Australian Journal of Earth Sciences (2001) 48, 183–192

INTRODUCTION

The Moranbah Coal Measures form the basal formation ofthe Blackwater Group, and their deposition is consideredto mark the initiation of the foreland stage of basinalevolution (Murray 1983; Fielding et al. 1993). These coalmeasures are the focus of contemporary mining in thenorth Bowen Basin and have been the subject of extensiveexploration. The Platypus Tuff Bed (new name) is widelydistributed in the north Bowen Basin (Figure 1) and hasbeen much utilised as a key bed, assisting in the recognitionof a lithostratigraphic framework across the basin fill.

The tuff was informally named the P-tuff by Koppe(1978), but has never been formally defined. It has beenwidely used as a key stratigraphic marker in the MoranbahCoal Measures for more than two decades (Johnson 1984;Falkner & Fielding 1993; Mallett et al. 1995). This contribu-tion formalises the Platypus Tuff Bed as a stratigraphicunit and documents its regional stratigraphic context, dis-tribution and attributes. Absolute age control for thePermo-Triassic Bowen Basin sequence is limited, and noradiometric ages are presently available for sequences inthe northern Bowen Basin or for the coal-bearingBlackwater Group. We have used the SHRIMP ion micro-probe to determine a U–Pb age for zircons from thePlatypus Tuff Bed. This new radiometric age constrains theage of the Blackwater Group and provides a useful per-spective on the sedimentation rates that prevailed duringits accumulation.

DEFINITION OF THE PLATYPUS TUFF BED

Derivation of name This distinctive tuff interval in theMoranbah Coal Measures is named after the PlatypusCreek, which is located close to the type section.

Synonymy The Platypus Tuff Bed has historically beeninformally named the P-tuff (Koppe 1978).

Type and reference sections The type section is definedfrom a 1.63 m-thick interval (257.0–258.63 m) in drillcorefrom Grosvenor NS 14 (AMG 8554E 049845N) located in theBroadmeadow area, ~20 km north-northwest of the town-ship of Moranbah (Figures 1, 2). The core is stored at theDepartment of Minerals and Energy Exploration DataCentre, Zillmere, Brisbane. The bed also crops out in thehighwalls of the adjacent Goonyella opencut, where it canbe followed laterally for approximately 5 km.

Lithology The Platypus Tuff Bed is pale yellow-brown tolight-grey, contains abundant, medium to coarse bronze-coloured biotite, and is soapy to touch due to a substantialclay content. In the terminology of Cas and Wright (1987) it is a tuffaceous mudstone, whereas the terminologyof Fisher and Schmincke (1984) and McPhie et al. (1993)places it as a fine (ash) tuff. Typically the bed is internallymassive, but at several locations has crude normal gradingin the basal ~10 cm, in which the basal few centimetres arecoarse grained. The uppermost (10–25 cm) part may exhibitsmall-scale cross-stratification. Traces of rootlets, includ-ing Vertebraria up to to 16 cm in length, commonly show abioturbated horizon from 15 to 50 cm from the top of thebed. In drillcore DDH20, from the Moranbah North area(Figure 3), the tuff is characterised by two superposed normally graded units.

The Platypus Tuff Bed has a moderately porphyritictexture (sensu McPhie et al. 1993), with phenocrysts evenly

Age and significance of the Platypus Tuff Bed, a regionalreference horizon in the Upper Permian Moranbah CoalMeasures, north Bowen BasinP. MICHAELSEN,1* R. A. HENDERSON,1 P. J. CROSDALE1 AND C. M. FANNING2

1School of Earth Sciences, James Cook University, Qld 4811, Australia.2Research School of Earth Sciences, Australian National University, ACT 0200, Australia.

The Platypus Tuff Bed in the Permian Moranbah Coal Measures provides a basin-wide marker horizontraceable for over 300 km along strike. The bed is a tephra event unit, the product of a large-scalevolcanic eruptive episode involving a pyroclastic volume > 10 km3. The relatively even thickness(~1–1.5 m) of the tuff across the entire northern Bowen Basin (~10 000 km2) implies a distant source.The tuff is ash-rich and its original geochemistry has been compromised by diagenetic alteration.Crystal content (10–15%) is dominated by quartz, suggesting a rhyolitic association. SHRIMP U–Pbanalysis of zircons indicates an age of 258.9 6 2.7 Ma for the Platypus Tuff Bed, confirming the LatePermian age that has generally been assigned to the Blackwater Group. The age framework nowapparent for the coal-bearing Blackwater Group suggests an average depositional rate ranging from~133 m/106 years for its eastern depocentre in the northern Bowen Basin to ~70 m/106 years in moremarginal settings to the west.

KEY WORDS: Bowen Basin, geochronology, Moranbah Coal Measures, Permian, Platypus Tuff Bed,uranium–lead dating.

*Corresponding author: Per.Michaelsen@jcu.edu.au

184 P. Michaelsen et al.

distributed, varying in size up to 1.1 mm, and constitutingapproximately 10–15% of the tuff. The dominant crystalcomponents consist of quartz as blocky subhedral grainsand elongate slivers (to 0.5 mm), altered biotite (to 0.8 mm),and albite (to 1.1 mm). Zircon, ilmenite and apatite areuncommon to rare accessories. A minority of quartz phe-nocrysts are rounded and corroded, probably the result ofmagmatic resorption prior to the volcanic eruption (Hess

1989; McPhie et al. 1993). Biotite is partly chloritised andsome grains have suffered compactional deformation. Theoverwhelming dominance of quartz phenocrysts relative to feldspar suggests a rhyolitic parentage (Fisher &Schmincke 1984; Hess 1989).

The lithic component consists of a fine, clay-rich matrixin which sporadic angular to subrounded grains, up to0.4 mm in size, can be discriminated. Most of these werevitric grains and are now chert. Other lithic grains havevague, rounded relict outlines. In some cases diageneticsiderite has replaced primary grains of unknown originalcharacter.

Results from whole-rock XRD analysis indicate that the strongly altered groundmass is dominated by illite,smectite (mainly dioctahedral), kaolinite, quartz, plagio-clase (albite and oligoclase), K-feldspars (microcline andorthoclase), and possibly Fe-oxides (goethite, limonite andhematite), calcite, dolomite and possibly minor graphite.

Distribution The Platypus Tuff Bed represents the onlythick, clearly recognised pyroclastic unit in the MoranbahCoal Measures, which form the basal formation of the coal-bearing Blackwater Group. The distinctive character of thePlatypus Tuff Bed and its consistency of stratigraphicposition in the Moranbah Coal Measures (Figure 3) allowsit to be easily and unequivocally recognised and correlated.The bed has been documented for over 300 km along strike,from an extensive drillcore record, near the western mar-gin of the north Bowen Basin, and an equivalent horizonis represented in the German Creek Formation from thenorthern part of the Denison Trough (Koppe 1978; Draper& Balfe 1985; Mallett et al. 1995). Recently the Platypus TuffBed has also been reported from the eastern sector of thenorthern Bowen Basin from exploration tenements west ofNebo (O’Reilly et al. 1995) and Kemmis Creek (Michaelsen1995). The bed thus appears to be represented in theMoranbah Coal Measures throughout the north BowenBasin (Figure 1).

Thickness Koppe (1978) described the Platypus Tuff Bed asapproximately 1 m thick. Regionally the tuff has a remark-ably uniform thickness. It is 1.63 m thick in the type sec-tion and to the east at South Kemmis Creek. At thenorthernmost limit of the Moranbah Coal Measures, nearCollinsville, it ranges from 1.09 to 1.56 m in thickness,whereas to the south, near the German Creek mine, it is1.17 m thick in drillcore GSQ Cairns County 34 (Figure 1).Isopachs based on drillcore intersections of the Platypus

Figure 1 Distribution of Moranbah Coal Measures in the north-ern Bowen Basin showing the location of 19 logged coreholes con-taining the Platypus Tuff Bed (PTB) and thickness of itsintersections. The type section is represented by core from theGeological Survey of Queensland Grosvenor 14 (GR 14). The mapalso shows the location of profiles A–A9 and B–B9 (Figure 2), andthe Moranbah North mining lease (Figure 3). BM 14,Broadmeadow 14; CC 34, GSQ Cairns County 34; DH 1, DabienHolding 1; DR 19, GSQ Drake 19; DR 26, GSQ Drake 26; GR 14, GSQGrosvenor 14; M 2, Moranbah 2; NE 1, North Ellensfield 1; NM 1,North Moranbah 1; PH 1, Phillips Creek 1; RS 1, Ripstone 1; RV2, River Paddock 2; SMC 1, Smoky Creek 1; SKC 1, South KemmisCreek 1; TB 1, Teviot Brook 1; WB 1, West Broadmeadow 1; WPD1, West Peak Downs 1.

Age of the Platypus Tuff Bed, Qld 185

Tuff Bed at district scale and based on drilling records fromthe Moranbah North mine area, indicate variation inthickness from 0.8 to 1.88 m with no systematic trend relatedto source (Figure 3). The relatively even regional thicknessof the tuff bed across the northern Bowen Basin (Figure1) implies a distant source. Given the tectonic settinginferred for the Bowen Basin during accumulation of theBlackwater Group (Veevers et al. 1994), an easterly ornortherly volcanic source is most likely.

Stratigraphic relationships In the northern Bowen Basinthe bed is located in the mid-section of the Moranbah CoalMeasures and its position relative to the base of the for-mation varies with the thickness of the unit (Figure 2a). Inthe south, where the MacMillan Formation progressivelyreplaces the upper part of the Moranbah Coal Measures,the Platypus Tuff Bed rises towards the top of the forma-tion. Further south, at the German Creek opencut, anequivalent horizon to the Platypus Tuff Bed is located nearthe top of the Moranbah Coal Measures (Figure 2b). Faciesof the Moranbah Coal Measures associated with thePlatypus Tuff Bed are detailed in Michaelsen andHenderson (2000a).Age SHRIMP U–Pb analysis of zircons indicates an ageof 258.9 6 2.7 Ma (Late Permian) for the Platypus Tuff Bed.

This confirms the Late Permian age that has generally beenassigned to the Blackwater Group.

DEPOSITION OF THE PLATYPUS TUFF BED

Despite the lithological uniformity illustrated by the distri-bution and grainsize of bronze-coloured biotite, thePlatypus Tuff is a composite deposit. The rootlet horizon,the presence of two graded intervals in an intersection inthe Moranbah North area, and subdivision of the tuff intotwo beds by a coal/carbonaceous claystone up to 80 cm thickat German Creek confirms the composite nature of thedeposit.

The Platypus Tuff is best described as a tephra eventunit (Fisher & Schmincke 1984), one that embodies both theeruptive emplacement of tephra and the immediate post-eruptive period of resedimentation (Orton 1996). The uni-formity of the Platypus Tuff Bed implies a commonparentage and it is interpreted here to represent the prod-uct of two or more related eruptions. The small-scale cross-stratification in its upper part in some core intersectionsis indicative of minor reworking as might be expectedfollowing the emplacement of a thick, areally extensivepyroclastic deposit. Variations in thickness, apparent at

Figure 2 Stratigraphic frame-work of the Platypus Tuff Bed inthe Moranbah Coal Measures,northern Bowen Basin. See Figure1 for drillhole locations. Note thesignificant increase in thicknessof the formation to the east andnorth and great lateral continuityof coal superseams. In the south,the Moranbah Coal Measuresinterfinger with the shallowmarine MacMillan Formation andGerman Creek Coal Measures.Only major coal seams (> 1 mthick) are shown.

186 P. Michaelsen et al.

district scale (Figure 3), are interpreted as reworking inresponse to slight palaeorelief developed on the accumu-lating Moranbah Coal Measures. The continuity and con-sistent thickness of the bed across the northern BowenBasin argue for its deposition over a short interval of time,which would be essentially instantaneous in the context ofLate Palaeozoic geochronology. The crystallisation age ofthe Platypus Tuff Bed thus approximates the depo-sition/redeposition age.

The Platypus Tuff Bed is considered to be the productof distal pyroclastic fallout related to a large-scale eruptivevolcanic episode. The preserved volume, based on its distribution and thickness in the northern and centralBowen Basin is estimated to be > 10 km3, but the completeeruptive volume could well have been as much as an orderof magnitude larger. As a comparison, the historic eruption at Krakatau in Indonesia in 1883 involved approximately 10 km3 of rhyolite ash (Izett 1981). At thetime of emplacement of the Platypus Tuff Bed, a peat precursor of the extensively developed P Seam coveredmost of the basin extending from Collinsville to GermanCreek (Figure 2).

GEOCHEMISTRY

The whole-rock geochemistry of the Platypus Tuff Bed wasinvestigated to better constrain its igneous association andto assist in its characterisation (Table 1: see Table 2 for

location of drillholes). Twenty-two samples were selectedfrom 17 drillhole intersections of the bed. Five sequentialsamples were taken from the 1.73 m-thick Platypus Tuff Bedintersection in Mitsubishi Gas Chemical ResourcesAustralia’s drillhole West Broadmeadow 1, located 1 kmeast of the type section (Figures 3, 4); this core is uniformin character and has no trace of internal layering. Thereis significant variation in major and trace-element contentsbetween these samples (Table 1). A similar range in vari-ation of major and trace elements is present in othersamples from across the northern Bowen Basin (Table 1).

The lithic component of the Platypus Tuff Bed has beennearly completely modified to clay minerals during diagen-esis. This has modified the major element contents,most clearly seen as the wide variation in SiO2 (50.5–77.5%)and Al2O3 (11.4–22.1%). Alkalis have depleted values and are likely to have been mobile. The low values of Fe (mean of 1.5%), Mg (mean of 1.6%) and TiO2 (0.2–0.57%,mean of 0.38%) fall within the range expected for acid magmas of calc–alkaline association (Le Maitre 1976;Ewart 1979).

The phenocryst assemblage in the Platypus Tuff is char-acteristic of rhyolites (Fisher & Schmincke 1984; Hess 1989;Smith & Braile 1994). Rhyolites, which occur in manytectonic environments, may exhibit large variations intrace-element contents (Hess 1989). For example, trace ele-ments in the 0.74 Ma Pleistocene Bishop Tuff in California,emplaced during a single massive eruption, vary by factorsranging from 2 to 50, notably, trace elements Ba, Sr, Ce, Eu,Zr and Ni (Bailey et al. 1976). This variation is thought toreflect crystal fractionation in the magma chamber(Hildreth 1979, 1981).

The significant variation in trace elements evident in the Platypus Tuff Bed (Table 1) could be the result ofcompositional zonation in a silicic magma chamber, ordiagenetic alteration, or a combination of both processes.The ubiquitous development of sericite after feldspar,chlorite after biotite and the general development of a clay assemblage in the matrix, indicates a substantialdiagenetic overprint involving hydration: it implies asubstantial throughput of formation water throughout thebed. Even so, the range of trace-element composition forthe Platypus Tuff Bed is comparable with that of rhyolitesfrom active margin settings in the western Pacific (Ewart1979).

GEOCHRONOLOGY

A sample for age dating was taken from drillcore MGCRAWest Broadmeadow (WB) 1, at between 297.85 and 298.25 m(Figure 4). The location of the drillcore is shown on Figures1 and 3. It is worth noting that the core is located only 1 kmeast of the type section of the Moranbah Coal Measures(Koppe 1978), and located close to the type section of thePlatypus Tuff Bed (Figure 3). Zircons were separated usingstandard crushing and heavy-liquid procedures. The grains

Figure 3 Isopachs of the Platypus Tuff Bed (PTB) in theMoranbah North mining lease, based on 32 drillhole inter-sections. Numbers in parentheses refer to PTB thickness.

Age of the Platypus Tuff Bed, Qld 187

Tab

le1

Res

ult

s fr

om g

eoch

emic

al a

nal

yses

of

wh

ole-

rock

maj

or a

nd

trac

e el

emen

ts o

f22

Pla

typu

s T

uff

Bed

sam

ples

fro

m 1

7 re

gion

ally

spa

ced

dril

lhol

es.

WP

D1

NM

1S

KC

1D

H1

M2

WB

#1

WB

#2

WB

#3

WB

#4

WB

#5

BM

aB

Mb

TB

1R

S1

RV

2W

O1

SM

C1

NE

1P

H1

DR

26

SiO

271

.860

.169

.870

.869

63.7

62.3

6765

.750

.571

.863

.475

.169

.361

.377

.560

.570

.460

.973

.5T

iO2

0.34

0.33

0.37

0.34

0.39

0.43

0.46

0.44

0.42

0.57

0.32

0.51

0.29

0.31

0.45

0.2

0.48

0.28

0.49

0.28

Al 2

O3

14.9

18.9

17.2

15.7

16.6

20.5

22.1

18.1

19.7

21.9

14.2

19.7

11.4

14.8

16.8

11.9

2112

.221

.212

.2F

e 2O

31.

371.

550.

551.

581.

111.

081.

031.

051.

212.

381.

471.

421.

061.

482.

110.

631.

722.

251.

772.

67M

nO

0.01

0.03

0.01

0.01

0.02

0.01

0.01

0.01

bd0.

080.

010.

020.

030.

020.

030.

020.

030.

050.

020.

02M

gO1.

021.

980.

591.

170.

921.

461.

451.

251.

462.

981.

941.

751.

141.

362.

770.

681.

261.

641.

322.

4C

aO0.

592.

690.

60.

31.

170.

380.

310.

580.

34.

280.

820.

892.

141.

682.

830.

991.

372.

730.

730.

8N

a 2O

1.09

1.07

1.68

0.84

1.35

1.67

1.69

1.72

1.34

1.54

1.45

1.92

0.8

1.06

1.36

0.68

1.31

1.4

2.19

0.48

K2O

2.63

3.17

2.13

3.12

2.61

1.96

1.95

22.

141.

791.

462.

061

2.58

1.83

1.9

2.93

1.04

3.62

1.67

P2O

50.

030.

060.

080.

070.

09bd

0.02

0.04

0.09

0.06

0.03

0.1

0.07

0.03

0.12

0.06

0.1

0.06

0.1

0.05

LO

I5.

8310

.14

6.5

6.06

6.74

8.15

8.5

7.14

7.75

13.4

66.

437.

867.

17.

1110

.05

5.56

8.73

7.79

7.06

5.64

Tot

al99

.61

100.

0299

.51

99.9

910

099

.34

99.8

299

.33

100.

1199

.54

99.9

399

.63

100.

1399

.73

99.6

510

0.12

99.4

399

.84

99.4

99.7

1

Ga

14.9

2218

.716

.816

.318

20.1

1520

.319

14.5

17.7

11.4

14.9

17.3

12.4

2113

.321

.512

.6Z

n43

.446

.253

.450

.553

.377

33.6

5534

.483

9750

.718

0.8

3746

.941

.773

.359

.667

.143

Ni

3.3

4.2

1.8

5.3

4.6

41.

93

5.2

62.

82.

82.

28.

35.

91.

54.

42.

812

.16.

3C

o4.

25.

13.

55.

87.

417

2.9

177.

520

9.3

6.4

6.7

3.9

6.8

46.

38.

96.

88.

7M

n48

.513

1.6

44.4

75.5

92.4

4038

.150

2450

971

84.4

161.

913

0.3

120.

513

5.8

134

301.

112

3.3

155

Cr

95.

37.

26.

34.

94

7.3

55.

36

5.3

5.4

4.3

5.9

6.1

58.

511

7.7

4T

i20

78.5

1979

.221

1720

7023

2426

1327

37.6

2596

2424

.134

7719

96.5

3501

.317

99.3

1878

.228

81.6

1152

.828

50.1

1774

.627

82.2

1734

.9S

c4.

83.

83.

78

6.4

78.

37

22.9

84.

47.

36.

65.

34.

63.

66.

64.

79.

15.

5V

15.8

18.6

17.9

29.5

21.7

2424

2510

9.2

3024

.426

.545

.720

24.1

1336

.219

.231

.923

.2A

s0.

81.

81.

91.

41.

82

1.6

26.

32

1.2

1.9

1.4

1.4

1.6

12.

41.

61.

50.

9P

b20

.729

24.3

19.5

25.8

3036

.731

18.3

2718

.426

.414

.819

.121

.422

30.1

16.9

37.4

18.3

Rb

8092

.662

.410

0.6

74.2

5964

.956

70.3

5647

.669

.930

77.5

66.7

55.6

97.5

34.7

103.

558

.3S

r43

1.1

483.

460

2.8

434.

946

8.1

515

491.

547

258

6.3

737

413.

263

4.8

343.

845

3.3

878

487.

359

9.2

424.

743

4.2

197.

9Y

21.8

36.1

25.1

3328

.334

3032

23.4

4023

.830

.120

.827

.831

.918

.235

.222

.932

.225

.5Z

r18

0.1

211.

520

4.5

211.

619

5.8

233

247.

324

114

8.1

257

162.

624

5.3

127

172.

219

8.8

107.

826

014

5.3

262.

714

6.2

Nb

8.2

9.1

108.

910

1310

.511

516

8.6

11.4

5.6

98.

85.

511

.76.

811

.48.

2

An

alys

es w

ere

con

duct

ed i

n t

he

Adv

ance

d A

nal

ytic

al C

entr

e at

Jam

es C

ook

Un

iver

sity

wit

h a

Sie

men

s S

RS

3000

seq

uen

tial

X-r

ay f

luor

esce

nce

spe

ctro

met

er.

bd,b

elow

det

ecti

on.

DR

19C

C3

74.1

66.4

0.27

0.39

12.4

16.9

1.93

1.97

0.01

0.01

2.47

1.98

0.53

0.8

0.1

1.97

1.1

1.47

bd0.

076.

66.

8799

.51

98.8

3

13.2

17.4

38.6

48.7

3.6

1.5

10.5

7.3

129.

154

.44.

36.

815

90.5

2403

.44.

96.

513

.117

.71.

21

18.5

25.7

36.7

47.7

258

430.

622

.330

.315

9.3

200.

58.

210

.1

188 P. Michaelsen et al.

are clear euhedral crystals with pyramidal terminations,tending to be elongate to equant in outline, or are fragmentsof such crystals.

Cathodoluminescence images reveal a relatively simplezoned magmatic internal structure to the grains. There isminimal evidence for complex structure, which may beinterpreted to represent inherited older zircon components.

Twenty-six analyses were made on 23 grains usingSHRIMP II at the Research School of Earth Sciences,Australian National University, following procedures and

data treatment similar to those outlined in Compston et al.(1992) and Williams (1998). Data assessment and calculationof all uncertainties have been carried out using SQUID(Ludwig 2000). The U–Pb ratios have been calibratedrelative to zircons from the Duluth Gabbro, sample AS3, asdescribed by Paces and Miller (1993); the U/Pb ratios forAS3 measured in conjunction with the Platypus Tuff Bed

Figure 4 Intersection of the Platypus Tuff Bed in WestBroadmeadow 1 drillhole showing geochronological and geo-chemical sample points.

Table 2 Locations of drillholes used in geochemical analysis ofthe Platypus Tuff Bed.

Drillhole Latitude (S) Longitude (E)

BM14 21°519 148°049

CC34 22°559 148°379

DH1 21°369 148°019

DR19 20°449 147°849

DR26 20°379 147°529

M2 22°019 148°059

NE1 21°409 148°109

NM1 21°549 148°079

PH1 22°269 148°229

RS1 22°169 148°179

RV2 22°079 148°109

SKC1 21°379 148°239

SMC1 21°549 148°129

TB1 21°509 148°059

WB1 21°519 148°039

WO1 21°559 148°109

WPD1 22°139 148°149

Table 3 Summary of SHRIMP U–Th–Pb analyses on sample WB1.

Grain U Th Th/U Pb* 204Pb/206Pb f206% Total Radiogenic Age (Ma)spot (ppm) (ppm) (ppm) 238U/206Pb 207Pb/206Pb 206Pb/238U 206Pb/238U

1.1 135 113 0.84 6 0.001084 1.35 24.73 6 0.37 0.0620 6 0.0014 0.0399 6 0.0006 252.2 6 3.82.1 139 111 0.79 7 0.001105 1.62 23.85 6 0.35 0.0644 6 0.0015 0.0413 6 0.0006 260.6 6 3.83.1 261 298 1.14 14 0.000394 0.44 24.34 6 0.40 0.0549 6 0.0014 0.0409 6 0.0007 258.5 6 4.24.1 143 103 0.72 7 0.001175 1.07 23.56 6 0.35 0.0601 6 0.0014 0.0420 6 0.0006 265.2 6 3.94.2 160 145 0.91 8 0.001398 0.59 23.45 6 0.34 0.0563 6 0.0013 0.0424 6 0.0006 267.7 6 3.95.1 179 110 0.62 8 0.000457 0.46 24.39 6 0.35 0.0551 6 0.0012 0.0408 6 0.0006 257.9 6 3.76.1 123 65 0.53 6 0.002087 1.53 23.92 6 0.36 0.0637 6 0.0015 0.0412 6 0.0006 260.0 6 3.97.1 75 61 0.81 4 0.001897 2.16 24.31 6 0.40 0.0686 6 0.0020 0.0403 6 0.0007 254.4 6 4.28.1 225 237 1.05 12 0.000501 0.51 23.87 6 0.33 0.0556 6 0.0010 0.0417 6 0.0006 263.3 6 3.68.2 92 60 0.65 5 0.002813 5.20 21.44 6 0.35 0.0936 6 0.0035 0.0442 6 0.0007 278.9 6 4.79.1 216 156 0.72 10 0.000680 1.07 24.50 6 0.42 0.0599 6 0.0011 0.0404 6 0.0007 255.2 6 4.39.2 93 56 0.60 4 0.001043 2.21 24.73 6 0.58 0.0689 6 0.0020 0.0395 6 0.0009 250.0 6 5.8

10.1 187 169 0.90 9 0.000628 0.80 24.18 6 0.38 0.0578 6 0.0012 0.0410 6 0.0006 259.2 6 4.111.1 138 142 1.03 7 0.001164 1.55 24.66 6 0.37 0.0637 6 0.0015 0.0399 6 0.0006 252.4 6 3.712.1 114 98 0.86 6 0.000891 0.67 23.70 6 0.45 0.0569 6 0.0015 0.0419 6 0.0008 264.7 6 4.913.1 206 235 1.14 11 0.000259 0.55 23.46 6 0.35 0.0560 6 0.0011 0.0424 6 0.0006 267.7 6 4.014.1 141 113 0.80 7 0.001017 1.34 24.24 6 0.35 0.0621 6 0.0014 0.0407 6 0.0006 257.2 6 3.715.1 75 65 0.87 4 0.002733 1.90 24.54 6 0.41 0.0665 6 0.0020 0.0400 6 0.0007 252.7 6 4.216.1 155 92 0.59 7 0.001706 1.35 23.96 6 0.44 0.0622 6 0.0013 0.0412 6 0.0007 260.1 6 4.717.1 143 137 0.95 7 0.000268 1.17 24.07 6 0.35 0.0608 6 0.0015 0.0411 6 0.0006 259.4 6 3.818.1 52 32 0.61 2 0.001625 4.60 23.98 6 0.44 0.0881 6 0.0029 0.0398 6 0.0007 251.5 6 4.619.1 145 131 0.90 7 0.001048 1.89 23.99 6 0.35 0.0665 6 0.0015 0.0409 6 0.0006 258.4 6 3.820.1 134 120 0.90 7 0.000884 1.64 23.47 6 0.35 0.0647 6 0.0015 0.0419 6 0.0006 264.6 6 3.921.1 50 33 0.67 2 0.003604 6.22 22.97 6 0.43 0.1012 6 0.0038 0.0408 6 0.0008 258.0 6 4.922.1 132 109 0.82 7 0.000236 0.95 24.09 6 0.36 0.0591 6 0.0015 0.0411 6 0.0006 259.7 6 3.923.1 266 279 1.05 14 0.000570 0.76 24.46 6 0.38 0.0574 6 0.0010 0.0406 6 0.0006 256.4 6 4.0

Uncertainties given at 1s level; f206% denotes percentage of 206Pb that is common Pb; correction for common Pb made using the measured238U/206Pb and 207Pb/206Pb ratios following Tera and Wasserburg (1972) as outlined in Compston et al. (1992).

Age of the Platypus Tuff Bed, Qld 189

zircons have an uncertainty of 0.82% (95% confidence limitson 12 of 14 analyses). The U–Th–Pb data are given in Table 3and shown on Figure 5, a composite Tera and Wasserburg(1972) concordia plot, cumulative probability plot andweighted mean age calculation using ISOPLOT (Ludwig1999). Note that uncertainties in individual analyses aregiven at the 1s level in Table 3, although uncertainties inage calculations are reported at 95% confidence limits. TheSteiger and Jäger (1977) decay constants have been used forall age calculations.

The U and Th concentrations in the areas analysed bothrange from approximately 50 to 300 ppm, but show a relatively high and uniform Th/U ratio between 0.5 and 1.1,reaffirming the magmatic origin for these zircons. On theTera and Wasserburg (1972) concordia plot of the calibratedtotal ratios the analyses form a coherent array between a360 Ma radiogenic end-member and common Pb (Figure 5).This is further highlighted on a cumulative probability plotof the radiogenic 206Pb/238U ages where the distribution canbe seen to form a simple bell curve. A weighted mean ofthe 206Pb/238U ages for all 26 analyses has slight excessscatter (MSWD 5 2.1, probability 5 0.1%) giving an age of259.5 6 3.2 Ma. Analysis 8.2 does not fit the weighted meanto within analytical uncertainty, and when rejected the agecalculation gives 258.9 6 2.7 Ma (MSWD 5 1.42, probability5 8%). Analysis 8.2 is slightly older at approximately275 Ma. The initial analysis of grain 8.1 is withinuncertainty of the weighted mean and close examinationof the cathodoluminescence images does not provideevidence for analysis 8.2 being of an area that may bestructurally older. All the areas analysed are of simplezoned magmatic zircon, but this does not preclude the

presence of slightly older zoned magmatic zircon as aninherited component in an otherwise simple zonedmagmatic zircon population. The well-defined weightedmean age of 258.9 6 2.7 Ma for these zircons gives the timeof magmatic crystallisation.

DISCUSSION

The fundamental importance of the Platypus Tuff Bed asa chronostratigraphic marker was first recognised byKoppe (1978) and the compilation presented here indicatesthat it is a key bed for correlation throughout the northBowen Basin. Isotopic dating of the tuff has provided a new calibration tool for the Upper Permian succession in the Sydney–Bowen Basin system (Wells et al. 1993).Furthermore, the new age provides a new tie point thatallows a more accurate calibration with global events in thePermian Period (Ross & Ross 1994). As Roberts et al. (1996)pointed out, almost all elements of the Permian biota ineastern Australia are exclusively Gondwanan and cannotbe used to correlate with the Permian System of the north-ern hemisphere. The Moranbah Coal Measures contains a low diversity macroflora, including Glossopteris,Gangamopteris, Cladophlebis, Vertebraria and Phyllotheca(Koppe 1978), which is typical of Gondwanan Permiansequences (Foster 1979; Johnson 1984). Koppe (1978)reported palynological data from the Rosella Creek section,near the northernmost extension of the basin, whichrepresents the middle part of the Moranbah Coal Measures. This section contains Didecitriletes ericianus,Dulhuntyispora parvithola and Praecolpatites sinuosus,

Figure 5 Tera and Wasserburg(1972) concordia plot of SHRIMPanalyses for sample WB1 showingthe total 207Pb/206Pb ratios vs thecalibrated 238U/206Pb ratios, uncor-rected for common Pb; analyses areplotted as 1s error ellipses. Insetsshow the weighted mean age calcu-lation and a cumulative probabilityplot, both using the radiogenic206Pb/238U ages. Analytical resultsare given in Table 3.

indicative of Late Permian stage 5 (sensu Price 1983). Thisassignment is consistent with the Late Permian SHRIMPage of the Platypus Tuff Bed.

Although the Blackwater Group has been universallyassigned to the Late Permian (Veevers et al. 1994; Mallettet al. 1995), the position of the Platypus Tuff Bed relativeto stage divisions of the Permian Period is uncertain. Asreviewed by Menning (1995), calibration of Late Permianstage divisions is presently deficient and it is not possibleto make a stage assignment for the Platypus Tuff Bed onthe basis of its geochronology. Palmieri et al. (1994) con-sidered that foraminifers from the Ingelara Formation,which is stratigraphically lower than the Platypus Tuff Bed(Figure 6), indicate a correlation with the Late PermianKazanian stage of Russian sections.

Roberts et al. (1996) reported SHRIMP ages of253 6 3.2 Ma and 249 6 2.2 Ma from tuff samples in marinestrata of the stratigraphically lower Ingelara Formation in the Denison Trough, and ages of 250.1 6 2.8 Ma and 250 6 2.2 Ma, from tuff samples in lacustrine deposits of thestratigraphically higher Black Alley Shale (Figure 6).Roberts et al. (1996) suggested diachronous relationships forthese two formations, to explain the overlapping dates.However, Draper and Fielding (1997) pointed out that thetwo formations are stratigraphically disjunct (Figure 6)rendering this explanation implausible and they concludedthat the discrepancies between dates from each formationand the overlap of dates between formations for whichstratigraphic relationships indicate a clear separation intime, imply either a problem with the SHRIMP techniqueor with the interpretation of results.

A summary of geochronological calibration for theBowen Basin is provided in Figure 6. The summaryassumes that the boundary between the Blackwater andRewan Groups, which represents a profound change inpalaeoenvironment, represents the Permian–Triassicboundary as has been widely accepted (Veevers et al. 1994;Morante et al. 1994; Roberts et al. 1996; Michaelsen et al. 1999;Retallack 1999).

For the northern Bowen Basin the Permian–Triassicboundary is expressed as a sharp change in sedimentary

regime at the contact between the Rangal Coal Measuresand the coal-barren Rewan Group (Figure 3). It marks the collapse of a long-lived (~9 million years) peat-mire ecosystem when almost all peat-producing plants becameextinct (Michaelsen et al. 1999). The boundary shift isunlikely to reflect change in the tectonic setting of theBowen Basin because there is little or no change in thedetrital character of terrigenous sediment supply in theboundary sequence. Michaelsen et al. (1999) attributed theboundary shift to a short-period climatic change accom-panied by an abrupt change in plant ecosystems. This isconsistent with recent findings for the Sydney Basin byRetallack (1999).

Considerable geochronological work has been under-taken on an ash bed in the east Chinese Meishan section,which contains the Permian–Triassic boundary. This bedlies approximately 10 cm below the biostratigraphicPermian–Triassic boundary and immediately below an iso-topic excursion taken to be the event boundary of the twosystems (Yin 1993; Bowring et al. 1998). Age estimates avail-able for the bed place it as 251.4 6 0.4 Ma on the basis of aconventional U/Pb zircon date (Bowring & Erwin 1998),251.2 6 3.4 Ma on the basis of a SHRIMP U/Pb zircon date(Claoué-Long et al. 1991), and 249.9 6 1.5 Ma on the basis ofan Ar/Ar date for sanidine (Renne et al. 1995). A boundarydate of 251 Ma is taken here to best represent a consensusof results.

The age span of the supra-Platypus Tuff BedBlackwater Group may be estimated using the boundaryestimate and the new date presented here. For the easternpart (Nebo Synclinorium) of the northern Bowen Basin,proximal to the foreland uplift to the east (Michaelsen &Henderson 2000b), the supra-Platypus Tuff Bed BlackwaterGroup is approximately 1050 m in thickness. An averagedepositional rate of 133 m/106 years is indicated. For thewestern margin of the basin, distal to the foreland uplift,the equivalent interval has a thickness of approximately550 m, indicating an average depositional rate of approxi-mately 70 m/106 years. Aitken et al. (1997) suggested thatsuch high depositional rates are common for coal-bearingforeland basin systems, the tectonic setting attributed to the

190 P. Michaelsen et al.

Figure 6 Stratigraphic framework of the Bowen Basin (modified from Fielding et al. 1993), with coal-bearing sequences shaded, showingthe context of the age determined for the Platypus Tuff Bed. Age estimates are available for formations marked with an asterisk. Seetext for discussion. Fm, formation; Sst, sandstone.

northern Bowen Basin (Fielding et al. 1993; Michaelsen &Henderson 2000b).

ACKNOWLEDGEMENTS

The authors thank Mike Rubenach (James CookUniversity) for petrographic advice, and Bill Koppe (AngloCoal Australia) and Steve Mattheson (Department of Minesand Energy) for useful discussions. Ian Withnall andAlbert Brakel are thanked for advice on stratigraphicnomenclature. We are grateful to Anglo Coal Australia foraccess to company data and for permission to publish. Thiscontribution benefited from comments and thoroughreviews from Finn Surlyk, Mike Blum and Jim Beeston,and reviewers John Roberts and John Claoué-Long. PMacknowledges support from the Danish Research Academy,Mitsubishi Gas Chemical Resources Australia, Shell CoalAustralia and BHP Coal.

REFERENCES

AITKEN J. F., GUION P. D. & QUIRK D. G. 1997. Controls on the depositionof major sandbodies in the Westphalian to Lower Westphalian Cof northern England. In: Rogers J. ed. 6th International Conferenceon Fluvial Sedimentology Abstracts, p. 2. University of Cape Town,Cape Town.

BAILEY R. A., DALRYMPLE G. B. & LANPHERE M. A. 1976. Volcanism,structure and geochronology of Long valley Caldera,Mono County, California. Journal of Geophysical Research81, 725–744.

BOWRING S. A. & ERWIN D. H. 1998. A new look at evolutionary rates indeep time: uniting paleontology and high-resolution geo-chronology. U/Pb zircon geochronology and tempo of the end-Permian mass extinction. GSA Today 8 (9), 1–7.

BOWRING S. A., ERWIN D. H., JIN Y. G., MARTIN M. W., DAVIDEK K. & WANG W. 1998. U/Pb zircon geochronology and tempo of the end-Permian mass extinction. Science 280, 1039–1048.

CAS R. A. F. & WRIGHT J. V. 1987. Volcanic Successions. Modern andAncient. Allen & Unwin, London.

CLAOUÉ-LONG J. C., ZICHAO Z., GUOGAN M. A. & SHAOHUA D. U. 1991. Theage of the Permian–Triassic boundary. Earth and PlanetaryScience Letters 105, 182–190.

COMPSTON W., WILLIAMS I. S., KIRSCHVINK J. L., ZICHAO Z. & GUOGAN M.A. 1992. Zircon U–Pb ages for the Early Cambrian time-scale.Journal of the Geological Society of London 149, 171–184.

DRAPER J. J. & BALFE P. E. 1985. Late Permian stratigraphy—westernBowen Basin, Queensland. In: 19th Symposium on Advances in theStudy of the Sydney Basin, pp. 106–109. Department of Geology,University of Newcastle.

DRAPER J. J. & FIELDING C. R. 1997. Discussion. SHRIMP zircon datingof the Permian System of eastern Australia. Australian Journalof Earth Sciences 44, 535–538.

EWART A. 1979. A review of the mineralogy and chemistry ofTertiary–Recent dacitic, rhyolitic and related volcanic rocks. In:Barker F. ed. Trondhjemites Dacites and Related Rocks, pp. 277–304.Elsevier, Amsterdam.

FALKNER A. J. & FIELDING C. R. 1993. Geometrical facies analysis of a mixed influence deltaic system: the Late Permian German Creek Formation, Bowen Basin, Australia.International Association of Sedimentologists Special Publication17, 195–209.

FIELDING C. R., FALKNER A. J. & SCOTT S. G. 1993. Fluvial response toforeland basin overfilling: the Late Permian Rangal Coal Measuresin the Bowen Basin, Queensland, Australia. Sedimentary Geology85, 475–497.

FISHER R. V. & SCHMINCKE H. U. 1984. Pyroclastic Rocks. Springer-Verlag,Berlin.

FOSTER C. B. 1979. Permian plant microfossils of the Blair Athol CoalMeasures, Baralaba Coal Measures, and basal Rewan Formation

of Queensland. Geological Survey of Queensland Publication 372(Palaeontological Paper 45).

HESS P. C. 1989. Origins of Igneous Rocks. Harvard University Press,London.

HILDRETH W. 1979. The Bishop Tuff. Evidence for the origin of compo-sitional zonation in silicic magma chambers. Geological Societyof America Special Paper 180, 43–75.

HILDRETH W. 1981. Gradient in silicic magma chambers: Implicationsfor lithosphere magmatism. Journal of Geophysical Research 86,10 153–10 192.

IZETT G. A. 1981. Volcanic ash beds: recorders of upper Cenozoic sili-cic pyroclastic volcanism in the western United States. Journalof Geophysical Research 86, 10 200–10 222.

JOHNSON D. P. 1984. Development of Permian fluvial coal measures, Goonyella. Australia. International Association ofSedimentologists Special Publication 7, 149–162.

KOPPE W. H. 1978. Review of the stratigraphy of the upper part of thePermian succession in the northern Bowen Basin. QueenslandGovernment Mining Journal 79, 35–45.

LE MAITRE R. W. 1976. The chemical variability of some commonigneous rocks. Journal of Petrology 17, 589–637.

LUDWIG K. R. 1999. User’s manual for Isoplot/Ex, Version 2.10: ageochronological toolkit for Microsoft Excel. BerkeleyGeochronology Center Special Publication No. 1a. 2455 RidgeRoad, Berkeley CA 94709, USA.

LUDWIG K. R. 2000. SQUID 1.00, a user’s manual. BerkeleyGeochronology Center Special Publication No. 2. 2455 RidgeRoad, Berkeley CA 94709, USA.

MCPHIE J., DOYLE M. & ALLEN R. 1993. Volcanic Textures. A Guide to theInterpretation of Textures in Volcanic Rocks University ofTasmania Press, Hobart.

MALLETT C. W., PATTISON P., MCLENNAN C., BALFE P. & SULIVAN D. 1995.Bowen Basin. In: Ward C. R., Harrington H. J., Mallett C. W. &Beeston J. W. eds. Geology of Australian Coal Basins, pp. 299–339.Geological Society of Australia Coal Geology Group SpecialPublication 1.

MENNING M. 1995. A numerical time scale for the Permian andTriassic periods: an integrated time analysis. In: Scholle P. A.,Peryt T. M. & Ulmer-Scholle D. S. eds. The Permian of NorthernPangea, pp. 77–97. Springer-Verlag, Berlin.

MICHAELSEN P. 1995. Cyclothemic motifs in the Upper PermianMoranbah Coal Measures. In: Follington I. L., Beeston J. W. &Hamilton L. H. eds. Bowen Basin Symposium, pp. 17–26. GeologicalSociety of Australia Coal Geology Group, Mackay.

MICHAELSEN P., FOSTER C. B. & HENDERSON R. A. 1999. Destabilisationand collapse of a long-lived (c. 9 My) peat mire ecosystem anddramatic changes of alluvial architecture: Permian–Triassicboundary, northern Bowen Basin, Australia. In: Yin H. & Tong J.eds. Proceedings of the International conference on Pangea and thePaleozoic–mesozoic Transition, pp. 137–140. Wuhan, China.

MICHAELSEN P. & HENDERSON R. A. 2000a. Facies relationships and cyclic-ity of high-latitude, Late Permian coal measures, Bowen Basin,Australia. International Journal of Coal Geology 44, 19–48.

MICHAELSEN P. & HENDERSON R. A. 2000b. Sandstone petrofacies expres-sions of multiphase basinal tectonics and arc magmatism:Permian–Triassic north Bowen Basin, Australia. SedimentaryGeology 136, 113–136.

MORANTE R., VEEVERS J. J., ANDREW A. S. & HAMILTON P. J. 1994.Determination of the Permian–Triassic boundary in Australiafrom carbon isotope stratigraphy. The APEA Journal 34, 330–336.

MURRAY C. G. 1983. Permian geology of Queensland. In: Symposiumon the Permian Geology of Queensland, pp. 1–32. GeologicalSociety of Australia, Queensland Division, Brisbane.

O’REILLY K. W., BUZACOTT R. & QUINN G. W. 1995. Geology of the Nebo West anthracite deposit. In: Follington I. L., Beeston J. W. & Hamilton L. H. eds. Bowen Basin Symposium, pp. 37–45,Mackay.

ORTON G. J. 1996. Volcanic environments. In: Reading H. R. ed.Sedimentary Environments: Processes Facies and Stratigraphy, pp.465–567. Blackwell Science, London.

PACES J. B. & MILLER J. D. 1993. Precise U–Pb ages of Duluth Complexand related mafic intrusions, northeastern Minnesota: geochrono-logical insights to physical, petrogenetic, paleomagnetic, andtectonomagmatic process associated with the 1.1 GaMidcontinental Rift System. Journal of Geophysical Research 98,13 997–14 013.

Age of the Platypus Tuff Bed, Qld 191

PALMIERI V., FOSTER C. B. & BONDAREVA E. V. 1994. First record of sharedspecies of Late Permian small foraminiferids in Australia andRussia: time correlations and plate reconstructions. AGSOJournal of Australian Geology & Geophysics 15, 359–365.

PRICE P. L. 1983. A Permian palynostratigraphy for Queensland. In:Symposium on the Permian Geology of Queensland, pp. 155–212.Geological Society of Australia, Queensland Division.

RENNE P. R., ZHANG Z., RICHARDS M. A., BLACK M. T. & BASU A. R. 1995.Synchrony and casual relations between Permian–Triassic bound-ary crises and Siberian flood volcanism. Science 269, 1413–1416.

RETALLACK G. J. 1999. Postapocalyptic greenhouse paleoclimaterevealed by earliest Triassic paleosols in the Sydney Basin,Australia. Geological Society of America Bulletin 111, 52–70.

ROBERTS J., CLAOUÉ-LONG J. C. & FOSTER C. B. 1996. SHRIMP zircon dat-ing of the Permian System of eastern Australia. AustralianJournal of Earth Sciences 43, 401–422.

ROSS C. A. & ROSS J. R. P. 1994. Permian sequence stratigraphy andfossil zonation. In: Embry A. F., Beauchamp B. & Glass D. J. eds.Pangea: Global Environments and Resources, pp. 219–231. CanadianSociety of Petroleum Geologists Memoir 17.

SMITH R. B. & BRAILE L. W. 1994. The Yellowstone hotspot. Journal ofVolcanological Geothermical Research 61, 121–187.

STEIGER R. H. & JÄGER E. 1977. Subcommission of geochronology:

convention on the use of decay constants in geo- and cosmo-chronology. Earth and Planetary Science Letters 36, 359–362.

TERA F. & WASSERBURG G. 1972. U–Th–Pb systematics in three Apollo14 basalts and the problem of initial Pb in lunar rocks. Earth andPlanetary Science Letters 14, 281–304.

VEEVERS J. J., CONAGHAN P. J. & POWELL C. MCA. 1994. Eastern Australia. In: Veevers J. J. & Powell C. McA. eds. Permian TriassicBasins and Foldbelts along the Panthalassan Margin ofGondwanaland, pp. 11–133. Geological Society of America Memoir184.

WELLS A. T., BRAKEL A. T., TOTTERDELL J. M., KORSCH R. J. & NICOLL M. G.1993. Sequence stratigraphic interpretation of seismic data northof 26°S, Bowen and Surat Basins, Queensland. AustralianGeological Survey Organisation Record 1993/51.

WILLIAMS I. S. 1998. U–Th–Pb geochronology by ion microprobe. In:McKibben M. A. & Shanks W. C. eds. Applications ofMicroanalytical Techniques to Understanding MineralizingProcesses, pp. 1–35. Reviews in Economic Geology 7.

YIN H. 1993. A proposal for the Global stratotype section and point (GSSP) for the Permian–Triassic boundary. Albertiana 11,4–30.

Received 20 December 1999; accepted 17 October 2000

192 P. Michaelsen et al.