Architecture, evolution history and controlling factors of the Baiyun submarine canyon system from the middle Miocene to Quaternary in the Pearl River Mouth Basin, northern South China Sea

Architecture, evolution history and controlling factors of the Baiyun submarine canyon system from the middle Miocene to Quaternary in the Pearl River Mouth Basin, northern South China Sea

Marine and Petroleum Geology 67 (2015) 389e407 Contents lists available at ScienceDirect Marine and Petroleum Geology journal homepage: www.elsevier...

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Marine and Petroleum Geology 67 (2015) 389e407

Contents lists available at ScienceDirect

Marine and Petroleum Geology journal homepage: www.elsevier.com/locate/marpetgeo

Research paper

Architecture, evolution history and controlling factors of the Baiyun submarine canyon system from the middle Miocene to Quaternary in the Pearl River Mouth Basin, northern South China Sea Wei Zhou a, b, *, Yingmin Wang a, c, **, Xianzhi Gao a, b, Weilin Zhu d, Qiang Xu e, Shang Xu a, b, Jianzhi Cao f, Jin Wu a, b a

State Key Laboratory of Petroleum Resources and Prospecting (China University of Petroleum, Beijing), Beijing 102249, China College of Geosciences, China University of Petroleum, Beijing 102249, China Ocean college, Zhejiang University, Hangzhou, Zhejiang Province, 310058, China d China National Offshore Oil Corporation Ltd (CNOOC), Beijing 100010, China e CNOOC Research Center, Beijing 100027, China f Petroleum Exploration & Exploration Co., Ltd. Sinochem, Beijing 100031, China b c

a r t i c l e i n f o

a b s t r a c t

Article history: Received 22 February 2014 Received in revised form 10 May 2015 Accepted 12 May 2015 Available online 22 May 2015

Since the middle Miocene, a series of northeastward unidirectionally migrating canyons (UMCs) have developed in the Baiyun Sag, Pearl River Mouth Basin. Seven closely spaced submarine canyons have been investigated in detail under the integrated analysis of 2D/3D seismic and well data. Five architectural elements of canyon-fill deposits are identified: basal lag (BL), slump and debris-flow deposits (SDFDs), canyon confined sheets (CCSs), laterally inclined packages (LIPs) and channel-levee complexes (CLCs). Three evolutional stages of canyon development are recognized for the first time: (1) the middle Miocene stage, during which U-shaped, slope-confined vertical aggradation-dominated UMCs developed; (2) the late Miocene stage, during which U-shaped, shelf edge-indented lateral migrationdominated UMCs developed; and (3) the Pliocene-Quaternary stage, during which slope-confined vertical aggradation-dominated UMCs developed. Extensive slope failures which developed during this final stage had a close relationship with the extensive release of focused fluid flow. The UMCs identified in all of these three stages suggest that along slope bottom currents have been active in the Baiyun Sag since the middle Miocene. Evolution of the Baiyun canyon system was likely controlled by sediment supply, regional tectonic activity, sea level fluctuations and paleo-ocean current. Based on the contrast of UMCs in this study area, two different types of UMCs are identified. The first type of UMCs with their heads coupling with areas of high coarse-grained sediment supply, usually indent the shelf edge, generating canyon-fills with sand-rich BL in the bottom grade upward into SDFDs and finally into LIPs, and downslope submarine fans/aprons. The second type of UMCs with their heads coupling with areas of starving sediment supply, usually do not indent the shelf edge, generating canyonfills with mud-rich BL in the bottom grade upward into SDFDs, thin CCSs and CLCs and finally into LIPs, which are dominated by retrogressive mass wasting processes, exhibiting highly aggradational morphologies, and absence of downslope fans/aprons. Both these two types of UMCs form U-shaped heads and result from the interaction between gravity flow and bottom current, however, the first type of UMCs are influenced by the surface current (mainly the South China Sea Warm Current) and the intermediate water current, but the second type of UMCs are only influenced by the intermediate water current. © 2015 Elsevier Ltd. All rights reserved.

Keywords: Unidirectionally migrating canyons Bottom currents Slope failures Focused fluid flow Pearl River Mouth Basin Northern South China Sea

1. Introduction * Corresponding author. State Key Laboratory of Petroleum Resources and Prospecting (China University of Petroleum, Beijing), 18 Fuxue Road, Changping District, Beijing 102249, China. ** Coresponding author. Ocean College, Zhejiang University, 866 Yuhangtang Road, Hangzhou, Zhjiang Province, 310058, China. E-mail addresses: [email protected] (W. Zhou), [email protected] com (Y. Wang). http://dx.doi.org/10.1016/j.marpetgeo.2015.05.015 0264-8172/© 2015 Elsevier Ltd. All rights reserved.

Submarine canyons are common features on continental margins and are generally considered to be major conduits for sediment transport from the shallow continental shelf and upper slope into the deep-water environments (e.g. Shepard, 1936, 1981; McHugh et al., 2002; Babonneau et al., 2002; Popescua et al., 2004; Harris

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and Whiteway, 2011; Gong et al., 2011; Jobe et al., 2011; Saller and Dharmasamadhi, 2012; He et al., 2013). During sea-level fluctuation, the activity of canyon-channel system is primarily controlled by the coupling between the canyon head and the coarse-grained sediment supply (Fildani and Normark, 2004; Paull et al., 2005; Covault et al., 2007; Harris and Whiteway, 2011; Jobe et al., 2011). It is mainly displayed that sandy sediments transported by longshore drifts can be captured by the canyon head and move down to the axial channel of the canyon even in a very shallow water environment (Paull et al., 2003, 2005; Covault et al., 2007). The submarine canyons with heads connecting with areas of high coarse-grained sediment supply transport sediments to the deepwater area to develop large submarine fans (McHugh et al., 1998; Babonneau et al., 2002; Popescu et al., 2004; Covault et al., 2007; Lastras et al., 2009; Jobe et al., 2011). Furthermore, the sand-rich deposits filled in these canyons can become important hydrocarbon reservoirs (Clark and Pickering, 1996; Stow and Mayall, 2000; Posamentier and Kolla, 2003; Mayall et al., 2006; Crossey et al., 2006; Anderson et al., 2006). In contrast, submarine canyons with heads connecting with areas of starving sediment supply, have very different morphologies and depositional processes and form the “starving” canyons without submarine fans (Covault et al., 2007; Jobe et al., 2011; Harris and Whiteway, 2011; Saller and Dharmasamadhi, 2012). In addition retrogressive mass failures and mass transport deposits are common in these “starving” canyons (Carlson and Karl, 1988; Normark and Carlson, 2003; Green and Uken, 2008; Saller and Dharmasamadhi, 2012). The Baiyun canyon system, named in this current study, lies in the Baiyun Sag of the Pearl River Mouth Basin, nowadays in the deep-water area of the northern continental margin of the South China Sea. Modern submarine observation shows that in the Baiyun Sag, there are seventeen regularly-spaced modern submarine canyons (Zhu et al., 2010; Li et al., 2013), lying in water depths ranging from 450 to 1500 m, most of which are NeS orientated and confined in the deep-water continental slope (Zhu et al., 2010). In the past few years, these canyons received a lot of attentions from the scientists focused on the interaction between down-slope gravity flow and along-slope bottom current because of their special characteristic of unidirectionally migration (Zhu et al., 2010; Gong et al., 2013; Li et al., 2013). The unidirectionally migrating canyons/channels are also studied in other basins worldwide (e.g. Rasmussen, 1994; Rasmussen et al., 2003; Viana et al., 1998, 1999; ranne and Abeign, 1999; Mulder et al., 2008; Biscara et al., Se 2010; He et al., 2013). Previous studies have mainly focused on the depositional architecture, process and genesis of the canyon, as well as their effect on the speculation or reestablishment of the modern and ancient ocean circulations, but little discussed the topography and depositional architecture of these unidirectionally migrating canyons based on different external factors (such as sediment supply, ocean circulation, sea level, etc.). Moreover, the evolution history of the Baiyun canyon system has remained unclear. In this paper, using newly available high-quality 3 dimensional (3D) seismic and well data, we are able to discuss (1) the detail morphology and internal architecture of modern and buried canyons, (2) the evolution history of the canyon system, (3) the focused fluid flow and their effects on the canyon system and (4) the external controlling factors on the canyon evolution. 2. Geological setting 2.1. Tectonic setting and basin evolution The Pearl River Mouth Basin (PRMB) is the largest basin in the continental margin of northern South China Sea and has a drainage

area of 17.5  104 km2 (Fig. 1) (Zhu et al., 2010). Similar to many other passive margin basins, the basin evolution of the PRMB experienced rifting (59 Ma ~32 Ma), transition (32 Ma ~23.8 Ma) and subsidence (23.8 Ma ~0 Ma) stages (Fig. 2). In particular, in the middle and late periods of the subsidence stage, multi-period of tectonic collisional phases took placed around the northern South China Sea. One of them was the NNW-directed underthrust of the Philippine Sea Plate at 10.5 Ma, named the Dongsha Event in the region (Li and Rao, 1994; Huang et al., 2001), which caused strong elevation and subsidence of the fault blocks and tenso-shear action of the strata in the PRMB. Two main collision phases between Taiwan and the continental margin of East China at 5e3 and 3e0 Ma ago may have resulted in two post-drift uplift events within the Dongsha region (Lüdmann and Wong, 1999; Lüdmann et al., 2001). In response to the basin evolution, the dominant sedimentary environment in the Baiyun Sag changed from lacustrine-fluvial in the early stage, through shallow neritic shelf, finally to deep-water continental slope in the late stage (Fig. 2). 2.2. Sea level change, sediment supply and continental margin shape The sea level curve of the PRMB has some difference with the global eustatic curve, and it mainly displays a character of ladderlike rising after the Oligocene (Fig. 2). Moreover, from the late Middle Miocene to Pliocene, the sea level never fell to below the shelf break (Fig. 2). The Pearl River is the one of the two most important rivers (the other one is the Red River) in the northern continental margin of South China Sea. The sediment transported by the Pearl River formed large-scale shallow water deltas and submarine fans during the subsidence phase (Pang et al., 2007a; Peng et al., 2004, 2005; Wang et al., 2012). In the Pleistocene, the Pearl River delta prograded to the shelf edge and formed thick clinoforms in the shelf margin area (Lüdmann et al., 2001, Fig. 3A and C). Nowadays, the Pearl River delta is confined in the estuary of the Pearl River (Wei and Wu, 2011), and the outer shelf is covered with sand waves (Wang, 2000; Zhuo et al., 2014). During the subsidence stage, the Baiyun Sag manifests as an inner-slope basin (Fig. 3A). Since 13.8 Ma, submarine canyons has been persistent within the Baiyun Sag (Fig. 4). The basic type of the modern shelf margin is Gaussian type based on the classification scheme of Adams and Schlager (2000), which shows some evidence of the modern continental margin has been suffered by the ocean-current-induced remodeling (Fig. 3A and B). 3. Data and methods The data used for this research include a conventional, industry acquired, 1600 km2 zero phase 3D seismic volume integrated with well data (Fig. 5). The 3D seismic volume has a dominant frequency of 40 Hz in the interval of interest (the middle Miocene e Quaternary interval). Line spacing is 25 m, trace spacing is 12.5 m and the sample rate is 4 ms. The exploration well B6-1-1 was drilled in the study area, but only limited well logs, biostratigraphy and lithology data were released. Although the secondary level seismic interfaces were not easily to trace because of the intensive development of canyons, fluid leakages and slope failures, the main seismic horizons, such as 13.8 Ma (T3), 10.5 Ma (T2), 5.5 Ma (T1), 1.9 Ma (T0) and 0 Ma (seabed), could still be traced and correlated in the wide range. Hence the essential seismic sequence stratigraphic framework was able to be established. The seabed morphology could be established by horizon automatic tracking using the software Landmark 2003.

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Figure 1. Map showing major geomorphologic features and modern ocean circulations (e.g., South China Sea Warm Current, intermediate and deep water contour currents) along the northern continental margin of the South China Sea. The South China Sea Warm Current shows the SWeNE oriented movement, straddling the shelf break region and is a special consistent northeastward-directed current among the complex surface currents along the northern continental margin of the South China Sea (Xue et al., 2004; Guan and Fang, 2006; Zhu et al., 2010; He et al., 2013). The circulations of intermediate and deep water contour currents are modified from Chen (2005), Zhu et al. (2010) and Li et al. (2013). Note the distribution of sandy sediments (marked A-D) on the modern seafloor (Wang, 2000; Zhuo et al., 2014). Abbreviations: PRMB ¼ Pearl River Mouth Basin; QDNB ¼ Qiongdongnan Basin; A ¼ sand ribbons; B ¼ sand waves; C ¼ sand sheets; D ¼ sand ridges.

To interpret the internal architecture of canyons, seismic facies analyses were operated on a group of typical seismic lines on the first step, and then interpretations were extended to other seismic lines based on the internal configuration and texture (amplitude and continuity) and the external form of seismic reflections. The focused fluid flow systems and shallow gas could be identified in the conventional seismic profiles as well as some specially processed profiles (e.g. the instantaneous frequency profiles) by means of acoustic anomalies.

4. Results

obvious axial erosive features on their modern wide and flat canyon thalwegs (Fig. 5B). Exponential type curves (Adams and Schlager, 2000) and very concave profile forms (Covault et al., 2011) along the axial thalwegs of these seven canyons suggest that the modern continental margin of the study area is immature and dominated by erosional sedimentary processes (Kertznus and Kneller, 2009; Covault et al., 2011) (Fig. 6A). Moreover, there are many different scale slope failures along the strike direction of continental slope and the canyon walls (Fig. 5B). The abundant canyon wall failures are shown by the complex geomorphologic shapes of the canyon flanks and the abrupt decrease of the vertical heights between the canyon flanks and canyon thalwegs (Fig. 5B).

4.1. General characterization of the seafloor On the modern seafloor, a series of U-shaped submarine canyons are found in water depths approximately ranging from 500 to 1800 m (using a time-depth conversion on the sea bottom by the seawater compressional wave velocity of 1500 m/s) (Figs. 3e5A). These canyons have a regular spacing of eight to 10 km; all of them do not erode the shelf edge and are confined in the continental slope environment. In this study, we focus on the seven submarine canyons therein, named as C1 to C7 in the threedimensional seismic dataset (Fig. 5B). The heads of the canyons are almost U-shaped, and incise deeply into the underlying formations (Fig. 5). The morphology of most canyons' heads are smooth (C1, C2, C3, C6 and C7; see Fig. 5B), but the others show phenomenon of strong mass wasting and headward erosion around the canyon heads (C4 and C5, see Fig. 5B). C6 and C7 show

4.2. Lithology of canyon-fills Although not directly intersecting submarine canyons, the well B6-1 still provides us some crucial information about the lithology of the canyon-fill deposits. The lithological information shown in B6-1-1 (Fig. 7) includes siltstone, mudstone, silty mudstone, limy mudstone and limy siltstone. When tied to the seismic profile, the well indicates that strong amplitude reflection represents sandy deposits and weak amplitude reflection represents muddy deposits (Fig. 7). 4.3. Interpretation of seismic facies within the submarine canyon Following Mayall et al. (2006) and Piper and Normark (2001), five main canyon-fill elements are identified within the canyons,

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Figure 2. Sequence-stratigraphic framework of the Baiyun Sag, deep-water area of the PRMB (modified from Pang et al., 2008). The red line marks the approximate location of the shelf break. Lithology and abbreviations: brown: basement; yellow: sandstone; gray: mudstone; light blue: carbonatite; jacinth: magmatic intrusive rock; ZHU I ¼ Zhu I Depression; PYLU ¼ Panyu Lower Uplift; BS ¼ Baiyun Sag; SUB ¼ Southern Uplift Belt. At 23.8 Ma the shelf break jumped to the northern margin of the BS from the SUB (Pang et al., 2007a,b). The sea-level curve of the PRMB and the global eustatic curve are respectively from Qin (1996) and Haq et al. (1987). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.).

which are named as basal lag (BL), slump and debris-flow deposits (SDFDs), canyon confined sheets (CCSs), laterally inclined packages (LIPs) and channel-levee complexes (CLCs). 4.3.1. Basal lag (BL) Most canyons have basal lags (Mayall et al., 2006). The basal lags were deposited in the canyon thalwegs and formed during the initial time when the canyon was being cut (Mayall et al., 2006). The mostly limited deposits in the basal lags are coarse sands, mudclast conglomerates and shale drapes, and the thickness of them is generally from a few decimeters to a few meters (Mayall et al., 2006). In this current study, the basal lag generally composes one seismic event, directly lays upon the canyon erosional surface and is deposited in the thalweg (Fig. 8A, D, F and G). when the basal lags contain more coarse sands, the seismic event are mainly characterized by a high-amplitude reflector (HARP) (Piper and Normark, 2001) (e.g. Fig. 8A, D, F and G); but when they contain more mudclasts or shale, the seismic amplitude reflection will be so weak that they could not be distinguished from the mass transport deposits (MTDs) (Fig. 8B and E). In geological time, the coarse-grained basal lags mainly developed in the late Miocene, and fine-grained base lags mainly developed in the middle Miocene, Pliocene and Quaternary (Figs. 4B, 8F and G).

4.3.2. Slump and debris-flow deposits (SDFDs) The slump and debris-flow deposits (SDFDs) are recognized in many canyons and are considered as the main depositional element of the canyon-fills. The involved sediments are slides, slumps and debris-flow deposits (Mayall et al., 2006). Some of them can be locally derived just from the collapse of canyon walls, but others can be seen far distal in the downstream experiencing long distance transport. They are more likely to be developed in the early stages of lowstand sequence in a sea-level change cycle (Posamentier and Kolla, 2003). In seismic facies, they are usually characterized by low amplitude chaotic reflections or transparent reflections (Posamentier and Kolla, 2003; Mayall et al., 2006). In this current study, the SDFDs are identified by low amplitude chaotic or transparent seismic reflections, and are usually developed above the base lags or even directly laid upon the canyon erosional surface in the thalweg, with sources from the southwestern canyon wall (Fig. 8A, B, F and G). In geological time, the SDFDs more developed during Pliocene-Quaternary. 4.3.3. Canyon confined sheets (CCSs) The canyon confined sheets are always rich in sand, lay upon the SDFDs and are confined in the broad and flat canyon thalweg. In seismic facies, they are manifested as high-parallel, high amplitude, high continuous and sheet-liked reflections, differed from the

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Figure 3. (A) Interpretation of the two-dimensional seismic profile through ODP1148 (modified after Zhu et al. (2010)). Location is shown in Fig. 1. The shelf break has remained stable along the northern margin of the Baiyun Sag since 23.8 Ma; Note the typical deep-water submarine canyons has developed in the Baiyun Sag since 13.8 Ma. T1 ¼ 5.5 Ma; T2 ¼ 10.5 Ma; T3 ¼ 13.8 Ma. (B) The overall shape of the modern continental margin belongs to the Gaussian type based on the math equation presented by Adams and Schlager (2000), indicating the modern continental margin has been suffered by the ocean-current-induced remodeling. (C) 2D seismic profile across shelf margin showing the Pleistocene shelf margin deltas and clinoforms as well as the canyons developed on the outer shelf in the Pliocene. Location is shown in Fig. 1.

leveed channels (Fig. 8B, C, F and G). Since 5.5 Ma, we can easily identify the canyon confined sheets laid upon the SDFDs in lower segments of paleo-canyon 5, 6 and 7 because of the broad and flat shape of the canyon thalwegs (Fig. 8F, G and 9G). The canyon confined sheets mainly developed during Pliocene-Quaternary periods. 4.3.4. Laterally inclined packages (LIPs) The laterally inclined packages (LIPs), which mark the systematic lateral migration of the channels in a highly aggradation setting, was described recently by Zhu et al. (2010). In our current study, the LIPs can be identified only on the southwestern flanks of the canyons, and they are made up of stacked, sigmoidal shaped inclined reflectors that build out from the southwestern canyon wall dip to the northeast and downlap onto either basal lags, slump and debris-flow deposits or even the basal erosional surface of a canyon, forming aggradational sediment wedges which extend from the southwestern canyon wall to the canyon thalweg (Figs. 8 and 9). Low amplitude, moderately continues transparent reflections suggest that the limited deposits of LIPs in this study are shale drapes (Fig. 8A, D, F and G). As shown in Figure 9, the LIPs are developed better and thicker close to the heads of the buried canyons. Moreover, around the heads of the buried canyons, the aggradational sediment wedge of the LIPs is always parallel with the overlaying basal erosion surface of a new canyon (Fig. 9A and B). However, around the middle and bottom cuts of the buried canyons, the aggradational sediment wedge of the LIPs are always truncated by the overlaying basal erosion surface of a new canyon (Fig. 9 CeG). The LIPs are considered to be formed by the

northeastward-directed bottom currents (Zhu et al., 2010; Gong et al., 2013; Li et al., 2013). In our seismic-stratigraphic framework, the LIPs prominently developed in the late Miocene but poorly developed in Pliocene and Quaternary (Figs. 8F, G and 9A-G). Furthermore, the LIPs are also identified in the middle Miocene section (Fig. 8F, G, 10B, C and D), in which they were not previously identified by Zhu et al. (2010) and Gong et al. (2013). 4.3.5. Channel-levee complexes (CLCs) The channel-levee complexes (CLCs) can be observed within larger channels/valleys/canyons (Kolla et al., 2007). The highly sinuous leveed channel is regarded as the final fill element of many large erosional channels, and its fills are mainly muddy (Mayall et al., 2006). Seismic facies of a channelelevee complex are generally of high amplitude, discontinuous reflections, whereas many of the associated levees have relatively low amplitude, continuous reflections (Kolla et al., 2007). In our current study, since 5.5 Ma, channel-levee complexes have developed in the buried canyons and even on the modern seafloor. In buried and modern canyons 6 and 7, the channel-levee complexes can be easily identified in the seismic cross sections and the submarine topographical map (Figs. 5B and 8E). Because of the gradual and wide thalwegs of the lower segments of paleo-canyons 6 and 7 (Figs. 5B, 6G and H), the channels were developed within the canyons, with low-amplitude levees (Figs. 5B, 8E, 9F and G). On the modern seafloor, we can also identify the channel-levee complexes in the lower segment of C5, as well as the downstream of the canyons C3 and C4, they are so small but can still be identified

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Figure 4. 2D seismic profile along the strike of continental slope showing the closely spaced development of unidirectionally migrating canyons (UMCs) between 13.8 Ma and 5.5 Ma. Note the UMCs are obviously truncated to the seismic horizon 5.5 Ma. LIPs ¼ laterally inclined packages.

through the tiny differences in the seismic reflections and the seabed topographic relief (Figs. 5B, 8F and G). 4.4. Stacking patterns of the paleo-canyons in the late Miocene The paleo-canyons in the late Miocene show prominent characteristics of continuous unidirectional migration (Fig. 9AeG). From section A to C, the width/depth ratios of each paleo-canyon are gradually reduced. From section C to E, the width/depth ratios of each paleo-canyon are basically consistent. In section F, the width/ depth ratios of paleo-canyons 3 and 4 are very small and these two paleo-canyons incise deeply into the underlying layers. In section G, the paleo-canyons basically disappear while vertically superimposed submarine fans are developed on the bottom. Figure 10 shows the trajectories of the thalwegs of paleo-canyons (C1eC6) between 10.5 Ma and 5.5 Ma. From Figure 10, we can find that for every paleo-canyon, from proximal to distal segment, the stacking patterns of the thalwegs show the upward reduction in the distance of lateral migration (e.g. paleo-canyon 3, unidirectional migration of up to 30 km in section A but 3 km in section G). And during the temporal evolution of a single canyon, most of the paleo-canyons have three evolutional phases: aggradation with slightly lateral migration in the early phase, oblique lateral migration in the middle phase and aggradation with slightly lateral migration in the last phase (e.g. paleo-canyons 2, 3, 4 and 5, see Fig. 10). 4.5. Focused fluid flow and shallow gas Focused fluid flow and shallow gas are widespread in the Baiyun Sag evidenced by the presence of seismic chimneys, mud diapirs,

mud volcanoes, pipes, normal faults, pockmarks and acoustic anomalies (Sun et al., 2012). Based on the observations from Sun et al. (2012), focused fluid flow structures and the shallow gas are studied in the present study. 4.5.1. Focused fluid flow structures Focused fluid flow structures in this current study mainly contain seismic gas chimneys, pipes, pockmarks and normal faults. The seismic gas chimneys are widespread in this study area and can be identified in seismic profiles through the seismic reflection turbidity, the pull-down characteristic of the local seismic events, the disorder and abnormal low instantaneous frequency seismic reflection (Figs. 5B, 11, 12B and D). The pipes are also abundant and can be identified by the extended upward tube-shaped amplitude anomaly in the vertical seismic profiles (Figs. 9, 12 A, B and D). The pockmarks is located on the modern seafloor, and appear circle in planform, 100 s of meters in diameter and up to 10 s of meters in depth (Figs. 5B and 12D). The normal faults are well developed in the northern and southern parts of the study area (Fig. 9A, B, 12A and D). From the relationship between the faults and faulted formations, we consider that the normal faults were mainly formed in the late Miocene and Pliocene (Fig. 9A, B, 12A and D). 4.5.2. Shallow gas Abundant acoustic anomalies observed in the 3D seismic data provide us the direct evidence of the occurrence of shallow gas across the study area. The acoustic anomalies mainly contain enhanced reflections, acoustic blanking and acoustic turbidity; and the acoustic blanking and acoustic turbidity are usually found below the enhanced reflections or associated with faults (Sun et al., 2012). The shallow gas

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Figure 5. (A) Bathymetric map of the study area, location is shown in Fig. 1. Note all of the submarine canyons is located in the water depth of more than 500 m. (B) Dip map of the study area calculated by the software Landmark. Modern canyons' thalwegs are marked in red dot lines. The focused fluid flow structures (e.g., gas chimneys, pockmarks) are abundant in this area. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.).

also can be identified in the instantaneous frequency seismic profile due to the abnormal low frequency reflections (Fig. 11B). 4.6. Slope failures in the Quaternary period There are many slope failures formed in the Quaternary period in this study area, including the slope failures along the strike direction of the continental slope and the slope failures along the canyon walls and heads. Two major slope failures along the slope strike direction are identified, named as Land Slide A and Land slide B. Land Slide A is located in the upper slope (Figs. 5B, 9A, and 13A). Land Slide B is located in the end of C4 and C5, which has a larger run-out distance than Land Slide A (Figs. 5B and 12A). Canyon wall failures are observed from the seafloor topography map (Fig. 5B). Most of the heads of the canyons have very smooth morphology suggesting weak headward erosion along the canyon heads (Green and Uken, 2008), except for C4 and C5 (Figs. 5B and 6BeH). 5. Discussion

speculation or reestablishment of the modern and ancient ocean circulations (Zhu et al., 2010; Gong et al., 2013; Li et al., 2013). But the evolution history of the Baiyun canyon system has remained unclear and not been discussed. In the current study, based on the interpretation of the 3-D seismic dataset, the evolution of the Baiyun canyon system can be separated into three stages for the first time: 1) The middle Miocene stage (13.8 Ma ~10.5 Ma), during which typical unidirectional migrating canyons (UMCs) developed. The canyons were all confined in the slope environment and not indented the shelf edge. 2) The late Miocene stage (10.5 Ma ~5.5 Ma), during which typical regularly-spaced unidirectionally migrating canyons developed. The canyons indented the shelf edge. 3) The Pliocene - Quaternary stage (5.5 Ma ~ nowadays), during which abundant slope failures and atypical unidirectional migrating starving canyons developed. In this stage the canyons were all confined in the slope environment and not indented the shelf edge.

5.1. Evolution history of the Baiyun canyon system As mentioned above, the previous studies on the Baiyun canyon system have mainly focused on the depositional architecture, the process and genesis of the canyon, as well as its effect on the

5.1.1. Middle Miocene stage (13.8 Ma ~10.5 Ma) In the beginning of this stage, the Pearl River submarine fan developed upon the seismic horizon 13.8 Ma (Wang et al., 2012).

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Figure 6. (A) Longitudinal depth profiles along the axial thalwegs of the modern submarine canyons C1eC7. All of heads of these seven canyons are located in water depths ranging from 500 to 1000 m, suggesting that all these seven modern canyons are confined in the continental slope environment and do not indent the shelf edge. Moreover, all of these seven profiles have a character of exponential type curve (Adams and Schlager, 2000), although there are some slope gradient and relief jumps along them. (B)e(H) Longitudinal depth profiles along the axial thalwegs and franks of the modern canyons C1eC7 showing the distribution of canyon wall failures along the axial directions of the canyons.

The siltstone lithology information (core) shown in the well B6-1-1 and the strong seismic amplitude reflectors suggest that the Pearl River submarine fan at 13.8 Ma was a sandy submarine fan (Figs. 8F, 13B and 13C). After that, a series of slope confined U-shaped UMCs developed therein, as shown by active vertically superimposed with minor lateral migrational features (Fig. 8F, G, 9 BeD and 13A). The unidirectional migration distance was up to 3 km near the

canyon head (Fig. 9B). The UMCs are regarded as resulting from the interaction of the gravity flow and bottom current (Rasmussen et al., 2003; Zhu et al., 2010; He et al., 2013, Li et al., 2013). And one cut-and-fill cycle of a single UMC undergoes three stages: erosion-dominated stage, erosion-deposition stage and depositiondominated stage (He et al., 2013; Li et al., 2013), and the persistent bottom current is the main cause to make the unidirectional

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Figure 7. Lithological features of the core in the well B6-1-1 and the calibration of core and seismic profile. The seismic profile is cut out from Fig. 8F.

migration of the canyon (Zhu et al., 2010; He et al., 2013; Li et al., 2013). So, the UMCs identified in this stage demonstrate that in the middle Miocene (13.8 Ma ~10.5 Ma), the northeastwarddirected bottom current was active in the Baiyun Sag. 5.1.2. Late Miocene stage (10.5 Ma ~5.5 Ma) In the late Miocene, the most significant sedimentary feature in this study area was the spectacular regularly-spaced UMCs (Fig. 13B). Nearly ten continuous cut-and-fill cycles are identified in this stage (Figs. 8G and 9BeE). Because of our observation of these UMCs indenting the shelf edge, we think that these UMCs were all influenced by the South China Sea Warm Current on an outer shelf (Guan, 1978; Hsueh and Zhong, 2004; Xue et al., 2004; He et al., 2013) and the intermediate water current on the deep-water slope (Zhu et al., 2010; Gong et al., 2013; Li et al., 2013) (see Figs. 3A, C, 9A and 12B for the paleo-canyons incised the shelf edge). From the trajectories of laterally migrating distances of canyons' thalwegs (Figs. 9AeG and 10), we could infer that, from the shallow water outer shelf to the deep-water continental slope, the evolution of a

single canyon was a continuous system in the space. In general, when the water was deeper, the laterally migrating distance of the UMC was shorter. 5.1.3. Pliocene-Quaternary stage (5.5 Ma ~ nowadays) From 5.5 Ma, the depositional architecture was distinctly different from the earlier stages. The main observation is that, in Pliocene, on the upper slope there developed an approximately 200 ms (TWTT) thickness of fine drape sediments (Figs. 3C, 12A and 13C), and on the middle and lower slope, there developed some Ushaped canyons (Figs. 9CeF, 12D and 13C); in the Quaternary, on the upper slope there developed an approximately 500 ms (TWTT) thickness of slope clinoform deposits (Figs. 3C, 12A, 12B and 13D), and on the middle and lower slope, there developed regularly spaced submarine canyons, abundant slope failures and mass transport deposits (Figs. 4, 5, 8e11, 12A, B, 12D, 13D and 13E). All these canyons were confined in the slope environment and not indented the shelf edge. Furthermore, the canyons in this stage were almost vertically superimposed with minor lateral migrating characteristic and the LIPs had a small amount of developments

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Figure 8. Seismic facies of the submarine canyon-fills and their corresponding interpretations, including (A) basal lag (BL), (B) slump and debris-Flow Deposits (SDFDs), (C) canyon confined sheets (CCSs), (D) laterally inclined packages (LIPs) and (E) channel-levee complexes (CLCs). (F) and (G) Detailed features of seismic facies of canyon-fills and their interpretations across well B6-1-1 along the strike direction of the slope, location is shown in Fig. 5.

(Figs. 9F, 13D and E). All these might suggest that the bottom current during the Pliocene-Quaternary stage were obviously weak relative to the effects of gravity flows. 5.2. The relationship among the focused fluid, the slope failure and slope-confined canyons The submarine slope failure is a widespread phenomenon around the passive and active continental margins from ancient to modern (Canals et al., 2004) and it plays a significant role in the evolution of continental margin (Pratson, 2001). Once a slope failure initiates, it may progress by means of numbers of mass movement processes, from translational sliding to fluidal flowage (Martinsen, 1994). Over the last two decades, the submarine slope failures were intensely investigated because of the assessing of the seabed stability, the deep-water material transportation and the exploration and exploitation of hydrocarbon resources in the deep sea (Campbell, 1999; McAdoo et al., 2000; Gee et al., 2001; Masson et al., 2002; Canals et al., 2004; Evans et al., 2005; Vanneste et al., 2006; Ercilla et al., 2008; Gong et al., 2014; Alfaro and Holz, 2014). And the main triggering agents or mechanisms for the submarine

slope failure were summarized as earthquakes, tsunamis, decomposition of gas hydrates, overpressure, fluid flow and sedimentary instability (Bugge et al., 1987; Mello and Pratson, 1999; Dugan and Flemings, 2000; Bünz et al., 2003, 2005; Canals et al., 2004; Evans et al., 2005; Mienert et al., 2005; Berndt, 2005; Greene et al., 2006; Brown et al., 2006; Gee et al., 2006, 2007; Vanneste et al., 2006; Paull et al., 2008; Lopez et al., 2010). During the recent years, in the northern continental margin of the South China Sea, scientists found abundant gas hydrate accumulations, especially in the Shenhu area (close to the current study area) (Zhang et al., 2002; Wang et al., 2011; Wu et al., 2007; Lin et al., 2009). Sun et al. (2012) identified two types of tectonically and stratigraphically controlled fluid flow related systems in the deep-water area of the Baiyun Sag adjacent to our current study area. After 5.5 Ma, the deep-water area of the PRMB might undergo strong overpressure fluid release event which was the result from the deep-buried over-matured source rocks or gas generation, and the formation of overpressure might be due to the high sedimentation rate of the last three million years in the northern South China Sea (Sun et al., 2012; and the references therein). However, the relationship between focused fluid system and the slope failure

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in this area is still not clear. In this current study, we find that the focused fluid structures have a close connection with the slope failures. The slope failures are more likely to take place in the area where the focused fluid structures exist, and when the focused fluid flow does not affect the stability of the strata, the local canyon walls or the slope formation are shown to be stable blocks (Figs. 5B, 9 and 12AeC). So the focused fluid system and its associated decompensated shallow gas could do favors in triggering the instabilities of the subcutaneous layers under the sea floor to form slope failures. What's more, the slope failure might have an important impact on the evolution of the canyon. This impact mainly displays in the equilibrium profile adjustment (Ross et al., 1994) along the canyon's axial thalweg. The equilibrium profile adjustment can trigger the re-cut and re-fill processes of the gravity flows along the canyon thalweg. On the modern seafloor, V-shaped lower segments of the

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C4 and C5 and the knickpoints along the axial thalwegs of C1eC5 have shown the adjustments of the equilibrium profiles of these canyons ((Figs. 5B, 6BeF and 9G). The equilibrium profile adjustment facilitates the retrogressive erosion around the canyon heads (C4 and C5, Fig. 5B) and the instabilities of the steep canyon walls (C1eC5, Figs. 5B, 6E and F). 5.3. Controlling factors for the evolution of the Baiyun canyon system 5.3.1. Sediment supply Submarine canyons connected with areas of high coarsegrained sediment supply transport sediments to the deep-water area to develop large submarine fans (McHugh et al., 1998; Babonneau et al., 2002; Popescua et al., 2004; Covault et al., 2007; Lastras et al., 2009; Jobe et al., 2011), such as the canyons

Figure 9. Interpretations of seismic profiles along the strike direction of the slope, locations are in Fig. 5B. These profiles show the changes in geomorphologies and stacking patterns of modern canyons C1eC7 and their corresponded buried paleo-C1epaleo-C7, from shallow water to deep water. Note the vertically superimposed submarine fans developed in the late Miocene and abundant developments of submarine slope failures, focused fluid structures (gas chimneys, pipes and normal faults) and shallow gas occurred within the study area. BL ¼ basal lag; LIPs ¼ laterally inclined packages; ES ¼ erosion surface.

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Figure 9. (continued).

developed in the late Miocene in this current study (Figs. 3B, 9G and 12). In this period, the canyons' heads were all connected with the coarse-grained sediment sources which were reworked by the SCSWC on the outer shelf. When the sediment supply is low, the continental margin is dominated by slope-confined valleys/canyons formed by slope failures, and the ends of the valleys/canyons contain broad areas with chaotic seismic reflections interpreted as mass transport complexes, and absence of downslope sand rich fans/aprons (Saller and Dharmasamadhi, 2012), such as the continental margin developed in the middle Miocene and PlioceneQuaternary periods in this current study (Figs. 5B, 12A and D). In these periods, the canyons' heads were all away from the coarse grained sediments which were reworked by the SCSWC on the outer shelf. What's more, the sediment supply is regarded as the main driver of shelf margin growth (Carvajal et al., 2009). In the current study, the shelf break has remained stable in the northern margin of the Baiyun Sag since 23.8 Ma (Figs. 2 and 3A), indicating that most of the sediments supplied by the Pearl River Delta has been deposited on the shallow water shelf therefore the deep-water slope has become “starving” meanwhile the shelf margin has got steeper (i.e., aggradation-dominated continental margin). While, the steep slope environment is beneficial to the developments of submarine canyons and slope failures.

5.3.2. Regional tectonic activity The Dongsha Event at 10.5 Ma and the collision event at 5 Ma between Taiwan and the continental margin of East China might cause significant effect on the ocean current activities of the northern South China Sea by the inference from the strong sedimentary architectural contrasts before and after 10.5 Ma and 5.5 Ma. We could identify the lower slope-confined aggradationaldominated canyons and the upper lateral migration-dominated canyons separated by the horizon 10.5 Ma; we could also identify the strong truncation under the seismic horizon 5.5 Ma, and seismic horizon 5.5 Ma separated the lower typically unidirectionally migrating canyons and the upper non-obviously laterally migrating slope-confined canyons (Fig. 4). Moreover, the seismic horizons 10.5 Ma and 5.5 Ma could be respectively correlated with the seismic horizons tracked as boundaries of the middle Miocene/ late Miocene and the late Miocene/Pliocene by He et al. (2013). Meanwhile, unidirectionally migrating canyons were simultaneously developed in the late Miocene on the shelf margin in the Qiongdongnan Basin, approximately 250 km southwest of our study area (He et al., 2013). So we consider that the tectonic activities at the ends of the middle Miocene and the late Miocene might affect the paleoceanographic condition of the bottom current system around the continental margin of the northern South China Sea as summarized below.

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Figure 10. Trajectories of canyon thalwegs of the paleo-canyons developed in the period of the late Miocene. The trajectories a to g are traced corresponding to the seismic profiles A to G shown in Fig. 9. Each trajectory of paleo-canyon thalwegs has almost the same stacking patterns from aggradation with slightly lateral migration, to oblique lateral migration and finally to aggradation with slightly lateral migration, separated by the green circle and red circle. From the proximal to the distal of paleo-canyons, the laterally migrating distances of every paleo-canyon get shorten which indicate the canyons tend to be more aggradational. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.).

5.3.3. Paleo-ocean current Firstly, the ocean circulation in northern South China Sea still remains controversy. The ocean circulation of the South China Sea can be subdivided into surface circulation (less than 350 m water depth), intermediate water circulation (350e1350 m), and deep water circulation (more than 1350 m) (Chen and Wang, 1998; Chen, 2005). There is a consistent northeastward-directed current among the complex seasonal surface current, which is named the South China Sea Warm Current (SCSWC) by Guan (1978), straddling the shelf break region (Guan, 1978; Hsueh and Zhong, 2004; Xue et al., 2004; Guan and Fang, 2006). Zhu et al. (2010) has considered that the intermediate water moves along the slope from SW to NE, which consists of the SCSWC and the Kuroshio Current. Li et al. (2013) has considered that the SCSWC and the Kuroshio Current are surface waters, and the northeastward-directed UMCs found at a water depth between ~200 m and ~1200 m in the Baiyun Sag are mainly originated from the interaction between the circulation of the intermediate water and gravity flows. Secondly, He et al. (2013) has documented a series of UMCs in the Qiongdongnan Basin and all of these canyons are confined in

the outer shelf. So He et al. (2013) has concluded that the unidirectional migration of those submarine canyons is due to the SCSWC, and since the late Miocene the SCSWC has been active. In our current study, we could identify the undirectionally migrating canyons developed across the outer shelf and the continental slope in the late Miocene, but we did not identify the undirectionally migrating canyons developed on the outer shelf during the middle Miocene, Pliocene and Quaternary (Figs. 3A, C, 5A and 9A), so the paleo-ocean current conditions (e.g. the intensity) may have changed from the middle Miocene to Quaternary, hence the conditions of paleo-ocean current might affect the evolution history of the canyon system. 5.3.4. Sea level fluctuations The intensity of bottom current may imply spatial, obviously depth-related in the steep deep-water slope environment, while the sea level fluctuations control the water depth. During the late Miocene, at the early high sea level stage, the migratory trajectories of the canyon thalwegs showed more vertically superposed features; at the middle low sea level stage, the trajectories showed

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Figure 11. 3D normal and its corresponding specially handled instantaneous frequence seismic profiles, location is in Fig. 5. The seismic reflection turbidity, the pull-down of the local seismic events and the disorder and abnormal low instantaneous frequency reflections show the existence of gas chimneys. What's more, the shallow gas can be identified by the enhanced reflections and abnormal low instantaneous frequency reflections just under the modern canyons.

more laterally migrated features; and at the late high sea level stage, the trajectories showed more vertically superposed features again (Fig. 10). Since tectonic event was absent in the late Miocene, we infer that the sea level fluctuations might have a certain influence on the intensity of the along slope bottom current, consequently influenced the stacking patterns of the canyons (Figs. 2 and 10). During the Pliocene, the sea level was so high (Fig. 2), and we could not identify typical LIPs deposited on the west canyon walls. In addition, when the sea level is low, the distance between the canyon head and the shallow water coarse-grained sediment supply zone is usually short, which may have introduced a condition in that the canyon head captures the coarse-grained detrital material to form downslope eroding gravity flows. When the sea level is high, the shallow water coarse-grained sediment supply zone may have been far away from the canyon head. Consequently the canyon head could not capture the coarse-grained detrital material to form downslope eroding gravity flows. These conditions can explain why the gravity flow deposits (e.g., Basal lags) are overlaid by LIPs within the submarine canyon (Fig. 8). So, the sea level fluctuations might control the evolution of the canyon system.

5.4. Two different types of undirectionally migrating submarine canyons Based on the above discussions, two types of undirectionally migrating canyons in this area are summarized (Fig. 14). The first type of undirectionally migrating submarine canyons (Fig. 14A) correspond to the canyons developed in the late Miocene. The morphology of the canyons is obviously asymmetric Ueshaped. They indent the shelf edge and with their heads coupling with areas of high coarse-grained sediment supply, generating canyon-fills with sand-rich basal lags overlaid by slump and debris-flow deposits and laterally inclined packages. During the early stage of the canyon development, only thin coarse-grained basal lags were deposited on the thalwegs of the canyons, and most of the materials fed by the ocean current transportation on the outer shelf were captured by the canyon head and transported through the canyon by the turbidity current of gravity flows into the lower-slope or deep-sea basin to form submarine fans/aprons. The slump and debris flow deposits mostly fed by the southwest canyon wall failures were deposited within the canyon upon the basal lags in the middle stage of the canyon development. During the last stage of the canyon development, the bottom current is dominant and

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Figure 12. Focused fluid structures and the overlying abundant slope failures around the canyon head and canyon wall, showing close relationship with each other. The pipes are shown as the dark dotted lines with the arrow. Locations of the profiles are shown in Fig. 5. (A) Seismic profile showing the canyon head headward-erosion and the canyon wall failure during the Pliocene-Quaternary stage. It also shows the Quaternary MTDs and the late Miocene submarine fans in the NE part of this seismic profile. The focused fluid structures in this profile contain normal faults and pipes. (B) and (C) Seismic profile and its corresponding geologic sketch map across the well B6-1-1. (D) Seismic profile showing the late Miocene superimposed submarine fans, upper Miocene paleo-canyons (C3 and C4), Quaternary MTDs and seafloor small scaled channels, as well as the abundant focused fluid structures (gas chimney, normal faults, pipes and pockmarks).

the LIPs are generated along the southwestern flank of a canyon. The activity of bottom current is strong in the upper-middle segment of the canyons consequently the thickness of LIPs is thick and the canyons is represented by long-distant unidirectional lateral migration. The second type of undirectionally migrating submarine canyons (Fig. 14B) correspond to the canyons developed in the middle Miocene, Pliocene and Quaternary. They do not indent the shelf edge and their heads are associated with areas of “starving” sediment supply, form highly aggradational, mostly less-obviously asymmetric U-shaped morphologies. During the early stage of the canyon development, there were none coarse basal lags deposited on the thalwegs of the canyons, most of the materials supplied by canyon head mass wasting and canyon wall failures were deposited locally on the canyon thalwegs or transported just nearby in the forms of slump and debris-flow deposits, and they were overlaid by thin canyon confined sheets and mud-rich channel-levee complexes in the middle stage of the canyon development. Moreover, there were no downslope fans/aprons

associated with them. During the last stage of the canyon development, the bottom current is dominant and the LIPs are generated along the southwestern flank of a canyon. The intensity of bottom current is relatively weak in the upper-middle segment of the canyons consequently the thickness of LIPs is thin and the canyons are almost aggradational with low laterally migration perpendicularly to the canyon axial direction. 6. Conclusions Using 2D/3D seismic and well data, the current study documents the geomorphology, sedimentary architecture, stacking pattern, evolution history and controlling factors of the Baiyun canyon system from the middle Miocene to Quaternary in Pearl Mouth Basin, and the results show that: (1) Seven regularly-spaced submarine canyons and their corresponding buried canyons, named C1 to C7 (and paleo-C1 to paleo-C7) have been detailedly investigated. Five

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Figure 13. The evolution of the Baiyun canyon system. Horizons 13.8 Ma, 10.5 Ma, 5.5 Ma, 1.9 Ma and seabed show the ancestral canyon systems in the middle Miocene, late Miocene, Pliocene, Quaternary and nowadays, respectively. Note only the canyon system in Fig. 13B indents the shelf edge. What's more, the lateral migrating distance of the canyon system in the late Miocene (Horizon 10.5 MaeHorizon 5.5 Ma) is significantly larger than other canyon systems.

architectural elements of canyon-fills are identified, including basal lag (BL), slump and debris-flow deposits (SDFDs), canyon confined sheets (CCSs), laterally inclined packages (LIPs) and channel-levee complexes (CLCs). (2) The evolution of Baiyun submarine canyon system could be separated into three stages. During the middle Miocene stage (13.8 Ma ~10.5 Ma), typical aggradation-dominated unidirectional migrating canyons developed. The canyons were all confined in the slope environment and not incised the shelf edge. During the late Miocene stage (10.5 Ma ~5.5 Ma), spectacular typical regularly-spaced unidirectionally migrating canyons were dominated and they incised the shelf edge. During the Pliocene-Quaternary stage (5.5 Ma ~ nowadays), abundant slope failures and nontypical unidirectional migrating starving canyons

dominated. In this stage, the canyons were all confined in the slope and the shelf edge was not incised by the canyons; moreover, the lateral migration characteristics of these canyons were not obvious. (3) The extensive activities of focused fluid during the Pliocene and Quaternary have facilitated the triggering of the different scaled submarine slope failures in the Baiyun Sag, and these slope failures controlled and influenced the topography of the buried and modern canyons in the Pliocene and Quaternary. (4) The evolution of the Baiyun canyon system was likely to be controlled by sediment supply, regional tectonic activity, paleo-ocean current and sea level fluctuations. (5) Two different types of undirectionally migrating canyons in this area are recognized. The first type canyons indent the

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Figure 14. Two types of undirectionally migrating canyons in the Baiyun Sag, named as Type-A and Type-B unidirectionally migrating canyons. Type-A canyon developed in the late Miocene, the Type-B canyon developed in the middle Miocene, Pliocene and Quaternary. Abbreviations of canyon-fills: BL ¼ basal lag; SDSFs ¼ slump and debris-flow deposits; LIPs ¼ laterally inclined packages; CCSs ¼ canyon confined sheets; CLCs ¼ channel-levee complexes.

shelf edge, form U-shaped heads which are associated with areas of high coarse-grained sediment supply, generating canyon-fills with sand-rich basal lags overlaid by slump and debris flow deposits and laterally inclined packages, and downslope submarine fans/aprons. The second type canyons do not indent the shelf edge, form U-shaped heads which are associated with areas of starving sediment supply, generating canyon-fills with mud-rich BL in the bottom grade upward into SDFDs, thin CCSs and CLCs and finally into LIPs, which are dominated by mass wasting processes, exhibiting highly aggradational morphologies, mud-rich fill, and absence of downslope fans/aprons. Both the two types of canyons result from the interaction between gravity flows and bottom current, however, the first type is influenced by the surface current (mainly the South China Sea Warm Current) and the intermediate water current, but the second type is only influenced by the intermediate water current.

Acknowledgments This study was funded by the National Natural Science Foundation of China (Grants NO. 41372115 and 40972077). We gratefully acknowledge the CNOOC Research Institute for providing the seismic and well data. Special thanks go to reviewers, Andrea Fildani and an anonymous reviewer and associate editor Makoto Ito for their insightful reviews and constructive suggestions to make the paper a better contribution.

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