Studies Of Transgressive Process And Transgressive System Tract On Continental Shelf Of The East China Sea

The continental shelf of the East China Sea (ECS) is characterized by shallow water, flat grade, sediments supplying enough and complex oceanic dynamics environments.The work of the thesis is mainly based on the formers' datum and studies and is processed earnestly and cautiously. The initial purpose is to discuss transgressive proceeding and the growing model and the sedimental principle of the transgressive system tract since last glacial maximum. But on the way to gain the parameters needed in modeling, a series of important options and conclusions, some maybe resolve existing scientific disputation on East China Sea sea-level changes and tectonic subsidence, emerges. These bypassing product include: (l)The mainly differences between Feng-Jin curve and Eustatic curve are dominated by rifting and tectonic subsidence; (2) The rifting is the archcriminal which has been disguising us from really known the location of the lowest sea-level during the Last Glacial Maximum(LGM) for a long time. The rifting make the ECS shelf below 150m water-depth fall at least 30m, which is the reason that Chinese scholars have been defining the belt between 150m and 160m water-depth ,corresponding time is 17cal.ka BP, as the location of the lowest sea-level of ECS. Through rifting analysis, proving that the lowest sea-level of ECS during LGM locates near present water-depth 140m; (3)the existing of shell layer, terrace and rifting company with relative sea-level indicated by shell layer near the water-depth 150m imply that the belt between150m and 160m is a relative height belt which is submerged just about 17cal.ka B.P.. Before it is submerged, the belt maybe is a natural shield between East China Sea and Okinawa trough, which may play important role in trapping and capturing sediment transmitted paleo-rivers in East China Sea shelf and its hinterland; (4)Water-isostacy results in relative sea-level changes; (5) Tectonic subsidence is background subsidence which regularly changes with water-depth and is an important factorpreventing sign of paleo-coastline from indicating the real changes in sea-level of East China Sea. The tectonic hinge-line of East China Sea shelf locates in water-depth near 50m and the tectonic subsidence center locates in water-depth near 110m; (6)Through analyzing and summarizing based on formers' datum, the author proposes three underwater sediment transmission models which change with water-depth. Those models are I large-scale river estuary seaward radiation sediment transmission model, II longshore diffuse and small-scale river runoff sediment transmission model and HI bedrock coast none sedimentation or weak sedimentation model.Through analyzing and disambiguating local geo-factor of East China Sea shelf, the parameters needed in modeling process of transgression and transgressive system tract are gained. Under a series of simplified conditions, relative complex but ordered transgressive process and transgressive system tract is modeled. On the base of contrasting modeling result with real stratum profile, analyzing and discussing on the possible growing model of transgressive system tract and it's sediment mechanism in East China Sea shelf are developed. Thus, some basic knowledge established on the knowledge structure mastered by the author is gained: (7) negative landform is filled preferentially, and filling effect is bigger than the result of modeling. The reason maybe lie in inherited filling which fills the depositional basin with sediments follows habitual filling but before the graded stream profile in the catchment basin finishing it's isostatic adjustment The sediment compaction resulted by sediment load is the main reason that makes the depositional areas keep on receiving sediment and results in relative thick stratum; (8) In the process of transgression, the inherited physiography of a basin is another main factor following the regime parameters such as sea-level change, tectonic subsidence and sediment supply, even acts at the same as them in controlling the sedimentary architecture. It's gradient decides sea dynamic setting directly and then decides itself sedimentary characters; (9)The physiograpy of East China Sea shelf controls the characters and architecture of transgressive stratum: a, high gradient causes high power water dynamic setting and then causes no transgressive sediment system or no seismic respondence in the setting, b, Low gradient shelf areas are onlaped extensively by landward-stepping transgressive sediment stratum. Sheet-like sand layers formed in the marine environment overlaps on the coastal plain faces. Sand bars and sand ridges formed inthe background of low gradient shelf are drown in place in the condition of sea-level rising rapidly, which proved the foundation for the extensively distributive tidal sand ridges on East China Sea shelf; (10) Since 7ka B.P., eustacy relatively halt. A great volume of sediment are gradually transmitted into the sea-basin, like revendication, sediment superposed transgressive tract formed in advance in style of aggradation and progradation in succession. Thus land expands seaward. At the same time, condense section are forming in pelagic sea because of sediment undercompensation.