| The South China Sea,which is called the natural structural laboratory by the geologists, is located in the intersection place of Eurasian plate, Indochina plate and Pacific plate, and it has complicated tectonic evolution history. As one of the most representative extensional basins in the world, the Qiongdongnan basin lies along the north margins of South China Sea, and is characterized by a double-layer structure. Its sedimentary environment has changed from continental to marine from Paleocene to Neocene. This work was conducted by the sequence stratigraphy and the sedimentary basin research theory, based on amount of full and high accuracy geological and geophysical data, and used Geoframe as the main technological means, then built up the district sequence stratigraphic framework in the post rifting stage of the study area. Based on the sequence stratigraphy analysis, this paper firstly recovered the structural subsidence history and the paleotopography of the key surfaces in the post rifting stage, moreover, clarified the control action of paleotopography to sedimentation and the space allocating relation between sedimentary system and paleotopography were revealed intuitive. The main achievements of this dissertation are follows:1. Establishment of the district sequence stratigraphic frameworkAccording to the seismic, well, logging, paleontology and other analysis data, twelve sequence boundaries were recognized in the postrifting strata and the sequence stratigraphic framework was built up. These provide a framework of isochronous stratigraphic correlation for the research of sedimentary filling and source-reservoir-cap assemblage. The post rifting stage of the Qiongdongnan basin began from 21Ma, and is bounded by the break-up unconformity (S60) at the bottom. The filling strata of this stage can be divided into two second-order sequences (the second-order sequenceâ…¡andâ…¢) and eleven third-order sequences. The second-order sequenceâ…¡is bounded by the S60 at the bottom and by S40 at the top, including 5 third-order sequences (â…¡-sy2,â…¡-sy1b,â…¡-sy1a,â…¡-ms2, andâ…¡-ms1). The second-order sequenceâ…¢is bounded by the S40 at the bottom and by S20 at the top, including 6 third-order sequences (â…¢-hl2,â…¢-hl1,â…¢-ygh2c,â…¢-ygh2b,â…¢-ygh2a, andâ…¢-ygh1). 2. Detailed analysis of subsidence mechanism in the post rifting stage Four typical seismic lines perpendicular to structural trend were selected and be used to obtain tectonic subsidence of the study area through back-striping analysis. The tectonic subsidence curves and the average tectonic subsidence rate curves all indicate that two phases fast subsidence were existed. The first period is the thermal subsidence stage from 21 Ma to 17.5Ma; the second period is accelerated subsidence stage from 10.5Ma to 4.2Ma, the maximum average tectonic subsidence rate is 280m/Ma. The characteristics of tectonic subsidence in the Qiongdongnan basin is obviously different from the subsidence pattern of the typical passive continental margin. The study area exist significant anomalous subsidence in the post rifting stage, which range from 300m to 1200m, since 5.5Ma, the value of anomalous subsidence increased evidently. There are three reasons for the anomalous subsidence:the influence of dynamic topography; the effects of the plate's activity surrounding the basin; and strong lithosphere thinning.3. Explanation of the relationship between the subsidence centers and depocenters and their evolutionAccording to the back-stripping analysis of over 100 seismic lines, the characteristics of tectonic subsidence in each third-order sequences has been studied. In the post rifting stage, the centers of subsidence has transported for 4 times, and the main transfer trend is from several centers to single, from west to east. Similarly, the depocenters also changed from several centers to single gradually. In the early post rifting stage, the depocenters distribute in the center of every sag in the western basin. In the Huangliu Formation deposition period, there is only one depocenter located at the Ledong sag in the southwest of the basin, after that, the depocenter always keeps the same position.In the early period of postrifting stage (Sanya and Meishan formations), the centers of subsidence and sedimentation is coincidence, which reflects the control of tectonic subsidence on sedimentation. In the period of Huangliu formation and Ying 2 member, the centers of tectonic subsidence distribute along the central sag zone, however the depocenters migrate toward the southwest of the basin and change from several to single, and separates the tectonic subsidence centers from depocenters gradually. Until the Ying 1 member, these two centers separates completely, the tectonic subsidence centers are located in the eastern basin and the depocenters are located in the western basin. The reasons for this phenomenon are the influence of sediment supply and structural activity.4. Restroration of paleotopography during the post rifting stage intuitively reveals the spatal configure relation between sedimentary systems and paleotopographyIn the Sanya Formation deposition stage, the shape of the basin basically retains the up warping- down warping pattern in the rifting stage, is characterized by zones distributed along north-south and blocks distributed along east-west. Lowstand fan sediments in this period is prone in northern slope near the sediment supply source, which are dominated by slope fans distributing along the central sag zone. These fans developed in the western basin where terrain slope is steeply, and its core features show thick interbeds of siltstones and mudstones. In the Meishan Formation deposition stage, the continental slope relief began to form. Because of weak structural activity, sea level change is taken as the main cause of paleotopography shapes variation. In the Mei 2 member, lowstand sediments mainly consist of slope fan and basin floor fan distributing in the Baodao and Ledong sags, the paleo-uplift in the southern basin is still local exposed.In the Huangliu formation deposition stage, sedimentary environments are pelagic and semi-pelagic environments and the continental slope relief has formed. Slope fans develops in the Huang 2 member and is comparatively rough granularity bearing gravel near the sediment supply source in the western basin; the central canyon controlled by tectonic activity and turbidity current erosion has began to developed in the eastern basin. In the Huang 1 member, less lowstand sediments developes in the basin and the central sag zone shows the semi- pelagic environment. In the Yinggehai formation deposition stage, the continental slope prevails in the whole basin and sediments progradation is faster in the western basin than that in the eastern basin, but in the Shenhu uplift of the eastern basin the shelf slope is broad and gentle. In this period, the lowstand sediments always distribute along the slope-break zones, then form turbidity current by secondary transportations and accumulate to the central canyon.5. Propose depositional models controlling by paleotopography in the postrifting stageDistinguish patterns of slope break develop in different evolution periods of basin. Based on different slope break types, the styles of paleotopography controlling on sedimentary pattern can be divided into three categories, named as tectonic break style, continent slope style and ramp style. During the early stage in the postrifting phase, fault actions control the paleotopography, and asymmetric tectonic action is the main factor causing the difference in local accommodation. Two kinds of break zones occur during this period, they are fault break zone and bend break zone. The accelerated subsidence phase, which is the main stage of continental slope relief forming for the South China Sea in the study area, has typical feature of shelf-slope topography and develop shelf-slope type sequence after S40. The ramp type sequences mainly develop in the area where the structural activity is weak.
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