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

181105.1.pdf Geochronology Record 1266

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

Location and sampling Zircon morphology Ajana (SG 50-13), Kalbarri (1742) Zircons isolated from this sample are mainly colourless, MGA Zone 50, 217735E 6926406N anhedral to subhedral, and strongly rounded. The crystals are up to 400 μm long, and equant to elongate, with aspect Sampled on 25 September 2003 ratios up to 4:1. Some crystals have pitted outer surfaces and, in cathodoluminescence (CL) images, most exhibit This sample was collected from a cliff adjacent to a concentric zoning truncated at grain edges, features walking trail at Rainbow Valley, about 11.0 km north- consistent with abrasion during sedimentary transport. A northeast of Bluff Point, 6.8 km southwest of Kalbarri CL image of representative zircons is shown in Figure 1. 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). Analytical details This sample was analysed on 21–22 October 2005 Tectonic unit/relations and 24–25 October 2005, using SHRIMP-A. Analyses 1.1 to 21.1 (spot numbers 1–23) were obtained during The unit sampled is a cross-bedded quartz sandstone the first session, together with 12 analyses of the of the Tumblagooda Sandstone (Hocking, 1991). The CZ3 standard. Significant drift of standard 238U/206Pb* formation consists mainly of fine-grained sandstone, minor dates during the session was addressed by fitting a coarse-grained and pebbly sandstone, and rare siltstone LOWESS curve (Cleveland, 1979) with a smoothing grading into very fine grained sandstone. An Ordovician – window of six analyses, implemented using the program earliest Silurian age (c. 485 to 420 Ma) is inferred for the Squid 2.50 (Ludwig, 2009; Wingate and Kirkland, formation based on Early Silurian conodont faunas in the 2015). The analyses indicated an external spot-to-spot overlying Ajana Formation (Mory et al., 1998). (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 Petrographic description 24–60) were obtained during the second session, together The sample is a medium- to coarse-grained quartz with 27 analyses of the CZ3 standard, which indicated sandstone, consisting of about 80–90% quartz, 10% illite an external spot-to-spot (reproducibility) uncertainty 238 206 or sericite, 3–5% K-feldspar, 3–5% lithic clasts, and of 1.51% (1σ) and a U/ Pb* calibration uncertainty minor zircon, tourmaline, altered biotite, and limonite. of 0.32% (1σ). Calibration uncertainties are included 238 206 Quartz is bimodal in size, with about 30% of the rock in the errors of U/ Pb* ratios and dates listed in occupied by rounded grains from 0.8 to 1.6 mm (coarse Table 1. Common-Pb corrections were applied to all to very coarse sand), commonly with narrow optically analyses using contemporaneous isotopic compositions continuous overgrowths, and more abundant grains from determined according to the model of Stacey and Kramers 0.1 to 0.5 mm in diameter (fine to medium sand), possibly (1975). Dates from analyses for which 204-corrected with optically continuous overgrowths and defining a 238U/206Pb* ratios indicate ages <1000 Ma are based on mosaic texture. Most of the coarser sand is composed of 207-corrected 238U/206Pb* ratios; those >1000 Ma are based single-crystal quartz grains with minor microcline and on 204-corrected 207Pb*/206Pb* ratios. 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 Results sand is more quartz-rich. Interstitial porous decussate illite Sixty analyses were obtained from 55 zircons. Results are or sericite is common, and there are disseminated voids, listed in Table 1, and shown in concordia diagrams (Figs 2 including crystal-lined cavities where there are euhedral and 3), and a probability density diagram (Fig. 4). 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 Interpretation also in optically continuous overgrowths. Altered biotite and patches of limonite-stained colloform clay are The analyses are concordant to strongly discordant (Figs 2 disseminated in the rock. and 3). Ten analyses >1000 Ma are >5% discordant, and

1 181105.1.pdf Geochronology Record 1266

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

0.24

3000 181105 60 analyses of 55 zircons 0.20

0.16 2500 * Pb 206 2000

Pb*/ 0.12 207

1500 0.08

700 400 300 0.04

0.00 0510 15 20 25 238 206 U/ Pb*

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).

2 181105.1.pdf Geochronology Record 1266 7. 7 7. 4 1. 6 3.7 2.4 0.9 3.2 9.3 0.2 2.6 2.0 -7.5 -1.2 -3.7 -4.6 14.8 38.6 23.5 -17.6 -31.7 -14.9 -18.1 -65.0 -46.2 -93.2 -28.8 -98.2 (%) Disc. -134.8 -124.6 -202.2

σ 80 46 84 58 84 43 87 35 29 58 81 35 81 96 54 43 38 34 25 52 Pb* 113 176 173 155 124 149 131 132 323 478 206 Pb*/ 219 376 170 916 487 463 207 825 302 438 555 554 349 267 405 443 489 540 289 686 789 777 828 1112 1116 1106 1121 1144 1076 1058 207 date (Ma) ±1 date

σ 4 6 7 5 7 5 8 4 4 7 8 11 11 11 11 12 12 12 13 12 13 15 19 12 28 25 24 25 24 28 Pb* 206 U/ 238 974 510 512 513 515 514 976 469 481 487 491 495 507 499 503 522 523 526 527 572 584 603 770 886 1167 1135 1121 1056 1078 1095 date (Ma) ±1 date 0.01012 0.00118 0.00115 Pb* 0.00109 0.00145 0.00171 0.00199 0.00164 0.00145 0.00130 0.00764 0.00213 0.00206 0.00381 0.00378 0.00203 0.00361 0.00357 0.00225 0.00278 0.00373 0.00090 0.00077 0.00298 0.00254 0.00275 0.00359 0.00422 0.00095 0.00202

σ 206 ± 1 Pb*/ 207 0.05411 0.07461 0.05158 0.07642 0.07666 0.07681 0.05688 0.05628 0.05026 0.05051 0.06659 0.05238 0.05563 0.05867 0.05864 0.05347 0.04947 0.05483 0.05577 0.05694 0.05827 0.05208 0.06234 0.06544 0.06508 0.06958 0.07528 0.06670 0.07700 0.07788 0.113 0.118 0.274 0.310 0.315 0.168 0.183 0.276 0.166 0.123 0.181 0.133 0.150 0.183 0.166 0.132 0.131 0.138 0.099 0.289 0.303 0.281 0.306 0.094 0.089 0.245 0.267 0.081 0.071 0.054 Pb* σ 206 ± 1 U/ 238 7.880 6.117 6.128 5.196 6.784 5.620 5.495 5.403 5.038 5.265 11.741 11.762 11.851 11.826 12.016 10.769 10.198 12.153 10.537 13.259 12.921 12.754 12.634 12.534 12.226 12.434 12.322 12.094 12.080 12.038 0.00112 0.00104 0.00105 0.00102 0.00109 0.00103 0.00109 0.00132 0.00145 0.00139 0.00175 0.00141 0.00141 0.00177 0.00129 0.00182 0.00168 0.00147 0.00074 0.00274 Pb 0.00213 0.00089 0.00081 0.00098 0.00681 0.00892 0.00070 0.00096 0.00089 0.00075

σ 206 ± 1 Pb/ 207 0.05910 0.06108 0.06108 0.05817 0.06141 0.05766 0.07813 0.07976 0.05947 0.06030 0.05835 0.06095 0.05902 0.06256 0.05924 0.06085 0.06337 0.06008 0.05859 0.05858 0.06495 0.06721 0.06456 0.06927 0.07934 0.07950 0.07831 0.07930 0.07829 0.07982 0.112 0.115 0.118 0.301 0.313 0.157 0.178 0.157 0.173 0.127 0.150 0.181 0.162 0.131 0.130 0.138 0.098 0.286 0.296 0.275 0.279 0.293 0.271 0.093 0.089 0.243 0.265 0.081 0.069 0.053

206

U/ ± 1

238 7.885 5.188 6.786 6.088 6.043 5.587 5.473 5.392 5.023 5.253 11.874 11.774 11.743 11.828 11.752 11.737 10.176 12.301 10.682 10.503 13.223 12.890 12.607 12.479 12.468 12.307 12.270 12.036 12.071 12.042 1.743 1.152 1.185 1.224 1.066 1.381 0.274 0.074 ƒ204 0.810 0.418 0.916 0.318 0.214 0.195 0.153 (%) 0.234 0.525 0.477 0.649 0.696 0.089 0.039 0.483 0.582 0.397 0.295 0.230 -0.665 -0.064 -0.037 Th U 1. 11 1. 01 1. 01 1. 12 1. 18 1. 15 1.50 1.06 1.09 1.25 1.94 1.09 1.06 1.24 232 0.17 0.99 2.47 0.39 2.09 0.80 2.23 0.75 0.93 0.52 0.40 0.75 0.90 0.92 0.95 0.62 238 Table 1. Ion microprobe analytical results for zircons from sample 181105: quartz sandstone, Rainbow Valley sample 181105: quartz sandstone, Rainbow from zircons for analytical results 1. Ion microprobe Table Th 98 83 84 72 40 45 30 72 96 97 116 141 148 180 145 142 141 155 182 203 358 239 246 302 234 478 241 657 354 239 232 (ppm) U 73 77 59 90 41 82 80 88 116 374 301 100 109 189 145 170 171 141 617 160 154 136 259 390 229 222 268 349 362 250 238 (ppm) 8.1 9.1 5.1 4.1 8.2 2.1 1 7. 1 51.2 21.1 41.1 18.1 15.1 15.2 14.1 54.1 53.1 48.1 29.1 55.1 52.1 28.1 20.1 38.1 24.1 39.1 23.1 36.1 49.1 46.1 50.1 spot Grain.

8 4 9 2 10 18 15 19 17 14 57 56 50 31 59 55 54 30 22 20 40 26 23 43 41 25 38 51 48 52 no. Spot Y S S S S S S S S S S S S S S S S S S S S S S S S S S S S S ID Group

3 181105.1.pdf Geochronology Record 1266 7. 3 7. 0 0.6 3.8 3.2 3.2 4.0 2.8 2.9 2.0 5.2 5.5 5.4 5.6 -0.5 -0.3 -3.2 -8.0 -6.7 -6.0 41.3 15.6 16.8 82.2 20.1 52.8 30.8 56.1 26.8 22.1 (%) Disc.

σ 5 11 13 14 12 19 17 56 23 32 27 29 26 77 69 68 31 21 36 28 56 25 20 Pb* 112 136 121 130 144 138 202 206 Pb*/ 423 751 607 464 391 641 931 942 1118 1152 1159 1190 1193 1146 1163 1317 1765 1054 1502 1656 1656 1781 1465 1263 1395 1544 1737 2454 2878 2862 207 date (Ma) ±1 date

σ 9 6 8 5 8 9 9 3 11 13 15 14 12 18 19 18 27 36 33 36 36 23 26 24 30 28 33 30 37 56 Pb* 206 U/ 238 260 338 354 356 392 403 443 554 681 929 977 1114 1017 1146 1151 1193 1610 1086 1275 1442 1664 1729 1233 1297 1459 1446 1669 2404 2885 2702 date (Ma) ±1 date 0.00110 0.00117 0.00118 Pb* 0.00129 0.00144 0.00140 0.00147 0.00191 0.00193 0.00132 0.00139 0.00127 0.00134 0.00074 0.00510 0.00613 0.00315 0.00221 0.00090 0.00083 0.00046 0.00372 0.00337 0.00307 0.00493 0.00086 0.00385 0.00222 0.00089 0.00099

σ 206 ± 1 Pb*/ 207 0.10171 0.10175 0.07445 0.10888 0.06104 0.10633 0.10795 0.15980 0.09190 0.07689 0.07822 0.07848 0.07971 0.08505 0.09371 0.20647 0.05526 0.06428 0.06009 0.05631 0.05448 0.08275 0.07007 0.07986 0.07047 0.07796 0.07865 0.08859 0.09582 0.20444 0.110 0.113 0.117 0.016 0.105 0.131 0.142 0.152 0.128 0.123 0.046 0.062 0.043 0.046 0.082 0.080 0.075 0.027 0.529 0.427 0.248 0.399 0.369 0.352 0.246 0.249 0.096 0.090 0.082 0.048 Pb* σ 206 ± 1 U/ 238 6.111 5.114 1.771 1.921 4.743 5.141 2.213 4.918 5.299 4.572 3.990 3.395 3.525 3.252 8.971 6.450 5.851 5.450 4.485 3.937 3.977 3.384 11.136 17.618 17.698 24.276 18.592 15.942 15.482 14.050 0.00111 0.00111 0.00112 0.00107 0.00100 0.00103 0.00108 0.00130 0.00134 0.00125 0.00195 0.00121 0.00134 0.00130 0.00074 Pb 0.00215 0.00216 0.00068 0.00094 0.00094 0.00070 0.00079 0.00045 0.00294 0.00062 0.00244 0.00095 0.00094 0.00089 0.00086

σ 206 ± 1 Pb/ 207 Table 1. continued Table 0.10142 0.11687 0.08013 0.08311 0.08211 0.09747 0.10463 0.10926 0.10787 0.10863 0.16006 0.08256 0.08073 0.08727 0.20821 0.06593 0.08467 0.06433 0.06552 0.06640 0.06637 0.08806 0.07986 0.07751 0.07935 0.07877 0.08256 0.08859 0.09667 0.20486 0.116 0.016 0.511 0.109 0.107 0.105 0.129 0.416 0.141 0.147 0.128 0.122 0.046 0.062 0.043 0.045 0.082 0.080 0.075 0.027 0.209 0.395 0.360 0.342 0.244 0.241 0.096 0.090 0.082 0.048

206

U/ ± 1

238 1.768 5.115 1.920 5.801 5.108 2.212 5.289 4.560 3.973 3.383 3.526 3.250 8.865 6.425 6.055 4.903 5.445 4.721 4.485 3.933 3.970 3.381 11.066 17.260 17.526 15.761 18.347 15.254 13.959 23.569 1.318 1.136 1.476 1.175 ƒ204 0.515 0.196 0.120 0.196 2.912 0.519 0.914 0.177 (%) 0.260 0.437 0.337 0.043 0.029 2.473 0.650 0.626 0.386 0.845 0.292 0.096 0.464 0.000 0.099 0.078 0.047 -0.038 Th U 1. 18 1. 12 1. 16 1.08 1.46 1.65 1.09 232 0.12 0.66 0.91 0.73 0.79 0.46 0.21 0.56 0.80 0.32 0.64 0.91 0.94 0.65 0.70 0.75 0.48 0.72 0.89 0.83 0.51 0.31 0.49 238 Th 76 50 37 89 95 37 32 73 93 52 118 174 174 105 102 149 155 121 127 316 122 164 152 261 433 351 204 287 391 275 232 (ppm) U 49 63 35 20 91 176 103 100 108 109 157 317 276 576 175 190 155 136 233 284 249 484 571 232 434 587 822 202 343 238 1398 (ppm) 7. 1 1. 1 3.1 6.1 11. 1 27.1 37.1 47.1 31.1 51.1 10.1 12.1 16.1 19.1 13.1 26.1 43.1 32.1 35.1 44.2 44.1 44.3 22.1 42.1 25.1 40.1 30.1 45.1 33.1 34.1 spot Grain.

7 5 3 1 6 11 12 16 13 33 28 45 34 37 58 21 46 53 60 24 44 27 42 29 32 47 35 36 39 49 no. Spot S S S S S S S S S S D D D D D D D D D D D D D D D D D D D D ID Group

4 181105.1.pdf Geochronology Record 1266

181105 0.10

Pb 0.08 206 / Pb

207 1000

800 0.06 600

400

0.04 468101214161820 238 206 U/ Pb

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.

9 181105 maxima 482 8 513 525 7 579 770 y 886 6 979 1130 requenc

f 1316 5 1502 1656 4 1781 2454 Histogram 2878 3

0 2800 2400 2000 1600 1200 800 400 Age (Ma)

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).

5 181105.1.pdf Geochronology Record 1266 ten analyses <1000 Ma indicate large corrections for Recommended reference for this common Pb (ƒ207 >1%). The dates obtained from these publication 20 analyses (Group D; Table 1) are considered not to be geologically significant. The remaining 40 analyses can be Wingate, MTD, Hocking, RM and Mory, AJ 2015, 181105: quartz divided into two groups, based on their 207Pb*/206Pb* and sandstone, Rainbow Valley; Geochronology Record 1266: Geological 238U/206Pb* ratios. Survey of Western Australia, 6p. Group Y comprises one analysis (Table 1), which yields a Data obtained: 25 October 2005 238U/206Pb* date of 468 ± 11 Ma (1σ). Data released: 30 June 2015 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.

References

Cleveland, 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.