Geochemical and Mineralogical Variations in The

Journal of the Geological Society, London, Vol. 150, 1993, pp. 67-75, 7 figs. Printed in Northern Ireland

Geochemical and mineralogical variations in the upper Mercia Mudstone Group (Late Triassic), southwest Britain: correlation of outcrop sequences with borehole geophysical logs

A. B. LESLIE l'3,B. SPIRO 2&M.E. TUCKER l 1Department of Geological Sciences, Durham University, South Road, Durham DH1 3LA, UK :NERC Isotope Geoscience Laboratories, British Geological Survey, Nicker Hill, Keyworth, Nottingham NG12 5GG, UK 3Present address: School of Environmental Sciences, University of East Anglia, Norwich NR4 7TJ, UK

Abstract: The Mercia Mudstone Group comprises up to 1200m of predominantly red mudstones and siltstones laid down in rift-related basins during a period of regional subsidence. The mudstones are commonly dolomitic and contain horizons of sulphate and halide salts. Up to 150 m of undifferentiated red mudstones of Norian and Rhaetian age (Late Triassic) were examined in coastal outcrops in Devon and Somerset, southwest Britain, and mineralogical and stable isotopic studies were carried out in order to identify any subtle changes in the succession. Gamma-ray measurements were taken at outcrop to provide correlation with published borehole logs in which the Mercia Mudstone Group has been subdivided on the basis of gamma-ray and sonic response. One of the borehole subdivisions was identified at outcrop within a sequence of otherwise undifferentiated red mudstones and was related to a transition in clay mineral assemblage and stable isotopic composition over 20 to 30 m of section. The red mudstones below this transition have a higher proportion of magnesian clay minerals and enriched carbonate oxygen isotope compositions, indicating deposition from Mg2+-rich marine-derived waters. Clay mineral assemblages in the succeeding red mudstones are dominated by illite, and oxygen isotopic compositions are relatively depleted, indicating a greater influence of K÷-rich continental-derived waters.

The Mercia Mudstone Group (Warrington et al. 1980) consists of up to 1200m of red and green mudstones and siltstones Palaeozoic basement which accumulated in a series of graben and half-graben (Whittaker 1975). In southwest Britain these flanked the major faults southern and eastern sides of the Welsh uplands and extended south and east into Devon and Somerset (Fig. 1). Inland, exposure is poor but good coastal outcrops permit the examination of most of the upper Mercia Mudstone succession (Fig. 2). N During the late Triassic there was negligible extension and fine-grained sediments were laid down in passively sub- siding basins (Chadwick 1985; Holloway 1985). The Mercia Mudstone Group in the study area is Ladinian to Rhaetian in age (Warrington et al. 1980) and reaches a maximum thickness of 800 m in the central Somerset basin. The exposures of interest, on the Devon and Somerset coasts, comprise beds in the uppermost 170 m of the Group which are Norian and Rhaetian in age. Lack of continuous exposure made sampling of underlying mudstones not possible. The exposures studied are marginal to the thicker basinal deposits of the Inner Bristol Channel, Somerset, and Devon/Dorset basins (Fig. 1). The deposition environment in which the mudstones formed Fig. 1. Location map showing the position of the two cliff sections has been the subject of discussion and several interpretations. samples in this study with respect to the major Upper Triassic Deposition of the mudstones in a hypersaline sea with basins in southwest Britain. restricted circulation of marine waters was proposed by Sherlock (1928), Evans et al. (1968), Warrington (1970) and Jeans (1978); whereas deposition in a playa-alluvial The transport of mud as sand-sized pedogenic aggregates in environment was favoured by Audley-Charles (1970) and fluvial systems has been proposed as one possible process Tucker (1977, 1978). Others have suggested that periods of forming extensive red mudstones (Nanson et al. 1988; Rust & extensive halite precipitation were the result of marine Nanson 1989). There is little evidence for the continual activity incursions into the basins (Wills 1970; Arthurton 1980; Taylor of fluvial systems in the Mercia Mudstone Group, and 1983; Holloway 1985). although such pedogenic processes may have provided some

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Subdivision of the Mercia Mudstone Group has been carried out in boreholes on the basis of variations in gamma-ray and Base / - Blue Anchor sonic log response (Lott et al. 1982; Penn 1987), and l formation correlations continued through several basins (Fig. 4). In particular, boundaries were identified within the mudstone units themselves which are not the result of obvious

m lithological changes. These are of most interest in this study 20-. . D 20- D since they could, in theory, be identified at outcrop using a ~D i portable gamma-ray spectrometer. ' 'O

Methods 40- ' " 40- Up to 150 m of red mudstones were examined from coastal outcrops in west Somerset and south Devon. Sections were logged for gamma-ray (m) (m) variations and samples for clay mineral and stable isotopic analyses were taken every one or two metres. 60- 60- Both areas provide relatively continuous coastal outcrop of the upper 100 m of the Mercia Mudstone Group. In Devon, the majority of samples were collected from Haven and Seaton Cliffs (SY275895- SY236893). Some samples were also taken from the coast between Coxe's Cliff (SY185881) and Berry Cliff (SY198882) to the west of 80- 80- Branscombe. The gap in vertical section between the Seaton and Branscombe sections was estimated by Jeans (1978) and Warrington & 85 metres Scrivener (1980) as approximately 55 m, based upon borehole evidence Base of Haven/Seaton from Lyme Regis 16 km to the east. In Somerset, a continuous section Cliffs Section was sampled from St Audrie's Bay (ST104443-ST119437). In the lower 100I mudstones in St Audrie's Bay there is minor faulting but this has not A B disrupted the continuity of the section.

Fig. 2. Sketch logs showing the distribution of green mudstone beds (black) within the red mudstones below the base of the Blue I OOl (IOA) Anchor Formation. D, massive dolomite-rich beds. (a) South ~ POOl (10.4A) Devon. (b) Somerset. / So0~l122A)/

fine-grained sediment to the basin (Wright et al. 1988; North 1989), it is unlikely to be a volumetrically important source. The relatively abundant (> 10%) silt-sized grains in the Mercia Mudstone Group suggest that there may have been a substantial aeolian input. Aeolian processes have been I suggested as a source of the carbonate within arid red bed environments (Allen 1986) and it is likely that significant /~ S-C (g)(14-15A) volumes of aeolian sediment were supplied to the Triassic s-c /1 cA,,,; ,/ basins (North 1989). • ~'~, i I

Correlation within the Mercia Mudstone Group The Mercia Mudstone Group in the study area comprises an uppermost Blue Anchor Formation, which is between 10 to 40m in thickness, and underlying undifferentiated red mudstones (Warrington et al. 1980). The red mudstones are generally unfossiliferous, poorly bedded and contain few ,1 sedimentary structures. Neither chrono- nor litho- stratigraphical correlation has been carried out in the mudstones on a regional scale, although local intrabasinal names have been introduced for sandstone and halite beds (Warrington et al. 1980). In the upper Mercia Mudstone Group prominent sandstone units have been identified in several basins (the Weston Mouth Sandstone Member in Devon, and ~. air-dried the Arden Sandstone member in Worcestershire, for example) glycolated and dated as late Carnian (Warrington & Williams 1984). The 550°C for 4hours overlying red mudstones, the subject of this study, have yielded no datable fossils but are assigned to the Norian and lower Fig. 3. X-ray diffraction peaks used in the identification of clay Rhaetian stages since they underlie the Blue Anchor mineral species. I, illite; C, chlorite; P, palygorskite; S, sepiolite; Formation which has yielded palynomorphs of Rhaetian age S-I, mixed-layer smectite-illite; S-C, mixed-layer smectite-chlorite. (Warrington & Whittaker 1984). (g), glycolated trace used in identification.

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SEABOROUGH BURTON ROW BRISTOL CHANNEL SOUTH DEVON SOMERSET 103/18-1

GR .-.- ~- BHCS

Formation i P°

lOOm r, D L DD 200m -

300m -

Fig. 4. Regional gamma-ray/sonic log 400m - correlation of the Mercia Mudstone Group in southwest Britain. PG, Penarth Group; SS, Sherwood 500m Sandstone Group. Mercia Mudstone Group units A to F taken from Lott et 21°3 o2 al. (1982). 1, taken from Lott et al. (1982); 2, taken from Penn (1987); 3, taken from British Geological Survey unpublished data. GR, gamma-ray intensity, increasing from left to right; BHCS, borehole compensated sonic log, increasing from right to left.

Gamma-ray logging was carried out using a Geometrics peak/background ratio was far superior to that achieved when the (G410A Spectrometer with a GPX 21 NaI (TI) detector. The sample was smeared onto a glass slide. ideal surface required for measurement (a planar surface Mudstone samples for stable isotopic analyses were crushed and perpendicular to bedding with a radius of 1 m) was rarely treated with 100% phosphoric acid at 55°C (Rosenbaum & Sheppard exposed naturally and sample spacing varied between 1 and 1986) for up to 24 hours, after the composition of the carbonate fraction 4m depending on outcrop suitability. Duplicate readings were was determined using XRD. Analyses were carried out using standard taken at every tenth station, and standard errors were under methods (Craig 1957). All samples were analysed in the British 10%. For the purpose of correlation the total gamma-ray count Geological Survey isotope geochemistry laboratories at Gray's Inn values were visually calibrated with standard API units in Road, London (now relocated at Keyworth, Nottingham). Results are order to achieve comparable logs. reported in %0 relative to the PDB standard. Reproducibility of the Analysis of clay mineralogy was carried out using the ~< 2 technique is +0.10%o for oxygen isotopes. /~m fraction of the mudstones. Samples were run after air- drying, glycolation and heating to 550°C; the peaks used in Sedimentology of the mudstones identification of the clay mineral species are shown in Fig. 3. For XRD analysis, a flocculent (CaCI2) was added to the The Mercia Mudstone Group underlying the Blue Anchor powdered samples in suspension which were then centrifuged directly Formation in the study area is lithologically homogeneous on to a glass slide. There has been some discussion regarding the and consists of mudstones and siltstones with few coarse sand validity of any quantitative XRD interpretation which involves the horizons. The mudstones comprise clay minerals, carbonates, preparation of samples by the settling of clay minerals through a water quartz, sulphates and iron oxides/sulphides. Few sedimentary column (McManus 1991). Gibbs (1965) and Towe (1974) both state structures are identifiable and bedding is defined by green- that smectites, which have smaller mean grain sizes than other clay coloured horizons which are the only recognizable mineral species, will settle more slowly and therefore be concentrated lithological change in the majority of the succession (Fig. 2). on the top surface of a sample, biasing the XRD analysis. Sandy horizons are uncommon and are at most 10cm in In this study, the addition of the flocculent (Taylor 1982) removes the thickness. Most are structureless although a few of the sand possibility of differential settling. If a sample was left to stand after layers show cross-lamination. addition of the flocculent, the entire clay fraction settled to the base of Nodular gypsum is abundant at outcrop, forming laterally the beaker (a vertical distance of 6 cm) in under three minutes, which is impersistent horizons. These are most common in the lower insufficient time for differential settling. It is very unlikely that during part of the Blue Anchor Formation and upper 30cm of red flocculation, clay minerals formed monomineralic flocs, or that such mudstone. The sulphate appears to have formed either as flocs would form which were proportional to the size of single clay surficial crusts or disseminated within the sediment (Curtis particles. In the centrifuge, the entire sample with flocculent settled in 1982; Leslie 1989), and has later been remobilized to form the under 30 seconds. There was no time for differential settling and the present nodular morphology. Gypsum whose formation can be

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SEABOROUGH BURTON ROW SOUTH DEVON SOUTH DEVON SOMERSET SOMERSET GR~ *- BHCS 3R~ GR--* *-- BHCS

GR-* Base Blue Anchor Base Blue Anchol formation Formation E . EI

100r D?I 100m Section D? of no exposure ...j

200~ 200m

300~ 300m

400r 400m

5001 (a) 5OOm (b)

Fig. 5. Correlation of outcrop gamma-ray measurements (on right hand side of ligure) with adjacent borehole logs from (a) the Devon/Dorset Basin and (b) the Somerset Basins. PG, Penarth Group; SS, Sherwood Sandstone Group. Mercia Mudstone Group units A to F taken from Lott et al. (1982). Seaborough borehole from Lott et al. (1982). Burton Row borehole from Penn (1987).

clearly attributed to pedogenesis is found in a number of Gamma-ray variations Mercia Mudstone outcrops (Wright et al. 1988; North 1989) but was not identified within the sections examined in this Use of gamma-ray studies as a tool for the examination of study. The nodular gypsum horizons may also be related to sediments at outcrop is becoming increasingly common. evaporation of localized, ephemeral pools. There is no Southworth (1987) used gamma-ray variations to correlate evidence for substantial halite beds or their solution parts of the southern North Sea Triassic with Mercia Mudstone residues; casts of simple cubes on bedding planes are the outcrops in quarries in Nottinghamshire. Other studies only evidence for the original presence of halite in the investigating the Mercia Mudstone Group have used only mudstones examined. borehole data (Lott et al. 1982; Penn 1987). A number of prominent green dolomitic horizons, which are The Triassic has been drilled extensively in the Devon/ most common in the Somerset section, occur in the upper 30 m Dorset Basin (Lott et al. 1982) and also in the Somerset, Bristol of red mudstones underlying the Blue Anchor Formation Channel and Celtic Sea Basins (Penn 1987). The Mercia (Fig. 2). These beds are between 0.2 and 0.9 m in thickness and Mudstone Group in these areas has been subdivided into six contain between 60 and 80"/,, carbonate. The dolomites are units on the basis of gamma-ray and sonic variations in laminated and contain asymmetric ripples which give a borehole geophysical logs. The six subdivisions of the Group palaeocurrent direction from the south-southwest. The top (Fig. 4) have been recognized in all four basins; except in the surface of each dolomitic bed is characterized by polygons up case of massive saliferous horizons of Carnian age (within unit to 0.2 m in diameter. The centre of each polygon is raised as C), and the coarse-grained Weston Mouth Sandstone Member, much as 5cm above the margins. The raised polygons and the changes in the geophysical parameters below the Blue laminated character suggest microbial influence during Anchor Formation have been attributed to subtle variations in deposition. Algal fragments have been identified in these grain size. dolomitic beds using UV microscopy (M. Talbot, pers. comm. At outcrop, the Blue Anchor Formation (unit F) is clearly 1989). distinguishable from the underlying red mudstones. The Pedogenesis has been cited as one of the most important boundary at the base of the Blue Anchor Formation was used processes in the sediments (Wright et al. 1988; North 1989). as a datum since it is recognized both at outcrop and in Obvious pedogenic structures such as those described by North borehole logs. Measurement of outcrops in Somerset and (1989) were not observed within the upper Mercia Mudstone Devon have proved successful in correlating the variation in successions studied in Devon and Somerset. Microscopic gamma-ray values with the lithological change at the base of structures such as coated grains were observed in some the Blue Anchor Formation (Fig. 5). The lower gamma-ray/ examples but it is unlikely that pedogenic processes were active sonic boundaries identified in boreholes within the Mercia throughout deposition of the upper part of the Mercia Mudstone Group are defined in terms of changes in grain size of Mudstone Group examined. the clastics and the presence of saliferous beds (Lott et al.

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1982), and to date no correlations of these boundaries with the comparable with those in the upper red mudstones, with < 10% equivalent outcropping mudstones have been carried out. mixed-layer clays. The successions studied in Devon and Somerset are In Devon, the transition between clay assemblages takes lithologically homogeneous below the Blue Anchor place between 45 and 65 m below the Blue Anchor Formation Formation. In Devon the base of the Mercia Mudstone Group (Fig. 6a). In Somerset, the transition is 50-70m below the sequence studied was a gypsum-rich mudstone containing some green mudstones and dolomites (Fig. 6b). The change is carbonate, although in Somerset there is no change in transitional but subdivides the upper Mercia Mudstone Group lithology throughout the 150m succession of red mudstones. into two units with distinct clay mineral assemblages. In boreholes from Devon, Dorset, Somerset and the Inner Bristol Channel Basin, the boundary between units E and D in the mudstones is placed between 50 and 140 m below the base Stable isotopes of the Blue Anchor Formation (Lott et al. 1982; Penn 1987). If the thicknesses of the upper Mercia Mudstone Group are Stable isotopic measurements of the carbonate fraction of the relatively consistent, then this boundary would be expected to Mercia Mudstone Group have been undertaken in few studies be present in the outcrops examined on the Devon and (Taylor 1983). The samples examined here contain both calcite Somerset coasts. and dolomite in contrast to the mudstones examined by Taylor Few sharp changes in gamma-ray intensity were observed (1983). It is probable that a proportion of the carbonate was in the red mudstones examined (Fig. 5), but there are gradual supplied to the basin by aeolian proceses (Allen 1986; North changes over 10 to 20m which correspond to the variations 1989). In the Devon and Somerset areas there are few sources of described by Lott et al. (1982) and Penn (1987). In the lower detrital carbonate in the basement rocks which were exposed part of the successions in both Devon and Somerset the during deposition of the Mercia Mudstone Group. The gamma-ray intensity decreases downwards, and in the lower carbonate which is present within the mudstones is an early 20m of the red mudstones in Devon, below the unexposed diagenetic precipitate, associated with the surficial evaporative section, gamma-ray intensity is relatively low (Fig. 5). The environment. There is no evidence from petrographic and SEM change in gamma-ray intensity, which takes place within the studies for any precipitation of carbonate during burial and it is unexposed section, probably corresponds to the boundary probable that precipitation took place in the topmost few between gamma-ray/sonic units E and D. In Somerset there is metres of sediment. A stable, ordered carbonate phase can a gradual downwards decrease over 60 m and the position of form during early diagenesis in the presence of depositional the boundary between units E and D cannot be precisely waters (Rosen et al. 1988). SEM examination of the mudstones fixed. shows few well developed carbonate crystals, and energy- dispersive X-ray analysis indicates that calcite and dolomite exist primarily as coatings around other clasts. Clay mineralogy There is no way of proving co-genesis of calcite and The clay mineralogy of the Mercia Mudstone has been studied dolomite in the Mercia Mudstone Group, although there is no by Dumbleton & West (1966), Jeans (1978), Mayall (1979, evidence of any burial diagenesis of the mudstones. The fine- 1981) and Taylor (1982, 1983). Several authors have proposed grained nature of the carbonate minerals made it impossible to a sequence of clay mineral variations related to movement of separate the phases for isotopic analysis. North (1989) stated waters across an extensive evaporative platform or basin that the dolomite in the Mercia Mudstone Group of the Bristol (Lucas & Ataman 1968 for the French continental Triassic; Channel area was primary and effectively unaltered. There has Jeans 1978 for the Mercia Mudstone Group of the UK). Taylor been some discussion as to whether dolomite and calcite can (1982) subdivided the upper Mercia Mudstone Group into two co-precipitate in equilibrium (Degens & Epstein 1964; Veizer & members on the basis of changes in clay mineralogy and Hoers 1976; Anderson & Arthur 1983). Although no single related these changes to differences in chemistry of the basin model can be universally applied, some enrichment of 6180 in waters. dolomite relative to calcite is expected (Land 1980). Forty seven samples of red mudstone were analysed from Samples with >90% dolomite in the Mercia Mudstone the coastal successions of Devon and Somerset. A further nine Group are enriched in 6180 by an average of +2.8%0, relative to samples were taken from the overlying Blue Anchor samples containing > 90% calcite. This lies within the range of Formation. The clay mineral assemblages of these samples +1.0 to +7.0% given by Land (1980). The dolomite/calcite consist predominantly of illite and chlorite, with relatively ratio may itself be related to changes in water chemistry, and so low proportions of other clays. Smectite and various other in Fig. 7, both uncorrected and corrected 6180 values are mixed-layer smectite varieties are present, and traces of shown. palygorskite and sepiolite have been identified (Leslie 1989). In Devon, a clear trend is seen in the 6180 values of the Variations in the clay mineral assemblage in both Devon mudstones, with heavier values of oxygen in the Blue Anchor and Somerset allow the subdivision of the Mercia Mudstone Formation and at the base of the sampled section, in the cliffs Group units into two, based on the proportion of smectite and to the west of Branscombe (Fig. 7a). Although one sample mixed-layer clays in the samples (Fig. 6). In the upper unit, the within the main body of red mudstone is relatively enriched in clay assemblage consists of >90% illite and chlorite, with 180, with an uncorrected value of 0.0%0, the majority of the red minor amounts of smectite and mixed-layer smectitic clays. mudstones between 0 and 80m below the base of the Blue Within the lower unit of red mudstone, the proportion of illite Anchor Formation are relatively depleted in 180 in both and chlorite is reduced to < 80% and mixed-layer clays (illite- uncorrected and corrected values (Fig. 7a). The variations in smectite in Devon and chlorite-smectite in Somerset) become 6180 value can be used to subdivide the mudstones into those more abundant. In Somerset, up to 3% smectite was also with a negative 6180/6160 ratio, and those more enriched in 180. recorded, associated with the mixed-layer clays. Within the This subdivision corresponds with the relatively 180-depleted Blue Anchor Formation clay mineral assemblages are evaporitic continental (6180 between-4 and 0%) and the 180-

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% clay mineral species % clay mineral species 60 70 80 90 100 60 70 80 90 100 Base Base m Blue Anchor Blue Anchor m Formation Formation

20- 20 m m

4.0 (m) 40- /~77~':~:::::::::::: 60- 60 ~r (m) ~r 80- 8O 55m NO EXPOSURE 100

~ Illite

-'---]Chlorite 120 ~ Mixed layer illite /smectite l Smectite 140 Palygorskite/sepiolite (a) (under 2%) [:~ IIlite ~-] Chlorite -~ Mixed layer chlorite/ smect~te l Smect,te Palygor skate/sepiolite (b) (under 2%)

Fig. 6. Clay mineral assemblages in the upper Mercian Mudstone Group from (a) South Devon and (b) Somerset.

enriched evaporitic marine (6180 between +1 and +5%) increased evaporitic effects during deposition of the mudstones dolomite fields of Taylor (1983) (Leslie 1989). The 6~80 variations in Somerset are similar to those in Devon, with relatively enriched values in the Blue Anchor Discussion Formation, and depletion of 180 values in the underlying red mudstones (Fig. 7b). At the base of the section there is some The mineralogical and geochemical variations described in the evidence for enrichment of the lowermost samples but this is mudstones give evidence for a number of changes in the not as marked as in Devon, and is within the range of values of depositional and early diagenetic environment for which there the samples corrected for 100% dolomite. Other than in is no lithological evidence. Although some lithological samples from the Blue Anchor Formation, the 6180 values in variations such as the green dolomitic horizons in the Somerset the Somerset succession fall within the evaporitic continental succession are evident, the majority of the Mercia Mudstones dolomite field of Taylor (1983). The isotopic variations sampled in this study is a lithologically homogeneous therefore suggest a predominantly continental source of waters sequence of predominantly red mudstones and siltstones. in the Somerset Basin, with a minor input of marine brine in the Individually, none of the three analytical techniques used early Norian. As in the Devon succession, one sample, 61 m provide conclusive proof of a significant change in conditions below the Blue Anchor Formation, is anomalously enriched in of deposition. All of the changes described are transitional and 180, which is most likely the result of a short-lived period of relatively subtle. However, the enrichment of 180, coupled

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significant long-term variations in the degree of evaporation ~18 0 PDB during deposition of the upper Mercia Mudstone Group. -5 -4 -3 -2 -1 0 1 2 3 I I I I I I I I Sulphate horizons are present throughout the succession and are more common in the mudstones with depleted ~80 values, which is the converse of what would be expected if evaporation ba .....-it- were the main control on oxygen isotope values. The single B./ 13 samples from the red mudstones which are anomalously enriched in 180 may, if they are genuine excursions, represent short periods of increased evaporation, but these are clearly short-term events which do not fit into the overall pattern of 40 variations. Taylor (1983) attributed the variations in 180 in the Mercia Mudstone Group of Nottinghamshire to the presence of (m) 180-rich marine brines during deposition and early diagenesis. 6O It is probable that the slight enrichment in 180 at the base of the Mercia Mudstone sections in Devon and Somerset reflects the \ \ residual presence of marine brines in the basins. \ 80- \ 55m NO The clay mineral variations can also be explained by \ < \ EXPOSURE >< \ reference to changes in water chemistry during deposition of the \ \ mudstones. A suite of degraded clay minerals, predominantly 140- illite and chlorite, was supplied to the basins as detritus. During transport, deposition and early diagenesis, these \ degraded minerals absorbed catons from the surrounding 160- waters, a process of transformational regradation (Lucas 1962; Lucan & Ataman 1968). The minerals formed during (a) regradation will depend on the chemistry of the depositional 180pD B waters (Taylor 1982). In all cases, illite absorbed K ÷ and -5 -4 -3 -2 -1 0 1 2 3 chlorite Mg 2+, to regrade to their original mineral type. If t i I i 1 i i ! i Mg 2÷ was abundant, however, then the most degraded illites BB also absorbed Mg 2÷ to become mixed-layer smectite varieties. base m The proportion of smectitic clay minerals within the B.A.F. assemblages can therefore be used to subdivide the upper Mercia Mudstone Group. The lower sections of red mudstones 20- sampled, which contain > 15% smectitic and mixed-layer clays, formed in waters which contained more Mg 2÷. The presence of waters which are relatively rich in Mg 2÷ can be 40- attributed to the waning influence of residual marine brines (m) within the Devon and Somerset basins during the late Norian. This marine influence gradually declined towards the end of 60- deposition of the red mudstones, when continental-derived waters relatively rich in K ÷ were dominant. The lower Blue Anchor Formation contains a similar suite of clays to that in 80-__ the underlying red mudstones, but the incursion of marine [] waters is recorded in the increase in smectitic clays in the upper 100 Blue Anchor Formation and the Penarth Group (Mayall 1979, 1981). The variations in clay mineralogy can be used to explain the 120 observed gamma-ray variations. There are few possible gamma-ray sources within the Mercia Mudstone Group which ? are sufficiently abundant to cause the variations observed both (b) in borehole logs and at outcrop. The most significant source is potassium, which is dominantly in the illite. Both uranium and Fig. 7. Oxygen isotopic variations in the upper Mercian Mudstone thorium are present within the mudstones but they are not Group from (a) South Devon and (b) Somerset. Filled squares, considered sufficiently abundant to affect the total count uncorrected values; open squares, values corrected for the presence gamma-ray curve which is used in this study. A trace of feldspar of calcite in the carbonate fraction. was detected in a few samples using XRD, but this amount (probably < 1%) is not sufficient to change the gamma-ray signature of a rock containing 15 to 60% illite. with the increase in mixed-layer smectite minerals at the base of The gamma-ray curve measured at outcrop shows a the sections analysed, indicate a significant change in the reduction in intensity in the lower section of red mudstones, geochemical environment during deposition of the upper which is most likely a reflection of the lower abundance of Mercia Mudstone Group. illite, and therefore potassium, in the rocks. This control on The enrichment in 180 can be attributed either to an increase gamma-ray intensity is applicable to the variations within the in evaporative effects or an increase in the input of marine- red mudstones where there are no other lithological variations derived waters into the basin. There is little evidence of which can be used to account for the changes. The overlying

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Blue Anchor Formation is lithologically heterogeneous and sedimentary structures and no features which allow gamma-ray variations can be attributed to a number of factors. correlation of the mudstones over distances greater than a few The duration of the incursion of marine waters into the kilometres. basins is difficult to assess. The most prolonged late Triassic Gamma-ray and sonic variations in borehole logs have marine incursion before the Rhaetian transgression took place allowed the subdivision of the mudstones into six units, the in the Carnian Stage when extensive halite horizons were uppermost of which is the Blue Anchor Formation. Although deposited in the Devon/Dorset and Somerset basins the boundaries of some units can be correlated with arenaceous (Warrington et al. 1980). It is possible, given the thickness of or saliferous horizons, other boundaries have no obvious the unexposed section, that the base of the measured section in lithological control. Outcrop gamma-ray studies suggest that Devon corresponds to the top of the Weston Mouth Sandstone one such boundary is exposed in coastal sections in south Member. The marine influence in the lower mudstones could Devon and north Somerset, although the boundary itself is be attributed to the waning effects of the late Carnian marine different in character at the two locations. incursion during which the sandstones were deposited This boundary, which is marked by a downwards reduction (Warrington & Williams 1984). in gamma-ray intensity, is associated with changes in the clay It is also possible, however, that the marine influence is the mineralogical and oxygen stable isotopic composition of the result of a later, Norian incursion, as the sandstones have been mudstones. An increase in mixed-layer smectitic clay minerals interpreted to be the result of a pluvial event (Simms & Ruffel and a relative depletion in heavy 6180 are associated with the 1990). In inland exposures from northern Somerset Ruffell reduction in gamma-ray values. (1990) has correlated the gamma-ray and sonic divisions of the The variations can be related to an incursion of marine- Mercia Mudstone Group with lithological variations observed derived, Mg2+-rich waters into the basins during the early at outcrop. Ruffell (1990) correlated the North Curry Norian. The upwards increase in 6180 and reduction in mixed- Sandstone Member of Somerset with the Weston Mouth layer clays is the result of the reduced influence of marine Member of Devon, as do others (Warrington et al. 1980). This brines and a concomitant increase in continental-derived sandstone, which probably underlies the section sampled in potassic waters. An increase in the incorporation of K + into this study, is taken to represent the top of the Carnian stage and clay mineral lattices during regradation is the cause of the its base forms the boundary between gamma-ray units D and C increased gamma-ray intensity in the upper red mudstones. (Lott et al. 1982). Ruffell has, however, also correlated the Although there is some lithological expression of this boundary between units E and D with a 'calcareous, commonly boundary in Devon, in Somerset the boundary is within an arenaceous orange to red nodular or rubbly siltstone'. This apparently homogeneous succession of red mudstones. calcareous siltstone, termed the Cotlake Member, is laterally impersistent (Ruffell 1990) and cannot be traced towards the coast, either to the north or south, although it has been Thanks are due to K. Gittins, P. Judge and S. Davies for assistance. identified in the Wessex Basin (Ruffell 1991). The description A.B.L. acknowledges receipt of NERC grant number GT/86/GS/27. K. of this bed is very similar to that of the rocks at the base of the Myers, C. North, C. Southworth and S. Taylor are thanked for useful succession studied in Devon. The lack of exposure along the discussions. The authors are grateful to A. Parker and an anonymous South Devon coast between Branscombe and Weston Mouth referee for reviews of the manuscript. The paper is published with the means that it is not possible to determine with certainty permission of the Director, British Geological Survey. whether the gypsiferous mudstones at the base of the measured section represent the Weston Mouth Sandstone Member or the References Cotlake Member. The abundance of gypsum, possibly of ALLEN, J. R. L. 1986. 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Received 11 February 1992; revised typescript accepted 27 April 1992.

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