华南埃迪卡拉纪—寒武纪过渡期标准层型剖面再研究

In the Ediacaran-Cambrian transition, there is a major mutation in the geological history of the Earth, a series of geological and biological events occurred in the environment changes and life evolution. New material of the important macrofossil biotas from South China shows that cross the boundary formations of Preambrian–Cambrian, so hyperactive biological evolution seems possible of a qualitative leap mainly from soft-bodied metazoans to shelled animals that the research on the phylogenetic and stratigraphic correlation of the major groups has important significance. The eastern Yunnan is located in the southwestern margin of Yangtze region, during the Precambrian-Cambrian transition period is an open platform to slope of shallow water environment, preserved the typical and continuous deposits of Upper Ediacaran to Cambrian bottom, and also contains a candidate stratotype of Chinese Sinian and Cambrian boundary. In recent years, herein newly discovered macrofossil algae, trace fossils and possible Ediacaran-type fossils with more sophisticated content (“Jiangchuan Biota”), as well as numerous of long-striped macrofossil Shaanxilithes occurred beneath the bottom of phosphate layer of Early Cambrian. Relevant boundary sections from Dengying Formation up to the bottom of phosphate sediments (Zhongyicun Member) exposed continuous deposition with diverse macrofossil records (assemblage zones) can be useful for the division in the Dengyingxian Stage of the upper Ediacaran, and articulated with Jinningian Stage and Meishucunian Stage of the lowermost Cambrian, by a potential standard stratotype section..It will be prior in this project to research the standard sections and Meishucun stratotype, Jingning in the eastern Yunnan, South China with occurrence of macrofossil assemblages including Jiangchuan Biota, then comparable to other neighboring supplementary sections and the typical successions of Upper Ediacaran and Lower Cambrian phosphorite sediments containing early animal fossils with phosphate mineral shell in South China, and in-depth study of biostratigraphy and evolutionary biology on the macrofossils and their assemblages, etc., in order to build the chronostratigraphic stage of late Ediacaran, contrast and establish stratotype, and provide more desirable index macrofossils to define the worldwide boundary between Precambrian and Cambrian.

新元古代与寒武纪之交，即埃迪卡拉纪—寒武纪过渡期是地球历史上的一个重大地质突变期，出现了一系列地质环境变化事件和生物快速演化事件。云南东部位于扬子地区西南缘，在前寒武纪—寒武纪过渡期属于开放式浅水台地至台缘斜坡环境，是我国埃迪卡拉(震旦)系顶部与底寒武统基本连续沉积的典型地区，也曾是中国震旦系与寒武系界线层型候选剖面所在地。近年来，在此新发现了更为丰富的宏体藻类化石、遗迹化石和埃迪卡拉生物群的疑似后生动物化石等(江川生物群)，以及大量以条带状 Shaanxilithes为代表的宏体化石层。相关剖面自下而上从灯影组至磷矿沉积层(中谊村段)出露有连续的宏体化石记录，重点研究后可以细分埃迪卡拉系顶部灯影峡阶的化石带，并与底寒武统的晋宁阶、梅树村阶对接，具备作为标准层型剖面的较大潜力，为埃迪卡拉纪末期年代地层单位的建阶分层、对比及确立层型剖面，并与寒武纪最早期年代地层的界线划分提供更为理想的标准。

“雪球事件”以来，早期生命呈现大规模多层次的快速演化。华南地区前寒武纪—寒武纪过渡期已知的丰富多样的化石记录是世界上其它国家和地区都无法比拟的，有助于解释和复原早期生命起源到寒武纪生命爆发的快速演替过程。. 本项目主要对华南地区伊迪卡拉系—寒武系地层中产出的多门类宏体化石进行较深入的分类和对比研究，并对该两系界线标志化石的选取提出新建议。通过详细测制滇东地区的若干剖面，分析各剖面宏体化石的产出层位及其分带性，归纳出以蠕形化石为主的分布规律及与其它宏体化石的组合特征。滇东地区蠕形化石自下而上产出三个化石带，分别是：1. Helminthopthes- Shaanxilithes组合带，2. Sabellidites带，3. ?Harlaniella带。新发现的第2、3化石带与小壳化石的第1组合几乎重合，化石产出层位稳定，生物延限短，综合小壳化石的组合特征，对滇东地区与华南等国内外其它地区进行更加精准的生物地层对比，确定伊迪卡拉系—寒武系过渡阶段的地层框架及界线地层划分与层型选择有如下进展：. (1)滇东地区梅树村剖面小壳化石的分带性在华南地区都有相似的特征，川西南、黔中、峡东和陕南的伊迪卡拉系—寒武系边界地层都可与滇东地区进行对比；.(2)条带状化石cf. Sabellidites在滇东地区出露于寒武系底部磷块岩之下、伊迪卡拉系顶部地层之中，易于识别对比并未见于寒武系，化石带延限短，其灭绝(末现)层位是明显的等时界线，可作为伊迪卡拉系—寒武系界线划分的标志化石；.(3) 稳定出露于寒武系底部粘土层以下的条带状cf. ?Harlaniella化石在时空上的延限分布需要更为深入地发掘研究，现阶段可以建议将原来定义跨界的小歪头山段划归灯影组；.(4)上述两类条带状化石与下伏旧城段或高家山段广泛分布的Shanxilithes和Cloudina标志化石相结合可以细分伊迪卡拉系的顶阶及寒武系底部地层；.(5)在滇东不同岩性地层中发现的蠕形Shanxilithes化石已被认为是实体化石的证据，与国内外不同地区的Shanxilithes化石完全可以对比，上覆地层富集保存的两类条带状化石形态上极相似于俄罗斯和北欧的Sabellidites和Harlaniella化石，但亲缘关系及系统归属还有待进一步的发掘与研究。