In the 1940s, the deepwater canyon-channel system was found on the passive continental margin of North America for the first time. Since then, it has been one of the key focuses in the petroleum industry (Heinio and Davies, 2007). Channel-levees, as one of parts of deepwater canyon-channel system, can act not only as good hydrocarbon reservoirs but also as important paths transporting terraneous components from lands to deep-sea basins, playing a role of classification and constraint to sediments. Deepwater canyon-channels developed mainly in the slope of continental margins. We study their genetic mechanisms and evolutions, not only consisting in finding deepwater hydrocarbon reservoirs and helping to establish their geological theories but also speculating the sedimentary sources of deep-sea basins according to their evolution characteristics, restoring their specific paleogeographic environments of these basins, and exploring the dynamic evolution mechanisms of continental margins.
Most modern deepwater canyon-channels in the world have been documented from large passive-margin fans, supplied by major rivers carrying huge volumes of dominantly fine-grained sediments (Wynn et al., 2007), e.g., Amazon (Damuth and Flood, 1985), Mississippi (Bouma et al., 1984), Zaire (Droz et al., 1996), Bengal (Emmel and Curray, 1985), Indus (Kolla and Coumes, 1987), and Rhone and Nile Fans (Wynn et al., 2007). This paper regards the deepwater canyons in Baiyun sag (BS) in the northern continental slope of South China Sea (SCS) as the research subject, which develop in the slope environment of marginal sea basin and has the unique characteristics of formation and sedimentary evolution processes. These are of great significance to explore their genetic mechanisms and sedimentary models, demonstrating the paleogeographic environments where they exist and improving the genetic theories of deepwater canyon-channels. Based on the multi-beam bathymetric data and 2D high-resolution, multi-channel seismic profiles, this study reveals the sedimentary characteristics of deepwater canyons. Also, referring to the lithological and paleoenvironmental information that is provided by the ODP1148 drilling and those oil boreholes located in BS and the shelf break, and combing biostratigraphy and sequence stratigraphy and regional comparative methods, this paper comprehensively analyzes and explores the formation and sedimentary evolution processes of deepwater canyons.
In the northern slope of BS, there occurs complicate submarine canyon-channel system, which develops below the shelf break and about 200 m WD. In the 3D topography map made by multi-beam sounding (Fig. 2), there are at least 21 canyons to be identified and four small ones of which in the west are located in the source of the Zhujiangkouwai Submarine Grand Canyon. However, it is very difficult to identify the four small canyons in the 2D seismic profiles (Fig. 3), which may be due to their too small size or too shallow erosion and should be gully in the grand canyon rather than canyons formed by the long-term eroding and filling of flows. Therefore, there are only 17 deepwater canyons numbered C1–C17 from west to east, located on the east of Zhujiangkouwai submarine grand canyon in the northern slope of BS, to be chosen as research subjects in this work (Figs. 2 and 3). All 17 canyons, showing linear-like characteristics, arranging in sub-parallel each other and extending in near N-S direction, are approximately oblique with the slope. In the downstream or near the end of canyons, there are abrupt changes from the south to the southeast in their strike directions, and some of them even turn to the near W-E direction. The canyons begin to originate below the shelf break and terminate at about 1 500 m WD in the northern slope of BS, and their main parts are at about 500–1 500 m WD (Fig. 2). It is concave in the axes of canyons whose thalweg morphology changes with the gradient, and especially C2 and C3 that show the most obvious characteristics.
Figure 2. Topography map of the 17 deepwater canyons in the study area. (a) 3D multi-beam bathymetric topography map; (b) the drawing line.
Figure 3. Cross-section profile of canyons in the study area (the seismic line is shown in Fig. 1). (a) Original seismic section; (b) the drawing line.
Yu and Chang (2009) have pointed out that a submarine canyon-channel developing in continental margins is still in its immature stage when it has not deeply eroded into the continental shelf. In contrast, a submarine canyon-channel eroding into the continental shelf can receive a lot of sediments from lands and strengthen its headward erosion into lands, making it further incise deeply and gradually reaches to its mature stage. The deepwater canyons in the study area have not eroded into the shelf and are not connected with rivers on the land receiving relatively few sediments and possessing weaker headward erosion (Fig. 2). Thus, these deepwater canyons are in an immature stage, but they become more mature in the west than in the east.
Interestingly, these 17 deepwater canyons in the study area show similar characteristics in the plane, but they can be divided into three groups according to the WD and the distance between their headwaters to the shelf break (Fig. 2). The first group begins to originate in the near shelf break at about 250 m WD, including C1–C3; the headwaters of the second group are farther away from the shelf at about 500 m WD, including C4–C9; and the third group are the farthest away from the shelf at about 900 m WD, including C10–C17.
Deepwater canyons C2–C3, originating in the vicinity of shelf break and ending at about 1 000 m WD, are straighter than other ones. As shown in the 2D high-resolution multi-channel reflection cross-section seismic profiles (Fig. 4), the flanks of canyons in the upper are steep and have a small aspect ratio (width/depth) showing V-type, which are dominated by eroding, while their flanks in the lower are gentle and have a large aspect ratio showing U-type, which are dominated by both eroding and sedimentation. They are similar with the typical turbidity canyon-channel—Amazon canyon-channels, which are most deeply researched in the world so far. In the cross-section, the Amazon canyon-channels show V-type in the upper and U-type in the lower, which are mainly due to the long-term eroding-filling of turbidity currents down the slope (Pirmez and Imran, 2003).
Figure 4. Cross-section seismic reflection profiles through C2–C3 (the measuring line is shown in Fig. 2). (a) Original seismic profile; (b) the drawing line.
Kolla et al. (2007) have suggested that there is only one distributary canyon-channel to be active in the same deepwater canyon-channel-levee system during a specified period. However, in the study area, all 17 deepwater canyons are very close to each other and show characteristics of migrating towards the northeast at the same time; then, it suggests that they are almost active simultaneously, so their genetic mechanism may vary from the typical deepwater turbidity canyon-channels in the world.
There are four stratigraphic boundaries to be identified in the 2D high-resolution multi-channel seismic profiles, including T1 (Quaternary/Pliocene), T2 (Pliocene/Upper Miocene), T3 (Upper Miocene/Middle Miocene), and T4 (Middle Miocene/Lower Miocene) (Table 1), and the canyons in the study area are developed in the Middle Miocene (after T4) (Fig. 3). All studied canyons show similar genetic characteristics and evolution processes, which include their bottom cross-section appearing as V-type, their axes migrating consistently to the northeast and the vertical overlay and filling characteristics. In their extending direction and the adjacent canyons at the same WD, seismic structural units show basically the same shapes and overlay patterns, that is, a single canyon changes gradually from lateral migration to the northeastern direction in the proximal upper slope to vertical overlay in the distal lower slope. Canyon C9, which is located in the center of the study area and in the middle stage of developing processes, can be representative of all studied 17 canyons. Then, canyon C9 is chosen to explore further as shown in Fig. 2.
Table 1. Stratigraphic division in the northern slope of BS
Based on seismic facies characteristic parameters (including external morphology, internal reflection texture, amplitude, and continuity) and seismic reflection termination, the seismic reflection units of deepwater canyons are divided into 10 classes (Zhu et al., 2010; Weimer and Slatt, 2007), including basal erosive surfaces (BES), thalweg deposits (TD), lateral migration packages (LMP), mass transport deposits (MTD), outer levee-overbank deposits (OL-OD), inner levee deposits (ILD), canyon margin deposits (CMD), drape deposits (DD), canyon-channel-lobe zones (CLZ), and lobe/sheet sandstone deposits (L/SSD), and their specific characteristics are shown in Table 2. The deepwater canyons in the study area are mainly made up of six seismic reflection units, including BES, TD, LMP, MTD, CMD, and DD. After analyzing their seismic facies, sedimentary facies, and lithofacies using the method by Prather et al. (1998), we find that fine-grained components mainly containing clays and silts are up to approximately 95%, which is concordant with the percentage of fine-grained sediments (mainly being silts) transported by turbidity currents (Pirmez et al., 2000).
Table 2. Characteristics of seismic reflection units in deepwater canyons
In the cross-section A (Fig. 5a), there are at least two normal faults dipping to the southwest under the bottom of canyons, and the two flanks on either side of a single canyon are both steep showing V-type and high relief. It shows characteristics of vertical overlay between the two single adjacent canyons, and there is a small amount of deposition in the thalweg of canyons, indicating that their axes are steep and dominant in erosion. The draping depositions are found at the top of canyons, which may be relevant to the near position away from their sources. In the cross-section B (Fig. 5b), the axis of a single canyon widens and its flanks become gradually gentle, showing U-type. The BES migrates conspicuously to the northeastern direction in a great amount of movement. The internal formation within LMP dips apparently towards the east. There exist a lot of draping depositions at the top of these canyons, which suggests that there are rich sediments coming here. In the cross-section C (Fig. 5c), by comparison to the cross-section B, the amount of lateral migration gradually decreased with time in BES and LMP and eventually transform from the lateral migration into the vertical overlay pattern. According to the ODP1148 borehole data, this transformation occurs in Pliocene (about 5 Ma). The single canyon is mainly in vertical aggradational filling each other. The draping depositions develop very well at the top and there are irregular-shaped MTD outside of canyons, including slide, slump, and debris-flow deposits.
Figure 5. Seismic reflection units of C9 in cross-section profiles (the location is shown in Fig. 2).