181105: quartz sandstone, rainbow...

1

181105.1.pdf Geochronology Record 1266

181105: quartz sandstone, Rainbow Valley(Tumblagooda Sandstone, Southern Carnarvon Basin)

Location and sampling

Ajana (SG 50-13), Kalbarri (1742)MGA Zone 50, 217735E 6926406N

Sampled on 25 September 2003

This sample was collected from a cliff adjacent to a walking trail at Rainbow Valley, about 11.0 km north-northeast of Bluff Point, 6.8 km southwest of Kalbarri town site, and 1.3 km south of Red Bluff. The sample comes from close to 36 m in the section measured there by Evans et al. (2007, Fig. 6).

Tectonic unit/relationsThe unit sampled is a cross-bedded quartz sandstone of the Tumblagooda Sandstone (Hocking, 1991). The formation consists mainly of fine-grained sandstone, minor coarse-grained and pebbly sandstone, and rare siltstone grading into very fine grained sandstone. An Ordovician – earliest Silurian age (c. 485 to 420 Ma) is inferred for the formation based on Early Silurian conodont faunas in the overlying Ajana Formation (Mory et al., 1998).

Petrographic descriptionThe sample is a medium- to coarse-grained quartz sandstone, consisting of about 80–90% quartz, 10% illite or sericite, 3–5% K-feldspar, 3–5% lithic clasts, and minor zircon, tourmaline, altered biotite, and limonite. Quartz is bimodal in size, with about 30% of the rock occupied by rounded grains from 0.8 to 1.6 mm (coarse to very coarse sand), commonly with narrow optically continuous overgrowths, and more abundant grains from 0.1 to 0.5 mm in diameter (fine to medium sand), possibly with optically continuous overgrowths and defining a mosaic texture. Most of the coarser sand is composed of single-crystal quartz grains with minor microcline and rare polycrystalline quartz as well as rare clasts of very fine grained sandstone containing quartz, microcline, and possible illite–kaolinite aggregates. The fine to medium sand is more quartz-rich. Interstitial porous decussate illite or sericite is common, and there are disseminated voids, including crystal-lined cavities where there are euhedral optically continuous overgrowths on larger grains. Zircon, up to 0.1 mm long, occurs as inclusions in quartz, partly in optically continuous overgrowths, with tourmaline also in optically continuous overgrowths. Altered biotite and patches of limonite-stained colloform clay are disseminated in the rock.

Zircon morphology

Zircons isolated from this sample are mainly colourless, anhedral to subhedral, and strongly rounded. The crystals are up to 400 μm long, and equant to elongate, with aspect ratios up to 4:1. Some crystals have pitted outer surfaces and, in cathodoluminescence (CL) images, most exhibit concentric zoning truncated at grain edges, features consistent with abrasion during sedimentary transport. A CL image of representative zircons is shown in Figure 1.

Analytical details

This sample was analysed on 21–22 October 2005 and 24–25 October 2005, using SHRIMP-A. Analyses 1.1 to 21.1 (spot numbers 1–23) were obtained during the first session, together with 12 analyses of the CZ3 standard. Significant drift of standard 238U/206Pb* dates during the session was addressed by fitting a LOWESS curve (Cleveland, 1979) with a smoothing window of six analyses, implemented using the program Squid 2.50 (Ludwig, 2009; Wingate and Kirkland, 2015). The analyses indicated an external spot-to-spot (reproducibility) uncertainty of 0.50% (1σ), which is included in the errors of 238U/206Pb* ratios and dates listed in Table 1. Analyses 22.1 to 44.3 (spot numbers 24–60) were obtained during the second session, together with 27 analyses of the CZ3 standard, which indicated an external spot-to-spot (reproducibility) uncertainty of 1.51% (1σ) and a 238U/206Pb* calibration uncertainty of 0.32% (1σ). Calibration uncertainties are included in the errors of 238U/206Pb* ratios and dates listed in Table 1. Common-Pb corrections were applied to all analyses using contemporaneous isotopic compositions determined according to the model of Stacey and Kramers (1975). Dates from analyses for which 204-corrected 238U/206Pb* ratios indicate ages 1000 Ma are based on 204-corrected 207Pb*/206Pb* ratios.

Results

Sixty analyses were obtained from 55 zircons. Results are listed in Table 1, and shown in concordia diagrams (Figs 2 and 3), and a probability density diagram (Fig. 4).

InterpretationThe analyses are concordant to strongly discordant (Figs 2 and 3). Ten analyses >1000 Ma are >5% discordant, and

2


Figure 1. Cathodoluminescence image of representative zircons from sample 181105: quartz sandstone, Rainbow Valley. Numbered circles indicate the approximate locations of analysis sites.

Figure 2. U–Pb analytical data for sample 181105: quartz sandstone, Rainbow Valley. Blue square indicates Group Y (youngest detrital zircon); red squares indicate Group S (older detrital zircons); crossed squares indicate Group D (discordance >5% or high common Pb).

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3


Gro

up ID

Sp

ot

no.

Gra

in.

spot

238 U

(pp

m)

232 T

h(p

pm

)

232 T

h23

8 Uƒ2

04(%

)

238 U

/206

Pb

± 1σ

207 P

b/2

06P

b±

1σ

238 U

/206

Pb

*±

1σ

207 P

b*/

206 P

b*

± 1σ

238 U

/206

Pb

*d

ate

(Ma)

±1σ

207 P

b*/

206 P

b*

dat

e (M

a) ±

1σD

isc.

(%)

Y57

54.1

189

203

1.11

0.27

413

.223

0.31

30.05

910

0.00

132

13.259

0.31

50.05

688

0.00

206

469

1148

780

3.7

S8

8.1

374

358

0.99

0.23

412

.890

0.09

80.05

817

0.00

089

12.921

0.09

90.05

628

0.00

118

481

446

346

-3.7

S4

9.1

100

239

2.47

1.15

212

.607

0.15

70.05

947

0.00

145

12.754

0.16

80.05

026

0.00

381

487

620

717

6-134

.8

S56

53.1

145

141

1.01

1.22

412

.479

0.30

10.06

030

0.00

139

12.634

0.31

00.05

051

0.00

378

491

1221

917

3-124

.6

S50

48.1

259

980.39

0.52

512

.468

0.28

60.05

835

0.00

104

12.534

0.28

90.05

411

0.00

203

495

1137

684

-31.7

S18

18.1

7314

82.09

-0.665

12.307

0.17

80.06

108

0.00

175

12.226

0.18

30.06

659

0.00

361

507

782

5113

38.6

S31

29.1

170

246

1.50

1.06

612

.301

0.29

60.06

095

0.00

141

12.434

0.30

30.05

238

0.00

357

499

1230

215

5-65.0

S59

55.1

390

302

0.80

0.41

812

.270

0.27

50.05

902

0.00

081

12.322

0.27

60.05

563

0.00

145

503

1143

858

-14.9

S55

52.1

229

234

1.06

0.47

712

.036

0.27

90.06

256

0.00

141

12.094

0.28

10.05

867

0.00

225

512

1255

584

7.7

S15

15.1

222

478

2.23

0.07

412

.071

0.112

0.05

924

0.00

098

12.080

0.113

0.05

864

0.00

115

513

555

443

7.4

S10

5.1

7783

1.12

0.91

612

.042

0.15

70.06

085

0.00

681

12.153

0.16

60.05

347

0.00

764

510

734

932

3-46.2

S9

4.1

171

180

1.09

1.18

511.874

0.115

0.06

108

0.00

105

12.016

0.12

30.05

158

0.00

278

515

526

712

4-93.2

S54

51.2

116

840.75

1.74

311.828

0.29

30.06

337

0.00

892

12.038

0.30

60.04

947

0.01012

514

1317

047

8-202

.2

S30

28.1

268

241

0.93

0.64

911.774

0.27

10.06

008

0.00

102

11.851

0.27

40.05

483

0.00

213

522

1240

587

-28.8

S22

20.1

5972

1.25

0.69

611.743

0.17

30.06

141

0.00

177

11.826

0.18

10.05

577

0.00

373

523

844

314

9-18.1

S19

15.2

349

657

1.94

0.08

911.752

0.09

30.05

766

0.00

074

11.762

0.09

40.05

694

0.00

090

526

448

935

-7.5

S20

8.2

362

354

1.01

0.03

911.737

0.08

90.05

859

0.00

070

11.741

0.08

90.05

827

0.00

077

527

454

029

2.4

S17

17.1

109

116

1.09

0.81

010

.682

0.12

70.05

858

0.00

129

10.769

0.13

30.05

208

0.00

298

572

728

913

1-98.2

S40

38.1

250

400.17

0.31

810

.503

0.24

30.06

495

0.00

109

10.537

0.24

50.06

234

0.00

171

584

1368

658

14.8

S26

24.1

9045

0.52

0.21

410

.176

0.26

50.06

721

0.00

182

10.198

0.26

70.06

544

0.00

254

603

1578

981

23.5

S23

21.1

141

145

1.06

-0.064

7.88

50.08

10.06

456

0.00

096

7.88

00.08

10.06

508

0.00

109

770

877

735

0.9

S43

41.1

617

239

0.40

-0.037

6.78

60.15

00.06

927

0.00

274

6.78

40.15

00.06

958

0.00

275

886

1991

681

3.2

S41

39.1

4130

0.75

0.48

36.08

80.18

10.07

934

0.00

213

6.117

0.18

30.07

528

0.00

359

976

2810

7696

9.3

S25

23.1

8272

0.90

1.38

16.04

30.16

20.07

813

0.00

168

6.12

80.16

60.06

670

0.00

422

974

2582

813

2-17.6

S2

2.1

8096

1.24

0.58

25.58

70.06

90.07

950

0.00

112

5.62

00.07

10.07

461

0.00

199

1056

1210

5854

0.2

S38

36.1

160

142

0.92

0.39

75.47

30.13

10.07

976

0.00

103

5.49

50.13

20.07

642

0.00

164

1078

241106

432.6

S51

49.1

154

141

0.95

0.19

55.39

20.13

00.07

831

0.00

109

5.40

30.13

10.07

666

0.00

145

1095

251112

381.6

S14

14.1

136

155

1.18

0.29

55.02

30.05

30.07

930

0.00

089

5.03

80.05

40.07

681

0.00

130

1167

111116

34-4.6

S48

46.1

301

182

0.62

0.15

35.18

80.118

0.07

829

0.00

075

5.19

60.118

0.07

700

0.00

095

1135

241121

25-1.2

S52

50.1

8897

1.15

0.23

05.25

30.13

80.07

982

0.00

147

5.26

50.13

80.07

788

0.00

202

1121

281144

522.0

Tabl

e 1.

Io

n m

icro

prob

e an

alyt

ical

res

ults

for

zirc

ons

from

sam

ple

1811

05:

quar

tz s

ands

tone

, Rai

nbow

Val

ley

4


Gro

up ID

Sp

ot

no.

Gra

in.

spot

238 U

(pp

m)

232 T

h(p

pm

)

232 T

h23

8 Uƒ2

04(%

)

238 U

/206

Pb

± 1σ

207 P

b/2

06P

b±

1σ

238 U

/206

Pb

*±

1σ

207 P

b*/

206 P

b*

± 1σ

238 U

/206

Pb

*d

ate

(Ma)

±1σ

207 P

b*/

206 P

b*

dat

e (M

a) ±

1σD

isc.

(%)

S33

31.1

103

118

1.18

0.51

55.115

0.12

90.08

256

0.00

130

5.14

10.13

10.07

822

0.00

221

1146

271152

560.6

S7

7.1

233

149

0.66

0.19

65.28

90.04

60.08

013

0.00

068

5.29

90.04

60.07

848

0.00

090

1114

91159

233.8

S11

11.1

100

105

1.08

0.12

05.10

80.06

20.08

073

0.00

107

5.114

0.06

20.07

971

0.00

129

1151

131190

323.2

S5

10.1

176

155

0.91

0.26

04.56

00.04

30.08

727

0.00

094

4.57

20.04

30.08

505

0.00

117

1275

1113

1727

3.2

S12

12.1

108

760.73

0.43

73.97

30.04

50.09

747

0.00

094

3.99

00.04

60.09

371

0.00

144

1442

1515

0229

4.0

S28

26.1

157

121

0.79

0.33

73.38

30.08

20.10

463

0.00

100

3.39

50.08

20.1017

10.00

140

1664

3616

5626

-0.5

S45

43.1

284

127

0.46

-0.038

3.52

60.08

00.1014

20.00

070

3.52

50.08

00.1017

50.00

074

1610

3316

5613

2.8

S34

32.1

249

500.21

0.04

33.25

00.07

50.10

926

0.00

079

3.25

20.07

50.10

888

0.00

083

1729

3617

8114

2.9

S16

16.1

484

261

0.56

0.02

92.21

20.016

0.16

006

0.00

045

2.21

30.016

0.15

980

0.00

046

2404

1424

545

2.0

S3

3.1

4937

0.80

0.19

61.76

80.02

70.20

821

0.00

134

1.77

10.02

70.20

647

0.00

147

2885

3628

7812

-0.3

D37

35.1

1398

433

0.32

2.91

223

.569

0.511

0.1168

70.00

294

24.276

0.52

90.09

190

0.00

372

260

614

6577

82.2

D58

44.2

571

351

0.64

1.31

818

.347

0.41

60.06

593

0.00

125

18.592

0.42

70.05

526

0.00

337

338

842

313

620

.1

D21

19.1

232

204

0.91

2.47

317.260

0.20

90.08

467

0.00

195

17.698

0.24

80.06

428

0.00

613

354

575

120

252

.8

D46

44.1

317

287

0.94

0.51

917.526

0.39

50.06

433

0.00

103

17.618

0.39

90.06

009

0.00

191

356

860

769

41.3

D53

51.1

276

174

0.65

1.13

615

.761

0.36

00.06

552

0.00

121

15.942

0.36

90.05

631

0.00

307

392

946

412

115

.6

D60

44.3

576

391

0.70

1.47

615

.254

0.34

20.06

640

0.00

111

15.482

0.35

20.05

448

0.00

315

403

939

113

0-3.2

D13

13.1

434

316

0.75

0.65

013

.959

0.10

90.06

637

0.00

111

14.050

0.113

0.06

104

0.00

193

443

364

168

30.8

D24

22.1

587

275

0.48

0.62

611.066

0.24

40.08

806

0.00

074

11.136

0.24

60.08

275

0.00

132

554

1212

6331

56.1

D44

42.1

6389

1.46

1.17

58.86

50.24

10.07

986

0.00

215

8.97

10.24

90.07

007

0.00

493

681

1893

114

426

.8

D27

25.1

822

950.12

0.38

66.42

50.14

10.08

311

0.00

062

6.45

00.14

20.07

986

0.00

086

929

191193

2122

.1

D1

1.1

3537

1.12

0.84

55.80

10.10

70.07

751

0.00

216

5.85

10.110

0.07

047

0.00

385

1017

1894

2112

-8.0

D6

6.1

2032

1.65

0.91

46.05

50.14

70.08

211

0.00

244

6.111

0.15

20.07

445

0.00

510

977

2310

5413

87.3

D42

40.1

175

122

0.72

0.29

24.90

30.116

0.07

935

0.00

095

4.91

80.117

0.07

689

0.00

139

1193

261118

36-6.7

D29

27.1

202

174

0.89

0.09

65.44

50.12

80.07

877

0.00

094

5.45

00.12

80.07

796

0.00

110

1086

241146

285.2

D32

30.1

9173

0.83

0.46

44.72

10.12

20.08

256

0.00

134

4.74

30.12

30.07

865

0.00

222

1233

301163

56-6.0

D47

45.1

190

930.51

0.00

04.48

50.10

50.08

859

0.00

089

4.48

50.10

50.08

859

0.00

089

1297

2813

9519

7.0

D35

33.1

155

164

1.09

0.09

93.93

30.09

60.09

667

0.00

112

3.93

70.09

60.09

582

0.00

127

1459

3315

4425

5.5

D36

34.1

343

102

0.31

0.17

73.97

00.09

00.10

787

0.00

086

3.97

70.09

00.10

633

0.00

099

1446

3017

3717

16.8

D39

37.1

136

152

1.16

0.07

83.38

10.08

20.10

863

0.00

108

3.38

40.08

20.10

795

0.00

118

1669

3717

6520

5.4

D49

47.1

109

520.49

0.04

71.92

00.04

80.20

486

0.00

130

1.92

10.04

80.20

444

0.00

134

2702

5628

6211

5.6

Tabl

e 1.

co

ntin

ued

5


Figure 3. Expanded view of U–Pb analytical data, not corrected for common Pb, for sample 181105: quartz sandstone, Rainbow Valley. Symbols as in Figure 2.

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Figure 4. Probability density diagram and histogram for sample 181105: quartz sandstone, Rainbow Valley. Thick curve, maxima values, and frequency histogram (bin width 25 Ma) include only accepted data (40 analyses of 38 zircons). Thin curve includes all data (60 analyses of 55 zircons).

6


ten analyses 1%). The dates obtained from these 20 analyses (Group D; Table 1) are considered not to be geologically significant. The remaining 40 analyses can be divided into two groups, based on their 207Pb*/206Pb* and 238U/206Pb* ratios.

Group Y comprises one analysis (Table 1), which yields a 238U/206Pb* date of 468 ± 11 Ma (1σ).

Group S comprises 39 analyses of 37 zircons (Table 1), which yield 207Pb*/206Pb* or 238U/206Pb* dates of 2878–481 Ma.

It is possible that all of the analyses are of unmodified detrital zircons, in which case the date of 468 ± 11 Ma (1σ) for the single analysis in Group Y represents a maximum depositional age for the sandstone. A more conservative estimate of the maximum depositional age can be based on the weighted mean 238U/206Pb* date of 489 ± 9 Ma (MSWD = 2.4) for the nine youngest analyses.

The data for combined Groups Y and S indicate significant age components at c. 1130, 979, 579, 525, 513, and 482 Ma, based on contributions from 9, 3, 3, 9, 12, and 6 analyses, respectively (Fig. 4). These are interpreted as the ages of zircon-crystallizing rocks in the detrital source region(s), or as the ages of detrital components within sediments that have been reworked into this rock.

ReferencesCleveland, WS 1979, Robust, locally weighted regression and smoothing

scatter plots: Journal of the American Statistical Association, v. 74, p. 829–836.

Evans, R, Mory, AJ and Tait, A M. 2007, An outcrop gamma ray study of the Tumblagooda Sandstone, Western Australia. Journal of Petroleum Science and Engineering, v. 57, p. 37–59.

Hocking, RM 1991, The Silurian Tumblagooda Sandstone, Western Australia: Geological Survey of Western Australia, Report 27, 124p.

Low, GH 1975, Yilgarn Block – Proterozoic rocks on or adjoining the Yilgarn Block: Geological Survey of Western Australia, Memoir 2, p. 71–81.

Ludwig, KR 2009, Squid 2.50, A User’s Manual: Berkeley Geochronology Centre, unpublished report, 95p.

Mory, AJ, Nicoll, RS and Gorter, JD 1998, Lower Palaeozoic correlations and thermal maturity, Carnarvon Basin, WA. In PG Purcell and RR Purcell (eds). The sedimentary basins of Western Australia: Proceedings, Petroleum Exploration Society of Australia, Western Australian Basins Symposium, Perth, p. 599–611.

Stacey, JS and Kramers, JD 1975, Approximation of terrestrial lead isotope evolution by a two-stage model: Earth and Planetary Science Letters, v. 26, p. 207–221.

Wingate, MTD and Kirkland, CL 2015, Introduction to geochronological information released in 2015: Geological Survey of Western Australia, 5p.

Recommended reference for this publicationWingate, MTD, Hocking, RM and Mory, AJ 2015, 181105: quartz

sandstone, Rainbow Valley; Geochronology Record 1266: Geological Survey of Western Australia, 6p.

Data obtained: 25 October 2005

Data released: 30 June 2015

181105: quartz sandstone, rainbow...

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