Characteristics And Controlling Factors On Gravity Flow Sedimentary Systems Development In Deep Water Area, Northwestern South China Sea

With the development of sedimentology and oil-gas exploration in deep water area in recent years, deep-water sedimentology has attracted much attention of sedimentologists and petroleum geologists. Based on the integration among seismic, core and logging data, this thesis focuses on the gravity flow sedimentary systems in Yinggehai and Qiongdongnan Basins(YQB), northwestern South China Sea (NSCS), especially on the sedimentary features and distribution of submarine fans and submarine channels, by using comprehensive methods of sequence stratigraphy, seismic sedimentology and deep-water sedimentary dynamics. Subsequently, the control factors on gravity flow sedimentary systems development, including relative sea level, tectonics and provenance system, and the developing processes of gravity flow sedimentary systems in NSCS are discussed. The several key findings are listed as follows:1. By the integration of seismic-log-core, the sedimentary characteristics of submarine fans is illustrated and five types of submarine fans are distinguished, including north multiple-source sand-rich submarine fan(NMSSF), south multiple-source sand-rich submarine fan(SMSSF), north multiple-source mud-rich submarine fan(NMMSF), west point-source sand-rich submarine fan(WPSSF) and composite-source submarine fan(CSF). The NMSSF mainly develops in the northern YQB in Early-Middle Miocene, and the SMSSF in southern YQB in Early Miocene, the NMMSF in northern YQB in most late Miocene and Pliocene, whereas the WPSSF only occurs in middle Pliocene and the CSF only deposits in late Miocene, southwestern QDNB.2, After the framework of high resolution sequence stratigraphy are set up, the evolution processes of submarine fan system in deep water area of the YQB are divided into three stages: (1)the sand-rich submarine fan stage in Early Miocene-Middle Miocene; (2)the sand-rich to mud-rich submarine fan stage in late Miocene and (3)the mud-rich submarine fan stage in Pliocene. In Early Miocene-Middle Miocene, numerous NMSSF and SMSSF develop in the basin, and they are characterized of large scale and sand-rich, both of them decrease as time goes on. In late Miocene, the submarine fans are characterized of transformation from the sand-rich ones to mud-rich ones. In Pliocene, a large numbers of mud-rich submarine fans develop with bread and thin beds, somewhere reformed by mass transport complexes (MTCs).3. The initial of the Central Canyon is confirmed at 11.6 Ma by boreholes, and it develops as heandless submarine channel. Most of the filled turbidite sediments are confirmed to be sourced from Vietnam provenance.4. Seismic sedimentology is applied for the research on the erosion and filling processes of the Central Canyon and the evolution process of the submarine channel system are recovered. During 8.2-5.7 Ma, the submarine channel system is composed of several small erosional channels from western Hainan Island, and then they converge into a main channel going through the YQB. When the channel flows through the slope, the erosional ability of turbidity flow increases resulting in a "V-shaped" transverse section and turbidity sediments finally deposit in Lingshui Depression. Mud debris flow from shelf of Hainan Island make the filling of the Central Canyon complexed in Songnan and Baodao Depressions, QDNB. During 5.7-4.2 Ma, the morphology of submarine channel system is similar, but the scale of erosionnal and depositional is smaller with a "box-shaped" transverse section. The turbidity sediments deposits backward to the Ledong Depression, QDNB and local erosional scale increases drastically with southern channels flowing into it. During 4.2-3.7 Ma, the submarine channel system activate as a type of "two-section, multiple-source", and finally the turbidity sediments deposit in Lingshui Depression, QDNB as a submarine fan with little erosion to underlie stratum during 5.7-4.2 Ma. In summary, the submarine channel systems atrophies from 8.2-4.2 Ma both in the scale of erosion and deposition.5. The tectonic activities in the basins are analyzed carefully using 2D seismic data, and the controlling of tectonic activities on gravity flow sedimentary system are discussed. The tectonic activities control the gravity flow sedimentary system in deepwater area of the YQB in three ways:continental shelf width, continental shelf gradient and accomondation. The No.1 fault activities in the Miocene in YGHB result in the formation of narrow shelf in this area, and control the growth of submarine fans and deepwater channels in the Yingghai Basin in Miocene. The No. 1 fault activities in the QDNB in the late Miocene construct the slope from YGB to QDNB, and supply a accomondation for the great submarine fan in the Ledong Depression. Multiple normal faults in northern Qiongdongnan Basin control the formation of high-angle continental shelf in north of the basin, which supply topographic condition for the submarine fans in the early Miocene and middle Miocene. No.2 fault activates as blind fault in Songnan Depression and Baodao Depression during early Miocene and middle Miocene control the steep slope, and result in the formation of a slope tilting to the north which supply topographic condition for axial transportation of submarine channels along the Qiongdongnan Basin.6. Based on the study of relative sea level changes in YQB, the controlling of relative sea level on gravity flow sedimentary system in long period and short period are discussed. The increasing of related sea level in the Miocene controls the transformation of sand-rich to mud-rich submarine fan systems in the basins. The low relative sea level in third-order sequence is beneficial to the transportation of terrestrial sediments to outer shelf and even to slope through rivers, and then form submarine fan sediments in the type of gravity flow deposits. In addition, the compulsory regressive event in late Miocene controls sediments transporting to the Ledong Depresion, QDNB from Vietnam and rivers of western Hainan Island extend to the slope directly.7. Synthesizing study results of provenance and deposits in the basin, we discussion the characteristics of provenance in NSCS and the controlling on gravity flow sedimentary system. The northwestern South China Sea is affected by various provenances:During early to late Miocene, the basin is mainly dominated by the Hainan Island, which controls the development of submarine fans depositing in northern part of the basin; During late Miocene, the enhancing of Vietnam provenance results in depositing of channel complexes in southern part of YGHB and large submarine fan in western part of QDNB; During Pliocene, the accelerating of sedimentation rate of Hainan Island controls development of submarine fan in northern part of QDNB.8. Characteristics of gravity flow deposits in various phase are summarized systematicly and the history of gravity flow sedimentary systems development is reconstructed as three phases:(1) sand-rich submarine fan dominate stage in Early-Middle Miocene, which is characterized by numerous sand-rich submarine fans developing; (2) submarine channel dominate stage in late Miocene, in which only a small number of submarine fans develop, and the YQB is dominated by a great submarine channel system and (3) submarine channels atrophying and mud-rich submarine fans dominate stage.