The geochemistry and mineralogy of the Permian red beds of Southwest England

Chemical Geology, 11 (1973) 31-47 © Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands

THE GEOCHEMISTRY AND MINERALOGY BEDS OF SOUTHWEST ENGLAND

OF THE PERMIAN RED

M.E. COSGROVE

Department of" Geology, The University, Southampton {Great Britain) (Accepted for publication December 18, 1972)

ABSTRACT Cosgrove, M.E., 1973. The geochemistry and mineralogy of the Permian red beds of Southwest England. Chem. Geol., 11: 31-47. Detailed mineralogical and chemical examination of some hundred rock samples representing the Permian red bed sequence of Southwest England has been carried out. Mineral-element correlations are presented and factor analysis of the chemical data shows that 80% of the variance can be explained by seven factors. Among these are recognised a clay factor; detrital minerals versus carbonate factor; halide and sulphate factors and a volcanic enrichment factor. The behaviour of elements in the strong oxidising environment associated with red bed deposition is discussed. In particular, zirconium is shown to be enriched in red bed clays, reflecting solution from the source area. Cerium, in its oxidised state as Ce4÷ is shown to be strongly correlated with the carbonate phase, indicating its tendency to form relatively stable carbonate complexes. Strontium is positively correlated with the clays and negatively correlated with carbonate, while sulphur (as SO, 2-) is associated with carbonate usually in the form of barytes. The origin of the red pigment is also discussed. INTRODUCTION The Permian red bed sequence of Southwest England rests with marked unconformity on the Palaeozoic complex of Devonian and Carboniferous rocks forming the Cornubian peninsula, the outcrop extending from the Tor Bay area in south Devon towards Watchet on the Somerset coast in the north (see Fig.l). The belt of Permian rocks, narrowed by sea erosion in the immediate south, is about 16-km wide in the Exmouth region but narrows towards the north until it is eventually overlapped between the Quantocks and Exmoor by Triassic rocks. Two prominent Permian-filled troughs stretch westwards from the main outcrop; these are the Crediton trough extending nearly 48 km to the Hatherleigh area north of Dartmoor, and the Tiverton trough extending westwards for about 12 km in the vicinity of Tiverton. Lithologically the Permian rocks are comprised of a conglomerate sequence at the base which passes upwards and outward into sandstones and eventually mudstones. They were deposited as a result of intensive weathering of the Cornubian highlands probably in a warm, alternating wet and dry climate. Lateral variation in the strata is very marked; a

32

M.E. COSGROVE

30 1

, \\

\

E X MOOR

¢,~'

6b

~:z~Hotherlebgh

¢

on

K~lierton

_t O

J~

0 0

10 5

[xmbuth ]~Slra~glnt Point Orcombe Rock~

20Km I0 Miles

Key to Syrnbo~

Tot

7

E N G L I S H

CHANNEL

i Pre- Permian

,

Post-Pe,m,an

j

i

30

i

J

Fig. 1. Map of eastern Southwest England showing the distribution of Permian rocks.

fact in keeping with the type of sedimentation on an old land surface (Laming, 1968). The conglomerates bear all the evidence of having been formed as alluvial fans on the western margin of a desert basin, the centre of which accumulated sandstones and siltstones of fluvial origin. Aeolian sands developed in places adjacent to the alluvial fans (Laming, 1966). The red sandstones develop towards the top of the succession. They have recently been shown to comprise several sedimentation units (cyclothems) showing fining upward characteristics indicative of a fluvial origin (Allen, 1965). Additional sedimentary structures strengthen the thesis that the mudstones were formed in a floodplain complex of a river of high sinuosity (Henson, 1970). Near the base of the Permian succession, some quite extensive lava masses occur forming what is commonly known as the "Exeter Volcanic Series". They extend as isolated outcrops from the Loxbeare area (Tiverton trough) in the north to Dunchideock in the south and as far west as the Hatherleigh area in the west (Crediton trough). They form an interesting group of potassium-rich shoshonitic rocks, often of lamprophyric character, the geochemistry of which has been described elsewhere (Cosgrove, 1972). Radiometric dating of two of the volcanics (279 +- 6 m.y. for the Killerton minette, 281 +- 11 m.y. for the Dunchideock shoshonite (Miller and Mohr, 1964) places them at or near the PermoCarboniferous boundary, generally accepted at 280 m.y. Since it is quite evident that red

PERMIAN RED BEDS OF S.W. ENGLAND

33

bed conglomerates were formed before the volcanicity in both the Torquay area (Laming, 1965) and in the Crediton trough (Hutchins, 1963), this implies that the oldest red-beds are not Permian but of Upper Carboniferous (Stephanian) age. The entire succession is essentially of "red bed" type, the red colouration being due to haematite coating of the individual clasts whether they be of boulder or clay size. Some lithological units, especially some sandstones, are completely free of red colouration; these are usually of pale buff colour. Other units, both sandstones and mudstones are of pale green colour and probably represent locally reduced conditions during the haematite formation. Yet other units are mottled red and green with all possible combinations between the "end" colours. Eighty-two surface outcrop samples of the Permian rocks were collected representing all lithologies and covering, as far as possible, the entire outcrop (sample numbers P1-P82) These were supplemented by fourteen borehole samples from the Exmouth junction boreholes (sample numbers By 5186-By 5200, less By 5199) kindly provided by the Institute of Geological Sciences. The object of this geochemical and mineralogical study was to investigate the element-mineral associations in rocks formed in, or subjected to, an oxidising environment, in contrast to the considerable research effort in the study of sedimentary rocks formed under reducing conditions (black shales). At the same time, it was of considerable interest to examine the influence of a volcanic phase on the sedimentary environment, and also it was hoped that this approach would throw some light on the problem of red bed pigmentation. ANALYTICALTECHNIQUES

Chemistry The major elements (Si, Ti, A1, Fe, Mg, Ca, Na and K) and the minor elements (P, S, CI, V, Mn, Ni, Cu, Zn, Ga, As, Br, Rb, St, Y, Zr, Nb, Sn, Ba, La, Ce, Nd, Pb, Th and U) were determined by X-ray spectroscopy on pure rock powder pellets. A 2 kW Philips PW 1212 spectrometer was used throughout, the standard error of estimate for the major elements ranging from about 2.0% for SiO2 to 0.04% for TiO2 at the average values of these elements for the mudstone samples. For the trace elements, the coefficient of variation is better than 5% for the middle atomic numbers (Ni-Y) and for P. The remainder have C of V values between 6 and 16 with the exception of C1 at 20%. The rapid methods of Shapiro and Brannock (1962) were used for the determination of CO2 and H20 +.

Mineralogy The mineralogy of the clay fraction of the rocks was determined by X-ray diffraction using a Philips diffractometer with a copper target X-ray tube. The clay fraction was

34

M.E. COSGROVE

obtained (taken to be less than 5/3 size) from a suspension of a slurry prepared from ultrasonically disintegrated rock chips. Untreated, glycollated and heated (550°C) clay mounts were prepared for each sample, enabling the individual clay minerals to be identified and quantified as parts per hundred of the total clay fraction. Such values although not necessarily absolute, are suitable for comparative treatment. In some cases, treatment of the rock with 0.1N acetic acid was necessary to remove excessive carbonate. Quartz was determined quantitatively by an X-ray diffraction method using the same rock pellets originally prepared for the X-ray spectrometer. The 4.26A peak was measured, correcting the intensity for the mass absorption of the specimen. This particular peak was chosen since it was the most intense available not overlapped by peaks of other possible minerals present. Details of this method, which depend upon the absence of amorphous silica in the samples, is described elsewhere (Cosgrove and Sulaiman, 1973). The quantity of carbonate, mostly calcite but occasionally dolomite, was estimated from the CO2 values. A conversion factor of 2.2 was used (CO2 value X 2.2) to give approximate absolute carbonate percentages since the factor for CO2 to CaCO3 is 2.273 and for CO2 to CaCO3"MgCO3 is 2.091. The approximate mineralogical composition for each sample was then computed on the basis: total clay + quartz + carbonate = 100%. Relative individual clay mineral abundances were then calculated by multiplying the parts per hundred estimate by total clay percentage. Undoubtedly the major source of error in this estimation is that no account is taken of the felspar content of the rocks. In some samples, particularly the arenites, felspar is relatively abundant as judged from the diffractometer traces. Organic carbon and sulphide phases are virtually absent, though in some cases evaporite minerals form minor rock constituents. MINERALCOMPOSITION Table I shows the average mineralogical composition of the samples studied. It is doubtful whether this analysis is truly representative, since weighted sampling is very difficult from surface outcrops of a sequence showing rapid lateral and vertical changes. However, all lithologies are included and hopefully it is a reasonable indication of the true mineralogical distribution. TABLE I Average mineralogicalcompositions of 96 Permian red bed samples, Southwest England Composition of rock samples (%)

Relativeabundance of individual clay minerals (%)

Quartz Carbonate Clays

Illite Kaolinite Chlorite Montmorillonite Mixed-layers

49 1 3 0.5 2.5

Total clay

56

33 11 56

PERMIAN RED BEDS OF S.W. ENGLAND

35

Kaolinite is rare, only occurring in some conglomerates of the Crediton trough and some sandstones o f the northern part of the outcrop towards the Quantocks. In both localities it represents rotting detrital felspar. Distinctive montmorillonite is even more rare being restricted to an horizon outcropping in the cliffs between Orcombe Rocks and Straight Point, and in two borehole samples from Exmouth junction (By Series) which may have intersected the same level. The carbonate most commonly found is calcite, though dolomite is not rare, and occasionally siderite or ankerite occur. CHEMISTRY The major element chemical analyses of the samples has produced no surprises, as can be appreciated from a study of Table II where averaged values for the Permian mudstones, sandstones and limestones have been compared with general averages o f these rock types. The selection of the samples into the necessary lithological groups was carried out on the basis of Q-mode factor plots to be discussed in a later section. TABLE II Average major element chemistry of red bed mudstones, sandstones and limestones compared with averages for general sedimentary rocks*

SiO2 TiO 2 AI~O3 Fe203 CaO MgO Na20 K20 CO: H:O

Red bed mudstones

Average shales

Red bed sandstones

Average sandstones

Red bed limestones

Average limestones

59.1 0.9 19.3 7.1 2.1 1.7 0.6 4.3 1.I 4.6

58.1 0.7 15.4 6.7 3.1 2.4 1.3 3.2 2.6 5.0

85.3 0.2 7.2 1.6 0.1 0.3 0.5 3.2 0.1 1.1

78.3 0.3 4.8 1.4 5.5 1.2 0.5 1.3 5.0 1.6

16.6 0.2 2.6 2.1 38.5 5.9 0.2 0.9 32.2 1.0

5.2 0.1 0.8 0.5 42.6 7.9 0.1 0.3 41.5 0.8

*All values in %. Average shale, sandstone and limestone from Mason (1966).

The red bed mudstone to shale comparison shows a very close similarity. The higher Al2Oa and K 2 0 values in the red beds must reflect the greater concentration of illite in them as compared with the average shale. In the same way, the higher MgO and CaO values in the average shale indicates more chlorite and/or montmorillonite in them, and coupled with the greater CO2 value, more calcite and/or dolomite also. The smaller amount of Na20 in the red beds probably mostly represents less alkali felspar than normal, there being usually little Na20 in the clay minerals. Surprisingly, the Fe203 value for the red beds is not appreciably higher than the general average (7.1% as compared with 6.7%).

36

M.E. COSGROVE The sandstones comparison shows greater variation; in particular the SiO2 value at

85.3% for the red beds being considerably higher than the 78.3% for the average sandstones. Higher A12Oa and K20 in the red beds again indicates predominant illite in the clay fraction, while higher CaO, MgO and particularly CO2 show that the average sandstone is relatively rich in carbonate minerals. The limestone figures merely reflect the greater clay and quartz impurity in the red bed rocks as compared with the average limestones. Comparisons between the trace-element abundances on the same basis show some remarkable similarities and differences (see Table III). In all rock types, for example, the values for V, Th and U are very close as between red bed types and the general averages Taking into account the greater clay content of the red bed sandstones and limestones, the P, Mn, Ni, Cu, Zn, Ga, Br, Rb, Sr, Y, Nb and Pb values are also not greatly at variance. On the other hand, S shows considerable depletion in red bed mudstones and sandstones, TABLE Ill Average trace-element abundances of red bed rnudstones, sandstones and limestones compared with averages for general sedimentary rocks*

P S CI V Mn Ni

Cu Zn Ga As Br Rb Sr Y Zr Nb Sn Ba La Ce Nd Pb Th U

Red bed mudstones

Average shales

Red bed sandstones

Average sandstones

Red bed limestones

Average carbonates

568 546 551 131 767 61 22 95 21 23 3 176 307 33 245 15 9 533 52 82 29 30 13 4

700 2400 180 130 850 68 45 95 19 13 4 140 300 26 160 11 6 580 92 59 24 20 12 4

236 93 809 20 282

170 240 10 20 X0

493 2906 769 19 1446

400 1200 150 20 1100 20 4 20 4 1 6 3 610 30 19 1 1 10 1 12 5 9 2 2

8

2

9

9 6 4 10 2 72 47 13 107 5 6 375 6 13 n.d. 13 n.d. n.d.

X 16 12 1 1 60 20 40 220 1 1 X0 30 92 37 7 2 1

2 80 n.d. 12 n.d. 21 213 12 75 4 8 4697 13 194 n.d. 11 n.d. n.d.

*All values in parts per million. Abundances for shales, sandstones and carbonates from Turekian and Wedepohl (1961). X indicates order of magnitude estimates; n.d. = not detected.

PERMIAN RED BEDS OF S.W. ENGLAND

37

and a marked increase in limestones as compared with the general values. This is easily accounted for on the grounds of the virtual absence of sulphide sulphur in the red bed mudstones and clay component of the sandstones, and the relative abundance of sulphate sulphur in the evaporite-bearing carbonate rocks. Zirconium shows enrichment in the red bed mudstones (245 p.p.m.) as compared with the average shales (160 p.p.m.), depletion in the sandstones (107 p.p.m.) as compared with the average value (220 p.p.m.) and again enrichment in the limestones (75 p.p.m, as compared with 19 p.p.m.). A reasonable interpretation of these figures seems to be that zirconium is removed from zircon in the source region during intensive weathering and is subsequently absorbed by the clay minerals. Degenhardt (1957) for example presented evidence for Zr substituting for AI in kaolinite and also suggested that the element could enter the montmorillonite lattice. Perhaps it can also enter the illite or chlorite lattices. Thus its distribution pattern would be reversed as compared to its "normal" behaviour when remaining as the resistate mineral zircon. Barium shows good agreement in the mudstone/shale comparison but considerable enrichment in the red bed sandstones. This probably relates to the relatively high K20 percentage for these rocks, the Ba proxying for K in the felspar lattice. The very large abundance figure for red bed limestones (4,697 p.p.m.) relates to the presence of barytes in many of these rocks. The comparative abundance figures for the three rare-earths is puzzling. The only common factor is that all three have low values in red bed sandstones as compared with averaged sandstones. Cerium is particularly rich in the red bed limestones (16 times more than the average limestone value). It is unlikely that Ce 4÷ (ionic radius 0.94A) would substitute for Ca 2÷ (ionic radius 0.99A) in calcite, but cerium in its oxidised state (as would be the case in a red bed environment) becomes clearly distinguished from the other light rare-earths and acquires a stronger tendency towards hydrolysis and complexing. In the latter connection, it forms relatively stable carbonate complexes (Ronov et al., 1967). GEOCHEMICALCORRELATIONS Correlations between the mineralogical and chemical data are shown in Table IV. At a glance it can be seen that the three mineral groups: clays, quartz and carbonate are antipathetically related to each other, the total clays showing the expected strong positive correlation with TiO2, A1203, Fe203, K20, H20, MgO and a wide range of trace elements, while quartz only correlates positively with SiO2, and CO2 (representing the carbonates) shows strong positive correlation with CaO and Ce and weaker correlation with Mn and S. In an attempt to differentiate element associations with individual clay minerals it was found that the matrix for all but illite and chlorite was meaningless since in so many samples kaolinite, montmorillonite or mixed-layers could not be detected and were thus recorded as zero. With so many zeros recorded for these variables, the correlations involving them became highly dubious. However, the correlations with illite and chlorite were valid since all samples contained illite and most contained chlorite. Compared with the element relationships shown for total clay in Table IV, illite gained

M.E. COSGROVE

38 TABLE IV Chemical and mineralogical correlations Confidence levels Positive

Total clay

5%

1%

0.01%

0.01%

1%

5%

Sn

MgO,Cu, Sr, U, As

TiO2,A1203, Fe~O3, K~O, H~O, P, V, Ni, Zn, Ga, Rb, Y, Zr, Nb, La, Nd, Pb, Th

CaO, CO2

-

Mn

SiO 2

TiOz, Fe203, CO~, P, Zn, Ga, Rb, Sn, La, Ce

A1203,CaO, MgO, H20, V, Ni, Nb, As, Y, Nd, Pb, Th, U

Mn, Cu

CaO, Ce

SiO2, A1203, K20, H20, Ga, Rb, Nd, Pb, Th

TiO2, Fe~O3, Na20, Y, Zr, Nb, La, V, Zn

Cu, Sr

Quartz

CO 2 (carbonate)

Negative

S

Mn

SiO 2 and Na20 at the 1% confidence level, lost U and uniquely retained Pb, Cu, Sr and As. Chlorite lost K20 and uniquely retained U. The other trace elements remained correlated with both illite and chlorite, there being only minor changes in confidence level for some of them. The element correlations are broadly as expected from similar investigation on sedimentary rocks (Gad et al., 1969; Hirst and Kaye, 1971) with a few very marked exceptions. Strontium, for example, is positively correlated with the clays (illite) and, perhaps even more meaningful, is negatively correlated with carbonate. Zirconium shows a strong positive correlation with the clays, an unlikely situation if it is all held essentially in the resistate phase as zircon. As already mentioned, there is evidence that some Zr might be removed in solution from the source area as the result of intensive chemical weathering. Cerium is strongly correlated (positive) with carbonate. The peculiar behaviour of the ceric ion is probably responsible for this association as already mentioned. Sulphur is correlated with the carbonate phase. In a red bed situation this must relate to the S, in the form of sulphate, tending to co-precipitate with carbonate. Only three elements originally determined do not figure in Table IV. These are C1, Br and Ba. On inspection of the correlation matrix, the former two are seen to correlate with each other at the 0.01% level, but show no relationship with any other element or mineral. Barium shows correlation with both K20 and S at the 1% level and, therefore, would appear to share its distribution between potassium substitution and formation of small quantities of barytes. It may be that the correlation of Ba with K20 withotit Ba appearing in the total clay or illite list reflects the presence of alkali felspar as a minor mineral phase.

PERMIAN RED BEDS OF S.W. ENGLAND

39

FACTOR ANALYSIS Further insight into the geochemistry is only possible by reducing the complex interrelationships between the large number of variables to simpler relationships between lesser variables. R-mode factor analysis achieves this, and since in geochemical investigations the factors produced are not likely to be independent, oblique promax rotation was used to produce correlating factors. The computer programme due to Mather (1970) was used for this study. The principles underlying the technique of factor analysis have been reviewed by Cattell (1965a, b) and several geochemical applications have been described, for example by Klovan (1966), Spencer et al. (1968) and Hirst and Kaye (1971). Q-mode factor analysis, where attention is focussed on the samples rather than the variables, was used to isolate the three main lithologies used in compiling Table 11 and 1II. Three factors were extracted leading to three groups of rock mudstones, sandstones and limestones. The computer programme due to Ondrick and Srivastava (1970) was used for this application. The mineralogical data was not used in the factor analyses since the values are relatively imprecise, and in the majority of samples individual constituents such as kaolinite and montmorillonite could not be detected. The R-mode factor analysis for the chemical data on the entire population of 96 samples produced seven factors which together account for 80% of the total variance, each factor explaining more than 3% of the variance of the data. These factors were rotated about orthogonal axes (varimax) and also a series of oblique axes (promax). The data shown in Table V and VI are for the promax matrix with K min = 4 (the degree of obliqueness). Factor 1

The very high loadings on TiO2, A1203, Fe203 and H20 coupled with the long list of loaded trace elements suggest that this factor should be designated "the clay factor". The elements V, Ga, Rb, Y, Nb, La, Nd and Th are all highly and uniquely loaded here, whereas other trace elements such as Zn, Zr, Sn and Pb, while showing their highest loadings on this factor evidently play a dual or triple role by showing significant loadings on other factors. Another group of elements, e.g., As, Sr and Ce, show loadings with this factor, but have higher affinities on others. The positive loadings on these elements is opposed by moderate negative loadings on CaO, CO2 and Mn, clearly indicating the carbonate phase antipathetically related to the clays. Fig.2 shows a plot of the factor scores for factor 1 against total clay content as determined by difference: 100 - (quartz + carbonate)%. Correlation is good, the line drawn is believed to represent the most realistic regression with the samples plotting well to the right of it being in error due to their relatively high felspar content. Since felspar is not considered in the above calculation, the total clay values will be too high by the" amount of felspar in the sample. It is noticeable from Fig.2, and predictable from the calculation method that the error in high-clay samples is small ( < 10%), is about 20% at 50% clay and

4O

M.E. COSGROVE

can increase to about 30% in low-clay samples.

Factor 2 This factor is truly bipolar with a high negative loading for SiO2 with associated low loaded Na20 and K 2 0 opposed to CaO, CO2 and Ce plus lower loaded Sn. This obviously reflects the antipathetic relationship between detrital minerals (especially quartz but also TABLE V Promax oblique factor pattern matrix* Factors 1

SiO2 TiO 2 AI~O3 F%Oa MgO CaO Na20 K20 CO 2 H~O P S C1 V Mn Ni Cu Zn Ga As Br Rb Sr Y Zr Nb Sn Ba La Ce Nd Pb Th U

2

3

4

5

6

7

-0.98 1.01 0.87 0.82 0.75 -0.33

-0.36 0.86 0.64

0.89 -0.35 -0.30 0.87

0.58 -0.61

0.89

-0.37

0.96 0.90 -0.38 0.54

0.54 0.53 0.68 0.30

0.32 -0.31

0.72 0.87 0.39

-0.64 0.97

0.74 0.49 0.90 0.79 0.94 0.37

0.63 0.54 0.35 0.33

0.98 0.38 0.94 0.54 0.91

-0.34 -0.84

0.98 0.33 -0.53

*Only factor loadings of 0.30 and greater are included.

PERMIAN RED BEDS OF S.W. ENGLAND

4

TABLE VI Promax factor correlations Factor

Factor 1 2 3 4 5 6 7

1

2

3

4

1.00 -0.16 0.05 0.04 -0.26 0.32 -0.21

1.00 -0.17 0.05 -0.03 0.14 0.11

1.00 0.02 -0.16 0.14 -0.09

0.29 0.09 -0.25

5

6

7

1.00 -0.11 0.02

1.00 -0.16

1.00

1.00

2.O-

/' / Jl

1.8

•

f

1.6 1.4 1.2

I~/••

1.0 0'8 0

°

%~".

0.6 •o

O4 I

0-2 • II~o

o

o

10

20

30

•

40 50,601

• ~o Total Clay

zo

80

9o

~> -0-2

-0"4 -0"6

../...:.

-0"8

oo °

-1.0

•

-1"2 -1"4

-1-6

°°

° o go

,I °

•

/,

Fig.2. Factor 1 scores against percentage total clay. felspars) on the one hand and precipitated minerals (especially calcite) on the other. The association of cerium (as Ce 4÷) with the carbonate phase has already been discussed in earlier sections. The presence of positively loaded Sn is difficult to explain, but its persistent appearance in three factors with low loadings just in excess of 0.30 and three

42

M.E. COSGROVE

other factors with loadings just below 0.30 is highly suggestive of a variable with very little range (low standard deviation) showing analytical error variance. A plot of the factor scores against percentage carbonate (Fig.3) shows strong correlation Only ten samples contain more than 10% carbonate and these are all plotted. Thirty randomly selected samples of less than 10% carbonate content have been added to complete the plot, since plotting all the samples would have caused considerable congestion

4.0

30

.

"'Y

Y

2.0

04

1.0

o ou co

Carbonate 60

S

7'0

8'0

9'0

-10

- 2-0

Fig.3. Factor 2 scores against percentage carbonate.

Factor 3 With the very high positive loadings on C1 and Br this factor justifies the name "halide factor". It is not clear as to the precise location of the halides as the abundances are small (average content C1 = 654 p.p.m.; Br = 3 p.p.m.), but one possibility would be halite. The fact that Na20 is not loaded with C1 and Br can be explained by the fact that only very small amounts of Na20 would be so associated, the bulk being concentrated elsewhere -perhaps as albite. The possibility that this factor results from modern sea spray was considered, but coastal samples did not show significantly higher CI values than inland exposure or the borehole samples.

PERMIAN RED BEDS OF S.W. ENGLAND

43

Factor 4

This factor shows high positive loadings on S and Sr together with moderate loadings on Ba and Cu against moderate to low negative loadings on U and Zn. The association of S with Sr and Ba is interpreted as representing a sulphate phase, and the presence of Cu in this situation is also not unexpected since traces of copper secondary minerals can be seen in some samples. Clearly, such a chemistry results from the oxidising conditions under which the rocks were formed. In contrast, the opposing elements U and Zn point to reducing conditions; the concentration of U in sedimentary rocks of such an environment being particularly well known. In the studied samples there are many examples of green spotting in the otherwise red rocks. These result from the inclusion of small fragments of organic matter which cause local reduction of the haematite and bring about the concentration of lower-solubility reduced state elements. Some samples represent beds of buff or green colouration believed to indicate times of incomplete oxidation or particularly abundant organic matter inclusion The presence of Zn in this situation is not unusual;it commonly being associated with the reducing environment. In some green calcareous samples traces of zinc blende have been observed. The oxidation-reduction contrast for the two halves of this factor leads to the proposal to designate it "the Eh factor". Factor 5

The high negative loading on Ba dominates this factor, and its close association with K20 reflects its ability to substitute for K in such minerals as potassium felspar and illite. The correlation between this factor and the clay factor 1 (-0.26) supports the role played by illite, but felspar must also play a vital part in the location of K and Ba as suggested by Fig.2. This is, to a great extent, substantiated by the factor 5 scores which show high negative values for high-silica rocks (siltstones and sandstones) with relatively high K20 quantities reflecting the presence of potash felspar. The low loading of sulphur along with K20 and Ba is at first sight puzzling, but when it is appreciated that this factor is correlated with factor 4 (0.29), this places the Ba-S relationship in opposition between the two factors. As already mentioned before Ba shows correlation between K20 and S at the 1% level. Factor 4 has isolated the B a - S relationship, whereas factor 5 has identified the B a - K 2 0 relationship, but is unable to split off the S completely. Factor 6

This factor shows positive loadings, in order of magnitude, on MgO, Cu, Mn, Ni, Pb and Zn and is correlated with the clay factor 1 (0.32). The factor scores are highest on those samples in contact with or in close proximity to the lavas. The loaded elements are those

44

M.E. COSGROVE

TABLE VII Element abundances in volcanic rock and red bed mudstone

Cu Mn Ni Pb Zn

Volcanics* (p.p.m.)

Mudstones** (p.p.m.)

70 1531 175 55 162

22 767 61 30 95

* From Cosgrove (1972). **From Table III. perhaps particularly susceptible to absorption by clays, with the loading on MgO possibly indicating a preference for chlorite. Certainly the volcanics were able to enrich the neighbouring sediments particularly in Cu, Mn, Ni, Pb and Zn as indicated by the average abundances of these elements in the volcanic rock as compared with red bed mudstones (see Table VII). Factor 6 is thus designated the "volcanic enrichment factor". Factor 7

A bimodal factor with moderate positive loadings on Na20 and Zr against negatively loaded As. This probably represents, on the one hand, the resistate phase of Zr as zircon along with Na20 in the form of albite. (It has already been suggested that most Zr is removed from zircon at source.) On the other hand, the As can probably be regarded as one of the few elements entirely taken into solution from the source area to be fixed in the sedimentary basin according to prevailing conditions. The correlation of this factor with the clay factor 1 points to its association (adsorbed or absorbed) with the clay mineral. At the same time, correlation with factor 4 suggests that some of the As is behaving as AsO43-. It is thus inferred that most of the trace elements particularly associated with the clay minerals are inherited. Arsenic, originating from the arsenic-rich source area as an independent phase, and some Mn, Ni, Cu, Zn and Pb volcanically derived being exceptions. THE ORIGIN OF THE RED BEDS The mineralogical evidence clearly shows the dominance of illite in the clay mineral fraction of these rocks (Table I) and this shows the unlikelihood of them being the products of reworked laterites whose clay fraction is exclusively kaolinite. As suggested by Millot (1970), the source of true red beds characterised by illite is initially the ferruginous soils and ardnes of still young uplands (the Cornubian Highlands) formed in a climate neither desertic nor tropical, but intermediate. This warm climate would be characterised by

PERMIAN RED BEDS OF S.W. ENGLAND

45

alternate wet and dry seasons; the wet season causing the release of iron by chemical weathering of ferromagnesian silicates, and the dry season causing the dehydration of iron compounds to form haematite. In this context, the geochemical behaviour of Zr is of interest; the evidence from this study being that it is, to a great extent, taken into solution by intensive chemical weathering to be later fixed by the clay minerals. A cycle very much in keeping with the proposed palaeoenvironmental model. The existence of potash felspar as a stable detrital mineral in these rocks (Factor 5) is not in keeping with the laterite origin theory. Certainly what little kaolinite that does exist seems to be derived from rotting felspar, but this is of extremely local formation and could even be of modern weathering origin. Further, although the volcanic rocks were extruded contemporaneously with the lower Permian red beds, many of the flows are less altered than one would expect for intensive haematite development in situ. For example, although most of the olivines are altered, the pyroxenes are often fresh. The whole geochemical picture presented by the factor analysis: a clay factor with an apparently largely inherited trace element chemistry; carbonate with strongly associated cerium (Ce4+); halide and sulphate factors; a potash felspar factor and an arsenic factor representing widespread enrichment of this element, points to the rocks being essentially of detrital origin with the mechanically deposited minerals being supplemented by precipitated oxy-salts (and perhaps halides) formed in water-filled basins of wet season origin. In this situation as far as the haematite pigment is concerned, the iron was supplied essentially syngenetically and the haematite formed essentially epigenetically. CONCLUSIONS The mineralogical studies of the Permian red bed sequence of southwest England clearly establishes the dominance of illite in the clay fraction. The carbonate mineral is chiefly calcite and minor amounts of sulphate and possible halide also occur. Alkali felspars are important constituents of some rocks and quartz is the ever present inert diluent. Haematite provides the red pigment. Chemical studies of the rocks show that they are broadly similar to average mudstones, sandstones and limestones, the greatest discrepancies being among the trace elements. Sulphur, for example, is depleted in the red mudstones (no sulphide) but is enriched in the red limestones (sulphate evaporites). Chlorine, As and Sn are enriched in all phases of the red beds. This probably reflects provenance in the case of As and Sn, both elements being particularly common in adjacent Cornubia. Chlorine may be of volcanic (solfataric) origin, or successively enriched by repeated dry season evaporation. Strontium correlates with S (celestite) but shows negative correlation with the carbonate. Cerium is notable for its consistent high correlation with calcite. The association of such an element as Zr with the clay minerals suggests that even minerals such as zircon will succumb to intensive chemical weathering. The contemporaneous volcanic episode can be detected in the sedimentary rocks by

46

M.E. COSGROVE

enrichment of such elements as Ni, Cu, Pb and Zn, apparently mainly attached to the clays. Such enrichment is not large (perhaps 3 to 5 times the average value) and can only be detected in sedimentary rocks formed in close proximity to the lavas. The evidence presented can be used to support the interpretation of conditions of formation for the red beds as being a warm climate with alternating wet and dry seasons. During the wet season intensive weathering produced ferruginous soils and ar~nes on the Cornubian highlands which were quickly flushed into the flanking basins, the haematite being produced by dehydration during the dry season. ACKNOWLEDGMENTS I wish to thank Messrs. R.G. Harvey, J.R. Merefield and R.A. Saunders for their great assistance in the experimental work. I am also most grateful to Mr. T. Clayton for helpful discussions in connection with the computer applications and factor analysis interpretation. Thanks are also due to Professor F. Hodson for his encouragement. I am most indebted to the Director of the Institute of Geological Sciences for permission to collect rock chips from selected borehole core samples held in the Institute's Borings Department. Some of the work was carried out during the tenure of a research grant from the Natural Environmental Research Council. REFERENCES Allen, J.R.L., 1965. Fining upward cycles in alluvial succession. Liverp. Manch. GeoL J., 4: 229-246. Cattell, R.B., 1965a. Factor analysis: an introduction to essentials, 1. Biometrics, 21: 190-215. Cattell, R.B., 1965b. Factor analysis: an introduction to essentials, 2. Biometrics, 21: 405-435, Cosgrove, M.E., 1972. The geochemistry of the potassium-rich Permian volcanic rocks of Devonshire, England. Contr. Mineral. Petrol., 36: 155-170. Cosgrove, M.E. and Sulaiman, A.M.A., 1973. A rapid method for the determination of quartz in sedimentary rocks by X-ray diffraction incorporating mass absorption correction. Clay Minerals (in press). Degenhardt, H., 1957. Untersuchungen zur geochemischen Verteilung des Zirkoniums in der Lithosphere. Geochim. Cosmochim. Acta, 11: 279-309. Gad, M.A., Catt, J.A. and Le Riche, H.H., 1969.. Geochemistry of the Whitbian (Upper Lias) Sediments of the Yorkshire Coast. Proc. Yorks. Geol. Soc., 38: 105-139. Henson, M.R., 1970. The Triassic rocks of south Devon. Proc. UssherSoc., 2: 172-177. Hirst, D.M. and Kaye, M.J., 1971. Factors controlling the mineralogy and chemistry of an Upper Visean sedimentary sequence from Rookhope, County Durham. Chem. Geol., 8: 37-69. Hutchins, P.F., 1963. The Lower New Red Sandstone of the Crediton Valley. Geol. Mag., 100:107-128 Klovan, J.E., 1966. The use of factor analysis in determining depositional environments from grainsize distributiQns. J, Sediment. Petrol., 36:115-125. Laming, D.J.C., 1965. Age of the New Red Sandstone in South Devonshire. Nature, 207: 624-625. Laming, D.J.C., 1966. Imbrication, palaeocurrents and other sedimentary features in the Lower New Red Sandstone, Devonshire, England. J. Sediment. Petrol., 36: 940-959. Laming, D.J.C., 1968. New Red Sandstone stratigraphy in Devon and west Somerset. Proc. UssherSoc., 2: 23-25. Mason, B., 1966. Principles of Geochemistry. Wiley, New York, N.Y., 329 pp. Mather, P.M., 1970. Principal Components and Factor Analysis. Computer Applications No. 10. Department of Geography, University of Nottingham, 53 pp.

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Miller, J.A. and Mohr, P.A., 1964. Potassium-argon measurements on the granites and some associated rocks from southwest England. Geol. J., 4: 105-126. Millot, G., 1970. Geology of Clays. Chapman and Hall, London. Trans. from: G~ologie des Argiles. Masson, Paris, 1964, 500 pp. Ondrick, C.W. and Srivastava, G.S., 1970. Corfan-Fortran IV computer program for correlation, factor analysis (R- and Q-mode) and varimax rotation. State Geol. Surv., Univ. Kansas, Lawrence, Kansas, Computer Contrib. 4 2 : 9 2 pp. Ronov, A.B., Balashov, Yu.A. and Migdisov, A.A., 1967. Geochemistry of rare earths in the sedimentary cycle. Geokhimiya, 1 : 3 - 1 9 (in Russian). Geochem. Int., 4 : 1 - 1 7 (English transl.). Shapiro, L. and Brannock, W.W., 1962. Rapid analysis of silicate, carbonate and phosphate rocks. U.S. Geol. Surv. Bull., 1144-A: 1-56. Spencer, D.W., Degens, E.T., Kulbicki, G., 1968. Factors affecting element distributions in sediments. In: L.H. Ahrens (Editor), Origin and Distribution of the Elements. Pergamon, Oxford, pp.981-998. Turekian, K.K. and Wedepohl, K.H., 1961. Distribution of the elements in some major rock units of the earth's crust. Geol. Soc. Am. Bull., 72: 175-192.