Sedimentary Records And Source To Sink Process In The Northern Part Of Liaodong Bay Since 1.2 Ma

Located in the north of the Bohai Sea,Liaodong Bay is the largest bay of the Bohai Sea and also a semi-closed bay at the highest latitude of China.The Quaternary geology of Liaodong Bay is an important component of the environmental evolution of the Bohai Sea.Recently,increasing research attention has been devoted to the Quaternary geology of Liaodong Bay.Due to the lack of long sequence sedimentary boreholes in Liaodong Bay,there is an imbalance in the regional data of the Bohai Sea,impacting upon the understanding of the evolutionary process of the sedimentary environment in the Bohai Sea and its surrounding areas.In recent years,the Quaternary geology of Liaodong Bay has received more and more attention,but there are three questions that need to be solved.First of all,previous studies have found that three transgressions occurred in Liaodong Bay in MIS5,MIS3 and MIS1 respectively.How to explain this result is inconsistent with the global sea level change trend?Secondly,where are the sources of sediments in the northern part of Liaodong Bay?The Yellow River material plays a dominant role in the distribution of sediments in the Bohai Sea.Is it possible that the Liaodong Bay will become a remote deposition area of the Yellow River material?Thirdly,what factors control the source to sink process of Liaodong Bay?Neotectonic movement is active on the east side of Liaodong Bay.What is the relationship between neotectonic movement and provenance and sea level changes?The Liaozhong depression in the middle of Liaodong Bay is the deepest area in the Bohai Sea except for the Bohai Strait.The thickness of the Quaternary is the thinnest area in the Bohai Sea,and the west is only about 200 m.The Tanlu fault zone runs through the eastern part of the Liaodong Bay and has many activities during the late Middle Pleistocene-early late Pleistocene and late late Pleistocene-early Holocene.This makes Liaodong Bay an ideal place to solve the above problems.This study uses the JXC-1 and YKC-2 boreholes drilled in the north of Liaodong Bay as the research object.The core sediments taken from the boreholes of JXC-1 and YKC-2 are dated with AMS¹⁴C and OSL,in addition to grain size analysis,magnetostraitigraphy,foraminifera and pollen identification,lithological characteristics and other testing results.Based on the paleomagnetic test results and dating results,this studied constructed a chronological framework for holes JXC-1and YKC-2 in the north of Liaodong Bay.Based on the identification of micropaleontology,the transgressions of the two boreholes were identified,and the sedimentary environment was reconstructed by combining the regional stratigraphic division and the deep-sea oxygen isotope change curve.The factor analysis of the major and trace elements in the sediments of hole JXC-1 was carried out,and the vertical changes of the rare earth element differentiation parameters were compared and analyzed,and the source response changes of different sedimentary environments were studied.Based on the single-channel seismic profile and sub-bottom seismic profile,the structural response mechanism of differential deposits in the study area is analyzed.Through research,several new understandings have been obtained,which are summarized as follows:(1)The paleomagnetic research results show that the JXC-1 hole recorded the deposition between Brunhes and the upper part of Matsuyama,and the age of the bottom of the borehole was about 1.2 Ma.The MBB is located at 45.18 m,and the depth of Jaramillo is 61.6?66.2 m in the JXC-1 borehole.The YKC-2 borehole mainly records the deposition of the upper part of Brunhes.Both of the results is consistent with the research results that the thickness of the Quaternary strata in the Liaodong Bay is only about 200m and the thickness of the Holocene strata gradually increased from southwest to northeast.(2)Influenced by climate fluctuation,large-scale transgressions occurred in the northwest of Liaodong Bay since the end of Middle Pleistocene,with three transgressions from the end of the Middle Pleistocene to the Holocene.The ages of the three transgressions from old to new are MIS7,MIS5,and MIS1,consistent with global sea level changes.The YKC-2 borehole in the northeastern of Liaodong Bay recorded the sedimentary environment since 150 ka B.P.,including two transgressions,which occurred in MIS5 and MIS1 from the old to the new.The two results of this study are different from previous researches in the Bohai Sea,which found three times of transgressions in MIS5,MIS3,and MIS1 respectively.There are two possibilities,either no transgression in the Bohai Sea,or the transgression did not enter Liaodong Bay during the time of MIS3.(3)Liaodong Bay has been a mixed sedimentary area since 1.2 Ma,and the sources of sediment vary significantly under different sedimentary environments.During the transgressive period since the end of the Middle Pleistocene,the Yellow River has been the main carrier of material transported into the sea.From the late Early Pleistocene to the mid-Middle Pleistocene,it was a lacustrine sedimentary environment.The high-water stage sediments mainly came from the Yellow River,and the low-water stage sediments mainly came from the Liaohe and Daliaohe.The mid-Pleistocene to the end of the Middle Pleistocene is a fluvial sedimentary environment with complex provenance,but the sediments on the depth of 32.40-25.70m is obviously affected by the Liaohe and Daliaohe rivers.All these studies suggest that the sediments coming from the Yellow River may be distributed northward up to40.5°N.(4)Since the end of Middle Pleistocene,the sedimentary thickness in the northern part of Liaodong Bay has been significantly differentiated controlled by two factors:provenance and neotectonic movement.On the west side,the provenance is dominated by the fine-grained material carried by the Bohai Sea current,and the sediment supply is insufficient,and it has been in a state of"starvation"for a long time;the control basin fracture is stable.On the east side,the provenance are dominated by near-source rivers with sufficient supply;the inheritant activities of the Tan-Lu fault zone controlled the accommodation space in the area.Affected by the early strong extensional background in the Bohai Sea,the Liaozhong sag,where the YKC-2 borehole is located,is located in the descending block of the Liaozhong No.1fault and the Liaozhong No.2 fault(normal fault in nature).Since Middle Pleistocene,the accommodation space has continued increasing,and the sag received a large amount of river sediments from the provenance near sources.The outcomes of this study will provide an important reference to the Quaternary geological events and processes in the Bohai Sea and their connections with the material migration of the Yellow River,as well as the circulation system in the Bohai Sea.