica, and competition for the magnesium in solution from carbonate and evaporite mineral precipitation.
The alternative hypothesis was developed in Germany. Echle (1961) set the scene by modelling the uptake of Mg that must have been involved in the formation of the Mg-rich clay minerals and carbonates as they developed in the hypersaline porewaters of the Triassic Steinmergel Keuper and Roten Wand. It was Lippmann (Fig. 2⇑) and coworkers (Lippman, 1954, 1956; Lippmann & Savascin, 1969; Lippmann & Schlenker, 1970; Lippman & Steiner, 1983; Lippmann & Zimmermann, 1983; Lippmann & Pankau, 1988; Lippmann & Berthold, 1992) who developed the neoformational hypothesis. They described petrographic evidence for the post-depositional neoformation of corrensite and chlorite. They recognized an inconsistent relationship between the development of these clay minerals, authigenic quartz and feldspar, and the distribution of possible parental materials (detrital kaolin, mica and smectite) necessary to provide the siliceous and aluminous components for the silicate-forming reactions. Lippmann & Savasçin (1969) suggested the following reactions:
Lippmann & Pankau (1988) made two important points. Firstly, they noted the absence of a consistent quantitative relationship between the authigenic silicates and the stoichiometric equations modelled for their derivation from the decomposition of mica, kaolin or smectite (pyrophyllite). This indicated that an additional source of silica was present, either from continental groundwaters finding their way into the hypersaline basin or from the dissolution of siliceous organic skeletons. Secondly, they observed the poor correlation between the development of the authigenic Mg-rich clay minerals and the evaporation status of the strata (judged by the type of evaporite minerals present) indicates that the magnesium concentration in the depositional waters or in the porewaters was not the controlling factor. This implies that the alkalinity played the controlling role in whether Mg-rich clay minerals were formed. Lippmann & Pankau (1988) suggest that the Permo-Triassic basins did not have frequent contact with an open-marine environment, and that their alkalinity was controlled by alkaline weathering solutions derived from the surrounding continental areas as well as by dilution from freshwater inputs.
Further support for the neoformational hypothesis came from the English Trias (Jeans, 1978) where a differentiation was made between the ‘background’ detrital clay mineral assemblage (mica alone or with minor chlorite) and the authigenic assemblages. No evidence for Lucas’ transformation reactions was found, although they could have occurred on a minor scale. In addition, a geological relationship was demonstrated between the typical authigenic clay assemblages of Central Europe (chlorite and corrensite) and those of England (sepiolite, palygorskite, smectite and irregular mixed-layer chlorite-smectite). The mechanism of neoformation suggested by Jeans (1978) was based upon a general, but not precise, correlation between the evaporite status of the strata and the pattern of authigenic clay minerals. The authigenic clays were presumed to result from the interaction between open-marine waters, spasmodically introduced into restricted basins, and basinal hypersaline water masses characterized by lateral variations in salinity. The Si and Al necessary for clay mineral authigenesis were considered by Jeans (1978) to have come from continental sources and post-Hercynian mineral springs. These elements were concentrated in the hypersaline environments that were hostile to the organisms normally involved in their removal. No evidence was found that the Si and Al came from the dissolution of the detrital clay minerals associated with the authigenic clay minerals. Subsequent research on the Permo-Triassic sequence in the Staithes No. 20 Borehole, NE Yorkshire (Jeans, 1995) suggested that the relationship between the type of authigenic clay and the evaporite status of the strata was weaker than that envisaged in Jeans (1978). Therefore, using the argument of Lippmann & Pankau (1988), the possibility of variation in alkalinity and rather than salinity within these hypersaline water masses is favoured as the controlling factor for the precipitation of the Mg-rich clay mineral assemblages.
The absence of recent or modern analogues of the Triassic Germanic facies with its rich assemblage of Mg-rich clay minerals has led to a third and quite different hypothesis concerning their origin. Hillier (1993) suggested that the mixed-layer smectite-chlorite minerals could be of metamorphic or hydrothermal origin, making reference to their occurrence in the very low-grade metamorphic Devonian strata in the Orcadian Basin of Scotland (Hillier, 1993; Hillier et al., 2006, this volume) and in basic and ultrabasic rocks and their detritus (see Jeans et al., 2005 for details) which have been altered by hydrothermal or low-grade metamorphism. Hillier envisaged that Mg-rich smectite (stevensite, saponite), which is a widespread authigenic clay mineral of recent saline and alkaline lakes, had reacted with Mg-rich solutions or Mg-rich minerals during very low-grade metamorphism to form various smectite-chlorite mixed-layer minerals. Bloodworth & Prior (1993) applied this hypothesis to the Triassic strata of the English Midlands. Those authors consider the Mg-rich clay assemblage to have originated from ‘precursor smectite’ and sepiolite formed in reactions between degraded detrital mica and the alkaline/saline groundwaters of the depositional environment. Subsequently the ‘precursor smectite’ has been modified to smectite-chlorite and corrensite by low-grade metamorphism with temperatures exceeding 100°C. Some additional support for this hypothesis comes from Weibel’s (1999) investigation of the Triassic Skagerrak Formation (Fig. 3⇓) in eleven boreholes in Denmark. Jeans et al.(2005) tested the metamorphic hypothesis by comparing the regional and stratigraphical patterns of variations in the Mg-rich clay mineral assemblage in the Trias of England with the predicted pattern of maximum palaeotempertures obtained by Spore Coloration Index measurements. They found no convincing match in either regional or stratigraphical patterns, and drew the conclusion that, on present available evidence, the metamorphic hypothesis was not favoured.
Another aspect of the exceptional nature of the Permo-Trias clay mineralogy is the presence of assemblages of pure mica. These are widespread (Spain, Morocco, Germany, Western Approaches, central part of North Sea, England) and they grade into the mica-dominated (with minor chlorite) typical detrital assemblage of Jeans (1978). Lippman & Berthold (1992) were first to investigate the nature of this assemblage in detail. They noted the enhanced Fe-content of the mica and its 1Md and 1M dioctahedral polytypism, suggesting a detrital origin from pre-existing sediments and low-grade metamorphic rocks. Subsequent research (Jeans et al., 1994) has shown its formation was coeval with sedimentation and that it is ferric-rich mica: a pedogenic origin in desert soils was suggested.
CLAY MINERAL ANALYSIS
Mica is used in this review in preference to the term illite (Mayall, 1979; Goodall, 1987; Bloodworth & Prior, 1993; Kemp, 1999) or clay mica (Jeans et al., 1994). It is a major component in all the Permo-Triassic clay assemblages from onshore Britain.
The clay mineral analyses discussed in this review come mainly from published work and to a much lesser extent from unpublished sources (Goodall, 1987; Bloodworth & Prior, 1993; Kemp, 1999; S. Kemp, pers. comm., see Appendix 2). The analyses are based largely on X-ray diffraction (XRD) analysis of oriented aggregates of the <2 μm clay fraction. There is little available information on the chemistry of individual clay minerals. This reflects the difficulty in separating pure components from the complex, naturally occurring mineral mixtures. The XRD methods used by various investigators have in common the examination of the clay fraction in the air-dried (untreated) state, then after glycerolation or glycolation, and then after heating to temperatures in the range of 375–550°C. Jeans (1978, 1995) based his analyses on the examination of samples in the untreated state, after glycerolation, and then when heated to 440°C (400°C after 1982) and 550°C. Mayall (1979) and Goodall (1987) based their analysis on the untreated, glycolated and 375°C heated samples, occasionally including a 550°C heated sample. Bloodworth & Prior (1993) restricted their analysis to untreated and glycolated samples and ‘where necessary’ heat-treated samples. Kemp (1999) analysed untreated, glycolated and 550°C-heated samples. In spite of these variations of method, the qualitative results have shown good general agreement. The following minerals have been identified: mica, smectite, irregular mixed-layer smectite-mica, Fe-, Mg- and Al-rich chlorite, irregular mixed-layer smectite-chlorite, corrensite, sepiolite, palygorskite and kaolin. In contrast to the Jurassic and Cretaceous clay assemblages, those of Permo-Triassic age do not contain major amounts of poorly defined collapsible minerals except at the very top of the Trias (Rhaetian). Small amounts occur widely in association with mica as is evident from the frequently observed narrowing and enhanced intensity of the 10 Å mica peak when the sample is heated to 400°C or 440°C. This open mica is often conspicuous in various beds from the Dunscombe, Weston and Bindon cycles of the Mercia Mudstone Group of SW England where the clay assemblages contain a 10 Å mica peak asymmetrical to low angles of 2𝛉 (Fig. 8d⇓). Mayall (1979) is the only author to quantify the proportion of poorly-defined collapsible minerals referring to it as illite-smectite.
Quantitative analysis of the Permo-Triassic clay mineral assemblages has proved to be difficult because of the considerable overlap that may occur with the most intense and diagnostic peaks of chlorite, corrensite, irregular mixed-layer chlorite-smectite and mica-smectite, smectite and sepiolite. Jeans (1978, Appendix 1) used an internal comparison of peak areas of the glycerolated and 550°C XRD patterns between 2 and 14°2𝛉. This method did not prove entirely satisfactory and in later work (Jeans, 1995) qualitative analysis was combined with a modified version of Griffin’s (1971) quantitative method. Mayall (1979) used the procedure of Bradshaw (1975). Bloodworth & Prior (1993) and Kemp (1999) applied the NEWMOD computer modelling and the reference intensity method of Moore & Reynolds (1989, 1997) to achieve quantitative analyses.
Two varieties of chlorite are recognized in the clay assemblages from the onshore areas. One is interpreted to be Fe-rich based on its peak intensity pattern for its basal XRD spacings of 001 (20), 002 (100), 003 (20) and 004 (50); this is referred to simply as chlorite. It occurs widely as a minor component in the detrital clay mineral assemblages of the Zechstein and Triassic strata, but the XRD pattern may be obscured by the presence of various authigenic clay minerals. The other is interpreted to be magnesian-rich based upon its peak intensity pattern for its basal XRD spacings of 001 (70), 002 (100), 003 (50) and 004 (80); this is referred to as Mg-chlorite. Both types of chlorite display considerable variation in the extent to which the relative intensities of their basal spacings (001) are affected by 550°C heating. In the central part of the North Sea three types of ‘chlorite’ have been identified in the cements from the sandstone reservoirs (Jeans, 1995, his fig. 18). Type 1 is a true chlorite, Type 2 is randomly interstratified serpentine-chlorite, Type 3 is probably an Al-rich chlorite. Further details are discussed in the section on the Central North Sea.
The Permo-Trias of the British Isles is dominated by poorly fossiliferous red-bed lithofacies containing deposits of evaporitic sulphates and halides. Local lithostratigraphical subdivision is mostly well established, hence the plethora of names (Fig. 3⇑). However, the general absence of fossils combined with rapid lateral variations in lithology mean that inter-regional correlations are not well established at many stratigraphical levels.
The distribution of Permian rocks is shown in Fig. 4⇓. In SW England, the West Midlands and the Cheshire Basin, the Permian is represented by continental red-bed strata consisting of rudaceous, arenaceous and argillaceous lithologies. To the east, in north Nottinghamshire and NE Yorkshire these lithologies interdigitate with limestones and sequences of sulphate and halide evaporite beds that together make up the Zechstein Group of Late Permian age. The evaporites were laid down in the giant salina basin that occupied the area of the present-day North Sea and extended eastwards through the Netherlands, northern Germany and into Poland (Taylor, 1998, his fig. 6.1).
The distribution of Triassic rocks is more widespread than for the Permian and is shown in Fig. 5⇓. The sedimentary succession can be considered as a major upward-fining red-bed cycle starting with the arenaceous Sherwood Sandstone Group passing up into the predominantly siltstones and mudstone lithologies of the Mercia Mudstone Group. Within the Mercia Mudstone Group (Keuper Marl of earlier authors) there are deposits of sulphate and halite which, unlike the Permian evaporites of northeast Yorkshire, display a regional arrangement suggesting the presence of strong lateral variations in salinity. The most prominent lithology of the Mercia Mudstone is a massive reddish-brown clay-rich silty-textured mudstone (Fig. 6⇓) which on disaggregation in the laboratory provides only a relative minor amount of <2 μm material (Dumbleton & West, 1966). Much of the >2 μm clay mineral material consists of resistant aggregates of fine clay particles that do not disperse easily – a feature which may be associated with argillized volcanic ash deposits. The possible significance of this is discussed later in this review.
The most widely accepted setting for the Triassic Germanic facies of western Europe is a broad, low-lying continental zone marginal to the open-marine Alpine facies of southern Europe (Ziegler, 1982, his figs 15⇓–17⇓⇓). This marginal zone extended to the north and west into the interior of the Pangea continental plate, particularly along rift zones. On occasions, in the Scythian, Anisian, Carnian and Rhaetian stages, the seaward parts of this marginal zone were inundated by major marine incursions which left their record in the sedimentary succession (Waterstones, Muschelkalk, Dunscombe Mudstone, Blue Anchor Fm., Westbury Fm.) when seawater flowed along the rifts into the hinterland. At times, in numerous rift basins, halite deposits developed in shallow salinas. The presence of sulphate deposits either in sabka facies of the seaward parts of the continental marginal zone or in interbasinal areas in the hinterland as groundwater deposits, is suggestive of strong lateral variations in salinity. Isotope studies (Taylor, 1983; Leslie et al., 1992, 1993) on the sulphate and carbonate minerals in the Mercia Mudstone Group of England indicate alternating periods of marine and continentally-derived waters from which these authigenic minerals precipitated
An alternative depositional setting has been proposed by Wright & Sandier (1994) for the Late Triassic Germanic facies of southwest England and Europe. This is based on a hydrogeological system without any contribution from marine waters, such as occurs in the arid playa-flood-plain-aeolian setting of the internally draining basins of the arid to semi-arid areas of Australia. There seems no reason why such hydrogeological groundwater systems were not locally dominant in parts of the huge areas over which the Germanic facies was deposited; however, the general palaeogeographic setting suggests a widespread marine influence. A similar setting was proposed by Talbot et al.(1994) who suggested that the Mercia Mudstone Group was deposited in a low-relief continental basin that had many features in common with the present arid and semi-arid interior of Australia. Their hypothesis was based upon comparisons between the Triassic strata on the Somerset coast at Watchet and St Audrie’s (Fig. 5⇑) with the late Quaternary sediments of Central and East Central Australia.
CLAY MINERAL STRATIGRAPHY
The Permo-Triassic strata of England exhibit clay mineral patterns suggesting both lithofacies, stratigraphical and regional control. The continental red-bed facies of the Lower Permian, including sediments of probably Permo-Carboniferous age (Keele and Enville Formations), are characterized by clay assemblages not dissimilar to those from the uppermost Carboniferous Barren Red Measures such as the Etruria Marl (Perrin, 1971). Kaolin, mica and smectite (or undifferentiated mixed-layer mineral) are the dominant components. Chlorite is locally present but only as a minor component. Our knowledge of the origin and geological controls of these assemblages is poor. In Late Permian times they are replaced by a distinctive two-component pattern of clay minerals which continues until the end of the Trias. A typical background detrital association dominated by mica, usually with minor chlorite (Fe-rich), occurs throughout all the sediments. This may be superimposed upon by an authigenic association containing a range of different clay species. This pattern is evident in the Upper Permian limestones and evaporites in the Staithes No. 20 Borehole, NE Yorkshire. It is also recognizable in the Upper Permian Edlington Formation of Teeside (Goodall, 1987) but in this instance there is evidence of detrital assemblages (with smectite and minor kaolin), intermediate between the Early Permian-Late Carboniferous and the Late Permian-Triassic assemblages which are associated with the typical authigenic association. The development of this two component pattern of clay assemblages is not linked closely to the occurrence of marine strata in NE England because the Early Permian-Late Carboniferous assemblage is still present in the marine Cadeby (Lower Magnesian Limestone) Formation of late Permian age. In SW England this changeover in clay patterns occurs at the top of the Dawlish Sandstone and presumed, but poorly dated, Permian continental red-bed sequence. The general loss of kaolin and smectite from the detrital clay assemblages in late Permian times may reflect synchronous climatic changes over the area affecting weathering and mineral development in soils; physical loss of the sources of these minerals by sediment burial or increasing transport distance, or the appearance of a new and dominant source of fine-grained detritus are also possible causes. In the youngest Triassic strata (Rhaetian) the clay mineral assemblages develop a Jurassic character with the incoming of detrital kaolin and poorly-defined collapsible minerals. Authigenic kaolin is present in some onshore and offshore Triassic sandstones where meteoric waters have replaced the original porewater of the beds. The regional variation in the Permo-Triassic clay mineral stratigraphy of the onshore and off-shore areas of the British Isles is described below using the stratigraphical scheme shown in Fig. 3⇑.
The Western Approaches Trough
Permo-Triassic red beds, anhydritic mudstone, halite and sandstone up to at least 1000 m thick, occur in the Western Approaches. Their biostratigraphy is poorly known, the position of the Triassic-Permian boundary is uncertain, and it is possible that they include strata of Late Carboniferous age. Their clay mineral stratigraphy has been studied in four off-shore wells (Fig. 7a,b; Fisher & Jeans, 1982; Jeans, 1995). In Well 72/10-1A (Melville Basin), there are two zones of authigenic clay minerals that correlate with the Dunscombe/Weston and Bindon cycles in the Triassic succession on the south Devon coast. Magnesian-rich chlorite occurs in beds containing halite which are of Carnian age. Corrensite occurs at the top of the section in strata of Rhaetian age which are equivalent to the Bindon Cycle (Blue Anchor Formation and the Penarth Group) on the south Devon coast (see below). The Permo-Triassic strata of these four off-shore wells contain the typical Triassic detrital clay mineral assemblage dominated by mica, usually associated with minor chlorite (Fig. 8⇓). Minor amounts of kaolin and smectite may occur in the clay assemblages of Wells 85/28-1 (St Mary’s Basin), 86/18-1 (St Mary’s Basin) and 87/12-1A (Plymouth Bay Basin) but their origin is unclear. The kaolin is possibly of authigenic origin as it tends to be preferentially associated with the more sandy parts of the succession. However, these smectite- and kaolin-bearing strata also have affinities with the Permian and Permo-?Carboniferous clay assemblages of SW England.
The Permo-Triassic strata of SW England (Fig. 3⇑) are particularly unfossiliferous. The lower and coarser-grained part of the succession is thick (3000 m) and is considered to be of Permian age, although it may contain beds of Late Carboniferous age at the base.
Sparse clay mineral data is available from the Permian strata. Cosgrove & Salter (1966) and Perrin (1971) record clay assemblages containing kaolin, mica and smectite (or smectite-mica) in varying proportions. Chlorite may be present as a minor component. Kaolin may be a dominant constituent, and was considered by Cosgrove & Salter (1966) to have been derived from the Exeter volcanics or surface manifestations of the Cornubian Granite. A marked change in the clay mineral assemblages occurs between the top of these kaolin-bearing Permian strata and the Triassic Aylesbeare Mudstone Group which is characterized by abundant mica and minor chlorite. Kaolin does not appear again in the Triassic section until the upper part of the Penarth Group: the only exception is the Budleigh Salterton Pebble Beds. Clay mineral data for the Aylesbeare Mudstone Group and the overlying Sherwood Sandstone Group, exposed in the cliffs between Exmouth and Sidmouth, has been published by Perrin (1971), Henson (1973) and Jeans (1978, 1994, 1995). There is general agreement that mica is the dominant component and that in the Aylesbeare Mudstone Group and the Otter Sandstone Formation it is accompanied by minor chlorite, whereas in the mudstone lenticles within the Budleigh Salterton Pebble Beds it is associated with minor kaolin. The presence of kaolin was related by Burley (1984, p. 430) to the late-stage diagenetic alteration of feldspar under meteoric conditions in these predominantly arenaceous/rudaceous beds.
The Mercia Mudstone Group and the overlying Penarth Group are excellently exposed in the cliffs between Sidmouth and to the east of Seaton in south Devon where their clay mineralogy and lithofacies have been studied in detail (Jeans, 1978; Mayall, 1979). This is summarized in Fig. 9⇓. Most of the Mercia Mudstone is characterized by massive, poorly bedded reddish brown mudstone (Fig. 6⇑), but in the middle there is a thin sandy unit (Sandstone Group of the Dunscombe/Weston Cycles (Jeans, 1978) as modified in Appendix 1; in part equivalent to the Lincombe Member of the Dunscombe Mudstone Formation), interpreted as marine or quasi marine facies (Jeans, 1978) or freshwater (Porter & Gallois, 2005). It is associated with and separated from the reddish brown mudstone by finely laminated mudstones, marls and limestones exhibiting pale grey, purple and black colours. This megafacies cycle is similar to that associated with the Blue Anchor Formation and the Penarth Group. The two cycles have been referred to by Jeans (1978, 1995) and by Fisher & Jeans (1982) as the Dunscombe (also Weston: see Appendix 1), and Bindon Cycles. The main difference between these two cycles is that there is no sandstone in the Bindon Cycle where it is replaced by marine shales (Westbury Fm.). Evaporites are poorly developed in the south Devon Mercia Mudstone Group. They are restricted to the reddish brown mudstone lithology, and are represented by occasional horizons of small gypsum nodules or, in one instance, a 6 m thick nodular gypsum unit (Red Rock Gypsum Member, Fig. 9⇓). More soluble salts may have been present as there are some thin horizons of collapsed strata in the Dunscombe Mudstone Formation (Jeans, 1978, his figs 34–35, 37–39; see Appendix 1) which could represent their penecontemporaneous or recent dissolution. Inland, thick halite-rich beds (30–130 m) in the Dunscombe Mudstone Formation occur in the subsurface of the central Somerset Basin in the Puriton and Burton Row boreholes and in the subsurface of the Wessex Basin in boreholes at Marshwood, Nettlecombe, Mappowder and Winterborne Kingston (Gallois, 2003).
Throughout the Mercia Mudstone Group, up to and including the Blue Anchor Formation, the clay assemblages contain the typical Triassic detrital association dominated by mica and minor chlorite (Fig. 8c⇑). Authigenic clay minerals (smectite-mica, smectite-chlorite, sepiolite, palygorskite; Figs 10⇓, 11⇓) are restricted to the middle and upper part where they occur in the reddish brown mudstones and the finely laminated mudstones and thin carbonates (carbonate groups of Jeans, 1978) adjacent to, but not within, the sandy unit of the Dunscombe Mudstone Formation. The sandy unit contains a normal detrital assemblage, sometimes with glauconite (sensu lato). In the reddish brown mudstones of the Seaton Mudstone Member, palygorskite occurs both within the matrix and as vein fillings, with calcite and celestite, of subhorizontal dilation fractures. In the overlying Haven Cliff Mudstone Member the clay assemblages then revert to the typical Triassic detrital association and this extends up to the top of the Branscombe Mudstone Formation. In the Blue Anchor Formation the detrital clay association is little changed although the mica is more open containing a greater proportion of collapsible interlayers (Fig. 8d⇑).
Kaolin first reappears in the Lilstock Formation (Penarth Group) of south Devon and north Somerset (Cosgrove & Salter, 1966, Mayall, 1979). On the south Devon coast the strata of the Blue Anchor Formation and the Lilstock Formation (Lower and Upper Carbonate groups of the Bindon Cycle (Jeans, 1978)), although similar in general lithofacies to the carbonate groups of the Dunscombe/Weston Cycle, are not associated generally with the occurrence of authigenic clay minerals: however, smectite and smectite-mica occur in the Westbury Formation and as traces in the Lilstock Formation (Mayall, 1979). At St Audries Bay in north Somerset (Fig. 5⇑) Mayall (1979) recorded corrensite in the Blue Anchor Formation.
Gloucestershire, Oxfordshire, Warwickshire and Worcestershire (South Midlands)
Clay mineral data (Keeling, 1956; Freeman, 1964; Dumbleton & West, 1966; Jeans, 1978) is restricted to the Triassic Penarth Group, Mercia Mudstone Group and the upper part of the Sherwood Sandstone Group. There is no information for the Permian strata. The stratigraphical pattern of the Triassic clay mineral assemblages is generally similar to the south Devon section, although there are some significant differences in the type and distribution of authigenic clay minerals. The most complete clay mineral sequences have been recorded from the Stowell Park and Upton boreholes (Figs 12⇓, 13⇓). The Mercia Mudstone Group in both these boreholes, as well as in surface exposures at Warndon (Jeans, 1978, his figs 53, 54) and at Bentley Heath, Bickenhill, Henley-in-Arden, Solihull and the Arden Forest area in Warwickshire (Jeans, 1978, his figs 48–52) is characterized by an abundance of authigenic clay minerals including sepiolite, smectite-mica and corrensite, associated with inter-bedded sandstones and variegated marls of the Arden Sandstone Formation. It is the presumed correlative in part of a similar zone rich in authigenic clay minerals associated with the Dunscombe Mudstone Formation on the south Devon coast: both formations have yielded fossils indicative of a Carnian age. Beneath this zone the patterns of clay assemblages differ. In the Stowell Park Borehole there is a corrensite-bearing section (899.2–1066.8 m), whereas farther east in the Upton Borehole, there is a smectite-mica-bearing section (251.5–281.9 m) in contrast to the typical Triassic detrital assemblage of the south Devon coast. The Blue Anchor and the Westbury formations at Westbury-on-Severn (Fig. 5⇑), differ from the same beds in north Somerset and south Devon by having typical Triassic detrital clay assemblages without any authigenic clay minerals (Jeans, 1978, his fig. 59).
Leicestershire and Nottinghamshire (Central Midlands)
Permian rocks appear at outcrop just north of Nottingham, and also form an extensive subcrop in the east of this region, where they are continuous with the more fully-developed Permian rocks in the North Sea. There are no clay mineral analyses from the Permian beds of Nottingham known to the author.
The clay mineral stratigraphy of the Triassic succession in Leicestershire and South Nottinghamshire is well known (Jeans, 1978; Bloodworth & Prior, 1993; Kemp, 1999) from the Ashfordby, Clipston (Blackberry Hill No. 7), Cropwell Bishop, Cropwell Bridge, Fulbeck No. 1, Keyworth (A, C), Owthorpe No. 11 and Radcliffe No. 2 boreholes (Fig. 5⇑) and from surface exposures (Jeans, 1978). Figure 14⇓ shows the composite clay mineral stratigraphy for Leicestershire and south Nottinghamshire. The stratigraphical scheme applied by Bloodworth & Prior (1993) and Kemp (1999) is used in the description of the clay mineral stratigraphy below.
The clay mineralogy of the Sherwood Sandstone Group, known from a single analysis (Kemp, 1999), consists of 85% mica and 15% chlorite. In the overlying Mercia Mudstone and Penarth groups there are two zones of strata that are characterized by the detrital clay mineral assemblage dominated by mica with minor chlorite. The lower zone includes the Sneinton Formation, the Radcliffe Formation and the basal part of the Gunthorpe Formation: it may be represented at Ibstock in a mudstone succession with many thin sandstones which contain the typical Triassic detrital clay mineral assemblage (Jeans, 1978, his fig. 60). The upper zone includes the upper two-thirds of the Cropwell Bishop Formation, the Blue Anchor Formation and the Penarth Group; major deposits of anhydrite and gypsum are restricted to this interval, occurring in the Cropwell Bishop Formation. There is clay mineral data from surface exposures of this zone at Leicester, Cropwell Bishop, Stanton-in-the-Vale and Bunny (Jeans, 1978). Corrensite occurs occasionally in the upper zone; minor amounts have been reported from the Ashfordby Borehole (Kemp, 1999) and from Stanton-in-the-Vale and Bunny (Jeans, 1978, his figs 65, 69). In parts of Leicestershire the mudstones of Mercia Mudstone Group, interbedded with locally derived sandstones rest unconformably on the Precambrian granites (Bosworth, 1912). At Croft, the mudstones banked up against the Precambrian granites contain minor amounts of corrensite within a typical Triassic detrital clay mineral assemblage (Jeans, 1978, his fig. 61).
Between the two zones of detrital clay mineral assemblages the Mercia Mudstone succession consists of the main part of the Gunthorpe Formation, the Edwalton Formation and the basal part of the Cropwell Bishop Formation. This part of the succession is characterized by clay assemblages rich in authigenic clay minerals – smectite, smectite-mica, smectite-chlorite, corrensite, chlorite and sepiolite – in addition to the normal detrital clay minerals (Fig. 15⇓). It can be divided into two. In the lower section, consisting of the Gunthorpe Formation and the basal part of the Edwalton Formation, corrensite and smectite-chlorite are the dominant authigenic clay minerals. The upper section is dominated by authigenic smectite, smectite-chlorite and sepiolite (absent in Ashfordby Borehole (Kemp, 1999)) and this is similar in its composition to the clay mineral assemblages broadly associated with the Arden Sandstone Formation and adjacent strata to the south, and to the Dunscombe Mudstone Formation and adjacent strata on the south Devon coast. Lithologically the beds are quite different, although sandstones (Hollywell Sandstone and Cotgrave Sandstone members) do occur in the Leicestershire and Nottinghamshire sections. Surface exposures of this part of the succession, rich in authigenic clay minerals, are known from Cotgrave and Edwalton (Jeans, 1978, his figs 67, 68).
Permian strata consisting of sandstone, marl and mudstone, up to ~700 m thick, are present in the Cheshire Basin (Plant et al., 1999, their fig. 11). Knowledge of their clay mineralogy is based on a small number of analyses from the Kinnerton Sandstone Formation (Fig. 3⇑) which straddles the Permo-Triassic boundary (Kemp in Plant et al., 1999, p. 91). The clay assemblages consist of mica, smectite and chlorite in varying proportions (Appendix 2).
The clay mineralogy of the overlying Triassic succession is known from Jeans (1978) and Kemp (in Plant et al., 1999). The lower sandy and pebbly part of the section, made up of the Chester Pebble Beds Formation, the Wilmslow Sandstone Formation and the Helsby Sandstone Formation (including the Grinshall Sandstone), contain clay mineral assemblages dominated usually by mica with lesser smectite and chlorite; one sample from the Wilmslow Sandstone Formation was dominated by smectite (possibly montmorillonite) with minor mica (Kemp in Plant et al., 1999, pp. 91–92). The clay mineralogy of the Mercia Mudstone Group and the overlying Penarth Group has been examined in the Wilkesley Borehole by Jeans (1978). More recently Kemp (in Plant et al., 1999) provided scattered analyses from a range of boreholes penetrating the Mercia Mudstone Group (Appendix 2) – and these essentially confirm the earlier results from the Wilkesly Borehole.
The clay mineral stratigraphy of the Wilkesley Borehole (Fig. 16⇓) shows that much of the section contains the authigenic clay mineral suite and this consists only of corrensite or Mg-chlorite, or mixtures of these two minerals (Figs 17a⇓, 18a⇓). The stratigraphical distribution of corrensite and Mg-chlorite displays no obvious lithological or depth control. The detrital clay mineral assemblage, dominated by mica with minor chlorite and lacking authigenic clays, is restricted to the Tarporley Siltstone Formation and the upper parts of the Bollins Mudstone, and the Wych Mudstone members, and short sections in the Wilkesley Halite Member and Branscombe Mudstone Formation.
NE Yorkshire and Southern North Sea Basin
The Permian strata of the Southern North Sea Basin and adjacent areas consist of the Zechstein, a succession of cyclic evaporites and limestone, that overly the predominantly sandy Rotliegend (Fig. 3⇑). When these strata are traced into NE Yorkshire and north Nottinghamshire, the lower sandstones thin and the overlying limestones and evaporites pass into continental mudstones. Knowledge of the clay mineralogy of these beds is limited. Offshore, there are considerable data from the Rotliegend (Ziegler, 2006, this volume) but nothing from the Zechstein. Onshore, there is a scattering of clay mineral analyses (in part reviewed by Perrin, 1971) including a summary of the clay mineral stratigraphy in the Upper Permian Zechstein succession in the Staithes No. 20 Borehole (Fig. 4⇑; Jeans, 1995).
The Basal Permian or Yellow Sands which may underlie the Zechstein succession in Yorkshire and further to the south contain clay mineral assemblages dominated by kaolin with minor mica (Perrin, 1971). Clay assemblages dominated by mica with minor kaolin and occasional vermiculite occur in the Cadeby (Magnesian Limestone) Formation (Perrin, 1971). Kaldi (1986) has reported euhedral kaolin crystals from stylolitic clay seams in the Cadeby Formation (Sprotbrough Member). Smith (1968) has recorded illite (possibly glauconite sensu lato) with minor chlorite as the main components in two thin green shaley mudstone seams in the Hampole Beds (Cadeby Formation).
The clay mineral stratigraphy of the Zechstein succession (top EZ2–EZ5) in the Staithes No. 20 Borehole is illustrated in Fig. 19⇓. Five clay mineral assemblages are recognized: (1) Mica (Brotherton Fm.) (Fig. 8b⇑); (2) Mica and chlorite (upper Fordon Fm.; Sleights Siltstone Fm.; Littlebeck Fm.; Roxby Fm.) (Fig. 18d⇑); (3) Mica, chlorite and corrensite (Billingham Fm.; part Fordon Fm.; upper Sneaton Fm.) (Fig. 17b⇑); (4) Mica, chlorite, corrensite, and Mg-chlorite (Boulby Fm., in various beds); (5) Mica, chlorite and Mg-chlorite (Boulby Fm.; Upgang Fm.; Sherburn Fm.; lower Sneaton Fm.) (Fig. 18b,c⇑). Minor detrital chlorite is assumed to be present in assemblages 4 and 5, but is obscured by the dominant Mg-chlorite of authigenic origin. The same two clay mineral associations present in the Triassic strata in England can be recognized. The detrital association consists of mica or mica with minor chlorite, and the authigenic association consists of Mg-chlorite and corrensite. Comparison between the general lithology of the Permian succession in the Staithes No. 20 Borehole and the clay mineral assemblages displays no consistent relationship, other than that the authigenic clay minerals are associated with the beds containing evaporitic sulphates and halides.
Triassic strata are widespread in the Southern North Sea Basin (Fisher & Mudge, 1998). They range in age from Rhaetian to Scythian (= Olenekian–Induan) (Fig. 3⇑). The lithological succession, containing the representative of the Muschelkalk and a number of evaporite formations, is a link between the Triassic strata of England and those of the Netherlands/Germany (Geiger & Hopping, 1968, Fisher & Mudge, 1998) – two regions where Triassic clay mineral stratigraphy is well established. Unfortunately there is no published or unpublished clay mineral data known to the author available from the southern North Sea Basin.
The clay mineral stratigraphy of the Triassic strata of NE Yorkshire is known from the Staithes No. 20 Borehole (summarized in Jeans, 1995) based on a ~1.5 m sample interval (Fig. 20⇓). Three clay mineral assemblages are present: (1) Mica, chlorite (Sherwood Sandstone Group; 601–634m, 532–560 m, 386–410 m in Mercia Mudstone Group and Penarth Group). (2) Mica, chlorite, corrensite, and possibly Mg-chlorite (601–634m, 580–594 m, 524–532 m, 474–492 m, 410–464 m Mercia Mudstone Group) and two minor occurrences between 570 and 580 m. (3) Mica, chlorite, poorly-defined collapsible minerals (mixed-layer mica-smectite-vermiculite) with kaolin (Penarth Group 378–386 m).
The detrital clay mineral association (assemblage 1) occurs throughout the succession whereas authigenic clay minerals are restricted to the Mercia Mudstone Group. Jeans (1995, his fig. 12) identified 12 clay mineral zones (nos 8–19) in the Triassic section of the Staithes No. 20 Borehole. These are shown in Fig. 20⇑. Raymond (1955) recorded swelling chlorite (irregular mixed-layer smectite-chlorite) in the Blue Anchor and Lilstock formations in north Yorkshire.
Central North Sea
The Triassic strata of the central area of the North Sea (Fig. 3⇑) differ lithologically from those of the Southern North Sea Basin and onshore UK (Fisher & Mudge, 1998, their fig. 7.14). Evaporite deposits are absent, and sandstones, infrequent in the lower part of the section (Smith Bank Formation), become increasingly abundant and more extensive in the middle and upper part (Skagerrak Formation). A tentative clay mineral stratigraphy has been assembled from studies of different wells (Fig. 21⇓; see also Jeans, 1995, his fig. 14). The clay assemblages of the fine-grained Smith Bank Formation are dominated by mica in association with varying amounts of chlorite, kaolin and smectite. Near the top of this formation kaolin disappears and a pattern of clay mineral assemblages appears similar to the onshore Triassic succession in NE Yorkshire which continues to the top of the Trias. There is a detrital assemblage of major mica with minor chlorite that occurs throughout. Authigenic corrensite is also present, particularly in the lower and middle part of the Skagerrak Formation.
The sandstones of the deeply buried Triassic successions in the Central North Sea area contain rather different clay mineral assemblages than the adjacent mudstones (Jeans, 1995, his fig. 15). Authigenic clay minerals, in particular chlorite and corrensite, tend to be more abundant in the sandstones. This difference in clay mineral assemblages between mudstones and sandstones is reported from the relatively shallow buried Triassic rocks in the Inner Moray Firth penetrated by Well 13/28-3 (Fig. 21⇑; Jeans & Atherton, 1989, their fig. 4). The assemblages consist of mica and chlorite, with chlorite a major component in the sandstones and a minor component in the mudstones. This is not a surprising relationship considering the much greater proportion of detrital clay minerals, dominated by mica, present in the mudrocks.
In the least altered Triassic sandstones of the Central North Sea, e.g. in well 29/6a-3, the earliest clay cement is represented by a thin pore-wall lining or skin (3–4 μm thick), typically stained by hematite and dominated by mica with minor chlorite and sometimes with corrensite in addition (Figs 22b⇓, 23c⇓; Jeans, 1995, his fig. 17). The clay crystals within this skin are arranged tangentially to the surfaces they coat. This assemblage is comparable in mineralogy and fabric to similar skins in onshore Triassic sandstones, where they would be considered to consist of detrital clay minerals with the addition of authigenic corrensite. The arrangement of the detrital clay particles as skins is considered to result from their translocation from the surface of the sediment by porefluids flowing downwards through the partially saturated sandstone. The presence of euhedral and filamentous clay crystals in the fine clay fraction (<0.5 μm) of the clay skins (Fig. 23d⇓) from Central North Sea Triassic sandstones suggests that some recrystallization of detrital clay minerals has occurred.
In many Triassic reservoirs of the Central North Sea there have been extensive precipitation of chloritic and mica cements after the formation of the initial clay skin. These chloritic cements have been most studied in Upper Triassic reservoir sandstones of the Marnock Field and adjacent areas (Humphreys et al., 1989; Purvis, 1990; Jeans, 1995). In the Marnock Sandstone Formation (Fig. 24⇓) the chloritic cements are associated with the widespread dissolution of detrital grains, particularly feldspar and mica. The development of these cements predates the formation of quartz overgrowths and dolomite cements, and its relative timing within the diagenetic scheme appears to be independent of the burial and thermal history. Chlorite in these cements display considerable variations in the XRD pattern, chemical composition, heat stability and crystal morphology. Three chlorite types are recognized from their XRD patterns (Jeans, 1995). Type 1 (Fig. 25a,b⇓) is a true chlorite with a composition of (Fe2.8Mg5.2Al4) (Si6.4Al1.6)O20(OH)16 (analytical transmission electron microsocpy (ATEM) analysis by W.J. McHardy). Type 2 (Fig. 25c,d⇓) is a chlorite which contains ~16–18% of randomly interstratified serpentine layers; its chemical composition is Fe3.8Mg4.4Al3.8)(Si6.2Al1.8)O20(OH)16 according to an ATEM analysis by W.J. McHardy. Both Types 1 and 2 exhibit more heat stable (variety A) and less heat stable (variety B) forms (Figs 25⇓, 26a⇓). Where it has been possible to petrographically separate these varieties, Type 1 is the first to form, followed by Type 2. These relationships are illustrated in Figs 27⇓ and 28⇓. Figure 29⇓ shows the different crystal form of Type 1 and Type 2 chlorite and their association with filamentous mica. Factors controlling the development of the two chlorite types are not clear. In individual wells (22/19a-1, Jeans, 1995, his fig. 16) their distribution pattern may suggest stratigraphical control. However, until much more information is available, all that can be assumed is that their precipitation reflects variation in the pore-fluid chemistry rather than temperature and pressure variations. Type 3 chlorite (Fig. 26b⇓) is probably Al-rich and is of rare occurrence.
FEATURES OF THE PERMO-TRIASSIC CLAY MINERALS IN THE BRITISH ISLES
The characterization of the various clay minerals found in the Permo-Triassic strata of the British Isles has mainly been to group level only. Little is known of their detailed chemistry, morphology or petrographic relationships. What data are available are restricted mainly to Triassic strata. The commercial interest in the hydrocarbon reservoirs of the Marnock Sandstone in the Central North Sea has provided detailed data on the nature of the chloritic cements. Other data, pertaining to the onshore occurrences of Permo-Triassic rocks, are considered below. The general distribution of the different clay minerals in the Triassic beds of England is summarized in Fig. 30⇓.
Detrital clay minerals
Two detrital clay mineral associations occur (Fig. 8⇑). One consists of mica alone; although this is of very limited occurrence it is widespread in the Permo-Triassic sections in the Western Approaches and adjacent offshore basins (Fig. 7⇑). The other detrital clay assemblage is of widespread occurrence, dominated by mica, but in association with minor chlorite. K/Ar dating of these mica-rich detrital clay mineral assemblages from the south Devon section suggests that they contain a mixture of coarse (1–2 μm) pre-Triassic mica and fine (<0.2 μm) penecontemporaneous mica (Jeans et al., 1994, their table 1; Jeans et al., 2001). The fine-grained penecontemporaneous mica is a ferric 1 Md or 1M dioctahedral mineral that typically occurs as irregular shaped crystals. Within the <0.2 μm fraction, these crystals have an average thickness of 90 to 180 Å. A typical example of the mica has the structural formula of (K0.61Na0.03Ca0.08) (Fe0.193+Ti0.02Mg0.26Mn0.002Al1.52)[Si3.48Al0.52] O10(OH)2 (Sample De352, table 2, Jeans et al. 1994). This type of mica in the south Devon section exhibits systematic changes in cation population with increasing depositional age of the strata. In the octahedral site, Fe3+ decrease from 0.39 to 0.19 and Mg2+ from 0.54 to 0.25 per formula unit, whereas Al increases from 1.08 to 1.55: in the tetrahedral site Al increases from 0.36 to 0.54 at the expense of Si (3.64 to 3.46). Sedimentological and petrographical evidence favours its deposition as fine-grained silicate detritus which had been transported, possibly as wind blown dust, from distant but stable upland areas where the mica was being neoformed in arid or semi-arid soil profiles. An alternative hypothesis is that volcanic ash may have been the parent material for the newly-formed ferric mica. This has little supporting evidence other than the peculiar manner, reminiscent of argillized ash deposits, in which the clay-rich reddish-brown Mercia Mudstone lithology disaggregates in water to silt and sand particles (Dumbleton & West, 1966). Where this mica has been buried deeply (>1500–2000 m), such as in the Western Approaches and adjacent basins, it is typically recrystallized and exhibits euhedral crystals without obvious change in its major element chemistry (Jeans et al., 1994). The ages of recrystallized micas are usually anomalous and may reflect the complete or partial resetting of the K/Ar system.
There is no detailed information on the nature or origin of the detrital chlorite associated with the mica; the XRD pattern suggests it is Fe-rich (Fig. 8c⇑). Differentiation between this detrital chlorite and the Mg-rich authigenic chlorite, when they are thought to occur together in the same sample, has not been possible by the analytical methods used by various authors. Jeans (1978) assumed that small amounts of detrital chlorite are present in all the clay mineral assemblages. Observed regional and stratigraphical variations in the relative abundance of detrital chlorite within the detrital assemblage could be interpreted as resulting from the varying contribution from a separate source or from differential settling of its slightly coarser grain size from wind-blown dust.
Authigenic clay minerals
In England, sepiolite is restricted to the upper part of the Triassic succession of the Midlands and regions to the south (Fig. 30⇑). Here it occurs in the envelope of strata, largely of Carnian age, surrounding the Lincombe Sandstone and Arden Sandstone. Sepiolite has not been found in sandstones. In mudstones, the sepiolite fibres occur as mats and irregular aggregates between grains or as very short fibres coating coarser clay particles (Fig. 11b⇑). In carbonate-cemented beds, sepiolite fibres appear to be intergrown with dolomite (fig. 28⇑ in Jeans 1978) or filling voids (Fig. 11a⇑). It is not clear whether the growth of sepiolite pre-dates or is synchronous with the carbonate cement. The distribution of sepiolite shows no preferential association with detailed lithology or carbonate types. It occurs predominantly in mudstones with either dolomite or a mixture of dolomite and calcite, but also occurs without any carbonate minerals.
Records of palygorskite in the Trias of Britain are rare. The most extensive occurrence is in the south Devon succession (Fig. 9⇑) where it is found in parts of the Branscombe Mudstone Formation of probable Norian age. The palygorskite may occur within the mudstone as bundles of fibrous crystals filling voids or as mats of closely associated fibres (Fig. 11d⇑). These mudstones may contain horizons of nodular gypsum and dolomite. Stratigraphically rather more extensive, but still within the Branscombe Mudstone Formation in the south Devon section, is a series of sub-horizonal fractures filled by a leathery textured mixture of palygorskite, calcite and celestite. Jeans (1994, p. 418) suggested that the precipitation of these vein minerals took place as the result of reduced critical supersaturation requirements for nucleation as porefluids moved from the fine-grained host mudstone into the space created by the developing fracture. This veinfill palygorskite and its associated minerals were precipitated possibly during the rapid uplift and erosion of the overlying Jurassic sediments in the late Jurassic or Early Cretaceous. A record of palygorskite in the Merica Mudstone Group of the Upton Borehole, Oxfordshire (Jeans, 1978, fig. 83), is a drafting error. There are records of palygorskite in the Trias of Leicestershire, Derbyshire and Staffordshire (Evans & King, 1962; King & Ford, 1969). King & Ford (1969) record it as a white clay infilling the pore space in the basal Triassic conglomerate at Enderbey Warren Quarry and also penetrating joints in the underlying diorite: it may occur also in the basal conglomerate at Newhurst Quarry (Shepshed). Taylor (1983) mentions that palygorskite occurs in the Fould Member of the Trent Formation in Staffordshire and South Nottinghamshire but provides no XRD or other data. It was not recorded by Jeans (1978) in the Mercia Mudstone Group of Nottingham, although small amounts are unlikely to be identified by XRD analysis of the <2 μm fraction.
Smectite, smectite-mica and smectite-chlorite
These minerals are recorded from the Edlington Formation of Teeside by Goodall (1987). They are associated with the hypersaline facies and the immediately adjacent red mudstones of the detrital alluvial facies. This author considered the smectite and smectite-mica to be of detrital origin and the smectite-chlorite to have developed by transformation from the smectitic minerals as the result of reaction with Mg-rich porefluids.
In the Triassic succession, smectite, smectite-mica and smectite-chlorite are restricted to strata of Late Triassic age in Central and South Midlands and further to the south (Fig. 30⇑). They are not known from the Cheshire Basin or NE Yorkshire. The distribution pattern is similar to that of sepiolite in being restricted to the sediment envelope surrounding the sandy unit (Sandstone Group of the Dunscombe/Weston Cycle) within the Dunscombe Mudstone Fm. and the Arden Sandstone. However these minerals are stratigraphically more widespread than sepiolite in their occurrence (Fig. 30⇑). Smectite, smectite-mica and smectite-chlorite do not occur within the Sandstone Group of the Dunscombe/Weston Cycle on the south Devon coast. The distribution pattern of individual clay minerals in this group shows a strong regional variation. In south Devon, smectite and smectite-mica, practically to the exclusion of smectite-chlorite, are characteristic. As this envelope of occurrence is traced northwards, smectite-chlorite replaces the smectite and smectite-mica (Fig. 30⇑).
Scanning electron micrographs of the smectite-mica show that it occurs as thin irregular crystals (Fig. 11c⇑). The absence of these three minerals from most of the Mercia Mudstone and its association with sepiolite in the envelope of sediment in and adjacent to the Dunscombe Mudstone and the Arden Sandstone formations leaves little doubt that it developed in the sediment after deposition.
Corrensite and Mg-chlorite.
Corrensite is well represented in the evaporitic sediments of the Late Permian both in Teeside and North East Yorkshire. Goodall (1987) figures an example of authigenic Corrensite as a porefilling in chert from the Edlington Formation.
Within the Triassic succession, corrensite is restricted to the Mercia Mudstone Group of the Midlands, the Cheshire Basin and NE Yorkshire. It exhibits no clear relationship to lithology although all the strata are evaporite-bearing in the general sense. The occurrence of corrensite extends south of the Midlands but not as far as south Devon. This mineral is not associated with the envelope of authigenic minerals related to the Dunscombe Mudstone and the Arden Sandstone formations. Corrensite-bearing clay assemblages may pass gradually into assemblages with only Mg-chlorite. Figure 30⇑ shows that smectite and smectite-chlorite are absent from detrital clay assemblages which are in juxtaposition with those bearing corrensite. Such relationships do not support a transformation origin for corrensite from smectite. The only apparent example of this transition is seen in the lateral passage of the authigenic clay assemblages associated with the Arden Sandstone Formation as they pass into assemblages containing corrensite as the Mercian Mudstone is traced northwards to the Cheshire Basin and NE Yorkshire.
Kaolin is a widespread detrital clay mineral in the Permian red beds of SW England and parts of Yorkshire (see earlier section on clay mineral stratigraphy). In the Permo-Triassic red beds in offshore areas to the west and southwest of England (Fig. 7b⇑) kaolin is present but its origin is uncertain. In the main part of the Triassic strata of England, kaolin is restricted to strata of the Sherwood Sandstone Group where it developed during late-stage diagenesis which occurred under the influence of meteoric water (Burley, 1984). Kaolin is present in the Budleigh Salterton Pebble Beds of south Devon (Perrin, 1971; Jeans et al., 1994, his fig. 5; Burley, 1984, pp. 430–431). Detrital kaolin reappears at the very top of the Triassic section in the Penarth Group (Rhaetian). In the central part of the North Sea, kaolin is generally absent from the Triassic beds; however, where they have been uplifted and their porewaters replaced by meteoric waters, kaolin cements may be present. For example, the Triassic sandstone sequence in Well 29/6a-3 (Fig. 21⇑) has been extensively uplifted and eroded, and is overlain by Jurassic sandstones (Jeans, 1995, his fig. 17). The uppermost ~9 m of the Triassic sandstone section contains, in addition to a clay skin cement of typical Triassic character, an extensive kaolin cement which fills much of the remaining primary and secondary porosity (Figs 22a⇑, 23a,b⇑). Regional geological evidence suggests that this kaolin cement is related to the replacement of the Triassic porewaters by meteoric waters during subaerial exposure in the early part of the Jurassic and is associated with conditions causing the dissolution of feldspar grains and the etching of earlier dolomite cement.
Two aspects of the geology and clay mineralogy of the Permo-Triassic strata of the British Isles need to be improved if further progress is going to be made in understanding the significance of the extraordinary clay mineral assemblages. The first is the development of a satisfactory chronostratigraphical framework in which the regional and stratigraphical variations in the clay mineral assemblages can be examined. This may be achieved by additional subsurface mapping using cored boreholes, closely spaced systematic sampling for palynological studies and the use of non-palaeontological methods of dating and correlation (e.g. magneto-stratigraphy (Hounslow et al., 2003); heavy mineral stratigraphy (Jeans et al., 1993)). At the present time, satisfactory correlation between the Permo-Triassic rocks in different parts of England is very limited. This has allowed the concept of correlation using clay mineral-forming events to develop without adequate constraints. Fisher & Jeans (1982) suggested that such correlation was possible using the zone of authigenic clays (Carnian-Norian) between south Devon and an offshore well (72/10-1A), some 500 km away in the Western Approaches (Fig. 7a⇑). What little palynogical evidence there is gave support. Similar correlation is possible between south Devon and the Midlands and Nottingham using the same zone of authigenic clays. However, farther north in the Cheshire Basin and NE Yorkshire, the distribution of authigenic clays in the Triassic sections is more complex (Figs 16⇑–20⇑⇑⇑⇑) and chronostratigraphy is very largely lacking. Until a satisfactory stratigraphical framework is available, it is not possible to deduce the extent to which the complex zones of authigenic clay minerals in northern England are related to geological events affecting the whole area of Western Europe or are an expression of much more local conditions.
The second topic that needs attention, and this is independent of having a satisfactory stratigraphical framework, is a better understanding of the range of structures and chemical compositions of the different Permo-Triassic clay minerals and how they relate to each other, to associated authigenic minerals (carbonates, evaporitic minerals, quartz, feldspar) and to the host rocks in which they occur. Various lines of investigation are promising and these include:
The analysis of the chlorites which are widespread in the clay assemblages. Is the detrital Fe-rich chlorite clearly differentiated from the Mg-rich authigenic chlorite? Can the detrital chlorite be differentiated when it occurs as a minor component in clay assemblages dominated by Mg-rich chlorite?
Testing of the hypothesis that the detrital penecontemporaneous mica could be of volcano-genie origin. A study of its pattern of rare earth elements might give evidence of such an origin.
Palaeotemperature estimates based upon measurement of the Spore Colour Index should be obtained from the main part of the Triassic succession in England. This could go some way to settling the controversy concerning the role of very low-grade metamorphism in forming the authigenic clay assemblages.
The description of the clay mineral stratigraphy of the Merica Mudstone Group (then referred to as the Keuper Marl) of the south Devon coast by Jeans (1978) was based on samples from a nearly continuous succession exposed in the cliffs between Sidmouth and Axmouth. The succession was divided into informal units based upon their megafacies and three facies cycles were recognized – the Dunscombe, Weston and Bindon cycles. However, it was not clear from the exposures available at that time whether the Dunscombe and Weston cycles were different stratigraphical levels or were the same cycle exhibiting marked lateral difference in detail (Jeans, 1978, Appendix 4). Subsequent investigations by Ramues Gallois have demonstrated that the Dunscombe and Weston cycles are the same succession of laterally varying beds. Once this is accepted – and it is supported by the palynological evidence (Fisher 1985; Fisher, pers. comm., 2004) which better fits this alternative – the detailed lithological succession displayed in figs 30–44 of Jeans (1978) require revision as well as the addition of the new formal stratigraphical scheme proposed by Gallois (2002) and Gallois et al.(2005). Details are given below.
Sequence in Otter Sandstone Fm. (Upper Keuper Sst) and in the Sid Mudstone and Salcombe Hill Mudstone members of the Sidmouth Mudstone Fm. (Mudstone I): Sid Mudstone/Otter Sandstone contact at 3.15 m above top of Upper Keuper Sst: Sid Mudstone/Salcombe Hill Mudstone contact at 0.43 m below sample De 138.
Sequence in the Salcombe Hill, Salcombe Mouth and Hook Ebb mudstone members of the Sidmouth Mudstone Fm. (Mudstone I): Salcombe Hill Mudstone/Salcombe Mouth Member contact at the base of sandstone at 0.57 m below sample De 208: Salcombe Mouth Member/Hook Ebb Mudstone contact at sample De 206.
Sequence in the Hook Ebb Mudstone Member of the Sidmouth Mudstone Fm. (Mudstone I).
Sequence in the top of the Hook Ebb Mudstone and Little Weston Mudstone members of the Sidmouth Mudstone Fm. (Mudstone I) and the overlying Dunscombe Mudstone Fm. (Dunscombe Cycle): Hook Ebb Mudstone/Little Weston Mudstone contact at the base of the laminated unit, 0.29 m above base of the section in this figure: Little Weston Mudstone Mudstone Member/Dunscombe Mudstone Fm. contact is at sample De 226.
Sequence in the Dunscombe Mudstone Fm. (Dunscombe Cycle) with the base of the Lincombe Member at sample De 231 and its top at 0.41 m below sample De 148.
Sequence in the Dunscombe Mudstone Formation (Upper Carbonate Group, Dunscombe Cycle).
Sequence at the top of the Little Weston Mudstone Member of the Sidmouth Mudstone Fm. (Mudstone II) and the base of the Dunscombe Mudstone Fm. (Lower Carbonate Group, Weston Cycle): the contact is at the base of the black bed in which sample De 234 is located.
Contact at the top of the Sidmouth Mudstone Fm. (Mudstone I) with the Dunscombe Mudstone Fm. (Lower Carbonate and Sandstone groups, Weston Cycle). The contact is at the base of black band (sample De 165). The Lincombe Sandstone Member has its base just above sample De 168 and its upper limit at the top of 8th sandstone (1 m above sample Del73).
Dunscombe Mudstone Fm. (Sandstone and Upper Carbonate groups, Weston Cycle).
Dunscombe Mudstone Fm. (Upper Carbonate Group, Weston Cycle) and its contact with the Littlecombe Shoot Mudstone Member of the Branscombe Mudstone Fm. (Mudstone III): contact at the top of the thin limestone at 0.48 m above sample De 53.
Littlecombe Shoot Member, Branscombe Mudstone Formation (Mudstone III).
Branscombe Mudstone Fm. (Mudstone III) with the Red Rock Gypsum Member (top at 0.57 m above Sample De275) at the base of the section overlain by the Seaton Mudstone Member.
The Branscombe Mudstone Fm. (Mudstone III) with the junction of the Seaton Mudstone Member with the overlying Haven Cliff Mudstone Member at 1.86 m below sample De 126.
The junction of the Haven Cliff Mudstone Member of the Branscombe Mudstone Formation (Mudstone III) with the Blue Anchor Formation (Lower Carbonate Group, Bindon Cycle) at 0.30 m below sample De 250.
Blue Anchor Formation (Lower Carbonate Group, Bindon Cycle).
Ninety clay mineral analyses from the Permo-Triassic rocks of the Cheshire Basin are listed in Table 1. They were carried out by Simon Kemp (BGS) and briefly summarized by Plant et al.(1999), but were not published in full. The analyses are essentially qualitative and various chlorite types have not been differentiated. The sample locations given in Table A2-1⇓ are shown in Fig. 2⇑ of Plant et al.(1999).
|Sample||Borehole||Sample depth (m) top||Sample depth (m) bottom||Stratigraphy||Clay mineralogy (<2 μm)|
|CHB4||Halewood ETW||71.23||71.43||Kinnerton SST||Smectite, illite, tr chlorite|
|CHB6||Halewood ETW||89.80||89.80||Kinnerton SST||Illite, chlorite, tr smectite|
|CHB7||Halewood ETW||88.05||88.24||Kinnerton SST||Illite, smectite, tr chlorite|
|CHB16A||Mickle Trafford BH||33.55||33.65||Wilmslow SST||Smectite, illite, tr chlorite|
|CHB16B||Mickle Trafford BH||33.55||33.65||Wilmslow SST||Smectite, illite, tr chlorite|
|CHB21||Mickle Trafford BH||58.50||58.50||Chester Feeble Beds||Smectite, illite, tr chlorite|
|CHB27||Mickle Trafford BH||77.74||77.86||Kinnerton SST||Smectite, illite, tr chlorite|
|CHB36||Thornton ETW||108.05||108.11||Helsby SST||Illite, tr chlorite, tr smectite|
|CHB37||Thornton ETW||108.90||108.90||Helsby SST||Smectite, illite, tr chlorite|
|CHB38||Thornton ETW||130.20||130.31||Helsby SST||Illite, smectite, chlorite|
|CHB54A||Thornton ETW||190.98||191.12||Helsby SST||Illite, smectite, chlorite|
|CHB54B||Thornton ETW||190.98||191.12||Helsby SST||Illite-smectite, illite, chlorite|
|CHB61||Stanlow OBH||27.50||27.60||Kinnerton SST||Smectite, illite, ?illite-smectite, tr chlorite|
|CHB64||Stanlow OBH||42.90||42.98||Kinnerton SST||Illite, smectite, tr chlorite|
|CHB65||Stanlow OBH||62.90||62.90||Kinnerton SST||Smectite, illite, chlorite|
|CHB82||Stanlow OBH||272.12||272.20||Kinnerton SST||Illite, smectite, tr chlorite|
|CHB83||Woodlane OBH||4.50||4.50||Tarporley SLST||Illite, chlorite, tr smectite|
|CHB84||Woodlane OBH||10.20||10.20||Tarporley SLST||Illite, smectite, chlorite|
|CHB85||Woodlane OBH||34.95||34.95||Tarporley SLST||Illite, smectite, chlorite|
|CHB86||Woodlane OBH||37.60||37.60||Helsby SST||Illite, chlorite, smectite|
|CHB87||Woodlane OBH||41.00||41.10||Helsby SST||Illite, smectite, chlorite|
|CHB88||Woodlane OBH||46.40||46.40||Helsby SST||Illite, smectite, tr chlorite|
|CHB89||Woodlane OBH||51.90||51.90||Helsby SST||Illite, tr smectite, tr chlorite|
|CHB93||Littleton BH||70.50||70.64||Chester Feeble Beds||Illite, smectite, chlorite|
|CHB95||Littleton BH||76.70||76.70||Chester Feeble Beds||Smectite, illite, chlorite|
|CHB101||Littleton BH||98.20||98.20||Chester Feeble Beds||Illite, tr chlorite, ?tr smectite|
|CHB114||Bootle Golf Course||84.00||84.00||?Helsby SST||Illite, smectite, tr chlorite|
|CHB120||Bootle Golf Course||49.60||49.60||?Helsby SST||Illite, tr smectite|
|CHB121||Bootle Golf Course||50.00||50.10||?Helsby SST||Illite, tr smectite, tr chlorite|
|CHB123A||Bootle Golf Course||75.58||78.80||?Helsby SST||Smectite, illite, tr chlorite|
|CHB123B||Bootle Golf Course||75.58||78.80||?Helsby SST||Smectite, illite, tr chlorite|
|CHB130||Bootle Golf Course||26.50||26.77||?Helsby SST||Smectite, illite, chlorite|
|CHB137||Deskins Quarry||Wilmslow SST||Smectite, illite|
|CHB141||Bridge Quarry||Tarporley SLST||????|
|CHB162||ICI Sports Gd, Widnes||105.00||105.00||Chester Feeble Beds||Smectite, illite, tr chlorite|
|CHB168||ICI Sports Gd, Widnes||172.00||173.00||Chester Feeble Beds||Illite, chlorite, smectite|
|CHB175||ICI Sports Gd, Widnes||218.50||218.50||Chester Feeble Beds||Illite, tr chlorite|
|CHB187||Winsford Salt Mine||Northwich Halite||Illite, chlorite + tr corrensite|
|CHB189||Winsford Salt Mine||Northwich Halite||Illite, chlorite + tr corrensite|
|CHB208||Crewe Heat Flow||51.30||51.30||Wych Mudstones||Corrensite, illite, chlorite|
|CHB215||Crewe Heat Flow||83.30||83.30||Wych Mudstones||Corrensite, illite, chlorite|
|CHB220||Crewe Heat Flow||111.70||111.70||Wych Mudstones||Illite, corrensite, chlorite|
|CHB227||Crewe Heat Flow||116.85||116.95||Wych Mudstones||Corrensite, illite, chlorite|
|CHB230||Crewe Heat Flow||152.40||152.40||Wych Mudstones||Corrensite, illite, chlorite|
|CHB232||Crewe Heat Flow||158.35||158.45||Wych Mudstones||Corrensite, illite, chlorite|
|CHB233||Crewe Heat Flow||160.30||160.30||Byley Mudstones||Corrensite, illite, chlorite|
|CHB238||Crewe Heat Flow||187.30||187.40||Byley Mudstones||Corrensite, illite, chlorite|
|CHB241||Crewe Heat Flow||208.80||208.80||Byley Mudstones||Corrensite, illite, chlorite|
|CHB245||Crewe Heat Flow||225.90||225.90||Byley Mudstones||Corrensite, illite, chlorite|
|CHB248||Crewe Heat Flow||244.30||244.30||Byley Mudstones||Corrensite, illite, chlorite|
|CHB253||Crewe Heat Flow||269.40||269.40||Byley Mudstones||Corrensite, illite, chlorite|
|CHB258||Crewe Heat Flow||289.50||289.50||Byley Mudstones||Corrensite, illite, chlorite|
|CHB261||Crewe Heat Flow||298.55||298.55||Byley Mudstones||Corrensite, illite, chlorite|
|CHB361||Weaverham BH1||10.85||11.05||Bollins Mudstones||Illite, chlorite|
|CHB363||Weaverham BH1||13.00||14.00||Bollins Mudstones||Illite, chlorite|
|CHB365||Weaverham BH1||15.40||15.60||Bollins Mudstones||Illite, chlorite|
|CHB367||Weaverham BH1||16.90||17.05||Bollins Mudstones||Illite, chlorite|
|CHB368||Weaverham BH1||18.90||19.10||Bollins Mudstones||Illite, chlorite|
|CHB369||Weaverham BH5||15.90||16.00||Bollins Mudstones||Illite, chlorite|
|CHB371||Weaverham BH6||15.90||16.00||Bollins Mudstones||Illite, chlorite|
|CHB373||Weaverham BH6||17.35||17.50||Bollins Mudstones||Illite, chlorite|
|CHB375||Weaverham BH6||20.75||20.85||Bollins Mudstones||Illite, chlorite|
|CHB547||Gallantry Bank||7.80||Helsby Sandstone||Illite only|
|CHB553||Gallantry Bank||37.10||Helsby Sandstone||Illite, ?tr smectite|
|CHB787||Mobberley Town 6||Northwich Halite||Corrensite, illite, chlorite|
|CHB789||Mobberley Town 6||Northwich Halite||Corrensite, illite, chlorite|
|CHB791||Arclid Bridge 2||Wilkesley Halite||Corrensite, illite, chlorite|
|CHB793||Arclid Bridge 2||Wilkesley Halite||Corrensite, illite, chlorite|
|CHB794||Lotus Ltd BH, Stafford||17.80||17.80||Brooks Mill Mudstones||Corrensite, illite, chlorite|
|CHB795||Lotus Ltd BH, Stafford||30.72||30.72||Brooks Mill Mudstones||Corrensite, illite, chlorite|
|CHB796||Lotus Ltd BH, Stafford||42.06||42.06||Brooks Mill Mudstones||Corrensite, illite, chlorite|
|CHB797||Lotus Ltd BH, Stafford||67.91||67.91||Stafford Halite||Corrensite, illite, chlorite|
|CHB798||Lotus Ltd BH, Stafford||73.61||73.61||Stafford Halite||Corrensite, illite, chlorite|
|CHB799||Lotus Ltd BH, Stafford||91.14||91.14||Stafford Halite||Corrensite, illite, chlorite|
|CHB800||Lotus Ltd BH, Stafford||108.81||108.81||Stafford Halite||Corrensite, illite, chlorite|
|CHB804||A2/57 BH||327.36||327.36||Manchester Marl||Illite, chlorite|
|CHB805||A2/57 BH||345.72||345.72||Manchester Marl||Illite, chlorite|
|CHB806||A2/57 BH||355.40||355.40||Manchester Marl||Illite, chlorite|
|CHB813||A556 BH446||Northwich Halite||Corrensite, illite, chlorite|
|CHB816||A556 BH446||Northwich Halite||Illite + chlorite|
|CHB835||A556 (M56-M6 mprov. BH80)||17.70||17.70||Bollins Mudstones||Illite, chlorite|
|CHB836||A556 (M56-M6 mprov. BH80)||21.00||21.00||Bollins Mudstones||Illite, chlorite (??corrensite)|
|CHB837||A556 (M56-M6 mprov. BH80)||25.80||25.80||Bollins Mudstones||Illite, chlorite|
|CHB838||A556 (M56-M6 mprov. BH80)||31.00||31.00||Bollins Mudstones||Illite, chlorite|
|CHB839||A556 (M56-M6 mprov. BH80)||48.80||48.80||Bollins Mudstones||Illite, chlorite (??corrensite)|
|CHB840||BrGyp Audlem BHAU17||40.60||40.60||Blue Anchor Fm.||Illite, tr chlorite|
|CHB841||BrGyp Audlem BHAU17||47.20||47.20||Brooks Mill Mudstones||Illite, chlorite|
|CHB842||BrGyp Audlem BHAU17||50.60||50.60||Brooks Mill Mudstones||Illite, chlorite|
|CHB843||BrGyp Audlem BHAU17||63.60||63.60||Brooks Mill Mudstones||Illite, chlorite|
|CHB844||BrGyp Audlem BHAU17||108.60||108.60||Brooks Mill Mudstones||Corrensite, illite, chlorite|
|CHB845||BrGyp Audlem BHAU17||121.00||121.00||Brooks Mill Mudstones||Corrensite, illite, chlorite|
|CHB846||BrGyp Audlem BHAU17||132.00||132.00||Brooks Mill Mudstones||Corrensite, illite, chlorite|
Manchester Marl Formation.
The oldest sedimentary rocks examined from the Cheshire Basin are represented by three samples from the A2/57 borehole. All three samples present a clay mineral assemblage predominantly composed of mica with minor chlorite.
Sherwood Sandstone Group
Kinnerton Sandstone Formation.
The Kinnerton Sandstone Formation, the lowest formation of the Sherwood Sandstone Group, is represented by a total of eight samples from three boreholes; Halewood ETW, Mickle Trafford and Stanlow OBH. The clay assemblage of these samples is composed of varying proportions of mica, smectite and chlorite.
Chester Pebble Beds Formation.
Seven samples from three boreholes (Mickle Trafford, Littleton and ICI Sports Ground, Widnes) were examined to determine the clay mineralogy of the Chester Pebble Beds Formation. These samples also present a mica, smectite and chlorite assemblage similar to the underlying Kinnerton Sandstone Formation although locally it appears more chloritic.
Wilmslow Sandstone Formation.
The Wilmslow Sandstone Formation is represented by three samples. Two samples were taken from the Mickle Trafford BH and represent adjacent red and grey-green sandstones. Both contain a similar clay mineral assemblage of smectite, mica with minor chlorite. A third sample, tentatively assigned to the Wilmslow Sandstone Fm., was taken from Deskins Quarry. The clay mineralogy of this sample is uniquely dominated by smectite with only minor mica. The 060 spacing was also measured for this assemblage in order to characterize the smectite species on the basis of octahedral sheet cation occupancy. The scan resulted in a broad reflection at 1.50 Å with a minor, sharp peak at 1.54 Å. Though not conclusive, this would appear to indicate that the smectite species is dioctahedral (1.50 Å spacing) and the mica to be trioctahedral (1.54 Å). The most common dioctahedral smectite-group mineral containing divalent interlayer cations is montmorillonite.
Helsby Sandstone Formation.
A total of 17 samples was examined from the Helsby Sandstone Formation. The majority of the samples contain a clay mineral assemblage dominated by mica with only trace quantities of smectite and chlorite. These include all the Woodlane OBH samples, all bar one sample from Thornton ETW and the samples above ~76 m in the Bootle Golf Course BH. The exceptional sample from Thornton ETW and deeper samples from Bootle Golf Course all contain a significantly greater proportion of smectite.
The outcrop samples from Gallantry Bank are again dominated by mica with a trace of smectite but lack any chlorite.
Mercia Mudstone Group
Tarporley Siltstone Formation.
Four samples of Tarporley Siltstones were analysed. Three of these were from the Wood Lane BH and these exhibit a clay mineralogy composed of mica with a variable proportion of smectite and chlorite. A further sample from Bridge Quarry has a <2 μm fraction which is dominated by ?corrensite with minor mica.
Bollins Mudstone Formation.
Fourteen samples from four boreholes (Weaverham BHs 1, 5 and 6 and the A556 (M56–M6) BH) were selected as representative of the Bollins Mudstone Formation. These are composed of mica with minor chlorite; corrensite may be present in samples CHB 836 and CHB 839 from the A556 Borehole.
Northwich Halite Member.
Two samples of Northwich Halite were examined from the A556 BH 446 together with two samples from the Mobberley Town BH6 and 2 samples from the Winsford Salt Mine. These samples show some variation. The Mobberley Town samples and the younger sample from the A556 BH exhibit a mica, corrensite and chlorite assemblage. The samples from Winsford display a mica and chlorite assemblage with only a trace of corrensite while the deeper sample from the A556 BH has an assemblage of mica and chlorite only.
Byley Mudstone Member.
Eight samples from the Byley Mudstone Formation were examined from the Crewe Heat Flow BH. These show a uniform clay mineral assemblage of mica, corrensite and minor chlorite.
Wych Mudstone Member.
Six samples of Wych Mudstone Formation were examined from the Crewe Heat Flow BH. As in the underlying Byley Mudstones, these show the same uniform clay mineral assemblage of mica, corrensite and minor chlorite.
Wilkesley Halite Member.
Two samples of Wilkesley Halite were examined from the Arclid Bridge BH2. These again show the same clay assemblage of mica, corrensite and minor chlorite.
Branscombe Mudstone or Brooks Mill Mudstone Formation.
The Brooks Mill Mudstone Formation is represented by 13 samples in this study from the Audlem 17 and Lotus Ltd, Stafford BHs. These show either the mica and chlorite assemblage or the mica, corrensite and chlorite assemblage.
Blue Anchor Formation.
One sample was examined from the Blue Anchor Formation, from the Audlem 17 BH. The clay mineral assemblage is formed of mica and chlorite only.
I am most grateful to the following: Keith Ambrose, Simon Kemp and Andrew Bloodworth of the British Geological Survey, for unpublished technical reports on the stratigraphy and clay mineral stratigraphy of the Mercia Mudstone Group; David Doff and Mark Wilkinson for literature references; Tony Kirkham for micrographs of the Triassic microtekites from Bristol; Bill McHardy for scanning electron micrographs and chemical analyses of chloritic and kaolinitic cements from the Triassic sandstone reservoirs of the Central North Sea; Ramues Gallois for keeping me abreast of the latest developments on the stratigraphy of the Mercia Mudstone Group of SW England; Cleveland Potash Ltd for providing samples from the Staithes No. 20 Borehole and for permission to publish the results; Christa Lippman, Jacques Lucas and Davy Rousset for photographs; Sandra Last for her invaluable skills and patience in preparing this paper for publication; Steve Reed for very considerable help in preparing the figures. This review has benefited greatly from constructive suggestions and editorial help from Michael Fisher, Ramues Gallois and Dick Merriman.