上奥陶统赫南特阶下扬子地区标准剖面(浙江省安吉县杭垓剖面)的地质特征及其意义

GEOLOGICAL CHARACTERISTICS OF THE HANGGAI SECTION IN ANJI,ZHEJIANG(THE REFERENCE SECTION OF UPPER ORDOVICIAN HIRNANTIAN STAGE FOR THE LOWER YANGTZE REGION)AND THEIR IMPLICATIONS

通过对浙江省安吉县杭垓地区晚奥陶世地层的区域地质调查、剖面测量以及系统化石采集、鉴定与分析样品测试研究,浙江省安吉县杭垓剖面被列为"上奥陶统赫南特阶下扬子地区标准剖面"。该剖面的赫南特阶和奥陶系-志留系界线地层出露完整,发育了厚度超过360m的笔石页岩和细碎屑岩的连续旋回沉积,化石丰富,主要界线附近未见构造破坏,较完整地记录了赫南特期的地质事件和生物序列,符合建立下扬子区赫南特阶标准剖面的要求。在该剖面的上奥陶统凯迪阶—赫南特阶—志留系兰多维列统鲁丹阶的连续地层中发现丰富的笔石、几丁虫和海绵动物化石,以及部分三叶虫、腹足类、腕足类、头足类等门类化石,自下而上共识别出Dicellograptus complexus,Paraorthograptus pacificus,Normalograptus extraordinarius,Normalograptus persculptus,Akidograptus ascensus,Parakidograptus acuminatus 6个笔石带和1个Songxites-Aegiromenella壳相化石组合。在赫南特阶地层中首次发现极为丰富多样的海绵动物群,为进一步认识全球赫南特期的生物圈面貌、寒武纪生命大爆发之后的海绵生物演化打开了重要窗口。

Recently the Hanggai Section in Anji, northwestern Zhejiang Province, was ratified as the u-nique ＂Reference Section of the Hirnantian Stage for Lower Yangtze Region, South China＂, based on an e- valuation of the proposal and related documents, and after a field investigation by the evaluation group or- ganized by the National Committee on Stratigraphy of China. The proposal includes the geological setting of the section, regional geology, a detailed description of the section, results of lithostratigraphy, bios- tratigraphy, sequence stratigraphy and chemostratigraphy, and the yielded abundant fossils. The section exposes continuous Upper Katian to Rhuddanian interval of graptolitic shale and fine-grain clastics depos- its, in which the Hirnatian Stage is ca. 360 m thick （Wenchang Formation）. No tectonic disturbances have been confirmed across the Hirnantian. The section contains fairly diverse fossils, including abundant grap- tolites, chitinozoans, and sponges, and less abundant brachiopods, trilobites and gastropods, etc. Bios- tratigraphically, six successive graptolite biozones and a benthic assemblage are identified, which are in as- cending order： （1） Dicellograptus complexus Biozone, （2） Paraorthograptus pacificus Biozone, （3） Normalograptus extraordinarius Biozone, （4） Songxites-Aegiromenella Assemblage, （5） Normalograp- tus persculptus Biozone, （6） Akidograptus ascensus Biozone, and （7） Parakidograptus acuminatus Bio- zone. The discovery of abundant and diverse sponges in the Hirnantian interval opens a new window to probe into the marine benthic community during the Hirnantian Extinction and sheds lights on the sponge evolution after the Cambrian Explosion.