U–Pb zircon age of the Walloon Coal Measures in the Surat Basin, southeast Queensland: implications for paleogeography and basin subsidence

The Jurassic Walloon Coal Measures of the Surat Basin were previously estimated to be of Middle Jurassic age, ranging from Aalenian to Callovian, based on an uncalibrated eastern Australian biostratigraphic framework. New U–Pb dates of 162.55 ± 0.05 Ma and 158.86 ± 0.04 Ma obtained from zircons in ash-fall volcanic tuffs now place the Walloon Coal Measures of the Surat Basin in the Upper Jurassic Oxfordian. The new dates have several implications for the interpretation of the Jurassic strata in the Surat Basin. First-order subsidence rates of 61 m/Myr for the Walloon Coal Measures are more akin to those of foreland basins than the previously assumed intracratonic setting. The dates also imply deposition of the Walloon coals in substantially higher latitudes than previously assumed and that they accumulated as peats in mires that experienced more than three months’ continual darkness each winter. Zircon dating of tuffs and associated geochemistry should assist with the correlation of the laterally impersistent coals, fluvial sandstone and mudstone of the Walloon Coal Measures, which are currently difficult to correlate over distances of more than a few kilometres. Dating of the palynostratigraphic zones APJ4.2 to APJ5 (<i>Aequitriradites norrisii</i> Association Zone to <i>Murospora florida</i> Association Zone) will also need to be recalibrated.

此前基于未校准的东澳大利亚生物地层框架（eastern Australian biostratigraphic framework），苏拉特盆地（Surat Basin）内的侏罗纪沃隆煤系（Jurassic Walloon Coal Measures）被推定形成于中侏罗世，时代跨度为阿伦阶（Aalenian）至卡洛维阶（Callovian）。本次研究从降落型火山凝灰岩（ash-fall volcanic tuffs）中的锆石（zircons）中获得两组新的铀-铅年代数据（U–Pb dates）：162.55 ± 0.05 Ma与158.86 ± 0.04 Ma，据此将苏拉特盆地沃隆煤系的形成时代修订为晚侏罗世牛津阶（Upper Jurassic Oxfordian）。
这一新年代数据对苏拉特盆地侏罗系地层的解释具有多重意义：其一，沃隆煤系的一级沉降速率达61 m/Myr，相较于此前假设的克拉通内盆地环境（intracratonic setting），其更趋近于前陆盆地（foreland basins）的沉降特征；其二，该年代数据表明沃隆煤系的沉积时期所处纬度远高于此前推测，且其泥炭（peats）形成于每年冬季持续黑暗超三个月的沼泽湿地（mires）中。
对凝灰岩的锆石测年及其配套地球化学分析，将有助于解决沃隆煤系中横向分布不连续的煤层、河流相砂岩及泥岩的地层对比难题——目前这类地层仅能在数公里范围内实现对比，跨距更远的对比仍存在困难。此外，孢粉地层带（palynostratigraphic zones）APJ4.2至APJ5（即诺氏均刺环藻（Aequitriradites norrisii）组合带至佛罗里达膜孢（Murospora florida）组合带）的年代学研究也亟需重新校准。