扬子台地西缘奥陶纪末-志留纪早期的几丁虫及综合地层学研究

The opinions on the subdivision and correlation of Silurian in the Yangtze Region have long been focus on debates when comparing with other systems which have almost been fixed. It is well accepted that the Llandovery in the main part yielded on the Yangtze Region while the Wenlock, Ludlow and Pridoli develop occasionally around the marginal areas. However, chitinozoan experts believe that the four series of Silurian are completely developed in the Yangtze Region. They believe that inverted strata, layerslip, intra- and inter- formational missing are common in the Silurian in the Yangtze Region. A few recently researches have reported the presence of Ludlow-Pridoli in the central part of the Yangtze Region, which partly confirm the view of chitinozoan experts that both Llandovery and post-Llandovery are developed in the main part of the Yangtze Region.. The western Yangtze Region is an ideal area for the studies of reconstructing the early Silurian chitinozoan sequences. It is because clastic and carbonate rocks occur together in the Ordovician-Silurian transitional interval, More importantly, the lowermost Silurian is characterized by non-condensed deposits. abundant graptolites and chitinozoans co-occur in the same beds make the study of biostratigraphy available. Based on the systematics and biostratigraphy of chitinozoans and graptolites from the western margin of the Yangtze Region, this application focuses on the end-Ordovician mass extinction-recovery. At the same time, integrated studies of chemostratigraphy and sedimentology will be conducted to improve the resolution of division and correlation of the Silurian in the western Yangtze Region. The present study will be a guidance of the Silurian division work in the Yangtze Region. Furthermore, it will favor the exploration and identification of oil and gas in South China. Finally, it will open a window of accessing the strategy of how the planktonic chitinozoans respond to the end-Ordovician mass extinction and the following survival-recovery-radiation events.

扬子区志留系划分对比意见分歧和变动大，为其他各系所罕见。比如，现在一般认为扬子区主要发育志留纪早期地层，志留纪中晚期只在周缘局部发育。而几丁虫学者认为扬子区主体内4统发育齐全，志留系存在倒序、脱顶、丢层等问题。近年已有多篇文献在扬子台地内部报道了志留纪晚期的沉积，部分证实了几丁虫学者提出的扬子区内部不仅发育志留纪早期沉积，还发育晚期沉积的观点。. 扬子区西缘奥陶纪末-志留纪初期地层同时发育细碎屑岩和灰岩，尤其是志留纪早期地层为非凝缩沉积，几丁虫、笔石丰富、相伴同层产出，是重建华南志留纪初几丁虫生物地层序列的理想区域。本申请立足奥陶-志留纪之交扬子台地西缘几丁虫、笔石等门类的系统古生物学、生物地层学这一古生物学研究的基础领域，紧扣奥陶-志留纪之交生物大灭绝-复苏的热点问题，同时借助化学地层学、沉积学等综合地层研究手段，提高华南志留系划分对比的精度。通过本申请研究，能够指导扬子区志留系的划分对比，助力华南油气藏勘探与评价，同时有望打开了解几丁虫这一海洋浮游生物如何应对奥陶纪末大灭绝，及其后的残存-复苏-辐射过程的窗户。

扬子区的志留系研究具有理论和实践的双重意义。自上世纪90年代后期，关于扬子区的志留系是否发育完整的问题在中国学者中出现了两种完全不同的观点的争论，持续至今。争议的核心在于扬子台地上是否发育温洛克世的地层。上扬子区下志留统龙马溪组的黑色页岩是我国南方页岩油气的主力烃源岩和储层层段，页岩油气藏已经进入工业化生产阶段。对扬子区西缘志留纪烃源岩分布地层的精细划分对比、对其沉积环境与油气资源关系的研究，有助于我国页岩油气资源勘探开发的后续拓展。. 本基金项目是基于上述两点而设立的，主要研究内容是扬子区西缘志留纪几丁虫生物地层和志留纪地层划分对比的综合研究，取得主要成果如下：. 1. 建立了扬子区西缘志留纪综合生物地层序列，为扬子区西缘志留纪地层精细划分对比奠定了坚实的理论基础，为该区域志留纪油气资源勘探开发工作提供了最基础的地层古生物数据。. 2. 扬子区西缘志留系与下伏奥陶纪地层在不同地区分别表现为整合接触或不整合接触。这与扬子区内部志留系龙马溪组与下伏奥陶纪地层在不同地区分别表现为整合接触和不整合接触的情况相同。. 3. 扬子区西缘的志留系顶部地层穿时。滇东北和川西大部分地区志留系顶部为兰多维列统特列奇阶，但在局部地区如雅安二郎山地区滇藏海与杨子海通道地区存在中、晚志留世的地层。. 4. 木厂沟剖面Hg浓度和有机碳同位素组成的高分辨率分析为火山作用、气候变化和晚奥陶世生物演化之间的关系提供了详细而新颖的见解。奥陶纪末期海洋生态危机与频繁的火山活动造成生物地球化学循环的一系列中断有关。根据汞通量计算，据估计，在Katian期火山活动向华南地区释放了超过1.1×1013吨的CO2。强烈火山活动对地表碳和汞循环造成了明显扰动。通过对奥陶纪-志留纪过渡地层的Hg元素浓度分析，发现晚奥陶世地球上频繁的火山活动可能是导致奥陶纪末期生物大灭绝的因素之一。