Internal Architectures And Depositional Model Of The Pearl River Submarine Canyon System

With the development of study on deep-water depositional system and hydrocarbon exploration in recent years, deep-water canyon system has attracted intense attention. A large number of related researches have reported on deep-water canyon system worldwide, but it is relatively weak in China in this area. The deep-water canyon which acts as the main conduit transporting sediments from shallow water to deep water is widely developed in the passive and active continental margins. The formation and evolution of canyon system record the information of ancient climate, sea level change and regional tectonic evolution, attracting broad attention by marine geologists. Based on the integration of core, logging and seismic data, this thesis describes the morphology characteristics and internal architectures of the Pearl River Submarine Canyon System (PRSC) in Pearl River Mouth Basin (PRMB), South China Sea, by using comprehensive methods of sequence stratigraphy, sedimentology and geophysical techniques. Subsequently, the relationship among depositional features of the canyon, sea level changes, tectonic background, micro-topographic features and sedimentary supply are discussed in order to unravel the forming mechanism of the PRSC. Together with the comparison of characters of different segments of the PRSC,3 stages were identified, and finally, the erosion-filling process of the PRSC was rebuilt. The main results in the thesis are as follows:1. The PRSC in South China Sea, occurs in the deep-water environment, and displays an "S-shaped" geometry with a length of 254 km from the northern upper reach in Baiyun Sag in the PRMB to the southern terminations in northern margins of Northwest Sub-basin of South China Sea. Results show that the PRSC can be separated into three segments with different orientations. The upper reach is NW-oriented with a shallowly incised course, whereas the middle and lower reaches, that are located mainly in the Baiyun Sag, have a broad U-shape and have experienced consistent deposition. Seventeen deeply-cut canyons have developed in the slope north of the Baiyun Sag, playing an important role in the sedimentary processes of the middle and lower reaches of the Pearl River Canyon. These canyon are NNW-oriented, Length of 30-60 km, width of 1-5.7 km, depth of about 50-300 m, have smooth and steep canyon walls on both sides. Filling Features of the buried Channels show the obvious ENE direction migration, indicate that these canyons may undergo a long-term, continuing rework of the bottom current which from the west to the east, leading to the canyon the mandatory migration to the east since 11.6 Ma. Quantitative statistical canyon geometry seven elements:Valley Length, Channel Length, Amplitude, Meander Wavelength, Meander Radius of Curvature, Average Width, Sinuosity. Thus characterize the plane shape of the canyons system.3 morphological types of profiles are identified:U-shape (US), V-shape (VS) and multiple-shape (MS). The well and seismic data were combined to calibrate the forming time of the PRSC.2. The infillings of the PRSC were composed of 2 main sedimentary types:turbidities and mass transport deposits (MTDs). Utilizing some geophysical techniques, some depositional units can be identified, such as turbidities channel, residual channel sand, branch channels, sheet sands, MTDs, deep-sea fine-grained sediments, collapse and so on. In different segments and different periods, the internal architecture and sedimentary composition were different.3. Three major development stages and its relative depositional models of the PRSC could be established.1). Early stage (23-21 Ma), canyons scarcely developed, main deposited were turbidities fan and mass transport deposits; 2). Medium stage (21-11.6Ma), canyons began to develop, gradually erosion the continental slope, forming a wide U-shaped valley, and fan deposition occurred at the end of the deep canyons. Upstream segment developed turbidities channel deposition, thalweg deposits, slumping. Downstream segment developed turbidities fan deposits.3). Later stage (11.6-OMa), diminished erosion capacity of canyon, then the main canyon almost buried by the MTDs.4. Summarized the main control factors of the PRSC. The main control factors are Cenozoic tectonic activity, sea level change, provenance system, palaeogeomorphology and current. ①The Cenozoic tectonic activity is a key control factor, it not only controlled the palaeogeomorphology, but also controlled the extension direction of partly slope canyon. The spreading of SCS began at 30Ma, caused the detrital material of terrigenous origin transported from continental shelf to submarine basin, the continental slope has been eroded and shaped many small linear channels during the transportation. In the passive continental margin, channel usually formed some channels like the rivers on land as the slope gradient decreased. Moreover, the layers under the head of the canyon and slope canyons of the northern Baiyun sag exist several faults, part of the faults are located in the buried channel or below the current valley. These faults have some direct relationship with the buried channel, the major tectonic units made by NS-SE faults may affect the deep water erosion and the formation of the canyon or channel in the study area. ②The continental margin basin of northern SCS has massive regional sea level decrease at 23 Ma,15.5 Ma,11.6 Ma,5.5 Ma,2.6 Ma, lead to the erosion of slope canyon at lowstand deposition period, conducive to develop the shelf incised canyon system which is made by upper slope complex canyons-middle slope main canyon-lower slope and abyssal plain submarine fan. Therefore, every regional sea level decrease corresponds to canyon erosion, the only difference is the erosion degree, it also an important factor to control the development of the canyon system. ③ The importation of terrigenous sediment comes from the Pearl River effected the formation and evolution of the canyon. The test of neodymium isotopes in the core which comes from the Voyage ODP184, found out that it has the similar sedimentation with PRD, and the provenance may comes from South China. These features not only illustrates the PRD provides mass sedimentation, but also these sedimentation can transport long distance.④The seafloor palaeotopography also has affection on the development of canyon system. The presence of the seafloor uplift or sea mountain can change the path of erosion and the morphological characteristics and evolution process of the canyon. From the seismic data, we found out that the submarine topography today determines the development size and path, also the spatial distribution of each sedimentary unit. ⑤The current of slope of northern SCS effect the filling of canyon. The directed migration of the canyons in the head of the Pearl River Submarine Canyon related to the middle ocean current which goes from west to east. The middle ocean current played a certain role in the filling and evolution.5. Three evolutionary phases of the PRSC were classified since Early Miocene.① Initial development phase (23-21 Ma), channel began to develop but canyon does not exist, submarine fan is the feature, constituted the channel-submarine fan pattern. In the Early Miocene, appeared two small channels in the lower slope area, since the suddenly northward migration of the shelf break because the Baiyun movement at 21 Ma, these canyons were soon covered by the turbidities fans and MTDs. ②Complex channel-submarine fan phase (21-11.6 Ma), constituted the upper slope complex channels-middle slope simple canyon-lower slope and abyssal plain fan pattern. Only MTDs and turbidities fans and some small channels on the surface of fans occurred since 21 Ma. Until the Middle Miocene (15.5 Ma), the incised channel appeared again, and it has the similar morphology of the modern PRSC. The regional sea level fall again, a lowstands period, the erosion of canyon and the sediment transport volume reached the maximum. The sediment delivered from the shelf directly to the deep sea basin through these canyons and formation of a massive turbidities fans system.③ Submarine canyon-submarine fan phase (11.6-0 Ma), developed upper slope complex channels-middle-lower slope canyon-submarine fan from Pliocene to now, affected by rich sediment supply and global persistent fall of sea level due to climate factor, considerable slope canyons were developed. These canyons transported abundant sediments into deep-sea, and form abyssal fan at the lower slope.