Sediment waves on the South China Sea Slope off southwestern Taiwan: Implications for the intrusion of the Northern Pacific Deep Water into the South China Sea

Sediment waves on the South China Sea Slope off southwestern Taiwan: Implications for the intrusion of the Northern Pacific Deep Water into the South China Sea

Marine and Petroleum Geology 32 (2012) 95e109 Contents lists available at SciVerse ScienceDirect Marine and Petroleum Geology journal homepage: www...

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Marine and Petroleum Geology 32 (2012) 95e109

Contents lists available at SciVerse ScienceDirect

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

Sediment waves on the South China Sea Slope off southwestern Taiwan: Implications for the intrusion of the Northern Pacific Deep Water into the South China Sea Chenglin Gong a, b, *, Yingmin Wang a, b, Xuechao Peng c, Weiguo Li d, Yan Qiu c, Shang Xu 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), Beijing 102249, China Guangzhou Marine Geological Survey, Guangzhou Province 510760, China d BP America Inc., Houston, TX 77079, USA b c

a r t i c l e i n f o

a b s t r a c t

Article history: Received 11 July 2011 Received in revised form 2 December 2011 Accepted 15 December 2011 Available online 24 December 2011

Using an integrated multi-beam bathymetry, high-resolution seismic profile, piston core, and AMS 14C dating data set, the current study identified two sediment wave fields, fields 1 and 2, on the South China Sea Slope off southwestern Taiwan. Field 1 is located in the lower slope, and sediment waves within it are overall oriented perpendicular to the direction of down-slope gravity flows and canyon axis. Geometries, morphology, and internal seismic reflection configurations suggest that the sediment waves in field 1 underwent significant up-slope migration. Field 2, in contrast, is located more basinward, on the continental rise. Instead of having asymmetrical morphology and discontinuous reflections as observed in field 1, the sediment waves in field 2 show more symmetrical morphology and continuous reflections that can be traced from one wave to another, suggesting that vertical aggradation is more active and predominant than up-slope migration. Three sediment wave evolution stages, stage 1 through stage 3, are identified in both field 1 and field 2. During stage 1, the sediment waves are built upon a regional unconformity that separates the underlying mass-transport complexes from the overlying sediment waves. In both of these two fields, there is progressive development of the sediment waves and increase in wave dimensions from the oldest stage 1 to the youngest stage 3, even though up-slope migration is dominant in field 1 whereas vertical aggradation is predominant in field 2 throughout these three stages. The integrated data and the depositional model show that the upper slope of the study area is strongly dissected and eroded by down-slope gravity flows. The net result of strong erosion is that significant amounts of sediment are transported further basinward into the lower slope by gravity flows and/or turbidity currents. The interactions of these currents with bottom (contour) currents induced by the intrusion of the Northern Pacific Deep Water into the South China Sea and preexisting wavy topography in the lower slope result in the up-slope migrating sediment waves in field 1 and the contourites as observed from cores TS01 and TS02. Further basinward on the continental rise, turbidity currents are waned and diluted, whereas along-slope bottom (contour) currents are vigorous and most likely dominate over the diluted turbidity currents, resulting in the vertically aggraded sediment waves in field 2. The results from this study also provide the further evidence for the intrusion of the Northern Pacific Deep Water into the South China Sea and suggest that this intrusion has probably existed and been capable of affecting sedimentation in South China Sea at least since Quaternary. Ó 2011 Elsevier Ltd. All rights reserved.

Keywords: Sediment waves Interaction between down- and along-slope processes South China Sea Slope off southwestern Taiwan Intrusion of the Northern Pacific Deep Water into the South China Sea

1. Introduction

* Corresponding author. College of Geosciences, China University of Petroleum, Beijing 102249, China. Tel.: þ86 15210726942. E-mail address: [email protected] (C. Gong). 0264-8172/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.marpetgeo.2011.12.005

Deepwater sediment waves are one of the most distinct and frequently described submarine bedforms (Wynn, 2000; Cattaneo et al., 2004; Wynn and Masson, 2008; Stow et al., 2009; Cartigny et al., 2011; Andrew, 2011). Over the last five decades, sediment waves have been observed and documented throughout the world,

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from shelf, to continental slope and rise, and to abyssal-plain environments (Ediger et al., 2002; Lewis and Pantin, 2002; LomKeil, et al., 2002; Mosher and Thomson, 2002; Wynn and Stow, 2002). Based on their processes of formation, sediment waves can be classified into four different groups, and these are: (1) Turbidity current sediment waves derived from down-slope processes. This type of sediment waves has been widely documented (Damuth, 1979; Lewis and Pantin, 2002; Migeon et al., 2004). They are commonly formed by confined flows within submarine canyons and/or channels or by expanding flows in channel-lobe transition zones and thus have limited distribution (Mulder and Alexander, 2001; Faugères et al., 2002; Wynn et al., 2002; Arzola et al., 2008). (2) Bottom current sediment waves induced by along-slope processes. The generation of bottom current sediment waves is related to bottom current activities, and they commonly cover very large areas of many slope and basin-floor settings where the persistent actions of bottom currents are active (Faugères et al., 2002; Lewis and Pantin, 2002; Masson et al., 2002; MacLachlan et al., 2008). Bottom current sediment waves could provide records of long-term bottom current circulation patterns, and the changes in the internal geometries and morphology of this type of sediment waves have been suggested as the evidence for the changes in bottom current activities (Cunningham and Barker, 1996; Lom-Keil, et al., 2002; Masson et al., 2002). (3) Sediment waves induced by sediment instability and failure (softsediment deformation features). This type of sediment waves usually has undulating or wavy morphology and occurs where slope gradients and sedimentation rate are high (Wynn and Stow, 2002). The development of this type of sediment waves can be triggered by a variety of mechanisms, including slope

failure, shear force instabilities, earthquake shaking, gravitational instabilities, gas escape, and constant strain (Faugères et al., 2002; Holbrook et al., 2002; Lee et al., 2002; Heifetz et al., 2005). (4) Interactions between down- and along-slope processes generated sediment waves (IDASPWs). Many continental margins are built up by the interactions between down- and along-slope processes (e.g. Gulf of Mexico, Antarctic Peninsula, Demerara continental rise, Bay of Biscay, and Gulf of Cadiz) (Robesco et al., 1996; Gonthier et al., 2002; Kenyon et al., 2002; Faugères et al., 2002; Hernández-Molina et al., 2006). The interactions between down- and along-slope processes account for many bedforms and sediment waves formed in deep-water settings (Hernández-Molina et al., 2006; Mulder et al., 2008). Similar to contourites, IDASPWs play a critical role in paleoceanographic studies, in that their architecture and geometries are argued as the direct responses to paleoceanographic circulation patterns (Stow, 2002). Despite their importance, IDASPWs are relatively poorly understood depositional features. Previous work on the first three types of sediment waves has considerably improved our knowledge of their geometries, morphology, and forming processes (Normark et al., 1980; Migeon et al., 2000; Wynn, 2000; Wynn and Stow, 2002; HernándezMolina et al., 2006; Wynn and Masson, 2008). There is, however, still much to learn about IDASPWs, particularly their initiation, evolution, and paleoceanographic implications (Migeon et al., 2000). The large sediment wave fields developed in the South China Sea Slope off southwestern Taiwan and the rich amount of data in this area provide an ideal opportunity to bridge the gap (Figs. 1 and 2). The present study focuses on two sediment wave fields,

Figure 1. Bathymetric map showing the location of the study area as well as the pathways (red arrows) for the intrusion of the Northern Pacific Deep Water into the South China Sea via the Bashi Channel and the Luzon Strait. The pathway of the Northern Pacific Deep Water is modified from Xie (2009). The inset rectangle illustrates the location of Figure 2. Map modified from the images provided by the Institute of Oceanography, National Taiwan University. NPDW ¼ the Northern Pacific Deep Water (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.).

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Figure 2. Detailed bathymetric image of the study area showing the locations of the studied sediment wave fields, fields 1 and 2, the piston cores TS01 and TS02, and other major geological elements in the area. The dashed lines show the locations of the high-resolution seismic profiles in Figures 4 and 5. The dotted boxes illustrate the multi-beam bathymetry surveys shown in Figures 3 and 9 (a) respectively. Bathymetric contours are in meters. See Figures 6e8 for detailed analyses of these two cores.

fields 1 and 2. Sediment wave field 1 is located to the north of field 2. Sediment wave field 2 was first identified by Damuth (1979), and the study interpreted the wave field as the result of turbidity currents. However, it has already been accepted that strong bottom (contour) currents induced by the intrusion of the Northern Pacific Deep Water into the South China Sea exist in the area (Chao et al., 1996; Qu et al., 2000, 2006; Li et al., 2008; Zhao et al., 2009; Xie, 2009). These currents could have a strong influence on the initiation and development of the sediment waves in field 2. The previous interpretation, thus, needs to be re-evaluated. Using newly available high-quality 2D seismic, multi-beam bathymetry, piston core, and 14C age dating data, the current study attempts to (1) describe the geometries and morphology of the sediment waves in fields 1 and 2; (2) investigate the initiation, evolution, and depositional processes of the sediment waves in these two fields; and (3) address the paleoceanographic implications of the studied sediment waves. 2. The study area and associated deepwater circulation The study area is located in the South China Sea Slope off southwestern Taiwan (Figs. 1 and 2). The Taiwan Island was formed by the oblique collision between the Luzon Arc and the passive Chinese margin since 5 Ma (Lallemand and Tsien, 1997; Huang et al., 2000; Lin et al., 2009). The study area is dissected by the Taiwan Canyon, the Penghu Canyon, and well-developed erosional gullies (Fig. 2). The Taiwan Canyon cuts deeply into the eastern segment of the northern South China Sea Slope and extends southeastwards over 110 km (Yu and Song, 2000). The Penghu Canyon is a multi-head submarine canyon and is elongated NeS

along the intersection between the South China Sea Slope and the Gaoping Slope (Yu and Chang, 2002). These two canyons gradually merge into the northern Manila Trench (Fig. 2). In addition, the upper slope of the is intensely eroded by NWeSE trending gullies (Fig. 2). These erosional gullies cut the upper slope deeply and act as conduits for the transport of large amounts of sediment to the toe of the South China Sea Slope off southwestern Taiwan where two sediment wave fields, fields 1 and 2, are observed. The South China Sea is the largest marginal sea in the western Pacific Ocean, and its ocean circulation is strongly influenced by the semi-enclosed basin physiography (Chao et al., 1996; Qu et al., 2000, 2006; Zhu et al., 2010). The intrusion of the Northern Pacific Deep Water into the South China Sea (Fig. 1) has strong impacts on sedimentation in South China Sea, and the evidence for the intrusion and its associated bottom (contour) currents include: (1) the occurrence of many bottom current induced deposits and bedforms, such as drifts (Lüdmann et al., 2005), contourite channels (Shao et al., 2007), and unidirectionally-migrating channels (Zhu et al., 2010); (2) density and dissolved oxygen (O2) measurements suggesting the intrusion of the cold dense Northern Pacific Deep Water into the South China Sea via the Bashi Channel (Qu et al., 2006; Tian et al., 2009); (3) reversal of the benthic foraminifera d13C gradient between ODP sites 1146 (2029 m) and 1148 (3294 m) in the South China Sea indicating an increase in the southward flux of low d13C (more than 2000 m) (Shevenell et al., 2004; Tian et al., 2009); (4) results of high-resolution circulation modeling strongly suggesting the existence of the deep South China Sea circulation (Qu et al., 2006); and (5) direct observations of deepwater circulation between the South China Sea and the Northern Pacific Ocean (Xie, 2009). These observations indicate

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that the Northern Pacific Deep Water enters the South China Sea via the Luzon strait with a mean velocity of 0.15 m s1, and maximum velocity of 0.3 m s1 (Xie, 2009). The study area, the South China Sea Slope off southwestern Taiwan, is thus most probably strongly affected by the westward flowing bottom (contour) currents associated with this intrusion (Fig. 1). 3. Data and methods Data used in this study were provided by the Guangzhou Marine Geological Survey, and include a full suite of multi-beam bathymetry, high-resolution seismic-reflection profiles, piston cores, and the results of AMS 14C dating. 3.1. Multi-beam bathymetry SIMRAD EM950 multi-beam echo-sounder data cover a large region of the study area (Figs. 1e3). The seafloor topography of the two sediment wave fields was imaged in detail using these data (Figs. 2 and 3). 3.2. High-resolution seismic-reflection profiles The high-resolution seismic-reflection profiles run obliquely through the sediment wave fields (Figs. 2, 4 and 5). These highquality seismic data were used to characterize the wavelengths, height, thickness, and migration directions of the sediment waves in fields 1 and 2. 3.3. Piston cores Many previous studies of sediment waves were restricted by the lack of core data. Two long piston cores, core TS01 (699 cm) and core TS02 (690 cm), were acquired from the study area (see Fig. 2 for core locations). And a series of analysis, including detailed

geological descriptions, grain size, micropaleontology, and heavy mineral were conducted, and the results were presented in Figures 6e8 and Tables 1e5. 3.4. AMS

14

C dating

AMS 14C dating was obtained from the single-species planktonic foraminifera of Globigerinoides sacculifer, and the work was conducted by Kiel University. Results of AMS 14C dating were present in Table 6. 4. Results and observations 4.1. Intrusion of the Northern Pacific Deep Water into the South China Sea Murray (2006) suggested that benthic foraminifera prefer living in certain bathymetric depth ranges closely associated with particular deepwater mass. Benthic foraminifera are, thus, widely used as water mass indicators. Planulina wuellerstorfi, Bulimina aculeate, and Eggerella bradyi have been commonly accepted as typical benthic foraminifera that live in the Northern Pacific Deep Water (Qiu et al., 2007; Zhao et al., 2009). As presented in Tables 2e5, all these three benthic foraminifera have been identified from cores TS01 and TS02 collected from the lower slope (see Tables 2e5 for foraminiferal contents). The occurrence of these three benthic foraminifera in cores TS01 and TS02, thus, can be taken as the solid evidence for the intrusion of the Northern Pacific Deep Water into the South China Sea along the lower slope and continental rise of the study area (Fig. 1; Tables 2 and 3). In addition, the foraminiferal assemblage identified in cores TS01 and TS02, as discussed below, suggest the Quaternary deposits. The intrusion of the Northern Pacific Deep Water into the South China Sea therefore have existed and been capable of affecting the sedimentation in the study area and the South China

Figure 3. (a) Multi-beam bathymetry map (plan view) showing two sediment wave fields. (b) Sketch of the wave crests shown in (a). Crests of the sediment waves in fields 1 and 2 are marked in black and red dotted lines respectively. Notice that crest lines of the sediment waves in field 1 are slightly sinuous and show regular bifurcation, and align perpendicular to the down-slope SE flowing gravity flows and canyon axis. In contrast, crests of the sediment waves in field 2 are straighter, and are parallel or subparallel to the bathymetric contours. See Figure 2 for the location of the bathymetric survey. swf ¼ sediment waves in field.

Figure 4. (a) High-resolution seismic-reflection profiles cross the sediment waves in field 1 showing the overall characteristics of the sediment waves in field 1. The inset rectangle illustrates the position of the enlarged image shown in Figure 4 (b). (b) The enlarged image showing the geometries, morphology, and internal reflection configurations of the sediment waves in more detail. Notice that individual sediment waves have an asymmetrical seismic reflection, a thicker up-slope flank and a thinner down-slope flank. Reflections on the up-slope sides are more continuous than those on the down-slope sides, and there is an overall up-slope increase in wave dimensions. Three evolution stages, stage 1 through 3, are identified and are correlated through successive sediment waves. The initiation surface is a regional unconformity that separates the mass-transport complexes below from the sediment wave deposits above. The solid arrow in (a) indicates up-slope migration of the sediment waves, and the arrowed lines in (b) indicate the growth directions of the sediment waves. See Figure 2 for locations of the seismic profiles.

Figure 5. High-resolution seismic profiles showing the difference in geometries, morphology, and seismic reflection configurations between the sediment waves in fields 1 and 2. The dotted boxes illustrate the location of Figures 5 (a) and (b). The profile in Figure 5 (a) is mostly located in field 1, but covers part of field 2 to the right. Sediment waves in field 1 are asymmetrical and have longer wavelengths, whereas those in field 2 are more symmetrical with shorter wavelengths. Seismic profile in Figure 5 (b) is located in field 2 and shows more details of the depositional characteristics of the sediment waves in it. The initiation surface and the three evolution stages 1 through 3 identified in field 1 can be traced to field 2. Notice that the transparent, chaotic seismic reflections are related to the mass-transport complexes, and end up right at the boundary between these two fields, suggesting that down-slope gravity flows most probably play minor role in the development of sediment waves in field 2.

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Figure 6. (a) Measured section and results of various analyses of the piston core TS01. A sandy and a silty facies are identified, the sorting coefficient shows that deposits in both of these two facies are well to very-well sorted. (b) Measured section and results of various analyses of the piston core TS02. Core TS02 mainly consists of clayed silts and sandy silts that are interpreted as silty contourites. C1 through C16 are the core sample positions. Results from these analyses, coupled with the grain size cumulative frequencies as shown in Figure 8, suggest that both down-slope turbidity currents and along-slope bottom (contour) currents are active and important in the study area at least since the Quaternary (2.5 Ma). PF abundance ¼ Planktonic foraminifera abundance, BF abundance ¼ Benthic foraminifera abundance, and R abundance ¼ Radiolaria abundance. See Figure 2 for the locations of the piston cores and text for more details.

Figure 7. Photos of the piston core TS01 showing the major facies and sharp upper contacts. The clean sands consist of abundant bio-skeletons and shell fragments as shown by the red dots in the inset photo and commonly have sharp upper contacts with the silty facies (red arrows) above. The sandy and the silty deposits are interpreted as sandy and silty contourites respectively. See Figure 2 for core location. Core depths are in centimeters (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.).

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Figure 8. Cumulative frequency diagrams showing the grain size characteristics of the sandy (a) and silty facies (b) in core TS01. See Figure 2 for core location and Figure 6 for positions of samples C1 through C16.

Sea at least since Quaternary. Previous studies, as discussed earlier, also suggest that there is the intrusion of the Northern Pacific Deep Water into the South China Sea (Shevenell et al., 2004; Lüdmann et al., 2005; Qu et al., 2006; Shao et al., 2007; Tian et al., 2009; Xie, 2009) (Fig. 1). The studied sediment waves, thus, are most probably significantly affected by the bottom (contour) currents resulted from this intrusion. 4.2. Location and morphology of the sediment waves observed from multi-beam bathymetry 4.2.1. Sediment waves in field 1 Sediment wave field 1 lies upon the lower slope of the South China Sea Slope off southwestern Taiwan at a water depth of 3000e3200 m. It covers an area of approximately 2880 km2 and is bounded to the east by the Penghu Canyon and to the west by the Taiwan Canyon. Additionally, sediment waves in field 1 are commonly developed immediately basinward of the mouths of slope channels and/or gullies (Figs. 2 and 9a). In plan view, the crests of the sediment waves in field 1 (see the black dotted lines in Figs. 3b and 9a) are generally oriented SWeNE, a direction roughly perpendicular to the down-slope SE flowing gravity flows and the canyon axis. The sediment waves can be continuous along strike for up to 55.2 km. Their crest lines are

Table 1 Thickness and abundance of the silty and sandy facies observed from piston core TS01 located in the Taiwan Canyon, the South China Sea Slope off southwestern Taiwan. For core location see Fig. 2. Core TS01

Composition Silty facies

Thickness (cm) Percentage

Sandy facies

Clayey silts

Sandy silts

Silts

Silty sands

Sands

456 90

30 6

22 4

53 28

138 72

either linear or mildly sinuous, and in many cases bifurcate (bifurcation points marked in Fig. 3b). 4.2.2. Sediment waves in field 2 Sediment wave field 2, first identified by Damuth (1979), is located along the right-hand side of the Manila Trench (looking down from Manila Trench), and covers an area of 2738 km2. The sediment waves in this field extend up dip to the 3200 m bathymetric contour and down dip to the 3400 m bathymetric contour (Figs. 2 and 9a). Due to limited coverage of the multi-beam bathymetry data, only part of the sediment waves in field 2 is imaged (see the red dotted line in Fig. 3b). Compared with those in field 1, the crest lines of the sediment waves in field 2 are straighter, and show less bifurcation and in general are oriented parallel or subparallel to bathymetric contours, although the orientation occasionally varies (Figs. 3b and 9a). 4.3. Geometries and reflection configurations of the sediment waves observed from high-resolution seismic profiles 4.3.1. Sediment waves in field 1 The sediment waves in field 1 display a variety of dimensions with wavelengths ranging from 0.5 km to 4 km (average 2.6 km) and wave heights between 7 m and 117 m (average 28 m) (Figs. 4 and 5a). High-resolution seismic-reflection profiles show that individual sediment waves usually display an asymmetrical morphology with thicker, steeper, and shorter up-slope flanks (Figs. 4 and 5a). Sediments on the up-slope flanks are commonly well-layered and show parallel or subparallel seismic reflections, whereas those on the down-slope flanks are thin and less well-layered, corresponding to discontinuous or truncated seismic reflections. Sediments are deposited preferentially on the up-slope flanks, and bypassing and erosion, in contrast, are more important on the down-slope sides. In response to up-slope wave migration, there is overall up-slope

102

Table 2 Abundance of benthic foraminifera associated with the Northern Pacific Deep Water in core TS01 at core depth of 0e360 cm (see Fig. 2 for core location). Taxa (samples/10 g)

Depth (cm), TS01 0w20 20w40 40w60 60w80 80w100 100w120 120w140 140w160 160w180 180w200 200w220 220w240 240w260 260w280 280w300 300w320 320w340 340w360

Planulina wuellerstorfi 0 Bulimina aculeata 0 Eggerella bradyi 0

72 0 0

0 15 0

0 24 6

16 0 0

0 0 8

0 0 4

5 3 0

2 5 0

0 0 72

0 0 0

0 0 24

0 0 18

144 0 0

0 0 0

0 0 0

0 0 0

0 0 0 C. Gong et al. / Marine and Petroleum Geology 32 (2012) 95e109

Table 3 Abundance of benthic foraminifera associated with the Northern Pacific Deep Water in core TS01 at core depth of 360e699 cm (see Fig. 2 for core location). Taxa (samples/10 g)

Depth (cm), TS01 360w380 380w400 400w420 420w440 440w460 460w480 480w500 500w520 520w540 540w560 560w580 580w600 600w620 620w640 640w660 660w680 680w699

Planulina wuellerstorfi 0 Bulimina aculeata 0 Eggerella bradyi 0

0 0 0

0 0 0

0 0 4

0 0 5

0 0 10

0 12 12

0 0 24

15 0 10

0 0 4

0 0 12

0 0 0

0 8 3

0 0 0

0 0 6

0 0 3

0 0 0

Table 4 Abundance of benthic foraminifera associated with the Northern Pacific Deep Water in core TS02 at core depth of 0e346 cm (see Fig. 2 for core location). Taxa (samples/10 g)

Depth (cm), TS02 0w20

20w36

36w63

63w83

83w100

120w148

148w167

167w190

190w194

220w230

230w238

238w257

260w286

289w300

328w346

Planulina wuellerstorfi Bulimina aculeata Eggerella bradyi

0 0 0

0 0 0

0 0 1

0 0 3

1 0 0

0 0 0

0 3 0

0 0 0

0 0 0

3 0 0

0 1 0

0 0 0

0 0 0

0 0 2

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Table 5 Abundance of benthic foraminifera associated with the Northern Pacific Deep Water in core TS02 at core depth of 346e690 cm (see Fig. 2 for core location). Taxa (samples/10 g)

Planulina wuellerstorfi Bulimina aculeata Eggerella bradyi

Depth (cm), TS02 346w367

367w376

376w392

392w400

420w445

445w470

470w500

520w538

538w560

560w580

580w600

620w640

640w660

660w670

670w690

0 0 0

2 128 0

0 0 1

0 0 1

0 256 64

0 6144 0

0 64 0

0 0 64

0 2 0

1 1 0

0 0 0

0 4 0

0 0 0

0 0 1

0 0 0

103

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Table 6 A summary of the depths of the samples in the piston cores TS01 and TS02 used for AMS 14C age dating and dating results. Core

Depth (cm)

Calendar age (yrs B.P)

TS01 TS01 TS02 TS02 TS02

260w280 360w387 328w346 367w376 420w450

16,380  80 19,020  100 15,190  70 12,160  60 14,250  70

increase in wave dimensions (Figs. 4 and 5a). In addition, the sediment waves in field 1 are dissected by the Taiwan and the Penghu canyons, as evidenced by the wide occurrence of truncations observed from seismic-reflection profiles (Figs. 4, 5 and 9a).

4.3.2. Sediment waves in field 2 The wavelength of the sediment waves in field 2 ranges from 1.5 km to 5.4 km with an average of 1.9 km. Wave heights of sediment waves in this field can be up to 110 m, with an average of 80 m (Fig. 5b). In contrast to sediment waves in field 1, these in field 2 are generally more symmetrical, and the associated sediments are better layered, resulting in more continuous, parallel or subparallel reflections on both flanks of the sediment waves. These continuous reflections can be traced from one wave to another. All these characteristics suggest that deposition occurs both on the up-slope and the down-slope flanks, and sediment waves in field 2 are dominated by vertical aggradation, rather than up-slope migration as observed in field 1. Similar to sediment waves in field 1, those in

Figure 9. (a) Three-dimensional bathymetric image showing the morphology and major depositional elements in the study area. Notice the wide occurrence of channels, canyons and gullies in the upper slope, suggesting significant erosion by gravity flows. Two sediment wave fields are shown up clearly in plan view. (b) Schematic diagram summarizing the major depositional processes in different region of the study area. The upper slope is characterized by gravity flow erosion and sediment bypass. Sediments shed from the upper slope are transported basinward into the lower slope where the interactions of down-slope turbidity currents and along-slope bottom (contour) currents induced by the intrusion of the Northern Pacific Deep Water into the study area (INPDW) result in the development of sediment wave field 1. Further basinward into the continental rise where sediment wave field 2 is develop, turbidity currents are diluted and along-slope bottom currents are the dominant processes. The directions of the bottom (contour) currents resulted from INPDW is inferred from the elongation directions and the cross-section geometries of the sediment waves as well as the lee-wave model (Flood, 1988). INPDW ¼ the intrusion of the Northern Pacific Deep Water into the study area, AEDSW ¼ average elongation direction of sediment waves. See Figure 2 for location of the 3D survey.

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field 2 are also incised by the Taiwan and the Penghu canyons (Fig. 9a). 4.4. Results from analysis of the piston core TS01 4.4.1. Lithofacies Piston core TS01 is 669 cm long, and was collected from the Taiwan Canyon at a water depth of 3284 m (see Fig. 2 for core location). The core is mainly composed of two lithofacies: a silty facies (up to 90% clayey silt and 10% sandy silt and silt) and a sandy facies (72% sand with abundant bio-skeletons or shell fragments and 28% silty sand) (Figs. 6a and 7; Table 1). 4.4.2. Key results observed from core TS01 and their interpretation Previous studies have shown the value of detailed core description and analysis in understanding the formation of sediment waves (Mulder and Alexander, 2001; Gonthier et al., 2002; Migeon et al., 2004; Arzola et al., 2008). Key results from analyses of the piston core TS01 are here summarized: (1) Micropaleontological analyses show three sandy zones (marked by the three pale yellow zones in Fig. 6a) that have relatively high concentrations of both planktonic and benthic foraminifera (Fig. 6a). Some of these planktonic and benthic foraminifera are commonly abraded and fragmented and are linked to species living in water depth less than 500 m. (2) The sandy and silty facies are mostly fine- to medium-grained, with a mean size of 2e7 F (Figs. 6ae8; Table 1). (3) The majority of the sediments in both the sandy and silty facies has a sorting coefficient of less than 2.5, and a fraction of the sediments have a sorting coefficient close to 1.2, suggesting that sorting processes are very efficient during deposition (Fig. 6a). (4) Compared with typical turbidites, no inversion of the ages has been observed at core depth of 260e389 cm (Fig. 6a; Table 6). (5) Each of the sandy facies at core depths of 20e25 cm, 191e200 cm, and 260e389 cm (marked by the three pale yellow zones in Fig. 6a) consists of matrix-poor, clean sands and have sharp upper contacts with the overlying silty facies as marked by the red arrows in Figure 7. (6) The core also contains a significant mount of radiolarians, particularly in the silty facies (Fig. 6a). (7) The sandy facies consist of a significant amount of bioskeletons and shell fragments (marked by the red dots in the inset photo of Fig. 7). (8) On the cumulative frequency plots, the sandy facies commonly show a tripartite subdivision into a coarser-grained bedload fraction moved by traction load, an intermediate fraction associated with saltation load, and a finer-grained fraction transported wholly in suspension (Fig. 8a). The cumulative frequency diagram of the silt facies (Fig. 8b), in contrast, consists of a minor coarser-grained fraction associated with saltation load and a major suspended fraction which is transported in suspension. (9) Micropaleontological analyses show that benthic foraminifera are dominated by five species, Brizalina spp., Amphistegina spp., Melonis spp., Bolivina spp., and Bulimina spp, and Planktonic foraminifera are represented by Globigerinoides quadrilobatus, G. ruber, G. conglobatus, G. sacculifer, Globorotalia tumida, G. inflata, G. ungulata, G. menardii, G. flexuosa, G. truncatulinoides, Pulleniatina obliquiloculata, Neogloboquadrina dutertrei, N. blowi, N. eggeri, Globigerina bulloides, Globigerinita glutinata, and Globogerinella aequilateralis. The foraminiferal assemblage as identified in core TS01 suggests Quaternary deposits.

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Interpretation. The occurrence of abraded and fragmented shallow water foraminifera in the core TS01 indicates the transportation of sediments from shelf and/or upper slope basinward into the core site by gravity flows. The sandy and silty facies in the core TS01, thus, can be interpreted as turbidites. The common occurrence of sharp upper contacts and the lack of age inversion in the core suggest that processes other than turbidity currents most probably have impacted deposition, because the contacts would be gradational and there would be age inversion if only gravity flows or turbidity currents are active (Shanmugam, 2008). The core sediments are commonly rich in radiolarians and bio-skeletons, and all these are in line with many of the observations in contourites (Shanmugam et al., 1993; Stow, 2002; Stow and Faugères, 2008). Grain size distribution patterns of both the sandy and silty facies share significant similarities to those observed in many other contourites (Stow et al., 2008). The occurrence of Planulina wuellerstorfi, Bulimina aculeate, and Eggerella bradyi, as discussed earlier, indicates that bottom (contour) currents associated with the intrusion of the Northern Pacific Deep Water into the study area were active during the deposition of core TS01 (Tables 2 and 3). The along-slope bottom (contour) currents are generally considered as the persistence of the processes over very long time interval, and commonly involve a significant amount of water mass in large area. The turbidites in core TS01, were most likely reworked substantially by bottom (contour) currents associated with the intrusion of the Northern Pacific Deep Water into the South China Sea, leading the generation of reworked turbidites. In addition, all the observations listed above share great similarities to those documented in many other deposits formed by the interactions between down- and along-slope processes (Shanmugam et al., 1993; Viana et al., 1998; Mulder et al., 2006, 2008; Hernández-Molina et al., 2006, 2009; Stow, 2002; Stow and Faugères, 2008; Shanmugam, 2008; Viana, 2008). The reworked turbidites in core TS01 are, thus, interpreted as contourites (Figs. 6 and 7). 4.5. Results from analysis of piston core TS02 Piston core TS02 is located in sediment wave field 1 and consists of clayey silts and sandy silts (Fig. 6b). Key results observed from the core are summarized in the following: (1) Both planktonic and benthic foraminifera are relatively abundant within the sandy facies in the core (marked by the four gray zones in Fig. 6b). Foraminifera in these facies, commonly fragmented and abraded, consist of significant proportions of shallow water species. (2) The clayey silts and sandy silts are commonly fine grained with a mean size of 4e8 F, and are well sorted with a sorting coefficient of 1.5e2 (Fig. 6b). (3) Similar to core TS01, foraminifera assemblage as recognized in TS02 core also shows Quaternary deposits. (4) Stratigraphic inversion is not observed in core TS02 (Table 6). Similar to the observations in TS01, the common occurrence of shallow water foraminifera in core TS02 suggest that down-slope turbidity currents were active during the deposition of core TS02. The clayey silts and sandy silts in core TS02 therefore represent the deposits of turbidity currents. Additionally, vigorous bottom (contour) currents are evidenced in cores TS01 and TS02 as discussed earlier. This, coupled with the fact that bottom current activities usually involve significant water mass in a large area and persist for very long time interval, suggest that the turbidites in core TS02 were therefore substantially

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reworked by persistent actions of bottom (contour) currents. The silty facies in core TS02, thus, can be interpreted as silty contourites (Fig. 6b). The observations from cores TS01 and TS02 suggest that both down-slope turbidity currents and along-slope bottom (contour) currents induced by the intrusion of the Northern Pacific Deep Water have been active in the study area at least since the Quaternary (2.5 Ma). The development of the studied sediment waves, initiated at about 1.2 Ma as discussed below, are thus most likely strongly affected by the bottom (contour) currents resulted from the intrusion of the Northern Pacific Deep Water into the South China Sea.

5. Discussions and the depositional model 5.1. The genesis of the sediment waves 5.1.1. The genesis of the sediment waves in field 1 The sediment waves in field 1: (1) are closely linked to the slope channels and erosional gullies in the study area (Figs. 2, 3 and 9a); (2) have wave crests that regularly bifurcate and are roughly perpendicular to the directions of down-slope gravity flows and canyon axis (Figs. 2, 3 and 9a); and (3) display an asymmetrical morphology and internal discontinuities that suggest up-slope migration (Figs. 2e5a and 9a). Based on the criteria proposed by Wynn and Stow (2002), the sediment waves in field 1 are interpreted as turbidity current sediment waves. The crest lines of overbank sediment waves commonly extend subparallel to the channel or canyon axis (Migeon et al., 2000, 2004; Normark et al., 2002). Those in the studied sediment waves, in contrast, align roughly perpendicular to the canyon axis, suggesting that the development of the studied sediment waves is not linked to the repeated spillovers from the head and/or the body of a single turbidity current within the Taiwan Canyon and/or the Penghu Canyon. Additionally, the studied sediment waves are commonly developed immediately basinward of the mouths of slope channels and/or gullies. All of these indicate that the development of sediment waves in field 1 is most probably associated with the turbidity currents confined within the slope channels and/ or erosional gullies (Figs. 3 and 9a). As discussed above, the deposition of the studied sediment waves is also influenced by the along-slope bottom (contour) currents associated with the intrusion of the Northern Pacific Deep Water into the South China Sea (Fig. 9). The formation of the sediment waves in field 1, thus, is interpreted as the result of the interactions between down- and along-slope processes. 5.1.2. The genesis of the sediment waves in field 2 In contrast, sediment waves in field 2: (1) are associated with deep ocean basins away from the influence of turbidity currents (Figs. 1 and 2); (2) have wave crests that are parallel or subparallel to bathymetric contours and rarely bifurcate (Fig. 3); and (3) are more symmetrical with continuous, parallel to subparallel internal seismic-reflection configurations, indicating active vertical aggradation rather than up-slope migration (Fig. 5b). These features are consistent with those observed from bottom current sediment waves (Faugères et al., 2002; Flood and Giosan, 2002; Lewis and Pantin, 2002; Masson et al., 2002; Hohbein and Cartwright, 2006; MacLachlan et al., 2008). Damuth (1979) also suggested that the sediment waves in field 2 shared many similarities with bottom current sediment waves. Without solid evidence for the occurrence of bottom (contour) currents in the studied area, however, Damuth (1979) interpreted the sediment waves as turbidity current sediment waves.

5.2. The initiation of the sediment waves The base and top of the studied wave fields are defined by a wavy initiation surface and the seafloor respectively (Figs. 4 and 5). Within field 1, the initiation surface is characterized by a continuous high-amplitude reflection that corresponds to a regional unconformity. Strata underlying the unconformity show transparent, chaotic seismic reflections with variable amplitudes and continuity, and are interpreted as mass-transport complexes (Figs. 4b and 5a). The overlying sediment waves show very different seismic reflection characteristics and are intensely incised by the Taiwan Canyon and Penghu Canyon. The initiation surface can be correlated from field 1 to field 2 where the underlying strata show parallel or subparallel and continuous seismic reflections (Figs. 4b and 5). Wang et al. (2008) suggested that the average sedimentation rate in the study area is about 16.65 cm/ka. Average thickness of the documented sediment waves is 210 ms TWTT (two-way travel time). Using a velocity of 1560 m/s for the water column, the initiation surface and the initiation of the studied sediment waves, thus, correspond roughly to 1.2 Ma, when the arc-continent collision culminated on the Taiwan Island (Lallemand and Tsien, 1997). 5.3. The evolution of the sediment waves The overall geometries, morphology, and internal seismicreflection configurations of the sediment waves suggest three stages of the sediment wave evolution (Figs. 4 and 5). During stage 1, sediment waves in field 1 are characterized by continuous to discontinuous, moderate- to high-amplitude, and low-frequency reflections with some wavy geometries (Figs. 4 and 5a). Sediment waves in field 2 deposited during this stage 1, in contrast, show higher reflection continuities and amplitudes, suggesting vertical sediment draping and aggradation over the preexisting wavy initiation surface (Fig. 5b). This stage represents the early growth of the sediment waves, and the resultant sediment waves are thin with indistinct wavy geometries. Further upward into stage 2, more sediment waves with wavy geometries are observed in field 1. These sediment waves, still with low wavelengths and wave heights, migrated slightly up-slope (Figs. 4 and 5a). Similarly, small-scale sediment waves also developed in field 2 during this stage, those, however, show more symmetry morphology and vertical wave aggradation (Fig. 5b). Sediment waves developed during stage 3 in field 1 have distinct asymmetric wavy geometries that suggest significant upslope migration (Figs. 4 and 5a). Sediment waves reach maximum thickness of 250 m, and in response to up-slope migration there is higher deposition rates on the up-slope flanks and lower rates and/ or erosion on their down-slope counterparts (Figs. 4 and 5a). Sediment waves deposited during stage 3 in field 2, in contrast, consist of more continuous, high-amplitude reflections, and show symmetrical geometries, suggesting that vertical aggradation dominates over up-slope migration (Fig. 5b). The sediment waves in the study area may be still migrating and aggrading today, considering that the canyons are still active and no hemipelagic drapes are identified either on seismic-reflection profiles or on cores (Figs. 4 and 5). Once the sediment waves started growing, they became “fixed” in position during stage 1, and then they began to up-slope migrate in field 1 and vertically aggrade in field 2 during the following stages. From stage 1 to stage 3, sediment waves in both fields show vertical increase in wave dimensions (Figs. 4 and 5). A similar pattern was also observed by Migeon et al. (2004), and was inferred as the result of flow asymmetry over the development of the sediment waves through time.

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5.4. Depositional model 5.4.1. Upper slope: erosion and sediment bypass Previous studies have shown that the slope configuration plays important roles in controlling deposition along the slope-basin profile (Armitage et al., 2009; Gong et al., 2011). The upper slope of the study area is overall steep and narrow with an average width of 33 km and an average slope gradient of 5.6 (Fig. 9a and b). When moving down slope under such a high gradient, gravity flows (e.g. slides, slumps, and debris flows) usually have high energy. As a result, only very limited amounts of sediment will spillover these conduits; most of the sediments are, thus, transported basinward into deepwater settings where sediment waves occur (Fig. 9). These transported sediments are the source for the studied sediment waves, particular those in field 1. 5.4.2. Lower slope: interactions between down- and along-slope processes The slope gradient in the lower slope decreases to 1.6 . Gravity flows may lose part of their energy and transform progressively down slope from plastic and/or semiplastic fluids into turbidity currents due to this decrease in slope gradient and increase in fluid content (Fig. 9a and b). These turbidity currents actively interact with bottom (contour) currents associated with the intrusion of the Northern Pacific Deep Water into the South China Sea. Due to the lee-wave effect proposed by Flood (1988), when turbidity currents and bottom currents flow over the preexisting wavy topography, internal lee waves generated. The down-slope turbidity currents and/or bottom currents decelerate on the up-current flanks of the lee waves whereas accelerate on the downcurrent flanks, leading to the preferential deposition on the up-current flanks and up-slope migration of sediment waves as identified in field 1 (Figs. 4, 5a and 7). 5.4.3. Continental rise: predominant along-slope bottom current processes Further basinward, at the toe of the South China Sea Slope off southwestern Taiwan and near the continental rise, average slope gradient gradually decreases to less than 0.3 . Due to this decrease in slope gradient and further increase in fluid content, turbidity currents are most probably diluted and only very fine-grained sediments are transported to the continental rise by diluted turbidity currents. Bottom (contour) currents induced by the intrusion of the Northern Pacific Deep Water into the South China Sea, in contrast, are vigorous and most likely dominate over the diluted turbidity currents. When stratified bottom currents flow over the preexisting wavy topography, internal lee waves generated. Due to the lower flow velocity of the bottom currents, vertical aggradation is more active and predominant, leading to the development of the symmetrical sediment waves as observed in field 2 (Figs. 2, 5b and 9). 6. Implications for paleoceanography The results of this study further support the existence of bottom (contour) currents induced by the intrusion of the Northern Pacific Deep Water into the South China Sea, as evidenced by the occurrence of the benthic foraminifera associated with the Northern Pacific Deep Water in cores TS01 and TS02. A preliminary estimation of the direction of the bottom (contour) currents in the study area is made based on the orientations and cross-section geometries of the sediment waves and the lee-wave model proposed by Flood (1988). Considering that the development of the studied sediment waves is probably controlled by the interactions of down-slope

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turbidity currents, along-slope bottom (contour) currents, and the lee-wave effects. The turbidity currents and/or the along-slope bottom (contour) currents should thus run perpendicular or oblique to the elongation of the sediment waves. The bottom (contour) currents, therefore, most probably have flowed from ENE to WSW, oblique to the NEeSW elongation of the studied sediment waves (Fig. 9). The lack of the detailed chronological calibration, however, prevents making further estimations of the paleoceanographic details in the study area. 7. Conclusions (1) Using a high-quality 2D seismic, multi-beam bathymetry, piston core, and AMS 14C dating data, this study identified two sediment wave fields in the South China Sea Slope off southwestern Taiwan. Sediment wave field 1, with an area of approximately 2880 km2, is located in the lower slope; sediment wave field 2, with an area of about 2738 km2, is further basinward near the continental rise. (2) The development of the sediment waves in field 1 is interpreted as the result of the interactions between down-slope turbidity currents and along-slope bottom (contour) currents. In contrast, the development of the sediment waves in field 2 is mainly controlled by the WSW flowing bottom (contour) currents induced by the intrusion of the Northern Pacific Deep Water into the South China Sea. (3) The studied sediment waves are built upon an initiation surface, a regional unconformity, across which there is a change in sedimentology from mass-transport complexes below into sediment waves above. Three stages of sediment wave evolution are identified, and from the oldest to the youngest stages sediment waves grow progressively in both of these two fields and increase in wave dimensions. The difference, however, is that sediment waves grow preferentially upslope in field 1 due to the lee-wave effect and the interactions between down- and along-slope processes, whereas those in field 2 are dominated by vertical aggradation as a result of bottom (contour) currents. (4) Data and the depositional model show that the upper slope of the study area, with high average slope gradient of 5.6 , is characterized by intense gravity flow erosion and sediment bypass. In the lower slope where average slope gradient drops to 1.6 , however, there are clear interactions between downslope turbidity currents and along-slope bottom (contour) currents, resulting in the development of the sediment waves in field 1. Further basinward onto the continental rise, turbidity currents are most probably diluted as a result of the further decrease in slope gradient down dip and increase in fluid content. Bottom (contour) currents associated with the intrusion of the Northern Pacific Deep Water into study area, in contrast, are the dominant processes, and probably dominate over the diluted turbidity currents, leading to the development of the sediment waves in field 2. (5) Results from this study provide further evidence for the activities of bottom (contour) currents induced by the intrusion of the Northern Pacific Deep Water into the South China Sea Slope off southwestern Taiwan. Preliminary estimations show that the bottom (contour) currents most probably have flowed from NNE to WSW. Acknowledgments We gratefully thank the Guangzhou Marine Geological Survey for providing the data and for their permission to publish the results of this study. The study is also supported by the National

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Natural Science Foundation of China (No. 40972077), the National Basic Research Program of China (No. 2009CB219407), the Map Series of Geology-geophysics of China Sea and Adjacent Areas, and the Marine Geological Survey of Shantou, South China Sea (Scale: 1:1,000,000). We also would like to appreciate editor Thomas Hadlari and two anonymous reviewers for their insightful reviews and constructive suggestions to make the paper a better contribution.

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