Clay Minerals (1985) 20, 455-466

T H E P R O G R E S S I V E I L L I T I Z A T I O N OF I N T E R S T R A T I F I E D I L L I T E - S M E C T I T E F R O M

C A R B O N I F E R O U S S E D I M E N T S OF N O R T H E R N E N G L A N D A N D ITS R E L A T I O N S H I P TO

O R G A N I C M A T U R I T Y I N D I C A T O R S

G. S M A R T AND T. C L A Y T O N

Department of Geology, University of Southampton, Southampton S09 5NH

(Received 28 March 1985; revised 3 June 1985)

A B S T R A C T : Interstratifled illite-smectites from Carboniferous sediments from the Alston and Askrigg blocks and the intervening Stainmore trough of the northern Pennines, UK, were examined by XRD. Both blocks are underlain by Caledonian granites. During late Carboniferous times sediments overlying the Alston block were intruded by the Whin Sill. The percentage smectite in illite-smectite (%S in I/S) varies from ~ 35% to < 5%; only homogeneous mudstones give consistent results. In the case of the Askrigg block and Stainmore trough, a good inverse correlation was found between %S in I/S and vitrinite reflectance. Both illitization of illite-smectite and vitrinite reflectance increase towards the centre of the block. This is thought to be related to high heat-flow centred about the granite basement. In the case of the Alston block, there is no direct relationship between clay and vitrinite data, vitrinite reflectance being controlled by the position and thickness of the Whin Sill. Except where close to the contact, the Whin Sill had no apparent effect on %S in I/S. As observed for the Askrigg block, %S in I/S is directly related to the position of the underlying granite. The progressive illitization of illite-smectites of low expandability is very slow when compared to vitrinite alteration in response to rapidly increasing temperature. Consequently, %S in I/S is a potential indicator of thermal maturation in situations where vitrinites, by virtue of their rapid response to increasing temperatures, fail to provide a regional view of heat-flow patterns.

The transformation of smectite to illite via interstratified illite-smectite occurs in a wide variety of geological environments and localities. The percentage of smectite layers in illite-smectite (%S in I/S) invariably decreases with increasing temperature. Potentially, therefore, the mineral is a sensitive indicator of thermal maturation. However, it is not known whether illite-smectite compositions represent equilibrium states, or whether they are kinetic products consequent upon a slow reaction rate. Experimental work is inconclusive in this respect (Eberl & Hower, 1976).

Most field evidence concerning the reaction comes from studies of burial diagenesis of clastic sediments, particularly from the Gulf Coast of the USA (Burst, 1959; Perry & Hower, 1970; Hower et al., 1976). In the majority of studies, the transformation appears to slow or cease at compositions of around 20%S in I/S, this composition persisting over a considerable temperature range. It represents either a particularly stable interstratified illite-smectite composition or a change in the rate-determining step of the reaction. In either

1985 The Mineralogical Society

456 G. Smar t and T. Clayton

case, it appears that the final part of the transformation towards 'pure' illite is substantially slower than the earlier part. Relatively little information is available regarding the conditions of formation of the highly illitic phases. However, work by McDowell & Elders (1980) on the Salton Sea geothermal system shows that, in shales, %S in I/S changes from around 10-15% at 185 o C to 5% at 210 o C, reaching 'pure' illite at around 275 ~ C. Inoue & Utada (1983) showed that in an alteration envelope surrounding a Kuroko-type ore deposit at Shinzan, Japan, only illite-smectites with very low smectite contents are observed above an estimated temperature of 200 ~ C.

Many other studies have concentrated on the mechanism of the reaction. Most of these propose some type of solid-state transformation involving the replacement of tetrahedral Si by A1 and the fixing of K in interlayer sites. Howard (1981) suggested that the reaction is a two-step process involving A1 for Si substitution to create a greater negative layer charge followed by K fixation, the latter being the rate-determining step. Lahann & Robertson (1980), however, claimed that uptake of K precedes and accelerates the transformation. In either case, it appears that the availability of K is a crucial factor in controlling the extent of reaction. Where K is unavailable, as for example in many bentonites, the %S in I/S is considerably higher than in the surrounding mudstones, and in extreme eases unreacted smectite still persists in the centre of the bed (~rodofi, 1979).

Nadeau et al. (1984) have proposed an alternative model for the transformation of smectite to illite. They regard the XRD pattern ascribed to interstratified illite-smectite as actually being due to interparticle X-ray diffraction effects from very thin illite particles. In their model, the apparent decrease of %S in I/S is caused by the precipitation or growth of thicker fundamental illite particles. Such growth would be expected to increase with increasing temperature. Although it may prove possible to interpret interstratified minerals in a different manner, at the present time the concept of %S in I/S remains a convenient empirical way of representing the differences between them.

The most widely accepted indicator of thermal maturation in sediments is vitrinite reflectance, although again considerable debate has arisen regarding the relative effects of temperature and time. Karweil (1956) and Lopatin (1971) have favoured models based on first-order reaction kinetics, in which long periods of heating at low temperatures are considered to be equivalent to shorter periods at higher temperatures. Other workers such as Neruchev & Parparova (1972), Ammosov et al. (1975) and Price (1983) have claimed that only short heating periods are required and that vitrinite reflectance is thus an indicator of absolute temperature. As a result of this uncertainty with respect to the effects of time, conversion of vitrinite reflectance values to maximum temperatures attained within the sediments is extremely tentative.

A number of studies have correlated coal-rank with the formation of authigenic silicate minerals. A comprehensive review is given by Kisch (1983). A few studies have attempted to relate %S in I/S to vitrinite reflectance. However, these have mainly been concerned with the earlier part of the illite-smectite transformation down to values of around 20%S in I/S (Pearson et al., 1982; Pearson et al., 1983). One notable exception is the study by ~rodofi (1979) of sediments from the Upper Silesian coal basin of Poland. grodofi shows an inverse correlation between %S in I/S and vitrinite reflectance. Reflectance levels of 0.85-0.90% correspond to the transition from IS-ordered to ISII-ordered mixed-layer type at around 15%S in I/S. The present study was undertaken to provide more information on the relationship between these two important indicators of diagenetic grade.

Illite-smectite transformation and organ& maturity 457

G E O L O G I C A L S E T T I N G

The geology of the northern Pennines is well documented and has been reviewed by Rayner & Hemingway (1974) and Robson (1980). The study area includes two fault-bounded blocks, the Alston and Askrigg blocks, which are separated by the Stainmore trough (Fig. 1). Both blocks consist of a granitic core intruded into Lower Palaeozoic basement rocks. The position and geometry of the granites has been established by geophysical methods (Bott, 1967). A thin cover (~1500 m) of cyclic Carboniferous sediments is draped over each block. This thickens to around 2500 m in the Stainmore trough. Although the granites are Caledonian in age, there is substantial evidence that post-emplacement thermal events have occurred. Many workers have postulated that such events could have been responsible for the anomalous high-rank coals in the area. Potassium-argon studies have shown that a significant thermal event occurred within the Wensleydale granite of the Askrigg block about 300 Ma ago (Dunham, 1974). The petrographic study by Creaney (1980) of vitrinites from the Alston block provides evidence of some form of re-activation of the underlying Weardale granite, resulting, in places, in temperatures > 185 ~ C within the overlying sediments. During late Carboniferous times (295 + 6 Ma; Fitch & Miller, 1967), the Whin Sill and associated dykes were intruded into the Carboniferous sediments

B ~ Bt ublick Faults ... - ' / ,, , '

% ", E .... . . . . . .

f ~ ~ Brough �9 / "~STAINMORE B ~ TROUGH / / ?

Z , , , 4oo , , -1

FIG. 1 Location map of the area studied including position of sampling traverses A and B. Dashed lines indicate the extent of granite basement, including cupola regions (after Bott, 1967).

458 G. Smart and T. Clayton

overlying the Alston block. The large N-S trending faulted monocline known as the Burtreeford Disturbance predates the Whin Sill and acted as a channel for discordant movement of magma (Dunham, 1948). Extensive fault-controlled mineralization is present on both blocks.

S A M P L I N G

The majority of the samples were taken from the Four Fathom Limestone cyclothem on the Alston block (Fig. 5) and its stratigraphic equivalent on the Askrigg block. This cyclothem is well defined, easy to recognize, and crops out extensively over most of the study area. On the Alston block, this cyclothem lies directly below the vitrinite sampling horizon of Creaney (1980), enabling direct comparison to be made between clay mineral and vitrinite data. Where the Four Fathom Limestone cyclothem fails to crop out, for example in the Stainmore trough and towards the eastern limits of the two blocks, samples were taken from as close as possible. Where exposure was adequate, a number of samples of varying lithotypes were collected. Samples were taken from several N-S trending traverses across both blocks, two of which are shown in Fig. 1 and Fig. 4. A total of 458 samples was collected, including 357 mudstones and shales, and 95 sandstones, siltstones and limestones.

M E T H O D S A N D E Q U I P M E N T

All samples were washed free of surface contaminants and the <1.4 gm (e.s.d.) clay fraction was separated using standard sedimentation procedures. The clay fractions were Mg-saturated, smeared onto glass slides and studied by XRD after air drying, glycolation and heating to 375 and 550~ In addition, several organic-rich mudstones and coal samples were mounted in resin, polished in propan-2-ol using alumina, and examined under a Zeiss Universal Standard microscope fitted with an EMI 8944A photomultiplier. Maximum vitrinite reflectance was measured at 546 nm under oil immersion. Special care was taken to avoid oxidized material (Creaney, 1977).

A N A L Y S I S OF X - R A Y D I F F R A C T I O N P A T T E R N S

The methods of Reynolds & Hower (1970) and Reynolds (1980) were used to estimate %S in I/S. The presence of discrete illite in every sample, coupled with the low percentage of smectite in the illite-smectites, severely limited the application of the method of ~rodofi (1980) for the precise identification of illite-smectite interstratifications. ~rodofi's method takes into account the effect of the starting thickness of the smectite-glycol layer and the influence of domain size on the positions of the reflections. Ignoring these factors may result in up to 30% error in the estimation of the smectite to illite ratio in the mixed-layer structure. However, errors of this magnitude are only apparent at large percentages of %S in I/S. In the present study, where %S in I/S rarely exceeds 30%, the maximum error likely to be introduced by ignoring these factors is probably of the order of +4%S in I/S. This degree of error is quite acceptable, since it is trends in %S in I/S, rather than absolute values, that are important. In most samples, the positions of ~rodofi's critical reflections in the regions 42-48 ~ 20 and 26-27 ~ 20 could not be determined with any precision. For this

Illite-smectite transformation and organic maturity 459

reason, the reflection between 16 ~ and 18 ~ 20 was found to be the most suitable for the estimation of %S in I/S. This peak is only slightly affected by domain size and the effects of ordering (Reynolds & Hower, 1970). It is also least affected by the presence of discrete illite, the peak intensity of which is relatively low at this position.

R E S U L T S

Illite and interstratified illite-smectite are the most abundant clay minerals present, with lesser and variable amounts of chlorite and kaolinite. Typical XRD patterns are shown in Fig. 2. The %S in I/S within a given section varies considerably between samples, often by as much as 20%. It is usually much higher in sandstones and limestones than in surrounding mudrocks. Moreover, %S in I/S commonly, varies between different sandstones and siltstones within the same section. Homogeneous mudrocks appear to give constitent results, although the presence of only a small number of coarser-grained laminae is usually sufficient to produce anomalous data. For the above reasons, only homogeneous mudrocks were used to establish the trends described below.

Percentage S in I/S varies from 35 to <5%. There is a good inverse correlation between %S in I/S and vitrinite reflectance for the Askrigg block and the Stainmore trough (Fig. 3a). The traverse B-B' (Fig. 4a) shows that %S in I/S decreases and vitrinite reflectance increases towards the centre of the block.

In the case of the Alston block, there is no direct correlation between the clay and vitrinite data (Fig. 3b). The traverse A - A ' shows that %S in I/S tends to decrease towards

17~ 9~

<5%Sinl/S = /

20 18 16 14 12 10 8 6 4 2

~ Cu-Kr

FIG. 2. XRD patterns of glycolated, <1.4 gm clay fractions. (A-C) Whin-Sill-affected; increasing distance from the Burtreeford Disturbance. (D-G) Non-Whin-SiU-affected; D, Alston

block: E, Askrizg block; F, Stainmore trough; G, northern margin of Alston block.

460 G. Smart and T. Clayton

35

30

25

~ S i n l / S

2e

15

10

5

, ~ a)ASKRIGG BLOCK I I \ AND \~ ~ STAINMORE TROUGH

\ \

35

30

25

20

15

10

5

4 1 0 , L

%Ro(max)

b) ALSTON BLOCK

. . . : " 5 .

~, "* �9 . o ~ o ~ ~

~ .%~ ~ ~

= i i ~ i i i i i 110 210 3!0 1'.0 2'-0 310 4'-0 '5-0 V i t r i n i t e data "Present s t u d y

�9 Creaney (1980)

F[•, 3. Correlation between %S in I/S and vitrinite reflectance.

O

10

%S in I / S

2 0

a) ASKRIGG BLOCK

B B"

�9 -0 t % Ro ( m a x )

1-O

b ) ALSTON BLOCK

FIG. 4. Relationship of%S in I/S and vitrinite reflectance to the position of the granite basement.

the centre of the block, whereas vitrinite reflectance shows no overall trend (Fig. 4b). In addition, %S in I/S falls dramatically where sample sites are very close to the Whin Sills (Fig. 5).

In samples containing small amounts of discrete illite, some indication of the type of ordering could be established using the method of grodofi (1980). All samples studied were ordered to some degree, ranging from less than half partial IS-ordering at 30-35%S in I/S to ISII-ordered below 15%. A well developed superlattice peak is present in samples containing > 30%S in I/S. The peak remained detectable down to ~ 17%S in I/S.

lllite-smectite transformation and organ& maturity 461

D I S C U S S I O N

At any particular sample site, %S in I/S varies considerably between different lithotypes. Several authors have described similar variability (Boles & Franks, 1979; Howard, 1981). Generally the sandstones show a higher %S in I/S than the surrounding mudrocks, although the reverse is sometimes the case. The inconsistent results obtained from sandstones and siltstones is attributed to their higher permeability. Since the presence of only a small number of coarser-grained laminae in mudrocks is sufficient to effect the %S in I/S values, it is concluded that these laminae acted as pathways for the migration of ions to and from the system. Thus the chemistry of the pore fluids was more likely to be influenced by external conditions, and hence the nature and rate of formation of mineral phases within these open or semi-open systems would be much more variable. In addition to these chemical differences, the physico-chemical conditions affecting ion transport and crustal nucleation/growth would be different in sandstones and mudrocks, and might be expected to lead to differences in reaction rate.

In mudrocks, where the permeability was low, the rocks probably acted as closed systems to most components (Hower et al., 1976). In this situation, the chemistry of the pore waters was primarily controlled by the chemistry of the solid phases present, which depended to a large extent on the composition of the original detrital assemblage. Semi-quantitative mineralogical analysis suggests that this was reasonably uniform on a regional scale, and that major differences in the initial pore-fluid chemistry were unlikely. In particular~ the ubiquitous presence of discrete illite and K-feldspar is believed to indicate the presence of a readily available and regionally uniform source of potassium ions for the reaction.

In the case of the Askrigg block and the Stainmore trough there is an excellent inverse correlation between %S in I/S and vitrinite reflectance (Fig. 3a). When the distribution of vitrinite reflectance and %S in I/S across the block is considered, it can be seen (Fig. 4a) that there is a marked decrease in %S in I/S coupled with an increase in vitrinite reflectance towards the centre of the block. This variation appears to be related to the position of the underlying granite basement, and may be explained in terms of enhanced heat flow consequent upon some form of re-activation of the granite at depth. Although the granite is of Caledonian origin, significant post-emplacement thermal events are known to have occurred (Dunham, 1974).

The minimum value of around 15%S in I/S observed over the centre of the block would be consistent with a maximum temperature of around 185~ if the data of McDowell & Elders (1980) from the Salton Sea geothermal system are applicable. Using the method of Karweil (1956), the vitrinite reflectance maximum of 2.0% observed would be consistent with such a temperature if a period of heating of ~30 Ma is assumed. The absolute temperature curves of Ammosov et al. (1975) and Price (1983) give maximum temperatures of 230 and 280~ respectively, which seem rather high.

In the case of the Alston block, there is no direct relationship between the clay mineral and vitrinite data (Fig. 3b). Creaney (1980) has shown that vitrinite reflectance levels on the Alston block are directly related to the position and thickness of the Whin Sill. Using the heat equations of Jaeger (1964) and geological data on Whin Sill position and thickness (Dunham, 1948; Francis, 1982; and references therein), estimates of relative maximum temperatures likely to have been attained at sample localities along the traverse were obtained (Fig. 5). For the purposes of the calculation, zero background temperatures were

462 G. Smart and T. Clayton

O- A A" ALSTON

10- p ~ j /

% S in I/S 20

30-

% Ro 4"0 "-~ )~-L /" (max)

2 '0 "l ~

Estimated maximum | r" ~ temp.OC 300-~ ! L i (zero background) |__. ~ "N ]

100] . . . . . . .~ , u<l E r Lst ~ G r e a t Lst . ;, ~Vitr i~it e somp[e horizon ~]~ I Four Fathom Lst , -Ctay sample hor izon L ttle / 01~. Iscar Lat , ' , [~'~,~'~/~'~'n'Sill i T,--ot'omL" ~1~-] .e,merb.~oar ".t"" 1 _ ~';" J <: -L-P,~L-AEO-ZOIC: - - "j~.~* lOOm

Basement

FiG. 5. Relationship of %S in I/S and vitrinite reflectance to the position and thickness of the Whin Sill. (For illustrative purposes, the sampling horizons are reconstructed to be horizontal. The apparent displacement of the Whin Sill is an artifact resulting from its discordant

transgression across the Burtreeford Disturbance.)

assumed. Although the values obtained will not represent true absolute temperatures, they give a good indication of the relative temperature distribution likely to have been produced by the sill. It can be seen from Fig. 5 that vitrinite reflectance closely parallels these temperatures. The relatively high vitrinite reflectance levels on the Alston block as compared with those on the Askrigg block (Fig. 4) are therefore a direct result of the intrusion of the sill. It is apparent from Fig. 5 that the Whin Sill had little, if any, effect on %S in I/S, with the exception of the region close to the contact. Preliminary work shows that this region extends for a distance of about 1.5 times the thickness of the sill, corresponding to temperatures almost certainly in excess of 250~ Samples within this region show vitrinite reflectance > 3.3 %.

Although %S in I/S values are not directly related to the sill, they do appear to be related to the position of the underlying granite, as was observed for the Askrigg block. The poor correlation between vitrinite and clay data in Fig. 3b can thus readily be explained. Except where close to the sill, the effect of the intrusion is to shift the data points to higher values of vitrinite reflectance by an amount dependant on the thickness and distance of the sill, without significantly affecting the %S in I /S values.

Outside the region close to the sill, minimum observed values of %S in I /S appear to be ~ 15%. This is similar to the Askrigg block and comparison with the data of McDowell & Elders (1980) suggests maximum background temperatures of around 185 o C. This would

Illite-smectite transformation and organ& maturity 463

be consistent with the pre-Whin Sill temperatures of around 180~ proposed by Creaney (1980) on the basis of petrographic studies of coals. The close similarity between the baselines of data points down to 15%S in I/S in Figs 3a and 3b suggests that in the case of the Alston block vitrinite values located close to this boundary may be free from the effects of the Whin Sill. Analysis of the locations of these samples shows that they are situated at a distance of at least six times the thickness of the sill from the contact.

Because of the many uncertainties regarding the parameters involved, the exact time/temperature relationships of the intrusion of the sill are not known. In this respect, the background temperatures at the time of intrusion are particularly important. If background temperatures were at a maximum during intrusion, both higher absolute temperatures and a longer duration of heating would have occurred. If, however, intrusion took place during a time of low background temperature, much of the duration of heating would have taken place at temperatures below those resulting from block-centred heating. Further work is being undertaken on the clay minerals within the Whin Sill aureole in the hope of distinguishing between these two possibilities. Even in the former case, the time- temperature curves of Lovering (1935) suggest that the thermal effects of the sill are unlikely to have been important for more than a thousand years.

Under these circumstances, it is obvious that the response of vitrinite to rising temperature must be very rapid--much more rapid than would be predicted by extrapolation of the curves of Karweil (1956). In this situation, the absolute temperature curves of Ammosov et al. (1975) and Price (1983) seem to be more applicable.

Unlike vitrinite reflectance, the smectite to illite transformation failed to respond to the relatively short duration of heating caused by the sill, except where close to the contact. This would support previous experimental work which suggests that the reaction is very slow (Eberl & Hower, 1976; Roberson & Lahann, 1981).

~rodofi (1979) reported another situation in which clays responded much more slowly than organic matter to relatively rapid increases in temperature. In this case, illite-smectite clays failed to reflect a major synsedimentary overthrust, whereas vitrinite reflectance increased markedly.

C O N C L U S I O N S

The present study has shown that unlike the alteration of vitrinite, the progressive illitization of illite-smectites of low expandability occurs too slowly to reflect the thermal effects of minor igneous intrusions, except where close to the contact. The transformation does, however, respond to longer-lasting thermal events of a more regional character. Consequently, any study attempting to relate %S in I/S to temperature must consider the rate and duration of heating involved. As a result of the large difference in the rates of reaction, combined plots of %S in I/S and vitrinite reflectance should yield useful information in this respect. A wide spread of points, as observed for the Alston block, might suggest the presence of at least two heating events of widely differing duration. Where a single well-defined curve is obtained, the position of the curve may give some information regarding the rate and duration of heating. For example, Fig. 6 shows the approximate position of the curve obtained by interpolation of the data of ~rodori (1979) from the Upper Silesian coal basin of Poland, as well as the data obtained for the Askrigg block in the present study. The curve for the Silesian coal basin is situated below that of the Askrigg block such that the former shows a greater degree of illitization at any given

464 G. Smart and T. Clayton

351

30

25

20

%S in I/S

15

10

\

ck

Srodofi (1979)

110 ' 210 3~0 4~0 ' 5~) %Ro(max)

FIG. 6. Relationship of %S in I/S and vitrinite reflectance for the Askrigg block (present study) and the Upper Silesian coal basin (Srodofi, 1979).

vitrinite value. Although it is appreciated that other effects such as the nature of the initial smectite or even systematic errors in the estimation of %S in I/S may be responsible for the difference, flrodofi's data may well reflect a longer, more regional, period of heating than envisaged for the Askrigg block. Further studies involving %S in I/S and vitrinite reflectance from a wide variety of geological environments should prove revealing.

The differences in reaction rate of vitrinite and clay minerals may be of value in regional basinal analysis. For example, if vitrinite reflectance values are elevated as a result of the effect of small intrusions, the true burial history of the rocks will be masked. The clay data, however, will reflect the more regional patterns of heat flow encountered during burial.

A C K N O W L E D G M E N T S

We thank Dr D. Moore for valuable discussion on the stratigraphy of the area, Dr J. E. A. Marshall for advice on the measurement of vitrinite reflectance and Dr I. M. West for his critical reading of the manuscript. Thanks are expressed to Robin Saunders for excellent technical assistance and to Jackie Skinner for typing the manuscript. One of us (G.S.) wishes to acknowledge financial support from NERC.

R E F E R E N C E S

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I l l i te-smecti te t rans format ion and organic matur i ty 465

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