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Clay minerals from the Lower New Red Sandstone of South Devon
Clay Minerals from the Lower New Red Sandstone of South Devon by M. R. HENSON Received 30 November 1972; taken as read 7 December 1973
CONTENTS
1. INTRODUCTION 2. 3. 4. 5. 6. 7.
ANALYTICAL METHODS IDENTIFICATION AND CHARACTERISTICS OF THE CLAY MINERALS GROUPS SEMI-QUANTITATIVE ANALYSIS DISTRIBUTION OF CLAY MINERAL GROUPS PERMIAN AND TRIASSIC CLAY MINERAL ASSEMBLAGES ... DEPOSITIONAL ENVIRONMENTS OF THE SUCCESSION 8. CONCLUSIONS ACKNOWLEDGMENTS REFERENCES
page 429 431 432 434 435 437 439 444
444 444
HENSON, M. R. 1973. Clay Minerals from the Lower New Red Sandstone of South Devon. Proc. Geol. Ass., 84 (4), 429-445. The clay mineral composition of 41 samples from localities within the South Devon Lower New Red Sandstone succession was determined using X-ray diffraction. The clay mineralogy of the succession is dominated by illite with subordinate amounts of kaolinite and chlorite, except in the Budleigh Salterton Pebble Beds where kaolinite is predominant. Swelling chlorite and a mixedlayer illite are restricted to the Exmouth Sandstones and Mudstones and Littleham Mudstones. Interpretation of depositional environments based on the clay mineral assemblages are consistent with the sedimentological interpretation of the succession as piedmont fan and fluviolacustrine complexes. • Institute of Geological Sciences, 5 Princes Gate, London SW7 IQN.
1. INTRODUCTION of the investigation was to determine the distribution of the various clay mineral groups through the Lower New Red Sandstone succession in South Devon and to compare the results with recent studies of clay minerals from Permo-Triassic rocks elsewhere. Millot (1949, 1951, 1964) and others have stressed the value of clay minerals as indicators of the geological environment in which they were formed. By studying the geochemical environments it was hoped to confirm and support the conclusions on the nature of the depositional environments reached from a study of the physical structures shown by the lithologies (Henson, 1970, 1971). The Permo-Triassic rocks considered in this study crop out to the west and east ofthe Exe estuary, South Devon (Fig. 1). Rocks currently regarded as Permian (Henson, 1971) are restricted almost exclusively to the area west of the estuary; they comprise in ascending order the Teignmouth 429
THE PURPOSE
M.R.HENSON
430
Breccias, Dawlish Sandstones and Exe Breccias (Henson, 1971). The Teignmouth Breccias are coarse to fine-grained immature sediments composed of sandstone, porphyry, slate and occasionally limestone clasts in a sandy mud matrix. The Dawlish Sandstones are aeolian and fluvially cross-stratified sands with intercalated breccia lenses. The overlying Exe Breccias are similar in character to the Teignmouth Breccias. The lowest of the Triassic formations (Henson, 1970, 1971) is the Exmouth Sandstones and Mudstones which is composed of thick fluvially cross-stratified sands and thin silty sand beds intercalated in mudstones. This formation is overlain by the Littleham Mudstones; mudstones with silty sand intercalations. Overlying these formations are the Budleigh Salterton Pebble Beds; distinctive metaquartzite cobbles and boulders in a muddy sand matrix. The succeeding Otter Sandstones are fluvially crossstratified sands in graded units with basal intraformational conglomerates. ---00 o
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NEW RED SANDSTONE OF SOUTH DEVON
431
The Upper (Keuper) Marls overlying the predominantly arenaceous Permian and Lower Triassic rocks were not sampled. Interpretations of the depositional environments by Laming (1966) and Henson (1970, 1971) have made possible the grouping of the succession into three major facies divisions related to the infilling of the basin. (1) Marginal deposits, the stream flood and sheet flood deposits of the breccias, late Carboniferous and Permian in age; (2) Fluvial basin deposits, sandstones and mudstones of the Lower and Middle Trias; (3) Marine basin deposits, mudstones of the Upper Trias. In the absence of palaeontological evidence the base of the Trias is arbitrarily taken at the base ofthe Exmouth Sandstones and Mudstones because of the marked hiatus at this point due to a change in depositional processes and source area (Henson, 1971). 2. ANALYTICAL METHODS A field sample of fresh unweathered material was collected from each locality; 109. of the sample were crushed and treated with acetic acid of pH5, buffered by sodium acetate solution, to remove any carbonates present. The sample was then dispersed by shaking with sodium hexametaphosphate for eight hours and treatment in an ultrasonic tank. The dispersed sample was then sedimented after the method of Mackenzie (1956) and the < 1·4 micron equivalent sedimentation diameter fraction was siphoned off. Oriented specimens for X-ray diffraction were prepared by placing a few drops of this suspension on a 24 X 24 mm. glass slide and allowing it to dry exposed to the air. The clays were examined in the Mg++ saturated condition. Four slides of each sample were prepared and X-rayed: (1) air dried; (2) treated with ethylene glycol, by placing the specimen in ethylene glycol vapour at 70° C. for 4 hours and allowing it to equilibriate in a desiccator for 12 hours before X-raying; (3) heated to 3350 C. for 4 hours; (4) heated to 5500 C. for 4 hours. The various pre-treatments are necessary for the identification of the clay mineral groups present. The behaviour of the lattice spacings under different conditions is a diagnostic feature of the mineral groups. After treatments 2, 3 and 4 the specimens were kept in a desiccator to prevent rehydration. X-Ray Routine A Phillips X-ray Diffractometer with nickel filtered CuKa radiation at
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M. R. HENSON
46 kV and 18 rnA was used throughout, after comparative trials between MoKa and CuKa radiation revealed that iron fluorescence with the CuKa radiation was negligible. The specimens were scanned at 1 28 per minute to 30 Counting was by Geiger Muller tube and the counts from 3 were recorded on a Leeds-Northrup Chart Recorder, with a chart speed of 25 mm. per minute. It was assumed that counter deadtime did not appreciably affect the results. 0
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3. IDENTIFICATION AND CHARACTERISTICS OF THE CLAY MINERALS Details of the X-ray characteristics used in identifying the mineral groups present are given in Brown (1961) and Warshaw & Roy (1962). The characteristics of the mineral groups identified in this study are summarised in Table 1. Illite was present in all the specimens examined, characterised by spacings of 9,9-10,07 A (002), 4'9-4·99 A (004) and 3-3-3-34 A (006). In most cases the peaks were sharp and narrow; however, several specimens from the Exmouth Sandstones and Mudstones showed a broadening of the base of the peak to 11'5 A; the apex of the peak remained at 9·9 A. The broadening collapsed on heating to the normal form of the illite peak and was not affected by glycolation. This behaviour is thought to indicate the presence of a mixed-layer illite. In all specimens heating resulted in sharpened and enhanced peaks. Kaolinite was present in most of the specimens, characterised by a basal reflection of7'07-7'17 A (001), and a reflection of3'5 A (021). The kaolinite 7·1 A (001) peak is indistinguishable from the 7·07 A (002) chlorite peak. On heating to 550 C. kaolinite is rendered amorphous and all the characteristic peaks disappear. In all the samples examined kaolinite was more abundant than chlorite and the 7·1 A peak was usually attributed to the presence of kaolinite. The presence of chlorite in a sample was indicated by peaks at 14·1 A and 4'7 A. Chlorite was present in many of the specimens, characterised by spacings of 14,1-14'2 A (001), 7·07 A (002), 4·72 A (003) and 3·54 A (004). These spacings and the form of the peaks were generally maintained in all the treatments, although there was a marked tendency for the (002), (003), and (004) reflections to show a decreased intensity after heating to 5500 C. An increase in the intensity of the 14 A (001) peak after heating to 550 C. was frequently observed, indicating that the chlorites are probably of the iron-rich types. This is also confirmed by the decrease in the (001) spacing to 13'8 A at 5500 ; Fe-chlorites typically show a greater shrinkage parallel to c than do Mg-chlorites. A number of specimens showed an expansion of the 14·2 A (001) spacing 0
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pronounced swellin g behaviour on glyco lation
no swelling on glyeolation
distinguishable from chlorite on its swelling behaviour
14 A peak enhanced and collapsed to 13'8 A on heating to 5500 C., indicative of Fe-chlorites that show shrinkage parallel to c
mixed-la yer illite in the Exmouth Sandstones and Mudstones showed an expanded (001) basal spac ing that collapsed on heating to 3350 C.
7 A peak stronger than 3·5 A peak, rendered amorphous on heating to 550 0 C.
Effects of the var ious pre-treatments on the basal spacings
I. Characteristics used for the identification of the clay minerals in the samples studied
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to 16-17 A on glycolation ; on heating to 550 C. the peak collapsed to 13·6-13·8 A, indicating that the swelling was not attributable to montmorillonite or vermiculite. A 24 A peak was not observed, indicating that it was neither a mixed-layer chlorite-montmorillonite nor a chloritevermiculite. The other peaks characteristic of chlorite were present . Such behaviour is characteristic of swelling chlorite. In all the specimens containing swelling chlorite the 14'1 A peak remained, indicating that both normal chlorite and swelling chlorite were present. Montmorillonite occurred in a few specimens, characterised by a basal reflection at 14·2 A (001) that expands on treatment with ethylene glycol to 17-7 A in the Mg++ saturated condition. On heating to 3350 C. the spacing collapsed to 10 A; at 5500 C. it further collapsed to 9-8 A, where it was indistinguishable from the illite (001) peak. Other clay minerals were not detected and quartz was not found in the fraction ( < 1'4p) examined. In a preliminary study haematite was identified in red clays but not in green and grey clays. 0
4. SEMI-QUANTITATIVE ANALYSIS Specified peak areas were compared directly and in ratio to contrast the proportions of the minerals present. Peak areas were measured in square centimetres but it is an arbitrary scale; the actual areas are governed by the instrument settings which were maintained constant throughout this study. The recalculation of the peak areas to percentages for each specimen was not attempted for two main reasons. Firstly clay minerals within geological environments will be imperfectly developed and many interstratified, therefore quantification based upon standard minerals would be unjustified (Brindley, 1961). Secondly no really satisfactory method of calculating the amounts of clay minerals in sediments exists and until methods are standardised peak areas should be used, so, if necessary, comparison of data could be made after suitable recalculation (Pierce & Siegel, 1969). In this study the actual percentage of any mineral present in a sample is not crucial, it is the assemblage that is important. Thus the rough method used to assess the relative proportions of minerals present in a sample is considered adequate. The proportion of illite present was measured by the area of the 10 A peak; the proportion of kaolinite was indicated by the area of the 7·1 A kaolinite and chlorite peak and that of chlorite by the 14'2 A peak. The area of the 17'5 A peak indicated the proportion of montmorillonite present and the 16-18 A peak the proportion of swelling chlorite. The relationship of kaolinite to illite is shown by the ratio 7: 10 A = kaolinite and chlorite:iIlite (ratio 1); and that of chlorite to kaolinite by 4-7:3'5 A = chlorite: kaolinite (ratio 2).
NEW RED SANDSTONE OF SOUTH DEVON
415
5. DISTRIBUTION OF THE CLAY MINERAL GROUPS The distribution of the clay minerals within the succession is summarised in Fig. 2. Illite is present in all the specimens analysed and except in the Budleigh Salterton Pebble Beds is always the most abundant mineral. The proportion is fairly uniform throughout the succession; it is, however, more abundant within the Exmouth Sandstones and Mudstones and Littleham Mudstones, and less abundant in the breccia formations. Significantly greater amounts of illite occur in the samples taken from the sand and green silty sand units of the Exmouth Sandstones and Mudstones than in the surrounding mudstones. Mixed-layer illite is restricted to the Exmouth Sandstones and Mudstones and Littleham Mudstones. Kaolinite is present in many of the specimens; it is less abundant within the breccia formations than the other units. The amount of kaolinite is generally one-fifth that of illite. Within the Exmouth Sandstones and Mudstones and Littleham Mudstones the proportion of kaolinite is relatively constant; but in the Budleigh Salterton Pebble Beds the amount of kaolinite drastically increases so that it is predominant-a very marked feature. In the Otter Sandstones the amount of kaolinite returns to near its previous level. Chlorite is present in many samples; it generally occurs more frequently within the Exmouth Sandstones and Mudstones and Littleham Mudstones. Within the Teignmouth Breccias, Dawlish Sandstones and Exe Breccias, chlorite, where present, is often in a greater amount than in the other formations. Swelling chlorite is restricted almost entirely to the Exmouth Sandstones and Mudstones and Littleham Mudstones; but is also found in the Dawlish Sandstones. Montmorillonite is the least abundant of the clay minerals found in the succession; it occurs in significant quantities mainly in the Teignmouth Breccias, the Exe Breccias and the Dawlish Sandstones. Above these formations traces occur in the sandstones of the Orcombe Point and Straight Point in the Exmouth Sandstones and Mudstones, and in the Otter Sandstones. When the ratio of kaolinite and chlorite to illite is plotted against the ratio ofchlorite to kaolinite, a distinct separation ofthe specimens into two groups is obtained (Fig. 1). The silts, mudstones and sands of the Exmouth and Littleham Formations and the Otter Sandstones form a group relatively poorer in kaolinite and richer in chlorite. The few results from the Teignmouth Breccias, Dawlish Sandstones and Exe Breccias show a relatively greater proportion of kaolinite but a similar chlorite content. The groups thus effectively separate the clay mineral assemblages of the marginal deposits from the fluvial basin deposits. Two specimens from the major sandstone bodies of Orcombe Point and Straight Point in the Exmouth PROC. GEOL. ASS., VOL. 84, PART 4, 1973
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NEW RED SANDSTONE OF SOUTH DEVON
437
Sandstones and Mudstones fall into the field of the marginal deposits. These sand units contain material derived from the breccias; sandstone and chert breccia clasts are present in the Straight Point sand, and reworked Carboniferous miospores were obtained from the Orcombe Point sand. It may therefore be concluded that these sand units should have some affinitieswith the breccia formations.
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6. PERMIAN AND TRIASSIC CLAY MINERAL ASSEMBLAGES The investigations of the following authors are cited to show the distribution of the clay mineral groups in the Permo-Triassic rocks of northwest Europe. Dr. G. Warrington (personal communication) examined the clay mineral assemblages of samples taken from the Bunter, the Keuper and the Rhaetic of the Midlands. Dlite and some kaolinite were found in the Bunter samples and illite (with accessory chlorite and kaolinite) was dominant in the lower part of the Keuper Sandstone. In samples from the
438
M.R.HENSON
upper part of the Keuper Sandstone, the Waterstones and the Keuper Marl, illite was predominant though montmorillonite, chlorite and kaolinite also occurred. Chlorite was, however, absent from the Arden Sandstone and montmorillonite was absent from the Tea Green Marl. Sepiolite was recorded from the Keuper Marl both above and below the Arden Sandstone. Material from the basal Rhaetic yielded only illite. Dumbleton & West (1966) investigated the clay mineralogy of samples taken from the Keuper Marl. Illite (30-60 per cent) and chlorite were present in all nineteen samples analysed and swelling chlorite in all but two (chlorite-l-swelling chlorite up to 40 per cent). Sepiolite and palygorskite were present in some samples. Davis (1967) determined the clay mineralogy of fifty samples scattered over the outcrop of the Keuper Marl of the Midlands; forty samples showed illite, corrensite and sometimes a trace of chlorite. Five samples contained illite only; these samples were mainly from the top of the Keuper Marl where it grades into the Tea Green Marl. Four samples from Solihull, Warwickshire, showed sepiolite, illite and swelling chlorite; the sepiolite was restricted to an 18·35 m. vertical section. Lucas & Bronner (1961) studied the clay mineral assemblages in boreholes from the Triassic of the Jura. Illite was present with chlorite throughout the basin, though in variable proportions. Kaolinite was present only as a trace in the greater part ofthe succession; within the basal conglomerate, however, it was abundant. Corrensite was present only in the central portions of the basin and it disappeared at the top of the Upper Keuper. Interstratified minerals occurred around the margins of the basin and in the upper and lower parts of the succession; it was apparent that they were derived from degraded illites and that they were altered to corrensite within the basin. Lucas (1962) investigated the clay mineralogy of the Triassic rocks in the Paris Basin and Lorraine. The clays were essentially illite and a 14 A interstratified montmorillonite--chlorite trending towards corrensite; true chlorite was rare. At the base of the succession illite was dominant with kaolinite and degraded chlorite; moving upwards the proportion of illite decreased while that of the 14 A complex increased. From the middle of the Upper Keuper the illite content rapidly increased until it became completely dominant or present with a small proportion of 14 A minerals not found in the middle part of the succession. Successions in Morocco, the Pyrenees and Spain were also examined and showed similar assemblages to these recorded in the Paris and Jura Basins. In all the investigations cited the minerals present are nearly always the same and they show a similar distribution in space and time. Within all the basins s1 udied the basal and marginal areas show assemblages dominated by illite accompanied by kaolinite, chlorite and interstratified minerals.
NEW RED SANDSTONE OF SOUTH DEVON
439
Higher in the successions and towards the centres of the basins illite is replaced by 14 A interstratified minerals--chlorite-montmorillonite, swelling chlorite, corrensite, sepiolite and palygorskite; true chlorite is never characteristic. Towards the top of the succession illite regains its dominance and is associated with chlorite. The minerals from the centre of the basin are less variable than those from the margins. Krumm (1969)in a comparison of the clay minerals of European Triassic sediments with their sedimentary position within the depositional basin similarly found a zonation of the clay minerals in a sequence identical to that detailed above. The zonation is attributed to physico-chemical differences in the sedimentary environments. The mineral assemblages obtained during the examination of the Permian and Trias of South Devon are comparable to those obtained from the Keuper Sandstone of the Midlands (Warrington, personal communication) and those from the basal and marginal areas of the Jura and Paris Basins (Lucas & Bronner, 1961; Lucas, 1962). However, they are unlike those obtained from the Keuper Marl. It is apparent that the assemblages in this study are typical of the marginal and basal regions of a basin; while the assemblages of the Keuper Marl are typical of the central basin area. If, as seems to be the case, the Upper (Keuper) Marls of Devon bear the same relationship to the fluvial basin deposits as the Keuper Marl does to the Keuper Sandstone of the Midlands, then the Upper (Keuper) Marls will show the same mineral assemblages as the Keuper Marl of the Midlands and the Keuper of France and Germany.
7. DEPOSITIONAL ENVIRONMENTS OF THE SUCCESSION (a) Clay minerals There is a controversy over the interpretation of geological environments from their clay mineral assemblages; it is based upon arguments as to whether clay mineral assemblages are detrital, that is inherited from the source area, or diagenetic in origin. Detrital assemblages can show no relationship to their environment of deposition but reflect the weathering regime in the source area. Slight facies differences are caused by varying properties of susceptibility to transport, settling velocity and flocculation; the lattice structure is not affected by the geochemical environment (Weaver, 1958). Grim (1958), Keller (1956, 1970) and Millot (1949, 1951, 1964) consider that clay minerals are the reactive response to the geochemical environment of the material in the environment. Source rocks react to their environment by weathering to the various clay minerals; these minerals will then react to any new environment into which they might be transported. This reaction may be substantial or non-existent
440
M. R. HENSON
depending on the difference between the geochemical environments. If this latter view is accepted, then clay minerals can be used as indicators of the geochemical and hence the sedimentary environments of which they are characteristic; and conversely the environment can be used to predict the clay minerals present. Millot (1949, 1951, 1964) showed that the major sedimentary facies had characteristic clay mineral assemblages, both in the proportion and the type of clay minerals present. The structure and chemistry of clay minerals coupled with their relatively high surface area make it likely that clay minerals will react with their environment if they are not in equilibrium with the environment. Cation substitution in the tetrahedral and octahedral sites and cation exchange in the interlayer positions are the most easily effected changes. Complete recrystallisation is possible and solution will occur if the minerals are sufficiently out of equilibrium with the environment. The pH, Eh and the concentration of reacting ions are the more important factors in the geochemical environment controlling any changes that may occur. The greater number of clay minerals are first formed by the weathering of a parent silicate; the environments most favourable to the formation of specific minerals, and hence to their preservation, have been investigated over a considerable period. Keller (1970) summarises the conditions most favourable to the production of the primary clay minerals. The formation of kaolinite is favoured by the removal of Ca++, Mg" ", Na -, K+ and Fe++ from the environment with the addition of H+. Precipitation must exceed evaporation giving strong leaching to remove the requisite cations; H + ions are obtained from the fresh water or waters rich in organic and carbonic acids and sulphur compounds. The AI+++ concentration must be high and the Al/Si ratio must also be high, thus necessitating the removal of Si02. Montmorillonite (smectite) requires the retention of Mg++, Ca ++, Fe ++ and Na r with H.SiO.; therefore evaporation must exceed precipitation and leaching must be ineffective. Such conditions are present in the stagnant waters of lakes, marine basins or semi-arid areas. Primary illite forms from the weathering of alumino-sil icates where the ionic concentrat ions of K + and H+ are high relative to the concentration of H4Si04. Illites are clay size mica minerals, so degradation by weathering of layer silicates will result in the production of illites. The stripping of K + ions from illites results in degraded illites; these minerals are more reactive than the normal ionically balanced illites and readily absorb K + from the environment to remedy the deficiency. Simple disaggregation of preexisting argillaceous rocks will yield abundant illite as most shales are composed of ilIitic clay that is reworked with little change except for K + stripping. A considerable amount of chlorite is similarly produced from the disaggregation of slates and phyllites. In areas of poor leaching with the retention of K+ and Mg++ it is possible to have chlorite formin g with
NEW RED SANDSTONE OF SOUTH DEVON
441
montmorillonite but the majority of chlorite is derived from pre-existing rocks. The parent material will have an overriding control of the minerals produced during weathering; if the requisite ions are not present then the characteristic mineral cannot be produced by the weathering conditions. Authigenic minerals will crystallise from solutions approximating to the ideal structural formulae in composition and in sufficient concentration when the pH and Eh conditions are optimum. Illite, kaolinite, montmorillonite, Al-chlorites, sepiolite and palygorskite have all been found as authigenic minerals in sedimentary successions. The assemblages obtained from the Teignmouth Breccias, Dawlish Sandstones and Exe Breccias, illite with kaolinite and chlorite and a little montmorillonite, are similar to those obtained from piedmont complexes (Millot, 1964). The minerals are entirely detrital in origin, and, except for kaolinite, produced from the disaggregation of the source rocks, they show no evidence of alteration. Deposition in this case is likely to be in an actively transporting situation with little concentration of bases in ephemerallakes and ponds, typically sheet floods, stream floods and high braided distributary systems such as are present in alluvial fans. The mixing of illite and kaolinite, minerals formed in contrasting weathering regimes, suggests that the source area underwent a seasonal climatic change; a hot dry season would be followed by a tropical rainy season. The assemblages are very similar to those described by Millot, Perriaux and Lucas (1961) from the Permo-Triassic red-bed sequence of the Vosges. Montmorillonite could well be the weathering product of clasts of Devonian tuff contained in the breccias. The assemblages of the Exmouth Sandstones and Mudstones and Littleham Mudstones (illite, kaolinite, chlorite and swelling chlorite) are similar to those obtained from fluvio-Iacustrine environments (Millot, 1964).The illite, kaolinite and chlorite are probably predominantly detrital, indicating that they have the same type of source area as the marginal deposits. The kaolinite and chlorite: illite and chlorite: kaolinite ratios indicate that the fluvial basin deposits are lower in kaolinite and higher in chlorite than the marginal deposits. This feature can be explained by a loss of kaolinite by solution and an increase in chlorite by alteration, indicating that Mg++,Fe++ and K + were being concentrated in the environment. The transformation of illite to a mixed-layer illite similarly shows that base concentration was significant. Swelling chlorite, an important accessory mineral, also indicates a concentration of bases, especially Mg++, in the environment. Ca ++ concentration is shown by the calcareous nature of the mudstones; euhedral calcite crystals are frequently present, and small crystals of dolomite and possibly gypsum are also found. From the foregoing evidence it appears that the environment was approaching supersaline conditions; which were most likely to occur during the evapora-
442
M. R. HENSON
tion of ephemeral lake areas, the depositional environment of the overbank silts and clays that comprise the mudstones. Neither palygorskite nor sepiolite was formed, indicating that extremes of supersaturation were not reached. The similarity of the kaolinite and chlorite: illite and chlorite: kaolinite ratios of the major sand beds to those of the marginal deposits shows that the channels were transporting material derived from the source area and possibly the outcrop of the marginal deposits. Illite is significantly higher in the silty sand beds and thick sandstone units intercalated with the mudstone sequences; this would seem to suggest a link between the silty sands and the channel sands, the poorly sorted silty sands being derived from material transported in the channels, hence they are the coarse overbank deposits. The clay mineral assemblage of the Budleigh Salterton Pebble Beds accords with assemblages of fluvial deposits but differs dramatically from those of the other formations. The kaolinite and chlorite:illite ratio is very much higher than in any other formation. This great increase in kaolinite is probably mainly due to leaching by pore waters in the recent weathering environment. Feldspar grains in the matrix and the scattered porphyry clasts are extensively altered to kaolinite. Interpretation of the original depositional environment usingclay minerals in these circumstances would be unrealistic. The Otter Sandstones maintain the high kaolinite content of the Pebble Beds and show a similar illite content. As in the Pebble Beds the effect of recent leaching producing kaolinite overrides the original Triassic assemblage. (b) Sedimentary Structures
The sedimentary features shown by the succession were described and discussed by Laming (1966) and Henson (1970, 1971). In these studies it was concluded that the Teignmouth and Exe breccias and the Dawlish Sandstones were deposited by stream floods and sheet floods as alluvial fans that coalesced into a piedmont fan adjacent to a mountain front. The Exmouth Sandstones and Mudstones and Littleham Mudstones were deposited as a flood-plain complex, the thick sand units being channel sands, the thin sand and green silty sand beds being the coarse overbank deposits, crevasse splays and levees, and the mudstones being the flood basin deposits. Coarse fining-upwards sequences characteristic of stream flood deposits are clearly discernible in the cobble and boulder conglomerate of the Budleigh Salterton Pebble Beds, suggesting deposition in an alluvial fan. This very marked change in the style of Triassic sedimentation is most easily explained as the result of a climatic change in both the source and depositional areas and a change in major source area. Overlying the Pebble Beds the Otter Sandstones show fining-upwards cycles typical of
high-sinuosity, low-braided river ; act ive meander belt PIEDMONT FAN COMPLEX mid-fan deposits
mudstones with silty sand beds
mudstones with thick cross-stratified sand units and silty sand beds
sandstone and porphyry breccias
cross-stratified aeolian and fluvial sands with breccia lenses
sandstone and porphyry brecc ias
Littl eham Mudstones
Exmouth Sandstones and Mudstones
Exe Breccias
Dawlish Sandstones
Teignmouth Breccias
mid-fan deposits
fan-base deposits
high-sinuosity, low-braided river; channels distant
stream flood deposited alluvial fan
cobble and boulder conglomerate
Budleigh Salterton Pebble Beds
high-braided, lowsinuosity streams
Depositional environment
cross-stratified sands in fining upward cycles
Lithology
illite , kaolinite and chlorite with montmorillonite
} ;WI<. kaolinite and chlorite with swelling-ehlorite
kaolinite a nd illite
illite and kaolinite with chlorite
Clay minerals
II. The clay mineral assemblages and their corresponding sedimentary environments
Otter Sandstones
Formation
TABLE
e
z
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tIl
o
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C
o
til
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o
tIl
Z
o
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til
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~
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Z tIl
444
M. R. HENSON
braided streams, reflecting the change in sedimentation as the source area retreated and the underlying fan aggraded.
8. CONCLUSIONS The distribution of the clay mineral groups and assemblages demonstrate that a piedmont complex and a fluvio-Iacustrine complex were the depositional environments for the lower part of the succession, confirming the conclusions of the sedimentological studies. They did not indicate the depositional environment of the Budleigh Salterton Pebble Beds or the Otter Sandstones because of the effects of recent weathering rendering any environmental interpretations unrealistic. The similarity of the clay mineral assemblages in the succession to those of the Keuper Sandstone of the Midlands and the marginal areas of the Permo-Triassic basins of Europe suggests that they were formed in similar environments under the same general climatic controls. Also the similarity of the clay mineral assemblages sequences through the Keuper to the Rhaetic show the interrelation of the Permo-Triassic successions throughout north-west Europe, and the control of clay mineral assemblages by the depositional sedimentary environment.
ACKNOWLEDGMENTS This work was carried out as part of the NERC-Institute of Geological Sciences contract with the Department of Geology, University of Exeter, for the re-mapping of Sheet 339 (Teignmouth), My grateful thanks are due to Professor S. Simpson, Mr. G. Bisson, Dr. E. R. Shephard-Thorn and Mr. B. Young for critically reading the manuscript; Dr. G. Brown and Mr. D. Dallow for helpful advice on X-ray diffraction techniques; and Dr. G. Warrington for permission to precis his unpublished results.
REFERENCES BRINDLEY, G. W. 1961. Quantitative Analysis of Clay Mixtures. In Brown, G. (Ed.), The X-ray Identification and Crystal Structures 0/ Clay Minerals, 489-514. BROWN, G. (Ed.), 1961. The X-ray Identification and Crystal Structures 0/ Clay Minerals. 2nd ed, Mineralogical Society, London. DAVIS, A. G. 1967. The Mineralogy and Phase Equilibrium of Keuper Marl. Quart. Jl Eng. Geol., 1, 25-38. DUMBLETON, M. J. & G. WEST. 1966. Studies of the Keuper Marl: Mineralogy. Min. ofTransport R.R.L., Report 40. GRIM, R. E. 1958. Concept of Diagenesis in Argillaceous Sediments. Bull. Am. Ass. Petrol. Geol., 42, 246-53. HENSON, M. R. 1970. The Triassic Rocks of South Devon. Proc. Ussher Soc., 2, 172-7. - - - . 1971. The Permo-Triassic Rocks 0/ South Devon. Ph.D. Thesis, University of Exeter.
NEW RED SANDSTONE OF SOUTH DEVON
445
KELLER, W. D. 1956. Clay Minerals as Influenced by Environments of their Formation. Bull. Am. Ass. Petrol. Geol., 40, 2689-710. - - - . 1970. Environmental Aspects of Clay Minerals. J. sedim. Petrol., 40, 788810. KRUMM, H. 1969. A Scheme of Clay Mineral Stability in Sediments Based upon Clay Mineral Distribution in Triassic Sediments of Europe. Int. Clay Conf., Tokyo, 1969,313-24. LAMING, D. J. C. 1966. Imbrications, Palaeocurrents and other Sedimentary Features in the Lower New Red Sandstone, Devonshire, England. J. sedim. Petrol., 36,940-59. LUCAS, J. 1962. La Transformation des Mineraux Argileux dans la Sedimentation. Etudes sur les Argiles du Trias. Mem. Servo Carte geol. Als. Lorr., 23. & A. M. BRONNER. 1961. Evolution des Argiles Sedimentaires dans le Bassin Triasique du Jura Francais, Bull. Servo Carte geol, Als. Lorr. 4 (4), 137-48. MACKENZIE, R. C. 1956. Methods for Separation of Soil Clays in Use at the Macaulay Institute for Soil Research. Clay Miner. Bull., 3 (4), 4-6. MILLOT, G. 1949. Relations entre la Constitution et la Genese des Roches Sedimentaires Argileuse. These Sci. Nancy and Geol, Appl, Prospec. Min., 2, nos. 2, 3,4,1-352. - - - . 1951. The Principal Sedimentary Facies and their Characteristic Clays. Clay Miner. Bull.,t (7), 235-7. - - - . 1964. Geologie des Argiles. Paris. - - - , J. PERRIAUX & J. LUCAS. 1961. Signification Clirnatique de la Couleur des Gres Permo-Triassiques et des Grandes Series Detritiques Rouges. Bull. Serv, Carte geol. Als, Lorr., 4 (4), 91-100. PIERCE, J. W. & F. R. SIEGEL. 1969. Quantification in Clay Mineral Studies of Sediments and Sedimentary Rocks. J. sedim. Petrol., 39, 187-93. WARSHAW, C. M. & R. Roy. 1962. Classification and a Scheme for the Identification of Layer Silicates. Bull. geol. Soc. Am., 72, 1455-92. WEAVER, C. E. 1958. Geologic Interpretation of Argillaceous Sediments. Part I. Origin and Significance of Clay Minerals in Sedimentary Rocks. Bull. Am. Ass. Petrol. Geol., 42, 254-71.
CONTENTS
1. INTRODUCTION 2. 3. 4. 5. 6. 7.
ANALYTICAL METHODS IDENTIFICATION AND CHARACTERISTICS OF THE CLAY MINERALS GROUPS SEMI-QUANTITATIVE ANALYSIS DISTRIBUTION OF CLAY MINERAL GROUPS PERMIAN AND TRIASSIC CLAY MINERAL ASSEMBLAGES ... DEPOSITIONAL ENVIRONMENTS OF THE SUCCESSION 8. CONCLUSIONS ACKNOWLEDGMENTS REFERENCES
page 429 431 432 434 435 437 439 444
444 444
HENSON, M. R. 1973. Clay Minerals from the Lower New Red Sandstone of South Devon. Proc. Geol. Ass., 84 (4), 429-445. The clay mineral composition of 41 samples from localities within the South Devon Lower New Red Sandstone succession was determined using X-ray diffraction. The clay mineralogy of the succession is dominated by illite with subordinate amounts of kaolinite and chlorite, except in the Budleigh Salterton Pebble Beds where kaolinite is predominant. Swelling chlorite and a mixedlayer illite are restricted to the Exmouth Sandstones and Mudstones and Littleham Mudstones. Interpretation of depositional environments based on the clay mineral assemblages are consistent with the sedimentological interpretation of the succession as piedmont fan and fluviolacustrine complexes. • Institute of Geological Sciences, 5 Princes Gate, London SW7 IQN.
1. INTRODUCTION of the investigation was to determine the distribution of the various clay mineral groups through the Lower New Red Sandstone succession in South Devon and to compare the results with recent studies of clay minerals from Permo-Triassic rocks elsewhere. Millot (1949, 1951, 1964) and others have stressed the value of clay minerals as indicators of the geological environment in which they were formed. By studying the geochemical environments it was hoped to confirm and support the conclusions on the nature of the depositional environments reached from a study of the physical structures shown by the lithologies (Henson, 1970, 1971). The Permo-Triassic rocks considered in this study crop out to the west and east ofthe Exe estuary, South Devon (Fig. 1). Rocks currently regarded as Permian (Henson, 1971) are restricted almost exclusively to the area west of the estuary; they comprise in ascending order the Teignmouth 429
THE PURPOSE
M.R.HENSON
430
Breccias, Dawlish Sandstones and Exe Breccias (Henson, 1971). The Teignmouth Breccias are coarse to fine-grained immature sediments composed of sandstone, porphyry, slate and occasionally limestone clasts in a sandy mud matrix. The Dawlish Sandstones are aeolian and fluvially cross-stratified sands with intercalated breccia lenses. The overlying Exe Breccias are similar in character to the Teignmouth Breccias. The lowest of the Triassic formations (Henson, 1970, 1971) is the Exmouth Sandstones and Mudstones which is composed of thick fluvially cross-stratified sands and thin silty sand beds intercalated in mudstones. This formation is overlain by the Littleham Mudstones; mudstones with silty sand intercalations. Overlying these formations are the Budleigh Salterton Pebble Beds; distinctive metaquartzite cobbles and boulders in a muddy sand matrix. The succeeding Otter Sandstones are fluvially crossstratified sands in graded units with basal intraformational conglomerates. ---00 o
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Fig. 1. Outcrops of the Permian and Triassic formations in the area studied, with sample locations
NEW RED SANDSTONE OF SOUTH DEVON
431
The Upper (Keuper) Marls overlying the predominantly arenaceous Permian and Lower Triassic rocks were not sampled. Interpretations of the depositional environments by Laming (1966) and Henson (1970, 1971) have made possible the grouping of the succession into three major facies divisions related to the infilling of the basin. (1) Marginal deposits, the stream flood and sheet flood deposits of the breccias, late Carboniferous and Permian in age; (2) Fluvial basin deposits, sandstones and mudstones of the Lower and Middle Trias; (3) Marine basin deposits, mudstones of the Upper Trias. In the absence of palaeontological evidence the base of the Trias is arbitrarily taken at the base ofthe Exmouth Sandstones and Mudstones because of the marked hiatus at this point due to a change in depositional processes and source area (Henson, 1971). 2. ANALYTICAL METHODS A field sample of fresh unweathered material was collected from each locality; 109. of the sample were crushed and treated with acetic acid of pH5, buffered by sodium acetate solution, to remove any carbonates present. The sample was then dispersed by shaking with sodium hexametaphosphate for eight hours and treatment in an ultrasonic tank. The dispersed sample was then sedimented after the method of Mackenzie (1956) and the < 1·4 micron equivalent sedimentation diameter fraction was siphoned off. Oriented specimens for X-ray diffraction were prepared by placing a few drops of this suspension on a 24 X 24 mm. glass slide and allowing it to dry exposed to the air. The clays were examined in the Mg++ saturated condition. Four slides of each sample were prepared and X-rayed: (1) air dried; (2) treated with ethylene glycol, by placing the specimen in ethylene glycol vapour at 70° C. for 4 hours and allowing it to equilibriate in a desiccator for 12 hours before X-raying; (3) heated to 3350 C. for 4 hours; (4) heated to 5500 C. for 4 hours. The various pre-treatments are necessary for the identification of the clay mineral groups present. The behaviour of the lattice spacings under different conditions is a diagnostic feature of the mineral groups. After treatments 2, 3 and 4 the specimens were kept in a desiccator to prevent rehydration. X-Ray Routine A Phillips X-ray Diffractometer with nickel filtered CuKa radiation at
432
M. R. HENSON
46 kV and 18 rnA was used throughout, after comparative trials between MoKa and CuKa radiation revealed that iron fluorescence with the CuKa radiation was negligible. The specimens were scanned at 1 28 per minute to 30 Counting was by Geiger Muller tube and the counts from 3 were recorded on a Leeds-Northrup Chart Recorder, with a chart speed of 25 mm. per minute. It was assumed that counter deadtime did not appreciably affect the results. 0
028
028.
3. IDENTIFICATION AND CHARACTERISTICS OF THE CLAY MINERALS Details of the X-ray characteristics used in identifying the mineral groups present are given in Brown (1961) and Warshaw & Roy (1962). The characteristics of the mineral groups identified in this study are summarised in Table 1. Illite was present in all the specimens examined, characterised by spacings of 9,9-10,07 A (002), 4'9-4·99 A (004) and 3-3-3-34 A (006). In most cases the peaks were sharp and narrow; however, several specimens from the Exmouth Sandstones and Mudstones showed a broadening of the base of the peak to 11'5 A; the apex of the peak remained at 9·9 A. The broadening collapsed on heating to the normal form of the illite peak and was not affected by glycolation. This behaviour is thought to indicate the presence of a mixed-layer illite. In all specimens heating resulted in sharpened and enhanced peaks. Kaolinite was present in most of the specimens, characterised by a basal reflection of7'07-7'17 A (001), and a reflection of3'5 A (021). The kaolinite 7·1 A (001) peak is indistinguishable from the 7·07 A (002) chlorite peak. On heating to 550 C. kaolinite is rendered amorphous and all the characteristic peaks disappear. In all the samples examined kaolinite was more abundant than chlorite and the 7·1 A peak was usually attributed to the presence of kaolinite. The presence of chlorite in a sample was indicated by peaks at 14·1 A and 4'7 A. Chlorite was present in many of the specimens, characterised by spacings of 14,1-14'2 A (001), 7·07 A (002), 4·72 A (003) and 3·54 A (004). These spacings and the form of the peaks were generally maintained in all the treatments, although there was a marked tendency for the (002), (003), and (004) reflections to show a decreased intensity after heating to 5500 C. An increase in the intensity of the 14 A (001) peak after heating to 550 C. was frequently observed, indicating that the chlorites are probably of the iron-rich types. This is also confirmed by the decrease in the (001) spacing to 13'8 A at 5500 ; Fe-chlorites typically show a greater shrinkage parallel to c than do Mg-chlorites. A number of specimens showed an expansion of the 14·2 A (001) spacing 0
0
all un affected
loA 4'9 A 3-3 A
14A 7 A,4'7 A 3'5 A
14A
14A
14 A
Illi te
Chlori te
Swelling chlorite
Vermiculite
Montomorillon ite
expands to 17·5 A
unaffected
shifts to c. 16-17 A
all unaffe cted
unaffected
Glycolated
7A 3'5 A
Air-dry ba sal spacing
all frequently sharpened
14 A peak enhanced, remainder diminished collapses to 1308 A collapses to c. l o A
all unaffected
all unaffected
unaffected collapses toe. loA
collapses to e. loA
both peaks vanish
unaffected
collapses to e. 10 A
Heated 4 hr at 5500 C.
Heated 4 hr at 3350 C .
i
Notes
pronounced swellin g behaviour on glyco lation
no swelling on glyeolation
distinguishable from chlorite on its swelling behaviour
14 A peak enhanced and collapsed to 13'8 A on heating to 5500 C., indicative of Fe-chlorites that show shrinkage parallel to c
mixed-la yer illite in the Exmouth Sandstones and Mudstones showed an expanded (001) basal spac ing that collapsed on heating to 3350 C.
7 A peak stronger than 3·5 A peak, rendered amorphous on heating to 550 0 C.
Effects of the var ious pre-treatments on the basal spacings
I. Characteristics used for the identification of the clay minerals in the samples studied
Kao linite
M ineral
TABLE
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w
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m
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o
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434
M. R. HENSON
to 16-17 A on glycolation ; on heating to 550 C. the peak collapsed to 13·6-13·8 A, indicating that the swelling was not attributable to montmorillonite or vermiculite. A 24 A peak was not observed, indicating that it was neither a mixed-layer chlorite-montmorillonite nor a chloritevermiculite. The other peaks characteristic of chlorite were present . Such behaviour is characteristic of swelling chlorite. In all the specimens containing swelling chlorite the 14'1 A peak remained, indicating that both normal chlorite and swelling chlorite were present. Montmorillonite occurred in a few specimens, characterised by a basal reflection at 14·2 A (001) that expands on treatment with ethylene glycol to 17-7 A in the Mg++ saturated condition. On heating to 3350 C. the spacing collapsed to 10 A; at 5500 C. it further collapsed to 9-8 A, where it was indistinguishable from the illite (001) peak. Other clay minerals were not detected and quartz was not found in the fraction ( < 1'4p) examined. In a preliminary study haematite was identified in red clays but not in green and grey clays. 0
4. SEMI-QUANTITATIVE ANALYSIS Specified peak areas were compared directly and in ratio to contrast the proportions of the minerals present. Peak areas were measured in square centimetres but it is an arbitrary scale; the actual areas are governed by the instrument settings which were maintained constant throughout this study. The recalculation of the peak areas to percentages for each specimen was not attempted for two main reasons. Firstly clay minerals within geological environments will be imperfectly developed and many interstratified, therefore quantification based upon standard minerals would be unjustified (Brindley, 1961). Secondly no really satisfactory method of calculating the amounts of clay minerals in sediments exists and until methods are standardised peak areas should be used, so, if necessary, comparison of data could be made after suitable recalculation (Pierce & Siegel, 1969). In this study the actual percentage of any mineral present in a sample is not crucial, it is the assemblage that is important. Thus the rough method used to assess the relative proportions of minerals present in a sample is considered adequate. The proportion of illite present was measured by the area of the 10 A peak; the proportion of kaolinite was indicated by the area of the 7·1 A kaolinite and chlorite peak and that of chlorite by the 14'2 A peak. The area of the 17'5 A peak indicated the proportion of montmorillonite present and the 16-18 A peak the proportion of swelling chlorite. The relationship of kaolinite to illite is shown by the ratio 7: 10 A = kaolinite and chlorite:iIlite (ratio 1); and that of chlorite to kaolinite by 4-7:3'5 A = chlorite: kaolinite (ratio 2).
NEW RED SANDSTONE OF SOUTH DEVON
415
5. DISTRIBUTION OF THE CLAY MINERAL GROUPS The distribution of the clay minerals within the succession is summarised in Fig. 2. Illite is present in all the specimens analysed and except in the Budleigh Salterton Pebble Beds is always the most abundant mineral. The proportion is fairly uniform throughout the succession; it is, however, more abundant within the Exmouth Sandstones and Mudstones and Littleham Mudstones, and less abundant in the breccia formations. Significantly greater amounts of illite occur in the samples taken from the sand and green silty sand units of the Exmouth Sandstones and Mudstones than in the surrounding mudstones. Mixed-layer illite is restricted to the Exmouth Sandstones and Mudstones and Littleham Mudstones. Kaolinite is present in many of the specimens; it is less abundant within the breccia formations than the other units. The amount of kaolinite is generally one-fifth that of illite. Within the Exmouth Sandstones and Mudstones and Littleham Mudstones the proportion of kaolinite is relatively constant; but in the Budleigh Salterton Pebble Beds the amount of kaolinite drastically increases so that it is predominant-a very marked feature. In the Otter Sandstones the amount of kaolinite returns to near its previous level. Chlorite is present in many samples; it generally occurs more frequently within the Exmouth Sandstones and Mudstones and Littleham Mudstones. Within the Teignmouth Breccias, Dawlish Sandstones and Exe Breccias, chlorite, where present, is often in a greater amount than in the other formations. Swelling chlorite is restricted almost entirely to the Exmouth Sandstones and Mudstones and Littleham Mudstones; but is also found in the Dawlish Sandstones. Montmorillonite is the least abundant of the clay minerals found in the succession; it occurs in significant quantities mainly in the Teignmouth Breccias, the Exe Breccias and the Dawlish Sandstones. Above these formations traces occur in the sandstones of the Orcombe Point and Straight Point in the Exmouth Sandstones and Mudstones, and in the Otter Sandstones. When the ratio of kaolinite and chlorite to illite is plotted against the ratio ofchlorite to kaolinite, a distinct separation ofthe specimens into two groups is obtained (Fig. 1). The silts, mudstones and sands of the Exmouth and Littleham Formations and the Otter Sandstones form a group relatively poorer in kaolinite and richer in chlorite. The few results from the Teignmouth Breccias, Dawlish Sandstones and Exe Breccias show a relatively greater proportion of kaolinite but a similar chlorite content. The groups thus effectively separate the clay mineral assemblages of the marginal deposits from the fluvial basin deposits. Two specimens from the major sandstone bodies of Orcombe Point and Straight Point in the Exmouth PROC. GEOL. ASS., VOL. 84, PART 4, 1973
29
M.R.HENSON
436
illite 8 7 35 29 26 2S
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lltholoJ)' red mud.tone redaand redaand red mud stone sand matrix sand bed sand matrix red mud .tone red mudstone red mudstone green sand ston e red mudstone green mud ston e red mud stone red land red muds tone sandy mudsto ne red muds tone red muds tone red muds ton e sreenliltyund red mudst one red mud stone red mudstollC red mudstone re4 sandy lilt .....n.Uty_cl
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Fig. 2. Distribution of Clay Minerals within the succession. Sample numbers are indicated on the columnar section in Fig. 1
NEW RED SANDSTONE OF SOUTH DEVON
437
Sandstones and Mudstones fall into the field of the marginal deposits. These sand units contain material derived from the breccias; sandstone and chert breccia clasts are present in the Straight Point sand, and reworked Carboniferous miospores were obtained from the Orcombe Point sand. It may therefore be concluded that these sand units should have some affinitieswith the breccia formations.
KEY
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+
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•
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6. PERMIAN AND TRIASSIC CLAY MINERAL ASSEMBLAGES The investigations of the following authors are cited to show the distribution of the clay mineral groups in the Permo-Triassic rocks of northwest Europe. Dr. G. Warrington (personal communication) examined the clay mineral assemblages of samples taken from the Bunter, the Keuper and the Rhaetic of the Midlands. Dlite and some kaolinite were found in the Bunter samples and illite (with accessory chlorite and kaolinite) was dominant in the lower part of the Keuper Sandstone. In samples from the
438
M.R.HENSON
upper part of the Keuper Sandstone, the Waterstones and the Keuper Marl, illite was predominant though montmorillonite, chlorite and kaolinite also occurred. Chlorite was, however, absent from the Arden Sandstone and montmorillonite was absent from the Tea Green Marl. Sepiolite was recorded from the Keuper Marl both above and below the Arden Sandstone. Material from the basal Rhaetic yielded only illite. Dumbleton & West (1966) investigated the clay mineralogy of samples taken from the Keuper Marl. Illite (30-60 per cent) and chlorite were present in all nineteen samples analysed and swelling chlorite in all but two (chlorite-l-swelling chlorite up to 40 per cent). Sepiolite and palygorskite were present in some samples. Davis (1967) determined the clay mineralogy of fifty samples scattered over the outcrop of the Keuper Marl of the Midlands; forty samples showed illite, corrensite and sometimes a trace of chlorite. Five samples contained illite only; these samples were mainly from the top of the Keuper Marl where it grades into the Tea Green Marl. Four samples from Solihull, Warwickshire, showed sepiolite, illite and swelling chlorite; the sepiolite was restricted to an 18·35 m. vertical section. Lucas & Bronner (1961) studied the clay mineral assemblages in boreholes from the Triassic of the Jura. Illite was present with chlorite throughout the basin, though in variable proportions. Kaolinite was present only as a trace in the greater part ofthe succession; within the basal conglomerate, however, it was abundant. Corrensite was present only in the central portions of the basin and it disappeared at the top of the Upper Keuper. Interstratified minerals occurred around the margins of the basin and in the upper and lower parts of the succession; it was apparent that they were derived from degraded illites and that they were altered to corrensite within the basin. Lucas (1962) investigated the clay mineralogy of the Triassic rocks in the Paris Basin and Lorraine. The clays were essentially illite and a 14 A interstratified montmorillonite--chlorite trending towards corrensite; true chlorite was rare. At the base of the succession illite was dominant with kaolinite and degraded chlorite; moving upwards the proportion of illite decreased while that of the 14 A complex increased. From the middle of the Upper Keuper the illite content rapidly increased until it became completely dominant or present with a small proportion of 14 A minerals not found in the middle part of the succession. Successions in Morocco, the Pyrenees and Spain were also examined and showed similar assemblages to these recorded in the Paris and Jura Basins. In all the investigations cited the minerals present are nearly always the same and they show a similar distribution in space and time. Within all the basins s1 udied the basal and marginal areas show assemblages dominated by illite accompanied by kaolinite, chlorite and interstratified minerals.
NEW RED SANDSTONE OF SOUTH DEVON
439
Higher in the successions and towards the centres of the basins illite is replaced by 14 A interstratified minerals--chlorite-montmorillonite, swelling chlorite, corrensite, sepiolite and palygorskite; true chlorite is never characteristic. Towards the top of the succession illite regains its dominance and is associated with chlorite. The minerals from the centre of the basin are less variable than those from the margins. Krumm (1969)in a comparison of the clay minerals of European Triassic sediments with their sedimentary position within the depositional basin similarly found a zonation of the clay minerals in a sequence identical to that detailed above. The zonation is attributed to physico-chemical differences in the sedimentary environments. The mineral assemblages obtained during the examination of the Permian and Trias of South Devon are comparable to those obtained from the Keuper Sandstone of the Midlands (Warrington, personal communication) and those from the basal and marginal areas of the Jura and Paris Basins (Lucas & Bronner, 1961; Lucas, 1962). However, they are unlike those obtained from the Keuper Marl. It is apparent that the assemblages in this study are typical of the marginal and basal regions of a basin; while the assemblages of the Keuper Marl are typical of the central basin area. If, as seems to be the case, the Upper (Keuper) Marls of Devon bear the same relationship to the fluvial basin deposits as the Keuper Marl does to the Keuper Sandstone of the Midlands, then the Upper (Keuper) Marls will show the same mineral assemblages as the Keuper Marl of the Midlands and the Keuper of France and Germany.
7. DEPOSITIONAL ENVIRONMENTS OF THE SUCCESSION (a) Clay minerals There is a controversy over the interpretation of geological environments from their clay mineral assemblages; it is based upon arguments as to whether clay mineral assemblages are detrital, that is inherited from the source area, or diagenetic in origin. Detrital assemblages can show no relationship to their environment of deposition but reflect the weathering regime in the source area. Slight facies differences are caused by varying properties of susceptibility to transport, settling velocity and flocculation; the lattice structure is not affected by the geochemical environment (Weaver, 1958). Grim (1958), Keller (1956, 1970) and Millot (1949, 1951, 1964) consider that clay minerals are the reactive response to the geochemical environment of the material in the environment. Source rocks react to their environment by weathering to the various clay minerals; these minerals will then react to any new environment into which they might be transported. This reaction may be substantial or non-existent
440
M. R. HENSON
depending on the difference between the geochemical environments. If this latter view is accepted, then clay minerals can be used as indicators of the geochemical and hence the sedimentary environments of which they are characteristic; and conversely the environment can be used to predict the clay minerals present. Millot (1949, 1951, 1964) showed that the major sedimentary facies had characteristic clay mineral assemblages, both in the proportion and the type of clay minerals present. The structure and chemistry of clay minerals coupled with their relatively high surface area make it likely that clay minerals will react with their environment if they are not in equilibrium with the environment. Cation substitution in the tetrahedral and octahedral sites and cation exchange in the interlayer positions are the most easily effected changes. Complete recrystallisation is possible and solution will occur if the minerals are sufficiently out of equilibrium with the environment. The pH, Eh and the concentration of reacting ions are the more important factors in the geochemical environment controlling any changes that may occur. The greater number of clay minerals are first formed by the weathering of a parent silicate; the environments most favourable to the formation of specific minerals, and hence to their preservation, have been investigated over a considerable period. Keller (1970) summarises the conditions most favourable to the production of the primary clay minerals. The formation of kaolinite is favoured by the removal of Ca++, Mg" ", Na -, K+ and Fe++ from the environment with the addition of H+. Precipitation must exceed evaporation giving strong leaching to remove the requisite cations; H + ions are obtained from the fresh water or waters rich in organic and carbonic acids and sulphur compounds. The AI+++ concentration must be high and the Al/Si ratio must also be high, thus necessitating the removal of Si02. Montmorillonite (smectite) requires the retention of Mg++, Ca ++, Fe ++ and Na r with H.SiO.; therefore evaporation must exceed precipitation and leaching must be ineffective. Such conditions are present in the stagnant waters of lakes, marine basins or semi-arid areas. Primary illite forms from the weathering of alumino-sil icates where the ionic concentrat ions of K + and H+ are high relative to the concentration of H4Si04. Illites are clay size mica minerals, so degradation by weathering of layer silicates will result in the production of illites. The stripping of K + ions from illites results in degraded illites; these minerals are more reactive than the normal ionically balanced illites and readily absorb K + from the environment to remedy the deficiency. Simple disaggregation of preexisting argillaceous rocks will yield abundant illite as most shales are composed of ilIitic clay that is reworked with little change except for K + stripping. A considerable amount of chlorite is similarly produced from the disaggregation of slates and phyllites. In areas of poor leaching with the retention of K+ and Mg++ it is possible to have chlorite formin g with
NEW RED SANDSTONE OF SOUTH DEVON
441
montmorillonite but the majority of chlorite is derived from pre-existing rocks. The parent material will have an overriding control of the minerals produced during weathering; if the requisite ions are not present then the characteristic mineral cannot be produced by the weathering conditions. Authigenic minerals will crystallise from solutions approximating to the ideal structural formulae in composition and in sufficient concentration when the pH and Eh conditions are optimum. Illite, kaolinite, montmorillonite, Al-chlorites, sepiolite and palygorskite have all been found as authigenic minerals in sedimentary successions. The assemblages obtained from the Teignmouth Breccias, Dawlish Sandstones and Exe Breccias, illite with kaolinite and chlorite and a little montmorillonite, are similar to those obtained from piedmont complexes (Millot, 1964). The minerals are entirely detrital in origin, and, except for kaolinite, produced from the disaggregation of the source rocks, they show no evidence of alteration. Deposition in this case is likely to be in an actively transporting situation with little concentration of bases in ephemerallakes and ponds, typically sheet floods, stream floods and high braided distributary systems such as are present in alluvial fans. The mixing of illite and kaolinite, minerals formed in contrasting weathering regimes, suggests that the source area underwent a seasonal climatic change; a hot dry season would be followed by a tropical rainy season. The assemblages are very similar to those described by Millot, Perriaux and Lucas (1961) from the Permo-Triassic red-bed sequence of the Vosges. Montmorillonite could well be the weathering product of clasts of Devonian tuff contained in the breccias. The assemblages of the Exmouth Sandstones and Mudstones and Littleham Mudstones (illite, kaolinite, chlorite and swelling chlorite) are similar to those obtained from fluvio-Iacustrine environments (Millot, 1964).The illite, kaolinite and chlorite are probably predominantly detrital, indicating that they have the same type of source area as the marginal deposits. The kaolinite and chlorite: illite and chlorite: kaolinite ratios indicate that the fluvial basin deposits are lower in kaolinite and higher in chlorite than the marginal deposits. This feature can be explained by a loss of kaolinite by solution and an increase in chlorite by alteration, indicating that Mg++,Fe++ and K + were being concentrated in the environment. The transformation of illite to a mixed-layer illite similarly shows that base concentration was significant. Swelling chlorite, an important accessory mineral, also indicates a concentration of bases, especially Mg++, in the environment. Ca ++ concentration is shown by the calcareous nature of the mudstones; euhedral calcite crystals are frequently present, and small crystals of dolomite and possibly gypsum are also found. From the foregoing evidence it appears that the environment was approaching supersaline conditions; which were most likely to occur during the evapora-
442
M. R. HENSON
tion of ephemeral lake areas, the depositional environment of the overbank silts and clays that comprise the mudstones. Neither palygorskite nor sepiolite was formed, indicating that extremes of supersaturation were not reached. The similarity of the kaolinite and chlorite: illite and chlorite: kaolinite ratios of the major sand beds to those of the marginal deposits shows that the channels were transporting material derived from the source area and possibly the outcrop of the marginal deposits. Illite is significantly higher in the silty sand beds and thick sandstone units intercalated with the mudstone sequences; this would seem to suggest a link between the silty sands and the channel sands, the poorly sorted silty sands being derived from material transported in the channels, hence they are the coarse overbank deposits. The clay mineral assemblage of the Budleigh Salterton Pebble Beds accords with assemblages of fluvial deposits but differs dramatically from those of the other formations. The kaolinite and chlorite:illite ratio is very much higher than in any other formation. This great increase in kaolinite is probably mainly due to leaching by pore waters in the recent weathering environment. Feldspar grains in the matrix and the scattered porphyry clasts are extensively altered to kaolinite. Interpretation of the original depositional environment usingclay minerals in these circumstances would be unrealistic. The Otter Sandstones maintain the high kaolinite content of the Pebble Beds and show a similar illite content. As in the Pebble Beds the effect of recent leaching producing kaolinite overrides the original Triassic assemblage. (b) Sedimentary Structures
The sedimentary features shown by the succession were described and discussed by Laming (1966) and Henson (1970, 1971). In these studies it was concluded that the Teignmouth and Exe breccias and the Dawlish Sandstones were deposited by stream floods and sheet floods as alluvial fans that coalesced into a piedmont fan adjacent to a mountain front. The Exmouth Sandstones and Mudstones and Littleham Mudstones were deposited as a flood-plain complex, the thick sand units being channel sands, the thin sand and green silty sand beds being the coarse overbank deposits, crevasse splays and levees, and the mudstones being the flood basin deposits. Coarse fining-upwards sequences characteristic of stream flood deposits are clearly discernible in the cobble and boulder conglomerate of the Budleigh Salterton Pebble Beds, suggesting deposition in an alluvial fan. This very marked change in the style of Triassic sedimentation is most easily explained as the result of a climatic change in both the source and depositional areas and a change in major source area. Overlying the Pebble Beds the Otter Sandstones show fining-upwards cycles typical of
high-sinuosity, low-braided river ; act ive meander belt PIEDMONT FAN COMPLEX mid-fan deposits
mudstones with silty sand beds
mudstones with thick cross-stratified sand units and silty sand beds
sandstone and porphyry breccias
cross-stratified aeolian and fluvial sands with breccia lenses
sandstone and porphyry brecc ias
Littl eham Mudstones
Exmouth Sandstones and Mudstones
Exe Breccias
Dawlish Sandstones
Teignmouth Breccias
mid-fan deposits
fan-base deposits
high-sinuosity, low-braided river; channels distant
stream flood deposited alluvial fan
cobble and boulder conglomerate
Budleigh Salterton Pebble Beds
high-braided, lowsinuosity streams
Depositional environment
cross-stratified sands in fining upward cycles
Lithology
illite , kaolinite and chlorite with montmorillonite
} ;WI<. kaolinite and chlorite with swelling-ehlorite
kaolinite a nd illite
illite and kaolinite with chlorite
Clay minerals
II. The clay mineral assemblages and their corresponding sedimentary environments
Otter Sandstones
Formation
TABLE
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z
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til
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tIl
Z
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til
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Z
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til
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tIl
~
=E
Z tIl
444
M. R. HENSON
braided streams, reflecting the change in sedimentation as the source area retreated and the underlying fan aggraded.
8. CONCLUSIONS The distribution of the clay mineral groups and assemblages demonstrate that a piedmont complex and a fluvio-Iacustrine complex were the depositional environments for the lower part of the succession, confirming the conclusions of the sedimentological studies. They did not indicate the depositional environment of the Budleigh Salterton Pebble Beds or the Otter Sandstones because of the effects of recent weathering rendering any environmental interpretations unrealistic. The similarity of the clay mineral assemblages in the succession to those of the Keuper Sandstone of the Midlands and the marginal areas of the Permo-Triassic basins of Europe suggests that they were formed in similar environments under the same general climatic controls. Also the similarity of the clay mineral assemblages sequences through the Keuper to the Rhaetic show the interrelation of the Permo-Triassic successions throughout north-west Europe, and the control of clay mineral assemblages by the depositional sedimentary environment.
ACKNOWLEDGMENTS This work was carried out as part of the NERC-Institute of Geological Sciences contract with the Department of Geology, University of Exeter, for the re-mapping of Sheet 339 (Teignmouth), My grateful thanks are due to Professor S. Simpson, Mr. G. Bisson, Dr. E. R. Shephard-Thorn and Mr. B. Young for critically reading the manuscript; Dr. G. Brown and Mr. D. Dallow for helpful advice on X-ray diffraction techniques; and Dr. G. Warrington for permission to precis his unpublished results.
REFERENCES BRINDLEY, G. W. 1961. Quantitative Analysis of Clay Mixtures. In Brown, G. (Ed.), The X-ray Identification and Crystal Structures 0/ Clay Minerals, 489-514. BROWN, G. (Ed.), 1961. The X-ray Identification and Crystal Structures 0/ Clay Minerals. 2nd ed, Mineralogical Society, London. DAVIS, A. G. 1967. The Mineralogy and Phase Equilibrium of Keuper Marl. Quart. Jl Eng. Geol., 1, 25-38. DUMBLETON, M. J. & G. WEST. 1966. Studies of the Keuper Marl: Mineralogy. Min. ofTransport R.R.L., Report 40. GRIM, R. E. 1958. Concept of Diagenesis in Argillaceous Sediments. Bull. Am. Ass. Petrol. Geol., 42, 246-53. HENSON, M. R. 1970. The Triassic Rocks of South Devon. Proc. Ussher Soc., 2, 172-7. - - - . 1971. The Permo-Triassic Rocks 0/ South Devon. Ph.D. Thesis, University of Exeter.
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KELLER, W. D. 1956. Clay Minerals as Influenced by Environments of their Formation. Bull. Am. Ass. Petrol. Geol., 40, 2689-710. - - - . 1970. Environmental Aspects of Clay Minerals. J. sedim. Petrol., 40, 788810. KRUMM, H. 1969. A Scheme of Clay Mineral Stability in Sediments Based upon Clay Mineral Distribution in Triassic Sediments of Europe. Int. Clay Conf., Tokyo, 1969,313-24. LAMING, D. J. C. 1966. Imbrications, Palaeocurrents and other Sedimentary Features in the Lower New Red Sandstone, Devonshire, England. J. sedim. Petrol., 36,940-59. LUCAS, J. 1962. La Transformation des Mineraux Argileux dans la Sedimentation. Etudes sur les Argiles du Trias. Mem. Servo Carte geol. Als. Lorr., 23. & A. M. BRONNER. 1961. Evolution des Argiles Sedimentaires dans le Bassin Triasique du Jura Francais, Bull. Servo Carte geol, Als. Lorr. 4 (4), 137-48. MACKENZIE, R. C. 1956. Methods for Separation of Soil Clays in Use at the Macaulay Institute for Soil Research. Clay Miner. Bull., 3 (4), 4-6. MILLOT, G. 1949. Relations entre la Constitution et la Genese des Roches Sedimentaires Argileuse. These Sci. Nancy and Geol, Appl, Prospec. Min., 2, nos. 2, 3,4,1-352. - - - . 1951. The Principal Sedimentary Facies and their Characteristic Clays. Clay Miner. Bull.,t (7), 235-7. - - - . 1964. Geologie des Argiles. Paris. - - - , J. PERRIAUX & J. LUCAS. 1961. Signification Clirnatique de la Couleur des Gres Permo-Triassiques et des Grandes Series Detritiques Rouges. Bull. Serv, Carte geol. Als, Lorr., 4 (4), 91-100. PIERCE, J. W. & F. R. SIEGEL. 1969. Quantification in Clay Mineral Studies of Sediments and Sedimentary Rocks. J. sedim. Petrol., 39, 187-93. WARSHAW, C. M. & R. Roy. 1962. Classification and a Scheme for the Identification of Layer Silicates. Bull. geol. Soc. Am., 72, 1455-92. WEAVER, C. E. 1958. Geologic Interpretation of Argillaceous Sediments. Part I. Origin and Significance of Clay Minerals in Sedimentary Rocks. Bull. Am. Ass. Petrol. Geol., 42, 254-71.