滇东晚二叠世-早三叠世陆相地层高分辨率沉积地球化学研究

The Latest Permian mass extinction was the severest bio-crisis on Earth in the geological history, which was coincident with the environmental stress resulted from several global events. Compared with those extensive studies on marine strata and marine ecosystem, relatively fewer workers focused on terrestrial stratigraphy and relevant ecosystem due to the absence of marine index fossils or unique geochemical indicators, causing that precise correlation between the terrestrial and marine PTB is very difficult. The Eastern Yunnan and Western Guizhou area is one of the most excellent regions related to the terrestrial Upper Permian and Lower Triassic in South China. The successive transition from marine to continental facies around PTB make it easier for direct correlation between marine and terrestrial stratum in one continued transitional sedimentary environment. This project is aimed at finding a new section containing integrated terrestrial sedimentary facies straddling P/T boundary at the background of previous works on paleontology. Taking advantage of high-resolution sedimentary petrology and geochemical studies, combined with radiogenic isotope dating of zircons in the deposited ash beds, provide more informative evidences about the correlations between marine and terrestrial strata around the Late Permian and Early Triassic transitional period. With those works projected, we expect more insights into the paleoenvironment while the evolution of terrestrial ecosystems during that important period.

晚二叠世末发生的生物大绝灭是地质历史上规模最大的一次生物危机事件，与之伴随的是诸多全球性异常环境问题。与二叠纪-三叠纪过渡时期海相地层及海洋生态系统的大量研究相比，目前国内外对该时期陆相地层及陆地生态系统的研究较为薄弱，重要原因之一是后者缺乏统一的古生物地层学或地球化学证据，导致不同剖面的地层对比较为困难。滇东黔西地区是华南陆相二叠-三叠系研究的重要地区之一，其从东到西海相、海陆过渡相和陆相地层连续过渡的特征有利于海、陆相地层的直接“追索式”对比。本项目拟在该区新选择一个发育连续、完整的PTB陆相剖面，结合已有的古生物学工作，利用沉积岩石学、地球化学等手段，开展高分辨率陆相地层研究；同时利用间歇分布的火山灰锆石放射性同位素测年，确定准确的地质年龄框架，为该区不同剖面在P/T过渡时期的陆相地层及海、陆相地层对比研究提供详实的沉积地球化学依据，为华南陆地生态系统演化过程中的古环境研究提供参考。

与二叠纪-三叠纪过渡时期海相地层及海洋生态系统的大量研究相比，国内外对该时期陆相地层及陆地生态系统的研究目前仍较薄弱。滇东黔西地区是尝试研究陆相P/T界线及海陆地层对比的理想地区。本项目选择滇东发育连续且出露较好的桃树井剖面开展精细的沉积学和地球化学研究，主要内容包括野外地质剖面调查、古土壤类型识别，XRD黏土矿物学和岩相学研究，并选择有代表性的样品开展了元素地球化学分析及锆石U-Pb年龄测试。.对该剖面的野外地质工作发现，从老到新的地层中古土壤发育类型表明了晚二叠世到早三叠世早期气候逐步干旱化：宣威组中下部发育Gelysols、Histosols，含大量植物根系或碎屑，代表了潮湿的热带沉积环境；宣威组上部到整个卡以头组发育Argillisols及混层黏土矿物，表明气候干旱化趋势的开始；东川组底部则发育大量河道沉积夹杂Vertisols，代表了强烈季节性降雨的半干旱环境。然而岩相学研究发现整个剖面的岩石受到广泛的后期成岩作用影响，对利用元素地球化学开展陆相古环境重建的设想带来了困难，例如，根据主量元素含量计算所得的化学风化指数在很大程度上受此影响，高岭石矿物的伊利石化、砂岩中的碳酸盐矿物胶结等，都使化学风化指数显著降低；不过化学风化指数的降低也可能与晚二叠世到早三叠世过渡时期陆地生态系统崩溃，物理风化作用强烈导致的基岩裸露有关。不仅如此，成岩作用还可能对利用有机碳同位素进行滇东黔西不同相区P/T剖面作为地层对比的指标提出了挑战。.以上研究所取得的初步认识正在撰写修改稿件，预计上半年投出。