上扬子地区五峰组-龙马溪组主要地质事件及岩相沉积响应

The Sedimentary Response to the Major Geological Events and Lithofacies Characteristics of Wufeng Formation-Longmaxi Formation in the Upper Yangtze Area

上扬子地区广泛发育上奥陶统五峰组和下志留统龙马溪组富有机质页岩,前人研究较少涉及其岩相类型和沉积过程对主要地质事件的响应.细粒沉积物高精度层序地层学研究表明,晚奥陶世至龙一段充填沉积期,经历了3次大的地质事件,形成了2个三级层序(五峰组层序Sq1和龙一段层序Sq2)、12个准层序组和13个笔石带.在生物地层约束的等时格架中,识别出火山喷发事件形成的富碳、富有机硅的硅质页岩相,底流侵入事件及陆源输入浊流事件沉积的贫碳、贫有机硅的混合质页岩相和泥质页岩相,建立了五峰组-龙马溪组富有机质页岩的成因模式,该套页岩主要为该时期的火山喷发(包括冰期和生物绝灭)、底流侵入和陆源输入等地质事件导致的古气候、古海洋等各要素协同作用的结果.

The organic rich marine black shale of the upper Ordovician Wufeng Formation-lower Silurian Longmaxi Formation is widely developed in the upper Yangtze area.However,there are few studies on lithofacies types and on the response of the deposition process to the main geological events.From the Late Ordovician to the end of sedimentary filling period of the first member of Longmaxi Formation,the upper Yangtze area had experienced three major geological events.Based on the high resolution sequence stratigraphy study on the fine-grained sediments,two 3rd order sequences Wufeng（Sq1） and Long1（Sq2） including 12 para-sequences and 13 graptolite zones are identified during this interval.Within the isochronal stratigraphic framework which was constrained by the biostratigraphy,we have demonstrated the characteristics and genesis mechanism of different shale lithofacies.The siliceous shale of high TOC is likely attributed to volcanic eruption.The mixed shale of low TOC is likely attributed to the bottom flow intrusion and the argillaceous shale of low TOC is likely due to the enhanced terrigenous input though turbidity current.Thus a genetic model has been established.We suggest that the deposition of the Wufeng Formation and the first member of Longmaxi Formation heterogeneous shale is a sedimentary response to the altered paleo-climate and ocean environment which are attributed to three geological events：volcanic eruption（including glacial and biological extinction）,bottom flow intrusion and terrigenous input.