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)

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.