Insights from the Devonian Adavale Basin on the Tectonic History of the Thomson Orogen

INSIGHTS FROM THE DEVONIAN ADAVALE BASIN ON THE TECTONIC HISTORY OF THE THOMSON OROGEN

Pascal Asmussen M.Sc.

Submitted in fulfilment of the requirement for the degree of

Doctor of Philosophy

Queensland University of Technology

Science and Engineering Faculty

School of Earth and Atmospheric Sciences

2020

Keywords

A-Type rhyolite Adavale Basin Darling Basin Zircon U-Pb geochronology Detrital rutile U-Pb geochronology Devonian Lachlan Orogen Sediment provenance Statistics Tectonomagmatic affinities Thomson Orogen Tasmanides Zircon inheritance

Abstract

Assessing the timing of stabilisation of non-collisional accretionary orogens is challenged by the inherent absence of the continental collision stage depicted in the Wilson Cycle in such settings. The Thomson Orogen forms part of the Australian Tasmanides and the tectonic setting and timing of stabilisation of the orogen is only poorly constrained due to very low proportions of exposed areas. This PhD thesis utilises Devonian-aged sedimentary basins covering the Thomson Orogen and the adjacent Lachlan Orogen, to study the timing and extent of crustal stabilisation by means of (i) the basal volcanic rocks of the Adavale Basin to constrain the timing of basin initiation and tectonomagmatic affinities, (ii) the nature and provenance of the sedimentary infill of the Adavale Basin to capture the tectonic setting of the basin throughout its lifespan and to aid in (iii) correlating with the Devonian Darling Basin in the Lachlan Orogen to constrain the extent of stabilisation across the Thomson and Lachlan orogens. The basal volcanic rocks of the Adavale are studied using U-Pb zircon geochronology, petrography and whole rock geochemistry. The provenance of sedimentary infill of the Adavale and Darling basins is investigated by means of stratigraphic logging, sandstone petrography, DZ and rutile U-Pb geochronology, and further integrates detrital grain morphologies and zircon/rutile chemistry to fingerprint sediment sources.

The results of this study show that the Adavale Basin was initiated during the Early Devonian at 398.2 ± 1.9 Ma (MSWD = 0.94, N=5, n = 93). Significant zircon inheritance in the volcanic rocks of the Gumbardo Formation record reworking of Ordovician and Silurian silicic igneous basement from the Thomson Orogen. In combination with the results from whole rock geochemical analsysis, the data indicate A-Type affinities and an intracratonic setting for the Adavale Basin is concluded. Sediment provenance in the Adavale Basin is characterised by (i) continuous input from a plutonic/volcanic Ordovician zircon source (~480 Ma), (ii) subordinate reworking of metasedimentary basement rocks with distinctive zircon grain morphologies (high sphericity) and (iii) an addition of a syn- depositional volcanic zircon source ~380 - 360 Ma, having high lateral variability across the basin. Sediment provenance of the Darling Basin is dominated by reworking of (meta)sedimentary basement evident from large proportions of rounded zircons exhibiting

a ‘Pacific-Gondwana’ age signature. A subordinate population of syn-volcanic DZ suggests input from distal extra-basinal volcanic sources.

It is concluded that the Adavale and Darling basin both represent intracratonic basins, recording the stabilisation of the Thomson and Lachlan orogens, respectively. Both basins record similar provenance signals in terms of reworking of their respective hinterlands and relatively distal syn-depositional volcanism. The correlation of sediment provenance proxies, however, suggest that the Adavale and Darling basins were not connected during the Devonian.

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Table of Contents

Keywords ...... I Abstract ...... II Table of Contents ...... IV List of Figures ...... VII List of Tables ...... XXI List of Abbreviations ...... XXIV Statement of Original Authorship ...... XXVI Acknowledgements ...... XXVII Chapter 1: Introduction ...... 1 1.1 Background ...... 1 1.1.1 Crustal growth during Earth’s history ...... 1 1.1.2 Stabilisation of continental crust ...... 3 1.1.3 Tracking stabilisation of accretionary orogens ...... 5 1.1.4 Detrital zircon provenance in intracratonic basins ...... 7 1.2 Area of Study ...... 9 1.2.1 The Thomson Orogen as part of the Tasmanides ...... 9 1.2.2 The Adavale Basin ...... 13 1.3 Research Questions ...... 14 1.4 Approach – Constraining timing of basin initiation and tectonomagmatic affinities of basin- related volcanism (Chapter 2) ...... 17 1.5 Approach – Provenance of sedimentary rocks in the Adavale Basin (Chapter 4) ...... 18 1.6 Approach - Provenance of sedimentary rocks in the Darling Basin and correlation with units from the Adavale Basin (Chapter 5) ...... 21 1.7 Thesis structure ...... 22 1.8 List of research outputs ...... 22 Chapter 2: Geochronology and geochemistry of the Devonian Gumbardo Formation 24 2.1 Introductory Statement ...... 24 2.2 Geological Background ...... 24 2.1 Methods ...... 25 2.1.1 Zircon geochronology and chemistry (LA-ICP-MS) ...... 29 2.1.2 Whole-Rock Geochemistry (XRF & ICP-MS) ...... 31 2.2 Results ...... 32 2.2.1 Volcanosedimentary lithologies of the Gumbardo Formation ...... 32 2.2.2 Geochronology and chronochemistry ...... 38 2.1 Discussion ...... 53 2.1.1 Age assignment of the volcanic rocks of the Gumbardo Formation ...... 53

2.1.2 Zircon inheritance ...... 57 2.1.3 Tectonomagmatic affinities and source of the Gumbardo Formation volcanics 59 2.2 Conclusions ...... 61 Chapter 3: Approach and Methodology ...... 63 3.1 Detrital U-Pb zircon geochronology ...... 63 3.1.1 Grain morphologies/measurements zircon/rutile ...... 66 3.1.2 Maximum depositional ages ...... 67 3.1.3 DZ and the issue of sample representativeness ...... 70 3.1.4 Sample Illustration: Histogram, KDE, PDP and density plots...... 71 3.1.5 Sample comparison methods ...... 73 3.2 Detrital U-Pb rutile geochronology ...... 76 3.3 Summary ...... 78 Chapter 4: Sediment Provenance of the Adavale Basin ...... 79 4.1 Introductory Statement ...... 79 4.2 Geologic Background ...... 80 4.2.1 Eastwood Beds ...... 82 4.2.2 Log Creek Formation ...... 83 4.2.3 Lissoy Sandstone ...... 85 4.2.4 Etonvale Formation ...... 86 4.2.5 Buckabie Formation ...... 87 4.3 Sandstone Petrography ...... 90 4.3.1 Sample locations and petrographic sample description ...... 90 4.3.2 Interpretation ...... 100 4.4 Detrital zircon U-Pb geochronology ...... 101 4.4.1 Comparison of maximum depositional ages and biostratigraphic ages ...... 112 4.4.2 Temporal trends of detrital zircon distributions ...... 117 4.4.3 Zircon trace element chemistry ...... 121 4.4.4 Detrital zircon morphologies ...... 125 4.4.5 Similarity assessment of detrital zircon frequency distributions ...... 127 4.5 Detrital rutile U-Pb geochronology ...... 130 4.6 Discussion ...... 140 4.6.1 Sources of detrital zircon ...... 140 4.6.2 Spatial and temporal trends in detrital zircon distributions ...... 151 4.6.3 Detrital rutile provenance ...... 159 4.7 Stratigraphic Revision ...... 161 4.8 Provenance changes in the Adavale Basin ...... 163 4.9 Conclusions ...... 165 Chapter 5: Sediment provenance of the Darling Basin ...... 167 5.1 Introductory statement ...... 167 5.2 Geologic Background ...... 168 5.2.1 Winduck Group ...... 170 5.2.2 “Lower” Mulga Downs Group ...... 171 5.2.3 “Upper” Mulga Downs Group ...... 173 5.2.4 Stratigraphic correlation with the Adavale Basin ...... 174

5.3 Methods and Samples ...... 176 5.3.1 Sample description and age control ...... 177 5.4 Detrital zircon U-Pb geochronology ...... 181 5.4.1 Wana Karnu Group ...... 181 5.4.2 Ravendale Formation ...... 183 5.4.3 Integration of previous U-Pb zircon geochronology data ...... 187 5.4.4 Zircon trace element chemistry ...... 193 5.4.5 Detrital zircon morphologies ...... 196 5.5 Detrital rutile U-Pb geochronology ...... 198 5.5.1 Wana Karnu Group ...... 199 5.5.2 Ravendale Formation ...... 199 5.5.3 Summary and rutile chemistry...... 200 5.6 Discussion ...... 202 5.6.1 Temporal evolution/Stratigraphic variation...... 203 5.6.2 Detrital zircon sources ...... 204 5.6.3 Primary igneous sources ...... 204 5.6.4 Recycled metasedimentary sources ...... 209 5.6.5 Detrital rutile sources ...... 210 5.7 Conclusions ...... 214 Chapter 6: Synthesis ...... 216 6.1 Sediment Provenance Comparison Between the Adavale and Darling Basins ...... 216 6.1.1 Comparison of synchronous units from the Adavale and Darling basins ...... 216 6.1.2 Devonian volcanic DZ sources ...... 217 6.1.3 Ordovician igneous basement sourcing ...... 220 6.1.4 Cambro-Ordovician sedimentary basement sourcing ...... 220 6.2 Hypothesis testing ...... 224 6.3 Implications for the tectonic history of the tHOMSON oROGEN ...... 227 References ...... 229 Appendices ...... 252

List of Figures

Figure 1.1. Elements of a convergent margin setting and their implications for crustal growth/preservation (green) vs. destruction/recycling (red, after Cawood et al. 2013, Cawood et al., 2011; Hawkesworth et al., 2010; Kemp et al., 2009; Condie, 2007; Dewey, 2005) ...... 2 Figure 1.2. Features of the orogen to platform transition for a collisional accretionary orogen with respect to the evolution of the relief, the predominant type of magmatism, and maturity of sediments (modified after Garfunkel, 1999)...... 7 Figure 1.3 Kernel Density Estimates (KDE) of detrital zircon age frequency distributions from Precambrian intracratonic basin. Data compiled from Cawood et al., 2004; Guadagnin et al., 2015; Khudoley et al., 2001; Rainbird et al., 2010. Orange line shows maximum depositional ages for the respective samples, number of analysed samples given as N...... 8 Figure 1.4 Tasmanide subprovinces and Devonian basin elements after Glen, 2005. Abbreviations: DB, Darling Basin; BT, Barrolka Trough; WT, Warrabin Trough; AB, Adavale Basin; BeB, Belyando Basin; BB, Burdekin Basin. .10 Figure 1.5. Location of the subsurface Adavale Basin and outcropping areas across the Thomson Orogen (modified after Glen 2005), highlighting the scarcity of exposure especially in the central and southern Thomson Orogen. Depth to basement highlights the rather shallow level of the Thomson Orogen at the southern and north-eastern margins compared to the central area where the Adavale Basin is located that resides >1 km beneath the surface. Locations of drill holes intersecting rock successions of the Adavale Basin are shown as black dots, and locations of broadly temporally overlapping igneous rocks in the wider area of the Thomson Orogen highlighted in red. Depth to basement contours created from drill hole data and seismic interpretations...... 12 Figure 1.6. General stratigraphy of the Adavale Basin modified after McKillop et al. (2008) and using the updated geologic time scale (Gradstein et al., 2012). Isotopic age control based on revised SHRIMP ages by Cross et al. (2018). The K-Ar age is from the well completion report for PPC Gumbardo-1 by Phillips Petroleum Company and Sunray DX Oil Company (1963). Palynological assemblages I, II & III (Ass. I, Ass. II and Ass. III) after Hashemi & Playford (2005)...... 15 Figure 2.1 Location of the subsurface Adavale Basin and outcropping units across the Thomson Orogen (modified after Glen, 2005), highlighting the scarcity of exposure especially in the central and southern Thomson Orogen. Depth to basement highlights the shallow level of the Thomson Orogen at the southern and north-eastern margins compared with the central area where the Adavale Basin is located and resides>1 km beneath the surface. Locations of drill holes intersecting rock successions of the Adavale Basin

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are shown as black points, and locations of broadly synchronous igneous rocks in the wider area of the Thomson Orogen highlighted in red. Depth to basement contours created from drill-hole data and seismic interpretations...... 26 Figure 2.2. Location of inspected wells within the Adavale Basin and major basin components. Wells intersecting volcanic rocks are marked with a star symbol...... 27 Figure 2.3. Compilation of well logs with sample locations (e.g., ETO-1 in PPC Etonvale-1), lithological information and interpreted emplacement ages for eruptive units of the Gumbardo Formation and underlying/ overlying units across a N–S transect. Inset map shows location of wells across the Adavale Basin. All depths in metres, scale breaks are not to scale...... 28 Figure 2.4. Core photographs of sampled Gumbardo Formation volcanic rocks showing lithic-bearing ignimbrites from BEA Allandale-1: (a) ALL-1A (2796 m depth) and (b) ALL-1B (3003 m depth). Figures (c–e) show two K-feldspar and plagioclase-phyric ignimbrites (c, GUM-1B, 3283 m depth; d, GUM-1F, 3700 m) and the coherent porphyritic rhyolite (e, GUM-1E, 3537 m) sampled from Gumbardo-1 well. (f) Ignimbrite intersected in the Cothalow-1 well (COT-1, 2500 m); (g) ignimbrite intersected in the Etonvale well (ETO-1, 3305 m); and (h) fine-grained ignimbrite from Carlow-1 (CAR-1, 3293 m)...... 34 Figure 2.5. Photomicrographs of selected volcanic rocks from the Gumbardo Formation. Abbreviations: XPL, crossed polarised light; PPL, plane polarised light. (a) GUM-1B, showing fragmented euhedral plagioclase phenocrysts (pl) and sericitised fiamme (fm) (XPL). (b) Ignimbrite sample GUM-1F showing sericitised, euhedral to subhedral K-feldspar phenocrysts (fsp) in ash matrix and adjacent to fiamme that have been largely replaced by secondary quartz aggregates (qz) and sericite (XPL). (c) Ignimbrite COT-1 with sericitised fiamme (fm) in a groundmass of vitric ash (XPL). (d) Ignimbrite GUM-1E showing intensely sericitised K- feldspars (fsp) in a micropoikilitic groundmass cross-cut by thin veinlets of quartz (XPL). (e) Ignimbrite CAR-2 with sericitised but well-preserved vitriclastic shard textures, and rare phenocrysts of K-feldspar (fsp) (XPL). (f) Ignimbrite ETO-1A that also has well-preserved and nonwelded vitriclastic shard textures (PPL)...... 35 Figure 2.6. Results of the core/rim analyses for individual zircons from (a) GUM- 1B, (b) COT-1, and (c) ETO-1 ignimbrites. 2σ-error bar pairs show core age (red) and rim age (green) of individual grains; the results highlight issues to resolve subtle zircon inheritance as ages for most core and rim domains lie within uncertainty of each other for the majority of the analysed grains. All ages are 206Pb/238U ages in Ma...... 44 Figure 2.7. SEM images showing selected grains targeted for core/rim ablations and 206 238 corresponding Pb/ U age, Th/U ratio and TZircTi (Watson et al., 2006) for individual ablation pits. (a) GUM-500, (b) GUM-419, (c) GUM-238, (d) COT-493, (e) COT-445, (f) COT-395, (g) ETO-382, and (h) ETO-521. All scale bars represent 20 µm...... 44

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Figure 2.8. Weighted averages of 206Pb/238U zircon ages defining the emplacement ages for the analysed samples based on data presented in Table 5. All bars represent 2σ-error bars, dates shown in analytical order...... 45 Figure 2.9. Detrital zircon 206Pb/238U age spectrum for epiclastic sandstone sample GUM-1A, displayed as kernel density estimates (bandwidth 10 Ma) and absolute numbers (bin width 10 Ma). Abbreviations: MDA, maximum depositional age...... 45 Figure 2.10. Autocrystic (orange) and inherited (green/blue) zircon populations and zircon trace-element geochemical data for the respective groupings showing subtle differences between the age populations. Bin width and bandwidth for KDE plot is 5 Ma. Boxes of the whisker plots (b–h) represent the middle 50% of the data, mean values represented by black dot, median by horizontal line, outliers are drawn as circles, far outliers as triangles...... 46

Figure 2.11. TZircsat vs. TZircTi plot after Siégel et al. (2018) suggesting an overall Zirc-Oversaturated magmatic regime for zircons recovered from volcanic rocks of the Gumbardo Formation, promoting the formation of autocrystic and antecrystic zircon...... 47 Figure 2.12. Zr/Ti vs. Nb/Y classification diagram for volcanic rocks (Pearce, 1996) showing volcanic rocks from the Gumbardo Formation collectively plotting in the field for rhyolite/dacite. Additional whole-rock geochemical data are shown for similar Silurian–Devonian (ca 420–410 Ma) rhyodacitic rocks of the Ural and Mt Hope Volcanics (Central Lachlan Orogen; Bull et al., 2008) and intermediate volcanic rock from drill hole Milcarpa-1 dated ca 396 Ma in the southern Thomson Orogen; Roach et al., 2018)...... 51 Figure 2.13. Tectonic affinities of the Gumbardo Formation volcanic rocks. (a) Tectonic discrimination diagram after Pearce et al. (1984), showing transitional affinities for the volcanic rocks of the Gumbardo Formation and temporally related units. Abbreviations: WPG, within-plate granite; VAG, volcanic arc granite; COLG, collision granites; ORG, ocean ridge granites. Supplementary data from igneous rocks across the Tasmanides (volcanic rocks marked with star): Ural Volcanics and Mt Hope Volcanics (Bull et al., 2008), Milcarpa-1 (Roach et al., 2018), AOD Balfour-1 (Siégel et al., 2018), Retreat Batholith (Withnall et al., 1995), Cape York Peninsula Batholith/Pama Igneous Association (Blue Mountains Adamellite and Flyspeck Granodiorite; Cooper et al., 1975). (b) Granite classification diagram after Whalen et al. (1987) showing A-type affinities for samples GUM-1B, CAR-2 and ETO-1. (c) Ba/Nb vs La/Nb plot after Ewart et al. (1992) showing the majority of the volcanic rocks of the Gumbardo Formation plotting in the field for intraplate (SE QLD high- silica rhyolites)...... 52 Figure 2.14. Multi-element diagrams for whole-rock geochemical data of the volcanic rocks of the Gumbardo Formation. (a) Trace-element data normalised to composition of the upper continental crust (Rudnick & Gao, 2014) showing relative depletions of Sr and Ti, as well as a stronger depletion of Ba for plagioclase-rich ignimbrite GUM-1B. Grey shaded

background shows composition of Silurian granites encountered in drill holes south west of the Adavale Basin (data from Siegel, 2015). (b) Rare earth element data of the volcanic rocks normalised to C1 chondrite (McDonough & Sun, 1995) highlighting similarities to the composition of the upper continental crust and higher LREE compared with lower continental crust composition (Rudnick & Gao, 2014). (c) Trace-element data normalised to MORB (Pearce et al., 1981). Light grey shaded field shows data from the Ural Volcanics and Mount Hope Volcanics in the Central Lachlan Orogen (late Silurian–Early Devonian; Bull et al., 2008). Dark grey shaded field shows data from the Chon Aike Province (Mapple Formation, Middle Jurassic, Antarctic Peninsula; Riley et al., 2001) as an example of intraplate rhyolitic volcanism. Note the relative enrichment of HFSE such as Zr, Hf, Y and Yb compared with MORB...... 52 Figure 3.1. Series of three selected detrital zircon grains shown in CL (left) and transmitted light (right), respectively. Red circle (30 µm diameter) shows selected analytical spot selection for each individual grain in the outermost zircon domain...... 66 Figure 3.2. Selected processing steps of the applied image analysis procedure for DZ and CL imagery in JMicroVision (Roduit, 2008). (A) Stitched input CL imagery in TIFF file format, (B) application of threshold technique to DZ grains, (C) object extraction and delineation of individual mineral phases and processing of morphology parameters...... 68 Figure 3.3. Four examples for illustrating an individual DZ geochronology sample using the default settings in DensityPlotter (Vermeesch 2012). (A) Histogram showing the absolute number of ages for individual bins, (B) PDP illustrating the probability density and highlighting the effect of oversmoothing for older DZ grains ( 1Ga), (C) KDE showing the effect of underestimation of the KDE bandwidth especially for older grains ( 1Ga) as opposed to the locally adaptive KDE shown in (D)...... 72 Figure 3.4. Schematic illustrations of different sample comparison methods using KDEs (A,B,C) and cumulative distributions (D) of a simplified synthetic DZ sample. Respective coefficient for each method shown in grey box (0, no similarities; 1, high similarity), Blue and red arrowheads for A and B indicate height of the modes is assessed for these methods, as oppose to C. Similarity (A, Gehrels, 2000) assessing overlapping modes and respective proportions, Cross Correlation Coefficient (B, Saylor et al., 2013) rating modes, absent populations, proportions and shape of the kernel. Likeness (C, Satkoski et al., 2013) as a complement of the area mismatch is focussed on the percentage of the KDE-overlap, neglecting position and proportions of modes. K-S test (D, Kuiper, 1960) utilises the cumulative age distribution for sample comparison (D-value given in grey box)...... 74 Figure 3.5. Probability Model Ensemble (PME) output as part of the Bayesian Population Correlation (BPC) procedure in Tye et al. (2019), assessing the uncertainties of DZ population proportions. Colour legend illustrates concentration of Probability Density Function (PDF) from retained models, red curve shows Kernel Density Estimate (KDE). P-value on Y-axis depicts relative probability, colour bar to the right shows concentration of

the PDF curves as a result of the resampling procedure (maximum of 10000 models)...... 75 Figure 3.6. Wetherill-concordia plots with concordia ages of secondary rutile standards R13, R210 and R10b for two analytical sessions on 13.09.2018 (left) and 27.03.2019 (right)...... 77 Figure 4.1. Updated stratigraphy of the Adavale Basin after McKillop et al. (2007), stratigraphic position of biostratigraphic assemblages I, II and III revised based on geological time chart of Gradstein et al. (2012). Age constraints for the Eastwood Beds, Log Creek Formation, Lissoy Sandstone and Etonvale Formation based in biostratigraphic assemblages (Hashemi & Playford 2007), age of the Gumbardo Formation based on U-Pb zircon SHRIMP ages (Draper, 2006)...... 81 Figure 4.2. Interpolated thickness and total modelled volume of selected sedimentary rock units of the Adavale Basin, based on formational thickness interpreted from well logs after McKillop et al. (2007). Red line shows interpreted location of the Warrego Fault (after Spampinato et al., 2015). Units in chronological/stratigraphic order from the Early to the Late Devonian, a) Eastwood Beds, b) Log Creek Formation, c) Lissoy Sandstone, d) Etonvale Formation and e) Buckabie Formation. Open circles represent well locations, where the respective formation has been intersected. Interpolation method: Angular Distance Weighted (ADW) grid interpolation for irregular distributed points after Shepard (1968), 13 km cell size, processed in SAGA (System for automated geoscientific analysis, Conrad et al. 2015)...... 88 Figure 4.3. Overview of drill hole locations with stratigraphic interpretation available (Geological Survey of Queensland, DNRME, 2014). Logged and sampled well locations highlighted in red with bold bore name label. Transects A-A’ and B-B’ referring to cross sections in Fig. 4.5...... 95 Figure 4.4. Samples in QFL diagram for sandstone classification after Folk 1970, Fields for Quartzarenite (1), Subarkose (2) and Sublitharenite (3). Light grey arrow indicates strong south-north trend in modal composition for samples from the upper section of the Buckabie Formation, which is strongly controlled by the abundance of volcanic rocks fragments (Table 4.3)...... 96 Figure 4.5. DZ/rutile geochronology sample locations in the context of the stratigraphy of the sampled wells, and schematic stratigraphic relationship between wells. For individual locations of depicted wells, see Fig. 4.4...... 96 Figure 4.6. Photographs of samples processed for DZ/rutile geochronology. Image width is 9 cm for all images except pictures of LOG-4 and GUM-6 (11cm), due to larger core diameter. Sample labels correspond to Table 4.3 and 4.4.97 Figure 4.7. Selected microphotographs from sampled rocks of the Eastwood Beds (A-C), Lissoy Sandstone (D) and Etonvale Formation (E-H). (A) overview of fine-grained sublitharenite CAR-13, (B) well developed quartz overgrowths in CAR-14, (C) Detrital rutile from ALL-6, (D) overview image of lithic arkose from LOG-4 (E) overview image of poorly sorted lithic arkose sample FAI-1 showing syntaxial calcite cement, (F) rounded

volcanic clasts from FAI-1, (G) overview image of well-sorted fine- grained subarkose sample GUM-7, (H) sub-rounded DZ grain in GUM-7. 98 Figure 4.8. Selected microphotographs from sampled rocks of the Etonvale Formation (A, B) and Buckabie Formation (C-H). (A) Overview of BUC-1 showing pore-lining clays throughout, (B) Detrital tourmaline from BUC- 1, (C) overview of very fine-grained feldspathic litharenite in BUC-3, (D) rounded volcanic clast showing plagioclase microlites in LOG-6, (E) well- rounded lithic fragment of siltstone in LOG-6, (F) overview image of FAI- 3 showing moderately sorted lithic arkose with nearly 25% rounded to sub- rounded igneous rock fragments, (G) detailed image of rounded volcanic rock fragment in FAI-3 exhibiting plagioclase microlites and phenocrysts, (H) well sorted medium-grained feldspathic litharenite from LOG-7...... 99 Figure 4.9. KDE plot of samples ALL-6 and CAR-13 (organised from north to south) from the Eastwood Beds showing prominent peaks at ~400 Ma, subordinate peak at ~480, ~580 Ma and 1.1 Ga (Grenvillian) for CAR-13. Both samples exhibit a DZ population ~1.5-1.7 Ga...... 103 Figure 4.10. KDE plot of samples CAR-14, LOG-4 and GUM-6 from the Lissoy Sandstone (organised from north to south) displaying major ~480 Ma peaks, a subordinate population ~580 Ma and some scattered Grenvillian ages (0.9-1.1 Ga) for all samples. Samples show varying proportions of ~400 Ma zircons and MDAs are consistently around 400 Ma...... 105 Figure 4.11. KDE plots of samples FAI-1, LOG-5, GUM-7, BUC-1 (organised from north to south) of the Etonvale Formation, showing relatively high proportions of ~480 Ma DZ in all samples and variable proportions of ~385 Ma aged zircons. A subordinate population at ~580 Ma is present in all samples. Note that the southernmost samples GUM-7 and BUC-1 exhibit higher abundance of Grenvillian aged DZ and also yielded significantly younger MDA, compared to the northern sample locations. .108 Figure 4.12. KDE plots of samples FAI-2, LOG-6 and BUC-3 (organised from north to south) from the lower section of the Buckabie Formation showing strong lateral variations in detrital age spectra. The proportions of zircons dating around 480 Ma peak increase from the north-eastern sample location to the southwest, whereas the younger grouping ~370 Ma is only weakly represented in the southernmost location and predominant in the middle and northern sample location. The population ~580 Ma is nearly absent in LOG-6 and forms a subordinate population in the other two samples. Older zircons >600 Ma are rare in FAI-2 and LOG-6 and slightly more abundant in the southernmost sample...... 110 Figure 4.13. KDE plots of samples FAI-3, LOG-7 and BUC-5 (organised from north to south) from the lower section of the Buckabie Formation. All samples are dominated by ~370 Ma zircons, whereas this population almost shows a unimodal distribution for the northernmost location FAI-3. Remaining sample locations exhibit additional DZ populations at ~480 and subordinate at ~580 Ma...... 112 Figure 4.14. Generalised stratigraphy of the Adavale Basin, modified after McKillop et al. (2007), integrated MDA displayed as YCσ2 (3+) plus

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associated uncertainties. Samples and associated MDAs are organised from south to north with respect to the sampled location in the basin...... 116 Figure 4.15. Comparison of YCσ2 (3+) MDA (x-axis) and biostratigraphic age assemblages (y-axis, Hashemi and Playford, 2005). Coloured bars indicate upper and lower boundaries of biostratigraphic assemblages I-III. Rocks of the Buckabie Formation lack reliable biostratigraphic age control, lower and upper age boundary are inferred after McKillop et al. (2007). MDA yielding model ages older than biostratigraphic ages plot in the upper left field (MDA consistent with biostratigraphy), and MDA younger than biostratigraphic ages in the lower right field (MDA inconsistent with biostratigraphy). MDA show overall good agreement with biostratigraphy. Only few MDA suggest younger depositional ages and are thus inconsistent with the biostratigraphic ages (e.g. GUM-7, BUC-1)...... 117 Figure 4.16. KDE plot showing the total DZ data acquired from the Adavale Basin, highlighting large proportions of Devonian and Ordovician ages DZ. Inset shows detailed data from 300 to 660 Ma (~70% of the total DZ data) with colour-coded geological periods. Abbreviations: Sil., Silurian; Ordov., Ordovician...... 119 Figure 4.17. KDE plots of DZ data aggregated per formation. Inset showing variability of proportions in the Ordovician age peak (~480 Ma), manifold Devonian populations (~400, 385, 370 Ma) and variability of the subordinate ~580 Ma population...... 120 Figure 4.18. Density plots, showing the relationship between DZ data (x-axis, aggregated per formation illustrated as heat maps) and depositional ages inferred from biostratigraphy (y-axis, section 4.2.3), allowing to track the persistence of DZ age populations through the stratigraphy of the Adavale Basin. Data illustrate the decrease of Precambrian-aged zircons up stratigraphy and large proportions of Palaeozoic ages (a). Subset (b) shows DZ ages from 350 – 650 Ma highlighting continuous contribution of Ordovician DZ ages (460-485 Ma), disappearance of DZ ages related to Adavale rift volcanics up-section (400 Ma) and distinctive Middle (380 Ma) and Upper Devonian (375-360 Ma) aged DZ successively being introduced up section...... 120 Figure 4.19. Proportions of selected intervals of DZ ages contributing to the DZ budget for each formation illustrating substitution of Precambrian ages by Palaeozoic ages up stratigraphy...... 121 Figure 4.20. Zr/Hf and Th/U ratios for Paleozoic DZ ages showing highest data density for Devonian and Ordovician U-Pb ages (A,B). Data indicate increase in Zr/Hf and Th/U over time. Ordovician aged DZ with low Th/U (<0.3, B) follow xenotime substitution (C, after Allen et al., 2018)) suggesting S-Type affinities of source rock lithologies (plot along trendline). Abbreviations: Carb., Carboniferous, Sil., Silurian, REE, rare earth elements...... 123 Figure 4.21. Trace element data and geochemical parameters grouped into selected age intervals to capture temporal trends in the trace element chemistry of DZ...... 124

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Figure 4.22. Histograms showing relationship between DZ ages and aspect ratio and sphericity for DZ ages from 350 - 2000 Ma (a,d), inset shows data from 350-650 Ma (b,e) and normalised to selected age bins (c, f; 350-430 Ma, 430-510 Ma, 510-650 Ma)...... 126 Figure 4.23. Examples of high aspect ratio zircons from sample FAI-2 (A) and high sphericity grains from BUC-1 (B)...... 127 Figure 4.24. Sample comparison coefficients for each possible pair of DZ distributions from the Adavale Basin data arranged in an individual coefficient matrix for each method. Sample labels exemplary shown for the cross correlation coefficient, samples compared within the same formation highlighted by a bold black outline. Coefficient scale for D-statistic of the K-S test and V-statistic of the Kuiper test (marked with *) is inverted, i.e. high similarity coefficient is 0, and low similarity is 1. Data shows varying degrees of variance between methods and tendency of some methods to over- (BPC, Similarity Value) or underestimate (p-values K-S and Kuiper Tests) similarity...... 129 Figure 4.25. KDE plot (A) and cumulative distribution (B) of sample comparison coefficients for all tested methods for the 15 DZ samples from the Adavale Basin, resulting in 105 coefficients per method from pairwise sample comparison. Coloured coefficient scale corresponds to Fig. 4.24 (0, low similarity; 1, high similarity). Data illustrates high variance for the CCC as opposed to other methods, which exhibit lower variance and are left or (extremely) right skewed. KDE bandwidth=0.1, normalised to 0.3. Abbreviations: KUIP, Kuiper Test (p-value); KSP, K-S Test (p-value); CCC, Cross Correlation Coefficient; LV, Likeness Value; SV, Similarity Value; 1-KUIV, Kuiper Test (1-[V-Value]); 1-KSD, K-S test (1-[D- Value]); BPC, Bayesian Population Correlation...... 130 Figure 4.26. Wetherill-discordia plots of detrital rutile U-Pb data for all analysed samples, highlighting overall high discordance of analysed rutiles. Discordia model-1 ages are calculated for the main population in all samples and results are given in the individual plot. Analysis from older age groupings are greyed out and not considered here for calculation of model ages. Concordant ages per sample shown as histogram and PDP with peak ages in the insets...... 136 Figure 4.27. Tera-Wasserburg plot of 207Pb/206Pb versus 238U/206Pb rutile data identifying common lead as a major source for high discordance of the analysed rutiles...... 137 Figure 4.28. Tera-Wasserburg plots for all detrital rutile U-Pb data grouped by individual grain sizes (determined by length of individual rutile grains), very fine sand (A), fine sand (B, C), and medium sand (D). Data shows consistently younging discordia model-1 ages with increasing grain size fraction...... 138 Figure 4.29. Summary of U-Pb and trace element data from detrital rutile analysis via LA-ICP-MS. (A) Age distribution of concordant U-Pb rutile ages plotted as KDE, (B) Zr-in-rutile temperatures calculated after Watson et al. (2006) for all analysed samples (black boxplots) and summarised for all

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data (grey boxplot). Median value indicated by horizontal line within the box comprising 50% of the data, averages shown as point, outlier indicated by open circles, far outliers by triangle symbols. (C) Nb/Cr classification to constrain rutile source lithology (Meinhold, 2010), plotted for the identified older (blue data points) and younger population (orange data points) showing similar source rocks for both age groups. (D) KDE plot of Zr-in-rutile temperatures for both age groups indicating virtually superposable temperature distributions for both age groups...... 139 Figure 4.30. Compiled DZ geochronology data from the Adavale Basin (only Precambrian ages, this study) in comparison with metasedimentary rocks from the Thomson Orogen. LN-EC group is shown in blue, Grenvillian plateau in orange and 1.5 Ga population in green. Data for Thomson Beds, Mt. Windsor, Les Jumelles Beds, Betoota Beds, Nebine Metamorphics (GSQ Mitchell 1) from Purdy et al. (2016), Nebine Metamorphics (GSQ Eulo 1) from A. Cross, unpublished data. DZ distributions for Adavale Basin and Thomson Beds are shown as locally adaptive KDE, as data density and nature of the distribution is adequate, remaining samples are plotted as PDP to avoid oversmoothing and allow visual comparison...... 148 Figure 4.31. Distribution of potential volcanic/plutonic source rocks locations for DZ in the Adavale Basin from drill holes and outcrop. All ages are U-Pb zircon emplacement ages complied after Jones et al., 2018 and Siegel, 2015, except Gem Park Granite (65, U-Pb monazite via SHRIMP) and granite in PPC Etonvale-1 (61, whole rock Rb-Sr). Detailed information on each sample location is documented in Appendix 4.8...... 149 Figure 4.32. Distribution of selected metasedimentary units in the Thomson Orogen after Purdy et al. (2016) showing the widespread distribution of the subsurface Thomson beds. Labelled drill holes and outcropping units correspond to samples with existing DZ data in Fig. 4.30...... 150 Figure 4.33. Sample comparison of DZ sample pairs using the Cross Correlation Coefficient (CCC) in a lateral context (A) and temporal context (B). Well locations are organised from north to south (left to right) with all samples and their formational context in the vertical axis. The colour of the square indicates the CCC value comparing the two neighbouring samples. Sample GUM-7 is shown here in the context of the upper part of the Buckabie Formation (according to the MDA from Section 4.4.1) instead of the Etonvale Formation. In addition to BEA Allandale-1, samples taken from PPC Gumbardo-1 and PPC Buckabie-1 are duplicated on the left side of (A) to allow for sample comparison of the respective northern- and southernmost well location for each formation. Data highlights high lateral similarity for the Lissoy Sandstone and Etonvale Formation (yellow and orange colours), compared to reduced similarity and increasing lateral heterogeneity in DZ distributions for both sections of the Buckabie Formation (green and blue colours). This shift is also observed between the Etonvale and Buckabie Formations in temporal trends (B) but seems ‘delayed’ in the southern portion of the basin (as indicated by grey arrow).155 Figure 4.34. Multi-dimensional scaling plot (MDS) of all analysed DZ samples plotted in IsoplotR allows to integrate the spatial and temporal

relationships for the DZ distributions (Vermeesch, 2018). Solid lines show nearest neighbour relationship; dashed lines show second nearest neighbour relationships between samples. In order to provide orientation in this otherwise dimensionless diagram, the three major DZ age populations identified in the Adavale Basin are included in the MDS plot as synthetic normally distributed samples (mean=360 Ma, 480 Ma and 580 Ma; n=150, standard deviation=2% of mean). Data show DZ distributions are dominated by basement derived ages for Lower and Middle Devonian units with subtle differences between the southern and northern basin. Increasing influence of ~380 Ma zircons causes a major provenance change in the basin as evident for Upper Devonian Buckabie Formation. Abbreviations: A.B. Adavale Basin...... 158 Figure 4.35. (a) Nb versus Cr classification for detrital rutile data (after Meinhold et al., 2008) from this study and data from the Thomson beds and Les Jumelles Beds (Siegel et al., 2017) indicating predominantly metapelitic sources for rutiles from both datasets (concordant and discordant datasets plotted). (b) KDE plots of concordant rutile U-Pb ages for the Cambrian- Ordovician Thomson beds (Siegel et al., 2017), Devonian Adavale Basin (this study) and the Carboniferous Drummond Basin (Sobczak, 2019), showing strong age overlap between Drummond Basin and Thomson beds and the older peak age for the Adavale Basin data...... 160 Figure 4.36. Comparison of revised stratigraphy of the Adavale Basin (left) based on the data and interpretations of this Chapter in combination with Chapter 2 and basin stratigraphy after McKillop et al. 2007. Revisions comprise shortening of the Gumbardo Formation based on U-Pb zircon geochronology and identification of potential zircon inheritance. Deposition of the Eastwood Beds is proposed to be extended into the Eifelian based on an Eifelian MDA for a sample from the Eastwood Beds (Section 4.4.1). Deposition of the Buckabie Formation most likely extended into the earliest Carboniferous based on MDA for upper sections of the Buckabie Formation (Section 4.4.1). Abbreviations: Miss., Mississippian; Tourn., Tournaisian...... 162 Figure 4.37. Illustration of two distinctively different sediment provenance phases for the Lower Devonian (A, C) and Upper Devonian (B, D) sedimentary successions. Coloured outlines are based on compiled U-Pb age data from Figs. 4.31, 4.32 and adapted from Rosenbaum (2018) showing distribution of DZ source locations in the Thomson and New England Orogens schematically. Colour coding also corresponds to highlighted age intervals in the KDE plots (C, D). The first phase (A) is characterised by reworking of proximally located Thomson Orogen basement rocks and potential contributions of more distal Ordovician plutonic rocks, whereas the second phase (B) is characterised by influx of syn-sedimentary Upper Devonian zircons from distal sources to the north-east of the Adavale Basin...... 164 Figure 5.1. Overview map of the Tasmanides subprovinces (after Glen, 2005) and location of the Darling Basin across the Lachlan and Delamerian orogens (a). Inset map showing the named sedimentary troughs comprising the Darling Basin. Boundaries for the Thomson, Lachlan and Delamerian

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orogens shown by red lines and sampled drill holes highlighted in bold (b). Abbreviations: DB, Darling Basin; BT, Barrolka Trough; WT, Warrabin Trough; AB, Adavale Basin; BeB, Belyando Basin; BB, Burdekin Basin.169 Figure 5.2. Lithostratigraphic units in the Darling Basin after Bembrick (1997), Cooney and Mantaring (2005). Informal basin-wide stratigraphy and predominant paleoflow directions after Bembrick (1997) and references therein. Abbreviations: Mt., Mount; GA, Geoscience Australia...... 170 Figure 5.3. Comparison of Adavale and Darling basin stratigraphy, biostratigraphic age control for selected units and overall timing of regional orogenic events. Data highlights the low precision of biostratigraphic age constraints for the Darling basin (primarily from fossils) versus higher precision age control in the Adavale Basin via palynology (Hashemi and Playford, 2005). Adavale Basin stratigraphy revised after McKillop et al. (2007), Darling Basin stratigraphy after Bembrick (1997) and Cooney and Mantaring (2005). Darling Basin biostratigraphy data from (Bembrick, 1997; Freeman, 1966; Glen and Powell, 1986; Macquarie University Centre for Ecostratigraphy and Palaeobiology (MUCEP), 2002; Neef et al., 1995; Sherwin, 1980). Abbreviations, AA, absolute age; BA, biostratigraphic age; OE, orogenic event; EB, Eastwood Beds; LCF, Log Creek Formation; LS, Lissoy Sandstone; BL, Bury Limestone; EF, Etonvale Formation; Miss., Mississippian; Dev., Devonian...... 176 Figure 5.4. QFL diagram after Folk (1970) for samples from the Ravendale Formation (red) and the Wana Karnu Group (blue) in comparison with sampled units in the Adavale Basin. Fields for quartzarenite (1), subarkose (2) and sublitharenite (3). Samples from both units in the Darling Basin are compositionally mature and very similar in composition to the Eastwood Beds in the Adavale Basin...... 179 Figure 5.5. Representative microphotographs of sampled sandstones from samples PAM-11 (a, Pamamaroo 1, 769.5 – 760.5 m, image from Barry, 2016), PON-21 (b, Pondie Range 1, ~450.5 m) and MOS-18 (d, Mossgiel 1, 610.9 – 611.5 m). Rutile bearing quartz grain from PON-21 (c), accessory minerals of zircon (Zr) and muscovite (Ms) from MOS-18 (e). Sample EMU-20 was sampled from ditch cuttings (f, Mount Emu 1, 372.0 – 396.0 m, image from Barry, 2016). All microphotographs were taken under crossed polarized light...... 180 Figure 5.6. DZ geochronology of samples from the Wana Karnu Group, showing core and rim ages. All data plotted as locally adaptive KDE (only rim analyses)...... 182 Figure 5.7. Transmitted light (upper row) and CL images (lower row) of the youngest concordant DZ from PAM-11 showing euhedral to subeuhedral morphologies and often containing melt inclusions. Analysis of core domain (green circle) for 11_96 yielded a discordant age...... 184 Figure 5.8. DZ geochronology data from two samples from the Ravendale Formation integrating concordant U-Pb ages from rim and core analysis. KDE (blue) and PDP (red) shown together with a histogram of the data, in order to capture all aspects of the DZ age distribution. Data highlights

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issues arising from broad age populations and the usage of KDE plots, PDP captures additional age cluster ~360 Ma for PAM-11 that otherwise remains undetected in the KDE plot. U-Pb ages for core domains not included in the overall data only displayed as circles on the bottom of the plot...... 185 Figure 5.9. Compiled concordant core and rim ages grouped for individual grains from all four analysed samples arranged in ascending order of U-Pb rim ages. Data show relatively high proportions of inherited U-Pb ages ~500- 600 Ma and subordinate for the Grenvillian age bracket (0.9-1.2 Ga)...... 186 Figure 5.10. KDE plots of previous DZ U-Pb data (blue, Barry, 2016), this study (red) and combined datasets (black) for analysed samples from the Ravendale Formation (A+B) and Wana Karnu Group (C+D)...... 191 Figure 5.11. DZ data combined for the two sampled formations, Wana Karnu Group (Emsian, Lower Devonian) and Ravendale Formation (Famennian, Upper Devonian). Age peaks exclusive to the respective unit written in colour, shared age peaks are illustrated in black...... 192 Figure 5.12. KDE plot showing the total DZ data acquired from the Darling Basin, highlighting a high proportion of Devonian (9%), Cambrian (17%) and Ediacaran (20%) DZ ages. Inset shows data from 300 Ma to 1.3 Ga Ma (~86% of the total DZ data) with colour coded geological periods. Abbreviations: Ca., Cambrian...... 192 Figure 5.13. Trace element data and geochemical parameters grouped into selected age intervals to capture trends in the trace element chemistry of DZ from the Darling Basin...... 195 Figure 5.14. REE subset vs P plot for all analysed DZ in this study highlights the absence of P and REE coupling for DZ from the Darling Basin, indicating absence of igneous source rocks with S-Type affinities in the Darling Basin (after Allen et al., 2018)...... 196 Figure 5.15. Histogram showing U-Pb DZ ages for all data from the Darling Basin integrated with morphology parameters aspect ratio (A) and sphericity (B). Stacked histograms show binned morphology parameter normalised for selected ages bins...... 198 Figure 5.16. Tera Wasserburg plots showing concordia (a) and discordia model-1 ages (b,c,d) for the major age population on a per sample basis for U-Pb rutile geochronology. Older grains not included in major population are displayed in grey...... 201 Figure 5.17. Summary of analytical results, (a) PDP plot of all concordant rutile U- Pb ages, (b) boxplots of Zr-in-rutile temperatures on a per sample basis (black boxplots) and summarised for all analyses (grey boxplot), (c) Nb/Cr classification for discordant and concordant data (after Triebold et al., 2007)...... 202 Figure 5.18. Tera-Wasserburg plots for all detrital rutile U-Pb data grouped by individual grain sizes (determined by length of individual rutile grains), very fine sand (A), fine sand (B, C). Data shows consistently younging discordia model-1 ages with increasing grain size fraction...... 203

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Figure 5.19. Spatial relationship between sampled locations with syn-basinal DZ ages (Ravendale Formation, black circles) in the Darling Basin (blue) and location potential synvolcanic source units (Orange). Note that both volcanic units lie in easterly direction to the sampled location, syn-volcanic zircons from explosive eruptions might have been transported by easterly atmospheric currents. Abbreviations: QLD, Queensland; NSW, New South Wales; SA, South Australia; VIC, Victoria; ACT, Australian Capital Territory...... 206 Figure 5.20. Overview of relevant (igneous) geological units in the Lachlan Orogen for sediment provenance in the Darling Basin. U-Pb ages have been extracted from the XplorPak 2016 database (New South Wales. Department of Industry. Division of Resources and Energy compiler, 2016). Black circles represent sampled locations. Abbreviations: QLD, Queensland; NSW, New South Wales; SA, South Australia; VIC, Victoria; ACT, Australian Capital Territory...... 208 Figure 5.21. Compiled DZ data from Cambro-Ordovician sedimentary rocks in the Delamerian and Lachlan Orogens (ST36, ST76, ST21, ST18, ST23, Squire et al., 2006; F03D02, Armistead and Fraser, 2015; KB22a/21/27/12/14/16/26, Johnson et al., 2016) and comparison with total DZ data from the Darling Basin (combined datasets of this study and Barry, 2016) showing interplay of ~580 (red line) and 500 Ma (green line) peaks in Cambrian and Ordovician sedimentary rocks. The ~580 Ma peak in Middle to Late Cambrian samples is replaced by a dominant ~500 Ma population during the Late Cambrian/Ordovician. Note that the Darling Basin data shows both peaks with nearly equal proportions...... 211 Figure 5.22. Spatial extent of selected Cambro-Ordovician sedimentary units in the Lachlan, Thomson and Delamerian Orogens, subsurface extent of the Darling Basin shown in light blue. Sample labels in brackets in the legend refer to DZ samples in Fig. 5.21...... 212 Figure 5.23. Concordant detrital rutile U-Pb ages, compiled from various Phanerozoic sedimentary rocks showing subtle variations and similarities between the datasets (a). Plot of discordia model ages vs median grain length on a per-sample basis, highlighting correlation between grain size and rutile U-Pb ages...... 213 Figure 5.24. Sediment provenance interpretation for the Lower to Middle Devonian (a) and Upper Devonian (b) successions in the Darling Basin in conjunction with KDE plots of the respective unit (lower section of a and b). Data highlights the predominance of Cambro-Ordovician (meta)sedimentary rocks (green) in the sedimentary budget of the Darling Basin. Younger age groups change from Lower Devonian (~410 Ma) to Upper Devonian (~380 Ma) up stratigraphy, whereas Upper Devonian volcanic sources are distal to the Darling Basin...... 215 Figure 6.1. Comparison of aggregated DZ distributions of coeval sedimentary successions in the Adavale and Darling basins in their respective stratigraphic context. Data emphasize the previous observations with

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distinctively different KDE peaks ages for Palaeozoic DZ ages and some similarities for Precambrian DZ data...... 219 Figure 6.2. Data compilation of Th/U for DZ data from Chapter 4 and 5 (sink) in comparison with Th/U data from syn-volcanic units in the Anakie Province and Lachlan Orogen (source). Data highlight differences in Th/U between Adavale and Darling basin, especially for ages between 350-370 Ma. Th/U of DZ from the Darling Data exhbit distinctive overlap (green bar) with zircons from the interpreted syn-volcanic sources (data compiled from Atton, 2013; Cross et al., 2015, 2009; Kemp et al., 2006)...... 220 Figure 6.3. KDE plots of total DZ U-Pb data from the Adavale Basin (blue, this study) and Darling Basin (a, orange, this study and Barry, 2016) showing high similarity of distributions for ages >525 Ma (b) and major differences in the spectra for DZ age <750 Ma (c)...... 222 Figure 6.4. Summary of provenance phases in the Darling and Adavale basins across the Thomson and Lachlan orogens. Both basins record sourcing from Cambro-Ordovician (meta)sedimentary rocks (green) throughout the Devonian and additionally first-order contributions from igneous basement units (Early Devonian [yellow] and Ordovician-aged [red] in the Adavale Basin; Early Devonian in the Darling Basin [orange]). Provenance in the Upper Devonian is characterised by ongoing sediment contribution from basement rocks and additionally derivation from syn-volcanic sources distal to the respective basin (blue)...... 223

List of Tables

Table 1.1. Comparison of signals for crustal stabilisation processes for the Archean and Proterozoic/Phanerozoic cratons (after criteria outlined by Pollack, 1986)...... 5 Table 1.2. Characteristic features of selected sedimentary successions related to intracratonic basins (Compiled after: Alkmim and Martins-Neto, 2012; Friend et al., 2003; Guadagnin et al., 2015; Khudoley et al., 2001, 2001; Rainbird et al., 2010; Soper et al., 1998)...... 9 Table 1.3. Biostratigraphically synchronous sedimentary rock units in the Adavale and Darling Basins (Bembrick, 1997; Khalifa, 2010)...... 19 Table 1.4. Logged wells, interpreted thickness and recovered and inspected core material for selected wells in the Adavale Basin. Bore names highlighted in bold were selected for geochronology sampling ...... 20 Table 2.1. Wells reported to intersect Gumbardo Formation in McKillop et al. (2007) and inspected Gumbardo intervals...... 29 Table 2.2. Sampled depths for volcanic rocks of the Gumbardo Formation. Hole deviations at sampled depth are extracted from the well completion reports and report the deviation from a vertical axis at the respective sampled depth...... 30 Table 2.3. Weighted averages of 206Pb/238U zircon ages of secondary standard Plešovice (mean ID-TIMS U–Pb age: 337.13 ± 0.37 Ma; Sláma et al., 2008), sample preparation method and analysed unknowns. Abbreviations: MSWD, mean squared weighted deviation; POF, probability of fit...... 30 Table 2.4. Petrographic assessment of volcanic rocks of the Gumbardo Formation, encompassing modal composition (based on point counting: 300 counts, 1 mm step length), phase descriptions and alteration assessment. Abbreviations: gm, groundmass; qz, quartz; K-fsp, K-feldspar; pl, plagioclase; lit, lithic clasts; juv, juvenile clasts; hb, hornblende; px, pyroxene...... 37 Table 2.5. U–Pb geochronology results documenting weighted averages for zircon population interpreted as emplacement ages and inherited populations. Abbreviations: WA, weighted average; MSWD, mean square weighted deviation...... 43 Table 2.6. Whole-rock geochemical data for sampled volcanic rocks of the Gumbardo Formation. Major elements in wt% normalised to 100% (volatile free), trace elements in ppm. Eu/Eu*, Ce/Ce* and (La/Yb)N normalised to chondrite after McDonough and Sun (1995)...... 54 Table 2.7. Comparison of major-element compositions of an average volcanic rock of the Gumbardo Formation (this study), average rhyodacite (Le Maitre, 1976) and quartz latite (Milner et al., 1992). All data in wt%, normalised to 100% and volatile free...... 55

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Table 2.8. Compilation of geochemical classifications and zircon inheritance data. Geochronology studies of samples marked with star have been analysed by Draper (2006) and reinterpreted by Cross et al. (2018). Magmatic affinity after Whalen et al. (1987), magnesian/ferroan rock classification after Frost and Frost (2008). Percentages of inheritance based on based on data from Table 5. Abbreviations: ASI, aluminium saturation index...... 55 Table 3.1. Summary weighted averages of secondary zircon standard “Plešovice” for all analytical sessions for detrital U-Pb zircon geochronology and analysed unknowns. Abbreviations: MSWD, mean squared weighted deviation; POF, probability of fit ...... 64 Table 3.2. Methods introduced in this Chapter and their application in detrital mineral provenance analysis in the following Chapters 4 and 5...... 78 Table 4.1. Sample locations and sample depths for palynology analysis, extracted from Hashemi and Playford (2005). For drill hole locations, see Fig. 4.3. ..82 Table 4.2. Wells intersecting selected sedimentary formations in the Adavale Basin with formational thickness interpreted from well log data (int. thick., interpreted thickness; Geological Survey of Queensland, DNRME, 2014; McKillop et al., 2007) and cumulative core material available (cum. core, cumulative core; extracted from the respective well completion reports and logging of selected wells)...... 89 Table 4.3. Petrographic data for all sampled sandstones from point counting analysis (300 counts per thin section sample). Abbreviations: qm, monocrystalline quartz; qp, polycrystalline quartz; ch, chert; kfsp, K- feldspar; pl, plagioclase; mcr, microcline; srf, sedimentary rock fragments; irf, igneous rock fragment, mrf, metamorphic rock fragment; ms, detrital muscovite; bt, detrital biotite; zirc., zircon; tour., tourmaline; rut., rutile. Abundant zircon and rutile presence in mineral separate indicated by “x”, minor abundance by “(x)”, absence indicated by “-“. Tourmaline presence in thin section indicated by “x”, absence by “-“...... 95 Table 4.4. DZ sample details. Sampled core section with respective well information and median values of selected morphology parameters. Abbreviations: len., length; Fm, Formation, AR, aspect ratio...... 101 Table 4.5. Comparison of YCσ1 (2+) and YCσ2 (3+) populations (Dickinson and Gehrels, 2009a) and statistical indicators for all samples. Note the differences between the weighted averages for YCσ1 (2+) and YCσ2 (3+) with YCσ2 (3+) exhibiting older ages. Abbreviations: Fm, Formation; MSWD, mean squared weighted deviation; POF, probability of fit...... 113 Table 4.6. Summary of basic statistical parameters for each conducted sample comparison method. Probability distributions are documented in Fig. 4.25. Underlying data is documented in Appendix 4.5 ...... 130 Table 4.7. Summarised data from U-Pb analysis of detrital rutile for 10 analysed samples. For sample LOG-7 an additional 7 analyses were rejected due to large analytical uncertainties (>50%). Discordia model-1 ages show generally good agreement with PDP peak ages, especially when number of concordant analyses is sufficient (n>5). Abbreviations: #, number of; anal.,

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analysed; conc., concordant; disc., discordant; PDP, Probability Density Plot...... 138 Table 5.1. Sampling details on analysed samples from the Darling Basin. All coordinates are reported in GDA94. Location of wells shown in Fig. 5.1. 179 Table 5.2. Sample details for detrital U-Pb zircon geochronology including information on core and rim analyses and grain morphology parameters. .186 Table 5.3. MDA calculated as YC1σ (2+) and YC2σ (3+) for U-Pb data of (i) this study, (ii) previous data from Barry (2016), and (iii) combined datasets. MDA in bold for the combined data are the preferred MDA. Abbreviations: MSWD, mean squared weighted deviation; POF, probability of fit...... 189 Table 5.4. Summarised data of U-Pb geochronology of detrital rutile for four samples from the Darling Basin highlighting two subtly different age populations for the Early Ordovician (~475 Ma) and Cambrian (~500-505 Ma)...... 201

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List of Abbreviations

ADW – Angular Distance Weighted ASI – Aluminium saturation Index BPC – Bayesian Population Correlation CARF – Central Analytical Research Facility CCC – Cross Correlation Coefficient CL – Cathodoluminescence DNRME – Department of Natural Resources, Mining and Energy DZ – Detrital zircon ECD – Equivalent Circular Diameter Ga – Billion years ago GSNSW – Geological Survey of New South Wales GSQ – Geological Survey of Queensland ICP-MS - Inductively Coupled Plasma Mass Spectrometer ID-TIMS - Isotope dilution-thermal ionization mass spectrometry HFSE – High field strength element HREE – Heavy rare element KDE – Kernel Density Estimate K-S test – Kolmogorov-Smirnov Test LA-ICP-MS – Laser Ablation-Inductively Coupled Plasma Mass Spectrometer LREE – Light rare earth element Ma – Million years ago MDA – Maximum depositional age MDS – Multidimensional Scaling MSWD - Mean square weighted deviation Myr(s) – Million year(s) NIST – National Institute of Standards and Technology POF – Probability of fit QLD - Queensland QUT – Queensland University of Technology PDF – Probability Density Function PDP – Probability Density Plot PME – Probability Model Ensembles PPL – Plain polarised light REE – Rare earth element SAGA – System for Automated Geoscientific Analysis SEM – Scanning Electron Microscopy SHRIMP – Sensitive High-Resolution Ion Microprobe TE – Trace element USGS – United States Geological Survey WA – Weighted Average XPL – Crossed polarised light XRF - X-ray fluorescence YC1/2σ – Youngest sigma 1/2 cluster YPP – Youngest peak age (Probability plot)

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YSG – Youngest Single grain

Bore name company prefixes

AAO – Associated Australian Oilfields NL AAR – Associated Australian Resources NL AGA – Australian Gasfields Ltd AMX – Ampol Exploration Ltd AOD – Alliance Oil Development NL AOP – American Overseas Petroleum Ltd ASO – Australian Sun Oil Co Ltd BEA – Beaver Exploration Australia Ltd EAL – Esso Australian Ltd IOD – Icon Oil NL HEP – Hartogen Explorations Pty Ltd LEA – LASMO Oil Company Australia Ltd LOL – Longreach Oil Ltd PPL – Pan Continental Petroleum Ltd PPC – Phillips Petroleum Company TEA – Total Exploration Australia Pty Ltd

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Statement of Original Authorship

With the exception of the referenced sections of Chapter 1 and 2, the work contained in this thesis has not been previously submitted to meet requirements for an award at this or any other higher education institution. To the best of my knowledge and belief, the thesis contains no material previously published or written by another person except where due reference is made.

Signature: QUT Verified Signature

Date: ______

XXVI Acknowledgements

First of all, I would like to thank my supervisory team Scott Bryan, Charlotte Allen and David Purdy for their support over the last four years. I further thank the technical support staff at QUT and CARF for they support on various aspects of this project, especially Karine Harumi Moromizato, Will Stearman, Alex Hepple, Gus Luthje and Crystal Cooper. I would like to thank Phil Gilmore, Ryan Dwyer and Janelle Simpson for their support in transferring samples across the state border. Thanks to Professor David Gust, Professor Balz Kamber, Sloss, David Murphy and David Flannery and Susan Fuller.

I would like to acknowledge the importance of the Exploration Data Centre in Zillmere for this project, maintained by the Department for Natural Resources, Mining and Energy (Geological Survey of Queensland). This facility has been of utmost value for this project and will be important for all projects working on Queensland geology legacy data. Special thanks to Chris Hansen, Lex, Tony and Berny from the EDC!

Thanks to the Geological Society of Australia for providing bursaries that allowed me to attend conference in Vienna and Adelaide in 2018 and 2019. I acknowledge the QUTPRA that provided the scholarship allowing me to pursue my studies.

I thank Charlie Verdel and Matt Campbell for sharing their expertise on detrital geochronology, statistical data treatment and for being great guys! Thanks to Oli Gaede and Chris Schrank for their advise, support and encouragement over the last years.

Special thanks to the QUT HDR group for making the time at QUT so great for me, thanks to Aidan Kerrisson, Gloria Awo, Kasia Sobczak, Coralie Siegel, Nick Dyriw, Daniel Wiemer, Kat Gioseffi, Adam Wright, Liz Elphick, Joe Knafelc, Robert Emo and Joe Austin.

Finally, I would like to thank my partner Dina, thank you for supporting me and believing in me.

XXVII

I started out with nothing and I still got most of it left.

Seasick Steve (2008)

XXVIII

1 Chapter 1: Introduction0F

1.1 BACKGROUND

1.1.1 Crustal growth during Earth’s history A fundamental difference exists between the principal processes of continental growth for the Archean and post-Archean settings, as a result of the growing influence of plate tectonic processes and Earth’s evolving thermal state (Cawood et al., 2013). Archean crustal growth was characterised by relatively fast additions of crustal material (300 km3/km/Myr) with cratonic material built up in <100 Myrs (Condie 2007). The main process responsible for crustal growth of Archean cratons operated predominantly in a vertical manner by mantle differentiation processes (Condie, 2007; Durrheim and Mooney, 1991; Kröner, 1984). Thickening of Archean lithosphere resulted from the generation of a sub-cratonic lithospheric upper mantle as a by-product of large scale melt extraction to produce the new continental crust (Pollack, 1986). Moreover, this large and early melt extraction from the upper mantle resulted in chemical depletion and changed the relative buoyancy of the lithosphere to the underlying asthenosphere. The lithospheric mantle beneath present-day Archean cratons is composed of buoyant and strong, but dehydrated and highly depleted mantle peridotite (Cawood et al., 2013; Griffin et al., 2003; Pollack, 1986), and forms the backbones of modern continents.

Plate convergence and subduction-related processes as a result of plate tectonics during the Proterozoic and Phanerozoic, in contrast, have provided an additional mechanism for crustal growth where new continental crust generated above subduction zones is added to the margins of earlier formed cratons (i.e. lateral growth). Compared to the Archean, crustal growth has been comparatively slower (70-200 km3/km/Myr) and lasting up to 250 Myrs (Condie 2007). This lateral growth and addition are a function of accretion/collision events where buoyant crustal material isolated in oceanic crust is added to the overriding plate rather than being subducted, as the subduction zones migrate oceanward over time.

1Some sections of this chapter have been published in the Australian Journal of Earth Sciences. Full citation: Asmussen, P., Bryan, S.E., Allen, C.M., Purdy, D.J., 2018. Geochronology and geochemistry of the Devonian Gumbardo Formation (Adavale Basin): evidence for cratonisation of the Central Thomson Orogen by the Early Devonian. Aust. J. Earth Sci. 65, 1133–1159.

Accretionary orogen is a term that has been proposed to capture one style of lateral growth of continents, and appears to characterise the Paleozoic margin of eastern Australia (e.g. Cawood et al., 2009; Sengör and Natal’In, 1996; Reymer and Schubert, 1984; Crook, 1974). In detail, crustal growth at convergent margins is determined by the overall preservation of crustal material, more specifically by the mass balance between the added volume of generated crust (e.g., igneous and sedimentary rocks) and loss of crustal material recycled back into the mantle such as through subduction erosion (Cawood et al., 2013). Key processes contributing to the addition and/or preservation of continental crust in accretionary orogen settings are the addition of juvenile mantle material through arc magmas, and lateral accretion (Cawood et al., 2011; Hawkesworth et al., 2010; Kemp et al., 2009; Condie, 2007; Dewey, 2005), along with production of granulite facies rocks, which are highly resistant to deformation processes, but are generally restricted to the back- arc and intraplate regions (Collins, 2002a; Kemp et al., 2009).

The addition of juvenile igneous rocks in an accretionary orogen occurs below the magmatic arc, where fluid-fluxing of the mantle wedge above the down-going slab causes partial melting of the mantle above it (Grove et al., 2012; Tatsumi, 2005). The initially basaltic magmas may evolve to higher silica compositions by fractional crystallisation, and/or partial melting of silicic, meta-igneous or meta-sedimentary rocks (Moyen et al.,

2017; Rudnick and Fountain, 1995). The last process plays an inferior role for crustal growth, as it primarily reworks pre-existing relatively small volume rocks, compared to the direct addition of basaltic arc magmas from the mantle wedge, which record a net addition of buoyant crustal material (Moyen et al., 2017). The arc magmas provide not only juvenile crustal material (Fig. 1.1) but promote crustal buoyancy by their relatively lighter density (Hawkesworth et al., 2013, 2010; Cawood et al., 2009; Lyatsky et al., 2006). Crustal material can also be added in a lateral manner by collision and accretion of oceanic arcs, oceanic plateaus and oceanic crust (Condie, 2007), that are introduced by the subducting oceanic plate (Fig. 1.1).

1.1.2 Stabilisation of continental crust Regardless of the vector of crustal growth (i.e. either vertical or lateral), over the long-term continental crust eventually becomes stabilised so that it forms part of the craton. Cratons are tectonically stable continental regions, which are further distinguished into shields and platforms (e.g. Burgess, 2019; Cawood et al., 2013; Hawkesworth et al., 2010). Continental shields are characterised by areas of exposed igneous or metamorphic rock (often of Archean or Proterozoic age), whereas in continental platforms the shield is overlain by sedimentary successions (commonly Phanerozoic) that appear relatively undeformed (Cawood et al., 2013); these sedimentary basins are referred to as cover basins.

According to Pollack (1986), stabilisation or cratonisation has been achieved for continental crust during the Archean when:

• There is an overall paucity of magmatic activity following a preceding history of intensive addition of juvenile magmatic material to the crust;

• Resistance to internal deformation processes occurs, preventing subsequent destructive processes affecting the crust (e.g. orogeny/mountain building); and

• Continental freeboard is maintained and so a buoyant craton is preserved above the base level of erosion defined by sea level.

In most cases, Archean crust is observed to have achieved this stability ~ 100 Myrs after the initiation of crustal growth (Condie, 2007), and it was the development of the thick, melt depleted underlying mantle lithosphere that enabled crust stability. Consequently, stabilised continental crust became a feature of the early surface of Earth.

For the Proterozoic and Phanerozoic, continental crust evolved in an active plate tectonic regime and the Wilson Cycle provides one framework for the mechanism of stabilisation through continental collision. Collision facilitates overthickenning of the crust and lithosphere (e.g. Himalayas), but does not lead necessarily to a thick underlying mantle lithosphere, due to a lack of major mantle melting and melt extraction (McKenzie and Priestley, 2008). For example, the former collisional belts of the Appalachians (United States) and Hercynian Belts (Europe) are now parts of their respective stabilised continental plates. However, there are also orogens that initially formed in convergent margin settings which are now stabilised and form part of continental platforms, but they did not follow the Wilson Cycle and were not stabilised due to crustal overthickenning of a continental collision (e.g. Tasmanides, Cawood et al., 2011; Glen et al., 2016). This raises the questions of how and exactly when they became part of the stabilised craton/continent when there is no collision event to overthicken the crust/lithosphere and also no clear marker for the beginning of the end of these mobile belts.

One way to approach the problem of constraining the timing of crustal stabilisation is to identify the termination of igneous activity, however the underlying processes are fundamentally different between Archean and Proterozoic/Phanerozoic terranes (Table 1.1). For Archean cratons stabilisation is described by termination of the following: Large- scale melting of the upper mantle, leading to transfer of less dense components into the continental crust and to depletion of the upper mantle. This results in a thick dehydrated lithosphere beneath Archean cratons, which is refractory mitigating any subsequent melting and thus any future igneous activity (Table 1.1). For convergent margin settings in post- Archean terranes, the termination of igneous activity refers to the cessation of subduction- related magmatism in terms of fluid flux melting in the mantle wedge and any associated decompression melting of mantle and associated crustal melting in the back-arc areas. However, magmatic activity in formerly stabilised terranes can be reactivated by large- scale continental rifting events, described as large igneous provinces (LIPs, Bryan and Ernst, 2008).

One of the critical indicators of crustal stabilisation is that the crust is strong and becomes resistant to deformation resulting from convergent plate boundaries (Burgess, 2019). Tectonic uplift, subsidence and deformation caused by epeirogenic processes, however, do not categorically preclude cratonic behaviour as they are not a direct consequence of plate

margin processes (Burgess, 2019). A feature in the geological record that can be used to define and constrain the timing of stabilisation for Archean and Proterozoic-Phanerozoic crust alike, is the presence of undeformed sedimentary cover basins also called platform or intracratonic basins, and these features indicate stabilised underlying crust (e.g. Burgess, 2019, Table 1.1). Intracratonic basins are an indicator of stabilised continental crust and either overlie crystalline basement or rifted/accreted continental lithosphere (Leighton, 1990). Intracratonic basins undergo differential subsidence relative to surrounding stable basement and can accumulate substantial amounts of sediment with thicknesses >4.5 km (e.g. Williston and Michigan basins, Burgess, 2019). Many intracratonic basins are initially developed as rifts (grabens or failed rifts) and typically have long lifespans of 150-600 Myrs (Leighton, 1990), and record very low extensional strain and stretch factors (Armitage and Allen, 2010). In comparison, basins formed on failed rifts are commonly shorter-lived (30-305 Ma, Ziegler, 1988). Dimensions of intracratonic (“cover” in this thesis) basins have been found to be highly variable in size covering areas between 100,000 km2 (Paris Basin) – 1,400,000 km2 (Paraná Basin) but commonly exhibit saucer-like shapes (Leighton, 1990).

Table 1.1. Comparison of signals for crustal stabilisation processes for the Archean and Proterozoic/Phanerozoic cratons (after criteria outlined by Pollack, 1986).

Signal for crustal Archean crust Proterozoic-Phanerozoic crust stabilisation

Tectonic abandonment of terrane: e.g. roll-back of Termination of igneous Elevated volatile-free solidus in the upper subducting oceanic plate activity mantle hampers subsequent melting Subduction termination by ultimate collision

tectonic abandonment of terrane Resistance to internal Thick refractory lithosphere by dehydrated cooling of the crust in the continental back-arc deformation mantle peridotite formation of resistant granulite facies production of buoyant lithospheric mantle Maintenance of freeboard by devolatilisation (Mg >Fe in the residual addition of buoyant igneous material mantle) anorogenic A-type magmatism Kimberlites, komatiites, A-Type Intraplate magmatism magmatism, dyke swarms extensional dyke swarms

terrestrial platform sedimentation (intracratonic setting) Cover basins relatively undeformed sedimentary successions

1.1.3 Tracking stabilisation of accretionary orogens The transformation of an accretionary orogen into a stabilised continental platform has been interpreted to comprise three major phases, where transformation is the result of continental

collision (Garfunkel, 1999, outlined in Fig. 1.2). During Phase 1 (Orogenic Stage) Magmatism during ongoing subduction is principally driven by fluid-flux melting of the mantle wedge (e.g. Grove et al., 2012; Pearce et al., 1995; Tatsumi, 2005; van Keken et al., 2011) and magma compositions should therefore be dominantly mafic to intermediate with ‘I-Type’ chemical affinities (e.g. Chappell and White, 2001). Sedimentation in adjacent basins during this stage is dominated by compositionally and texturally immature sediments with proximal volcanic (arc) sources (Garzanti, 2016). Following collision, the subduction zone mechanism is switched off and the transitional stage begins (Phase 2, Fig. 1.2). During the transition stage, orogen relief and elevation are reduced, ultimately developing a continental platform, characterized by low-relief and an extensive drainage system, capable of far-travelled sediment transport across the platform (Garfunkel, 1999). Igneous activity switches to anorogenic ‘A-Type’ affinities throughout the transitional and platformal stages, as the effects of subduction and fluid-flux drivers for mantle melting cease (Fig. 1.2). During the orogenic stage, compositionally immature sediments tend to be deposited in proximal basins (e.g. foreland, intra-arc or intramontane basins, e.g. Garzanti et al., 2004), whereas the transition into a continental platform (Phase 3) facilitates spacious sedimentation with deposition of more compositionally mature terrestrial sediments in more extensive basin systems settings (e.g. intracratonic basins, Folk, 1980).

The above transformation was proposed for orogens that terminate by continental collision, but collision is not the only process that can promote this transformation. The stabilisation of non-collisional accretionary orogens can also occur by progressive abandonment through retreat of the convergent plate boundary over time (e.g. Cawood et al., 2011). Another proposed mechanism is promoted by the initial sediment load associated with rift valleys or grabens. Crustal sediment load causes an increase in lithostatic pressure at depth resulting in phase changes in the lithosphere which ultimately increases the density of the crust (Sleep et al., 1980).

The change of sedimentation from more restricted, to more widespread, platform-style sedimentation has the potential to identify the cratonisation of a crustal element at a regional scale. Cover basins are developed in intracratonic terranes, which are characterised by relatively stable, thick lithosphere inboard of the continental margin (e.g. Allen and Armitage, 2012; Burgess, 2019; Cawood et al., 2013; Folk, 1980; Sleep et al., 1980). The associated depositional environments are characterised by terrestrial to shallow marine

depositional environments only, illustrating the maintenance of continental freeboard. Sedimentary basins, preserved in the geological record of cratonised accretionary orogens, can thus provide valuable information about the timing of the stabilisation process. If a sedimentary basin can be identified as a relatively undeformed intracratonic basin, featuring compositionally and texturally mature sedimentation, a stabilised crustal substrate for the platform can be interpreted. Moreover, anorogenic magmatic affinities of igneous rocks can be taken as further evidence for the stabilisation of the terrane, precluding the influence of an active subduction zone (e.g. Mišković and Schaltegger, 2009).

Figure 1.2. Features of the orogen to platform transition for a collisional accretionary orogen with respect to the evolution of the relief, the predominant type of magmatism, and maturity of sediments (modified after Garfunkel, 1999).

1.1.4 Detrital zircon provenance in intracratonic basins

As consequence of widespread sedimentation during the platformal stage, sediment provenance in intracratonic basins is commonly dominated by reworking of large areas of hinterlands. The associated DZ frequency distributions are dominated by a variety of older age populations, predating the timing of sediment deposition by tens or hundreds of million years and predominantly record sedimentary recycling or reworking of exposed (crystalline) igneous basement (Cawood et al., 2012). Intracratonic basins are usually located remote from centres of magmatic activity (arc-magmatism) and commonly show a lack of contemporary zircons from syn-basinal volcanic source areas (Cawood and Nemchin, 2001; Cawood et al., 2007, 2012).

The paucity or even absence of syn-depositional DZ ages in intracratonic basins hinders statistically robust age constraints from maximum depositional ages, as often just

a handful of individual young zircon ages are available from a given sample. This especially challenges age constraints for terrestrial Precambrian-aged strata, which inherently lack age control from the fossil record and thus maximum depositional ages provide the only age constrain for sediment deposition (Dickinson and Gehrels, 2009a). Figure 1.3 shows a compilation of DZ age data from Precambrian sedimentary successions related to intracratonic basins in Russia, Canada, Brazil and Scotland (Table 1.2). The DZ frequency distributions are dominated by a variety of older age populations, commonly predating the deposition of the sedimentary rocks (Cawood et al., 2004; Guadagnin et al., 2015; Khudoley et al., 2001; Rainbird et al., 2010). The average number of individual DZ analyses per sample, however, are relatively low (n=32 – 72 per sample) for the compiled examples, which is problematic with respect to small DZ populations, especially DZ with syn-depositional ages. Low numbers of individual analysis potentially lead to undetected and/or underestimated proportions of small age populations (Andersen, 2005; Vermeesch, 2004). The low number of individual analysis thus hampers assignment of a statistically robust maximum

Figure 1.3 Kernel Density Estimates (KDE) of detrital zircon age frequency distributions from Precambrian intracratonic basin. Data compiled from Cawood et al., 2004; Guadagnin et al., 2015; Khudoley et al., 2001; Rainbird et al., 2010. Orange line shows maximum depositional ages for the respective samples, number of analysed samples given as N.

depositional age which is critical to constrain timing of sediment deposition and can be used for stratigraphic correlation.

Table 1.2. Characteristic features of selected sedimentary successions related to intracratonic basins (Compiled after: Alkmim and Martins-Neto, 2012; Friend et al., 2003; Guadagnin et al., 2015; Khudoley et al., 2001, 2001; Rainbird et al., 2010; Soper et al., 1998).

Basin Lifespan Thickness Opening style Infill References Predominantly siliciclastic Rae domain, Intracontinental sag Paleoproterozoic sedimentary rocks, intercalated Churchill Province >1km basin/Incipient rift Rainbird et al., 2010 (2.45 - 1.90 Ga) mafic volcanics, platformal (Canada) basin carbonates Paleo- Predominantly siliciclastic Guadagnin et al., Espinhaco Basin Mesoproterozoic 2.5 km rift-sag successor basin sedimentary rocks, some 2015; Alkmim and (Brazil) (1.8-0.9 Ga) intercalated mafic volcanics Martins-Neto, 2012 Moine Supergroup Neoproterozoic Predominantly siliciclastic Friend et al., 2003; >5km rift basin (half-graben) (Scotland) (1.0-0.87 Ga) sedimentary rocks Soper et al., 1998 Riphean-Vendian Paleo- Siliciclastic sedimentary rocks, basin, SE Siberia Neoproterozoic 12-14km rift basin carbonates and intercalated Khudoley et al., 2001 (Russia) (1.6 -0.54 Ga) mafic sills and flows

1.2 AREA OF STUDY

1.2.1 The Thomson Orogen as part of the Tasmanides The Tasmanides of eastern Australia record substantial and sustained lateral crustal growth over the last 500 Myrs in an overall convergent margin setting (e.g. Cawood et al., 2011; Glen, 2005). Five major orogens record lateral crustal growth; the Delamerian, Thomson, Lachlan, Mossman and New England orogens (Fig. 1.4). The Delamerian Orogen (e.g. Foden et al., 2006) separates the Australian Precambrian cratons from the largely Palaeozoic Lachlan, Thomson and Mossman orogens. These crustal elements reflect additions to the Rodinian rifted margin, which is also the edge of the older Precambrian Australian craton. The outboard New England Orogen records the most recent orogenic activity that terminated in the Middle Triassic (Cawood et al., 2011; Fergusson and Henderson, 2015; Glen, 2005).

The Thomson Orogen is the largest of the orogens by area (886 689 km2) and crustal volume (41 x 106 km3; calculated after Kennett et al., 2011), and was tectonically active from the late Neoproterozoic to the Carboniferous (Glen, 2005; Murray and Kirkegaard, 1978). It is also the most concealed of the Tasmanide orogens with exposed areas representing just under 2.5% of the total orogen area (Fig. 1.5). Exposures are principally limited to the northern and north-eastern margins of the orogen in the Anakie, Greenvale

and Charters Towers provinces with only a few restricted exposures occurring in the southern Thomson (Hungerford and Eulo Ridge Granite, Tibooburra and Warratta Inliers; Fig. 1.5). The orogen is mostly buried beneath sedimentary basins ranging in age from Cambrian to Cretaceous (Fergusson et al., 2013).

At a regional scale, the Thomson Orogen comprises four main lithological groups: (i) outcropping upper greenschist to amphibolite grade metasedimentary rocks dated between 580 and 495 Ma (Fergusson et al., 2007) in the northern Thomson Orogen (Anakie and

Figure 1.4 Tasmanide subprovinces and Devonian basin elements after Glen, 2005. Abbreviations: DB, Darling Basin; BT, Barrolka Trough; WT, Warrabin Trough; AB, Adavale Basin; BeB, Belyando Basin; BB, Burdekin Basin.

Charters Towers provinces), (ii) Ordovician–Silurian low-grade marine siliciclastic metasedimentary rocks (Purdy et al., 2016a), (iii) terrestrial sedimentary cover basins including the Devonian Adavale Basin (Fig. 1.4, McKillop et al., 2007) and Carboniferous Drummond Basin (Olgers, 1972; Henderson and Blake, 2013), and (iv) granites and related volcanic rocks dated between 510 and 360 Ma (Siegel, 2015; Siegel et al., 2018a).

Within this broad geologic subdivision, a number of fundamental changes are apparent at the Silurian–Devonian boundary, potentially as a result of the late Bindian Orogeny (Collins and Hobbs, 2001). These include: (i) a change in deformation style and intensity with Ordovician metasedimentary rocks being folded and tilted and in unconformable contact with flat-lying, weakly deformed sedimentary rocks of the Devonian Adavale Basin System; (ii) changes from marine to terrestrial depositional environments; and (iii) changes in exposure levels, with Silurian igneous rocks being predominantly plutonic (Siegel, 2015; Siégel et al., 2018) and volcanic rocks being mainly Early Devonian and younger in age. Tectonic activity in the Thomson Orogen, in terms of contractional events and magmatic activity, had concluded by the Carboniferous, when activity shifted eastwards to the New England Orogen. Consequently, the tectonic evolution of the Thomson Orogen can be viewed as one of progressive cratonisation (Cawood and Buchan, 2007; Cawood et al., 2011) and tectonic abandonment from the Silurian to Carboniferous.

The Devonian Adavale Basin is important in recording these first-order changes as it occupies a central position in the Thomson Orogen (Figs. 1.4, 1.5). The few previous studies on the Adavale Basin, however, have developed no consensus on the tectonic setting of the basin, suggesting either more tectonically active and plate boundary proximal basin settings (back-arc or foreland; Passmore and Sexton, 1984; Remus and Tindale, 1988; Evans et al., 1990) or basin development on more stable crust and remote from any active plate boundary (intracratonic or epicratonic basin, i.e. cover-type basin; Finlayson et al., 1988; Hoffmann, 1988; Murray, 1994). Most of the proposed tectonic models are exclusively based on structural interpretations of large-scale seismic surveys, which reveal an extensional initiation phase producing a half-graben system within the Adavale Basin (Evans et al., 1990; Finlayson et al., 1988; Hoffmann et al., 1988; Passmore and Sexton, 1984; Remus and Tindale, 1988).

Figure 1.5. Location of the subsurface Adavale Basin and outcropping areas across the Thomson Orogen (modified after Glen 2005), highlighting the scarcity of exposure especially in the central and southern Thomson Orogen. Depth to basement highlights the rather shallow level of the Thomson Orogen at the southern and north-eastern margins compared to the central area where the Adavale Basin is located that resides >1 km beneath the surface. Locations of drill holes intersecting rock successions of the Adavale Basin are shown as black dots, and locations of broadly temporally overlapping igneous rocks in the wider area of the Thomson Orogen highlighted in red. Depth to basement contours created from drill hole data and seismic interpretations.

1.2.2 The Adavale Basin The subsurface Devonian Adavale Basin blankets the Thomson Orogen in central and southwestern Queensland and has a preserved extent of ~60,000 km2 (Figs. 1.4, 1.5). Combined with the Devonian Warrabin Trough to the west, these two main depressions form the Adavale Basin System (Draper et al., 2004) with a combined extent of 90,000 km2 (Fig. 1.5). This basin system underlies younger sedimentary mega-sequences of the Galilee Basin (late Carboniferous to Middle Triassic) and Eromanga Basin (Early Jurassic to Cretaceous; Evans et al., 1990) with a cumulative preserved thickness of 1–3 km blanketing the Adavale Basin (Passmore & Sexton, 1984). Isolated Devonian basin elements are widespread across the Thomson Orogen (Barrolka Trough, Belyando Basin, Burdekin Basin, Paka Tank Trough) and in the adjacent Lachlan Orogen (Darling Basin System), but their connectivity during the Devonian is poorly understood (Fig. 1.4, Tanner, 1968; Murray and Kirkegaard, 1978; Draper et al., 2004).

As interpreted from seismic data, the basin is bounded to the west by the high standing Canaway Ridge, with a west-dipping crustal detachment fault that developed during the initial rifting phase (Finlayson et al., 1988; Hoffmann et al., 1988; Evans et al., 1990). To the east, the Adavale Basin is bounded by the Pleasant Creek Arch, which developed as a half-graben shoulder in the north (Draper et al., 2004) and a horst block in the south (Finlayson and Collins, 1986). The present-day shape of the Adavale Basin is a structural remnant of a larger Adavale Depression (Passmore and Sexton, 1984), modified by deformation and erosion during the Carboniferous (Evans et al., 1990). Deformation structures are especially pronounced in the southern and eastern parts of the basin, resulting in various synclinal sub-troughs in these areas (Quilpie Trough, Cooladdi Trough, Westgate Trough; Draper et al., 2004; Passmore and Sexton, 1984).

Sedimentary rocks in the basin are generally flat-lying and undeformed as evidenced from drill hole intersections, but seismic profiles reveal some thrust faults and post-depositional long-wavelength folding (tens of kilometres; Draper et al., 2004), with fold axes oriented in an east-west direction. Much of the contractional deformation is interpreted to be Carboniferous in age because the structures do not persist into overlying Galilee Basin strata, and thus is thought to be regionally related to the 350–330 Ma Kanimblan Orogeny (Finlayson, 1993). This event has locally been referred to as the Quilpie Orogeny for the Adavale Basin System (Finlayson et al., 1990), which coeval presumably genetic linked to

the last phase of the Alice Springs Orogeny in central Australia during the middle Carboniferous (Haines et al., 2001). The Carboniferous sedimentary rocks in the Drummond basin are interpreted to record erosion from the upper sections of the Adavale Basin and the southern areas of the Thomson Orogen (Sobczak, 2019).

Murray (1994) suggested that the Adavale Basin developed as a rift basin in the Early Devonian as indicated by the presence and structure of the silicic volcanic rocks of the Gumbardo Formation. The volcanic rocks were deposited in a half-graben system that appears to be confined to an NNE–SSW-trending rift corridor, defining the main basin depocentre (Finlayson, 1993). Despite various intersections and cored intervals of the volcanic rocks at the base of the basin, no petrographic or geochemical analysis has been conducted to date.

The bulk of the subsequent basin infill (3–4 km) comprises fluvial siliciclastic sedimentary rocks (Eastwood Beds, Etonvale Formation, Lissoy Sandstone, Buckabie Formation; Fig. 1.5) with marine transgressions affecting the eastern parts of the basin (Log Creek Formation, Bury Limestone; Fig. 1.6). Significant unconformities developed during the Eifelian between the Eastwood Beds and the Log Creek Formation, and during the mid- late Givetian between the Boree Salt Member and the Etonvale Formation (Fig. 1.6). Stratigraphic control within the basin is hampered by an absence of surface outcrops, the lack of dense drill core coverage and incomplete coring of wells. McKillop et al. (2007) developed a revised basin stratigraphy based on: (i) wireline-log interpretations, (ii) palynology of the Eastwood beds, Log Creek and Etonvale formations (Hashemi & Playford, 2005), and (iii) two U–Pb SHRIMP (Sensitive High-Resolution Ion Microprobe) dates from basal volcanic rocks (Fig. 1.5; Cross et al., 2018; Draper, 2006).

1.3 RESEARCH QUESTIONS

The Adavale Basin exhibits a number of features that suggest it developed during (or shortly after) the transitional stage to continental stabilisation (Phase 2, Fig. 1.2). The basin comprises a thick sequence of relatively undeformed siliciclastic sedimentary infill (3-4 km) and a basal silicic volcanic sequence, possibly linked to initiation of the basin. The basin is one of several coeval basins developed along the eastern margin of the Tasmanides in the Devonian (Fig. 1.4). This raises the dual questions of: 1) what tectonic setting was

conducive to the extensive development of sedimentary basins over 3000 km strike length along, and over 500 km width across, the margin (Fig. 1.3), and 2) what connectivity existed between these basins? The overarching hypothesis to be tested in this thesis is:

The Adavale Basin represents a remnant of a once larger, contiguous platform basin across the Tasmanides.

Figure 1.6. General stratigraphy of the Adavale Basin modified after McKillop et al. (2008) and using the updated geologic time scale (Gradstein et al., 2012). Isotopic age control based on revised SHRIMP ages by Cross et al. (2018). The K-Ar age is from the well completion report for PPC Gumbardo-1 by Phillips Petroleum Company and Sunray DX Oil Company (1963). Palynological assemblages I, II & III (Ass. I, Ass. II and Ass. III) after Hashemi & Playford (2005).

The consequence of this hypothesis, if proved, is that the Thomson Orogen had largely been stabilised (i.e. was now the platform to the craton) prior to basin initiation in the Early Devonian. From the literature review, the following unknowns regarding the Adavale Basin have been identified:

• No consensus exists on the plate tectonic setting of the Adavale Basin; current interpretations are that it formed in any of foreland, back-arc, epicratonic or intracratonic settings (Murray, 1994; Evans et al., 1990; Finlayson et al., 1988; Remus and Tindale, 1988; Passmore and Sexton, 1984).

• With up to 8,000 m of siliciclastic sediment accumulating in the basin (McKillop et al., 2007), no provenance studies have been undertaken to constrain the source of this vast sediment volume and how sediment sourcing may have changed over time.

• Devonian cover basins across the Thomson Orogen (Adavale Basin) and Lachlan Orogen (Darling Basin) have been postulated to represent remnants of a larger basin system across the Tasmanides (Alder et al., 1998; Bembrick, 1997; Cooney and Mantaring, 2005; Draper et al., 2004; Khalifa, 2010), but this is based on very broad stratigraphic correlations and seismic unconformities recognised in seismic sections. Moreover, this model lacks supporting evidence from a defined provenance of sedimentary rocks to confirm correlations and basin connectivity.

The research questions below provide the focus for this PhD research on the role of the Devonian basins in recording the stabilisation of the Thomson Orogen and will form the basis for three body chapters of this PhD thesis.

Research Question 1: What are the ages and tectonomagmatic affinities of the basal volcanics within the Adavale Basin, and what constraints do these place on the tectonic setting of volcanism and basin initiation? (Chapter 2)

Research Question 2: What is the provenance of the sedimentary units within the Adavale Basin, and where were the main source areas supplying sediment into the basin and across the Thomson Orogen? (Chapter 4)

Research Question 3: Were the Adavale and Darling basins connected during the Devonian such that these basin remnants formed part of a larger platform basin system across the Thomson and Lachlan Orogens? (Chapter 5 and 6)

The exploration and study of these research questions will generate new knowledge and understanding of: (i) the depositional history of the Adavale Basin, (ii) the tectonic setting of the Adavale Basin, and refined timing constraints on basin initiation and potentially the timing of stabilisation of the Thomson Orogen, and (iii) the connectivity (or lack thereof) of remnant Early Devonian basins developed across the Thomson and neighbouring Lachlan Orogens. The above research questions are addressed in three body chapters, and an approach for each research question/body chapter is outlined in the following.

1.4 APPROACH – CONSTRAINING TIMING OF BASIN INITIATION AND TECTONOMAGMATIC AFFINITIES OF BASIN-RELATED VOLCANISM (CHAPTER 2)

Chapter 2 focuses on the basal volcanics of the Adavale Basin and constrains emplacement ages for the basal volcanics from all available locations in the Adavale Basin. This will lead to a better understanding of the timing of basin initiation and also in distinguishing the volcanic rocks of the Gumbardo Formation from potentially older volcanic basement units. The volcanic successions of the Gumbardo Formation have been logged to provide stratigraphic constraints and lithological information as a framework for sampling for U- Pb zircon geochronology and whole-rock geochemistry. Available petroleum wells intersecting the Devonian successions have been reviewed by means of core inspections, and review of well completion reports to identify those wells intersecting the basal volcanic successions. U-Pb zircon geochronology is used to constrain the timing of volcanism and delimit the volcanic rocks associated with the Adavale Basin from older volcanic basement rocks. Petrographic analysis of the sampled volcanic units has been undertaken to characterize the lithologies and for alteration assessment with respect to geochemical analysis. Whole-rock geochemistry is used to gain insight into the magmatic affinities of the rocks. The integration of the stratigraphic, lithologic, petrographic, geochronology and geochemical data provide robust constraints on the tectonomagmatic affinities and timing of initial volcanism within the basin and aid in constraining the tectonic setting of the basin.

Petrographic studies of the rocks have been examined for: (i) mineralogical characteristics for constraints on the geochemical composition of the sampled rocks, (ii) alteration assessment as quality control on geochemical analysis, and (iii) quantification of visual descriptions and assessment of textural features. The results of the petrographic studies guided sample selection for subsequent geochemical and geochronological analyses. Whole-rock geochemical analyses have been conducted for major elements (via X-ray fluorescence spectrometry, XRF), trace and rare earth elements (via laser ablation inductively coupled plasma mass spectrometry, LA-ICP-MS). Major and trace element data are used for geochemical characterisation principally focusing on immobile elements (e.g. Winchester and Floyd, 1977) for tectonic discrimination (e.g. Pearce et al., 1984) and to test the volcanic rocks for intraplate, A-Type and other anorogenic geochemical signatures (e.g. Frost et al., 2001; Frost and Frost, 2008; Whalen et al., 1987).

Geochronology via U-Pb zircon geochronology is be used to constrain timing and duration of volcanism related to basin initiation, whereas the timing of events coupled with tectonomagmatic affinities has temporal importance for the synchronicity of the Darling and Adavale Basins (Fig. 1.4, Research Question 1). Dating of zircons by LA-ICP-MS (undertaken at the Central Analytics Research Facility, CARF, Queensland University of Technology) facilitates cost-effective dating of many individual grains, and chemical information of the ablated zircons are gathered synchronously. Together with cathodoluminescence (CL) images of the zircon samples, these data are used to discriminate autocrystic and inherited grains for more accurate age assignment (Siégel et al., 2018b).

1.5 APPROACH – PROVENANCE OF SEDIMENTARY ROCKS IN THE ADAVALE BASIN (CHAPTER 4)

Chapter 4 focuses on four selected terrestrial to marginal marine sedimentary units in the Adavale Basin, aiming to characterise the rocks for the first time in terms of sediment provenance characteristics, and use the findings for intrabasin (and later, inter-basin) correlations (Research Question 2). Detailed age constraints from the basal volcanic rocks in the Adavale Basin (Chapter 2) provide a baseline for the study of the sedimentary rock successions in terms of maximum depositional ages, as the overlying sediments must be younger than the basal volcanics. DZ age populations indicating reworking of the basal

volcanics can therefore be identified confidently, and separated from other DZ age groups. The methodologies and techniques utilised in Chapter 4 are outlined in detail in Chapter 3.

Based on correlations of seismic marker horizons and depositional age data from biostratigraphy, three sedimentary units of the Adavale Basin have been assumed to represent correlatives to the contemporary Darling Basin in the Lachlan Orogen (Table 1.3, Fig. 1.4; Khalifa, 2010; McKillop et al., 2007; Cooney and Mantaring, 2005; Bembrick, 1997). These three units are: Eastwood Beds, Etonvale and Buckabie formations, which have been logged and sampled along with the Middle Devonian unit of the Lissoy Sandstone. Thus, it is possible to continuously track temporal trends in sediment provenance during the lifespan of the basin from these cores. Each formation was sampled in multiple locations of the basin to study the spatial provenance characteristics for each unit. This aims to test if sediment provenance is laterally consistent within the respective formations along a north-south transect in the Adavale Basin (indicative of sheet-like geometries in a platform basin), or inconsistent across the basin, reflecting restricted basin geometries and sediment pathways and potentially an orogenic basin type.

This work involved drill core logging and stratigraphic analysis for constraints on sampling units, supported by a review of existing biostratigraphic constraints (Hashemi and Playford, 2005) to aid in establishing correlations and depositional synchronicity. Maximum depositional ages are constrained from DZ geochronology and tested against the established biostratigraphic age constraints. For units with no or insufficient biostratigraphic age control (i.e. Buckabie Formation), maximum depositional ages complement age control. DZ and rutile geochronology using LA-ICP-MS, integrated with grain morphology data are utilised to fingerprint source areas of sediments (Dickinson and Gehrels, 2009b; Fedo et al., 2003).

Table 1.3. Biostratigraphically synchronous sedimentary rock units in the Adavale and Darling Basins (Bembrick, 1997; Khalifa, 2010).

Adavale Basin (Thomson Orogen) Darling Basin (Lachlan Orogen) Unit Stage Lithology Unit Stage Lithology quartz sandstone, Buckabie Frasnian- shale and Formation Famennian Upper conglomerate Ravendale Givetian - arenite, shale, Devonian Formation Famennian conglomerate Etonvale quartz sandstone, Frasnian Formation mudstone, shale

quartz to Wana quartz Lower Eastwood Emsian- Late Emsian feldspathic Karnu sandstone, Devonian Beds Eifelian sandstone Group shale

In combination with depositional ages provided by biostratigraphic ages and maximum depositional ages (Coutts et al., 2019; Dickinson and Gehrels, 2009b), the gathered DZ geochronology data potentially allows for constraints on tectonic setting of the Adavale Basin, indicated by presence or absence of syn-sedimentary igneous sources (Cawood et al., 2012). The new detrital zircon U-Pb age data are assessed using visual and statistical sample comparison methods to investigate temporal and spatial trends in DZ provenance during the lifespan of the Adavale Basin and across its main depocentre. In addition, detrital rutile U-Pb geochronology can give insight into metamorphic source rocks contributing to sedimentation in the basin (Triebold et al., 2012; Zack et al., 2004). The obtained data are subsequently used as a basis for correlation with samples from the Darling Basin in the northern Lachlan Orogen (Chapter 5).

Table 1.4. Logged wells, interpreted thickness and recovered and inspected core material for selected wells in the Adavale Basin. Bore names highlighted in bold were selected for geochronology sampling

Log Creek Lissoy Etonvale Buckabie Total Eastwood Formation Sandstone Formation Formation total Operator Well Depth [m] Beds [m] [m] [m] [m] [m] [m] ASO Fairlea-1 3035.0 189.0 - 14.4 328.0 470.0

Core recovery [m] 1.0 - - 3.5 6.1 10.6 PPC Carlow-1 3420.0 1227.0 - 78.0 199.4 -

Core recovery [m] 17.7 - 2.2 3.2 - 23.1 PPC Log Creek-1 3976.0 - 463.0 43.0 332.0 1514.5

Core recovery [m] - 45.3 5.2 4.2 11.1 65.8 PPC Gumbardo-1 3904.0 - 307.0 59.9 385.0 906.8

Core recovery [m] - 6.4 2.9 4.6 7.8 21.7 PPC Buckabie-1 2686.8 - - - 330.8 765.6

Core recovery [m] - - - 8.1 35.5 43.6 BEA Allandale-1 2772.8 624.0 - - - -

Core recovery [m] 10.0 - - - - 10.0 AGA Phfarlet-2 3962.0 - 249.7 39.2 405.3 1270.5

Core recovery [m] - 18.0 25.0 - - 43.0 PPC Gilmore-2 3775.9 - ? 34.2 467.1 1240.1

Core recovery [m] - 46.5 31.0 - - 77.5 PPC Stafford-1 3024.2 - ? - 607.3 439.1

Core recovery [m] 0.5 - 12.8 4.4 17.7

cumulative recovery per formation [m] 28.7 116.7 66.3 36.4 64.9 313.0

Sampling Strategy

Nine partly cored wells across the Adavale Basin were selected for logging and six were ultimately selected for sampling with respect to recovered core material (Table 1.3). The sandstone successions of the Eastwood Beds, Lissoy Sandstone, Etonvale Formation and Buckabie Formation have been sampled in six individual drill holes across the Adavale Basin along a ~250 km NNE-SSW transect. The Eastwood Beds could only be sampled at two locations in the northern basin as this unit is regionally restricted (McKillop et al., 2007). Due to the large thickness of the Buckabie Formation, the unit was sampled in a lower and upper section for each individual location, to better constrain the depositional ages and potential provenance changes during deposition of each formation.

1.6 APPROACH - PROVENANCE OF SEDIMENTARY ROCKS IN THE DARLING BASIN AND CORRELATION WITH UNITS FROM THE ADAVALE BASIN (CHAPTER 5)

Establishing the extent, synchronicity and connectivity of Devonian basin elements will help to test and establish the existence and extent of platform basins across the Thomson and potentially Lachlan orogens, and by inference the stability and extent of stabilisation of the Thomson Orogen in the Devonian (Research Question 3).

Chapter 5 builds on the approach and results for Chapter 4 and focusses on two sedimentary formations in the Darling Basin that have been assumed to be correlative with the Adavale Basin formations (Table 1.3), the sandstones of the Wana Karnu Group (formerly Snake Cave Sandstone, Greenfield et al., 2010) and Ravendale Formation (Table 1.2). Logging and sampling of the targeted intervals were conducted by the Regional Mapping and Exploration Geosciences unit of the Geological Survey of New South Wales (GSNSW) at the W.B. Clarke Geoscience Centre in Londonderry, NSW. The study was targeted at Devonian sandstone lithologies with biostratigraphic age control across central and western NSW, conducting DZ analysis via LA-ICP-MS (Barry, 2016).

Sandstone composition and provenance is assessed by means of sandstone petrography. DZ and rutile U-Pb ages are used to fingerprint sediment source areas, and, in combination

with existing biostratigraphic age constraints (e.g. Roberts et al., 1972), form the major correlation tool between the respective formations in the Adavale and Darling Basins.

1.7 THESIS STRUCTURE

This PhD thesis comprises an introductory chapter (Chapter 1), three research chapters (Chapter 2, 4 and 5) and a methods and approach chapter providing in more detail the utilised tools and concepts for detrital mineral analysis and interpretation (Chapter 3) prefacing Chapter 4 and 5. Chapter 1 outlines the global significance of the research topic and identifies gaps in knowledge for the specific research area. The basal volcanics in the Adavale Basin are utilised in Chapter 2 to constrain the timing of basin initiation and the tectonomagmatic affinities of the rift-related volcanic rocks to better understand the tectonic setting of the basin during initiation. Chapter 2 is published in the Australian Journal of Earth Sciences (Asmussen et al., 2018). Chapter 3 introduces the methodologies and concepts used to study the provenance of the sedimentary rock successions in both investigated sedimentary basins. The focus here relies on the approach of detrital U-Pb zircon geochronology and a brief review of the associated methods that ultimately aid in constraining sediment provenance in a spatial and temporal context. The results of the provenance study of the sedimentary successions of the Adavale Basin are presented in Chapter 4 in a spatial and temporal context. Chapter 5 presents a provenance study of the Darling Basin, and the data are correlated and synthesized with the outcomes of the previous chapter in Chapter 6 to evaluate the existence of a continental-scale Devonian sedimentary basin system.

1.8 LIST OF RESEARCH OUTPUTS

Referred Journal Articles

Asmussen, P., Bryan, S. E., Allen, C. M., & Purdy, D. J. (2018). Geochronology and geochemistry of the Devonian Gumbardo Formation (Adavale Basin): evidence for cratonisation of the Central Thomson Orogen by the Early Devonian. Australian Journal of Earth Sciences, 65(7-8), 1133-1159.

Conference Abstracts

Asmussen, P., Bryan, S. E., Allen, C. M., & Purdy, D. J. (2019): Investigating spatial and temporal detrital zircon trends in sedimentary basins: A provenance study of the subsurface Adavale Basin, Queensland, Australia. European Geoscience Union (EGU) General Assembly 2019, Vienna, Austria, abstract/oral presentation (https://meetingorganizer.copernicus.org/EGU2019/EGU2019-16210.pdf)

Asmussen, P., Bryan, S. E., Allen, C. M., & Purdy, D. J. (2018): Geochronology and geochemistry of the Early Devonian Gumbardo Formation: Evidence for Silurian basement rocks beneath the Adavale Basin. Australian Geoscience Council Convention 2018 (AGCC), Adelaide, Australia, abstract/oral presentation

(https://www.agcc.org.au/wp-content/uploads/2019/03/clickheretodownloadabstract.pdf)

Chapter 2: Geochronology and geochemistry of the Devonian Gumbardo 2 Formation1F

2.1 INTRODUCTORY STATEMENT

To understand better the tectonic evolution of the Thomson Orogen and its transition to a (potentially) stabilised cratonic element of Australia (Fig. 2.1), this chapter focusses on the basal volcanic successions of the Adavale Basin. Previous studies on the Adavale Basin have focussed on the petroleum potential of the Middle Devonian successions (e.g. Draper et al., 2004) and despite usage as a seismic marker horizon, the Gumbardo Formation has generally been neglected. Geochemical assessment of volcanics within the Gumbardo Formation has not been conducted in previous works and compositional statements are based simply on petrographic observations (Murray, 1994). Age control for the Gumbardo Formation was first established by two SHRIMP U-Pb zircon dates (Draper, 2006) and reinterpreted based on revised constants (Cross et al., 2018) to 402.9 ± 2.9 Ma for a section of drill core from PPC Gumbardo-1 (ignimbrite, in a middle section of the formation, 3700 m depth) and 408.1 ± 3.1 Ma for core from PPC Carlow-1 (ignimbrite from a top section of the formation, 3292 m depth, Figs. 2.2, 2.3). In the following, U–Pb zircon geochronology is utilised to constrain the timing of basin inception, and whole-rock geochemistry to gain insights into the tectonic affinity of magmatism across the orogen in the Early Devonian to clarify the tectonic setting of the basin.

2.2 GEOLOGICAL BACKGROUND

The initial extensional phase in the Adavale Basin led to a series of half-grabens with blocks rotating from the northeast towards the basin axis and was accompanied by deposition of

2 Material presented in this Chapter has been published in the Australian Journal of Earth Sciences. Full citation: Asmussen, P., Bryan, S.E., Allen, C.M., Purdy, D.J., 2018. Geochronology and geochemistry of the Devonian Gumbardo Formation (Adavale Basin): evidence for cratonisation of the Central Thomson Orogen by the Early Devonian. Aust. J. Earth Sci. 65, 1133–1159.

the volcanic rocks of the Gumbardo Formation (McKillop et al., 2007). Based on wireline- log interpretations, the formation is intersected in ten wells (Table 2.1, Fig. 2.2, McKillop et al., 2007) along the main depocentre of the Adavale Basin. A more extensive distribution of volcanic rocks interpreted to be the Gumbardo Formation, thickening eastwards from the Warrabin and Quilpie troughs has been suggested based on a highly reflective seismic sequence (Passmore and Sexton, 1984). However, outside the main depocentre, potential rocks of the Gumbardo Formation are below drilled depths (Evans et al., 1990).

The entire Gumbardo Formation has been intersected in three wells (McKillop et al., 2007), with a maximum formational thickness observed in PPC Gumbardo-1 (~500 m) and a minimum of 103 m in PPC Carlow-1 (Table 2.1). Basement in these wells is made up of Lower Ordovician volcanic rocks while Silurian granite forms basement in PPC Etonvale- 1, dated at 429 Ma based on the Rb/Sr method on whole-rock and a feldspar concentrate (Lewis and Kyranis, 1962). The Gumbardo Formation is locally overlain by different sedimentary units as a result of lateral variation, isolated rift sedimentation and the presence of large Carboniferous thrust faults (Fig. 2.3, McKillop et al., 2007).

Early studies reported olivine basalt, andesite, andesitic tuff and silicic volcanics (dacitic and rhyolitic ignimbrites) from the Gumbardo Formation (Murray, 1994). The olivine basalt in PPC Gumbardo-1 and a brecciated rhyolite in PPC Carlow-1 have since been excluded from the Gumbardo Formation (McKillop et al., 2007) based on Early Ordovician U-Pb zircon SHRIMP dates for the brecciated rhyolite in PPC Carlow-1 (Draper, 2006) and an Early Ordovician K-Ar pyroxene date for the basalt in PPC Gumbardo-1 (Phillips Petroleum Company and Sunray DX Oil Company, 1963). Silicic ignimbrites are the most widespread volcanic rock type reported for the Gumbardo Formation (BEA Allandale-1, PPC Carlow-1, PPC Etonvale-1, PPC Cothalow-1, PPC Gumbardo-1), whereas the overlying upper Gumbardo Formation is dominated by feldspathic and lithofeldspathic sandstones (PPC Cothalow-1, Galloway, 1970).

2.1 METHODS

Using the existing stratigraphic framework (McKillop et al., 2007), available drill core material of the Gumbardo Formation was logged and sampled at the Exploration Data

Figure 2.1 Location of the subsurface Adavale Basin and outcropping units across the Thomson Orogen (modified after Glen, 2005), highlighting the scarcity of exposure especially in the central and southern Thomson Orogen. Depth to basement highlights the shallow level of the Thomson Orogen at the southern and north-eastern margins compared with the central area where the Adavale Basin is located and resides>1 km beneath the surface. Locations of drill holes intersecting rock successions of the Adavale Basin are shown as black points, and locations of broadly synchronous igneous rocks in the wider area of the Thomson Orogen highlighted in red. Depth to basement contours created from drill-hole data and seismic interpretations.

Figure 2.2. Location of inspected wells within the Adavale Basin and major basin components. Wells intersecting volcanic rocks are marked with a star symbol.

Centre in Zillmere (Brisbane, QLD). The inspected wells are situated in a 211 km long NNE-SSW striking corridor with spacing between the well locations ranging from 30 to 67 km (Fig. 2.2). Cored sections containing rocks of the Gumbardo Formation range from 1.5 m to 5.2 m of continuous core, whereas gaps between the cored intervals range from 60 m to 250 m. The wide well spacing, and incomplete coring hinders stratigraphic correlations between the sampled wells.

A total of 10 samples of volcanic units previously defined as belonging to the Gumbardo Formation (Table 2.2) were collected for petrographic, geochemical and geochronological analysis. Following petrographic examination, a subset of samples was selected for whole- rock geochemical analysis and zircon mineral separation. Petrographic examination was supplemented by existing thin sections prepared by the Geological Survey of Queensland for previous studies (Murray, 1994). Modal composition was acquired using a PELCON point counter with step length of 1 mm and 300 counts.

Figure 2.3. Compilation of well logs with sample locations (e.g., ETO-1 in PPC Etonvale-1), lithological information and interpreted emplacement ages for eruptive units of the Gumbardo Formation and underlying/ overlying units across a N–S transect. Inset map shows location of wells across the Adavale Basin. All depths in metres, scale breaks are not to scale.

Table 2.1. Wells reported to intersect Gumbardo Formation in McKillop et al. (2007) and inspected Gumbardo intervals.

Total depth Reported thickness Cored Gumbardo Total Well Lat °S Long °E [m] Gumbardo [m] interval [m] [m] PPC Gumbardo-1 -25.979 144.695 3944.1 755.5 3278.4-3941.3 662.9 PPC Etonvale-1 -25.159 144.995 3464.9 206.1 3304.3-3305.8 1.5 PPC Carlow-1 -24.839 145.431 3666.1 140.0 3292.1-3395.4 103.3 PPC Cothalow-1 -25.724 144.390 2613.1 273.0 2275.3-2502.7 227.4 BEA Allandale-1 -24.415 145.905 3004.1 236.0 2794.1-3004.1 210 AOP Boree-1 -24.757 145.577 2676.4 60.0 2616.4-2618.8 2.4 PPC Leopardwood-1 -25.617 144.671 4184.0 280.0 - - AAO Eastwood-1 -24.774 145.350 3385.1 100.0 - - PPC Bury-1 -25.042 145.606 2744.4 10.0 - - AOD Yongala-1 -25.506 143.929 3105.0 25.6 - -

2.1.1 Zircon geochronology and chemistry (LA-ICP-MS) Depending on sample availability, 1.3 to 5 kg of rock sample was processed with low to good zircon yields (Table 2.2). Zircon mineral separation was undertaken by Geotrack International where rock samples were initially crushed with a jaw crusher and then by a disc pulveriser. Frantz isodynamic magnetic separators and heavy liquid mineral separations were used to separate mineral grains by magnetic susceptibility and gravity, respectively. For one sample (ALL-1B) electromagnetic separation was performed as an additional procedure, as the zircon concentrate for this sample contained sulphides, which were extracted by passing the concentrate through a high voltage field (25KeV). Details on mineral separation techniques and ICPMS settings are documented in Section 3.1 of Chapter 3 and Appendix 2.1.

U-Pb dating of zircons separated from six volcanic rocks and one epiclastic sandstone sample was performed using the Agilent 8800 laser ablation inductively coupled plasma mass spectrometer (LA-ICP-MS) at the Central Analytics Research Facility (CARF) at QUT. Zircons in two epoxy mounts were analysed in three sessions over three days. Zircon concentrates from two samples (ALL-1A and B) yielded very small and fractured zircon fragments, hampering sample preparation in a polished resin mount and overall, zircon rim domains were too thin with respect to the laser spot size. To overcome these issues a fourth session was conducted where zircons were ablated directly on adhesive tape without polishing according to the method introduced in Campbell et al. (2005). This sample preparation ensures ablations of rim domains via depth profiling allowing acquisition of distinct zircon rim ages that can be used to constrain the emplacement age.

Table 2.2. Sampled depths for volcanic rocks of the Gumbardo Formation. Hole deviations at sampled depth are extracted from the well completion reports and report the deviation from a vertical axis at the respective

sampled depth.

Recovery

Sample Sampled Sample sampled Hole deviation at

yield

weight geochemical Sample analysis[g] weightfor mineral separation [g] Zircon name Well name depth (m) section (m) section sample depth (°) Sample

ALL-1A BEA Allandale-1 2795.9 2794-2797 90% 6.50 238 2140 low-good ALL-1B BEA Allandale-1 3003.8 3003-3004 50% 4.50 238 1530 good GUM-1A PPC Gumbardo-1 3940.8 3941-3940 100% 5.50 244 6100 good GUM-1B PPC Gumbardo-1 3282.1 3278-3283 100% 3.25 310 5700 good GUM-1E PPC Gumbardo-1 3537.8 3535-3540 90% 7.25 527 - - GUM-1F PPC Gumbardo-1 3701.2 3700-3702 90% 5.50 692 - - COT-1 PPC Cothalow-1 2500.3 2499-2502 75% 2.75 182 1700 low 10+ YON-1 AOD Yongala-1 3052.9 3052-3053 50% 2.00 214 1330 good ETO-1 PPC Etonvale-1 3304.9 3304-3305 80% 4.50 470 2760 low-good CAR-2 PPC Carlow-1 3292.8 3292-3293 100% 2.75 502 - -

Table 2.3. Weighted averages of 206Pb/238U zircon ages of secondary standard Plešovice (mean ID-TIMS U–Pb age: 337.13 ± 0.37 Ma; Sláma et al., 2008), sample preparation method and analysed unknowns. Abbreviations: MSWD, mean squared weighted deviation; POF, probability of fit.

Session 206Pb/238U zircon age "Plešovice" sample preparation analysed unknowns 1 343.6 ± 3.3 Ma (MSWD=1.3, n=23) polished mount ETO-1, COT-1, GUM-1B 2 348.9 ± 4.2 Ma (MSWD=1.3, n= 14) polished mount YON-1, GUM-1A 3 337.1 ± 5.3 Ma (MSWD=1.7, n=14) polished mount ETO-1, COT-1, GUM-1B 4 339.3 ± 3.9 Ma (MSWD=1.7, n=13) tape mounted ALL-1A, ALL-1B

The primary natural zircon standard Temora-2 (416.78 ± 0.33 Ma, Black et al., 2004) and the NIST 610 glass standard were used to calibrate U-Pb systematics, and trace element compositions, respectively, and natural zircon standard Plešovice (337.13 ± 0.37 Ma, Sláma et al., 2008) was treated as an unknown, and used to check accuracy (Table 2.3). Samples were analysed in the following sequence: Temora-2, Plešovice, NIST 610 and then 11 unknowns, whereas unknowns were analysed in a round–robin procedure. The laser ablation system NWR 193 by ESI New Wave Research equipped with a Truelink Mk 2 non-cantilevered small volume ablation cell and a wavelength of 193 nm was used for the analysis. Laser dwell time was 30 seconds at 2.0 J/cm2 and at a rate 7 Hz with a laser cell helium flow of 600 ml/min, with a spot size of 25 µm during the first two sessions, and a reduced spot size of 20 µm for selective analysis of zircon rims in the third session. A 25 second gas blank between ablations was used to determine background. Data reduction was

performed using Iolite (Paton et al., 2011), filtering and checks for concordance with and without a 208-based common Pb correction and trace element calculations were done in excel. Statistical assessment of the collected age data was conducted in Isoplot 4.15 (Ludwig, 2003). Zircon trace element compositions assume a stoichiometric Si content of 15.22 wt% and lanthanide anomalies (Ce and Eu) are calculated after Ballard et al., 2002 using an Onuma plot strategy because of the limited number of rare earth elements analysed while simultaneously collecting isotopes for dating.

2.1.2 Whole-Rock Geochemistry (XRF & ICP-MS) Rock powders were prepared at the QUT Banyo Pilot Plant Precinct by crushing 182 to 692g (depending on the amount of core available and/or not required for zircon mineral separation) to a coarse sand to fine pebble fraction in a steel jaw crusher. The primary crushed samples were subsequently milled with an agate mill to powder and grainsize <10 μm. Samples were processed as fused glass disks using ~1.15g milled sample powder and

~8.85g lithium tetraborate flux (50% Li2B4O7, 50% LiBO2). Fused glass disks were used to obtain major element composition via XRF and trace element composition via LA-ICP- MS, respectively.

Whole-rock geochemistry was acquired by PANalytical AXIOS Wavelength X-ray Fluorescence (WD-XRF) Spectrometer for major elements and Agilent 8800 laser ablation inductively coupled plasma mass spectrometer (LA-ICP-MS) for trace elements at the Central Analytical Facility (CARF) at Queensland University of Technology (QUT). LA- ICP-MS was used for trace (TE) and rare earth element (REE) analysis following the method introduced by Günther et al. (2001). Fused disks were cut in half and the trimmed edge was ablated with 80 µm spot size for REE and 105 µm for TE. Dwell time was 30 seconds at 2.0 J/cm2 at a rate of 10 Hz for both methods allowing the acquisition of 32 elemental compositions. Data quality control was ensured by analysis of primary standard USGS reference materials BCR-2, SRM 612 and 610 after every ten ablations of the unknowns. SiO2 was used as internal standard based on previous major element analysis with XRF.

2.2 RESULTS

2.2.1 Volcanosedimentary lithologies of the Gumbardo Formation Here a summary of the lithologies of the Gumbardo Formation with a focus on the eruptive units is provided. Table 2.4 and Appendix 2.2 provide more detailed sample descriptions for each well intersecting the formation.

Recovered core material containing volcanic and epiclastic rocks of the Gumbardo Formation was inspected and logged in five wells (Table 2.1, Fig. 2.3). From the available core, three main rock types were identified: 1) ignimbrites of rhyodacitic composition, 2) coherent porphyritic rhyodacite, and 3) epiclastic sandstones (lithic arkose to arkose). Formational thickness is constrained by well log interpretations (McKillop et al., 2007) and has been verified based on cutting descriptions and core logs from the associated well completion reports (Lewis, 1961; Lewis and Kyranis, 1962; Phillips Petroleum Company and Sunray DX Oil Company, 1963; Kyranis, 1966; Leslie, 1971). Our revision of this material confirmed formational thickness for PPC Gumbardo-1 (755.5 m), PPC Carlow-1 (140 m) and PPC Cothalow-1 (273 m). For two wells, only minimum thicknesses are constrained (Fig. 4, BEA Allandale-1, >236 m; PPC Etonvale-1, >206.1 m). Epiclastic sandstones of the upper Gumbardo Formation are intersected in PPC Cothalow-1 (~110 m) and PPC Etonvale-1 (~100 m) in the central part of the Adavale Basin.

Volcanic lithologies The volcanic rocks of the Gumbardo Formation comprise predominantly dacitic to rhyolitic ignimbrites (Fig. 2.3, encountered in all logged wells) with porphyritic coherent rhyolite intersected in one well (Fig. 2.4d, PPC Gumbardo-1, 3540 – 3535 m). The ignimbrites are predominantly crystal-rich (>20%), one unit shows slightly lower crystal abundance (ETO-1, 18%), and one crystal-poor ignimbrite is intersected in PPC Carlow-1 (5%). Phenocryst assemblages of the ignimbrites are dominated by K-feldspar (up to 21.7%, present in all sampled units, Fig. 2.5b, d), plagioclase (up to 25%, present in sampled units, but absent in three units, Fig. 2.5a) and quartz (up to 4.9 %, absent in two units, Table 2.4). In PPC Gumbardo-1, where stratigraphic variation is able to be assessed,

a decrease in K-feldspar phenocryst abundance is observed up section across three consecutive units (bottom to top; GUM-1F: 21.7%, GUM-1E: 18.2%, GUM-1B: 8%).

Juvenile clasts in the ignimbrites are usually welded forming fiamme with phenocrysts of K-feldspar, plagioclase and quartz (Fig. 2.5c), but also occur as nonwelded, vesiculated clasts in one ignimbrite (ALL-1A). The ash matrix in all ignimbrites exhibits variably sericitised, moderately welded vitriclastic shard textures (Fig. 2.5e, f). Lithic fragments are only present in three samples (ALL-1A, ALL-1B and COT-1) comprising a variety of igneous lithologies including plagioclase-bearing lava clasts showing trachytic texture (inferred to be of intermediate composition), devitrified aphyric silicic lava clasts, granitic clasts, hornblende- and pyroxene-phyric pilotaxitic lava clasts and ignimbrite clasts (Table 2.4). The ignimbrites of the Gumbardo Formation are generally flat-lying to gently dipping (eutaxitic fabrics at 65-90° to the core axis) but core material was not oriented, thus bedding measurements remain uncorrected for the deviation angles and should be viewed as semi-quantitative (Table 2).

One porphyritic rhyolite unit is intersected in PPC Gumbardo-1 situated in between two ignimbrite units. The rock exhibits a phenocryst content of ~26%, and K-feldspar proportions are similar to the ignimbrite units (18.2%), but shows a higher abundance of quartz (8%). Crystal sizes are comparable to the interbedded ignimbrites, but the rock shows stronger alteration of K-feldspar crystals (Fig. 2.5d) compared to the ignimbrites (e.g. Fig. 2.5b). Microfractures with infills of sericite, quartz aggregates and carbonate are abundant in this unit. In contrast to the other volcanic rocks samples, vitriclastic shard textures and fiamme are absent in this coherent volcanic rock.

Sedimentary lithologies Epiclastic sedimentary rocks occur near the base of the Gumbardo Formation in PPC Gumbardo-1 and stratigraphically below the volcanic units, but also overlie the ignimbrite eruptive units in PPC Cothalow-1 and PPC Etonvale-1. The epiclastic sandstone in PPC Gumbardo-1 nonconformably overlies basaltic basement rocks (Fig. 2.3), and comprises detrital plagioclase (23%), quartz (15%) and K-feldspar (6%) and lithic grains (13% aphyric lava clasts, 7% basaltic lava clasts). The sandstones overlying an ignimbrite eruptive unit in PPC Cothalow-1 between 2353 and 2275 m are moderately sorted and are dominated by subangular K-feldspar (24 – 54%) and quartz (6 – 10%; resorption

embayments abundant) and minor subangular plagioclase (up to 7%). Lithic grains are present as ignimbrite clasts (7 – 14%) often with phenocrysts of quartz and some exhibiting relict vitric glass shard textures.

Figure 2.4. Core photographs of sampled Gumbardo Formation volcanic rocks showing lithic-bearing ignimbrites from BEA Allandale-1: (a) ALL-1A (2796 m depth) and (b) ALL-1B (3003 m depth). Figures (c–e) show two K-feldspar and plagioclase-phyric ignimbrites (c, GUM-1B, 3283 m depth; d, GUM-1F, 3700 m) and the coherent porphyritic rhyolite (e, GUM-1E, 3537 m) sampled from Gumbardo-1 well. (f) Ignimbrite intersected in the Cothalow-1 well (COT-1, 2500 m); (g) ignimbrite intersected in the Etonvale well (ETO-1, 3305 m); and (h) fine-grained ignimbrite from Carlow-1 (CAR-1, 3293 m).

Figure 2.5. Photomicrographs of selected volcanic rocks from the Gumbardo Formation. Abbreviations: XPL, crossed polarised light; PPL, plane polarised light. (a) GUM-1B, showing fragmented euhedral plagioclase phenocrysts (pl) and sericitised fiamme (fm) (XPL). (b) Ignimbrite sample GUM-1F showing sericitised, euhedral to subhedral K-feldspar phenocrysts (fsp) in ash matrix and adjacent to fiamme that have been largely replaced by secondary quartz aggregates (qz) and sericite (XPL). (c) Ignimbrite COT- 1 with sericitised fiamme (fm) in a groundmass of vitric ash (XPL). (d) Ignimbrite GUM-1E showing intensely sericitised K-feldspars (fsp) in a micropoikilitic groundmass cross-cut by thin veinlets of quartz (XPL). (e) Ignimbrite CAR-2 with sericitised but well-preserved vitriclastic shard textures, and rare phenocrysts of K-feldspar (fsp) (XPL). (f) Ignimbrite ETO-1A that also has well-preserved and nonwelded vitriclastic shard textures (PPL).

Interpretation The majority of the sampled ignimbrites have preserved vitriclastic shard textures, confirming the volcanics have not been subjected to texturally destructive alteration and further, the clastic textures and welded fiamme confirm the pyroclastic origin for these rocks. Based on the absence of coarse and abundant accidental lithics in most of the ignimbrites, we interpret an outflow facies rather than an intra-caldera or proximal facies setting for the ignimbrites. Contact relationships with the underlying and overlying ignimbrite units are not preserves for the single coherent volcanic rock intersected in PPC Gumbardo-1, hampering a further genetic interpretation of this unit (i.e. distinction between lava or subvolcanic intrusion).

All the sampled volcanic rocks are notably devoid of mafic and hydrous minerals, suggesting an overall silicic and relatively anhydrous magmatic composition. K-feldspar is the predominant phenocryst phase, with only two samples dominated by plagioclase phenocrysts. The predominance of K-feldspar is distinctively different from volcanic suites in continental margin arc settings, where phenocryst assemblages are commonly dominated by plagioclase and contain mafic mineral phases (Ewart and Barker, 1979), which are absent in the volcanic rocks of the Gumbardo Formation.

In contrast, the Ordovician volcanic rocks forming basement to the Adavale Basin are a variety of volcanic rock types and associated phenocryst assemblages range from plagioclase-rich vesciculated basalt (PPC Gumbardo-1) and plagioclase-rich porphyritic andesite (PPC Cothalow-1), to dacite/rhyolite lavas (PPC Carlow-1 and AOD Yongala-1). Middle to Late Devonian volcanic rocks in the Anakie Inlier (e.g. Theresa Creek Volcanics, Dunstable Volcanics, Greybank Volcanics) are predominantly mafic (basaltic to andesitic) with a phenocryst assemblage of plagioclase, hornblende and pyroxene (Blake et al., 1995). The Late Devonian Silver Hills Volcanics are dominated by rhyolitic to dacitic ignimbrites and lavas (Fig. 2.1, Blake et al., 2013). Petrographically, the volcanic rocks of the Gumbardo Formation are different from adjacent igneous rock units in the Thomson Orogen, by containing high proportions of K-feldspar phenocrysts and only subordinate quartz and plagioclase.

Table 2.4. Petrographic assessment of volcanic rocks of the Gumbardo Formation, encompassing modal composition (based on point counting: 300 counts, 1 mm step length), phase descriptions and alteration assessment. Abbreviations: gm, groundmass; qz, quartz; K-fsp, K-feldspar; pl, plagioclase; lit, lithic clasts; juv, juvenile clasts; hb, hornblende; px, pyroxene.

Sample gm qz K-fsp pg lit juv phenocrysts groundmass lithics juvenile clasts phenocrysts alteration rounded pg-bearing trachytic subeuhedral to anhedral clasts (intermediate), micropoikilitic qz K-fsp (1-2mm), volcanic devitrified aphyric K-fsp moderately altered to clay ALL-1B 51.7 2.3 18.8 - 27.2 - groundmass, some absent quartz showing intermediate-silicic clasts (sericite and kaolinite) areas with vitric ash resorption embayments (some with K-fsp phenocrysts) juvenile clasts (3-4mm) showing K-fsp and pg bearing granitic welding, partly with devitrified K-fsp and pg moderately altered subeuhedral to anhedral sericitised relicticic ALL-1A 54.1 4.9 14.0 4.0 5.7 17.3 lithic clasts (one large clast, spherulites, intensely altered to to sericitise and kaolinite, few K-fsp and pg (1-2 mm) vitric ash 9mm) chlorite, containing K-fsp (intensely secondary clay filled veins sericitised) euhedral to subeuhedral moderately welded central rims of pg crystals subtly (pg 0.5-2mm), showing sericitised fiamme showing bending GUM-1B 57.0 - 8.0 25.0 - 10.0 sericitised absent sericitised (dusted with sericite), zonation and where adjacent to pg phenocrysts groundmass secondary qz veins abundant fragmentation micropoikilitic qz K-fsp intensely sericitised, subeuhedral K-fsp (0.5- groundmass, some lustrous secondary veins with GUM-1E 73.8 8.0 18.2 - - - absent absent 2mm) poikilitic domains sericite, zeolithe, qz and with minor sericite carbonate mm to cm long fiamme partly bearing euhedral K-fsp forming eutaxitic subeuhedral K-fsp (1- micropoikilitic texture, anostomosing solution K-fsp moderately sericitised and GUM-1F 63.7 0.0 21.7 - - 14.6 absent 3mm) groundmass seams modified with sericite and zeolitised secondary cryptocristalline quartz mineralisations euhedral to subeuhedral vitric groundmass lithics of silicic composition K-fsp and pg (0.5- COT-1 64.3 3.3 8.7 10.0 5.0 8.7 with some serictised with micropoikilitic textures sericitised fiamme K-fsp and pg weakly sericitised 2.5mm), partly areas bearing qz & K-fsp (2-4mm) fragmented subeuhedral K-fsp (0.5- sericitised relicticic sericitrised fiamme and juvenile K-fsp moderately serictised, ETO-1 77.0 3.7 14.3 - - 5.0 absent 1mm) vitric ash clasts sericitised vitric ash slightly welded subeuhdral K-fsp and pg sericitised relictic juvenile clasts bearing qz and pg, CAR-2 91.0 2.0 2.0 1.0 - 4.0 (predominantly <0.5 mm, vitric ash with some absent showing welding and devitrified K-fsp and pg weakly sericitised few 1-2mm) sericitised spherulites spherulites subangular qz (0.5-1cm), subeuhedral to anhedral ash-rich, slightly welded bearing GUM-1A 34.7 15.3 6.2 23.0 - 20.8 pg (0.5-1mm), absent euhedral phenocrysts of K-fsp subeuhedral K-fsp (0.5- 1cm)

2.2.2 Geochronology and chronochemistry Zircon mineral separates from six volcanic rocks and one epiclastic sandstone sample were analysed by LA-ICP-MS principally to obtain U-Pb ages. The primary standard Temora-2 used as the dating standard is similar to the unknowns in age and grossly similar in U and Th content. The weighted averages of the secondary zircon standard and the respective analysed unknowns are summarised in Table 2.3. The first three analytical session were conducted on two polished mounts, whereas in the third session the same zircon mounts were retargeted and ablated with a reduced spot size (20µm), focusing on analysing zircon rim domains to better constrain final crystallisation ages, as well as interior domains to investigate zircon inheritance. In a fourth session zircons from two samples were directly ablated on adhesive tape in order to improve ablation of rim domains to improve sampling statistics and calculation of crystallisation ages.

U-Pb zircon geochronology Data for the six samples are summarised in Table 2.5. Three samples (ETO-1, GUM- 1B and COT-1) were investigated with carefully targeted ablations of zircon interior and rim domains for a number of individual grains (Fig. 2.7). SEM images of the ablated zircons were assessed prior to, and after each analytical session to monitor the position of the ablation spot on each zircon grain (Fig. 2.8). Rim and interior domains of zircons were ablated in order to identify any potential zircon inheritance (Storm et al., 2011). In contrast to zircon core domains, interior domains are inconspicuous in terms of CL-response and morphology, but can exhibit subtle inheritance information (Storm et al., 2011). Here we focus on ablation sites that can be clearly assigned to an interior or rim domain of a zircon, whereas ablations positioned in intermediate locations between interior and rim have not been included in the final age assignment because of the possibility that these ablations are mixtures (Siégel et al., 2018b). The weighted averages of the secondary standard Plešovice overall conform with the standard age, however the weighted average age for the second analytical session is elevated and results from this session might be affected by an age drift (Table 2.3). All analysed zircon data and their population groups are given in Electronic Appendix A and are also available in the data repository of the Australian Journal of Earth Sciences, Appendix 2.3 contains Wetherill Concordia plots of all analysed igneous samples.

GUM-1B

Zircons recovered from this welded plagioclase-rich ignimbrite are largely fragmented with diameters ranging from 40 to 120 µm and aspect ratios between 1.2 and 5.4. The zircons show oscillatory and planar zoning and 20% of the analysed grains contain melt inclusions (see Fig. 2.6b and 2.6g). Sixty-two grains were mounted for this sample, with 61 yielding results meeting analytical criteria (concordant and inclusion free). Nineteen analyses were not included in the final age assignment due to spot locations in intermediate areas of grain domains. Out of the selected 42 analyses, 18 ablations were conducted on zircon rims and 24 analyses were performed on interior domains. One age population was identified at 398.1 ± 4.4 Ma (MSWD=0.54, n=22, Fig. 2.8a), which is predominantly defined by zircon rim domains (n=15) but also some interior domains (n=7). An older age population of 420.1 ± 4.5 Ma (MSWD=0.35, n=19) is restricted to predominantly interior domains (16 analyses) and in a few zircon rim domains (3 analyses). One older inherited grain (an ablation spot in an interior domain) yielding an age of 447 ± 18 Ma was additionally identified for this sample. For 9 out of the 30 grains analysed in this sample, we undertook further detailed interior and rim domain analyses on the same grain, confirming zircons with older interior and younger rim domains (Figs. 2.7a, 2.6a, b, c). The emplacement age for this ignimbrite is interpreted to be 398.1 ± 4.4 Ma (MSWD=0.54, n=22) based on the weighted average of the youngest age population mostly recorded by zircon rims.

COT-1

Sixty-four largely fragmented zircons were recovered from this plagioclase and K-feldspar- rich ignimbrite sample. The zircons all show oscillatory zoning, with grain diameters ranging from 25 to 100 µm and aspect ratios from 1.3 to 3.8. For this sample 64 grains were mounted and 52 analyses yielded concordant results with no inclusions. Twenty-five analyses were not included in the final age assignment as they were conducted in intermediate grain locations. Two remaining concordant ages from interior domains gave anomalously young ages at 367.7 ± 15 and 373.9 ± 14 Ma, and are interpreted to have lost Pb, however, Th (143 and 268 ppm) and U (175 and 248 ppm) concentrations, as well as alpha dose (0.378 and 0.573·1018 particles per gram) are not outstanding in this regard. Out of the remaining 25 analyses, 13 analyses were conducted in rim locations and 12 in interior domains. The largest population was identified at 398.9 ± 5.8 Ma (MSWD=1.5, n=16, Fig.

2.8b) and is based on analyses mainly from rim domains (12 analyses) and a few interior domains (4 analyses). Another age subpopulation was identified at 431.9 ± 7.4 Ma (MSWD=0.92, n=6), exclusively in interior domains. Three clearly inherited older grains were analysed and yielded ages of 526 ± 27, 661 ± 28 and 995 ± 35 Ma, all from zircons with resorbed core domains. Detailed analysis of interior/rim domain pairs on 7 individual grains exhibited a general overlap of ages between zircon interiors and their respective rims for the majority of analyses (Figs. 2.7b, 2.6d, e, f). Additionally, inherited ages are restricted to interior and core domains for this sample. This suggests that the majority of zircons in this sample belong to the magma (autocrystic) and are not inherited or accidental. The emplacement age for this ignimbrite is interpreted to be 398.9 ± 5.8 Ma (MSWD=1.5, n=16) based on the younger age population.

ETO-1

Zircons from this K-feldspar-rich ignimbrite show predominantly planar zoning and acicular morphologies with high aspect ratios of up to 8. The zircons recovered from this sample are largely fragmented with diameters ranging from 25 to 80 µm and aspect ratios from 1.5 to 8. Sixty-eight zircon grains were mounted for this sample, with 62 analyses yielding results meeting analytical criteria. Assessment of SEM images after ablation identified 27 spots in intermediate grain locations. Out of the 35 remaining analyses, 15 zircon rims and 20 interior domains were analysed, yielding two dominant age populations. The younger age population is 398.3 ± 4.6 Ma (MSWD=0.54, n=14, Fig. 2.8c) with individual ages obtained from 8 rim and 6 interior domains. A distinctly older age population is identified at 422.5 ± 4.5 Ma (MSWD=1.15, n=21), predominantly identified in interior domains (14 analyses) and a few rim domains (7 analyses). ETO-1 contains no inherited grains older than Silurian, yet exhibits the largest population of inherited Silurian zircons across all samples. The interpreted emplacement age for this ignimbrite is 398.3 ± 4.6 Ma (MSWD=0.54, n=14), obtained from the weighted average age of the youngest population in this sample.

ALL-1A

The majority of zircons recovered from this lithic-bearing ignimbrite are small acicular grains showing planar zonation and aspect ratios of ~4. Zircons were ablated directly on adhesive tape, without polishing prior to the analysis in order to ensure analysis of rim domains and to best constrain eruption ages. Zircon crystal diameters range from 25 to 90 µm, and aspect ratios between 1.4 and 5.5. For this sample 32 zircons were mounted and 30 analyses met the analytical criteria. Six analyses were rejected due to intermediate ablation spot locations. From the remaining 24 analyses, 23 were in zircon rim domains and one in an interior domain. Three zircon populations are identified in this sample with the largest group yielding an age of 398.3 ± 6.2 Ma (MSWD=1.6, n=14, Fig. 2.8d), restricted to rim domains. There are 3 grains with ages of ~420 Ma (all from rim domains), five zircons at ~450 Ma (4 from rim domains and one analyses in an interior domain) and two older inherited grains at 908 ± 29 and 1084 ± 76 Ma (both from rim domains). The emplacement age for this ignimbrite is interpreted to be 398.3 ± 6.2 Ma (MSWD=1.6, n=14), based on the weighted average of the largest age population.

ALL-1B

Zircons from this lithic-rich ignimbrite show oscillatory zoning and are largely fragmented. Diameters range between 20 and 70 µm and aspect ratios between 1.3 and 4.0. As for sample ALL-1A, adhesive tape-mounted zircons were analysed to ensure analyses in rim locations. Out of 35 mounted zircon grains, 32 analyses yielded results meeting analytical criteria. Four grains yielded ages of ~420 Ma from two rim and two interior analyses, one older grain was identified at 680 ± 41 Ma (rim domain). The predominant population yields a weighted average of 397.6 ± 3.5 Ma (MSWD=0.95, n=27, 25 analyses in rim domains and two analyses in interior zircon domains, Fig. 2.8e) and this is interpreted as the emplacement age of this ignimbrite.

YON-1

Zircons from this sample are significantly larger than in the other samples (100-200 µm long), and fewer grains are broken or fragmented. Aspect ratios range from 1.2 to 3.6, with diameters from 25 to 100 µm, and the zircons show oscillatory zoning. In this

porphyritic rhyolite, 76 out of 86 analyses meeting analytical criteria yielded an age of 491.0 ± 3.6 Ma (MSWD=2.3, n=76). The high MSWD of 2.3 given the number of analyses indicates the sample does not represent a single age population. Three of the youngest ages were obtained from rim domains and yielded ages of 487 ± 20, 470 ± 20 and 487 ± 23 Ma. The unmixing function in Isoplot (Sambridge and Compston, 1994) was applied to identify potential multiple age population in this sample, yielding a younger population at 488.6 ± 14 Ma (MSWD=0.64, n=23, Fig. 2.8f) and a slightly older age cluster at 507.9 ± 7.0 Ma (MSWD=0.31, n=8). This sample shows a significant inherited population ~600 Ma (n=4), and single inherited zircon ages of 841 ± 70, 950 ± 45, 1150 ± 140 and 1636 ± 88 Ma. The emplacement age for this rhyolite is interpreted to be Late Cambrian/Early Ordovician at 488.6 ± 14 Ma (MSWD=0.64, n=23) based on the youngest population, and therefore is unrelated to, and excluded from the Gumbardo Formation.

GUM-1A

This sample is situated in the lowermost section of the Gumbardo Formation, unconformably overlying an Early Ordovician basalt, and is identified as epiclastic arkosic sandstone. DZ recovered from this sample show planar and oscillatory zoning and are predominantly euhedral but fragmented. Aspect ratios range from 1.4 to 3.3 and diameters from 30 to 80 µm. 58 out of 76 analyses yielded concordant results, whereas 39 analyses were assigned to either rim or interior domains (Fig. 2.9). Following the approach in Dickinson and Gehrels (2009) to constrain the maximum depositional age of this sandstone, the youngest single grain (YSG 1σ) is identified at 398 ± 8.5 Ma, the youngest 1σ cluster (YC1σ) at 401.9 ± 5.4 (MSWD=0.18, n=9) and the youngest 2σ cluster (YC2σ) at 403.6 ± 4.8 Ma (MSWD=0.3, n=12). Apart from the assessment of maximum depositional age, two dominant age groupings have been identified at 404.6 ± 4.6 Ma (MSWD= 0.47, n=13) and 430.8 ± 4.1 Ma (MSWD=0.64, n=20) using the unmixing function in Isoplot (relative misfit: 0.894). Five significantly older grains were dated between 500 and 600 Ma (494 ± 20, 524 ± 23, 534 ± 19, 570 ± 25 and 596 ± 29 Ma), and one grain at 1000 ± 44 Ma. These ages are similar to DZ cluster ages obtained from metasedimentary rocks across the Thomson Orogen (e.g. 520, 565, 580, 1070 and 1085 Ma populations from the Thomson Beds in Purdy et al., 2016). The interpreted maximum depositional age of this epiclastic sandstone is 401.9 ± 5.4 Ma based on the youngest 1σ cluster.

Table 2.5. U–Pb geochronology results documenting weighted averages for zircon population interpreted as emplacement ages and inherited populations. Abbreviations: WA, weighted average; MSWD, mean square weighted deviation.

rejected total WA inherited rejected WA emplacement age inherited older younger concordant Sample Lithology population mixed [Ma] grains [Ma] analyses analyses [Ma] ages [Ma] (n)

420.1 ± 4.5 398.1 ± 4.4 GUM-1B ignimbrite (MSWD=0.35, 20 447±18 (n=1) - 61 (MSWD=0.54, n=22) n=19)

431.9 ±7.4 526±27 (n=1), 367.7±15, 398.9 ± 5.8 COT-1 ignimbrite (MSWD=0.92, 25 661±28 (n=1), 373.9±14 51 (MSWD=1.5, n=16) n=6) 995±35 (n=1) (n=2)

422.5 ± 4.5 398.3 ± 4.6 ETO-1 ignimbrite (MSWD=1.15, 27 - - 62 (MSWD=0.54, n=14) n=21)

443.6± 25, 444±14, 419.5 ± 9.7 450 ± 17, 465± 15, 398.3 ± 6.2 ALL-1A ignimbrite (MSWD=0.02, 6 480.9± 19 (n=5); - 30 (MSWD=1.6, n=14) n=3) 908± 29, 1084± 76 (n=2)

421.6 ± 9.7 397.6 ± 3.5 ALL-1B ignimbrite (MSWD=0.04, 0 680±41 (n=1) - 32 (MSWD=0.95, n=27) n=4)

522.8±21, 543.6±23 494 ± 5.3 (n=2); 570±22, porphyritic 481.0 ± 12.0 YON-1 (MSWD=2.1, 34 581±23, 603±31, - 78 rhyolite (MSWD=0.92, n=3) n=31) 616±23 (n=4); 841±70, 950±45, 1150±140 (n=3); 1636±88 (n=1) Zircon chronochemistry Based on the U-Pb zircon dating, three main groups of zircon ages are identified in the volcanic rocks of the Gumbardo Formation: 1) syn-emplacement (“autocrystic”) zircons between 390 and 405 Ma, 2) subtly inherited zircons with ages between 415 and 450 Ma, which can be subdivided into two further subpopulations, and 3) distinctively older inherited zircons (450 – 1600 Ma; Fig. 2.10a). Despite the abundant zircon inheritance, cathodoluminescence imaging did not reveal the presence of texturally distinct core domains yielding these older ages. The existence and magnitude of inheritance was not noted in two previous SHRIMP zircon ages (Draper, 2006), but recognized to some degree in a revision of the SHRIMP ages (Cross et al. 2018). Zircon chemistry is examined here to test whether there are distinct chemical differences between the age populations to help distinguish autocrystic vs inherited (antecrystic/xenocrystic) zircons. Zircon inheritance is promoted in Zr-saturated magmas where inherited zircons can develop new rim growths of zircon when immersed in the melt as the magmas crystallise. A potential test of zircon inheritance is to compare the Zr saturation temperature for the magma (rock) with estimates

Figure 2.7. SEM images showing selected grains targeted for core/rim ablations and corresponding 206 238 Pb/ U age, Th/U ratio and TZircTi (Watson et al., 2006) for individual ablation pits. (a) GUM-500, (b) GUM-419, (c) GUM-238, (d) COT-493, (e) COT-445, (f) COT-395, (g) ETO-382, and (h) ETO-521. All scale bars represent 20 µm.

Figure 2.6. Results of the core/rim analyses for individual zircons from (a) GUM-1B, (b) COT-1, and (c) ETO-1 ignimbrites. 2σ-error bar pairs show core age (red) and rim age (green) of individual grains; the results highlight issues to resolve subtle zircon inheritance as ages for most core and rim domains lie within uncertainty of each other for the majority of the analysed grains. All ages are 206Pb/238U ages in Ma.

Figure 2.9. Detrital zircon 206Pb/238U age spectrum for epiclastic sandstone sample GUM-1A, displayed as kernel density estimates (bandwidth 10 Ma) and absolute numbers (bin width 10 Ma). Abbreviations: MDA, maximum depositional age.

of magmatic temperature (Siégel et al., 2018b). If the host magma composition is Zr undersaturated and zircons are present, then this can be a strong indication that the zircons are inherited and have not crystallised from the host rock, and that the obtained age information may not provide accurate constraints on to the emplacement age.

TZircTi has been has been calculated to constrain crystallisation temperatures of the analysed zircons (assuming Ti activity of 1) and to provide some constraint on magmatic temperature. Assessment of TZircTi and TZircsat (Fig. 2.11, after Siégel et al., 2018a), based on Ti in zircon and whole rock geochemistry shows TZircTi < TZircsat for the majority of analyses, suggesting an overall Zr oversaturated magmatic regime across analysed interior and rim domains. All samples exhibit a high variation of up to 300°C for TZircTi, but for the middle 50% of the data (based on median value), TZircTi ranges between 710 – 775 °C for the two younger populations, and is distinctively lower for the older population (430 – 450 Ma; 650 – 750 °C for the middle 50% of the data). Together with the observation of

Figure 2.10. Autocrystic (orange) and inherited (green/blue) zircon populations and zircon trace-element geochemical data for the respective groupings showing subtle differences between the age populations. Bin width and bandwidth for KDE plot is 5 Ma. Boxes of the whisker plots (b–h) represent the middle 50% of the data, mean values represented by black dot, median by horizontal line, outliers are drawn as circles, far outliers as triangles.

significant proportions of older (Silurian) zircon interior domains, which lack resorbed grain boundaries, this indicates the erupted magmas of the Gumbardo Formation were Zr saturated and a significant entrainment of zircon into the magmas was preserved.

Assessment of zircon geochemistry across all Gumbardo Formation volcanic rock samples has been conducted for the autocrystic, emplacement related population (~390 – 405 Ma) and two inherited zircon populations (~415 – 425, ~425 – 448; Fig. 2.10). The

median values for Th/U, Zr/Hf and TZircTi are collectively lower for the inherited populations than for the Early Devonian autocrystic population, but the differences are subtle (Fig. 2.10b-d). Ce/Ce* shows significantly higher average values for the oldest population, whereas the two younger populations are very similar. This is consistent with

increasing TZircTi average values over time, which are slightly lower for the oldest population and again very similar for the two younger age groupings (Fig. 2.10e,f; Trail et al., 2012). The chemical differences between the different aged zircons are overall very subtle, suggesting persistent magmatic environments of zircon crystallisation over time, which is further reflected in the unresorbed nature of the zircons.

Whole Rock Geochemistry

Major element geochemistry

Eight volcanic rocks of the Gumbardo Formation were analysed for major and trace elements (Table 2.6). Given that geochemical analysis has been undertaken on whole-rock samples of the ignimbrites, the chemistry will not strictly match the original magma chemistry due to some crystal enrichment and ash elutriation resulting from pyroclastic density current processes (Walker, 1972). All analysed samples have been affected to some extent by hydrothermal alteration. Plagioclase and K-feldspar preservation in the rocks ranges from fresh to cloudy with clay and/or sericite alteration. Petrographic assessment of samples ALL-1A and ALL-1B yielded lithic fragment contents >5% (intermediate to silicic volcanics in composition) and identifiable lithic fragments were removed during sample preparation for geochemical analysis. However, it is possible that minor lithic clast contamination exists for these two samples and further, these two samples do show evidence of whole rock alteration (e.g. high Na2O contents and anomalously low Rb contents, Table 2.6). Interpretation of these two samples are treated with caution in the following examination of the geochemical data. The remaining six samples have LOI values that range from 2.3 to 4.8 wt.% and are negatively correlated with K2O implying some fluid-rock interaction.

The sampled volcanic rocks have intermediate to high SiO2 contents from 66.6 to 74.8 wt.% and plot collectively in the field for dacite/rhyolite using immobile element classification diagrams (Fig. 2.12). All samples have relatively low Na2O contents from 0.8 wt.% to 2.7 wt.% but elevated K2O contents ranging from 3.6 to 5.5 wt.%, with the highest concentrations of K2O (~ 5 wt.%) from the K-feldspar rich ignimbrites GUM-1E, GUM-

1F and ETO-1 (Tables 2.4, 2.6). Silicic volcanic rocks with similar high K2O contents have been referred to as quartz latites (e.g. Milner et al., 1992), describing rocks that tend to straddle the boundary of the dacite, trachydacite and rhyolite field in the TAS diagram.

Compared to an average rhyodacite, quartz latites exhibit higher K2O, higher FeOT and lower Al2O3 concentrations (Table 2.7). The average of six samples from the volcanic rocks of the Gumbardo Formation (excluding ALL-1A and B, due to significant alteration) has higher K2O, higher FeOT and slightly lower Al2O3 than an average rhyodacite, and the

volcanic rocks of the Gumbardo Formation show a geochemical resemblance to quartz latites.

Two ignimbrites (ETO-1, CAR-2) straddle the boundary between magnesian and ferroan rocks, and at a more regional scale, coeval Thomson Orogen granitoids (425 – 380 Ma, outcropping in the southern Thomson Orogen and in the Lolworth-Ravenswood Province, as well as granitic rocks encountered in stratigraphic drill holes in the central Thomson Orogen) are predominantly ferroan (Siegel, 2015; Siégel et al., 2018b). The aluminium saturation indices (ASI = molecular Al2O3/(CaO+K2O+Na2O) after Zen, 1986) for the volcanic rocks range from 1.3 to 2.5, whereas samples CAR-2 and ETO-1 show the highest values (2.5 and 2.2, Table 2.8). However, the apparently strongly peraluminous character is more likely a consequence of alteration and the mobilisation of K and Na, rather than being a primary magmatic feature.

Trace element geochemistry

The K-feldspar-rich ignimbrites collectively show pronounced depletions for Sr and

TiO2 and minor depletions for Ba, when normalised to an average upper continental crust composition (Fig. 2.13a). Sample GUM-1B has the greatest abundance of plagioclase phenocrysts amongst all sampled volcanic rocks and shows a strong depletion of Ba and only minor depletion of Sr, compared to the K-feldspar-rich samples (Table 2.6, Fig. 2.13a).

The overall TE-pattern with characteristic depletions in Sr and TiO2 is similar to temporally related Late Silurian/Early Devonian granitic rocks southwest of the Adavale Basin (Fig. 2.13a, Siegel, 2015). The strong depletions in Sr and the weak negative Eu anomalies for the majority of the samples may indicate plagioclase fractionation, but that feature may be a source characteristic given a general lack of other evidence for extended crystal fractionation (e.g., Eu/Eu* values of zircons, relatively elevated MgO, whole rock Eu/Eu* and Rb/Sr ratios; Table 8, Figs. 11d, 14b).

The Gumbardo ignimbrites have LaN/YbN values from 3.9 to 6.7, values distinctly lower than the porphyritic rhyolite from PPC Gumbardo-1 (GUM-1E, LaN/YbN = 14.5, Table 6). This suggests relative light rare earth element (LREE) enrichment and heavy rare earth element (HREE) depletion for all the volcanic rocks of the Gumbardo Formation (Fig.

2.13b). The magnitude of negative Eu anomalies correlates with increasing SiO2 content, with more prominent negative Eu anomalies observed in 5 samples (0.54 to 0.69 in GUM- 1E, GUM-1F, COT-1, ETO-1, CAR-2), and only a weak anomaly in 1 sample (0.85 in GUM-1B). Generally, the REE concentrations of the volcanics follow the composition of the upper continental crust (Rudnick and Gao, 2003), but with collectively higher HREE than the bulk upper crust for CAR-2, ETO-1, GUM-1B and COT-1 (Fig. 2.13b). The normalised values for the LREE are below the composition of the upper continental crust for COT-1 and GUM-1F but are still distinctively different from lower continental crust LREE values (Fig. 2.13b). When normalised to MORB, the trace element data depict enrichment of the most incompatible elements Ba, Th, Ta and little to no enrichment of the least incompatible elements Y and Yb (Fig. 2.13c). This pattern is characteristic of other rifts in intraplate settings, e.g. the Mt Hope Volcanics and Ural Volcanics in the Central Lachlan Orogen (Fig. 2.1, Bull et al., 2008) and the Jurassic Chon Aike Province in Patagonia/Antarctic Peninsula (Riley et al., 2001).

Tectonic discrimination diagrams for granitic rocks have been utilized to investigate the tectonomagmatic affinities of the volcanic rocks of the Gumbardo Formation and other coeval igneous units. In terms of tectonomagmatic affinity, the analysed samples straddle the boundary between the fields of within-plate granites and volcanic arc granites (Fig. 2.14a; Pearce et al., 1984). Samples ETO-1 and CAR-2 consistently plot in the field for within-plate granites for all three discrimination diagrams (i.e. Y/Nb, Yb/Ta, Y+Nb/Rb). Discrimination of magmatic affinity after Whalen et al. (1987) suggests A-Type affinities for ETO-1, CAR-2 and GUM-1B, whereas the remaining samples plot in the field for other granites (Fig. 2.14b). Compared to the composition of the upper continental crust, these three samples with A-Type signatures show collectively higher concentrations of REE, particularly for the HREE. These samples also show the highest concentrations of Fe2O3 and Al2O3 amongst all sampled volcanic rocks. The majority of the volcanic rocks of the Gumbardo Formation plot in the field for SE Queensland high silica rhyolites (Cenozoic), which exemplify intraplate settings (Ewart et al., 1992). Samples GUM-1E and GUM-1F plot outside the field as they exhibit higher concentrations of Ba than the remaining samples (511 and 585 ppm, Table 2.6). The same accounts for the coeval volcanic rock intersected in Milcarpa-1, whereas the coeval granite intersected in AOP Balfour-1 and the Silurian- Devonian Mt Hope Volcanics and Ural Volcanics plot in the transitional field.

Figure 2.12. Zr/Ti vs. Nb/Y classification diagram for volcanic rocks (Pearce, 1996) showing volcanic rocks from the Gumbardo Formation collectively plotting in the field for rhyolite/dacite. Additional whole-rock geochemical data are shown for similar Silurian–Devonian (ca 420–410 Ma) rhyodacitic rocks of the Ural and Mt Hope Volcanics (Central Lachlan Orogen; Bull et al., 2008) and intermediate volcanic rock from drill hole Milcarpa-1 dated ca 396 Ma in the southern Thomson Orogen; Roach et al., 2018).

Fig. 2.13. For figure caption see next page

Figure 2.14. Multi-element diagrams for whole-rock geochemical data of the volcanic rocks of the Gumbardo Formation. (a) Trace-element data normalised to composition of the upper continental crust (Rudnick & Gao, 2014) showing relative depletions of Sr and Ti, as well as a stronger depletion of Ba for plagioclase-rich ignimbrite GUM-1B. Grey shaded background shows composition of Silurian granites encountered in drill holes south west of the Adavale Basin (data from Siegel, 2015). (b) Rare earth element data of the volcanic rocks normalised to C1 chondrite (McDonough & Sun, 1995) highlighting similarities to the composition of the upper continental crust and higher LREE compared with lower continental crust composition (Rudnick & Gao, 2014). (c) Trace-element data normalised to MORB (Pearce et al., 1981). Light grey shaded field shows data from the Ural Volcanics and Mount Hope Volcanics in the Central Lachlan Orogen (late Silurian– Early Devonian; Bull et al., 2008). Dark grey shaded field shows data from the Chon Aike Province (Mapple Formation, Middle Jurassic, Antarctic Peninsula; Riley et al., 2001) as an example of intraplate rhyolitic volcanism. Note the relative enrichment of HFSE such as Zr, Hf, Y and Yb compared with MORB.

Figure 2.13. Tectonic affinities of the Gumbardo Formation volcanic rocks. (a) Tectonic discrimination diagram after Pearce et al. (1984), showing transitional affinities for the volcanic rocks of the Gumbardo Formation and temporally related units. Abbreviations: WPG, within-plate granite; VAG, volcanic arc granite; COLG, collision granites; ORG, ocean ridge granites. Supplementary data from igneous rocks across the Tasmanides (volcanic rocks marked with star): Ural Volcanics and Mt Hope Volcanics (Bull et al., 2008), Milcarpa-1 (Roach et al., 2018), AOD Balfour-1 (Siégel et al., 2018), Retreat Batholith (Withnall et al., 1995), Cape York Peninsula Batholith/Pama Igneous Association (Blue Mountains Adamellite and Flyspeck Granodiorite; Cooper et al., 1975). (b) Granite classification diagram after Whalen et al. (1987) showing A- type affinities for samples GUM-1B, CAR-2 and ETO-1. (c) Ba/Nb vs La/Nb plot after Ewart et al. (1992) showing the majority of the volcanic rocks of the Gumbardo Formation plotting in the field for intraplate (SE QLD high-silica rhyolites).

2.1 DISCUSSION

2.1.1 Age assignment of the volcanic rocks of the Gumbardo Formation Five silicic ignimbrites sampled at the base of the Gumbardo Formation from different wells yield very similar emplacement ages at ~398 Ma. Pooling all ages interpreted as emplacement ages across all samples yields a weighted average age of 398.2 ± 1.9 Ma (MSWD=0.94, n=93). These ages are stratigraphically consistent with the MDA (401.9 ± 5.4 Ma) determined from an epiclastic sandstone overlying the unconformity in PPC Gumbardo-1 (Fig. 2.3). The DZ spectrum of the sandstone suggests an onset of regional volcanic activity between 410 and 405 Ma (Fig. 2.9). Importantly, the uppermost ignimbrites from the Gumbardo Formation have similar emplacement ages of ~398 Ma. The similarity in eruption ages from the base to top of the Gumbardo Formation indicate a short-lived phase of volcanism at the initiation of basin opening, which is consistent with the relatively thin thickness of the formation (< 1 km). Two SHRIMP zircon ages from a previous study (Draper, 2006, and revised by Cross et al. 2018, this volume) from a middle section of the formation in PPC Gumbardo-1 (402.9 ± 2.9 Ma) and PPC Carlow-1 (408.1 ± 3.1 Ma) near the top of the formation lie outside the newly identified span of emplacement ages, although the younger age from PPC Gumbardo-1 lies within error.

The SHRIMP age in PPC Gumbardo-1 is further challenged by the MDA of 401.9 ± 5.4 Ma identified for the sandstone overlying the basal unconformity (Fig. 2.9). Based on the significant but often subtle inheritance of zircons with ages of ~410-430 Ma (Table 2.5), the slightly older SHRIMP ages obtained from volcanic rocks of the Gumbardo Formation in PPC Carlow-1 and PPC Gumbardo-1 are likely to be affected by this subtle zircon age inheritance. This can only be clarified by reassessment of the SHRIMP ages, and particularly an assessment of the analytical spot locations, which is crucial to confidently identify subtle inheritance, as our data demonstrate. Based on our approach the presence of a zircon population ~408 Ma cannot be ruled out, but for the data presented in this paper, this age grouping would exclusively represent an inherited age population, as all samples show a population of ~398 Ma that is representative of the latest zircon growth phase.

Table 2.6. Whole-rock geochemical data for sampled volcanic rocks of the Gumbardo Formation. Major elements in wt% normalised to 100% (volatile free), trace elements in ppm. Eu/Eu*, Ce/Ce* and (La/Yb)N normalised to chondrite after McDonough and Sun (1995).

Sample ALL-1A ALL-1B GUM-1B GUM-1E GUM-1F COT-1 ETO-1 CAR-2 porphyritic ignimbrite ignimbrite ignimbrite ignimbrite ignimbrite ignimbrite ignimbrite Lithology rhyolite Long 145.905 145.905 144.696 144.696 144.696 144.391 144.996 145.431 Lat -24.415 -24.415 -25.980 -25.980 -25.980 -25.725 -25.160 -24.839 Depth [m] 3003.8 2795.9 3282.1 3537.8 3701.2 2500.3 3304.9 3292.8 XRF major elements wt% oxide

SiO2 65.01 64.93 63.19 70.90 69.15 70.79 65.78 69.80

TiO2 0.57 0.51 0.62 0.24 0.29 0.31 0.34 0.21

Al2O3 15.13 14.01 17.54 13.74 14.18 12.86 16.16 15.64

Fe2O3 4.19 3.79 2.74 1.79 1.95 2.22 4.12 2.98 MnO 0.11 0.13 0.03 0.03 0.05 0.03 0.08 0.06 MgO 2.10 1.63 2.29 1.50 1.26 2.30 1.75 1.28 CaO 1.42 2.57 2.62 0.67 1.24 0.97 0.44 0.21

Na2O 3.45 5.52 2.18 2.38 2.55 1.13 0.72 0.95

K2O 2.77 1.34 3.43 5.18 4.94 3.84 5.25 4.27

P2O5 0.12 0.11 0.10 0.03 0.04 0.06 0.02 0.02 LOI 4.67 5.10 4.70 2.79 2.28 4.70 4.77 4.06 Total 99.89 99.80 99.56 99.38 98.07 99.28 99.53 99.58 ICP-MS trace & REE elements ppm Sc 13.6 13.5 13.2 4.9 5.5 7.4 7.6 14.1 V 59.4 51.4 33.7 21.2 24.2 13.1 9.8 10.6 Cr 14.4 13.8 10.4 9.6 11.7 8.9 9.8 9.6 Ni 7.3 6.7 4.9 7.4 7.2 4.6 5.8 6.0 Cu 9.2 7.6 9.9 12.2 10.7 11.3 6.9 7.6 Zn 57.0 60.6 19.1 19.6 22.1 31.3 44.7 69.4 Rb 112.7 40.7 130.6 163.9 200.3 167.8 303.1 277.0 Sr 81.3 150.8 273.1 86.4 149.8 104.4 44.6 55.2 Y 25.5 25.7 35.4 23.5 23.4 30.6 42.9 44.4 Zr 170.5 141.6 293.9 162.4 185.3 183.7 231.3 239.0 Nb 7.9 6.4 13.6 9.2 10.4 10.4 15.3 17.8 Cs 19.1 2.2 20.6 5.9 8.7 14.8 41.3 12.9 Ba 269.4 182.1 132.5 511.8 585.4 357.6 400.8 418.1 La 17.5 12.5 31.9 48.4 21.7 24.9 38.2 44.0 Ce 41.5 27.4 72.6 110.1 50.6 57.6 74.4 86.5 Pr 4.8 3.1 8.1 10.7 5.4 6.4 8.9 10.1 Nd 19.8 12.3 32.1 34.5 19.2 25.0 32.8 39.4 Sm 4.3 2.9 7.0 5.3 3.5 5.1 6.6 7.3 Eu 1.1 0.9 1.9 1.0 0.7 1.1 1.4 1.3 Gd 3.8 3.5 6.3 4.2 3.3 5.1 5.8 6.9 Tb 0.6 0.6 1.0 0.6 0.5 0.8 0.9 1.1 Dy 4.2 4.3 5.9 3.7 3.5 4.9 5.8 7.4 Ho 0.9 0.9 1.3 0.8 0.7 0.9 1.1 1.7 Er 2.5 2.6 3.6 2.2 2.1 2.7 3.3 5.3 Tm 0.4 0.4 0.5 0.4 0.3 0.4 0.5 0.8 Yb 2.9 2.9 3.6 2.2 2.1 2.7 3.8 5.8 Lu 0.5 0.4 0.5 0.3 0.3 0.4 0.5 0.9 Hf 4.2 3.7 7.4 4.1 5.0 4.6 5.0 7.0 Ta 0.5 0.4 1.0 0.7 1.0 0.9 1.7 1.5 Pb 4.4 3.9 12.4 6.5 8.8 5.1 10.8 5.7 Th 5.6 4.7 12.3 16.2 15.3 11.6 20.5 18.3 U 1.2 1.1 2.8 5.4 3.5 2.3 3.0 3.3

REE Tot 105 75 176 224 114 138 184 219 Eu/Eu* 0.83 0.85 0.85 0.64 0.60 0.65 0.69 0.54 Ce/Ce* 1.11 1.08 1.11 1.24 1.19 1.13 1.00 1.00

(La/Yb)N 3.95 2.84 5.93 14.53 6.78 6.23 6.72 5.02

Table 2.7. Comparison of major-element compositions of an average volcanic rock of the Gumbardo Formation (this study), average rhyodacite (Le Maitre, 1976) and quartz latite (Milner et al., 1992). All data in wt%, normalised to 100% and volatile free.

Average Gumbardo volcanic rock Average Rhyodacite Average Lower Tafelberg quartz latite (n=6) (n=100) (n=49) SiO2 70.27 67.69 69.05 TiO2 0.34 0.60 0.86 Al2O3 15.47 15.57 13.06 FeOT 4.88 4.02 5.39 MnO 0.04 - 0.08 MgO 1.74 1.68 1.19 CaO 1.04 3.36 2.47 Na2O 1.66 3.91 2.91 K2O 4.51 3.17 4.70 P2O5 0.04 - 0.30

Magmatic Magnesian/ferroan % Silurian % Ordovician % Precambrian Sample affinity rocks Eu/Eu* ASI inheritance inheritance inheritance

GUM-1E I-Type magnesian 0.64 1.4 n/a n/a n/a

GUM-1F* I-Type magnesian 0.60 1.3 4 - -

COT-1 I-Type magnesian 0.65 1.9 24 - 6

ETO-1 A-Type ferroan 0.69 2.2 60 - -

GUM-1B A-Type magnesian 0.89 1.8 45 2 -

CAR-2* A-Type ferroan 0.54 2.5 - 19 2

Igneous rocks that are coeval with the identified emplacement age for the volcanic rocks of the Gumbardo Formation have been encountered in drill holes in the central Thomson Orogen in APC Thunderbolt-1 (392.5 ± 2.5 Ma, Purdy pers. comm. 2014), located ~200 km north of the Adavale Basin and AOP Balfour-1 (396.0 ± 2.2 Ma, Siégel et al., 2018), ~100 km to the east of the basin. In the southern Thomson Orogen, plutonic rocks of similar age are intersected in the drill holes TFB002 (398.0 ± 2.8 Ma, Fraser et al., 2014) and TRI-RMD08-01 (401.8 ± 3.1 Ma, Bodorkos et al., 2013). A maximum depositional age of 402.0 ± 5.2 Ma is interpreted for a volcaniclastic sandstone of the Louth Volcanics ~400 km south of the study area (Dwyer et al., 2018). Outside of the Thomson Orogen zircon ages similar to those of the Gumbardo Formation have been assigned to granitic rocks in the Yeoval Complex, Eastern Lachlan Orogen (397 ± 4 Ma, 398 ± 4 Ma

and 399 ± 4 Ma, Black, 1998), as well as in the Mossmann Orogen for rocks of the Pama Igneous Association (e.g. Flyspeck Supersuite, 398 ± 10 Ma, Black et al., 1992).

Overall, these Early Devonian igneous and volcanosedimentary rocks are spatially and temporally distinct from the Middle Devonian emplacement ages for plutonic rocks in the southern Thomson Orogen (e.g. Eulo Ridge Granite, 385 ± 2.5 Ma, Fig. 2.1; Currawinya Granite, 381.5 ± 2.5; Cross et al., 2015), Anakie Inlier (Retreat Batholith, Mount Newsome Granodiorite, 392.4 ± 10.2 Ma ,Wood and Lister, 2013), northern New England Orogen (Mount Morgan Trondhjemite, 379.9 ± 4.27 Ma; Pomegranate Tonalite, 369.0 ± 4.2 Ma, (Murray et al., 2012) and Late Devonian to Early Carboniferous (~360 – 340 Ma) volcanic rocks to the east of the Adavale Basin (e.g. Campwyn Volcanics, Bryan et al., 2004; Silver Hills Volcanics of the Drummond Basin, Henderson et al., 1998; Cross et al., 2008).

Dating of a porphyritic rhyolite in AOD Yongala-1 yielded an emplacement age of 488.6 ± 14 Ma and is therefore excluded this Early Ordovician/Late Cambrian rock from the Gumbardo Formation. In accord with this age determination, the absence of Early Devonian rocks in the westernmost area of the basin was previously predicted based on regional seismic interpretations (Passmore and Sexton, 1984). The sampled porphyritic rhyolite lies in vicinity of the Canaway Ridge associated with the predominant crustal detachment fault identified from seismic studies. This section is interpreted as an uplifted basement horst. The exclusion of the volcanic rock at the base of AOD Yongala-1 from the Gumbardo Formation emphasizes and narrows the NNE-SSW trending rift corridor confined by volcanics of the Gumbardo Formation during basin initiation (Fig. 2.2). The rhyolite from AOD Yongala-1 is overlain by a section of dolomitic sandstone, which has previously been interpreted as the Middle Devonian Cooladdi Dolomite (McKillop et al., 2007) but findings of Late Silurian conodonts (Nicoll, 1995) in this section led to confusion regarding the stratigraphic interpretation of this well (McKillop et al., 2007). The Late Cambrian age of the underlying rhyolite allows a Late Silurian age for the overlying dolomite, consistent with the fossil evidence, meaning the dolomitic sandstone is not the Cooladdi Dolomite of the Adavale Basin.

2.1.2 Zircon inheritance Our results show zircon inheritance in the volcanic rocks of the Gumbardo Formation is both subtle in terms of age distinction, and widespread. Careful spatial targeting of different domains in zircon grains has been undertaken by means of interior and rim ablations on individual grains and depth profiling on tape mounted zircons to intersect thin autocrystic rims. 73% of all interior ablations show older inherited ages and surrounded by new melt-precipitated rims (Fig. 2.6). CL imaging revealed little textural evidence for any resorption prior to the crystallisation of the new rims (Fig. 2.6), suggesting the inherited zircons were entrained into largely Zr saturated magmas, in accord with assessment of

TZircTi and TZircsat relationships (Fig. 2.11).

A dominant autocrystic zircon population is identified ~398 – 400 Ma in all volcanic rock samples as well as a dominant detrital population in an epiclastic sandstone in a lower section of the Gumbardo Formation. Zircons older than 415 Ma are considered inherited and two distinguishable populations have been identified at ~415 – 425 Ma and 430 – 450 Ma (Fig. 2.10a). Distinctly older inherited zircons between 450 and 1600 Ma are also present, but far less abundant than the aforementioned age populations. Despite the relatively subtle age inheritance with regards to the autocrystic population chemical distinctions are also subtle (Fig. 2.10).

The younger zircon population (~400 Ma) is common in zircon rim domains, which is representative of the latest zircon growth phase. The identified older populations are commonly restricted to the interior zircon domains. For some analyses, however, the older populations are observed in the rim domains. These zircons apparently did not develop an autocrystic magmatic rim during the latest zircon growth phase, which has previously been interpreted as an effect of asynchronous crystallisation in the source system (Storm et al., 2011). The most prominent inherited population ~420 Ma is found in four ignimbrite units of the Gumbardo Formation. Only one plutonic basement rock of Silurian age is reported from within the Adavale Basin at PPC Etonvale-1 (Lewis and Kyranis, 1962), but the associated core material is lost and the age date cannot be replicated. Magmatism of Late Silurian age is voluminous in the southern Thomson Orogen although the igneous rocks are relatively distal to the Gumbardo Formation rift corridor (e.g. Tibooburra Suite, Hungerford Granite, granites within the Ella Belt – see Purdy et al., 2018). Specifically, the ages of the prominent inherited grouping at ~420 Ma defined here (Fig. 2.10a) correlates

well with S-type granites intersected in DIO Wolgolla 1 and TEA Roseneath 1 (Siégel et al., 2018c) and the outcropping Hungerford Granite (Fig. 2.1, Cross et al., 2015). Intrusions of similar age are also found in the extreme northern Thomson Orogen where they are grouped into the Pama Igneous Association and represent a major component of the Ravenswood Batholith (Fig. 2.1, Hutton et al., 1994). In addition to the abundant Silurian zircon inheritance, two older inherited zircon subpopulations were also identified yielding Late (~450 Ma, GUM-1B, n=1; ALL-1A, n=3) and Early Ordovician age (~470 Ma, ALL- 1A, n=2; PPC Carlow-1, n=11; Cross et al. 2018). The occurrence of these inherited zircon ages is largely restricted to the north-eastern area of the Adavale Basin. Early Ordovician silicic volcanic basement rocks have been intersected in wells from this area (PPC Carlow- 1, Fig. 2.3) and could be the source for the inherited zircons. An Early Ordovician/Late Cambrian age was assigned to a volcanic rock intersected in AOD Yongala-1.

The abundance of zircon inheritance and the ages of inherited zircons in volcanic rocks of the Gumbardo Formation suggest extensive reworking of igneous basement rocks and geographically distinct inheritance age patterns can be identified. Zircon inheritance in the volcanic rocks suggests Ordovician silicic volcanic basement rock was reworked in the north-eastern sector of the Adavale Basin (samples ALL-1B and CAR-2, Table 2.5), consistent with silicic volcanic basement rocks intersected at the base of PPC Carlow-1. In contrast, Ordovician zircon inheritance is absent in the southern part of the basin where Ordovician mafic to intermediate volcanic basement rocks (zircon poor) are intersected (PPC Gumbardo-1 and PPC Cothalow-1), the abundance of Silurian zircon inheritance is conspicuous in these wells (Table 2.5).

Igneous rocks coeval to those of the Gumbardo Formation are rare in the Thomson Orogen, occurring in scattered locations. A granitic rock is intersected in AOP Balfour-1 east of the Adavale Basin, with an interpreted emplacement age of ~396 Ma (Fig. 2.1, Siégel et al., 2018). Only limited inheritance of Silurian aged zircons has been detected in this intrusion compared to the volcanic rocks of the Gumbardo Formation (24 – 60% Silurian inheritance). The Tinchelooka diorite in the southern Thomson was dated 401.8 ± 3.1 Ma and one inherited Silurian zircon grain (Bodorkos et al., 2013), and some Cambrian to Neoproterozoic inherited zircons have been analysed. An intermediate porphyry intersected in drill hole TFB002 in the southern Thomson Orogen was dated 398.0 ± 2.8 Ma and revealed no Silurian inheritance, but one analysis of Neoproterozoic age (Fig. 2.1,

Fraser et al., 2014). Another intermediate to felsic porphyritic rock has been intersected in Milcarpa-1 (Figs. 2.1, 2.14, southern Thomson Orogen), and a preliminary age of ~396 Ma was assigned (Roach et al., 2018). The coeval plutonic rocks in the Thomson Orogen thus show generally a greater diversity of inherited zircon ages, but inherited zircons are less abundant compared to the Gumbardo Formation.

The volcanic rocks of the Gumbardo Formation lack inherited zircons having ages between 500 and 600 Ma, which is prominent DZ age in metasedimentary rocks of the Thomson Orogen (Fergusson et al., 2007; Purdy et al., 2016a). Cambrian zircon inheritance in the volcanic rocks of the Gumbardo Formation is restricted to one sample from PPC Cothalow-1 (526 ± 27 Ma). The remaining older inherited zircons (n=5) can be grouped into two Neoproterozoic populations, ~670 Ma (COT-1, ALL-1B) and ~1000 Ma (COT-1, ALL-1A), with the latter age common in metasedimentary rocks of the Thomson Orogen (Purdy et al., 2016a). The lack of significant proportions of inherited zircons from the two age groups, coupled with the clear alignment of these rocks as I-Type or A-Type, suggests that metasedimentary rocks have not been a major source for the volcanic rocks of the Gumbardo Formation.

2.1.3 Tectonomagmatic affinities and source of the Gumbardo Formation volcanics The lack of zircon inheritance from metasedimentary rocks within the volcanic rocks of the Gumbardo Formation indicates little to no contribution of metasedimentary rocks in the petrogenesis of Gumbardo rhyolitic volcanic rocks. This is also consistent with inheritance signals from contemporary plutonic rocks in the Thomson Orogen (Siegel, 2015; Siégel et al., 2018b). In fact, the majority of inherited zircons found in the volcanic rocks of the Gumbardo Formation are probably derived from upper crustal Silurian and Ordovician igneous sources, and relatable to subsurface and outcropping igneous units in the Thomson Orogen. The overall geochemical signatures of the volcanic rocks of the Gumbardo Formation are consistent with derivation from upper crustal materials (Fig. 2.13b).

The relatively high percentage of zircon inheritance found in our study (24 – 60%, Table 2.5), suggests crustal partial melting of igneous rocks was the dominant process in generating the silicic magmas. This has consequences for the interpretation of the

geochemical affinities of the volcanics, which will reflect more the composition and nature of the (Silurian/Ordovician) source material, than the tectonic processes operating at the time of volcanism (e.g., Bryan et al., 2014; Roberts and Clemens, 1993). The magmas do not show strong signals for fractionation processes as demonstrated by their dacitic to low- silica rhyolitic character and overall weak negative Eu-anomalies. Some particular element depletions (Ba, Sr and Ti) indicate fractional crystallisation, but may more likely be a compositional feature of the source materials (Fig. 2.13a, cf. Siégel et al., 2018).

Overall, the geochemical indicators are that the volcanics of the Gumbardo Formation along with other contemporaneous igneous units have transitional I- to A-Type affinities (Fig. 2.14b). The volcanics show similarities with the Silurian Ural Volcanics and Mount Hope Volcanics in the Cobar Basin (Central Lachlan Orogen, Bull et al., 2008), which have been interpreted as A-Types in an intracratonic setting (Fig. 2.14b). The identified A-Types exhibit generally lower SiO2 contents, but higher Al2O3, FeOT and ASI compared to the I- Types (Table 2.8). The degree of Silurian and/or Ordovician zircon inheritance is significantly higher for A-Types (47 – 60% cumulative) compared to the I-Type samples (24%) potentially indicating a higher degree of reworking of igneous crustal rocks in their petrogenesis (Table 2.8). The significant portions of inherited zircons indicate a strong crustal reworking signal, which may be isotopically primitive given relatively young (Ordovician-Silurian) inheritance ages and a lack of metasedimentary or older Precambrian crustal contributions.

Within the context of the Tasmanides of eastern Australia, a continuous belt of Silurian-Devonian granites widening from Cape York, across the Thomson Orogen into the Lachlan Orogen has been suggested (Henderson et al., 2013). The extent of this system in the Thomson Orogen is not constrained due to thick sedimentary cover sequences, and besides the exposed Retreat Batholith (Fig. 2.1) in the north-eastern Thomson Orogen, only few igneous rocks of Silurian-Devonian age have been intersected in drill holes to the east (AOP Balfour-1) and south (Milcarpa-1) of the Adavale Basin (Fig. 2.1). These rocks show transitional I- to A-Type affinities and plot in proximity to the volcanics of the Gumbardo Formation (Fig. 2.14a). The significant Silurian zircon inheritance identified in the Gumbardo Formation rocks indicates voluminous Silurian silicic igneous rocks beneath the Adavale Basin in the central Thomson Orogen.

The silicic volcanics of the Gumbardo Formation and contemporary granites and silicic volcanic rocks surrounding the Adavale Basin (Cooper et al., 1975; Roach et al., 2018; Siegel, 2015; Siegel et al., 2018; Withnall et al. 1995) have geochemical signatures transitional between “within-plate” and “convergent margin” fields on trace element discrimination diagrams (Fig. 2.14a), which is a feature of other rift-related and intraplate rhyolites (e.g., Bryan, 2007; Bull et al., 2008; Riley et al., 2001). Therefore, an intracratonic or a setting very remote from the active plate boundary is suggested at that time for the Adavale Basin during basin initiation in the Early Devonian.

2.2 CONCLUSIONS

The Gumbardo Formation is mainly comprised of ignimbrites and interbedded crystal-lithic sandstones reflecting reworking and resedimentation of the contemporaneous volcanic material. The welded ignimbrites are dominantly K-feldspar-phyric and lack mafic or hydrous phenocryst phases. Compositionally, the ignimbrites are dacitic to low- silica rhyolites. The ignimbrites appear to be relatively thin and generally lack abundant and large lithic fragments suggesting they are distal to their eruptive sources. A decline in the modal abundance of K-feldspar phenocrysts was observed up-section. The Gumbardo Formation volcanics unconformably overlie either Ordovician volcanic rocks or Silurian granites, which potentially had been exhumed during the Bindian Orogeny, and represent rocks of an age similar to that which was melted to form the ignimbrites

Zircon U-Pb dating of available volcanic rock units from the Gumbardo Formation define a relatively short-lived magmatic episode at ~398 Ma. Two SHRIMP ages from a previous study differ slightly (Draper, 2006), but one is within analytical uncertainty of the new ages obtained In this study. Significant Silurian zircon inheritance is identified for the volcanics and the recognition of this subtle inheritance is important, and may have contributed to the slightly older SHRIMP age of ~408 Ma (Cross et al., 2018) for an ignimbrite at the top of the formation, which is not fully consistent with stratigraphic constraints and the new, and carefully spatially resolved zircon age data presented here. Due to analytical resolution of the chosen approach, an inherited population at ~408 Ma cannot explicitly ruled out, but the age seems unlikely to represent an emplacement age of a volcanic rock of the Gumbardo Formation as other interpreted emplacement ages are

collectively younger for all analysed rocks. A previously included rock type encountered in the westernmost part of the basin (AOD Yongala-1) is now excluded from the Gumbardo Formation, as its emplacement age has been interpreted to be Early Ordovician/Late Cambrian. This provides evidence for a more restricted distribution to a NNE-SSW trending half-graben rift system within the Adavale Basin.

The geochronology study on the Gumbardo Formation volcanics has revealed significant proportions of inherited zircons with ages around the Devonian/Silurian boundary. The trace element geochemistry of the volcanic rocks shows similarities to granitic rocks west of the Adavale Basin, with emplacement ages around the Devonian/Silurian boundary. The rhyodacitic rocks of the Gumbardo Formation are interpreted as products of partial melting of mainly upper crustal rocks, such as those represented by plutonic rocks of Silurian age in the Thomson Orogen. The restricted zircon inheritance age range of the Gumbardo Formation records reworking of relatively young continental crust in the central section of the Thomson Orogen during rifting of the Adavale Basin, and a lack of involvement of the metasedimentary crust.

The tectono-magmatic affinities, syn-rift setting and generally undeformed nature of the Gumbardo Formation are consistent with a more intraplate/rift setting for basin development. In contrast to the coeval extensive rift phase in the Darling Basin in the adjacent Lachlan Orogen, the syn-rift volcanic phase in the Adavale Basin appears rather brief, indicated here by a short phase of volcanism and development of a half graben system. This suggests the Adavale Basin is a cover-type basin that developed as the Thomson Orogen became stabilised and magmatically inactive, following a major contractional deformation event in the late Silurian (Bindian Orogeny) that deformed the metasedimentary rocks and exhumed granitic rocks.

Chapter 3: Approach and Methodology

The approach and methods utilised to study the provenance of the sedimentary infill of Devonian basins in the Thomson (Adavale Basin, Chapter 4) and Lachlan orogens (Darling Basin, Chapter 5) are presented in this chapter. Sandstone petrography in combination with DZ (Section 3.1) and rutile geochronology (Section 3.2) form the baseline to investigate the sedimentary provenance in both basins. The obtained detrital rutile and zircon U-Pb geochronology datasets are complemented by comprehensive grain morphology data for each individual analysis (Section 3.1.1). Subsequently, the concept of modelling maximum depositional ages from DZ data is introduced in Section 3.1.2, followed by an abridgement of statistical treatment for DZ data (Sections 3.1.3, 3.1.4). Statistical sample comparison methods allow for detailed data investigation of large DZ geochronology datasets and are utilised in this study to better understand spatial and temporal trends in DZ provenance (Section 3.1.5).

3.1 DETRITAL U-PB ZIRCON GEOCHRONOLOGY

Heavy mineral separation for 15 samples from the Adavale Basin was undertaken by Geotrack International where ~2.0 – 2.5 kg of sampled rock material was crushed with a jaw crusher and then further processed in a disc pulveriser. Mineral grains were then further separated using Frantz isodynamic magnetic separation according to their magnetic susceptibility and gravity. Four samples from the Darling Basin were processed in a similar fashion at the Geological Survey of New South Wales in Maitland, NSW and in the Earth Sciences Department and Analytical and Biomolecular Research Facility (ABRF) laboratory at University of Newcastle, NSW.

U-Pb dating of zircon separated from 19 sedimentary rocks was performed using the ESI New Wave Laser and the Agilent 8800 Laser Ablation Inductively Coupled Plasma Mass Spectrometer (LA-ICP-MS) at the Central Analytical Research Facility (CARF) at Queensland University of Technology (QUT). Zircons were hand-picked from zircon concentrates, by picking all available grains regardless of preservation, morphology, size, degree of rounding and discolorations from a single sample aliquot before proceeding to

the next aliquot. This approach ensures to minimise the introduction of selection bias by, for example, only picking the most pristine grains which can ultimately lead to bias in the total DZ distribution (e.g. Markwitz and Kirkland, 2018; Sláma and Košler, 2012). About 200 zircons were mounted per sample and polished in three 1” epoxy mounts and were analysed in five sessions over 5 consecutive days for the samples from the Adavale Basin, and in one single session for samples from the Darling Basin. Detailed information on the mineral separation procedure and ICP-MS settings are given in Appendix 2.1. CL-imagery was utilised to identify the outermost domains in the DZ and laser ablation spots were placed in these areas.

The primary natural zircon standard Temora-2 (416.78 ± 0.33 Ma; Black et al., 2004) and the NIST 610 glass standard were used to calibrate U-Pb systematics, and trace element composition, respectively. Natural zircon standard Plešovice (337.13 ± 0.37 Ma; Sláma et al., 2008) was treated as an unknown and used to check accuracy of each analytical session (Table 3.1). Samples were analysed in a round-robin fashion, with a 25 second gas blank between individual ablations to determine background. Laser dwell time was 30 seconds at 2.0 J/cm2 at a rate of 7 Hz with a laser cell helium flow of 600 ml/min. Laser spot size was chosen consistently for all analysed unknowns and standards at 30 µm. Element concentrations of Si, P, Ti, Zr, Nb, La, Ce, Nd, Eu, Dy, Lu, Hf, Ta, 206Pb, 207Pb, 208Pb, Th and U were measured simultaneously for each analytical session. The major and trace element data are used to (i) determine U-Pb ages, (ii) for quality control purposes and (iii) integrated with final U-Pb ages to investigate temporal trends in trace element compositions and geochemical parameters calculated from the data. Data reduction was performed using Iolite (Paton et al., 2011), filtering and checks for concordance and trace-element compositions were performed in an internal excel spreadsheet.

Table 3.1. Summary weighted averages of secondary zircon standard “Plešovice” for all analytical sessions for detrital U-Pb zircon geochronology and analysed unknowns. Abbreviations: MSWD, mean squared weighted deviation; POF, probability of fit session date analysed samples 206Pb/238U zircon age ‘Plešovice’

3/09/2018 CAR-14, CAR-13, LOG-7, LOG-6, LOG-5 340.5 ± 1.6, MSWD=0.97, POF=0.53, n=53

4/09/2018 CAR-14, CAR-13, LOG-7, LOG-6, LOG-5 334.5 ± 3.2, MSWD=1.01, POF=0.44, n=21

5/09/2018 GUM-6, LOG-4, FAI-3 341.9 ± 2.0, MSWD=0.73, POF=0.91, n=45

6/09/2018 BUC-3, FAI-2, FAI-1, BUC-1 337.0 ± 1.4, MSWD=0.89, POF=0.72, n=61

7/09/2018 GUM-7, ALL-6, BUC-5 338.2 ± 1.7, MSWD=0.45, POF=0.99, n=44

26/03/2019 PAM-11, PON-21, MOS-18, EMU-20 336.5 ± 2.5, MSWD=0.47, POF=1.00, n=58

All DZ ages are reported as 206Pb/238U ages for zircons younger than 950 Ma and 207Pb/206Pb age for zircons older than 950 Ma, with all uncertainties are reported as propagated 2-sigma uncertainties. For quality control purposes, P, Ti and La are monitored in addition to transmitted light and cathodoluminescence (CL) images to identify accidental ablations of mineral inclusions in the targeted zircons (e.g., apatite and titanite). Analyses that yield concentrations above threshold values for either P >2000 ppm (indicative of apatite inclusions), Ti >120 ppm (indicative of titanite inclusions), La >10 ppm (due to the low distribution coefficient in zircon) or >2% common 206Pb are rejected. Analyses are treated as concordant if the 206Pb/238U age plus uncertainty is within the age range plus uncertainty of the respective 207Pb/235Th age, and the 207Pb/235Th 2-sigma uncertainty is <20% with respect to the 207Pb/235Th age.

Spot selection for laser ablation analysis has been conducted offline prior to the analytical session based on scaled CL-images for ~150 DZ grains per sample (Corfu et al., 2003). Ablation spots were selected in zircon domains that were free of melt inclusions, fractures and mineral inclusions. In order to obtain the age of the youngest magmatic addition to the zircon grains, spots were placed in domains that represent the outermost rim area of the zircon grains, often indicated by magmatic oscillatory zoning in CL-imagery. This approach is critical to consistently obtain U-Pb ages from zircon rim domains and aids in avoiding analyses of mixed age domains which might blur detrital age populations (cf. Chapter 2, geochronology of the volcanic rocks of the Gumbardo Formation). Assessment of the CL-imagery for the analysed 15 DZ samples shows that this approach is equally important for studies of DZ and zircon from igneous rocks (Fig. 3.1). DZ that are encountered in samples as rounded and sub-rounded show preferential abrasion on one side of the zircon grain with respect to the central domain of the grains, as indicated by oscillatory zoning (Fig. 3.1). Hence, spot selection solely based on transmitted and/or reflected light images will inevitably lead to ablation of mixed zircon domains resulting in acquisition of mixed U-Pb ages, which is an issue of larger magnitude when analysing primary igneous grains to constrain emplacement ages (Siegel et al., 2018). DZ inherently undergo mechanical transport prior to deposition potentially leading to grain abrasion, causing mechanical removal of the outermost growth rims from igneous zircons (Morton and Hallsworth, 1999). This already leads to analytical fuzziness causing subtly mixed U- Pb ages, the approach outlined above, however, minimises the effects of mixed ages in DZ geochronology studies (Siegel et al., 2018). In addition, grain morphologies can provide

insight into the extent of grain recycling, which can aid in determining involvement of recycled (meta)sedimentary zircon sources (Shaanan et al., 2018).

Figure 3.1. Series of three selected detrital zircon grains shown in CL (left) and transmitted light (right), respectively. Red circle (30 µm diameter) shows selected analytical spot selection for each individual grain in the outermost zircon domain.

3.1.1 Grain morphologies/measurements zircon/rutile

Integrating grain morphologies of DZ with U-Pb age data is a commonly used tool in provenance studies and combines information about degree of rounding, sphericity or aspect ratios of the analysed DZ with the isotopic age information for each analysis (e.g. Malusà et al., 2013; Naipauer et al., 2010; Shaanan et al., 2018; Shaanan and Rosenbaum, 2018; Stevens et al., 2010). Morphology assessments are commonly performed by manually tracing grain boundaries based on CL or transmitted light images of mounted zircons. Automated approaches utilising image analysis software are much less common in the literature (e.g. Markwitz and Kirkland, 2018), but provide a more objective reproducible and time efficient alternative to manual measurements or classification schemes.

For all analysed for DZ and rutile in this study, grain measurements have been carried out using a semi-automatic image analysis approach. Based on CL imagery for DZs and transmitted light images for detrital rutile, grains were digitized using colour threshold techniques as part of the object separation procedure in the free image analysis software JMicroVision (Roduit, 2008). This approach allows integration of grain size and morphology data with U-Pb geochronology data, which can be utilised to investigate grain morphologies for specific age populations, and ultimately give additional insight into the provenance of the DZ and rutile in addition to the obtained U-Pb ages alone.

As part of the developed procedure, stitched images (Fig. 3.2a) comprising the sample spot label for every analysed phase are initially imported into the software and transformed to the .tiff format to reduce image file size to enhance processing speeds of the object separation procedure. The scale bar from the imagery annotation was used to spatially reference the imagery for every sample. Object separation was performed using the colour threshold of the respective mineral phases, which shows high contrast to the dark background of the CL imagery for zircons (Fig. 3.2b), and vice versa, good contrast of the dark brown and often opaque rutile grains in contrast to the translucent resin puck. The threshold technique also excludes carbon filled cavities (e.g. air bubbles and other impurities in the resin mount), as these will reflect very bright in the CL imagery. For zircon and rutile image analysis, this leads to good results in the automatic delineation, with only minimal manual object extraction using the 2D measurement function in JMicroVision (Fig. 3.2c).

Once the automatic and manual measurements are integrated, the software provides morphological information for all analysed minerals, e.g. length of the long and short grain axis (Fig. 3.2c), as well as the area represented by the sectioned mineral grain. These data are then used to calculate further basic morphological parameters like aspect ratio, sphericity and ellipticity for all analysed grains. These data have subsequently been integrated with the individual geochronological data for each analysis and provides a powerful tool to assess morphological properties of selected age populations of DZ geochronology presented in Chapters 4 and 5.

3.1.2 Maximum depositional ages

DZ geochronology can aid in determining the depositional age of a sedimentary rock, especially where fossils are absent or biostratigraphic control is not yet established for a given unit (Gehrels, 2014). In theory, the age of the youngest DZ recovered from a sedimentary rock has to predate its deposition, or if sourced from a contemporary volcanic eruption, can have the same age as the sedimentary rock (Andersen, 2005; Fedo et al., 2003; Nelson, 2001). If a zircon grain with an isotopic age close to the depositional age is crystallised and also immediately deposited in a sedimentary system (short lag time), the isotopic zircon age allows for excellent constraints on the depositional age of the rock

Figure 3.2. Selected processing steps of the applied image analysis procedure for DZ and CL imagery in JMicroVision (Roduit, 2008). (A) Stitched input CL imagery in TIFF file format, (B) application of threshold technique to DZ grains, (C) object extraction and delineation of individual mineral phases and processing of morphology parameters.

(Cawood et al., 2012; Daniels et al., 2018). However, if the isotopic age of the youngest deposited zircon significantly predates the deposition of a sedimentary rock, the age of the youngest zircon can only serve as a maximum depositional age, i.e. the depositional age of the rock can be younger than the isotopic age of the zircon, but not older.

The usage of a single isotopic DZ age raises issues like the inherent lack of reproducibility of an individual isotopic age, or the risk of this one zircon grain being affected by lead loss, which results in an underestimation of the crystallisation age (Dickinson and Gehrels, 2009a). Additionally, contamination of DZ samples in the laboratory or field might introduce zircon grains from foreign sources (e.g. Coutts et al., 2019; Fedo et al., 2003). To overcome these difficulties arising from the use of a single

grain age, Dickinson and Gehrels (2009) introduced a series of measures merging multiple (young) isotopic ages in a given sample into a statistically more robust age population. The measures comprise age determination by the youngest peak age in a probability density plot (YPP), weighted averages of a series of individual zircon ages within uncertainty of the youngest individual isotopic age (YC1σ (2+), YC2σ (3+)), and an approach involving statistical resampling to build a robust age cluster (YDZ).

YC1σ (2+) and YC2σ (3+) yield statistically robust ages and are commonly utilised when a biostratigraphic framework is available in a sedimentary basin (Coutts et al., 2019). Ultimately, any population of aggregated young DZ ages depends on the reliability of the youngest individual isotopic age, as it constitutes the foundation of the youngest age cluster. This requires careful monitoring of all zircon trace element data available, especially for the youngest grains in a given sample in order to ensure the quality of those isotopic ages. Lead loss affects the isotopic age by lowering the true Pb concentrations in a sample and thereby makes it isotopically younger than it is (Gehrels et al., 2008; Horstwood et al., 2016). Pb loss can occur in response to structural damage of the crystal lattice, which can be monitored by calculating alpha doses from the isotopic age of the zircon and the respective U and Th contents using Equation 3.1 (Holland and Gottfried, 1955)

퐷 = 8 ∗ 238푈[exp(휆 238푈 ∗ 푡) − 1] + 7 ∗ 235푈[exp(휆235푈 ∗ 푡) − 1] + 6 ∗ 232푇ℎ[exp(휆232푇ℎ ∗ 푡) − 1]

(Equation 3.1)

Where D is the dose in alpha-decay events/mg; 238U, 235U and 232Th are the measured concentrations in atoms/mg; their respective decay constants 휆 238U, 휆 235U and 휆 232Th in years-1. Based on natural abundances in zircon, 235U is assumed to be (1/139)238U. In addition to alpha decay, Th/U ratios can be monitored as an indicator of presence of metamorphic fluids during zircon crystallisation, as well as high U zircons which have an elevated susceptibility to Pb loss (e.g. Rubatto et al., 2001; Williams, 2001).

The methodology YC1σ (2+) for YC2σ (3+) as outlined in Dickinson and Gehrels (2009) is lacking some specifications with respect to the treatment of isolated young grains. The following procedure regarding isolated young grains has been applied to the samples in this thesis to ensure robust statistics for the youngest detrital age cluster: If no grain dates

within the 2σ uncertainty (for calculating a YC2σ (3+)) of the youngest grain, the analysis is rejected regardless of the quality of the age. The procedure is likewise for the YC1σ (2+), if there is no grain within range of the 1σ uncertainty of the youngest individual analysis it is rejected. If only one analysis is dated within the 2σ/1σ uncertainty, the next youngest age forms the base of the YC2σ (3+), but the youngest analyses is still included in the population.

3.1.3 DZ and the issue of sample representativeness

The question of how many zircon grains need to analysed in a sample to achieve a required or desired level of statistical adequacy is debated in the literature (Dodson et al., 1988; Vermeesch, 2004). Ultimately, the “right” number of analysed grains depends on the purpose or goal of a study and the nature of the total DZ distribution (Vermeesch, 2004). Unless there are previous studies serving as a basis for sample selection, the total DZ distribution of a yet to be analysed sample is unknown and cannot be predicted, making it difficult to anticipate the DZ distribution prior to the conducted analysis. For the sedimentary rock units in the Adavale Basin, no prior DZ geochronology was available, and the DZ age distributions and their proportions were unknown. For samples from the Darling Basin, DZ data were available serving as a guideline for the conducted analysis (Barry, 2016), but the total number of concordant analyses was relatively low (44 – 67).

For the 15 samples from the Adavale Basin, ~150 grains were targeted per sample, in anticipation of acquiring at least 117 concordant, high-quality analyses per sample, passing the quality control criteria outlined in Section 3.1. The relatively high number of 150 analysed grains leaves room for ~20% of discordant or low-quality analyses (i.e. misplaced analytical spots or ablation of inclusions). Assuming the worst case scenario, a uniform age distribution where each age fraction is of the same size, this approach ensures that no population > 5% of the total DZ distribution is missed with a confidence of 95% (given a total of n=117 per sample is achieved, Vermeesch, 2004). The maximum probability of missing a population >5% drops to 0.9% using 117 analyses when considering a more realistically distributed DZ sample (e.g., Nubian Sandstone in Vermeesch, 2004).

3.1.4 Sample Illustration: Histogram, KDE, PDP and density plots

Several approaches to visualize the distribution of DZ ages in a given sample can be employed to interrogate U-Pb geochronology data. The simplest form of visualisation is a histogram with a chosen bin width for the isotopic ages on the y-axis and the absolute abundance of individual ages for each bin on the y-axis (Fig. 3.3a). More sophisticated visualisation methods, such as Probability Density Plot (PDP, Fig. 3.b) and Kernel Density Estimation (KDE, Fig. 3.3c,d) attempt to estimate the relative likelihood of the different ages in the population using the Probability Density Function (PDF, Vermeesch, 2012). Both approaches produce continuous curves by stacking a normal distribution for each measurement, with the isotopic ages on the x-axis and the relative probability for every age continuously on the y-axis.

The major difference between PDP and KDE is the determination of the bandwidth, which ultimately dictates the relative likelihood of the PDF. For the PDP, the bandwidth is controlled by the analytical uncertainty of each individual measurement, whereas the bandwidth of the KDE is determined by the data density. For the KDE, the bandwidth can either be selected constantly across the entire range of ages (‘kernel density estimation’, Fig. 3.3c) or variable according to the local density of individual ages (‘adaptive kernel density estimation’, Fig. 3.3d, Vermeesch, 2012). Both methods are integrated in the Java- based DensityPlotter program (Vermeesch, 2012). A major criticism of the PDP is the effect of oversmoothing the distribution, especially for older ages > 1.0 Ga, as the absolute analytical uncertainties are inherently larger for those older ages and tend to oversmooth the PDP (see Fig. 3.3b and compare e.g. to Fig. 3.3d). KDE plotted in this thesis are preferably displayed as adaptive kernel density estimation as they consider the local density of the detrital age data, which allows a more precise determination of age clusters in areas of high data density. On the other hand, the adaptive kernel density estimations (Fig. 3.3d) do not underestimate the bandwidth for areas of low data density (see Fig. 3.3c) without oversmoothing the low data density areas like the PDP does (Fig. 3.3b).

Figure 3.3. Four examples for illustrating an individual DZ geochronology sample using the default settings in DensityPlotter (Vermeesch 2012). (A) Histogram showing the absolute number of ages for individual bins, (B) PDP illustrating the probability density and highlighting the effect of oversmoothing for older DZ grains ( 1Ga), (C) KDE showing the effect of underestimation of the KDE bandwidth especially for older grains ( 1Ga) as opposed to the locally adaptive KDE shown in (D).

In addition to KDE, density plots are utilised in this thesis specifically to illustrate and assess trends in DZ populations between samples with respect to the depositional age (e.g. Wissink et al., 2018, Fig. 4.18 of Chapter 4). The density plots can either be used to simply illustrate the obtained data, or in combination with a statistical resampling approach, be used to normalise samples with deviating number of analyses. Assessing temporal trends by vertically stacking KDE or PDP (e.g. Gehrels et al., 2011; Ireland et al., 1998; Wissink et al., 2018) can be challenging as the proportions of individual detrital age populations are displayed in the y-dimension, which is also utilised to illustrate the temporal trends by stacking multiple curves. The proportions of individual detrital populations in the density plots are colour coded by a heatmap and the y-dimension is reserved to illustrate the temporal trends between samples and the potential changes in DZ populations by means of their proportions.

3.1.5 Sample comparison methods

Sample comparison tools provide objective insight into the similarity or dissimilarity of DZ distributions and provide robust statistical assessment in addition to more subjective visual comparison methods (Vermeesch 2013, Saylor and Sundell, 2016). A high- frequency sampling across the stratigraphy of a sedimentary package in combination with depositional age constraints independent from MDA (i.e. biostratigraphy) can give insight into not only temporal, but also spatial trends in sedimentary systems. Comparison based on the visual assessment of KDE or PDP plots can be subjective or even overwhelming if a lot of samples are being compared. A variety of different statistical methods is available and their suitability for comparing DZ samples has been tested in Saylor and Sundell (2016).

The Kolmogorov-Smirnov (K-S) and Kuiper Tests evaluate the null hypothesis that the two samples chosen for comparison are drawn from the same parent population and that the variation between the samples is within the expected variation for randomly drawn samples (Saylor and Sundell, 2016). The two tests are based on the cumulative distributions of two given samples (Fig. 3.4d), and respectively feature differing sensitivities for similarities/dissimilarities within the distribution; The K-S test is more sensitive around the median value and exhibits low sensitivity for the extreme ends of the distributions (i.e. for very old and very young ages). The Kuiper test is a modification of the K-S test and equally sensitive for all sections of the age distribution (Kuiper, 1960). A major misunderstanding regarding the usage of p-values as a similarity measure for K-S and Kuiper tests exists in the DZ community, as these are unsuitable to measure similarity for DZ samples with n1000 leading to excessive rejection of the null hypothesis (Vermeesch 2013, Saylor and Sundell 2016). D-values for K-S and V-values for Kuiper tests depict an appropriate measure for sample similarity and should instead be utilised for sample comparison (Vermeesch 2013, Saylor and Sundell 2016).

The Similarity Coefficient is based on the comparison of either a pair of KDE or PDP and assesses overlapping modes and proportions of individual age populations (Fig. 3.4a; Gehrels, 2000). Sample similarity using Likeness is assessed by the complement of the area mismatch of two KDE or PDP (Fig. 3.4c; Satkoski et al., 2013). Both methods show only minimal variation when comparing samples and do not use the full range of coefficients

from 0 to 1 (Saylor and Sundell, 2016a). The Cross Correlation Coefficient (Saylor et al., 2013) assesses the degree of similarity between samples based on either KDE or PDP. The method complements the Similarity coefficient by not only assessing the modes and the proportions of age populations, but also considering the absence of age populations and the shape of age peaks. In contrast to the aforementioned Similarity and Likeness methods, the Cross-correlation coefficient utilises the full range of coefficients and is suitable to compare samples for their similarities but also dissimilarities. All methods introduced above are implemented in DZstats, a MATLAB-based graphical user interface, which is used to calculate comparison matrices (Saylor and Sundell, 2016).

Figure 3.4. Schematic illustrations of different sample comparison methods using KDEs (A,B,C) and cumulative distributions (D) of a simplified synthetic DZ sample. Respective coefficient for each method shown in grey box (0, no similarities; 1, high similarity), Blue and red arrowheads for A and B indicate height of the modes is assessed for these methods, as oppose to C. Similarity (A, Gehrels, 2000) assessing overlapping modes and respective proportions, Cross Correlation Coefficient (B, Saylor et al., 2013) rating modes, absent populations, proportions and shape of the kernel. Likeness (C, Satkoski et al., 2013) as a complement of the area mismatch is focussed on the percentage of the KDE-overlap, neglecting position and proportions of modes. K-S test (D, Kuiper, 1960) utilises the cumulative age distribution for sample comparison (D-value given in grey box).

The introduced Similarity coefficient, Similarity and Cross-correlation coefficient imply that the compared samples are representative of the total DZ distribution (i.e. all DZ ages in a given rock sample). However, none of the methods assesses the representativity of a sample as part of the method based on the concepts in Section 3.1.3. Low numbers of

n often result in high uncertainties regarding the proportions of individual age peaks (Andersen, 2005), which are ultimately one of the criteria to assess sample similarity. The Bayesian Population Correlation (Tye et al., 2019) assesses the sample representativity by subsampling each DZ sample and provides Probability Model Ensembles (PME, Fig. 3.5). Bayesian Population Correlation then evaluates the similarity of two samples considering the parent population inference reflected by PMEs. As a result, each sample comparison is endowed with an uncertainty, reflective of the representativity of the analysed samples. The computationally intense BPC tool is implemented in a MATLAB-based graphical user interface (Tye et al., 2019).

All of the methods discussed above facilitate sample comparison by means of a sample pair, but becomes problematic when attempting to compare multiple sample pairs, and requires further processing in order to analyse and properly compare a large dataset. Multidimensional scaling (MDS) is a superset of the principal component analysis and is a dimension-reducing method that can display multiple samples and their similarity or dissimilarity in a two-dimensional plot (Vermeesch, 2013, Fig. 4.34 of Chapter 4). The similarity of two samples is expressed as the Euclidian distance between two sample points, i.e. the closer two points plot together, the more similar are the DZ distributions, and vice versa. Synthetic normally distributed age populations in MDS plots are a helpful addition to give orientation in these otherwise dimensionless plots (e.g. Matthews et al., 2017). The

Figure 3.5. Probability Model Ensemble (PME) output as part of the Bayesian Population Correlation (BPC) procedure in Tye et al. (2019), assessing the uncertainties of DZ population proportions. Colour legend illustrates concentration of Probability Density Function (PDF) from retained models, red curve shows Kernel Density Estimate (KDE). P-value on Y-axis depicts relative probability, colour bar to the right shows concentration of the PDF curves as a result of the resampling procedure (maximum of 10000 models).

MDS plots are constructed using the browser based online version of IsoplotR (Vermeesch, 2018), which translates the D-values of the above introduced K-S statistic into Euclidean distances in the plot (Vermeesch 2013).

3.2 DETRITAL U-PB RUTILE GEOCHRONOLOGY

Heavy mineral separation for rutile followed the same procedures for zircon (Section 3.1.; Appendix 2.1). U-Pb dating of rutiles separated (according to the outlined workflow in Section 3.1) from 10 sedimentary rocks from the Adavale Basin and 4 sedimentary rocks from the Darling Basin was performed using the ESI New Wave Laser and the Agilent 8800 Inductively Coupled Plasma Mass Spectrometer (LA-ICP-MS) at the Central Analytics Research Facility (CARF) at Queensland University of Technology (QUT).

Rutile grains were hand-picked from two different mineral separates, zircon concentrates and the zircon fraction. The zircon fraction comprises slightly magnetic grains and contained the majority of detrital rutile for the examined samples. In contrast to zircon, the abundances of rutile grains is very low in all mineral separates, hence all available rutile have been picked and mounted (23-51 grains). Rutiles were mounted and polished in three 1” epoxy mounts and were analysed in two sessions over two days. Transmitted light and SEM images were taken to characterise morphologies and heterogeneities of the mounted rutiles prior to analysis.

The primary natural rutile standard Wodgina and the NIST 610 glass standard were used to calibrate U-Pb systematics, and trace element composition, respectively. Natural rutile standards R10b (1090.0 ± 5.0 Ma, Luvizotto and Zack, 2009), R13 (504.0 ± 4.0 Ma, Schmitt and Zack, 2012) and R210 (1085.0 ± 14.0 Ma, Hartwein, 2014) were treated as unknowns and used to check accuracy of each analytical session. The results for the secondary standards are reported as concordia ages in Fig. 3.6, and show satisfactory accuracy for the two analytical sessions.

Samples were analysed in a round-robin fashion, with a 25 second gas blank between individual ablations to determine background. Laser dwell time was 30 seconds at 1.6 J/cm2 at a rate of 6 Hz with a laser cell helium flow of 600 ml/min. Laser spot size was chosen

consistently for all analysed unknowns and standards at 50 µm. Element concentration for Al, Ti, V, Cr, Mn, Fe, Cu, Zr, Nb, Ta, 206Pb, 207Pb, 208Pb, Th and U are analysed simultaneously and used for (1) U-Pb age assignment, (2) quality control of individual analyses and (3) assessment of rutile chemistry for source lithology classification (Meinhold, 2010) and calculation of Zr-in-rutile thermometry to constrain crystallisation temperatures (Watson et al., 2006). Data reduction was performed using Iolite (Paton et al., 2011), filtering and checks for concordance and trace-element compositions were performed in an excel spreadsheet. Major element monitoring encompasses a threshold of >10,000 ppm Fe and <15,000 counts per second of Ti to identify accidental ablations of

Figure 3.6. Wetherill-concordia plots with concordia ages of secondary rutile standards R13, R210 and R10b for two analytical sessions on 13.09.2018 (left) and 27.03.2019 (right).

iron-oxides (e.g., Ilmenite). Rutile with U contents < 2 ppm are generally rejected as these analyses yield extremely high individual uncertainties.

3.3 SUMMARY

The methods introduced in this Chapter are used in the following Chapters 4 and 5 to investigate sediment provenance in the Adavale and Darling basins. For DZ geochronology interpretation, assessment of age frequency distributions is preferably based on the locally adaptive KDE, while the representativity of samples is visually assessed using the Probability Model Ensemble (PME). The KDE then feeds into sample comparison methods to assess the similarity or dissimilarity of DZ samples by means of the pairwise sample comparison (Section 3.1.5). Zircon chemistry is used initially to assess data quality of individual ages, which is especially important for the youngest DZ ages in a given sample, as the youngest ages form the baseline for assigning MDA to a sample. Zircon chemistry is then utilised to chemically fingerprint DZ age populations. In addition, DZ morphology data are integrated to gain insight into aspects of DZ recycling or general igneous source rock properties (i.e. plutonic vs. volcanic sources). A subset of the methods is also applied to the detrital rutile geochronology data, integrating rutile grain morphology data with chemistry data to investigate rutile provenance. The compilation in Table 3.2 summarizes the applications of those methods for the detrital rutile and zircon geochronology.

Table 3.2. Methods introduced in this Chapter and their application in detrital mineral provenance analysis in the following Chapters 4 and 5.

Method/Attribute Approach Data analysis tools/Parameters Zircon U-Pb geochronology Assessment of frequency distributions KDE, PDP, Density Plot, Tera-Wasserburg Plots Sample representativity PME Maximum depositional ages YSG, YC1σ (2+), YC2σ (3+) e.g. K-S test, Bayesian Population Correlation, Cross Sample comparison methods Correlation Coefficient Zircon chemistry Data quality control Alpha dose, Th/U, La, P, Ti Fingerprinting DZ for provenance analysis e.g. Th/U, Zr/Hf, REE vs. P, TZircTi Zircon grain internal features Spot selection, zircon origin CL-imaging Zircon grain morphologies Assessment of source lithologies and recycling Grain size, aspect ratio, sphericity

KDE, PDP, Tera-Wasserburg Plots, discordia model Rutile U-Pb geochronology Assessment of frequency distributions ages Rutile chemistry Data quality control Ti, Fe, U

Fingerprinting source rocks Nb/Cr, Zr-in-rutile thermometry Rutile grain morphologies Assessment of recycling Grain size, sphericity

Chapter 4: Sediment Provenance of the Adavale Basin

4.1 INTRODUCTORY STATEMENT

The tectonomagmatic affinities of the basal volcanic rocks of the Gumbardo Formation are indicative of a rift basin in an intracratonic setting, implying that the subsequent sedimentary infill of the Adavale Basin reflects a post-orogenic basin exhibiting characteristics of a platform basin (Chapter 2). This basin type is generally characterised by relatively undeformed sedimentary infill that has not undergone major metamorphism and is dominated by craton-derived sediment, indicative of an established widespread sediment dispersal system supplying sediment to the basin (Allen and Armitage, 2012; Burgess, 2019; Cawood et al., 2013; Folk, 1980; Sleep et al., 1980).

The Devonian Adavale Basin occupies a central position within the Thomson Orogen and has been identified as a transitional basin type, recording both, the end of the orogenic phase of the Thomson Orogen and the transformation of the terrane into a tectonically stable craton (Doutch and Nicholas, 1978). The sedimentary infill of the Adavale Basin is predominantly terrestrial, shows only subordinate marine influence, and exhibits block faulting and gentle folding (Draper et al., 2004; McKillop et al., 2007; Passmore and Sexton, 1984). The lines of evidence regarding the role of the Adavale Basin as a transitional basin, however, are rather conceptual and based on large scale observations and interpretations. No comprehensive study of the composition of the sedimentary rocks and their provenance has been undertaken to identify sources of the terrestrial sedimentary infill and its importance in the context of the stabilisation of the Thomson Orogen in this subsurface basin.

This chapter focusses on the sedimentary formations to characterise the nature of the basin fill and assesses overall compositional maturity of sediment. Potential lateral and temporal trends in sediment characteristics are investigated. A detrital geochronology provenance study in conjunction with a review of source rocks across the Tasmanides can give insight into the extent of the sediment dispersal system during the Devonian. DZ U-Pb geochronology is used to identify primary igneous and reworked (meta)sedimentary source rocks and the approach is complemented by detrital rutile U-Pb dating to identify potential

contributions from medium to high-grade metamorphic rocks. This approach aims to test whether the sedimentation style is widespread as expected for a platform basin (Allen and Armitage, 2012; Burgess, 2019; Cawood et al., 2013; Folk, 1980; Sleep et al., 1980). Sandstone compositions for a continental platform basin would be expected to be compositionally and texturally mature sediments reflective of a widespread catchment and predominantly craton-derived sediments (Dickinson, 1985; Dickinson and Suczek, 1979; Dickinson et al., 1983). DZ and rutile U-Pb ages for such a basin-type would be expected to reflect reworking of old cratonic or basement material, exhibiting typical Australian DZ and rutile frequency distributions (i.e. ‘Pacific Gondwana’ or ‘Syn-Peterman’ signatures, e.g. Fergusson et al., 2001; Ireland et al., 1998; Purdy et al., 2016b; Siegel et al., 2017).

The obtained data from the Adavale Basin then serve as a baseline for comparison with the Devonian Darling Basin in the Lachlan Orogen in the next chapter (Chapter 5) and utilised to test connectivity of relict sedimentary systems over large distances (~500 km).

4.2 GEOLOGIC BACKGROUND

The sedimentary fill of the Adavale Basin has been divided into seven formations, based on wireline-log interpretations and lithofacies interpretation from drill core material (Fig. 4.1, McKillop et al., 2007). Biostratigraphic age control is constrained from palynology assemblages, establishing depositional ages for the Eastwood beds (Assemblage I), Log Creek Formation, Lissoy Sandstone (Assemblage II) and Etonvale Formation (Assemblage III) across multiple locations in the basin (Table 4.1, Hashemi & Playford, 2005). The bulk of the sedimentary basin infill (3–4 km) comprises fluvial siliciclastic sedimentary rocks (Eastwood Beds, Etonvale Formation, Lissoy Sandstone and Buckabie Formation) with shallow marine to marine units mainly present in the eastern parts of the basin (Log Creek Formation, Bury Limestone). Significant disconformities are thought to have developed during the Eifelian between the Eastwood Beds and the Log Creek Formation, and during the mid-late Givetian between the Boree Salt Member and the Etonvale Formation. The disconformities are interpreted on the basis of absent palynological assemblages (Fergusson et al., 2013; Hashemi and Playford, 2005; McKillop et al., 2007) but lack conclusive evidence from seismic data (Passmore and Sexton, 1984) or contact relationships in drill core.

Onset of siliciclastic sedimentation in the Adavale Basin is marked by the Eastwood Beds, interpreted to be a fluvio-deltaic sandstone succession, exclusive to the north-eastern part of the basin (Boreham and De Boer, 1998; Draper et al., 2004). Deposition of the subsequent Log Creek Formation marks a marine transgression initially affecting the entire basin, but marine conditions persisted only in the eastern basin (Bury Limestone), whereas the western areas became more terrestrial (Lissoy Sandstone) during the Middle Devonian. In the mid-Givetian, widespread deposition of the marine Cooladdi Dolomite occurred in the basin and deposition of the Boree Salt Member in the eastern basin, indicative of arid and evaporitic conditions (De Boer, 1996). After a late Givetian (palynology) hiatus, depositional environments became terrestrial throughout the entire Adavale Basin, marked by deposition of the Etonvale (continental red-bed facies of interbedded sandstone and

Figure 4.1. Updated stratigraphy of the Adavale Basin after McKillop et al. (2007), stratigraphic position of biostratigraphic assemblages I, II and III revised based on geological time chart of Gradstein et al. (2012). Age constraints for the Eastwood Beds, Log Creek Formation, Lissoy Sandstone and Etonvale Formation based in biostratigraphic assemblages (Hashemi & Playford 2007), age of the Gumbardo Formation based on U-Pb zircon SHRIMP ages (Draper, 2006).

shale) and Buckabie Formations (coarse-grained, high-energy sandstones, Remus and Tindale, 1988). Most sedimentary units show a widespread distribution across the Adavale Basin with relatively consistent unit thickness (Fig. 4.2), which suggests overall sheet-like basin geometries. At the same time, isolated synclinal throughs with varying interpreted sediment thickness are observed in the Quilpie (~3,000 m), Cooladdi (~9,500 m) and Westgate Trough (~3,000 m) in the southern part of the Adavale Basin (Draper et al., 2004; Passmore and Sexton, 1984). A detailed summary of the five terrestrial sedimentary units in the Adavale Basin is provided in the following section.

Table 4.1. Sample locations and sample depths for palynology analysis, extracted from Hashemi and Playford (2005). For drill hole locations, see Fig. 4.3.

Assemblage Unit Drill hole Sampled depth [m] Core # I Eastwood Beds BEA Allandale-1 2339.4/2338.5 7 I Eastwood Beds PPC Carlow-1 2255.5 12 I Eastwood Beds AOP Ravensbourne-1 1934.4 3 II Log Creek Formation PPC Carlow-1 2154 11 II Lissoy Sandstone PPC Carlow-1 2050.1 10 II Log Creek Formation PPC Etonvale-1 2928.9/2572.8 23/18 II Lissoy Sandstone PPC Etonvale-1 2523.2/2455.6 17/16 II Bury Limestone PPC Bury-1 2561.9 14 III Etonvale Formation AOP Boree-1 1455.9 10 III Etonvale Formation PPC Bury-1 1661.3 5 III Etonvale Formation PPC Stafford-1 2128.3 8 III Etonvale Formation PPC Etonvale-1 2133.4 13

4.2.1 Eastwood Beds This unit was named by Paten (1977) but was first described by Logan (1977) based on the first intersection of this unit in drill hole PPC Carlow-1. The Eastwood Beds comprise a restricted set of lithologies being dominated by grey to grey-green carbonaceous, fine to very fine-grained feldspathic sandstones that are interbedded and interlaminated with black mudstone (Appendix 4.1, Logan, 1977). Fine plant detritus preserved as carbon films are present in the mudstone and sand layers (Logan, 1977), and signs of bioturbation are apparent from a section in PPC Carlow-1 (Appendix 4.1). Based on wireline-log interpretations, sandstones of the Eastwood Beds are intersected in seven wells limited to the northeastern sector of the Adavale Basin, with a maximum formational thickness in PPC Carlow-1 (1227 m) and a minimum of 32 m in EAL Brynderwin-1 (McKillop et al., 2007). Cored intervals are present in three wells with a cumulative 31.3

m of cored material available (Table 4.2, based on well completion reports, core inspections and data from Geological Survey of Queensland, DNRME, 2014).

The Eastwood Beds are disconformably overlain by the Lissoy Sandstone in three wells, the Cooladdi Dolomite in one well and sedimentary rocks of the Permian-Triassic Galilee Basin in three wells. The stratigraphically overlying Log Creek Formation is absent in all wells where the Eastwood Beds are intersected, suggesting these units could potentially be lateral equivalents. The Eastwood Beds overlie the Gumbardo Formation in four wells, and unconformably overlie metasedimentary basement rocks of the Thomson Orogen in two wells (Geological Survey of Queensland, DNRME, 2014). The depositional age of the formation is constrained from palynoflora Assemblage I, and a Late Emsian age was assigned to the unit (Hashemi and Playford, 2005). This is substantiated by the new U- Pb ages of the underlying Gumbardo Formation, which yields a pooled emplacement age of ~398 Ma (Mid Emsian, Chapter 2, Asmussen et al., 2018). A disconformity is interpreted between the Eastwood Beds and the overlying Log Creek Formation based on the general absence of Early Eifelian palynoflora in sedimentary rocks of in the Adavale Basin, but remains uncertain due to sampling gaps in the core material that potentially could contain palynomorphs from this stage (Hashemi and Playford, 2005; McKillop et al., 2007) and a tentative assignment of an Eifelian palynoflora in an earlier study of the unit (McGregor and Playford, 1993). The depositional environment of the unit is deltaic to fluvial-lacustrine based on the absence of tidal structures and marine fossils (Paten, 1977). This interpretation, however, is based on lithological observations of limited cored material from three wells.

4.2.2 Log Creek Formation Rocks of the Log Creek Formation were previously included in an all-encompassing Etonvale Formation, which comprised all Middle to early Late Devonian units in the Adavale Basin and subdivided into D1/2/3/4 (Heikkila, 1965, McKillop et al., 2007). Based on the interpretation of an unconformity within this Etonvale Formation, the lowermost, unit D4, was separated out and named the Gilmore Formation (Tanner, 1968), and later renamed into Log Creek Formation (Galloway, 1970). In the western part of the Adavale Basin, Log Creek Formation is dominated by siliciclastic sedimentary rocks and merges into a marine facies in the southeast (Webb and Shaw, 1996). The alluvial facies comprises

coarse-grained feldspathic, lithic and quartz sandstones interbedded with silt- and mudstones, thin conglomerates and minor limestone (Appendix 4.1, De Boer, 1996). Fossiliferous marine shale and minor limestone are characteristic for the marine facies of the unit (Appendix 4.1, Paten, 1977).

Rocks of the Log Creek Formation are widespread across the central and southern part of the Adavale Basin (Fig. 4.2), yet are not apparent in the northeastern area and reported as absent where the stratigraphically underlying Eastwood Beds are intersected (McKillop et al., 2007). This might suggest the Log Creek Formation, and Eastwood Beds are complementary, however, the two units were assigned to different palynological assemblages. Thus they are not temporally equivalent based on the biostratigraphic data (Hashemi and Playford, 2005). The formational thickness of the unit varies from 36.8 m in PPC Leopardwood-1 to 694.5 m in PPC Etonvale-1 (Table 4.2), and the high variability has been interpreted as the result of differential subsidence and unstable basement tectonics during deposition (Galloway, 1970). The thickest intervals have been intersected in the area of the Cooladdi Trough where the Log Creek Formation comprises a lower sandstone unit and an upper marine unit, yet the unit is not entirely intersected in these wells, and thus only a minimum thickness is constrained here (PPC Quilberry-1, 1332.0 m; PPC Dartmouth-1, 755.3 m; Geological Survey of Queensland, DNRME, 2014). Evidence for marine facies also exists in the central part of the basin where marine fauna are present in PPC Log Creek-1, PPC Etonvale-1 (Day and McKellar, 1962) and PPC Gilmore-1 (Boreham and De Boer, 1998). Overall, rocks of the Log Creek Formation are cored in 9 wells with a cumulative 149.5 m of cored material (based on well completion reports, core inspections and data from Geological Survey).

The Log Creek Formation overlies the Gumbardo Formation, evident from wells where the unit is intersected completely. Rocks of the Log Creek Formation are conformably overlain by the Lissoy Sandstone in the central part of the basin and by the Bury Limestone in the eastern Adavale Basin (McKillop et al., 2007). The unit onlaps basement in the western part of the basin (Paten, 1977). Palynoflora of the Log Creek Formation have been assigned to Assemblage II together with the overlying Lissoy Sandstone and Cooladdi Dolomite, suggesting a mid-late Eifelian to earliest Givetian age for the Log Creek Formation (Hashemi and Playford, 2005; McKillop et al., 2007). Boreham and De Boer (1998) interpreted a full regressive cycle represented by rocks of the

Log Creek Formation in the central part of the Adavale Basin, composed of a sequence of marine sedimentary rocks followed by shallow marine sandstones and grading upwards into alluvial fan deposits in PPC Gilmore-1. Paten (1977) suggested an overall southward migrating delta system succeeding the deltaic Eastwood Beds.

4.2.3 Lissoy Sandstone The Lissoy Sandstone was defined by Paten (1977) and is made up of predominantly fine to coarse-grained sandstones, subordinate siltstones and conglomerates with dolomitic, argillaceous and minor siliceous cement. The sandstones are thinly bedded and show laminated beds, cross-bedding, soft-sediment deformation and occasionally rip-up clasts (Appendix 4.1, McKillop et al., 2007). The Lissoy Sandstone is a reservoir for the Gilmore Gas Field exhibiting average porosities of 12% and an average permeability of 136 md (Webb and Shaw, 1996). Rocks of the Lissoy Sandstone are intersected in a narrow NNE- SSW trending corridor along the central Adavale Basin where the unit is completely intersected in 13 wells (Fig. 4.2, McKillop et al., 2007). Webb and Shaw (1996) suggested a more widespread distribution to the east and additionally identified the unit in HEP Alva- 1, PPC Stafford-1 and PPC Bury-1. Minimum formational thickness is apparent in the northeastern (14.4 m in ASO Fairlea-1), and southwestern (15.0 m in IOD Paradise-1) margins of the basin, whereas the maximum thickness is observed in the central part of the Adavale Basin (AGA Phfarlet-1, 263 m; PPC Etonvale-1, 165.7 m). For the remaining wells, formation thickness ranges between 20 and 80 m (Geological Survey of Queensland, DNRME, 2014). Rocks of the Lissoy Sandstone are cored in 9 wells with a cumulative 88.5 m of core available (Table 4.2, based on well completion reports, core inspections and data from Geological Survey of Queensland, DNRME, 2014).

Based on different palynoflora assemblages, the Lissoy Sandstone (Assemblage II) disconformably overlies the Eastwood Beds (Assemblage I) in the northern part of the basin, and conformably overlies the Log Creek Formation (Assemblage II) in the remaining areas of the basin. The Lissoy Sandstone is conformably overlain by the Cooladdi Dolomite and merges laterally with the Bury Limestone to the east (McKillop et al., 2007). The Lissoy Sandstone, together with the underlying Log Creek Formation and the overlying Cooladdi Dolomite, have been assigned to the late Eifelian to early Givetian palynology Assemblage II. The relative ages of these units can only be constrained from their

stratigraphic relationships, and the age intervals have been assigned arbitrarily based on the stratigraphy (McKillop et al., 2007). The Lissoy Sandstone has been interpreted as the continental to marginal marine counterpart of the contemporary Bury Limestone (Paten, 1977). The marine influence of this unit is further supported by findings of microplankton cysts (Hashemi, 1997). Algal laminates and decreasing bioturbation in the upper sections of the Lissoy Sandstone, however, suggest marine conditions became restricted at some point (de Boer 1996).

4.2.4 Etonvale Formation The Etonvale Formation initially encompassed four members, D1-4 (Heikkila, 1965), whereas D3 is now assigned to the Cooladdi Dolomite and D4 to the sandstone member of the Log Creek Formation. The Etonvale Formation now comprises a lower sandstone member (D2) and an upper shale and siltstone member (D1). Rocks of the lower D2 member comprise quartz sandstones, whereas D1 rocks comprise mudstone and shale (Appendix 4.1, McKillop et al., 2007). Rocks of the Etonvale Formation are predominantly intersected along the main depocentre mostly west of the Warrego Fault with a formational thickness ranging between 200 and 450 m for the majority of wells (Fig. 4.2). Minimum thickness is observed in AAR Mount Morris-1 (40.4 m) in the southeastern part of the basin, east of the Warrego Fault, whereas the maximum thickness is intersected in PPC Stafford-1 (607.3m) just west of the Warrego Fault. Rocks of the Etonvale Formation are cored in 13 wells across the Adavale Basin with a cumulative 76.4 m of core available (Table 4.2, based on well completion reports, core inspections and data from Geological Survey of Queensland, DNRME, 2014). Rocks of the Etonvale Formation are also intersected in wells intersecting the Warrabin Trough west of the Adavale Basin (Pinchin and Senior, 1982).

Due to its wide distribution in the Adavale Basin, the Etonvale Formation overlies several units, reflecting the lateral variation of the underlying Middle Devonian strata and expanse of the Adavale Basin over the course of the Devonian. The unit disconformably overlies the Cooladdi Dolomite in the majority of wells along the main depocentre, halite rocks of the Boree Salt in the central-eastern basin and the Bury Limestone in the southeastern Adavale Basin. In the northern part of the basin, the Etonvale Formation disconformably overlies the Lissoy Sandstone in one well. Rocks of the Etonvale

Formation are conformably overlain by the Buckabie Formation in the majority of wells, and in some wells in the eastern basin, by rocks of the Galilee and Eromanga Basins (Geological Survey of Queensland, DNRME, 2014; McKillop et al., 2007). A latest Givetian/Early Frasnian age for the Etonvale Formation is indicated by palynology Assemblage III and is further supported by dating of conodonts, corals and brachiopods (Hashemi and Playford, 2005). A Givetian hiatus prior to deposition of the Etonvale Formation is interpreted based on the absence of mid Givetian palynofloras in the Adavale Basin (McKillop et al., 2007). The unit constitutes a continental red bed facies with interbedded sandstones and shales (Remus and Tindale, 1988) and is interpreted to reflect an arid fluvial to marginal-marine depositional environment (McKillop et al., 2007).

4.2.5 Buckabie Formation The Buckabie Formation was introduced by Tanner (1962) and comprises quartz sandstone, shale and conglomerate with dolomitic, calcareous and red clay cement (McKillop et al., 2007). The formation is intersected in 19 wells west of the Warrego Fault, and is absent east of the Warrego Fault and in the northeastern Adavale Basin (Fig. 4.2). Maximum thickness is observed in the central basin, where the unit is thicker than 1000 m (e.g. PPC Leopardwood-1; 1681.7m). The formation thins to the north-east and south-west, but formational thicknesses still range above 400 m. Only two wells show thinner sections of the Buckabie Formation in the central and eastern parts of the basin (PPC Bonnie-1, 113.1 m; PPC Etonvale-1, 101.8 m). A major fault striking NW-SE has been interpreted adjacent to PPC Etonvale-1, and the base of the Buckabie Formation lies substantially higher at ~2,000 m compared to locations south-west of these wells (~3,000 m), suggesting that the top of the Buckabie Formation has been eroded here (Passmore and Sexton, 1984). Rocks of the Buckabie Formation are intersected and cored in 10 wells with cumulative cored material of 104.3 m available (Table 4.2, based on well completion reports, core inspections and data from Geological Survey of Queensland, DNRME, 2014). The Buckabie Formation is also intersected in wells penetrating the Warrabin Trough west of the Adavale Basin (Pinchin and Senior, 1982).

Rocks of the Buckabie Formation represent the uppermost unit in the Adavale Basin and are overlain by a regional unconformity. In the area of the Adavale Basin, the Buckabie Formation is overlain by the Permian-Triassic successions of the Galilee Basin, and by

strata of the Cooper Basin in the area of the Warrabin Through to the west (McKillop et al., 2007). The Buckabie Formation conformably overlies rocks of the Etonvale Formation; in one well in the central Adavale Basin the formation unconformably overlies rocks of the Log Creek Formation (HEP Grey Range-1).

The unit has been described as a continental red bed succession (Paten, 1977), however, the unit lacks interbedded shales or mudstones in the logged intervals. Rip-up clasts that could be indicative of mudstones in between the cored intervals are occasionally observed (McKillop et al., 2007). The unit is nearly devoid of micro- and macrofossils, and a Givetian or Frasnian age was broadly interpreted from one fossiliferous sample (De Jersey, 1966). Due to the large formational thickness of the Buckabie Formation (up to

Figure 4.2. Interpolated thickness and total modelled volume of selected sedimentary rock units of the Adavale Basin, based on formational thickness interpreted from well logs after McKillop et al. (2007). Red line shows interpreted location of the Warrego Fault (after Spampinato et al., 2015). Units in chronological/stratigraphic order from the Early to the Late Devonian, a) Eastwood Beds, b) Log Creek Formation, c) Lissoy Sandstone, d) Etonvale Formation and e) Buckabie Formation. Open circles represent well locations, where the respective formation has been intersected. Interpolation method: Angular Distance Weighted (ADW) grid interpolation for irregular distributed points after Shepard (1968), 13 km cell size, processed in SAGA (System for automated geoscientific analysis, Conrad et al. 2015).

1680 m) compared to other units in the Adavale Basin, it has been speculated that deposition of the Buckabie Formation continued into the Carboniferous and was coeval with sedimentation in the Drummond Basin to the east (Heikkila, 1965; Tanner, 1968). However, Vine (1972) concluded that the poorly sorted sedimentary rocks of the Buckabie Formation were deposited rapidly, and sedimentation would have ended prior to the onset of deposition in the adjacent Drummond Basin. McKillop et al. (2007) further noted the absence of a marine transgression in the Adavale Basin during the Famennian, as opposed to transgressive events affecting the easternmost parts of the Drummond and Burdekin Basins to the east during that time (Draper et al., 2004), concluding deposition ended prior to the Late Famennian (latest Devonian) for the Buckabie Formation.

Table 4.2. Wells intersecting selected sedimentary formations in the Adavale Basin with formational thickness interpreted from well log data (int. thick., interpreted thickness; Geological Survey of Queensland, DNRME, 2014; McKillop et al., 2007) and cumulative core material available (cum. core, cumulative core; extracted from the respective well completion reports and logging of selected wells).

Buckabie Etonvale Lissoy Formation Formation Sandstone Eastwood Beds Bore int. cum. int. cum. int. cum. int. cum. ID Well name thick. core thick. core thick. core thick. core 707 PPC Carlow-1 199.4 3.2 78.0 2.2 1227.0 17.7 20324 PPC Log Creek-1 1514.5 11.1 332.0 4.2 43.0 5.2 708 AAO Eastwood-1 681.0 0.0 248.1 0.0 29.0 7.0 106.0 0.0 50184 AGA Phfarlet-1 1289.4 0.0 417.0 0.0 263.0 0.0 50185 AGA Phfarlet-2 1270.0 0.0 405.3 0.0 39.2 25.0 20318 PPC Collabara-1 1230.0 0.0 383.0 0.5 30.8 0.3 644 PPC Leopardwood-1 1681.7 0.0 454.6 2.1 22.0 0.0 57388 IOD Paradise-1 509.5 0.0 243.0 0.0 15.0 0.0 709 ASO Fairlea-1 470.2 6.1 328.3 0.0 14.4 0.0 189.5 0.0 640 PPC Etonvale-1 101.8 2.8 284.1 5.7 165.7 1.9 2858 PPC Gilmore-2 1240.1 0.0 467.1 0.0 34.2 31.0 740 PPC Quilberry-1 385.5 5.7 769 PPC Dartmouth-1 1277 AAR Rosebank-1 200.0 0.0 1259 AAR Mount Morris-1 40.4 0.0 643 PPC Gumbardo-1 906.8 7.8 385.0 4.6 59.9 2.9 645 PPC Lissoy-1 1253.4 4.4 241.4 0.0 39.6 13.0 642 HEP Grey Range-1 1086.9 0.0 636 HEP Alva-1 841.1 0.0 379.0 0.0 637 PPC Bonnie-1 113.1 7.3 407.8 0.0 796 AOP Boree-1 565.4 12.6 163.0 3.6 639 PPC Cothalow-1 425.4 10.9 248.4 5.6 647 PPC Stafford-1 439.1 4.4 607.3 12.8 658 PPC Bury-1 160.0 4.3 768 PPC Buckabie-1 765.6 35.5 330.8 8.1 793 AOD Yongala-1 639.7 14.0 280.2 7.0 797 EAL Brynderwin-1 31.9 0.0 795 BEA Allandale-1 624.0 10.0 Total [m] 104.3 76.4 88.5 31.3

4.3 SANDSTONE PETROGRAPHY

In this section, the results from the multimethod provenance study for selected sedimentary units of the Adavale Basin are presented. Petrographic information along with the spatial distribution of sampled units is discussed first (Section 4.3.1). Results of DZ (Section 4.4) and rutile (Section 4.5) U-Pb dating for 15 samples from the four main formations are presented together with a summary of grain morphology data. The DZ data are used to constrain maximum depositional ages (MDA), which are summarised together with existing biostratigraphic age constraints in Section 4.4.1. The DZ U-Pb age data are summarised in Section 4.4.2 and results from zircon trace element data is presented in the subsequent Section 4.4.3. The statistical sample comparison methods introduced in Chapter 2, and their application to the obtained DZ dataset are presented in Section 4.4.4, where the suitability of different approaches for the data is evaluated. Finally, the results of the detrital rutile U-Pb geochronology are presented in Section 4.5.

4.3.1 Sample locations and petrographic sample description Modal abundances have been assessed using petrographic thin sections of representative sample sections. Point counting was performed with 300 points per sample and a step length of 1.7 mm using a Pelcon automatic point counter. Sandstone classification followed Folk (1980) with quartz assemblage including mono- and polycrystalline quartz varieties, lithic fragments comprising igneous, metamorphic and sedimentary rock fragments and chert. Modal abundances for individual samples are summarised in Table 4.3 and Fig. 4.4, and detailed core logs of all inspected wells can be found in Appendix 4.2 and supplementary photographs of inspected core material in Appendix 4.1. Very fine to medium-grained sandstones have been targeted for sampling across all units to allow for comparison of DZ/rutile of similar host sediment grain size. Microphotographs of representative thin sections have been captured using a Zeiss Axio Scope polarisation microscope at the DNRME, Geological Survey of Queensland, Brisbane. The regional distribution and synthesized results of the petrographic assessment are summarised below per sampled formation.

Eastwood Beds This formation was sampled from the north-eastern part of the Adavale with samples collected from PPC Carlow-1 (CAR-14, 2552 m) and BEA Allandale-1 (ALL-6, 2339 m) (Fig. 4.6, McKillop et al., 2007). The sampled wells are 70 km apart. Both samples are taken from an upper section of the formation (Fig. 4.5) and are classified as moderately well-sorted, fine-grained (CAR-13, Q89F0L11, Fig. 4.7a) to very fine-grained (ALL-6,

Q91F4L5) sublitharenite. Quartz grains in both samples exhibit low-sphericity and are sub- rounded to rounded, show predominantly clear extinction with a few exhibiting a slight undulatory extinction. Fluid inclusions are very common in the monocrystalline quartz grains. Minor polycrystalline quartz is present in both samples. CAR-13 is devoid of feldspar, whereas ALL-6 shows some small grains of K-feldspar, plagioclase and microcline. Rounded low-sphericity fragments of siltstone are present in both samples. Accessory minerals include muscovite, biotite and zircon in CAR-13 and muscovite, tourmaline and rutile in ALL-6 (Fig. 4.7c). Silica cementation in the form quartz overgrowth is more pronounced in CAR-13 (Fig. 4.7b) than in sample ALL-6, which contains more argillaceous sedimentary matrix. Both samples show minor syntaxial calcite cement.

Lissoy Sandstone Rocks of the Lissoy Sandstone have been sampled along a 140 km NNE-SSW transect in three sample locations; in the north (PPC Carlow-1, CAR-14, 2051 m), central (PPC Log Creek-1, LOG-4, 3941 m) and southern part of the basin (Fig. 4.6, PPC Gumbardo-1, GUM-6, 2791 m). The sampled rocks comprise moderately sorted medium to coarse-grained, subarkose to arkose sandstones (average: Q74F21L6). Low to medium sphericity quartz grains are sub-rounded to rounded, and show predominantly straight extinction. However, the quartz grains in sample LOG-4 (Fig. 4.7d) from PPC Log Creek- 1 show more pronounced undulatory extinction, and may be an effect of deeper burial (3941 m) of these rocks compared to the other two locations (2791 and 2051 m). Feldspar detrital grains are dominated by K-feldspar with minor plagioclase and microcline in LOG-4. Accessory minerals include muscovite, tourmaline and zircon. Lithic siltstone fragments are rounded and highly altered to sericite. All samples show silica cementation in the form of a well-developed syntaxial quartz cement. Calcite cement is present as micritic rhombic

dolomite cement (CAR-14) and syntaxial calcite cement (LOG-4, GUM-6), minor clay cement is present in all samples. Fluid trail inclusions in quartz are common in both samples from the Lissoy Sandstone, suggesting a plutonic or vein origin similar to the underlying Eastwood Beds. It should be noted that the Lissoy Sandstone is significantly more feldspathic than the Eastwood Beds.

Etonvale Formation Four samples were collected along a 200 km NNE-SSW transect from the Etonvale Formation; in the northern Adavale Basin (ASO Fairlea-1, FAI-1, 2571 m), in the central basin (PPC Log Creek-1, LOG-5, 3810 m), in the southern basin (PPC Gumbardo-1, GUM- 7, 2540 m) and in the southwestern basin (Figs. 4.3, 4.6, PPC Buckabie-1, BUC-1, 2606 m). The samples collected from the Etonvale Formation comprise sandstones with a variety of modal compositions and grain sizes, and are described individually in the following.

Sample FAI-1 is a poorly sorted, fine to medium-grained lithic arkose (Fig. 4.7e,

Q68F21L11). Sub-rounded to rounded medium sized, monocrystalline quartz (showing clear extinction to slightly undulatory extinction) dominates the modal assemblage in the overall fine-grained rock. Fluid trail inclusions are common in the quartz grains and is a feature of all four samples from the Etonvale Formation. Feldspar detrital grains are comprised of K- feldspar (with some perthitic textures) and plagioclase. Lithic fragments encompass siltstone clasts, chert and volcanic rock fragments with plagioclase microlites showing pilotaxitic texture (Fig. 4.7f). Accessory minerals are zircon, biotite and muscovite. Syntaxial calcite cement is abundant, minor poikilotopic, and silica cementation from quartz overgrowths.

A well-sorted medium-grained arkose (LOG-5, Q53F37L10) was collected from PPC Log Creek-1. Monocrystalline quartz grains are sub-rounded to rounded, although minor polycrystalline quartz is present. The sample contains a high modal abundance of K- feldspar (32%), minor plagioclase (4%) and few grains of microcline. Sericitised siltstone clasts are rounded and have high sphericity. The only accessory mineral type is biotite. Quartz cement is well-developed in the form of quartz overgrowths, fluid inclusions are generally absent, hindering identification of the primary quartz grain morphologies to assess the degree of rounding.

Sample GUM-7 is a well-sorted fine-grained, subarkose with almost 90% monocrystalline quartz (Fig. 4.7g, Q89F7L4). Quartz grains show straight to slightly undulose extinction and appear rounded to well-rounded, but well-developed quartz overgrowths hinder the assessment of primary quartz grain morphologies. Lithic fragments make up well-rounded siltstone clasts. Feldspars are present as K-feldspar and plagioclase. Accessory minerals are zircon (Fig. 4.7h) and tourmaline. Silica cement is well-developed, some syntaxial calcite cementation is apparent.

BUC-1 was collected from PPC Buckabie-1 in the southwestern Adavale Basin (Fig. 4.8a). The sample is a medium to coarse-grained, moderately well-sorted subarkose

(Q78F12L10). Quartz grains appear sub-rounded to rounded, show slightly undulatory extinction and commonly fluid inclusion trails. Feldspar detrital grains are dominated by K-feldspar, subordinate plagioclase and microcline. Igneous rock fragments show signs of decomposition forming pseudo-matrix. Tourmaline is the only observed accessory mineral. (Fig. 4.8b). Clay cement is developed as pore-lining clay coatings of illite/smectite.

Buckabie Formation A total of six samples have been collected in three locations along a 200 km NNE- SSW transect from rocks of the Buckabie Formation. Formational thickness varies from 470.2 m in ASO Fairlea-1, 765.6 m in PPC Buckabie-1 to 1514.5 m in PPC Log Creek-1 (Fig. 4.6). To investigate the vertical compositional trends within the Buckabie Formation, two stratigraphic levels were sampled for each location, comprising a basal and a top sample; in the northern basin (ASO Fairlea-1, FAI-2, 2414 m; FAI-3, 2106 m), in the central Adavale Basin (PPC Log Creek-1, LOG-6, 2493 m; LOG-7, 2554 m) and in the southwestern part of the basin (Fig. 4.3, PPC Buckabie-1, BUC-3, 2230 m; BUC-5, 1677 m).

The three samples collected from the lower section of the Buckabie Formation are composed of very fine (FAI-2, Q46F25L28; BUC-3, Q75F17L8; Fig. 4.8c) to medium-grained

(LOG-6, Q33F50L17), moderately sorted feldspathic litharenite. Quartz grains are subangular to sub-rounded in the very fine-grained samples and sub-rounded to rounded in the medium-grained sandstone sample. Samples LOG-6 and BUC-3 are dominated by common quartz with fluid trail inclusions and subordinate volcanic quartz showing clear extinction. In contrast, the northernmost sample FAI-2 is dominated by volcanic quartz and

subordinate common quartz varieties. Detrital feldspar assemblages are dominated by K- feldspar (15-45%) and subordinate plagioclase (2-6%). The sampled rocks comprise a range of lithic fragments including rounded to well-rounded lithic fragments of chert and siltstone (Fig. 4.8e), recrystallised aphyric (rhyolitic) clasts and pilotaxitic texture (Fig. 4.8d). Accessory minerals comprise tourmaline, zircon, muscovite and biotite. Silica cement from quartz overgrowths, grain coating iron oxide-stained clay cement and syntaxial calcite cement is present in all samples.

Samples from the upper section of the Buckabie Formation are very fine to medium- grained moderately sorted lithic arkose (FAI-3, Q34F30L36, Fig. 4.8f; BUC-5, Q71F12L16) and well-sorted medium-grained feldspathic litharenite (LOG-7, Q53F13L34, Fig. 4.8h). Monocrystalline quartz grains are sub-rounded and minor polycrystalline and sub-rounded chert are present in all samples. Quartz varieties are dominated by volcanic quartz in FAI- 3 and BUC-5, showing clear extinction and some subeuhedral quartz grains. Quartz in sample LOG-7 commonly shows fluid inclusion trails, and only subordinate volcanic quartz. Feldspar assemblages are dominated by K-feldspar with subordinate plagioclase. Lithic fragments comprise siltstone and volcanic lithic fragments bearing plagioclase microlites with pilotaxitic textures and subordinate trachytic textures (Fig. 4.8g). Sample FAI-3 contains a variety of volcanic clasts comprising devitrified rhyolitic volcanic clasts with quartz phenocrysts (some with flow banding) and dark (potentially mafic) volcanic lithics showing mafic minerals (e.g. hornblende). Trace amounts of detrital hornblende and plutonic lithics are also present in this sample. The modal abundance of the volcanic rock fragments varies significantly between the different samples from the upper section of the Buckabie Formation (3 to 25%) and is correlated to the modal abundance of feldspars (12 to 30%), showing a strong north-south trend (Fig. 4.4). Accessory minerals comprise muscovite, biotite and tourmaline.

Table 4.3. Petrographic data for all sampled sandstones from point counting analysis (300 counts per thin section sample). Abbreviations: qm, monocrystalline quartz; qp, polycrystalline quartz; ch, chert; kfsp, K- feldspar; pl, plagioclase; mcr, microcline; srf, sedimentary rock fragments; irf, igneous rock fragment, mrf, metamorphic rock fragment; ms, detrital muscovite; bt, detrital biotite; zirc., zircon; tour., tourmaline; rut., rutile. Abundant zircon and rutile presence in mineral separate indicated by “x”, minor abundance by “(x)”, absence indicated by “-“. Tourmaline presence in thin section indicated by “x”, absence by “-“.

Sample Formation qm qp ch kfsp pl mcr srf irf mrf ms bt zirc. tour. rut. FAI-3 Buckabie Fm (upper) 28.3 5.7 6.7 24.0 5.7 0.0 4.7 24.7 0.0 0.3 0.0 x - - LOG-7 Buckabie Fm (upper) 47.0 5.0 11.3 11.3 2.0 0.0 11.3 11.0 0.0 0.3 0.7 x - x BUC-5 Buckabie Fm (upper) 67.3 4.0 4.7 8.7 3.7 0.0 8.7 3.0 0.0 0.0 0.0 x x (x) FAI-2 Buckabie Fm (lower) 44.7 1.7 6.7 18.3 6.7 0.0 18.7 3.0 0.0 0.3 0.0 x x x LOG-6 Buckabie Fm (lower) 30.3 2.3 3.7 45.3 4.7 0.0 9.3 4.3 0.0 0.0 0.0 x x x BUC-3 Buckabie Fm (lower) 74.7 0.0 0.7 15.0 2.0 0.0 7.7 0.0 0.0 0.0 0.0 x - x FAI-1 Etonvale Formation 67.1 0.0 2.8 20.6 0.7 0.0 3.5 4.5 0.0 0.0 0.3 x - - LOG-5 Etonvale Formation 50.7 1.7 1.0 32.3 4.0 0.3 9.3 0.0 0.0 0.3 0.3 x - (x) GUM-7 Etonvale Formation 88.0 1.0 0.3 6.3 1.0 0.0 3.3 0.0 0.0 0.0 0.0 x x x BUC-1 Etonvale Formation 76.0 2.0 1.0 10.0 1.3 1.0 3.3 5.3 0.0 0.0 0.0 x - x CAR-14 Lissoy Sandstone 77.0 0.7 2.3 16.7 0.0 0.3 3.0 0.0 0.0 0.0 0.0 x x (x) LOG-4 Lissoy Sandstone 61.7 1.0 1.0 26.7 0.3 0.7 7.7 0.0 0.0 1.0 0.0 x x x GUM-6 Lissoy Sandstone 79.0 0.7 0.7 16.3 1.0 0.0 2.0 0.0 0.0 0.3 0.0 x - (x) CAR-13 Eastwood Beds 86.0 2.4 4.9 0.0 0.0 0.0 6.1 0.0 0.0 0.6 0.0 x - x ALL-6 Eastwood Beds 88.6 1.2 2.9 2.9 0.5 0.2 2.4 0.0 0.0 1.2 0.0 x x x

Figure 4.3. Overview of drill hole locations with stratigraphic interpretation available (Geological Survey of Queensland, DNRME, 2014). Logged and sampled well locations highlighted in red with bold bore name label. Transects A-A’ and B-B’ referring to cross sections in Fig. 4.5.

Figure 4.4. Samples in QFL diagram for sandstone classification after Folk 1970, Fields for Quartzarenite (1), Subarkose (2) and Sublitharenite (3). Light grey arrow indicates strong south-north trend in modal composition for samples from the upper section of the Buckabie Formation, which is strongly controlled by the abundance of volcanic rocks fragments (Table 4.3).

Figure 4.5. DZ/rutile geochronology sample locations in the context of the stratigraphy of the sampled wells, and schematic stratigraphic relationship between wells. For individual locations of depicted wells, see Fig. 4.4.

Figure 4.6. Photographs of samples processedFigure for DZ/rutile4.6. geochronology. Image width is 9 cm for all images except pictures of LOG-4 and GUM-6 (11cm), due to larger core diameter. Sample labels correspond to Table 4.3 and 4.4.

Figure 4.7. Selected microphotographs from sampled rocks of the Eastwood Beds (A-C), Lissoy Sandstone (D) and Etonvale Formation (E-H). (A) overview of fine-grained sublitharenite CAR-13, (B) well developed quartz overgrowths in CAR-14, (C) Detrital rutile from ALL-6, (D) overview image of lithic arkose from LOG-4 (E) overview image of poorly sorted lithic arkose sample FAI-1 showing syntaxial calcite cement, (F) rounded volcanic clasts from FAI-1, (G) overview image of well-sorted fine-grained subarkose sample GUM-7, (H) sub-rounded DZ grain in GUM-7.

Figure 4.8. Selected microphotographs from sampled rocks of the Etonvale Formation (A, B) and Buckabie Formation (C-H). (A) Overview of BUC-1 showing pore-lining clays throughout, (B) Detrital tourmaline from BUC-1, (C) overview of very fine-grained feldspathic litharenite in BUC-3, (D) rounded volcanic clast showing plagioclase microlites in LOG-6, (E) well-rounded lithic fragment of siltstone in LOG-6, (F) overview image of FAI-3 showing moderately sorted lithic arkose with nearly 25% rounded to sub-rounded igneous rock fragments, (G) detailed image of rounded volcanic rock fragment in FAI-3 exhibiting plagioclase microlites and phenocrysts, (H) well sorted medium-grained feldspathic litharenite from LOG-7.

4.3.2 Interpretation The examined sandstones are generally well-sorted and were open framework upon deposition, allowing for quartz reprecipitation and cementation during burial and diagenesis as evident from prevalent quartz overgrowths. The well-sorted character of the sandstones suggests deposition under relatively dilute flow conditions with rounding of quartz grains, implying significant transport distances (e.g. Folk, 1980). The sediments are overall relatively texturally mature, and are compositionally mature in the lower successions (Eastwood Beds and Lissoy Sandstone), but become compositionally less mature towards the Buckabie Formation.

The detrital feldspar mineralogy of the sedimentary successions in the Adavale Basin is consistent with the observed mineralogy in the basal volcanic rocks of the Gumbardo Formation (K-feldspar dominated), however, the sandstones are most likely not predominantly derived from reworking the volcanic rocks of the Gumbardo Formation. The very quartz-rich Eastwood Beds overlying the Gumbardo Formation require a strong compositional shift and different provenance from the volcanic rocks of the Gumbardo Formation, which are relatively quartz poor (cf. Section 2.4.1). The Lissoy Sandstone, in contrast, is significantly more K-feldspar rich than the underlying Eastwood Beds that would be consistent with reworking of volcanic rocks of the Gumbardo Formation, to some degree, during deposition. The petrographic data show a long-term trend in sandstone compositions from more quartz-rich sandstones (Eastwood Beds and Lissoy Sandstone) to more feldspathic and lithic sandstones (Buckabie Formation). Rocks of the Buckabie Formation appear to be more volcanically influenced as evident from higher proportions of volcanic quartz, a significantly higher content of volcanic lithics and angular grain morphologies. Some samples show a bimodal composition of volcanic lithics (i.e. mafic and silicic volcanic rock fragments), which is potentially reflective of bimodal volcanism in the source region (Dickinson and Suczek, 1979; Garzanti, 2016).

The presence of accessory rutile suggests some (ultimate) metamorphic rock contribution to sediment provenance. Lithic clasts of siltstone are present in all samples and indicate contribution of reworked (meta)sedimentary sources to the sedimentary rocks in the Adavale Basin. Fluid trail inclusions in quartz and undulous extinction are observed in all samples, suggesting sourcing of quartz predominantly from plutonic rocks. The abundance of volcanic quartz is significantly higher in the sandstones of the Buckabie

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Formation compared to the underlying units and suggests, in conjunction with a high abundance of volcanic lithic clasts, a significant provenance evolution in the Upper Devonian.

4.4 DETRITAL ZIRCON U-PB GEOCHRONOLOGY

A total of 15 samples from four different sedimentary formations in the Adavale Basin were analysed for DZ geochronology, and a summary of the sampled sections is given in Table 4.4. The results are reported per individual sample from each formation. Reported age peaks of the most prominent age populations have been assigned using the locally adaptive KDE (Vermeesch, 2012). Grain morphology data are integrated for every individual analysis according to the approach outlined in Section 3.1.1 and is summarized as median length (grain long axis), aspect ratio (length/width) and sphericity (width/length) for individual samples (Table 4.4). In addition, a qualitative assessment of zircon grain morphologies was conducted based on transmitted light images. MDA are calculated as YC1σ (2+), and YC2σ (3+) after Gehrels (2009), but only the statistically more robust YC2σ (3+) is reported in the text. The full dataset with integrated U-Pb ages, trace element and grain morphology data is provided in the Electronic Appendix A. Appendix 4.3 contains transmitted light and SEM images of all analysed zircon grains and Tera- Wasserburg plots showing concordant and discordant analyses on a per sample basis are documented in Appendix 4.4.

Table 4.4. DZ sample details. Sampled core section with respective well information and median values of selected morphology parameters. Abbreviations: len., length; Fm, Formation, AR, aspect ratio. From Weight Zircon len. Sphericity Sample Well name [m] To [m] [g] yield Unit [µm] AR CAR-13 PPC Carlow-1 2552.1 2552.7 2329 good Eastwood Beds 195 1.7 0.56 ALL-6 BEA Allandale-1 2339.6 2339.9 2186 very good Eastwood Beds 170 2.1 0.48 CAR-14 PPC Carlow-1 2051.3 2051.6 2526 low-good Lissoy Sandstone 158 1.8 0.53 LOG-4 PPC Log Creek-1 3941.4 3941.7 2780 very good Lissoy Sandstone 202 1.8 0.53 GUM-6 PPC Gumbardo-1 2790.7 2791.1 2465 low Lissoy Sandstone 227 1.7 0.57 FAI-1 ASO Fairlea-1 2571.0 2571.3 2553 very good Etonvale Formation 193 1.9 0.52 LOG-5 PPC Log Creek-1 3809.7 3810.0 2452 very good Etonvale Formation 179 1.7 0.58 GUM-7 PPC Gumbardo-1 2539.6 2540.2 2257 good Etonvale Formation 133 2.2 0.45 BUC-1 PPC Buckabie-1 2605.7 2606.0 1928 very good Etonvale Formation 213 1.8 0.56 LOG-6 PPC Log Creek-1 3493.3 3493.6 2421 very good Buckabie Fm (lower) 194 1.8 0.55 BUC-3 PPC Buckabie-1 2230.5 2230.8 2251 very good Buckabie Fm (lower) 190 1.7 0.58 FAI-2 ASO Fairlea-1 2414.0 2414.3 2200 very good Buckabie Fm (lower) 176 1.9 0.51 FAI-3 ASO Fairlea-1 2105.9 2106.2 2074 very good Buckabie Fm (upper) 194 1.9 0.52 BUC-5 PPC Buckabie-1 1677.6 1677.9 2212 very good Buckabie Fm (upper) 184 1.9 0.53 LOG-7 PPC Log Creek-1 2553.9 2554.2 2448 very good Buckabie Fm (upper) 232 1.8 0.56

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Eastwood Beds

CAR-13 The sample yielded predominantly pitted, sub-rounded to rounded and subordinate sub-euhedral to euhedral zircons. Grains are mostly clear to slightly red-brown and purple- red discoloured. Sub-euhedral to euhedral grains are commonly colourless with lath-shaped inclusion minerals (apatite). The zircons show moderate contrasts under cathodoluminesence (CL), and oscillatory zoning is overall common, whereas high aspect ratio zircons show predominantly laminated zonation. Minor inherited cores and melt inclusions are visible in CL. One hundred fifty-five zircon grains were analysed from CAR- 13, where 132 yielded concordant ages, and 23 analyses yielded discordant ages. The four youngest analyses (ranging between 356 and 376 Ma) show elevated (negative) common 206Pb (>2%), indicating Pb-loss and have thus been removed from the dataset. The YC2σ (3+) cluster is then based on the next youngest grain at 385.0 ± 21 Ma and suggests a MDA of 397.1 ± 4.5 Ma (n=13, POF=0.79, MSWD=0.66). The sample shows the most prominent zircon cluster at ~400 Ma and another significant peak at ~480 Ma. A subordinate zircon cluster is identified at ~580 Ma, and a widespread population ranging from ~900 Ma to ~1200 Ma has a pronounced peak at ~1100 Ma. Another at ~1450 Ma defines a small grouping which extends to ~1700 Ma (Fig. 4.9).

ALL-6 The zircons in this sample are smaller compared to those from CAR-13 but have significantly higher median aspect ratios (2.1) and lower sphericity (0.48) compared to CAR-13 (Table 4.4). Zircons from this sample are overall colourless comprising sub- rounded and pitted grains, as well as subeuhdral to euhedral clear crystals. Oscillatory zoning is relatively weakly pronounced, melt inclusions are common and resorbed core domains are uncommon in CL. Of 160 analysed grains, 116 yielded concordant results, with 44 analyses discordant and were rejected. The five youngest grains (360-375 Ma) collectively show elevated (negative) common 206Pb (>2%) and were rejected from further consideration. The YC2σ (3+) is based on the youngest grain at 382.0 ± 16 Ma and provides an MDA of 389.8 ± 3.0 Ma (n=21, POF=0.94, MSWD=0.57). The most prominent age

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cluster in this sample is identified at ~400 Ma and a subordinate cluster occurs between 1400 and 1600 Ma with a pronounced peak at ~1500 Ma (Fig. 4.9).

Figure 4.9. KDE plot of samples ALL-6 and CAR-13 (organised from north to south) from the Eastwood Beds showing prominent peaks at ~400 Ma, subordinate peak at ~480, ~580 Ma and 1.1 Ga (Grenvillian) for CAR-13. Both samples exhibit a DZ population ~1.5-1.7 Ga.

Lissoy Sandstone

CAR-14 Zircons extracted from this sample are relatively small compared to the other samples with a median length of 158 µm (Table 4.X). This sample contains a high proportion of sub-euhedral to euhedral colourless and clear zircons (~40%). Sub-rounded and minor rounded grains (60%) are pitted and commonly show yellow-brown discolorations. Oscillatory zoning is common, melt inclusions are abundant, and a few resorbed core domains are identifiable in CL. From CAR-14, 156 zircons were analysed with 114 analyses yielding concordant results and 42 analyses are rejected because of discordancy. The youngest individual analysis from this sample returned an age of 366.2 ± 16 Ma and has been rejected due to an elevated alpha dose value of 2.3 x 1018 α /g, which indicates damage to the crystal lattice causing potential Pb-loss (e.g. Rubatto et al., 2001; Williams, 2001). The next youngest grain was dated at 389.0 ± 20 Ma, and the YC2σ (3+) suggests a MDA at 398.7 ± 5.4 Ma (n=9, POF=0.95, MSWD=0.34), coinciding with a significant age

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clustering at ~400 Ma The sample shows its most distinctive age peak at ~480 Ma, with an additional minor peak at ~580 Ma, a broad population between 900 and 1200 Ma and another broad subordinate cluster ~1500 Ma (Fig. 4.10).

LOG-4 The majority of zircons from this sample are sub-rounded to rounded grains with only minor discolorations (cumulative ~80%) whereas subeuhedral to euhedral colourless clear grains are subordinate (20%). Oscillatory zoning is prevalent and small melt inclusions are common. A small proportion of the grains (5%) exhibit noticeable inherited cores with a bright CL response. Out of 160 analysed grains, 120 yielded concordant results and 40 analyses were rejected as discordant. Two young grains dated at 372.0 ± 17 Ma and 385.3 ± 15 Ma have been rejected, the older grain shows an elevated alpha dose of 2.9 x 1018 α/g potentially indicating Pb-loss. For the younger grain, no analyses were dated within the 2σ uncertainty, and this was the basis for excluding this grain. The next youngest grain was dated at 391.3 ± 17 Ma, forming the basis for YC2σ (3+) at 404.6 ± 5.4 Ma (MSWD=0.28, POF=0.99, n=11). The most dominant DZ cluster is identified at ~480 Ma, while a minor shoulder in the KDE indicates a small population ~400 Ma (also indicated by the YC2σ (3+)), which is significantly smaller than observed in CAR-14. A subordinate cluster is identified at ~580 Ma and a few scattered analyses between 850 and 1700 Ma (Fig. 4.10).

GUM-6 Zircons extracted from this coarse-grained sandstone sample have a median size of 227 µm and are amongst the largest zircons analysed in this study (Table 4.4). The zircons are predominantly sub-rounded (~70%), although some grains are subeuhedral (~30%). The zircons show almost exclusively oscillatory zoning, dark CL core and melt inclusions are very common. Of 160 analysed zircons, 137 analyses yielded concordant results and this is the highest rate of concordancy (85%) across all analysed samples. Twenty-three analyses were rejected as discordant. The three youngest grains yielded ages of 254 ± 16 (#4_70), 359 ± 17 (#4_30) and 364 ±29 Ma (#4_59). Analyses 4_70 and 4_59 show extremely high negative common 206Pb (>50%) and have thus been rejected. Analysis 4_30 has been rejected due to suspiciously high Ti (52 ppm), suggesting partial ablation of a titanite inclusion in the analysed grain. The next youngest grain is dated at 382.0 ± 16 Ma,

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but only one analysis falls into the 2σ uncertainty. Hence, the next youngest grain dated at 395.0 ± 17 Ma forms the basis for the YC2σ (3+) and the analysis at 382.0 ± 16 Ma is included into this population giving an age of 402.2 ± 5.1 (MSWD=0.88, POF=0.56, n=12). The predominant age cluster for GUM-6 is identified at ~480 Ma analogous to the two previous samples from the Log Creek Formation. A minor cluster is identified at ~580 Ma and some scattered grains date between 800 and 1600 Ma, 400 Ma zircons are present as evident from the MDA, but do not form a significant cluster as in CAR-14 (Fig. 4.10).

Figure 4.10. KDE plot of samples CAR-14, LOG-4 and GUM-6 from the Lissoy Sandstone (organised from north to south) displaying major ~480 Ma peaks, a subordinate population ~580 Ma and some scattered Grenvillian ages (0.9-1.1 Ga) for all samples. Samples show varying proportions of ~400 Ma zircons and MDAs are consistently around 400 Ma.

Etonvale Formation

FAI-1 The zircon grains from this sample are predominantly colourless, clear and sub- rounded (90%) with minor pitted rounded grains (~5%) and show only minor orange-brown discolorations. The few sub-euhedral to euhedral zircons (5%) are colourless and clear.

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Some suspicious dark CL grain interiors suggest inherited zircon cores. Most zircons show oscillatory magmatic zoning and melt inclusions are common. From FAI-1, 155 zircon grains were dated with only 67 analyses yielding concordant results (42%). Overall, zircons from this sample exhibit elevated common 206Pb with average values of 4.5% and a median of 2.5%. A total of 88 analyses yielded discordant results. The youngest single grain is dated at 369.4 ± 20 Ma, and based on this age, the YC2σ (3+) age is 378.2 ± 5.4 Ma (MSWD=0.49, POF=0.82, n=7). The most prominent age cluster is identified at ~480 Ma. Two other populations are apparent at ~385 Ma and ~580 Ma and minor populations around 1.1 and 1.3 Ga (Fig. 4.11). Three grains > 3.0 Ga were dated from this sample at 3051 ± 44, 3110 ± 280 and 3226 ± 24 Ma, which are the only concordant zircons dated older than 3.0 Ga across all samples. Zircons with ages ~400 Ma are very rare in in this sample (n=4 between 395-405 Ma).

LOG-5 The sphericity of zircons from LOG-5 is amongst the highest for all analysed grains with a median value of 0.58 (Table 4.4). The grains are predominantly sub-rounded (~80%), subordinate pitted and rounded (10%), with minor subeuhedral zircons (~10%). Under transmitted light, the zircons appear predominantly clear and colourless with some grains showing red-brown discolourations. CL images reveal melt inclusions are common, especially in the larger oscillatory zoned zircons with lower aspect ratios, whereas acicular zircons with higher aspect ratios are less abundant. Out of 156 analysed grains, 115 analyses yield concordant results and 41 are discordant. One young grain dated at 312.0 ± 22 Ma has been rejected due to a high negative common 206Pb percentage of 7%, another three grains between 360 and 372 Ma show elevated U (>1000 ppm) and an elevated alpha dose (1.7 to 3.9 x 1018 α /g), indicating damage in the crystal lattice in these grains potentially causing Pb-loss. The next youngest grain was dated at 375.0 ± 16 Ma, forming the basis of the YC2σ (3+). The data suggests an MDA of 381.4 ± 5.9 Ma (MSWD=0.37, POF=0.94, n=9) for this sandstone, based on YC2σ (3+). A very dominant DZ cluster is identified at ~480 Ma, minor populations are apparent at ~385 Ma and ~580 Ma (Fig. 4.11). A few scattered grains are dated around 900 Ma and 1.4 Ga, and zircons ~400 Ma are virtually absent in this sample (n=1).

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GUM-7 Zircons from this sample are very small with a median length of 133 µm, and further exhibit a very high median aspect ratio of 2.2 and very low median sphericity of 0.45 (Table 4.X). Despite the high median aspect ratios of the grains, they are commonly sub-rounded (~90%) with only a few sub-euhedral to euhedral crystals (~10%) present. The grains are overall colourless and clear, with very few zircons showing red-brown discolourations. Grains show commonly oscillatory zoning in CL. For this sample, 174 grains were analysed, and 99 analyses yielded concordant results, 75 are discordant. Two analyses were rejected due to an elevated alpha dose (~1.0 x 1018 α/g) and high negative common 206Pb percentage (>5%). The youngest grain is identified at 354.2 ± 13 Ma and forms the basis for the YC2σ (3+) at 360.0 ± 4.5 Ma (MSWD=0.33, POF=0.96, n=9), which is interpreted as the MDA for this sample. The most dominant zircon population is identified at ~385 Ma, another significant grouping at ~480 Ma, ~400 Ma zircons are rare (n=3). This sample also shows age peaks at ~580 and pronounced age peaks at ~800 and 1100 Ma (Fig. 4.11).

BUC-1 Zircons from this medium to coarse-grained sandstone are fairly large with a median length of 213 µm. The grains are exclusively sub-rounded (~80%) to rounded (~20%), pitted and commonly maroon-red in colour. Under CL, the grains reveal oscillatory zoning and some relatively small melt inclusions. Resorbed zircon core domains are common and show either bright or dark CL-response in 10% of the imaged grains. Of 158 analysed grains, 120 yielded concordant results for this sample, 38 analyses are discordant. The youngest grain dated at 354.0 ± 17 Ma was rejected due to a very high negative common 206Pb percentage of 9.4% (208Pb/232Th age <<206Pb/238U). The next youngest grain was dated at 366.5 ± 13 Ma. Based on this individual analysis, the YC2σ (3+) is identified at 368.6 ± 8.8 Ma (MSWD=0.2, POF=0.82, n=3), which is interpreted as the MDA for this sandstone. The sample shows a pronounced population at ~480 Ma and smaller populations at ~385 and 580 Ma (Fig. 4.11). Two subordinate groupings are identified at ~950 and ~1200 Ma. Grains dated ~400 Ma are very rare in this sample (n=4).

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Figure 4.11. KDE plots of samples FAI-1, LOG-5, GUM-7, BUC-1 (organised from north to south) of the Etonvale Formation, showing relatively high proportions of ~480 Ma DZ in all samples and variable proportions of ~385 Ma aged zircons. A subordinate population at ~580 Ma is present in all samples. Note that the southernmost samples GUM-7 and BUC-1 exhibit higher abundance of Grenvillian aged DZ and also yielded significantly younger MDA, compared to the northern sample locations.

Buckabie Formation (lower part)

FAI-2 Mounted zircons from this sample are predominantly sub-rounded (80%), with minor rounded (10%) and subeuhedral (10%) grains. Stubby grains with lower aspect ratios exhibit commonly oscillatory zoning. Sub-rounded to subhedral lath-shaped zircons with high aspect ratios are abundant and show oscillatory zoning and often small melt inclusions. Inherited core domains with conspicuous CL response, as well as melt inclusions are commonly found in the grains with low aspect ratios. From FAI-2, 154 zircons were

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analysed with 128 yielding concordant results, 26 analysis are discordant. The YC2σ (3+) is based on the youngest grain dated at 352.1 ± 15 Ma and forms a population at 363.0 ± 3.8 Ma (MSWD=0.44, POF=0.95, n=13), which is interpreted as the MDA of this sample. This sample exhibits a pronounced population at ~370 Ma with 50% of all zircons falling between 350 and 380 Ma, wheras ~400 Ma zircons are rare (n=4). A minor peak occurs at ~480 Ma and a subordinate grouping at ~580 Ma (Fig. 4.12).

LOG-6 The zircons recovered from LOG-6 are predominantly sub-rounded (~90%) to rounded (~10%), and commonly show pitting and a red-brown discolouration. The CL- response is generally low and oscillatory zoning is common for the majority of zircons from this sample. The grains also exhibit small melt inclusions and resorbed core domains with conspicuous CL-response. Out of 155 analysed grains, 97 yielded concordant results, and 58 analyses yielded discordant results. The youngest grain is dated at 362.4 ± 17 Ma, but the trace element data for this analysis shows elevated contents for P (504 ppm), Th (577), Dy (358 ppm) and Lu (212 ppm), indicative of a partial ablation of an apatite inclusion. The next youngest grain is dated at 367.6 ± 18 Ma and forms the basis for the YC2σ (3+) at 377.9 ± 2.9 Ma (MSWD=0.36, POF=1.00, n=26) which is interpreted as the MDA for this sample. This sample shows two main detrital age peaks, the largest population is ~370 Ma and a smaller population at ~480 Ma (Fig. 4.12). Only a dozen grains are dated >500 Ma and does not cluster; some ~400 Ma zircons are present (n=7).

BUC-3 Zircons from BUC-3 have a median sphericity value of 0.58, which is amongst the highest of all analysed samples (Table 4.4). The grains are pitted and sub-rounded (90%) to rounded (~10%), and generally colourless, although a few grains show purple-red or red- brown discolouration. Under CL the zircons show oscillatory zoning, and inherited core domains are rare, and melt inclusions occur sporadically. A total of 156 grains were analysed, 114 analyses yielded concordant results and 42 analyses showed discordant results. The youngest zircon is dated at 370.2 ± 16 Ma and forms the base for the MDA based upon the YC2σ (3+) at 371.9 ± 7.0 Ma (MSWD=0.21, POF=0.89, n=4). The largest

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DZ population in this sample is identified at ~480 Ma, and a minor grouping is observed at ~370 Ma, which is consistent with the MDA of this sandstone. Grains dated ~400 Ma are virtually absent (n=1) and subordinate populations are identified at ~580 Ma, ~1200 Ma and ~1450 Ma (Fig. 4.12).

Figure 4.12. KDE plots of samples FAI-2, LOG-6 and BUC-3 (organised from north to south) from the lower section of the Buckabie Formation showing strong lateral variations in detrital age spectra. The proportions of zircons dating around 480 Ma peak increase from the north-eastern sample location to the southwest, whereas the younger grouping ~370 Ma is only weakly represented in the southernmost location and predominant in the middle and northern sample location. The population ~580 Ma is nearly absent in LOG-6 and forms a subordinate population in the other two samples. Older zircons >600 Ma are rare in FAI-2 and LOG-6 and slightly more abundant in the southernmost sample.

Buckabie Formation (upper part)

FAI-3 Zircons from this very fine to medium-grained sandstone have a relatively high aspect ratio with a median value of 1.9 (Table 4.4). The grains are predominantly sub-rounded with only a few subeuhedral zircons. Lath-shaped zircons with high aspect ratios are present but often broken. Oscillatory zoning is well developed in most grains; only very few grains show inherited core domains with conspicuous CL response. A total of 150

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grains were analysed for FAI-3 with 123 analyses yielding concordant results and 27 analyses yielded discordant results. The youngest analysed concordant grain is dated at 344.0 ± 19 Ma, but was rejected due to distinctly low U content (81.5 ppm) and low 206Pb concentrations (17 ppm). The next youngest grain is dated at 355.4 ± 16 and forms the base of the very robust YC2σ (3+) at 367.7 ± 2.7 Ma (MSWD=0.27, POF=1.0, n=35) which is interpreted as the MDA of this sandstone. The DZ distribution is nearly unimodal for this sample with the major peak at ~370 Ma, and where 85% of all analyses zircons are dated between 355 and 400 Ma. A minor population is identified at ~480 Ma and single grains at ~1.0 and ~2.6 Ga Ma, respectively (Fig. 4.13).

LOG-7 Zircons recovered from this medium-grained sandstone are quite large compared to the other samples with a median length of 232 µm (Table 4.4). The majority of grains appears sub-rounded, and a few grains are subhedral with at least two pristine crystal faces preserved. Large compact zircons with well-developed oscillatory zoning exhibit multiple small melt inclusions or single large inclusions. Inherited core domains are rare in this sample. Of 155 analysed grains 130 yielded concordant results, whereas 25 analyses are identified as discordant. The youngest individual zircon was dated at 359.2 ± 14 Ma and this analysis is the basis of the YC2σ (3+) at 366.2 ± 5.0 Ma (MSWD=0.28, POF=0.99, n=11) which is the interpreted MDA of this sandstone. The largest population is identified close to the MDA at ~370 Ma, and another significant population is observed at ~480 Ma. Subordinate age populations also occur at ~580 Ma, ~850 Ma and 1.1 Ga (Fig. 4.13).

BUC-5 Zircons recovered from this sample have a high median aspect ratio of 1.9. There are two different size fractions identified in this sample, with larger zircons tending to be more sub-rounded (~70%) and small zircons predominantly sub-rounded to subeuhedral (~30%). CL-response is very variable in this sample, however, oscillatory zoning is common and melt inclusions are found in the larger zircon grains. Out of 152 analyses for this sample, 105 yielded concordant results. A total of 47 analyses shows discordant results. The youngest individual analysis yielded an age of 350.2 ± 13 Ma, providing the base for the YC2σ (3+) at 358.4 ± 4.8 Ma (MSWD=0.48, POF=0.85, n=8) which is taken as the MDA

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for this sample. BUC-5 shows the same populations and proportions of those from LOG-7 at ~370 Ma and ~480 Ma. The population at ~580 Ma is only subtle, and the sample shows higher abundances of older zircons at ~850, ~1150, ~1500, ~2500 Ma compared to LOG- 7 (Fig. 4.13).

Figure 4.13. KDE plots of samples FAI-3, LOG-7 and BUC-5 (organised from north to south) from the lower section of the Buckabie Formation. All samples are dominated by ~370 Ma zircons, whereas this population almost shows a unimodal distribution for the northernmost location FAI-3. Remaining sample locations exhibit additional DZ populations at ~480 and subordinate at ~580 Ma.

4.4.1 Comparison of maximum depositional ages and biostratigraphic ages

MDA are calculated as YCσ2 (3+) and YCσ1 (2+) after Dickinson and Gehrels (2009). The two measures are rather conservative approaches to determine MDA and are commonly used when a (bio)stratigraphic framework already exists for a sedimentary succession (Coutts et al., 2019). The ages are calculated as weighted averages and reported with the associated uncertainties, as well as statistical parameters for the respective populations presented in Table 4.5. Statistical parameters (MSWD, n, POF) for the YCσ2 (3+) are overall more robust compared to the YCσ1 (2+), as a higher number of individual

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analyses are usually included in the population. This has a positive effect on the absolute error of the age population, and the MSWD values are slightly greater (but still < 1) with higher degrees of freedom (n) compensating for the effects of statistical overfitting. As a consequence, the YCσ2 (3+) was chosen here for the final assessment of MDA.

Biostratigraphic ages are from the Late Emsian to Frasnian period are based on palynology (Hashemi and Playford, 2005). These are used here as a chronostratigraphic framework for comparison with MDA obtained from the DZ data (Table 4.5) In addition, the new U-Pb geochronology from the volcanic rocks of the Gumbardo Formation yielding a combined U-Pb age of ~398 Ma (Chapter 2, Section 2.5.1) provides a baseline for constraining MDA and identifying a potential contribution of DZ from reworking of the basal volcanics.

Rocks of the Eastwood Beds are assigned to biostratigraphic Assemblage I (Late Emsian, 400 – 393 Ma; Hashemi and Playford, 2005). For CAR-13, an MDA of 397.1 ± 4.5 Ma is consistent with the Assemblage I age (Figs. 4.14, 4.15). The MDA for ALL-6 is slightly younger than Assemblage I (389.8 ± 3.0 Ma), but almost within error (Figs. 4.14, 4.15). The analysed sample from BEA Allandale (ALL-6) was taken in the same core interval (2339.6 – 2339.9 m) as the palynology sample (2339.4 m). The statistics of this model age are very robust (MSWD=0.57, POF=0.94, n=21), indicating that the depositional age of Eastwood Beds might be better constrained by the MDA, suggesting

Table 4.5. Comparison of YCσ1 (2+) and YCσ2 (3+) populations (Dickinson and Gehrels, 2009a) and statistical indicators for all samples. Note the differences between the weighted averages for YCσ1 (2+) and YCσ2 (3+) with YCσ2 (3+) exhibiting older ages. Abbreviations: Fm, Formation; MSWD, mean squared weighted deviation; POF, probability of fit.

sample unit YCσ1 (2+) MSWD n POF YCσ2 (3+) MSWD n POF CAR-13 Eastwood Beds 389.3 ± 8.2 0.40 4 0.75 397.1 ± 4.5 0.66 13 0.79 ALL-6 Eastwood Beds 384.0 ± 4.6 0.15 9 1.00 389.8 ± 3.0 0.57 21 0.94 CAR-14 Lissoy Sandstone 394.5 ± 8.5 0.19 5 0.94 398.7 ± 5.4 0.34 9 0.95 GUM-6 Lissoy Sandstone 400.4 ± 8.9 0.18 4 0.91 402.2 ± 5.1 0.88 12 0.56 LOG-4 Lissoy Sandstone 393.6 ± 9.6 0.25 3 0.78 399.2 ± 7.3 0.72 6 0.61 LOG-5 Etonvale Formation 378.2 ± 7.2 0.09 6 0.98 381.4 ± 5.9 0.37 9 0.94 FAI-1 Etonvale Formation 375.4 ± 6.5 0.16 5 0.96 378.2 ± 5.4 0.49 7 0.82 BUC-1 Etonvale Formation 368.6 ± 8.8 0.20 3 0.82 368.6 ± 8.8 0.2 3 0.82 GUM-7 Etonvale Formation 357.0 ± 6.1 0.12 5 0.97 360.0 ± 4.5 0.33 9 0.96 LOG-6 Buckabie Fm (lower) 373.5 ± 4.7 0.16 10 1.00 377.9 ± 2.9 0.36 26 1.00 BUC-3 Buckabie Fm (lower) 371.9 ± 7.0 0.21 4 0.89 371.9 ± 7.0 0.21 4 0.89 FAI-2 Buckabie Fm (lower) 355.1 ± 8.2 0.14 3 0.87 363.0 ± 3.8 0.44 13 0.95 FAI-3 Buckabie Fm (upper) 359.5 ± 6.5 0.12 6 0.99 367.7 ± 2.7 0.27 35 1.00 BUC-5 Buckabie Fm (upper) 352.5 ± 8.5 0.19 3 0.83 358.4 ± 4.8 0.48 8 0.85 LOG-7 Buckabie Fm (upper) 361.5 ± 8.2 0.13 4 0.94 366.2 ± 5.0 0.28 11 0.99

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deposition continued into the Eifelian. Palynoflora of the Eastwood Beds had previously been tentatively assigned to an Eifelian biostratigraphic assemblage (McGregor and Playford, 1993), which has been challenged by Hashemi and Playford (2005) and instead lead to the interpretation of an Eifelian hiatus in the Adavale Basin (McKillop et al., 2007). In conjunction with the Eifelian MDA from ALL-6, this suggests that deposition of the Eastwood Beds did continue into the Eifelian, and that no Eifelian hiatus may exist. This further suggests that the Eastwood Beds are either conformably overlain by the Log Creek Formation or even synchronous to some degree as indicated by the complementary thickness distribution of the units (Fig. 4.2).

The three MDAs calculated for the Lissoy Sandstone are collectively older (8.7 – 12.2 Myrs) than the assigned biostratigraphic age (Assemblage II, 390 – 385 Ma), indicating a lack of contemporary volcanic zircon supply in this unit (Figs. 4.14, 4.15). The MDAs are very consistent across all three sampled locations, vary only by 3.5 Myrs, and are all within error of the combined U-Pb age for the volcanic rocks of the Gumbardo Formation at ~398 Ma (Table 4.5). The overlap in DZ ages in conjunction with very high proportions of K-feldspar in all samples from the Lissoy Sandstone, suggests reworking of the K-Feldspar-rich volcanic rocks of the Gumbardo Formation has contributed to the Lissoy Sandstone. Volcanic rocks of the Gumbardo Formation are only known from a narrow NNE-SSW trending corridor within the Adavale Basin (cf. Chapter 2, Section 2.2). The high contribution of ~400 Ma zircons in the Lissoy Sandstone following deposition of a considerable amount of sediment covering the Gumbardo Formation (200-300 m thickness of Eastwood Beds/Log Creek Formation) suggests a more widespread distribution of the Gumbardo Formation may have originally existed, which is conclusive with previous interpretations on the spatial extent of the unit from gravity data (Abdullah and Rosenbaum, 2018; Frogtech Geoscience, 2018).

A lower Frasnian depositional age is assigned to rocks of the Etonvale Formation (Assemblage III, 382 – 377 Ma). Four MDA have been determined for rocks sampled from the Etonvale Formation: two samples yield ages coinciding with the biostratigraphic age (LOG-5, FAI-1) but two MDA are significantly younger than Assemblage III (GUM-7, BUC-1; Figs. 4.14, 4.15). The MDA for BUC-1 exhibits a relatively large uncertainty (368.6 ± 8.8 Ma, MSWD=0.2, POF=0.82, n=3) and the biostratigraphic assemblage lies just within error of the age. In contrast, the MDA for GUM-7 is statistically robust at 360.0

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± 4.5 Ma (MSWD=0.33, POF=0.96, n=9) and is clearly younger than the biostratigraphic age by 17 Myrs. The MDA for LOG-5 and FAI-1 perfectly coincide with the biostratigraphic age (Figs. 4.14, 4.15) and the MDA for BUC-1 does not yield a statistically robust model age (Table 4.5), whereas the MDA for GUM-7 from a middle section of the interpreted Etonvale Formation is statistically robust and conflicts with the biostratigraphic age assignment (Table 4.5, Figs. 4.14, 4.15), suggesting that the sampled section might be part of the stratigraphically younger Buckabie Formation instead. It is noteworthy that there is no direct biostratigraphic age control from the sampled well PPC Gumbardo-1, and the biostratigraphic age for the Etonvale Formation is constrained from other drill core intersections (Table 4.1). The sandstone petrography data show a relatively quartz-rich modal composition for GUM-7, which contrasts with other sandstone compositions from the Etonvale and Buckabie Formations (Fig. 4.4). In addition, GUM-7 lacks the characteristic reddening of the Buckabie Formation. Interpretations of seismic lines at the drill location of PPC Gumbardo-1 indicate a fault-controlled fold at this location (Finlayson and Collins, 1986), which possibly caused uplift and erosion of the Etonvale Formation at the sampled location.

The biostratigraphic Assemblages I, II and III (Figs. 4.14, 4.15, Hashemi and Playford, 2005) only constrain the depositional ages for the Eastwood Beds (Assemblage I), Lissoy Sandstone (Assemblage II) and Etonvale Formation (Assemblage III). A lack of palynomorphs in rocks of the Buckabie Formation hampers a biostratigraphic age assignment for this unit, yet a low-yield palynoflora suggested a Givetian or Frasnian depositional age (De Jersey, 1966). The MDAs for the lower part of the Buckabie Formation (samples BUC-3, LOG-6, FAI-2) broadly confirm the upper age boundary of Assemblage III in the Mid Frasnian, which has been inferred as the lower age boundary of the Buckabie Formation (McKillop et al., 2007). The upper limit of the Buckabie Formation can now be constrained by the youngest MDA for BUC-5 at 358.4 ± 4.8 Ma aligning with previous interpretations which suggest a Late Devonian to Early Carboniferous age, and some deposition coeval to the Drummond Basin to the east of the Adavale Basin (Figs. 4.14, 4.15, Heikkila, 1965; Tanner, 1968; Vine, 1972).

In conclusion, the MDA values obtained from four sedimentary formations in the Adavale, in combination with the existing biostratigraphic framework provide an improved chronostratigraphic framework for the Adavale Basin. The obtained MDA give insight into

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provenance trends during the lifespan of the basin, from MDA controlled by reworking of basement rocks during the Lower Devonian and syn-depositional volcanic DZ sources in the Upper Devonian, which is consistent with the overall observations from sandstone petrography in Section 4.3.2. The data highlight some issues with the overall age assignment of the Eastwood Beds, suggesting deposition of this unit preceded into the Eifelian as indicated in earlier versions of the biostratigraphic framework (McGregor and Playford, 1993). This further challenges the existence of the interpreted early Eifelian disconformity (Hashemi and Playford, 2005; McKillop et al., 2007); no direct geological observation of a disconformity is possible as no contacts between the Eastwood Beds and

Figure 4.14. Generalised stratigraphy of the Adavale Basin, modified after McKillop et al. (2007), integrated MDA displayed as YCσ2 (3+) plus associated uncertainties. Samples and associated MDAs are organised from south to north with respect to the sampled location in the basin.

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overlying units are preserved in cored material. In addition, no major differences in bedding angles, indicative of tectonic tilting between deposition of the Eastwood Beds and overlying units have been observed in drill core.

Figure 4.15. Comparison of YCσ2 (3+) MDA (x-axis) and biostratigraphic age assemblages (y-axis, Hashemi and Playford, 2005). Coloured bars indicate upper and lower boundaries of biostratigraphic assemblages I-III. Rocks of the Buckabie Formation lack reliable biostratigraphic age control, lower and upper age boundary are inferred after McKillop et al. (2007). MDA yielding model ages older than biostratigraphic ages plot in the upper left field (MDA consistent with biostratigraphy), and MDA younger than biostratigraphic ages in the lower right field (MDA inconsistent with biostratigraphy). MDA show overall good agreement with biostratigraphy. Only few MDA suggest younger depositional ages and are thus inconsistent with the biostratigraphic ages (e.g. GUM-7, BUC-1).

4.4.2 Temporal trends of detrital zircon distributions When all the concordant DZ ages analysed from the Adavale Basin are compiled (n=1712), almost 70% of the analyses yield Palaeozoic ages (Fig. 4.16). Two prominent DZ age peaks are identified: Devonian (370 – 385 Ma), and broadly syn-depositional, and Ordovician (465 – 480 Ma). A third, but significantly smaller Ediacaran age peak (560 – 590 Ma) is also evident. DZ ages older than 600 Ma do not form any distinct peaks, but a broad plateau

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of ages ranging from ~0.8 to 1.2 Ga (Grenvillian) and a somewhat more restricted age peak at ~1.5 Ga. DZ older than 2 Ga are almost absent (3% of all DZ) and no grain older than 3.2 Ga was encountered. Precambrian DZs in the Adavale Basin make up for just over 30% of all dated grains.

Ordovician-aged DZ with peak ages between 465 and 480 Ma are a feature in all sampled units. However, proportions of this population vary strongly across the formations (Figs. 4.17, 4.18). The proportions of DZ between 440 and 500 Ma progressively decrease from the Lissoy Sandstone (57%) up stratigraphy, with the lowest proportions in the upper section of the Buckabie Formation (20%). Ordovician-aged DZ are the least abundant in the Eastwood Beds (13%). The proportions of ~580 Ma zircons are significantly smaller compared to the Ordovician peaks within each successive formation, and also show significant variations between formations, with the smallest proportions recorded for the Buckabie Formation (3.5 and 3.0%; lower and upper sections, respectively) with the greatest abundances in the Lissoy Sandstone (9.3%) and Etonvale Formation (9.7%). In contrast, the proportions of Precambrian DZ ages show a more consistent pattern stratigraphically, decreasing in abundance, forming nearly 55% of the dated detrital population for the Eastwood Beds and only 12.5% in the upper section of the Buckabie Formation. Contribution of DZ ~400 Ma shuts off in the Middle Devonian, whereas Ordovician- and Ediacaran-aged DZ consistently contribute to the DZ budget of the Adavale Basin, contributing during the entire lifespan of the basin. From the Middle Devonian onwards, these DZ get overwhelmed by the contributions of syn-basinal populations (380-360 Ma).

Major differences between the sampled formations are observed with regards to broadly syn-basinal DZ populations. More precisely, multiple subtly different age populations are apparent and show a systematic younging trend up stratigraphy (Fig. 4.18b). Whereas the youngest visual peak age for the Eastwood Beds and Lissoy Sandstone are at ~400 Ma, the overlying Etonvale Formation shows a skewed KDE peak age towards ~385 Ma, indicating the introduction of a new DZ population at ~380 Ma, however, the youngest dominant DZ group is still at ~400 Ma (Fig. 4.17). For the Buckabie Formation, the KDE skews further towards younger ages and shows a peak age at ~370 Ma for the upper and lower sections of the formation. The proportions of Upper Devonian DZ increase from the Etonvale Formation up stratigraphy from 7.8% to 37.6% for the upper section of the Buckabie

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Formation. This younging trend is also reflected in the MDAs (presented in the previous section) and shows systematic younging up stratigraphy (Figs. 4.14, 4.15).

The observed temporal DZ trends in combination with sandstone petrography data show initial deposition of compositionally relatively mature sediments with DZ spectra exhibiting ages predating sediment deposition or reflecting input from the basal volcanic rocks of the Gumbardo Formation (Chapter 2). Sandstone composition then shifts towards more volcanically influenced compositions (Section 4.3.2) and syn-basinal DZ ages successively gaining influence, and older age populations become overwhelmed by contribution of syn-depositional DZ in the Upper Devonian (Figs. 4.18, 4.19). This temporal trend seems at odds with the proposed platform-basin type proposed for the Adavale Basin, where sediments are expected to be compositionally mature and DZ age spectra should be dominated by older DZ ages predating depositional ages, reflecting a wide-basin catchment accompanied by increasing contributions of older craton-derived detritus. To further investigate if the decrease of older DZ ages up stratigraphy is caused by swamping with syn-volcanic DZ potentially overwhelming the older DZ populations, detrital rutile is utilised in Section 4.5 as an additional thermochronometer.

Figure 4.16. KDE plot showing the total DZ data acquired from the Adavale Basin, highlighting large proportions of Devonian and Ordovician ages DZ. Inset shows detailed data from 300 to 660 Ma (~70% of the total DZ data) with colour-coded geological periods. Abbreviations: Sil., Silurian; Ordov., Ordovician.

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Figure 4.17. KDE plots of DZ data aggregated per formation. Inset showing variability of proportions in the Ordovician age peak (~480 Ma), manifold Devonian populations (~400, 385, 370 Ma) and variability of the subordinate ~580 Ma population.

Figure 4.18. Density plots, showing the relationship between DZ data (x-axis, aggregated per formation illustrated as heat maps) and depositional ages inferred from biostratigraphy (y-axis, section 4.2.3), allowing to track the persistence of DZ age populations through the stratigraphy of the Adavale Basin. Data illustrate the decrease of Precambrian-aged zircons up stratigraphy and large proportions of Palaeozoic ages (a). Subset (b) shows DZ ages from 350 – 650 Ma highlighting continuous contribution of Ordovician DZ ages (460-485 Ma), disappearance of DZ ages related to Adavale rift volcanics up-section (400 Ma) and distinctive Middle (380 Ma) and Upper Devonian (375-360 Ma) aged DZ successively being introduced up section.

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Buckabie Fm (upper)

Buckabie Fm (lower)

Etonvale Fm

Lissoy Sst

Eastwood Beds

0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%

350-370 370-395 395-405 405-440 440-500 550-600 >1600

Figure 4.19. Proportions of selected intervals of DZ ages contributing to the DZ budget for each formation illustrating substitution of Precambrian ages by Palaeozoic ages up stratigraphy.

4.4.3 Zircon trace element chemistry Most analysed DZ in this study show oscillatory zoning to some degree, suggesting they are of predominantly igneous origin. Zircon geochemistry can be utilised to investigate properties of the parental magma composition and in combination with the U-Pb age information aid to identify igneous source rock lithologies in the context of a sediment provenance study (e.g. Grimes et al., 2015; Belousova et al., 2002; McKenzie et al., 2018).

In addition to 206Pb, 207Pb, 208Pb, Th and U that are required for U-Pb dating of zircon, every ablated zircon was analysed for 13 additional elements with Si used as an internal standard element (Section 3.4). The trace element data are integrated here with the U-Pb ages and explored for temporal trends in trace element concentration and derived parameters/element ratios to provide further insight on provenance and potential source changes over time. The two largest DZ age populations are the Devonian and Ordovician with n=600 DZ for each group. Average Th/U and Zr/Hf are subtly higher for the Devonian DZ (Zr/Hf: 57; Th/U: 0.7) compared with Ordovician zircon (Zr/Hf: 50; Th/U: 0.6; Fig. 4.20a, b). Within the Ordovician age population is a distinct zircon subpopulation with low Th/U (<0.3) and enrichment of REE and P and co-variation indicating a xenotime-type substitution mechanism (Speer, 1980, Fig. 4.20c). Due to acquisition of only 6 “essential”

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REEs for these zircons (La, Ce, Nd, Eu, Dy, Lu) and not a full REE set, the trend line describing the coupled enrichment with P has been modified according to the approach utilised in Allen et al. (2018). The data suggest the small group of Ordovician-aged zircons exhibiting xenotime-rich compositions might be derived from S-Type granites as typified in the Lachlan Orogen (Fig. 4.20c, Allen et al., 2018).

The data have been further subdivided to explore trends (i) for Precambrian, (ii) Cambrian, Ordovician, Silurian and (iii) Devonian aged zircons (Fig. 4.21). Average and median values for Zr/Hf ratios show a constant increase during the Palaeozoic (from 50 to 57) and seem to stagnate during the Middle/Late Devonian, which is controlled by decreasing Hf concentrations in the same timeframe (averaged abundances decreasing from 12,000 ppm to 10,500 ppm), while average Zr concentrations remain relatively constant. Th/U show a steady increase for the Palaeozoic and a pronounced increase during the course of the Devonian (from 0.6 to 0.8). Precambrian Th/U are more variable compared to the Palaeozoic and tend to include very high ratios up to 2. Zircons between 550-600 Ma show significantly lower Lu concentrations compared to the remaining age groups.

TZircTi has been calculated after Watson et al. (2006) to constrain crystallisation temperatures of zircons, assuming a Ti activity of 1 (i.e. all temperatures are slight underestimates). The data show relatively high average temperatures for Precambrian zircons (~750°C) and significantly lower temperatures for Palaeozoic ones (~700-710°C). Upper Devonian (latest Famennian) to Early Carboniferous zircons (350-360 Ma) exhibit significantly higher TZircTi (~740°C) than Early to Late Devonian grains (360-400 Ma, 700- 710°C), suggesting some differences in the petrogenesis of Upper Devonian/Early Carboniferous igneous source rocks compared to the Early to Late Devonian DZ. This group further shows lower U, higher Eu/Eu* and higher Th/U compared to the remaining zircons of Devonian age.

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Figure 4.20. Zr/Hf and Th/U ratios for Paleozoic DZ ages showing highest data density for Devonian and Ordovician U-Pb ages (A,B). Data indicate increase in Zr/Hf and Th/U over time. Ordovician aged DZ with low Th/U (<0.3, B) follow xenotime substitution (C, after Allen et al., 2018)) suggesting S- Type affinities of source rock lithologies (plot along trendline). Abbreviations: Carb., Carboniferous, Sil., Silurian, REE, rare earth elements.

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Figure 4.21. Trace element data and geochemical parameters grouped into selected age intervals to capture temporal trends in the trace element chemistry of DZ.

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4.4.4 Detrital zircon morphologies All grains analysed for U-Pb geochronology were digitized using the approach outlined in Section 2.5.1 of Chapter 2 and integrated with the DZ age data. From the morphological data, aspect ratio and sphericity were calculated and plotted in histograms integrating the age frequency distribution and the respective morphological parameter (Fig. 4.22). Almost all analysed DZ show rounding to some degree, allowing the inversion of the value for aspect ratio (long axis/short axis) as a discriminator for sphericity of grains. Three significant age populations are identified for the total DZ data from the Adavale Basin, a subordinate Ediacaran age population at ~560-590 Ma, a major group of Ordovician aged zircons (~465-485 Ma) and a Middle to Upper Devonian age group (~370- 385 Ma). These age groups feature distinctive morphological characteristics that are normalised for comparison and discussed in following.

The overall proportions of DZ with very high aspect ratios (>3, Fig. 4.23a) are very low (3.8%), medium (2-3, 32.6%) and low aspect ratio zircons (1-2, 63.6%) are more common (Fig. 4.22a). The highest proportions of high aspect ratios are observed for the Middle to Upper Devonian group (6%, n=102), whereas Ordovician or Ediacaran zircons have lower proportions of high-aspect ratio zircons (~2%, Fig. 4.22c). Within the Devonian group, the proportions of medium and high aspect ratio grains (Fig. 4.22b) increase towards the Upper Devonian and exhibit the highest proportions between 380 and 360 Ma. High aspect ratio zircons are predominantly derived from volcanic rocks where zircons are thought to crystallise more rapidly in shallow magma chambers prior to eruption producing more acicular zircons with high aspect ratios (e.g. Corfu et al., 2003).

The proportion of highly spherical zircons (Fig. 4.23b) increases with age of the zircon where > 50% of the zircons in the Cambrian/Ediacaran group and over 40% of DZ older than 650 Ma have high sphericity values >0.6 (Fig. 4.22d). The highest proportions of very high and high sphericity DZ (>0.6) are observed between 580 and 560 Ma (Fig. 4.22f), indicating that DZ of this age group are derived from (meta)sedimentary rocks, which have undergone enhanced recycling resulting in higher abrasion and rounding of the grains (e.g. Shaanan and Rosenbaum, 2018). In contrast, the Silurian/Ordovician and Devonian groups show significantly lower proportions of spherical zircons (25-30%). In addition, Ordovician-aged zircons exhibit high values for equivalent circular diameter (ECD), which is a measure for overall DZ grain size. This grouping shows higher

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proportions of large zircons (45%) compared to the other two groups for ECDs >150 µm (33.1, 34.4%), which is indicative for contribution of zircons from plutonic sources, crystallising from slow cooling magmas and producing overall larger zircons (Corfu et al., 2003).

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Figure 4.23. Examples of high aspect ratio zircons from sample FAI-2 (A) and high sphericity grains from BUC-1 (B).

4.4.5 Similarity assessment of detrital zircon frequency distributions Several sample comparison methods for the correlation of DZ age spectra (introduced in Chapter 3 Section 3.5.5) have been carried out to assess the similarity for all DZ sample pairs from the Adavale Basin in order to diagnose “like sources”. The suitability of the individual methods to compare DZ age frequency distributions is debated in the literature and commonly assessed using synthetic datasets (e.g. Saylor and Sundell, 2016; Vermeesch, 2013, 2018). Here, the adequacy of the six most common sample comparison coefficients is evaluated using the DZ data acquired from samples of the Adavale Basin, to find the most suitable method for the dataset (Fig. 4.24). In this approach, the main measure for suitability is represented by variance of the data, illustrating how well the respective approach utilises the entire range of coefficients from 0 (no similarity) to 1 (high similarity) and is summarised together with median, minimum and maximum coefficients for each individual method in Table 4.6. The distribution of coefficients is illustrated in Fig. 4.25 for each method and the complete dataset of coefficients for each method can be found in Appendix 4.5.

For the K-S and Kuiper tests, the data illustrate the issues in using p-values as a measure of similarity for samples with n < 1000 (Vermeesch, 2013). The variance of the coefficients from both methods is extremely low (0.007 and 0.008), and most sample pairs

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are declared as very dissimilar with a median coefficient of 0 resulting in extremely right skewed distributions for both tests (Figs. 4.24, 4.25). V-values from the Kuiper Test, show a variance of 0.028, and slightly higher variance is achieved by the D-values of the K-S Test (0.035), demonstrating that these coefficients retained from the K-S and Kuiper Tests are more suitable for sample comparison of samples with n < 1000 as outlined in Vermeesch (2013).

As discussed in Section 3.3.5, Saylor and Sundell (2016) highlighted the low variance of the coefficients obtained from the Likeness and Similarity methods, which is also observed for the DZ data from the Adavale Basin (0.022 and 0.017). In addition, the coefficients from the Bayesian Population Correlation also show fairly low variance (0.027) and a slightly left-skewed coefficient distribution, questioning the suitability of this method for the Adavale data (Figs. 4.24, 4.25). The Cross Correlation Coefficient yields a significantly higher variance of 0.073 and seems to be the most suitable approach to discuss the data, as it uses nearly the full range of available coefficients, indicated by a minimum value of 0 and a maximum of 0.96 across the dataset. Consequently, the coefficients obtained from the Cross Correlation Coefficient are utilised in Section 4.3.2 to discuss spatial and temporal trends in DZ distributions.

The Bayesian Population Coefficient might not constitute the most favourable comparison method due to the low variance and the relatively high median coefficient values (Figs. 4.24, 4.25). However, in addition to the individual coefficients for each sample pair, the method also provides uncertainties for each individual coefficient. These are based on the Probability Model Ensembles (PME), obtained from subsampling each DZ sample and inform the representativeness of a given sample (see Appendix 4.6). The uncertainties for the Adavale Basin samples yield uncertainty values between 0.031 and 0.057 with a median of 0.038, or 3.2 – 13.1% and a median of 4.95%. Notably, the coefficients involving sample FAI-1 show the highest absolute uncertainties, because the sample yielded by far the smallest number of concordant ages (n=64), which impacts the representativeness of this sample and results in higher uncertainties.

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Figure 4.24. Sample comparison coefficients for each possible pair of DZ distributions from the Adavale Basin data arranged in an individual coefficient matrix for each method. Sample labels exemplary shown for the cross correlation coefficient, samples compared within the same formation highlighted by a bold black outline. Coefficient scale for D-statistic of the K-S test and V-statistic of the Kuiper test (marked with *) is inverted, i.e. high similarity coefficient is 0, and low similarity is 1. Data shows varying degrees of variance between methods and tendency of some methods to over- (BPC, Similarity Value) or underestimate (p-values K-S and Kuiper Tests) similarity.

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Table 4.6. Summary of basic statistical parameters for each conducted sample comparison method. Probability distributions are documented in Fig. 4.25. Underlying data is documented in Appendix 4.5

Method Variance Minimum Maximum Median Cross Correlation Coefficient 0.073 0.00 0.96 0.42 Likeness Value 0.022 0.12 0.80 0.48 Similarity Value 0.017 0.30 0.91 0.73 K-S Test (p-value) 0.008 0.00 0.58 0.00 K-S Test (D-value) 0.035 0.09 0.88 0.33 Kuiper Test (p-value) 0.007 0.00 0.51 0.00 Kuiper Test (V-value) 0.028 0.15 0.88 0.44 Bayesian Population Correlation 0.027 0.26 1.02 0.80

Figure 4.25. KDE plot (A) and cumulative distribution (B) of sample comparison coefficients for all tested methods for the 15 DZ samples from the Adavale Basin, resulting in 105 coefficients per method from pairwise sample comparison. Coloured coefficient scale corresponds to Fig. 4.24 (0, low similarity; 1, high similarity). Data illustrates high variance for the CCC as opposed to other methods, which exhibit lower variance and are left or (extremely) right skewed. KDE bandwidth=0.1, normalised to 0.3. Abbreviations: KUIP, Kuiper Test (p-value); KSP, K-S Test (p-value); CCC, Cross Correlation Coefficient; LV, Likeness Value; SV, Similarity Value; 1-KUIV, Kuiper Test (1-[V-Value]); 1-KSD, K-S test (1-[D-Value]); BPC, Bayesian Population Correlation.

4.5 DETRITAL RUTILE U-PB GEOCHRONOLOGY

Detrital rutile grains were mounted for 10 out of 15 assessed samples from the Adavale Basin. The concordance rate of rutile from individual samples (concordant analyses/total analysed rutiles) ranged from 4-51%. Discordance is the result of large proportion of common Pb in almost all grains. No common Pb correction was attempted but instead model-1 discordia ages (Ludwig, 1998) are calculated for visually coherent age clusters based on the assessment of Tera-Wasserburg (207Pb/206Pb vs 206Pb/ 238U, Fig. 4.27, Tera and Wasserburg, 1972) and Wetherill-concordia diagrams (207Pb/235U vs 206Pb/238U, Fig. 4.26

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Wetherill, 1956). This permits incorporation of the large number of discordant analyses on a single sample basis, however, it is important to state that the detrital rutiles commonly show a spread in ages which is not fully represented by the discordia age. The obtained discordia model-1 age can be interpreted as an equivalent to peak ages in the KDE or PDP of concordant ages. Based on transmitted light imagery, grain morphologies of the analysed detrital rutile grains have been quantified using threshold techniques within the object separation procedure described in Section 3.1.1 in Chapter 3. Median values of the longest grain axis are given together with a summary of the assigned discordia ages in Table 4.7. The full dataset with integrated U-Pb ages, trace element, and grain morphology data is given in the electronic Appendix B. Appendix 4.7 comprises transmitted light and SEM images of all analysed rutile grains.

Eastwood Beds Detrital rutile grains are present in both heavy mineral separates from the two sample locations of the Eastwood Beds in the north-eastern Adavale Basin. Backscatter imagery shows rutiles from both samples appear rather unaltered and are nearly free of inclusions. Recovered grains are rounded to sub-rounded and similar in size, with a median length of 175 µm in CAR-13 and 165 µm in ALL-6. Out of 43 analysed rutiles for CAR-13, 33 analyses were discordant, and 4 analyses were rejected due to very low U concentrations (< 2 ppm; meaning very poor counting statistics for an age calculation); six analyses were concordant resulting in an overall concordance rate of 14%. One concordant analysis yielded an age of ~630 Ma, and 5 analyses scatter between 1.7 and 2.5 Ga. In the Wetherill- discordia plot (207Pb/235U vs 206Pb/238U) two age populations are apparent, ~500 Ma and ~1.5 Ga (Fig. 4.26). The major younger grouping yields a discordia model-1 age of 492.1 ± 6.4 Ma (n=33) and an older population is identified at 1651.3 ± 43.2 Ma (n=7). For the second sample (ALL-6) 51 rutiles were analysed, 19 analyses yielded discordant results, 6 analyses were rejected due to low U contents, and 26 analyses yielded concordant results (51% concordance rate). The majority of the concordant ages fall between 520 and 770 Ma (n=16) with a peak age at ~630 Ma, and 10 grains yielded older ages between 1190 Ma and 3360 Ma (Fig. 4.26). Three main populations are apparent from the discordia plot, the major population at ~600 Ma, a minor cluster at ~2.0 Ga and a subordinate cluster ~1.3-1.5 Ga. A discordia model-1 age yields an age of 631.6 ± 4.8 Ma (n=29) for the youngest cluster.

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Lissoy Sandstone Only one mineral separate out of three sampled sandstones from the Lissoy Sandstone yielded any rutile (LOG-4), where the other two samples were barren of any rutile with the relevant heavy mineral separates dominated by pyrite (CAR-14) and iron-oxides (GUM- 6). Rutiles recovered from LOG-4 are fairly large with a median length of 228 µm, grains appear rounded to sub-rounded, and a few grains show some minor alteration. Out of 47 analysed grains, 44 analyses returned discordant results and one analysis was rejected due to a low U concentration. Two grains dated at 567 and 590 Ma yielded concordant results. From the discordia plot, only one age population is apparent, yielding a discordia model-1 age of 540.2 ± 4.1 Ma (n=46, Fig. 4.26).

Etonvale Formation Three out of four samples from the Etonvale Formation yielded rutile in the heavy mineral separate, although one separate was swamped with detrital pyrite and contained no rutile (FAI-1). The three samples show high variations in median grain length with very small grains in BUC-1 (97 µm), medium-sized grains in GUM-7 (131 µm) and the largest grains in LOG-5 (192 µm). Some rutiles from LOG-5 show heterogeneities on backscatter images and some minor alteration. For BUC-1, out of 40 analysed rutiles, 19 yielded discordant results and 5 analyses were rejected due to low U concentrations (<2ppm). Sixteen analyses yielded concordant data, resulting in an overall concordance rate of 40%. Concordant grains were dated between 562 and 830 Ma, forming two populations at ~615 and ~700 Ma. Excluding the two oldest concordant analyses at 830 and 784 Ma, the sample yields a discordia model-1 age of 648.1 ± 5.0 Ma (n=33), which lies between the two concordant populations (Fig. 4.26). Out of 37 analysed rutiles for GUM-7, 29 analyses were discordant, one analysis was rejected due to low U concentration (<2ppm), and 7 analyses yielded concordant results (19% concordance rate). Concordant grains fall in two clusters, a major one ~600 (n=5) and two older analyses at 1570 and 1860 Ma. The discordia model- 1 age at 610.9 ± 5.0 Ma (n=33) aligns with the younger population indicated by the concordant ages. Rutiles from LOG-5 are substantially larger than the two previously presented samples from the Etonvale Formation (Table 4.7). Out of 38 analysed grains, 30 yielded concordant results, 3 analyses were rejected due to low U contents (<2ppm). Five

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grains yielded concordant results (concordance rate 13%) and form a population at ~580 Ma, whereas the discordia model-1 age suggests an age of 553.2 ± 6.4 Ma (n=35).

All three analysed samples from the Etonvale Formation show significantly differing age populations at ~650 , ~610 and ~550 Ma, which is correlated with the median rutile grain size in the respective samples (Table 4.7). The oldest discordia age corresponds to the smallest median grain size (BUC-1, 97 µm, 648.1 ± 5.0 Ma) and the largest median grain size to the youngest discordia age (LOG-5, 192µm, 553.2 ± 6.4 Ma).

Buckabie Formation (lower section) Rutile was analysed from 2 out of 3 investigated samples. Rutile was absent in LOG- 6 where ilmenite was instead present in the heavy mineral fraction of this sample. BUC-3 yielded above average and FAI-2 below average. Rutiles from FAI-2 and BUC-3 have a median length of 112 and 127 µm and appear sub-rounded and unaltered in backscatter images (Appendix 4.7). From the heavy mineral separate prepared for FAI-2, 23 rutile grains were recovered and analysed, 13 analyses returned discordant ages, 2 grains were rejected due to low U concentrations, and 8 analyses yielded concordant results, resulting in a concordance rate of 35%. The 8 concordant ages form a cluster at ~625, which is further supported by a discordia model-1 age of 616.7 ± 5.1 Ma (n=20, Fig. 4.26). One older discordant analysis at ~2.0 Ga is identified in the discordia plot. For BUC-3, 46 grains were analysed with 42 analyses returning discordant ages, one analysis was rejected due to low U contents and 3 concordant ages were obtained resulting in another low concordance rate (7%). The concordant analyses yielded ages of 575, 614 Ma and 2.7 Ga. The discordia model-1 age indicates a rutile population age of 618.5 ± 5.9 Ma (n=44), and one discordant age is an outlier at ~2.2 Ga.

Buckabie Formation (upper section) For the upper section of the Buckabie Formation, rutile was recovered from BUC-5 and LOG-7, but was absent in sample FAI-3. Rutile yields were below the overall average of all analysed samples for BUC-5 and LOG-7. Rutiles from this formation appear sub- rounded and appear moderately fresh in backscatter images. Median grain lengths are very variable between the two samples, with larger rutiles in LOG-7 (232 µm) and significantly smaller grains in BUC-5 (142 µm). Twenty-seven rutiles have been analysed for LOG-7,

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and 16 analyses yielded discordant ages, 7 analyses were rejected due to very high uncertainties for 207Pb/235U and 206Pb/238U, respectively. Two analyses were rejected based on low U concentrations and only two analyses yielded concordant results giving ages of 1.6 and 1.8 Ga (7% concordance rate). Additionally, a younger population which is not represented by any concordant ages is apparent from the discordia diagram and exhibits a discordia model-1 age of 455.5 ± 10.7 Ma (n=16, Fig. 4.26). From BUC-5, 35 rutile grains were analysed, 22 yielded discordant ages, 3 analyses were rejected due to low U contents, and 10 analyses showed concordant ages, ranging from 535 – 774 Ma with a peak age of ~640 Ma. The discordia model-1 age of 640.2 ± 5.2 Ma (n=31) overlaps with this peak concordant rutile age. One older discordant grain is dated at ~2.0 Ga.

Summary For the Adavale Basin, a total of 387 detrital rutiles from 10 sandstone samples were analysed, where 85 analyses provided concordant ages (22% concordance rate overall). Rutile grains are predominantly rounded and the median rutile sizes correlate overall with the median zircon sizes presented in Section 4.4 (on a per sample basis). Rutile yields are very low in the northernmost sample location ASO Fairlea-1 with rutile being absent or yields below average, whereas in the southern location (PPC Buckabie-1) yields are above or around average, regardless of the sampled formation. Overall rutile yields are noticeably lower for all samples of the Buckabie Formation compared to the other sampled formations. Concordant U-Pb ages range from 523 to 784 Ma with a major age population at 615 Ma and a subordinate age cluster at ~1.8 Ga but part of a subpopulation of rutile grains ranging in age from 1.7 to 1.9 Ga (Fig. 4.29a). Isolated concordant grains were dated at 830 Ma, 1.19, 1.57, 2.18, 2.19, 2.33, 2.54, 2.69 and 3.36 Ga. Discordia ages on a single sample basis show that besides the older age cluster (610-650 Ma, also represented in the major peak of the concordant data), a younger grouping between 450-550 Ma is observed in some samples (Table 4.7). The oldest grouping at ~1.8 Ga forms a substantial population in the two samples from the Eastwood Beds, and isolated grains in one sample from the Etonvale Formation (GUM-7) and a number of samples from the Buckabie Formation (FAI-2, BUC- 5, BUC-3, LOG-7). Discordia ages of the younger grouping (450-550 Ma) are observed for all sampled formations in PPC Log Creek 1 and apart from this location, only in CAR-13 (Eastwood Beds).

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Integration of discordia age and grain size data on a single sample basis shows a relationship between the two major discordia age groups and median rutile grain size in the corresponding sample (Fig. 4.26). Rutiles from samples that yielded model-1 discordia ages from the older grouping (610-650 Ma) have median lengths of 100-160 µm, whereas the median lengths for the samples from the younger population (450-550 Ma) range between 190 and 230 µm, indicating smaller median grain sizes for the rutile grains (Table 4.7). Further investigation of the relationship between grain size and age population on the basis of individual analysis is hampered by the very low concordance rate. To further investigate the correlation between grain size and U-Pb age of detrital rutile, all analysed rutiles were grouped, regardless of sample affiliation, into grain size fractions and discordia ages were assigned to each size fraction (Fig. 4.28). The data show a consistent pattern of discordia ages decreasing from small to large grain size fractions, confirming the correlation of age and size apart from sample affiliation. This correlation could not be reproduced by grouping U-Pb ages by sphericity values. However, all samples exhibit elevated MSWD values for model-1 discordia ages, and a visible spread of concordant and near concordant analyses in the Tera-Wasserburg plot reveals some statistical inaccuracies with the population assignment based on detrital rutile grain sizes.

Detrital rutile compositions Trace element data obtained from LA-ICP-MS analysis of all analysed rutiles are utilised to investigate rutile source lithologies (mafic/pelitic) using the updated Cr/Nb classification (Meinhold et al., 2008). Rutiles with negative log(Cr/Nb) values are indicative of metapelitic (felsic) sources (with the exception of rutiles with Nb contents <800 ppm), and analysis with positive log(Cr/Nb) values or Nb contents > 800 ppm are derived from metamafic sources (Triebold et al., 2007). According to the Cr/Nb classification, the majority of the Adavale Basin rutiles (~75%) has metapelitic sources, and 25% metamafic sources (Fig. 4.29c). The proportions of metapelitic and metamafic rutiles for the two dominant age populations (i.e. 610-650 Ma and 450-550 Ma) are both consistent with the ratios for the bulk data. Hf concentrations for the analysed rutiles range from ~1 to ~150 ppm with a mean concentration of 35 ppm and a median value of 28.6 ppm, which is in the typical range for rutile (Meinhold et al., 2008).

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Figure 4.26. Wetherill-discordia plots of detrital rutile U-Pb data for all analysed samples, highlighting overall high discordance of analysed rutiles. Discordia model-1 ages are calculated for the main population in all samples and results are given in the individual plot. Analysis from older age groupings are greyed out and not considered here for calculation of model ages. Concordant ages per sample shown as histogram and PDP with peak ages in the insets.

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Figure 4.27. Tera-Wasserburg plot of 207Pb/206Pb versus 238U/206Pb rutile data identifying common lead as a major source for high discordance of the analysed rutiles.

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Figure 4.28. Tera-Wasserburg plots for all detrital rutile U-Pb data grouped by individual grain sizes (determined by length of individual rutile grains), very fine sand (A), fine sand (B, C), and medium sand (D). Data shows consistently younging discordia model-1 ages with increasing grain size fraction.

Table 4.7. Summarised data from U-Pb analysis of detrital rutile for 10 analysed samples. For sample LOG- 7 an additional 7 analyses were rejected due to large analytical uncertainties (>50%). Discordia model-1 ages show generally good agreement with PDP peak ages, especially when number of concordant analyses is sufficient (n>5). Abbreviations: #, number of; anal., analysed; conc., concordant; disc., discordant; PDP,

Probability Density Plot.

[µm]

length length

#anal.

# disc. # #conc.

#low U #low discordia age main PDP peak older

%conc. median Sample Unit pop. [Ma] age [Ma] ages CAR-13 Eastwood Beds 43 4 6 33 14 492.1 ± 6.4 (n=32) 630 (n=1) n=7 175 ALL-6 Eastwood Beds 51 6 26 19 51 631.6 ± 4.8 (n=29) 630 (n=13) n=16 165 LOG-4 Lissoy Sst 47 1 2 44 4 540.2 ± 4.1 (n=46) 580 (n=2) - 228 BUC-1 Etonvale Fm 40 5 16 19 40 648.1 ± 5.0 (n=33) 655 (n=16) n=2 97 GUM-7 Etonvale Fm 37 1 7 29 19 610.9 ± 5.0 (n=34) 600 (n=5) n=2 131 LOG-5 Etonvale Fm 38 3 5 30 13 553.2 ± 6.4 (n=38) 580 (n=5) - 192 FAI-2 Buckabie Fm (l) 23 2 8 13 35 616.7 ± 5.1 (n=20) 625 (n=8) n=1 112 BUC-3 Buckabie Fm (l) 46 1 3 42 7 618.5 ± 5.9 (n=38) 600 (n=2) n=1 127 LOG-7 Buckabie Fm (u) 27 2 2 16* 7 455.5 ± 10.7 (n=16) - n=2 232 BUC-5 Buckabie Fm (u) 35 3 10 22 29 646.9 ± 5.8 (n=31) 640 (n=10) n=1 142

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Zr-in-rutile temperatures are calculated after Watson et al. (2006) using Equation 4.1, assuming ~10kbar of pressure during rutile genesis.

4470 푇(°퐶) = (푍푟 [푝푝푚]) − 273 7.36 − 푙표𝑔10 (Equation 4.1) The Zr-in-rutile temperatures for concordant and discordant rutiles lie between 482 and 897°C disregarding extreme outliers (Fig. 4.29b). The data show a unimodal peak at ~700°C for both age populations (mean value 696°C, median 698°C, Fig. 4.29d). Overall, the obtained Zr-in-rutile temperatures suggest the analysed rutiles were formed under upper

Figure 4.29. Summary of U-Pb and trace element data from detrital rutile analysis via LA-ICP-MS. (A) Age distribution of concordant U-Pb rutile ages plotted as KDE, (B) Zr-in-rutile temperatures calculated after Watson et al. (2006) for all analysed samples (black boxplots) and summarised for all data (grey boxplot). Median value indicated by horizontal line within the box comprising 50% of the data, averages shown as point, outlier indicated by open circles, far outliers by triangle symbols. (C) Nb/Cr classification to constrain rutile source lithology (Meinhold, 2010), plotted for the identified older (blue data points) and younger population (orange data points) showing similar source rocks for both age groups. (D) KDE plot of Zr-in- rutile temperatures for both age groups indicating virtually superposable temperature distributions for both age groups.

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amphibolite, mid-eclogite to lower granulite facies conditions. Dispersion across the fields for metamafic and metapelitic in the Cr/Nb classification has been suggested to be indicative of amphibolitic sources because their protoliths can have a wide range of source protoliths (Meinhold, 2010).

4.6 DISCUSSION

The results presented above highlight a number of important aspects of the dataset, and have significance for the interpretation of sediment provenance for the Adavale Basin. Assessment of DZ morphology highlights subtle morphological differences between the major age populations (Section 4.4.4), which is potentially indicative of predominant source rock lithologies that are discussed in the context of the DZ data at a regional scale in Section 4.6.1. Subtle spatial and very distinctive temporal trends are evident from the dataset, implying changing sediment provenance over time and potentially changing sediment composition across the basin indicating downstream effects (e.g. Cawood et al., 2003; Niemi, 2013). If downstream effects are significant, this could affect intrabasinal stratigraphic correlations, and, at a more regional scale, basin correlations over larger distances. The results from the DZ sample comparison methods applied in Section 4.4.5 are utilised in Section 4.6.2 to investigate the spatial and temporal trends of DZ distributions based on individual samples. The detrital rutile U-Pb age data show some conspicuous correlations between grain size (smaller) and U-Pb age (older). Potential sources of detrital rutile in the Adavale Basin are discussed based on integrated U-Pb geochronology and chemical classification data (Section 4.6.3).

4.6.1 Sources of detrital zircon Ages of igneous units from the Mossman, New England and Thomson orogens have been compiled using existing U-Pb age compilations (Jones et al., 2018, Purdy et al., 2018, Siegel 2015), and organised in a GIS database. References for the individual interpreted emplacement ages are given in the text together with an identification number referring to the location of the respective U-Pb age on Fig. 4.31. All compiled ages are U-Pb zircon ages (SHRIMP or LA-ICP-MS) unless stated otherwise. In addition, DZ age populations that are potentially derived from recycled metasedimentary rocks are discussed in the last

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part of this section. The data utilised in this section in Appendix 4.8 includes all isotopic ages, references and coordinates of sample locations.

Devonian volcanic sources (350-400 Ma)

Zircons of Lower Devonian to Lower Mississippian age (420 – 360 Ma) account for ~35% of the total DZ in the Adavale basin, with the highest proportions having ages between 370 and 400 Ma. MDA of ~400 Ma are constrained for rocks of the Lower and Middle Devonian units of the Eastwood Beds and the Lissoy Sandstone (Section 4.4.1). Silicic volcanic source lithologies are confined to the central part of the Thomson Orogen as evident from the basal volcanic rocks of the Gumbardo Formation at the base of the Adavale Basin with ages between 402.9 ± 2.9 Ma (97, 98; Draper, 2006; Cross et al., 2018) and 397.6 ± 3.5 Ma (110-113, 115; Asmussen et al., 2018). Apart from the Gumbardo Formation volcanic rocks of this age are only found in a crystal-rich, welded ignimbrite in drill core 200 km north of the Adavale Basin in APC Thunderbolt 1 dated at 392.9 ± 2.7 Ma (95, Kositcin et al., 2015). Other igneous sources of this age are exclusively intrusive, and were most likely not exposed at the time to contribute to the Lower and Middle Devonian sedimentary rocks in the Adavale Basin. A granite dated at 396.0 ± 2.2 Ma is intersected in AOP Balfour, just east of the Adavale Basin (24; Siégel et al., 2018). Apart from the central Thomson Orogen, intrusive rocks between 410 and 390 Ma have also been identified in the southern Thomson Orogen. A micromonazite from the Tincheloooka Diorite was dated at 401.8 ± 3.1 Ma (28; Bodorkos et al., 2013) and a granite from the Conlea Porphyry at 398.0 ± 2.8 Ma (Fraser et al., 2014). A large group of Lower Devonian plutonic rocks is also identified in far north Queensland across the Mossmann Orogen with emplacement ages between 395.0 ± 4.0 and 409.0 ± 7.0 Ma (23, 25, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41; Black et al., 1992).

It is likely that ~400 Ma DZ populations as evident from MDA (Section 4.4), as well as a major population from the Eastwood Beds and a subordinate population from the Lissoy Sandstone (Section 4.4) are derived predominantly from the volcanic rocks of the Gumbardo Formation; a high modal abundance of K-feldspar in all samples from the Lissoy Sandstone further supports this assertion (Section 4.3.1). The Eastwood Beds, however, are virtually devoid of any feldspar, and volcanic rock fragments are absent in samples from

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both formations (Section 4.3.1). The proportions of 400 Ma zircons decrease precipitously up stratigraphy and grains of this age are almost totally absent in the samples of the Etonvale Formation and are rare in the Buckabie Formation (Fig. 4.18). During deposition of the Etonvale Formation the ~400 Ma zircons are replaced by a younger age signal ~380 Ma initiating a change in DZ frequency distributions in the basin during the Middle Devonian. In the Upper Devonian, another younger DZ population with ages 360-370 Ma is introduced (Buckabie Formation) shown by the youngest age peak in the KDE plots and the MDA, respectively (Section 4.4).

Igneous rocks ages between 390 and 360 Ma are absent in the central part of the Thomson Orogen, apart from a volcanic rock intersected in AAE Towerhill 1, 250 km north of the Adavale Basin, dated at 381.7 ± 5.7 Ma (94; Cross et al., 2018). Further north, in the Mossman Orogen, the Mount Formartine Granite has been dated between 375.2 ± 1.9 and 378.8 ± 2.7 Ma (15, 16, 17; Kositcin et al., 2015a). South of the Adavale Basin, granitic rocks in this age bracket have been intersected and dated in drill hole TEA Tickalara 1 (7, 360.0 ± 8.9 Ma; Siégel et al., 2018), PPL Omicron 1 (13, 369.1 ± 7.9 Ma; Siégel et al., 2018) and PPL Noccundra 1 (14, 373.3 ± 9.3 Ma; Siégel et al., 2018). The outcropping Currawinya Granite is dated at 381.5 ± 2.4 Ma (19, Cross et al., 2015) and Eulo Granite is 385.0 ± 2.5 Ma (21, Cross et al., 2012). To the east of the Adavale Basin, the Scalby Granite, situated on the subsurface Nebine Ridge, is dated at 368.4 ± 2.5 (10, Kositcin et al., 2015b), and further east in the area of the Roma Shelf, two granites were dated at 362.3 ± 3.1 and 363.8 ± 1.6 Ma in drill hole Santos Javel 2 (8, 9; Siegel, 2015). Any contribution of the aforementioned Middle and Upper Devonian intrusive rocks, which have been emplaced synchronous to deposition of the sediments in the Adavale Basin, seems unlikely, as they were unexposed during the Devonian. The sandstones contain very few plutonic lithic clasts.

Silicic volcanic rocks of Upper Devonian age are observed from the Anakie Province and further east in the northernmost part of the New England Orogen. In the Anakie Province volcanic rock samples of the Silver Hills Volcanics have been dated 363.0 ± 2.7 Ma (89; Cross et al., 2009), 365.3 ± 4.6 and 371.2 ± 6.1 Ma (91, 92; Henderson et al., 1998), but these ages might be slightly skewed due to inherited zircons with ages between 370-380 Ma as indicated by subtle zircon inheritance in the Campwyn Volcanics in the New

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England Orogen exhibiting emplacement ages between 354.7 ± 3.3 and 364.6 ± 3.4 Ma (90, 93; Bryan et al., 2004).

In addition, the Bimurra Volcanics in the northern part of the Anakie Province were dated at 360.0 ± 2.5 Ma (88, Cross et al., 2009). Middle to Upper Devonian (Frasnian) igneous units appear to be much less voluminous and emplacement ages are poorly constrained from an MDA of 382.0 ± 7.0 Ma for the Theresa Creek Volcanics outcropping in proximity to the Silver Hills Volcanics in the southern Anakie Province (104, Cross et al., 2015).

Upper Devonian to Early Carboniferous DZ (370-350 Ma) in the Adavale Basin are contained in large proportions in rocks from the Etonvale (8%) and Buckabie Formations (25 – 38%) and were most likely sourced from the Middle to Upper Devonian volcanic rocks in the Anakie Province and silicic volcanic source rocks in the northernmost part of the New England Orogen (i.e. Bimurra Volcanics, Silver Hills Volcanics, Campwyn Volcanics). The U-Pb zircon ages of the volcanic source rocks are synchronous to the deposition of the Buckabie Formation, which is evident from the biostratigraphic age assignment and the MDA (Section 4.4.1). The Middle Devonian DZ population ~380 Ma introduced during deposition of the Etonvale Formation is distinctively different to the younger 370-350 Ma grouping and is evident from peak ages in the Etonvale Formation and large zircon populations forming MDA for LOG-5, FAI-1 and LOG-6 (cf. Table 4.5). Volcanic rocks of Middle Devonian age are only known from the Theresa Creek Volcanics in the southern Anakie Inlier. Furthermore, emplacement of the Retreat Batholith occurred during the Middle Devonian (Cross et al., 2018), but contribution from the intrusions seems very unlikely as it was presumably not exposed. While the youngest MDA in the Upper Devonian successions of the Adavale is ~355 Ma, indicating that reworking of the lowermost volcanic successions in the area of the Drummond Basin played a role during deposition of the uppermost part of the Buckabie Formation, the youngest major age peaks remain at ~370 Ma potentially reflecting a mixture of inheritance in the Silver Hill Volcanics and ongoing contribution from volcanic source rocks ~380-370 Ma.

Volcanic lithic clasts are abundant in rocks of the Etonvale and Buckabie formations (Section 4.3.2). For the Etonvale Formation, the northern- and southernmost sample locations show ~5% volcanic lithics in the modal abundance (FAI-1, BUC-1) and are absent in the other central basin sample locations. In the lower section of the Buckabie Formation about 3 - 4% volcanic lithic grains are present in the northernmost (FAI-2) and in the central

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sample location (LOG-6), and no volcanic lithics are observed in the southernmost location (BUC-3). A strong spatial trend in the modal abundance of volcanic lithics is observed for the upper section of the Buckabie Formation with proportions increasing towards the northern part of the Adavale Basin, which is closest to a potential source region in the Anakie Province (Fig. 4.4; 3.0% in BUC-5, 11.0% in LOG-7 and 24.7% in FAI-3). Volcanic lithic fragments in the Buckabie and Etonvale Formations comprise a variety of lithologies, including rhyolitic, intermediate and mafic compositions (Section 4.4) and the Anakie Inlier could have supplied such given the Silver Hills Volcanics exhibit a dacitic to rhyolitic composition (Blake et al., 2013), and the Theresa Creek Volcanics, Dunstable Volcanics and Greybank Volcanics, which are mafic to andesitic in composition (Blake et al., 1995).

Silurian plutonic/volcanic sources Silurian to Lower Devonian igneous rocks with emplacement ages between 440 and 410 Ma in the Thomson Orogen are predominantly plutonic and primarily encountered in the southern Thomson Orogen. A Silurian granite was dated at 429 Ma in the central part of the Adavale Basin using the Rb-Sr method for whole rock and feldspar concentrate (64, PPC Etonvale 1; Lewis and Kyranis, 1962). Volcanic rocks are only present in the eastern portion of the southern Thomson Orogen, represented by the Louth Volcanics dated 411.0 ± 6.3 Ma (106; Dwyer et al., 2018) and the Warraweena Volcanics dated at 414.0 ± 4.0 and 417.0 ± 3.5 Ma (108, 109; Hack et al., 2018). Silurian granitic rocks in the western part of the southern Thomson Orogen are grouped into the Tibooburra Suite (60, 62, 57 Armistead and Fraser, 2015; 50, 59; Black, 2007; Vickery, 2010), Wolgolla Granite (45, 47; Siégel et al., 2018), Ella Granite (58; Cross et al., 2018; Draper, 2006) and Warrata Group (53, 55 Black, 2006). In the central part of the southern Thomson Orogen, the Hungerford Granite yielded an age of 419.1 ± 2.5 Ma (46: Cross et al., 2018; Purdy et al., 2016) and the Brewarrina Granite dates 420.9 ± 2.3 Ma (51: Bodorkos et al., 2013). Outside of the southern Thomson Orogen, an unnamed granodiorite in the southernmost Anakie Province was dated 419.5 ± 3.7 Ma (48, Cross et al., 2015), another spatially isolated intrusion is intersected in LOL Longreach 3 and dated at 422.6 ± 10.4 Ma (54; Siégel et al., 2018).

The proportions of DZ of Silurian age are very small in the Adavale Basin, especially for ages between 440 and 420 Ma (Section 4.4.2). Silurian igneous rocks in the central part of

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the Thomson Orogen are rare, however high proportions of inherited Silurian zircons in Early Devonian zircons from volcanic rocks of the Gumbardo Formation indicate a more extensive distribution of Silurian igneous basement rocks than represented by two drill core intersections of Silurian granitic rocks in the central Thomson Orogen (Asmussen et al., 2018). The relatively small proportions of Silurian DZ can generally be explained by the predominantly intrusive nature of exposed Silurian igneous rocks, which were potentially not extensively exposed during sedimentation in the Adavale Basin during the Devonian.

Ordovician volcanic and plutonic sources Zircons of Ordovician to Late Silurian age (490 – 440 Ma) account for ~35% of total DZ in the Adavale Basin with the highest proportions in the age interval between 485 and 465 Ma and are present in all sampled formations (Section 4.4.2). Intrusive and subordinate extrusive igneous rocks of Ordovician age are widely distributed in the Thomson Orogen and are present in the subsurface of the central Thomson Orogen, as well as outcropping in the southern Thomson Orogen, Anakie Province, Charters Towers and Greenvale Province and may represent potential sources for the Ordovician aged DZ in the Adavale Basin (Fig. 4.31).

In the Anakie Province, the Mooramin Granite was dated 463.0 ± 15.0 Ma (73; Fergusson et al., 2013), the Coquelicot Tonalite 471.0 ± 3.8 Ma in the northern part of the Anakie Province (76; Cross et al., 2018) and the Gem Park Granite in the southern Anakie Province at 443.3 ± 6.2 Ma (65; U-Pb monazite SHRIMP age, Fergusson et al., 2013). In the Charters Towers Province, Ordovician granites are present in the Fat Hen Creek Complex (64, 66; Hutton, 2004), Grass Hut Granite, Charters Towers Metamorphics, Sunburst and Schreibers Granodiorite as part of the Ravenswood Batholith (72, 74, 81, 83; Hutton and Rienks, 1997) and Mount Windsor Volcanics (102, 103; Kositcin et al., 2016). In the Greenvale Province, metaporphyries from the Balcooma Metavolcanic Group were dated 471.0 ± 4.0 Ma and 478.0 ± 5.0 Ma (79; Withnall et al., 1991) and the Saddington Tonalites dates 488.0 ± 2.7 Ma (82; Henderson et al., 2013). The Granite Springs Granite in the southern Thomson Orogen was dated 455.6 ± 5.4 Ma (68; Cross et al., 2015).

Ordovician silicic rocks, both plutonic and volcanic, are abundant in the subsurface of the central Thomson Orogen, either underlying the Adavale Basin or occurring in proximal to the basin (100 – 150 km). The informal Maneroo Volcanics comprise lower Ordovician

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crystal-rich rhyolitic ignimbrites and porphyritic shallow-level intrusions (Carr et al., 2014) in three drill holes clustered in the northern part of the central Thomson Orogen (99, GSQ Maneroo 1; 100, BEA Coreena 1, 101, PPC Carlow 1; Cross et al., 2018; Draper, 2006; Purdy et al., 2016). A porphyritic rhyolite intersected in AOD Yongala 1 at the boundary between the Adavale Basin and Warrabin Through was dated 488.6 ± 14 Ma (Section 2.1.1 of Chapter 2, Asmussen et al., 2018) and might be associated with the Maneroo Volcanics. Granitic rocks of Ordovician age are intersected in LEA Albilbah 1 (453.8 ± 8.4 Ma), BEA Valetta 1 (458.6 ± 8.4 Ma), PPC Lissoy 1 (462.0 ± 19.2 Ma), AMX Toobrac 1 (465.7 ± 5.3 Ma), AOD Budgerygar 1 (470.3 ± 3.6 Ma) and LOL Stormhill 1 (477.9 ± 2.0 Ma; 67, 69, 70, 75, 78, 80; Siégel et al., 2018).

The Ordovician-aged plutonic and volcanic rocks underlying the Adavale Basin potentially represent a major source for the abundant Ordovician DZ in the basin. In addition to the various intersections in boreholes, a more widespread distribution of Ordovician igneous basement rocks has been interpreted from gravity and magnetic subsurface data (Frogtech Geoscience, 2018). Ordovician basement rocks were presumably extensively exposed during the development of the half-graben system forming the initial Adavale Basin in the Early Devonian. Potentially, intrusive Ordovician rocks from the Charters Towers, Greenvale and Anakie Provinces may have contributed as semi-distal sources to the basin. In addition, a dominant Ordovician population is evident from a single DZ sample from the Nebine Metamorphics from GSQ Eulo 1 (Fig. 4.30, A. Cross, unpublished data), indicating that DZ of Ordovician age might also be derived from metasedimentary rocks.

The trace element data for a group of Ordovician DZ are characterised by conspicuously low Th/U (<0.3) and show characteristics of xenotime substitution that is indicative of an S-Type granite affinity (Section 4.4.3, Allen et al., 2018). Ordovician granites with S-Type affinities are evident from many locations, e.g. Fat Hen Creek Complex, Charters Towers Province (Hutton, 2004), the Mooramin Granite (Crouch et al., 1994) and Gem Park Granite (Fergusson et al., 2013b) in the Anakie Province, AMX Toobrac 1, in the subsurface central Thomson Orogen (Champion and Bultitude, 2013) and Granite Springs Granite, southern Thomson Orogen (Purdy et al., 2016b).

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Recycled (meta)sedimentary sources Late Neoproterozoic to Cambrian metasedimentary rocks are widespread in the Thomson Orogen and are encountered as upper greenschist- to amphibolite-grade metasedimentary rocks in the Anakie and Charters Towers Provinces and low-grade marine siliciclastic metasedimentary rocks in boreholes as basement rocks of the Thomson Orogen. The two dominant DZ age peaks in the metasedimentary rocks are identified between 0.9 - 1.3 Ga (Grenvillian age) and 650 - 500 Ma (Late Neoproterozoic to Early Cambrian, LN-EC). Based on the respective proportions of these two distinct DZ age populations, the source metasedimentary rocks have been assigned into two groups. The ‘Syn-Petermann’ signature with a dominant Grenvillian-aged DZ population , and the ‘Pacific-Gondwana’ signature with a dominant LN-EC group and a less dominant Grenvillian population (e.g. Ireland et al., 1998; Purdy et al., 2016b). The ‘Syn-Petermann’ signature is apparent from the Machattie Beds at the boundary between the North Australian Craton and the Thomson Orogen, as well as in the Cape River, Argentine and Bathampton Metamorphics in the Anakie and Charters Towers Province (Fergusson et al., 2001; Purdy et al., 2016a). Metasedimentary rocks bearing the ‘Pacific-Gondwana’ signature are more widespread in the Thomson Orogen and include the Mt. Windsor Volcanics and Les Jumelles Beds in the northern Anakie Province, the Thomson Beds in the subsurface central Thomson Orogen, the Nebine Metamorphics in the Nebine Ridge and the Betoota Beds in the westernmost Thomson Orogen (Figs. 4.30, 4.32, Purdy et al., 2016b). A substantial number of (meta)sedimentary rocks in the southern part of the Thomson Orogen feature a more significant DZ population ~490-500 Ma in addition to the Ediacaran population in the Thomson beds (Fraser et al., 2019). These two age peaks are also very prominent in the Adavale Basin, indicating that the southern Thomson Orogen (or now eroded rocks that previously were overlying the Thomson beds in the central Thomson Orogen) played a role in sediment provenance of the Adavale Basin.

The majority of DZ in the Adavale Basin exhibit Palaeozoic ages; however, 30% of the ages are Precambrian and older. The most prominent population for Cambrian and Precambrian zircons in the Adavale is identified at ~490 Ma and between 560 – 590 Ma, a rather broad plateau of DZ ages between ~0.8 and 1.2 Ga and a more defined age peak at ~1.5 Ga. Overall, the distribution of DZ ages older than 490 Ma resembles a ‘Pacific- Gondwana’ signature showing a pronounced population of LN-EC ages and a plateau

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comprising Grenvillian ages, which seems to be extended to 750-800 Ma (Fig 4.30). Zircons with ages between 750-800 Ma are not a typical age group in the metasedimentary rocks of the Thomson Orogen and are only apparent as small populations in the samples from the Nebine Metamorphics (GSQ Eulo 1 and GSQ Mitchell 1, Purdy et al., 2016b, A.

Figure 4.30. Compiled DZ geochronology data from the Adavale Basin (only Precambrian ages, this study) in comparison with metasedimentary rocks from the Thomson Orogen. LN-EC group is shown in blue, Grenvillian plateau in orange and 1.5 Ga population in green. Data for Thomson Beds, Mt. Windsor, Les Jumelles Beds, Betoota Beds, Nebine Metamorphics (GSQ Mitchell 1) from Purdy et al. (2016), Nebine Metamorphics (GSQ Eulo 1) from A. Cross, unpublished data. DZ distributions for Adavale Basin and Thomson Beds are shown as locally adaptive KDE, as data density and nature of the distribution is adequate, remaining samples are plotted as PDP to avoid oversmoothing and allow visual comparison.

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Figure 4.31. Distribution of potential volcanic/plutonic source rocks locations for DZ in the Adavale Basin from drill holes and outcrop. All ages are U-Pb zircon emplacement ages complied after Jones et al., 2018 and Siegel, 2015, except Gem Park Granite (65, U-Pb monazite via SHRIMP) and granite in PPC Etonvale- 1 (61, whole rock Rb-Sr). Detailed information on each sample location is documented in Appendix 4.8.

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Figure 4.32. Distribution of selected metasedimentary units in the Thomson Orogen after Purdy et al. (2016) showing the widespread distribution of the subsurface Thomson beds. Labelled drill holes and outcropping units correspond to samples with existing DZ data in Fig. 4.30.

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Cross, unpublished data). Silicic igneous rocks of this age are confined to the Boucaut Volcanics in the in the Adelaide Rift Complex (e.g. Preiss, 2000), constituting a potential source for zircons of this age in the Nebine Metamorphics. A small population of DZ ages between 1.3 and 1.4 Ga is apparent from the combined Adavale data, but these ages are practically absent in all metasedimentary units and only evident from the Nebine Metamorphics in GSQ Mitchell 1 (Fig. 4.30, Purdy et al., 2016b).

The pronounced ‘Pacific-Gondwana’ background signature in the Adavale Basin (Fig. 4.30) in conjunction with the presence of ubiquitous rounded lithics of siltstone (Section 4.4.1, Table 4.3) and the high proportions of Ediacaran high sphericity DZ (Section 4.5.4) suggest a significant volume of sediment was indeed sourced from the Thomson Orogen metasedimentary rocks exhibiting ‘Pacific-Gondwana’ DZ signatures. The relatively low proportions of Grenvillian aged zircons compared to LN-EC ages, in conjunction with a peak at 1.5 Ga in the Adavale Basin indicate that sources with ‘Syn-Peterman’ signatures (e.g. Machattie Beds, Cape River, Argentine and Bathampton Metamorphics) contributed to a lesser degree to the DZ budget, and the recycled sedimentary sources exhibit predominantly ‘Pacific-Gondwana’ signatures.

4.6.2 Spatial and temporal trends in detrital zircon distributions

Multi-sampling DZ geochronology studies can be utilised to gain insight into the evolution of sediment dispersal patterns in ancient sedimentary systems (e.g. Blum and Pecha, 2014; Link et al., 2005; Raines et al., 2013). Reconstructing the paleoflow regime of the Adavale Basin in the context of a sediment provenance study is challenging as the basin is only present in the subsurface. As a consequence, no paleocurrent data are available and no data on core orientation was acquired in the framework of the drilling campaigns that might aid in reconstructing flow directions. Using lateral grain size grading trends to constrain overall flow directions is hampered by incomplete coring of all sedimentary units across well locations in the Adavale Basin. Here, the differences and similarities of DZ frequency distributions from multiple sample locations across various units in the basin are used to constrain potential sediment transport directions and sediment source locations. Assuming the proportions of a specific DZ population are higher closer to the specific source and proportions laterally decrease for more distal sample locations, this approach

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can indicate sediment supply directions, especially when syn-basinal volcanic sources contribute to the sediment budget as indicated in Section 4.6.1 (Cawood et al., 2003; Niemi, 2013). This section utilises the results of the inter-sample comparison by means of the Cross Correlation Coefficient (CCC) in Section 4.4.5 and aims to elucidate temporal trends, indicative of provenance changes, sedimentary hiatuses or changes in tectonic setting (Cawood et al., 2012). Lateral trends between synchronous samples are investigated to assess lateral variability in the DZ distributions and gain insight into potential basin geometries (sheet-like versus restricted basin geometries, Section 1.5, Chapter 1) and potential downstream changes.

Relative proportions of DZ populations may only poorly reflect the ‘true’ proportions (Andersen, 2005), depending on the number of analyses per sample and the complexity of the DZ distribution (Vermeesch, 2004). The statistical “goodness” of the discussed data has been assessed using the Probability Model Ensembles (PME) as part of the Bayesian Population Correlation in Section 4.4.4. Sample comparison coefficients are used in the following to investigate spatial and temporal trends in DZ distributions. Fig. 4.33a shows CCC between samples from corresponding formations and highlights lateral DZ trends, whereas Fig. 4.33b utilises CCC to compare temporal trends between samples throughout the lifespan of the basin from the same or nearby sampled borehole location. Most samples are assigned to formations based on the lithostratigraphy and wireline properties (Geological Survey of Queensland, DNRME, 2014; McKillop et al., 2007), however, sample GUM-7 yielded a MDA that significantly conflicts with the assigned biostratigraphic age (Section 4.4.1) and the sample is reassigned here to the lower section of the Buckabie Formation to be more consistent with the MDA.

Lateral trends Only one sample pair is available for comparison of the Eastwood Beds, showing a moderate similarity (CCC=0.52). The samples both exhibit a dominant population at ~400 Ma with variation appearing in the relative proportion of ~480 Ma zircons, generating a moderate similarity. The samples are also compositionally similar resembling sublitharenites with > 85% quartz (Section 4.3.1).

Samples from the Lissoy Sandstone show a very high similarity across the basin with CCC values of 0.81 – 0.96, even when comparing the northernmost and southernmost

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samples, which are ~150 km apart. This suggests a very homogenous DZ distribution across the basin (Fig. 4.33a). The DZ distributions are dominated by Ordovician zircons (~480 Ma), with subordinate ~400 and ~580 Ma zircons. These populations have been attributed to primary igneous basement derived (~480 Ma, Ordovician igneous basement; ~400 Ma, reworking of the exposed basal volcanic rocks of the Gumbardo Formation, Section 4.6.1) and erosion of basement metasedimentary rocks from the central Thomson Orogen (~580 Ma, Section 4.6.1). MDA for the three samples are consistent across all sample locations showing only minimal variation suggesting synchronous deposition of the sediments (Sections 4.3.1 and 4.4.1). The QFL data show high proportions of K-feldspar (>16%) in all samples and samples are generally quartz rich (>60%, Section 4.3).

Lateral similarity is also high in the overlying Etonvale Formation ranging from 0.75 – 0.81, even when comparing samples BUC-1 and LOG-5 that are sampled in drill holes ~225 km apart from each other (Fig. 4.33a). The DZ distributions of the remaining samples from the Etonvale Formation are dominated by Ordovician-aged zircons (~480 Ma) and subordinate ~580 Ma zircons, similar to the underlying Lissoy Sandstone. A new ~380 Ma DZ population, which is not evident in the underlying Lissoy Sandstone, appears during deposition of the Etonvale Formation and shows the highest proportions in sample FAI-1, which represents the northernmost out of the three sample locations. The ~380 Ma DZ population represents a new and additional sediment contribution to the basin, supplementing the dominant basement-derived DZ populations at ~480 and ~580 Ma, which also dominated Lissoy Sandstone samples. Combining known locations of middle Devonian volcanic rocks to the north and northeast of the Adavale Basin (see Fig. 4.31), and the apparent lateral grading trends with ~380 Ma zircons being more abundant in the northern Adavale Basin, this suggests this contemporary volcanic-derived sedimentary component was sourced from regions to the north of the basin. Potential volcanic source rocks of this age have been discussed in Section 4.6.1.

Samples from the lower section of the Buckabie Formation show high lateral variability based on CCC values of 0.07 – 0.62, with the lowest value (0.07) for the comparison of the northernmost (FAI-2) and southernmost sample (BUC-3) and the highest similarity evident from comparing the northernmost (FAI-2) and central sample location (LOG-6). The dissimilarities for this unit are controlled by extremely variable proportions of ~370 Ma zircons, showing a strong north-south trend for the proportions of zircons

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between 360 and 380 Ma (FAI-2: 46%, LOG-6: 18.7%, BUC-3: 3.5%, samples ordered from north to south, see also Fig. 4.12 in Section 4.4). The observed north-south trend is consistent with the spatial distributions of volcanic source rocks with ages between 350 and 370 Ma to the northeast of the Adavale Basin in the Anakie Province and the northernmost part of the New England Orogen (Bimurra Volcanics, Silver Hills Volcanics and Campwyn Volcanics (Fig. 4.31, Section 4.6.1, Bryan et al., 2004; Cross et al., 2009; Henderson et al., 1998). The proportions of basement-derived Ordovician zircons between 460 – 480 Ma exhibit an inverse relationship to the proportions of Upper Devonian zircons (360 – 380 Ma) with proportions of Ordovician ages decreasing from north to south (FAI-2: 9%, LOG- 6: 17.7%, BUC-3: 35.9%, Fig. 4.12, Section 4.4). Comparison of samples in the upper section of the Buckabie Formation show overall moderate similarity with CCC values from 0.60 – 0.71 across the entire basin. The proportions of zircons between 360 – 380 Ma remain high (FAI-3: 55%, LOG-7: 18.5%, BUC-5: 25.7%) and are now more dispersed across all sample locations (Fig. 4.13, Section 4.4). QFL data from both sections of the Buckabie Formation show high lateral variability in the modal composition of the rocks which is reflective of the syn-depositional volcanic input (Section 4.3.1).

Temporal trends Temporal trends in the DZ frequency distributions are investigated in the following based on up-section comparison of different formations from the same, or nearby sample location (Fig. 4.33b). The data show moderate to high similarity of the DZ spectra between the Lissoy Sandstone and Etonvale Formation with values from 0.74 – 0.92. Even though the additional age population of ~380 Ma zircons appears in the Etonvale Formation, the predominance of basement-derived DZ in both formations results in moderate to high similarity coefficients between the two formations.

From the Etonvale Formation to the lower section of the Buckabie Formation, the similarity between samples decreases significantly to values between 0.25 – 0.36, and only remains high in the southernmost locations (0.81). This is consistent with the introduction of Upper Devonian ~370 Ma zircons from northerly sources into the Adavale Basin (Section 4.6.1). The data further suggest that during deposition of the lower part of the Buckabie Formation this new age signal had not yet propagated through the entire basin as

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shown by the very low proportions of ~370 Ma zircons in BUC-3 compared to the other two samples (Fig. 4.12, Section 4.4).

Comparison of individual samples from the lower and upper sections of the Buckabie Formation exhibits very high similarity for the northern (0.95, ASO Fairlea-1) and central location (0.94, PPC Log Creek-1) and very low similarity for the southernmost location (0.34, PPC Buckabie-1). The high similarity between ASO Fairlea-1 and PPC Log Creek- 1 is predominantly controlled by the consistently high proportions of Upper Devonian ~370 Ma zircons in the northern and central areas of the basin. The dissimilarities in the southern well are reflective of the difference in proportions of Upper Devonian ~370 Ma zircons between deposition of the lower and upper section of the Buckabie Formation.

Figure 4.33. Sample comparison of DZ sample pairs using the Cross Correlation Coefficient (CCC) in a lateral context (A) and temporal context (B). Well locations are organised from north to south (left to right) with all samples and their formational context in the vertical axis. The colour of the square indicates the CCC value comparing the two neighbouring samples. Sample GUM-7 is shown here in the context of the upper part of the Buckabie Formation (according to the MDA from Section 4.4.1) instead of the Etonvale Formation. In addition to BEA Allandale-1, samples taken from PPC Gumbardo-1 and PPC Buckabie-1 are duplicated on the left side of (A) to allow for sample comparison of the respective northern- and southernmost well location for each formation. Data highlights high lateral similarity for the Lissoy Sandstone and Etonvale Formation (yellow and orange colours), compared to reduced similarity and increasing lateral heterogeneity in DZ distributions for both sections of the Buckabie Formation (green and blue colours). This shift is also observed between the Etonvale and Buckabie Formations in temporal trends (B) but seems ‘delayed’ in the southern portion of the basin (as indicated by grey arrow).

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The data suggest the drastic change in sample similarity between the Etonvale and Buckabie formations is connected to the influx of an Upper Devonian and contemporary DZ population, beginning at ~380 Ma. Comparison of the lower and upper section of the Buckabie Formation records the establishment of Upper Devonian aged zircons across the entire Adavale Basin, but also reflects the delayed influx of this age population in the southern basin (Fig. 4.33b). This is most likely a consequence of the input of Upper Devonian zircons from the area of the Drummond Basin and New England Orogen from the northeast of the Adavale Basin (Section 4.6.1). The DZ in the Adavale Basin record the onset of volcanism in those areas, and the strong north-south trend of Upper Devonian zircons in the lower part of the Buckabie Formation potentially illustrates the lag time for the sedimentary system of the Adavale Basin. During deposition of the upper section of the Buckabie Formation, the signal had propagated through the entire basin and samples show moderate similarity and the samples exhibit a strong compositional trend with volcanic influence depicting a gradient from north to south (Figs. 4.33a, 4.4).

Integration of spatial and temporal DZ trends Sample comparison using multi-dimensional scaling (MDS, introduced in Section 3.1.5) allows integration of the spatial and temporal trends in DZ frequency distributions. The MDS plot in combination with normally distributed unimodal synthetic age populations for the three main DZ populations identified in Section 4.4.2 demonstrates the following key findings for spatial and temporal trends in DZ provenance for the Adavale Basin (Fig. 4.34). Basement-derived DZ with ages groupings at 400, 480 and 580 Ma dominate the DZ distributions of the Lower and Middle Devonian formations (Eastwood Beds, Lissoy Sandstone, Etonvale Formation), with slightly higher contributions of 480 Ma populations in the southern basin as opposed to the northern basin. At the same time, the MDS plot reveals the northern sample locations exhibit a subtly higher contribution of 580 Ma zircons compared to the southern basin (Fig. 4.34). A major change in DZ frequency distributions depicted by influx of Upper Devonian (Frasnian) aged zircons between deposition of the Etonvale and Buckabie formations is evident from both, temporal and lateral trends in sample similarity (Figs. 4.33a, b), as well as by a major shift in modal sandstone composition caused by an increased volcanic influence (Fig, 4.4, Section 4.3.1). The onset of syn-depositional volcanism indeed occurred prior to/or during deposition of

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the Etonvale Formation as evident from a shift in modal compositions from the Lissoy Sandstone to Etonvale Formation caused by a subtle volcanic influence (Table 4.3, Fig, 4.4, Section 4.3.1), but the differences in DZ frequency distributions provoked by influx of Middle Devonian aged zircons are too subtle to be detected as significant by the sample comparison methods. A major change in DZ spectra occurs between the Etonvale and Buckabie Formations and is expressed by high lateral heterogeneity of DZ distributions in the lower part of the Buckabie Formation and to a lesser extent in the upper section of the Buckabie Formation controlled by the unidirectional influx of Upper Devonian-aged zircons (~370 Ma). The lateral variability abates through time, illustrated by a lower spatial variability in the upper section of the Buckabie Formation, but the influence of Upper Devonian (Frasnian) zircons remains high especially in the northern basin causing prolonged lateral variability in the DZ age distributions as evident from moderate similarity of sample from the Upper Buckabie Formation.

In addition to the changes observed from the major DZ populations, the modelled MDA for the Etonvale and Buckabie Formations show systematic younging up stratigraphy, indicating that in the upper successions of the basin Upper Devonian (Famennian) to earliest Carboniferous DZ also contribute to the DZ budget of the Adavale Basin (Section 4.4.1). These populations, however, are too small to form significant age peaks and thus have no significant impact on sample comparison methods. At the same time, the modal composition of rocks from the upper part of the Buckabie Formation show very high volcanic influence and in contrast to the lower Buckabie and Etonvale Formations, a higher abundance of silicic volcanic rock fragments (Section 4.3.1). In summary, these observations suggest that the uppermost units in the Adavale Basin indeed received detritus from the voluminous Upper Devonian (Famennian) to earliest Carboniferous syn-basinal volcanic units in the area of the Drummond Basin and the northern New England Orogen (Bryan et al., 2004; Henderson et al., 1998), but the contribution is primarily evident from the MDA and the composition of volcanic lithic clasts and not expressed in peak ages in the DZ frequency distributions.

Lateral changes of DZ frequency distributions have previously been studied on modern and ancient river systems of different scale (e.g. Cawood et al., 2003; Garzanti et al., 2018; Link et al., 2005; Saylor et al., 2013). These studies collectively observe downstream changes in DZ distributions caused by a change in outcropping lithologies

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along a fluvial pathway, which modify or even overwhelm a DZ signature from an upstream compared to a downstream sample. In this DZ study of the Adavale Basin, lateral trends and temporal trends in the DZ frequency distributions are integrated to track provenance through space and time. This approach reveals some subtle differences in DZ distributions between northern and southern sample localities for Lower and Middle Devonian units, potentially caused by a difference in exposed basement rocks contributing to the DZ budget. During deposition of the Upper Devonian sediments, DZ spectra indicative of reworking of basement rocks were overwhelmed by synvolcanic input and integration of spatial and temporal changes in DZ distributions indicate a northerly source for this new

Figure 4.34. Multi-dimensional scaling plot (MDS) of all analysed DZ samples plotted in IsoplotR allows to integrate the spatial and temporal relationships for the DZ distributions (Vermeesch, 2018). Solid lines show nearest neighbour relationship; dashed lines show second nearest neighbour relationships between samples. In order to provide orientation in this otherwise dimensionless diagram, the three major DZ age populations identified in the Adavale Basin are included in the MDS plot as synthetic normally distributed samples (mean=360 Ma, 480 Ma and 580 Ma; n=150, standard deviation=2% of mean). Data show DZ distributions are dominated by basement derived ages for Lower and Middle Devonian units with subtle differences between the southern and northern basin. Increasing influence of ~380 Ma zircons causes a major provenance change in the basin as evident for Upper Devonian Buckabie Formation. Abbreviations: A.B. Adavale Basin.

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signal. This is further confirmed by lateral trends of sandstone composition within the respective formations (Section 4.4).

4.6.3 Detrital rutile provenance

Precise interpretation of the detrital rutile U-Pb age data is challenged by generally low concordance rates. This results in very small sample sizes, even when aggregating data for entire basin successions. Concordance rates of detrital rutile U-Pb ages are similar between the Drummond (30%, Sobczak, 2019) and Adavale Basins (22%), as well as samples analysed from metasedimentary rocks in the Thomson Orogen (19%, Siegel et al., 2017). The obtained concordant detrital rutile U-Pb age data from the Adavale Basin shows some age overlap with detrital rutile data from the Thomson beds and Les Jumelles Beds (Siegel et al., 2017), and the Carboniferous Drummond Basin (Sobczak, 2019). However, the peak age of the youngest Neoproterozoic age population at 615 Ma is substantially older than the peak age of the combined data from metasedimentary rocks from the Thomson Orogen (530 Ma) and Drummond Basin (537 Ma, Fig. 4.35b). The subordinate population at ~1.8 Ga from the Adavale data is only evident from one individual analysis in the Thomson beds and absent in the data from the Drummond Basin. Rutiles of this age form a major population in Paleoproterozoic sandstones from the Reynolds Range in Central Australia (Rösel et al., 2014) and potentially display an ultimate source for rutile of this age.

The compiled Nb/Cr compositions show that rutile from the Adavale Basin and Thomson beds is predominantly sourced from metapelites (70-75%). Rutiles from the Adavale Basin exhibit slightly higher proportions of rutiles from (meta)mafic sources (~25%) compared to the Thomson beds (~20%, Fig. 4.35a). The metasedimentary rocks of the Thomson Beds underlying the sedimentary units in the Adavale Basin display a potential source for the younger proportion of the detrital rutile populations (i.e. 450-550 Ma). The subtly older age population (600-750 Ma), however, is not apparent from the available data from the metasedimentary rocks in the Thomson Orogen (Siegel et al., 2017). This population might either not be present in the metasedimentary rocks, or small grain sizes (< 200µm, identified predominantly for the older population in Section 4.5) were not sampled and/or analysed in the previous study. The older grouping can also not be attributed to an ultimate source of meta-igneous rocks in the Musgrave province recording cooling to 585–560 °C by 498– 472 Ma (Walsh et al., 2013) after the Peterman Orogeny (530-600 Ma, Raimondo et al.,

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2010) and neither to the Ross-Delamerian Orogeny (520-480 Ma), as both orogenies post- date the ages. The Adavale and Drummond basins reveal an older age component with a substantial proportion of ages between 600-750 Ma, which cannot be explained by any recognized orogenic event on the eastern margin of Gondwana. These ages might be related to formation of granulite facies rocks during the break-up of Rodinia between 750 – 600 Ma (e.g. Li et al., 2008). It should be noted that the peaks for older populations are distinctively different between the Adavale (1.8 Ga) and Drummond Basin (1.5 Ga, Sobczak, 2019).

Figure 4.35. (a) Nb versus Cr classification for detrital rutile data (after Meinhold et al., 2008) from this study and data from the Thomson beds and Les Jumelles Beds (Siegel et al., 2017) indicating predominantly metapelitic sources for rutiles from both datasets (concordant and discordant datasets plotted). (b) KDE plots of concordant rutile U-Pb ages for the Cambrian-Ordovician Thomson beds (Siegel et al., 2017), Devonian Adavale Basin (this study) and the Carboniferous Drummond Basin (Sobczak, 2019), showing strong age overlap between Drummond Basin and Thomson beds and the older peak age for the Adavale Basin data.

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For data from the Adavale Basin, a correlation between rutile grain sizes and discordia ages was found, suggesting overall older ages (~630 Ma) for rutile grains in the very fine sand fraction and younger ages (~545 Ma) in the medium-sized grain fraction (Section 4.5). In the context of metamorphic source rocks, the closure temperature for U-Pb diffusion is dependent on the size of the rutile crystal and closure temperatures are generally higher for larger grains than for smaller crystals (Cherniak, 2000; Mezger et al., 1989, 1991). Assuming the detrital rutile in this study are derived from the same metamorphic source rock, and the detrital grain sizes are proportional to the original crystal sizes, the observed trend conflicts with the concept of size-dependent volume diffusion. Assuming the observed U-Pb ages are proportional to exhumation and (i.e. older grains were exhumed and exposed earlier) the relationship could be a consequence of higher abrasion during the course of mechanical transport and longer transport distances. The available data, however, is limited by low concordance rates and fuzziness of the discordia model-1 ages. A larger dataset with more concordant datasets, integrated data from U-Pb geochronology, grain morphologies and geochemical classification are required to further determine correlation of grain size with detrital rutile U-Pb age.

4.7 STRATIGRAPHIC REVISION

Two disconformities have been interpreted for the sedimentary succession of the Adavale Basin based on the absence of palynoflora assemblages in the early Eifelian and late Givetian, respectively (Hashemi and Playford, 2005; McKillop et al., 2007). The contact relationships of these proposed nonconformities are not preserved in drill core material, and no significant changes in bedding angles have been observed stratigraphically above and below the interpreted unconformities in the wells inspected for this study (cf. stratigraphic logs in Appendix 4.2). In addition, a statistically robust MDA of Eifelian age is modelled for a sample of the Eastwood Beds (Section 4.4.1, ALL-6), suggesting the Eifelian hiatus is not present, and the contact with overlying units is conformable (Fig. 4.36). The Eastwood Beds and the overlying Log Creek Formation appear to be complementary units based on their interpreted distribution in drill holes, suggesting synchronicity between the two units (Section 4.2).

The late Emsian through the entire Middle Devonian appears to be characterised by volcanic quiescence in the catchment of the Adavale Basin (apart from the DZ population

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forming the MDA for ALL-6). In addition, MDAs obtained for samples from the Lissoy Sandstone are older than the biostratigraphic age assignment, implying syn-volcanic contribution was absent during this time (Section 4.4.1). A tectonically controlled basin restriction causing evaporitic conditions in the Adavale Basin has been postulated for the Givetian (McKillop et al., 2007), however, no major changes in bedding angles are evident from units below and above the apparent unconformity.

The depositional age of the Buckabie Formation was previously only broadly constrained based on sparse biostratigraphic data. The newly acquired DZ data now allow to better

Figure 4.36. Comparison of revised stratigraphy of the Adavale Basin (left) based on the data and interpretations of this Chapter in combination with Chapter 2 and basin stratigraphy after McKillop et al. 2007. Revisions comprise shortening of the Gumbardo Formation based on U-Pb zircon geochronology and identification of potential zircon inheritance. Deposition of the Eastwood Beds is proposed to be extended into the Eifelian based on an Eifelian MDA for a sample from the Eastwood Beds (Section 4.4.1). Deposition of the Buckabie Formation most likely extended into the earliest Carboniferous based on MDA for upper sections of the Buckabie Formation (Section 4.4.1). Abbreviations: Miss., Mississippian; Tourn., Tournaisian.

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constrain depositional ages. Deposition in the Adavale Basin lasted at least into the Earliest Carboniferous as evident from the youngest MDA from the upper section of the Buckabie Formation with YCσ2 (3+) at 358.4 ± 4.8 Ma and YCσ1 (2+) at 352.5 ± 8.5 Ma, confirming the speculations of Heikkila (1965) and Tanner (1968) of a temporal overlap of the base of the Drummond and preserved top of the Adavale Basin (Fig. 4.36).

4.8 PROVENANCE CHANGES IN THE ADAVALE BASIN

The integration of petrographic and U-Pb zircon and rutile geochronology data in a spatial and temporal context provides interesting insights into the sediment provenance of the Adavale Basin. The data record onset of syn-basinal volcanism to the northeast of the basin in two stages which significantly affects the DZ inventory and modal composition. On the other hand, the results of the detrital rutile U-Pb geochronology indicate that the overall sediment provenance did not change.

During the Lower and Middle Devonian, DZ and rutile provenance is characterised by basement-derived detritus mainly from the Thomson Orogen, recording reworking of metasedimentary basement rocks, Ordovician volcanic and plutonic basement rocks and reworking of the exposed volcanic rocks of the Gumbardo Formation (Fig. 4.37a). Contributions of DZ populations from metasedimentary basement are slightly greater in the northern basin, whereas Ordovician volcanic and plutonic source rocks show higher proportions in the southern basin. Apart from these subtle differences, the Eastwood Beds and Lissoy Sandstone are laterally consistent across the basin in terms of a high similarity of DZ distributions (Section 4.6.2) and relatively low variations in the QFL inventory (Section 4.3.1). Quartz assemblages are dominated by common quartz in both units, which is consistent with plutonic/vein quartz provenance.

Rocks of the Etonvale Formation mark a transitional phase, where (i) DZ show some influx of contemporary volcanic zircons at ~380 Ma, (ii) some volcanic influence on sandstone composition with modal compositions subtly shifting towards more feldspathic and lithic- rich compositions, (iii) an increase in the proportions of volcanic quartz is observed, along with (iv) an increase in the abundance of volcanic lithics. Lateral DZ trends are relatively homogenous and also very similar to those of the Eastwood Beds and Lissoy Sandstone. However, they differ in that syndepositional zircons at ~380 Ma mix with basement derived

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zircons ~400 Ma from the Gumbardo Formation and subtly skew the age distribution and shape of the youngest DZ peak.

A pronounced change in provenance is evident from rocks of the Buckabie Formation compared to the Lower and Middle Devonian successions and the transitional phase represented by the Etonvale Formation. DZ distributions are dominated by synsedimentary zircons ~370 Ma and temporal trends reveal a lag time in the propagation of this new age signal through the basin from north to south within the Buckabie Formation. Volcanic sourcing from the area of the Drummond Basin and New England Province (Fig. 4.37b) is

Figure 4.37. Illustration of two distinctively different sediment provenance phases for the Lower Devonian (A, C) and Upper Devonian (B, D) sedimentary successions. Coloured outlines are based on compiled U-Pb age data from Figs. 4.31, 4.32 and adapted from Rosenbaum (2018) showing distribution of DZ source locations in the Thomson and New England Orogens schematically. Colour coding also corresponds to highlighted age intervals in the KDE plots (C, D). The first phase (A) is characterised by reworking of proximally located Thomson Orogen basement rocks and potential contributions of more distal Ordovician plutonic rocks, whereas the second phase (B) is characterised by influx of syn-sedimentary Upper Devonian zircons from distal sources to the north-east of the Adavale Basin.

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evident from: (i) the north-south propagation of contemporary volcanic zircons of ~370 Ma in the lower Buckabie Formation, (ii) a strong shift in modal composition of sandstones to more feldspathic and (volcanic) lithic-rich (QFL) exhibiting north-south trends, (iii) a substantial increase in volcanic quartz compared to common quartz varieties.

Proportions of basement-derived Ordovician and especially Ediacaran DZ contributing to the sediment budget decrease in response to the syn-sedimentary zircon population, but at the same time, U-Pb ages of detrital rutile remain virtually unchanged compared to the lower successions of the Adavale Basin which are dominated by basement derived detritus. This indicates that contribution of basement derived sediment is persistent throughout the lifespan of the Adavale Basin, but DZ provenance is disturbed by the onset of contemporary volcanism outside of the basin.

4.9 CONCLUSIONS

Sediment provenance in the Adavale Basin was studied here to test if the sedimentary infill is indicative of a post-orogenic platformal basin supporting the hypothesis that the Thomson Orogen was stabilised prior to the Devonian.

The initial (Lower to Middle Devonian) successions exhibit (i) compositionally and texturally relatively mature sediments dominated by common quartz with (ii) DZ spectra reflective of reworking of a variety of the basement lithologies in the Thomson Orogen and (iii) detrital rutile ages recording reworking of metasedimentary basement rocks. In conclusion, all provenance data (i, ii, iii) indicate a relatively wide catchment with sediment sources encompassing the dominating lithologies encountered in the subsurface of the Thomson Orogen. Based on the evidence from sandstone petrography and DZ and detrital rutile geochronology, the Lower and Middle Devonian deposits in the Adavale Basin exhibit characteristics typical of infill of a platform basin.

In the Upper Devonian sediment provenance is disturbed by a successively increasing volcanic influence on the sedimentary units, which affects both, modal sandstone composition and DZ age spectra. The predominant LN-EC aged detrital rutile population remains virtually unchanged between the Lower and Middle to Upper Devonian units, yet the proportions of distinctively older populations decrease. The DZ age spectra for Phanerozoic DZ ages show a dramatic change from the Lower and Middle to the Upper

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Devonian, which has been attributed to the onset of syn-basinal volcanism occurring regionally. The persistence of the DZ background signal of Precambrian ages, however, indicates that overall sediment provenance did not change, but was overwhelmed by the syn-basinal volcanism outside of the Adavale Basin.

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Chapter 5: Sediment provenance of the Darling Basin

5.1 INTRODUCTORY STATEMENT

In addition to the Adavale Basin, a number of other sedimentary basins were operating along the eastern margin of the Tasmanides during the Devonian (Draper et al., 2004; Fergusson et al., 2013a), including the Belyando Basin, Burdekin Basin, and Warrabin and Barrolka throughs in the Thomson Orogen (Fig. 5.1), as well as the Darling Basin in the Lachlan Orogen (Fig. 5.1). The infills of these basin elements show overlaps in depositional ages, sedimentary facies and compositional characteristics, raising the question whether the basin elements might have been once connected, forming a larger, contiguous basin (e.g. Bembrick, 1997; Draper et al., 2004; McKillop et al., 2007). In contrast, if they were structurally separate basin entities with individual basin records and tectonic histories this would indicate significant tectonic variability along the eastern margin of the Tasmanides. This chapter focusses on the Darling Basin, the largest of the Devonian basin elements, situated along strike to the south of the Adavale Basin.

The aim of this study is to test the hypothesis if the provenance of the basin fill is the same between the Adavale and Darling basins, then the two were most likely interconnected. Two scenarios are conceivable if this hypothesis is rejected:

• The basin fills are synchronous, yet sediment provenance is different, but indicative of a similar type of sediment source region (e.g. craton derived or arc-related). This suggests the basins were coevally developed but structurally separated reflecting similar basin forming processes and tectonic settings.

• The basin fills may or may not be synchronous and no relationships can identified, suggesting the basins are separate unrelated entities reflecting different tectonic settings and drivers, and would reinforce a lack of connection between the Thomson and Lachlan Orogens at this time.

The focus of this chapter is to provide a review of the (chrono)stratigraphy of the Darling Basin to assess the synchronicity of the overall basin fill and the timing of tectonic events affecting the sediment provenance in both basins. In this study, the sedimentary provenance

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of the Darling Basin infill is established for the first time by means of sandstone composition in conjunction with detrital rutile and zircon U-Pb geochronology. The findings will then be utilised in the following chapter (Chapter 6) to assess similarities of the provenance indicators from the Adavale Basin (Chapter 4) to test the hypothesis of a potential connectivity of the Darling and Adavale basins.

5.2 GEOLOGIC BACKGROUND

The Darling Basin is a large terrestrial sedimentary basin system covering the Lachlan Orogen and parts of the Delamerian Orogen in the Tasmanides of eastern Australia. It lies in the western part of New South Wales and occupies an area of about 150,000 km2 (Cooney and Mantaring, 2005; Neef et al., 1996). The basin has an east-west extent of approximately 400 km and is bounded to the east by the deep marine Devonian Cobar Basin (Glen, 1990) and to the west by Precambrian basement rocks of the Broken Hill Province (Bembrick, 1997; Cooney and Mantaring, 2005). The Darling Basin comprises a number of sub-basins and troughs (Fig. 5.1):

• the Bancannia Menindee and Wentworth troughs in the west,

• Booligal Trough in the south, and

• Pondie Range, Blantyre, Poopelloe Lake, Neckarboo and Nelyambo troughs in the eastern part.

The sedimentary infill of up to 8,000 m is predominantly Devonian in age and dominated by terrestrial to shallow marine depositional environments (Bembrick, 1997; Cooney and Mantaring, 2005), which are generally similar to those recorded in the Adavale Basin (Fig. 5.1a, McKillop et al., 2007). The Darling Basin stratigraphy is recorded by two formal Groups, the Lower Devonian Winduck Group and the Lower to Upper Devonian Mulga Downs Group (Ray, 1996; Scheibner, 1987), both of which form part of the Cobar Supergroup (Fig. 5.2, MacRae, 1987). A regional unconformity has been identified based on seismic interpretations between the Mulga Downs Group and Winduck Group, which is believed to correspond to the Bindian event (Bembrick, 1997). Another unconformity is

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Figure 5.1. Overview map of the Tasmanides subprovinces (after Glen, 2005) and location of the Darling Basin across the Lachlan and Delamerian orogens (a). Inset map showing the named sedimentary troughs comprising the Darling Basin. Boundaries for the Thomson, Lachlan and Delamerian orogens shown by red lines and sampled drill holes highlighted in bold (b). Abbreviations: DB, Darling Basin; BT, Barrolka Trough; WT, Warrabin Trough; AB, Adavale Basin; BeB, Belyando Basin; BB, Burdekin Basin. interpreted within the Winduck Group, possibly associated with the Tabberabberan event (Evans, 1977; Neef, 2012). In an attempt to establish a basin-wide stratigraphy for the Darling Basin including subsurface intersections and outcropping rocks, three informal intervals have been proposed (Fig. 5.2, Winduck, Snake Cave and Ravendale Interval, Bembrick, 1997).

Initiation of the Darling Basin has been linked to crustal extension in the Lachlan Orogen from the Late Silurian to Middle Devonian, leading to widespread basin opening from the Broken Hill block in the west to Cobar in the east (Glen et al., 1996). More recently, the opening of Devonian Basins has been subdivided into an Early Devonian phase for the Menindee and Wentworth troughs in the southwestern Darling Basin and the Cobar Basin to the east, and initiation in the Silurian for the Bancannia Trough and the remaining eastern sub-basins in the Darling Basin (Rosenbaum, 2018). Both interpretations are based on the structural trends in the Darling Basin sub-basins. Rocks associated with magmatic activity related to the opening of the Darling Basin are not reported, thus the timing of initiation is only poorly constrained (Cooney and Mantaring, 2005).

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Figure 5.2. Lithostratigraphic units in the Darling Basin after Bembrick (1997), Cooney and Mantaring (2005). Informal basin-wide stratigraphy and predominant paleoflow directions after Bembrick (1997) and references therein. Abbreviations: Mt., Mount; GA, Geoscience Australia.

5.2.1 Winduck Group Rocks of the Winduck Group are widespread across the Darling Basin and attain a formational thickness of over 6000 m in outcrops in the western part of the basin in the Mount Daubeny Formation (Fig. 5.2, Bembrick, 1997; Neef et al., 1989). Formational thickness in the eastern basin is up to 3000 m in outcrops in the McCullochs Range north of the Blantyre Trough, whereas subsurface intersections are substantially thinner (~400 m in the central part of the Bancannia Trough and over 1000 m in the eastern Blantyre Trough, Fig. 5.1, Bembrick, 1997).

Major lithofacies types of the Mount Daubeny Formation are horizontally stratified quartzarenite with current laminations, indicative of sheet flood deposits and tabular crossbeds indicating braided stream environments (Neef and Bottrill, 1991; Neef et al., 1989). Distal equivalents of the unit have been interpreted from drill core intersections in the

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Pondie Range Trough (Fig. 5.1, Bembrick, 1997). A shallow open marine shelf environment is apparent from outcrops and subsurface intersections in the eastern part of the Darling Basin (Glen and Powell, 1986; Scheibner, 1987). Lithologies in the Mount Daubeny Formation comprise quartzarenites, minor siltstone and conglomerates, whereas in the Pondie Range to the east, rocks have been interpreted as fluvial sandstones and minor conglomerates showing high-angle crossbeds (Bembrick, 1997; Neef et al., 1989). In general, the sedimentary rocks of the Winduck Group range from fluvial depositional environments in the west to fluvio-deltaic and shallow shelf environments in the east (Alder et al., 1998; Bembrick, 1997). Based on predominant paleoflow directions from the west, sediment supply is thought to be from the Wonominta and Broken Hill Blocks west of the Darling Basin (Fig. 5.1, Bembrick, 1997). This interpretation, however, has not been tested with sediment provenance data such as sandstone petrography, heavy mineral assemblages and detrital mineral dating.

Biostratigraphic age control for the Winduck Group is based on: 1) a Lochkovian Baragwanathia flora in the west from the Mount Daubeny Formation (Bembrick, 1997; Freeman, 1966), and 2) Lochkovian to Pragian bivalve faunas in the central and eastern part of the Darling Basin (Buckambool/Gundaroo Sandstone, Fig. 5.2, Sherwin, 1980). Emsian ages, constraining the upper boundary of the Winduck Group are only observed from rocks representing shallow shelf and marginal marine depositional environments in the Neckarboo Trough (Fig. 5.1, Glen and Powell, 1986; Scheibner, 1987).

5.2.2 “Lower” Mulga Downs Group The lower part of the Mulga Downs Group has been informally termed Snake Cave Interval (Bembrick, 1997). The eponymous Snake Cave Sandstone, however, has been superseded by the Wana Karnu Group (Sharp, 2004), which has not been formally assigned to the Cobar Supergroup (Geoscience Australia and Australian Stratigraphy Commission., 2019) leaving open how to formally name rocks of late Emsian to Eifelian age in the Darling Basin. For the purpose of the stratigraphic overview the formal term ‘Mulga Downs Group’ is used here and prefaced with “lower” to address rocks of middle Emsian to Eifelian age and ‘upper’ for units of Givetian to Famennian age (Fig. 5.2, Bembrick, 1997).

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The ‘lower’ Mulga Downs Group is one of the most extensive units in the Darling Basin, with a unit thickness over 1300 m in outcrop, and over 1000 m in drill holes indicating thickness is relatively uniform across the basin (Bembrick, 1997). The unit has been identified in all wells intersecting the Bancannia Trough and is also present in outcrop to the east and west of the Bancannia Trough, represented by the Coco Range and Snake Cave sandstones, respectively (Figs. 5.1, 5.2, Bembrick, 1997; Neef et al., 1996).

Lithologies in the Coco Range Sandstone comprises fine-grained to pebbly (quartz) arenite (Neef et al., 1996), whereas the Wana Karnu Group contains conglomerates in addition to pebbly quartz sandstones and silty sandstones (Fig. 5.2, Bembrick, 1997; Sharp, 2004). A shallow marine depositional environment has been suggested around the area of the Blantyre Trough in the central part of the basin, which interestingly lies in proximity to the Emsian shallow marine rocks of the upper Winduck Group (Bembrick, 1997). Elsewhere, planer bedded and tabular cross-bedded strata are dominant suggesting braid plain and high energy distal alluvial fan environments (Neef et al., 1996). Depositional environments change to lower energy environments up-section with speculated meandering streams and aeolian deposition evident from laminae of fine and coarse sand (Neef and Bottrill, 1996). Sedimentation of the ‘lower’ Mulga Downs Group was terminated by a brief but widespread brackish to marine incursion at the beginning of the Givetian, indicated by widespread occurrences of skolithos burrows (Bembrick, 1997 and references therein).

Paleocurrent data indicate that sediment was predominantly transported from the west, and subordinate volumes from more south-west to north-western directions. These general directions are based on ~160 paleocurrent measurements (comprising planar crossbeds, streaming lineations and trends of trough cross-beds) in the Snake Cave Sandstone southwest of the Koonenberry fault and 234 paleocurrent trends over a ~50 km transect in the Coco Range Sandstone in the western part of the Bancannia Trough (Bembrick, 1997; Glen and Powell, 1986; Neef, 2004; Neef et al., 1996). Sediments of the Wana Karnu Group are assumed to be derived from reworking of the underlying Cambro-Ordovician Mutawintji Group (Sharp, 2004) and from the Broken Hill Province to the west, indicated by westerly paleoflow directions and heavy mineral assemblages (tourmaline, rutile, zircon and leucoxene) matching rocks in the Broken Hill region (Neef et al., 1996). The interpretation on provenance is solely based on the presence of those mineral phases. No

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compositional or isotopic age data for heavy minerals is available to further test this hypothesis.

Biostratigraphic age control for the “lower’ Mulga Downs Group is generally limited (Alder et al., 1998; Bembrick, 1997), however, depositional ages are constrained from rare occurrences of fish fossils (Wuttagoonaspais sp.) near the base of the Coco Range Sandstone indicating an Emsian-Eifelian age to the unit (Neef et al., 1995). Fish fossils found in drill hole Mossgiel-1 in the Booligal Trough indicate a Pragian to Emsian age for rocks of the Wana Karnu Group (MUCEP, 2002).

5.2.3 “Upper” Mulga Downs Group The “upper” Mulga Downs Group represents the topmost unit in the Darling Basin, with thickness commonly >1000 m in drill holes in the Bancannia Trough and outcrops to the east and west of the Bancannia Trough and in the Dolo Hills between the Pondie Range and Bancannia troughs where it has been called the Nundooka Sandstone (Figs. 5.1, 5.2, Bembrick, 1997; Neef et al., 1995). Lithologies comprise fine-grained quartz sandstone to sublitharenite in the Nundooka Sandstone and conglomerate and fine-grained quartz sandstone in the Bancannia Trough (Bembrick, 1997; Neef et al., 1996). In the eastern Darling Basin, rocks of the ‘upper’ Mulga Downs Group crop out as the Bundycoola Formation (conglomeratic) and the overlying Crowl Creek Formation (Fig. 5.2, sublitharenite and quartz arenites, Glen and Powell, 1986).

Paleocurrent data from the Nundooka Sandstone indicate flow directions from the west and north-west and the orientations of paleocurrent trends is indicative of deposition in low-angle alluvial fans and distal braid plains (Fig. 5.2, Neef et al., 1996, 1995). A total of 164 paleocurrent trends from the Nundooka Sandstone have been measured across a ~50 km transect in the western Bancannia Trough (Neef et al., 1996). Areas of major exposure in the eastern part of the Darling Basin, however, show more complex paleoflow directions indicating a change of 180° from easterly to westerly flow directions upsection in the Crowl Creek Formation (Fig. 5.2, Bembrick, 1997).

Depositional ages from palynological data are only available for the Ravendale Formation and commonly limited to the top sections of the formation, which suggest a Famennian age (Neef et al., 1995), while the base of the Ravendale Formation is considered to extend down

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into the Frasnian (Alder et al., 1998; Bembrick, 1997). Age control for the units in the eastern part of the basin is only poorly constrained by one occurrence of a plant fossil suggests a late Givetian or younger age (Glen and Powell, 1986).

5.2.4 Stratigraphic correlation with the Adavale Basin Correlation of the Adavale and Darling basins has been suggested based on (i) similar depositional ages based on biostratigraphic control (Hashemi and Playford, 2005; McGregor and Playford, 1993; McKillop et al., 2007; Playford, 2004), (ii) broad similarities in sedimentation style and depositional history (Alder et al., 1998; Bembrick, 1997; Fergusson et al., 2013; McKillop et al., 2007; Veevers, 1984; Webby, 1972) and (iii) the correlation of seismic unconformities (Khalifa, 2010). Correlation of the two basin-fill histories is illustrated in Fig. 5.3.

The timing of initiation of the Darling Basin is not well constrained but presumably predates the Adavale Basin. This is recently confirmed in this study with the new depositional ages of the basal rift-related volcanic rocks of the Gumbardo Formation marking initiation of the Adavale in the mid-Emsian (~398 Ma, Chapter 2). In contrast, Lochkovian to Pragian floras and bivalve faunas record deposition occurring earlier in the Darling Basin (Bembrick, 1997; Freeman, 1966; Sherwin, 1980). Sedimentation of the Winduck Group thus terminated around the time of Adavale Basin inception. (Fig. 5.3). The ‘lower’ Mulga Downs Group appears to be coeval with the Eastwood Beds based on biostratigraphic constraints (Bembrick, 1997; Hashemi and Playford, 2005) although the temporal overlap based on the biostratigraphy is restricted to the second half of the Emsian (Fig. 5.3). The proposed extension of the Eastwood Beds into the Eifelian based on modelled MDA from detrital U-Pb zircon analysis (Sections 4.4.1, 4.7), however, suggests the Eastwood Beds and the “lower” Mulga Downs Group have greater time equivalence (Fig. 5.3). The transition from the ‘lower’ to ‘upper’ Mulga Downs Group in the Darling Basin is marked by a brief marine incursion at the end of the Eifelian (Neef, 2009, 2012), and coincides with the marine transgression in the Adavale Basin marked by the Log Creek Formation during the Late Eifelian (Chapter 4, Section 4.2.2).

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Fig. 5.3. For figure caption see next page next page

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Figure 5.3. Comparison of Adavale and Darling basin stratigraphy, biostratigraphic age control for selected units and overall timing of regional orogenic events. Data highlights the low precision of biostratigraphic age constraints for the Darling basin (primarily from fossils) versus higher precision age control in the Adavale Basin via palynology (Hashemi and Playford, 2005). Adavale Basin stratigraphy revised after McKillop et al. (2007), Darling Basin stratigraphy after Bembrick (1997) and Cooney and Mantaring (2005). Darling Basin biostratigraphy data from (Bembrick, 1997; Freeman, 1966; Glen and Powell, 1986; Macquarie University Centre for Ecostratigraphy and Palaeobiology (MUCEP), 2002; Neef et al., 1995; Sherwin, 1980). Abbreviations, AA, absolute age; BA, biostratigraphic age; OE, orogenic event; EB, Eastwood Beds; LCF, Log Creek Formation; LS, Lissoy Sandstone; BL, Bury Limestone; EF, Etonvale Formation; Miss., Mississippian; Dev., Devonian.

(Fig. 5.3). The continental red-bed units of the Etonvale and Buckabie formations (Frasnian and Famennian) are coeval with the ‘upper’ Mulga Downs Group and both are dominated by overall fluvial depositional environments, but rocks of the Mulga Downs Group generally lack evidence for red-beds (Bembrick, 1997; Neef, 2012). In summary, the existing biostratigraphic framework highlights the overall synchronicity of the two basins and to some extent the synchronicity of their depositional history, however, the potential connectivity of the two has not been adequately tested.

5.3 METHODS AND SAMPLES

The samples for the provenance evaluation and then basin comparison are those from a previously conducted DZ study by the Regional Mapping and Exploration Geosciences unit of the Geological Survey of New South Wales (GSNSW) at the W.B. Clarke Geoscience Centre in Londonderry, NSW (Barry, 2016). The sampling campaign aimed to build a reference dataset for Devonian sandstones with known biostratigraphic age control which will aid in future stratigraphic correlations with other Devonian units that lack biostratigraphic age control by means of DZ provenance and MDA (Barry, 2016). For the purpose of this study, samples were preferentially selected from cored sections with immediate biostratigraphic control, to ensure good age control that is critical for subsequent correlation of provenance indicator from Chapter 4. Sample details are summarised in Table 5.1.

The precision of biostratigraphic age constraints differs significantly between the Adavale and Darling basins (Fig. 5.3). Detailed work on palynological assemblages by Hashemi and Playford (2005) limits depositional ages relatively precisely in the Adavale (±5 Myrs). Biostratigraphic constraints for most units in the Darling Basin are based on limited findings of fish and plant fossils, hampering the identification of floral or faunal

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biostratigraphic assemblages. As a result, depositional ages for most units in the Darling Basin are imprecise and encompass multiple geological stages within the Devonian (Fig. 5.3). The Wana Karnu Group and Ravendale Formation were selected for sampling as their depositional age is relatively well constrained.

Samples were analysed using the approach outlined in Chapter 4 and the methods in Chapter 3. This includes DZ U-Pb geochronology (Section 5.4) including integration of zircon trace element chemistry and DZ morphology data (Sections 5.4.4, 5.4.5) and integration of the previously generated U-Pb DZ age dataset (Barry, 2016, Section 5.4.3). Results from detrital rutile U-Pb geochronology and compositional data are presented in Section 5.5. Potential DZ sources are discussed in Section 5.6, including the critical test of comparison with the DZ distributions from the Adavale Basin in a temporal context (Section 5.6.3). Sources for DZ are discussed in Section 5.7. Details on the mineral separation procedure performed at the University of Newcastle for these Darling Basin samples is provided in Appendix 5.1.

5.3.1 Sample description and age control Wana Karnu Group

A 533 m section of the Wana Karnu Group is intersected in Mossgiel-1 between 226- 759 m (Fig. 5.1, Table 5.1, Matti and Moffitt, 2001). Sample MOS-18 was taken from a depth of 610.9 - 611.5 m from a massive, fine- to medium-grained sandstone (Fig. 5.5d, Barry, 2016). Fish fossils at ~610 m indicate a Pragian to Emsian age for the sampled section (MUCEP, 2002). Isotopic age control for the sampled section is provided by an underlying tuff at 661.7 m with an interpreted depositional age of 409.77 ± 8.1 Ma, and overlying claystone at 658.45 m 393.32 ± 7.84 Ma with K-Ar analysis of illite separates (Zwingmann, 2001). These ages have been determined from whole rock separates with high illite concentrations (~80%) in the <2 µm fraction, however no alteration assessment has been conducted reducing confidence of the quality of these ages. In addition, no thermal history data were taken into account for the age assignment and the ages are potentially compromised by thermal disturbance of the K-Ar system (Dodson, 1973) by later orogenic events (i.e. Tabberabberan event, 390-370 Ma, Black et al., 2005; VandenBerg, 2000). This section of the Wana Karnu Group is underlain by the Mossgiel Granite at 1740m, which

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was dated as Silurian (U-Pb SHRIMP age: 427 ± 7 Ma, Wilde, 2000 [unpublished] cited in Barry, 2016).

Sample MOS-18 has been point-counted as part of this study and is classified as a well-sorted sublitharenite (Q85F0L15, Fig. 5.4), containing medium-grained subrounded to rounded monocrystalline quartz and more angular fine-grained quartz. Quartz grains commonly show fluid inclusion trails indicative of plutonic or vein quartz origin and are intensely fractured. Feldspars are almost totally absent in this sandstone, with only trace amounts of plagioclase and microcline observed. Rounded to well-rounded lithic fragments comprise sericitised sedimentary lithics with some lithics showing schistose fabrics and subordinate chert. Silica cement is developed as syntaxial quartz overgrowth and pore- lining dark brown clays. Accessory minerals comprise muscovite, biotite and zircon (Fig. 5.5e).

EMU-20 was collected from drill core cuttings of fine sandstones and subordinate siltstone between 372 – 396 m depth in the Mount Emu-1 well in the southern Blantyre Trough (Figs. 5.1, 5.5f, Table 5.1). The cuttings comprise fine- to medium-grained sand and lithic fragments of fine-grained sandstone, as well as detrital muscovite (Barry, 2016). An Emsian to Eifelian age was assigned to a biostratigraphic assemblage of palynomorphs at 1088 – 1387 m (Playford, 2004).

In summary, the sampled sandstones from the Ravendale Formation and Wana Karnu Group are compositionally and texturally very mature, and quartz assemblages are dominated by common quartz, indicative of derivation from plutonic or vein sources. Overall low modal proportions of feldspar and a complete absence of volcanic lithic clasts in the sampled rocks suggest a lack of contemporary volcanic influence during the Middle to Late Devonian in the Darling Basin. On the other hand, in Mossgiel-1 an interbedded tuff underlying the sample sections is reported (Matti and Moffitt, 2001; Zwingmann, 2001), and might be indicative of some syn-volcanic influence in the Wana Karnu Group.

Ravendale Fm

PAM-11 was sampled from a well-sorted medium to coarse-grained quartz arenite between 759.5-760.5 m depth in Pamamaroo-1 southwest of the Dolo Hills (Fig. 5.1, Table 5.1, Barry, 2016). Biostratigraphic control is provided by Late Famennian to earliest

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Carboniferous megaspores at a depth of 758.9 m just above the sampled location (Moreton, 1983). Petrographic data for this sample are extracted from a GSNSW report, as the petrographic thin section is lost (Barry, 2016). The sandstone (Q90F0L10, Fig. 5.4) is moderately to well-sorted, where quartz grains are subrounded to rounded and commonly show fluid inclusions indicating plutonic or vein quartz provenance. Lithic clasts comprise polycrystalline quartz and foliated sericitised sedimentary clasts. Accessory detrital minerals comprise muscovite, zircon and tourmaline.

Sample PON-21 was taken at ~450.5 m depth in drill hole Pondie Range-1 in the northern Blantyre Trough from a section of interlaminated fine- to medium-grained sandstone and siltstone (Figs. 5.1, 5.5b, c, Table 5.1, Barry, 2016). Age control is provided from Famennian miospores at 450.8 m and fish plates at 449.0 and 450.5 m (Bembrick, 1997; Jessop, 1967). The fine- to medium-grained sandstone is moderately sorted

Figure 5.4. QFL diagram after Folk (1970) for samples from the Ravendale Formation (red) and the Wana Karnu Group (blue) in comparison with sampled units in the Adavale Basin. Fields for quartzarenite (1), subarkose (2) and sublitharenite (3). Samples from both units in the Darling Basin are compositionally mature and very similar in composition to the Eastwood Beds in the Adavale Basin.

Table 5.1. Sampling details on analysed samples from the Darling Basin. All coordinates are reported in GDA94. Location of wells shown in Fig. 5.1.

Sample name Well name Unit Latitude Longitude Sampled depth [m] Comment PAM-11 Pamamaroo 1 Ravenwood Formation -32.171 142.496 759.5 - 760.5 core PON-21 Pondie Range 1 Ravenwood Formation -31.478 143.116 ~450.5 core MOS-18 Mossgiel 1 Wana Karnu Group -33.372 144.654 610.9 - 611.5 core EMU-20 Mount Emu 1 Wana Karnu Group -32.331 143.544 372.0 - 396.0 cuttings

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(Q87F3L10, Fig. 5.4), where quartz grains are sub-rounded to rounded and commonly show fluid trail inclusions suggesting plutonic or vein quartz origin. Modal abundance of feldspar is overall low (~3%), with sporadic grains of plagioclase, K-feldspar and microcline. Lithic clasts occur as intensely sericitised silt- to fine sand-grade sedimentary rock fragments and commonly show a schistose fabric. Minor lithics of chert and polycrystalline quartz are also present. The sample exhibits well-developed quartz cementation, subordinate syntaxial

Figure 5.5. Representative microphotographs of sampled sandstones from samples PAM-11 (a, Pamamaroo 1, 769.5 – 760.5 m, image from Barry, 2016), PON-21 (b, Pondie Range 1, ~450.5 m) and MOS-18 (d, Mossgiel 1, 610.9 – 611.5 m). Rutile bearing quartz grain from PON-21 (c), accessory minerals of zircon (Zr) and muscovite (Ms) from MOS-18 (e). Sample EMU-20 was sampled from ditch cuttings (f, Mount Emu 1, 372.0 – 396.0 m, image from Barry, 2016). All microphotographs were taken under crossed polarized light.

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calcite cementation and poikilotopic calcite cement in some domains. Detrital accessory minerals in this sample comprise detrital muscovite and well-rounded zircon.

5.4 DETRITAL ZIRCON U-PB GEOCHRONOLOGY

Two samples each from the Ravendale Formation and Wana Karnu Group, have been analysed for DZ and rutile U-Pb geochronology following the same methods as outlined in Chapter 4, including the approach for grain size measurements and morphology parameters. DZ from the Darling Basin show significantly more indications of inherited core domains in CL-imagery. Thus, in addition to ablation spot placement in rim domains, as carried out for the DZ from the Adavale Basin, apparent inherited zircon core domains were also targeted for selected grains from all samples (Table 5.2, Fig. 5.9). The full dataset with integrated U-Pb ages, trace element and grain morphology data are provided in the Electronic Appendix A, which also comprises the integrated DZ U-Pb data acquired via LA-ICP-MS from Barry (2016). Appendix 5.2 contains transmitted light and CL images of all analysed zircon grains, Tera-Wasserburg plots showing concordant and discordant analyses on a per sample basis are documented in Appendix 5.3. Data for the monitored secondary standard material Plešovice (Sláma et al., 2008) are documented in Table 3.1 of Chapter 3.

5.4.1 Wana Karnu Group MOS-18

Zircons from MOS-18 appear clear to brownish in colour under transmitted light, and subrounded, rounded and subeuhedral zircons are present in approximately equal proportions. Subhedral to euhedral grains show high aspect ratios (~3) compared to the subrounded to rounded grains. DZ from MOS-18 have a median length of 148 µm, a median aspect ratio of 1.6 and median sphericity of 0.61 (Table 5.2). Resorbed core domains are rare (<5%), and zircons show an overall intermediate to low CL-response. Oscillatory zoned zircons indicating an igneous origin are most common, a subordinate number of grains exhibit complex or planar zonation. Out of 145 targeted grains, 111 analyses yielded concordant results, 34 analyses were discordant (76% concordance rate, Table 5.2). Core domains were analysed for 10 grains with 7 analyses yielding concordant results and 3

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discordant ages, resulting in 4 concordant core-rim pairs (Fig. 5.9). Very pronounced age populations for MOS-18 are apparent at ~410, ~510 and ~575 Ma, with subordinate peaks at ~690 Ma and 1.5 Ga, and a well-defined plateau of Grenvillian ages (~0.9-1.2 Ga, Fig. 5.6).

EMU-20

Zircons from EMU-20 are the largest out of the four samples analysed from the Darling Basin with a median length of 168 µm and a maximum grain length of just under 400 µm. The median aspect ratio for zircons from this sample is 1.6 and grains exhibit a median sphericity of 0.64 (Table 5.2). Zircons are predominantly clear in colour, and some show an orange-red staining. The grains are commonly pitted, and subrounded to rounded,

Figure 5.6. DZ geochronology of samples from the Wana Karnu Group, showing core and rim ages. All data plotted as locally adaptive KDE (only rim analyses).

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subeuhedral grains with preserved crystal faces are rare. The mounted DZ show intermediate to good CL response, where oscillatory zoning is very common, and very few grains show planar or complex zoning. Resorbed core domains are observed for ~10% of the mounted grains. For sample EMU-20, 152 grains were targeted for U-Pb dating, 75 yielded concordant results, and 77 analyses were discordant resulting in a very low concordance rate of 49% (Table 5.2). Fourteen grains were additionally targeted with an ablation spot in core domains, whereas 6 grains yielded concordant and 8 grains yielded discordant results (total of 5 concordant core-rim age pairs). A broad major age population is identified at ~525 Ma and a plateau of Grenvillian-aged zircons between ~850 Ma and 1.2 Ga is also present (Fig. 5.6).

5.4.2 Ravendale Formation PAM-11

DZ from PAM-11 have median lengths of 155 µm, show median sphericity of 0.57 and median aspect ratios of 1.7 (Table 5.2). In transmitted light the grains appear clear in colour, are predominantly rounded to sub-rounded and are commonly fractured. Rounded grains commonly show pitting, and only a few grains are subhedral showing preserved crystal faces. The grains exhibit intermediate CL-response, oscillatory zoning is common, and some grains show a complex zonation. Resorbed core domains with conspicuous CL- response are identified for ~10% of the mounted zircons. A few grains show thin rims with a bright CL-response, which is common for metamorphic overgrowths on zircons (Corfu et al., 2003). A total of 147 grains were analysed for PAM-11, and 111 analyses are concordant, 34 analyses yielded discordant results, and two very young age results were rejected (282.0 ± 21, 316.3 ± 20 Ma, Table 5.2). All monitored trace elements and chemical parameters are inconspicuous for these two anomalously young analyses, suggesting they might be contaminants from overlying younger Permian and Carboniferous units in the drill hole (Moreton, 1983). In addition to the 147 analysed rim domains, 14 core domains were analysed and yielded 7 concordant and 7 discordant analyses. The overall concordance rate for this sample is 73% (including core and rim analysis). A total of 3 grains yielded concordant dates for rim and core analysis and are included in Fig. 5.9. The sample shows a major age population between 500 and 560 Ma and a plateau of Grenvillian aged zircons between 0.9 and 1.2 Ga (Fig. 5.8). A subordinate population is identified at 360 Ma (~7%

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of all analyses), and zircons from this age group are subhedral to euhedral with preserved crystal faces and show commonly oscillatory zoning suggesting a contemporary volcanic origin (Fig. 5.7).

Figure 5.7. Transmitted light (upper row) and CL images (lower row) of the youngest concordant DZ from PAM-11 showing euhedral to subeuhedral morphologies and often containing melt inclusions. Analysis of core domain (green circle) for 11_96 yielded a discordant age.

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PON-21

The grains from this sample are clear under transmitted light and predominantly sub- rounded to rounded, where only a few zircons are subhedral. DZ from PON-21 have a median length of 142, median aspect ratios of 1.7 and median sphericity of 0.58 (Table 5.2). CL-response is intermediate to low, and observed internal zonation comprises oscillatory zoning and conspicuous core domains are observed for ~10% of the mounted grains. 149 grains were analysed from PON-21, with 121 analyses yielding concordant results, and 27 returned discordant analyses (Table 5.2). One analysis yielded a conspicuously young age (340.2 ± 20 Ma) and was rejected due to very high U contents (>1,000 ppm) and corresponding high calculated alpha dose (>2.0 x 1018 g), indicating likely radiation damage and ensuing Pb-loss in this grain. In addition to analysed rim

Figure 5.8. DZ geochronology data from two samples from the Ravendale Formation integrating concordant U-Pb ages from rim and core analysis. KDE (blue) and PDP (red) shown together with a histogram of the data, in order to capture all aspects of the DZ age distribution. Data highlights issues arising from broad age populations and the usage of KDE plots, PDP captures additional age cluster ~360 Ma for PAM-11 that otherwise remains undetected in the KDE plot. U-Pb ages for core domains not included in the overall data only displayed as circles on the bottom of the plot.

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domains, core domains of 15 grains were analysed yielding 13 concordant ages and 2 discordant analyses, resulting in 11 concordant core-rim pairs (Fig. 5.9). Zircons from PON-21 yield the highest concordance rate of the analyses samples from the Darling Basin with 81% concordance. A major DZ population is identified at ~560 Ma, and DZ ages of Grenvillian age are observed between 0.8 and 1.2 Ga (Fig. 5.8). A subpopulation at ~425 Ma is only evident from the PDP and minor indistinct populations are identified at ~1.6 and 2.3 and 2.7 Ga (Fig. 5.8).

Figure 5.9. Compiled concordant core and rim ages grouped for individual grains from all four analysed samples arranged in ascending order of U-Pb rim ages. Data show relatively high proportions of inherited U-Pb ages ~500-600 Ma and subordinate for the Grenvillian age bracket (0.9-1.2 Ga).

Table 5.2. Sample details for detrital U-Pb zircon geochronology including information on core and rim analyses and grain morphology parameters.

rims cores morphology [median values]

concordant discordant rejectedyoung analysedgrains concordant discordant conc.rim/core pairs [µm] length AR sphericity sample analysedgrains PAM-11 147 111 34 2 14 7 7 3 155 1.7 0.57 PON-21 149 121 27 1 15 13 2 11 142 1.7 0.58 MOS-18 145 111 34 0 10 7 3 4 148 1.6 0.61 EMU-20 152 75 77 0 14 6 8 5 168 1.6 0.64

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5.4.3 Integration of previous U-Pb zircon geochronology data DZ used in this study were picked and mounted from mineral separates used in the study of Barry (2016). The goal here was to increase the total number of analyses (n) to enhance the statistical robustness of the data and investigate the effects on MDA and overall DZ frequency distributions (i.e. the position of age peaks and shape of peaks) in this context. In the following, the results presented in the previous section are discussed in the context of the U-Pb data reported in Barry (2016). The implications for modelled MDA and DZ age frequency distributions are not only evaluated by directly comparing the legacy data and the newly acquired data, but also by integrating the two datasets.

Maximum depositional ages MDAs were calculated according to the method outlined in Section 3.4.2 of Chapter 3, and reported as per the DZ data from the Adavale Basin in Section 4.2.3. MDAs have been recalculated for three scenarios to investigate the statistical effects of the selected resampling strategy, using (i) the previous data from Barry (2016), (ii) the newly acquired data presented in the previous section and (iii) a combination of both datasets (Table 5.3). Trace element data, particularly Th/U ratios, U concentrations and calculated alpha dose were monitored for both datasets in order to identify and reject individual analyses that potentially indicate Pb-loss. In the following, the integration of the previous and new data and the implications for assignments of MDAs are discussed. An MDA is considered as improved for the combined data when a statistically robust model age yields a significantly younger MDA compared to an MDA based on (i). For simplification purposes, only the results from the usually statistically most robust YC2σ (3+) are discussed here.

The MDA for PAM-11 remains essentially unchanged after integrating the new data from this study with the previous dataset, with the combined data giving an MDA of 361.0 ± 4.5 Ma compared to 362.6 ± 5.2 Ma from the previous study. The statistics for the YC2σ (3+) however, were improved with a significantly higher number of degrees of freedom determining the population (n=13). For PON-21, the MDA was significantly improved with regards to the calculated age and its associated statistical parameters. The previous age was poorly constrained by the YC2σ (3+) with an assigned MDA of 429.0 ± 30.0 Ma (MSWD=5.9, POF=0.003, n=3), whereas integration of the new data indicates an age of 419.1 ± 5.7 Ma with improved statistics (MSWD=0.92, POF=0.49, n=8). It is noteworthy

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that the YC1σ (2+) are significantly younger for all three scenarios presented for PON-21 in Table 5.3. Four young isolated grains were dated at 351.2 ± 7.7, 362.2 ± 8 Ma, 370.0 ± 24 and 377.4 ± 9.3 Ma in the previous study (Barry, 2016), but elevated alpha doses of >1.5*1018 α /g for three of the analyses indicate the grains might have been affected by Pb- loss. The remaining analysis at 377.4 ± 9.3 Ma is unremarkable in terms of trace element data, but is isolated from the next youngest group at ~405 Ma.

The MDA for EMU-20 is significantly improved by integrating the new data (from 447.1 ± 8.4 Ma to 420.7 ± 7.6 Ma), suggesting an end-Silurian rather than Upper Ordovician MDA for this sample. After integrating the previous dataset with the new one, the MDA for MOS-18 remained unchanged given uncertainties, however, the statistics were significantly improved (from n=3 to n=28).

The revised MDA obtained from YC2σ (3+) of the combined datasets show overall good agreement with the existing biostratigraphic framework. The MDA for PAM-11 lies within the Famennian/Earliest Carboniferous interval as indicated by biostratigraphy (Moreton, 1983). A Famennian age is determined from fossils both immediately above and below the sampled depth for PON-21 (Bembrick, 1997; Jessop, 1967). The MDA obtained from the YC2σ (3+) predates the biostratigraphic ages by ~40 Myrs, however, one isolated U-Pb zircon ages has been reported by Barry (2016) at 377.4 ± 9.3 Ma. This isolated date might be indicative of the true depositional age of the sample and is agreement with the biostratigraphic age constraints. The date also agrees with the MDA for the second sample from this unit (PAM-11, Table 5.3), but on its own, is not sufficient to conclude a much younger MDA. As a consequence, it is suggested to use the younger YC1σ (2+) to constrain the MDA for this sample instead of the more conservative (older) YC2σ (3+).

The integration of both datasets further highlights some caveats for calculating MDA from DZ data. The YC2σ (3+) yield consistently older ages for the combined dataset compared to the newly acquired dataset, as a result of the higher number of analyses included in the YC2σ (3+) in the combined dataset. This effect is most likely a result of mixing in of older populations (in part illustrated by higher POF values) resulting in skewing of the modelled ages (Table 5.3). On the other hand, the YC1σ (2+) model ages are not affected by this effect to the same degree, as less additional U-Pb dates are included, resulting in identical or just slightly older model ages (Table 5.3).

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Table 5.3. MDA calculated as YC1σ (2+) and YC2σ (3+) for U-Pb data of (i) this study, (ii) previous data from Barry (2016), and (iii) combined datasets. MDA in bold for the combined data are the preferred MDA. Abbreviations: MSWD, mean squared weighted deviation; POF, probability of fit.

Sample Unit YC1σ (2+) MSWD n POF YC2σ (3+) MSWD n POF

PAM-11 Ravendale Formation 358.6 ± 8.1 0.21 3 0.81 362.6 ± 5.2 0.38 7 0.890

PON-21 Ravendale Formation 422.1 ± 6.8 0.1 2 0.75 429.0 ± 30.0 5.90 3 0.003

Berry(2016) EMU-20 Wana Karnu Group 443.0 ± 10.0 0.08 2 0.77 447.1 ± 8.4 0.92 3 0.400

MOS-18 Wana Karnu Group 407.7 ± 8.2 0.12 2 0.72 414.1 ± 4.3 0.53 8 0.810

Sample Unit YC1σ (2+) MSWD n POF YC2σ (3+) MSWD n POF

PAM-11 Ravendale Formation 355.0 ± 10.0 0.15 5 0.96 356.2 ± 9.0 0.21 6 0.960

PON-21 Ravendale Formation 404.0 ± 13.0 0.05 4 0.99 412.0 ± 11.0 0.75 6 0.580 this study this EMU-20 Wana Karnu Group 412.0 ± 11.0 0.13 5 0.96 415.0 ± 10.0 0.49 6 0.790

MOS-18 Wana Karnu Group 405.9 ± 7.4 0.11 10 0.99 412.1 ± 5.4 0.43 19 0.980

Sample Unit YC1σ (2+) MSWD n POF YC2σ (3+) MSWD n POF

PAM-11 Ravendale Formation 354.9 ± 8.1 0.12 6 0.99 361.0 ± 4.5 0.40 13 0.960

PON-21 Ravendale Formation 404.0 ± 13.0 0.048 4 0.99 419.1 ± 5.7 0.92 8 0.490 EMU-20 Wana Karnu Group 412.0 ± 11.0 0.13 5 0.97 420.7 ± 7.6 0.83 7 0.540 combined data

MOS-18 Wana Karnu Group 406.8 ± 5.5 0.11 12 1.00 414.1 ± 3.2 0.55 28 0.970

Comparison and integration of previous DZ U-Pb geochronology data for frequency distributions The newly obtained data show overall good agreement with the previous dataset for most major age populations (Fig. 5.10). Comparing the two datasets for samples of the Ravendale Formation in detail reveals some differences in the proportions of the ~360 Ma population for PAM-11 (Fig. 5.10a). The proportions of ~360 Ma are higher in the combined data compared to the previous dataset, highlighting the importance of a sufficient number of analysis in DZ studies to detect small DZ populations (Vermeesch, 2004). In this case this has consequences for the determination of MDA as outlined in the previous section.

The DZ data for the samples from the Lower Devonian Wana Karnu Group and the Upper Devonian Ravendale Formation show that both units exhibit a large proportion of DZ ages between 500-575 Ma (Fig. 5.10). The data from MOS-18 show two distinctive age peaks within the bracket of ‘Pacific Gondwana’ ages (e.g. Ireland et al., 1998; Purdy et al., 2016b), which is represented by a more broad age population in the remaining samples. It is noteworthy that a substantial increase in the number of individual analyses

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does not change the shape of the more broad ‘Pacific Gondwana’ age group, as demonstrated with the integrated datasets for PAM-11, PON-21, EMU-20 (Fig. 5.10a, b, d). The combined data exhibit a more plateau-shaped age population for the Grenvillian ages (Fig. 5.10a, b) which is a characteristic shape of DZ distributions from this age group (e.g. Ireland et al., 1998; Purdy et al., 2016b). In addition, the new data reveal a DZ subpopulation at ~1.6 Ga for MOS-18 and at ~2.7 Ga for EMU-20, both of which were not evident from the previous dataset of Barry (2016).

In summary, a total of n=417 concordant DZ ages from this study, in combination with 251 DZ U-Pb dates from the previous study of Barry (2016) yield a combined total of n=668 concordant ages for the Darling Basin. The aggregated data indicate five prominent age populations: (i) Late Devonian (370 Ma), (ii) Early Devonian to Silurian (400 – 430 Ma), (iii) Cambrian (500 – 520 Ma), (iv) Ediacaran (550 – 600 Ma) and (v) a large group of Grenvillian ages DZ (~25%, 0.9-1.2 Ga, Fig. 5.11). Just over half of the total data yield Palaeozoic ages (54%), and DZ ages older than 2.0 Ga make up ~6.5% of DZ in the samples from the Darling Basin.

Temporal trends Comparing the two sampled stratigraphic levels by means of the combined data for the Lower Devonian Wana Karnu Group and the Middle to Upper Devonian Ravendale Formation highlights some changes in the DZ frequency distributions over time, which have implications for the provenance evolution of the Darling Basin.

The Wana Karnu Group data show a very pronounced age peak at 410 Ma (Early Devonian which is replaced by a Late Devonian population (~375 Ma) in the Ravendale Formation (Fig. 5.11). The remaining data depict a typical ‘Pacific Gondwana’ signature with a pronounced age peak 500-575 Ma and a subordinate Grenvillian age population. Two separate age peaks are visible especially in the aggregated data for the Wana Karnu Group and to a lesser degree in the Ravendale Formation (Fig. 5.11). At the same time, the proportions of Grenvillian aged DZ are substantially higher in the Ravendale Formation. In addition, the Wana Karnu Group exhibits an age population at ~690 Ma (Cryogenian), which appears more minor for the Ravendale Formation and disappears between the ‘Pacific Gondwana’ and Grenvillian age groups.

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Figure 5.10. KDE plots of previous DZ U-Pb data (blue, Barry, 2016), this study (red) and combined datasets (black) for analysed samples from the Ravendale Formation (A+B) and Wana Karnu Group (C+D).

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Figure 5.11. KDE plot showing the total DZ data acquired from the Darling Basin, highlighting a high proportion of Devonian (9%), Cambrian (17%) and Ediacaran (20%) DZ ages. Inset shows data from 300 Ma to 1.3 Ga Ma (~86% of the total DZ data) with colour coded geological periods. Abbreviations: Ca., Cambrian.

Figure 5.12. DZ data combined for the two sampled formations, Wana Karnu Group (Emsian, Lower Devonian) and Ravendale Formation (Famennian, Upper Devonian). Age peaks exclusive to the respective unit written in colour, shared age peaks are illustrated in black.

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Overall the data suggest some provenance evolution in the Darling Basins over the course of the Devonian depicted by a shift of the youngest age peak from ~410 Ma to 375 Ma. The remaining DZ ages (i.e. >500 Ma) indicate overall unchanged DZ provenance, however the proportions of the Grenvillian population appear to increase substantially in the younger unit. This is consistent with the similarities in sandstone composition between the two units, which are both quartz-rich with subordinate proportions of sedimentary lithic fragments and very low abundance of feldspars (Section 5.3.1). The apparent continuity in sediment provenance between the Wana Karnu Group and Ravendale Formation casts some doubt on the regional unconformity being substantial between the “lower” and “upper” Mulga Downs Group (Section 5.2).

5.4.4 Zircon trace element chemistry Trace element data are integrated here with DZ U-Pb ages, as undertaken for the Adavale Basin in Section 4.4.3. In addition to the zircon trace element data acquired in this study, Th, U and Th/U are also available from the legacy dataset (Barry, 2016) and have been integrated with the new data. The data are subdivided into the six major age groups to study compositional distinctions between age populations. An ultimate magmatic origin of virtually all analysed DZ is indicated by oscillatory zoning, thus zircon chemistry gives information about the parental magma composition and in combination of the U-Pb age information aid to identify igneous source rock lithologies in the context of a sediment provenance study (e.g. Grimes et al., 2015; Belousova et al., 2002; McKenzie et al., 2018).

Zr/Hf ratios are relatively restricted (~45) throughout the Precambrian, and the earliest Palaeozoic, but are higher for the Devonian age populations (~45-50, 350-400 ppm) reflecting a distinctive decrease in Hf contents in the same age bracket (Fig. 5.13). Averages of Eu/Eu* and Ce/Ce* and Lu concentrations are relatively consistent over time, while Lu shows a very low average for Neoproterozoic zircon ages. The U concentrations of the analysed zircons depict a steady increase in median and average values from the Precambrian into the Silurian, while Silurian U concentrations are distinctively higher (550-600 ppm) compared to the remaining age groups (~250-400 Ma). Th/U ratios are relatively restricted throughout all age groups with averages around 0.4-0.6. TZircTi values for DZ from the Darling Basin show overall decreasing temperatures over the course of the Precambrian and earliest Palaeozoic and a sudden increase in calculated temperatures to a

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median of 750 °C and temperatures ranging from 700-800°C for the interquartile range during the Devonian (Fig. 5.13).

The zircon trace element data highlight three distinctive features that might have implications for DZ provenance:

• Devonian-aged zircons (440-400 Ma) exhibit elevated TZircTi (700-800°C, interquartile range) in comparison with all older aged zircons. The elevated temperatures are indicative of a volcanic origin of the DZ as opposed to plutonic source rock, which can show relatively lower temperatures (Siegel et al., 2018). It is noteworthy that Ti-in-zircon thermometry requires large a upward correction depending on composition and petrology of the igneous host rock (e.g. Schiller and Finger, 2019). The applicability of TZircTi in DZ is also compromised by the fact that DZ are inherently detached from their igneous host rock.

• Silurian-aged DZ exhibit distinctively high U concentrations (400-750 ppm, interquartile range). Considering that high U zircons are inherently prone to higher radiation damage (e.g. Marsellos and Garver, 2010), the preservation of these zircons as detrital components in a sandstone might have implications for the provenance of this age group (e.g. proximal sources with shorter transport distances as well as its relative preservation).

• The acquired data show no distinctive enrichment of REE and P (Fig. 5.14), suggesting the absence of xenotime-type substitution mechanisms for the majority of analysed DZ grains (Speer, 1980). This indicates that plutonic zircon sources with S-Type affinities, such as the Silurian (435-425 Ma) S-Type granites in the central and eastern Lachlan Orogen (Chappell and White, 1992; Keay et al., 1999) did not contribute to the DZ in the Darling Basin, which is not surprising given that Silurian aged DZ (435-425 Ma) in the Darling Basin are rare (<2% of total DZ ages).

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Figure 5.13. Trace element data and geochemical parameters grouped into selected age intervals to capture trends in the trace element chemistry of DZ from the Darling Basin.

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Figure 5.14. REE subset vs P plot for all analysed DZ in this study highlights the absence of P and REE coupling for DZ from the Darling Basin, indicating absence of igneous source rocks with S-Type affinities in the Darling Basin (after Allen et al., 2018).

5.4.5 Detrital zircon morphologies The analysed DZ from two formations in the Darling Basin were digitized following the approach outlined in Section 2.5.1. Virtually all analysed grains show rounding to some degree; thus the inversion of the aspect ratio calculated from the long and short axis for each individual grain is used here to calculate the sphericity of DZ grains on a range from 0 (low sphericity) to 1 (high sphericity). The DZ age data are subdivided into five age groups, corresponding to the major age populations identified in Section 5.4.3. The morphology parameters are normalised per age group, to allow comparison between the substantially smaller Devonian (350-400 Ma, n=15) and Siluro-Devonian populations (400-440 Ma, n=34) with the large Cambro-Ordovician (440-540 Ma, n=81), Ediacaran (540-650 Ma, n=85) and Grenvillian populations (0.9-1.2 Ga, n=78, Fig. 5.15).

DZ with high aspect ratios (>3, n = 4, ~1.5%), are generally very rare in the analysed samples, with most DZ having low aspect ratios (1-2, n =321, ~77%). The high to medium aspect ratio grains (2-3) characterise the Siluro-Devonian (~50%) and Devonian age

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populations (~30%), whereas the older age populations have much lower proportions of medium aspect ratio grains (10-25%, Fig. 5.15a). DZ with high sphericity (0.8-1.0) account for about 10% of all analysed grains, and the highest cumulative proportions of medium to high sphericity (>0.6-1) grains are evident from the Ediacaran and Grenvillian populations (>50%, Fig. 5.15b). For the Paleozoic age populations, medium and high sphericity grains are generally <40%, and the Siluro-Devonian age populations have the lowest proportions (~25%).

Another way of characterising grain shape is by equivalent circular diameter (ECD). Using ECD informs about the overall grain size by using the area of the analysed grain object, constructing a circle with area equivalent to the grain area and states the diameter of the circle. The highest proportions of large grains (ECD > 150 µm) are found in the Cambro- Ordovician and Ediacaran populations (~20%). Grain sizes of Grenvillian age group are substantially smaller with <4% of grains exhibiting ECD >150 µm, implying additional recycling for DZ of this age.

Summary The integrated U-Pb age and grain morphology data show an increase in the medium and high sphericity morphologies with increasing age suggesting DZ >440 Ma are predominantly derived from (meta)sedimentary sources and have undergone at least one recycling event (Fig. 5.15, e.g. Malusà et al., 2013; Naipauer et al., 2010; Shaanan and Rosenbaum, 2018; Stevens et al., 2010). Cambro-Ordovician and Ediacaran grains exhibit substantially higher ECD than Grenvillian aged DZ indicating a higher number of recycling cycles for Grenvillian DZ. High aspect ratio zircons (>3) are virtually absent, but the highest proportions of medium to high aspect ratios (>2) are found for the Silurian and Devonian (350-440 Ma), whereas the 350-400 group represents syn-volcanic DZ (Section 5.4.2) and the 400-440 Ma group appears to be dominated by medium and high aspect ration DZ, indicative of predominantly volcanic zircon origin (e.g. Corfu et al., 2003).

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Figure 5.15. Histogram showing U-Pb DZ ages for all data from the Darling Basin integrated with morphology parameters aspect ratio (A) and sphericity (B). Stacked histograms show binned morphology parameter normalised for selected ages bins.

5.5 DETRITAL RUTILE U-PB GEOCHRONOLOGY

Detrital rutiles were hand-picked and mounted from the same heavy mineral separates that were used for DZ. Rutile yields are very good for samples PON-21, MOS-18 and EMU- 20, where for each sample, ~150 rutile grains were mounted. Rutile yields for sample

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concordant analysis was able to be measured for sample PAM-11. As in the approach for detrital rutile analysis from the Adavale Basin samples (Section 4.2.7), model-1 discordia ages (Ludwig, 1998) were calculated for visually coherent clusters and illustrated in Tera- Wasserburg plots (207Pb/206Pb vs 206Pb/238U, Tera and Wasserburg, 1972), which additionally allow calculation of common Pb compositions. Data for the monitored standard material are given in Section 3.2 (Chapter 3). The results are presented below per sample and are summarised in Table 5.4. The full dataset with integrated U-Pb ages, trace element, and grain morphology data is documented in the Electronic Appendix B, and transmitted light images of all analysed grains are illustrated in Appendix 5.3.

5.5.1 Wana Karnu Group For sample MOS-18, 149 rutiles were analysed and 127 analyses yielded discordant results. Grains from this sample are subrounded to rounded, comprise large, low sphericity rutiles with smaller grains exhibiting higher sphericity. Seven analyses exhibit concordant ages that range from 410 to 622 Ma (n=5) and single grain ages at 1.6 and 2.8 Ga. Four analyses were excluded due to extreme 207Pb/235U values (>100), 3 grains exhibited very high uncertainties for 207Pb/235U ages (>50%), 6 grains show low U contents (<2 ppm) and another 2 analyses were rejected due to poor error correlation for 207Pb/235U and 206Pb/208Pb (<0.3). A discordia model-1 age of 475.5 ± 3.9 Ma was assigned to the main rutile age population (n=132, Fig. 5.16c, Table 5.4), and the two older concordant grains at 1.6 and 2.8 Ga represent the only older grains in this sample. Rutiles from EMU-20 are similar in size and morphology to MOS-18. A total of 124 rutiles was analysed for this sample, with 70 analyses returning discordant ages and 3 grains yielded concordant results at 554, 569 Ma and 3.8 Ga (Table 5.4). A great number of analyses are excluded from the assessment here due to extreme 207Pb/235U values (n=35), high uncertainties for 207Pb/235U or 206Pb/238U ages (n=12) and low U contents (n=4). A discordia model-1 age of 506.5 ± 3.6 Ma is assigned to the major age cluster (n=67, Fig. 5.16d, Table 5.4) and the few scattered older discordant analyses do not form a coherent age cluster.

5.5.2 Ravendale Formation Rutiles from PAM-11 are large with a median grain length of 162 µm (Table 5.4). Only 11 rutile grains were recovered from the mineral separate. Nine analyses yielded

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discordant results, and no concordant analysis was measured; one analysis was rejected due to a very high uncertainty for the obtained 207Pb/235U age (>50%), and another one due to low U contents (<2 ppm). The data show no clear trend on the Tera-Wasserburg plot (Fig. 5.16a), however, a concordia age was assigned to approximate a peak age for the sample (599.9 ± 5.7 Ma).

Detrital rutile grains from the second sample (PON-21) are significantly smaller with a median grain length of 132 µm and are subrounded to rounded. A total of 140 rutiles was analysed for sample PON-21, with 121 discordant ages, and 6 concordant grain ages obtained (concordance rate 5%, Table 5.4). The concordant ages form a cluster of 3 analysis at ~1.6 Ga and isolated ages at 2.2, 2.7 and 3.8 Ga. Five analysis were excluded due to extreme 207Pb/235U values (>100), 2 grains exhibit very high uncertainties for 207Pb/235U ages (>50%), 4 grains show low U contents (<2 ppm) and another 2 analyses were rejected due to poor error correlation for 207Pb/235U and 206Pb/208Pb. A discordia model-1 age was assigned to the major cluster (n=109) in this sample, yielding an age of 473.0 ± 3.9 Ma (Fig. 5.16b). A second cluster of mainly discordant ages is identified ~1.1 Ga.

5.5.3 Summary and rutile chemistry From the total of 424 detrital rutiles, concordance rates are extremely low for all analysed samples, and only 16 analyses yielded concordant U-Pb dates (Table 5.4). The consolidated concordant data forms an age cluster from 410-622 Ma with peak ages at ~430 Ma (n=7) and ~1.6 Ga (n=4, Fig. 5.17a). Discordia model-1 ages were assigned on a single sample basis to include the large amounts of discordant analyses (Fig. 5.16). The modelled ages range from ~473-506 Ma and lie exclusively within the younger cluster identified from the concordant ages. Median grain sizes are relatively consistent across the samples, and the data show no correlation between U-Pb ages and median grain length on a per- sample basis (Table 5.4), however, grouping the data by grain size fractions and assignment of discordia model-1 ages reveals a correlation between age and grain size, i.e. larger grains show younger discordia ages (Fig. 5.18). The overall trend of grain size vs. discordia age is the same as observed for the detrital rutile data from the Adavale Basin (Section 4.5).

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Rutile trace element data are used to determine Zr-in-rutile temperatures (Watson et al., 2006) and identify rutile source lithologies using the Nb/Cr classification diagram (Triebold et al., 2007). For the combined data, Zr-in-rutile temperatures range from 530- 926 °C (disregarding extreme outliers) with 50% of the data between 684 and 790 °C and an average of 732 °C (Fig. 5.17b). Average temperatures are slightly higher for rutiles from

Figure 5.16. Tera Wasserburg plots showing concordia (a) and discordia model-1 ages (b,c,d) for the major age population on a per sample basis for U-Pb rutile geochronology. Older grains not included in major population are displayed in grey.

Table 5.4. Summarised data of U-Pb geochronology of detrital rutile for four samples from the Darling Basin highlighting two subtly different age populations for the Early Ordovician (~475 Ma) and Cambrian (~500-505 Ma).

discordia median # rutiles #anal. # # age main main older length Sample unit analysed criteria conc. disc. %conc. pop [Ma] pop. ages [µm] PON-21 Ravendale Formation 140 13 6 121 5 473.0 ± 3.9 n=109 n=18 132 PAM-11 Ravendale Formation 11 2 0 9 0 499.9 ± 5.7 n=9 - 162 MOS-18 Wana Karnu Group 149 15 7 127 6 475.5 ± 3.9 n=132 n=2 152 EMU-20 Wana Karnu Group 124 51 3 70 4 506.5 ± 3.6 n=67 n=5 152

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the two samples of the Wana Karnu Group (~750 °C) and slightly lower for PON-21 (~710 °C) but the differences are not significant. The Nb/Cr classification suggests rutile in the Darling Basin are predominantly sourced from metapelites (~80%), and subordinate proportions were contributed from metamafic sources (~20%, Fig, 5.17c).

Figure 5.17. Summary of analytical results, (a) PDP plot of all concordant rutile U-Pb ages, (b) boxplots of Zr-in-rutile temperatures on a per sample basis (black boxplots) and summarised for all analyses (grey boxplot), (c) Nb/Cr classification for discordant and concordant data (after Triebold et al., 2007).

5.6 DISCUSSION

The DZ and DR data are combined here to focus on stratigraphic variations and temporal evolution of the basin fill, and then to identify potential source rocks and regions supplying sediment to Darling Basin.

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5.6.1 Temporal evolution/Stratigraphic variation The dataset presented in this Chapter highlights some interesting insights into the stratigraphic variation of the investigated provenance indicators. Sandstone composition appears not to change between the Lower Devonian Wana Karnu Group and the Upper Devonian Ravendale Formation, as indicated by a very similar bulk detrital grain composition (Section 5.3.1, Fig. 5.4). DZ are dominated by (ultimately) igneous zircon in

both units and the age frequency distributions are very similar between the two sampled units, apart from the disappearance of some small subpopulations at ~410 Ma and the appearance of contemporary volcanic zircons at ~370 Ma (Section 5.4.3). In addition, the acquired U-Pb data from detrital rutile record predominantly Cambro-Ordovician ages and rutile chemistry indicates derivation from metapelitic source rocks for both units (Section

Figure 5.18. Tera-Wasserburg plots for all detrital rutile U-Pb data grouped by individual grain sizes (determined by length of individual rutile grains), very fine sand (A), fine sand (B, C). Data shows consistently younging discordia model-1 ages with increasing grain size fraction.

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5.5.3). The integrated data suggests that sediment provenance between the “lower” and “upper” part of the Mulga Downs Group did not change dramatically. The similarities in sediment provenance between both sampled stratigraphic levels within the Mulga Downs Group questions the magnitude of the proposed unconformity in this section of the Darling Basin (Bembrick, 1997; Khalifa, 2010).

5.6.2 Detrital zircon sources CL-imaging of DZ shows oscillatory zoning for most analysed grains suggesting the analysed zircons are ultimately igneous in origin. The combination of DZ age and morphology information can be used to discriminate DZ sourced directly from igneous materials (i.e. first cycle contemporary volcanic contributions and eroded igneous rock) versus recycled DZ from (meta)sedimentary rocks. Five prominent age populations have been identified in Section 5.4.3: (i) Late Devonian (370 Ma), (ii) Early Devonian to Silurian (400 – 430 Ma), (iii) Cambrian (500 – 520 Ma), (iv) Ediacaran (550 – 600 Ma) and (v) a broad group of Grenvillian ages DZ (0.9-1.2 Ga, Fig. 5.11). Potential source rocks for these zircon age populations are investigated.

5.6.3 Primary igneous sources Upper Devonian sources (370-360 Ma) DZ of Upper Devonian age are exclusive to samples of the Ravendale Formation and evident from a distinctive population in sample PAM-11. Biostratigraphic data indicates Famennian (372.2 to 358.9 Ma) depositional ages for the Ravendale Formation (Section 5.2.3), which is consistent with a MDA of 361.0 ± 4.5 Ma for PAM-11 (Section 5.4.3). Volcanic rocks with emplacement ages between 370-360 Ma are unknown or not reported from the immediate surroundings of the Darling Basin and the detrital grain compositions of the sampled rocks are not affected by any contemporary volcanic influence (Section 5.3.1). For the Ravendale Formation no interbedded tuffs are reported and the proportions of syn-volcanic DZ are very limited, indicating that the DZ are potentially sourced from distal ash fallouts from silicic explosive (plinian) eruptions (e.g. Barham et al., 2016). Potential source units are displayed in Figures 5.19 and 5.20 and are discussed in the following.

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Southeast of the Darling Basin, pyroclastic volcanic rocks associated with caldera formation in the Central Victorian Magmatic Province are widespread (Birch, 2003; Gray and Foster, 2004, Figs. 5.19, 5.20). The Violet Town Volcanics, part of the Strathbogie Igneous Complex is one of the few units dated via U-Pb zircon geochronology and yields an emplacement age of 373 ± 3 Ma (Kemp et al., 2006, Fig. 5.20). This age appears to be coeval with rhyolitic to dacitic ignimbrites in the Marysville Igneous Complex (Taggerty Subgroup, Fig. 5.20) that were dated at 373 ± 4 Ma using the K-Ar method (Richards and Singleton, 1981). The age of the Ryans Creek Rhyolite as part of the Tolmie Igneous Complex is constrained via Rb/Sr (368 Ma) and K-Ar (Fig. 5.20, 348 ± 8 Ma, VandenBerg et al., 2004). A Frasnian age for the Rose River Volcanics in the Wellington Volcanic Group is inferred from stratigraphic relationships and biostratigraphic constraints from overlying sedimentary units (Fig. 5.20, Long and Werdelin, 1986; O’Halloran and Gaul, 1997). Average Th/U for DZ with ages between 360-380 Ma from PAM-11 (0.65) are similar to values reported for the Violet Town Ignimbrite (0.5-0.6, Kemp et al., 2006).

In the eastern Lachlan Orogen, ~500 km to the east of the sampled location of PAM-11, zircons from the Dulladerry Volcanics as part of the Rocky Ponds Group have been dated between 385-375 Ma and represent another potential source for contemporary DZ in PAM- 11 (Fig. 5.19). The unit comprises welded rhyolitic ignimbrites, rhyolite lavas and volcanic breccias (Pogson and Watkins, 1998). A crystal-rich quartz feldspar porphyry from the ‘Caloma dyke’ has been dated at 381.8 ± 2.1 Ma (Th/U 0.63, Bodorkos et al., 2013), a quartz-phyric rhyolitic lava from the Warraberry Member at 377.3 ± 4.7 Ma (Th/U 0.53) and flow banded lavas from the Coates Creek Member at 385.4 ± 5.1 Ma (Th/U 0.53, Atton, 2013).

In conclusion, there are no signs of proximal contemporary volcanism and sources to supply Upper Devonian aged zircons to the Darling Basin. A number of silicic volcanic provinces located in the eastern Lachlan Orogen, that overlap in ages and are characterised by explosive eruptions and pyroclastic rocks, could have produced tall plinian columns for distal ash fallout into the Darling Basin. Regardless of the respective volcanic source of contemporary DZ, these volcanic provinces are all ~700 km upwind (Fig. 5.19). Sourcing from distal ash fallouts would help explain the minor presence of syn-depositional aged

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euhedral/angular zircon but without any volcanic disturbance to the detrital grain compositions of the sandstones.

Siluro-Devonian sources (440 – 400 Ma) The combined DZ data from the Wana Karnu Group show a clear Early Devonian peak at ~410 Ma (400-420 Ma, 4.7%) and slightly lower proportions of Silurian aged zircons (420- 440 Ma, 3.6%, Fig. 5.11), indicating that Early Devonian igneous rocks contributed to a higher degree to the DZ budget than Silurian igneous units. Silurian to Early Devonian DZ do not form a distinctive peak for the combined data from the Ravendale Formation but are indeed present between the predominant Late Cambrian and Middle Devonian age peaks (Fig. 5.11).

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Plutonic rocks of Silurian to Early Devonian age and co-magmatic volcanic rocks are widespread east of the Darling Basin and are exposed in broad north-south trending belts in the eastern and central Lachlan Fold Belt (e.g. Black, 2006, 2005; Ickert and Williams, 2011; Lyons et al., 2000, Fig. 5.20). Igneous rocks in the area exhibit predominantly Silurian emplacement ages (435 – 425 Ma) which are dominated by S-Type granites, and subordinate Early Devonian ages (420 – 400 Ma) are commonly associated with granitic I- Type rocks (Collins and Hobbs, 2001; Ickert and Williams, 2011). Apart from the widespread distribution in the central and eastern Lachlan Orogen, a narrow belt of intrusive rocks is apparent in the westernmost part of the Delamerian Orogen east of the Bancannia Menindee and Wentworth Troughs and west of the Pondie Range and Blantyre troughs of the Darling Basin (Wilkurra Granite, Wilde, 2000; unnamed granite and shallow intrusive rocks, Black, 2006, Figs. 5.1, 5.20). In the southern Thomson Orogen, to the north of the Darling Basin, Late Silurian intrusive and volcanic rocks crop out in the Tibooburra and Warrata Inliers and near Dynamite Tank (Black, 2007, 2006; Greenfield et al., 2010; Vickery, 2010, Fig. 5.20). Volcanic rocks of Early Devonian age have also been identified in the eastern part of the southern Thomson Orogen (Dwyer et al., 2018; Hack et al., 2018, Fig. 5.20), in the subsurface of the central Thomson Orogen (Gumbardo Formation, Chapter 2, Asmussen et al., 2018) and granitic rocks of late Silurian age are exposed in the central part of the southern Thomson Orogen (Bodorkos et al., 2013; Cross et al., 2018; Purdy et al., 2016, Fig. 5.20).

Paleoflow directions are dominated by easterly flow directions indicating westerly sediment sources for rocks of the “lower” Mulga Downs Group (specifically for the Coco Range Sandstone, Section 5.2.2, Fig. 5.2), however, flow directions are not directly constrained for rocks of the Wana Karnu Group. Sourcing of DZ from volcanic and potentially plutonic rocks from occurrences of igneous basement rock in the area of the Darling Basin and from igneous units to the west seems most likely based on the predominant paleoflow directions (Fig. 5.20). Sourcing from locations to the east of the Darling Basin, however, cannot be ruled out as flow directions are not directly constrained for rocks of the Wana Karnu Group. The proportions of Siluro-Devonian aged zircons noticeably decrease up-section from the Wana Kanu Group (14.5%) to the Ravendale Formation (3.0%), while paleoflow directions remain generally unchanged from the “lower” to the “upper” Mulga Downs Group (Section 5.2.2, Fig. 5.2). This potentially

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Figure 5.20. Overview of relevant (igneous) geological units in the Lachlan Orogen for sediment provenance in the Darling Basin. U-Pb ages have been extracted from the XplorPak 2016 database (New South Wales. Department of Industry. Division of Resources and Energy compiler, 2016). Black circles represent sampled locations. Abbreviations: QLD, Queensland; NSW, New South Wales; SA, South Australia; VIC, Victoria; ACT, Australian Capital Territory.

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indicates that Siluro-Devonian igneous basement rocks were the main source for DZ ~410 Ma and became covered-up over the course of the lifespan of the basin.

5.6.4 Recycled metasedimentary sources The high proportions of medium and high sphericity grains of Cambrian and Ediacaran age (Fig. 5.15b) together with the overall lack of igneous lithic clasts and low abundances of detrital feldspar in all samples (Section 5.3.1) implies igneous rocks were not a major source for the Darling Basin sedimentary rocks. In the following section, Cambro- Ordovician sedimentary units in the Lachlan and Delamerian orogens are discussed as a potential sediment source for recycled DZ.

Two major age peaks at 500 and 580 Ma are identified for the combined DZ data from the Darling Basin, and these Cambrian and Ediacaran DZ populations encompass about 40% of all DZ ages (Fig. 5.11, Section 5.3.3). As outlined in the section above, igneous rocks of Cambrian (~500 Ma) and Ediacaran (~580 Ma) age are rare in the Lachlan and Delamerian Orogens. More importantly, the Darling Basin DZ have high proportions of medium and high sphericity grain morphologies (~60%, Section 5.4.5) of these specific ages suggestive of (meta)sedimentary origin. The age peaks at 500 and 580 Ma are characteristic of Middle to Late Cambrian sedimentary sequences in the Delamerian Orogen and Ordovician sedimentary rocks in the Lachlan Orogen (Fig. 5.21). In addition, Cambro-Ordovician sedimentary rocks commonly possess a Grenvillian age population, which is also observed in the samples from the Darling Basin. Grain morphologies of Grenvillian aged DZ (0.9-1.2 Ga) show high proportions of medium and high sphericity grains (>50%, Section 5.4.5), indicating a recycled provenance from (meta)sedimentary sources rather than primary igneous sources.

Broad populations of Delamerian-aged DZ (500 – 600 Ma) and Grenvillian aged DZ have been reported from the Kanmantoo Group (Ireland et al., 1998) and are further recognized in the Glenelg River Complex in western Victoria (Fig. 5.22, Haines et al., 2009). Quartz-rich sandstones in the south-eastern Lachlan Orogen in the Kiandra Volcanic belt exhibit slightly more pronounced populations at 500 and 560 – 580 Ma (Fig. 5.22). Early to Late Cambrian sandstones from the Teltawongee and Ponto groups in the Koonenberry Belt (Fig. 5.22) to the northwest of the Darling Basin exhibit a distinct

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population at ~580 Ma and a characteristic Grenvillian age plateau (Johnson et al., 2016, Fig. 5.21). Cambrian and Ordovician metasedimentary rocks from the eastern Delamerian and western Lachlan Orogen show a pronounced overall shift from DZ distributions dominated by ~580 Ma DZ during the Cambrian to distributions dominated by ~500 Ma ages (Fig. 5.21, Squire et al., 2006). A dominant detrital population at 500 Ma and a minor grouping at ~580 Ma is also evident from the Warrata Group in the southern Thomson Orogen (Armistead and Fraser, 2015; Purdy et al., 2016b, Figs. 5.21, 5.22). DZ from Late Ordovician quartz turbidites show a broad Delamerian cluster and subordinate contribution of Grenvillian ages (Fergusson and Fanning, 2002).

In conclusion, the dominant ~500 and ~580 Ma DZ populations in combination with a subordinate plateau-shaped Grenvillian age population in the Devonian sedimentary rocks of the Darling Basin is typical of a ‘Pacific Gondwana’ DZ age signature (Ireland et al., 1998; Purdy et al., 2016a). General paleoflow directions indicate sediment sourcing from the west of the Darling Basin, and this points to sediment sourcing from the Ponto Group and Teltawongee Group in the Koonenberry Belt (Fig. 5.22). The DZ frequency distribution for these units shows similar proportions of ‘Pacific Gondwana’ and Grenvillian ages compared to the Darling Basin, however, they are lacking the pronounced peak at ~500 Ma (Fig. 5.21). This indicates that (meta)sedimentary rocks of Late Cambrian to Ordovician age additionally contributed to the sediment budget in the Darling Basin (e.g. Warratta Group, Figs. 5.20 and 5.21). The characteristic ‘Pacific Gondwana’ age signature in combination with high proportions of rounded medium to high sphericity grains associated with this DZ age signature (Section 5.4.5) strongly suggest that a mixture of Cambrian and Ordovician (meta)sedimentary rocks are the dominant sediment source for the Darling Basin.

5.6.5 Detrital rutile sources Detrital rutile from the Darling Basin exhibit extremely low concordance rates, with 16 concordant ages obtained from 424 analysed grains due to low U contents, and high common Pb contents. This suggests exercising caution in the interpretation and comparison of concordant data alone. The concordant analyses show some overlap with detrital rutile

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Figure 5.21. Compiled DZ data from Cambro-Ordovician sedimentary rocks in the Delamerian and Lachlan Orogens (ST36, ST76, ST21, ST18, ST23, Squire et al., 2006; F03D02, Armistead and Fraser, 2015; KB22a/21/27/12/14/16/26, Johnson et al., 2016) and comparison with total DZ data from the Darling Basin (combined datasets of this study and Barry, 2016) showing interplay of ~580 (red line) and 500 Ma (green line) peaks in Cambrian and Ordovician sedimentary rocks. The ~580 Ma peak in Middle to Late Cambrian samples is replaced by a dominant ~500 Ma population during the Late Cambrian/Ordovician. Note that the Darling Basin data shows both peaks with nearly equal proportions.

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Figure 5.22. Spatial extent of selected Cambro-Ordovician sedimentary units in the Lachlan, Thomson and Delamerian Orogens, subsurface extent of the Darling Basin shown in light blue. Sample labels in brackets in the legend refer to DZ samples in Fig. 5.21.

from the Adavale Basin (Chapter 4) and the Thomson beds (Siegel et al., 2017), but also reveals a proportion of younger ages ~430 Ma which has so far only been recorded from the Carboniferous Drummond Basin to the north of the Thomson Orogen (Fig. 5.23a, Sobczak, 2019). A ~1.6 Ga population in the Darling Basin data suggests further similarities with the Drummond Basin, which also contains a population between 1.5 and

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1.6 Ga, whereas rutiles of this age are virtually absent in the data from the Adavale Basin and Thomson beds (Fig. 5.23a). In addition, very old grains were dated for the Darling Basin (~2.8 and ~3.8 Ga), which are absent in all of the above-mentioned datasets. For the younger main age cluster (470-510 Ma), a correlation between grain size and discordia model ages was found analogous to Section 4.5 in Chapter 4, however, the data do not correlate with the model ages from the Adavale Basin that are distinctly older (600-650 Ma) for grain sizes between 130-170 µm (Fig. 5.23b).

In summary, U-Pb dating of detrital rutile shows some similarities between the Devonian Darling Basin and the Carboniferous Drummond Basin, indicating that the Darling Basin might play a role in the rutile provenance of the Drummond Basin, which is further supported by a pronounced Ediacaran and Grenvillian DZ population (‘Pacific Gondwana’ signature) in the Drummond Basin (Sobczak, 2019). Comparison of the data for the Adavale and Darling basins reveals similar discordia model ages on a per-sample basis (470-520 Ma). The question remains open as to what the immediate source for detrital rutile in the Darling Basin is. Considering the very high rutile yields and predominant zircon sourcing from (meta)sedimentary rocks in the Lachlan and Delamerian orogens, it seems likely that these rocks also were the immediate source of rutile in the Darling Basin. However, no detrital rutile data from Ordovician or Cambrian sedimentary rocks are available from this area to test this hypothesis.

Figure 5.23. Concordant detrital rutile U-Pb ages, compiled from various Phanerozoic sedimentary rocks showing subtle variations and similarities between the datasets (a). Plot of discordia model ages vs median grain length on a per-sample basis, highlighting correlation between grain size and rutile U-Pb ages.

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5.7 CONCLUSIONS

The DZ budget in the Darling Basin is dominated by Ediacaran and Cambrian DZ populations that are interpreted here to have been sourced from Cambrian and Ordovician sedimentary basement rocks in the Lachlan and Delamerian orogens. This is consistent with sandstone petrography showing a contribution from sedimentary source rocks and a lack of evidence for igneous source lithologies. In addition, the Cambro-Ordovician sedimentary rocks are most likely the major source of detrital quartz in the Darling Basin. Detrital rutile U-Pb ages overlap with the timing of the Ross-Delamerian Orogeny (541-490 Ma in the Delamerian Orogen, Foden et al., 2006), which potentially is the ultimate source of the rutiles. Cambro-Ordovician sedimentary successions in the Lachlan and Delamerian Orogens contain rocks that are a potential immediate source of these rutiles, but a lack of published detrital rutile U-Pb data from these successions hinders further interpretation.

Temporal trends in DZ frequency distribution between the Lower Devonian Wana Karnu Group and the Upper Devonian Ravendale Formation indicate predominant sediment supply from Cambro-Ordovician sedimentary rocks (~95%, Fig. 5.24). Sediment provenance remains unchanged over the course of the Devonian, and this observation challenges the interpreted unconformity within the Mulga Downs Group, which is based on a brief marine incursion and interpretations of seismic unconformities associated with the Tabberabberan Orogeny (Alder et al., 1998; Bembrick, 1997; Khalifa, 2009).

Younger DZ populations (i.e. younger than Silurian), however, highlight the introduction of an additional source from in Ravendale Formation, interpreted to be induced by covering of igneous basement rocks and introduction of a syn-volcanic DZ population (Fig. 5.24). The Lower Devonian (Emsian/Eifelian) Wana Karnu Group exhibits a contribution from Lower Devonian (Lochkovian) DZ sources that predate the (biostratigraphic) depositional age by ~15 Myrs. Extensive belts of Lower Devonian plutonic and volcanic rocks lie to the east of the Darling Basin, but paleoflow directions indicate the sediment sources lie within the area of the Darling Basin and to the west of it (Fig. 5.24). The DZ spectra from the Upper Devonian Ravendale Formation feature an Upper Devonian (Frasnian) population and biostratigraphic constraints in combination with modelled MDA indicate a contribution from syn-volcanic sources. Potential sources of these zircons are distal to the Darling Basin, ~700 km to the southeast (Central Victorian Igneous Province) and east (Dulladerry Volcanics, Figs. 5.19, 5.24). Contemporary volcanic sources in the New England Orogen

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might also have contributed to the Darling Basin, but these are much further away (>1000 Km, e.g. Offler and Murray, 2011). The sandstone of the Ravendale Fm shows no indications of a contemporary volcanic influence on sandstone compositions which is consistent with a remote source for contemporary volcanism.

Figure 5.24. Sediment provenance interpretation for the Lower to Middle Devonian (a) and Upper Devonian (b) successions in the Darling Basin in conjunction with KDE plots of the respective unit (lower section of a and b). Data highlights the predominance of Cambro-Ordovician (meta)sedimentary rocks (green) in the sedimentary budget of the Darling Basin. Younger age groups change from Lower Devonian (~410 Ma) to Upper Devonian (~380 Ma) up stratigraphy, whereas Upper Devonian volcanic sources are distal to the Darling Basin.

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Chapter 6: Synthesis

The integration of biostratigraphic data and modelled MDA from newly acquired DZ U-Pb data for the Adavale and Darling basins indicate an overall synchronicity of sedimentation. In addition, the depositional environments of the Adavale and Darling basins appear to be broadly correlated. This synthesis chapter focusses on returning to the overarching hypothesis of this thesis that the Adavale and Darling Basins were connected and part of a larger intracratonic basin system established in the Devonian, reflecting the stabilisation of the Thomson and Lachlan orogens that had now switched to become the continental platform. A connection of the two basins would imply the existence of a large Devonian continental-scale sedimentary system that would then indicate the Thomson and Lachlan orogens had a shared tectonic history by the Devonian, and both orogens had become stabilised by this time.

6.1 SEDIMENT PROVENANCE COMPARISON BETWEEN THE ADAVALE AND DARLING BASINS

6.1.1 Comparison of synchronous units from the Adavale and Darling basins Comparison of DZ age spectra can be made at two comparative stratigraphic levels - the Wana Karnu Group and Eastwood Beds (Lower Devonian) and the Ravendale and Buckabie Formations (Upper Devonian), for the Darling and Adavale basins, respectively. This allows a comparison at each time slice, but also to evaluate temporal changes and whether those changes are similar in each basin. The comparison focuses on the main detrital age populations as informed by KDE peaks (Fig. 6.1).

The integrated age constraints from biostratigraphy and MDA determined from the newly acquired DZ U-Pb data indicate that the Wana Karnu Group temporally overlaps with the Eastwood Beds in the Adavale Basin (Sections 4.4.1, 5.2.4, 5.4.3). These Lower Devonian units share a similarly shaped Grenvillian age population (0.9-1.2 Ga), and an Ediacaran population is evident in both samples (Fig. 6.1). However, the proportions of both the Grenvillian and Ediacaran are much smaller in the Adavale Basin. While the Wana Karnu Group and Eastwood Beds exhibit pronounced KDE peaks for the Early Devonian,

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the peak ages are sufficiently different (i.e. 400 Ma versus 410 Ma) and are interpreted here to record different regional primary igneous sources (cf. Section 5.5.1 and 4.6.1). The Early Ordovician (~475 Ma) and Mesoproterozoic (1.5-1.6 Ga) age peaks in the Eastwood Beds are interpreted to record local sourcing in the Thomson Orogen (Section 4.6.1) and are almost absent in the samples from the Wana Karnu Group (Fig. 6.1). On the other hand, the Eastwood Beds of the Adavale Basin are virtually devoid of Cambrian DZ (~500 Ma), precluding sourcing from metasedimentary rocks from the Lachlan and Delamerian orogens (Section 5.6.4).

Comparison of the Upper Devonian Buckabie (Adavale) and Ravendale (Darling) formations reveals a lower degree of similarity than observed for the Lower Devonian units. The first major difference is the presence (or continuation) of a Grenvillian age DZ population in the Ravendale Formation, but which is absent in the Buckabie Formation. The Ravendale data further shows pronounced Ediacaran (~575 Ma) and Cambrian (~500 Ma) age peaks that are barely present in the Buckabie Formation (Fig. 6.1). Samples from both formations exhibit an Upper Devonian age peak, which is significantly more pronounced in the Buckabie Formation than in samples from the Ravendale Formation. While the Adavale and Darling basins share an ~370 Ma age population, the zircon morphologies (i.e. higher aspect ratio grains in the Adavale Basin, cf. Section 4.4.4, 5.4.2 and 5.4.5) and trace element compositions (specifically higher Th/U for DZ 350-370 Ma in the Adavale Basin, cf. Section 4.4.3 and 5.4.4, Fig. 6.2) are distinctively different, suggesting they are derived from different syn-volcanic sources. Three major DZ source signals are identified across the Adavale and Darling basins are summarised in the following in the context of the Tasmanides.

6.1.2 Devonian volcanic DZ sources The Adavale Basin records a variety of different Devonian volcanic sources, including (i) reworking of the basal volcanic rocks of the Gumbardo Formation (~400 Ma, Chapter 2 and Asmussen et al., 2018), (ii) a contemporary volcanic source supplying zircon at the beginning of the Upper Devonian (~380 Ma) and (iii) a second pulse of contemporary volcanic zircon in the Famennian (~370-360 Ma) that reflects a more substantial supply of zircon and resedimented volcanic material into the basin (Fig. 6.3, Section 4.6.1). Upper Devonian syn-volcanic sources occur in the Anakie Inlier and northern New England

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Orogen, 300-600 km northeast of the Adavale Basin (e.g. Silver Hill Volcanics, Theresa Creek Volcanics, Campwyn Volcanics; Blake et al., 2013, 1995; Bryan et al., 2004; Cross et al., 2015, 2009; Henderson et al., 1998). In comparison, Lower Devonian volcanic basement rocks within the Mount Daubeny Formation are substantially older in and around the Darling Basin (~410-420 Ma, Greenfield et al., 2010) and this signal is mostly recorded in the Lower Devonian section of the Basin. A synvolcanic DZ population (~370 Ma) is identified in the Upper Devonian part of the Darling Basin, but this source does not modify the sandstone towards more lithic and feldspathic compositions, suggesting a more remote volcanic source where zircon was most likely supplied by distal ash fallout. Potential explosive volcanic sources have been identified ~500km to the south and east of the Darling Basin (Central Victorian Igneous Province, Dulladerry Volcanics, Birch, 2003; Gray and Foster, 2004; Pogson and Watkins, 1998).

Comparison of syn-volcanic DZ populations from the Adavale and Darling basins reveals distinctions between the two: - Significant differences in zircon trace element compositions for syn-basinal Upper Devonian DZ, especially with regards to Th/U for zircons with ages between 350- 370 Ma (Fig. 6.2, cf. Section 4.4.3 and 5.4.4)

- Different zircon morphologies in the form of high aspect ratio DZ in the Adavale Basin (>3) as opposed to aspect ratios ~2 in the Darling Basin (cf. Section 4.6.3 and 5.4.2)

The differences in overall abundance, zircon chemistry and grain morphology suggest Upper Devonian syn-volcanic zircons are not from the same source eruptions as the coeval volcanic-derived Upper Devonian DZ in the Adavale Basin, even though a southward sediment transport through the Adavale is indicated. Despite overlapping Upper Devonian ages, the differing zircon chemistry indicates the Darling and Adavale basins were both influenced by contemporary volcanism, occurring regionally to the east of the basins along the eastern margin of Gondwana. The differing proportions of contemporary volcanic zircon indicate the Adavale Basin was relatively more proximal to these volcanic sources than the Darling Basin.

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Figure 6.1. Comparison of aggregated DZ distributions of coeval sedimentary successions in the Adavale and Darling basins in their respective stratigraphic context. Data emphasize the previous observations with distinctively different KDE peaks ages for Palaeozoic DZ ages and some similarities for Precambrian DZ data.

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Figure 6.2. Data compilation of Th/U for DZ data from Chapter 4 and 5 (sink) in comparison with Th/U data from syn-volcanic units in the Anakie Province and Lachlan Orogen (source). Data highlight differences in Th/U between Adavale and Darling basin, especially for ages between 350-370 Ma. Th/U of DZ from the Darling Data exhbit distinctive overlap (green bar) with zircons from the interpreted syn- volcanic sources (data compiled from Atton, 2013; Cross et al., 2015, 2009; Kemp et al., 2006).

6.1.3 Ordovician igneous basement sourcing A persistent Ordovician DZ population is apparent throughout the entire lifespan of the Adavale Basin and DZ are most likely derived from Ordovician plutonic and volcanic basement rocks in the central part of the Thomson Orogen (Maneroo Volcanics, Carr et al., 2014; Cross et al., 2018; Draper, 2006). In the Darling Basin, Ordovician-aged grains make up <5%, reflecting the general absence of Ordovician silicic magmatism in the southern Tasmanides, apart from A-Type magmatism during the earliest Ordovician in the southern Delamerian Orogen (Foden et al., 2002; Rosenbaum, 2018). The absence of Ordovician DZ in the Darling Basin negates sediment transport from the Adavale Basin into the Darling Basin and highlights differences in the basement geology between the Thomson and Lachlan orogens (e.g. Rosenbaum, 2018, Fig 6.4).

6.1.4 Cambro-Ordovician sedimentary basement sourcing Both basins feature DZ spectra reflecting reworking of Cambrian and/or Ordovician (meta)sedimentary basement rocks with ‘Pacific Gondwana’ signatures (e.g. Ireland et al., 1998; Purdy et al., 2016b; Squire et al., 2006), but the DZ frequency distributions of these

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source rocks exhibit some obvious differences between the Lachlan and Thomson Orogens. Sedimentary sources in the Lachlan and Thomson orogens feature the key elements of a ‘Pacific Gondwana’ source signature with a distinct Ediacaran Peak (~580 Ma) and a Grenvillian aged population (0.9-1.2 Ga, e.g. Fergusson and Fanning, 2002; Ireland et al., 1998; Purdy et al., 2016; Squire et al., 2006). However, differences exist in the occurrence of 1.5 Ga zircons that are consistently present in (meta)sedimentary rocks in the Thomson Orogen (Purdy et al., 2016b) but are only sporadically apparent in the Ordovician and Cambrian sedimentary rocks in the Lachlan and Delamerian Orogens (Fig. 6.3b, Johnson et al., 2016; Squire et al., 2006). In addition, units with Late Cambrian to Ordovician depositional ages in the Lachlan Orogen show an additional Late Cambrian age population (~500 Ma, Armistead and Fraser, 2015; Squire et al., 2006) that is significantly less developed or even absent in (meta)sedimentary units in the Thomson Orogen (Purdy et al., 2016a). Recently published DZ data from Late Cambrian to Ordovician (meta)sedimentary rocks in the southern Thomson exhibit ‘Pacific Gondwana’ signatures that show features of both (meta)sedimentary units in the Lachlan and Thomson orogens (i.e. ~500 Ma, 560- 580 Ma, 0.9-1.2 Ga and ~1.5 Ga populations, Fraser et al., 2019).

The Cambro-Ordovician (meta)sedimentary rocks are also considered to represent the immediate source for detrital rutile populations in the Adavale and Darling basins. The chemical composition indicates predominantly metapelitic sources for detrital rutiles from both basins and grain morphology indicate extensive sedimentary recycling of those minerals. The metamorphic grades of the Cambro-Ordovician (meta)sedimentary rocks in the Thomson and Lachlan orogens (greenschist to lower amphibolite conditions, e.g. Fergusson et al., 2007; Johnson et al., 2016; Purdy et al., 2016) are not high enough to facilitate rutile crystallisation at approximately 650-750°C, implying the rutile was ultimately crystallised under higher amphibolite to lower granulite conditions (Sections 4.5, 5.5.3). A robust age assignment for the detrital rutile age population is challenged by a very low concordance rate for all analysed samples, hampering explicit identification of ultimate source regions (Sections 4.6.3, 5.6.5). The majority of U-Pb rutile ages in this study range from ~470-650 Ma presumably reflecting cooling ages of a variety of major orogenic events associated with various stages of Gondwana assembly (e.g. Peterman and Ross Delamerian orogenies, Foden et al., 2006; Walsh et al., 2013).

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Figure 6.3. KDE plots of total DZ U-Pb data from the Adavale Basin (blue, this study) and Darling Basin (a, orange, this study and Barry, 2016) showing high similarity of distributions for ages >525 Ma (b) and major differences in the spectra for DZ age <750 Ma (c).

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Figure 6.4. Summary of provenance phases in the Darling and Adavale basins across the Thomson and Lachlan orogens. Both basins record sourcing from Cambro-Ordovician (meta)sedimentary rocks (green) throughout the Devonian and additionally first-order contributions from igneous basement units (Early Devonian [yellow] and Ordovician-aged [red] in the Adavale Basin; Early Devonian in the Darling Basin [orange]). Provenance in the Upper Devonian is characterised by ongoing sediment contribution from basement rocks and additionally derivation from syn-volcanic sources distal to the respective basin (blue).

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6.2 HYPOTHESIS TESTING

The overall proportions of older DZ grains (>525 Ma) are significantly greater in the Darling Basin (~70%) compared to the Adavale Basin (~30%), indicating that reworking of sedimentary basement rocks probably played a relatively greater role in the DZ provenance of the Darling Basin (Fig. 6.3a). This is further confirmed by overall higher concentrations of rutile in the Darling Basin that have been interpreted to be derived from the metasedimentary basement. Contribution of Thomson Orogen (meta)sedimentary rocks to the DZ budget of the Darling Basin cannot be categorically ruled out, but seems unlikely due to a relative lack of 1.5 Ga DZ in the Darling Basin (Fig. 6.3b). It is noteworthy that overlap between the two distributions exists (depicted by purple mix colour between blue and orange) in between Devonian and Cambrian ages, but the peak ages describing the major populations for each basin are significantly different, especially for DZ ages <750 Ma.

Based on the sediment provenance tools in this study, no strong evidence for a direct connection between the Adavale and Darling basins can be concluded. The results underline that sourcing for the basins was from their respective hinterlands and reflect the characteristic basement make-up of the Thomson and Lachlan orogens, respectively (Fig. 6.4). While a direct connection between the two seems unlikely, both basins were remote from zones of active volcanism (and thus remote from plate margin), and occupy a similar tectonic position (i.e. continental platform). This is consistent with a relatively quartz rich sandstone composition, high textural and compositional maturity of sediment and only very distal volcanic influence.

Sediment transport from the Adavale to the Darling Basin is highly unlikely, as the Darling Basin lacks Ordovician-aged DZ (465 Ma age peak) that prevail in high abundance throughout the entire lifespan of the Adavale Basin. In addition, the prominent contemporary Upper Devonian DZ signature in the Adavale Basin does not persist into the Darling Basin. Coeval Upper Devonian-aged DZ are both attributed to contemporary volcanism, however, it is unlikely this has been supplied through the Adavale Basin, or from the same sources for the Adavale Basin (Fig. 6.4). Vice versa, sediment transport from the Darling into the Adavale Basin cannot be categorically ruled out due to high similarity of DZ signatures of the (meta)sedimentary basement in the Thomson and Lachlan orogens, but paleoflow directions in the Darling Basin indicate predominant sediment transport from

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the west rather than from to the north. The shared Ediacaran and Grenvillian age populations are a feature of the common ‘Pacific-Gondwana’ signature constituting the Cambro-Ordovician sedimentary basement rocks in the Thomson Orogen and Lachlan/Delamerian Orogen, respectively (Fig. 6.3b, Fergusson et al., 2007; Ireland et al., 1998; Johnson et al., 2016; Purdy et al., 2016b; Squire et al., 2006). The same accounts for the overlap in detrital rutile U-Pb ages, as both basins are supplied with Cambro-Ordovician recycled sedimentary detritus that potentially bears detrital rutile crystallised during the Cambrian Ross-Delamerian Orogeny.

The research presented in this thesis aimed to test the hypothesis: “The Adavale and Darling basins represent a remnant of a once larger, contiguous platform basin across the Tasmanides”. In order to address the topic of utilising provenance of sedimentary cover basins to investigate the timing of crustal stabilisation processes in non-collisional accretionary orogens, three major research questions were framed and addressed in three body chapters (Chapters, 2, 4 and 5).

The basal volcanic rocks of the Gumbardo Formation are likely associated with rifting of the Adavale Basin and are predominantly K-feldspar-phyric and rhyodacitic in composition. Based on U-Pb zircon geochronology, the volcanics were emplaced during a brief period (~2 Myrs) in the Lower Devonian (~398 Ma). In addition, all analysed samples revealed significant inheritance of Silurian and/or Ordovician-aged zircons, indicating crustal remelting of igneous basement rocks played a significant role in the petrogenesis of the volcanic rocks. The tectonomagmatic affinities of the volcanics are transitional I- to A- Type, while the nature of zircon inheritance precludes (meta)sedimentary sources and is rather indicative of A-Type affinities (Eby, 1992; Whalen et al., 1987). Based on petrographic and geochemical assessment, a tectonic setting associated with an active subduction zone (i.e. back-arc or foreland basin, Henderson et al., 2013; Rosenbaum, 2018) seems unlikely during initiation of the Adavale Basin. A comparative study based on whole

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rock geochemistry composition with typical intracratonic (silicic) rift volcanics suggests an intracratonic rift setting for the volcanic rocks of the Gumbardo Formation.

Research Question 2: What is the provenance of the sedimentary units within the Adavale Basin, and what were the main source areas supplying sediment into the basin and across the Thomson Orogen? (Chapter 4)

The integrated data from sandstone petrography, DZ and rutile geochronology in conjunction with zircon trace element and morphology data emphasizes sediment provenance for the Adavale Basin was from the Thomson Orogen throughout the Devonian, and additionally from the New England Orogen during the Upper Devonian. Cambrian metasedimentary basement and Ordovician volcanic/plutonic basement in the Thomson Orogen persistently contributed detritus throughout the lifespan of the Adavale Basin. Investigation of temporal trends in DZ provenance and sandstone composition highlight an influx of detritus from syn-volcanism in the Middle and Upper Devonian, while lateral trends in DZ frequency distributions and sandstone composition point to north-easterly volcanic sources, and therefore an overall southward sediment dispersal direction across the Adavale Basin.

While the Adavale and Darling basins coexisted during the Devonian, there is no evidence for direct sediment exchange between the two basins. The DZ age spectra indicate local sediment source regions, with sourcing of the Adavale from the Thomson Orogen and sediment sources located in the Lachlan and Delamerian orogens for the Darling Basin. Cambro-Ordovician (meta)sedimentary rocks in the southern Thomson Orogen might be a source for both basins. Similarities in detrital rutile and zircon provenance are attributed to similarities in the Cambro-Ordovician (meta)sedimentary basement lithologies in both terranes, rather than being indicative of connectivity. Nonetheless, both Devonian basins show characteristics of platform basins featuring relatively undeformed and widespread successions of compositionally and texturally relatively mature sedimentary infill.

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6.3 IMPLICATIONS FOR THE TECTONIC HISTORY OF THE THOMSON OROGEN

The provenance and nature of the sedimentary infill of the Devonian Adavale and Darling basins have implications for the timing of tectonic activity of the southern Thomson Orogen. While the majority of the southern Thomson Orogen lies under relatively shallow cover, prominent east-west striking structural features are apparent from gravity and magnetic subsurface data (e.g. Burton, 2010). The timing of tectonic activity and the tectonic significance for the interaction of the Thomson and Lachlan orogens is debated (Burton, 2010; Doublier et al., 2018; Fergusson and Henderson, 2015; Glen, 2013; Glen et al., 2014; Purdy et al., 2016b; Rosenbaum, 2018). Most geodynamic models imply that tectonic activity in the southern Thomson Orogen predates deposition in the Adavale and Darling basins during the Devonian, but suggest that reactivation of the structures might have occurred into the Carboniferous (Burton, 2010; Doublier et al., 2018; Glen, 2013; Verdel et al., 2016). This is supported by (U-Th)/He data from zircon and apatite for the Ordovician Granite Springs Granite that indicate very slow cooling and exhumation from the Upper Devonian through the Carboniferous (Purdy et al., 2016b).

The Adavale and Darling basins are situated on either side of the southern Thomson Orogen and their widespread and generally uniform sedimentary infill is characterised by compositionally and texturally mature siliciclastic sedimentary rocks. Sediment sources in both basins appear to be continuous and unchanged throughout the Devonian, apart from a new volcanic influence during the Upper Devonian in both basins. Overall, sediment provenance does not appear to change significantly in response to reported orogenic events like the Devonian Tabberabberan Orogeny (Black et al., 2005; VandenBerg, 2000) that might have had the potential to reactivate structures in the southern Thomson Orogen. Nevertheless, the southern area of the Thomson Orogen was most likely a topographic barrier to some degree during the Devonian, effectively preventing sediment exchange between the Adavale and Darling basins.

The treatment of the DZ age data for the Darling Basin in Section 5.4.3 highlight an important aspects that has initially been introduced in Sections 1.1.4 and 3.1.3 of Chapters 1 and 3, respectively; The detection of small DZ populations is critical for a statistically robust assignment of depositional ages, especially in intracratonic settings, where absolute numbers of syn-depositional DZ ages are inherently low. This study has demonstrated how

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the statistical parameters associated with maximum depositional ages and the overall age frequency distribution benefit from an increased number of individual U-Pb zircon ages, especially in intracratonic settings.

According to the classification scheme for the tectonic setting of sedimentary basins (Cawood et al., 2012), the DZ age spectra of the Adavale Basin with its large proportions of syn-depositional ages is at odds with a “typical” DZ frequency distribution of an intracratonic basin. The source of the syn-volcanic contribution, however, is distal to the basin, located in area of the adjacent Drummond Basin and Anakie Inlier, and is not associated with an active magmatic arc. The nature of the sedimentary infill in the Adavale Basin in conjunction with a rather uniform distribution and thickness of the sedimentary units is conclusive with an intracratonic setting of the basin. Furthermore, the tectonomagmatic affinities (A-Type) of the rift-related Gumbardo Formation preclude the influence of a magmatic arc and therefore argue against a tectonic setting in proximity to arc related magmatism.

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Appendices Appendix 2.1 Mineral separation and U-Pb LA-ICP-MS dating analytical procedures

Heavy mineral (>2.95 g/cm3) separation of the samples collected from the field was performed by Geotrack International Pty Ltd based in Melbourne. Samples are initially reduced in size using a jaw crusher, followed by a disc pulveriser, designed to disaggregate the rock, rather than crush the constituent mineral grains. Fine material is removed by hand washing and the resulting fine-sand sized material is dried in a low temperature oven. Frantz isodynamic magnetic separators and heavy liquid mineral separations are then used to separate mineral grains by gravity and magnetic susceptibility. The following separates were extracted:

• zircon concentrate (the purest, non-magnetic material),

• zircon fraction (often more impure and contains more magnetic material),

• magnetic fraction.

Heavy liquid mineral separations were used to separate grains by specific gravity. The minerals were consequently separated with a 25° forward slope and 2° side angle and full scale on the Frantz isodynamic magnetic separators. Each sample contained zircon and rutile (found in the zircon concentrate and fraction separates).

Specifications laser ablation system:

Make, model and type ESI New Wave Research NWR 193 Ablation cell and volume Trueline Mk 2, non-cantilevered small-volume ablation cell

Laser wavelength 193 nm Pulse width 5 ± 1 ns

Fluence 2.0 J cm-2

Repetition rate 7 Hz

Ablation duration 30 s

Spot diameter Variable, see respective chapter Sampling mode Static spot ablation Cell carrier gas flow 0.6 K min-1 He

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Typical lens settings for 8800 Agilent single-quad, no-gas mode:

RF power 1350 W Sample depth 4.5 mm Extract 1 1V Extract 2 -170 V Omega Bias -105V Omega Lens 9V Q1 Entrance -2V Q1 Exit -1V Cell Focus 0V Cell Entrance -40V Cell Exit -60V Deflect 12.5V Plate Bias -50V Q1 bias -6V Q1 pre-filter bias -12V Q1 post-filter bias -22V

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Appendix 2.2 Petrographic descriptions (Gumbardo Formation)

BEA Allandale-1

In this well 236 m of the Gumbardo Formation has been intersected, with two lithic bearing ignimbrites being cored at the bottom of the interval and at the top respectively (Fig. 2.X). Existing cuttings descriptions (Leslie, 1971) suggest consistent lithology between cored sections. Basement rocks have not been intersected in this well. The lower ignimbrite is a massive maroon, relatively lithic-rich ignimbrite (sample ALL-1B, Fig. 2.X). Angular lithic fragments are abundant (27%, 2-10 mm) comprising plagioclase-phyric trachytic-textured lava clasts, and largely aphyric clasts with some containing K-feldspar phenocrysts. The phenocryst assemblage includes subhedral to anhedral phenocrysts of K-feldspar (1-2 mm, 19%) and minor embayed volcanic quartz (1 mm, 2%). The groundmass consists of micropoikilitic quartz with abundant relict glass shards visible in some parts. K-feldspars are sericitised and kaolinised throughout. Overall, the proportion of clasts (lithics and phenocrysts) is high (38% cumulative) but a grain supported fabric is not apparent. In contrast, at 2797.1 m is a massive, relatively crystal-rich grey-green ignimbrite (ALL-1A; Fig. 2.X). The rock contains subhedral to anhedral phenocrysts of K-feldspar (14%, 1-2 mm) plagioclase (4%, 1-2 mm) and quartz (5%, 1 mm) in a sericitised groundmass of relict vitric ash, with visible shard textures in the groundmass. Juvenile clasts are low-moderately vesciculated, devitrified and altered to chlorite (11%, 3-4 mm) and are K-feldspar phyric. Lithics are present as pilotaxitic lava fragments, quartz-, K-feldspar and plagioclase phyric plutonic clasts and minor ignimbrite lithics (cumulative 12%, 2-6 mm). A few secondary clay-filled veins are present in the groundmass.

PPC Gumbardo-1

This well features the thickest interval of the Gumbardo Formation with 755.5 m intersected. Volcanic Ordovician basement is intersected at 3941.3m. Three different eruptive units of the Gumbardo Formation (3702.4 - 3278.4 m) and an epiclastic sandstone (3941.3 - 3940.7 m, sample GUM-1A) are cored in this section (Fig. 2.X). The epiclastic sandstone occurs near the base of the Gumbardo Formation. It is purple to dark grey in colour and moderately sorted with quartz (1 mm, largely subangular, partly euhedral with resorption embayments, 15%), K-feldspar (0.5-1 mm, 6%) and plagioclase (0.5-1 mm, 23%). Lithic grains are predominantly rounded aphyric lava clasts, partly with plagioclase phenocrysts (1-2 mm,

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13%), and intensely chloritized, amygdaloidal basaltic clasts of pilotaxitic plagioclase (1-3 mm, 7%). Three cored intervals above the epiclastic sandstone contain volcanic rocks. In the lower part, a maroon ignimbrite with large, dark brown fiamme (10%, dipping at 80-85º from core axis) is intersected (GUM-1F, Fig. 2.X). The ignimbrite contains euhedral to subhedral phenocrysts of K-feldspar (22%) in a micropoikilitic quartz groundmass (Fig. 2.X). Fiamme are present throughout (15%, mm to several cm in size), some containing euhedral K-feldspar phenocrysts, and define a eutaxitic foliation to the rock. The fiamme are largely replaced by secondary cryptocrystalline quartz and sericite (Fig. 2.X). The K-feldspar phenocrysts are altered to sericitised carbonate. The ignimbrite is devoid of lithic fragments. Small secondary sericite veins are present in the groundmass. The cored section within the middle of the formation consists of a massive, pink-maroon porphyritic rhyolite (GUM-1E, Fig. 2.X). The rhyolite contains subhedral sericitised K- feldspar phenocrysts (17%, 0.5-2 mm, Fig. 9d) and minor quartz phenocrysts (9%, 1-2mm). The groundmass is dominated by micropoikilitic quartz with some poikilitic quartz domains and minor sericite present. The rock is highly fractured with lustrous veins bearing iron stains (hematite), sericite, quartz and carbonate. In contrast to other cored samples, this rock shows no pyroclastic textures and the even distribution of phenocrysts through the rock suggest a coherent volcanic texture. This rhyolite is interpreted to represent a lava or a subvolcanic intrusion, but the lack of contact relationships prevents further discrimination. The uppermost section comprises a welded brown ignimbrite (GUM-1B, Fig. 2.X). The ignimbrite appears gently dipping based on welding foliations being 65-70º from the core axis. In thin section, the rock is a plagioclase-rich (25%, 0.5-2 mm) ignimbrite with a sericitised welded ash shard matrix (Fig. 2.X). The plagioclase phenocrysts are subhedral to euhedral showing zonation and are fragmented with jig-saw fit textures. Sericitisation of plagioclase is moderate and concentrated in the centres of the crystals with some additional sericite dusting. The rock is fractured and brittle, but no secondary vein-mineralisation is present.

PPC Cothalow-1

This well intersects 273 m of the Gumbardo Formation, and basement is intersected and cored (Fig. 2.X). This well features a section of the eruptive sequence of the formation (~2550 – 2400m) and is overlain by lithic arkose to arkose sandstones of the upper Gumbardo Formation (~2400 – 2290m), which is cored in three sections (Fig. 2.X). At the base of the interval a shallow-dipping (80-85º from core axis), brown ignimbrite is intersected (COT-1,

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Fig. 2.X). In thin section, this ignimbrite is poorly sorted with phenocrysts of plagioclase (10%, 0.5-2.5 mm), K-feldspar (9%, 0.5-2.5 mm) and minor quartz (4%, 0.5-1.5mm) present. Phenocrysts of plagioclase and K-feldspar are euhedral to subhedral, partly broken and strongly sericitised. Fiamme (8%, 3-5 mm) are well-preserved and show sericite alteration (Fig. 2.X). Some quartz and K-feldspar-phyric devitrified coherent volcanic rock fragments (5%, 2-4 mm) of intermediate to silicic composition are present. The groundmass is dominated by very fine vitric ash shards, showing moderate welding and some sericitisation.

PPC Etonvale-1

206.1 m of the Gumbardo Formation are reported to be intersected in this well, comprising volcanic rock in the lower section (~3400 – 3300m) and interbedded arkose sandstone and mudstone in the upper section (~3300 – 3200 m). However, no volcanic fragments are not reported from the upper section (Lewis and Kyranis, 1962). A Silurian granite is forming basement in this well, but the associated core material is lost. At the base of the interval, a massive, maroon ignimbrite is intersected (ETO-1, Fig. 2.X). The rock is generally very brittle and the core material in poor condition with only fragments of cm-size preserved. The ignimbrite contains subhedral phenocrysts of sericitised K-feldspar (14%, 0.5-1 mm) and minor quartz (4%, 0.5-1 mm) in a sericitised groundmass of vitric ash (Fig. 2.X). Some sericitised fiamme are present (5%, 2-3 mm long). Minor secondary Fe-Ti oxide blebs and illite are observed in the groundmass.

PPC Carlow-1

A section 140 m of the Gumbardo Formation is intersected in this well, which is tuffaceous throughout the section according to the cuttings descriptions in the associated well completion report (Kyranis, 1966). The formation is underlain by a metasedimentary sandstone, which overlays Ordovician basement (Fig. 2.X). A diffusely stratified (85-90º from core axis) maroon ignimbrite is intersected in the lower section of this well (CAR-2, Fig. 2.X). This ignimbrite has a low abundance of very small phenocrysts (predominantly <0.5 mm, a few 1-2 mm) and is dominantly ash-rich (91%). Phenocrysts comprise quartz (2%), K-feldspar (2%) and plagioclase (1%) and few juvenile clasts (5%, 1-2 mm) containing quartz and plagioclase phenocrysts in a welded matrix. Lithic fragments are absent. The sericitised groundmass consists of fine vitric ash (Fig. 2.X). Rare subhedral K-feldspars and plagioclase are moderately sericitised and fragmented.

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Appendix 2.3 Wetherill concordia plot for U-Pb zircon ages

Appendix 2.3. Wetherill concordia plots for volcanic rocks of the Gumbardo Formation (ALL-1A, ALL-1B, COT-1, ETO-1, GUM-1B) and Ordovician volcanic unit (YON-1) showing concordant analyses. Blue error ellipses show population interpreted as emplacement ages, green ellipses depict inherited Silurian populations and beige ellipses show older inherited grains.

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Appendix 4.1 Photographs of drill core material from sedimentary rock units in the Adavale Basin

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Appendix 4.1. Selected photographs of core sections showing flaser-bedding (a), bioturbation (b) and thinly interbedded sand- and mudstone in the Eastwood Beds. Brachiopod fossil (d), cross-bedded sandstone (e) and interbedded conglomerates (f) in rocks of the Log Creek Formation. Mudstone rip-up up clasts in sandstone (g), cross-bedded sandstone (h), massive sandstone (i) and cross bedded sandstone with rip-up clasts (j) in the Lissoy Sandstone. Fine grained sandstone with medium- to coarse grained interbeds (k), very coarse grained interbeds in medium grained sandstone (l), well-sorted fine grained sandstone (m), coarse grained massive sandstone (n) and fine grained mottled red-maroon/buff sandstone (o) from the Etonvale Formation. Cross bedded medium grained sandstone (p), mottled dark brown/buff sandstone (q) and brown- maroon subtly cross bedded sandstone (r) from the Buckabie Formation. All cored sections are 9 cm in width, (h) approximately 7cm wide, pen width is 7 mm.

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Appendix 4.2 Lithostratigraphic logs from drill holes:

• BEA Allandale-1 • ASO Fairlea-1 • PPC Carlow-1 • PPC Etonvale-1 • PPC Log Creek-1 • PPC Cothalow-1 • PPC Gumbardo-1 • PPC Buckabie-1

Legend

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264

265

266

267

268

269

270

271

272

273

274

275

276

277

278

279

280

Appendix 4.3 Transmitted light and cathodoluminescence images detrital zircon Adavale Basin samples

281

282

283

284

285

Appendix 4.4 Tera-Wasserburg diagrams of 207Pb/206Pb vs 206Pb/ 238U for detrital zircon samples from the Adavale Basin

286

287

Appendix 4.4. Tera-Wasserburg plots showing DZ U-Pb data from Adavale Basin sandstone samples. Yellow error ellipses show concordant data, red ellipses display discordant analyses

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Appendix 4.5 Sample comparison coefficients

Cross Correlation

Coefficient

ALL-6 CAR-13 0.52 CAR-14 0.22 0.66 GUM-6 0.08 0.43 0.82 LOG-4 0.04 0.37 0.81 0.96 LOG-5 0.10 0.44 0.84 0.91 0.92 FAI-1 0.20 0.53 0.74 0.63 0.64 0.75 BUC-1 0.20 0.55 0.84 0.90 0.88 0.89 0.77 GUM-7 0.44 0.63 0.45 0.31 0.26 0.38 0.60 0.50 LOG-6 0.46 0.32 0.32 0.24 0.24 0.36 0.49 0.41 0.47 BUC-3 0.04 0.25 0.63 0.90 0.88 0.83 0.52 0.81 0.23 0.26 FAI-2 0.20 0.09 0.07 0.04 0.05 0.11 0.25 0.14 0.35 0.62 0.07 FAI-3 0.22 0.07 0.03 0.00 0.01 0.05 0.17 0.06 0.30 0.60 0.02 0.95

BUC-5 0.23 0.23 0.26 0.27 0.25 0.35 0.45 0.41 0.58 0.60 0.34 0.69 0.60 LOG-7 0.49 0.44 0.41 0.30 0.29 0.42 0.61 0.50 0.63 0.94 0.28 0.65 0.61 0.71 ALL- CAR- CAR- GUM- LOG- LOG- BUC- GUM- LOG- BUC- BUC- 6 13 14 6 4 5 FAI-1 1 7 6 3 FAI-2 FAI-3 5

Likeness

value

ALL-6 CAR-13 0.54 CAR-14 0.49 0.67 GUM-6 0.32 0.50 0.63 LOG-4 0.32 0.46 0.60 0.80 LOG-5 0.35 0.43 0.55 0.74 0.73 FAI-1 0.36 0.56 0.63 0.53 0.52 0.56 BUC-1 0.42 0.56 0.67 0.72 0.70 0.71 0.64 GUM-7 0.48 0.65 0.62 0.47 0.43 0.46 0.62 0.57 LOG-6 0.41 0.38 0.46 0.46 0.44 0.50 0.46 0.52 0.39 BUC-3 0.30 0.42 0.52 0.70 0.68 0.69 0.48 0.64 0.40 0.43 FAI-2 0.34 0.33 0.35 0.29 0.29 0.35 0.39 0.39 0.40 0.57 0.30 FAI-3 0.27 0.20 0.16 0.14 0.12 0.20 0.23 0.22 0.28 0.55 0.15 0.72

BUC-5 0.39 0.45 0.48 0.44 0.41 0.46 0.50 0.52 0.54 0.56 0.43 0.57 0.45 LOG-7 0.45 0.49 0.54 0.52 0.48 0.51 0.57 0.58 0.54 0.75 0.44 0.59 0.51 0.63 ALL- CAR- CAR- GUM- LOG- LOG- BUC- GUM- LOG- BUC- BUC- 6 13 14 6 4 5 FAI-1 1 7 6 3 FAI-2 FAI-3 5

289

Similarity

value

ALL-6 CAR-13 0.79 CAR-14 0.78 0.89 GUM-6 0.61 0.76 0.83 LOG-4 0.61 0.75 0.82 0.91 LOG-5 0.63 0.70 0.79 0.88 0.87 FAI-1 0.62 0.79 0.81 0.76 0.75 0.77 BUC-1 0.70 0.79 0.84 0.87 0.86 0.88 0.85 GUM-7 0.72 0.83 0.83 0.72 0.71 0.72 0.83 0.80 LOG-6 0.68 0.63 0.66 0.69 0.68 0.80 0.70 0.75 0.66 BUC-3 0.58 0.69 0.77 0.87 0.85 0.87 0.75 0.87 0.68 0.72 FAI-2 0.58 0.55 0.55 0.50 0.52 0.65 0.63 0.67 0.66 0.82 0.61 FAI-3 0.49 0.41 0.36 0.31 0.30 0.50 0.49 0.47 0.52 0.79 0.42 0.86

BUC-5 0.66 0.70 0.69 0.67 0.64 0.73 0.74 0.77 0.79 0.78 0.73 0.79 0.70 LOG-7 0.68 0.71 0.71 0.74 0.73 0.82 0.79 0.82 0.79 0.88 0.74 0.82 0.77 0.85 ALL- CAR- CAR- GUM- LOG- LOG- BUC- GUM- LOG- BUC- BUC- 6 13 14 6 4 5 FAI-1 1 7 6 3 FAI-2 FAI-3 5

K-S test (p-

value)

ALL-6 CAR-13 0.013 CAR-14 0.000 0.301 GUM-6 0.000 0.000 0.000 LOG-4 0.000 0.000 0.006 0.277 LOG-5 0.000 0.000 0.000 0.106 0.007 FAI-1 0.002 0.035 0.250 0.000 0.012 0.000 BUC-1 0.000 0.000 0.001 0.366 0.090 0.005 0.004 GUM-7 0.000 0.004 0.003 0.001 0.000 0.000 0.005 0.032 LOG-6 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 BUC-3 0.000 0.000 0.000 0.579 0.016 0.145 0.000 0.146 0.000 0.000 FAI-2 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 FAI-3 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000

BUC-5 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.001 0.003 0.000 0.000 0.000 LOG-7 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.430 0.000 0.000 0.000 0.033 ALL- CAR- CAR- GUM- LOG- LOG- BUC- GUM- LOG- BUC- BUC- 6 13 14 6 4 5 FAI-1 1 7 6 3 FAI-2 FAI-3 5

290

K-S test D

statistic

ALL-6 CAR-13 0.192 CAR-14 0.246 0.117 GUM-6 0.317 0.305 0.256 LOG-4 0.320 0.286 0.207 0.120 LOG-5 0.423 0.409 0.378 0.146 0.202 FAI-1 0.226 0.172 0.123 0.266 0.192 0.375 BUC-1 0.304 0.295 0.242 0.111 0.150 0.208 0.213 GUM-7 0.288 0.214 0.219 0.243 0.283 0.308 0.209 0.173 LOG-6 0.432 0.466 0.485 0.513 0.512 0.427 0.464 ### 0.380 BUC-3 0.312 0.329 0.312 0.094 0.188 0.138 0.310 0.138 0.275 0.485 FAI-2 0.523 0.609 0.623 0.633 0.624 0.547 0.521 0.540 0.404 0.328 0.609 FAI-3 0.731 0.816 0.831 0.878 0.875 0.791 0.747 0.776 0.644 0.460 0.847 0.262

BUC-5 0.333 0.367 0.391 0.414 0.398 0.322 0.386 0.320 0.231 0.217 0.378 0.250 0.479 LOG-7 0.402 0.385 0.412 0.439 0.472 0.388 0.392 0.355 0.308 0.105 0.444 0.331 0.483 0.173 ALL- CAR- CAR- GUM- LOG- LOG- BUC- GUM- LOG- BUC- BUC- 6 13 14 6 4 5 FAI-1 1 7 6 3 FAI-2 FAI-3 5

Kuiper test (p-

value)

ALL-6 CAR-13 0.000 CAR-14 0.000 0.019 GUM-6 0.000 0.000 0.000 LOG-4 0.000 0.000 0.000 0.505 LOG-5 0.000 0.000 0.000 0.359 0.070 FAI-1 0.000 0.003 0.023 0.000 0.000 0.000 BUC-1 0.000 0.000 0.003 0.118 0.209 0.006 0.001 GUM-7 0.000 0.012 0.014 0.000 0.000 0.000 0.001 0.000 LOG-6 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 BUC-3 0.000 0.000 0.000 0.471 0.061 0.066 0.000 0.002 0.000 0.000 FAI-2 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 FAI-3 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000

BUC-5 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 LOG-7 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.315 0.000 0.000 0.000 0.000 ALL- CAR- CAR- GUM- LOG- LOG- BUC- GUM- LOG- BUC- BUC- 6 13 14 6 4 5 FAI-1 1 7 6 3 FAI-2 FAI-3 5

291

Kuiper test (V

statistic)

ALL-6 CAR-13 0.341 CAR-14 0.412 0.228 GUM-6 0.585 0.450 0.312 LOG-4 0.595 0.476 0.290 0.146 LOG-5 0.649 0.516 0.378 0.159 0.202 FAI-1 0.437 0.258 0.225 0.415 0.337 0.437 BUC-1 0.499 0.369 0.258 0.190 0.175 0.248 0.284 GUM-7 0.334 0.237 0.234 0.442 0.429 0.503 0.273 0.341 LOG-6 0.445 0.469 0.489 0.524 0.522 0.481 0.464 ### 0.473 BUC-3 0.621 0.508 0.384 0.149 0.205 0.204 0.410 0.269 0.518 0.521 FAI-2 0.539 0.617 0.631 0.641 0.639 0.587 0.521 0.540 0.447 0.383 0.630 FAI-3 0.739 0.816 0.831 0.878 0.884 0.799 0.747 0.776 0.679 0.460 0.855 0.296

BUC-5 0.401 0.400 0.441 0.488 0.460 0.480 0.386 0.396 0.301 0.388 0.485 0.341 0.597 LOG-7 0.425 0.393 0.425 0.447 0.495 0.428 0.392 0.362 0.377 0.163 0.460 0.341 0.483 0.307 ALL- CAR- CAR- GUM- LOG- LOG- BUC- GUM- LOG- BUC- BUC- 6 13 14 6 4 5 FAI-1 1 7 6 3 FAI-2 FAI-3 5

BPC values

ALL-6 CAR-13 0.925 CAR-14 0.873 0.980 GUM-6 0.685 0.873 0.925 LOG-4 0.646 0.822 0.914 1.020 LOG-5 0.641 0.753 0.849 0.973 0.963 FAI-1 0.686 0.858 0.904 0.833 0.801 0.819 BUC-1 0.716 0.870 0.939 0.976 0.942 0.959 0.954 GUM-7 0.760 0.860 0.863 0.742 0.729 0.756 0.912 0.860 LOG-6 0.719 0.680 0.732 0.726 0.718 0.799 0.789 ### 0.768 BUC-3 0.595 0.765 0.856 0.991 0.958 0.963 0.838 0.947 0.755 0.757 FAI-2 0.614 0.604 0.627 0.593 0.581 0.656 0.760 0.725 0.803 0.891 0.650 FAI-3 0.411 0.299 0.303 0.275 0.255 0.418 0.483 0.427 0.582 0.818 0.383 0.909

BUC-5 0.717 0.781 0.792 0.759 0.737 0.795 0.888 0.864 0.928 0.877 0.830 0.936 0.742 LOG-7 0.707 0.749 0.749 0.808 0.761 0.842 0.889 0.884 0.869 0.986 0.814 0.920 0.803 0.939 ALL- CAR- CAR- GUM- LOG- LOG- BUC- GUM- LOG- BUC- BUC- 6 13 14 6 4 5 FAI-1 1 7 6 3 FAI-2 FAI-3 5

292

BPC uncertainties

ALL-6 CAR-13 0.035 CAR-14 0.040 0.039 GUM-6 0.035 0.034 0.038 LOG-4 0.035 0.035 0.039 0.034 LOG-5 0.035 0.033 0.036 0.032 0.031 FAI-1 0.050 0.048 0.053 0.047 0.049 0.047 BUC-1 0.038 0.035 0.037 0.035 0.035 0.035 0.050 GUM-7 0.038 0.037 0.039 0.037 0.036 0.035 0.053 0.039 LOG-6 0.041 0.040 0.046 0.042 0.040 0.038 0.057 ### 0.043 BUC-3 0.039 0.036 0.041 0.035 0.038 0.037 0.054 0.038 0.039 0.044 FAI-2 0.038 0.037 0.040 0.036 0.037 0.038 0.056 0.040 0.042 0.044 0.041 FAI-3 0.035 0.033 0.036 0.035 0.033 0.033 0.049 0.036 0.037 0.039 0.038 0.037

BUC-5 0.038 0.037 0.044 0.036 0.039 0.037 0.055 0.041 0.041 0.044 0.041 0.041 0.038 LOG-7 0.034 0.033 0.036 0.032 0.032 0.032 0.049 0.033 0.036 0.038 0.034 0.035 0.031 0.038 ALL- CAR- CAR- GUM- LOG- LOG- BUC- GUM- LOG- BUC- BUC- 6 13 14 6 4 5 FAI-1 1 7 6 3 FAI-2 FAI-3 5

293

Appendix 4.6 Probability Model Ensembles (PME)

294

295

296

297

Appendix 4.6. Probability Model Ensemble plots showing KDE (red curve) and density distribution (yellow=high density, blue=low density) of random subsamples for each DZ sample from the Adavale Basin.

298

Appendix 4.7 Transmitted light images of detrital rutile

299

Appendix 4.8 U-Pb zircon age data compilation for igneous units

300

Type

ID Longitude Latitude Unit Name/Drill Hole Province Lithology A/I/S Intrusive/extr usive Age[Ma] Ageunc. [Ma] 2sigma Dated mineral Method Reference 1 146.020 -18.944 Kallanda Granite Charters Towers granite i 340.6 2.3 zircon SHRIMP Kosticin et al., 2015 2 148.235 -20.006 Unnamed Granite (b) New England Orogen granite i 341.3 2.4 zircon SHRIMP Kosticin et al., 2015 3 148.841 -26.450 AAO Quibet 1 Roma Shelf granite i 341.8 3.7 monazite LA-ICP-MS Siegel, 2015 4 150.782 -25.404 Donore Granite Gneiss New England Orogen granitic gneiss i 343.0 4.0 zircon SHRIMP Withnall et al., 2009 5 145.337 -18.724 Unnamed Granite (a) Greenvale Province monzogranite i 344.2 2.4 zircon SHRIMP Kosticin et al., 2015 6 142.451 -20.299 Gladevale Downs 1 Granite North Australian Craton granite i 347.0 2.6 zircon SHRIMP Neumann and Kosticin, 2011 7 141.409 -28.323 TEA Tickalara 1 Southern Thomson Orogen monzogranite i 360.0 8.9 zircon LA-ICP-MS Siegel et al., 2018 8 149.154 -26.424 Santos Javel 2 Roma Shelf granite S i 362.3 3.1 zircon LA-ICP-MS Siegel, 2015 9 149.154 -26.424 Santos Javel 2 Roma Shelf granite S i 363.8 1.6 zircon SHRIMP Siegel, 2015 10 147.397 -26.863 Scalby Granite Nebine ridge granite S i 368.4 2.5 zircon SHRIMP Kosticin et al., 2015 12 150.536 -23.893 Pomegranate Tonalite New England Orogen tonalite i 369.0 4.2 zircon SHRIMP Murray et al. 2012 13 141.248 -28.685 PPL Omicron 1 Southern Thomson Orogen granite i 369.1 7.9 zircon LA-ICP-MS Siegel et al., 2018 14 142.639 -27.925 PPL Noccundra 1 Southern Thomson Orogen granite i 373.3 9.3 zircon LA-ICP-MS Siegel et al., 2018 15 145.698 -16.783 Mount Formartine Granite Mossmann granite i 375.2 1.9 zircon SHRIMP Kosticin et al., 2015 16 145.620 -16.700 Mount Formartine Granite Mossmann granodiorite i 376.0 3.0 zircon SHRIMP Kosticin et al., 2015a 17 145.581 -16.915 Mount Formartine Granite Mossmann granite i 378.8 2.7 zircon SHRIMP Kosticin et al., 2015a 18 150.420 -23.704 Mount Morgan Trondhjemite/R New England Orogen tonalite i 380.0 4.3 zircon SHRIMP Murray et al. 2012 19 144.469 -28.780 Currawinya Granite Southern Thomson Orogen monzogranite I i 381.5 2.4 zircon SHRIMP Cross et al., 2015 20 141.634 -19.223 Unnamed Biotite Granite North Australian Craton granite i 382.0 2.9 zircon SHRIMP Carson et al., 2011 21 145.019 -28.170 Eulo Granite Southern Thomson Orogen monzogranite S i 385.0 2.5 zircon SHRIMP Cross et al., 2012 23 143.364 -14.798 Ebagoola Granite Mossmann granite i 395.0 4.0 zircon SHRIMP Black et al., 1992 24 146.620 -25.622 AOP Balfour 1 Central Thomson Orogen granite i 396.0 2.2 zircon SHRIMP Siegel et al., 2018 25 143.318 -14.599 Flyspeck granodiorite Mossmann granodiorite i 398.0 10.0 zircon SHRIMP Asmussen et al. 2018 26 144.580 -30.035 Conlea porphyry Southern Thomson Orogen granite i 398.0 2.8 zircon SHRIMP Fraser et al., 2014 28 144.944 -29.839 Tinchelooka Diorite Southern Thomson Orogen Micromonzonite i 401.8 3.1 zircon SHRIMP Bodorkos et al., 2013 29 143.177 -13.894 Blue Mountains Adamellite (a) Mossmann granite i 405.0 13.0 zircon SHRIMP Black et al., 1992 30 143.179 -13.905 Blue Mountains Adamellite (b) Mossmann granite i 405.0 13.0 zircon SHRIMP Black et al., 1992 31 143.454 -14.627 Kintore Granite Mossmann granite i 405.0 9.0 zircon SHRIMP Black et al., 1992 32 143.581 -14.979 Flyspeck granodiorite Mossmann granodiorite i 406.0 10.0 zircon SHRIMP Black et al., 1992 33 142.942 -13.435 Morris Adamellite Mossmann monzogranite i 407.0 8.0 zircon SHRIMP Black et al., 1992 34 142.954 -13.442 Morris Granite Mossmann granite i 407.0 8.0 zircon SHRIMP Black et al., 1992 35 143.151 -13.774 Lankelly Granite (a) Mossmann granite i 407.0 7.0 zircon SHRIMP Black et al., 1992 36 143.169 -13.769 Lankelly Granite (d) Mossmann granite i 407.0 7.0 zircon SHRIMP Black et al., 1992 37 143.153 -13.768 Lankelly Granite (b) Mossmann granite i 408.0 6.0 zircon SHRIMP Black et al., 1992

301

38 143.163 -13.767 Lankelly Granite © Mossmann granodiorite i 408.0 6.0 zircon SHRIMP Black et al., 1992 39 143.362 -14.396 Kintore Granite Mossmann granite i 408.0 10.0 zircon SHRIMP Black et al., 1992 40 143.131 -13.553 Blue Mountains Adamellite Mossmann granite i 409.0 7.0 zircon SHRIMP Black et al., 1992 41 143.386 -12.619 Weymouth Granite Mossmann granite i 409.0 6.0 zircon SHRIMP Black et al., 1992 43 143.865 -16.289 Lukinville granodiorite granodiorite i 412.0 4.0 zircon SHRIMP Kosticin et al., 2015a 44 145.304 -30.471 Louth Volcanics Southern Thomson Orogen Dolerite i 414.0 9.3 zircon LA-ICP-MS Dwyer et al., 2018 45 141.317 -28.179 Wolgolla Granite (a) Southern Thomson Orogen granite S i 416.4 4.5 zircon SHRIMP Siegel et al., 2018 46 144.462 -28.996 Hungerford Granite Southern Thomson Orogen monzogranite S i 419.1 2.5 zircon SHRIMP Purdy et al., 2016; Cross et al. 2018 47 141.239 -28.168 Wolgolla Granite (b) Southern Thomson Orogen granite S i 419.4 6.9 zircon LA-ICP-MS Siegel et al., 2018 48 147.537 -24.154 Unnamed granodiorite Anakie Province granodiorite i 419.5 3.7 zircon SHRIMP Cross et al., 2015 49 141.896 -29.444 Dynamite Tank granodiorite (a) Diorite i 420.2 3.4 zircon SHRIMP Black, 2007; Vickery, 2010 50 141.983 -29.431 Tibooburra granodiorite Southern Thomson Orogen granodiorite I i 420.6 3.3 zircon SHRIMP Black, 2007; Vickery, 2010 51 146.844 -29.984 Brewarrina Granite Southern Thomson Orogen granite S i 420.9 2.3 zircon SHRIMP Bodorkos et al., 2013 52 144.260 -19.211 Dumbano Granite Greenvale Province granite i 421.0 8.0 zircon SHRIMP Withnall et al., 1997 53 141.817 -29.587 Warrata Gp. (b) Southern Thomson Orogen granodiorite i 421.3 2.0 zircon SHRIMP Black, 2006 54 144.249 -23.440 LOL (Longreach) 3 Central Thomson Orogen granite i 422.6 10.4 zircon LA-ICP-MS Siegel et al., 2018 55 141.832 -29.551 Warrata Gp. (a) Southern Thomson Orogen granodiorite i 423.3 2.1 zircon SHRIMP Black, 2006 56 143.838 -18.274 White Springs granodiorite North Australian Craton granodiorite i 424.0 11.0 zircon SHRIMP Donchak et al., 2013 57 143.876 -30.506 Unnamed intrusion (b) Southern Thomson Orogen Granitic dyke i 424.5 4.9 zircon SHRIMP Armistead and Fraser, 2015 58 142.423 -28.088 Ella Granite Southern Thomson Orogen granite I i 425.4 6.6 zircon SHRIMP Draper, 2006; Cross et al., 2018 59 141.896 -29.440 Dynamite Tank granodiorite (b) Southern Thomson Orogen granodiorite i 427.7 2.3 zircon SHRIMP Black, 2007; Vickery, 2010 60 143.875 -30.506 Unnamed intrusion (a) Southern Thomson Orogen granite i 428.2 3.1 zircon SHRIMP Asmussen et al. 2018 61 144.996 -25.160 PPC Etonvale 1 Central Thomson Orogen granite i 429.0 whole rock Rb-Sr Asmussen et al. 2018 62 144.331 -30.643 Unnamed intrusion © Southern Thomson Orogen Quartz diorite i 429.1 8.5 zircon SHRIMP Asmussen et al. 2018 63 144.435 -19.147 Dido Tonalite Greenvale Province tonalite I i 431.0 7.0 zircon SHRIMP Cross et al., 2018 64 145.018 -20.179 Fat Hen Creek Complex Charters Towers granite S i 441.0 10.0 zircon SHRIMP Hutton, 2004 65 147.677 -23.441 Gem Park Granite Anakie Province granite S i 443.3 6.2 monazite SHRIMP Fergusson et al., 2013 66 145.203 -20.364 Fat Hen Creek Complex Charters Towers granite S i 452.0 7.0 zircon SHRIMP Hutton, 2004 67 144.244 -24.806 LEA Albilbah 1 Central Thomson Orogen granite i 453.8 8.4 zircon LA-ICP-MS Siegel et al., 2018 68 144.547 -28.331 Granite Springs Granite (a) Southern Thomson Orogen granite S i 455.6 5.4 zircon SHRIMP Cross et al., 2015 69 146.601 -25.046 BEA Valetta 1 Central Thomson Orogen monzogranite i 458.6 8.4 zircon LA-ICP-MS Siegel et al., 2018 70 144.884 -25.154 PPC Lissoy 1 Central Thomson Orogen monzogranite i 462.0 19.2 zircon LA-ICP-MS Siegel et al., 2018 71 144.553 -28.335 Granite Springs Granite (b) Southern Thomson Orogen granodiorite S i 463.0 11.0 zircon SHRIMP Cross et al., 2015 72 146.663 -19.946 Grass Hut Granite Charters Towers granite i 463.0 3.0 zircon SHRIMP Hutton and Rienks, 1997 73 147.919 -22.534 Mooramin Granite Anakie Province granite S i 463.0 15.0 zircon SHRIMP Fergusson et al., 2013 74 146.319 -20.058 Charters Towers Metamorphics/M Charters Towers granodiorite i 464.0 5.0 zircon SHRIMP Hutton and Rienks, 1997 75 143.766 -23.591 Amx Toobrac 1 Central Thomson Orogen granite i 465.7 5.3 zircon LA-ICP-MS Siegel et al., 2018 76 146.899 -21.137 Coquelicot Tonalite Anakie Province Tonalite I i 471.0 3.8 zircn SHRIMP Cross et al., 2018

302

77 143.766 -23.591 AMX Toobrac 1 Central Thomson Orogen granite i 471.8 9.1 zircon LA-ICP-MS Cross et al., 2018 78 144.356 -25.406 AOD Budgerygar 1 Central Thomson Orogen monzogranite i 470.3 3.6 zircon LA-ICP-MS Siegel et al., 2018 79 144.713 -18.772 Balcooma Metavolcanic Group Greenvale Province porphyry i 471.0 4.0 zircon SHRIMP Withnall et al., 1991 80 143.586 -24.144 Lol Stormhill 1 Central Thomson Orogen monzogranite i 477.9 2.0 zircon SHRIMP Siegel et al., 2018 81 146.291 -20.076 Sunburst granodiorite Charters Towers granodiorite i 482.0 8.0 zircon SHRIMP Hutton and Rienks, 1997 82 144.890 -19.185 Saddington Tonalite Greenvale Province tonalite i 488.0 2.7 zircon SHRIMP Henderson et al., 2013 83 146.164 -20.256 Schreibers granodiorite Charters Towers granodiorite i 490.0 6.0 zircon SHRIMP Hutton and Rienks, 1997 84 149.435 -22.851 Clive Creek Volcanics New England Orogen ignimbrite e 349.0 4.8 zircon SHRIMP Withnall et al., 2009 85 148.257 -20.012 Edgecumbe Beds New England Orogen ignimbrite e 349.0 2.8 zircon SHRIMP Cross et al., 2012 86 149.813 -22.393 Charon Point Rhyolite Member New England Orogen ignimbrite e 352.0 4.9 zircon SHRIMP Withnall et al., 2009 88 147.380 -21.033 Bimurra Volcanics Anakie Province rhyolite e 360.0 2.5 zircon SHRIMP Cross et al., 2009 89 147.448 -21.428 Silver Hills Volcanics Anakie Province rhyolite e 363.0 2.7 zircon SHRIMP Cross et al., 2009 90 149.298 -21.330 Campwyn Volcanics New England Orogen Resed. pyrocl. e 364.6 3.4 zircon LA-ICP-MS Bryan et al., 2004 91 147.518 -23.477 Silver Hills Volcanics Anakie Province flow e 365.3 4.6 zircon SHRIMP Henderson et al., 1998 92 147.280 -23.046 Silver Hills Volcanics Anakie Province Ignimbrite e 371.2 6.1 zircon SHRIMP Henderson et al., 1998 93 149.302 -21.400 Campwyn Volcanics New England Orogen Ignimbrite e 373.1 2.6 zircon LA-ICP-MS Bryan et al., 2004 94 144.682 -21.717 AAE TOWERHILL 1 Central Thomson Orogen volcanic e 381.7 5.7 zircon SHRIMP Cross et al., 2018 96 145.003 -22.366 APC THUNDERBOLT 1 Central Thomson Orogen ignimbrite e 392.9 2.7 zircon SHRIMP Kosticin et al., 2015 97 144.696 -25.980 PPC GUMBARDO 1 Central Thomson Orogen Rhyolite e 402.0 2.1 zircon SHRIMP Draper, 2006 98 145.431 -24.839 PPC CARLOW 1 Central Thomson Orogen Ignimbrite e 408.0 2.4 zircon SHRIMP Draper, 2006 99 143.468 -23.382 GSQ MANEROO 1 Central Thomson Orogen felsic volcanics e 473.0 2.7 zircon SHRIMP Draper, 2006 100 145.390 -23.313 BEA COREENA 1 Central Thomson Orogen felsic volcanics e 478.0 2.6 zircon SHRIMP Draper, 2006 101 145.431 -24.839 PPC CARLOW 1 Central Thomson Orogen Rhyolite e 484.0 5.9 zircon SHRIMP Draper, 2006 102 146.065 -20.411 Mount Windsor Volcanics Charters Towers Rhyolite e 481.0 5.0 zircon SHRIMP Kosticin et al., 2016 103 146.065 -20.411 Mount Windsor Volcanics Charters Towers Rhyolite e 485.0 5.0 zircon SHRIMP Kosticin et al., 2016 104 147.556 -22.969 Theresa Creek Volcanics Anakie Province dacitic ignimbrite e 382.0 7.0 zircon SHRIMP Cross et al., 2015 105 141.971 -29.428 Warratta Gp., Easter Monday Fm. Southern Thomson Orogen Volcaniclastic e 497.2 2.6 zircon SHRIMP Black, 2006; Greenfield, 2010 106 145.304 -30.471 Louth Volcanics Southern Thomson Orogen Volcaniclastic e 411.0 6.3 zircon LA-ICP-MS Dwyer et al., 2018 107 146.181 -30.081 Gumbercoo zone? Southern Thomson Orogen Volcanic e 411.0 4.5 zircon LA-ICP-MS Hack et al., 2018 108 146.288 -29.940 Warraweena Volcanics Southern Thomson Orogen Volcanic e 414.0 4.0 zircon LA-ICP-MS Hack et al., 2018 109 146.286 -29.943 Warraweena Volcanics Southern Thomson Orogen Volcanic e 417.0 3.5 zircon LA-ICP-MS Hack et al., 2018 110 144.391 -25.725 PPC Cothalow-1 Central Thomson Orogen rhyolitic ignimbrite e 398.9 5.8 zircon LA-ICP-MS Asmussen et al. 2018 111 144.996 -25.160 PPC Etonvale-1 Central Thomson Orogen rhyolitic ignimbrite e 398.3 4.6 zircon LA-ICP-MS Asmussen et al. 2018 112 145.905 -24.415 BEA Allandale Central Thomson Orogen rhyolitic ignimbrite e 398.3 6.2 zircon LA-ICP-MS Asmussen et al. 2018 113 145.905 -24.415 BEA Allandale Central Thomson Orogen rhyolitic ignimbrite e 397.6 3.5 zircon LA-ICP-MS Asmussen et al. 2018 114 143.930 -25.506 AOD Yongala-1 Central Thomson Orogen Rhyolite e 488.6 4.0 zircon LA-ICP-MS Asmussen et al. 2018 115 144.696 -25.980 PPC GUMBARDO 1 Central Thomson Orogen rhyolitic ignimbrite e 398.1 4.4 zircon LA-ICP-MS Asmussen et al. 2018

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Appendix 5.1 Mineral separation procedure for samples from the Darling Basin

Logging and sampling of the targeted intervals were conducted by the Regional Mapping and Exploration Geosciences unit of the Geological Survey of New South Wales (GSNSW) at the W.B. Clarke Geoscience Centre in Londonderry, NSW in September and November 2015.

Mineral Separation

Whole rock samples were split into ~5 cm blocks using a hydraulic splitter, subsequently crushed into ~1–2 cm chips using a Retsch BB200 Jaw Crusher, and finally thoroughly washed and dried in a 65°C oven. Rock chips were then milled to ~300 μm. A Wilfley Table was used for density separation to remove less dense minerals, reducing the sample size significantly, then thoroughly washed and dried again. Highly magnetic minerals were removed from the samples using hand magnets before being processed in a Frantz Isodynamic Mineral Separator to remove other magnetic minerals. Remaining minerals underwent heavy liquid mineral separation using Lithium Heteropolytungstate solution (liquid density of 3.2 g/cm3) to extract the minerals with higher densities from the sample. Zircon grains were then hand-picked under a binocular microscope and soaked in acetone to remove surface contaminants.

304

Appendix 5.2 Transmitted light and cathodoluminescence images of detrital zircon samples from the Darling Basin

305

Appendix 5.3 Tera-Wasserburg diagrams of 207Pb/206Pb vs 206Pb/ 238U for detrital zircon samples from Darling Basin samples

Appendix 5.3. Tera-Wasserburg plots comprising all DZ U-Pb data from Darling Basin sandstone samples. Yellow error ellipses show concordant data, whereas red ellipses display discordant analyses.

306

Appendix 5.4 Transmitted light images of detrital rutile samples from the Darling Basin

307