New U-Pb zircon dates reveal a late Norian age for the upper part of the Red Cliff Coal Measures, New South Wales, Australia—implications for biostratigraphy

The upper part of the Red Cliff Coal Measures in northern New South Wales hosts a well-preserved Late Triassic flora that, previously, has not been dated precisely. New high-precision U-Pb zircon dating (U-Pb CA-ID-TIMS) from a bentonite immediately overlying the fossiliferous beds yields a maximum depositional age of 208.66 ± 0.33 Ma. This new chronological constraint demonstrates that at least the upper Red Cliff Coal Measures, and the correlative Blackstone Formation of the Ipswich Basin, are Norian in age rather than Carnian, as suggested previously. Palynological assemblages, although thermally altered and dominated by degraded phytoclasts, include spores and pollen assignable to the <i>Craterisporites rotundus</i> Oppel Zone of de Jersey (1975) and are representative of the high-latitude ‘Ipswich microflora’ phytoprovince. The macroflora is well preserved in the upper part of the Red Cliff Coal Measures and is dominated by the umkomasialean ‘seed fern’ <i>Dicroidium odontopteroides</i> but incorporates at least 11 other gymnosperm and two fern taxa typical of the well-documented Late Triassic <i>Dicroidium</i>-dominated floras of southeastern Gondwana. The fossil foliage is typically preserved in dense accumulations reminiscent of modern autumnal leaf mats. These leaves host a moderate array of herbivory and oviposition damage attesting to the presence of a varied insect fauna that is not represented by body fossils. The sedimentary facies associations, ranging from coarse-grained alluvial fan and fluvial channel deposits to finer lacustrine and peat-mire successions, together with numerous scour surfaces and lateral interdigitation of bedforms indicate deposition in a dynamic landscape typical of a small fault-bound continental basin. Collectively, these data refine the stratigraphic framework of the Red Cliff area and correlative components of the Ipswich Basin, and clarify the composition of the Late Triassic palaeovegetation and palaeoenvironments along the eastern margin of Gondwana. <i>Stephen McLoughlin* [[e-mail]] and Vivi Vajda [[e-mail]], Department of Palaeobiology, Swedish Museum of Natural History, Stockholm, Sweden; James L. Crowley [[e-mail]] Isotope Geology Laboratory, Boise State University, Boise, ID, USA</i>.

新南威尔士州北部的红山煤系上部产出保存完好的晚三叠世植物群，此前该植物群尚未获得精确年代限定。本次研究采用高精度U-Pb锆石定年（U-Pb CA-ID-TIMS）方法，对紧邻化石层之上的膨润土样品进行测年，获得的最大沉积年龄为208.66±0.33 Ma。这一新的年代学约束条件表明，至少红山煤系上部以及伊普斯威奇盆地的对应地层布莱克斯通组，其时代应为诺利期（Norian），而非此前所提出的卡尼期（Carnian）。尽管孢粉组合经历了热变质作用，且以降解的植物碎屑为主，但其中仍包含可归属于de Jersey（1975）建立的圆形凹坑孢（*Craterisporites rotundus*）奥佩尔组合带的孢子与花粉，代表了高纬度的‘伊普斯威奇微植物群’植物省。红山煤系上部的大植物群保存完好，以乌马科夫目类‘种子蕨’齿叶二歧蕨（*Dicroidium odontopteroides*）为优势类群，同时包含至少11个裸子植物类群与2个蕨类类群，这些类群均为冈瓦纳东南部晚三叠世以二歧蕨（*Dicroidium*）为主导的知名植物群的典型分子。该化石叶片通常以密集堆积的形式保存，其形态令人联想到现代秋季的落叶层。叶片表面可见多种植食性损伤与产卵痕迹，证明当时存在多样的昆虫类群，尽管这些昆虫并未以实体化石的形式保存下来。本次研究涉及的沉积相组合涵盖粗粒冲积扇、河流河道沉积，以及细粒湖相和泥炭沼泽沉积序列；加之广泛发育的冲刷面与床形的侧向交错叠置特征，表明该地层形成于小型断裂控制的陆相盆地典型的动态沉积环境中。综上，本研究数据完善了红山地区以及伊普斯威奇盆地相关地层的年代地层格架，并阐明了冈瓦纳东缘晚三叠世古植被与古环境的组成特征。*斯蒂芬·麦克劳克林* [[e-mail]]与维维·瓦亚达 [[e-mail]]，瑞典自然历史博物馆古生物部，瑞典斯德哥尔摩；詹姆斯·L·克劳利 [[e-mail]]，博伊西州立大学同位素地质实验室，美国爱达荷州博伊西