Geochronology and sediment provenance of the Precipice Sandstone and Evergreen Formation in the Surat Basin, Australia: implications for the palaeogeography of eastern Gondwana - Geochronology data
A new view of the palaeogeography and tectonic evolution of eastern Gondwana during the late Mesozoic is emerging, driven largely by the detrital zircon record of sedimentary basins comprising the Great Australian Superbasin (GAS). The updated model suggests that a long-lived magmatic arc was present on the eastern margin of Australia, which is in stark contrast to some previous views that eastern Australia was situated well-inland of a plate boundary. However, the active magmatic arc model is derived from Late Jurassic-Cretaceous strata, with tectonic processes and sediment pathways operating in the Early Jurassic still poorly understood. The Lower Jurassic Precipice Sandstone in the Surat Basin – an important stratigraphic component of the GAS – affords an opportunity to fill such a knowledge gap. The Precipice Sandstone is also economically important due to its aquifer capacity and prospectivity for CO2 sequestration. Previous work has hypothesized that the formation contains deposits sourced from multiple terranes, yet this has not been rigorously tested. This geochronology dataset comprises two parts: detrital zircon LA-ICP-MS data and CA-TIMS ages from tuff samples. These radiometric dating methods were used to 1) constrain depositional ages and add resolution to the stratigraphic framework for the lower part of the Surat Basin succession, and 2) characterise the sedimentary provenance of the Precipice Sandstone and the overlying Evergreen Formation. The new U-Pb zircon dates provide some of the first radiometric depositional age constraints for the Lower Jurassic part of the GAS succession, thus far based on palynology and lithostratigraphy. Our analysis revealed mixed provenance with multiple source terranes within the broader Tasmanides region. The dataset is dominated by Grenvillian (~900-1200 Ma), Pacific-Gondwanan (~500-650 Ma) and Permian-Triassic (240-266 Ma) age populations. Smaller proportions of other Palaeozoic and Proterozoic ages, as well as a small but prominent syn-depositional Early Jurassic population, are also present. This age profile indicates that the bulk of sediment was derived from the Thomson Orogen, with a lesser contribution from the New England Orogen. We found no evidence for sediment derived from the Lachlan Orogen, despite a distinct northward palaeocurrent component. Syn-depositional volcanic material within the succession corroborates the notion of continued arc magmatism along the eastern Gondwana margin, which contributed sediment to the GAS throughout the Early Jurassic. Radiometric ages place the Precipice Sandstone in the Lower Jurassic Sinemurian-Pliensbachian and the Evergreen Formation in the middle Pliensbachian-topmost Toarcian, and suggest diachronous deposition across the basin. Despite pronounced variations in formation thickness, the Precipice Sandstone shows relatively uniform provenance across the basin, showing little evidence for multiple depocenters.
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LA-ICP-MS: U-Pb Isotopic dating was performed using Teledyne Analyte Excite laser and Agilent 7900 ICP-MS at the CARF-QUT. The samples were analysed during 8 sessions with the following instrument settings: fluence 1.73-2.51 J/cm², rep rate 9 hz, dwell time 30 sec, spot size 40 μm (25 μm for sample T17-D1). Temora-2 was used as a primary standard. Plešovice zircon and 91500 were used as age control standards. The synthetic silicate glass NIST 610 was used as a trace element standard. Concentrations of the following major and trace elements were measured in all sessions except session 7: Si, P, Ti, Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Dy, Er, Yb, Lu, Hf, 206Pb, 207Pb, 208Pb, Th and U. Element concentrations measured during session 7 were: Si, P, 206Pb, 207Pb, 208Pb, Th and U. The element concentrations, especially P and La, were used as one of the analytical criteria for individual analyses to be included/excluded from the final dataset. For zircon grains older than 950 Ma, the 207Pb/206Pb ages were used. The 206Pb/238U ages were used for zircon grains younger than 950 Ma. A 208Pb-based common Pb correction was calculated and applied to grains <950 Ma to check for improved concordance after Cumming and Richards (1975) model. The data were processed using the Iolite software. CA-TIMS: U and Pb were separated from the zircon matrix using an HCl-based anion-exchange chromatographic procedure, eluted and dried with 2 µl of 0.05 NH3PO4. Pb and U were loaded on a single outgassed Re filament in 5 µl of a silica-gel/phosphoric acid mixture and isotopic measurements made on a GV Isoprobe-T multicollector TIMS equipped with an ion-counting Daly detector. Pb isotopes were measured by peak-jumping all isotopes on the Daly detector for 160 cycles and corrected for 0.16 ± 0.03%/a.m.u. (1σ) mass fractionation. Transitory isobaric interferences due to high-molecular-weight organics disappeared within approximately 30 cycles, while ionization efficiency averaged 104 cps/pg of each Pb isotope. Linearity (to ≥1.4 x 106 cps) and the associated deadtime correction of the Daly detector were determined by analysis of NBS982. U was analyzed as UO2+ ions in static Faraday mode on 1012 ohm resistors for 300 cycles and corrected for isobaric interference of 233U18O16O on 235U16O16O with an 18O/16O of 0.00206. Ionization efficiency averaged 20 mV/ng of each U isotope. U mass fractionation was corrected using the known 233U/235U ratio of the ET535 tracer solution. Ages and uncertainties were calculated using algorithms of Schmitz and Schoene (2007), ET535 tracer solution with calibration of 235U/205Pb = 100.233, 233U/235U = 0.99506, and 205Pb/204Pb = 11268, and U decay constants recommended by Jaffey et al. (1971) and 238U/235U of 137.818. 206Pb/238U ratios and dates were corrected for initial 230Th disequilibrium using DTh/U = 0.2 ± 0.1 (2σ) and the algorithms of Crowley et al. (2007), resulting in an increase in the 206Pb/238U dates of ~0.09 Ma.