Quaternary Seismic Sequence And Stratigraphy In The Central South Yellow Sea Basin

The Yellow Sea is a semi-enclosed shallow shelf sea located in the eastern shelf ofChina, and its shelf is a good target area for studying Quaternary environmentalchanges. In the Yellow Sea, plenty of material supply came from the Yangtze, Yellowand other small rivers; tectonic subsidence made abundant space to accommodatesediment; and global climate cycles caused frequent and large sea-level fluctuation.These internal and external agents worked together with the complex hydrodynamicsystem in the formation of strata containing abundant information of land, ocean andtheir interaction. In this study, we acquire shallow seismic profiles and some core dataacquired and collected in previous studies are analyzed to illustrate the structure anddistribution pattern, the chronological framework, rebuild the evolution mode ofQuaternary stratigraphy in central South Yellow Sea Basin (SYSB).In central SYSB,14seismic reflective interfaces are detected by using seismicstratigraphy, corresponding to a series of conformal and unconformable surfaces.These surfaces are used to divide the strata into13major seismic stratigraphic units(SU1~13in descending order). The seismic stratigraphic age is calibrated by the agedata of the related core, and these13seismic units were formed since1.95Ma B.P..The Quaternary stratigraphy in central SYSB are characterized by marine-terrigenousalternate deposition and well correlated to the “three-uplift and two-basin” tectonicpattern. Quaternary strata in the study region has an average thickness of91.2m, andthe average thickness of the Lower Pleistocene, Middle Pleistocene, UpperPleistocene and Holocene is11.8m,38.1m,34.9m and5.7m respectively.Sea-level fluctuation is considered as a key factor of causing the marine-terrigenousalternate deposition mode. A series of unconformable interfaces resulting fromsea-level fluctuations make up the framework of Quaternary strata. The scale and of sea-level fluctuation determines the scale of the strata to some extent. The scale ofmarine strata developed before0.9Ma B.P. is not large and mostly littoral sedimentcorresponding to the dominant glacial cycle represented by the high-frequency andlow-amplitude sea level fluctuation. While after128ka B.P., responding to the sealevel fluctuation in the low-frequency and high-amplitude, large-scale marine stratahave been formed. However, during900-128ka B.P., several sea-level highstands didnot lead to large-scale development of marine strata, and this unusual deviationbetween stratigraphic development and sea-level fluctuations should be caused bytectonic movements.Tectonic movement and its inherited factors controlled the development anddistribution mode of the strata in the study region before the late Pleistocene. TheZhemin-Lingnan Uplift has been a major obstacle on the seawater transgressivechannel before the Late Pleistocene. A synchronal relative uplifting occurred in theSouth Yellow Sea Middle Uplift (MU) before the late Pleistocene, and this upliftinggradually had been weakened in the Middle Pleistocene and stopped before the LatePleistocene. Before the Late Pleistocene, the strata in the central SYSB presents the“thick in the north and south and thin in the middle” pattern, while since the LatePleistocene, the development and distribution of strata could no longer bounded bythe MU.In the study area, the major sedimentary sources are the river material, while theYellow River is the greatest one in all the rivers contributing to the sediments in studyarea. Because river material contributed less than65.9%,55.9%and69.0%respectively of the strata formed during MIS1, MIS3and MIS5, the material from theerosion, transform and redistribution of the seabed also are the important sedimentarysource. Ocean power is a key factor in the process of sediment transport,transformation and deposition, however, its effect was not revealed until the twohighstand periods occurred since the late Pleistocene. The average deposition rate ofSU1~13respectively is54.6、46.8、30.2、6.6、9.2、3.5and5.1cm/ka, which display a sharp increasing of deposition rate. This phenomenon not only relate to the growthof sediment discharge of the Yellow River and Changjiang River causing by theTibetan Plateau uplift and the special warm climate in MIS5e, but also relate to theability strengthen of the marine dynamic system in sediment transport andredistribution causing by the stable high sea level.The development and evolution of Quaternary strata in the study region was mainlycontrolled by sea-level fluctuations and tectonic evolutions; in addition, sedimentsupply and marine dynamic environment also played important roles. The dominantfactors affecting strata development and distributrion have been changed at thebeginning of the Late Pleistocene: tectonic movements before the late Pleistocene, butsea-level fluctuations and ocean hydrodynamics since the late Pleistocene.