Calibrating the middle and late Permian palynostratigraphy of Australia to the geologic time-scale via U–Pb zircon CA-IDTIMS dating

The advent of chemical abrasion-isotope dilution thermal ionisation mass spectrometry (CA-IDTIMS) has revolutionised U–Pb dating of zircon, and the enhanced precision of eruption ages determined on volcanic layers within basin successions permits an improved calibration of biostratigraphic schemes to the numerical time-scale. The Guadalupian and Lopingian (Permian) successions in the Sydney, Gunnedah, Bowen and Canning basins are mostly non-marine and include numerous airfall tuff units, many of which contain zircon. The eastern Australian palynostratigraphic scheme provides the basis for much of the local correlation, but the present calibration of this scheme against the numerical time-scale depends on a correlation to Western Australia, using rare ammonoids and conodonts in that succession to link to the standard global marine biostratigraphic scheme. High-precision U–Pb zircon dating of tuff layers via CA-IDTIMS allows this tenuous correlation to be circumvented—the resulting direct calibration of the palynostratigraphy to the numerical time-scale highlights significant inaccuracies in the previous indirect correlation. The new data show: the top of the <i>Praecolpatites sinuosus</i> Zone (APP3.2) lies in the early Roadian, not the middle Kungurian; the top of the <i>Microbaculispora villosa</i> Zone (APP3.3) lies in the middle Roadian, not the early Roadian; the top of the <i>Dulhuntyispora granulata</i> Zone (APP4.1) lies in the Wordian, not in the latest Roadian; the top of the <i>Didecitriletes ericianus</i> Zone (APP4.2) lies in the first half of the Wuchiapingian, not the latest Wordian; the <i>Dulhuntyispora dulhuntyi</i> Zone (APP4.3) is exceptionally short and lies within the Wuchiapingian, not the early Capitanian; and the top of the <i>Dulhuntyispora parvithola</i> Zone (APP5) lies at or near the Permo-Triassic boundary, not in the latest Wuchiapingian.

化学研磨-同位素稀释热电离质谱（chemical abrasion-isotope dilution thermal ionisation mass spectrometry，CA-IDTIMS）的问世彻底革新了锆石U-Pb定年技术，而对盆地沉积序列内火山岩层喷发年龄的高精度测定，可实现生物地层格架与数值时间标尺校准精度的显著提升。悉尼盆地、冈尼达盆地、鲍恩盆地与坎宁盆地内的瓜德鲁普统与乐平统（二叠系）沉积序列多为非海相沉积，且赋存大量降落凝灰岩单元，其中多数含有锆石。澳大利亚东部的孢粉地层格架是当地多数地层对比的核心依据，但当前该格架与数值时间标尺的校准，需依赖与西澳地层的对比——通过该序列中产出的罕见菊石与牙形石，将其与全球标准海相生物地层格架建立关联。通过CA-IDTIMS对凝灰岩开展高精度锆石U-Pb定年，可规避这种牵强的对比方式；由此实现的孢粉地层直接校准至数值时间标尺的结果，凸显了此前间接对比方法中存在的显著误差。新数据揭示了以下结论：波纹前扁形粉（<i>Praecolpatites sinuosus</i>）带（APP3.2）的顶界位于罗德阶早期，而非孔谷阶中期；毛微棒孢（<i>Microbaculispora villosa</i>）带（APP3.3）的顶界位于罗德阶中期，而非罗德阶早期；粒状杜尔亨蒂孢（<i>Dulhuntyispora granulata</i>）带（APP4.1）的顶界位于沃德阶，而非罗德阶末期；埃里克西得三缝孢（<i>Didecitriletes ericianus</i>）带（APP4.2）的顶界位于吴家坪阶前半段，而非沃德阶末期；杜尔亨蒂孢（<i>Dulhuntyispora dulhuntyi</i>）带（APP4.3）异常短暂，且赋存于吴家坪阶内，而非卡匹敦阶早期；细柱杜尔亨蒂孢（<i>Dulhuntyispora parvithola</i>）带（APP5）的顶界位于或接近二叠-三叠纪界线，而非吴家坪阶末期。