I L L I T E / S M E C T I T E D I A G E N E S I S IN D E V O N I A N L A C U S T R I N E M U D R O C K S FROM N O R T H E R N S C O T L A N D A N D ITS R E L A T I O N S H I P TO ORGANIC MATURITY INDICATORS S. H I L L I E R
Department of Geology, Universityof Southampton, Southampton S09 5NH (Received 20 June 1988; revised 10 February 1989) A B S T R A C T : Devonian rocks from the Orcadian Basin show an extreme range of organic
maturity. Mean vitrinite reflectance ranges from 0-6 to 10.5~ and spore colour from yelloworange to black. Illite/smectite (I/S) is abundant in the clay fraction of lacustrine mudrocks and shows variation in expandability from 30 to < 5~. A correlation is observed between expandability and vitrinite reflectance, but the specific relationship is different in the Moray Firth and the Caithness-Orkney areas. Illite 'crystallinity' measurements in the CaithnessOrkney area are also correlated with vitrinite reflectance. Geological evidence favours an I/S origin from precursor smectite, and mineral and chemical analyses of lacustrine shales show no evidence for regional differences in composition that might affect iUitization rates. The different relationships within each region are believed to result from the different relative response of organic maturity indicators and clay minerals to temperature and reaction time. The lower I/S expandabilities in the Caithness-Orkney region at a given vitrinite reflectance are attributed to relatively longer burial times.
Although organic indicators such as vitrinite reflectance and spore colour are used routinely to establish thermal maturity, clay minerals have only been used in the broadest sense. Most attention has focused on the use of mixed-layer illite/smectites (I/S) and illite 'crystallinity'. With increasing temperature, smectite is converted to illite via an intermediate series of progressively less expandable I/S (Perry & Hower, 1970). Hence, the percentage of expandable layers in I/S is potentially a useful indicator of diagenetic grade, particularly where organic indicators are unreliable or absent, as for example in red beds. However, there is increasing evidence that illitization is kinetically controlled (Perry & Hower, 1970; Srodon & Eberl, 1984) and that residence time at temperature can have a significant affect on the extent of illitization (Ramseyer & Boles, 1986). In organic metamorphism, temperature is recognized as the dominant control by all workers. The precise role of time is disputed, but recent work suggests that models such as those of Karweil (1956) and Lopatin (1971) over-emphasize the effects of time. Statistical analyses of large numbers of borehole samples of all ages show a strong correlation between vitrinite reflectance and temperature but a negligible correlation with time (Barker, 1983; Price, 1983; Robert, 1985; Barker & Pawlewicz, 1986). Thus, vitrinite reflectance is interpreted as being controlled essentially by maximum temperature and, apart from an initial 'stabilization' period, it is not significantly altered by geological time. Several combined studies of organic maturity indicators and illitization have compared 9 1989 The Mineralogical Society
S. Hillier and T. Clayton
regions known to have had different thermal histories (Wolf, 1975; Smart & Clayton, 1985; Robert, 1985). The results of these investigations demonstrate the importance of time in illitization. Organic maturity indicators respond much more rapidly to increasing temperature than illitization. The more short-lived the heating event, the greater is the extent to which illitization appears to 'lag' behind the changes in organic maturity indicators. Previous studies of the clay mineralogy of Devonian rocks from the Orcadian Basin have been made by Burollet et al. (1969) and by Wilson (1971). Both studies found the Middle Old Red Sandstone (MORS) of Caithness to be dominated by iUite and chlorite with sporadic occurrences of kaolinite and minor montmorillonite. This assemblage was interpreted by Wilson (1971) to be largely detrital and inherited from micaceous and chloritic rocks in the hinterland. Wilson (1971) also examined a number of samples from the MORS bordering the Moray Firth and found them to be dominated by I/S and kaolinite. The present paper describes the results of a regional study of organic maturity indicators which has provided a palaeotemperature framework for the Orcadian Basin. This is used to constrain the evidence for I/S diagenesis throughout the basin.
In NE Scotland, large areas of Devonian sedimentary rocks occur around the margins of the Moray Firth, in Caithness, in Orkney and in Shetland (Fig. 1). Collectively, these are the remnants of the Orcadian Basin, an extensive Old Red Sandstone continental sedimentary basin in which thick sequences of non-marine sediments of Early, Middle and Late Devonian age were deposited (Mykura, 1983). The MORS forms by far the thickest and most extensive sequences throughout the basin. In Caithness, the MORS is about 4000 m thick and is dominated by lacustrine sediments known as the Caithness Flagstones (Donovan et al., 1974). Organic-rich facies comprise a large part of the succession and are of interest as potential hydrocarbon source rocks (Marshall et al., 1985). In Shetland and in the Moray Firth region, fluvial sediments predominate but lacustrine intervals occur throughout both successions, these representing periods when the Orcadian lake transgressed the surrounding alluvial plains. The MORS Achanarras horizon represents a time of maximum lake expansion and it can be correlated throughout the basin due to its distinctive fish fauna. The Old Red Sandstone succession of Caithness and Orkney was deposited in a system of westerly-tilted half-graben structurally continuous with those seen on seismic sections in the offshore West Orkney Basin (Enfield & Coward, 1987). In the Moray Firth region, the pattern of sedimentation was controlled similarly by an extensional half-graben system (Rogers, 1987). In the Wails Basin, Shetland, Devonian sedimentation ended following the intrusion of the Sandsting Granite Complex (Astin, 1982) of late Devonian age.
All the samples examined in this study were lacustrine mudrocks collected at outcrop. The majority were grey-green shales, organic-rich grey-black shales or lacustrine carbonate laminites. Virtually all samples came from the MORS. Several tufts from the Achanarras interval on the island of Hoy, Orkney, were also examined.
Illite/smectite diagenesis and organic maturity
SHETLAND Melby1 ~ Foula,~.~ Walls Sandst Granite
'~ ,~' ORKNEY CAITHNESS ~ ~0G~ ~MOR
~T" TarbatNets ~;~
~ , . ~ - l n ' / G r e a t Glen
SHETLAND REGIONS I~ CAITHNESS ~-~ORKNEY [~MORAY FIRTH
_ A 1~'~7~
I 30km FIG. 1. Generalized map of the regional variation in expandability (percentage of swelling layers) of I/S in the Orcadian Basin, based on representative samples. For illustrative purposes the value of 3% is used to denote all samples of < 5% expandability. Also shown are the various regions of the Orcadian Basin referred to in the text.
Organic maturity indicators Kerogen concentrates were obtained by demineralizing samples in HC1 and H F without heating or oxidation. F o r vitrinite reflectance, only Type I I I kerogens rich in humic organic
S. Hillier and T. Clayton
matter were used in order to avoid the problem of reflectance suppression associated with samples rich in amorphous organic matter (Price & Barker, 1985). Concentrates were mounted and polished using the method of Hillier & Marshall (1988) and mean vitrinite reflectance (Ro) was measured at 546 nm with a Zeiss UMSP microscope linked to an IBM PC. In all, 468 samples were processed, of which only 170 contained suitable Type III material for vitrinite reflectance measurement. For most samples, R0 was calculated from between 15 and 55 individual vitrinite reflectance measurements. The standard deviation was generally of the order of 10% of the mean value. Where Ro was determined on several different samples from the same locality, the average Ro for that locality was used. Spore colour was measured in transmitted light on kerogen-strew slides by visual comparison with the Phillips Petroleum Pollen/Spore Colour Standard (Pearson, 1984). Spores were present in many samples not containing vitrinite, and consequently it proved possible to determine spore colour on 240 samples.
Clay mineralogy Crushed shale samples were dispersed in distilled water and the <2/zm clay fraction separated by settling under gravity. The clay fraction was Mg-saturated, smeared on glass slides and examined by X-ray diffraction (XRD) in an air-dried state, after glycolation (vapour pressure method), and after heating to 375~ A total of 360 samples was examined. Estimates of the relative abundance of clay minerals in the < 2/tm fraction were made for selected samples using the method of Biscaye (1965) with the addition of the procedure used by Schultz (1960) for I/S.
Expandability of illite/smectite The method of Reynolds (1980) was used to estimate the expandability of I/S down to values of 10%,'and results are reported to the nearest 5%. Due to overlap of I/S reflections with those of discrete illite, changes in the whole diffraction pattern in response to the various treatments were also considered in estimating expandabilities of <10%. Calculated diffraction patterns of ordered I/S with ~ 10% expandable layers show the presence of a peak at about 10.8 A in the air-dried trace and two peaks either side of 10 A in the glycolated trace (Hower, 1981). At lower expandabilities these peaks migrate towards 10 A. Thus, a distinct shoulder at about 10.8 A in the air-dried trace, substantial broadening on both sides of the combined 10 A peak after glycolation, and a considerable increase in both the height and the sharpness of the 10 A peak after heating, were taken to indicate the presence of an I/S component with ~ 10% expandability. Samples which showed smaller, but equivalent, changes in peak profiles and the absence of a distinct shoulder on the 10 A peak in air-dried traces were taken to indicate the presence of an I/S with around 5% expandability. Samples with considerably narrower diffraction peaks and only very slight changes in peak profile between the air-dried, glycolated and heated traces were considered to contain an I/S component with < 5% expandable layers. No claim is made that the results presented represent precise measurements of expandability, but it seems reasonable to assume that they correctly reflect real trends. The more accurate methods of Srodon (1984) for the estimation of I/S were not used mainly because of the low intensities and interference by discrete illite.
Illite/smectite diagenesis and organic maturity
Illite 'crystallinity' IUite 'crystallinity' was measured as the Kubler Index (peak width at half-peak height of the 10 A peak) in ~ using the air-dried preparations. Each sample was scanned three times using a scan speed of 0-5~ 20/min, and divergence, receiving and anti-scatter slits of 0.5~ 0.2 mm and 0.5~ respectively.
Mineral and chemical analysis Semi-quantitative analyses of the whole-rock mineral composition of a selected number of samples were made by XRD of random powders using aluminium powder as an internal standard. The same samples were prepared as fused beads and analysed for major elements by X-ray fluorescence spectrometry (XRF). RESULTS
Organic maturity data The results of the organic maturity study show that a wide range of maturity exists within the basin. Vitrinite reflectance ranges from 0-6% to 10.5% Ro. Most values lie between 0"8~o and 5% Ro and this range of maturity is present in all regions (Fig. 2). Spore colour data show a wide range from orange-yellow to black and highly carbonized, and corroborate the maturity variations shown by vitrinite reflectance.
Clay mineralogy The clay minerals illite, illite/smectite, kaolinite, chlorite, chlorite/smectite, chlorite/ vermiculite, and minor amounts of vermiculite were identified. On a regional scale, the main variation in these assemblages is in the expandability of I/S and the distribution of kaolinite and chlorite minerals. Only the occurrence and diagenesis of I/S is discussed in this paper; the distribution and diagenesis of kaolinite and chlorite minerals will be considered elsewhere. Ordered mixed-layer I/S with expandabilities ranging from 30% to < 5% were observed. The regional variation in expandability is shown in Fig. 1 and representative diffraction patterns are shown in Fig. 3. In the more expandable samples, where I/S peaks are wellresolved from those of discrete illite, I/S was estimated to comprise between 60 and 80% of the < 2/am fraction. In samples with lower expandability, increasing peak overlap made meaningful division of the diffracted intensity between I/S and discrete iUite impossible. However, the change in peak profile of the 10 A peak after glycolation and the increase in peak-height after heating indicate that all samples contain an I/S component (Fig. 4). The tufts from the MORS of Orkney contain both illite and I/S. They show a higher proportion of I/S than shales from this region but show similar expandabilities of ~ 10% (Fig. 4).
Relationship of expandability to maturity In the Moray Firth area, many samples show between 20 and 25% expandability (e.g. SH647 of Fig. 3) and these are associated with vitrinite reflectance values of around 1% R0. In a composite 3500 m section exposed along the SE-facing coastline of the Tarbat Ness peninsula (Rogers, 1987), expandability decreases from about 25% in the Upper Old Red
S. Hillier and T. Clayton
I-B ~ 14)
MORAY FIRTH OQ 1.0
FIG. 2. Generalizedmap of the regionalvariation in vitrinite reflectancein the Orcadian Basin, based on 170 samples. Sandstone at the top of the section to 10% in the Middle Old Red Sandstone at the base. Over most of the section, however, I/S contains between 20 and 25% expandable layers and maturity levels show little variation, ranging between 0.8 and 1%. A particularly high-rank area occurs to the east of the Great Glen fault (Fig. 2) with the most extreme value of 10.5% R0 being recorded here from rocks exposed along the shore of Loch Ness. Samples from this area show very sharp 10 A peak profiles with only slight traces of expandable layers (e.g. SH862, Fig. 3).
I l l i t e / s m e c t i t e diagenesis a n d organic m a t u r i t y
S. Hillier and T. Clayton
,J S22165_~ ~
.... G .... J
+20Cu Ko( FIG. 4. Comparison of XRD patterns of the < 2 #m fraction of low-expandability samples. SH968: mudrock from the Moray Firth area with ~ 10~ expandable layers. $22165: air-fall tuff from Orkney with ~ 10% expandable layers. SH927: mudrock from Orkney with ~ 10% expandable layers. SH465 mudrock from Orkney with ~ 5% expandable layers.
In the Caithness-Orkney area the highest expandabilities (10%) observed in shales and tufts occur at the top of the succession and correspond to vitrinite reflectance values of ~ 1% (e.g. SH927, Fig. 3). In samples from lower in the succession with higher maturities, expandabilities are lower and no distinct I/S peaks can be seen. In Fig. 5, vitrinite reflectance is plotted against percentage expandability for data from all areas. This shows that, at a given vitrinite reflectance, the percentage of expandable layers is much lower in rocks from Caithness and Orkney than it is in those from the Moray Firth area. The difference is most marked at low vitrinite reflectance values. In Shetland, shales from the island of Foula contain I/S with ~ 10% expandable layers and shales from the Melby region of Mainland Shetland contain I/S with ~ 15% expandable
lllite/smectite diagenesis and organic maturity
9 Moray Firth 9 W. Shetland 9 Caithness-Orkney
FIG. 5. Expandability of I/S vs. vitrinite reflectance for samples from various areas of the Orcadian Basin.
layers. Data from these areas plotted in Fig. 5 appear to be similar to data from the Moray Firth area. In the regions of Walls and SE Shetland, all samples show very sharp 10 A peaks with only slight traces of expandability and this is consistent with the very high maturity of these areas (Fig. 2).
Relationship of illite 'crystallinity' to maturity Measurements of the Kubler Index on samples from Caithness and Orkney show a wide range from 0.9 ~ to 0-18~ and are clearly correlated with maturity (Fig. 6). In these lowexpandability samples, illite 'crystallinity' measurements appear to be a more sensitive indicator of maturity-related changes than the simple categorization of I/S into ~ 5% and < 5% expandable layers used above.
Mineralogy and chemistry Table 1 shows the average chemical and mineral composition of 30 grey-green shales from the two major areas of this study. The results show that shales from the Moray Firth area have slightly higher quartz and lower total-clay contents than those from Caithness and Orkney, and this is reflected in the higher s e e 2 and lower A1203 contents. Although there is a wide
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% * o
%Ro FIG. 6. Relationship between the Kubler Index and vitrinite reflectance for samples from Caithness and Orkney.
TABLE1. Whole-rock average chemical and mineral composition of grey-green shales. A: samples from the Moray Firth region (n = 15). B: samples from Caithness and Orkney (n = 15).
x o x
T i O 2 A1203 Fe203
57"25 4"91 51.97 3"42
0.69 0"16 0"74 0"07
0"06 0-02 0"07 0-01
3"65 1.23 4.52 0-75
4.73 3-54 4.98 1.79
1-37 0"74 1"52 0"64
4"17 1-19 5"03 0"79
0.13 0"03 0.13 0"02
0.46 0-44 0"08 0.15
7.83 3-95 8"11 2"22
15"03 2"67 16.82 1.78
4.64 1"24 6-05 1.99
30.3 6"2 20"3 6"5
6-0 5"5 8.4 3"8
7.3 2.5 6-9 2"4
6-1 9"0 11.7 8-1
4.3 5.2 2-8 2"7
0.0 0-0 0"5 2"0
45.0 8"9 51.0 8.1
0.7 1.1 0.1 0"5
variation in feldspar content between samples, the mean is similar in both areas and notably high in comparison to 'average shale' values (cf. Pettijohn, 1975). The average K 2 0 content is higher in shales from Caithness and Orkney, but the m e a n K20/A1203 ratios are not significantly different. Carbonate content shows a wide variation between samples.
Organic maturity data Using the absolute geothermometer of Barker & Pawlewicz (1986), the general range of vitrinite reflectance values of 0.8-5% Ro observed in the Orcadian Basin corresponds to a
Illite/smectite diagenesis and organic maturity
palaeotemperature range of 120~176 If a geothermal gradient of 30~ is assumed, the lowest Ro values observed'regionally of around 0.8% imply uplift and erosion of at least 3 km of overlying sediments. High organic maturity values of 5% R0 or more in the Walls Basin, Shetland, result from the contact metamorphic effects of the Sandsting Granite complex, and comparable maturity levels elsewhere in the basin (Fig. 2) are probably related to similar but unexposed igneous intrusions (Hillier & Marshall, in preparation).
Diagenesis and origin of illite/smectite In the Moray Firth area, there is a good correlation between I/S expandability and vitrinite reflectance (Fig. 5). The most expandable I/S occurs in the areas with the lowest maturity and there is a progressive decrease of expandability with increasing maturity. Clearly, within the Moray Firth area, the percentage of expandable layers in I/S is related to thermal maturity. In the Caithness--Orkney area of the Orcadian Basin, variation in the percentage of expandable layers in I/S is also correlated with maturity (Fig. 5). The conventional methods of estimating expandability are difficult to apply precisely to such low expandability samples as occur in this region. In many ways, the diagenetic trend is better illustrated by the correlation of the illite 'crystaUinity' values with maturity (Fig. 6). In hydrothermal I/S Eberl et al. (1987) have shown that the Kubler Index is correlated with expandability. In terms of the interparticle diffraction model of Nadeau et al. (1984), both the expandability of I/S and the Kubler Index are related to the mean thickness of 'illite' particles. Although in sedimentary rocks the Kubler Index is affected by the presence of detrital illite, it will be largely controlled by the expandability of I/S if this component is abundant. In both the Moray Firth and the Caithness-Orkney areas the range of vitrinite reflectance values is similar, yet the relationship between vitrinite reflectance and expandability is different. It is therefore important to consider the possible origins of I/S in the Orcadian Basin and, in particular, if there is any evidence for different origins in these two areas. The formation of ordered I/S from precursor detrital smectite (or random highly-smectitic I/S) during shale diagenesis is well-documented from many sedimentary basins (Srodon & Eberl, 1984). Although the presence of precursor smectite in shales from the Orcadian Basin cannot be demonstrated directly, palaeoclimatic and provenance evidence favour its original presence. Palaeomagnetic evidence places the Orcadian basin in tropical latitudes at between 22~ 15~south of the equator throughout the Devonian (Tarling, 1985) and consequently a tropical climatic regime would be expected. Calcretes are known from a few localities around the basin (Parnell, 1983) pointing to semi-arid climatic conditions, whilst the cyclicity of the lacustrine facies sequences indicate repeated fluctuations between periods of semi-arid and periods of more humid climatic conditions. During the wetter periods, the Orcadian lake transgressed the surrounding alluvial plains and extended up to the basin margins. Weathering under these climatic conditions would be expected to favour the neoformation of smectite and kaolinite in amounts depending on variation in precipitation and drainage (Tardy et al., 1973; Singer, 1984). Evidence for provenance from conglomerate clast assemblages indicates that the firstcycle metamorphic and granitic rocks of the Caledonian basement were the major source of sediments (Mykura, 1983; Rogers, 1987). Clasts of volcanic rocks are often recorded in these assemblages throughout the Orcadian Basin and weathering of penecontemporaneous volcanics is likely to have been an important source of smectite. Indeed, direct evidence of
S. Hillier and T. Clayton
volcanism is provided by sequences of volcanic rocks and tufts which are preserved at several localities within the basin itself (Mykura, 1983). The tufts from Orkney contain abundant I/S which is almost certainly derived from precursor smectite. The fact that the I/S in these tufts show similar expandabilities to the I/S observed in the shales is further support for the original presence of smectite in the shales. Besides a smectite precursor, the possibility of a detrital origin for the ordered I/S must also be considered. Although ordered I/S probably does not form in soils (Wilson & Nadeau, 1985), it may be derived by erosion of pre-existing sedimentary rocks. Clasts of sedimentary rocks are not abundant in Orcadian conglomerate assemblages and, therefore, substantial amounts of detrital ordered I/S seem unlikely. Irrespective of the I/S precursor, there is no evidence for major differences in the bulk mineralogy and chemistry of shales from the different areas (Table 1). In addition, the XRD patterns of samples from the different areas with comparable expandabilifies of ~ 10% show similar 10 A peak profiles (Fig. 4), indicating that the ratio of discrete iUite to I/S is about the same. Apart from the mineralogical evidence, palaeogeographic reconstructions show that the Caithness-Orkney region formed the centre of the lacustrine basin and was broadly distal to the Moray Firth area. Sedimentological and faunal evidence indicates that the lacustrine intervals of the Moray Firth succession represent the periodic incursions of the Caithness Lake into this area. It is, therefore, doubtful that there would have been major differences in the clay mineralogy of the original lacustrine muds. From all of the above considerations it seems most probable that smectite was an original constituent of Orcadian lake muds and was the precursor of the I/S that is observed today. Several workers have described mechanisms for the low temperature illitization of smectite. Illitization in recent sediments of Lake Albert has been described by Singer & Stoffers (1980). This lake, however, is characterized by unusually high K+/Na + ratios, abundant zeolites, and drains a volcanic source area. Although evaporite pseudomorphs after gypsum are recorded in the Orcadian Basin, clastic and carbonate deposition predominated and there is no evidence for such a K-rich chemistry. Wetting and drying cycles have also been suggested as a mechanism for low-temperature iUitization, but this process appears only to form random I/S (Srodon & Eberl, 1984).
Regional variation in illite/smectite diagenesis Within each region of the Orcadian Basin there is a good correlation between vitrinite reflectance and the degree of illitization, but for each region the specific relationship is quite different. Shales with Ro values of ~ 1% in the Moray Firth region contain I/S with 20-25% expandable layers whereas, at the same level of maturity in the Caithness-Orkney region, I/S contains only 5-10% expandable layers. Comparable data from other sedimentary basins also show different relationships (Fig. 7) and it seems clear that a single universal correlation between organic maturity indicators and I/S diagenesis does not exist. Such regional variation could arise in two ways. First, because illitization is dependent on external geochemical controls, the absolute rate of reaction at a given temperature may vary from one sedimentary sequence to another. Secondly, because of the different relative response of organic maturity indicators and iUitization to temperature and time, their specific correlation in any region will be dependent on the thermal history of that region.
Illite/smectite diagenesis and organic maturity
~S;;~r~:agbcO~Basi n MF Moray Firth
CA Caithness 20
15 EXP % 10
%Ro FIG. 7. Comparison of relationships between I/S expandability and vitrinite reflectance from the Orcadian Basin with those of Srodon (1979) for the Silesian Coal Basin, Poland, and that of Smart & Clayton (1985) for the Askrigg Block, northern England.
Smectite-to-illite reaction rates In recent mechanistic models of the smectite-to-illite reaction (Nadeau et al., 1984; Alan & Peacor, 1986), expandability of ordered I/S is a function of the mean thickness of 'illite' particles in the c* direction. This implies that the reaction will be kinetically dependent on factors controlling supply and transport of material and the growth of illite in this direction. The relative importance of such factors is difficult to assess theoretically but empirical evidence indicates a variety of controls. Apart from temperature and time, the chemical composition of the rock and pore-fluid system is obviously important. In particular, the rate of illitization has been shown to be sensitive to the availability of K + ; low K + activity leads to suppression of the reaction in K-deficient bentonites (Altaner et al., 1984) and to apparent cessation of the reaction in Gulf Coast shales following complete K-feldspar dissolution. All shales from the Orcadian Basin contain considerable amounts of K-feldspar (probably reflecting the first-cycle nature of the original detritus), thus providing an abundant potential source of K +. An abundant source of K § ions may influence reaction rates but this would apply equally to all regions in the Orcadian Basin. The presence of ions such as Mg 2+ and Ca 2+ has been shown to inhibit reaction rates (Robertson & Lahann, 1981). The activities of these ions in Orcadian Basin shales would be dependent on the carbonate content of the samples. Although carbonate content fluctuates considerably between samples from any one
S. Hillier and T. Clayton
locality, expandability appears to be unaffected. Chemical differences in the starting smectite are also thought to affect reaction rates (Huff & Turkmenoglu, 1981) but, as discussed above, there is little evidence for major differences in the original mineralogy of the sediments.
Thermal history In the Orcadian Basin, the lack of evidence for regional differences in illitization rates suggests that the different relationships between I/S expandability and organic maturity may be due to different thermal histories. Since vitrinite reflectance is relatively more responsive to temperature and relatively less responsive to time than illitization (Smart & Clayton, 1985; Robert, 1985), longer heating times would be expected to lead to lower expandabilities at any given vitrinite reflectance value. Vitrinite reflectance values of between 0.8 and 1~o which, using the absolute geothermometer of Barker & Pawlewicz (1986) correspond to temperatures of 120~176 are associated with I/S of 20-25~ expandability in the Moray Firth region and I/S of 5-10~ expandability in the Caithness--Orkney region. The relatively lower expandabilities in the Caithness-Orkney region would imply a longer residence time at temperature for this region than for the Moray Firth region. There is very little geological evidence with which to constrain the thermal histories of the two areas. Offshore in the inner Moray Firth Basin, the Devonian Orcadian succession is presently buried beneath a several-kilometre-thick Mesozoic sequence. Obviously, this part of the original Orcadian Basin has had a post-Permian burial and thermal history very different from both the onshore outcrop areas. Onshore remnants of the Mesozoic succession are substantially thinner (Craig, 1983) and the post-Permian burial histories have probably been relatively similar for both the Caithness-Orkney and Moray Firth areas. Devonian to Permian burial and thermal histories are essentially unconstrained. However, different histories would be likely in view of the basin-margin versus basin-centre character of the two successions. The presence of a Permian dyke swarm in the Caithness-Orkney area provides some evidence for such differences. CONCLUSIONS In the Orcadian Basin, I/S expandability is related to thermal maturity. In different regions of the basin the specific relationship of vitrinite reflectance to illitization is probably controlled by the thermal history. In comparing such relationships with those of other sedimentary basins, kinetic factors besides temperature and time must also be considered. A simple universal correlation between organic maturity indicators and the degree of illitization does not exist. The relationships obtained in individual regions, however, demonstrate that it is feasible to use the smectite-to-illite reaction to determine maturity levels provided that it is 'calibrated' for the area of interest. In the Caithness--Orkney region, I/S with low expandabilities (5-10~) associated with vitrinite reflectance values of around 1~ are believed to have formed through long residence times at temperatures of the order of 120~176 In relatively young sequences, such low expandabilities are associated with higher temperatures. An independent control on temperature such as vitrinite reflectance provides a way to assess the importance of time in illitization, particulady in Palaeozoic sequences where the thermal history is often poorly constrained.
Illite/smectite diagenesis and organic maturity
ACKNOWLEDGEMENTS SH gratefully acknowledges receipt of an NERC CASE studentship with BP Petroleum Development Ltd., Aberdeen. John Marshall is thanked for his contribution to the study and Andy Leonard for supervision of work at BP. Dave Rogers provided many samples and much unpublished data on Orcadian stratigraphy. Thanks are also due to Tim Astin and John Parnell for the provision of samples. The technical assistance of Robin Saunders is greatly appreciated.
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