浙西北上奥陶统文昌组和堰口组沉积特征及其意义

Sedimentary characteristics of the Upper Ordovician Wenchang and Yankou Formations in northwestern Zhejiang Province

晚奥陶世赫南特期是地史上的一个重要转折时期,在这一短暂的时期内发生了生物大灭绝、全球性冰川和全球性海平面剧降等事件。在南半球冈瓦纳大陆上以北非为中心的大陆冰盖急剧扩增,在国外许多地区和块体上常有冰碛岩和海退序列报道,但中国对该时期地层的沉积学研究还不多。浙西北地区晚奥陶世的文昌组和堰口组为一套海退型沉积,上段发育有华南该时期一套独特的砾岩和含砾砂岩沉积,根据该沉积及上下地层所含的笔石动物群分析,该套砾岩层的层位相当于赫南特阶Normalograptus extraordinarius笔石带,在形成时间上与奥陶纪末的全球冰川事件一致。当前研究表明,这套砾岩代表水下河道沉积,其砾石主要来自东南侧的华夏古陆。文昌组和堰口组砂岩成分统计分析显示,文昌组和堰口组的物源区具有再循环造山带性质,且砂岩的成分成熟度和结构成熟度均较低,与华夏古陆构造隆升紧密相关。文昌组和堰口组的沉积序列可与北非、中东、英国、瑞典、捷克等地的同期序列进行对比,是赫南特期全球性冰川事件在华南的响应证据。

Based on new collections of fossils from the Upper Ordovician Wenchang,Yankou and Anji Formations in northwestern Zhejiang Province,southeastern China,especially those significant graptolites,the Wenchang Formation is considered to be equivalent to the Yankou Formation,and is largely of Hirnantian Age.In Hirnantian Age,the Gondwana ice sheet,centered in the North Africa,reached its maximum,as evidenced by that more and more glacial deposits（tillites or diamictites）of the age have been reported across the world.The Wenchang and Yankou Formations yield well-developed conglomerates,which are unique in South China and might be related with the glaciation event.Regarding the origination of the conglomerates,there have been some debates and viewpoints from different perspectives.Based on sedimentary studies,these conglomerates display different sedimentary characters and mechanisms among localities and sections in the region.They represent several types of channel depositions in shallow water.A detailed regional lithofacies study of the Wenchang and Yankou Formations in northwestern Zhejiang Province indicates that a Zhe-Wan Basin was developed in the Hirnantian Age,with Cathaysian Land located to its southeast and served as the clastic source.The basin becomes deeper northwestwardly,following the palaeogeographic pattern of pre-Hirnantian.Petrological analysis of the sandstones in the Wenchang and Yankou Formations displays a low compositional maturity along with a low textural maturity,and a provenance of recycled orogenic belt,which indicate a probable sustaining uplift of the Cathaysian Land.The regressional sequence recognized in the Wenchang and Yankou Formations in northwestern Zhejiang Province can be well correlated with that of other continents or regions,such as North Africa,Middle East,U.K.,Sweden and Czech etc.