中扬子地区嘉陵江组的牙形石序列及其年代地层学意义

Ecological environment recovery after the end-Permian mass extinction event is the hot topic in the Earth history, especially Early Triassic Olenekian and Middle Triassic Anisian period. A series of major breakthroughs were mainly obtained from the lower Yangtze and Nanpanjiang basin areas in the south China. The Jialingjiang Formation, mainly corresponding to the Olenekian period, widely distributed in the central and upper Yangtze area. It is one key carrier to study ecological environment recovery but remains much controversy about biostratigraphy and geochronology. As for conodont sequence of this Formation in the central Yangtze area, it is not clear until now because of the limited lithology and other reasons; so far conodont study was only reported from Lichuan area of Hubei Province and a few other places. Depending on over ten years geological survey for the central Yangtze area, three sections including Jialingjiang Formation were carefully selected, which featured good outcrop, rich in limestone and successive sections. Moreover, the research we had done have already supplied for some information about conodont and carbon isotope and so on. Through setting up integrated conodont sequence combined with carbon isotope data and microfacies analysis，it is in favor of correcting multiple stratigraphical classification and correlation of Jialingjiang Formation, confirming the timing of the Nanzhang-Yuan'an fauna, probing into paleooceangraphical environmental changes in the Olenekian period and guiding the oil and gas field exploitation.

二叠纪末生物大灭绝之后的生态环境复苏已成为国际上研究热点之一，其研究重点聚焦在早三叠世奥伦尼克期至中三叠世安尼期，并已经在我国华南下扬子地区和南盘江地区进行了成功的尝试。早三叠世晚期的嘉陵江组碳酸盐岩广泛分布于中、上扬子区，是研究该地区生物复苏的重要载体。然而涉及到嘉陵江组牙形石序列的研究，由于受困于岩性和剖面等条件约束，进展缓慢，制约了生物地层学的研究和对生态环境复苏时限的标定。本文拟在中扬子地区十几年地质调查的基础上，选择了其中三条出露良好、剖面连续且有一定前期工作基础的嘉陵江组剖面，通过牙形石序列、碳同位素组成变化曲线和沉积相的研究，重新厘定嘉陵江组的多重划分对比系统，标定以多门类海生爬行动物繁盛为特色的南漳-远安动物群的时代，探讨早三叠世晚期海洋环境演变特点以及服务于油气的地质调查和勘察开发等。

在中扬子地区，关于嘉陵江组的地层划分目前还处于较为混乱的阶段，对于牙形石序列的研究也较为薄弱。此次研究利用微相、牙形石、稳定同位素同时结合前人资料对该地区的嘉陵江组地层进行了详细研究。此次工作仅在嘉陵江组地层中发现牙形石的枝形分子及极少且破碎的梳型分子，为下一步的研究提供了重要数据。研究通过对南漳和远安一带9个剖面嘉陵江组地层的测量和对比，厘定了南漳-远安动物群赋存层位及上下层位的岩石地层划分特征。动物群赋存层位之下主要为一套滑塌角砾岩，之上依次为火山凝灰岩、灰黄色泥岩、白云质灰岩和岩溶角砾岩。动物群则主要赋存在一套纹层状泥晶灰岩之中，该套地层厚度在剖面中均在30米左右，动物群产出层位常伴随着藻席发育。通过文献综合及大量剖面实测，文中恢复了中扬子地区南漳-远安动物群时期的古地理图，该时期南漳-远安地区和桑植小溪-利川一带分别位于总体背景为局限台地的两个洼地中。通过详细探讨分析，本文认为南漳-远安动物群时期扬子台地北缘的秦岭洋依然存在，并作为主要通道连接南漳-远安动物群和安徽巢湖鱼龙动物群，但在Spathian中晚期，沉积环境发生改变，造成两个动物群之间的地理隔离，这种隔离最终造就了安徽巢湖鱼龙动物群以鱼龙为主而南漳-远安动物群以湖北鳄类化石为主的动物群面貌的形成。研究初步建立了中扬子地区典型地质剖面嘉陵江组碳同位素曲线对比格架，并将其划分为4个阶段，对比分析认为南漳-远安动物群赋存地层的主要时代为Spathian亚阶晚期。探讨了南盘江地区“绿豆岩”与早中三叠世界线分子Chiosella timorensis的关系及对中扬子地区出露“绿豆岩”地层的启示。