Sedimentary Sequence And Differential Tectonic Subsidence Since MIS3 In Fuzhou Basin

Fuzhou basin,located on the west side of Taiwan Strait,is a fault basin developed in late Pleistocene.Its formation and evolution are influenced by river,ocean,climate and geological structure.Quaternary marine facies,marine continental transitional facies and continental sediments are widely distributed in the study area,recording a lot of paleoclimate and environmental evolution information,which provides abundant information for the study of paleoenvironmental evolution in the study area.Meanwhile,the sedimentary layer also records rich information about the tectonic evolution of the basin.Fuzhou basin is located in the southeast coastal seismic belt,which is one of the five major seismic belts in China,and there are still potential shallow earthquakes with magnitude 6in the area.Therefore,it is of great significance to strengthen the study of sedimentary environment evolution in the area and structural evolution information in the basin for urban economy,resource development and building safety.With the support of the National Natural Science Foundation of China,Fuzhou Survey Institute and Fujian Geological Survey Institute,913 boreholes from 25 sites were collected.In this study,taking the ZKCS and ZK004 as representative borehole,AMS¹⁴C and OSL dating tests and detailed lithology identification,diatom identification,grain size measurements,geochemical parameters,loss on ignition and electric conductivity were carried out to divide the sedimentary facies and construct the transgressive-regression sequence since MIS3 of the basin.On this basis,the distribution characteristics of the two transgressive layers in the basin of MIS3 and Holocene were extracted by integrating all the drilling data collected in the basin.Using SPSS correlation analysis and ArcGIS inverse distance weight interpolation analysis,combined with the distribution of proven fault zones in the area,to explore the different tectonic subsidence activities and the migration trend of sedimentary center in the basin.Finally,combined with the reconstructed global sea-level change and global ice evolution,the main reasons for the two period of transgressions are explored.Then,obtains the following conclusions:1)Based on the lithology analysis,grain size measurements,geochemical parameters,loss on ignition and electric conductivity,combined with diatom identification of representative borehole ZKCS,it can be divided into seven sedimentary facies:fluvial facies,lacustrine facies and lakeshore facies,gulf facies,fluvial facies,coastal marsh facies,shallow marine facies,lacustrine facies,which constitute two complete transgression-regression cycles.2)Based on the AMS¹⁴C and OSL geochronology of representative core,and the comparison with the existing studies,it is confirmed that ZKCS borehole recorded two transgressions in the basin.The first transgression occurred in the early MIS3 stage,corresponding to the"Fuzhou transgression",the second transgression occurred in the Holocene,corresponding to the"Changle transgression",which is consistent with the transgression layer in the basin constructed by predecessors.The Fuzhou transgression intensity of MIS3 is smaller than that of Holocene.3)The buried depth,thickness and other parameter information of the MIS3 and Holocene transgressive layers in nearly 1000 boreholes in the basin are extracted,and use the SPSS correlation analysis and ArcGIS inverse distance weight interpolation analysis.The results show that the Gushan piedmont fault was active before the MIS3 transgression.During the MIS3 transgression to the Holocene transgression,there are differential tectonic subsidence occurred in the basin due to the influence of Bayi Reservoir Shanggan fault in the study area.There has been no differential tectonic subsidence in the basin since Holocene.The sedimentary center of the basin moved westward.Through comparative analysis of the distribution of transgressive silt layers in the same horizon,it is found that there are different subsidence activities between the north and south of Minjiang River,with relative uplift in the South and relative subsidence in the north.4)The"Changle transgression"is mainly related to the climate warming in the post glacial period.The main reason for the"Fuzhou transgression"is related to the tectonic activity of the basin.Due to the different subsidence activities in the north and south of Minjiang River,the ending time of MIS3 Fuzhou transgressive layer in different positions in the basin is quite different.In the later stage of sedimentation,the sedimentary thickness and burial depth of MIS3 Fuzhou transgressive layer in the basin are inconsistent due to the different tectonic subsidence activities caused by the internal fault activities in the basin.