The Study Of Sedimentary Response To The Late Ordovician And Early Silurian Tectonic Transition Process In Dabashan Area

The Dabashan area located in the northern Sichuan basin tectonically belongs to the joint region between the upper Yangtze block of the Yangtze blocks and Qinling arc-basin system of Qin-Qi-Kun orogenic belt,which mainly is composed of southern thrust belt of Micangshan-Dabashan basement and northern rift belt of South Qinling passive continental margin,and is adjacent to the Mesozoic foreland basin of middle Sichuan.Therefore,the Dabashan region is the important window to study the early Paleozioc Qinling ocean(also called Shangdan ocean)between the Yangtze and North China block and orogenic process of Qinling collision as the nowadays basin-mountain transformation zone from Sichuan basin to Qinling orogenic belt.The sedimentary basin in the northern margin of the Yangtze block recorded the entire process of the open and closure of Paleo-Qinling ocean and Qinling collision orogeny during the early Paleozoic.The Dabashan region was undergoing continuous subduction of Paleo-Qinling ocean,and local extension in the southern margin of the North Qinling which resulted in a short-lived continental rift in the Yangtze passive continental margin and large-scale magmtic activities such as basic-rock intrusion and alkaline volcanic eruption.The tectonically transitional process would undoubtedly bring important and profound influence on the late Ordovician-early Silurian depositional basin properties,filling process and sedimentary facies distribution,even biological evolution and redox environment of sedimentary watermass during the deposional period.Through comprehensive studies of the late Ordovician-early Silurian strata to the Paleontology,lithosfacies,sedimentary facies,ourcrop sequence strata with isotopic and lithogeochemisty,isotopic chronology and geochemisty of volcanic rocks in the Dabashan area,some primary results and findings are achieved in this paper.Systematically sedimentary studies of the late Ordovician-early Silurian strata such as outcrop sequence stara,lithosfacies,sedimentary facies,conbined with Paleontologic characters and field detailed observation were developed in South Dabashan area,it is thought that two distinct regression events ever occurred in the South Dabashan during the late Ordovician-early Silurian period: Linxiang regression event(NRE)between Linxiang Formation and Wufeng Formation and Hirnantian regression event(HRE)between Wufeng Formation and Guangyinchiao member,and these two regression event could be compared regionally.The mainly control factors to the two regression events are analyzed and their dynamic mechanism is discussed according to regionally geologic setting.Zircon U-Pb ages of alkaline bentonites in this area are firstly obtained,and their tectonic environment are defined according to regionally geologic setting,alkaline bentonites older than 450 Ma stemed from island arc-genesis volcanic material and were related to the Yangtze block subduction to the North China block;however,those younger than 450 Ma originated from volcanic eruption of inner-plate tectonic environment and had to do with volcanic eruption of continental rift in the context of local extention in South Qinling during plated subduction period.After statistics of published ages of alkaline bentonites from the peripheral Yangtze blocks and the southern North China,it considered that there was a long and continous period of volcanic eruption from early Ordovician Tremadocian to late Silurian Ludfordian,but mainly focused on three periods of late Ordovician Sandbian and Katian and early Silurian Rhuddanian.Stable isotopic and lithogeochemical study are systematically developed and regionally correlated from late Ordovician Wufeng Formation and early Silurian Longmachi Formation,and sedimentary evolution model during the late Ordovician and early Silurian transition is built in South Dabashan.Stable isotopes such as carbon,sulfate sulfur and nitrogen in South Dabashan area like other places in South China demonstrate that isotopic excursions in Hirnantian Guanyingchiao Member generally take place,which is temporally identical to Hirnantian glaciation.Depositional bottom watermass of Wufeng Formation and Longmachi Formation was subjected to an anoxic-suboxic(oxic)-anoxic transition.It was a strong stratificated and anoxic environment during the Wufeng Formation and Longmachi Formation,otherwise,a weak stratificated and oxic or local suboxic environment during the Guanyingchiao Member which was related to metal particles stemed from seafloor volcanic eruption in passive continental margin resulted in mantle plume and brought by Paleo-Qinling ocean current to depositional area in South Dabashan.On the basis of previous researches and the combination of latest datum of isotopic chronologies and lithosgeochemistries,tectonic evolution model during early Paleozoic is reestablished in northern margin of the Yangtze block.Based on this,sedimentary evolution model during late Ordovician and early Silurian transition is reconstructed in South Dabashan.Anoxic environment is mostly dominated by seafloor relief,glacial sea-level eustasy and uprising ocean current.These results in this paper not only have an importantly geological significance to sedimentary and tectonic evolution during early Paleozoic in the basin-mountain transitional region of northern margin of the Yangtze block,but also an vast theoretical significance to the shale-gas exploration of late Ordovician Wufeng Formation and early Silurian Longmachi Formation in Dabashan area.