Characteristics Of Sea Level Change In The South China Sea Since 18.5Ma

As an important balance surface of the Earth,sea level has important guiding significance for exploring the causes and mechanisms of the earth's tectonic evolution,studying the distribution law of natural mineral resources,and predicting the future trend of sea level change;By understanding the laws of changes in sea level changes in the past,we can "explore the future with the ancient theory;" At the same time,the global sea level change curve drawn by Haq does not use the data of the South China Sea,so the curve has certain defects.Based on this,this paper uses the drilling data and seismic data of the Zhujiangkou Basin in the northern South China Sea and the Zengmu Basin in the southern South China Sea to identify and calibrate the main sequence interface,and divide the sequence,According to the sedimentary facies change of the drilling sequence and the position of the uplap-point moves on the seismic section.Using the phase sequence method and the uplap-point method,At the same time,combined with the sea level change curve made by the predecessors,qualitatively compounding the sea level curve of the South China Sea since 18.5Ma,and the curve characteristics are described;Then,the characteristics of the special geological events are described in combination with the structure and climate;Finally,the similarity and difference and their causes are analyzed in comparison with Haq's global sea level curve,oxygen isotope curve and sea level curve in other parts of the South China Sea.Through the above work,the following understanding is obtained:(1)PY33-1-1 drilling 21~10.5Ma identified 7 third-order sequence boundaries and divided 6 third-order sequences.Each drilling sequence developed a Transgression systems tract and a Highstand systems tract.Since 21 Ma,the seismic profiles of the northern and southern parts of the South China Sea have identified 17 third-order sequence boundaries,dividing 16 third-order sequences.(2)The characteristics of the sea level change curve in the South China Sea have been rising step by step since the 18.5Ma composite production.The overall second-order sea level change cycle has a time limit of 18.5~13.8Ma,13.8~8.2Ma and 8.2~0Ma,respectively.15 complete plus an incomplete cycle third-order sea level lifting cycle,each level third-order sea level lifting cycle is first rise and then fal,but the difference between the rise and fall.(3)The sea level of the South China Sea reached the maximum transgression at 15.5 Ma,and then the sea level decreased,which was related to the end of the Antarctic expansion;the sea level dropped to the lowest at 13.8 Ma,corresponding to the occurrence of MMCT events,climate quenching,and Antarctic ice sheet formation events;At 8.2 Ma,the maximum transgression position was reached again,and then the sea level fell again,corresponding to the Xishan movement and the Arctic ice sheet formation event;after 3.0Ma,the sea level rise and fall into the oscillation period,which is considered to be related to the alternation of the ice period and the interglacial period.,(4)Compared with the Haq global sea level curve and the oxygen isotope curve,there are differences between the first sea level and the second sea level in the South China Sea.This difference is the result of special tectonic movements in the area since the South China Sea since 32 Ma;The retreat-seaward cycle of the third-order sequence is globally comparable,and the third-order sea level change in the South China Sea is not affected by tectonic movement,but is the result of global glacier action.Compared with the sea level change curves in other parts of the South China Sea,it is concluded that the first-order sea level is an overall rise;There is a difference between the number of sea level rise and fall times and the time point of the second-order sea level descent event.The second-order sea level change trend also differs between 18.5Ma~13.8Ma and 13.8Ma~8.2Ma,which may be related to the tectonic movement,sedimentation,the method of making each curve and the data used in the region.There are also differences in the number of sea level rise and fall cycles of the third-level sea level;In this paper,the number of cycles is the most,and the curve accuracy is higher.