中扬子地区早三叠世“错时相”沉积与生物复苏

The Permian-Triassic mass extinction has catched our eyes by the most devastating extinction and the longest recovery in the Phanerozoic Eon. The marine ecosystems were severely affected by the end-Permian mass extinction events, at the same time the sedimentary systems also changed dramatically, from a normal marine sediments in the Late Permian to the unique "reef gap" on platform, "chert gap" in basin, "coal gap" on land, and "anachronistic facies" widespread in the shallow seas of the Early Triassic. "Anachronistic facies" is a distinctive sedimentary record including flat-pebble conglomerates, vermicular limestones, subtidal wrinkle structures, microbialites, carbonate seafloor fans, vermicular limestone, and zebra limestone-mudstones. These sediments were characteristic phenomena of the Cambrian and Precambrian rocks and disappeared from the normal shallow marine settings since the Ordovician but occurred only in some high-pressure environments devoid of metazoans. And such, these sediments are regarded as the "anachronistic facies" when they re-occured in the post-Cambrian normal shallow marine deposits. The "anachronistic facies" occurred in the immediately aftermath of the end-Permian mass extinction and disappeared with the final establishment of the Mesozoic marine ecosystems in the Anisian. The dramatic synchronization between the recurrence and disappearance of "anachronistic facies" sediment and the end-Permian mass extinction and subsequent recovery reflects that the sedimentary systems had responded to specific environmental conditions and ecosystems regime during the Paleozoic-Mesozoic transition. This project aims to answer the question about the background, duration and cause of "anachronistic facies" based upon diverse sedimentary phenomena in the middle Yangtze region, furthermore, provide new materials to explore the causes of delayed recovery, the pattern and process of the biotic recovery, and new thinking about how to understand the co-evolution between organisms and environment in the geological history.

P/Tr灭绝事件以导致显生宙最大生物灭绝和最长的生物复苏而引人注目。在生态系遭受毁灭性打击的同时，沉积体系也发生剧变，导致全球早三叠世"缺礁、缺硅、缺煤"，正常浅海中则表现为"错时相"沉积广泛再现。"错时相"沉积紧接生物大灭绝后出现，并随中生代海洋生态系在安尼晚期重建基本完成时退出正常浅海环境，其出现、消失与生物灭绝、复苏如此同步绝非偶然，这种耦合关系正是沉积体系对古-中生代之交地质突变期"特异"的环境状况和生态系状况之自然响应。本项目通过中扬子地区"错时相"沉积的类型、时空分布、盛衰变化、地球化学特征等研究，分析"错时相"沉积形成背景、时限与机制，提取"错时相"沉积的环境和生物指示意义，建立"错时相"沉积-环境变化-生物复苏三者协同演化的模式，从而为探索早三叠世生物迟缓复苏的原因、生物复苏的型式与过程提供新材料，为正确认识地质历史上生物与环境的协同演化提供新思路。

P/Tr灭绝事件以导致显生宙最大生物灭绝和最长的生物复苏而引人注目。在生态系遭受毁灭性打击的同时，沉积体系也发生剧变，导致全球早三叠世"缺礁、缺硅、缺煤"，正常浅海中则表现为"错时相"沉积广泛再现。中扬子地区“错时相”沉积以微生物岩、蠕虫状灰岩、扁平砾石砾岩、巨鲕和薄层泥晶灰岩为代表，分布于二叠系/三叠系界线和下三叠统中。研究区微生物岩以利川地区为代表，二叠纪末大绝灭主灭绝线正位于微生物岩底部，而P/Tr界线位于微生物中距顶部约25cm处。扁平砾石砾岩不发育，随机出现于大冶组和嘉陵江组中，其出现有两方面的指示意义值得关注，一是风暴盛行的极端气候背景，二是利于快速成岩的海洋化学条件。蠕虫状灰岩和薄层泥晶灰岩是研究区最为发育的"错时相"沉积。蠕虫状灰岩野外露头上的表现特征复杂多样，大致可分为层状或似层状、粒状、短柱状和椭球状四种类型，且四种类型在垂向序列上的分布具有明显规律。笔者认为蠕虫状灰岩具机械、化学、生物等多种成因，是古生代–中生代地质转折期的特殊海洋环境状况和以“蠕形动物”等机会生物为主导的特异底栖生态系面貌的体现。薄层泥晶灰岩因内部缺乏垂向生物扰动且很少存在生物化石，而层面上保存不同数量的水平遗迹有别于正常泥晶灰岩，其形成体现了一种缺乏具钙质骨骼或钙质壳的动物，而软体动物或蠕形动物发育，造迹生物垂向扰动能力低下和缺乏捕食者的生态系面貌。. "错时相"沉积紧接生物大灭绝后出现，并随中生代海洋生态系在安尼晚期重建基本完成时退出正常浅海环境，其出现、消失与生物灭绝、复苏如此同步绝非偶然，这种耦合关系正是沉积体系对古-中生代之交地质突变期"特异"的环境状况和生态系状况之自然响应。系列的环境异常事件导致了生物的灭绝和生态系、沉积体系的转变，恶化环境在早三叠世的频繁波动导致了丰富的特殊沉积持续出现和二叠纪末大灭绝后生物的迟滞复苏。特殊沉积在中三叠世初的消失可作为生态环境逐步得到改善和中生代生态系重建基本完成的标志。