Stratigraphy, biostratigraphy and C-isotopes of the Permian-Triassic non-marine sequence at Dalongkou and Lucaogou, Xinjiang Province, China
Measured lithostratigraphic sections of the classic Permian–Triassic non-marine transitional sequences covering the upper Quanzijie, Wutonggou, Guodikeng and lower Jiucaiyuan Formations at Dalongkou and Lucaogou, Xinjiang Province, China are presented. These measured sections form the framework and reference sections for a range of multi-disciplinary studies of the P–T transition in this large ancient lake basin, including palynostratigraphy, vertebrate biostratigraphy, chemostratigraphy and magnetostratigraphy. The 121 m thick Wutonggou Formation at Dalongkou includes 12 sandstone units ranging in thickness from 0.5 to 10.5 m that represent cyclical coarse terrigenous input to the lake basin during the Late Permian. The rhythmically-bedded, mudstone-dominated Guodikeng Formation is 197 m and 209 m thick on the north and south limbs of the Dalongkou anticline, respectively, and 129 m thick at Lucaogou. Based on limited palynological data, the Permian–Triassic boundary was previously placed approximately 50 m below the top of this formation at Dalongkou. This boundary does not coincide with any mappable lithologic unit, such as the basal sandstones of the overlying Jiucaiyuan Formation, assigned to the Early Triassic. The presence of multiple organic δ¹³C-isotope excursions, mutant pollen, and multiple algal and conchostracan blooms in this formation, together with Late Permian palynomorphs, suggests that the Guodikeng Formation records multiple climatic perturbation signals representing environmental stress during the late Permian mass extinction interval. The overlap between the vertebrates Dicynodon and Lystrosaurus in the upper part of this formation, and the occurrence of late Permian spores and the latest Permian to earliest Triassic megaspore Otynisporites eotriassicus is consistent with a latest Permian age for at least part of the Guodikeng Formation. Palynostratigrahic placement of the Permian–Triassic boundary in the Junggar Basin remains problematic because key miospore taxa, such as Aratrisporites spp. are not present. Palynomorphs from the Guodikeng are assigned to two assemblages; the youngest, from the upper 100 m of the formation (and the overlying Jiucaiyuan Formation), contains both typical Permian elements and distinctive taxa that elsewhere are known from the Early Triassic of Canada, Greenland, Norway, and Russia. The latter include spores assigned to Pechorosporites disertus, Lundbladispora foveota, Naumovaspora striata, Decussatisporites mulstrigatus and Leptolepidites jonkerii. While the presence of Devonian and Carboniferous spores and Early Permian pollen demonstrate reworking is occurring in the Guodikeng assemblages, the sometimes common occurrence of Scutasporites sp. cf. Scutasporites unicus, and other pollen, suggests that the Late Permian elements are in place, and that the upper assemblage derives from a genuine transitional flora of Early Triassic aspect. In the Junggar Basin, biostratigraphic data and magnetostratigraphic data indicate that the Permian–Triassic boundary (GSSP Level) is in the middle to upper Guodikeng Formation and perhaps as high as the formational contact with the overlying Jiucaiyuan Formation.
High precision U-Pb isotopic ages of Permian-Triassic events in eastern Australia: a chronological framework for energy resources
The Middle Permian-Early Triassic (MP-ET) of Eastern Australia hosts extensive black coal reserves of major economic importance but contains predominantly endemic biota precluding precise international correlation. MP-ET stage boundaries, and end-Guadalupian and end Permian mass extinction levels are poorly constrained. Attempts to calibrate the MP-ET of Australia using SHRIMP resulted in controversial radioisotopic ages with percent-level uncertainty and compromised accuracy. We here report more than 40 new high-precision (most at the ±0.05 myr level) using U-Pb CA-IDTIMS single zircon techniques for tuffs in the Sydney, Gunnedah and Bowen basins. These dates provide vital international timescale tie points and allow us to correlate individual tuff beds at the intra and inter basin levels. The youngest dates from the Garie Fm Sydney Basin (c. 247.7 Ma, c. 248.0 Ma) give a late Early Triassic (late Spathian) age. An age of c. 252.2 Ma from the top Bandanna Formation, Bowen Basin equates with the Permian-Triassic boundary. The oldest dates obtained are c. 263.4 Ma from the Broughton Fm and c. 271.4 Ma for the Rowan Fm, Sydney Basin, older than the Guadalupian-Lopingian boundary of c. 260 Ma. Implications of these ages for calibration of stratigraphy, local and regional correlations, placement of mass extinction levels, sedimentation rates, dating of environmental and climate change (including glaciation) and as a chronological framework for energy resources are presented.
Late Permian U-Pb CA-IDTIMS isotope geochronology of the Bowen Basin, Eastern Australia
High-resolution geochronology with an age resolution at the permil or sub-permil levels has the potential to internationally calibrate Late Permian peri-Gondwanan sequences in the Bowen Basin. Previous attempts at such calibration have relied on carbon-isotope records and very tenuous biostratigraphic and lithostratigraphic correlations here deemed unsuitable for a high-resolution chronostratigraphic framework for the basin. Existing SHRIMP U-Pb ages for some tuffs in the basin have been compromised by inaccuracy and low precision when compared to the CA-IDTIMS method. High-precision CA-IDTIMS ages also allow us to correlate individual tuff beds across the basin, to estimate rates of climatic and biotic change between the late Guadalupian and late Changhsingian major Permian mass extinctions. We here report seven new U-Pb CA-IDTIMS ages for ash-tuffs in the Late Permian of the Bowen Basin.
The Late Permian-Early Triassic palynology of the Bowen and Sydney basins: results and implications of new CA-IDTIMS isotopic ages
Palynology is the principal biostratigraphic tool employed to correlate among the thick fluvial to shallow marine successions of the Permian-Early Triassic of the Bowen and Sydney basins of eastern Australia. The regional palynofloras can be utilised for intra-continental comparisons but are only broadly correlative across Gondwana and rarely applicable as stage or sub-stage level of global tie-points. High precision CA-IDTIMS dating of Middle Permian-Early Triassic ashfall tuffs in these basins has provided a unique opportunity to confidently tie the endemic fossil biota to the international timescale. Carbonaceous siltstones and coals bracketing the tuff beds have been processed for their palynological content thus enabling precise chronometric ages to be ascribed to the fossil biotas. Tying these biozones to the internationally accepted Geologic Timescale will greatly enhance the event and biozonation correlation to areas outside Australia.
New Middle Permian - Early Triassic U-Pb zircon CA-IDTIMS isotopic ages of tuffs in the Sydney Basin, Australia: International calibration of stratigraphy and biostratigraphy
The Middle Permian-Early Triassic (MP-ET) of Eastern Australia contains a predominantly endemic biota precluding precise correlation with standard international biozones and System/Stage boundaries. The Permian-Triassic boundary, other MP-ET stage boundary levels, and the major end-Guadalupian and end-Permian mass extinction levels in Australia remain poorly constrained. Attempts to calibrate the MP-ET of Australia using Sensitive High Resolution Ion Microprobe techniques have resulted in controversial radioisotopic ages with percent-level uncertainty and compromised accuracy due to the use of an unsuitable standard. We here report eighteen new high-precision (<±0.05 my) U-Pb CA-IDTIMS ages for tuffs in the Sydney Basin based on isotopic dating of chemically abraded individual zircons. These dates provide vital international timescale tie points for the MP-ET of Australia.
Late Permian-Early Triassic palynology of the Bowen and Sydney basins: more CA-IDTIMS
Palynology is the principle biostratigraphic tool employed to correlate thick fluvial to shallow marine successions of the Permian-Early Triassic of the Bowen and Sydney basins of eastern Australia. The regional palynofloras can be utilised for intra-continental comparisons but are only broadly correlative across Gondwana and rarely applicable as stage or sub-stage level global tie-points. High-precision CA-IDTIMS dating of Middle Permian-Early Triassic ashfall tuffs in these basins has provided a unique opportunity to confidently tie the endemic fossil biota to the international timescale. Carbonaceous siltstones and coals bracketing the tuff beds have been processed for their plant microfossil content thus enabling precise chronometric ages to be assigned to the identified palynozones. Tying these biozones to the internationally accepted Geologic Timescale will greatly enhance correlation to areas outside of Australia, allowing specific global events to be recognised. Results from the Sydney Basin, previously published, suggested significant modifications to the ages currently assigned to some of the Late Permian and Early Triassic palynozones. These results are now coupled with results from the Bowen Basin to provide a regional synthesis of eastern Australian basin palynofloras.
New U-Pb CA-IDTIMS isotopic age tie points for the Lightjack Formation, Canning Basin, Western Australia
Ash-fall tuffs are extremely rare in the Permian of Western Australia, but recent coal exploration boreholes in the Fitzroy Trough of the Canning Basin have intersected several in the ~30-300 m thick Lightjack Formation. We here report four high-precision U-Pb zircon CA-IDTIMS ages of these tuffs. The formation contains marine to non-marine facies, and extends along the major depocentre of the Canning Basin (Fitzroy Trough - Gregory Sub-basin) and its margins, where it is sporadically exposed at localities including Lightjack Hill (type section), the Noonkanbah area, Liveringa Ridge and in Shore Range. The formation is dominated by siltstone and calcareous to ferruginous sandstone, with minor coal and fossiliferous beds near its base. It has previously been assigned a Roadian-Wordian age based on ammonoids ('Daubichites goochi' and 'Bamyaniceras australe') and brachiopods ('Neochonites (Sommeriella) afanasyevae' Zone), whereas palynomorphs ('Dulhuntyispora granulata' to 'D. parvithola' Zones) indicate a slightly younger age for the upper part of the formation.