Search

Submarine canyons are the major channels for the migration of terrigenous sediments towards deep seas and an important geomorphological unit in continental shelves/slopes. With the development of multibeam bathymetry technology, a method of quickly identifying and accurately extracting the characteristics of submarine canyons from large datasets is urgently required. In this paper, under manual supervision, based on the principles of hydrological analysis and slope analysis, we established a method to quickly identify and extract the characteristic elements of submarine canyons from digital elevation model (DEM) data via the data modeling tools in ArcGIS, according to the topographic features such as U and V shaped downcutting at the bottom of the submarine canyon and high-relief steep canyon walls. The analytical results of the Shenhu Canyons in the northern slope of the South China Sea show that this method can rapidly determine the development position and characteristic elements of submarine canyons and obtain the canyon head and mouth water depth, canyon thalweg length and canyon range (area of canyon floor). To obtain the optimal parameter set applicable to the study area, the factors affecting canyon identification, such as the canyon morphology, reclassification threshold and data resolution, are discussed and analyzed. The results show that the canyon morphology has little influence on the accuracy of recognition results for straight, parallel or sub-parallel canyons. The reclassification threshold of the zero flow accumulation and the spatial resolution of the DEM data determine the accuracy of canyon thalweg recognition, which are two important factors affecting the accuracy of canyon identification results. For the Shenhu Canyons area, a spatial resolution of 200 m and reclassification threshold of 0.4 are the optimal parameters for identifying and extracting features from submarine canyons, and the identification results of automatic and manual extraction of canyon parameters show that the heads water depth differences all are less than 210 m and the thalwegs length difference of the canyon are within 7 km.

Although the structural characteristics and triggering mechanisms of submarine landslides have been investigated, little attention has been paid toward the submarine landslides in canyons. In the Baiyun canyon, there are many small submarine landslides that could be an important sediment transport process of the canyons, but few have been studied. This study aims to examine the characteristics and mechanisms of submarine landslides in the Baiyun canyon on the northern continental slope of the South China Sea using multibeam depth data, acoustic sub-bottom profiles, and borehole data. The results suggest that the submarine landslides are mainly distributed in the head and side valley walls of the canyons. The surface morphology of the landslides is broken-stepped, tongued, or round, and the area of the landslides varies between 0.04 and 15.13 km2. Nevertheless, most of the landslides have an area of less than 2 km2. The scarp height of most landslides is between 20 and 40 m. Based on the internal acoustic structures of the landslides, the submarine landslides can be divided into brittle and plastic. The brittle landslides have clear landslide walls or sliding surfaces, whereas the interior of the landslides in the upper slope area of 650–740 m near the source of submarine canyon has the original stratigraphic structure. The interior of the plastic landslides is typically chaotic and has no clear landslide walls or sliding surfaces. The plastic landslides mainly occur in the head and sides of the canyon. We determined the slope stability using the software GeoSlope and concluded that the strength of the soil on the seabed slopes and the slope gradient are the main factors that control the landslides in the study area. Landslides occur when the slope gradient is greater than 9°. In addition, when the horizontal seismic load is not zero, slopes with a gradient of less than 9° are unstable. This study will be helpful in geological risk assessments and understanding of sediment transportation in the northern margin of the Northern South China Sea.

Utilizing the collected high-resolution seismic dataset and accompanying borehole and bathymetric data, we systematically evaluated the morphology, architecture, sedimentology, and evolution of erosional mud ridges within the inner East China Sea (ECS) shelf. We identified 20 mud ridges, i.e., seismic reflection profile crossings of exposed or buried mud ridges, which are 3.0–30.1 km in width and 2.5–17.3 m in height. The mud ridges are composed predominantly of gray clayey silt, and on seismic profiles contain parallel to subparallel reflectors. They formed around ~ 10–12 ka BP within an estuarine environment. Scouring features of some mud ridges on the eastern part of the study area can be recognized. Consideration of the relative positions of mud ridges, together with the topographical features, enables us to map four linear mud ridges (LMRs). The SE-NW oriented LMRs are > 50 km in length, 3.0–9.5 km in width and running parallel to each other. They also display asymmetric shapes, with steeper slopes to the SW. The eastern segments of some LMRs are exposed on the present seafloor whereas other segments are mainly overlain by the mid- and late Holocene strata. Since the LMRs share similarities with the modern tidal sand ridges in shape and orientation, we hypothesize that they are formed under a uniform tidal current. Seismic data highlight that the internal reflectors of sand ridges consist of dipping clinoforms and are significantly different from LMRs, a feature which is largely due to the difference in grain-size composition of sediments between the inner and mid-outer ECS shelf. The mid- to outer ECS shelf is capped by coarser-grained sediments (i.e., medium to fine-grained), which were reworked and deposited at locations near the erosional areas under a polycyclic tidal current, thus forming multiphase sand ridges. However, fine-grained sediments (i.e., silty clay and clayey silt) overlain on the inner ECS shelf with light mass were carried far away from the erosional areas by the tidal currents, and the relict mud ridges were built. Evidence shows that the LMRs were formed by the early Holocene strata being scoured by a persistent southeasterly flowing current during ~ 8–10 ka BP. The evolution of the LMRs occurred in three stages: (a) formation of the transgressional strata at ~ 10–12 ka BP, (b) strong trough erosion at ~ 8–10 ka BP, and (c) local adjustment (i.e., local erosion slightly and preservation) after ~ 8 ka BP.

The sub-bottom and collected borehole data provide insight into the transport and accumulation processes of the Yangtze-derived sediment in the study area since ~11 kyr BP. Five seismic units were identified according to six major acoustic surfaces. The sedimentary strata consist of fluvial, estuarine and deltaic systems from the bottom up, characterized by two different trends in sediment accumulation rates, i.e., low-high-low, and high-low-high. On the inner shelf of the East China Sea, the terrain with trough and ridge was formed by the Early Holocene transgression strata (formed in ~10 to 12 kyr BP) scoured by the later rectilinear tidal current due to postglacial sea-level transgression, and the sharply protruding seismic units are interpreted to be bedrocks outcropping on the seafloor. An analysis of the sedimentary characteristics in the boreholes and such factors as difference in accumulation rates, and tectonic subsidence led us to conclude that the paleo-coastline was located not far away from and to the east of Core ZK09 at ~9 kyr BP, and the southern bank of the Yangtze River estuary was located to the south of Core ZK09. At ~9 kyr BP, the Yangtze-derived sediments were transported eastwards along the southern bank of the Yangtze River and the barrier due to the influence of the paleo-coastal current from the north, the direction of the Yangtze-derived sediment transport was split on the northeast of the Zhoushan archipelago, and the sediments covered the terrain with trough and ridge. During the high sea level period (7 kyr BP–present), the eastward migration of paleo-coastline had resulted in the increase in accumulation rate. We also conclude that the sharp increase in accumulation rate near the Yangtze River estuary after ~2 kyr BP was not primarily caused by human activities. The position shifts of the estuary caused by the paleo-coastline migration and sea level oscillations since the Holocene is the main cause controlling the Yangtze-derived sediment distribution, and the difference in accumulation rate at different locations in the study area.

The South Yellow Sea (SYS) is a semi-closed epicontinental sea, where voluminous material input and regional tectonic subsidence facilitate preservation of depositional strata, making it an ideal place for studying the regional responses to global sea-level changes during the Quaternary. Based on high-resolution single-channel seismic data, we conducted a detailed study of the SYS shelf in terms of its stratigraphic architecture and seismic facies. In combination with borehole data, we analysed the depositional processes of the SYS shelf since the Quaternary. A minimum of 14 seismic sequences were identified on the seismic profiles, which primarily show four typical seismic reflection facies: progradational reflection facies, parallel reflection facies, chaotic reflection facies and incised valley facies, with the former two seismic facies usually alternating with the latter two vertically. Borehole and seismic data revealed that the SYS shelf has been basically controlled by global sea-level changes since the Quaternary, and three significant transgression events had occurred over the SYS shelf since the Middle Pleistocene (ca. 730 ka BP–Present), taking place in the early Middle Pleistocene, the early Late Pleistocene and at the end of the Late Pleistocene–Holocene, respectively, with their products corresponding to seismic sequences U90, U50 and U10, respectively. Seismic data showed that, in a previous division of the main boundaries for holes EY02-2, NHH01 and QC2, the Holocene and the Upper Pleistocene bottom boundaries are concordant, while the Middle Pleistocene bottom boundary is discordant, and our study indicated that the division scheme for the Middle Pleistocene bottom boundary related to holes EY02-2 and NHH01 is more reasonable. The turning point of variation in the depositional environment of the SYS is at the end of the Middle Pleistocene, before which global sea-level change and tectonic subsidence controlled the shelf deposition. The shelf was inclined westwards and the tectonic topography of the southern margin of the shelf-forced seawater to invade via passages during sea-level rise. This resulted in a limited stratigraphic distribution formed by short-duration transgression events with low-amplitude sea-level rise; thereafter, the shelf deposition was predominated by global sea-level and rivers, and the shelf sediments were mainly derived from mainland China on the west of the shelf and were primarily accumulated on the middle and western parts of the shelf. Copyright © 2016 John Wiley & Sons, Ltd.

On the basis of newly collected multibeam bathymetric data, chirp profiles and existing seismic data, we presented a detailed morphological interpretation of a series of slope-confined canyons in water depths of 300–2000 m in the Baiyun deep-water area, northern margin of the South China Sea. Although these canyons are commonly characterized by regular spacing and a straight-line shape, they vary in their lengths, starting and ending water depths, canyon relief, slope gradients, wall slope gradients and depth profiles along the axis. The eastern canyons (C1–C8) have complex surface features, low values in their slope gradient, canyon relief and wall slope gradient and high values in their length and starting and ending depth contrasting to the western ones (C9–C17). From the bathymetric data and chirp profiles, we interpret two main processes that have controlled the morphology and evolution of the canyons: axial incision and landsliding. The western part of the shelf margin where there were at least four stages of submerged reefs differs from the eastern part of the shelf margin where sedimentary undulations occurred at a water depth of ~650 m. We consider that the variation in morphology of submarine canyons in the study area is the result of multiple causes, with the leading cause being the difference in stability of the upper slope which is related to the submerged reefs and sedimentary undulations.

The process of mass movements and their consequent turbidity currents in large submarine canyons has been widely reported, however, little attention was paid to that in small canyons. In this paper, we document mass movements in small submarine canyons in the northeast of Baiyun deepwater area, north of the South China Sea (SCS), and their strong effects on the evolution of the canyons based on geophysical data. Submarine canyons in the study area arrange closely below the shelf break zone which was at the depth of–500 m. Within submarine canyons, seabed surface was covered with amounts of failure scars resulted from past small-sized landslides. A complex process of mass transportation in the canyons is indicated by three directions of mass movements. Recent mass movement deposits in the canyons exhibit translucent reflections or parallel reflections which represent the brittle deformation and the plastic deformation, respectively. The area of most landslides in the canyons is less than 3 km2. The trigger mechanisms for mass movements in the study area are gravitational overloading, slope angle and weak properties of soil. Geophysical data indicate that the genesis of submarine canyons is the erosion of mass movements and consequent turbidity currents. The significant effects of mass movements on canyon are incision and sediment transportation at the erosion phases and fillings supply at the fill phases. This research will be helpful for the geological risk assessments and understanding the sediment transportation in the northern margin of the SCS.

Comprehensive research was performed to investigate the existence of submerged reefs on the continental shelf edge and upper slope outside the Pearl River estuary in the northern South China Sea to analyze the morphology and distribution of the coral reefs, identify their ages, and discuss their paleoenvironmental significance. The results show that the submerged reefs are distributed on the topographic high of the continental shelf and upper slope at a water depth of 140–420 m, with an outcropping area of over 700 km2. The submerged reefs are mainly topographically characterized by low mound protrusions, several to 20 m above the modern seafloor, and they exhibit three typical acoustic facies in the acoustic profile: punctate reflection, planar reflection, and mound reflection. Based on various geophysical data, we propose at least four stages (I–IV) of submerged reefs on the continental shelf edge and upper slope: The oldest stage I submerged reefs are covered by loose sedimentary strata of >100 m thickness, a minority of stage II submerged reefs outcrop on the modern seafloor, and most stage III and IV submerged reefs outcrop on the seafloor. The submerged reefs developed on the top of the sandy or muddy substrate on the continental shelf edge and upper slope and formed during a period of low sea level. Sea-level changes controlled periodic reef formation events, with stage I, II, and IV reef formation events corresponding to three low-sea-level events, i.e., at 1450, 600, and 20 ky B.P. Based on the maximum water depth (∼220 m) where the last glacial submerged reefs appeared, we estimate that the sea level during the last glacial maximum fell by ∼132–152 m. Further geophysical surveys and sampling need to be performed to clarify the actual distribution, type, and age of the submerged reefs in the study area.

An optimization inversion model of the ground stress field of Bohai Sea and its adjacent area is established by combining Hooke-Jeeves optimization algorithm and finite element method. The study area was simplified into two 2D sections, a shallow and a deep section, for optimization inversion analysis in consideration of the spatial distribution of the measuring points. The main faults in the study area were simulated by 2D surface to surface contact model, in which the friction state between hanging wall and footwall was modeled by Coulomb friction model. The objective function is established by least squares method, according to the difference values between calculated ground stress data and measured ones. The results show that a plausible ground stress field distribution of Bohai Sea and its adjacent area can be obtained with less data by the optimization inversion method. When there are less measuring points, the optimization inversion result can be easily affected by the distribution of measuring points and the distance of each measuring point to the model boundary; when there are more measuring points, the inversion errors at the checkpoints can be obviously reduced and a more reasonable integrated inversion result can be obtained.

The AHP-GDM model is used for the assessment of structural stability, with the Bohai Sea area as an example. In this model, the credit degree of each expert is calculated through the assessment matrix based on the similarity and diversity of vector. The comprehensive opinions of expert panel are quantitatively obtained by considering the effect of credit degree. According to the geological structural setting, the Bohai Sea is divided into twelve assessment zones of structural stability by non-uniform element method. The structural stability grade of each zone is obtained on the basis of the latest geophysical data, earthquake statistical data, and the information of fault activities, current stress field and crustal deformation. The results show that there are one relatively stable area, three relatively sub-stable areas, six relatively sub-unstable areas and two relatively unstable areas. The assessment results of non-uniform element method are very close with those of uniform grid method with size of 0.25° in longitude direction and 0.14° in latitude direction. However the workload of non-uniform element method is only 1/16 of the latter. Compared with traditional assessment methods of structural stability, a more objective and reliable assessment result can be obtained by combining non-uniform element method and AHP-GDM model.