The thermal history of the Bowen Basin, Queensland, Australia: vitrinite reflectance and clay mineralogy of Late Permian coal measures

Tectonophysics 323 (2000) 105–129 www.elsevier.com/locate/tecto

The thermal history of the Bowen Basin, Queensland, Australia: vitrinite reflectance and clay mineralogy of Late Permian coal measures I. Tonguc¸ Uysal *, Miryam Glikson, Suzanne D. Golding, Frank Audsley Department of Earth Sciences, The University of Queensland, Brisbane Qld 4072, Australia Received 25 June 1999; accepted for publication 31 March 2000

Abstract The thermal history of the Bowen Basin (Queensland, Australia) has been investigated using vitrinite reflectance data and clay mineralogy. Vitrinite reflectance data combined with a study of clay mineral reactions indicates that the maximum temperatures which induced organic maturation of the Bowen Basin coals and extensive clay mineralisation are not related to deep burial metamorphism during the latest Middle Triassic–earliest Late Triassic as previously believed. The results of the present study indicate that the development of a zone of high heat flow in the latest Late Triassic had a major control on the thermal history of the Bowen Basin. High palaeogeothermal gradients estimated in the northern Bowen Basin are interpreted to result from convective heat transfer during a hydrothermal event. Variable heat distributions due to localised fracture-enhanced permeable zones acting as hot reservoirs in the deeper part of the basin may have been responsible for some significant local thermal anomalies in the lower coal measures. The estimated palaeogeothermal gradients in the southern Bowen Basin also indicate high heat flow in the lower sections of the stratigraphy. Sections in the southern Bowen Basin, however, are believed to reflect a rock dominated semi-closed system with low water/rock ratio, where rocks are impervious to circulating fluids and thus heat transfer may have occurred by conduction. The correlation between vitrinite reflectance and clay mineralogy shows a delay in illitisation reaction relative to organic maturity for many illite/smectite (I/S) mixed-layer clays in the northern Bowen Basin. This phenomenon can be explained as a result of insufficient time for the completion of mineral reactions and a variable potassium supply in relatively impermeable rocks. The relationship between I/S expandability and vitrinite reflectance for the Bowen Basin data compared to basins with known tectonic regimes suggests a thermal history in a rift setting for the Bowen Basin. The effect of thin igneous intrusions on clay mineral reactions is very limited. Intensive illitisation due to heating of intrusions can only be observed in narrow zones immediately adjacent to intrusive bodies. This further demonstrates that mineral reactions are too slow to record the effect of extremely short heating duration, in contrast to organic maturity indicators. These differences between mineral and organic parameters aid in the identification of local contact metamorphic effects. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Bowen Basin; clay minerals; illite and chlorite crystallinity; illitisation; thermal history; vitrinite reflectance

* Corresponding author. E-mail address: [email protected] (I.T. Uysal ) 0040-1951/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S0 0 4 0- 1 9 51 ( 0 0 ) 0 00 9 8 -6

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1. Introduction In recent years, many studies have emphasised the role of short-term thermal events in organic maturation and mineral reactions in sedimentary basins (e.g. Barker, 1988, 1991; Hillier and Marshall, 1992; Zhao et al., 1996; Rantitsch, 1997). A detailed review ( Robert, 1988) of published studies of basins from different tectonic settings has shown that most of the sedimentary basins have been subjected to hyper- or hypothermal events of greater or lesser intensity linked to periods of regional tectonic activities. Therefore, the traditional concept, which assumes that the maximum temperature in a basin is achieved at the time of maximum burial, is not appropriate as a sole mechanism in the reconstruction of thermal histories of many sedimentary basins. As such, when reconstructing the thermal history of a basin, it is important to consider not only sedimentation and stepwise burial, but also the geotectonic evolution of the region. Hyperthermal anomalies may occur before or after maximum burial, and can be a major cause of maximum palaeotemperatures that lead to mineral reactions and organic maturation. These palaeothermal events, characteristic of rift systems and arc/back-arc areas (Robert, 1988), may be associated with hydrothermal fluid circulation (e.g. Barker, 1983, 1991; Person and Garven, 1992), large igneous intrusions (e.g. Hillier and Marshall, 1992; Ebner and Sachsenhofer, 1995; Zhao et al., 1996), or regional high heat flow related to thin crustal thickness (e.g. Teichmu¨ller and Teichmu¨ller, 1986). Vitrinite reflectance has been proven to be a most useful technique in recording maximum palaeothermal events (Barker and Goldstein, 1990). A number of studies have shown that organic matter is very sensitive to temperature (Robert, 1988; Wolf, 1988). Organic maturation or coalification as expressed by ‘rank’ (Stach et al., 1982) is the most reliable indicator for geothermometry, because it is irreversible and therefore not affected by retrograde metamorphism ( Teichmu¨ller, 1987; Wolf, 1988). In addition to this organic thermal indicator, clay mineral geothermometry has been utilised in evaluating the thermal and tectonic history of sedimentary basins

(e.g. Pollastro, 1993). However, as clay mineral reactions depend on many variables (water/rock ratio, potassium availability, rock composition, and time) in addition to temperature, their combined use with vitrinite reflectance data has great advantage in thermal maturation studies (e.g. Pearce et al., 1991; Essene and Peacor, 1995; Sachsenhofer et al., 1998; Corrado et al., 1998). Because of different reaction rates effecting organic and inorganic components, correlation between these two maturity indicators may provide valuable information about thermal processes and geotectonic settings (e.g. Hillier et al., 1995; Rantitsch, 1997). The aim of this paper is to evaluate the vitrinite reflectance and clay mineralogy data, in terms of thermal maturation in the Late Permian coal measures of the Bowen Basin. Furthermore, the correlation between the two maturity parameters is used to provide constraints on the thermal history and geotectonic setting of the Bowen Basin. Applications for clay geothermometry most commonly reported in the literature are:(1) illite/smectite ( I/S) reaction (e.g. Hoffman and Hower, 1979; Pollastro, 1993); (2) illite and chlorite crystallinity (e.g. Frey, 1987; Yang and Hesse, ´ rkai, 1991); (3) illite and chlorite polytyp1991; A ism (e.g. Frey, 1987; Walker and Thompson, 1990; Yang and Hesse, 1991; Hillier, 1993); and (4) chlorite composition (e.g. Cathelineau, 1988; Hillier and Velde, 1991). The present study has utilised the first three approaches.

2. Geological setting The Permo-Triassic Bowen Basin forms the northern part of the ca. 2000 km long Bowen– Gunnedah–Sydney Basin System in eastern Australia (Fig. 1). The northern half of the Bowen Basin is exposed and extends southwards to ca. 25°S, where it is overlain by Jurassic–Cretaceous sediments of the Surat Basin ( Fig. 1). The latter is a part of the much larger Great Artesian Basin system, which also comprises the Eromanga and Clarence–Moreton Basins (Fig. 1A). A three phase model has been proposed for the tectonic evolution of the Bowen Basin by Fielding

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Fig. 1. (A) Simplified geological map showing the major tectonic elements of north-eastern Australia (after Murray, 1990 and Holcombe et al., 1997a) and position of study area. (B) Map showing locations of boreholes referred to in Table 2 and text. GR, Grosvenor, DR, Drake, WH, Wodehouse; C, Cairns County; HL, Hillalong; KL, Killarney; BL, Blackwater; ST, Struan; SL, Sunlight; TA, Taroom; GL, Glenhaughton; TH, Theodore; CK, Cockatoo Creek.

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et al. (1997): (1) an Early Permian period of extensional subsidence with associated volcanic activity; (2) an early Late Permian passive thermal subsidence phase; and (3) a Late Permian to Middle Triassic phase of foreland thrust loadinduced subsidence. The Bowen–Gunnedah–Sydney Basin System runs along the entire western margin of the New England Fold Belt ( Fig. 1A). The New England Fold Belt developed in a convergent plate margin related to a Palaeozoic–Early Mesozoic westdipping subduction zone ( Korsch et al., 1990). The associated calc-alkaline dominated LateCarboniferous volcanism is transitional into Early Permian bimodal volcanism which is interpreted to be related to the extension that initiated the formation of the Bowen Basin ( Holcombe et al., 1997a). The contractional Hunter–Bowen Orogeny, occurring from Late Permian to Middle Triassic, overprinted the extensional and sag phases of the basin system (Holcombe et al., 1997b). The formation of the latest Permian coal measures was associated with high-energy alluvial conditions close to the basin margin during the Late Permian–Middle Triassic phase of foreland loading, where coarse clastic sediment from the rising, volcanically active orogen was shed westward into the basin (Fielding et al., 1993). Sediment accumulation in the Bowen Basin terminated at ca. 235– 230 Ma as a result of westward migrating thinskinned thrust deformation, uplift and erosion of the basin fill (Fielding et al., 1997). The foreland loading period corresponds with the maximum burial of Bowen Basin sediments as determined from subsidence modelling by Baker and de Caritat (1992). Following the uplift of the New England Fold Belt and the Bowen Basin in the Late Triassic, the tectonic evolution of eastern Australia was marked by a transition from convergence to an extensionrelated event. The latter involved silicic granite intrusions and development of a number of small, north–south elongate fault-bounded basins accommodating bimodal lavas along the New England Fold Belt ( Fielding, 1996). The widespread occurrence of intrusions (dykes, plugs and sills) mainly of Cretaceous age (Hamilton, 1985; Marshallsea,

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1988) within the Bowen Basin is the latest thermal event of potential relevance to this study. The Cretaceous magmatism has been interpreted as the products of rifting associated with formation of the Coral and Tasman seas and the fragmentation of the Gondwana super continent (Bryan et al., 1997).

3. Sampling and analytical methods Samples were selected from the Late Permian sedimentary rocks (mudrocks, sandstones and bentonites) at various depths with as much stratigraphic coverage as possible in boreholes distributed basin-wide ( Fig. 1B). The sampled stratigraphic sections in the northern part of the basin comprise the Moranbah, Fort Cooper and

Rangal Coal Measures ( Fig. 2). The sections in the Theodore area in the southeast consist of the Baralaba Coal Measures, which are equivalent to the Rangal Coal Measures in the northern part of the basin. The section sampled in borehole Cockatoo Creek 1 (CK 1) in the south contains an interval of sediments from the Baralaba Coal Measures at 1127 m to Barfield Formation at 3681 m (Fig. 2). All samples used in the current study were obtained from drillcores. Vitrinite reflectance data for Bowen Basin coals from different stratigraphic sections have been previously reported by Beeston (1981). In the present study, these were mainly used as R max o values relevant to authigenic mineral distributions in selected boreholes. The corresponding R max o values for the rock samples have been obtained by interpolation and extrapolation from Beeston’s

Fig. 2. Lithostratigraphic correlations for the Bowen Basin (after Draper et al., 1990).

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I. T. Uysal et al. / Tectonophysics 323 (2000) 105–129 Table 1 Vitrinite reflectance values measured in the present study %R range o

%R mean o

STD

No. of measurements

%R maxa o

Northern Bowen Basin WH2-1 Siltstone WH2-3 Sandstone WH2-11 Mudstone GR71-1 Mudstone GR71-6 Mudstone GR71-8 Mudstone C11-5 Mudstone C11-6 Mudstone C11-8 Mudstone C38-3 Mudstone C38-5 Mudstone GR75-4 Mudstone GR75-5 Siltstone GR75-7 Mudstone HL1-5 Sandstone C13-5 Bentonite DR24-1 Sandstone C16-9 Mudstone C16-10 Sandstone GR7-1 Bentonite DR26-9 Sandstone GR67-1 Sandstone

1.66–1.75 1.29–1.50 1.93–2.33 1.31–1.79 2.40–3.67 2.37–5.1 1.06–1.18 1.17–1.55 1.14–1.99 1.32–1.57 1.55–1.93 2.01–2.15 2.16–2.97 3.0–3.2 1.85–2.58 1.33–3.05 1.0–1.55 0.98–1.17 1.08–1.16 0.95–1.18 1.94–2.23 1.1–1.75

1.7b 1.41 2.20 1.58 2.99c 3.76c 1.13 1.33 1.52 1.43 1.66 2.10 2.53 3.10 2.31b 1.84b 1.41 1.10 1.12 1.07 2.1c 1.48c

0.026 0.059 0.11 0.106 0.402 0.475 0.049 0.105 0.257 0.073 0.099 0.047 0.182 0.082 0.252 0.468 0.094 0.044 0.025 0.06 0.063 0.167

27 26 28 35 35 30 7 11 15 19 14 14 20 11 31 12 38 25 11 24 32 24

1.43 1.64 2.31 1.61

Southern Bowen Basin TH4-3 Coal TH4-9 Coal TH5-1 Sandstone TH5-5 Coal TH6-3 Coal TH6-4 Coal TH6-5 Coal TH6-6 Coal TH6-9 Coal TH7-1 Mudstone TH7-2 Coal TH7-3 Coal TH7-4 Mudstone TH7-5 Sandstone TH7-6 Coal TH7-8 Coal TH7-9 Coal TH7-10 Mudstone TH7-11 Coal TH7-12 Coal

0.55–0.63 0.42–0.50 0.90–1.1 0.50–0.65 0.50–0.62 0.67–0.77 0.58–0.65 0.59–0.65 0.61–0.77 0.71–0.78 0.48–0.66 0.48–0.62 0.54–0.69 1.3–1.44 0.70–0.77 0.57–0.68 0.58–0.72 0.87–1.05 0.56–0.73 0.59–0.73

0.55 0.45 1.05b 0.58 0.55 0.65 0.55 0.55 0.65 0.75b 0.50 0.50 0.55 1.35b 0.75 0.55 0.60 0.90b 0.60 0.60

0.071 0.071 0.069 0.05 0.071 0.071 0.071 0.071 0.071 0.071 0.1 0.1 0.071 0.044 0.071 0.071 0.1 0.1 0.1 0.1

25 34 28 30 25 27 28 25 25 7 32 35 30 29 26 38 34 26 45 32

Sample

Lithology

1.27 1.34 1.52 1.53 1.67 2.65 2.99 3.15 0.98 1.17

a *%R max values are interpolated from depth/reflectance profiles of Beeston (1981). o b Oxidised organic matter. Such values are not used as thermal maturity indicator. c Sample taken near intrusion contact.

depth/reflectance profiles. These values were converted to random reflectance (R ) using the equao tion of Ting (1978). In addition, in the current

study, random reflectance was measured on dispersed organic matter in some sandstones and mudrocks as well as on coal samples from the

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Theodore area in the southern part of the Bowen Basin ( Table 1). Samples were embedded in polyester resin and polished using standard techniques for coal petrology (Stach et al., 1982). A Leitz MPV-2 photomicroscope was used both for observations and reflectance measurements. The number of reflectance measurements taken varied between 7 and 45, depending on the organic matter concentration and size of the organic particles. Calibration of the photometer was carried out using polished glass standards. Vitrinite reflectance values were used to estimate palaeotemperatures based on the equation of Barker and Pawlewicz (1986) ( Table 2). The clay mineral analyses were carried out on a Philips PWI840 XRD unit operated at 40 kV and 25 mA at a scanning rate of 1.2°2h min−1 with CoK radiation. Oriented clay specimens were recorded from 2 to 40°2h in an air-dried state, after ethylene glycol treatment, and for some selected samples after heating to 375 and 550°C. Illite content in I/S mixed-layer clays was

determined using the method proposed by Moore and Reynolds (1989). In this method, the precise angular difference between the I/S 001/002 peak and 002/003 peak is used to estimate the percentage of illite in a given I/S phase. The analytical error is estimated at ±5% (Moore and Reynold, 1989). Estimates of the relative abundance of clay minerals were carried out using the techniques described by Biscaye (1965) and Schultz (1964). A detailed description of sample preparation and analytical procedures for clay mineral analyses is given in Uysal (1999) and Uysal et al. (2000a). Several methods have been proposed to quantify illite crystallinity [for a review see Frey (1987)]. The most common indicator is the Ku¨bler index (or crystallinity index), defined as the width of the ˚ peak) at half first order illite basal reflection (10 A height and expressed usually in D2h values. The Ku¨bler index decreases with increasing illite crystallinity, and temperature is considered to be the most important factor controlling this process

Table 2 Vitrinite reflectance (from Beeston, 1981) and palaeotemperature data for the Late Permian formations Borehole

Formation

%R

Killarney 1 Wodehouse 2 Grosvenor 71 Grosvenor 75 Hillalong 1 Drake 17 Drake 18 Drake 12A Drake 9 Grosvenor 6 Grosvenor 3 Grosvenor 2 Grosvenor 12 Grosvenor 14 Cairns County 11 Cairns County 10 Cairns County 13 Cairns County 34 Blackwater 38 Cairns County 38 Taroom 10 Sunlight 1 Struan 1 Cockatoo Ck. 1 upper Cockatoo Ck. 1 lower

Moranbah C.M Rangal–Fort Cooper C.M. Fort Cooper–Moranbah C.M. Fort Cooper–Moranbah C.M. Rangal–Fort Cooper C.M. Fort Cooper C.M. Moranbah C.M. Rangal–Fort Cooper C.M. Fort Cooper–Moranbah C.M. Rangal–Fort Cooper C.M. Fort Cooper–Moranbah C.M. Moranbah C.M. Rangal–Fort Cooper C.M. Fort Cooper–Moranbah C.M. Rangal–Fort Cooper C.M. Fort Cooper–Moranbah C.M. Fort Cooper–Moranbah C.M. Burngrove–German Ck. Fm. Burngrove–Fair Hill Fm. Fair Hill–German Ck. Fm. Bandana–Cattle Ck. Fm. Bandana–Fair Hill Fm. Bandana Fm. Reids Dom Beds Baralaba C.M.–Gyranda Fm. Gyranda–Flat Top Fm.

2.98–3.52 1.52–2.51 1.59–2.38 2.34–3.55 0.87–1.08 0.88–1.18 1.17–1.55 0.94–1.10 1–1.39 0.91–1.10 0.91–1.17 1.07–1.45 0.94–1.30 1.02–1.41 1.22–1.46 1.55–2 1.40–1.72 1.41–1.87 1.25–1.48 1.53–2.07 0.46–0.92 0.73–0.94 0.62–1.30 0.70–0.89 0.89–1.60

max

range

T °C range max

%R /100 m max

T °C/km max

253–270 183–235 187–229 228–271 124–147 126–156 156–185 133–149 139–173 129–149 129–156 146–178 133–167 141–175 160–179 185–211 174–196 175–204 162–180 183–215 58–130 106–133 89–167 102–127 127–188

0.248 0.169 0.16 0.19 0.069 0.074 0.106 0.055 0.121 0.074 0.074 0.159 0.096 0.154 0.102 0.135 0.18 0.108 0.108 0.117 0.044 0.049 0.042 0.028 0.058

80 90 87 72 74 75 82 56 106 76 75 132 80 102 79 78 119 69 83 70 66 60 47 30 49

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(Frey, 1987). In this study, illite crystallinity ( Ku¨bler index) values were measured on samples of <2 mm fraction containing only insignificant amounts of expandable layers, as deduced from the small changes in the peak positions between the air-dried and ethylene glycol solvated samples. ˚ peak width at half height is Chlorite (002) 7 A most suitable to determine chlorite crystallinity because the presence of I/S, chlorite/smectite (C/S ) and vermiculite/chlorite mixed-layers may affect ˚ reflection, but has little or the chlorite (001) 14 A ˚ no effect on the 7 A peak ( Yang and Hesse, 1991; ´ rkai, 1991). In this study, chlorite crystallinity A ˚ peak of the <2 mm clay was measured on the 7 A fractions. To avoid the interference problem caused by the 001 kaolinite peak, samples containing this mineral were not used for chlorite crystallinity measurements.

4. Results 4.1. Vitrinite reflectance Reflectance values range from 0.45% R in the o Baralaba Coal Measures (Rangal Coal Measures equivalents) in the southern Bowen Basin to >3.5% R in the Moranbah Coal Measures in the o northern Bowen Basin. At intrusion contacts, they can be as high as 6.5% R max (Scott, 1987). o Isoreflectance contours reported in earlier studies (Beeston, 1986) show that the coalification in the Bowen Basin generally increases towards the eastern margin of the basin ( Fig. 3). It is apparent from the isoreflectance contours that reflectances in the Rangal Coal Measures reach the maximum values ca. 2.7% R max along a NW–SW directed o axis east of Bluff. Another area of high coal rank is south of Nebo in the northern part of the Basin, namely the northern projection of the isoreflectance axis near Bluff. The reflectance values decrease gradually to the north and east in the northern Bowen Basin, and south-easterly in the southern Bowen Basin. Lowest reflectance values are obtained in the Theodore area. In the southernmost part of the Bowen Basin where it is overlain by the Jurassic and Cretaceous sediments of the Surat Basin, increase in coalification occurs

Fig. 3. Isoreflectance map of the top of the Rangal Coal Measures equivalents in the Bowen Basin (after Beeston, 1986). Also shown is the relation of distribution of isoreflectance contours with depocentre of Triassic strata in the Bowen Basin. The information of the Triassic depocentre is based on isopach maps of Kassan (1993) for the Rewan and Clematis Groups.

towards the centre of the Taroom Trough, although, the Rangal Coal Measures equivalents do not exceed reflectance values of 1% R max. o Depth/reflectance curves reported for different areas across the Bowen Basin (Beeston, 1981) indicate increasing reflectance with depth, and a higher reflectance gradient in boreholes with higher

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reflectance values ( Table 2). The same study, however, also shows some significant local variations in reflectance gradients. For example, sediments (boreholes Drake 9, Grosvenor 2 and Grosvenor 14) on the western margin of the northern Bowen Basin exhibit anomalously high reflectance gradients over a relatively low reflectance regime ( Table 2). In the southern Bowen Basin, on the other hand, reflectance profiles of some boreholes (Cockatoo Creek 1, Glenhaughton 1 and Westgrove 3) are characterised by a succession of two different gradients. For example, in the upper part of borehole Cockatoo Creek 1 representing the Baralaba Coal Measures, a gradient of 0.028%/100 m is obtained, whereas in the deeper section, a change in coalification gradient is obtained with 0.058%/100 m. Similarly, in other boreholes in the southern Bowen Basin, shallow sections represented by the Rangal Coal Measures equivalents (Baralaba Coal Measures and Bandana Formation) and the Jurassic-Cretaceous sediments of the Surat Basin exhibit very low reflectance gradients, as low as 0.019%/100 m. 4.2. Clay mineralogy Clay mineralogical data based on X-ray diffraction ( XRD) and petrographic analyses of 256 mudrocks, sandstones and bentonites are presented in Uysal (1999) and Uysal et al. (2000a). Clay mineral separations of samples from the northern Bowen Basin consist mainly of R=1 and R≥3 mixed-layered I/S, chlorite and kaolinite. In the southern Bowen Basin, samples from the higher stratigraphic intervals are characterised by the occurrence of randomly ordered (R=0) I/S mixedlayers, and kaolinite and chlorite in smaller amounts. Deeper samples consist of R≥3 I/S, chlorite and chlorite-rich C/S mixed-layers. 4.2.1. Illite/smectite mixed-layers In Fig. 4(A), I/S expandability is plotted against vitrinite reflectance for the whole Bowen Basin data set including those of Baker (1989) from the Denison Trough in the south-west. While a trend of decreasing expandability of I/S with increasing reflectance values is indicated for the samples from the southern Bowen Basin, no clear correlation exists for the R=1 and R≥3 I/S

Fig. 4. (A) Expandability of I/S versus vitrinite reflectance for samples from various areas of the Bowen Basin. (B) Also shown is a comparison of the data with a theoretical range of correlations between vitrinite reflectance and I/S diagenesis determined by Hillier et al. (1995) for basins with different thermal and tectonic histories.

samples from the northern Bowen Basin. At a given vitrinite reflectance, many samples in the north contain much higher proportion of expandable layers in I/S than those in the south. In the north, there is no consistency of reflectance values at which the transition from R=1 to R≥3 ordering occurs. Many samples show that the R=1 I/S can persist up to 3% R and that R≥3 I/S can be o associated with vitrinite reflectance values of as high as 3.32% R , and higher for those samples o immediately adjacent to the intrusions.

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Fig. 5. Relationship between illite crystallinity and vitrinite reflectance values for samples from the Bowen Basin.

4.2.2. Illite and chlorite crystallinity The illite crystallinity values of the Bowen Basin samples show a great scatter, ranging from 0.44

to 1.03° 2Dh (Co), and there is no correlation with the vitrinite reflectance values (r=−0.29) ( Fig. 5). Chlorite crystallinity values range from 0.23 to

Fig. 6. Plot of chlorite crystallinity values versus present depth from boreholes Cockatoo Creek 1 in the southern Bowen Basin and Wodehouse 2 in the northern Bowen Basin.

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Fig. 7. Correlation between chlorite crystallinity (ChC ) and vitrinite reflectance (%R ) for samples from the Bowen Basin. max

0.69° D2h (Co) for the <2 mm clay fractions. Chlorite crystallinity measurements for clay fractions from the selected boreholes are shown in Fig. 6. The most continuous borehole, Cockatoo Creek 1, exhibits a regular increase in chlorite crystallinity with increasing depth ( Fig. 6). Chlorite-rich C/S mixed-layer minerals show higher crystallinity values and data fall along a separate trend. A general trend of increasing chlorite crystallinity with increasing vitrinite reflectance values is observed throughout the study area (Fig. 7). However, the samples from the borehole GR 75, located near a large intrusion (Bundarra Granodiorite), show higher coalification at the same values of chlorite crystallinity and plot away from the linear trend.

In the present study, chlorite polytypes can be determined from the random powder analysis of chloritic minerals from the borehole Cockatoo Creek 1 (Fig. 8). These minerals with small amounts of expandable layers occur as a single clay mineral phase, so avoiding any interference problem caused by the presence of illitic clay minerals. The sample CK.1-6 is identified as Ib (b=90) polytype, whereas the samples CK.1-12 and CK.1-13 from the deeper section of the borehole, show well defined diagnostic peaks of the IIb polytype.

4.2.3. Illite and chlorite polytypism In the randomly oriented powder diffraction pattern of I/S samples from the study area, no diagnostic polytype reflections can be detected. This may indicate the presence of structurally disordered, 1 Md polytype, which is characterised by very weak and diffuse reflections ( Yoder and Eugster, 1955; Brindley, 1980). However, weak ˚ in the sample WH2-8, reflections at 3.06 and 3.66 A which is a highly illitic, R≥3 type I/S ( Uysal, 1999), suggest the presence of 1 M illite polytype.

Several authors have suggested that thermal maturation of the Late Permian coals in the Bowen Basin was achieved during the Middle–Late Triassic, when the strata were buried to a maximum depth of 3000–3500 m (e.g. Beeston, 1986; Mallett et al., 1990; Baker and Caritat, 1992). The depositional and subsidence history of the Triassic sediments in the Bowen Basin has been studied extensively by Kassan (1993), who concluded that the basin underwent rapid subsidence during the Early Triassic. The same author documented the

5. Discussion 5.1. Vitrinite reflectance in relation to burial history

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Fig. 8. Random powder XRD patterns of chloritic minerals from borehole Cockatoo Creek 1 for samples (A) CK1-6 (2574 m), (B) CK1-12 (3680 m) and (C ) CK1-13 (3681 m).

depocentre as having been located in the southeast of the basin at the time ( Fig. 3). If thermal maturation of the Late Permian coals was the result of deep burial during the Triassic, then the maximum vitrinite reflectance values can be expected to coincide with the depocentre of the Triassic sediments. However, it is apparent from Fig. 3 that there is no systematic correlation between the isoreflectance contours with maximum deposition. Maximum coalification occurs in an area from Bluff in the central Bowen Basin to south of Nebo in the northern Bowen Basin, while the basin depocentre is located east of Theodore in the south-eastern part of the basin. Thus, thermal maturation of the Late Permian Coals seems unlikely to be the result of Triassic burial. 5.2. Estimation of palaeotemperatures and geothermal gradients In contrast to the traditional concepts, which consider organic maturation to be a function of

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both maximum burial temperature and effective heating time ( Karweil, 1956; Waples, 1980), the time-independent approach (Price, 1983; Barker and Pawlewicz, 1986) has gained general acceptance recently for interpreting thermal histories of areas characterised by complex geological histories (e.g. Barker, 1983; Pollastro and Barker, 1986; Barker, 1991; Hillier and Marshal, 1992; Laughland and Underwood, 1993; Zhao et al., 1996; Akande and Erdtmann, 1998). While the application of time–temperature models is limited to first-cycle sedimentary basins with well known burial and thermal histories, the time-independent method based on several statistical correlations between vitrinite reflectance and peak temperature can be applied to complicated orogenic sequences (e.g. Underwood et al., 1993; Laughland and Underwood, 1993) and hydrothermal systems (e.g. Barker, 1983, 1991). Excellent correlations between fluid-inclusion homogenisation temperature and vitrinite reflectance confirm temperature as the major control on organic maturation (Aizawa, 1989; Barker and Goldstein, 1990). The effect of heating duration has been shown to be negligible from extensive studies documenting organic maturation and hydrocarbon generation in hydrothermal systems (Simoneit, 1985, 1994). The estimated maximum palaeotemperatures range from 60°C in the southern Bowen Basin to 333°C near an intrusion contact in the northern Bowen Basin. Except for the deeper parts of the basin (e.g. in borehole Cockatoo Creek 1), sediments in the south and south-east record palaeotemperatures <130°C, whereas the estimated temperatures in the northern part of the Basin are higher. Reflectance gradients and estimated palaeogeothermal gradients in the Bowen Basin are presented in Table 2. Reflectance gradient is dependent mainly on degree of coalification as a result of geothermal gradient ( Teichmu¨ller and Teichmu¨ller, 1986). Therefore, significant changes in reflectance gradients between different locations in the northern Bowen Basin may be explained as indicative of variations in the palaeogeothermal gradient. The estimated palaeogeothermal gradients can be obtained from the slope of regression equations between the calculated maximum palaeotemper-

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Fig. 9. Estimated maximum palaeotemperatures and their variation with depth of boreholes Grosvenor 2 and Grosvenor 3.

atures and depths (Fig. 9). Extremely high palaeogeothermal gradients in excess of 100°C km−1 are estimated for the lower coal measures ( lower part of the Fort Cooper Coal Measures and Moranbah Coal Measures) as evident in some boreholes. These gradients are anomalously high compared to data from boreholes in other areas in the equivalent stratigraphic levels ( Table 2). palaeotemperature–depth profiles for boreholes Grosvenor 2 and Grosvenor 3 in the same area (Fig. 1B) are shown in Fig. 9. It is evident that the deeper section (Moranbah Coal Measures) as represented in borehole Grosvenor 2 renders a maximum palaeogeothermal gradient of ca. 132°C km−1, whereas the stratigraphically higher coal measures (Fort Cooper Coal Measures) intersected in borehole Grosvenor 3 have a gradient of ca. 75°C km−1. The above results indicate that excessive local palaeo-heat flows must have existed in deeper parts of the northern Bowen Basin, which have induced such anomalously high geothermal gradients in the lower coal measures. The inflection in the reflectance gradient

observed at ca. 1700 m in borehole Cockatoo Creek 1 in the southern Bowen Basin may indicate a significant change in the thermal regime. As illustrated in Fig. 10, the Baralaba Coal Measures (Rangal Coal Measures equivalents) in the upper part have an estimated palaeogeothermal gradient of ca. 30°C km−1, whereas in the deeper section a much higher gradient of ca. 49°C km−1 is obtained. Together with similar reflectance gradients observed in some other deep boreholes in the south (Beeston, 1981), it can be concluded that a high heat flow existed in deeper sediments in the southern Bowen Basin. Vertical and local variations in geothermal gradient commonly reflect changes in the heat conductivity of the sections, where poorly conductive shaly formations, coals and carbonaceous mudstones are characterised by high geothermal gradients and high heat flow (e.g. Robert, 1988; Pollack and Cercone, 1994; Cercone et al., 1996). However, abundant lithology in the Bowen Basin coal measures consists of high conductivity coarse-grained sediments including sandstone, siltstone and con-

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Fig. 10. Vitrinite reflectance–depth profile for borehole Cockatoo Creek 1 (modified from Beeston, 1981).

glomerate. Coal seams and carbonaceous mudstones occur as relatively thin layers, and have no significant thickness, which would explain these variations in the heat flow pattern. Therefore, lithologic changes in themselves are inadequate in explaining the anomalies observed in palaeogeothermal gradients in the northern Bowen Basin. In the southern Bowen Basin, in contrast, low palaeothermal gradient is estimated in the Baralaba Coal Measures which contain abundant coarse-grained sediments, whereas the underlying Back Creek Group consisting of shales with minor sandstone (McClung, 1981) exhibit much higher palaeogeothermal gradient. On the basis of stable isotope data of authigenic clay minerals ( Uysal et al., 2000c), sections in the southern Bowen Basin have been interpreted to reflect a semi-closed system with low water/rock ratio. In such an environment, where rocks are impervious to circulating fluids, the effect of convective heat transfer cannot explain the sharp increase in geothermal gradient in the lower sections (Back Creek Group). Rather, the observed heat flow pattern is consistent with lithologic controls where shales absorb the

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heat due to low conductivity and act as a barrier to the vertical transfer of deep heat flows towards Baralaba Coal Measures. Therefore, it is reasonable to conclude that, in the southern Bowen Basin, heat flow and consequently, variations in geothermal gradient may have been controlled by different rock conductivities. Heat flow anomalies and local variations in geothermal conditions have been reported in recent hydrothermal systems, for example, in the Upper Rhine Graben, Germany and France (Doebl and Teichmu¨ller, 1979; Teichmu¨ller, 1979; Aquilina et al., 1997) and in the Dead Sea rift valley (Gvirtzman et al., 1997). These anomalies have been attributed to the effects of ascendant thermal waters that flow along fault planes and fracture zones, and create a local heat maxima. A similar process may be considered to be responsible for the local geothermal anomalies in the northern Bowen Basin. The northern part of the Bowen Basin was subjected to an extensional tectonic activity during the Late Triassic when the illitic clays precipitated ( Uysal et al., 2000b) and organic maturation took place. The combination of relatively high temperatures for clay and carbonate mineral precipitation in the northern Bowen Basin, together with considerably depleted oxygen and hydrogen isotopic compositions of these phases ( Uysal et al., 2000c) is compatible with hydrothermal processes in a fracture-enhanced permeable system. Deep regional faults and fracture zones, which would be associated with an extension zone, could enable deep penetration of fluids of meteoric origin (Fig. 11). Fluids that enter a zone of high heat flow related to a geothermal anomaly, would have absorbed heat and been carried by convection upwards into permeable fracture zones. These zones acting as hot reservoir units would have been responsible for an increase in geothermal gradients in the less permeable, overlying rocks. A similar explanation is given by Younker et al. (1982) for heat transfer in the Salton Sea Geothermal Field. However, in the Salton Sea Geothermal Field, an underlying magma chamber is considered to be the heat source, whereas no evidence exists for a Late Triassic igneous activity in the Bowen Basin region during organic maturation. In summary, it can be concluded that varia-

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Fig. 11. Schematic reconstruction (not to scale) showing the heat and fluid flow processes during the latest Late Triassic in the Bowen Basin. Deep normal faults which formed in the northern Bowen Basin as a result of the Late Triassic extensional tectonics are interpreted to have enabled deep penetration of meteoric waters and initiated a hydrothermal process. Hot fluids are believed to have been focused upwards along fracture and fault zones which formed during the Hunter–Bowen Orogeny and occur extensively in the north of the basin.

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tions in palaeogeothermal gradients in the northern Bowen Basin may indicate variable heat distributions due to the different locations and extent of permeable zones as hot reservoirs. The above conclusion is consistent with the interpretation of the mineralogical and oxygen isotope data presented by Uysal et al. (2000a,c). As suggested by the authors, clay mineral distributions in the northern Bowen Basin sections have been controlled by variable water/rock ratios which depend on permeability. For example, high fluid rates in permeable fracture zones within less permeable sedimentary sequences may be responsible for the observed narrow mineral zonation of highly illitic clays (R≥3 I/S) and chlorite (Fig. 11), and depletion in oxygen isotopic compositions of illitic clays ( Uysal et al., 2000c). In addition, potassium-rich fluids that were involved in highly illitic clay mineral precipitation in the narrow fracture zones have been suggested to have originated from deeper parts of the basin, i.e. the underlying hot reservoir units. 5.3. Comparison of the palaeogeothermal gradients in the Bowen Basin to those in other basins The Late Permian coal measures in the northern Bowen Basin indicate palaeogeothermal gradients generally over 70°C km−1 and as much as 132°C km−1 ( Table 2). In the southern Bowen Basin, the gradients are lower, but still above 46°C km−1. Similar palaeogeothermal gradient values have also been reported by Baker (1989) and Faraj (1995) for the southern part of the Basin. Except for Jurassic and Cretaceous sections in the south, ‘normal’ gradients of 25–30°C km−1 do not occur in any part of the Basin. Regions with average geothermal gradient ca. 25–35°C km−1, are characterised by a simple burial and thermal history (Robert, 1988). Examples for typical tectonic settings of such areas are passive oceanic margins (e.g. Gabon basin and the Gulf Coast area) and platforms (e.g. Algerian Sahara). On the other hand, convergent orogenic zones (hypothermal basins, Robert, 1988) are characterised by very low geothermal gradients. For example, the low present-day geothermal gradients of 19–23°C km−1 in the foredeeps of the

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Alps and northern Apennines are the result of the thickening of the cold continental crust ( Teichmu¨ller and Teichmu¨ller, 1986; Sachsenhofer, 1992). Regions characterised by very high heat flow and anomalous geothermal gradients (hyperthermal areas, Robert, 1988) are arc or back-arc regions and divergent zones (oceanic and continental rifts). The abnormally high values for the estimated palaeogeothermal gradients in the Bowen Basin are similar to those reported for continental rift basins. For example, the presentday geothermal gradients in the Red Sea area attain values up to 50°C km−1 (Robert, 1988). In the Dead Sea valley, a maximum geothermal gradient of 48°C km−1 was measured (Gvirtzman et al., 1997). In the Upper Rhine Graben, they generally range between 40 and 90°C km−1 (Robert, 1988), but locally, can be as high as 120°C km−1 ( Werner and Doebl, 1974). The Ruhr Basin in Germany provides evidence for a high palaeogeothermal gradient of 60–80°C km−1 during the Carboniferous, which was result of the high heat flow and thin crustal thickness at that time ( Teichmu¨ller and Teichmu¨ller, 1986). 5.4. Correlation between clay mineralogy and vitrinite reflectance 5.4.1. Illite/smectite mixed-layers Random (R=0) I/S samples from shallow sections in boreholes in the southern Bowen Basin are associated with vitrinite reflectance ranging from 0.50 to 0.75% R . Using the equation of o Barker and Pawlewicz (1986), these reflectance values correspond to a palaeotemperature range of ~75–115°C. In the northern Bowen Basin, random I/S is only observed in one bentonite sample from borehole Hillaliong 1. This sample correlates with a reflectance value of ca. 0.9% R max, whereas immediately above and below the o bentonite, the mudstones and sandstones at almost the same reflectance values show R=1 ordering. Transition from random I/S to short range ordered I/S with increasing depth is observed in samples from borehole Cockatoo Creek 1, and it takes place in association with a reflectance value of ca. 0.75% R max and a corresponding palaeotempero

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ature of ~115–120°C. This is in agreement with the data by previous workers presented for burial diagenesis (e.g. Hoffman and Hower, 1979; Jennings and Thompson, 1986) and hydrothermal systems (e.g. Muffler and White, 1969; McDowell and Elders, 1980; Harvey and Browne, 1991). The lowest reflectance value, at which R≥3 I/S is observed, has been found to be ca. 0.9% R max o (sample CK.1-4) in the southern Bowen Basin. The delay in the illitisation reaction relative to organic indicators for the northern Bowen samples (Fig. 4A) can be ascribed to many causes, including the initial composition of I/S (Huff and Tu¨rkmenoglu, 1981), the presence of an inhibiting ion (magnesium) concentration in the fluid (Roberson and Lahann, 1981; Huang et al., 1993), potassium availability (Pearce et al., 1991), and time (Ramseyer and Boles, 1986; Hillier and Clayton, 1989). The effect of the initial I/S composition can be ruled out, because samples showing difference in degree of illitisation reaction have similar volcanogenic parent rock compositions ( Uysal et al., 2000a). Inhibition by magnesium is also not regarded as the cause of the delay in illitisation, because many samples containing substantial ankerite indicate no evidence of retardation in the illitisation reaction ( Uysal, 1999). The importance of potassium activity in illitisation reaction has been documented by several studies (e.g. Hower et al., 1976; Pearce et al., 1991; Huang et al., 1993). In the northern Bowen Basin, the irregular changes in I/S compositions in relation to depth have been explained by differences in water/rock ratio and potassium supply related to permeability ( Uysal et al., 2000a). An external source, such as a flux of potassium-rich fluids from deeper part of the basin, was assumed to be the most likely explanation for the variable potassium supply. Hot potassium-rich fluids, introduced over a very short time period, caused a rapid increase in thermal gradient, and are considered to have induced the precipitation of highly illitic clays in permeable zones, whereas outside these zones, at similar or even higher organic maturation level, illitisation has not advanced. In these less permeable parts, rocks were less affected by potassiumrich fluids. As detected by XRD and petrographical observations, micas and K-feldspar are present

in variable amounts in many samples. These detrital minerals, as a potential source of potassium, appear not to have been utilised for the illitisation reaction, as they persist in samples containing less illitic I/S at relatively high reflectance values. These results contrast with those from studies by Hower et al. (1976) and Pearce et al. (1991) that relate the illitisation process to alteration of potassiumbearing minerals. Thus, a factor, such as a short period of time available for mineral reactions may be responsible, in part, for the relative delay in illitisation in the northern Bowen Basin. Many studies have pointed out the importance of time for clay mineral reactions and demonstrated that organic maturity is more advanced, relative to clay mineral reactions, when rocks are subjected to rapidly increasing temperature ( Wolf, 1975; Kisch, 1987; Robert, 1988). The opposite is true for old shallow sedimentary sequences with low heating rates and low geothermal gradients, according to the kinetic models of I/S reactions by Velde and Vasseur (1992) and Hillier et al. (1995). Hillier et al. (1995) determined a theoretical range of correlations between vitrinite reflectance and I/S diagenesis by utilising kinetic models for the I/S reaction and vitrinite maturation. The theoretical curves corresponding to heating rates for different sedimentary basins as illustrated in the diagram of Fig. 4B may help to identify different types of thermal histories and geotectonic settings. By plotting the data obtained from this study and an earlier study (Baker, 1989) on this diagram, the correlation between vitrinite reflectance and I/S expandability from the Bowen Basin can be compared with those from different types of basins characterised by different thermal and tectonic histories. Accordingly, the Bowen Basin data are similar to those expected for rift basins. Most samples from the northern Bowen Basin plot to the right of expected correlations (Fig. 4A), which, as discussed above, is due to variable water/rock ratios controlling potassium availability during illitisation and the time dependency of mineral reactions. Also interestingly, the data (borehole Grosvenor 75) obtained from proximity to the contact of a Cretaceous igneous intrusion (Bundarra Granodiorite) are shifted towards the right-hand side along a vector parallel to the R o

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axis, which is similar to the ‘anomalous’ data obtained in proximity to igneous intrusions documented by Hillier et al. (1995). Due to the different responses of vitrinite reflectance and illitisation under variable heating rates, there is no universally applicable correlation between them. As such, this correlation can differ with sedimentary basins or among individual regions or sub-basins (e.g. Hillier and Clayton, 1989; Hillier et al., 1995; Sachsenhofer et al., 1998). Discrepancies between mineral parameters and vitrinite reflectance are most pronounced in areas of rapid heating, such as rift basins and in the vicinity of igneous intrusions. For given vitrinite reflectance values, higher I/S expandabilities can be expected at higher geothermal gradients (Robert, 1988; Hillier et al., 1995). This, for example, seems to be the case in the northern part of the Bowen Basin, where the estimated palaeogeothermal gradients are very high and the ‘lag’ of I/S reaction behind coalification is more pronounced. 5.4.2. Kaolinite Several studies have documented the disappearance of kaolinite due to increasing burial in sedimentary basins (e.g. Boles and Franks, 1979; Jennings and Thompson, 1986). However, kaolinite may persist under low-grade metamorphic conditions in geothermal systems, where it is controlled by available reaction time and chemistry of the environment (Barker et al., 1986). Similarly, Kisch (1987) and Robert (1988) have reported a wide range of coalification from 0.9 to 3.3 R for o the disappearance of kaolinite, dependant on thermal histories of individual areas, as well as fluid and rock compositions in the system. The results of the present study confirm the conclusions of these earlier studies. As in the case of I/S ordering, disappearance of kaolinite varies greatly, and it persists over a maturation range of ca. 0.9–2.3% R max. Kaolinite may persist at much higher o reflectance values near Cretaceous intrusions. A negative correlation exists between the abundance of kaolinite and illite content in I/S, which has been attributed to the effect of the K+/H+ activity ratio, rather than temperature ( Uysal et al., 2000a). The persistence of kaolinite at high level organic maturation and in very close proximity to

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intrusions supports the notion that despite the significant increase in temperature, potassium concentration in the fluid, is related to permeability, and duration of its availability both of which control kaolinite occurrence. 5.4.3. Chlorite Chlorite can be found at reflectance values from ca. 0.5% R , indicating its formation over a wide o. range of temperature. Only Fe-rich chlorite, which is associated with less illitic I/S and kaolinite ( Uysal et al., 2000a), occurs at lower organic maturity levels (<0.9% R max). Chlorite, when o associated with highly illitic R≥3 I/S and illite, appears at reflectance values of 0.9% R max and o higher. According to Barker and Pawlewicz (1986), this suggests that chlorite formation with R≥3 I/S may begin at temperatures as low as 135°C. This estimated temperature for the first appearance of the chlorite/illite mineral association is lower than those reported by many other authors. Previous studies suggest that chlorite/illite mineral assemblages usually form at temperatures of ca. 180–200°C (e.g. Dunoyer de Segonzac, 1970; Velde, 1985; Jennings and Thompson, 1986). In the southern Bowen Basin (borehole Cockatoo Creek 1), chlorite-rich C/S occurs in association with reflectance values of ca. 1.4% R max (1.3% R ) at 2574 m, and 2% R max (1.9% o o o R ) at 3680 m. According to Barker and Pawlewicz o (1986), such reflectance values indicate temperatures of ca. 175 and 215°C, respectively. In this part of the basin, C/S mixed-layers represent the last step of smectite-to-chlorite transformation and contain small amounts of expandable layers (≤10%) which decrease in abundance with depth to form discrete chlorite ( Uysal et al., 2000a). Temperature is an important control on the transformation of smectite (saponite) to chlorite through corrensite according to Inoue and Utada (1991), who have estimated the temperature for the first appearance of corrensite at 100°C. The upper temperature limit for the stability of corrensite, which varies as a function of local chemical conditions, has been reported to be in the range of 170–250°C (Inoue, 1995). Laumontite coexists with C/S mixed-layers in borehole Cockatoo Creek 1, and its reported upper limit of thermal stability

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is considered as 200–220°C (Tomasson and Kristmannsdotir, 1972; Inoue and Utada, 1991). 5.4.4. Illite and chlorite crystallinity Determination of illite crystallinity has been proven to be a very useful method as a palaeotemperature indicator in advanced stages of diagenesis and anchi-epimetamorphic environments ( Teichmu¨ller and Teichmu¨ller, 1979; Yang and Hesse, 1991; Underwood et al., 1993; Barrenechea et al., 1995). In the present study, there appears to be no correlation between illite crystallinity and vitrinite reflectance ( Fig. 5). This is probably due to the variable effect of minor amounts of expandable layers in the illitic samples, but could also indicate that illite crystallinity may depend on other variables in addition to temperature (e.g. Kisch and Frey, 1987; Yang and Hesse, 1991). Although chlorite crystallinity has not been as extensively used as illite crystallinity, many authors have found a good correlation between the two and have shown chlorite crystallinity to be a reliable thermal maturation indicator (e.g. Duba and William-Jones, 1983; Yang and Hesse, 1991; ´ rkai, 1991). More recently, A ´ rkai and Ghabrial A (1997) documented strong correlations between chlorite crystallinity values and temperature, and concluded that the chlorite crystallinity technique may be useful in the estimation of metamorphic temperatures. Chlorite crystallinity can be applied as supplementary to illite crystallinity, or it can be used alternatively, if reliable illite crystallinity data are not available (e.g. Deutloff et al., 1980). Overall, chlorite crystallinity appears to be a function of thermal maturity, as indicated by the good negative correlations between chlorite crystallinity, depth and vitrinite reflectance (Figs. 6 and 7). However, sharp reverse patterns occur locally in the northern Bowen Basin, which display significantly greater chlorite crystallinity values relative to depth (e.g. the sample at 318 m in borehole Wodehouse 2, Fig. 6). Similar behaviour of chlorite crystallinity is also observed in some other samples from different boreholes in the northern Bowen Basin ( Uysal, 1999). Such a phenomenon can be explained by the effect of transient fluid movements and dissolution/precipitation processes under relatively high water/rock ratio conditions, rather than by the effect of temperature changes.

5.4.5. Chlorite polytypism Two polytypes, type-I (Ib, b=90 and b=97) and type-II (IIb), are known to be characteristic of chlorites (Bailey and Brown, 1962). The type-I polytypes are considered to be diagenetic, whereas type-II occurs as the stable structural form in high temperature metamorphic rocks. Hayes (1970) suggested that transition from type I to type II occurs between 150 and 200°C, although more recent studies ( Walker and Thompson, 1990; Walker, 1993) have shown that type II structure may form at temperatures as low as 135°C. The sample CK1-6 identified as Ib ( b=90) polytype at 2574 m in borehole Cockatoo Creek 1 ( Fig. 8) is associated with a vitrinite reflectance of ca. 1.4% R max, corresponding to a maximum palaeotemo perature of ca. 175°C (Barker and Pawlewicz, 1986). The samples CK1-12 and CK1-13 at 3680– 3691 m have extrapolated vitrinite reflectance values of ca. 2% R max (1.9% R ) which indicates o o maximum temperatures ca. 215°C. These results are consistent with those of Hayes (1970) and suggest that, similar to the other parameters of clay mineral reactions (I/S expandability and chlorite crystallinity), increasing temperature due to increasing depth is the major control on changes in chlorite polytypism in the southern Bowen Basin.

6. Effect of igneous intrusions on clay mineral reactions Many parts of the northern Bowen Basin are characterised by the extensive occurrence of narrow dykes and sills typically <15 m in maximum thickness ( Uysal, 1999). However, the XRD patterns of the clay separates indicate that the intrusions had no effect on the clay mineral reactions within the mudstones and sandstones in their vicinity. Samples close to intrusion contacts at a distance less than the intrusion thickness exhibit no increase in illite content in I/S and in chlorite to kaolinite ratio ( Uysal, 1999). Clay mineral distributions in relation to the heat effect of igneous intrusions in two representative boreholes, Killarney 5 ( KL 5) and Grosvenor 71 (GR 71), are shown in Figs. 12 and 13. Only the samples

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Fig. 12. Distribution of clay minerals and variation of I/S layer composition and ordering type in relation to the heat effect of igneous intrusions in borehole GR 71. Stratigraphic column modified after Scott (1987).

Fig. 13. Distribution of clay minerals and variation of I/S layer composition and ordering type in relation to the heat effect of an igneous intrusion in borehole KL 5. Stratigraphic column modified after Scott (1987).

immediately adjacent to the intrusions were found to contain illite and highly illitic (>%95 illite) I/S mixed-layer minerals. K–Ar ages of these illitic clays are consistent with the published age data of Cretaceous intrusions in the northern Bowen Basin ( Uysal et al., 2000b). By contrast, a bentonite sample at 459 m, near a 15 m thick intrusion (Fig. 12) and a sandstone sample at 865 m, above a relatively thick intrusion (30 m thick, Fig. 13),

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have not undergone intensive illitisation. These samples display the I/S R=1 structure with a composition of ~70 and ~80% illite, respectively, and record the K–Ar ages of an earlier (Late Triassic–Early Jurassic) thermal event ( Uysal et al., 2000b). The results presented above clearly demonstrate that the effect of relatively thin igneous intrusions in the Bowen Basin coal measures is very limited in terms of clay mineral reactions, and thus confirm the conclusions reached by earlier studies (e.g. Smart and Clayton, 1985; Esposito and Whitney, 1995). Intensive illitisation of I/S occurs only in narrow zones immediately adjacent to intrusions, because clay mineral reactions are too slow to record the effect of extremely short lived heating events. In contrast, organic matter has been shown to be much more sensitive to rapid temperature increases caused by igneous intrusions. For example, Teichmu¨ller et al. (1979) have studied the correlation between coalification and illite crystallinity in the contact aureole of an Upper Cretaceous large igneous intrusion of the Bramsche Massif in NW-Germany. The authors found that higher temperatures are needed to advance mineral reactions in comparison to organic maturation. The influence of a large intrusive body, similar to that in the Bramsche Massif, on thermal maturation levels can also be observed in the Bowen Basin. As demonstrated on the diagrams in Figs. 4 and 7, combined plots of smectite content in I/S and chlorite crystallinity versus vitrinite reflectance for the samples from borehole Grosvenor 75 enable the recognition of ‘anomalous’ data related to a igneous intrusion, in the sense of Hillier et al. (1995). The borehole Grosvenor 75 is located ca. 7–8 km from the Bundarra Granodiorite ( Fig. 1B), the largest intrusion within the Bowen Basin, covering an outcrop area of ca. 88 km2 (Hamilton, 1985) and dated as Early Cretaceous (Marshallsea, 1988). Similar to the Bundarra Granodiorite, contact metamorphic effects of other relatively large intrusive bodies in the northern part of the Bowen Basin may be responsible for the local occurrence of high level organic maturation and overprinting of coal rank set earlier during the Late Triassic regional thermal event. Examples from the Bowen Basin as documented

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in this study and from the literature (e.g. Nadeau and Reynolds, 1981; Bu¨hmann, 1992) indicate that the contact effects of igneous intrusions on clay mineral reactions and organic maturation depend on distance from the intrusive body and its thickness. Thus, the effect of contact metamorphic environments characterised by conductive heat transfer on maturation level in sedimentary basins is limited compared to that of hydrothermal environments (He´roux and Tasse´, 1990; Summer and Verosub, 1987, 1992). In the latter case, intrusive activity takes place in environments with relatively high porosity–permeability and associated fluid movements. Large volumes of fluid can efficiently transport the required heat, resulting in more widespread thermal effects, which may be of longer duration (>105 years) than the transient contact heating effects (Duddy et al., 1994). However, despite the widespread occurrence of Cretaceous igneous intrusions in the northern Bowen Basin no evidence for associated hydrothermal alteration zones in sandstones and mudrocks of the Late Permian coal measures were found during the present study. This may be due to the lack of sufficient porosity–permeability, which may have been reduced as a result of extensive clay and carbonate mineralisation during the Late Triassic regional fluid movement and thermal event. Nevertheless, indication of hydrothermal alteration, which is restricted to Cretaceous intrusive bodies, is evident from extensive clay, zeolite and calcite mineralisation ( Uysal, 1999). Illite/chlorite and corrensite/laumontite are the characteristic mineral associations in the hydrothermally altered intrusions. Formation temperatures of these minerals can be estimated roughly, based on comparisons with previous studies, which have been discussed earlier in this paper. Accordingly, illite/chlorite mineral assemblage may be indicative of temperatures ca. 200°C. A reasonable temperature range of corrensite/laumontite mineral association would be between 100 and 250°C.

7. Summary and conclusions Evaluation of the vitrinite reflectance data for the Bowen Basin has led to the conclusion that

thermal maturation of the Late Permian coal measures is related to the development of a regional high palaeo-heat flow zone in the latest Late Triassic rather than to the heat effect of deep burial during the Early to Middle Triassic, as previously believed. This conclusion is based on the following findings: 1. Coalification maxima do not coincide with a depocentre for the Triassic sequence. Maximum vitrinite reflectance and estimated palaeotemperature values are observed in the northern part of the basin, whereas the thickest sedimentary sequence and Triassic depocentre are located in the southeast. 2. The Late Permian sections have abnormally high, but variable (especially in the north) coalification and estimated palaeogeothermal gradients of >45°C km−1 in the south and 70°C km−1 in the north, which are characteristic of continental rift regions. By contrast, the Bowen Basin was a foreland basin at the time of maximum burial. High geothermal gradients estimated in the northern Bowen Basin are interpreted as being the result of a convective heat transfer in hydrothermal conditions. Variable heat distributions due to the different locations and extent of fracture-enhanced permeable zones acting as hot reservoirs in deeper parts of the basin are considered to be responsible for some significant local thermal anomalies in the lower coal measures. High heat flow is also apparent in the lower sections of the southern Bowen Basin, as inferred from the estimated palaeogeothermal gradients. However, on the basis of stable isotope data of authigenic clay minerals, sections in the southern Bowen Basin have been interpreted to reflect a semi-closed system with low water/rock ratio. In such an environment, where rocks are impervious to circulating fluids, heat transfer by conduction has occurred in low conductivity rocks. Based on the correlation between clay mineralogy and vitrinite reflectance, it is possible to gain further insight into the thermal history of the Bowen Basin. Such a correlation reveals a delay in illitisation reaction relative to organic maturity for many I/S samples from the northern Bowen Basin, in comparison with I/S from the southern Bowen Basin. In the northern Bowen Basin clays, kaolinite also persists at high organic maturity

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levels. The short time period for the mineral reactions and variable potassium supply in relatively impermeable rocks are considered to be responsible for the persistence of I/S with higher smectite content and kaolinite to temperatures past their expected stability range. This phenomenon observed for the northern Bowen Basin clays can be attributed to very intense heating by convection in a short period of time, and are consistent with a hydrothermal system. The correlation between I/S expandability and vitrinite reflectance for the Bowen Basin data can be compared with those from different types of basins characterised by different tectonic settings. This comparison suggests that the thermal history of the Bowen Basin resembles those of rift basins. The effect of thin igneous intrusions on clay mineral reactions is very limited. Intensive illitisation due to heating of intrusions can only be observed in narrow zones immediately adjacent to intrusive bodies. This is attributed to the fact that mineral reactions are too slow to record the effect of extremely short heating duration, in contrast to organic maturity indicators. These differences between mineral and organic parameters aid in the identification of local contact metamorphic effects. The Cretaceous intrusions are characterised by intensive hydrothermal alteration; however, despite their widespread occurrence in the northern Bowen Basin, there is no evidence for associated hydrothermal alteration zones in the sandstones and mudrocks of Late Permian coal measures.

Acknowledgements The authors thank K. Kyser, M. Coleman and T. Frank for constructive comments on an earlier version of this paper. Comments by P. Robert, S.O. Akande and an anonymous reviewer are also gratefully appreciated. Financial support for this project from the Queensland Transmission and Supply Corporation and Energy Research and Development Corporation is gratefully acknowledged. Thanks are also due to the Queensland Mines Department, The Shell Company Australia, Newlands, German Creek and Yarrabee Coal Companies for access to drill-core material.

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